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 Z. SMITH REYNOLDS 
 FOUNDATION 
 
 COLLECTION IN 
 SCIENCE AND TECHNOLOGY 
 
 G & 
 
 ^, 
 
 HEXRYB.H.BKArKOY. F.R.S. 
 
Digitized by tine Internet Arciiive 
 
 in 2010 witii funding from 
 
 NCSU Libraries 
 
 Iittp://www.arcliive.org/details/panoramaofscienc01smit 
 
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 Zabonoux //>,■ ^;/. /.;•/>/,;> ,f JLc/umun/ a,nt Sru/myi'c Inn ntivn: 
 
PANORAMA 
 
 OF 
 
 SCIENCE AND ART; 
 
 EMBRACINC 
 
 OF 
 
 AEROSTATION, AGRICULTURE AND GARDENING, ARCHITECTURE, ASTRONOMY, 
 
 CHEMISTRY, ELECTRICITY, GALVANISM, HYDROSTATICS AND HYDRAULICS, 
 
 MAGNETISM, MECHANICS, OPTICS, AND PNEUMATICS : 
 
 OF 
 
 Building, Brewing, Bleacliing, Clockwork, DiHtillation, Dyeing, Drawing, Engraving, 
 
 Gilding and Silvering, Ink-making, Japanning, Lacquering, Millwork, Moulding 
 
 and Casting in Plaster, Fainting, Staining Glass,^Stuiuijig Wood, 
 
 and \ arnisliing : 
 
 THE METHODS OF 
 
 WORKING IN WOOD AND METAL, 
 
 APPLICABLE IN 
 
 ANNEALING, BORING AND DRILLING, FILING, GRINDING, TEMPERING STEF.L, MAKIN1 
 CREWS, SOLDERING, COMMON AND ELLIPTIC TURNING, &C. 
 
 AND A 
 
 MISCELLANEOUS SELECTION 
 
 OF 
 
 INTERESTING AND USEFUL PROCESSES AND EXPERIMENTS. 
 
 BY JAMES SMITH. 
 
 fflSait^ ifortg=niur illiistratibe iSnfftaDtngfl, tij) ftm'ncnt arttsts, 
 
 IN TWO VOLUMES.— VOL. I. 
 
 EDITION. 
 
 LONDON: 
 
 PRINTED AT THE CAXTON PRKSS, BY HENRY FISHER, 
 
 (Printer in Ordinary to his Majesty.) 
 
 Sold by all Uie Booksellers in the United Kingdom. 
 
 1823. 
 
PREFACE. 
 
 Among the earliest indications of a mind sus- 
 ceptible of attaining general knowledge, appears that 
 which consists in the delight afforded by mechanical 
 performances. With this disposition, opportunities 
 of acquiring the means of imitative execution, and of 
 perceiving the advantages derivable from the posses- 
 sion of scientific principles, eminently tend to establish 
 that activity of mind, and that predilection for the 
 acquisition of useful knowledge, which beneficially 
 influence the character through life, and present un 
 failing sources of enjoyment for the hour of solitude 
 and leisure. 
 
 If those who are ingenious and skilful in the per- 
 ception of scientific truth, were practically conversant 
 with the manufacture of every instrument they use, 
 the progression of the sciences would be materially 
 accelerated. The theorist, however excellent and 
 original his views, inevitably suffers many of his plans 
 to perish, when mechanical dexterity is requisite ta 
 
 63. Vol. I. b 
 
iv PREFACE. 
 
 verify them, because he cannot contend with the 
 expense or the difficulty of having them executed by 
 others, or lias no adequate knowledge of the degree 
 in which they may be realized. The science of 
 chemistry, in the course of a few years, has risen to 
 a pre-eminent station of utility and splendour ; and 
 among the leading causes of its advancement, it is 
 observable, that the mechanics who have performed 
 the operations of the laboratory, and contrived and 
 executed the novel part of the apparatus, were the 
 very individuals, who, as men of science, were ac- 
 tuated by expanded views and the zeal of research. 
 Dr. Herschels observations and discoveries are the 
 most valuable which a single individual has ever 
 made in the departments of astronomy, requiring the 
 aid of instruments ; and we find upon investigation, 
 that the telescopes constructed by himself excel all 
 others : even his telescope of forty feet, which is one 
 of the most splendid trophies of mechanical skill 
 existing, was wholly executed under his own direc- 
 tion, by common artisans or labourers. 
 
 In adverting to these considerations, the Editor 
 concluded to allot the first portion of these volumes, 
 to the most general and radically important opera- 
 tions in working wood and metal. Architecture and 
 Building follow, after which, to the page which ter- 
 minates the essay on Gardening, the sciences and their 
 correlative arts are taken up according to the order 
 of succession in which they may be conveniently 
 studied. By this arrangement. Drawing and its 
 dependents are stationed near the conclusion of the 
 
PREFACE. 
 
 work, which is closed by a selection of Miscellanies, 
 comprising subjects adapted rather for reference than 
 regular perusal. 
 
 Historical sketches of the rise and progress of the 
 sciences have been introduced, when they appeared 
 calculated to interest the general reader, to inculcate 
 useful truths, or to facilitate the apprehension of sub- 
 sequent facts and principles. 
 
 Of subjects within the scope of this publication, 
 where minuteness of investigation could not be en- 
 tered upon, an attempt has been made to delineate 
 the most important features : of those which had yet 
 received no popular illustration, the Editor has been 
 especially solicitous to seek new information; and in 
 aid of this object, he has been favoured by a friend 
 with the communication of an entire essay. He 
 alludes to the treatise on Architecture, which is the 
 production of Thomas Rickman, of Liverpool, whose 
 system of English architecture he cannot doubt will 
 be acceptable to those who are interested in the sub- 
 ject, or who have pleasure in contemplating the mo- 
 numents of vernacular skill. 
 
 The Editor has occasionally availed himself of the 
 language of the ablest writers, where it appeared most 
 conducive to his purpose. Of entire extracts, he has 
 had the pleasure of specifying the source ; of those 
 which have been assimilated to his views by altera- 
 tions, or taken in a detached and limited manner; 
 where the information was of general notoriety, or its 
 
Vi PREFACE. 
 
 origin uncertain, acknowledgment became superfluous 
 or impracticable. 
 
 Convinced that no permanent advantages accrue 
 from attempts to deprecate censure, or to angle for 
 praise, the Editor waves all digression from the ge- 
 neral exposition of his plan ; but aware that a pub- 
 lication comprising so large a variety of subjects, can- 
 not be free from imperfections, he terminates these 
 prefutory remarks with assuming the liberty to ob- 
 serve, that the communication of hints for emendations 
 will receive his thanks. 
 
CONTENTS OF VOL. I. 
 
 Page 
 MECHANICAL EXERCISES. 
 
 Of Iron 1 
 
 Forging and welding 10 
 
 The tools u&ed in forging iron, and the working of 
 
 metals generally 14 
 
 Boring and drilling 18 
 
 Filing 22 
 
 Grinding 32 
 
 Annealing 36 
 
 The straight-edge and square 37 
 
 Screws 39 
 
 Of the most common metals. — Copper, 41— Tin, 44 
 
 — Lead, 46 — Zinc 49 
 
 Soldering , , , , , . , 50 
 
 Of TURNING 53 
 
 Of the foot-lathe, 53 — chucks, 60 — The boring and 
 mandrel collars, 61 — The hand -lathe or turning 
 bench, 62 — The pole-lathe, Q5 — The tools used in 
 turning, 67 — The parallel rest, 69 — Cutting screws 
 in the lathe, 72-:r-Elliptic turning, 76 — Miscellaneous 
 remarks on turning 79 
 
 Of TIMBER, 85 
 
 Of Saw-mills, 98 — Curving wood, 99 — An advanta- 
 geous method of converting the trunks of trees 
 into square timber, 102 — The tools used in work- 
 ing wood, 104 — Saws, 105 — Planes, 107 — Chisels, 
 112— Boring tools, 113— The Hammer, Mallet, 
 Square, Bevel, Mitre-square, Gauge, Straight-edge, 
 Winding-sticks, 116 — Glue, 120 — The mortise and 
 tenon, 122 — Reflections 123 
 
 ARCHITECTURE. 
 
 English Architecture 129 
 
 The first, or Norman Style. 
 Of Norman doors, 134 — Windows, 135 — Arches, - 
 136— Piers, 136— Buttresses, 137— Tablets, 137 
 — Niches, 137— Ornaments, 137 — Steeples, 138 
 
 The Second, or early English Style. 
 Of early EngHsh doors, 139 — Windows, 140 — Arches, 
 141— Piers, 141— Buttresses, 142— Tablets, 142 
 Niches, 143 — Ornaments, 144--Steeples, 144 
 
VUl CONTEXTS. 
 
 ARCHITECTURE, conUnued. 
 English Architecture. 
 
 Tfie tlurd, or Decorated Style. 
 Decorated English doors, 145 — Windows, 147 — 
 Arches, 149— Piers, 149— Buttresses, 149— Ta- 
 blets, 150 — Niches, 150 — Ornaments, 151^ 
 Steeples, 151 
 
 The fourth, or Perpendicular Style. 
 
 Perpendicular English doors, 153 — Windows, 153 
 — Arches, 155 — Piers, 155 — Buttresses, 156 — 
 Tablets, 157 — Niches, 157 — Ornaments, 158 — 
 Steeples, 159 — Miscellaneous remarks on perpen- 
 dicular buildings 1 59 
 
 Of Battlements 160 
 
 Roofs 161 
 
 Miscellaneous remarks 165 
 
 Description of plates I. and II 167 
 
 Oi Gkecian Akchitectvrb, 170 — Tuscan order, 
 173 — Doric order, 175 — Grecian Doric, 175 — 
 Roman Doric, 176 — Ionic order, 177 — Corinthian 
 order, 179 — Composite order, 180 
 
 BtriLDlNG 182 
 
 Of Bricks, 182 — The tools used in bricklaying, 192 
 — Foundations, 196 — Cements or mortar, 198 — 
 
 Brick bond and walling 209 
 
 Masonry 216 
 
 Of the mason's tools, 216 — Stones, 218 — Cements, 
 221 — Stone walls, 222 — Sloue columns, 224 — 
 
 Stone bridges 227 
 
 Miscellaneous remarks relative to building 241 
 
 Of the situation and plan of houses, 241 — Rooms, 
 243 — Chimneys, 245 — Doors, 251 — Windows, 
 251— Stairs, '253— Roofs, 254— floors, 258— 
 The proportions of timbers for small buildings, 
 260 — The proportions of timber for large 
 
 buildings 261 
 
 Building Act, 263 — Kvternal walls, 264 — Party walls, 
 
 265— Chimneys, 267 
 
 MECHANICS. 
 
 Of Matter 269 
 
 Motion 281 
 
 Laws of motion 289 
 
 Central forces 290 
 
 Centre of gravity 290 
 
 The mechanical powers, 293 — The lever, 295 — The 
 balance, 300 — The steel-yard, 305 — The pulley, 
 306 — The wheel and axle, 311 — The inclined 
 plane, 316 — The wedge, 317 — The screw, 319 
 —The endless screw, 321 
 
CONTENTS. ix 
 
 MECHANICS, continued. "^^ 
 
 Of compound machines 323 
 
 Fly wheels 334 
 
 Friction , 337 
 
 The application of men and horses, as moving powers 
 
 in machinery, 343 
 
 Mill-work 345 
 
 Practical rules and observations relative to mills, 347 
 — The millwright's table, 348 — Method of setting 
 out wheels, 353 — Bevei-g-eer, 354 — Hooke's uni- 
 versal joint, 355 — The cycloid and epicycloid, 
 and the. formation of the teeth of wheels, 355 — 
 Wipers for raising stampers and hammers, 362 — 
 
 Description of a corn-mill, 363 
 
 Of Wheel-carriages 368 
 
 Clock-work 374 
 
 Mode of dividing the circumference of circles, 383 — 
 Proportioning the diameters of wheels and pinions, 
 385 — The pendulum, 389 
 
 Abstract of mechanics 396 
 
 OPTICS. 
 
 Historical remarks 407 
 
 Definitions 411 
 
 Of Light, 412— The refrangibility of light, 420— The 
 reflexibility of light, 431 — The different refrangi- 
 bility of the rays of light, 439 — The rainbow, and 
 the natural phenomena dependent upon the refran- 
 gibility of light, 443 — The inflection and deflection 
 
 of light, 447 
 
 The eye and the general phenomena of vision. ..... 451 
 
 Optical instruments. 
 
 Of spectacles, 460 — Burning lenses and mirrors, 463 
 — Camera obscura, 466 — Magic lantern and phan- 
 tasmagoria, 469 — The single microscope, 470 — 
 Ellis's single aquatic microscope, 475 — Hand me- 
 galscope, 476 — Double or compound microscope, 
 477 — The solar microscope, 478 — The opaque 
 microscope, 479 — Microscopic objects, 479 
 
 Of telescopes, 483— The astronomical telescope, 484 
 — The night-glass, 486— The terrestrial telescope, 
 or perspective glass, 487— The Galilean telescope, 
 487 — Achromatic and aplanatic object-glasses, 488 
 — The reflecting telescope, 491 — Gregorian tele- 
 scope, 492 — Newtonian telescope, 493— Herschel's 
 telescope, 494— Cassegrainian telescope, 495— Of 
 the magnifying power of telescopes, 495— Miscella- 
 neous remarks on telescopes, 496 
 
 Micrometers 500 
 
 Abstract of optics , 502 
 
 . 64.— Vol. Lb 
 
X CONTEXTS. 
 
 J 
 
 Page 
 
 ASTRONOMY. 
 
 Hisiorical remarks 505 
 
 Detiiiitions 510 
 
 Of the apparent motions of the heavenly bodies 513 
 
 The figure, motion, and magnitude of the earth — 
 Magnitude and distance of the sun and moon, 
 and of planetary motion 519 
 
 Of the solar system. 531 — The sun, 532 — Mercury, 
 538— Venus', 539— The earth, 541— The moon, 
 546 — Mars, 552 — Vesta, 554 — Juno, 554 — Ceres 
 Ferdinandea, 554 — Pallas, 555 — Jupiter, 556 — 
 Satiim, 558— Herschel, 5G2 
 
 Synopsis of the solar system. — Distances of the planets 
 from the earth, 563 — Diameter of the sun and 
 planets, 563 — Density of the sun and planets, 564 
 — Quantity of matter in the planets, 564 — Number 
 of feet through -which a heavy body would fall at 
 the surfaces of the planets, 564 — Rotation of the 
 sun and planets round their axes, 564 — Tropical 
 revolutions of the planets, 565 
 
 Of comets 565 
 
 Fixed stars 571 — 585 
 
 Constellations of the zodiac 575 
 
 Constellations on the north side of the zodiac .... 575 
 Constellations on the south side of the zodiac .... 576 
 
 The tides 586 
 
 The harvest moon and hunter's moon 595 
 
 The horizontal moon 600 
 
 The aberration of light ^. 602 
 
 Nutation of the eailh's axis 605 
 
 Precession of the equinoxes 605 
 
 Eclipses 6(17 
 
 Echpses of the moon, 607 — Eclipses of the sun, 
 609 — Description of a total eclipse of the sun, 613 
 
 Division of lirae. 617 
 
 Of the day, 617— The week, 619— The month, 619 
 — The year, 620 — The lunar and solar cycles 
 and indiction, 622 — Epochs and eras, 623 
 
 Abstract of Astronomy 624 
 
Directions far placing the Plates in Vol /. 
 
 MECHANICAL EXERCISES Plate .. to face page 96 
 
 II 96 
 
 III 9U 
 
 IV..... 102 
 
 ARCHITECTURE ,... I 168 
 
 II ., 168 
 
 III 172 
 
 IV 180 
 
 V 180 
 
 Bl'ILDING 1 254 
 
 II 254 
 
 MECHANICS I 306 
 
 II 312 
 
 III 328 
 
 IV...'. 360 
 
 V 374 
 
 VI , 394 
 
 OPTICS 1 432 
 
 II 432 
 
 m 438 
 
 IV 464 
 
 V 476 
 
 VI , 488 
 
 vn 500 
 
 ' ASTRONOMY (solar system) r ......1 532 
 
 II 528 
 
 m 547 
 
 IV 604 
 
 V 612 
 
Directions for placing the Plates in Vol. II. 
 
 PNEUMATICS Plate T. to face page 24 
 
 II 26 
 
 III 40 
 
 HYDROSTATICS AND HYDRAULICS....! 88 
 
 II 116 
 
 III 128 
 
 STEAM ENGINE 140 
 
 AEROSTATION— MAGNETISM 186 
 
 ELECTRICITY... I 218 
 
 II 230 
 
 III 268 
 
 GALVANISM 290 
 
 CHEMISTRY I 310 
 
 II 322 
 
 BREWING— DISTILLING 582 
 
 DRAWING I 712 
 
 II 716 
 
 III 720 
 
 IV 720 
 
 MISCELLANIES 816 
 
THE 
 
 PANORAMA 
 
 OF 
 
 s^mawcsia Amm Aiim^ 
 
 MECHANICAL EXERCISES. 
 
 Of Iron. 
 
 V/F all metallic substances, iron isthe most abundantly dif- 
 fused, and the most intrinsically valuable. It may be detected 
 in plants, and in animal fluids ;* it is the chief cause of colour 
 in earths and stones; sands, clays, the waters of rivers, springs, 
 rain, and snow, are seldom perfectly free from it; and in several 
 parts of the world, whole mountains are composed of iron ore. 
 If the use of this metal were lost to mankind, the arts and 
 sciences would dwindle into insii;nificance, and civilization it- 
 self become rapidly retrogressive. An inquiry, therefore, into 
 the properties and means which render it subservient to such 
 important purposes, will not, it is presumed, prove uninterest- 
 ing to the general reader; while the prudent artisan, whose first 
 care is generally to provide himself with tools adapted to his 
 labours, will attentively review every hint which may improve 
 his knoAvledge of that material, the proper choice and manage- 
 ment of which constitute the first step towards success in 
 mechanical pursuits. 
 
 Iron is employed in three states, viz. that oicast iron, wrcmght 
 iron, and steel. Cast iron is the metal in its first state, rendered 
 fusible by its combination with those two substances which 
 chemists distinguish by the names of carbon and oxygen. In 
 the great ironworks, the ore, broken into small pieces, and mixed 
 with a portiont)f limestone to promote its fusion, is thrown into 
 the furnaces, which are from sixteen to thirty feet high. Baskets 
 of charcoal or coke, in due proportion, are thrown in along with 
 it. A part of the bottom of the furnace is filled with fuel only. 
 This being kindled, the whole is raised by the blast of the 
 
 * Metallic grains of iron have been found in strawberries ; and one- 
 twelfth part of the weight of dried oak wood is said to consist of this 
 metal. — The blood contains much iron, to wliich it owes its red colour, 
 1.— Vol. I. B 
 
MECHANICAL EXERCISES. 
 
 Munufacture of cast and wrought iron. 
 
 great bellows, to a most intense heat. The metal, as it is re- 
 duced, sinks down through the fuel, and collects at the bot- 
 tom of the furnace. More ore and fuel are supplied above, 
 and the operation goes on till the melted metal, increasing in 
 quantity, rises almost to the aperture of the blast ; a passage 
 is then made for it at the side of the furnace, and it is run into 
 what are called pigs of cast iron. A furnace will furnish daily 
 from two to five tons of iron, according to the richness of the 
 ore, and the skill with which the operation is conducted. 
 Ores of iron combined with magnesia, are very refractory, and, 
 as well as those which contain sulphur and arsenic, require to 
 be roasted before they are cast into the smelting furnace. 
 
 Pig iron is of very different qualities; that which is called 
 No. 1, and the fracture of which is of a dark colour, runs so 
 fluid as to be admirably suited for grates, and ornamental 
 work. Cast-iron cutlery is manufactured from it, as no other 
 would run fine enough for the purposes to which it is applied, 
 such as forks and small scissars, fish hooks and needles. 
 These articles obtain by annealing a considerable degree of 
 malleability, and are even rendered capable of being welded. 
 When great strength is required, as for large wheels, beams, 
 pillars, or railways, the iron which contains a smaller propor- 
 tion of carbon is preferable, as that called No. 2. The propor- 
 tion of carbon in cast iron varies in the different sorts, from 
 one-fifteenth to one-twenty-fifth. Cast iron also frequently 
 contains a portion of the phosphuret of iron, in which case it 
 breaks of a white colour, and must, from its excessive hardness, 
 be rejected for purposes which require it to be filed, or turned, 
 or cut with the chisel. It may be observed, that the whiter 
 the metal is, the harder it is also; whether these properties are 
 owing to its quality, or the mode of its management. Cast 
 iron expands in passing frcm the fluid to the solid state ; it 
 consequently assumes the exact figure of the mould into which 
 it is poured, a circumstance whicn adds greatly to its value 
 for casting. 
 
 Crude or cast iron is converted into wrought iron, by keep- 
 ing it in a state of fusion for a considerable time, and repeat- 
 edly stirring it in the furnace ; the oxygen and carbon which 
 it contains, unite, and fly off in the state of carbonic acid gas, 
 and as this takes place the iron becomes more infusible ; it 
 gets thick or stiff in the furnace ; and the w^orkmen know, by 
 this appearance, that it is time to submit it to the repeated 
 action of the hammer, or the regular pressure of large steel 
 rollers, by which the parts which still partake of the nature of 
 crude iron so much as to retain the fluid state, are forced out, 
 *nd the metal is rendered malleable, ductile, more closely com- 
 
MECHANICAL EXERCISES. 
 
 Qualities of wrouglit iron. — Wanufaoture of blistered steel. 
 
 pacted, of a fibrous texture, and totally infusible. In this state 
 it is known in commerce by the name of bar iron. The loss 
 of weight sustained by iron, in the process of refining, is 
 considerable, generally amounting to one-fourth, and some- 
 times to one-half. 
 
 Forged, like cast iron, varies greatly in quality. Thus 
 some of it is tough and malleable both when it is hot and 
 when it is cold. This is the iron in common use, and it is 
 the best and most useful. It maybe known generally by the 
 equable surface of the forged bar, which is free from trans- 
 verse fissures, or cracks in the edges; and by a clear, white, 
 small grained, or rather fibrous texture. The best and tough- 
 est iron is that which has the most fibrous texture, and is of 
 a clear grayish colour. This fibrous appearance is given bv 
 the resistance which its particles make to separation. The 
 texture of the next best iron, which is also malleable in all 
 temperatures, consists of clear whitish small grains, intermix- 
 ed with fibres. Another kind is tough when it is heated, but 
 brittle when cold. This is called cold-short iron, and is dis- 
 tinguished by a texture consisting of large shining plates, 
 without any fibres. It is less liable to rust than any other 
 description of forged iron. A fourth kind of iron, called red- 
 short, or hot-short, is extremely brittle when hot, and malle- 
 able when cold. On the surface and edges of the bars of 
 this kind of iron, transverse cracks or fissures may be seen, 
 aiid its internal colour is dull and dark. 
 
 The quality of iron may be much improved by violent com- 
 pression, as by forging and rolling, especially when it is not 
 long exposed to violent heat, which injures and at length 
 destroys its metallic properties. But though iron is rendered 
 malleable by hammering, this 0|)eration may be continued so 
 long, as to deprive it of its malleability. 
 
 Steel is made of the ))urest malleable iron, by a process 
 called cementation. In this operation, layers of bars of malle- 
 able iron, and layers of charcoal, are placed one upon another, 
 in a proper furnace, the air is excluded, the fire raised to a 
 considerable degree of intensity, and kept up for eight or ten 
 days. If upon the trial of a bar, the whole substance is con- 
 verted into steel, the fire is extinguished, and the whole is 
 left to cool for six or eight days longer. Iron thus prepared 
 is called blistered steel, from the blisters which appear on its 
 surface. In England, charcoal alone is used for this purpose; 
 but Duhamel found an advantage in using from one-fourth to 
 one-third of wood ashes, especially when the iron was not of 
 so good a quality as to afford steel possessing tenacity of 
 body as well as hardness. These ashes prevent the steel- 
 
MECHANICAL EXERCISES. 
 
 Teraperin^j steel hy colour. 
 
 makinur process from being effected so rapidly as it would 
 othf rwise be, and give the steel pliability without diminishing 
 its hardness. The blisters on the surface of the steel, under 
 this manatjement, are smaller and more numerous. He also 
 found that if the bars, when they are put into the furnace, be 
 Bpriukled with sea salt, this ingredient contributes to give 
 bodv to the steel. If the cementation be continued too long, 
 the steel becomes porous, brittle, of a darker fracture, more 
 fusible, and incapable of being welded. On the contrary, 
 steel cemented with earthy infusible powders, is gradually 
 reduced to the state of forged iron again. Excessive or re- 
 peated heatino; in the forge is attended with the same effect. 
 
 The properties of iron are remarkably changed by cementa- 
 tion, and it acquires a small addition to its weight, which con- 
 sists of the carbon it has absorbed from the charcoal, and 
 amounts to about the hundred and fiftieth or two hundredth 
 part. It is much more brittle and fusible than before ; though 
 it may still be welded like bar iron, if it has not been fused, 
 or over cemented ; but by far the most important alteration 
 in its properties, is, that it can be hardened or softened at 
 pleasure. If it be made red hot, and instantly cooled, it at- 
 tains a decree of hardness, which is sufficient to cut almost 
 every other substance ; but if heated and cooled gradually, it 
 becomes nearly as soft as pure iron, and may with much the 
 same facility be manufactxired into any determinate form. A 
 rod of good steel, in its hardest state, possesses so little te- 
 nacity that it may be broken almost as easily as a rod of glass 
 of the same dimensions. This brittleness can only be dimi- 
 nished by diminishing its hardness, and in the proper manage- 
 ment of this point, for difierent purposes, consists the art of 
 tempering. The colours which successively appear on the 
 surface of steel slowly heated, are, yellowish white, yellow or 
 straw colour, gold colour, brown, purple, violet, and deep blue. 
 These sij^ns direct the artist in reducino; the hardness of steel 
 to any particular standard. If steel be too hard, it will not 
 be proper for tools which are intended to have a fine edcre, 
 because it will be so brittle, that the edge will soon become 
 notched ; if, on the contrary, it be too soft, it is evident that 
 the edge will turn or bend. Some artists inclose the tools to 
 be hardened, in an iron case or box, and slowly heat them to 
 ipnition ; they then take the case out of the fire, and drop the 
 pieces into water, in such a manner as will allow them to come 
 as little as possible into contact with the air. This method 
 answers two good purposes ; it causes the heat to be more 
 equally ap])lied, and prevents the scaling occasioned by the 
 contact of the air. When the work has been pohshed, and 
 
MECHANICAL EXERCISES. 
 
 ManuTacture of cast steel. 
 
 well defended from the air, it is, when hardened, nearly as 
 clean as before. If the tool be unpolished, they brighten 
 its surface upon a stone ; it is then laid upon burning char- 
 coal, or upon the surface of melted lead, or upon an ignited bar 
 or plate of iron, till it acquires the desired colour; at which 
 instant they plunge it into cold water. The yellowish white 
 indicates a temper so little reduced as to be used for few edge 
 tools; the yellow or straw colour, the gold colour, and the 
 brown, are used for penknives, razors, and gravers; the pur- 
 ple for tools used in working upon metals, especially iron; the 
 violet for springs, and for instruments used in cutting soft sub- 
 stances, such as cork, leather and the like; but if the last blue 
 be waited for, the hardness of the steel will scarcely exceed 
 that of iron. When soft steel is heated to any one of these co- 
 lours, and then plunged into water, it does not acquire nearly 
 so great a degree of hardness, as if previously made quite hard, 
 and then reduced by tempering. The degree of ignition re- 
 quired to harden steel is different in the dift'ereut kinds. The 
 best kinds require only a low red heat. It has been inge- 
 niously supposed, that the hardness of steel depends on the 
 intimate combination of its carbon; and, on this supposition, 
 it follows, that the heat which effects this is the best, and that 
 a higher degree will be injurious. 
 
 The texture of steel is rendered more uniform by fusion. 
 When it has undergone this operation, it is called cast steel ; 
 which is wrought with more difficulty than conmion steel, be- 
 cause it is more fusible, and is dispersed under the hammer, 
 if heated to a white heat. The cast steel of England is made 
 from the fragments of the crude steel of the manufactories and 
 steel works. A crucible, about ten inches high, and seven 
 inches in diameter, is filled with these fragments, and ])laced 
 in a wind furnace, like that of the founders, but smaller, be- 
 cause intended to contain one pot only. It is likewise fur- 
 nished with a cover and chimney, to increase the draught of 
 air. The furnace is entirely filled with coke, and five hours 
 are required for the perfect fusion of the steel. It is then cast 
 into ingots, and afterwards forged in the same manner as other 
 steel, but with less heat and more precaution, as it is more 
 liable to break. Cast steel is about thirty per cent, dearer 
 than other good steel. Its uniformity of texture is for many 
 works an invaluable advantage. It is daily more and more 
 used in this country, but must necessarily be excluded from 
 many works of considerable size, on account of the difficulty 
 of welding it, and the facility with which it is degraded in the 
 fire. Cast steel takes a fine firm edge, and receiving an ex- 
 quisite polish, of which no other sort of steel is, in so high & 
 
MECHANICAL EXERCISES. 
 
 Tempering steel willi tallow, oil, £>.c. 
 
 degree, susceptible, it is made use of for all the finest cutlery ; 
 but though it may be cast into ingots, it is too imperfectly 
 fluid to be cast into small wares. The tenacity of steel ham- 
 mered at a low heat, or even when cold, is considerably in- 
 creased ; but the effect of this hammering is taken off by strong 
 io-nition. Tools, therefore, made of cast steel, and intended 
 to sustain a good edge, for cutting iron and other metals, are 
 not afterwards softened, but the ignition is carefully regulated 
 at first, as the most useful hardness is produced by that degree 
 of heat which is just sufficient to eflect the purpose. Cast 
 steel, annealed to a straw colour, is softened nearly as much 
 as other kinds to a purple or blue. 
 
 A convenient mode of tempering a great number of articles 
 at once, and of heating them uniformly, however irregular their 
 shape, is to put them into a proper vessel with as much oil or 
 tallow as will cover them, and then to place them over the fire, 
 or the flame of a lamp, until a sufficient heat is given. Clock 
 and watch pinions, watch verges, &c. are tempered in this man- 
 ner, sometimes many dozens at once, as expeditiously as a single 
 article. The requisite temper may be known by the following 
 circumstances : when the tallow is first observed to smoke, it 
 indicates the same temper, as that called a straw colour. This 
 will reduce the hardness but little ; but if the heat be continued 
 till the smoke becomes more abundant, and of a darker colour, 
 it will be equal to a brown, and indicates a temper that may be 
 ■wrought, that is, turned or filed, though with difficulty, and 
 only when a mild sort of steel is employed. If the tallow be 
 heated so as to yield a black smoke, and still more abundant, 
 this will denote a purple temper ; and if the steel be good, it 
 will now work more pleasantly, though still hard enough to wear 
 well in machinery. The next degree of heat may be known by 
 the tallow taking fire, if a lighted body be presented to it, but 
 yet not so hot as to continue to burn when the light is w ith- 
 drawn; and this will denote a blue. If the whole of the tallow 
 be allowed to burn away, or burn dry, as termed by the workmen, 
 it imparts the temper which clockmakers mostly use for their 
 work. Further tallow is useless ; a small degree of heat more 
 would just be seen in a dark place, or the lowest degree of red 
 heat. Any single article, to spare the trouble of brightening its 
 surface, may be smeared with oil or tallow, and its temper, 
 when heated, ascertained in a similar manner. Small articles, 
 such as ])endulum and other springs, need not be dropped into 
 water, but only made to pass through the air, by tossing them 
 out, and letting them full to the ground, which will make them 
 hard enough for most purposes. Small drills may be hardened 
 by holding their points in the flame of a candle, and, when suf- 
 
MECHANICAL EXEKCISES. 
 
 Effects of saline liquids aud mercury, in hafdeiiing steel. 
 
 ficiently hot, suddenly plucking them out ; the air will harden 
 them ; or they may be laid upon a plate of cold iron or lead, and 
 another plate upon them. They may be tempered, if found too 
 hard, by taking a little of the tallow upon their point, then 
 passing them through the flame at about half an inch above the 
 point, and holding them there till the tallow begins to smoke. 
 
 Solid tallow is an excellent material for hardening drills, and 
 other articles, which require considerable hardness, but must 
 not be made brittle ; tallow differs from oil, in the absorption oi 
 heat for its fusion. Oil is found to harden the surface of stei^l 
 much more than its internal part, so that it resists the file, but 
 is much less easily broken by the hammer. This effect is owing 
 to its imperfectly conducting quality, and the elevated tem- 
 perature it demands to be converted into the vaporous state ; 
 a covering of coal is also formed round the steel by the burned 
 oil, which greatly retards the transmission of the heat. Any 
 other fluid which covers the steel in like manner, for instance 
 water holding soap in solution, produces a similar effect. Hence 
 the vehicle in which ignited steel is plunged, is of great con- 
 sequence. The colder it is, the more effectually it hardens the 
 metal. Various artists avail themselves of different substances 
 for this pvirpose. Some use urine, others water charged with 
 common salt, nitre, or sal ammoniac. Saline liquids produce 
 rather more hardness than common water, and aquafortis, in 
 particular, possesses this property in an eminent degree. Files 
 are covered with the grounds of beer and common salt, and 
 dipped while wet in a powder made of burned or parched horn, 
 leather, or other coaly animal matter. By this means they are 
 not only defended from scaling, by the fused salt, and animal 
 coal, which covers them on all sides, but even rendered rather 
 more steely on the surface, by the absorption of carbon. They 
 are taken out as soon as they have acquired the low red heat 
 called cherry red, and instantly plunged into pure cold water. 
 
 When steel is required to possess the greatest possible de- 
 gree of hardness, it may be quenched in mercury, which will 
 render it so hard as to cut glass like the diamond ; but this 
 fluid, it is obvious, can only be used on a small scale. 
 
 Wrought iron may be hardened, in a small degree, by igni- 
 tion and plunging in water, but the effect is confined to the 
 surface ; except, as very often happens, the iron contain veins 
 of steel. These are no small impediments to the filing and 
 working of this material. 
 
 The general method of choosing steel for edge tools, is to 
 break a bar, observe its fracture, and select the closest grained. 
 But this mode is not always certain, as a variation in the frac- 
 ture will be occasioned by the diff'erence of its temper, and the 
 
MECHANICAL EXERCISES. 
 
 Choice of steel. — Case-hardenine. 
 
 greater or less heat at which it has been hararaered ; and some 
 steel breaks ota verv close crai^i thoutrh otiudiflerent quality. 
 The surest method is, to have one end of the bar drawn out 
 under a low heat, such as an obscure red, and then to plunge 
 it suddenly, at this heat, into pure cold water. If it prove hard, 
 for instance, if it will easily cut glass, and requires a great force 
 to break it, whatever its fracture may be, it is good, the excel- 
 lence of steel being always proportionate to the degree of its 
 tenacity in its hard state. In general, a neat curved line 
 fracture, and even gray texture, denotes good steel, and the 
 appearance of threads, cracks, or brilliant specks, is a proof 
 of the contrary. 
 
 If diluted nitrous acid (aquafortis) be applied to the sur- 
 face of steel previously brightened, it immediately produces 
 a black spot; but if applied to iron in like manner, the metal 
 remains clean. By this test it will be easy to select such 
 pieces of iron or steel as possess the greatest degree of uni- 
 formity; as the smallest vein of either upon the surface, will 
 be distinguished by its peculiar sign. 
 
 The hardness and polish of steel may be united, in a certain 
 degree, with the firmness and cheapness of malleable iron, by 
 what is called case-hardening, an operation much practised, 
 and of considerable use. It is a superficial conversion of iron 
 into steel, and only ditiers from cementation in being; carried 
 on for a shorter time. Some artists pretend to great secrets 
 in the practice of this art, using saltpetre, sal ammoniac, and 
 other fanciful in2;redients, to which they attribute tlieir success. 
 But it is now an established fact, that the greatest effect may 
 be produced by a perfectly tight box, and animal carbon alone. 
 The goods intended to be case-hardened, being previously 
 finished with the exception of polishing, are stratified with 
 animal carbon, and the box containing them luted with equal 
 parts of sand and clay. They are then placed in the fire, and 
 kept at a light red heat for hah" an liour, when the contents of 
 the box are emptied into water. Delicate articles may be pre- 
 served, like files, by a saturated solution of common salt, with 
 any vegetable mucilage to give it a pulpy consistence. The 
 carbon nere spoken of, is nothing more than any animal mat- 
 ter, such as horns, hoofs, skins, or leather, just sufficiently 
 burnt to admit of beino- reduced to powder. The box is com- 
 monly made of iron, but the use of it, for occasional case- 
 hardening upon a small scale, may be easily dispensed with ; 
 as it will answer the same end to envelope the articles with 
 the composition above directed to be used as a lute, drying 
 it gradually, before it is exposed to a red heat, otherwise it 
 will probably crack. It is easy to infer, that the depth of the 
 
MECHANICAL EXERCISES. 
 
 Bluing of steel. — Expansion of steel in hardening 
 
 Steel induced by case-hardening, will vary with the time the 
 operation is continued. In half an hour it will scarcely be 
 the thickness of a sixpence, and therefore will be removed by 
 violent abrasion, though sufficient to answer well for fire irons, 
 &c. in the common usage of which its hardness prevents its 
 being easily scratched, and its polish is preserved by friction 
 with so soft a material as leather. 
 
 The bluing of steel has a remai-kable influence on its elas- 
 ticity. This operation consists in exposing steel, the surface 
 of which has been brightened, to the regulated heat of a plate 
 of metal, or of a fire or lamp, till the surface has acquired a 
 blue colour. If this blue coal, so commonly considered rather 
 as ornamental than useful, be partially or Avholly removed by 
 grinding, or in any other manner, the elasticity is proportion- 
 ately impaired, and the original excellence of this property can 
 only be restored by bluing the steel again. Saw-jnakers first 
 harden their plates in the usual way, in which state they are 
 brittle and warped ; they then soften them by blazing, which 
 consists in smearing the plate with oil or grease, and heating 
 it till thick vapours are emitted, and burn off with a blaze 
 They then hammer them flat, and afterwards blue them on a 
 hot iron, which renders them stiff and elastic, without alter 
 ing their flatness. 
 
 It may be useful to apprise the inexperienced, that the hard- 
 ening of steel increases its dimensions ; so that such pieces of 
 work as are finished with nicety in their soft state, will not fit 
 their places when hardened. The amount of this expansion 
 cannot be accurately stated, as it varies in different sorts of 
 steel, and even in the same steel, operated upon at different 
 degrees of heat. Rinman found that bars of steel six inches 
 long, six lines wide, and half an inch thick, were lengthened 
 at least one line, after hardening at a whitish red heat, which 
 is about one-seventieth part of the linear dimensions; but, 
 according to the experiments made by others, the expansion 
 is not so considerable. It is also a curious fact, that intense 
 cold has an unfavourable effect upon steel; so that, in severe 
 frosts, workmen often find their tools incapable of receiving 
 the temper they wish. 
 
 A slender rod of wrought iron may be expeditiously con- 
 verted into steel, by plunging it into cast iron in fusion ; — a 
 satisfactory proof that cast iron contains the steel-making prin- 
 ciple, which, we need not repeat, is carbon. In fact, as it is 
 principally in the superabundance of its carbon that it differs 
 from steel, many attempts (and not wholly without success) 
 have been made to convert it into the latter, without the inter- 
 mediate operation of rendering it malleable. But the best 
 1.— Vol. I. C 
 
10 MECHANICAL EXERCISES. 
 
 Hardening cast iron. — Forging and welding. 
 
 Steel inadtt pursuant to this idea, is very imperfect. It is, how- 
 ever, not unimportant to observe, that all cast iron so far re- 
 sembles steel, as to be hardened, in a high degree, by sudden 
 coolincr, which imparts to it, at the same time, whiteness of 
 colour^brittleness, and closeness of texture. This property 
 of crude iron may be advantageously employed on many oc- 
 casions ; for instance, in the fabrication of a.vles and collars 
 for wheels, which are easily turned or tiled in their soft state, 
 and may afterwards be hardened, so as to wear admirably 
 well. 
 
 The heat applied to cast iron, previously to its being plunged 
 into water to harden, is greater than that to which steel is sub- 
 jected for the same purpose ; it should be little short of a white 
 heat. Cast iron, also, when once hardened, admits not, like 
 steel, of that hardness being reduced, by various gradations, 
 to any specific degree; to soften it materially, it must be sub- 
 mitted, for some time, to complete ignition, and very gradu- 
 ally cooled. 
 
 The smaller ramifications of cast iron work, and those por- 
 tions of metal which have the furthest to run from the git, are 
 often found so extremely hard, that the best file will make no 
 impression upon them, while the remainder of the casting is 
 sufficiently soft and manageable. This effect is owing to the 
 heat carried oft" by evaporation from the moistened sand of 
 the mould, by which the portions of metal, under the circum- 
 stances alluded to, are suddenly cooled. To increase the 
 number of gits, and to use the sand as dry as possible, are 
 the obvious means of preventing this defect ; but when it has 
 taken place, annealing is the only remedy. 
 
 The chemical properties of iron, and the best methods of 
 preserving it from rust, to which it is so liable, we shall speak 
 of hereafter. The mechanical management of it, which con- 
 stitutes our present subject, now requires us to proceed with 
 the operations of 
 
 Forerjncr and Wehlins. 
 
 In forging, the fire must be regulated by the size of the 
 work; and in heating the iron, the workmen, when the flame 
 begins to break out, beat the coals, round the outside of the 
 fire, close together with the slice, to prevent the heat from 
 escaping. To save fuel, they damp their coals, and throw- 
 water on the fire, if it extend beyond its proper limits. To 
 ascertain the state of the work, they draw it partly out of the 
 fire, and thrust it quickly in again, if not hot enough. The 
 heat the iron receives in forging, is judged of by the eve, and 
 is not commonly distinguished into more than these three 
 degrees, viz. the blood or cherry-red heat, the white flame 
 
MECHANICAL EXERCISES. 11 
 
 "White flame heat. — Welding heat. — Use of sand in forging. — Upsetting. 
 
 heat, and the sparkling or welding heat. The cherry-red heat 
 is used when it is only intended to smooth the surface of the 
 iron, or stitien it in a small degree, operations which are per- 
 formed by striking evenly with the hand hammer ; with light 
 blows, when smoothness of surface only is wanted, and using 
 considerable force, when it is desirable, at the same time, 
 rather to harden the work. When stifihess alone is required, 
 the iron is usually hammered cold, by which means it may be 
 rendered considerably elastic Bell-springs are rarely made 
 of any thing else than sheet iron thus managed. 
 
 In changing the form of iron, the white flame heat is used, 
 and, according to the size of the work, it is battered by one, 
 two, or more men, with sledge hammers, the largest size of 
 which, called About Sledges, are slung entirely round, with 
 both hands nearly at the extremity. When the iron is nearly 
 reduced to the proposed form and size, it is finished with the 
 hand hammer, the dexterous use of which will save much 
 trouble in filing. 
 
 When the iron is required to be doubled, or two or more 
 pieces consolidated, the sparkling or welding heat is used, by 
 which the metal is brought nearly to a state of fusion, and ap- 
 pears to be covered with a strong glaze or varnish. This var- 
 nish is still more abundant in steel. As soon as the two pieces 
 of iron to be united, have attained the welding heat, they are 
 taken out of the fire with the utmost dispatch, the scales or dirt, 
 which would hinder their incorporation, scraped oft, placed in 
 contact at the heated part, and hammered until no seam or fis- 
 sure remain. If they have not been sufiiciently incorporated, 
 the heating and hammering ought to be repeated, until the work 
 is perfectly sound. Workmen differ very considerably in the 
 care they bestow in this respect; and when axletrees and other 
 parts of machinery give way, a defective forging is generally 
 very apparent. To make the iron come sooner to a welding heat, 
 stir the fire with the hearth staff", and throw out the clinkers, 
 as well as the cinders upon which the iron may have run, as 
 they will prevent the coals from burning. The fire for welding 
 should be free from sulphur ; and the rods may, in part, be 
 prevented from wasting, by taking care to supply them, at the 
 heated part, with powdered glass, or sand ; or a mixture of sand 
 and the scales which fly from ignited iron in hammering. Care 
 must also be taken to prevent the iron from running, which 
 will make it so brittle as to prevent its forging, and so hard as 
 to resist the file. If this accident occur, the whole of the iron 
 supposed to be injured by the extreme heat to which it has 
 been exposed, must be cut oft' and rejected. 
 
 When it is required to thicken any part of a bar of iron with- 
 
12 MECHANICAL KXEUCISES. 
 
 Necessity of blencling sleel and iron. — Damascus steel. 
 
 out \vt'lclin;j;, the operation called upsetting must be resorted to. 
 This consists in e;iving it the white flame heat at the part to be 
 thickened, and, while one end rests upon the anvil, hammer- 
 ino at the other till the required size is produced. 
 
 In t"or'>ino" steel, great care must be taken not to use a higlier 
 deo-ree otheat than is absolutely necessary to eft'ect the desired 
 purpose, as well as to use the fewest heats possible. To unite 
 steel to iron completely, without injuring the former, is an 
 operation that demands a nicety of management which work- 
 men are not often very anxious to display. Those, therefore, 
 whose purposes require it to be well performed, will only em- 
 ploy men on whom they can depend. It is not always merely 
 for economy, that steel is welded to iron, but often principally 
 with the view of miiting the opposite qualities of the metal in 
 each state. If the mandrel of a lathe were made of the best 
 steel, sufficiently hard to wear well in the collar, it would be 
 snapped by a sudden check : and an axe, wholly of steel, if 
 soft, would be useless ; and if hard, would probably neither bear 
 the shock of a violent blow, nor the t^visting to which such 
 tools are subjected. But by uniting a proper quantity of iron 
 with the steel, the inconvenience, and even danger, resulting 
 from such accidents, are avoided. In applying them to each 
 other, regard must be paid to the manner in which the tool 
 will be used. For an axe, the edge of which is formed by 
 grinding both sides, the steel is placed in the middle, between 
 two plates of iron.; the blade of a plane, which is ground only on 
 one side, requires the iron also to be only on one side, namely, 
 the back of it; and for that part of a mandrel which works in 
 the collar of a lathe, the steel must encircle the iron. 
 
 Damascus was anciently famed for the excellence of the steel 
 goods manufactured there, especially its swords, which are said 
 to possess all the advantages of flexibility, elasticity, and hard- 
 ness. These united distinctions are supposed to have been 
 effected by blending alternate portions of iron and steel; the 
 latter, by repeatedly drawing out, doubling, and welding the 
 work, being diff"used throughout the former, almost as com- 
 pletely as a drop of ink is diffused, by intermixture, with a 
 glass of water. But the best attempt which we are at present 
 aware of having been made in England, to imitate Damascus 
 steel, according to the plan here pointed out, did not perfectly 
 succeed, the mass produced having cracked in tempering. It 
 appears probable, that the desired imitation may be effected 
 with much greater advantage by the use of steel alone, the iron 
 from which it is made being judiciously selected, and after- 
 wards very carefully cemented and forged. It is Swedish iron 
 that is mostly converted into steel ; but that kind called ola 
 
MECHANICAL EXERCISES. 13 
 
 Walby's forge hammer. — ^W^ elding cast steel. 
 
 sable) whichjwe believe, comes froniRussia,) possesses, in point 
 of tenacity, according to the experiments of an ingenious phi- 
 losopher, a very decided superiority over every other kind. It 
 would, doubtless, therefore, be suitable for the purpose ; the 
 properties of steel being influenced, as will easilv be supposed, 
 by the properties of the iron from which it is manufactured : 
 and, in confirmation of what has been said of the advantages 
 of good forging, we may here take notice of the forge hammer,* 
 invented by George Walby, of London, for which invention, 
 the Society for the Encouragement of Arts, Sec. rewarded him 
 with their silver medal and forty guineas. Although it weighs 
 seventy pounds, it may be wrought by one man, with the 
 greatest accuracy and ease, at the rate of thirty blows per 
 minute, and performs the work of two or three men. The in- 
 ventor states to the Society, that the steel is kept in better 
 temper by this hammer, and fewer heats are required for the 
 same work, than in the common way ; that the trowels made 
 with it by him, will bear any pressure of bending, and return 
 by their elasticity to their original shape, and they will even 
 cut a chip from a bar of iron, without hurting their edge; they 
 also are lighter and more handy than common trowels, and 
 serve much longer in use. 
 
 The steel which contains the smallest proportion of carbon, 
 as, for example, shear steel, is the most easily welded; but it 
 is by fusion, which entirely destroys its fibrous texture, that 
 it is rendered incapable of being welded to itself, and some 
 maintain that genuine cast steel has never been united even to 
 iron by welding. Yet others have stated, that the means by 
 W'hich this may be accomplished, consist in placing between 
 the iron and the steel another kind of steel, in the form of fil- 
 ings, or a thin plate, the iron being brought to a welding heat, 
 and the cast steel made as hot as can be done with safety. 
 Such, however, are the difficulties of the operation, and so fre- 
 quently imperfect the work when finished, that other means of 
 effecting the union have been resorted to. One of them, for 
 which a patent has been granted, has been brought into com- 
 mon use, for the blades of joiners' planes, and many other pur- 
 poses ; it consists in uniting the steel to the iron with soft sol- 
 der or tin. In this process, the cast steel, not being exposed 
 to much heat, loses none of its good properties; but the union 
 is not so substantial as that afforded by welding. 
 
 * For the description of this hammer, illustrated by an engraving, see 
 the Transactions of the Society for the Encouragement of Arts, Manufac- 
 tures, and Commerce, or the Repertory of Arts, vol. 7, second series, 1805. 
 
14 MECHANICAL EXERCISES. 
 
 The forge bellows. — The anvil. — ^The vice. 
 
 Of forging Iron, and the Tools used in the working of Metals. 
 
 Minutely to describe the various tools made use of in forging- 
 iron, and in the working of metals generally, would be more 
 likely to tire than to please our readers, to whose information 
 on such points, a volume of the most elaborate description 
 would not add so much as a few moments' inspection of work- 
 shops, which may be seen in every village of the kingdom. We 
 shall, therefore, on this, as on other occasions of the kind, 
 confine our remarks to particulars which are, for the most 
 part, either not generally practised, or not often communicated 
 by workmen, or not the most likely to catch the eye of the 
 looker-on. 
 
 • The best position for the belloivs is on a level with the fire- 
 place, but they are frequently placed higher, and the blast com- 
 municated through a bent tube, for the purpose of gaining 
 room near the floor. The small end of the pipe of the bellows 
 passes through the back of the forge, where it is fixed in a 
 strong iron plate, called a tue iron, or patent back, in order 
 to preserve the bellows from injury, and the back of the forge 
 from requiring frequent repair. 
 
 The anvil is a substantial mass of iron, to the surface of 
 which a plate of steel is firmly welded, and made sufficiently 
 hard to withstand the file, or the blow of a hammer. It is usu- 
 ally made, for forging iron upon, with one or two projecting 
 arms, and is then called a beak iron. These arms are useful in 
 giving the requisite form to various sorts of work : when there 
 is only one, it is preferred of a conical shape; when there are 
 two, one of them is pyramidical. They are afiixed length- 
 ways, a little below the surface of the body of the anvil, and 
 rather inclining upwards towards the point. In Birmingham, 
 where attic rooms are frequently converted into workshops, 
 the block upon which the anvil is fixed, is placed upon a stra- 
 tum of sand, which prevents the vibrations that would other- 
 wise be communicated to the floor, and much of the noise 
 which would incommode the inhabitants of the room below. 
 The contrivance is simple, and susceptible of other applica- 
 tions. Clock-makers use very small anvils or beak irons, 
 which they fix in the vice when in use. The anvils of tin- 
 plate workers are of various sizes, and are often made with 
 concavities and projections upon them, by the help of which 
 they can readily communicate difi^erent shapes to their work. 
 
 The large vice must be firmly fixed to the side of the work- 
 bench, to the edge of which its chaps must be parallel, their 
 upper surface being at the same time exactly horizontal. The 
 best elevation for a vice, is that of the workman's elbow, when 
 
MECHANICAL EXERCISES. 15" 
 
 Hand vice. — Hammers. 
 
 the upper arm is held vertically against the side ; and the 
 lower arm, for the sake of trying the height, is held at right 
 angles thereto. In filing, if the vice or the work be above 
 this position, which is seldom heeded, or even thought of, the 
 stroke will not be so powerful as the same exertion would 
 otherwise make it ; and, whether higher or lower, it will be 
 found exceedingly difficult to carry the file in a horizontal 
 direction. As the teeth on the inner surface of the chaps 
 would mar fine work, if pressed against it sufficiently hard to 
 keep it steady, they are, as often as the occasion requires, 
 covered with plates of lead, about the eighth of an inch thick. 
 These plates must be large enough to extend about half an 
 inch on each side beyond, and an inch above the chaps, to 
 each of which, when screwed tight, one of them is secured by 
 hammering down the projecting parts. 
 
 The hand vice is used to hold small articles in the act of 
 filing ; it is held in the left hand, and the parts of the iron, 
 while pressed upon the end of the bench, or upon a bit of 
 wood or bone in the large vice, are successively turned to the 
 file, which is held in the right hand. A nick is made in the 
 wood or bone, to keep the work from being carried aside by 
 the file. 
 
 Hammers, like anvils, are faced with steel, in a state of 
 considerable hardness. Their handles are almost always made 
 of nearly a uniform thickness in every part, or if they differ 
 from such figure, it is not for any specific purpose. Hence 
 the vibrations of the hammer head are communicated to the 
 hand, to which they occasion very unpleasant sensations, 
 i.nd the workman is tired before he has much exerted his 
 strength. If the handle of the hammer, at a little distance 
 from its upper end, be made considerably smaller, for a short 
 space, than in any other part, tlie alteration will be found a 
 decisive improvement. Such a hammer will, as it is techni- 
 cally termed, ya// well; diminishing, at the same time, the 
 workman's fatigue, and convincing him that his blows are 
 solid and effectual. Fig. 1. pi. III. will clearly designate this 
 construction: it represents a hammer for chipping iron; for 
 which purpose, the head need not be more than sixteen ounces 
 in weight, and the handle about twelve inches long. In a 
 hammer of any given shape, calculated to give the hardest 
 blows with the least weight, and, consequently, with the least 
 fatigue, the quantity of iron in the head should be equal on 
 the opposite sides of a line supposed to be drawn perpendi- 
 cular to the centre of the face. Hammers, therefore, made 
 for the purpose of drawing nails, with claws, which lean back- 
 wards from this line, are not calculated to produce the best 
 
IG MECHAMCAL EXERCISES. 
 
 Ri»eiin:r.- futlin? metals v,iih shears — chisels — saws. 
 
 ertect in striking. Clockmakers, tin-plate workers, and bra- 
 ziers, face their planishing hammers upon a grindstone; they 
 then rub them upon a soffdeal board, covered with emery and 
 oil, until the scratches of the stone are removed; they next 
 use a Turkev hone and oil, and finish them with putty, or col- 
 cothar and water, upon a smooth board. Watch-makers and 
 silversmiths take still more pains with theirs, selecting them 
 free from every flaw, removing every scratch, and giving them 
 an exquisite lustre with colcothar or putty. 
 
 In riveting two pieces of metal together, if the head of the 
 rivet is not intended to project, the hole must be widened a 
 little at the top and bottom. One of the heads of a rivet 
 should be made before it is put into its place, in which it is 
 secured, by striking the edge of the other end of the shank 
 (previously filed flat) with light blows, till it is evenly 
 spread all round, when heavier blows may be used, till it 
 is sutficientlv firm. When the head of a rivet or screw is 
 on a level with the surface of the work, it is said to be 
 countersunk. 
 
 In cutting sheet iron or brass, and even bars of the same 
 metals, shears are used. They are frequently made three or 
 four feet long ; one handle is screwed fast in the vice, or se- 
 cured to the bench, and the uppermost only is moveable. The 
 harder, and the greater the thickness of the metal they are 
 designed to cut, the more obtuse the angle by which the edge 
 is formed. 
 
 A chisel is often used instead of a pair of shears, and though 
 it does not cut with so much rapidity, it is, on many occasions, 
 more convenient, as it can be made of different figures, guided 
 in various directions, and stopped at any given point. Plates 
 of metal to be cut with a chisel, are laid, during the operation, 
 upon a mass of lead, or upon an anvil ; if the latter be used, they 
 are not cut quite through, to prevent injuring the chisel, vet thev 
 are so nearly divided that the separation can be effected bv 
 striking them with the hammer while held on the edcre of the 
 anvil, or by wriggling them with the hand or in the vice. 
 
 Saws for cutting metals, are made very narrow, (see fig. 2. 
 pi. Ill :) and stretched by a screw at one end ; thev are made 
 rather thicker on the edge than at the back ; the teeth are 
 small, and are not bent like those for joiners' use. Clock and 
 watch-makers often make their saws of broken watch-springs, 
 the temper of which is suitable for the purpose, and the metal 
 commonly excellent. In sawing malleable iron and steel, oil 
 must be used; cnide iron and brass require no oil, but for the 
 latter, a ver)- sharp saw is necessary, and it may also be rather 
 barder than for iron. 
 
MECHANICAL EXERCISES. 17 
 
 The chipping chisel. — The punch. 
 
 Metals are sometimes wrought by chipping. This operation 
 not only often produces the intended effect in an expeditious 
 manner, but saves much expense in the files which would other- 
 wise be required. It is most frequently applied to cast iron, 
 the dark rind or outside of which, taken as it comes from the 
 mould, is always harder than the rest, and frequently so very 
 hard, that it would spoil the best file in a few minutes, while, 
 at no greater depth than the twentieth part of an inch, or even 
 1-ess, it is nearly as soft as brass. The chisel will penetrate 
 this hard crust, and afterwards, as may be easily understood, 
 its edge need only be made to act upon the soft part. The 
 chisel, for this description of work, need not be more than 
 seven inches long, but it ought to be made of the best cast 
 steel. Fig. 3. pi. III. represents such a chisel. — No. 1, show- 
 ing the front, and No. 2, the side of it, to point out the nature 
 of its edge. The hammer to be used with it has been already 
 mentioned. It is held in an anoie of about fortv-five degrees, 
 and the blows of the hammer are given in quick succession. 
 Some dexterity, certainly, which can only be acquired by 
 practice, is requisite, to preserve a tolerably equable surface, 
 but the art is not of difficult acquirement. A pellicle of iron 
 may, by the chisel, be taken from a surface of a hundred 
 square inches, in four or five hours, and w^hen it has been well 
 done, the file very speedily levels the inequalities which it 
 leaves. When much exactness is required, it is advisable to 
 examine the work before the chipping is commenced, and if 
 improper protuberances or hollows appear in it, the chisel 
 must be struck deeper, or not so deep, at such places, as the 
 circumstance dictates. 
 
 Malleable iron, in a state of ignition, is easily perforated 
 with a steel punch, which is made of the size and shape of 
 the hole required, except that it must always be tapered more 
 or less towards the lower end, to facilitate drawdng it out. 
 It is seldom pointed at the extremity, which is hardened 
 without tempering, as the heat of the iron will soften it suffi- 
 ciently, and sometimes too much ; to check the latter effect, 
 it is plunged into water as often as it is supposed to be con- 
 siderably heated. The hole may be finished with a file, or 
 by hammering it at a low heat upon a smooth mandrel or pin, 
 or by a well tempered triangular square, or octagonal bar, 
 fixed to a handle, and wrought the same way as a carpenter's 
 auger. A tool of this description is called a rimer, and is 
 made to taper a little from the handle to the lower end- In 
 using it the motion must be slow. The triangular and square 
 form answer well for brass, and the softer metals ; but the 
 octagonal one is much more suitable for iron ; as the other 
 1.— Vol. I. D 
 
18 ^MECHANICAL EXERCISES. 
 
 Old luetliod of boring cylinders — new method. 
 
 would take hold so deeply as to break with the force requisite 
 to turn them round. A sharp-pointed punch will penetrate a 
 piece of cold iron, not exceeding the tenth of an inch in thick- 
 ness, sufficiently deep to cause a projection on the under side; 
 when this projection is filed oft', if the hole does not appear, a 
 repetition of the punching will immediately produce it ; and 
 it may be widened by the octagonal tool above-mentioned. 
 Brass may be managed in the same way, with still more fa- 
 cility: the plate of metal to be pierced, should be laid upon 
 lead ; or the under surface, opposite the point of the punch, 
 should be placed over a hole in an anvil. 
 
 As punching is not applicable to cast iron, nor to small 
 and deep, or verv large holes in any metal, and is, besides, apt 
 to throw the piece out of shape, mechanics have recourse, 
 according to the nature of the work they have in hand, to 
 the ditierent methods of 
 
 Borins and Drillius. 
 
 The steam engines of the present day are not more indebt- 
 ed for their excellence to modern improvements in their con- 
 struction, than to the new methods which have been adopted 
 to render them faultless in point of workmanship. In the 
 latter respect, the boring of the cylinder presents one of the 
 most remarkable features of difference from the old plans. 
 The way usually was, at some of the first founderies, to put 
 it upon a carriage, insert the cutter block, set the mill to 
 work, hang a cloth at the open end to keep in the dust, and 
 let it bore away, which it would be doing, on a large cylin- 
 der, for three weeks or a month ; and if it was tolerably 
 smooth, it was said to be well done. As the cylinder is cast 
 hollow, though the moulder pursues the most correct method 
 his art is capable of, yet it is impossible to be certain that, 
 when the mould has received the metal from the furnace, it 
 shall come out quite straight; and if it come out crooked, it 
 must remain so, for this despicable mode of boring will never 
 remedy that imperfection. It is not like boring a solid piece 
 of metal, as in boring ordnance, &.c. All that this old boring 
 can do to a cylinder, is to make it round and smooth, for 
 there is nothing to conduct the boring bit in its progress, but 
 the form given it by the moulder, whose best exertions cannot 
 ensure success : it complies, therefore, with the twistings of 
 its road, and the cylinder is inaccurate. If the metal be 
 harder on one side than another, it produces an additional 
 source of imperfection. 
 
 The new method of boring originated with John Wilkin- 
 son, iron master, and the cylinders were executed in a man- 
 
MECHANICAL EXERCISES. 19 
 
 New metbod of boring cjlinders. 
 
 ner which has not since admitted of improvement. When 
 the process is conducted by an intelligent workman, if the 
 cylinder should be cast ever so crooked, or ever so thick on 
 one side more than another, he can take out the redundancy 
 from that side, and scarcely touch the other. This will rea- 
 dily be admitted, when it is understood, that the cutting ap- 
 paratus is conducted along a tool (called by the workmen a 
 borijtg: bar-) which is itself a masterpiece of workmanship, a 
 perfect cylinder. Hence, whatever is carried along this bar, 
 parallel to its axis, must move in a right line. When, there- 
 fore, it has been turned with the utmost care and precision, it 
 is to have two grooves cut opposite each other in this direc- 
 tion. A cast iron socket is then bored and ground upon the 
 bar, so as to fit it in the most exact manner. The external 
 part of this socket is made conical, with four or six studs 
 upon the base of it, to receive the cutter block ; and fillets 
 fastened upon the inside of it, and falling into the grooves, 
 while they allow it to slide along the bar, prevent its being 
 carried round, unless the bar be carried round at the same 
 time. To give a progressive motion to the socket and cutter 
 block, while the bar is turning on its own axis, a collar of 
 metal is fitted on the socket, and that collar is connected with 
 two racks, long enough to reach through the cylinder, and 
 communicate with a pair of pinions, which being acted upon 
 by two levers, carrying a sufficient weight to overcome all 
 resistance in the operation, the socket is drawn along the 
 boring bar, and the cutters fastened in it effectually perform 
 their work. 
 
 In fitting up the boring apparatus, some diversity of prac- 
 tice prevails. By some, a hole, to admit a single rod, is 
 drilled through the whole length of the bar, and a groove is 
 sunk entirely through one side of it, so as to come into the 
 hole thus drilled. A branch from the internal part of the 
 socket is fitted into the groove, with an eye to receive the 
 end of the rod, to which it is secured by a screw, so that 
 when the rod is drawn along, the socket moves at the same 
 time in the same direction. A weight, with a rope over a 
 pulley, is applied to give the progressive motion to the socket 
 upon the bar. This mode of constructing the bar is the best 
 way for the boring of small cylinders, as there is no incum- 
 brance upon the socket; and if the bar is sufliciently strong,. 
 it will move with great steadiness. 
 
 Ordnance were formerly cast hollow ; they are now always 
 cast solid, and afterwards bored by machinery. The gun to 
 be bored lies with its axis parallel to the horizon, and in that 
 position, moving in a collar fixed at each end, it is turned 
 
20 MECHANICAL EXERCISES. 
 
 Boring of ordnance. — Drilling with the lathe — the bow. 
 
 round its axis. The borer is laid truly horizontal, in the di- 
 rection of the axis of the gun, and is incapable of motion in 
 any direction except that of its length ; and in this direction 
 it is constantly moved, so as to pierce and cut the gun, by 
 means of rack-work, a lever and weight, applied in the man- 
 ner represented by fig. 4. pi. Ill, where it is obvious, that if 
 the weight at the upper end of the lever is sufficient to over- 
 come the resistance, the pinion will turn till the lever rests 
 on the ground ; at this moment therefore, or a little before, 
 the workman who attends the machinery, taking it out of the 
 hole on the axis of the pinion, into which it is hooked, inserts 
 it in another higher up, and this he continues to do at the 
 proper interval, till the work is finished. The outside of the 
 gun is smoothed at the same time by men, with instruments 
 fit for the purpose, while it revolves, so that the bore may be 
 exactly in the centre of the metal. 
 
 Boring difters from drilling only in being commonly applied 
 to laroer works. Drilling may be effected in a lathe with 
 great facility. The drill is screwed, or otherwise fastened, 
 upon the spindle, so that its point shall turn exactly opposite 
 the point of the screw in the right hand pupp«t. The piece 
 to be drilled is then slightly pierced with a punch, where the 
 drilling is to commence, and also where it is intended to come 
 out. Against the latter puncture, the point of the screw in 
 the right hand puppet is directed, and gradually pressed for- 
 ward as the drill, on turning the wheel, is found to cut. The 
 motion of the wheel must be slow, especially for iron. The 
 rest, or any temporary support, may be used to keep the 
 work steady, which may then be perforated with expedition 
 and accuracy. A short lever, with a weight at the end of it, 
 may be applied to advance the screw, so as to leave both 
 hands of the workman at liberty for other matters. 
 
 Small drills, used by clock-makers and others, are usually 
 made of a single piece of steel wire, u])on which, about the 
 middle, a pulley or drill barrel is driven, (see fig. 5. pi. IlL) 
 Sometimes, a shank or small mandrel is used, with a square 
 hole, about half an inch deep, at the end of it, into which 
 drill bits of various sizes can be alternately inserted. The 
 disadvantage of this construction is, that the drill bit is 
 seldom held true, which causes it to perform indifferently. 
 It is, therefore, but little used by those workmen who can 
 readily furnish themselves with the other kind as they want 
 them. When these small drills are used, they are held hori- 
 zontally, and pressed against the work by a breast-piece, 
 which is sometimes made of wood, and sometimes of sheet 
 iron J but, in either case, is rather concave on its inside, to 
 
MECHANICAL EXERCISES. 21 
 
 The liand-drill. — Precaution in lenipering drills. 
 
 rest more steadily upon the breast, and in tlie centre of the 
 outside is fixed a ])it of steel, for the blunt end of the drill to 
 work in. The drill is turned by drawing- backwards and for- 
 wards an elastic bow, the string of which is coiled once round 
 its pulley. The best bows are made of steel, and the strings 
 of catgut; the strength of them must be jiroportioiied to the 
 size of the drill. A piece of stout cane makes no bad sub- 
 stitute for a steel bow. 
 
 To make large holes, more force is required than can be 
 given by the bow and string, instead of which a brace, not 
 very unlike that used by joiners, is em])loyed, and the drill 
 itself is fitted as a bit ; but instead of the stock which, in the 
 joiner's tool, remains stationary, while the rest is turning, we 
 have here a long tapering spindle, which being nothing more 
 than a continuation of the brace, is necessarily carried round 
 at the same time. The upper end of the spindle works in an 
 iron or steel plate, which is fixed on the under side of a beam, 
 called the drill beam. One end of the beam turns upon a 
 transverse pin between two uprights, pierced with various 
 holes, to fix it at difl'erent elevations ; the other end, which is 
 pressed down by a weight, passes, when great steadiness is 
 wanted, between two other uprights. The point of the bit 
 being then placed upon the part of the metal to be drilled, 
 the brace is revolved by the hand, and a hole to any required 
 depth may be made. The bit should be well fitted to the 
 brace, though, as very small holes are not made with this ap- 
 paratus, the disadvantage of its shaking a little is not of so 
 much moment as in the breast drill. The drill is commonly 
 fitted up so that the work to which it is applied can be fixed 
 in the vice. Fig. 6. pi. 111. represents the manner of fitting 
 up the hand drill, and fig. 7. one of the bits separately. 
 
 The vertical part of the crank, by which the hand revolves 
 the drill, ought to be very smooth, or, what is still better, it 
 may be covered with a loose handle. If this handle be made 
 of iron, it may be bent round and soldered ; if of wood, it may 
 be made out of a hollow cylinder, cut in two pieces, between 
 which the vertical part of the crank may be enclosed, and it 
 may then be fastened by glue, or by a hoo|) at either end, the 
 diameter of the lioop being made large enough to pass over 
 any part of the brace. 
 
 Drills ought to be made of the best steel, and the cutting 
 part only should be hard ; they are therefore tempered by 
 keeping the lower end out of the fire, but heating the rest 
 considerably, till the point attains the desired colour, when it 
 is instantly cooled in the usual maimer. By this means, the 
 cutting part of the bit may be tempered to a straw colour. 
 
22 MECHANICAL tXERClSES. 
 
 Use of a fly in drilling. — Filing. 
 
 while the rt-st is not higher than blue, so that its liability to 
 break when in use, is greatly diminished. We may observe, 
 in passinix, that this mode of tempering from the back of the 
 tool, so as to have the edge only in a state of great hardness, 
 is observed as a general principle in the art. 
 
 The application of a tly wheel to the upper part of the 
 large hand drill, would be a considerable improvement ; not 
 merely on account of its weight, but because its centrifugal 
 force would tend greatly to keep the drill exactly vertical. 
 
 In drilling, as in sawing, forged iron and steel require oil, 
 but to brass and cast iron none must be used. For brass, 
 also, the drill bit is made thinner, harder, and the cutting 
 edge formed by a more acute angle than for iron. 
 
 Filing. 
 
 In the working of metals, there is no operation more com- 
 mon than that of Jiling, and perhaps there is none so little 
 understood. A file is an instrument too familiar to every 
 one to require description. To use it well, generally proves 
 one of the most difficult tasks which the practical mechanic 
 has to encounter, and this difficulty is owing more to the 
 want of a proper plan in setting about the work, than to any 
 other cause. Plane surfaces, for instance, for the plates of 
 air-pumps, and a thousand other purposes, are of indispens- 
 able use ; but a knowledge of the manner in w hich they may 
 be readily and completely executed, is confined to very 
 few ; and a workman, aware of the exactness required from 
 him, can rarely be found who will undertake to execute 
 them. Grinding is the common and dernier resort of those 
 who wish to produce, on such occasions, the last degree of 
 accuracy ; but tw'o surfaces of metal may be ground together 
 for ever without being made plane, unless, by some previous 
 operation, all their cross-uindings are completely removed 
 In the execution, however, of this previous operation, nearly 
 the whole difficulty of the business lies. In what must it 
 consist? Grinding has a tendency to perpetuate any regular 
 convexity or concavity which either surface may have, and 
 even to produce one or other of these forms on each piece, 
 although both were plain to begin with. The application of 
 turning to the production of plane surfaces, (for which see 
 the section on Turning,) is not an easy undertaking, and re- 
 quires an expensive apparatus; and often the mere fixing 
 (upon the chuck) the metal to be turned, takes as much time 
 as ought to be required for the completion of the work. We 
 would incite, therefore, the ingenious artist to place confi- 
 dence in the^Vc, with which, we hesitate not to assure him. 
 
MECHANICAL EXERCISES. 23 
 
 Value of the art of filing. — General reflections. 
 
 that more beautiful and accurate workmanship may be exe- 
 cuted, than most of those who are, in other respects, very 
 respectable mechanics, are either apprised of or disposed to 
 consider possible. In this line of exertion, we have witnessed, 
 with admiration, the performances of Thomas Jerome, who 
 now holds a situation in the Royal Mint ; which has enabled 
 him to introduce into the coinage, and machinery for coining, 
 several valuable improvements. With the file alone, as his 
 cutting and polishing tool, he has not only produced speci- 
 mens of workmanship which challenge all competition, and 
 the severest scrutiny, but effected his purpose with a degree 
 of expedition, and consequent economy, of which no other 
 method would admit. The work (the appearance of which, 
 though remarkably fine, was only a secondary consideration,) 
 required the exact parallelism of its several sides, some of 
 which presented a surface of not less than fifty or sixty square 
 inches; and in his hands the file did all this, in such a man- 
 ner as to set at defiance the elegant art of turning, and to 
 render the dirty and tedious process of grinding wholly un- 
 necessary. How often, in provincial towns especially, have 
 embryo inventions been kept back, for the want of workmen 
 of sufficient skill to execute the proposed contrivance ; and 
 how often would inventors themselves carry into effect their 
 designs, if they were not filled with the apprehension that the 
 acquirement of a competent share of manual dexterity was 
 too difficult a task to be attempted ! Those who have had the 
 most ample opportunities of observation, will not consider 
 these idle surmises; they cannot but be sensible, that the in- 
 ventions which become publicly known, are few in compari- 
 son with those which spring up in the minds of ingenious 
 men, and perish from such obstacles as have been just stated, 
 often, perhaps, with the hour which gave them birth. What 
 one man has accomplished, let not another despair of accom- 
 plishing also. Superior opportunities of experience are often 
 vanquished by superior exertions ; and if these remarks on 
 the excellence attainable in an art of the first importance to 
 the practical mechanic, should stimulate one person to the 
 improvement of his skill, they will not be useless. 
 
 The practical directions belonging to this subject now claim 
 our attention. Here the general principle, upon the proper 
 application of which success depends, may in the first plr.ce 
 be noticed; it is simply this, that if a plane surface, alrea ly 
 known to be true, could be made use of so as to shew, with 
 perfect facility and correctness, the errors of another upon 
 which the artist may be employed, as often as he wishes to 
 ascertain the state of his work, a file, or any tool by which all 
 
24 MECHANICAL EXERCISES. 
 
 The ditJVrent forms and names of files. — Choice of files. 
 
 the projfctions may be removed without reducing the other 
 parts, will enable him at length to bring the latter surface to 
 an exact correspondence with the former. Such a surface is, 
 therefore, indispensably necessary in the art of flat filing: and 
 we mav add to it another implement of almost equal utility, 
 thouo^li verv little used, namely, a perfectly straight steel ruler, 
 for which we shall adopt the technical term, by calling it a 
 simicrht eds^e. On the production or procuring of these two 
 thincrs, we shall speak in a future section; at present we shall 
 suppose them to be obtained; then an assortment of files follows 
 of course, as also a vice, or some other method of steadily sup- 
 portino; the metal upon which the file is intended to operate. 
 
 Files are ditlerently formed, and of various sizes for ditferent 
 purposes, their sections b?ing either square, oblong, triangular, 
 or segmental; the files of these sections are respectively deno- 
 minated square, flat, three square, or half round. That sort of 
 file called the safe-edge, (on account of its not being cut on one 
 edge,) which is flat on both sides, and of equal or nearly equal 
 breadth in every part, is the best for every purpose to which 
 its form admits of its being applied, and is particularly to be 
 recommended for flat filing. 
 
 In chusing files, some degree of attention is requisite, and 
 will save much subsequent trouble; a file, the surface of which 
 is twisted in various directions, (a circumstance which often 
 happens in hardening,) will constantly deceive the workman, as 
 it will produce nothing but false strokes. They must, therefore, 
 be chosen free from such imperfection, but a small decree of 
 regular convexity is not detrimental. The goodness of a file, 
 so far as its shape is concerned, may be readily determined 
 by the eye. 
 
 It is perhaps too obvious to require remark, that the scratches 
 made by a file will be proportionate to the size of its teeth ; and 
 that the larger these are, the greater will be the efl'ect which an 
 adequate force will produce at one stroke : hence the verv evi- 
 dent propriety of commencing the work with the coarsest file 
 intended to be used; and afterwards, in regular oradation, em- 
 ploying finer and finer ones, as it approaches to the finished 
 state. — Files may be obtained, the teeth of which are so ex- 
 tremely fine, that they will leave the surface of metal, especi- 
 ally if it be brass, almost as smooth as an oil-stone. These 
 are, however, seldom necessary ; and for most purposes, files 
 of three or four degrees of fineness, are quite sufficient. 
 
 As most of the articles of manufacture, to which the file 
 can be applied, are composed of flat surfaces; as he who can 
 file a flat surface well, will find no difficulty in executing what- 
 ever the file will enable him to do; we shall detail the progress 
 
MECHANICAL EXERCISES. 25 
 
 Particular directioD.s for filing. 
 
 of a block of metal taken rough from the founderv, till it is 
 brought to a finished state ; and supposino- a rectanojular figure 
 to be aimed at, its surfaces will then be trulv flat, and, accord- 
 ing to their situation, either exactly parallel, or exactly at 
 right angles to each other. As somewhat greater difficulties 
 occur in filing iron than brass, and as cast iron is not so easy 
 to manage as the other descriptions of the same metal, we 
 shall suppose it to be a block of cast iron. Merely for the 
 sake of having definite ideas of our subject, as we go alono;, 
 let us suppose it to be nine inches in length, seven in breadth, 
 and one in thickness. On receiving it, the first step is, to 
 examine the state of the metal, whether it be hard or soft, 
 warped or tolerably straight, perfectly solid, or interspersed 
 with cavities. If it prove very hard, which may be known by 
 trying it with a file, it will be advisable to anneal it ; which 
 wall greatly facilitate our work ; but the outside will still be 
 somewhat harder than the internal part, owino- principally to 
 some of the sand of the mould closely adhering to it; this out- 
 side, or rind, some workmen remove by chipping, in the man- 
 ner already spoken of; others, who have the convenience, take 
 it off with a large grindstone turned by machinery ; and others, 
 again, use the file immediately, taking the precaution only of 
 using, in the first instance, a file that is already rather worn, 
 as a new one would quickly be spoiled. Chipping is upon 
 the whole the most economical and convenient process, and 
 when, for the removal of imperfections, or anv other purpose, 
 it is requisite to reduce the block materially, it is decisively to 
 be preferred. If, after the outside has been removed, there 
 appear any cavities or other imperfectiuiis, which are not 
 likely to be removed by the file, and which will unfit the 
 piece for its destination, they may be drilled out, and the 
 holes made by the drill filled with rivets. Small imperfec- 
 tions may be removed by drilling to the depth of about 
 half an inch, and then driving in a plug made of wire, 
 which may be fitted sufficiently tight to bear any degree 
 of hardship, and sufficiently correct to avoid the slightest 
 appearance of a flaw, without the trouble, as in rivet- 
 ing, of making the top of the hole wider than the rest. 
 With a view, however, to complete security, some tap the 
 hole they have drilled, and then screw in a pin which 
 exactly fits it ; but when this is done, and the screw has 
 a fine thread, in filing the surface level, that part of the 
 thread which is nearest to the surface is apt to break off, 
 to the extent of a semicircle, and thus leave the work im- 
 perfect ; whereas, when the plug or the rivet is well fitted 
 in, the place cannot aftervvards be distinguished from the 
 2.— Vol. I. E 
 
2G MECHAKICAL EX F.UCIS E>;. 
 
 Particular directions for filing. 
 
 Other parts of the block, by any other circumstance than the 
 superior brightness of the malleable iron. 
 
 As the holes in a piece of cast iron, which are occasioned 
 either by stagnated air, or the falling in of part of the mould, 
 have mostly not onlv very rough surfaces, but are wider inter- 
 nally than at the outside, they may be tilled with melted lead, 
 pewter, or some other soft metal, which they will retain: type- 
 metal will answer extremely well, as, from the antimony it 
 contains, it expands in passing from the fluid to the solid state. 
 This mode is applicable when levelness of surface is the prin- 
 cipal object in view, and it is not necessary to regard the uni- 
 formity of its appearance, the equal hardness of its several parts, 
 or its being able to bear a strong heat. If we w^ere speaking of 
 a piece of metal, eventually to be subjected to considerable 
 stress, we might here observe, that thus to fill up the hollows 
 it contains, will greatly increase its capability of resistance. 
 
 Let us now suppose that the block we have in hand, is com- 
 pletely freed from its hard black scurf, and, as far as may be 
 thought necessary, from every imperfection which the subse- 
 quent operations with the file are incapable of removing. We 
 now select the file we intend to use first, and in doing this we 
 pitch upon a safe-edge one, about fourteen inches long, an inch 
 and a half broad, and containing about fourteen rows of teeth 
 in each inch of its length. In the act of filing, the file is held 
 by the handle, and pushed forward by the right hand ; while 
 the left hand, near the wrist, pressing upon its lower end, 
 gives effect to the stroke, which must be directed as nearly 
 horizontal as possible. By the occasional application of the 
 Straio;ht-edcre to the surface we are filiuu, in various dii'ections, 
 but in particular, diagonally, we easily ascertain the state of 
 our work, and remove in succession the elevated parts. The 
 inequalities at length become so small, that it would be tedious 
 to apply the straight-edge to discover them ; but being pro- 
 vided with a surface which we know to be true, (and which we 
 shall designate by calling it a table, as it ought always to be 
 larger than the work we are filing, and for general purposes, 
 may with much advantage contain several square feet,) we now 
 make use of it, for the detection of the remaining imperfec- 
 tions, in the following manner : we mix finely washed red chalk 
 or ochre, with olive or any other oil which is not viscid, and 
 we rub this mixture upon it with a piece of cloth, so as to cover 
 the whole of it very thinly and evenly. If the surface we are 
 filing be then turned down upon it, and moved a few times 
 backwards and for^vards, it will be everv-where equally co- 
 vered with ochre from the table, provided it be equally level. 
 But as this will never happen at the first trial, those parts 
 
MECHANICAL EXERCISES. 27 
 
 Particular directions for filing. 
 
 which are highest will alone be reddened, and they must be 
 reduced by the re-application of the file. As soon as the marks 
 left by the ochre have disappeared, and we think we have re- 
 moved the inequalities they pointed out, we again try the state 
 of our work as before, and continue to repeat the same pro- 
 cess till it is finished. When it approaches nearly to a perfect 
 plane, the ochre will redden a great number of places in small 
 spots or strips, and then we not only, agreeably to the remark 
 already made, use a fine file, but hold it rather differently. In- 
 stead of pressing it down, as when we began, with the broad 
 part of the hand, we now merely press upon it with two or 
 three of our fingers, by which means we are enabled to observe 
 more distinctly the spot upon which we bear, and to move 
 with more expedition from one part to another. 
 
 Before we begin to finish our work with much nicefcy, we 
 carefully attend to one thing : turning that side of the block 
 we have been filing down upon the table, we strike the back 
 of it, at the corners, centre, and various other parts at pleasure, 
 with a mallet, or the end of the handle of a hammer held per- 
 pendicularly. If a dead sound, such as would be heard on 
 striking the table itself in a similar manner, be produced, we 
 feel gratified by the assurance thus afforded, that we have 
 none of those twistings of the surface which are technically 
 termed cross-windings, to remove ; but if a sharp chinking sound 
 be produced, it is evident that the surfaces of the table and 
 the block do not coincide, for the blow of the hammer has^ 
 pressed one part of the block lower down than it was before, 
 and raised another part; and to the action of the surfaces 
 upon each other thus occasioned, the ringing sound is attri- 
 butable. If the corner of the block, to the extent of a square 
 inch, or even much less, be lower than the remainder of the 
 surface, in no greater degree than the common thickness of a 
 sheet of writing paper, this mode of trial will make the imper- 
 fection very distinctly perceptible. If, therefore, the block 
 will not stand the test of this examination, we immediately 
 proceed, by the use of the ochre, to detect the extent of the 
 elevated parts ; and in moving the block upon the table for 
 this purpose, we are careful to press only on those parts under 
 which we know, by our previous trial with the hammer, they 
 are comprised. Having obtained the marks we desire, we file 
 away, to the best of our judgment, the convexities they indi- 
 cate, and repeat the experiment and filing, till the block will 
 lie perfectly solid upon the table. This object, so essential 
 to good work, being obtained, and it ought always to be ob- 
 tained as early as possible in our progress, we shall approach 
 with surer steps, to the successful accomplishment of our task. 
 
28 MECHANICAL EXERCISES. 
 
 Use of llie callipers and gauge in filing. 
 
 The practitioner, however, will soon discover, that although 
 the test by the hammer answers an important purpose, in prov- 
 ino- the existence or non-existence of cross-windings, yet its 
 ap^ilication extends but little further ; the depression of any 
 particular part, before it can point it out, must not only extend 
 to the edi>e of the block, but must embrace a small portion at 
 least of tvvo sides. Without, tjierefore, expecting from it what 
 it cannot afford us, w'e use it merely as a collateral help ; the 
 use of the ochre simply is our universal test ; but if we wish 
 to know the measure of any particular imperfection, we resort 
 to a o-ood straight-edge, the application of the arris of which, 
 to any part we chuse to try, gives us, with the utmost preci- 
 sion, the information we are seeking. If the surface tried be 
 perfectly true, no light will pass between it and the straight- 
 edo-e ; but if any hollow be present, the breadth and depth of 
 the line of light which appears, betrays its extent. — Arris is 
 a common term in the arts, applied to signify the line of con- 
 .ourse or meeting of two surfaces. 
 
 : Let us now suppose that one surface of the block will bear 
 examining in the different ways above mentioned; it will then 
 coincide with the table so exactly, that when laid upon it, the 
 finest hair could not be drawn out, or even moved, at whatever 
 part between the two planes a portion of it were placed. Not- 
 withstanding this, the surface, though very smooth, has not 
 been nicely polished ; the polishing we leave, if not to the 
 last, at leak till the opposite side, to which we now proceed, 
 is equally advanced. Here we have an additional object to 
 attend to ; we have not only to make the second side as level 
 as the first, but also to make it parallel with it at the same 
 time. The flatness is obtained by a repetition of the means 
 adopted to bring the first surface to that state, and the pa- 
 rallelism of the two sides is a necessary consequence of mak- 
 ing the block every-where equally thick. Having, therefore, 
 set a pair of callipers to the thickness intended, or adopted 
 some other equivalent mode of measurement, we frequently 
 examine it with respect to this particular. Callipers, in ex- 
 perienced hands, may be made to answer for this purposf; 
 very well, but they are apt to mislead the unwary, as they 
 afford different indications with slight differences in the man- 
 ner of holding them. In using them, therefore, we always 
 hold the centre of the head in such a manner that a line pass- 
 ing through it, and exactly midway between the points, shall 
 be parallel with the surfaces they inclose. Callipers are often 
 superseded by what is called a gauge, which is nothing more 
 than a piece of sheet iron, steel, or brass, cut in the manner 
 shown by fig. 8. pi. III. so that the distance between the 
 
MECHANICAL EXERCISES. 29 
 
 Advantage of using two gauges.— Effect of heat produced in filing. 
 
 legs A B, which ought to be exactly parallel with each other, 
 will exactly take in the proposed thickness of the block. It 
 is much easier to file correctly with the assistance of a gauge 
 than a pair of callipers ; and as the width of the former always 
 remains the same, we have an additional reason for preferring 
 it, when it is probable we shall often have occasion to measure' 
 like dimensions ; callipers, even if we wish to keep them to one 
 extent, being easily deranged by a fall or other common acci- 
 dent, and the frequent resetting of them frittering away 
 our time. 
 
 Those who wish to avail themselves of the utmost refinement 
 of artificial help, will not be displeased by the mention of 
 another expedient belonging to this subject. Two gauges may 
 be made, one of them of the true width, and the other a very 
 little wider ; the block may then be filed down to the latter 
 with rather a coarse file, and afterwards to the former with a 
 fine one. Those who think fit to take this pains, can scarcely 
 fail to succeed to their wish. Another hint deserves a place: 
 we are attentive to make the block fit the gauge tightly ; for 
 of the degree of tightness we can correctly judge; but if we 
 make them slack to each other, we can hardly determine the 
 degree of that slackness with even tolerable accuracy. 
 
 When we discontinued filing the first side, we have remark- 
 ed, that we left it unfinished or unpolished. The reason for 
 this requires explanation; labour bestowed in polishing, at 
 the time alluded to, would have been thrown away. The 
 heat produced by the strokes of a large coarse file, expands 
 the surface upon which they act, renders it convex, and the 
 opposite one necessarily concave. These effects remain in 
 part, after the equilibrium of temperature is restored. While 
 we are employed upon the first side, they are overlooked, 
 but when, after having nearly finished the second side, we 
 find upon trial with the ochre, that the other no longer af- 
 fords the same indications of correctness which it did before, 
 we are convinced of the propriety of having postponed the 
 finishing of it. In a block eight or ten inches long, the error 
 seldom exceeds the five-hundredth part of an inch, and there- 
 fore not having begun to polish when it occurs, we can use a 
 file, by which it will quickly be removed. 
 
 Having now so far accomplished our purpose as to have 
 rendered the two principal surfaces of our block correctly 
 plane and parallel with each other, we immediately direct our 
 attention to the four which yet remain in the rough state ; 
 these, for the sake of distinction, we may call the edges. We 
 begin upon one of the two longest of them, and file it true, in 
 the same manner as we did in the first example, except that 
 
30 MECHANICAL EXERCISES. 
 
 Prtrcaulioo in luiae; li>c square. — Manuer of fastening the block to be filed. 
 
 we make use of a square, applied alternately from the two 
 sides already filed, in order to assist us in keeping it exactly 
 ut ri'^ht anijles with them. As soon as this edge is true, we 
 make the opposite one parallel with it by a suitable gauge, 
 checking the chance of error by applying the square, which 
 \an quickly be run along the whole length of the edge ; and 
 ascertaininor, as usual, the general tlatness of the whole sur- 
 face, by the use of the ochre and table. As soon as two 
 of the edsces have been made true, the remaining two are 
 brou;j"ht to the same state, by a repetition of exactly the same 
 means. 
 
 If we are provided with a rectangular bar of iron, or any 
 hard metal, tlie sides of which are very smooth, and exactly 
 perpendicular when it is placed upon the table, we may make 
 use of it, in the filing of these edges, as follows : cover one 
 side with the ochre and oil, place upon the table either of 
 the sides at right angles w ith the one thus coated, opposite 
 which place that edge of the block which is to be tried ; 
 press the block and the bar down upon the table and against 
 each other at the same time, moving one of them while they 
 are in contact, backwards and forwards two or three times. 
 By the marks left upon the block, we detect at once all its 
 deficiencies. This mode of trial would also completely suc- 
 ceed in other cases ; for example, if we had to tile the inside 
 of a frame such as printers use to fasten their types in, to 
 which no other method would be so advantageously appli- 
 cable. 
 
 We pass one of our smoothest files along the arris of the 
 two surfaces upon which we are going to apply the square, in 
 order to take off that extreme sharpness, and those overiiano;- 
 ing particles of iron, produced by filing, and which would 
 prevent that instrument from afibrding a correct indication of 
 the angle examined. 
 
 As our block is too broad to be held between tlie chaps of 
 the vice, we placed it, before we began to file the principal 
 surface, upon a piece of stout board, in breadth about an inch 
 each way larger than itself. Close to the edge of the block, 
 we drove a strong nail here and there into the board, so as to 
 prevent its horizontal motion, but not its being lifted up and 
 taken off perpendicularly. By a square piece of wood, about 
 two inches broad, being firmly screwed to the under side of 
 tlie board, and fastened in the vice, a steady and convenient 
 supDort is obtained for our work. But as soon as the filing 
 of the edges was commenced, tiiis board was discarded, and 
 the vice alone, its teeth only being covered with lead, was 
 used to hold the block. 
 
MECHANICAL EXERCISES, 31 
 
 Ecoaoniicul way of using files. 
 
 Experienced plane-makers and others, who use files to 
 smooth their wood-work, select those, the teeth of which are 
 not jagged by cross cutting, and we find that upon iron, files 
 of this sort answer better for polishing than any other. We 
 accordingly use them of such a degree of fineness as will effect 
 our purpose, if that can be effected by a file : the last degree of 
 smoothness can only be obtained by grinding. We always 
 take care, when using a fine file, to spread the ochre so thin 
 as hardly to colour the table, otherwise we should presently 
 choke up its teeth, and not finish our work with so much ex- 
 actness. Ochre is too soft a material to injure a file, and when 
 it does choke up the teeth, it may be removed with a brush. 
 
 We have now detailed the means by which a rectangular 
 prism of cast iron, possessing a remarkable degree of correct- 
 ness of figure, may be produced. On the uses for which such 
 a block may be required, on the application of the general plan 
 to other sorts of work, and on the importance and multiplied 
 advantages of correct filing in general, we presume it is un- 
 necessary for us to enlarge. If there be those who are more 
 attentive to authority than reason, and who inquire by whom 
 a process is used, rather than what is its merit, w'e assure 
 them that the method of filing here pointed out, is adopted in 
 the far-famed manufactory of Bolton and Watt, at the Soho, 
 near Birmingham, 
 
 Workmen who have various sorts of metal to work, have an 
 economical mode of management in the use of files, which 
 deserves to be noted. They use all their new files to brass 
 in the first instance ; when the original keenness of the teeth 
 has been diminished by this metal, they lay them aside, to be 
 ready for filing cast iron ; and when they cease to be sharp 
 enough for cast iron, they use them to malleable iron, for 
 which they will serve tolerably well awhile longer. Let this 
 order be reversed ; let a new file be used first to malleable 
 iron, then to cast iron, and lastly to brass, it will hardly do 
 more than half the service ; the teeth strike into the malleable 
 iron, and the best portion of them is broken off; the file is 
 then of little value for cast iron ; and glides over brass, which 
 requires a keen edge, almost without effect, unless seconded 
 by a great exertion of strength. 
 
 The last uses of a file may be to smooth w ood or metal re- 
 volving in the lathe ; some keep them for a short time red hot 
 in the open fire, and then retemper them before they use them 
 in this way; the scale which they cast leaving them somewhat 
 sharper than they were previously : others make such of their 
 old files as have stood well into screw-plates and chisels, on 
 the presumption that they cannot have better steel. 
 
32 MECHANICAL EXERCISES. 
 
 General remarks. — Method of finding a true plane. 
 
 Grinding. 
 
 We restrict the signification of this term to the abrading of 
 metals and other substances, by rubbing them against each 
 other. In general, some hard body, such as emery, in a pul- 
 verulent state, and mixed with water, oil, &c. is interposed 
 between the pieces ground together; but when one or both of 
 them, as in the example of grit-stone, is composed of particles 
 which will cut, and motion alone separate, the powder, and 
 even the fluid, are often dispensed with. When grinding is 
 only employed to produce a smooth shining surface, it is 
 called polishing. 
 
 An observation or two has already been thrown out on the 
 subject of grinding ; but this method of working metals, re- 
 quires a little further elucidation. If the artist be not fully 
 aware of the general principle upon which he must proceed, 
 and of what grinding will really enable him to do, he will 
 often incur considerable expense and disappointment. 
 
 In (rrindino; two surfaces tos^ether, the usual sources of error 
 are, that the cutting powder is unequally spread between them, 
 that they are not every where equally hard, and that some 
 parts receive a greater number of strokes than others. 
 
 Suppose we have two similai blocks of iron, and wish to 
 make one of them, at least, perfectly tiat, from which state, as 
 far as the eye can judge, let it be granted that they are 
 already not very remote. Let us grind them together with 
 emery. Having done this for some time, they assume a very 
 smooth appearance; but we find, upon examining them, that 
 we have not attained our object — they are not plane. If we 
 have been grinding long enough, we shall generally find, 
 that one of them is pretty regularly concave, and the other 
 correspondently convex ; and we may be assured that, with- 
 out some further device, no variety of rectilinear, circular, or 
 elliptical strokes, will ever bring either of them to the state 
 we desire. But suppose we are provided with a third 
 block of a similar size, and concave in about the same 
 degree as one of the two in question ; if we grind these two 
 concave surfaces together, they will mutually correct the 
 defects of each other. Here then we are furnished with the 
 key to success hi this art : — to grind one surface perfectly 
 flat, it is indispensably necessary to grind three at the same 
 time. In practice, we do not ascertain the state of each sur- 
 face by experiment, with the view of bringing similar defects 
 together, (as just stated in order to render the explanation 
 more evident,) but proceed by grinding them interchangeably, 
 often reversing their position, pressing upon every part with 
 
MECHANICAL EXERCISES. 33 
 
 Methods of grinding. 
 
 an equal weight, spreading the emery, or whatever may be used 
 as the cutting powder, as evenly as possible, sometimes mov- 
 ing diagonally and sometimes from side to side, with an in- 
 termixture of rectilinear, circular, and elliptical strokes. To 
 execute good work in this art, so far at least as plane surfaces 
 are concerned, manual labour can alone be depended upon. 
 Though glass is much mere easily ground true than most of 
 the metals, yet the plate glass, which is ground by machinery, 
 is perhaps never so perfect as it might be made by the hand, 
 and sometimes parts of looking-glasses called plane, are de- 
 monstrably possessed of the properties of concave or convex 
 mirrors. 
 
 Emery is the substance most commonly employed as the cut- 
 ting powder in the grinding of metals. The artist should be 
 provided with it of various degrees of fineness to be used in 
 succession ; and as it is necessary that the finest sort should 
 be entirely free from the admixture of coarse grains, it is ad- 
 visable, on this account, to keep it at a distance from the 
 other kinds. It is the practice of some, not to polish in the 
 same room, or even in the same clothes, they used for the 
 rougher part of their operations. As soon as the emery 
 thrown upon the work, is found to have little or no effect, 
 being converted into mud, it must be entirely washed away, 
 and the supply renewed. When iron or brass have been 
 ground with emery, it is difficult to file them ; the file wears 
 very fast, and produces but a slight effect, till the emery which 
 has entered into the pores of the metal is removed. 
 
 Water, having the strong recommendation of cheapness, is 
 the fluid most commonly used in grinding ; but with oil, metals 
 are cut more evenly, expeditiously, and fewer scratches are 
 produced. It is therefore most used to finish and polish with. 
 
 The softer the three bodies intended to be ground, the 
 greater may be their inequalities at the commencement of this 
 labour, with an equal chance of making them perfect in the 
 end. Silicious substances, such as emery and sand, take 
 such powerful hold upon calcareous fossils, to which class of 
 substances belong almost all the hardest stones in common 
 use, that they may be ground, from rather a rough state, till 
 they become very true. The stone and marble masons care 
 nothing about that high degree of accuracy which is here 
 contended for; but if any one who may be disposed to follow 
 the directions given in the last section for filing, be supplied 
 by one of these artists with three slabs of marble, or some 
 close-grained homogeneous stone, made true in their best 
 manner, he may afterwards, with proper care, finish them, ac- 
 cording to the principle here laid down, so as to possess a 
 2.— Vol. I. F 
 
34 MECHANICAL KXEUCISES. 
 
 Methods of ^indin^. 
 
 table, with which he will probably have little reason to be 
 dissatisfied. This must be understood to be the resort, when 
 it is impossible or inconvenient to procure an iron table, which 
 is always to be preferred, and may in fact be manufactured 
 with ease, especially if no extraordinary size is required, by 
 the help of one of stone, obtained as above recited. 
 
 The metals are of a much more unyielding nature than 
 stone ; and when they are the subjects of our labour, a different 
 line of policy must be pursued. The grinding- of them should 
 always be comparatively a short operation. In the removal of 
 considerable inequalities, the file will often do more in a quarter 
 of an hour, than grinding in a whole day; but when the imper- 
 fections extended over a surface become exceedingly numerous 
 and minute, such, in short, as a file or any tool which acts only 
 on a small spot at once, is necessarily disposed to leave, and 
 which scarcely at all effect the general level of the whole, the 
 case is reversed, and the file will not produce so finished an 
 effect in a day, as grinding in a quarter of an hour. If any 
 one of three plates or slabs, which have been ground together, 
 will adhere so closely to either of the other two, as to exclude 
 the atmosphere from the surfaces in contact, and allow both 
 to be lifted up by taking hold of one of them, a strong pre- 
 sumption is afforded that all of them are true. 
 
 The common grind.'itone, upon which tools are sharpened, 
 requires no description; yet it may be useful to observe, that it 
 will be much easier to grind plane-irons, chisels, Sec. upon it, if 
 the circumference in the direction of the axis be kept a little 
 convex. When these and similar tools are ground, the stone 
 should also turn towards the person holding them, so as to run 
 against their edge. Grindstones are often turned by machinery. 
 When the velocity is very great, they do not cut well, and will 
 also sometimes break, a circumstance which has occasioned the 
 most serious accidents. It was formerly the invariable practice 
 to hang them on an axis passing throvigh a piece of wood 
 which occupied a square hole cut through the centre of them; 
 when this wood became wet, the swelling of it powerfully se 
 conded the tendency of rapid motion to break them. The prac- 
 tice of securing them by a circular plate screwed firmly against 
 each side, was therefore adopted, and is becoming more general. 
 The stones upon which cutlery is ground, are carried at the 
 rate of about six hundred feet per second. At Wickersley, 
 near Sheffield, stones are obtained which heat so little that 
 they admit of being used dry. When clogged, they are 
 cleared with a bar of soft iron. This process of dry grinding 
 is of the most destructive nature to the men employed ; the 
 
 sharp particles of iron constantly flying about, find their 
 
MECHANICAL EXERCISES. 35 
 
 The glazor. — The polisher. — The brush. — Grinding copper-plates. 
 
 way to the lungs, and ultimately produce incurable com- 
 plaints. 
 
 In the manufacture of cutlery, the use of the stone is fol- 
 lowed by that of the lap or glazor. This is formed of a num- 
 ber of pieces of wood, in such a manner that the edge or face 
 may always present the end-way of the wood, or it would 
 change its figure. Some glazors are covered with strong 
 leather, others with an alloy of lead and tin. After the face 
 of them is turned to the proper form and size, it is covered 
 with rakes or notches, which are filled up with emery and tal- 
 low. The glazor is carried at the rate of fifteen hundred feet 
 per second. 
 
 The polisher, in the same branch of art, is a circular piece of 
 wood, running upon an axis like the stone and glazor. It is 
 covered with buff leather, and its surface is from time to time 
 replenished with colcothar. The polisher is not allowed to 
 move quicker than about seventy or eighty feet per second. 
 
 A brush, consisting of a circular piece of wood fitted upon 
 an axis, and set upon the face with strong bristles, is used to 
 polish those parts which have been filed, and which the lap 
 and the polisher cannot touch. 
 
 Copper-plates are prepared for the engraver, by placing 
 them upon a board forming an inclined plane, and rubbing 
 them with a piece of sharp grit-stone, first in the direction of 
 their length, and then in that of their breadth, with rectilinear 
 strokes, till the marks of the planishing hammer and other de- 
 fects are taken out. The lower edge of the plate may lie in a 
 trough of water, so that the necessary supply of this fluid will 
 be carried over the plate without stopping to throw it on. The 
 scratches left by the grit-stone are then removed by rubbing in 
 the same way wdth a piece of pumice stone. Charcoal is next 
 used with water, to remove the scratches left by the pumice 
 stone, and the operation is finished with the same substance and 
 a little oil. If the piece of charcoal, when it is tried, glide over 
 the surface with little or no effect, another piece must be se- 
 lected. Its fitness for the purpose may be distinguished by its 
 making no scratches, yet, when wet and rubbed upon the cop- 
 per, seeming rough, and makingalow murmuringnoise. Clock- 
 makers finish the plates between which the wheels of clocks are 
 enclosed, in a similar manner, except that they do not take so 
 much pains to remove every scratch, but obtain a high gloss 
 by rubbing with colcothar or putty laid upon leather. A piece 
 of old hat makes a good polisher, as does also paper rolled up 
 till it forms a cylinder of sufficient size, when the end, being 
 first cut straight, must be used. If the hat, in the dying of 
 which iron is employed, be immersed a few minutes in sulphu- 
 
36 MECHANICAL EXERCISES. 
 
 Tripoli. — Polishing rouge. — Bnrnisher. — Annealing. 
 
 ric acid, the iron will pass to the state of red oxide, and it then 
 answers the purpose still better than before. 
 
 A species of the clay genus, called tripoli, or rotten stone, is 
 much used to prepare metals, marble, and glass, for receiving, 
 after the use of emery or sand, the highest polish of which 
 thev are capable. It is of a yellow colour, tastes like common 
 chalk, and is rouoh or sandy between the teeth, although no 
 sand can be separated from it ; and its particles are in fact so 
 fine and soft, as to leave no perceptible scratches. 
 
 Goldsmiths, to give the last polish to their wotk, which 
 thev commonlv call colouring it, employ what is termed po- 
 lishino; rouge.' This powder is said to be a very pure native 
 red oxide of iron. Sometimes it is of a red inclining to pur- 
 ple, and has the appearance of very fine colcothar ; but this 
 sort is of inferior quality. 
 
 Burnishing is too nearly allied to the above methods of po- 
 lishing, to require a separate section. The burnisher used by 
 the makers of spurs and bits, &c. is partly iron, partly steel, 
 and partly wood. It consists of an iron bar, with a wooden 
 handle at one end, and a hook at the other, to fasten it to 
 another piece of wood held in the vice, while the operator is 
 at work. In the middle of the bow, withinside, is what is 
 properlv called the burnisher, being a triangular piece of steel 
 with a tail, whereby it is riveted to the bow. 
 
 The iron ore called red haematites, or blood-stone, is much 
 used for burnishing metals, but steel burnishers are more com- 
 mon for this purpose than any other sort. They are much va- 
 ried in shape accordino- to the fancy of the user, or the work 
 for which thev are intended. The form shown at fig. 9. pi. III. 
 or some near resemblance of it, is most frequently adopted. 
 The steel of which a burnisher is made should be in a very 
 hard state, entirely free from flaws, and exquisitely polished. 
 
 Ainiealing. 
 
 In a considerable number of instances, bodies which are ca- 
 pable of undergoing ignition, are rendered hard and brittle by 
 sudden cooling. Glass, cast iron, and steel, are the most 
 remarkably atiected bv this circumstance ; the inconveniences 
 arising from which are obviated by cooling them very gradu- 
 ally, and this process is called atmealing. Glass vessels are 
 carried into an oven over the great furnace, called the leer, 
 where they are permitted to cool, in a greater or less time, ac- 
 cording to their thickness and bulk. Steel is most effectually 
 annealed by making it red hot in a charcoal fire, which must 
 completely cover it, and be allowed to go out of its own accord. 
 Cast iron, which may require to be annealed in too large a 
 
MECHANICAL EXERCISES. 37 
 
 Annealing. — The straight-edge. — Square. 
 
 quantity, to render the expense of charcoal very agreeable, 
 may be heated in a turf or cinder fire, which must completely 
 envelope and defend the pieces from the air till they are cold. 
 The fire need not be urged so as to produce more than a red 
 heat; a little beyond this, bars and thin pieces would bend, if 
 destitute of a solid support; and would even be melted without 
 any vehement degree of heat. If it be required to anneal a 
 number of pieces expeditiously, and the fire is not large enough 
 to take more than one or two of them at once ; or if it be 
 thought hazardous to leave the fire to itself, from an appre- 
 hension that the heat might increase too much, the fol- 
 lowing scheme may be adopted : heat as many of the pieces 
 at once as may be convenient, and as soon as they are red 
 hot, bury them in dry saw-dust. Cast iron, when annealed, 
 is less liable to warp by a subsequent partial exposure to 
 moderate degrees of heat, than that which has not undergone 
 this operation. 
 
 The above methods of annealing render cast iron easy to 
 work, but do not deprive it of its natural character. Cast iron 
 cutlery is therefore stratified with some substance containing 
 oxygen, such as poor iron ores, free from sulphur, and kept in 
 a state little short of fusion for twenty-four hours. It is then 
 found to possess a considerable degree of malleability, and is 
 not unfit for several sorts of nails and edge-tools. 
 
 Copper forms a remarkable exception to the general rule of 
 annealing. This metal is actually made softer and more flexi- 
 ble by plunging it when red hot into cold water, than by any 
 other means. Gradual cooling produces a contrary effect. 
 
 The Straight-edge and Square. 
 
 A steel ruler, made, by filing and grinding, perfectly true, is 
 called, by mechanics, a straight-edge, and is an instrument of 
 great use and value to the workman. A straight-edge is not 
 made to any fixed length, which must be varied according to 
 the work to which it is intended to be applied. Unless very 
 short, its breadth is commonly from one to two inches, and its 
 thickness should in all cases be sufficient to support its own 
 weight. To have this property, it must be thicker than would 
 be generally supposed. If it be made thirty inches long, it 
 should not be less than half an inch thick ; and if forty inches 
 long, its thickness should be five-eights of an inch. If made 
 materially thinner of these lengths, it will be found to sink in 
 the middle, when supported at the ends ; a fact easily ascer- 
 tained by trying it with the arris of another straight-edge. 
 
 This instrument is not even known by name, to a great 
 number of provincial artists, who might be laenefited by its use 
 
38 MFXHANICAL EXERCISES. 
 
 Slrajght-edge. — Square. 
 
 It should be made of the best cast steel, and filed, with all pos- 
 sible exactness, agreeably to the directions given on that sub- 
 ject. Three must be made at the same time, and they must 
 be ground with strict attention to the general directions for 
 that operation. If they be true, in whatsoever manner they 
 be laid flat upon each other, no light can pass between them. 
 Straight-edges are commonly bevelled on one side, so as to 
 make one edge considerably thinner than the other, in the 
 same manner as common wooden riders are generally made. 
 
 The use of a straight-edge in filing has already been shown. 
 In turning, it is equally useful, where great accuracy is wanted ; 
 for example, in turning a cylinder, or a cone, the application 
 of it will instantly show the irregularities of the figure. 
 
 Two rectangular prisms, joinetl at one of their extremities, 
 so as to form a right angle both internally and externally, con- 
 stitute the instrument called a square. See pi. III. fig. 10. A 
 good square is not often met with; those pieces of metal sold 
 under that name, by the clock and watch tool makers, are ge- 
 nerally worthless things, no two of them corresponding with 
 each other. A square, therefore, fit to serve as a guide in the 
 filing of metals, must be the work of the individual artist by 
 whom it is required ; or at least he must finish it, — any com- 
 mon workman will make it true to the fiftieth part of an inch. 
 It should be made of good steel, and as it is not to be tem- 
 pered, it should be well hammered, so as to be rendered stiff' 
 and elastic. Like the straight-edge, also, it should be filed 
 with the greatest accuracy, and ground and polished on every 
 side. 
 
 To try whether the square is accurate or not, lay it on a 
 level plate of metal at the least as broad as itself, and twice its 
 length, provided also with a ledge perpendicular to it, and 
 known to be truly flat. Against this ledge, so as to reach to 
 about the middle of it, press the external edge of one limb of 
 the square. Hold it steadily there, and with a graver, or some 
 finely pointed steel instrument, draw a line upon the plate, as 
 close to the external edge of the other limb as possible. Now 
 turn over the square, so that it shall lie on its other side, and 
 ])reiis the same edge against the other half of the ledge. While 
 thus pressed against the ledge, Ijring it up to the line just 
 drawn, and accurately examine if another line can be made 
 exactly coincident with it. If this can be done, the square is 
 true ; if it cannot, as soon as any part of the square touches 
 the line, draw another line ; an acute angle will be produced, 
 and half the breadth of this angle, at any given distance from 
 the ledge, shows the deviation of the instrument from a right 
 angle, at the same distance from the same point. 
 
MECHANICAL EXERCISES. 39 
 
 Different methods of cntting screws. 
 
 Screws. 
 
 Screws are cut principally by means of screw-plates, by 
 stocks or dies, and in the lathe. The part left standing be- 
 tween the spiral groove of a screw, is called the thread. The 
 pin by which the spirals of a screw nut are formed, is called a 
 tap. When it is cylindrical, or of the same diameter in every 
 part, it is called a plu^ tap ; and when it forms the portion of 
 a cone or a pyramid, it is aptly enough called a taper tap, or, 
 as it is the most frequent form, it is simply denominated a 
 tap. The screw cut by a tap is called an inside or concave 
 screw; the tap itself, and all such screws as are or can be 
 formed by plates or dies, are, for necessary contradistinction, 
 called outside or convex screws. The screw-plates on sale at 
 the ironmongers' shops, are, in effect, generally worn out 
 before they are used. In a good tap, the groove is sharp at 
 the bottom, as the thread is at the top, and in proportion as 
 they vary from this configuration they may be considered bad. 
 The longitudinal section of ordinary taps and plates, instead 
 of being bounded by a zigzag line, and the thread appearing 
 like a series of triangles touching each other at the base, is 
 bounded by an undulating line, and the groove, though seldom 
 perhaps so bad as to correspond with the segment of a circle, 
 very often comes no nearer a triangle than a portion of the 
 smaller end of an oval. Hence those who require perfect 
 screws for any piece of mechanism, will perceive the necessity 
 of examining the taps and plates intended to be used in mak- 
 ing them. 
 
 A screw-plate is a cheap and handy instrument for making- 
 screws; but as it cuts at once the whole depth of the groove 
 which it will make, it is apt, from the force necessarily em- 
 ployed in forcing it forward, to twist the pin upon which it 
 operates. This is a serious inconvenience, when the pin has 
 been turned, and is required to be quite straight. The best 
 outside screws are therefore cut with what are called stocks 
 or dies, of which we have given a plan in pi. III. fig. 11. 
 although, like screw-plates, they are almost too well known to 
 require notice, unless it be for the purpose of laying before 
 those who are unaccustomed to such things, such hints as may 
 enable them to prevent their work from being spoiled, by the 
 ignorance or carelessness of those whom they employ for its 
 execution. A square frame, A B, is welded to two handles, 
 C D, and on the inside of the opening, next each handle, is an 
 angular projection to the extent shown by the dotted lines. 
 This projection takes hold in a corresponding groove cut in two 
 pieces of steel, bb, the edge of one of which is shown at fig. 12. 
 
40 MECHANICAL EXERCISES. 
 
 Screw stocLs Tafs. 
 
 These pieces are put in at the part c, where there is no 
 antrular projection; they are then slipped down to their pro- 
 per place, and a piece of iron put over them, so as to oc- 
 cupy a great part of the space c. Upon this piece of iron 
 is pressed the end of the screw ^ by which the pieces, bh, are 
 brought and kept as near to each other as may be required. 
 Thev are then tapped, so that one-half of the concave screw 
 formed is upon each of them. Before they are tempered, 
 they are often ground or filed so as to make the aperture, 
 when they meet, less than a circle ; by which means they can 
 be used to cut a smaller screw than they were originally tap- 
 ped with. In using them, the screw^" can be turned back, 
 so that the aperture will easily admit the pin intended to be 
 cut ; thus is obtained the advantage of making the groove 
 very shallow in the first instance, and of deepening it gradu- 
 ally by turnincr further in the screw /". 
 
 The bolt or pin intended to be tapped, either with a screw- 
 plate or stocks, is tapered in a small degree at the extremity, 
 previous to the operation. If a screw-plate be employed, it is 
 then pressed downwards upon it, with a force proportionate to 
 its size, and turned, at the same time, with a progressive and re- 
 trosressive motion, always, in advancing, (to effect the neces- 
 ar)' revolution,) going over a greater space than in returning. 
 Stocks are used with a similar motion, but they are not press- 
 ed downwards except at the commencement, before a thread 
 of one revolution has been formed upon the pin. Screw- 
 plates and stocks are often ruined by being used to iron rough 
 from the forofe, and covered with scales, which, from their hard 
 gritty nature, grind away the threads. 
 
 As in makins: outside screws, the pin is tapered at the ex- 
 tremity, that the operation may be more readily commenced ; 
 so, in making inside screws, (which, when made for screwing 
 upon bolts, are called nuts,) the taper tap is, for the same rea- 
 son, used first, and aftei-wards the plug tap to finish with. 
 
 The tap-^crench is simply a lever, with a hole in or about 
 the middle of it, to admit the rectangular head of the tap, for 
 the purpose of turning it round, in the same manner as the 
 plate and stocks are turned. 
 
 The taps used to cast iron are made square ; for brass they 
 ere sometimes square, and sometimes triangular. As with 
 these shapes the thread can only be on each arris, they clear 
 their way better ; and there is room in the hole for the particles 
 which are rapidly cut from these metals ; and to clear away 
 which, if this provision ^^ere not made, the tap must very fre- 
 quently be drawn out ; or it will be broken by the force ap- 
 plied to overcome the accumulated obstruction to its progress. 
 
MECHANICAL EXERCISES. 41 
 
 Properties of copper. 
 
 Taper taps for malleable iron are frequently squared ; but plug 
 taps for the same metal are only channelled, in the direction 
 of their length, in two or three places, so as to take out the 
 thread where the grooves are made. 
 
 The method of cutting screws in the lathe, will be found in 
 the section on Turning. 
 
 Copper. 
 
 We refer to the article of Chemistry, for a minute enumera- 
 tion of the whole of the known metals ; but in this place we 
 shall, with the exception of iron, which has already been no- 
 ticed rather at length, introduce a general practical view of 
 the properties, applications, and combinations with each 
 other, of those most frequently occurring in common arts 
 and common life. Making this our plan, the first object 
 claiming our attention is copper. 
 
 Copper is a very brilliant, sonorous metal, of a fine red co- 
 lour, possessing a considerable degree of hardness, and elasti- 
 city. It is extremely malleable, and may be reduced to leaves 
 so 'fine, that they may be carried about by the wind. Its 
 tenacity is very great. A wire of one-tenth of an inch in 
 diameter, will support a weight equal to 300/^5. avoirdupoise 
 without breaking. It does not melt till the temperature is 
 elevated to about 27° of Wedgwood, or (by estimation) 1450'^ 
 of Fahrenheit. When rapidly cooled, it exhibits a granulated 
 and porous texture. When the temperature is raised beyond 
 what is necessary for its fusion, it is sublimed in the form of 
 visible fumes. Its greatest malleability is at a low red heat. 
 None of the malleable metals is so difficult to file or turn 
 smooth as copper; but it is cut by the graver, or ground by 
 gritty substances, with great ease. 
 
 When miners wish to know whether an ore contains copper, 
 they drop a little nitric acid upon it ; after a little time, they 
 dip a feather into the acid, and wipe it over the polished blade 
 of a knife ; if there be the smallest quantity of copper in it, 
 this metal will be precipitated upon the knife, to which it will 
 impart its peculiar colour. — Roman vitriol, much used by 
 dvers, and in many of the arts, is a sulphate of copper. A 
 solution of this salt is used for browning fowling-pieces and 
 tea-urns. 
 
 In domestic economy, the necessity of keeping copper 
 vessels perfectly clean, cannot be too strongly inculcated; but 
 it is worthy of remark, that fat and oily substances, and vege- 
 table acids, do not attack copper while hot ; and therefore 
 copper vessels may be used, for culinary- purposes, with perfect 
 safety, if no liquor be ever suffered to grow cold in them. The 
 2.— Vol. L G 
 
4% MECHANICAL EXERCISES. 
 
 .Maoufactnre of br»«s 
 
 mere tinning of copper and brass vessels does not afford com- 
 plete security, as it is never so perfect as to cover every part. 
 
 Compounds formed by the mixture of two or more different 
 metals, are called alloys. The alloys of copper, especially 
 those in which this metal predominates, are more numerous 
 and important in the arts than those of any other metal. 
 Many of them are perfectly well known, and have been imme- 
 morially in use. The exact composition, and particularly the 
 mode of preparing several, are kept as secret as possible. By 
 the aid of chemistry, we may detect the precise composition 
 of an alloy ; yet we may not always be able, by common me- 
 thods, to produce a mixture having all the excellencies, which, 
 perhaps, mere accident has taught the possessor of the secret 
 to combine. 
 
 Brass is the most important of all the alloys of copper. It 
 is more fusible than copper, less liable to tarnish from ex- 
 posure to the atmosphere, and its fine yellow colour is more 
 agreeable to the eye. It is much more malleable than copper 
 when cold, but less malleable when hot ; at a low red heat, it 
 crumbles under the hammer. Sieves of extreme fineness are 
 woven with brass wire, after the manner of cambric weaving, 
 which could not possibly be made with copper wire. Three 
 parts of copper and one of calamine, or native carbonate of 
 zinc, constitute brass. The calamine is first pounded in a 
 stamping mill, and then washed and sifted, in order to sepa- 
 rate the lead with which it is mixed. It is then calcined on a 
 broad, shallow, brick hearth, over an oven heated to redness, 
 and frequently stirred for some hours. In some places, it is 
 calcined in a kind of kiln, filled with alternate layers of cala- 
 mine and charcoal, and kindled from the bottom, where a suf 
 ficient quantity of wood has been deposited for the purpose. 
 When tlie calamine has been thoroughly calcined, it is ground 
 in a mill, and mixed at the same time with a third or a fourth 
 part of charcoal, and is then ready for the brass furnace 
 Being put into crucibles with the requisite proportion of grain 
 copper, copper clippings, or refuse bits of various kinds, the 
 whole is covered with charcoal, and the crucibles luted up 
 with a mixture of clay or loam and horse dunor. The heat 
 employed, is, for a considerable time, not sufficient to melt 
 the copper, which it is at length raised so as to fuse, and the 
 compound metal is then run into ingots. 
 
 In general, the extremes of the highest and lowest propor- 
 tion of zinc are from twelve to twenty-five per cent, of the 
 brass. Even with so much as twentv-five per cent, of zinc, 
 brass is perfectly malleable, if well manufactured, though zinc 
 itself scarcely yields to the hammer at common temperatures. 
 
MECHANICAL EXERCISES- 43 
 
 Pinchbeck. — Tombac. — Prince's metal. — Bronze. — Bell-metal. 
 
 Good brass, when received from the foundery, is nearly 
 inelastic, but exceedingly flexible, and when polished, the 
 naked eye cannot discover any pores, which are frequently 
 observable in the brass made in the country. The liberal use 
 of the hammer imparts a considerable portion of elasticity to 
 brass, and renders it at the same time less flexible. Clock- 
 makers, watch-makers, and all artists who employ this metal, 
 in forms that admit of the operation, hammer it well before 
 they turn or file it, otherwise their work would wear indiflPer- 
 ently, and a trifling cause injure its figure. Brass is not mal- 
 leable when ignited. 
 
 Hammering is found to give a magnetic property to brass, 
 perhaps occasioned by the minute particles of iron separated 
 from the hammer and the anvil during the process, and forced 
 into its surface. This circumstance makes it necessary to 
 employ unhammered brass for compass boxes and similar 
 apparatus. 
 
 Five or six parts of copper and one of zinc, form pinchbeck. 
 Tombac has still more copper, and is of a deeper red than 
 pinchbeck. Princes' metal is a similar compound, excepting 
 that it contains more zinc than either of the former. 
 
 The alloys of copper with diff'erent proportions of tin, are of 
 great importance in the arts. They form compounds which 
 have distinct and appropriate uses. Tin renders copper more 
 fusible, less liable to rust, harder, denser, and more sonorous. 
 Copper and tin separately, are not more remarkable for their 
 ductility, than, when united, the compounds they form are for 
 their brittleness. 
 
 Eight to twelve parts of tin, combined with one hundred 
 parts of copper, form bronze, which is of a greyish yellow 
 colour, harder than copper, and the usual composition for 
 statues. The customary proportions for bell-metal are, three 
 parts of copper and one of tin. The greater part of the tin 
 may be separated by melting the alloy, and then throwing a 
 little water upon it. The tin decomposes the water, is oxidiz 
 ed, and thrown upon the surface. The proportion ot' tin in 
 bell-metal is varied a little at different founderies, and for 
 different sorts of bells. Less tin is used for church bells than 
 clock bells ; and in very small bells, a trifling quantity of zinc is 
 used, which renders the composition more sonorous, and it is 
 still further improved in this respect, by the addition of a 
 little silver. A small quantity o( antimony is occasionally 
 found in bell-metal. When copper, brass, and tin, are used to 
 form bell-metal, the copper is from seventy to eighty per cent, 
 including the proportion contained in the brass, and the re- 
 mainder is tin and zinc. When tin is nearly one-third of the 
 
44 MECHANICAL EXERCISES. 
 
 Properties and uses of tin mod it* allovs. 
 
 alloy, it is then beautifully white, with a lustre almost like 
 mercury, extremely hard, close-grained, and brittle; but when 
 the proportion of tin is one-half, it possesses these properties 
 in a still more remarkable degree, and is susceptible of so ex- 
 quisite a polish, as to be admirably adapted for the speculums 
 of telescopes. If more tin be added than amounts to half the 
 weio-ht of the copper, the alloy begins to lose that splendid 
 whiteness for which it is so valuable as a mirror, and becomes 
 of a blue grey. As the quantity of tin is increased, the texture 
 becomes rough-grained, and totally unfit for manufacture. 
 
 Of Tin. 
 
 Tin is a metal of considerable importance in the arts. It is 
 of a silver white colour, very ductile, malleable, and gives out, 
 while bending, a peculiar crackling noise. Its specific gravity 
 is 7.291; a cubic foot weighs about olG/Z^s. avoirdupois. Its 
 purity is in proportion to its levity. It melts at the 400th de- 
 gree of Fahrenheit's thermometer, and promotes the fusibility 
 of the metals with which it is mixed. Two parts lead and one 
 of tin form plumber's solder, which melts sooner than either 
 of the metals separately. Eight parts of bismuth, five of 
 lead, and three of tin, form a metal which melts at a heat not 
 exceeding that of boiling water. Tea-spoons are made of this 
 alloy, to surprise those unacquainted with their nature : they 
 have the appearance of common tea-spoons, but are melted in 
 hot tea. 
 
 Tin is used to form boilers for dyers, and worms for recti- 
 fiers' stills. The common mixture for pewter, is 112 pounds of 
 tin, 15 pounds of lead, and six pounds of brass. But the name 
 of pewter is given to any malleable white alloy, into which tin 
 largely enters, and perhaps no two manufacturers employ the 
 same ingredients in the same proportions. The finest kinds of 
 pewter contain no lead Avhatever, but consist of tin with a 
 small quantity of antimony, and sometimes a little copper. 
 Pewter may be used for vessels containing wine, and even vine- 
 gar, provided the tin constitutes aboui three-fifths of the alloy. 
 
 The consumption of tin, in the operation called tinning, is 
 very considerable. The principal secret in tinning, is to pre- 
 serve the tin and the surface of the metal to which it is intended 
 to be applied, peifectly clean, and in a pure metallic state. 
 Thin plates or sheets of iron, which when coated with tin are 
 so well known under the name of tin-plates, white iron, or lat- 
 ten, are prepared by scouring them with sand. They are then 
 immersed in water acidulated with sulphuric acid, in which 
 they are kept for twenty-four hours, being occasionally turned 
 during that time, so that they may rust equally in every part. 
 
MECHANICAL EXERCISES. 45 
 
 Tinning sheet iron — copper and iron vessels. 
 
 When taken out, they are again scoured and made perfectly 
 clean. They are then dipped in pure water, and kept there 
 till wanted for tinning. The tin is melted in an iron crucible, 
 narrow, but deeper than the length of the iron plates, which 
 are plunged in downright, so that the tin swims over them. 
 The surface of the tin, to prevent its oxidation, is covered 
 with some oily or resinous matter. Reaumur states, that the 
 Germans cover it with suet, previously prepared by frying and 
 burning, which surprisingly puts the iron in a condition to 
 receive the tin. The melted tin must also have a certain 
 degree of heat ; if not hot enough, it will not adhere to the 
 iron ; and if it be too hot, the coat will be very thin, and the 
 plates discoloured. Plates intended to have a very thick 
 coat, are first dipped into the crucible when the tin is very 
 hot, and afterwards when it is cooler. For the second dip- 
 ping, the suet must not be prepared, but used in its common 
 state. The tin not only adheres to the surface of iron plates, 
 but penetrates and intimately combines with them. 
 
 Copper is tinned after it has been formed into utensils. If 
 the copper be new, its surface is first scoured with salt and 
 diluted sulphuric acid. Pulverized resin is then strewed 
 over the interior of the vessel, into which, after heating it to 
 a considerable degree, a sufficient quantity of melted tin is 
 poured, and spread upon it by means of a roll of hard twisted 
 flax, which renders the coating uniform. Pure tin is rarely 
 used for this purpose ; it is generally, though injuriously, 
 alloyed with a small proportion of lead. The use of the resin 
 is important ; for the heat given to the copper is sufficient to 
 oxidize its surface in some degree, and an alteration of this 
 sort, however slight, would prevent the perfect adhesion of 
 the tin. The resin is equally useful, in preventing the partial 
 oxidation of the tin, or in reviving the small particles of oxide 
 which may be formed during the operation. 
 
 For tinning old vessels a second time, the surface is first 
 scraped clean and bright with a steel instrument, or scoured 
 with iron scales, then pulverized sal ammoniac is strewed over 
 it, and the melted tin is rubbed on the surface with a solid 
 piece of sal ammoniac. 
 
 The process for covering iron vessels with tin, coi'responds 
 with that last described ; but they ought to be previously 
 cleaned with the muriatic acid, instead of being scraped or 
 scoured. Iron nails which cannot be conveniently tinned in 
 a bath, are easily covered with tin by including them, with a 
 due proportion of tin and sal ammoniac, in a stone bottle, 
 and agitating them while heating and cooling. 
 
 The following method of tinning is highly esteemed for its 
 
46 MECHANICAL EXERCISES. 
 
 Tinning of pins. — Properties of lead. 
 
 permanency and beauty ; the utensil is cleaned in the usual 
 manner; its inner surface is beaten on a rough anvil, or 
 scratched with a wire-brush, that the tinning may adhere more 
 closely to the copper; and one coat of pure tin is then laid oa 
 with sal ammoniac, as above directed for tinning old copper. 
 A second coat, consisting of two parts of tin and three of zinc, 
 must next be uniformly applied with sal ammoniac, in a simi- 
 lar manner: the surface is now to be beaten ; scoured with 
 chalk and water ; smoothed with a proper hammer ; exposed 
 to a moderate heat ; and lastly dipped in melted tin. This 
 sort of tinning effectually prevents the utensils from rusting. 
 
 Pins are whitened by filling a pan with alternate layers of 
 them and grain tin ; a solution of supcr-tartrate of potass 
 (cream of tartar) is then poured upon them, and they are 
 boiled for four or five hours. The tartaric acid first dissolves 
 the tin, and then gradually deposits it on the surface of the 
 pins, in consequence of its greater affinity for the zinc which 
 enters into the composition of the brass wire. 
 
 There are two kinds of tin known in commerce, viz. block 
 tin and grain tin. Block tin is procured from the common 
 tin ore; grain tin is found in small particles, in what is called 
 stream tin ore. It owes its superiority, not only to the purity 
 of the ore, but to the care with which it is washed and re- 
 fined 
 
 Of Lead. 
 
 Lead unites with most of the metals. It has little elasticity, 
 and is the softest of them all. Gold and silver are dissolved 
 by it in a slight red heat, but when the heat is much increas- 
 ed, the lead separates, and rises to the surface of the gold, 
 combined with all heterogeneous matters. This property of 
 lead is made use of in the art of refining the precious metals. 
 
 If lead be heated so as to boil and smoke, it soon dissolves 
 pieces of copper thrown into it ; the mixture, w'hen cold, is 
 brittle. The union of these two metals is remarkably slight, 
 for upon exposing the mass to a heat no greater than that in 
 which lead melts, the lead almost entirely runs oif by itself. 
 This process, which is peculiar to lead with copper, is called 
 eliquation. It has lately been discovered, that a certain pro- 
 portion of lead may be mixed with the metal formerly used 
 for white metal buttons, without injurino; the appearance; 
 thus affording a considerable addition of profit to the manu- 
 facturer. 
 
 The consumption of lead for water pipes, cisterns, and to 
 cover buildings, is very extensive. Sheet lead is made by suf- 
 fering the melted metal to run out of a box through a long 
 
MECHAXICAL EXERCISES. 47 
 
 Type metal. — Manufacture of small shot, 
 
 horizontal slit, upon a table prepared for the purpose. The 
 table is generally covered with sand, and the box is drawn over 
 it by appropriate ropes and pulleys, leaving the melted lead 
 behind to congeal in the desired form. The requisite unifor- 
 mity and thinness are given to these sheets, by rolling them 
 between two cylinders of iron acting upon the same principle 
 as the copper-plate printing press. 
 
 The alloy of lead and antimony is used for printers' types. 
 Chaptal made a great variety of experiments to ascertain the 
 best proportions of these metals to each other for this use. He 
 always found four parts of lead to one of antimony form the 
 most perfect composition. But if the antimony be pure, one 
 part of it, to seven or eight of lead, form an alloy too brittle 
 to be extended under the hammer, and as hard as the genera- 
 lity of types. To give hardness to the lead, is not the only 
 use of antimony in this composition. It renders the lead more 
 fusible, more fluid when melted, and as it expands in passing 
 to a solid state, it is calculated to produce a sharper impres- 
 sion of the mould, than could be easily obtained by lead alone. 
 Antimony (which in trade is commonly called regulus of anti- 
 mony, or regulus only,) requires, when alone, much more heat 
 for its fusion than lead, in combining with which metal, as it 
 is little more than half its weight, it rises to the surface, and 
 requires to be well stirred before it will incorporate. Different 
 parts of the same block of type-metal, often possess very dif- 
 ferent degrees of hardness. Stereot}"pe plates are' almost 
 always harder on the face than on the back. 
 
 The method of granulating lead in the making of small shot, 
 is curious. In melting the lead, a small quantity of arsenic is 
 added, which disposes it to run into spherical drops. When 
 melted, it is poured into a cylinder, w^hose circumference is 
 pierced with holes. The lead streaming through the holes, 
 soon divides into drops, which fall into water, where they 
 congeal. They are not all spherical ; therefore those that are 
 so, must be separated ; which is done by an ingenious contri- 
 vance. The whole are sifted on the upper end of a long, 
 smooth, inclined plane, and the grains roll down to the lower 
 end. But the pear-like shape of the bad grains makes them 
 roll down irregularly, and they waddle as it were to a side, 
 while the spherical ones roll straight on, and are afterwards 
 sorted into sizes by sieves. The manufacturers of the patent 
 shot have fixed their furnace, for melting the metal, at the top 
 of a tower one hundred feet high, and obtain a much greater 
 quantity of spherical grains, by letting the lead fall into the 
 water from this height, as the shot is gradually cooled before 
 it reaches the water. The arsenic is generally added in ex- 
 
48 MECHANICAL EXERCISES 
 
 Mciliod* of r#Tivui^ the oxiiles of lemd. 
 
 cess to a small quantity of lead, which is covered and closely 
 luted till the incoq)oration is complete. The compound is 
 called slag or poisoned metal. Ingots of this slag are then 
 added to soft pig lead, in such proportion as is found upon 
 trial to cause it to drop in a globular form. The smaller the 
 shot, the less the height from which it requires to be dropped. 
 The smallest kind need not fall from an elevation of more 
 than eight or ten feet. If the lead, at the time of casting, be 
 too hot, the shot will be apt to crack ; and if it be too cold, 
 it w ill stop up the holes in the cylinder. Instead of the cylin- 
 der, a plate of copper, about the size of a trencher, is used in 
 the making of small quantities, with a hollowness in the 
 middle, about three inches compass, pierced with thirty or 
 forty holes, according to the size of the shot wanted. The 
 part containino; the holes may be thin ; but the thicker the 
 brim, the better it will retain the heat. 
 
 The surface of melted lead, as every one knows, becomes 
 quickly covered with a skin or pellicle, often assuming dif- 
 ferent lively hues at first, and subsequently increasing in quan- 
 tity and darkness of colour. This eft'ect, termed by chemists 
 oxidation, as it is occasioned by the action of the oxygen of 
 the atmosphere, the activity of which is greater in proportion 
 to the heat of the lead, wastes the metal so fast, that it be- 
 comes an object of importance to those who melt much lead, 
 to check its formation, or to convert it, when formed, by the 
 cheapest process, into the metallic state again. A thick 
 coating of ashes of anv kind, will check the formation of the 
 oxide, and may be easily^ pushed back, when a quantity of 
 lead must be taken out of the crucible or melting pan. Char- 
 coal, which is also a good covering; for lead in the pan, will 
 convert dross into metal, when assisted by a sufficient heat ; 
 fat, oily, and bituminous substances in general, have a similar 
 effect. Common resin answers exceedingly well ; thrown in 
 powder upon melted lead, and stirred about, it immediately 
 converts the oxide into metal, causes the surface to shine 
 like mercury, and if any thing remains, it is only a black 
 dirt, containing little or no lead. But in taking off this 
 dirt, small globules of pure lead, skimmed off at the same 
 time, get mixed with it ; by throwing it into water, stii 
 ring it thoroughly, and pouring off all that does not im- 
 mediately sink, these grains may be separated. If part 
 of what has appeared to be dirt, is found to be so heavy as 
 instantly to sink to the bottom of water, it may be suspected 
 to be true dross or oxide, and may be revived by mixing it with 
 charcoal, and exposing it to a considerable heat. It is al- 
 ways, however, more prudent and economical, to use means of 
 
MECHAXICAL EXERCISES. 49 
 
 Observations on lea-^. — Zinc. 
 
 preventiuo- the formation of oxide, than to bestow much time 
 upon its revival. 
 
 Lead becomes less fluid every time it is melted, and by 
 much or frequent exposure to a hioh temperature, a state in 
 which it is said to be rotten, is superinduced. To use new 
 lead, and not to melt it oftener, or expose it to a greater heat 
 than is indispensable, are necessai-y precautions to preserve this 
 metal in its best state. Plumbers, when they cast it into sheets, 
 strew common salt upon the table, to facilitate its spreading, 
 when they are not using new lead, and are for that, or any 
 other reason, apprehensive that it will not run well. 
 
 The observations above recited on the management of lead, 
 applv, with equal propriety, to tin, antimony, zinc, bismuth, Sec. 
 and all the alloys of these metals with lead or each other. In 
 fact, as lead is so much cheaper than the other metals just 
 enumerated, the object of saving it from destruction is pro- 
 portionately of less consequence. 
 
 Of Zinc. 
 
 Zinc is a very combustible metal, of a bluish, brilliant white 
 colour. It seems to form the link between the brittle and mal- 
 leable metals. It is a modern discovery, that at a temperature 
 of from 210"^ to 300'^ of Fahrenheit, it yields to the hammer, 
 may be drawn into wire, or extended into sheets. After having 
 been thus annealed, it continues soft, flexible, and extensible, 
 and does not return to its partial brittleness ; thus admitting of 
 being applied to many uses for which zinc was formerly deemed 
 unfit. Hobson and Sylvester, of Sheffield, have taken out a 
 patent, securing to themselves the benefits accruing from the 
 application of this discovery to the arts. 
 
 There can now be no difficulty in forming zinc into sheath- 
 ing for the bottoms of ships, into vessels of capacity, water- 
 pipes, and utensils for various manufactories. As an internal 
 lining for culinary vessels, instead of tin, it has already been 
 applied with success. It is much harder and cheaper than tin, 
 and may be spread very uniformly. 
 
 Zinc, at a very elevated temperature, may be pulverized. 
 It may also, like several other metals, be minutely divided, 
 by pouring it, when in fusion, into water. These are the 
 most convenient means of reducing it into small particles. 
 Files have no oousiderable action upon it ; besides, it wears 
 and chokes them up in a short time. Zinc, in filings or small 
 particles, is used to produce those brilliant stars and spangles 
 which are seen in the best artificial fire-works ; but the fil- 
 ings of cast iron produce, at a cheaper rate, an effect scarcely 
 inferior. 
 
 3.— Vol. I. H 
 
50 MECHANICAL EXERCISES. 
 
 .Solder for platioa — gold — silver. 
 
 Calamine, or lapis calaminaris, used in converting copper 
 into brass, is found both in masses and in a crystallized state, 
 and is generally combined with a large portion of silex. It is 
 a native oxide of zinc, combined with carbonic acid. Zinc is 
 also found in an ore called blende, or, as the miners term it. 
 Black Jack. It is a sulphuret of zinc : in Wales it was 
 employed till lately for mending the roads. 
 
 Soldering. 
 
 To unite two pieces of the same or of different metals, by 
 fusing some metallic substance upon them, is called soldering. 
 
 It is a general rule, that the solder should be easier of 
 fusion than the metal to be soldered by it. It is, in the next 
 place, desirable, though seldom absolutely necessary, nor 
 always attempted, that the solder and the metal to which it is 
 intended to be applied, should be of the same colour, and of 
 the same degree of hardness and malleability. 
 
 Solders are distinguished into two principal classes, viz. the 
 hard and the soft solders. For the hard solders, which are 
 ductile, and admit of being hammered, some of the same sort 
 of metal as that to be soldered, is, m the greater number of 
 instances, alloyed with some other which increases its fusi- 
 bility. Some of the facts already detailed, respecting the 
 metals, prove that the addition made with this view need not 
 always be itself easier of fusioii. 
 
 The solder for platina is gold, and the expense of it will, 
 therefore, contribute to hinder the general use of platina 
 vessels, even in chemical experiments. 
 
 The hard solder for gold, is composed of gold and silver ; 
 gold and copper ; or gold, silver, and copper. Goldsmiths 
 usually make four kinds ; viz. solder of eight, in which, to 
 seven parts of silver, there is one of brass or copper ; solder 
 of six, where only a sixth part is copper ; solder of four, and 
 solder of three. But many who may have occasionally to 
 solder gold, cannot encumber themselves with these varieties. 
 For general purposes, therefore, the following composition 
 may be provided : melt two parts of gold, with one of silver 
 and one of copper ; stir the mass well to make it uniform, 
 add a little borax in powder, and pour it out immediately. . 
 If cast into very thin narrow slips, it will be the more handy 
 for subsequent use. To cleanse gold which has been soldered, 
 heat it almost to ignition, let it cool, and then boil it in urine 
 and sal ammoniac. 
 
 The hard solder for silver may be prepared by melting two 
 parts of silver with one of brass. It must not be kept long in 
 fusion, lest the zinc of the brass fly off iii fumes. If the silver 
 
:\IECHAXICAL EXERCISES. 51 
 
 Solder for copper — brass — iron. 
 
 to be soldered, be alloyed with much copper, the proportion 
 of brass may be increased : for example, the following com- 
 position may be used ; four parts of silver and three of brass, 
 rendered easier of fusion by the addition of a sixteenth part of 
 zinc. Silver which has been soldered, may be cleaned by 
 heating it, and letting it cool, as directed' for gold, but it 
 must be boiled in alum water. 
 
 The hard solder for copper and brass, is a soft fusible sort 
 of granulated brass, well known to artists under the name of 
 spelter. It consists of brass mixed with an eighth, a sixth, 
 or even one-half of zinc. The brazier's use no other kind of 
 hard solder, and it is commonly sold by them. As spelter 
 melts sooner than common brass, it serves for the solder of 
 the latter as well as for copper. 
 
 Standard silver makes an excellent solder for brass. It is 
 more fusible than spelter, proportionately easier to manage, 
 and equally as durable. A slight demand for silver solder may, 
 to many, be supplied at an easy rate, in consequence of the 
 number of small silver articles in common use, and which are 
 frequently wearing out ; no one need to be at a loss for it, 
 while they can provide themselves with a worn-out silver 
 thimble or toothpick. 
 
 Iron may be soldered with copper, brass, gold, or silver. 
 Brass or spelter is most comn>only used, and the operation is 
 then called brazing; but a carburet of the same metal, viz. the 
 dark srey or most fusible sort of pig iron, called Xo. 1. is the 
 most durable solder that can be used. The pig iron loses 
 some of its brittleness, and the malleable metal becomes 
 harder in the proximity of the parts soldered. 
 
 The parts upon which hard solder is intended to operate, 
 are touched with finely powdered borax moistened with water. 
 They must also, as in all soldering and tinning operations, be 
 perfectly clean. The borax, qiyckly running into a kind of 
 glass, promotes the fusion of the solder, and preserves from 
 oxidation the surfaces to which it is applied. The pieces in- 
 tended to be soldered, are fastened together with iron wire, or 
 secured by some contrivance having the same effect. Spelter 
 being composed of so many grains, is apt to spread when the 
 borax boils up: but just as itbecomesfused, the workmen bring 
 it to the place where it is wanted, by a slender iron rod. The 
 flame of a lamp, directed by the blow-pipe against the solder 
 covering the intended joint, which must be.laid upon charcoal, 
 is sufficient for small things. For large work, a common culi- 
 nary fire may be made to effect the desired fusion, though a 
 forge is still more convenient. The fire should not touch the 
 work, nor the ashes be allowed to fall upon it. 
 
52 MECHANICAL EXERCISES. 
 
 Solder for lead — tin — line brass worii. 
 
 The so^t soklers melt easily, but are partly brittle, and there- 
 fore cannot be hammered. The solder lor lead is usually com- 
 posed of two parts lead and one of tin. Its goodness is tried 
 by melting it, and pouring about the size of a crown piece upon 
 a table ; little shining stars will arise upon it if it be good. By 
 diminishing the proportion of lead, we form what is called strong 
 solder; we may also increase the proportion, which is advis- 
 able when we wish to solder vessels for containing acids; be- 
 cause lead is not so easily corroded or dissolved as tin. 
 I The metal with which tea-chests are lined, familiarly called 
 tea-lead, is an alloy principally composed of lead and tin. It 
 is generally sold at so low a rate, that it is bought by manu- 
 facturers who require alloys of lead, as a cheap method of 
 obtaining that metal already somewhat enriched. In some of 
 it, the proportion of tin is very small : other parcels contain 
 so much tin that they make excellent solder for lead without 
 any further admixture. No solder can be obtained at so low 
 a price as this. Tl/ese valuable portions of tea-lead may be 
 distinguished by their brilliancy, having suffered little from 
 oxidation ; also, when they principally consist of tin, by 
 that crackling noise while bending, which is peculiar to this 
 metal and some of the alloys into which it largely enters. 
 We may observe, in passing, that tea-lead, when mixed 
 with antimony, generally produces a compound of less firm- 
 ness when cold, and when melted^ of less fluidity, than if 
 pure new lead had been used. It is not improbable, that 
 when manufactured into sheets, it is occasionally spoiled by 
 too much heat. 
 
 The solder for tin may consist of four parts pewter, one of 
 tin, and one of bismuth, or two parts of tin and one of lead: 
 the latter is the composition mostly used. 
 
 The soldering-iron of tlie tin-pla.te workers, is an ino-ot of 
 copper, flattened at the point, in a pyramidical form ; it is 
 screwed or riveted to an iron stem driven into a wooden 
 handle. The copper is seldom more than four or five inches 
 long, and when it is worn away, the same stem and handle 
 are used for another piece. The bar of copper is prepared 
 for use, by filing it bright, and tinning it; when sufficiently 
 hot, it will melt and take up the solder, so as to aftord a ready 
 means of applying it to the intended juncture. Powdered 
 resin, and sometimes pitch, is used along with the soft sol- 
 ders, to preserve the metals employed from oxidation. 
 
 Tin-foil, applied between the joints of fine brass-work, first 
 wetted with a strong solution of sal ammoniac, and held firmly 
 together while heated, makes an excellent juncture, care being 
 taken to avoid too much heat. 
 
MECHANICAL EXERCISES. 53 
 
 Description of the foot laliie. 
 
 TURNING. 
 
 Turning is an art universally admired; the simplicity of the 
 operation, the facility with which precision in performing it is 
 attained, the agreeable exercise it affords to the mind, the 
 beauty and utility of its products, have drawn, for the amuse- 
 ment of a leisure hour, as well as for objects of real importance, 
 men of all ranks into the number of its practisers. It is an 
 art of great antiquity, but when or by whom it was first 
 adopted, we must leave the antiquarian to decide. In this 
 place it claims notice, on account of its contributing so essen- 
 tially to the perfection of several other arts. 
 
 The machine in which turning is performed, is called a Lathe. 
 Lathes differ very considerably in their general form, their size, 
 and the materials of which they are made. They are commonly 
 classed according to the manner in which they receive their 
 motion. Hence we have the Wke^l Lathe, the Foot Lathe, the 
 Hand Lathe or Tur)ting Bench, and the Pole Lathe. For very 
 large work, there are other lathes used, which are wrought by 
 horses, water wheels, or steam engines. 
 
 We shall, in the first instance, describe the foot lathe, which 
 may be converted into a wheel lathe, as often as occasion de- 
 mands ; as, for this end, it is only necessary to fix it in such 
 a situation as to admit of the requisite addition of a large 
 wheel, turned by one or more men. 
 
 Fig. 1. pi. I. is the perspective view of a foot lathe made 
 entirely of iron, excepting the wheel. The shears or cheeks, 
 which constitute the bed of the lathe, and one of which, 
 BB, is seen in front, are fastened, at one end, by two bolts, 
 c d, to the upright O ; and at the other end by the bolts 
 a b, to the upright Q. These bolts pass entirely through the 
 shears and the iiprights, and are each of them screwed at 
 the eaid, so as to be drawn perfectly tight by a nut at the 
 back. The heads are countersunk, so that they lie level in 
 front, and can occasion no inconvenience. The feet P R 
 are cast in the same piece with their respective uprights 
 O Q. They are fastened to the floor by screws passing 
 through them, and are large enough to afford a solid bearing 
 for the lathe. The shears are parallel, and enclose a space 
 for the puppets, or, as they are sometimes called, the head- 
 stocks, to slide in. 
 
 On the rim of the great wheel or fly, K, are three annular 
 grooves, gradually narrowing to the bottom, the section of each 
 exhibitino; an anoular indentation, which form is the most suit- 
 able to take efiectual hold of the band, and give it more power 
 to turn the mandrel E, the pulley F of which has three corre* 
 
54 MECHANICAL EXERCISES. 
 
 The foot latlie. 
 
 sponding grooves, of difierent diameters, to give it difterent 
 velocities. The fly wheel, the grooves of which are also of 
 different diameters, has its least diameter upon the same side 
 as the greater diameter of the pulley. The band is made of 
 strong catgut ; in order to make it suit, when applied to all the 
 ditlerent grooves of the pulley, the wheel K can be elevated or 
 depressed by means of the screws p p, each of which acts upon 
 a sliding piece in the uprights. The axle of the wheel, at one 
 end, works in the sliding piece of the upriglit Q ; but at the 
 other in the end of the screw^", which passes through the slider 
 of the upright O, and afibrds the convenience, not only of 
 making the motion of the wheel steady, but of taking it out, or 
 putting it in, without disturbing the other parts of the machine. 
 The axle of the wheel is of iron, except at each extremity, 
 whicli is steeled, left very hard, and of a conical figure. At L 
 it is bent so as to form a crank, receiving one hook of the con- 
 necting rod M, the other hook of which is attached to the 
 treadle frame N. The hooks of the conne-cting rod being 
 screwed in, can be lengthened or shortened as the wheel is 
 elevated or depressed. The treadle frame is made of wood ; to 
 each end of the back part q q, is affixed a conical piece of steel, 
 one of which works in a corresponding hole in the foot P, and 
 the other in R, upon the same plan as the axle of the fly. 
 
 The puppets C D are cast in one piece. On the under side 
 they have a projecting part, pointed uut by dotted lines near B, 
 exactly filling in breadth the space between the shears, upon 
 which they are drawn down by the screws w m, which enter 
 this projecting part, and render them immoveable at pleasure. 
 The puppet G is fastened in a similar manner, by the screw /. 
 The spindle or mandrel E, runs in a brass or bell-metal collar 
 in each of the puppets C D. The collar in the puppet C is of 
 a single piece, very carefully drilled ; but that in the puppet D 
 is in two pieces, w hich are fitted so as to slide down an angular 
 groove, and further secured by a plate /• on the top of it, w hich 
 plate is fastened by two screws. The part E is called the 
 nose of the spindle, and the screw upon it is intended to re- 
 ceive the chucks. The spindle, where it works in the collars, 
 must be steel, welded round the iron part, and turned cylin- 
 drical in the most accurate manner. It is supplied with oil by 
 small holes, one of which is drilled from the top of each pup- 
 pet through the collars. It is intended to be used occasionally 
 as a traversing mandrel, but when not used for this purpose, 
 it has a groove, for a piece of steel on the puppet C to fall 
 into, so as to prevent its horizontal motion. By slackening 
 The screws / m m, it is evident that the puppets are at liberty 
 to slide horizonlallv, and that D and G can be fixed at various 
 
MF.CHAXICAL EXERCISES. 00 
 
 The foot lathe. 
 
 distances from each other. G is the puppet moved to suit the 
 length of different work, which requires the use of the point of 
 the screw H as a centre. An accommodation of a few inches 
 is obtained by screwing H further through or out of the head- 
 stock. When the screw H has been brought to its proper place, 
 it is fastened by turning the screw s. To prevent the screw s 
 from damaging the thread of H, a small piece of brass inter- 
 cepts the end of it, and is pressed by it upon the screw. 
 
 The rest, I, can be affixed at any necessary distance from 
 the axis of the work, by the bolt i, which is drawn tight by 
 the nut A', the peculiar shape of which is very convenient in 
 practice, as it admits, in turning it, of the ready application 
 either of the hand or any sort of a lever. As the opening 
 which admits the bolt / is not stopped otf at one end, the rest 
 can be drawn from the lathe without taking off the nut A- from 
 the bolt /, which, when the rest is withdrawn, immediately 
 falls from between the cheeks. The cross part, g, of the rest, 
 having a cylindrical stem, which exactly fits a hole passing 
 perpendicularly through the pillar in the fore part of the rest, 
 admits of being moved entirely round, or of being raised or 
 lowered, and it may be fixed in any situation which can c 
 necessary, by the screw h. 
 
 The pulley F is commonly made of mahogany, or some 
 hard wood which will stand well. On the face, it is covered 
 with a brass plate, upon w-hich are a number of concentric 
 circles, each divided into a difterent number of equal parts 
 by small holes. There is a stop t, consisting of a stout 
 cylindrical piece that can be moved round on a screw on the 
 puppet D ; to the end of it is affixed a thin piece of steel, so 
 that when turned up, and a short point, near the top of it, 
 inserted in any of the divisions of the pulley, it has sufficient 
 spring to keep it there. This contrivance is used to divide 
 the circumference of any thing turned into equal parts. It 
 may also be applied to the cutting of the teeth of wheels, so 
 as to make the lathe not inferior, for this purpose, to the 
 clockmaker's engine, which is often more costly than a com- 
 plete lathe. To use it in this wav, let the part g of the rest 
 be drawn oat, and a strong plate of iron, with a stem of the 
 same kind, substituted. Upon this plate, let there be a slid- 
 ing piece, to be pushed forward by a spring, and containing 
 a small circular cutter, with teeth on its circumference like 
 those of a file. The cutter runs horizontally ; on its axis 
 must be a pinion, turned by a wheel, of some considerable 
 diameter, and working in the same sliding piece. This wheel 
 has a small winch, bv the turning of which the teeth are cut, 
 the stop t being first fixed in one of the holes of the brass 
 
56 MECHANICAL EXERCISES. 
 
 The foot lathe lued as a wheel lathe. 
 
 plate, the circle used being determined by the number of teeth 
 wanted, and the cutter brousht to the circumference of the 
 wheel in the lathe, upon which it is to operate. When one 
 tooth is cut, the stop is slipped into another hole, and the 
 process repeated till all are completed. 
 
 The wheel, and consequently the pulley with which the band 
 connects it, is put in motion by pushing down the treadle ; the 
 crank, previously to the first push, being brought nearly to its 
 highest elevation. By the momentum which the wheel has thus 
 acquired, the motion is continued, when, as the treadle and 
 crank rise, the workman can exert no power on the machine- 
 As soon as the crank begins to descend, the treadle receives 
 another impulse from the foot, by the repetition of which, at 
 each revolution of the wheel, the force destroyed by friction, 
 and bv the tool in the act of turning, is constantly renewed. 
 The wheel is frequently made of iron; it is not then solid, but 
 the rim, which is of considerable weight, is connected by fear 
 or five spokes or arms with a centre-piece, through which 
 there is a square hole, for the axis to pass, and it is fastened 
 by thin wedges. When an iron wheel is used, the arms and 
 centre-piece should be light, that nearly its whole weight may 
 be accumulated in its rim or circumference. The arms, also, 
 should present a thin edge in that direction in which they 
 revolve, to lessen the resistance of the air. 
 
 The foot-lathe has not power enough for the turning of very 
 heavy work. It is therefore converted into the wheel lathe by 
 using a large fly wheel, (five, six, or seven feet in diameter, for 
 instance, standing in a separate frame, at the distance of several 
 feet, or even yards, from the pulley, and turned by one or two 
 men, who use both hands at once to the winch. The band, to 
 give it greater power over the pulley, is crossed ; that part of 
 it which came from the top of the wheel consequently goes 
 directly to the bottom of the pulley, and the men who turn 
 can face the lathe, to assist them in knowing when it is proper 
 to stop, which is necessary when no contrivance is resorted to 
 for stopping the mandrel, without stopping the great wheel. 
 The following description of an ingenious, though more com- 
 plex, method, adopted by ^laudslay, an eminent turner in Lon- 
 don, in applying the power of the larire wheel, is given in Gre- 
 gory's able treatise on mechanics • The large fly wheel which 
 the men turn, works by a strap, on another wheel fixed to the 
 ceiling, directly over it ; on the axis of this wheel is a larger 
 one, which turns another small wheel or pulley fixed to the 
 ceiling directly over the mandrel of the lathe ; and this last 
 has on its axis a larger one which works the pulley F by a 
 band of catgut. These latter wheels are fixed in a frame of 
 
MECHANICAL EXERCISES. 57 
 
 Manner of using the foot lathe as a wheel lathe. 
 
 cast iron, moveable on a joint; and this frame has always a 
 strong tendency to rise up, in consequence of the action of a 
 heavy weight ; the rope from which, after passing over a pul- 
 ley, is fastened to the frame : this weight not only operates to 
 keep the mandrel-band tight,whenappliedtoany of the grooves 
 therein, but always makes the strap between the two wheels on 
 the ceiling fit. As it is necessary that the workman should be 
 able to stop his lathe, without the men stopping who are turn- 
 ing the great wheel, there are two pulleys or rollers, (on the 
 axis of the wheel over the lathe,) for the strap coming from the 
 other wheel on the ceiling ; one of these pulleys, called the 
 dead pidiey , is fixed to the axis, and turns with it, and the other, 
 which slips round it, is called the live puUeif : these pulleys are 
 put close to each other, so that by slipping the strap upon the 
 live puUei/, it will not turn the axis ; but if it is slipped on the 
 other it will turn with it : this is effected by a horizontal bar, 
 with two upright pins in it, between which the strap passes.. 
 This bar is moved in such a direction as will throw the strap 
 upon the live pulley, by means of a strong bell spring ; and in 
 a contrary direction it is moved by a cord fastened to it, which 
 passes over a pulley, and hangs down within reach of the 
 workman's hand : to this cord is fastened a weight, heavy 
 enough to counteract the bell-spring, and bring the strap up 
 to the dead pulley, to turn the lathe ; but when the weight is 
 laid upon a little shelf, prepared for the purpose, it will be 
 stopped by the action of the spring. 
 
 It may be considered a difficult and expensive undertaking 
 to make an iron lathe upon the plan represented by fio". 1 ; but if 
 the cheeks be cast in such a manner, that the transverse section 
 of them, when placed as they are to lie in the lathe, shall be 
 like fig. 2, the projecting parts opposite each other, and on the 
 top, which are all that need be filed flat, will be so narrow that 
 the making of them true will be very materially expedited. 
 In metallic lathes, it must not be concealed, there is an elastic 
 tremor which is disadvantageous ; but they admit of and retain 
 so much greater exactness in their workmanship, they are so 
 much more compact and durable than wood, that they merit a 
 decided preference, and the use of them is becoming so general, 
 that they will probably, in a short time, supersede eTery other 
 kind. In its first cost, if the business be set about in a proper 
 manner, an iron lathe need not be made to exceed the price of 
 a really good wooden one ; a small quantity of the cheapest sort 
 of wood will make the patterns for a large lathe ; one pattern, 
 for example, it is clear, will serve for both the uprights, and 
 the same may be observed of the cheeks, which are also both 
 alike. When a wooden lathe is intended for nice work, it 
 3.— Vol. I. I 
 
58 MECHANICAL Exercises. 
 
 Requisites of a good latbe. 
 
 ouo^ht to be made of mahogany ; but whether wood or iron be 
 the material, it is recommeudable to make the cheeks of con- 
 siderable depth. 
 
 The stronger and the more correct the workmanship of a 
 lathe, the easier it is to execute work in it, with expedition 
 and truth ; but good work may be performed with an inditi'erent 
 lathe, by takinsi; care to cut so little at a time, that the parts 
 of the enoine mav never be shaken out of contact. 
 
 It is essential to a good lathe, that the centres of the collars 
 of the mandrel, and the centre of the screw in the moveable or 
 right hand puppet head, be in one line, parallel to the bed or 
 shears. If the collars and the mandrel be truly formed, the 
 latter, when the rotation of it is slowly made by the hand, 
 will be equally stitf in every stage of its progress, and the 
 wearing parts, when examined, will have, every-where, the 
 same appearance. When the mandrel is fitted with accuracy, 
 the head of the moveable puppet, which must previously be 
 finished in that part which slides between the shears, may be 
 drilled in the foUowino; manner : screw firmly upon the mandrel 
 a piece of steel ; turn it true ; let the right hand end of it be 
 made conical like the extremities of the axle of the wheel K, 
 fig. 1 ; from the base of this conical end, to at least the same 
 extent as the thickness of the moveable puppet, at the part to 
 be drilled, let it be turned somewhat smaller than the broadest 
 part of the cone. File the cone, so as to leave two or three 
 edges untouched by the file, but which will cut like those of 
 a drill. Now bring up the moveable puppet, and bore it with 
 this tool; — the axis of the hole produced, will be in the same 
 line as the axis of the mandrel. While the puppet is drilling, 
 the screw which fastens it to the cheeks, should be tight 
 enough to prevent its shaking, but not so tight as to prevent 
 its being impelled forward as the tool cuts. 
 
 It is not customary, in foot lathes for general purposes, to 
 make the centre of the collars more than eight or nine inches 
 above the bed. As one means of ensuring perfect steadiness, 
 it is always proper to make them as low as the work intended 
 to be done will admit ; but in the lathe above described, an 
 arrangement may be made for the occasional turning of work 
 of extraordinary diameters. That part of the puppets C D 
 which slides between the cheeks, does not extend to the front 
 of D ; so that when these puppets are turned round, and D 
 stands where C now is, the nose of the mandrel is even with 
 the outside of the upright Q. Hence the body to be turned will 
 be relieved from the interference of the cheeks or the upright, 
 by the chuck, or whatever else is used to effect the rotation. 
 When this plan is adopted, the method of fixing the rest, and 
 
MECHAXICAL EXERCISES. 59 
 
 Jlethod of stopping the foot lathe. — The standards. — The mandrel. 
 
 also the right hand centre, when two centres are wanted, must 
 be left to the judgment of those who may require its assist- 
 ance, as different workmen will have different conveniences 
 for accomplishing the same end ; the possession of a substan- 
 tial bench will, in this case, relieve one person from embarrass- 
 ment, while the proximity of a wall will afford stability to the 
 fixtures of another. 
 
 The method of stopping the motion of the foot lathe, is 
 extremely simple ; a considerable piece of thick canvass or 
 leather, is suspended near the circumference of the fly, on 
 that side next the workman, who presses his knee upon the 
 rim, with the canvass or leather interposed, and the momen- 
 tum of the wheel is too inconsiderable not to be instantly- 
 overcome without difficulty. 
 
 Fig. 3, is an elevation of one of the standards and feet of 
 the foot lathe, shewn endways. The ribs a b are merely in- 
 tended to afford additional strength to the standard, which, if 
 made a little stronger than the proportion here represented, 
 will not require their aid. The screw by which the sliding 
 piece (for receiving the extremity of the axle of the fly,) is 
 elevated or depressed, is attached by a collar, in which the 
 upper end easily turns round. That part of the standard itself, 
 in which the screw works, is cast with a hole in the centre, in 
 which a brass nut is afterwards inserted. 
 
 Fig. 4, is the mandrel separately. When the mandrel is 
 not used as a traversing one, it has generally, besides the 
 shoulder against which the chucks are screwed, another 
 shoulder which is pressed against the collar of the middle 
 puppet by a screw ; for an example of this arrangement see 
 fig. 13. When the mandrel is not provided with a shoulder 
 of this sort, it is, at that part which works in the collar of the 
 middle headstock, of a conical figure, so that pressing it for- 
 ward with the screw at the back, will at all times produce the 
 effect of making it fit the collar. But when this form is given 
 to the mandrel, the friction in the collar is prodigiously in- 
 creased by the slightest pressure of the back screw, from its 
 operating like a wedge ; and it is difficult to regulate it in such 
 a way, that the mandrel is not so tight as uselessly to destroy 
 any part of the power applied to the machine, or so slack as 
 to be unfit for use. The nose of a mandrel is almost always 
 screwed externally, for the reception of chucks, and is besides 
 often furnished with a square hole for the ready insertion of 
 small arbors, boring tools, &c. An inside screw is much 
 better than a square hole, and, in lathes of the best construc- 
 tion, is also more common. 
 
60 MECHANICAL EXERCISES. 
 
 Mt-ans of fixini; the work in tnmif,-. 
 
 Of Chuck:>. 
 
 The chucks of a lathe are pieces of wood or metal, screwed 
 or otherwise fastened to the nose of the mandrel, and used to 
 sustain the work in its rotation. Their construction is varied 
 very considerably, to suit different purposes. When made of 
 metal, brass is most commonly selected. The work, as the 
 nature of it renders most convenient, is fastened to a wooden 
 chuck by cement, or by glue, or screwed into it, or gently 
 driven into a cell prepared to receive and hold it fast. To 
 sustain the latter operation, or any other violence, wooden 
 chucks are hooped with brass or iron. As it would be almost 
 impossible to screw a wooden chuck upon the convex nose of 
 a mandrel, and take it off as occasion required during the pro- 
 cess, without altering the position, it is found much the best to 
 make the medium of fastening it to the mandrel of brass. This 
 may be done in two ways : one-half of a solid cylinder of 
 brass, somewhat thicker than the nose of the mandrel, may be 
 reduced so as to leave a shoulder, and properly screwed to 6t 
 the concave screw of the mandrel ; the remaininor part may then 
 be screwed with a coarse thread, for the reception of the chuck, 
 which is to be tightly screwed upon it ; the screw and the 
 chuck may then be removed toecethpr from the mandrel, as 
 often as may be necessar>% and again screwed on exactly to 
 the same bearing. It is proper to use a little oil upon the 
 thread of the screw which enters the mandrel, lest it be so fast 
 that the position of the wooden chuck may alter in a small 
 degree when it is taken out. The other way of adoptincr the 
 intervention of metal, in securing a wooden chuck, is still more 
 secure, and crenerally used for large work : it consists in usino- 
 a short hollow cylinder of brass, internally screwed with the 
 same thread as the external screw of the mandrel, upon which 
 it can consequently be screwed with facility at any time, and 
 as easily taken off. Upon one end of it is soldered a circular 
 plate of iron or brass, so as to form upon it what is called a 
 flanch, the diameter and strength of which must be propor- 
 tioned to the magnitude of the chuck, and the weight of the 
 work. \\ hen the chuck is six inches in diameter, the diame- 
 ter of the flanch ought not to be less than two and a half or 
 three inches. Four or five holes are made near the circum- 
 ference of the flanch, for screws to pass through, in order to 
 fasten the brass to the wood. — A wooden chuck is so liable to 
 warp, from changes of. temperature, that even after they have 
 once been made true, it is customary to turn them \x little 
 every time they are used. 
 
MECHANICAL EXERCISES. 61 
 
 Means of fixing the work in turning. 
 
 Besides the chucks above described, there are several other 
 kinds, considerably different in their form ; some of the most 
 useful we shall describe. Fig. 5, is a universal chuck. It 
 consists of a short, hollow cylinder, one end of which can be 
 screwed upon the nose of the spindle, and on the circum- 
 ference, at equal distances from each other, and from the edge 
 of the other end, are inserted four screws. As these screws 
 may be altered to any distance within the cylinder, they can 
 be screwed upon work of any size which the cylinder will 
 admit, so as to hold it while it is turned. 
 
 Fig. 6, is one part of another contrivance for holding work 
 of various sizes; it is used in conjunction with the centre, fig, 
 7, and the chuck fig. 8. Suppose we have a bar of iron to 
 turn into a cylinder; let the chuck fig. 8, be screwed upon the 
 nose of the mandrel, and the centre, fig. 1, into it; then put 
 the bar a little way through the tool, fig. 6, which must be 
 fastened thereon by its screw, so that if it be carried round, 
 the bar miist revolve at the same time. Suspend the bar, by 
 punch holes, at one end on the centre fig. 7, and at the other 
 on the point of the screw on the right hand puppet, the arm 
 a of the tool fig. 6, being first put into the nick of the chuck 
 fig. 8; the bar may then be revolved upon the two centres, 
 and turned with the greatest facility, in every part except that 
 which is immediately under and on the left hand of the tool 
 fig. 6. It can often be so contrived, that the part thus left 
 unturned may be cut off either in the lathe or afterwards ; but 
 if this be inconvenient, nothing more is necessary, when this 
 part is all that remains undone, than to reverse the position 
 of the bar, and fasten the tool fig. 6, upon the finished end ; 
 we are then at liberty to complete our work. When there is 
 an inside screw at the end of the mandrel, the centre fig. 7, 
 and the chuck fig. 9, are screwed to fit it ; but when there is 
 only a square hole, they must necessarily possess a corre- 
 spondent form. 
 
 The chuck, fig. 9, is most commonly used for revolving long 
 pieces of wood. Its prongs, which are sometimes made to 
 stand in a line, and sometimes at equal distances from each 
 other close to the circumference, are driven into the wood ; it 
 is then screwed into the nose of the mandrel, and the other 
 end of the wood being supported, by its centre, on the screw, 
 as in the last instance, it is ready for turning. 
 
 Of the Boring and Mandrel Collars. 
 
 Fig. 10 is a boring collar : the holes are conical, and their 
 centres are all precisely at the same distance from the axis of 
 the collar. This collar is used to support the end of any long 
 
G2 MECIIAXICAL EXERCISES. 
 
 The boring collar. — The hand lathe. 
 
 body which must be turned hollow, and which could not be 
 kept steady by a chuck. It is usually made of iron or brass. 
 In using it, remove the common right hand puppet, and pro- 
 vide another considerably lower. Through this low headstock, 
 drill a hole of the same size as that in the centre of the boring 
 collar. The centre of this hole must be in the same perpen- 
 dicular line as the centre of the mandrel. The collar is at- 
 tached to it, so as to face the end of the mandrel, by a screw 
 with a button head, tightened at the back with a nut. When 
 properly fixed, the centre of the hole in the boring collar is 
 exactly opposite the axis of the mandrel, and when the largest 
 hole is used, and is therefore uppermost, it entirely clears the 
 top of the headstock to which it is affixed. The end of the 
 cylinder to be bored, being placed in the hole which fits it, 
 the boring tool is held upon a rest against its centre, and the 
 boring may then be performed, with great accuracy, to any 
 required depth. 
 
 Fig. 11, is a boring coll-ar with one aperture, capable of ad- 
 justment by turning the screw at the top. As for work of dif- 
 ferent sizes, it requires to be fixed at different heights ; the open- 
 ing for the screw which fastens it, is not circular, but of suffi- 
 cient length to admit of the required variation. A cylinder re- 
 volving in this boring collar, will always touch one point of 
 each side of the triangular aperture, which is sufficient to pro- 
 duce the requisite steadiness while the boring is executed. 
 
 Fig. 12, is the collar of the puppet D. Betore it is divided, 
 which is done with a fine saw, the hole for the mandrel is very 
 carefully turned. It is very common to make the collars of 
 lathes with steel ; but when formed of an alloy containing thir- 
 teen or fourteen parts of copper, to two or three of tin, they have 
 much less friction, and will last a very long time, several years 
 perhaps, in constant use, without requiring to be renewed. To 
 make the mandrel and its collar perfectly fit, they may be around 
 together a little in their places, with finely powdered Turkey 
 hone : emery must not be used, as it would enter the pores of the 
 metals, which could never be freed from it by cleaning. 
 
 The Hand Lathe, or Turning Bench. 
 
 The wheel of the hand lathe is so situated, that it can be 
 turned by the left hand, while the rioht is employed in hold- 
 ing the chisel. It is seldom more t1ian eighteen or twenty 
 inches in diameter, and is placed just behind the pulley of the 
 mandrel, so that the winch, at its greatest distance, is within 
 an easy reach to the workman. But the wheel is not an in- 
 separable part of the hand lathe; in which the rotary motion 
 of the work is sometimes produced by the drill bow. 
 
MECHAXICAL EXERCISES. 63 
 
 The hand lathe. 
 
 Fig. 13, is an elevation of a hand lathe of the most approv- 
 ed construction. C C are two metallic pillars, firmly screwed 
 to a board or table at the bottom, and at the top to tJie trian- 
 gular bar B B. One of the flat sides of this bar is parallel to 
 the table, consequently an angular edge is uppermost. The 
 puppets D E F have angular openings, which exactly fit the 
 bar ; two of them, D F, are exactly alike, and an elevation, 
 frontwise, of one of these, is given in fig. 14. These puppets 
 can be taken oif the bar, without unscrewing the pillars C C ; 
 there is an aperture on both sides of each of them, a little 
 below the under side of the ba-r; a plate of iron being placed 
 across each puppet, through these apertures, a screw is then 
 passed perpendicularly through the centre of it from below. 
 By pressing this screw against the bar, the puppet is rendered 
 immoveable. The shoulder of the mandrel is pressed against 
 the collar in the puppet E by the screw a, the end of which, 
 like that of the mandrel, is convex, and they therefore touch 
 each other only in a point. 
 
 The support, H, for the rest, G, is flat on the top ; it is 
 moveable along the bar, and fixed exactly in the same manner 
 as the puppets. The piilar and upper part of the rest, resem- 
 ble the same parts in the foot lathe ; but the plate, I, to the 
 end of which the pillar is fixed, has no aperture through it for 
 the admission of a bolt. The under side of it is, however, 
 grooved in the direction of its length, so that the section of it, 
 the narrow way, resembles fig. 16. Hence it admits the button 
 head of a bolt c, fig. 15, which passes down the back part of 
 the support H, at the bottom of which it is tightened by a nut. 
 The plate of the rest, from its peculiar formation, can not only, 
 like the plate of the common rest, be drawn out, in a direction 
 at right angles to the axis of the mandrel, but can be turned 
 round upon its support H, a motion which is occasionally ad- 
 vantageous. It may also be observed, that by slackening only 
 the screw which fastens the support H, the rest may be slid- 
 den to another part of the bar, without the liability, as in the 
 usual construction, of altering its position otherwise than in 
 the direction of the length of the bar. 
 
 The pin b, of the puppet F, has a convex centre at one 
 end, and a concave centre at the other. It is perfectly cylin- 
 drical, and fits equally well either end foremost. It is not itself 
 screwed, but is fixed at the place required, by a screw from the 
 top of the puppet. It is not unusual to force forward this pin 
 by a screw, which acts upon it exactly in the same way as the 
 screw a that prevents the mandrel from receding. This mode 
 of using an unscrewed pin, in the right hand puppet, is used in 
 lathes of all sizes, particularly in those of the very best de- 
 
G4 MECIIAXICAL EXERCISES. 
 
 The hand lallie. 
 
 scription. It should be so perfect a cylinder, as to be air-tight 
 in its socket, either end foremost. It is, however, not ap- 
 proved by some workmen, who consider a screw to be, upon 
 the whole, more convenient. 
 
 All the screws, in the lathe fig. 13, by which the puppets 
 and collars are tightened, it will be noticed, have rings to turn 
 them by. This form is more convenient than the common 
 thumb screw, which has no perforation, as either the hand or 
 any small lever that happens to be within reach, can with equal 
 facility be applied, according to the tightness with which they 
 must be fixed. 
 
 When a wheel is used to this lathe, it is placed between two 
 standards fixed to a board six or eight inches broad, which, in 
 the direction of its length, lies at right angles to the bar, and 
 slides in an angular groove at each side, in w^hich it has a range 
 of a i'cw inches, for the purpose of tightening the band. When 
 the drill bow is used to eii'ect the rotation of the work, the 
 mandrel is commonly withdrawn, and a centre pin substituted 
 in the puppet D, similar to that in the puppet F. The me • 
 thod of turning upon two centres, whether the bow^or the 
 wheel produce the revolution, admits of great truth and pre- 
 cision. 
 
 The vibrations of a mandrel, of a short one especially, are 
 much to be dreaded ; and cannot be too carefully guarded 
 against by accuracy of workmanship. In proportion as the 
 length of a mandrel is increased, the consequence of a given 
 quantity of vibration is diminished. It is therefore a general 
 rule to make the mandrel as long as will be convenient. In 
 a foot lathe, its length should never be less than fourteen or 
 fifteen inches. 
 
 The turning benches of the clock and watch makers, are 
 almost always constructed with a rectangular bar for the pup- 
 pets to slide upon ; but a bar of this sort is much inferior to a 
 triangular one. These artists, also, have frequently no screw 
 \ipon the nose of the mandrel, but merely a square hole in the 
 end of it. They have arbors of various sizes, one of the ends 
 of which fits this hole, and the other extremity runs upon a 
 centre. These arbors are turned true, but not polished, and they 
 are slightly conical in their figure. To turn wheels and other 
 work which is perforated through the axis, it is only neces- 
 sary to drive them, with a few light blows, upon these arbors, 
 which, though driven into a cylindrical hole, become suffici- 
 ently tight upon them for the operation. 
 
 For small lathes, which are exposed to no very extraordinary 
 strain, the mandrel may be made of cast steel, which wears 
 longer and more uniforndy than any other kind of steel 
 
MECHAXICAL EXERCISES. 65 
 
 Use and construclion of the pole lathe. 
 
 The Pole Lathe. 
 
 This lathe is commonly remarkable for the extreme simpli- 
 city of its construction; but the manner in which the body to 
 be turned in it receives motion, forms its essential character- 
 istic. It can be easily made by any common artist, at a small 
 expense; it can be wrought by one man, and possesses consi- 
 derable power ; but notwithstanding these important recom- 
 mendations, it is gradually sinking into disrepute and disuse. 
 As in turning, the tool can only be applied to cut, while the 
 body upon which it is intended to operate revolves in one di- 
 rection, the pole lathe, which produces a regular alternation of 
 contrary revolutions, wastes the time of the workman. This 
 sort of motion, which forms the principal disadvantage inse- 
 parable from the nature of the machine, when used to^nforce 
 the action of edge-tools, is a very useful one in some kinds of 
 grinding and polishing. Stop-cocks and valves mav more 
 quickly and eft'ectually be made water and air-tight in a pole 
 lathe than in any other kind of a lathe. This hint mav be of 
 considerable use to experimentalists, who may require the tem- 
 porary use of a lathe motion, for purposes of this kind, when 
 they may not have an opportunity of procuring it without more 
 trouble and expense than is convenient. Let a short cylindri- 
 cal pin be driven into a wall, or fixed piece of upright timber, 
 such as the cheek of a door ; place upon it a pulley which is 
 thick enough to hang a little over the end of it. The pole, the 
 treadle, and a temporary rest, are easily arranged. Let the 
 pulley be used as a chuck ; they may then give their work the 
 best motion that can be contrived for it. It is not even ne- 
 cessary that the pulley they use should be turned. Three cir- 
 cular pieces of thin board, one of them about an inch in dia- 
 meter less than the other two, may be united by nails or glue, 
 the small one being placed in the middle, and thev will form a 
 very tolerable pulley, if bored in the centre so as exactly to 
 fit the pin. If not sufficiently near true on the face, the pul- 
 ley may either be turned with almost any tool that can be ob- 
 tained, or the work may be accommodated to it by applying a 
 greater thickness of cement on one side than on another. To 
 prevent the pulley from grazing the wall, a small circular piece 
 should be put on the pin before it, to serve as a washer. 
 
 The pole lathe, see fig. 17, pi. I. consists of two wooden, 
 horizontal parallel cheeks, with a space between them for the 
 insertion of the lower part or tenon of the puppets, and sup- 
 ported by legs or standards upon the same plan as the foot 
 rathe. The puppets, of which there are only two, reach some 
 inches belew the chucks, and are fastened at any part of the 
 3.— Vol, I. K 
 
66 MECHANICAL EXERCISES. 
 
 Description of the pole lathe. 
 
 bed, by wedges, n n, passing through mortises near the lower 
 ends of them. The mortises extend u little within the cheeks, 
 otherwise the wedges would not draw the puppets tight. The 
 puppet L contains a fixed point or centre, and the puppet M 
 a moveable centre or regulating screw. Between these centres 
 the work is siipported. K is a long elastic pole, fixed at one 
 extremity to the ceiling, and at the other attached to a cord 
 which is coiled once or twice round the body to be turned, or 
 round a pullev which carries that body along with it, and then 
 passes on to the treadle, to which tlie lower end of it is fas- 
 tened. The cord passes to the treadle on the same side that 
 it came from the pole. It is of such a length, that when coiled 
 round the work, the pole, if left to itself, will keep the treadle 
 as elevated as the workman finds convenient. When the 
 treadle is pressed down by the foot, the re-action of the pole 
 elevates it again, the moment that pressure is forborne, and 
 rhe backward and forward revolutions of the piece, round 
 which the cord is coiled, necessarily follow. 
 
 As it is evident that the workman, in pressing down the 
 treadle, has to overcome the resistance of the pole, the strength 
 of the latter should not be more than sufficient to produce the 
 immediate elevation of the treadle, otherwise he will uselessly 
 waste his strength. 
 
 When the pole lathe is used, a groove for the cord must be 
 turned at one end of the work as early as possible ; or a perfo- 
 rated pulley may be placed upon the fixed centre, and the 
 work connected with it in the manner already described for 
 connecting the tool, fig. 6. w ith the chuck fig. 8. As simplicity 
 of workmanship so generally distinguishes this lathe, a nail, 
 of suflScient size, bent in the middle to a right angle, one end 
 of it passing into a nick in the pulley, and the other driven 
 into the piece to be turned, may be mentioned as a contrivance 
 still more in character, and not unfrequently used by the 
 humble provincial artist. 
 
 Though the pole lathe has been described, as it is generally 
 found, made of the cheapest materials, and in the simplest 
 manner, yet it must be obvious to the reader, that there is no 
 impediment to its being made of metal, with all the excellence 
 of workmanship so often bestowed upon other lathes, but that 
 of incurring too much expense for a machine of circumscribed 
 utility. 
 
 The rest mostly used to this lathe, is similar to that for the 
 foot lathe, fig. 1. 
 
MECHANICAL EXERCISES. 67 
 
 Haudles for tools. — Tlie ^ouge. — The chisel. 
 
 0/ the Tools used in Turning. 
 
 To describe the various tools used among the different de- 
 scriptions of turners, would be impossible. They are so infi- 
 nitely diversified in form and size, according to the necessities, 
 the ingenuity, and often, no doubt, even the stupidity, of those 
 who use them, that a volume would not suffice to do them jus- 
 tice. Mechanics are apt to consider the tools of their own 
 invention the best of any, and this attachment, not to say 
 bigotry, is often accompanied with a silly attempt to conceal 
 from their neighbours the benefits of their amazing discoveries 
 as to the best form of a chisel. But though we may not ex- 
 hibit many of those peculiar forms of tools, upon which par- 
 ticular artists are disposed to pride themselves, we shall not 
 omit those most essential and adequate to the execution of 
 every description of work. 
 
 The handles of turning tools, it may be premised, must be 
 varied in size according to the manner in which they are 
 intended to be held. For heavy work, when the lathe is turned 
 by machinery, or by men at the great wheel, they must be 
 long enough to reach to the shoulder, upon which one end of 
 them must rest in the act of turning, besides being held by 
 both hands at the same time. In using the foot lathe, the 
 tools are held by the two hands only, and their handles are 
 seldom more than half the length necessary in the former in- 
 stance. In using the hand lathe, as one hand is employed in 
 turning the wheel, the handles of the tools need not be longer 
 than what can be comprised in the other. 
 
 In turning wood, the gouge, fig. 13, pi. III. is the first tool 
 used, as no other will so quickly reduce its irregularities. It3 
 cutting edge is rounded; in using it, the rest is generally on a 
 level with the axis of the work, and its handle is inclined 
 downwards, so that its cutting edge is considerably above the 
 axis. As gouges are made of all sizes, they are useful in 
 making a variety of concave mouldings. 
 
 The chisel follows the use of the gouge ; its cutting edge is 
 oblique to its sides, and formed by bevelling both the upper and 
 under surface, so as to make it in the middle of the thickness of 
 the tool. In using it, the rest is elevated considerably above 
 the axis of the work, so that, though held with a less inclination 
 than the gouge, its edge operates on a higher part of the surface. 
 The cutting edge of broad chisels is commonly made a little con- 
 vex. The gouge and the chisel are the only tools held above 
 the level of the axis ; and the chisel is the only tool, the edge of 
 which is formed by its being bevelled on both sides. Fig. 14 
 No. 1, is the front, and No. 2, the profile of the chisel. 
 
68 MECHANICAL EXERCISES. 
 
 Tools as«(l in tumiog. 
 
 Yicr. 15, is a right-side tool, with two cutting edges, viz. a 
 side edge and an end edge, so as to cut at once both the bot- 
 tom and the side of a cavity. When it is used, the bevel 
 which fonns the edsre is downwards. 
 
 A /eft-side tool, the side-edge of which is opposite to that 
 last mentioned, is useful in smoothing the left external sur- 
 face of spheres, and other rotund figures. 
 
 Fig. 16, the round tool. Though the gouge is generally 
 used to form and sometimes to finish concave mouldings, yet, 
 as it is very difficult, in whettin^, to give it a regular con- 
 vexity, the round tool, w hich may be formed with considerable 
 exactness, is a useful assistant for the same purpose. 
 
 Some workmen chuse to have point tools, see fig. 17, for 
 makino^ nicks or small mouldings, as well as finishing the 
 shoulders and flat ends of work ; while others, instead of 
 them, use the sharp corners and arrises of the chisels. 
 
 The tools included under the general term, inside tools, for 
 turning out hollows and cups of all descriptions, exhibit the 
 greatest variety of shapes, and exercise most the judgment of 
 the artist, in suiting them to his wants. The forms exhibited 
 in fig. 18, 19, 20, 21, and 22, are some of the most useful. 
 
 The parting tool, fig. 23, is used for making deep incisions, 
 and cutting off any part of the work. 
 
 The drill, for making holes, is variously applied. It may 
 be used alono; with the borins: collar ; or it may be screwed 
 upon the mandrel, and the work held against it ; or the latter 
 arrangement may be reversed, and the work being fastened to 
 a chuck, the drill may be held against it, while the tool is 
 steadied upon the rest. The drill is sometimes formed so as 
 to resemble half a hollow cylinder, but its section is a cres- 
 cent, and its edges are moderately sharp. Its end is nearly 
 like the mouth of a tea-spoon, and its hollow is greater than 
 that of a gouge of the same breadth. This sort of a drill is 
 used for small holes, but it is ant to turn so ill upon its point 
 as to bore awry. The drill, fig. 24, is therefore used in pre- 
 ference, especially for large holes. 
 
 The use of the outside screic-tool, fig. 25, and the inside screiv- 
 tool, fig. 26, are explained in the section on turning screws. 
 It is obvious that the work ought to be turned very true, 
 before they are applied. 
 
 Turning gravers, triangular and square tools, with various 
 other nameless sorts, the contrivance of individual skill, are 
 used in turnincj hard bodies, such as bone, ivorv, and metals. 
 When the turning graver, fig. 27, is used, it is the first tool 
 taken to iron and steel. The tool, fig. 28, of which No. 1 is 
 the front and No. 2 the profile, is often used instead of the 
 
MECHAXICAL EXERCISES. 69 
 
 Tools used in turniug. — Parallel rest. 
 
 graver, and cuts extremely well, but is not so easily managed 
 with one hand. Triangular and square tools, are denominated 
 from their respective sections being of these figures. They are 
 flat at the end. The former have three cutting edges, viz. each 
 arris in the direction of their length; the latter, which are 
 mostly used for turning brass, have four, viz. each arris at the 
 extremity. In a powerful lathe, the tool, fig 29, is a very 
 useful one for iron. It is flat at the end, and has four cutting 
 edges, all alike. It cuts too keenly for a small lathe, and 
 requires, besides, two hands to hold it; but with two hands, 
 as one edge can be pressed upon the rest, it may be held 
 remarkably steady. 
 
 To describe the use of the tool, fig. 30, may afford a fami- 
 liar illustration of the advantage attendii:ig a proper adaptation 
 of the figure of the tool to the work. It is intended to turn 
 the flat heads of the small brass nails used in fastening the 
 sheathing upon ships. These nails are little more than an 
 inch long, and as a very trifling charge is made for turning tlie 
 heads, the operation requires to be performed with propor- 
 tionate expedition. A square hole, which just admits the 
 shank of the nail, is made in the end of the mandrel; the nail 
 is placed in this hole, the fly wheel set a-going, and the tool, 
 fig. 30, applied so that the notch shall touch the rim, and the 
 lower part of the tool the face of the head ; then as the parts 
 of the tool thus applied, have both cutting edges, the rim and 
 face of the head are both instantly turned, while the nail is 
 prevented from flying out of the lathe. — When one nail is 
 done, another is inserted without stopping the fly wheel, and 
 thus several hundreds may be done in an hour. 
 
 Of the Parallel Rest. 
 
 If a tool, opposed to any body revolving in a lathe, be 
 drawn alono-, parallel to the axis of the mandrel, a cylinder 
 will be produced ; if it move in a line forming an angle with 
 the axis of the mandrel, a conical figure will be obtained; and 
 if it operate at right angles to the same axis, a flat surface will 
 be the result. The machine in which a turning tool is fixed, 
 to produce these effects, as it is frequently applied to the 
 turning of parallel surfaces, has occasionally obtained the 
 name of the Parallel Rest. 
 
 Fig. 31, pi. III. is a perspective view of the parallel rest, 
 which must be wholly made of iron. It consists of two parallel 
 cheeks, like a lathe, but the standards B B, are very short, as 
 the whole height of the machine to the chisel F, must not be 
 more than that of the axis of the mandrel from the bed of the 
 lathe in which it is intended to be used. The standards, B B, 
 
MECHANICAL EXERCISES, 
 
 The piirallel reiU 
 
 are firmly screwed to a strong plate C C, or, what is still better, 
 cast in the same piece with that plate. The cheeks are united 
 to the standards by tour screws, in the same manner as those 
 of the foot lathe already described. The lower part of the 
 headstock D fits the space between them, in the most ac<;u- 
 rate manner, so that it cannot be thrown askew, but yet is not 
 so tio-ht as to prevent its free horizontal motion. If at any 
 time it is found too slack, it may be tightened by a screw a, 
 at the bottom of it. 
 
 The lower part of the interior side of each cheek is cham- 
 fered, and the plate through which the screw a passes, to act 
 upon the headstock, slides between the chamfered portion of 
 the cheeks. This arrangement is shewn in the section, fig. 32; 
 a is the headstock; b b the cheeks; c the plate through which 
 the screw passes, and in which there is no screw thread re- 
 quired; d the tightening screw. On the top of the headstock 
 1), fig. 31, there is a rectangular groove for the chisel F, which 
 is fixed in anv part of it by a plate or cap on the top of the 
 headstock. Tliis plate or cap is fastened down by screws, 
 passing perpendicularly through it; two screws, in this situa- 
 tion, are sufficient for a small machine, but four will be re- 
 quired for a large one. 
 
 To effect theliorizontal motion of the headstock D, a screw 
 E, the nut of which is in the standard B, is connected with it 
 by means of a collar, and by turning the winch-6 of this screw, 
 the motion required is produced. The screw must, be long 
 enough to push the headstock to the further end of the space 
 between the cheeks, and it is advisable to make its thread a 
 fine one, particularly when iron is intended, to. be. turned with 
 the machine, in order that the motion communicated to the 
 headstock may with greater facility be made very slow and 
 uniform. 
 
 In adjusting the machine to the lathe, the strong iron plate 
 H I, fig. 33, is made use of. There are two wide gropves in it, 
 g g, by which, with the help of two bolts, it is fast^ed upon 
 the lathe, in the manner of a rest. The heads of the bolts must 
 not projrrf:t above the surface of the plate; therefore the grooves 
 must be considerably wider at the top than the bottom, and the 
 heads may then be countersunk. On the plate HI are also 
 three immoveable cylindrical pins, h i k, all of them. screwed at 
 the top and fitted with nuts. In using the machine, the plate 
 C C, fig. 31, is placed on the plate H I, fig. 33, so that the 
 pins of the latter enter the apertures I m n of the former. The 
 middle pin exactly fills the middle aperture, and upon it as a 
 centre, from the width of the other apertures, the machine has, 
 without moving the lowermost plate, fig. o3, a short range 
 
MECHANICAL EXERCISES. 71 
 
 The parallel rest. 
 
 back and forward, until the nuts in it are screwed down upon 
 the upper plate C C, fig. 31. Now suppose a bar of iron, in- 
 tended to be formed into a cylinder, be put into the lathe, let 
 the machine be fixed so that its cheeks may be, as nearly as 
 can be ascertained, parallel to the axis of the intended cylin- 
 der, at the left hand end of which place the tool so that it shall 
 take off a slender shaving. Set the lathe to work, and slowly 
 turn the winch b, till the tool has completely traversed the 
 bar. Repeat the operation, if necessary, till the irregularities 
 in the figure of the bar are removed, and the tool has touched 
 every part of the surface; then, with a pair of callipers or a 
 guage, examine whether the bar, at both ends, is of equal dia- 
 meter ; if any inequality appear, the rest has not been set pa- 
 rallel to its axis, and it, consequently, is not a cylinder. The 
 matter is rectified by slackening the nuts of the pins hik, and 
 pushing in that side of the rest, which is opposite the thick 
 end of the bar, just half the extent of the error. The nuts 
 being then screwed down, and the tool made to traverse the 
 surface again, the cylinder will be completed. 
 
 As it has been supposed that the bar was, in the first instance, 
 by some oversight or other, turned rather conical, the method 
 of making a regular cone with the parallel rest, when occasion 
 requires, needs no explanation. When a flat surface is to be 
 turned, it must be well secured to a chuck, the machine fixed 
 across the bed of the lathe, and the cutting edge of the chisel 
 F precisely on a level with the axis of the mandrel, or some 
 part of the centre will remain unfinished. The tool may be 
 made to cut with so much exactness, that if the face of a rect- 
 angular block of cast iron we?e attempted to be turned flat 
 with it, the edges will not be jagged, when the circle of revo- 
 lution, extending beyond the shorter diameter of the piece, is 
 not complete. 
 
 When the chisel F, in the parallel rest, requires to be moved 
 a little further in, some chuse to alter it by percussion, and 
 keep it only so tight that the blow of a moderately sized ham- 
 mer will di-ive it in ; others think it better to regulate it by a 
 screw, and provide a frame for the back of it, similar to that 
 at the back of the puppet D, fig. 13, pi. I. in which the screw 
 a acts upon the end of the mandrel. 
 
 ; The section of the cheeks of the parallel rest, to abridge 
 the labour of filing, may be made to resemble that proposed 
 for the cheeks of the foot lathe, fig. 2, pi. I. 
 
72 MECHANICAL EXERCISES. 
 
 I .-e of tuc screw-tools. — Traversing mandrt-l. 
 
 Of cutting Screws in the Lathe. 
 
 The art of cutting screws in the lathe, constitutes one of 
 the most curious and useful branches in the art of Turning. 
 Accordincrlv, it generally proves one of the most interesting 
 exercises to the young practitioner, who is further stimulated 
 by the celebrity of those who can cut every description of 
 screw with facility, — an attainment commonly considered, 
 among turners, one of the most decisive proofs of skill that 
 can be exhibited. 
 
 In proportion as the art of cutting screws has been culti- 
 vated, the methods by which the object might be accomplished, 
 have been diversified. We shall notice some of those contri- 
 vances which are least expensive, most easily reducible to 
 practice, and most suitable for general use. 
 
 If the screw-tool, fig. 25, be opposed to a cylinder revolv- 
 ing in a lathe, and at the same time be moved along the rest, 
 with a regular horizontal motion, it will cut a screw on that 
 cylinder, the threads of which will fill the angular spaces 
 between the teeth of the tool. Fisf. 25, is an outside screw- 
 tool ; if the cylinder had been hollow, and intended to be 
 screwed internally, the inside screw-tool, fig. 26, must have 
 been emploved, which, when pressed against the side of the 
 cavity, while drawn out horizontally as the cylinder revolved, 
 would have produced the desired effect. There is some 
 difficulty in acquiring the art of cutting screws in this 
 manner, thou2;h the process is in very general ixse among 
 experienced turners. To obviate every disadvantage which 
 attends it, and ensure perfect precision in the operation, 
 was the object of the invention of the traversing man- 
 drel. Of this ingenious contrivance, we shall next, thera- 
 fore, endeavour to give the reader a description. At tiie 
 end of the mandrel E, at e, fig. 1, pi. I. there is a screw 
 about two inches long, the thread of which is like that intend- 
 ed to be made. Upon this screw, called the guide, is fitted a 
 piece of wood, the motion of which is entirely prevented by 
 any mode of fastening which may be found convenient. The 
 piece of steel on the headstock C, which falls into the groove 
 of the mandrel, and hinders its horizontal movement, being 
 then withdrawn, and the great wheel turned, the mandrel 
 assumes at once a rotarv and rectilinear motion, which is con- 
 tinued till it has gone so far, that the screw e can no longer 
 turn in the piece of wood. If, as soon as this circumstance 
 occurs, or a little sooner, the great wheel be turned the con- 
 trary way, the rotary and rectilinear motion of the mandrel im- 
 mediately takes place again, but in a reversed direction. This 
 
MECHANICAL EXERCISES. 73 
 
 Traversina: mandrel. — Traversing chuck. 
 
 compound motion of the mandrel, is precisely what is wanted 
 to facilitate the use of the screw-tool, which, while it is going 
 on, only requires to be held steadily upon the rest, against 
 the revolving- body, and the screw will be produced. The 
 teeth of the screw-tool must correspond with the screw upon 
 the mandrel, as if it had been made by holding it against that 
 screw revolving in a lathe. 
 
 It is customary to cut three or four screws, of different 
 threads, one behind another, upon a traversing mandrel, as a 
 single one would only be of limited use. But even as three 
 or four screws are often insufficient to meet the wants of the 
 artist, aad the length of so many together is awkward and 
 inconvenient, it is better to make a concave screw in the end 
 of the mandrel, to which any variety of convex or guide screvirs 
 may then be alternately attached. — The revolution of the guide 
 screw, without the mandrel, may be prevented by a screw z, 
 near the end of the latter. 
 
 In cutting screws, the proper motion cannot be commun: 
 cated from a fly wheel to the mandrel by means of the foot 
 acting upon a treadle. If a fly wheel be used, it must be 
 turned backwards and forwards by a winch, through a space 
 proportionate to what the guide screw will allow, so that two 
 persons will be required for the operation. But to cut screws 
 m a foot lathe, the fly of which is unprovided with a winch, 
 and to render one person adequate to the performance, a cord 
 descending from a spring, as in the pole lathe, is coiled round 
 the pulley of the mandrel, and attached to the treadle, the 
 range of which may be suited to the occasion. 
 
 With respect to the mode of fastening the wood in which the 
 guide screw turns, a word may be expected. Let a stock or 
 horizontal piece, w, be screw^ed to or cast along with the head- 
 stock C; let the end of it be tapped to receive the screw x, 
 which must be taken out previously to fixing the wood upon 
 the guide screw e. When the wood is in its proper place upon 
 the guide, it must hang down over the end of the stock w, and 
 there must be a hole in it just large enough to admit the 
 screw X, by which it can then be made perfectly secure. 
 
 The use of the traversing maudrel will probably in a little 
 time give way to that of the traversing chuck, which was 
 invented by Robert Healy, A. B. of Dublin, and a description 
 of it, conamunicated by him, inserted in the Philosophical 
 Magazine. On the common mandrel A, fig. 1, pi. IV. is 
 screwed the chuck B, to which may be screwed the chucks 
 of the lathe, as R. On the outside of this chuck B, is 
 turned a screw, which is fitted to an inside screw worked 
 in the circular block C, from which block extends an arm D, 
 4.— Vol. I. L 
 
74 MECHANICAL EXERCISES. 
 
 Traversing chuck. 
 
 as long as may be thought fit for the purpose of permitting 
 another arm E, to slide up and down it; a piece of iron should 
 be screwed to the circular block C, of such a lengtli as to be 
 capable of moving in a groove that may be cut in the collar, 
 or adapted to it. This piece of iron should be regular in its 
 shape, and well fitted to the groove; it is intended to prevent 
 the block C. from being turned round, and to allow it only a 
 steady rectilinear motion. The rest, G F O, must not stand 
 as usual parallel to the work, in cutting an outside screw; but 
 at right angles, as when an inside screw is to be cut, in order 
 that the further arm of the rest F, may be joined to the end of 
 the second or intermediate arm E. It is necessary that this 
 second or intermediate arm E, be capable of fastening firmly 
 the first arm D, to any part of the rest, G F; it must also have 
 a joint at each end to admit, in a horizontal plane, its free 
 play. Thus, as the lathe turns to us or from us, the arms must 
 traverse forwards or backwards ; which gives a similar motion 
 to the tool H, that is held steadily or fixed with a screw on 
 the further arm F, of the rest ; and thus a screw is cut with a 
 tool of a single point. It is unnecessary to mention, that no 
 joggling should arise from the motion of the arms, as that 
 would cause a failure in cutting a perfect screw. If the centre 
 of the rest should be drawn nearer to us, and by that means 
 bring the tool closer to the intermediate arm E, then a screw 
 of a much larger size will be cut :. for as the rest, turning 
 within its socket (the thumb-screw for fixing it in the pillar, 
 being in this operation always withdrawn,) moves on a 
 centre, the further the tool is moved from this centre, the 
 greater will be the radius of the circle described, and conse- 
 quently the coarser will be the screw ; and, vice versa, the 
 nearer the tool is brought to the centre, the smaller will be 
 the radius of the circle, and thus the screw will be finer. 
 Should the intermediate arm E, be connected w ith the nearer 
 arm of the rest G, and the tool held on the further one 
 F, then a left-handed screw will be cut, of a thread the 
 distance between the turns of which will varj' accord- 
 ing to the distance of the point at which the tool is held 
 between the centre and extreme end ; for, as the lathe 
 turns to us, the arms receive a forward motion, except 
 the further arm F, of the rest, which receives a backward 
 motion ; but when the lathe turns from us, then the further 
 arm receives a forward motion ; and as the tool meets the 
 wc^od, so it cuts a left-handed screw. 
 
 It may be apprehended that a piece of wood so far removed 
 from the collar K, might spring its motion ; but this may be 
 ooviated by not making use of die traversing chuck B, till the 
 
MECHANICAL EXERCISES. 75 
 
 Traversino; chuck. 
 
 screw is to be turned ; for as the cutting of it is light work, 
 there will be little resistance, and, of course, but little spring; 
 or the traversing screw, B, may be turned on the mandrel A. 
 Another disadvantage would seem to arise from the impossi- 
 bility of cutting screws when the puppet head is made use of, 
 to prevent the springing of a long piece of wood. But this 
 may be obviated by lengthening the intermediate arm E, to 
 the part where we intend cutting the screw, and thus we have 
 the same screw as that of the traversing one : if a finer or 
 coarser screw should be required, then, by having an arm of 
 the rest to slide in and out, and the intermediate arm to be 
 connected with the centre of the rest, we have just the same 
 power of turning screws as in the former case. A socket S, 
 is represented, the lower part of which slides on the rest, and 
 may be fastened firmly to it by a screw : the upper part, that 
 turns on a pivot, admits the intermediate arm to slide through 
 it, which arm is held stationary in it by a screw. 
 
 If the rest were to make a right angle with the piece of wood 
 on which the screw was to be turned, at the commencement of 
 the process, and to become parallel to it when the screw waa 
 finished, an approximation would take place from a larger thread 
 to a smaller, or vice versa ; but it is impossible for the rest to 
 become parallel to the work, from the connection of the arms. 
 Now let the traversing arm D, lie in the centre of the screw B, 
 on which it plays, and let the rest make a right angle with the 
 wood on which we intend to cut the screw. The rest may- 
 traverse thirty degrees on either side of the right angle ; which 
 will not cause any sensible approximation in the thread, and 
 will admit a motion sufficiently extensive for turning the com- 
 mon length of screws. But as the method answers for a short 
 screw of a few turns, that is sufficient for every purpose ; for, 
 in order to make a long screw, tliere may be three diff'erent 
 ways of accomplishing our object : 
 
 1st, At the commencement, the rest stands at right angles 
 with the wood on which the screw is to be cut; by its describ- 
 ino- an arch of a few degrees, a short screw is cut ; then by 
 brtnging back the rest to its original angle, the right one, and 
 sliding forward the single pointed tool to the last thread of 
 the screw that was just cut, we proceed to any length by re- 
 peating the same process. 
 
 2ndly, When one or two threads of a screw are cut by mak- 
 ing use of a common screw tool, the most unskilful haiid will 
 be able to continue the screw .to any length. 
 
 3dly, Should a side tool with many teeth, instead of the 
 single pointed one, be made use of, a screw of any length 
 may be cut, the rest describing its usual arch. 
 
76 MECHANICAL EXERCISES. 
 
 Apparatus for elliptic turuiog. 
 
 Elliptic Turning. 
 
 PI. II, exhibits perspective views and elevations of the appa- 
 ratus for elliptic, or (as it is vulgarly called) oval turning. An 
 oval is smaller at one end than the other, like an egg, and there- 
 fore dift'ers essentially from an ellipse. In all the different 
 figures, 1, 2, 5, 6, 7, and 8, the same letters refer either to the 
 same things, or to corresponding parts of the machine. 
 
 Fig. 1, is a front view of the machine. I K is the principal 
 iron plate, to which all the subordinate parts of it, except the 
 ring, (afterwards described,) are affixed. A short screw W, is 
 riveted or soldered to this plate, upon which the material to 
 be turned must be fastened, either uith or without the inter- 
 vention of a chuck, as the nature or form of it dictates. This 
 screw, to raise the back or lower part of it above the level of 
 the plate I K, has a shoulder, which affords a useful bearing 
 to whatever is screwed upon it, and prevents the inconve- 
 nience which would otherwise arise from the projecting heads 
 of the screws x x x x. 
 
 Fig. 2, exhibits the various parts of the machine on the back 
 of the plate 1 K, at each of the four corners of which there is 
 a short square pillar, on the summits of which are placed the 
 letters of reference, d d d d. Within these pillars, are placed 
 two narrow side ribs or pieces of steel, ff, which reach the 
 whole length of the plate I K. Each of these pieces, on the 
 side opposite the other, is bevelled, so that when placed on the 
 plate it forms an angular groove in the direction of its length. 
 The two angular grooves formed by the pieces^y, are filled by 
 the chamfered sides of the slider E F, which is capable of a 
 free longitudinal motion between them. When the slider has 
 been put in its place, two end ribs or pieces of steel m m, are 
 placed within the pillars, d d dd, parallel to each other ; they 
 bear upon the side pieces^/'y, to which, and to the plate I K, 
 they are firmly held by the screws x x x x. The nut L, is cast 
 in the same piece with the slider E F; in using the apparatus, 
 it is screwed upon the nose of the mandrel, and its size must 
 accordingly be proportioned to the mandrel, for which it is 
 intended. When the end pieces, m m, are fixed, the slider 
 cannot be thrown out of its situation in the grooves, as they 
 limit its only motion, the longitudinal one to the space between 
 them, because the nut L acts as a stop, at either end to which 
 the slider may be impelled. This effect, however, though a 
 necessary consequence of the construction, is not essential to 
 the excellence of the machine ; the principal use of the end 
 pieces, m m, is of a different nature. The space between them 
 18 exactly equal to the diameter of the ring O P, fig. 3, upon 
 
MECHANICAL EXERCISES. 77 
 
 Apparatus for elliptic turning. 
 
 the outside of which they revolve, when the nut L is screwed 
 upon the mandrel. Two arras, R S, are connected with this 
 ring, and in each of them there is a groove, nearly their whole 
 length. These grooves are exactly straight and opposite each 
 other, and a line carried from one to the other, along the 
 middle of them, would intersect the centre of the ring. The 
 projection of the ring above the arms, is clearly shown by the 
 elevation, fig. 4. 
 
 The elliptic machine is connected with the lathe, and its pe- 
 culiar motion obtained in the following manner: let E F G H, 
 fig. 9, represent the upper part of a headstock, through which 
 two holes have been drilled, at a little distance from the collar, 
 the centres of which holes are precisely in a line with the centre 
 of the mandrel M ; let the ring be fastened to the headstock by 
 means of two screws 1 1, with button heads, the shanks of which 
 pass through its grooves, and through the two holes made in 
 the headstock, at the back of which they can be drawn tight 
 by nuts. When the ring is in this situation, it will be per- 
 ceived, that it can only be moved from side to side, and its 
 centre, from what has been said of the position of its grooves 
 and that of the holes through which the screws that fasten it 
 pass, must always be in the same horizontal line with that of 
 the mandrel. Tighten the nuts of the screws 1 1. Now let 
 the apparatus A B C D, fig. 2, be united to the mandrel, by 
 screwing upon it the nut L ; the inner surface of the end ribs 
 or pieces m m, will fall at the same time upon the outside of 
 the ring. The plate I K, if the ring have been set so that its 
 centre exactly coincides with that of the mandrel, will, when 
 motion is communicated to it, revolve in a circle ; but if the 
 centre of the ring be in the smallest degree on one side of that 
 of the mandrel, it will revolve in an ellipse, the difference be- 
 tween the conjugate and the transverse, or long and short, dia- 
 meters of which, will be double the distance between the centre 
 of the ring and that of the mandrel. When, therefore, the 
 Work is fastened to the screw W, as in common turning it is 
 to the nose of the mandrel, it becomes as easy to turn an 
 iellipSe as in other cases it is to turn a cylinder. 
 
 The slider E F ought to move with great steadiness, and at 
 the same time with freedom; — requisites which cannot be com- 
 bined without considerable accuracy of workmanship. To ob- 
 tain an easy mode of making the wearing parts of the machine 
 fit, and also to lessen the friction in some degree, several little 
 arrangements are made, to some of which it may not be im- 
 proper to advert. The slider is made of bell-metal, or a com- 
 position similar to that already recommended for the collar of 
 a lathe, and only a narrow strip on each side of it touches the 
 
78 MECHANICAL EXERCISES. 
 
 Apparatus for elliptic turning. 
 
 back of the plate I K ; these narrow surfaces of friction are 
 shown by a lighter shade of the engraving, on the right and 
 left of E, tig. 1 ; the rest of the surface is cast about the twen- 
 tieth part of an inch lower than these strips, and it therefore 
 requires but little labour to make the slider true. The other 
 or upper side of the slider E F, fig. II, is not so thick as to 
 touch the end ribs, m m, when sliding under them. 
 
 In each of the four pillars, d d d d, are two screws. Four of 
 these screws, n n n n, press upon the end ribs m m, which, by- 
 screwing them in, can at any time be brought nearer to each 
 other, for the purpose of correctly fitting the ring. The side 
 ribs, in like manner, to embrace the slider tightly, may be 
 brought nearer to each other, by the four other screws, only 
 two of which, r r, can be seen in fig, 2. The four screws, x x x x, 
 the square heads of which are seen in fig. 1, and their ends in 
 fig. 2, are screwed only into the end ribs mm; they are rather 
 smaller than the holes in the plate I K, and those in the side 
 ribs,^"y/ through which they pass, otherwise the screws in the 
 pillars would not enable us to drive the ribs further in. 
 
 Fig. 5, and 6, are two elevations of the machine, which ex- 
 hibit the form and relation of some parts of it to each other, 
 more clearly than the perspective views. Fig. 5, is the side, 
 and fig. 6, the end of the machine. The same letters of refer- 
 ence being, as previously observed, placed upon the same 
 things, the position of which only is varied in the different 
 figures, much further description would be superfluous. It 
 may be observed, however, that fig. 6, shows distinctly the 
 bevel given to the sides of the slider E F, as well as that given 
 in a contrary direction, to the side ribs fj', in order to form 
 the grooves which receive the slider. 
 
 Fig. 7, represents one of the side ribs, and fig. 8, one of the 
 end ribs, separately. 
 
 The form given to the ring, fig. 3, though eligible for a 
 small machine, would be unsuitable for a large one. The limit 
 of its propriety is determined by the breadth of the puppet for 
 which it is intended; it cannot be wrong, when the puppet is 
 broad enough to admit the holes for the screws 1 1, fig. 9, to 
 be placed so as to allow the ring its full lateral range, in which 
 case it can be brought close up to the mandrel. But when 
 the shape here delineated is inadmissible, the grooves or open- 
 ings for the screws, may be formed within the ring, by two 
 stout ribs on each side of its centre, and it may then be fast- 
 ened to a puppet of the customary dimensions. 
 
MECHANICAL EXERCISES. 79 
 
 Earl Stanhope's turning rest. — Smart's rest. 
 
 Miscellaneous Remarks on Tunibig. 
 
 The parallel rest, already described, is similar in principle to 
 a machine invented by Earl Stanhope, for turning flat surfaces. 
 This patriotic Nobleman found, that the method of stereotyp- 
 ing invented by him, as w^ell as that construction of the print- 
 ing-press which is distinguished by his name, could not be 
 carried to the perfection he had in view, without devising some 
 method of making large surfaces of cast iron accurately flat. 
 He has always had in view the important object of rendering 
 the successful application of his inventions as independent as 
 possible of manual dexterity ; and, accordingly, the rest or tool 
 which he has invented to perfect his stereotype labours, is, if 
 we judge by its effects, and the manner in which it is wrought, 
 commensurate at once to his genius and his wants. Massive 
 blocks of iron, presenting a surface of eight or nine square feet, 
 are turned by it with wonderful facility, and with a degree of 
 exactness which will probablynever be exceeded. It is, like the 
 parallel rest, peculiarly adapted to the turning of metals, and 
 doubtless exceeds, in the excellence of its performance, every 
 thing of the kind before attempted. — Of the contrivances which 
 have been adopted to facilitate, upon similar principles, the 
 expeditious and correct turning of wood, that by Smart, of 
 Ordnance Wharf, Westminster, deserves particular notice and 
 approbation. The apparatus which this ingenious mechanic 
 has invented, is used in his manufactory, and when applied to 
 a common lathe, enables one man, with the assistance of two 
 labourers at the great wheel, to turn sis hundred poles, each 
 of them a very accurate cylinder, and five and a half feet long, 
 in the course of twelve hours. His mandrel revolves twelve 
 hundred times in a minute, in which short space of time one 
 pole is finished. The means by which he attains his object, 
 have the merit of not only being efiicacious, but of possessing 
 great simplicity. The gouge for roughing out the work is 
 fastened in a block, or cutter-frame, which is nothing more 
 than a piece of wood, containing a cylindrical hole, large 
 enough to be shoved over, without touching, the work to be 
 turned. The gouge passes through the block into this cavity, 
 where its edge projects, just as the blade of a joiner's plane 
 projects from the bottom of the block in which it is fixed. 
 The chisel, which succeeds the use of the gouge, is fastened 
 in another frame of a similar description. The gouge and 
 chisel are held in their respective places by screws. The re- 
 maining part of the apparatus consists of two strong wooden 
 cheeks; and as these must always be as long as the work, it is 
 best to make them, at once, the. full length of the bed of the 
 
80 MECHANICAL EXERCISES. 
 
 Smart's turning rest. 
 
 lathe, measured from the middle puppet. They may form a 
 separate frame, or they may be fastened one on each side of 
 the middle and right hand puppets, parallel to one another, 
 and their upper surfaces in the same horizontal plane. Their 
 position will then be similar to that of the cheeks of the lathe 
 Itself. To give them additional steadiness, if found necessary, 
 they may be supported, in one or two places, by feet resting 
 upon the lathe, or by short puppets of sufficient breadth to 
 reach under them. On the bottom of the gouge and chisel 
 frames, there are two grooves, the same size and distance from 
 each other as these additional cheeks, which they are intended 
 to admit into them. The cheeks must be of such a height, 
 that the cutter-frames can be slidden along upon them, with 
 the axis of the holes, into which the gouge and chisel project, 
 coincident with the axis of the mandrel, and consequently 
 coincident with that of the work. The projection of the man- 
 drel, and that of the screw in the right hand puppet, is always 
 ratlier more than the breadth of both the cutter-frames toge- 
 ther, in order that the latter, when in use, may be made en- 
 tirely to clear the ends of the pole. At the commencement 
 of the process, after the cheeks upon which the cutter-frames 
 slide have been fixed, let the cutter-frames themselves, (that 
 containing the gouge being outermost,) be placed against the 
 right hand puppet, the screw of which, if far enough out, will 
 then extend through the centre and a little beyond them. 
 The pole to be turned, which we will suppose already pre- 
 pared, by hewing it octagonally, or somewhat rounding it, is 
 fixed to the lathe in the customary manner, and the gouge 
 frame, the men having begun to turn the great wheel, is 
 pushed along its whole length ; and the tool being previously 
 adjusted so as to take off a shaving of sufficient thickness, as 
 soon as it has cleared the end of the pole, it is left over the 
 mandrel. The chisel frame is next pushed along in like man- 
 ner, and thus it is that one minute suffices to complete a very 
 smooth and accurate cylinder. It is obvious, that the best 
 position for the gouge and chisel in their frames, will be that 
 which gives each of them the same inclination to the surface 
 of the work, that is found most advantageous in turning by 
 hand. This mode of turning may be classed, perhaps, among 
 those inventions, which every one requiring their aid is apt to 
 wonder he has not thought of, or to believe that he could 
 have thought of; and the very simplicity of which, while, in- 
 stead of disparaging, it enhances their value, and the debt 
 which the public owe to their authors, is one of the main ob- 
 stacles to the discovery of them. 
 ^ A numberless variety of figures may be produced in the 
 
MECHANICAL EXERCISES. 81 
 
 Miscellaaeoas remarks on tuining. 
 
 lathe, by regulating the action of the tool, in its advance to, 
 or recess from, the face of the piece exposed to its action. 
 Medallions, and other similar pieces, have been executed by 
 enthusiasts in the art, who have spared neither expense in pro- 
 curing the necessary apparatus, nor patient perseverance in 
 the use of it. In the British Museum, there is a profile in 
 bass-relief, of Sir Isaac IS'ewton, wholly made by turning; the 
 resemblance is considered very correct, and the place in which 
 it is deposited may be considered a sufficient proof of the dif- 
 ficulty with which it has been executed. 
 
 Watch-cases, snuff-boxes, and various sorts of trinkets, are 
 sometimes formed by what is called rose-work. Plates of 
 iron or brass, indented or waved on the edge, in any curve or 
 form which may be desired, are screwed upon the mandrel. 
 These plates, which are called roves, serve as guides to regu- 
 late the action of the tool, in producing a correspondent form 
 on the work. 
 
 The copper cylinders used at Manchester in printing calicoes 
 by machinery, atford specimens of turning, or rather of en- 
 graving in the lathe, of the most curious and interesting kind. 
 No production of the art can be more beautiful than the work- 
 manship of many of them, nor more admirable than the effect 
 produced by their use. A whole web or piece of calico is 
 printed by them in three minutes. Thev are, in the first place, 
 turned accurately cylindrical, and polished with as much care 
 as the copperplates for common engraving; the pattern is then 
 cut upon them, and it is this part of the process, in preparing 
 them for use, which most remarkably exercises the genius of 
 the artist. Two methods of executing it present themselves— 
 the graver and the lathe. As the latter, for every pattern to 
 which it can be applied, is so much more expeditious and ac- 
 curate than the former, a desire to make use of it in preference 
 naturally follows; and accordingly it is made use of in cutting 
 a vast variety of beautiful patterns, to the production of which 
 few would consider any tool but the graver, directed by the 
 most complete manual dexterity, in any degree adequate. The 
 identical methods pursued by different artists, in this branch 
 of turning, are but little known, but a general idea of the na- 
 ture and possibility of the thing is not of difficult comprehen- 
 sion. Let the pattern intended for the copper be cut upon the 
 circumference of a small steel wheel, which must be made to 
 revoh^ upon an axis. Let this wheel be held against the cop- 
 per cylinder, which, when revolving in a lathe, will carry it 
 round, and receive from it an impression of the pattern on its 
 circumference. When one ring of the pattern impressed by 
 the wheel has beeu obtained, it is plaiii, tJiat by repeating the 
 4— Vol. I. M 
 
82 MECHANICAL EXERCISES. 
 
 Miioellaiieoiis remarks on turniu);. 
 
 operation, others may be made at any required distance from 
 each other. As the wheel operates ahiiost like a punch, the 
 marks made by it are seldom square at the edge; but these in- 
 equalities are soon reduced by the common process of turning 
 and polishing. It may also be observed, that the cavities made 
 by the wheel must be left rough or notched at the bottom, in 
 order that they may better retain the ink till they meet the 
 calico. To cut the wheel, so as to produce this effect, faci- 
 litates its entrance into the cylinder. 
 
 Practical directions for turning might easily be multiplied, 
 but the necessity for much further minuteness of detail will be 
 removed by a little observation and experience ; yet we are 
 unwilling to refuse a place to a few remaks which may be ac- 
 ceptable to the novice. In turning a hollow sphere, the con- 
 vex surface is first completed, and then perforated with a 
 centre-bit to admit of the tool, fig. 20, pi. Ill, by which the 
 interior may be cut away. But as it may not be desirable to 
 make a very large hole for the admission of the tool, it is cus- 
 tomary to perforate the sphere in six places, each hole being 
 made in a direct line to the centre, to which they must all ap- 
 proach within half their diameter. They are also bored at 
 equal distances from each other, and each hole is at right 
 angles with all the rest except one, to which it is exactly op- 
 posite. Hence the points at which these holes ought to be 
 made, may be obtained by drawing circles upon the sphere 
 which divide it into quarters ; the points of intersection of 
 these circles, are the places sought. Place the sphere in a 
 chuck, with the axis or middle of any two of the holes in the 
 same line with that of the mandrel. Turn out a portion of the 
 interior from the hole in front; then in succession bring every 
 other hole to the front, and proceed in like manner. The par- 
 titions dividing the several excavations will at length be cut 
 through, or will be made so thin that they may be cut from 
 the interior surface of the sphere by a bent saw. 
 
 To turn one sphere within another, cylindrical holes as in 
 the last case are required, but the thickness left between each 
 pair of opposite ones, must at least be equal to the diameter 
 of the inner sphere intended to be left. The tool, fig. 21, or 
 that fig. 22, is used to make the excavation or space between 
 the concentric surfaces, by entering in rotation, as before, the 
 six holes. By using the tool represented by fig. 18, or that 
 of fig. 19, a cube might have been turned instead of the inner 
 sphere. 
 
 To turn a series of spheres within each other, the depth of 
 the cylindrical holes must be such as to leave the thickness 
 betwe«n each pair no more, or but very little more, than the 
 
MECHANICAL EXERCISES. 83 
 
 Miscellaneous remarks on turning. 
 
 diameter of the smallest sphere intended to be left. With 
 forming this innermost sphere, the operation may be most 
 properly begun, and afterwards continued in regular progres- 
 sion to the larger, till at last the one next the exterior sphere 
 is completed. If it be thought desirable, the spheres may have 
 their cylindrical holes proportioned to their respective sizes, by 
 boring at first a little way with a large centre-bit, and after- 
 wards using a smaller and a smaller one, in approaching the 
 centre. 
 
 When the band of a lathe is crossed, and the wheel is 
 turned with great velocity, it is apt to wear by rubbing against 
 itself. This effect may be prevented by interposing a pulley 
 at the crossing place. The pulley may turn upon a pin at the 
 end of apiece of wood rising aslant, and attached to the cheek 
 of the lathe, or fastened to the floor. 
 
 The number of turns which the mandrel ought to make in a 
 given time, must be varied according to the nature of the mate- 
 rial in the lathe. The velocity of rotation, for wood, can 
 scarcely be too swift ; it must be rather slow for lead, pewter, 
 brass, and bell-metal; still slower for cast iron, and slowest of 
 all for forged iron and steel. The reason for these limits ap- 
 pears to be, that a certain time, varying with the material, is 
 requisite for the act of cutting to take place, and that the tool 
 itself, if much heated, will instantly become soft, and cease to 
 cut. In a lathe turned by the foot, three turns of the pulley 
 of the mandrel, for one of the fly wheel, will be found suffici- 
 ently quick for iron; four or five turns of the pulley, for one of 
 the fly, may be allowed to brass ; and ten or twelve will not be 
 too much for wood. As, however, in turning a large fly wheel 
 very slowly, to produce the required slow motion of the man- 
 drel, would occasion the loss of the power accumulated by its 
 increasing momentum when swiftly revolved, a small wheel 
 may be placed on the same axis. This small wheel may re- 
 ceive the band, and the size of it may be so proportioned, that 
 it may be turned with the large one as swiftly as we please, 
 without making the velocity of the mandrel too great. 
 
 The temper of the tools employed in turning iron and steel, 
 when they are annealed, is not commonly higher than purple. 
 But when the inequalities of steel have been reduced, and it 
 is required to cut it extremely clean, the use of a sharp hard 
 tool will be advantageous. Steel and cast iron at a high tem- 
 per also require a very hard tool, and the angle forming the 
 edge of the tool must be considerable ; if equal to seveaty or 
 eighty degrees, it will not be too obtuse. Steel and iron are 
 cut more freely, if kept constantly wetted with water, or 
 thinly covered with oil or tallow. 
 
S4 MECHANICAL EXERCISES. 
 
 Miscellaneous remarks on fuming. 
 
 When iron und steel are lo he turned in a small lathe of 
 little power, as soon as the work is fixed, it is with many a 
 practice to nick it all round with a graver, in one or two places ; 
 if the nick be deeper on one side than another, a file is em- 
 ployed to make it every-where alike ; and when this has been 
 done, the power employed to finish it may be very inconsider- 
 able, without much loss of time. 
 
 The bank, or upper part of the rest, upon which the tool is 
 held, is short, broad, unpolished, and very strong for metallic 
 work, in order that the tool may be steadied upon it in the 
 most etiectual manner; but in turning wood, it is long, narrow 
 on the top, and polished, so that the tool can be swiftly glided 
 from end to end. To a foot lathe, the bank of the rest, for 
 metal, is seldom made more than three inches ; while for wood 
 its length is twelve or fourteen inches. In turning bed-posts 
 and other things of great length, a piece of wood, the whole 
 length of the work, is made use of as a rest, to prevent the 
 trouble of those frequent removals which the common rest 
 would require. With a like view of saving time, the rest, for 
 globular work, is often made of a semi-circular form. 
 
 When the work is too long and slender to bear the action 
 of the tool without yielding, it is supported in its proper place, 
 from the back, by a frame, which, like the rest, can be fixed 
 in any situation, and carries an arm with a semi-circular ex- 
 cavation in front. This opening admits the work, to which it 
 is brought close up, and which it prevents from being bent or 
 broken by the tool. 
 
 When the material which has been turned is wood, and it 
 has been made as smooth as possible with the chisel, or other 
 edge-tool which its peculiar form may have required, it may 
 be polished with shark's skin, or Dutch rushes. The shark's 
 fekin, being obtained from different species of that fish, is some- 
 times of a reddish and sometimes of a dark grey hue, without 
 any material difference of quality; it is too rough for polishing 
 at first, and therefore must be worn a little by using it to the 
 rougher states of work, before it is applied to this purpose. 
 The Dutch rush is the equisetum hyemale of Linnaeus; it grows 
 in moist places among the mountains, and is remarkable for 
 having flinty particles in the substance of its leaves, w^hich 
 tender it so useful in polishing. It has a naked, simple, and 
 round stem, about the thickness of a goose quill. The oldest 
 plants are the best. Before they are used, they should be 
 tnoistened a little with water, otherwise they presently break, 
 fend become almost useless. The use of the rush is particularly 
 proper for hard woods, such as box, lignum-vitae, or ebony. 
 After having well smoothed up the work, it should be rubbed 
 
MECHANICAL EXERCISES. 85 
 
 Oak timber — beecb. 
 
 gently either with wax or olive oil, then wiped clean, and 
 finished by rubbing it with a cloth or its own shavings. 
 
 Ivory, horn, silver, and brass, may be smoothed with files, 
 or with pulverized pumice-stone, laid upon leather or a cloth a 
 little moistened. When a very fine surface is required, tripoli, 
 and afterwards oxide of tin, may succeed the use of the no 
 mice-stone. To polish iron and steel, after they have been 
 made as smooth as possible with files, very fine flour of emery 
 is mixed with oil, and laid upon two pieces of soft wood, be- 
 twee-n the surfaces of which, thus coated, the work is pressed 
 and made to revolve. 
 
 Of TIMBER. 
 
 From a view of the properties, uses, and principal modes of 
 working the metals in most common use, we now proceed to 
 details of a similar kind respecting timber. One description 
 of the mechanical operations belonging to this subject, have, 
 indeed, already been anticipated by the sections devoted to the 
 mixed art of Turning, to which both wood and metals are alike 
 extensively subjected ; but much yet remains to be adverted to, 
 which forms a proper part of the knowledge of the turner, 
 though more particularly subservient to the purposes of the car- 
 penter, the joiner, the millwright, and the cabinet-maker. We 
 shall commence with an enumeration of the most useful sorts 
 of wood, with a few remarks on their properties, their uses, 
 and the best methods of seasoning timber in general. 
 
 Of all the different kinds of timber produced in this country, 
 oak is the best for building, and for almost every purpose of 
 rural and domestic economy, particularly for staves, laths, and 
 spokes of wheels. Even when it lies exposed to air and water, 
 it is preferable to almost all other woods; and as it is, besides, 
 hard, tough, tolerably flexible, and not very apt to splinter, 
 its excellence for ship-building is unequalled. Its quality is 
 improved, if the tree be suffered to stand three or four years 
 after it has been barked, as it thus becomes perfectly dry ; 
 the inspissated sap renders it much stronger than the heart of 
 those trees which have not been stripped, and its hardness, 
 weight, and durability, are also increased. 
 
 Beech is also a wood of great utility ; it is very tough and 
 white when young, and of great strength ; but liable to warp 
 very much when exposed to the weather, and to be worm-eaten 
 when used within doors. Its greatest use is for planks, bed- 
 steads, chairs, and other household goods ; and for these 
 purposes, it seems almost as necessary to the cabinet-makers 
 
86 MECHANICAL EXERCISES. 
 
 B«ecb timber — elm— chesout. 
 
 and turners of the metropolis, as oak is to the ship-builder. 
 The worm bv which it is so liable to be destroyed, is supposed 
 principally to feed on the sap that remains in the wood, con- 
 sequently the best method of preserving it, is to extract the 
 food on which the worm subsists. For this purpose, scantlings 
 of beech, when large, should be laid to soak in a pond for seve- 
 ral weeks, according to the size of the timber, and the season 
 of the year. In the heat of summer the desired effect is more 
 speedily produced. Generally, beams and thick planks should 
 remain about twenty weeks in water; joists and rafters about 
 twelve weeks ; and the thinner boards about two months. As 
 the planks or boards are in danger of warping, when exposed 
 to dry, they should be sheltered from the sun and rain ; laths 
 ought to be placed at intervals between the boards, to prevent 
 their contact, and the whole pressed by a considerable weight. 
 If they are large pieces, for beams, joists, &.c. They need only 
 be left to dry gradually under sheds. Beech, by these means, 
 will be rendered as good and durable as elm ; but when it is used 
 for building, it is advisable to prepare that part of the timber 
 which touches the brick-work with a thick coat of pitch, to 
 guard it against the effects of moisture. If the wood have been 
 felled in the heat of summer, the sap, of which it is then full, 
 may be more readily extracted than if it be felled in winter. 
 When this wood is intended for small work, such as chairs, tur- 
 nery, the handles of saws, and the blocks of joiners' planes, it is 
 recommended to boil it in water for two or three hours. This 
 mode of preparing it extracts all the sap; makes it work more 
 smoothly; and renders it more beautiful and durable. 
 
 The consumption of elm is very considerable ; it is very 
 tough, pliable, and the best kinds of it are very hard ; it does 
 not readily split, and bears the driving of bolts and nails into 
 it better than any other wood. It is used for making axle- 
 trees, mill-wheels, keels of boats, water-pipes, chairs, and cof- 
 fins. It is frequently changed by art, so as to make an ex- 
 cellent resemblance of mahogany. For this purpose, planks 
 of it are stained with aquafortis, and rubbed over with a tinc- 
 ture, of which alkanet root, aloes, and spirit of wine, are the 
 principal ingredients. 
 
 Wild chesnut timber is very durable, and is by many esteem- 
 ed as good as oak. It seems to have been much used formerly 
 in building; it excels oak in two respects, namely, it grows 
 faster, and the sappy parts of it are more firm, and less Tiable 
 to corruption. At the age of eighteen or twenty munms, it 
 may be cut for hop-poles, for which it is very serviceable. It 
 is superior to elm for jambs, and several other purposes of 
 house-carpentry; but on account of its possessing a precarious 
 
MECHANICAL EXERCISES. 87 
 
 Ash timber — walnut tree — pine. 
 
 brittleness, which renders it unsafe for beams, it ought not to 
 be employed in any situation where it will have to support an 
 uncertain weight or strain. Perhaps this is the principal rea- 
 son why it has rather fallen into disuse for building. It is 
 generally agreed, that it is peculiarly excellent for casks, as it 
 neither shrinks, nor changes the taste or colour of the liquor. 
 It is often converted into articles of furniture, and may be 
 made to imitate mahogany, by rubbing it over with alum wa- 
 ter, then brushing it with a hot decoction of logwood, and 
 lastly with a decoction of Brazil wood. 
 
 Ash is a very useful wood, which possesses the very remark- 
 able property of being almost equally good, whether cut 
 young, or at full maturity. It is tough, hard, and more 
 elastic than most woods. It answers well in buildings, and 
 for any other use, when screened from the weather. It is 
 much used for making implements of husbandry, particularly 
 hop-poles, also for handspikes, oars, and the handles of tools, 
 such as the axe, adze, &cc. 
 
 Walnut-tree is excellent for the joiner's use, and was in 
 great request for all the best articles of furniture, till super- 
 seded by mahogany ; it is still in repute for the best grained 
 and coloured wainscot ; with the gunsmith, for stocks ; with 
 the coach-maker, for wheels and the bodies of coaches ; with 
 the cabinet-maker, for inlayings, especially the firm and close 
 timber about the root. To render this wood the better co- 
 loured, joiners put the boards into an oven, or lay them in a 
 warm stable ; and when they work it, polish it over with its 
 own oil very hot, which makes it look black and sleek. The 
 oldest wood is the most valued. 
 
 The wild pine-tree, called in this country the Scotch fir, 
 from its growing naturally in the mountains of Scotland, fur- 
 nishes the best red or yellow deal, so much employed in the 
 making of masts, floors, wainscots, tables, boxes, and for num- 
 berless other purposes. The wood is very resinous, and the 
 most durable of any of the kinds of fir yet known. The wood 
 of the black and white spruce firs, is very light, and decays 
 when exposed to the air for a considerable length of time ; it 
 is chiefly employed for packing cases, musical instruments, 
 and the like. 
 
 Deals, or common fir boards, may be much hardened and 
 improved, by immersing them, as soon as they are sawn, in 
 salt water for three or four days, care being taken to turn them 
 frequently during that time. They should then be dried by 
 exposing them to the sun and air ; but neither this, nor any 
 other mode of preparing them yet known, will prevent their 
 shrinking. The shrinking is greatest transversely ; in the 
 
88 MECHANICAL EXERCISES. 
 
 Larch timber — poplar — birch. 
 
 direction of the length of their fibres, when once well sea- 
 soned, they shrink so little, that a piece of deal is highly 
 esteemed for the rod of a pendulum. 
 
 The timber of the larch tree is possessed of many very 
 valuable properties. It is exempt from the depredations of 
 worms ; it is peculiarly calculated for ships' masts and the 
 buildins: of vessels, or for strengthenino- tlie wooden frame- 
 work of bridges ; for it is capable of supporting a much 
 greater weight than the oak itself, and almost petrifies 
 under water. It also resists the infiuence of our climate, 
 and is excellent for gates, pales, shingles, and other works 
 which are exposed to all the vicissitudes ef the weather. 
 Houses constructed with larch timber, have a whitish cast 
 for the first two or three years ; after which the outside 
 becomes black, while all the joints and crevices are firmly 
 closed with the resin extracted from the pores of the wood 
 by the heat of the sun ; and which, being hardened by the 
 air, forms a kind of varnish, not inelegant in its appearance. 
 Buildings constructed with it, have been observed to remain 
 sound two hundred years. No wood affords more durable 
 staves for casks, and the flavour of the wine is at the same 
 time preserved. The charcoal made from the larch is not 
 only much superior in quality, but in quantity, to that made 
 from a like measure of the fir-tree. 
 
 The common cedar (of Lebanon) is a species of the pine-tree. 
 The character of its wood is well known, and firmly established. 
 Its uniformity and softness recommends it to the use of the 
 manufacturer of black-lead pencils; while its neat appearance, 
 and its not being liable to the depredations of worms, occa- 
 sions a great demand for it from the cabinet-maker, for the 
 drawers and interior divisions of desks, See. where great 
 strength is not required. It is admirably calculated to with- 
 stand the effects of moisture. 
 
 The different species of poplar trees, supply timber much 
 used instead of fir; it looks better, and is tolerably tough 
 and hard. Poplar wood is not very subject to the ravages of 
 worms, nor to warping or shrinking. It is advantageously 
 employed for wainscoting and floors, as well as for water- 
 pipes, packing boxes, and turnery wares. For bedsteads 
 it is deemed unsuitable, as it is considered more liable than 
 other woods to be infested with bugs. It is a very incom- 
 bustible wood, and therefore admirably adapted for the 
 floors of workshops where ignited bodies are apt to be 
 thrown down. 
 
 The wood of the iircli or alder tree, differs not very mate- 
 rially from that of the poplar, either in quality or use. The 
 
V 
 MECHANICAL EXERCISES, 89 
 
 Maple. — Pear-tree. — Cherry-tree. — Yew. — Holly. 
 
 soles of what are called wooden shoes, and women's shoe 
 heels, are made of it. It is also much used for hoops, and by 
 the turner, for bowls and dishes of all sorts. 
 
 The whitest and finest kinds of maple, are used by cabinet- 
 makers for inlaying. On account of the lightness of this 
 wood, it is frequently used for musical instruments. It is 
 excellent for the use of the turner; cups made of it may be 
 turned so thin as to transmit light. These characteristics be- 
 long more particularly to English maple, which is also well 
 known under the name of sycamore or plane tree, and when 
 grown on favourable situations, is almost as hard, white, and 
 beautiful, as holly. The American maple is mostly of a dull 
 light brown coloiir ; it is a softer wood, and its grain has 
 some slight resemblance to that of the cherry-tree. 
 
 Pear-tree wood is smooth, light, and compact ; very suitable 
 for the turner's use, and when stained black, makes a good 
 imitation of ebony, for picture frames, and similar articles. 
 It is sometimes engraven upon, but for this purpose it is 
 much inferior to box, and even to holly. 
 
 The wood of the cherry-tree is smooth, tough, yellowish, 
 with a grain like the commoner kinds of mahogany, of which, 
 with a very little assistance from staining, it makes an excel- 
 lent imitation. It is therefore used for the middling qualities 
 of furniture, by the cabinet, chair, and bedstead makers. 
 
 The yew-tree was formerly cultivated for the manufacture of 
 bows, but since the decline of the demand for it on this 
 account, it has been much neglected. The wood is, however, 
 valuable, though, like the two last kinds, not very abundant; 
 it is hard and smooth, beautifully veined with red streaks, 
 admits of a fine polish, and is almost incorruptible. It is 
 employed by the turner and cabinet-maker, the millwright 
 and the engraver. It may therefore be seen in tables, chairs, 
 cups, spoons, toys, ornamental tea-caddies, urns, &c. Con- 
 verted into axletrees, cogs for mill wheels, and floodgates for 
 fish-ponds, it is found very proper and durable. By the en- 
 graver it is only used for large and coarse work. 
 
 Of all hard woods, that of the holly is the whitest, and is 
 much used for inlaying, especially under thin plates of ivory. 
 It is excellent for the use of the turner, and is highly prized 
 by the millwright for the cogs of wheels ; it also makes the 
 best handles and stocks for tools, flails, the best riding rods 
 and carters' whips, bowls, sheaves, and pins for blocks. 
 ; Next to box, it is the most suitable wood for the use of the 
 ^/^n^aver. It is tougher, but not so hard as box. 
 
 Box is the only European wood which will sink in water. 
 Its closeness of grain, hardness and toughness, are such, as 
 4,— Vol. I. N 
 
90 ilECHAN'ICAL EXERCISES. 
 
 Box-wood. — Ebon J. — Lignum-Tita-. 
 
 to render it admirably adapted to the purposes of the engraver 
 on wood. The engraving is always made on the end of the 
 wood, so that the fibres stand perpendicularly. When cut in 
 this manner, the graver will make a clean stroke in every di- 
 rection, and the piece is liable to warp but little. When cut 
 plankwise, boxwood is e.xtremely apt to warp, unless very well 
 seasoned. No kind of wood turns smoother than this, and its 
 yellow colour, when it is well polished, is very beautiful ; it is 
 much used for pulleys ; for shuttles ; for the bottoms of joiners' 
 planes, especially their moulding planes ; and, in short, is 
 valuable for every purpose requiring wood which will bear 
 friction well. When ivory would be too expensive, or cannot 
 be obtained of the requisite dimensions, boxwood is commonly 
 the substitute for it. Hence the consumption of it for combs, 
 button-moulds, knife handles, and particularly for mathema- 
 tical instruments, is very considerable. The bitter quality of 
 boxwood secures it from the attacks of worms. 
 
 Ebom/is an exceedingly hard and heavykind of foreign wood, 
 of a very smooth even grain, susceptible of a remarkably fine 
 polish, and on that account used in mosaic and inlaid works, 
 for toys, Jkc. It is of various colours, most usually black, 
 brown, red, and green. The black is the kind most generally 
 known, and preferred to that of other colours. The best is a 
 jet black, free from veins and rind, very massive, astringent, 
 and of an acrid pungent taste. Ebony is not in so much de- 
 mand as fomierlv, from the improvements which have been 
 made in giving other hard woods, especially the holly, a black 
 colour. It is used for parallel rulers, and other mathematical 
 instruments not requiring to be marked with fio-ures, which 
 the darkness of its colour would prevent from being distinctly 
 seen. It is brought from ^ladagascar, the Mauritius, and the 
 W^est Indies. The tree of the West India kind is seldom more 
 than eighteen feet high, and the trunk five or six inches in 
 diameter. 
 
 Lignum-vifce is another foreign wood, firm, solid, ponderous, 
 very resinous, of a blackish yellow colour in the middle, and a 
 hot aromatic taste. It is a native of the West Indies, and the 
 warmer parts of America. It is of considerable utility in the 
 arts: by the sugar planters it is manufactured into wheels and 
 cogs for sugar mills. The sheaves or puUevs in ship blocks 
 are mostly made of this wood ; which is also frequently formed 
 into bowls, mortars, and various utensils. It is much used 
 by the turner, making excellent castors, handles for tools, and 
 other small ware; but as it is expensive, hard to work, and 
 not very remarkable for its beauty, it is in little demand for 
 the cabinet-maker. 
 
MECHANICAL EXERCISES. 91 
 
 Mahogany. 
 
 Mahogany is one of the most valuable woods imported i-nto 
 this country, and the tree, growing in full maturity, is one of 
 the noblest productions of nature, attaining often the majestic 
 height of one hundred feet. ^Mahogany balks are often three 
 or four feet in diameter, and the diameter of one lately im- 
 ported into Lancaster measured five feet. Mahogany varies 
 very much in quality ; that grown on rocks is the hardest, 
 heaviest, closest in the grain, and most beautifully veined; 
 and Jamaica wood is preferable to that obtained on the coast 
 of Cuba and the Spanish INIain, on account of its being mostly 
 found on rocky eminences, while the latter is cut in swampy 
 soils near the sea-coast, and is light, porous, pale coloured, 
 and open grained. On soils neither rocky nor swampy, the 
 wood is of a medium excellence. Hence a good idea of the 
 
 - value of a parcel of mahogany may be formed, if we know cor- 
 rectly the nature of the soil upon which it grew. Different 
 parts, however, of the trunk of the same tree, vary somewhat in 
 quality ; and in felling the timber, the most beautiful portion 
 of it is commonly left behind. The negro workmen raise a 
 scaffolding of four or five feet elevation from the ground, and 
 
 - hack up the trunk, which they cut into balks. The part below, 
 j extending to the root, is not only of larger diameter, but of a 
 
 closer texture than the other parts, most elegantly diversified 
 with shades or clouds, or dotted like ermine with spots. This 
 part is only to be come at by digging below the spur, to the 
 depth of two or three feet, and cutting it through ; an operation 
 too laborious to be often attempted. — The remark just made, 
 with respect to the superiority of the wood of the mahogany- 
 tree, near the earth, is applicable to timber in general, and 
 ought not to escape the observation of those who are desirous 
 of selecting the choicest and most ornamental portions for par- 
 ticular purposes. The exquisite beauty of the finer kinds of 
 mahogany, the incomparable lustre of which it is susceptible, 
 < exempt also from the depredations of worms, — hard, durable, 
 
 - warping and shrinking very little, it is pre-eminently calculated 
 to suit the work of the cabinet-maker. Accordingly, these 
 admirable properties, added to its abundance, and the largeness 
 of its dimensions, have occasioned it to be manufactured into 
 every description of furniture. From its being so little subject 
 to shrink and warp, it is particularly excellent and much used 
 for the patterns of iron and brass founders, especially for the 
 patterns of wheel-work and other things which require the 
 greatest nicety. It is the commoner sorts of mahogany which 
 are generally wrought up in this way. The commoner : i ds 
 also are often stained black, and made to look to great advan- 
 tage, for small turnery wares, such as picture frames. 
 
92 MECHANICAL EXERCISES. 
 
 Felline of timber. 
 
 The preceding catalogue of woods nught easily be enlarged, 
 but not, perhaps, with advantage. Several woods of home 
 growth, wnich seldom come into consumption, and are alike, 
 from their qualit>- and their scarcity, of limited utility, as well 
 as many of foreign production, such as dyewoods, which are 
 rarelv employed in the mechanic arts, we pass over as unneces- 
 sarv in this enumeration. It will be more useful to subjoin a 
 few' remarks on the mode of seasoning and properties of timber 
 in ireneral. The goodness of timber not only depends on the 
 soil and situation in which it stands, but likewise on the sea- 
 ifion in which it is felled. With respect to the latter point, 
 considerable disagreement of opinion prevails ; some are for 
 having it felled as soon as its fruit is ripe, others in spring, and 
 raanv in autumn. But as the sap and moisture of timber is 
 certainly the cause fnat it perishes much sooner than it other- 
 wise would do, it evidently seems to be a good general rule, 
 that timber should be felled when there is the least sap in it, 
 viz. from the time that the leaves begin to fall, till the trees 
 begin to bud. The only plausible objection to a practice found- 
 ed upon this idea, is, that the sap in winter is thicker than at 
 any other season., and therefore may be of more diflficult ex- 
 traction, in the subsequent seasoning, than if the tree had 
 been cut when it was more abundant and more fluid. In Eng- 
 land, the work of felling timber usually commences about the 
 end of April, because the bark then rises more freely; and where 
 a quantity of oak timber is to be felled, the statute requires 
 it to be done at that time, for the advantao-e of tanning. 
 
 The age at which timber is cut, is a matter of great import- 
 ance; if cut too old or too young, it will not be so durable as 
 when cut at a proper age. It is said, that oak should not be 
 cut under sixty years old, nor above two hundred. It is easy 
 to offer as a general rule, that timber trees should be cut in 
 their prime, when almost fully grown, and before they begin 
 to decay ; but when it is inquired, how these particulars are to 
 be determined, we are compelled to refer to judgment and ex- 
 perience as the only guides which can be depended on. Dif- 
 ference of soil, situation, and climate, hasten or retard the growth 
 of timber so much, that the age at which any particular kind 
 of tree arrives at maturity, cannot be correctly assigned. Mar- 
 shall observes, that poplars may stand from thirty to fifty 
 years ; ash and elm-trees, from fiftv to a hundred. 
 
 With respect to the best mode of seasoning timber after it 
 has been sawed, a variety of opinions are entertained ; but 
 practical men, who seldom regard the notions of the speculatist, 
 and who require a process not only effectual but convenient 
 upon a large scale, consider no method better than that of 
 
MECHANICAL EXERCISES. 93 
 
 Seasoning of timber. 
 
 exposing the timber they intend to season, for a sufficientlength 
 of time to a free current of air, and every vicissitude of wind 
 and weather. Boards from one to two inches in thickness, 
 which have undergone this exposure twelve or eighteen months, 
 they deem fit for use. They observe, that the mere drying of 
 timber is not the same thing as seasoning it, and that unless it 
 is frequently wet, while exposed, it receives little benefit. This 
 is only stating, in other words, the necessity of extracting the 
 sap. Some persons, however, advise the planks to be laid up 
 in a dry airy place, out of the wind and sun, or at least free 
 from the extremes of either; and that they may not decay, but 
 dry evenly, they recommend them to be daubed over with cow- 
 dung. They must not be piled on their ends, as in the common 
 way, but one plank must be laid over another, small pieces of 
 wood being interposed, to prevent the contact which would 
 otherwise occasion an injurious mouldiness. This mode, 
 though not so convenient, nor probably so effectual as the 
 former, or common one, is certainly more rational than that 
 recommended by others, of burying the timber in the earth. 
 
 To season timber in a short time, it may be laid in a pool, 
 or running stream, in order to extract the sap, and afterwards 
 dried in the sun or air. On a small scale, boiling water, as di- 
 rected for beech, may be employed, by which the seasoning of 
 green wood may be accomplished with the greatest expedition. 
 The process will be found useful to turners and cabinet-makers. 
 But whatever mode of seasoning be adopted, against shrinking, 
 more or less, there is no remedy. The principal disadvantage 
 occasioned by the shrinking of timber, is from the diminution 
 of its transverse dimensions. When this kind of shrinking takes 
 place in casks, for example, in any considerable degree, the 
 hoops drop off, and these vessels fall to pieces; perhaps as great 
 a number of them are destroyed by this cause as by any other. 
 To obviate its effects, G. Smart has lately taken out a patent for 
 constructing casks on a new principle : before he puts them to- 
 gether, he presses the wood into a smaller compass than it would 
 ever be reduced to by drying : 32,000 vessels made according 
 to this plan, have verified its utility in the most ample manner, 
 not one of them having leaked, under circumstances that ren- 
 dered those made in the common way useless. 
 
 The Venetians are supposed to be the first in modern times, 
 Hvho adopted the method of seasoning timber by charring it, 
 which was done by exposing the piece to be seasoned to a 
 'strong fire, in the flame of which it was continually turned by 
 an engine, till it was completely covered with a black coally 
 ■:«<irust, when it was taken out and fit for use. By this means, 
 it became so hardened, as to resist the efl'ects of earth, air, and 
 
94 MECHANICAL EXERCISES. 
 
 PreserTing timber from decav. 
 
 water, for centuries. The beams of the theatre at Herculaneum, 
 were converted into charcoal by the lava which overHowed that 
 city, and after a lapse of more than seventeen hundred years, the 
 charcoal is as perfect as if it had been formed but yesterday. 
 Casks charred in the inside are used to preserve water uncor- 
 rupted, and are particularly to be recommended for long voyages. 
 Charring, also, is the best preparation which piles, or any kind 
 of stakes, intended to be driven into the ground, can receive. 
 
 When boards or planks have been properly seasoned, with- 
 out charring, additional care becomes necessary to preserve 
 tliem against the depredations of worms, the etlects of air, 
 moisture, &.c. For this purpose, Evelyn directs common sul- 
 phur to be put into a glass retort, with as much nitrous acid 
 as will cover it to the depth of about two inches. The whole 
 must be distilled to dryness, and rectified two or three times. 
 The remaining sulphur is then to be exposed to the open air 
 on a marble, or in a shallow glass vessel, where it will liquefy 
 into a kind of oil, with which the timber must be rubbed over. 
 This mixture, he asserts, will not only infallibly prevent the 
 attacks of worms, but also preserve every kind of wood from 
 decay, either in air or water. Two or three coats of linseed 
 oil may also be used to defend timber from the influence of 
 air or moisture ; and some have recommended the wood-work 
 of buildings to be painted, but this ought always to be defer- 
 red, till it is thoroughly dry. 
 
 If the wainscoting or other timber of a building be used too 
 green, and has in consequence riven or cracked, it has been 
 strongly recommended to cover it immediatelywith a solution of 
 beef-suet, which will often close the crevices so effectually, that 
 the defect will be scarcely perceptible. Some carpenters close 
 the crevices with a composition of grease and fine saw-dust. 
 
 The timber employed in building, without due precaution, 
 is extremely liable to destruction from the dry rot, which ap- 
 pears, by some late communications to the Society of Arts, &.c. 
 to be occasioned by a plant. It will destroy half-inch deal 
 wainscoting in a year. The plant is of the creeping kind, and 
 cannot rise above two inches; so that wood, in all cases, must 
 be in contact with the earth to support it. To preserve wood, 
 then, from its eflPects, it must be charred, painted, or prevented 
 from touching the earth by bricks and mortar. It is never 
 observed to commence in the middle of floors, so that it will 
 probably be found sufficient to secure the ends of beams or 
 joists. The plant has no adhesive powers but in contact with 
 wood. Timber thoroughly impregnated with brine, or a solu- 
 tion of common salt, has been found by experiment to be 
 secure from its destructive effects. 
 
MECHANICAL EXERCISES. 95 
 
 General observations on timber. 
 
 The experiments which have been made to ascertain the 
 strength and quality of timber, under different circumstances, 
 have been attended with results widely different from each 
 other; but as it would be unsuitable to the plan of this work to 
 enter upon the lengthened statements by which contradictory 
 results are supported, we shall merely collect a few remarks, 
 which seem to have obtained general assent. The wood next 
 the bark of a tree, called the white, or alburnum, is much 
 weaker than the rest. The wood of the north side of all trees 
 which grow in Europe, is the weakest, and that of the south- 
 east side is the strongest ; this difference is most remarkable 
 in hedge-row trees, and such as grow singly. The heart of a 
 tree is never in its centre, but always nearer to the north side, 
 and the annual coats of wood are thinner on that side. In 
 conformity with this, it is a general opinion of carpenters, that 
 timber is stronger whose annual plates are thicker. The tra- 
 chea, or air-vessels, are weaker than the simple ligneous fibres. 
 These air-vessels are the same in diameter and number of rows 
 in trees of the same species, and they make the visible separa- 
 tion between the plates, or annual layers. Therefore the thicker 
 these plates are, the greater the proportion they contain of the 
 simple ligneous fibres, and the better the timber. A contrary 
 opinion is nevertheless prevalent, and wood with a fine grain, 
 or thin annual layers, is preferred. The tenacity of wood is 
 greatest when it is green, and diminishes with drying. No per- 
 son, perhaps, has made experiments on wood with so much mi- 
 nuteness as Buffon, who observes, that he invariably found the 
 heaviest pieces to be the strongest, and he recommends an at- 
 tention to this circumstance as the surest guide in the choice 
 of timber. 
 
 Banks is of opinion, that beams should be strong enough to 
 bear twenty times the force they have to resist, or they will 
 probably bend, and in time break. The same author also ob- 
 serves, that one piece of wood is much stronger than another, 
 not only cut out of the same tree, but out of the same rod ; or 
 a piece of a given length, planed equally thick, and cut into 
 several equal parts, these pieces will be broken with different 
 weights. From a great number of experiments which he made 
 on the strength of wood, he found that the worst or weakest 
 piece of dry heart of oak, one inch square, and one foot long, 
 bore six hundred and sixty pounds, though much bent, and 
 two pounds more broke it. The sti'ongest piece he tried of the 
 same dimensions, broke with nine hundred and seventy-four 
 pounds. The worst piece of deal, bore four hundred and sixty 
 pounds, but broke with four pounds more ; the best piece bore 
 six hundred and ninety pounds, but broke with a little more. 
 
9G MECHAN UAL EXERCISES. 
 
 Geueral ob:iervalions ou timber. 
 
 The fibres of timber requiring so great a force to tear them 
 asunder in a vertical (lirt-ctiou, and being easily broken by a 
 transverse strain, when compared to that of a rope carrying 
 nearly an equal weight in all directions, opens a wide field for 
 useful experiments. All timber trees havo their annual circles 
 or urjowths, wiiich vary greatly according to the soil and expo- 
 sure to tlie sun. The north-east side of the trees (being much 
 smaller in the grain than the otlier parts, which are more 
 exposed to the sun,) is strongest for any column that has a 
 weight to support in a vertical direction; because its hard cir- 
 cles, or tubes, are nearer each other, and the area contains a 
 greater quantity of them; nor are they so liable to be com- 
 pressed by the weight, or to slide past each other, as when 
 they are at a greater distance. On the other hand, this part 
 of the tree is not fit for a transverse strain ; because the nearer 
 the hard circles are to each other, the easier the beam will 
 break, there being so little space between them, that one forms 
 a fulcrum to break the other upon; but that part of a tree, the 
 tubes of which are at a Greater distance, or of larg-er strain, is 
 more elastic, and requires a greater force to break it ; because 
 the outside fibre on the convex side cannot snap till the next 
 one is pressed upon it, which forms the fulcrum to break it 
 on. It is generally observed in large timbers, such as masts, 
 that the fracture is seldom on the convex, but usually on the 
 concave side ; which is owing to the fibres on the concave side 
 being more readily forced past each other, and those on the 
 convex side being so difficult to be torn asunder, that they 
 cannot snap, in consequence of the largeness of the segment 
 of the circle they describe when on the strain. The curve 
 described by the inner layers of the wood being so large, and 
 indeed little less than a straight line, cannot form a fulcrum 
 to break the outer ones upon ; and as the convex side, or that 
 on which the fibres are extended, ought to be always free from 
 any mortise or incision on the outside, the strength decreases 
 as it approaches the centre. 
 
 In early periods, the trunks of trees were split with wedges 
 into as many and as thin pieces as were required, in a manner 
 similar to that used by lath-cleavers at the present day. The 
 saw, though so convenient and beneficial, has not been able en- 
 tirely to banish the practice of splitting timber used in building, 
 orin making furniture and utensils. To be aware of the peculiar 
 advantagesofsplittingtimber, may be useful to artisans in wood 
 generally. By its advantages, we' do not so much allude to its 
 being more expeditiously performed than sawing, as to the cir- 
 cumstance that split timber possesses greater strength and elas- 
 ticity than that which has been sawn ; for the fissure follows 
 
MECHAKICAI. EXERCISES. 
 
 fL.I 
 
 Paaislied "bv 3eiiir Fisier Caxcon, londoii, i8z2. 
 
Mechanical zxercises. 
 
 PL.U. 
 
 EoHLslied V Heaiy Fisier Caiton.. Londoa.iSzz. 
 
Mecelvnic^vl. exercises 
 
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 Fig.n. JUUgU^ 
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 ?ahH«lied W Hear/ fisier. Cartoa. Lo-uAoa.iSSff . 
 
MECHANICAL EXERCISES. 97 
 
 Difference between metal and wooden springs. — Measurement of round timber. 
 
 the grain of the wood, and leaves it whole ; whereas the saw, 
 which proceeds in the line chalked out for it, divides the fibres, 
 and therefore lessens its cohesion and solidity. Though split 
 timber turns out often crooked or warped, this fault, which may 
 sometimes be amended, is, on many occasions, not prejudicial. 
 The fibres retaining their natural length and direction, thin 
 boards, particularly, can be bent much better. This is a great 
 advantage in making staves, or sieve frames, and in forming 
 various articles of the like kind. 
 
 There is a curious point of difference between wood and metal, 
 when employed as springs, which deserves to be noted, as it 
 is not very generally known, though the information may 
 occasionally prove very useful to the engineer. A metallic 
 spring, if it has nothing to stop against, but is suffered to vi- 
 brate after performing the requisite action, will, in a short 
 space of time, if the action be frequently repeated, either break 
 or set. A wooden spring, when the vibration cannot be avoid- 
 ed, is the best substitute which can be employed, as, in the 
 property alluded to, it is the reverse of a metallic one ; if stop- 
 ped in its vibrations, it soon sets or breaks ; but if permitted 
 to vibrate, its temper or elasticity suffers not the smallest di- 
 minution. The best wood for the purpose, is clean-grained 
 deal, perfectly free from knots. 
 
 To measure round timber, let the mean circumference be found 
 in feet and decimals of a foot : square it, multiply this square by 
 the decimal, 0.79577, and the product by the length. Example : 
 suppose the mean circumference of a tree be 10.3 feet, and the 
 length 24 feet. Then 10.3 x 10.3 x 0.079577 x 24=202.615, the 
 number of cubical feet in the tree. The foundation of this rule 
 is, that when the circumference of a circle is 1, the area is 
 0.0795774715, and that the areas of circles are as the squares 
 of their circumferences. But the common way used by arti- 
 ficers, for measuring round timber, differs widely from this in 
 its result: they measure the girth, and reckon one-fourth of it 
 equal to the side of a square, whose area is equal to the area of 
 the section of the tree. They therefore square this estimate of 
 one-fourth of the girth, and multiply the product by the length 
 of the tree. According to this method, the tree of the last exam- 
 ple would only exceed by a small remainder 159 cubical feet; 
 for one-fourth of 10.3, or 2.575 x 2.575 x 24=159.135000. 
 
 In measuring hewn or square timber, the custom is to take 
 the breadth in the middle, by placing two rules against the sides 
 of the tree, and measuring the distance between them. In like 
 manner they measure the breadth the other way, and if the two 
 measurements are unequal, they are added together, and half 
 their sum is taken for the true side of the square. 
 5.— Vol. I. O 
 
98 MECHANICAL EXERCISES. 
 
 Mechanism of a saw-mill. — Smart's saw-mill. 
 
 Oj Saiv-mills. 
 
 The sawing of timber is accomplished by manual labour 
 and by machinery. Sawing by manual labour is so familiar 
 to every one, as to require no particular description, in this 
 place. With respect to sawing by machinery, the mills for the 
 purpose are not numerous in Great Britain, nor does the utility 
 of them appear to be so properly appreciated as in America, 
 Norway, and other countries. A general description of the 
 objects to be attained by the mechanism of a saw-mill on the 
 largest scale, may be comprised in a few words : the saw is 
 drawn up and down as long as is necessary, by a motion com- 
 municated (commonly by water) to the wheel ; the piece of tim- 
 ber to be cut into boards is advanced by a uniform motion to 
 receive the strokes of the saw; for here the wood is to meet 
 the saw, and not the saw to follow the wood, therefore the 
 motion of the wood and that of the saw ought immediately to 
 depend the one on the other ; and when the saw has cut through 
 the whole length of the piece, the machine should stop and 
 remain immoveable; lest, having no obstacle to surmount, the 
 the force of the moving power should turn the wheel with too 
 great rapidity, and break some part of the machine. 
 
 Circular saws, for ripping up boards or scantlings of mode- 
 rate thickness, are not so generally used by artists, as would 
 be found advantageous. We shall therefore particularly notice 
 the construction of a circular saw-mill, invented by Smart, and 
 used in his manufactory. Like his improvement in the art of 
 turning cylinders, already described, it is distinguished by its 
 simplicity and utility. A B, fig. 2, pi. IV. is a strong table, 
 made of planks firmly braced together in the form of a joiner's 
 bench. In the middle of tliis bench, a longitudinal opening, 
 r 0, admits the circular saw, F. which is made of well-tempered 
 steel plate. G is a pulley on the same axis w ith the saw, and 
 a rapid motion is communicated to it by means of an endless 
 strap from a large fiy wheel, turned by horse power. The saw 
 is fixed on its spindle D, (fig. 3,) by a shoulder d, against 
 which it is held by another moveable shoulder e, pressed 
 against it by a nut, k, screwed on the end of the spindle, which 
 is tapped for the purpose. The hole in the centre of the saw 
 must tit the spindle exactly, and may be either square or cir- 
 cular. If it be circular, it must have a small notch in it, to 
 fit a fillet on fhe spindle, that the saw and the spindle may 
 revolve together. The ends of the spindle are turned off to 
 cones, in the customary manner for working in centres. The 
 cone or point nearest the saw, works in the end of a screw, c, 
 fig. 2, screwed into the bench; the other point works in a 
 
MECHANICAL EXERCISES. 99 
 
 Curving of wood. 
 
 similar screw, screwed through a cross beam H, mortised be- 
 tween two vertical beams, KK, extending from the floor to the 
 ceiling. The cross beam, H, can be raised or lowered in its mor- 
 tises throuoh the beams, KK, by wedges, n n, above its tenons, 
 and two others below them. A long straio'ht piece of wood, LL, 
 called the guide, is connected with the bench by joints similar 
 to those of a parallel ruler. It can be set at any distance from 
 the saw, and tixed by screws passing through circular sjrooves, 
 d d, cut through the bench. The front of the guide, LL, must 
 be perpendicular to the plane of the bench ; and it mav then 
 be made use of to set the plane of the saw also perpendicular 
 to the same plane. In using the machine, the workman slides 
 the end of the piece of wood to be cut against the saw as it 
 turns round, and presses its edge against the guide, LL, at the 
 same time, so that it m'^.y be cut straight. 
 
 When the saw is blunted by use, the centre screw, c, or 
 that in the cross piece, H, must be turned back; the spindle 
 and saw can then be removed ; and by taking ofl^" the nut A", 
 fig. 3, the saw will be loose, and another may be put on, or it 
 may be sharpened, in the same manner as any other saw, 
 while fixed in a vice. The teeth of the saw are set, that is, 
 bent out of the plane of the saw, one tooth on one side, the 
 next on the other, and so on alternately all round ; the out- 
 sides of the teeth are not filed to leave a surface perpendicu- 
 lar to the plane of the saw, but inclined to it, and in the same 
 direction that each tooth so filed is bent in the setting. By 
 this means the saw, when cutting, first takes away the wood 
 at the two sides of the kerf, or passage which it makes, leav- 
 ing an angular ridge in the middle of it, the use of which is 
 to keep the saw steady in a right line, that it may not have 
 so much tendency to get out of the straight, in any place 
 where the wood is harder on one side than on the other. 
 
 On giving a curved form to Wood. 
 
 Curved wood is frequently purchased by millwrights at a 
 very high price ; and such is its scarcity, that ver\' imperfect 
 pieces are frequently made use of, not only from motives of 
 economy in the first cost, but from necessity. The inequali- 
 ties also of wood which is naturally curved, are often so con- 
 siderable as materially to impede the workmen; but when it is 
 intended to curve it artificially, it may be dressed in its 
 straight state, so as to require very little labour afterwards. 
 The following observations, therefore, on the curving of wood, 
 from a Treatise on Carpentry, by J. H. Hassenfratz, will be 
 interesting to many, and will develope some principles with 
 
100 JIEClIANICxVL EXERCISES. 
 
 Curving of wood. 
 
 respect to the manageiuent of wood, which are not very gene- 
 rally known. 
 
 \V'hen trees are young and tender, their stems may be bent 
 either by cords, or by poles, stakes, or frames. They are kept 
 in this situation so long that they retain the curvature intend- 
 ed to be given them, even when disengaged from the ob- 
 stacles by which they are held. 
 
 Of all the methods of bending trees, that applied to young 
 and growing wood is the most easy and convenient; their sup- 
 pleness and their elasticity admit of their assuming any form 
 that is desired. There are few, to which, with proper care, 
 the most singular forms may not be given; but, it is true, they 
 are often reduced to a state of constraint and disease, preju- 
 dicial to their growth. 
 
 The curving of wood after it has been cut, though more 
 difficult, is, however, more customary, because such pieces 
 may be selected as are best adapted to the objects for which 
 they are intended, and a suitable curvature may be given them 
 immediately. 
 
 The process generally employed, is founded on the property 
 possessed by caloric, of augmenting the elasticity of wood by 
 penetrating it, and of diminishing its elasticity when it retires. 
 
 Accordingly, to give a curvature to thin pieces of wood, 
 such as pipe staves, and the planks that cover the sides of 
 boats, they are heated in the part where the curve is required, 
 and they are gradually bent as they become hot. 
 
 But caloric, applied to a ])articular portion of the wood, 
 while the other is in contact with the air, heats it unequally, 
 and only partially increases its elasticity : in curving, some 
 parts become stiff, and others bend, which produces an in- 
 equality of curvature, and sometimes cracks in the interior, 
 and splinters on the surface of the wood. The only method 
 of correcting this inequality, is to heat the wood alike in 
 every part. 
 
 Ovens and stoves, gradually heated, facilitate the curvature 
 of wood, by procuring an equal heat; but the risk of injuring 
 the wood in a dry heat is very considerable. The elasticity 
 of wood, also, is in proportion not only to its temperature, 
 but likewise its humidity. At an equal temperature, the same 
 pieces of wood have different degrees of elasticity, according 
 to the quantity of water by which they are impregnated ; in 
 the same manner as, with an e([ual degree of humidity, they 
 are the more elastic the more they are heated. 
 
 We have an example of the two-fold influence of humidity 
 and of caloric, in putting together two pieces of wood, as the 
 tenon and mortise, in which the mortise is only one-third of 
 
MECHANICAL EXERCISES. 101 
 
 Cui-ving of wood. 
 
 the size of the piece that is to go into it. This manner of join- 
 ing, apparently so extraordinary, is considered such an im- 
 portant invention, that most of those by whom it is practised 
 keep the method a secret. It was the process employed in 
 producing this effect, that suggested the method now employ- 
 ed for curving with facility the thickest and most obstinate 
 pieces of timber; the whole art consists in impregnating the 
 pieces of wood with humidity, by procuring them a uniform 
 temperature, then to bend them, and suffer them to cool, in 
 the form they are intended to assume. 
 
 To heat and to impart humidity to wood, three different 
 processes are employed ; the first is by boiling water, the 
 second by steam, and the third by wet heated sand. 
 
 The boiling of the wood in water, is attended with the 
 inconvenience of dissolving part of the substance of the wood ; 
 at least, on drying again, it shrinks both in thickness and in 
 length; its strength and elasticity are considerably diminished. 
 The next method tried was the vapour stove, which was a 
 chest proportioned in size to the wood to be curved, formed, 
 of thick planks, firmly joined together. The wood intended 
 to be submitted to the action of the vapour, is placed upon 
 supports. For small chests, a boiler may be placed at one of 
 the extremities, the wood being introduced by a door at the 
 other. In large chests, the boiler is placed in the middle, 
 and the wood is introduced at both ends. The boilers com- 
 municate in the interior of the chest by means of a pipe. The 
 vapour formed by the ebullition of the water, impregnates the 
 wood with humidity, augments its elasticity, and renders it 
 fit to be curved. Vapour stoves require little care or expense; 
 but they can only be used for planks of a certain thickness, be- 
 cause they cannot impart to wood a higher temperature than 
 that of boiling water, which is insuflficient to give thick pieces 
 the elasticity they require in order to be curved. 
 
 These considerations led to the invention of the sand stove, 
 which is formed of four walls of stone or brick, having in 
 the middle two fire-places that communicate with several 
 circular flues to convey the caloric, the heated air, and smoke, 
 to the two chimnies at each end. Over these flues are 
 plates of metal, forming the bottom of the chest in which 
 the sand is put. The flame and the smoke circulating in 
 the flues, heat the plates, which communicate heat to the 
 sand. This stove is an imitation of the sand baths, which 
 have been immemorially employed in a great number of che- 
 mical operations, and in various manufactories, for affording 
 an equal heat. 
 
 As sand is capable of being heated to a much higher tern- 
 
102 MECHANICAL EXERCISES. 
 
 Sujurt's uiflhod of converting the trunks ol' trees into square timber. 
 
 perature than boiling water, the wooil placed in this kind of 
 stove may be subjected to a much more powerful heat ; but if 
 there was nothing in the stove but tiie sand and the wood, the 
 heat might disengage from the latter, the gaseous substances 
 which compose it, and convert it into charcoal. To prevent 
 this, one or two boilers, filled with water, are placed in the 
 middle of the stove. The steam created by their ebullition 
 impregnates the sand with humidity, which likewise pene- 
 trates the wood, and the heat evaporates only the water which 
 is continually supplied by that which is disengaged ; by these 
 means, the constituent parts of the wood are preserved. 
 
 It may be supposed that in this operation, some portion of 
 the component parts of the wood are evaporated, and that it 
 is consequently liable to a commencement of deterioration : 
 but if care be taken to remove it as soon as it is sufficiently 
 hot and humid, the injury is imperceptible. 
 
 The sand stove is covered throughout its whole length, to 
 impede evaporation and the loss of heat. 
 
 The wood is introduced lengthways into tiie stove, at the 
 two ends, and being placed on gratings fixed for the purpose, 
 it is covered with sand. 
 
 When the wood has been rendered sufficiently hot and 
 humid, it must be bent upon a surface making the desired 
 curve. The force which produces the curvature may act by 
 means of cords, pulleys, and even capstans. The piece hav- 
 ing assumed the desired form, it must be left, with the force 
 still acting upon it, to cool and dry. 
 
 When the piece of wood is not thick, the pressure of men, 
 or even of weights, will frequently afford sufficient force to 
 produce the necessary curvature. 
 
 Advantageous Method of converting the Trunks of Trees into 
 square Timber. 
 
 The method we are about to describe, of converting all tim- 
 ber that is straight, and intended for square beams, to great 
 advantage in general use, is included in Smart's patent for 
 hollow masts ; but the ingenious patentee, as far as relates to 
 lessening the consumption of English oak, and introducing 
 the larch and firs of our own growth into general use, has 
 liberally yranttd licences to all who choose to apply to him for 
 them, with some trilling exceptions with respect to masts, 
 yards, bowsprits, &c. 
 
 Fig 4, is a section of the but-end of a tree, two feet in 
 diameter, sawn or chopped diagonally. Fig. 5, is the other 
 end, sawn square, one foot each side : cut it exactly through 
 
ME C H.\>' ICAL EXK RC TSE ! 
 
 K fie/ J 
 
 Fio S 
 
 J JffujA «i/ . 
 
 PnHhslici ~9jr "E^f^xrj Tizhsrc. Ca x tu i r, i,raidaiL, lS22. 
 
MECHANICAL EXERCISES. 103 
 
 Smart's method of converting the trunks of trees into square timber. 
 
 the centre in two cross-cuts, a b, d e; it will produce four 
 pieces, which are put together, as in fig. 6 and 7, with the 
 centre turned outwards, the but-end of one' piece with the 
 small end of the other, and dowel and holt them together as 
 in fig. 8. A beam will then be formed, whose section is shewn 
 in fig. 6 and 7, regular from one end to the other, with the 
 advantage of having the heart of the tree in the place where 
 the hardness and strength are most wanted, viz. in the cor- 
 ners, which form the abutments ; whereas the same tree squar- 
 ed into a parallel beam, would have been much smaller, and 
 the soft or sappy parts of the wood exposed to the action of 
 the air and moisture. In flush framing, it is observable, that 
 the failure of all timber in old buildings has commenced much 
 sooner than it otherwise would have done, owing to the sappy 
 wood being at the corners of the principal beams. This sappy 
 wood soon decays, as its spongy quality attracts the moisture; 
 whereas the heart, especially of oak, will be as sound as the 
 first day it was used. 
 
 As beams, taking their weight horizontally or on any trans- 
 verse bearing, have their principal strain on the upper and 
 lower surface, every workman ought to guard against having 
 sap in beams, because if they do not immediately decay, they 
 shrink, so as to loosen all the framing, and soon cripple the 
 building or machine; but on the above plan, the sappy or 
 worst part of the wood is excluded from what would cause its 
 decay, and the timber increased in quantity so as considerably 
 to overbalance the extra labour and expense. A tree of oak, 
 forty feet long, two feet in diameter at the but-end, and one 
 foot at the top, when put together on this plan, will have its 
 sides each eighteen inches square, and will contain ninety 
 feet; whereas on the old plan, forty feet would be the contents 
 of a square beam cut from the same tree ; fifty cubic feet would 
 have been cut off as slabs, or chopped up for the fire. Esti- 
 mating the expense of thus putting together a beam, of the 
 dimensions in question, to cost £3, (and it would probably not 
 amount to more where the price of labour is highest,) and the 
 fifty feet saved to be worth no more than £6; the proprietor 
 would save £3 by each beam so converted. The dowels ought 
 not to go through, as that would weaken the timber. In an 
 eighteen-inch beam, the dowels should come within three 
 inches of the outside ; but where a mortise is cut in place of 
 a dowel, it is proper to have an iron screw bolt to prevent the 
 joint opening with the pressure of the tenon; and the work 
 ought to be put together with screw clamps, for nails or ham- 
 mers bruise the wood, and weaken or destroy the cohesion of 
 its fibres for a considerable depth. 
 
104 MECHANICAL EXERCISES. 
 
 The axe. — The adze. 
 
 Oj the Tools used in the uorlcing of ^yood. 
 
 We do not here intend to speak of the tools used in turning 
 wood, they having been already noticed. We allude more par- 
 ticular! v to the tools required by the carpenter, joiner, and ca- 
 binet-maker, and which differ not from each other so much in 
 their general construction and name, as in size, and the varieties 
 necessary for coarse and large, or ornamental and small work. 
 We otter the enumeration of the tools used by these artisans, 
 not with the hope that tiie mere enumeration will convey any 
 information of importance, but to attord an opportunity of 
 makino^ a few remarks on the mode of using^ and choice of 
 them, and on those peculiarities in their form, upon which 
 their excellence chiefly depends. 
 
 The principal tools employed in the working of wood, are, 
 the axe, the adze, various sorts and sizes of saws, planes, 
 chisels, hamniers, and boring tools. 
 
 The construction of the axe and the adze, and their use in 
 chopping or hewing, scarcely require any remark. In grinding 
 these tools for use, the general rule observed in grinding all 
 other edge-tools must be attended to, viz. that of suiting their 
 edge to the work for which they are intended. When the 
 wood they are to cut is hard and knotty, the part ground off 
 to form the edge must be short, so as to leave the tool rather 
 thick and strong near the edge ; on the contrary, for soft, 
 clean-grained stuff", the part alluded to may be brought to an 
 acute angle, so as to form a thin wedge. A workman applied 
 his axe to the chopping of bones, and thought the metal it 
 was made of very bad, or badly tempered, because it became 
 notched or gapped, almost at every stroke, till the edge was 
 gone. The fact was, that it was ground to so acute an angle 
 as only to be fit for cutting fir. Such mistakes, however gross, 
 are not uncommon. The tool in question should have been 
 ground so as to have had an edge almost like that of the 
 chisel for chipping iron. The axe is ground on both sides to 
 form the edge : the adze is a much thinner and lighter tool 
 than the axe, it is ground only on one side, namely, the inner; 
 the part ground is at a right angle to the handle ; and the 
 blade is arched to the portion of a circle, the radius of which 
 is somewhat less than the length of the handle. The adze is 
 much used by the cooper, as well as the carpenter and joiner; 
 it will pare away very thin slices or chips, and leave a much 
 smoother surface than the axe, which, besides, cannot, like 
 it, be applied to a surface in a horizontal position. That part 
 of an axe, adze, or any other tool, which is ground off to form 
 the edge, is called the basil. 
 
MECHANICAL EXERCISES. 105 
 
 General remarks on saws. 
 
 Of Saws. 
 
 Saws are made of plates of steel; if they possess great elas- 
 ticity, and bend equally in a bow, they are judged to be well 
 tempered, and evenly ground. The edge in which the teeth 
 are cut, is thicker than the back, that the back may readily 
 follow the edge. The teeth are cut and sharpened with a tri- 
 angular file, the blade of the saw being first fixed in a whet- 
 ting block or vice. After the teeth have been filed, they are 
 set, that is, turned out of the right line, that they may make 
 the kerf or fissure wider than the thickness of the saw-plate, 
 and thus prevent the friction which would otherwise impede 
 the motion of the tool. If the first tooth be bent to the left, 
 then the next is turned to the right, and so on ; and it may 
 also be remarked, that the extremity of each tooth, at the out- 
 side corner, is left higher than on the inside corner, which 
 tends to facilitate the cutting. Hence it will be observed, 
 that the teeth of Smart's circular saw, which have been par- 
 ticularly noticed, are formed in the same way as those of the 
 common saw, and for the same reasons, in addition to those 
 stated on page 99. 
 
 The instrument with which the teeth of saws are bent, is 
 usually a piece of iron or steel, five or six inches long, with 
 several nicks in the edge, at right angles to its length, and of 
 different sizes. The tooth intended to be bent, being slipt into 
 the nick which it will exactly fill, and the saw in the mean time 
 being held fast, the effect of bending the tooth may readily be 
 produced by twisting the instrument up or down. The teeth of 
 a saw are made larger for coarse cheap stuff, than for hard and 
 fine, in cutting which large teeth would make too great a re- 
 sistance. The plate of a saw should be quite straight, or it 
 cannot be depended upon for making a straight kerf. 
 
 In large towns, there are men who earn a portion of their 
 livelihood by sharpening saws, and those who perform the 
 business well, receive considerable praise for their ingenuity, 
 from journeymen, not always of the most clumsy and idle class, 
 who employ them without reflecting that the skill they admire 
 is easily attained by a little attention ; that the time employed 
 in carrying, looking after, and fetching their tools, is generally 
 equal to what would be required for repairing them at home ; 
 and that they can therefore seldom call themselves gainers, 
 even if they had nothing to pay for the work they have had 
 done, or if it were no inconvenience to be for a time deprived 
 }f a tool almost constantly required to be at hand. The teeth 
 ••>f saws have a proper degree of acuteness, when comprising 
 an angle of about sixty degrees. In sharpening them, the 
 5.— Vol. I. P 
 
106 MECHANICAL EXERCISES. 
 
 The whip-saw. — Hand-saw. — Pannel-saw. — Franie-saw. — Tenoo-saw. 
 
 whole of tlie outer arris of each tooth, should be made sharp; 
 this can only be done by moving the file in a straight direc- 
 tion, which will make the slanting sides of the teeth flat. 
 Saws used for dividing wood longitudinally, or in the direc- 
 tion of its fibres, may have the front edge or apex of each 
 tooth standing almost as forward as the base of the tooth on 
 that side next the lower end of the plate. But this form, in 
 transverse or cross-cutting, would be inconvenient, as it 
 would hinder the workman from pushing forward the saw ; 
 tenon-saws are therefore usually made so that the apex 
 of the tooth is not more forward than the centre of its 
 base. 
 
 The saws in most common use are the following, viz. the 
 pit-saw, which is a large two-handed saw, for sawing timber 
 in pits, chiefly used by the sawyers. 
 
 The whip-saw, which is also two-handed, is used in sawing 
 such large pieces of timber as the hand-saw will not easily 
 reach. 
 
 The handsale, which is made for a single man's use ; the 
 length of the plate is about twentv-six inches, and it is gene- 
 rally made with about four teeth in an inch. It is used in 
 cutting wood across, as well as in the direction of its fibres. 
 The teeth toward the lower end of it are rather smaller than 
 those at the upper end, or broadest part of the plate, which 
 facilitates the working of the saw in that part of its course, 
 when the workman has the least power upon it, and the wood, 
 on the surface and at the sides of the kerf, particularly in 
 cross-cutting, are not so much torn as they would be if the 
 teeth were all of equal size. 
 
 The plate of the painiel-satc is about the same length as that 
 of the hand saw, but it contains about half as many more 
 teeth in the same compass. It is used for cutting very thin 
 boards in any direction which may be required. 
 
 The buiv or frame-saw is furnished with cheeks; by the 
 twisted cords from the upper parts of these cheeks, and the 
 tongue in the middle of them, the upper ends are drawn closer 
 together, and the lower set further apart; so as to tighten the 
 plate, which is too long and narrow to be kept straight with- 
 out a frame. 
 
 The tenon-saw is used for cutting across the fibres of wood, 
 and derives its name from its use in fonjiing the slioulders of 
 tenons. The smallest saw to which this name is given, is 
 about fourteen inches, and the largest about twenty inches 
 long. The number of teeth in an inch are from eight to ten, 
 according to the length of the plate, the larger sizes of which 
 have the fewest teeth in the same compass. 
 
MECHAXICAL EXERCISES 107 
 
 The sasU-saw. — Dove-tail-saw. — Compass-saw. — Key-:hole-saw. — Planes. 
 
 The saw used in cutting the tenons of sashes, is called a 
 sash-saw; the plate is about eleven inches in length, and the 
 number of teeth in the inch about fourteen. 
 
 The dove-tail-saw is used by joiners and cabinet-makers in 
 dove-tailing drawers, &c.; the plate is about nine inches long, 
 and the number of teeth in the inch about fifteen. — The plates 
 of the tenon-saw, the sash-saw, and the dove-tail-saw, are so 
 thin, that the back of them is let into a stout piece of iron or 
 brass, to keep them from bending, and as they are not intend- 
 ed to cut into wood their whole breadth, this addition is no 
 disadvantage. 
 
 The compass-saic is used for cutting a circular or any other 
 compass kerf: its formation is peculiar; the teeth are not set, 
 as the setting of a saw has a tendency to keep it in a right 
 line ; the teeth are small, about five in an inch ; the plate 
 narrow, about an inch in the broadest part, and gradually di- 
 minishing to about a quarter of an inch at the lower end; the 
 cutting edge is thick, and the back very thin, so that it may 
 have a compass to turn in. The sides of this saw should 
 either be correctly flat, or a little concave like a razor, other- 
 wise it will not work well. 
 
 A small kind of compass-saw, called a key-hole-saiv, is used 
 for quick curves, such as key-holes. The handle is long, and 
 in shape similar to that of a chisel, but perforated through its 
 whole length, in order that the saw, which is fixed by a screw, 
 may be set at any distance within the handle. Hence in cut- 
 ting the smallest curves to which it can be applied, or at the 
 commencement of the work, or when it is only wanted to saw 
 through an inconsiderable depth, but a small portion of the 
 saw is allowed to project from the handle; by which means 
 the springing or unsteadiness of sawing with the end of a 
 long narrow blade is avoided, and more force can be applied, 
 without the hazard of breaking the plate. 
 
 Of Planes. 
 
 Planes of different kinds form a very important part of the 
 tools of artizans in wood. A few remarks in explanation of 
 technical terms, will enable us subsequently to be more concise 
 and intelligible in noticing this class of tools. The block of 
 wood in which the blade or chisel of a plane is fixed, is called 
 the stock ; it is mostly made of beech or some other hard wood, 
 exceedingly well seasoned. The blade or chisel is called the 
 iron ; it is composed of iron and steel welded together, the 
 fore part of the lower half of it, when in the stock, containing 
 the steel. The under-side of the stock is called the sole. 
 The height or depth of a plane are synonymous terms, signify- 
 
103 MECHANICAL EXERCISES. 
 
 General remarks on planes. 
 
 ing the dimension from the sole to the upper surface. The 
 handle of a plane is called the tote. That part of the aperture 
 in the stock upon which the iron is laid and secured by the 
 wedge, is called the bed, which is a plane surface, making a 
 different angle with a line perpendicular to the sole, according 
 to the use for which the plane is intended. For the jack-plane, 
 the trying and the smoothing-planes, the angle of the bed is 
 usually from forty-two to forty-five degrees; for moulding 
 planes about thirty-five, and for those planes which operate 
 by scraping, it is almost perpendicular, not making an angle of 
 more than five or six degrees. The angle which the iron makes 
 with the perpendicular alluded to, is called its pitch, and the 
 greater this angle, the lower is said to be the pitch of the iron. 
 The basil of the iron fomis an acute angle with the steel side, 
 which is not ground, but always kept level. In grinding and 
 whetting plane irons, the basil must be made as flat as pos- 
 sible, or in a small degree concave, otherwise it will not seem 
 to be sharp when in use. 
 
 Planes are generally about three inches and one-eighth 
 deep ; the jack-plane is sometimes rather more, and the smooth- 
 ing-plane is mostly rather less. The blades of planes are, in 
 many cases, made double, a simple expedient of remarkable 
 utility in planing cross-grained stuff. The addition made to 
 the blade for this purpose, consists of a piece of iron of the 
 same breadth as the blade, with its lower end very thin, and 
 of the same shape at the edge as the edge of the blade. This 
 piece of iron, usually called the top-iron, is connected by a 
 screw with the blade, at any necessary distance from the edge 
 of whicli it can be fixed. The top-iron, the edge of which 
 should never extend below the sole, is fastened upon the front 
 or steel side of the blade, and the space between its edge and 
 that of the blade, determines the thickness of the shaving. 
 It is always necessary to make the top-iron fit the blade so 
 correctly that no shaving can get between them; for this end, 
 it is arched a little towards the lower end, and the concave 
 side of this arch being turned inwards, the screw necessarily 
 makes the edge fit closely the level siuface of the blade. The 
 top-iron is generally employed in the jack-plane, and almost 
 always to the trying-plane, the long-plane, and the jointer. 
 To the smoothing-plane, and the various sorts of moulding- 
 planes, it is not used. 
 
 If the iron of a plane project too far, the blow of a hammer, 
 on the fore end of the stock, will slacken the wedge and raise 
 it in a small degree. In tiiis case, the wedge must be re- 
 fastened by driving it down with a light blow or two before 
 the plane is used again. A smart blow on the fore end of a 
 
MECHANICAL EXERCISES. 109 
 
 The jack-plane. — Tr^-ing-plane. — Long-plane. — Jointer. — Strike-block. 
 
 plane will loosen the wedge so much that the iron may easily 
 be withdrawn by the hand. Instead of striking the fore end, 
 for these purposes, some workmen strike the upper surface of 
 the stock, near the orifice for the shavings. 
 
 The jack-platie used by joiners, is generally about seventeen 
 inches in length. Its use is to take off the greater irregulari- 
 ties of the stuff, left by the axe, the adze, or the saw, and it 
 is therefore the first plane employed. To suit the coarseness 
 of its work, the cutting edge of the iron rises with an arch of 
 considerable convexity in the middle, and the opening or 
 mouth which admits the shavings through the stock, is wider at 
 the sole than that of any other plane. The iron is often used 
 without a cover; the quantity of its projection must be regu- 
 lated by the texture of the stuff, in proportion to the hardness 
 or knottiness of which it must be lessened; the due degree of 
 it is easily ascertained by trial: it must be adjusted so as not, 
 on one hand, to require hard pressing down, or many strokes 
 to be made in reducing the wood; nor, on the other, to fatigue 
 the workman and tear the stuff, by taking hold too keenly. 
 The convexity of the cutting edge of the iron, prevents the 
 corners from entering the wood, which ought never to occur, 
 as the effect would be to spoil the work and impede the pro- 
 gress of the artisan. 
 
 When a piece of stuff has been nearly reduced to the intend- 
 ed form by the jack-plane, the tri/ing-plane is made use of to 
 produce a higher degree of regularity and smoothness. It is 
 four or five inches longer than the jack-plane, and its iron is 
 broader, set with a less projection, not so convex on the edge, 
 and, like the two following sorts of planes, always used double. 
 This plane, in taking off a shaving, is pushed along the whole 
 length of the stuff, whereas the strokes given with the jack- 
 plane are only within arms' length. 
 
 The third plane made use of in facing a piece of stuff with 
 the utmost exactness, is the long-plane, which is four or five 
 inches longer than the trying-plane, and proportionately 
 broader, while the projection and convexity of the iron are 
 somewhat less. 
 
 The jointer is the longest plane of all ; its edge is very fine, 
 and scarcely stands out above a hair's breadth ; it is chiefly 
 used for shooting the edges of boards perfectly straight, so 
 that when joined together their surfaces will exactly coincide, 
 and the juncture be hardly discernible. The jointer is made 
 about thirty inches long. 
 
 As the last-mentioned plane, from its extraordinary dimen 
 sions, would be unhandy in shooting short blocks, a short 
 kind of jointer, called the strike-block, is also in common use. 
 
iiC MECHANICAL EXERCISES. 
 
 Soaootbiag-plaiie. — Tuotli-plaiie. 
 
 It is much employed in planing the ends of boards, across the 
 fibres, and the inclined plane forming the bed, is lower than 
 that of the jointer. When employed for soft wood, the angle 
 is only increased two or three degrees ; but for fine cabinet 
 work, when the stuff is hard, it is often considerably more, so 
 as to make an angle with the perpendicular of fifty-five or even 
 sixty degrees, in the latter case, the position of the basil is 
 reversed, so as to be in front, or next the fore end of the stock. 
 The usual length of the strike-block is eleven or twelve inches. 
 
 The smoot/iing-plane is about seven inches in length, it has 
 no tote or handle, and otherwise differs in shape from any of 
 the planes yet mentioned. The sides of the stock are convex, 
 and its whole figure resembles that of a coffin. The inclina- 
 tion of the bed is similar to that of the jointer, which it also 
 resembles in the set of the iron. It is the last plane used in 
 finishing off the surface of wood, and from its smallness can 
 easily be applied to smooth any small part which the large 
 planes cannot touch ; and the direction in which it is wrought 
 can also be varied with facility, so as to suit cross-grained stufl'. 
 To secure these advantages more effectually, it is wrought, 
 like the jack-plane, with short strokes. From this description 
 of its use, it will be obvious, that though it is used in finish- 
 ing off wood, it is smootlt/iess, and not straightiiess of surface, 
 ■which it is calculated to produce. But if the work be well 
 managed, the inequalities which it leaves are not perceptible 
 to the eye, and are therefore left with impunity in tables, 
 bureaus, desks, and other furniture, even of the best kinds. 
 
 Though the double iron is an excellent invention, and the 
 use of it is, in fact, the best general remedy known against the 
 curling or cross-grained stutl of ordinary quality; yet, without 
 some other assistance, the planing of many of the finest spe- 
 cimens of mahogany, and many other woods, among which 
 fustic may be particularly mentioned, would be to the last 
 degree a difficult and perplexing operation to the workman. 
 Hence a plane, the stock of which is usually made of the 
 shape and size of the smoothing-plane, is fitted up so as to 
 act by scratching or scrajnng. The blade, or iron, on the 
 steel side of it, is covered with rakes or small grooves close to 
 each other, and all of them in the direction of its length : 
 when therefore it is ground, and the basil formed, its edge 
 presents a series of teeth like those of a fine saw ; the bed of 
 the stock intended to receive it is inclined only about six de- 
 grees, and consequently when the iron is fixed it is almost per- 
 pendicular. On account of these teeth in the iron, the plane 
 obtains the name of the loot h -plane. With this kind of a jilane, 
 however hard the stufi" may be, or however cross and twisted 
 
MECHANICAL EXERCISES. Ill 
 
 Compass and Forkstaff planes. — Round-sole. — Rebating-plane. — Plough. 
 
 its grain, the surface may be made every-where alike, and will 
 not be rougher than if it had been rubbed with a piece of new 
 fish-skin. This roughness may be effectually removed with 
 the scraper, which is a thin plate of steel, like part of a com- 
 mon case-knife, the back of it being let into a piece of wood, 
 as a handle. 
 
 To form the concave or convex surfaces of the rims of car- 
 riage-wheels, or the top rails of a camp-bedstead, and work of 
 a similar nature, the sole of the plane must be curved in the 
 same degree as the concavity or convexity to be produced. 
 Planes of this description are called compass-planes ; they re- 
 semble the smoothing-plane in size, and also in shape, except- 
 ing so far as regards the curve of the sole. A plane of the 
 size and shape in question, with a concave sole, is also dis- 
 tinguished by the name o^diforkstaff-plane; and one which is 
 convex, is sometimes called a roiinid-sole. 
 
 The rabbet or rebating plane, is employed in taking away (by 
 shavings) from the edge of a board, a piece in the form of a 
 square or rectangular prism, so as to leave a groove consisting 
 of two surfaces at right angles to each other. This mode of 
 reducing the stuff is required for some cornices, and various 
 sorts of ornamental work. The groove formed by the rebating- 
 plane, is also employed to receive the edge of another board 
 cut in a similar manner, so that the two lap over each other to 
 the breadth of the rebate, and form one even surface. Rebat- 
 ing-planes deliver their shavings at the side, and not at the 
 top, like the planes hitherto described. They are also of va- 
 rious kinds; some of them are provided with a fence which 
 regulates the horizontal breadth, and others with a stop, which 
 determines the vertical extent or depth of the rebate ; while 
 some have both stop and fence, and others neither. Rebating- 
 planes without a fence have the iron the whole breadth of the 
 sole; some of them have the cutting edge of the iron only on 
 the side, and others only on the bottom of the stock ; these 
 are employed for dressing and finishing with exactness, sepa- 
 rately, either side of the rebate. 
 
 The plane by which a square groove is taken out of the edge 
 of a board, so as to leave a ridge on each side, is called a 
 plough, and the operation of cutting with it is called ploughing. 
 To prevent the necessity of having, for grooves of different 
 sizes, a great number of ploughs, which would be cumbersome 
 and expensive, a tool of this description, called a universal 
 plough, is manufactured. The stop and fence of the universal 
 plough are moveable, and it admits alternately, according to 
 the extent of the groove desired, ten or a dozen different sizes 
 of irons. 
 
H2 MECHANICAL EXERCISES. 
 
 MnuMing-plants. — The ^nge. — The firmer chisel. — Mortise chisel. 
 
 Miiultiing-pUuies admit of the greatest diversity of contour, 
 which is necessarily the reverse of the mouhling nroduced. 
 The figure of the edge of the iron and that of the sole, should 
 exactly corresjiond; in whetting the iron, great care must be 
 taken not to injure its form : the whole of the sole, or at least 
 the ridci;es of the moulding, especially if narrow at the base, 
 should be made of box-wood, which unites, in a greater de- 
 gree than perhaps any other wood, the valuable properties of 
 hardness, toughness, smoothness, and durability. 
 
 Of CImels. 
 
 The yen*' large chisels used by carpenters, millwrights, and 
 others, for heavy coarse work, are generally composed of iron 
 and steel, welded together, — the steel forming but a small 
 portion of the whole mass of metal, as it seldom extends higher 
 than the broad part of the tool, and often constitutes no more 
 than a third of the thickness. The small and middle-sized 
 chisels of the best kind, are always made of cast steel. As 
 all chisels, not exclusively employed in turning, are driven 
 more or less by percussive force, they are (except the socket 
 chisel) provided with a shoulder, which abuts against the ena 
 of the handle into which the tang is driven, and prevents it 
 from being split by blows. The basil of chisels is on one 
 side, and if well formed should be quite flat. 
 
 The gouge used by the joiners and cabinet-makers is similar 
 to that of the turner, though not always sharpened in the same 
 way. The edge is generally, by joiners and cabinet-makers, 
 for small work, made straight across the end, and not convex 
 like the turner's gouge. The millwrights, again, often make 
 the basil on the hollow or concave side of the gouge, in order 
 to cut with it perpendicularly. 
 
 The thin broad chisel, the sides of which are parallel for a 
 certain length, and which afterwards becomes narrower to- 
 wards the shoulder, obtains the name of the /?;7«er c/?/>y/ when 
 driven by the mallet, and of the paring chisel, when the hand 
 only is employed in cutting with it. 
 
 The common mortise chisel, the section of which is a rect- 
 angle approaching almost to a square, is, as its name im- 
 plies, employed in making mortises; the basil is made on 
 one side of its narrow sides. It is, from its form, very strong, 
 which is necessary, not only on account of its havins: to sus- 
 tain extremely heavy blows with the mallet, but because it is 
 partly used as a lever to get out the pieces of wood as they 
 are severed, in the course of cutting the mortise. 
 
MECHANICAL EXERCISES. 113 
 
 The socket-chisel. — The auger. 
 
 The socket-chisel is distinguished from other chisels by its 
 having a conical socket, instead of a tang and shoulder, to re- 
 ceive the handle. It is used for the same purposes as the 
 mortise-chisel, but is not so thick in proportion to its breadth 
 It is much used for very large work. 
 
 The upper end of the handles of chisels which are driven 
 by percussion, should be made convex, as they will then be 
 least liable to be split or injured by blows. 
 
 Boiifis Tools. 
 
 The largest of the borino- tools for wood, is the au<icr. The 
 oldest construction of the auger, which is yet in common use, 
 in various parts of the country, cannot be wrought till a small 
 excavation has been made, which is mostly done with a gouge, 
 at the place where the hole is to be; and till the auger arrives 
 at a considerable depth, the motion of it is very unsteady. 
 This old auger is shaped like a gimblet, except at the point, 
 which is like that of a nose-bit. An improved construction 
 of the auger, by Phineas Cooke, appeared to possess so much 
 merit, that the Society for the Encourao-ment of Arts present- 
 ed thirty guineas to the inventor. This is called the spiral 
 auger, for it consisted of a rectangular bar of steel, twisted in 
 the shape of a bottle-screw, terminating in a short taper screw, 
 with a double worm like a gimblet. The upper part, like that 
 of the common auger, is formed into a large ring, in which the 
 handle is inserted, at right angles to the length of the auger. 
 That part of the screw adjoining the spiral, presents an edge 
 which cuts the wood. This auger is not very commonly used, 
 but it pierces the wood much truer than the common one ; no 
 picking is necessary before it can be wrought, nor does it re- 
 quire to be drawn out to discharge the chip. It is, however, 
 better adapted to the boring of soft wood than hard. Its use 
 being on this account more limited than workmen like, besides 
 its being not cheap in its first purchase, and if not made of 
 good metal and very carefully tempered, easily changing its 
 form, it will probably not regain the character it once acquired. 
 The latest construction of the auger has been found to answer 
 so well, that it will probably, ere long, nearly supersede the 
 use of the spiral and common auger. Like the spiral one it 
 terminates with a gimblet-screw, which draws it down into 
 the wood, while the workman turns it round and presses upon 
 it; and another peculiar advantage of which is, that its point 
 can be set precisely upon the centre marked for the perfora- 
 tion, the proper direction of which there is then a good chance 
 of preserving, while the broad-ended auger is apt to deviate 
 considerably at its very commencement. Immediately above 
 5— Vol. I Q 
 
114 MECHANICAL EXERCISES. 
 
 DJ!>advaiit.i(re!« of the auger. — -Stock and hit. 
 
 the Spiral screw, it is, for a short length, rather of a prismoidal 
 shape, tapering a little upwards, like the socket chisel below 
 the conical part. The prismoidal part has one cutting edge 
 which cuts the sides of the hole, and anotiier which cuts the 
 bottom. The core rises as the act of boring goes on, in the 
 form of a spiral shaving. Above the prismoidal part, the shaft 
 may be of any shape at pleasure, that possesses sufficient 
 strength, taking the obvious precaution of making its diameter 
 less than that of the bore. The general disadvantage of au- 
 gers with gimblet points, is, that when they encounter knots 
 or hard places in the wood, they are apt to break. 
 
 Every one who makes use of an auger in the usual way by 
 hand, knows by experience that he never can so completely 
 exert his strength in this operation, as when he bores down 
 perpendicularly, with his body leaning over his work ; and it 
 is very evident that by every degree of the auger's elevation 
 from this situation, his power is of less effect, consequently 
 his labour is increased, and his work so much retarded, that 
 in the former position he can bore four holes for one in 
 the latter. In hand boring, also, the unsteady and irregular 
 motion of the auger, (.particularly when the common old-shaped 
 one is used,) at its first entrance into the wood, occasions the 
 holes to be bored very crooked, often larger without than 
 within, and very wide of the direction aimed at, especially if 
 the wood proves hard and knotty, and the holes are deep. Re- 
 garding the prevention of these disadvantages as a matter of 
 considerable consequence to ship-builders, and a variety of 
 other artists, the Society for the Encouragement of Arts, &c. 
 presented the sum of fifty pounds to Vv' illiam Bailey, for his 
 invention of a machine for boring auger-holes, by the use of 
 which the force of the workman, and consequently the dispatch 
 of his operations, are equally exerted in all directions. It is 
 unavoidable also, in the usual way of boring, for the action of 
 the auger to be discontinued twice in every revolution; but 
 with the machine the motion is continued with equal force and 
 velocity, till the auger has bored to the depth required. A 
 description of this machine, illustrated by a plate, may be seen 
 in Bailey's Advancement of Arts ; our limits will not allow us the 
 further notice of it here, but the fact of such a contrivance hav- 
 ing been executed, being mentioned, the ingenious mechanic 
 will not perhaps find itvery difiicultto contrive one for himself. 
 
 The contrivance for boring next entitled to notice, is the 
 stock, which is in effect a crank, not unlike the hand-drill, and 
 frequently made of iron, though generally of wood, defended 
 by brass, at the parts most subject to wear. Where the crank 
 terminates two short limb? project from it, in a line with each 
 
MECHANICAL EXERCISES. 115 
 
 Tli'j 'gouge-bit. — Centre-bit. — Countersink. — Gimblet. 
 
 other, and parallel with that part of it by which it is revolved. 
 In the end of one of these limbs, which is called the pad, the 
 piece of steel by which the boring is performed, is inserted ; 
 the other limb is connected with a broad head, rather convex 
 externally, which head is placed against the breast, and is 
 stationary while all the other parts are revolved. 
 
 The piece of steel inserted in the stock is called the bit ; as 
 it can readily be taken out or put in, the same stock serves for 
 bits of all sizes. They are differently shaped, according to 
 their use. The gouge-bit is best adapted for boring small holes 
 in soft wood; it is shaped nearly like the turner's gouge, but 
 is rather more pointed like a spoon at the extremity; the basil 
 is made in the inside, and the sides are brought to a cutting 
 edge, like those of a gimblet. The centre-bit has a small co • 
 nical point projecting from the lower end ; this point entering 
 the wood first, keeps the tooth of the bit from wandering out 
 of its proper course, and the hole is bored straight with great 
 ease. The taper shell-bit is used for widening holes ; it differs 
 from the gouge-bit chiefly in tapering gradually from the pad 
 to the lower extremity. 
 
 The bit for widening the upper part of a hole, to admit the 
 head of a screw, is called a counters'mk. The head of the 
 countersink is conical, and the cutting edge is single when 
 made for wood alone, and stands out a little from the side of 
 the cone. Joiners and cabinet-makers, however, are generally 
 provided with countersinks for brass, and these, which have 
 ten or a dozen teeth on the surface running slantwise from the 
 base up the sides of the cone, they frequently make use of for 
 wood, especially when it is hard, and they are anxious to 
 avoid tearing it; for the teeth of the brass countersink act like 
 those of a file. 
 
 The gimblet is a boring implement too well known to re- 
 quire any explanation of its construction ; but with respect to 
 its management, it may not be wholly useless to remind the 
 novice, that, like other boring tools of a similar conformation, 
 it requires to be withdrawn to remove the core as often as the 
 cup or groove is filled, and this will be sooner or later, not 
 only in proportion to the depth penetrated, but the density of 
 the wood. Indeed, in boring such wood as lignum-vitae, 
 which clogs the tool, it is advisable to withdraw the gimblet, 
 to clear away the core, before the cup is full. The auger 
 gives warning of the time to stop, by the difficulty of turning 
 it, when surcharged with shavings, and is too strong a tool to 
 be in danger of being twisted ; but the smallness of the gimb- 
 let renders it liable to be twisted and broken before the work- 
 man is awar«, if not often enough withdrawn and emptied. 
 
116 MECHANICAL EXERCISES. 
 
 Spriy-bit. — Hammers. 
 
 Gimblets which are broken-pointed, or bhinted ou tlie arris of 
 the screw, are generally thrown aside, it beinti; tedious, and 
 laborious also when they are large, to work with them in such 
 a state; but we may observe, that though the grindstone can- 
 not be employed to sharpen the worm, a file may, so that a 
 few minutes' labour will render them fit for use again. 
 
 The smallest sort of boring tool is a kind of bodkin, called 
 the brad-awl, or sprig-bit, as it is chiefly used in making the 
 
 Eerforation to admit those small slender nails, which have no 
 ead except a trifling projection on one side, and are called 
 brads in some parts of the country, and sprigs in other parts. 
 The sprig-bit is generally made with a shoulder where tiie tang 
 terminates; below the shoulder it is cylindrical, to within a 
 short distance of the extremity, which is flattened, and thereby 
 made rather broader than the diameter of the cylindrical part; 
 but so thin at the same time towards the end, as to form an 
 edge. Unlike other boring tools, the sprig-bit takes away no 
 part of the substance of the wood, nor is it turned entirely 
 round in making a hole, but merely wrought backwards and 
 forwards about half round before the motion is reversed. 
 
 The Hammer — Mallet — Square — Bevel — Mitre-square — 
 Gauge — St ra ight-edge — Winding-sticks . 
 
 Though hammers, of various sizes, are indispensable in the 
 working of wood, yet we may pass over the consideration of 
 them in this place, with little more than referring to what has 
 been already said with respect to them, in treating of the work- 
 ing of metals. The object of having the head of the hammer 
 perfectly well secured to the handle is certainly well worth 
 attention, from the serious accidents which may attend the 
 neglect of it; but to attain it, we recommend not the use of 
 those hammers which have plates of iron extending from th?. 
 head, forming a kind of socket for the reception of the handle . 
 These plates, it is true, afl'ord ample means of uniting tl.o 
 head and the handle ; but they render the latter inflexible at 
 the very part where it is desirable there should be some spring. 
 Hence those hammers are best, in which the handle simply 
 passes through a perforation in the head, wooden wedges being 
 driven in at the end of it, after it has been tightly fitted, to 
 make it fast. If the aperture in the head be rather wider at 
 the back than where the handle enters, and the wedges be 
 dipped in glue before they are driven in, the fastening may 
 be made complete. To admit the wedges, the end of the 
 handle is cut a little way down with a saw. 
 
 The ma/let is in eflfect a hammer, but is made of wood ; it 
 consequently does not damage substances struck with it so 
 
MECHANICAL EXERCISES. 117 
 
 Mallet. — Square. 
 
 much as the hammer, while presenting a large surface with an 
 equal weight, it is more easy to hit the ends of the chisels, &c. 
 with it. Mallets are made of the soundest and toughest wood 
 which can be found ; either ash or beech, or the hardest kind 
 of elrn, is usually preferred. They are mostly made rather 
 concave on that side which the handle enters, and convex on 
 the other; this is done because it is customary, or because it 
 is supposed to look best : the diameter of the convex end, 
 measured at right angles to the handle, is greater than that of 
 the concave one, consequently the ends with which objects are 
 struck are not parallel with the handle, but inclined to it and 
 to each other; this is done for convenience, for the mallet is 
 generally used in such a manner that the end with which the 
 blow is made, notwithstanding its obliquity with respect to 
 the handle, is parallel with the surface struck. 
 
 The squares used by joiners and cabinet-makers are fie- 
 quently manufactured by these artists for their own use, in 
 which case they are made of wood ; but these wooden squares 
 are always so much inferior in point of durability, and gene- 
 rally in point of correctness, to those sold by the ironmongers, 
 at very reasonable rates, and which are made partly of wood 
 and partly of metal, that we shall only notice the latter kind. 
 The blade is a thin plate of steel, at a spring temper, of equal 
 thickness in every part, and the opposite edges, or at least the 
 two edges in the direction of its length, are correctly parallel 
 with each other. The stock or wooden part is of considerable 
 thickness, seldom less than half an inch in the smallest 
 squares, such, for example, as are only three or four inches 
 long, and the blades of which are not above a twelfth of that 
 thickness. The blade is let into the stock so as to form a 
 right angle with it both internally and externally. The mor- 
 tise or kerf which receives the blade, is made at one end, in 
 tile middle and entirely across its breadth ; great care is taken 
 to make it parallel with the sides of the stock, but it is not 
 cut so deep as to take in the whole breadth of the blade, the 
 part left out being partly designed to admit of the outer edge 
 being repaired, when worn. The inner edge of the stock, 
 which forms one side of the interior square, is faced with 
 brass. The stock, from its thickness, forms, on each side of 
 the blade, a shoulder, which being pressed against one edge 
 of a piece of wood, acts as a guide or stop to keep the blade 
 (which is extended over the adjoining surface) at right angles 
 to the arris, while a line is drawn alono; its outer edoe. The 
 interior square is mostly used for examining the squareness 
 of a piece of stuff, and not for drawing lines. In this case, 
 the sides of the square are not held parallel with, but perpen- 
 
118 MECHAMCA L EXERCISES. 
 
 Bfvel. — Mitre-square. — Gau^'e. 
 
 dicular to the piece examined. The mode of ascertaining the 
 correctness of the square, used in the working of metals, has 
 been already detailed, and will probably suggest the proper 
 method of trying the square now described. Here we have 
 no occasiun for a ledge, the shoulder is pressed against the 
 edge of a rectangular jjiece of stutF, and a line drawn close to 
 the blade ; the square is then turned over, and another line 
 drawn as in the former case, and consequently if the instru- 
 ment deviates from a correct figure, the error is detected upon 
 the same principle as before. 
 
 The bevel consists of a blade and stock similar to those of 
 the square, from which it diiiers only in one particular, viz. that 
 the blade is moveable, and can therefore be set at any angle 
 which may be required. The joint should be stiff, otherwise 
 the bevel cannot be depended on for remaining as it has been 
 set. Though rarely practised among the workers in wood, it 
 certainly would be easy in all cases to adapt a screw to the 
 bevel, so as to hold the blade firmly at any angle desired. 
 The stone-masons generally take this precaution. 
 
 The mitre-square is a bevel, the blade of which is immove- 
 ably fixed in the stock, and is commonly set for marking an 
 angle of forty-five degrees, this angle being more frequently 
 required in joinery than any other angle except the right angle. 
 Pieces of stuff bevelled at their extremities, and joined by 
 placing two of the bevelled surfaces together, are said to be 
 mitred. Mitring is often used in plane work ; but when the 
 pieces to be joined at an angle, are moulded, and the continua- 
 tion of the mouldings is not to be broken, it is absolutely ne- 
 cessary. Hence its frequent use; instances of it occur at the 
 corners of rooms, where the skirting boards of two diflferent 
 sides meet, in the surbase under the same circumstances, at 
 the angles of picture-frames, ccc. 
 
 The gauge is an instrument consisting of a stem, usually in 
 the form of a square prism, with a small steel point, nearly at 
 the end of one of the surfaces in the direction of its length, 
 and just projecting enough to mark distinctly when pressed 
 upon wood ; tlie stem passes at ri^ht angles through a mortise 
 in the middle of a piece of wood called the head, to which it 
 is about equal in point of thickness ; but those surfaces of 
 the head through which the mortise passes, should not be 
 less than three times the diameter of the stem. The head 
 can be set at any distance required from the steel point, and 
 there secured by a small wedge, passing through a mortise 
 in one of its sides, and beariu'j; upon the stem. The use of the 
 gauge is to draw lines parallel to the arris of a piece of stufl', 
 to serve as a guide for the saw, the plane, or the chisel. In 
 
MECHAXICAL EXERCISES. 119 
 
 Mortise-gauge. — Straight-edge. — Winding-sticks. 
 
 drawing the line, it is necessary to keep that side of the head 
 which is next the steel point rather firmly pressed against the 
 edge of the stuft, otherwise the point will be apt to deviate 
 from its proper course, if it meet with knots or irregularities 
 in the grain. 
 
 A gauge made with two points projecting on the same side, 
 and one of which (being moveable in a groove or mortise) can 
 be placed at any distance from the other, is called a mortise- 
 gauge ; it is used alike in gauging mortises and tenons. 
 
 Though the steel straight-edge is hardly known even by 
 name to a great majority of the artists who work in metal, yet 
 the wooden straight-edge is familiar enough to most who work 
 in wood. Wooden straight-edges should be made of stuff 
 exceedingly well seasoned ; it is usual to make two at the 
 same time ; the sides are first made true, and each piece of 
 equal thickness ; they are then placed against each other, and 
 fastened in the cheeks of the bench-screw, in which situa- 
 tion their upper edges are planed true with the assistance of 
 a straight-edge which can be depended on, or as nearly true 
 as can be determined by the eye, if we cannot readily obtain 
 a straight-edge for the trial. When the pieces are supposed 
 to be true, they are taken out of their situation, and the edges 
 last planed are placed upon each other. If the surfaces coin- 
 cide so exactly than no light can pass between them, the 
 straight-edges are finished ; but if any error be detected, the 
 planing with the jointer must be renewed in the same manner 
 as before, and the examination repeated till the result is satis- 
 factory. The use of the straight-edge, in ascertaining the 
 straightness of other surfaces, is not inconsiderable. 
 
 Winding-sticks are always \ised in pairs, and the use of them 
 constitutes another contrivance for determining the levelness 
 of any given surface, that the workman may reduce it more or 
 less in any particular part, in order to make it true. Straight- 
 edges are customarily finished only on one edge; but if both 
 edges were finished, and they were made correctly rectangu- 
 lar, that is, of equal breadth through their whole length, they 
 would ansv/er the end of winding-sticks, which are used in 
 the following manner : one of them is placed at each end of 
 the surface to be examined, the eye is then directed from the 
 uppermost edge of the nearer one to that perpendicular side 
 of the further one, which is next the observer. If the eye be 
 elevated or depressed, till one end of the nearer winding-stick 
 intercepts the view of the opposite perpendicular side of the 
 other ; and the other two ends are observed to have the same 
 relative situation, the ends of the surface examined are already 
 in the same plane. But if the nearer winding-stick will not 
 
120 MF.CHAMC AL KXERCISES 
 
 Draw-bore pins. — Glue. 
 
 iuteicf j>l ;in equal portion at each end of the further one, 
 the part found to be too hiah must he reduced till this is the 
 case. It is always proper to examine the surface by placing 
 the windinfj-sticks in various situations, but especially across 
 near the corners, so that the eye may look at them diago • 
 nally. 
 
 Drair-bore pins are used in forcing a tenoned piece into its 
 proper place in the mortise. They are made with tangs and 
 shoulders, and fitted into handles like chisels. Below the 
 shoulder thev are round, and taper slightly to within a short 
 distance of the point, where they are turned off to cones, like 
 the extremities of an axis running in hollow centres. To use 
 them, bore a hole through the mortise-cheeks, at the place 
 where the pin intended to fasten the mortised and tenoned 
 piece together will be required, and which is generally made 
 nearer the shoulders than the end of the tenon. Insert the 
 tenon, and when it is as nearly in its proper place as it can be 
 driven, mark it on both sides through the hole in the mortise- 
 cheeks. Take out the tenon, and bore it through, a little 
 nearer its shoulders than the centre of these marks ; insert it 
 attain in the mortise, and use the draw-bore pins by entering 
 them at the holes, to draw its shoulders against the abutments. 
 The use of the draw-bore pins, also, condenses or hardens 
 the wood on the sides of the holes, and when the wooden peg 
 is driven in, it has, on this account, a better hold. This is, in- 
 deed, their principal use at present, as the cramping-frame, 
 which acts by means of a screw, is much more powerful in 
 forcing the shoulders of tenons against their abutments ; but 
 the draw-bore pins, or some substitute for them, will obviously 
 be convenient to those who may occasionally require means 
 to force tenons to their bearings, without possessing a com- 
 plete apparatus. 
 
 Of Glue. 
 
 To prepare glue, it must be steeped for a number of hours, 
 over night, for instance, in cold water, by which means it will 
 become very considerably swelled and softened. It must then 
 be gently boiled, till it is entirely dissolved, and of a consistence 
 not too thick to be easily brushed over wood. About a quart of 
 water may be used to half a pound of glue. The heat employed 
 in melting glue should not be more than is required to make 
 water boil; and to avoid burning it, the joiners. Sec. as is well 
 known, suspend the vessel containing it in another vessel con- 
 taining only water, which latter vessel is generally made of 
 copper, in the form of a common tea-kettle without a spout; 
 and alone receives the direct influence of the fire. 
 
MECHANICAL EXERCISES. 121 
 
 Directions for using glue. — Glue which resists water. 
 
 The circumstances most favourable to the best effect which 
 glue can produce, in uniting two pieces of wood, are the fol- 
 lowing: that the glue should be thoroughly dissolved, and used 
 boiling hot at the first or second melting ; that the wood should 
 be warm and perfectly dry ; that a very thin covering of glue be 
 interposed at the juncture, and that the surfaces to be united, 
 be strongly pressed together, and left in that state, in a warm 
 but not hot situation, till the glue is completely hard. In ve- 
 neering, and for all very delicate work, the whole of these re- 
 quisites, as they not only ensure the strongest joint, but the 
 glue sets the soonest, should be combined in the operation ; 
 but on some occasions this is impossible, and therefore the 
 most essential must be regarded, such as the hotness of the 
 glue, and the dryness of the wood. When the faces of joints, 
 particularly those that cannot be much compressed, have been 
 besmeared with glue, which should always be done with the 
 greatest expedition, they should be rubbed lengthwise one 
 upon another two or three times, to settle them close. 
 
 When glue, by repeatedly melting it, has become of a dark 
 and almost black colour, its qualities are impaired; when newly 
 melted, it is of a light ruddy brown colour, nearly like that of 
 the dry cake held up to the light ; and while this colour remains, 
 it may be considered fit for almost every purpose. 
 
 Though glue which has been newly melted is the most suit- 
 able for use, other circumstances being the same, yet that 
 which has been the longest manufactured is the best. To try 
 the goodness of glue, steep a piece three or four days in cold 
 water; if it swell considerably without melting, and when taken 
 out resumes, in a short time, its former dryness, it is excel- 
 lent. If it be soluble in cold water, it is a proof that it wants 
 strength. 
 
 A glue which does not dissolve in water, may be obtained by 
 melting common glue with the smallest possible quantity of 
 water, and adding by degrees linseed oil rendered drving by 
 boiling it with litharge ; while the oil is added, the ingredients 
 must be well stirred to incorporate them thoroughlv. 
 
 A glue which will resist water, in a considerable degree, is 
 made by dissolving common glue in skimmed milk. 
 
 Finely levigated chalk added to the common solution ot 
 glue in water, constitutes an addition which strengthens it, 
 and renders it suitable for sign-boards, or other things which 
 must stand the weather. 
 
 A glue that will hold against fire or water, may be prepared 
 
 by mixing a handful of quick lime with four ounces of linseed 
 
 oil : thoroughly levigate the mixture, boil it to a good thickness, 
 
 and then spread it on tin plates in the shade ; it will become 
 
 6.— Vol. I. R 
 
122 MECHANICAL EXERCISES. 
 
 Pro|)ortious of mortises and tenous. 
 
 exceedingly hard, but may be dissolved over a fire, as ordinary 
 glue, and is then fit for use. 
 
 Several glues, such as that of isinglass, which, for a variety 
 of reasons^ are not used in the common course of business 
 amon<y joiners and cabinet-makers, we shall speak of particu- 
 larly under the head Cements. 
 
 Of the Mortise and Tenon. 
 
 The proportion which mortises and tenons ought to bear to 
 the wood, under different circumstances, has never been demon- 
 strated by experiments made for that purpose. It is common 
 practice, therefore, alone, which dictates the rules to be ob- 
 served. In general the tenon is one-third of the thickness of 
 the stuff; but when the mortise and tenon are intended to lie 
 horizontally, and the juncture will be unsupported, the tenon 
 should not be more than one-fifth of the thickness of the stuff, 
 otherwise a strain or weight on the upper surface of the tenon- 
 ed piece would probably split off the under cheek of the mor- 
 tise, while the tenon itself remained sound. 
 
 In joining two pieces of timber, so that the tenoned piece 
 shall not pass the end of the mortised piece, to prevent the 
 necessity of cutting open the side of the mortise, the tenon 
 must be reduced one-third or at least one-fourth of its breadth. 
 In either case, the mortise will still be so near the end of the 
 piece in which it is made, as to be split by a small force in 
 driving in the tenon. To prevent this accident, it is customary, 
 on such occasions as admit of the precaution being taken, to 
 make the end beyond the mortise considerably longer than it 
 is intended to remain; the tenon may then be driven tightly 
 in, and the superfluous wood afterwards cut off. 
 
 The above proportions for the mortise and tenon, refer to the 
 joining of timber, like dimensions of which are of the same 
 strength, and therefor* it will be necessary to vary them, ac- 
 cording as one piece is weaker or stronger than the other. 
 
 In making deep mortises, especially in hard wood, it is cus- 
 tomary to shorten the labour, by commencing it with boring 
 a number of auger holes, the compartments between which 
 are speedily cut away with the chisel. 
 
 In neat work, before a saw is employed to cut the shoulder 
 of a tenon, nick the place with a paring chisel ; the saw will 
 not then tear the wood, and the line of its entrance may be 
 correctly determined. 
 
 When the mortise is to pass entirely through a piece of stuff, 
 the space allotted for it is gauged on both sides with the great- 
 est precision, and when it has been half cut from one side, the 
 remaining half is cut from the opposite one. By this means. 
 
MECHANICAL EXERCISES. 123 
 
 Concluding reflections. 
 
 if there be any error in the direction of the chisel, it is of little 
 consequence, the irregularity being confined to the middle of 
 the mortise. The sides of the mortise should, however, be 
 made as even as possible, that they may every-where come in 
 contact with the sides of the tenon. 
 
 The sides of a mortise that passes through the stuff, should be 
 inclined to each other a little towards the shoulders of the te- 
 non, because the latter, after it is driven in, is expanded by 
 wedges. 
 
 We cannot close these details of the primary operations by 
 which metals and timber are fitted for mechanical purposes, 
 without offering a few sentiments to the consideration of the 
 young artist interested in them, whether he is one who is 
 anxious to excel in a particular branch of art, as affording the 
 means of honourable livelihood ; or claims merely the appella- 
 tion of an amateur, who studies mechanical operations from the 
 love of knowledge, the desire of amusement, or the hope of 
 celebrity in making discoveries or improvements. Let him 
 not be discouraged by the failure of first attempts. Instead 
 of losing his time in uselessly regretting his disappointment, 
 let him examine into the cause of it, and promptly repeat his 
 experiments with more precaution. It is a mistaken idea, that 
 manual dexterity is absolutely dependent upon length of prac- 
 tice ; uncommon are the cases in which it fails to be the early 
 reward of those who unite perseverance — patient and prompt, 
 with an attention ever alive to avail themselves of every new 
 light which the liberality of others, or the course of their own 
 experience, supplies. But those who postpone ardent exertion, 
 by satisfying themselves with the hope, that length of prac- 
 tice will perfect them, will in the end regret their delusion, 
 and may ineffectually try to recover their loss, when habitual 
 languor, and other injurious habits, have rendered the mind 
 averse to observe, and the hand unable to perform. 
 
 Much might be said on this subject, but we forbear a length- 
 ened disquisition; yet we cannot omit observing, that those 
 who take an early delight, and attain an early proficiency in 
 mechanical arts, are preparing an excellent groundwork for 
 investigations of a higher order. They are acquiring, in the 
 disguise of amusement, that dexterity of hand, and facility of 
 contrivance, that readiness in supplying their casual wants, 
 and habit of methodical arrangement, which will afterwards 
 qualify them to explore the paths of knowledge through the 
 medium of philosophical experiment ; and in proportion as 
 
124 MECHANICAL EXERCISES. 
 
 Conclading reflections 
 
 these interest their attention, they will be disposed to study 
 in earnest the fundamental principles of science. 
 
 It may be well for the young artists we are addressing, to 
 be aj)prized, that to make any useful proficiency in mechanical 
 pursuits, to be distinguished for skill and promptitude of exe- 
 cution, requires a degree of patient assiduity, of which few 
 ■who have been brought up at the desk or behind the counter, 
 can form any adequate idea; and who, therefore, would be 
 uneasy and unhappy, under a degree of exertion which they 
 must learn to display without entertaining a sentiment of its 
 hardship. Let them not, however, be disheartened at the pros- 
 pect; the habits indispensable to their full success, if acquired, 
 allowed their due influence, and guided by moral prudence, 
 are of inestimable value : they are extending the means of 
 multiplying their comforts ; they are increasing the power of 
 the head to contrive as well as of the hand to execute, and 
 the steadiness of attention superinduced, will be beneficial to 
 them in every action of their lives. 
 
 All know, but few act as if they believed, that, accident out 
 of the question, success in the general issue of our exertions, 
 is made up of success in little matters, or individual operations. 
 Let us then be allowed to impress this truism upon the reader's 
 mind, and to elucidate it by stating, that if two workmen have 
 each fifteen thousand motions to make in ten hours, he who 
 shall perform each motion half a second more quickly than the 
 other, will terminate his labour two hours sooner! Two hours 
 of spare time may be employed to great advantage ; yet what 
 is half a second considered by itself? and by how many may 
 this economy of time be practised, without a perceptible ad- 
 dition of fatiofue? The quantity of labour is no criterion of 
 the fatigue it will occasion, even to those who are alike in cor- 
 poral power. The sentiments by which we are actuated in the 
 performance of a task, have a paramount influence ; languor of 
 mind prematurely exhausts the body, and the weariness expe- 
 rienced is often disproportioned to the animal strength pos- 
 sessed. But our sentiments are much under our control, and 
 it surely should be our study to cherish those, which by inspir- 
 ing alacrity of exertion, are essentially conducive to our in- 
 terest. To the labourer, there is the prospect of commanding 
 respect, and of bettering his condition; to the amateur, be- 
 sides the common, daily advantages of knowledge, and the 
 indeprivable gratification which the acquirement of it affbrds, 
 there is the animating hope, that if discoveries crown his ex- 
 ertions, he may be considered a benefactor to mankind. 
 
ARCHITECTURE. 125 
 
 Introductory remarks. — Ancient architecture. 
 
 ARCHITECTURE. 
 
 The science of Architecture may be considered, in its most 
 extended application, to comprehend building of every kind ; 
 but at present we must consider it in one much more restricted, 
 according to which. Architecture may be said to treat of the 
 planning and erection of edifices, which are composed and 
 embellished after certain long established rules, accordino- to 
 two principal modes, 
 
 1st, the English or Gothic, 
 
 2nd, the Antique, or Grecian and Roman. 
 
 We shall treat of these modes in distinct dissertations, 
 because their principles are completely distinct, and indeed 
 mostly form direct contrasts, But before we proceed to treat 
 of them, it will be proper to make a few remarks on the dis- 
 tinction between mere house-buildino-, and that hioher charac- 
 ter of composition in the Grecian and Roman orders, which is 
 properly styled Architecture ; for though we have now very 
 many noble architectural houses, we are much in danger of 
 having our public edifices debased, by a consideration of what 
 is convenient as a house, rather than what is correct as an 
 architectural design. 
 
 In order properly to examine this subject, we must consider 
 a little, what are the buildings regarded as our models for work- 
 ing the orders, and in what climate, for what purposes, and 
 under what circumstances, they were erected. This may, per- 
 haps, lead to some conclusions, which may serve to distinguish 
 that description of work, which, however rich or costly, is 
 still mere house-building, in point of its composition. 
 
 It is acknowledged, on all hands, that our best models, in 
 the three ancient unmixed orders, — the Doric, Ionic, and Co- 
 rinthian, are the remains of Grecian temples. Most of them 
 were erected in a climate, in which a covering from rain was 
 by no means essential, and we shall find this circumstance very 
 influential; for as the open space within the walls was always 
 partially, and often wholly open, apertures in those walls for 
 light, were unnecessary ; and we find, also, in Grecian struc- 
 tures, very few, sometimes only one door. The purpose for 
 which these buildings were erected, was the occasional recep- 
 tion of a large body of people, and not the settled residence of 
 any. But, perhaps, the circumstances under which they were 
 erected, have had more influence on the rules which have been 
 
126 ARCHITECTURE. 
 
 Ancient architecture. — Difl'erence of mere baiidin^ and arclutectund design. 
 
 handed down to us, as necessary to be observed in composiu'^ 
 architectural designs, than either the climate or their use. It 
 is now prettv generally agreed, that the Greeks, if they were 
 acquainted with the mathematical properties of the arch, did 
 not use it till it was introduced by the Romans, Here then 
 vre see at once a limitation of the intercolumniation, which 
 must be restrained by the necessity of finding stones of sutii- 
 cient length to form the architrave. Hence the smaller com- 
 parative intercolumniations of the Grecian buildings, and the 
 constant ase of columns ; and hence the propriety of avoiding 
 arches, in compositions of the purer Grecian orders. 
 
 The Romans introduced the arch very extensively, into 
 buildings of almost even,- description, aird made several altera- 
 tions in the mode of working the orders they found in Greece, 
 to which thev added one order, by mixing the Corinthian and 
 Ionic, and another by stripping the Doric of its ornaments. 
 Their climate, also, was so far ditierent as to require more 
 general roofing, but still, from the greater necessity of provid- 
 ing a screen from the heat of the sun, than apertures to admit 
 the light, it does not appear that large windows were in general 
 use, and hence an important difference in modern work. Al- 
 though, by roofs and arches, much more approximated to mo- 
 dern necessities than the Grecian models, still those of Rome 
 which can be regarded as models of composition, are temples, 
 or rather public edifices, and not domestic buildings, which, 
 whenever thev have been found, appear variously unadapted 
 to modern wants, and therefore unfit for imitation. 
 
 In a few words we may sum up the grand distinctions be- 
 tween mere building and architectural design : — the former 
 looks for convenience, and though it wiU doubtless often use 
 architectural ornaments, and preserve their proportions, when 
 used as smaller parts, yet the general proportion may vary very 
 widely from the orders, and yet be pleasing, and perhaps not 
 incorrect; — but all this is modern building, and not architec- 
 ture in its restricted sense ; in this the columns are essential 
 parts, and to them and their proportions all must be made sub- 
 servient ; and here we may seek, with care and minuteness, 
 amongst the many remains yet left in various parts, (and of 
 which the best are familiar to most, from the valuable delinea- 
 tions we possess of those who have accurately examined them,) 
 for models, and in selecting and adopting these, the taste and 
 abilities of the architect has ample space. 
 
 As an introduction to the dissertations, it may not be amiss 
 to take a hasty sketch of the progress of Architecture in Eng- 
 land. 
 
 Of the British architecture, before the arrival of the Ro- 
 
ARCHITECTURE. 127 
 
 British architecture. 
 
 mans in the island, we have no clear account; but it is not 
 likely it difFered much from the ordinary modes of uncivilized 
 nations ; the hut of wood with a variety of coverings, and some- 
 times the cavities of the rock, were doubtless the domestic 
 habitations of the aborignal Britons ; and their stupendous 
 public edifices, such as Stonehenge and others, still remain to 
 us. The arrival of the Romans was a new era ; they introdu- 
 ced, at least in some degree, their own architecture, of which a 
 variety of specimens have been found; some few still remain, 
 of which, perhaps, the gate at Lincoln is the onlv one retain- 
 ing its original use. Although some fine specimens of work- 
 manship have been dug up in parts, yet by far the grreatest part 
 of the Roman work was rude, and by no means comparable 
 with the antiquities of Greece and Italy, though executed by 
 the Romans. ^Vllen they left the island, it was most likely 
 that the execution of the Britons was still more rude, and en- 
 deavouring to imitate, but not working on principle the Roman 
 work, their architecture became debased into the Saxon and 
 early Norman, intermixed with ornaments perhaps brought in 
 by the Danes. After the conquest, the rich Norman barons, 
 erecting; very magnificent castles and churches, the execution 
 manifestly improved, though still with much similarity to the 
 Roman mode debased; but the introduction of shafts, instead 
 of the massive pier, first began to approach that lighter mode 
 of building, which, by the introduction of the pointed arch, and 
 by an increased delicacy of execution, and boldness of compo- 
 sition, ripened, at the close of the twelfth centurv, into the 
 simple, yet beautiful early English style. At the close of an- 
 other century, this style, from the alteration of its windows, by 
 throwing them into large ones, divided by mullions, introducing 
 tracery in the heads of windows, and the general use of flow- 
 ered ornaments, together with an important alteration in the 
 piers, became the decorated English style, which may be con- 
 sidered as the perfection of the English mode. This w^as very 
 difficult to execute, from its requiring flowing lines where 
 straight ones were easier combined; and at the close of the 
 fourteenth century, we find these flowing lines giving wav to 
 perpendicular and horizontal ones, the use of which continued 
 to increase, till the arches were almost lost in a continued series 
 of pannels, which, at length, in one building, the chapel of 
 Henry the VII. covered completely both the outside and inside, 
 and the eye, fatigued by the constant repetition of small parts, 
 sought in vain for the bold grandeur of design which had been 
 so nobly conspicuous in the preceding style. The Reformation, 
 occasioning the destruction of many of the buildings the most 
 celebrated, and mutilating others, or abstracting the fund* 
 
{'2^ ARCHITECTURE. 
 
 iirttisli arcliitt^uturc. 
 
 necetisaiy for their repair, seems to have put an end to the 
 working of the English styles on principle. The square pan- 
 nelled nuillioned windows, and wooden pannelled roofs and 
 halls, of the great houses of the time of queen Elizabeth, seem 
 rather a debased English than any thiiig else; but during the 
 reign of her successor, the Italian architecture began to be 
 introduced, first only in columns of doors, and other small 
 parts, and afterwards in larger portions, though still the general 
 style was this debased English. Of this introduction, the most 
 memorable was the celebrated portico of the schools at Oxford, 
 where, into a building adorned with pinnacles, and having 
 mullioned windows, the architect has crowded all the five or- 
 ders over each other. Some of the works of Inigo Jones are 
 little removed beyond this barbarism. Longleat, in Wiltshire, 
 is a little more advanced, and thebanqueting-house, Whitehall, 
 seems to mark the complete introduction of Roman workman- 
 ship. The close of the seventeenth century produced Sir 
 Christopher Wren, a man whose powers, confessedly great, 
 lead us to regret he had not studied the architecture of his 
 English ancestors, with the success which he did those of 
 Rome; for while he has raised the most magnificent modern 
 building we possess, he seems to have been pleased to dis- 
 figure the English edifice he had to complete. While his 
 works at St. Mary, Aldermary, Bow-lane, and St. Dunstan in 
 the East, prove how well he could execute imitated English 
 buildings, when he chose, though even in them he has variously 
 departed from the true English principles. By the end of the 
 seventeenth century, the Roman architecture seems well esta- 
 blished, and the works of Vitruvius and Palladio successfully 
 studied; but Sir John Vaubrugh and iS'icholas Hawksmoor 
 seem to have endeavoured to introduce a massiveness of style 
 which happily is peculiar to themselves. The works of Pal- 
 ladio, as illustrated by some carpenters, seem to have been the 
 model for working the orders during the greatest part of the 
 eighteenth century, but in the early and middle part of it, a 
 style of ornament borrowed from the French was much intodu- 
 ced in interiors, the principal distinctions of which were the 
 absence of all straight lines, and almost of any regular lines. 
 
 The examples of this are now nearly extinct, and seem to 
 liave been driven out by the natural operation of the advance 
 of good workmanship in the lower class of buildings. All or- 
 namental carvings were difficultly executed in wood, and were 
 very expensive; but towards the latter end of the eighteenth 
 century, the Adams's introduced a style of ornament directly 
 contrary to the heavy carving of their predecessors. This was 
 so fiat, as to be easily worked in plaster and other compositions 
 
ARCHITECTURE. 129 
 
 Four styles of English architecture. 
 
 and ornament being sold very cheap, was profusely used in car- 
 penter's work. This flatness was more or less visible in many 
 considerable buildings; but near the close of the century, the 
 magnificent works of Stuart and Revet, and the Ionian anti- 
 quities of the Dilletante Society, began to excite the public 
 attention, and in a few years a great alteration was visible; the 
 massive Doric, and the beautiful plain Grecian Ionic, began 
 to be worked, and our ordinary door-cases. Sec. soon began to 
 take a better character. The use of the simple, yet bold mould- 
 ings and ornaments of the Grecian models, is gradually spread- 
 ing, and perhaps we may hope, from the present general inves- 
 tigation of the principles of science, that this will continue 
 without danger of future debasement, and that a day may come 
 when we shall have Grecian, Roman, and English edifices, 
 erected on the principles of each. 
 
 As the earliest in point of execution in England, we shall 
 begin with the dissertation on 
 
 ENGLISH ARCHITECTURE. 
 
 In a work like the present, there will be little propriety in 
 lengthened disquisitions on the origin of this mode of build- 
 ing ; but much service may be rendered to individuals, by a 
 clear detail of those distinctions, which, being once laid down 
 with precision, will enable persons of common observation, to 
 distinguish the difference of age and style in these buildings, 
 as easily as the distinctions of the Grecian and Roman 
 orders. 
 
 During the eighteenth century, various attempts, unaer the 
 name of Gothic, have arisen in repairs and rebuilding of eccle- 
 siastical edifices; but these have been little more than making 
 clustered columns and pointed windows, every real principle 
 of English architecture being, by the builders, either unknown 
 or totally neglected. 
 
 English architecture, then, which has been too long called 
 Gothic, may be divided into four distinct periods, or styles, 
 which may be named, 
 
 1st, the Norman style, 
 
 2nd, the Early English style, 
 
 3rd, the Decorated English style, and 
 
 4th, the Perpendicular English style. 
 
 The dates of these styles we shall state hereafter, and it may 
 
 be proper to notice, that the clear distinctions are now almost 
 
 entirely confined to churches ; for the destruction and alteration 
 
 of castellated buildings has been so great, from the alterations 
 
 6.— Vol. I. S 
 
130 AKCIIITECTLRE. 
 
 Dtrtinitioaa. 
 
 of the modes of warfare. &c. that, in them, we can hardly tell 
 which is original and which addition. 
 
 Before we enter on a description of the styles separately, it 
 will be necessary to explain a few terms which are made use 
 of in describing churches, &c. and without understanding 
 which, it will be impossible to comprehend the subject 
 clearly. 
 
 Most of the ancient ecclesiastical edifices, when considered 
 complete, were built in the form of a cross, with a tower, lan- 
 tern, or spire erected at the intersection. The interior space 
 was usually thus divided : 
 
 The space westward of the cross is called the nave. 
 
 The divisions outward of the piers are called aisles. 
 
 The space eastward of the cross is ger^erally the choir. 
 
 The part running north and south is called the cross or 
 transept . 
 
 The choir is generally enclosed by a screen, on the western 
 part of which is usually placed the organ. 
 
 The choir, in cathedrals, does not njenerally extend to the 
 eastern end of the building, but there is a space behind the 
 altar, usually called the lacli/ chapel. 
 
 The choir is only between the piers, and does not include 
 the side aisles, which serve as passages to the lady chapel, 
 altar, &c. 
 
 The transept has sometimes side aisles, which are often sepa- 
 rated by screens for chapels. 
 
 Chapels are attached to all parts, and are frequently addi- 
 tions. 
 
 The aisles of the nave are mostly open to it, and in cathe- 
 drals both are generally without pews. 
 
 In churches not collegiate, the eastern space about the altar 
 is called the chancel. 
 
 To the sides are often attached small buildings over the 
 doors, cdWedi porches, which have sometimes vestries, schools, 
 &c. over them. 
 
 Ih&font is generally placed in the western part of the nave, 
 but in small churches its situation is very various. 
 
 In large churches, the great doors are generally either at 
 the west end, or at the end of llie transepts, or both; but in 
 small churches, often at the sides. 
 
 To most cathedrals are attached a chapter-house and cloisters, 
 which are usually on the same side. 
 
 The chapter-house is often multangular. 
 
 The cloisters are generally a quadrangle, with an open spare 
 in the centre; the side to which is a series of arches, originally 
 glazed, now mostly open. The other wall is generally one side 
 
ARCHITECTURE. 131 
 
 Defini lions. 
 
 of the church or other buildings, with which the cloisters com- 
 municate by various doors. The cloisters are usually arched 
 over, and formed the principal communication between the 
 different parts of the monastery. 
 
 The lady chapel is not always at the east end of the choir; 
 at Durham it is at the west end of the nave ; at Ely, on the 
 north side. 
 
 The choir sometimes advances westward of the cross, as at 
 Westminster. 
 
 The spaces in the interior, between the arches, sue piers. 
 
 The windows above the arches, which appear on the out 
 side over the roof of the aisles, are called clerestory windoivs. 
 
 Any building above the roof may be called a steeple. If it 
 be square-topt, it is called a tower. 
 
 A tower may be round, square, or multangular. The tower 
 is often crowned with a spire, and sometimes with a short 
 tower of light work, which is called a lantern. An opening 
 into the tower, in the interior, above the roof, is also called a 
 lantern. 
 
 Towers of great height in proportion to their diameter, are 
 called turrets ; these often contain staircases, and are some- 
 times crowned with small spires. 
 
 Large towers have often turrets at their corners, and often 
 one larger than the others, containing a staircase ; sometimes 
 they have only that one. 
 
 The projections at the corners, and between the windows, 
 are called buttresses. 
 
 The walls are crowned by sl parapet, which is straight at the 
 top, or a battleynent, which is indented; both maybe plain or 
 sunk, pannelled or pierced. 
 
 Arches are round, pointed, or mixed : 
 
 A semi-circular arch has its centre in the same line with its 
 spring. 
 
 A segmental arch has its centre lower than the spring. 
 
 A horse-shoe arch has its centre above the spring. 
 
 Pointed arches are either, equilateral — described from two 
 centres, which are the whole breadth of the arch from each 
 other, and form the arch about an equilateral triangle; or drop 
 arches, which have a radius shorter than the breadth of the arch, 
 and are described about an obtuse-angled triangle ; or lancet 
 arches, which have a radius longer than the breadth of the arch, 
 and are described about an acute-angled triangle. 
 
 All these pointed arches may be of the nature of segmental 
 arches, and have their centres below their spring. 
 
 Mixed arches are of three centres, which look nearly like 
 elliptical arches; or of four centres, commonly called the Tudor 
 
192 ARCHITECTURE. 
 
 Definitions. 
 
 arch; this is tlat for its span, and has two ot its centres in or 
 near the spring, and the other two far below it. 
 
 The ogee, or contrasted arch, has four centres; two in or near 
 the span, and two above it, and reversed. 
 
 The spaces included between the arch and a square formed 
 at the outside of it, are called spaudrells, and are often orna- 
 mented. 
 
 Windows are divided into lights by muUioris. 
 
 The ornaments of the divisions at the heads of windows, 
 &c. are called tracery. Tracery is either Jioxving, where the 
 lines branch out into leaves, arches, &c.; or perpendicular, 
 where the muUions are continued through in straight lines. 
 
 The horizontal divisions of windows, &.c. are called tran- 
 soms. 
 
 The parts of tracery are ornamented with small arches and 
 points, which is called feathering or foliation, and the small 
 arches cusps, and according to the number in immediate con- 
 nection, they are called trefoil, quatrefoil, cinqiiefoil, &c. for 
 which see the plate. 
 
 The cusps are sometimes again feathered, and this is called 
 dau ble fea the ring . 
 
 Tablets are small projecting mouldings, or strings, mostly 
 horizontal. 
 
 The tablet at the top, under the battlement, is called a cor- 
 nice, and that at the bottom a basement, under which is gene- 
 rally a thicker wall. 
 
 The tablet running round doors and windows, is called a 
 drip-stone, and if ornamented, a canopy. 
 
 Bands are either small strings round shafts, or a horizontal 
 line of square, round, Js.c. pannels, used to ornament towers, 
 spires, &.c. 
 
 Niches are small arches, mostly sunk in the wall, and orna- 
 mented often very richly with buttresses, canopies, &.c. 
 
 A corbel is an ornamented projection from the wall, to sup- 
 port an arch, niche. Sec. and is often a head or part of a figure 
 or animal. 
 
 A pinnacle is a small spire, jxenerallv square and ornamented, 
 which is usually placed on the tops of buttresses, both exter- 
 nal and interned. 
 
 The small bunches of foliage ornamenting canopies, pinna- 
 cle*, &.C. are called crockets. 
 
 The larger bunches on the top are called finials, and this 
 term is sometimes applied to the w hole pinnacle. 
 
 The seats for the dean, canons, Sec. in the choirs of colle- 
 giate churches, are called stalls. 
 
 The bishop's seat is called his throne. 
 
ARCHITECTURE. 133 
 
 Definitions. — Division of the subject. 
 
 The ornamental open work over the stalls, and in general 
 any minute ornamental open work, is called tabernacle work. 
 
 In some churches, not collegiate, there yet remains a screen, 
 with a large projection at the top, between the nave and chan- 
 cel, on which was anciently placed certain images; this was 
 called a rood loft. 
 
 Near the entrance door is sometimes found a small niche, 
 with a basin, which held, in catholic times, their holy water; 
 these are called stoups. 
 
 Near the altar, or at least where some altar has once been 
 placed, there is sometimes found another niche, distinguished 
 from the stoup by having a small hole at the bottom to carry 
 off the remains of the consecrated wine ; this is called ?i pis- 
 cina ; it is often double, with a place for the bread. 
 
 On the south side, at the east end of some churches, are 
 found stone walls, either one, two, or three; of which the uses 
 have been much contested. 
 
 Under several laro;e churches, and some few small ones, are 
 certain vaulted chapels, these are called crypts. 
 
 In order to render the comparison of the different styles 
 easy, we shall divide the description of each into the following 
 sections : 
 
 Doors, 
 
 Windows, 
 
 Arches, 
 
 Piers, 
 
 Buttresses, 
 
 Tablets, 
 
 Niches, and ornamental arches, or pannels. 
 
 Ornamental carvings. 
 
 Steeples; 
 
 And at the end of the styles will be noticed, in one series, 
 the sections of battlements and roofs. 
 
 We shall first give, at one view, the date of the styles, and 
 their most prominent distinctions, and then proceed to the 
 particular sections as described above, 
 
 1st, the Norman Style, which prevailed to the end of the 
 reign of Henry II, in 1189; distinguished by its arches being 
 generally semi-circular, and not pointed, with bold and rude 
 ornaments. This style seems to have commenced before the 
 conquest, but we have no remains really known to be more 
 tiian a very few years older. 
 
 2nd, the Early English Style, reaching to the end of the 
 reign of Edward I, in 1307; distinguished by pointed arches, 
 and long narrow windows, without mullions ; and a peculiar 
 toothed ornament, more fully described hereafter. 
 
134 ARCHITECTURE. 
 
 Norman doors. 
 
 3d, Decorated English, reaching to the end of the reign of 
 Edward III, in 1377, and perhaps from ten to fifteen years 
 longer. This style is distinguished by its large windows, 
 which have pointed arches divided by mullions, and the tra- 
 cery in flowing lines of circles, arches, &c. and not running 
 perpendicularly ; its ornaments rich, and very delicately carv- 
 ed ; and ornament used to very great extent, yet seldom 
 crowded. 
 
 Perpendicular English. This is the last style, and appears 
 to have been in use, though much debased, even perhaps as 
 far as to 1630 or 1640, but only in additions. Probably the 
 latest whole building is not later than Henry VIII. The 
 name clearly designates this style, for the mullions of the win- 
 dows, the ornamental pannelling, &c. run in perpendicular 
 lines, and form a complete distinction from the last style, and 
 the richer buildings are often so crowded with ornament, as 
 to destroy the beauty of the design. The carvings are gene- 
 rally very delicately executed. 
 
 It may be necessary to state, that though many writers speak 
 of Saxon buildings, those which they describe as such, are 
 either known to be Norman, or are so like them, that there is 
 no real distinction. But it is most likely, that in some obscure 
 country church tower, &c. some real Saxon work of a much 
 earlier date may exist; hitherto, however, none has been as- 
 certained to be of so great an age. 
 
 We shall now begin to trace 
 
 The First, or Norman Style. 
 Norman Doors. 
 
 There seems to have been a desire in the architects who 
 succeeded the Normans, to preserve the doors of their prede- 
 cessors, whence we have so many of these noble, though, in 
 most cases, rude efforts of skill, remaining. In many small 
 churches, where all has been swept away, to make room for 
 even perpendicular alterations, the Norman door has been suf- 
 fered to remain. They are varied, yet there is no prominent 
 distinction to make it necessary to subdivide them. The arch 
 is semi-circular, and the mode of increasing their richness, was 
 by increasing the number of bands of moulding, and of course 
 the depth of the arch. Shafts are often used, but not always, 
 and we find very frequently, in the same building, one door 
 with shafts, and one without. When shafts are used there is 
 commonly an impost moulding above them, before the arch 
 mouldings spring. These mouldings are generally much 
 ornamented, and the wave or zigzag ornament in some of its 
 
ARCHITECTURE. 135 
 
 Norman windows. 
 
 diversities, is almost universal, as is a large round moulding, 
 with the heads on the outer edge projecting their beaks over 
 this moulding. There are also mouldings with a series of 
 figures enclosed in a running ornament; and at one church at 
 York, these figures are the zodiacal signs. The exterior 
 moulding often goes down no lower than the spring of the 
 arch, thus forming an apparent dripstone, though it does not 
 project so as really to form one. The door is often square, 
 and the interval to the arch filled with mouldings. Amongst 
 the great variety of these doors in excellent preservation, it is 
 difficult to point out particulars, but Iffley church, near Ox- 
 ford, is perhaps the best specimen, as it contains three doors, 
 all of which are different ; and the south door is nearly unique, 
 from the flowers in its interior mouldings. South Ockenden 
 church, in Essex, has also a door of uncommon beauty of de- 
 sign and elegance of execution. Durham, Rochester, Wor- 
 cester, and Lincoln cathedrals, have also fine Norman doors. 
 In these doors, almost all the ornament is external, and the 
 inside often quite plain. 
 
 There does not appear to have been any double Norman 
 doors. 
 
 Norman Windows. 
 
 The windows, in this style, are diminutive doors as to their 
 ornaments, except that, in large buildings, shafts are more fre- 
 quent, and often with plain mouldings. The size of these win- 
 dows is generally small, seldom, except in very large buildings, 
 sc large as even a small door; there are no raullions; the 
 arch is semi-circular, and if the window is quite plain, gene- 
 rally sloped sides, either inside or out, or both ; the bottom 
 often nearly horizontal. The proportions of the Norman win- 
 dows are generally those of a door, and very rarely, if ever, 
 exceed two squares in height, of the exterior proportions, in- 
 cluding the ornaments. 
 
 The existing Norman windows are mostly in buildings re- 
 taining still the entire character of that style ; for in most they 
 have been taken out, and others of later styles put in, as at 
 Durham and many other cathedrals. 
 
 There are still remaining traces of a very few circular win- 
 dows of this style; the west window at Iffley was circular, but 
 it is taken out ; there is one in Canterbury cathedral, which 
 seems to be Norman ; and there is one undoubtedly Norman 
 at Barfreston, rendered additionally singular, by its being di- 
 vided by grotesque heads, and something like mullions, though 
 very rude, into eight parts. 
 
 There seems to have been little if any attempt at feathering 
 
136 ARCHITECTURE. 
 
 Norman arches, — piers 
 
 or foliating the heads of Norman doors or windows : but there 
 is a singular door in the cloisters at Chester, with a semi-cir- 
 cular head, that has an ornament of this kind, but from its 
 situation, and the alterations which have been made, it is un- 
 certain whether it is original or not. 
 
 Korrnan Arches. 
 
 The early Norman arches are semi-circular, and in many 
 instances this form of the arch seems to have continued to the 
 latest date, even when some of the parts were quite advanced 
 into the next style; of this the Temple church is a curious 
 instance; here are piers with some of the features of the next 
 style, and also pointed arches with a range of intersecting 
 arches, and over this the old round-headed Norman window. 
 But though the round arch thus continued to the very end of 
 the style, the introduction of pointed arches must have been 
 much earlier, for we find intersecting arches in buildings of 
 the purest Norman, and whoever constructed them, constructed 
 pointed arches : but it appears as if the round and pointed 
 arches were for nearly a century used indiscriminately, as was 
 most consonant to the necessities of the work, or the builder's 
 ideas. Kirkstall abbey has all its work exteriorly round arches, 
 but the nave has pointed arches in the interior. There are 
 some Norman arches so near a semi-circle as to be only just 
 perceptibly pointed, and still with the rudely carved Norman 
 ornaments. 
 
 There are are a few Norman arches of very curious shape, 
 being more than a semi-circle, or what is called a horse-shoe, 
 and in a few instances a double arch. These arches have 
 sometimes plain faces, but are much oftener ornamented with 
 the zigzag, and other ornaments peculiar to this style. 
 
 Norman Piers. 
 
 These are of four descriptions, 1st, The round massive co- 
 lumnar pier, which has sometimes a round, and sometimes a 
 square capital; they are generally plain, but sometimes orna- 
 mented with channels in various forms, some plain zigzag, 
 some like network, and some spiral. They are sometimes met 
 with but little more than two diameters high, and sometimes 
 are six or seven, and those with square-headed capitals are 
 generally the tallest. 
 
 2nd, A multangular pier, much less massive, is sometimes 
 used, generally octagonal, and commonly with an arch more 
 or less pointed. 
 
 3d, The common pier with shafts ; these have sometimes 
 plain capitals, but sometimes much ornamented with rude 
 
ARCHITECTURE. 13? 
 
 Norman buttresses — tablets — niches — ornaments. 
 
 foliage, and occasionally animals. The shafts are mostly set 
 in square recesses. 
 
 4th, A plain pier, with perfectly plain round arches in two 
 or three divisions. 
 
 In some cases, the shafts are divided by bands, but the in- 
 stances are very few. 
 
 Normmi Buttresses. 
 
 These require little description; they are plain broad faces, 
 with but small projection, often only a few inches, and often 
 running up only to the cornice tablet, and there finishing un- 
 der its projection. Sometimes they are finished with a plain 
 slope, and in a few instances are composed of several shafts. 
 Bands or tablets running along the walls, often run round the 
 buttresses. 
 
 Norma?} Tablets. 
 
 In treating of tablets, that which is usually called the cor- 
 nice is of the first consideration; this is frequently only a plain 
 face of parapet, of the same projection as the buttresses; but 
 under it there is often placed a row of blocks, sometimes 
 plain, sometimes carved in grotesque heads, and in some in- 
 stances the grotesque heads support small arches, in which case 
 it is called a corbel table. A plain string is also sometimes 
 used as a cornice. 
 
 The next most important tablet is the dripstone, or outer 
 moulding of windows and doors; this is sometimes undistin- 
 guished, but oftener a plain round or square string, frequently 
 continued horizontally from one window to another round the 
 buttresses. 
 
 The other tablets under windows, 8vc. are generally plain 
 slopes above or below a flat string. In the interior, and in 
 some instances in the exterior, these are much carved in the 
 various ornaments described hereafter. 
 
 Norman Niches, $jc. 
 
 These, if so they may be called, are a series of small arches 
 with round and often with intersecting arches, sometimes 
 without, but oftener with shafts. Some of these arches have 
 their mouldings much ornamented ; but very few, if any, ap- 
 pear to have been intended for statues. 
 
 Norman Ornaments. 
 
 The ornaments of this style consist principally of the differ- 
 ent kinds of carved mouldings surrounding doors and windows, 
 and used as tablets. The first and most frequent of these is the 
 6.— Vol. I. T 
 
138 ARCHITECTURE. 
 
 Norman steeples. 
 
 ziorzag or chevron moulding, which is generally used in great 
 profusion. The next most common on door mouldings, is the 
 beak-head moulding, consisting of a hollow and a large round; 
 in the hollow are placed heads of beasts or birds, whose tongues 
 or beaks encircle the round. After these come many varieties, 
 almost every specimen having some difference of composition; 
 a good collection of these may be seen in the Archaeologia, 
 and King's Munimenta Antiqua. 
 
 The capitals of piers and shafts are often very rudely carved 
 in various grotesque devices of animals and leaves ; but in all, 
 the design is rude, and the plants are unnatural. 
 
 Norman Steeples. 
 
 The Norman steeple was mostly a massive tower, seldom ris- 
 ing more than a square in height above the roof, and often not 
 so much. They are sometimes plain, but often ornamented by 
 plain or intersecting arches, and have generally the flat but- 
 tress, but that of St. Alban's runs into a round turret at each 
 corner of the upper stage; and at St. Peter's, Northampton, 
 there is a singular buttress of three parts of circles, but there 
 is some doubt if it is not an addition. It does not seem likely 
 we have any Norman spires, but there are some turrets crowned 
 with large pinnacles, which may be Norman — such is one at 
 Cleve in Gloucestershire, and one of the towers at the side of 
 the west front of Rochester cathedral. 
 
 Having gone through the parts, it remains to speak of the 
 general a])pearance of the Norman buildings, of which we have 
 very few, if any, remaining unaltered. Almost all the west 
 fronts, and many transept ends, have had new windows, but 
 some small churches remain nearly entire. These present ap- 
 pearances of great solidity, but not much beauty ; the exterior 
 doors being generally the best portion. But though heavy and 
 dark, from the smallness of the windows, some of the large 
 Norman edifices, when complete, were very magnificent. 
 Amply to show this, enough remains at Durham, Southwell, 
 Gloucester, Rochester, Sec. In imitating or restoring build- 
 ings of this style, the work should not be polished too highly, 
 as all the Norman work remaining, is, however difficult of ex- 
 ecution, still rude, and not finely polished. 
 
ARCHITECTURE. 139 
 
 Early English doors. 
 
 Of the Second, or Early English Style. 
 
 Earli/ Eiiglish Doors. 
 
 As the Norman doors may be said to be all of semi-circular 
 arches, these may be said to be all pointed, at least all the ex- 
 terior ornamented ones; for there are small interior doors of 
 this style with flat tops, and the sides of the top as it were 
 supported by a quarter circle from each side. The large doors 
 of this style are mostly double, the two being divided by either 
 one shaft or several clustered, and a quatrefoil or other ornament 
 over them. These doors are often as finely recessed as the Nor- 
 man, but the bands and shafts are more numerous, being smaller ; 
 and in the hollow mouldings they are frequently enriched with 
 the peculiar ornament of this style, a singular toothed projec- 
 tion, which, when well executed, has a fine effect. But although 
 this ornament is often used, (and sometimes a still higher en- 
 riched moulding or band of open work flowers,) there are 
 many doors of this style perfectly plain. Of this kind the 
 door of Christchurch, Hants, is a fine specimen. 
 
 The dripstone is generally clearly marked, and often small, 
 and supported by a head. In many doors, a trefoil and even 
 cinquefoil feathering is used, the points of which generally 
 finish with balls, roses, or some projecting ornament. The 
 principal moulding of these doors has generally an equilateral 
 arch, but from the depth and number of the mouldings, the 
 exterior becomes often nearly a semi-circle. In interiors, and 
 perhaps sometimes too in the exterior, there are instances of 
 doors with a trefoil-headed arch. The shafts attached to these 
 doors are generally round, but sometimes filleted, and they 
 generally, but not always, stand quite free in a hollow mould- 
 ing. They have a variety of capitals, many plain, but many 
 with delicate leaves running up and curling round under the 
 cap moulding, often looking like Ionic volutes. The bases 
 are various, but a plain round and fillet is often used, and the 
 reversed ogee sometimes introduced. All these mouldings are 
 cut with great boldness, the hollows form fine deep shadows, 
 and the rich bands of open-work leaves are as beautiful as at 
 any subsequent period, being sometimes entirely hollow, and 
 having no support but the attachment at the sides, and the 
 connection of the leaves themselves. Of these doors, though 
 they are not so numerous as the Norman, many still remain in 
 perfect preservation; York, Lincoln, Chichester, and Salis- 
 bury'-, have extremely fine ones; and Beverley minster one, of 
 which the mouldings are bolder than most of them. The door 
 of the transept at York, and those of the choir screen at 
 
40 AKCHITECTURE. 
 
 Earl V English wiiiiio» : 
 
 Lincoln, have bands of the richest execution — and there is a 
 fine double door at St. Cross. 
 
 There are many wooden doors, both of this style and the 
 Norman, which seem to be of the same age as the stone work, 
 and some very curiously ornamented with ramifications of 
 iron-work from the hinges, &c. 
 
 Early English Windows. 
 
 These are, almost universally, long, narrow, and lancet- 
 headed, generally without feathering, but in some instances 
 trefoiled. 
 
 From this single shape of windows, a variety of appearance 
 results from their combination. At Salisbury, one of the 
 earliest complete buildings remaining, there are combinations 
 of two, three, five, and seven. Where there are two there is 
 often a trefoil, quatrefoil, &c. between the heads; and in large 
 buildings, where there are three or more, they are often divid- 
 ed by so small a division as to seem the lights of a large win- 
 dow, but they are really separate windows, having their heads 
 formed from individual centres, and in general separate drip- 
 stones. This is the case even at Westminster, where they ap- 
 proach nearer to a division by mullions, from having a small 
 triangle pierced beside the quatrefoil, and a general dripstone 
 over all. In small buildings, these are generally plain, with 
 the slope of the opening considerable, and in some small cha- 
 pels the windows are very narrow and long. In larger build- 
 ings they are often ornamented with very long and slender 
 shafts, which are frequently banded. Most of our cathedrals 
 contain traces of windows of this character, but some, as at 
 Durham, have tracery added since their original erection. 
 Salisbury, Chichester, Lincoln, Beverley, and York, still re- 
 main pure and beautiful; at York north transept are windows 
 nearly fifty feet high, and about six or eight wide, which have 
 a very fine effect. Although the architects of this style work- 
 ed their ordinary windows thus plain, they bestowed much care 
 on their circles. Beverley minster, York, Durham. and Lincoln, 
 have all circles of this style peculiarly fine, and there may be 
 many others ; that of the south transept at York, usually called 
 the marygold window, is extremely rich, but the tracery of 
 the circles at Westminster is of a much later date. 
 
 There is in all the long windows of this style, one almost 
 universal distinction; from the straight side of the window 
 opening, if a shaft is added, it is mostly insular, and has sel- 
 dom any connection with this side, so as to break it into faces, 
 though the shafts are inserted into the sides of the doors, so 
 as to give great variety to the opening. 
 
ARCHITECTURE. 141 
 
 Earij English arches — piers. 
 
 At Westminster abbey, there are a series of windows above 
 those of the aisles, which are formed in spherical equilateral 
 triangles. 
 
 Early English Arches. 
 
 The window arch of this style being generally a lancet arch, 
 and some persons having considered the shape of the arch to 
 be a very distinguishing feature of the different styles, it may 
 be necessary in this place to say a few words on arches gene- 
 rally. If we examine with care the various remains of the dif- 
 ferent styles, we shall see no such constancy of arch as has 
 been apprehended ; for there are composition lancet arches, 
 used both at Henry the VII. 's chapel, Westminster, and at Bath; 
 and there are flat segmental arches in the early English part 
 of York ; and upon the whole it will appear, that the architect 
 was not confined to any particular description of arch. The 
 only arch precisely attached to one period, is the four-centered 
 arch, which does not appear in windows, &:c. if it does in the 
 composition of groins, before the perpendicular style. In 
 large buildings, the nave arches of the early English style 
 were often lancet, but in some large and many small ones, 
 they are flatter, some of one-third drop, and perhaps even 
 more, and sometimes pointed segmental. 
 
 At Canterbury, in the choir, are some curious pointed horse- 
 shoe arches, and perhaps, though not common, they may be 
 found in other places. 
 
 The architraves of the large arches of rich buildings are 
 now beautifully moulded like the doors, with rich, deep, hol- 
 low mouldings, often enriched with the toothed ornament. 
 Of this description, York transepts, and the nave and tran- 
 septs of Lincoln, are beautiful specimens ; Salisbury is worked 
 plainer, but not less really beautiful, and Westminster abbey 
 is (the nave at least) nearly plain, but with great boldness of 
 moulding. 
 
 The arches of the gallery in this style, are often with tre- 
 foiled heads, and the mouldings running round the trefoil, 
 even to the dripstone; Chester choir is a fine specimen, and 
 there are some beautiful plain arches of this description in 
 Winchester cathedral. 
 
 Early English Piers. 
 
 Of the piers of large buildings of this style, there are two 
 distinguishing marks ; first, the almost constant division of the 
 shafts which compose them, by one or more bands in their 
 length, and secondly, their being ranged circularly round the 
 centre. In general they are few, sometimes only four, some- 
 
142 ARCHITECTURE. 
 
 Early Eoe;lisli buKresses 
 
 times eight, set round a large circular one; such are the piers 
 of" Salisbury and of Westminster abbey ; there are sometimes 
 so many as nearly to hide the centre shaft, as at Lincoln and 
 York; but the circular arrangement is still preserved, and 
 there are some few, as at the choir at Chester, which come so 
 near the appearance of decorated piers, as to be almost alone 
 distinguishable by this circular arrangement. 
 
 The capitals of these shafts are various; in many, perhaps 
 the greater number of buildings, they are plain, consisting of 
 a bell with a single or double annulet under it, and a sort of 
 coping, with more annulets above, and these mouldings are 
 continued round the centre pier, so as to form a general capital. 
 The dividing bands are also formed of annulets and fillets, and 
 are often continued under windows. Sec. as tablets, and are, 
 like the capitals, continued round the centre shaft. Another 
 and richer capital is sometimes used, which has leaves like 
 those in the capitals of the door shafts. This kind of capital 
 is generally used where the shafts entirely encompass the 
 centre one, as at York and Lincoln, and has a very fine effect, 
 the leaves being often very well executed. The bases used are 
 frequently near approaches in contour to the Grecian attic 
 base, but the reversed ogee is sometimes used. There is an- 
 other sort of pier in buildings that appear to be of this style, 
 ■which is at times very confusing, as the same kind of pier 
 seems to be used in small churches even to a very late date ; 
 this is the plain multangular (generally octagonal) pier with 
 a plain capital, of a few very simple mouldings, and with a 
 plain sloped arch. Piers of this description are very frequent, 
 and it requires great nicety of observation and discrimination 
 to refer them to their proper date ; but a minute examination 
 will often, by some small matter, detect their age, though it 
 is impossible to describe the minutiae without many figures. 
 
 Early English Buttresses. 
 
 These are of four descriptions. 1st, The old Norman flat 
 buttress is often used, but it is not always as broad, and its 
 tablets, &c. are more delicate. 
 
 2nd, A buttress not so broad as the flat one, but nearly of 
 the same projection as breadth, and carried up, sometimes 
 with only one set-off", and sometimes without any, and these 
 have often their edges chamfered from the window tablet. 
 They sometimes have a shaft at the comer, and in large rich 
 buildings are occasionally pannelled. 
 
 3d, A long slender buttress, of narrow face, and great pro- 
 jection in few stages, is used in some towers, but is not very 
 common. 
 
ARCHITECTURE. 143 
 
 Early English tablets — niches. 
 
 4th, Towards the latter part of this style, the buttress in 
 stages was used, but it is not very common, and is sufficiently 
 distinguished by its triangular head, the usual finish of this 
 style, which can hardly be called a pinnacle, though some- 
 times it slopes off from the front to a point. 
 
 Early English Tablets. 
 
 The cornice is now become sometimes rich in mouldings, 
 and often with an upper slope, making the face of the parapet 
 perpendicular to the wall below; there are cornices of this 
 style still resembling the Norman projecting parapet, but they 
 consist of several mouldings. The hollow moulding of the 
 cornice is generally plain, seldom containing flowers or carv- 
 ings, but under the mouldings there is often a series of small 
 arches resembling the corbel table. 
 
 The dripstone of this style is various, sometimes of several 
 mouldings, sometimes only a round with a small hollow. It 
 is, in the interior, occasionally ornamented with the toothed 
 ornament, and in a few late instances, as the interior of the 
 choir at Westminster, with flowers. In a few buildings, the 
 dripstone is returned, and runs as a tablet along the walls. 
 It is in general narrow, and generally supported by a corbel, 
 either of a head or a flower, &.c. There are frequently, in large 
 buildings, in the ornamented parts, bands of trefoils, quatre- 
 foils, &.C. some of them very rich. Although a sort of straight 
 canopy is used over some of the niches of this style, yet it 
 does not appear to have been used over windows or doors. 
 In some few buildings where they are found, they appear to 
 be additions. The tablets forming the base mouldings are 
 sometimes a mere slope, at others, in large buildings, are of 
 several sets of mouldings, each face projecting farther than 
 the one above it ; but the reversed ogee is very seldom used, 
 at least at large and singly. 
 
 Early English Niches. 
 
 The most important niches are those found in chancels, in 
 the walls of the south side, and of which the uses do not yet 
 appear to be decided. Of these there are many of all stages 
 of Early English; there are sometimes two, but oftener three, 
 and they are generally sunk in the wall, and adapted for a seat, 
 the easternmost one often higher in the seat than the others. 
 They are sometimes a plain trefoil head, and sometimes orna- 
 mented with shafts, &.C.; they are generally straight-sided. 
 The statuary niches, and ornamental interior niches, mostly 
 consist of a series of arches, some of them slope-sided, and 
 some with a small but not very visible pedestal for the statue. 
 
144 ARCHITECTURE. 
 
 Early En^isb ornaments — steeple- 
 
 They are often grouped two under one arch, with an ornamen- 
 tal opening between tlie small arches, and the large one like 
 the double doors ; a straight-sided canopy is sometimes used, 
 and a plain finial. These niches, except the chancel stalls, 
 and the stoup and piscina, are seldom single, except in but- 
 tresses, but mostly in ranges. 
 
 £ar/y EfigUsh Ornaments. 
 
 The first ornament to be described, is that already noticed 
 as the peculiar distinction of this style, to which it seems 
 nearly, if not exclusively confined ; it is the regular progression 
 from the Norman zigzag to the delicate four-leaved flowers so 
 common in Decorated English buildings. Like the zigzag it 
 is generally straight-sided, and not round like the leaves of a 
 flower, though, at a distance in front, it looks much like a small 
 flower. It is very difficult to describe it, and still more so to 
 draw it accurately; it may perhaps be understood by consider- 
 ing it a succession of small open p^Tamids of four legs, which 
 are formed of half a cube, and set on the edges of a hollow 
 moulding. This ornament is used very profusely in the build- 
 ings of this style, in Yorkshire and Lincolnshire, and frequently 
 in those of other counties. 
 
 Another ornament, which, though not peculiar, in small 
 works, to this style, was seldom but during its continuance 
 practised to so large an extent; this is the filling of the spaces 
 above the choir arches with squares, enclosing four-leaved 
 flowers. This is done at Westminster and at Chichester, in 
 both of which the workmanship is extremely good, and it has 
 a very rich effect. 
 
 In many parts, as in the spandrells of door arches, and other 
 plain spaces, circles filled with trefoils and quatrefoils, with 
 flowered points, are often introduced. In the early part of the 
 style, crockets were not used, and the finial was a plain bunch 
 of three or more leaves, or sometimes only a sort of knob; but 
 in small rich works, towards the end of the style, the beauti- 
 ful finials and crockets of the next style were used. 
 
 Early English Steeples. 
 
 The Norman towers were short and thick, the Early English 
 rose to a much greater height, and on the tower they placed 
 that beautiful addition the spire. 
 
 Some of our finest spires are of this age, and the proportions 
 observed between the tower and spire, are generally very good. 
 Salisbury, which stands unrivalled in height and beauty, and 
 Chichester, are of this age, as are the towers of Lincoln minster. 
 Wakefield has a fine steeple, as to proportion, though plain, and 
 
ARCHITECTURE 145 
 
 Early English steeples. — Decorated English doors. 
 
 it is singular for its machicolations, in the top of the tower. 
 The towers are flanked by octagonal turrets, square flat but- 
 tresses, or, in a few instances, with small long buttresses, and 
 generally there is one large octagonal pinnacle at tlie corners, 
 or a collection of smaller niches, ^c. There often is no para- 
 pet, but the slope of the spire runs down to the edge of the wail 
 of the tower, and finishes there with a tablet; and there is a 
 double slope to connect the corners with the intermediate faces. 
 The spire is often ornamented by ribs at the angles, sometimes 
 yi'ith crockets on the ribs, and bands of squares, filled with 
 quatrefoils, &c. surrounding the spire at different heights. 
 There are many good spires of this style in country churches. 
 
 Of this style we have the great advantage of one building 
 remaining, worked in its best manner, of great size and in ex- 
 cellent preservation ; this is Salisbury, which cathedral gives 
 a very high idea of improvement on the Norman style: magni- 
 ficent without rudeness, and rich tliough simple, it is one uni- 
 form whole. The west front is ornamented, but by no means 
 loaded, and the appearance of the north side is perhaps equ?il 
 to that of any cathedral in England. The west front of Lin- 
 coln is fine, but the old Norman space is too visible not to 
 break it into parts. Peterborough and Ely have perhaps the 
 most ornamented fronts of this style. Westminster is spoiled 
 by additions, but its north transept end is fine, as are both 
 the transepts of York. Interiorly, after the simple Salisbury, 
 the transepts of York are perhaps the best specimens, though 
 there are parts of many other buildings deserving much at- 
 tention. 
 
 Not much has been done in either restoring or imitating thi.s 
 style ; it is certainly not easy to do either well, but it deserves 
 attention, as in many places it would be peculiarly appropriate, 
 and perhaps is better fiUed than any for small country churches. 
 It may be worked almost entirely plain, yet if ornament is used, 
 it should be well executed; for the ornaments of this style are 
 in general as well executed as any of later date, and the toothed 
 ornament and hollow bands equal, in difficulty of execution, the 
 most elaborate perpendicular ornaments. 
 
 Of the Third, or Decorated English Style> 
 
 Decorated English Doors. 
 
 The large doors of the last style are mostly double, and 
 there are some fine ones of this, but they are not so common, 
 there being more single doors, which are often nearly as large 
 as the Early English double ones, and indeed but for the orna- 
 ments they are much alike, having shafts and fine hollow 
 7.— Vol. I. U 
 
146 ARCHITECTURE. 
 
 Decoratfil Etitrllsli doors 
 
 mouldiiiii's; in small doors there are often no shafts at all, but 
 the arch mouldings run down the side, and often almost to the 
 ground without a base. The shafts do not in this style generally 
 stand free, but are parts of the sweep of mouldings, and instead 
 of being cut and set up lengthways, all the mouldings and 
 shafts are cut on the arch stone, thus combining great strength 
 with all the appearance of lightness. The capitals of these 
 shafts differ from the Early English, in being formed of a 
 woven foliage, and not upright leaves; this, in small shafts, 
 generally has an apparent neck, but in larger ones often ap- 
 ])ears like a round ball of open foliage. The bases to these 
 shafts mostly consist of the reversed ogee, but other mouldings 
 are often added, and the ogee often made in faces. Although 
 the doors in general are not so deeply recessed, as the Nor- 
 man and Earl}^ English, yet in many large buildings they are 
 very deep. The west doors of York, and the later west doors 
 of Beverley, are of the richest execution, and very deep. To 
 the open work bands of the last style, succeeds an ornament 
 equally beautiful, and not so fragile ; this is the flowery hol- 
 low moulding; there are often three or four in one door-way, 
 and to the toothed ornament succeeds a flower of four leaves, 
 in a deep moulding, with considerable intervals between. This 
 flower, in some buildings, is used in great profusion to good 
 efl'ect, and a perforated ball in other buildings in equal abun- 
 dance. Over these doors, there are several sorts of canopies; 
 the dripstone is generally supported by a corbel, which is 
 commonly a head ; in some instances a plain return is used, 
 but that return seldom runs horizontally. The canopy is some- 
 times connected with the dripstone, and sometimes distinct. 
 The common canopy is a triangle, the space between it and 
 the dripstone is fdled with tracery, and the exterior ornament- 
 ed with crockets, and crowned with a finial. On the side of 
 the doors, small buttresses or niches are sometimes placed. 
 The second canopy is the ogee, which runs about half up the 
 dripstone, and then is turned the contrary way, and is finished 
 in a straight line running up into a finial. This has its inter- 
 mediate space filled with tracery, &c. and is sometimes crock- 
 eted and sometimes not. Another sort of canopy is an arch 
 running over the door, and unconnected with it, which is 
 doubly foliated; it has a good effect, but is not common. 
 
 In small churches, there are often nearly plain doors, hav- 
 ing only a dripstone and a round moidding on the interior 
 edge, and the rest of the wall a straight line or bold hollow, 
 and in some instances a straight slope side only. In some 
 doors of this style, a series of niches with statues are carried 
 up like a hollow moulding ; and in others, doubly foliated 
 
ARCHITECTURE. 147 
 
 Decorated English windows. — Two descriptions of tracery. 
 
 tracery hanging free from one of the outer mouldings, give 
 a richness superior to any other decoration. The south door 
 of the choir at Lincoln is perhaps hardly any where equalled 
 of the first kind ; and the west doors at Beverley are good 
 illustrations of the other. 
 
 : Decorated English Windoivs. 
 
 In these the clearest marks of the style ace to be found, 
 and they are very various, yet all on one principle : an arch is 
 divided by one or more muUions, into two or more lights, and 
 these muUions branch into tracery of various figures, but do 
 not run in perpendicular lines through the head. In small 
 churches, windows of two or three lights are common, but in 
 larger four and five lights for the aisles and clerestory windows, 
 five or six for transepts and the end of aisles, and in the east 
 and west windows seven, eight, and even nine lights, are used. 
 Nine lights seem to be the extent, but there may be windows 
 of this style containing more. The west window of York, and 
 the east window of Lincoln cathedrals, are of eight lights each ; 
 the west window of Exeter cathedral is of nine, and these are 
 nearly, if not quite, the largest windows remaining. 
 
 There may be observed two descriptions of tracery, and 
 although, in different parts, they may have been worked at the 
 same time, yet the first is generally the oldest. In this first 
 division, the figures, such as circles, trefoils, quatrefoils, &c. 
 are all worked with the same moulding, and sometimes do not 
 regularly join each other, but touch only at points. This may 
 be called geometrical tracery; of this description are the win- 
 dows of the nave of York, the eastern choir of Lincoln, and 
 some of the tracery in the cloisters at Westminster abbey, as 
 well as most of the windows at Exeter, which contains, per- 
 haps, the richest variety of windows of any cathedral in Eng- 
 land, and some of them are of such admirable workmanship as 
 to almost belong to the second division. 
 
 The second division consists of what may be truly called 
 flowing tracery. Of this description, York minster, the min- 
 ster, and St. Mary's, at Beverley, Newark church, and many 
 northern churches, as well some southern churches, contain 
 most beautiful specimens. The one engraved is from the west 
 end of the south aisle of Newark, and is perhaps one of the 
 most beautiful in its composition. The great west window 
 at York is, perhaps, the most elaborate. In these windows, 
 various wheels are sometimes introduced. In the richer 
 windows of this style, and in both divisions, the principal 
 moulding of the mullion has sometimes a capital and base. 
 
148 ARCHITECTURE. 
 
 Decornted Ent;lisli windows. 
 
 and tlius becomes a shiift. One great cause of the beauty 
 of fine riowing tracery, is the intricacy and delicacy of the 
 mouldings ; tlie principal moulding often running up only one 
 or two muUions, and forming only a part of the larger design, 
 and all the small figures being formed in mouldings, which 
 spring from the sides of the priiicipal. This is a distinction 
 the plate was too small to admit, which takes much from the 
 beauty of the window. The architraves of windows of this 
 style are now much ornamented with mouldings, which are 
 sometimes made into shafts. The dripstones and canopies of 
 windows are the same as in the doors, and have been described 
 under that head. Wherever windows of this style remain, an 
 artist should copy them ; the varieties are much greater than 
 might be supposed, for it is very difficult to find two alike in 
 dinerent buildings. 
 
 It does not appear that the straight horizontal transom was 
 much if at all used in windows of this style ; wherever it is 
 found there is generally some mark of the window originating 
 after the introduction of the perpendicular style ; but it may 
 have been used in some places, and there are a very few 
 instances of a light being divided in height by a kind of 
 canopy, or a quatrefoil breaking the mullion ; the church of 
 Dorchester, in Oxfordshire, has some very curious windows 
 of this kind. In some counties, where flint and chalk are 
 used, the dripstone is sometimes omitted. The heads of the 
 windows of this style are most commonly the equilateral 
 aich ; though there are many examples both of lancet and 
 drop arches ; but the lancet arches are not very sharp, perhaps 
 never exceeding one-third of the equilateral. There are a 
 few windows of this style with square heads ; but they are 
 not very common. 
 
 The circles of this style are some of them very fine ; there 
 are some very good ones in composition at Exeter and Chi- 
 chester, and the east window of old St. Paul's was a very fine 
 one; but perhaps the richest remaining is that of the south 
 transept at Lincoln, which is completely flowing. 
 
 Towards the end of this style, and perhaps after the com- 
 mencement of the next, we find windows of most beautiful 
 icomposition, with parts like the perpendicular windows, and 
 sometimes a building has one end decorated, the other per- 
 pendicular: such is Melrose abbey, whose windows have been 
 extremely fine, and, indeed, the great east window of York, 
 which is the finest perpendicular window in England, has 
 still some traces of flowing lines in its head. 
 
ARCHITECTURE. 149 
 
 Decorated English arches — piers — buttresses. 
 
 Decorated English Arches. 
 
 Though the arch most commonly used for general purposes 
 in this style is the equilateral one, yet this is by no means con- 
 stant. At York this arch is used, but at Ely a drop arch. The 
 architrave mouldings of interior arches do not differ much from 
 those of the last style, except that they are, perhaps, more fre- 
 quently continued down the pier without being stopt at the 
 line of capitals. The dripstones are of delicate mouldings, 
 generally supported by heads. The arches of the galleries are 
 often beautifully ornamented with foliated heads, and often 
 fine canopies ; and in these arches the ogee arch is sometimes 
 used, as it is freely in composition in the heads of windows. 
 
 Of this style, or perhaps of the next, is that singular yet 
 beautiful reversed arch in the nave of Wells' cathedral. 
 
 Decorated English Piers. 
 
 A new disposition of shafts marks very decidedly this style 
 in large buildings, they being arranged diamondwise, with 
 straight sides, often containing as many shafts as will stand 
 close to each other at the capital, and only a fillet or small 
 hollow between them. The shaft which runs up to support 
 the roof, often springs from a rich corbel between the outer 
 architrave mouldings of the arches ; Exeter is a fine example. 
 The capitals and bases of these shafts are much the same as 
 those decribed in the section on doors. Another pier of the 
 richest effect, but seldom executed, is that at York minster, 
 where the centre shaft is larger than those on each side, and 
 the three all run through the spring of the roof. Three also 
 support the side of the arch ; these shafts are larger in propor- 
 tion than those of Exeter, Sec. and stand close without any 
 moulding between. 
 
 Another pier, common towards the end of this style, and the 
 beginning of the next, is composed of four shafts, about two- 
 fifths engaged, and a fillet and bold hollow half as large as the 
 shafts between each; this makes a very light and beautiful 
 pier, and is much used in smaller churches. All these kinds 
 of piers have their shafts sometimes filleted, as are also often 
 some of the architrave mouldings. In small countrv churches, 
 the multangular flat-faced pier seems to have been used. 
 
 Decorated E)iglish Buttresses. 
 
 These, though very various, are all more or less worked in 
 stages, and the set-off's variously ornamented, some plain, 
 some moulded slopes, some with triangular heads, and some 
 with pannels ; some with niches in them, and with all the 
 
150 ARCHITECTURE. 
 
 Decorated English t:il>let!> — niches. 
 
 various degrees of ornament. The corner buttresses of this 
 style are often set diagonally. In some few instances small 
 turrets are used as buttresses. The buttresses are variously 
 finished, some slope under the cornice, some just through it; 
 some run up through the battlement, and are finished with 
 pinnacles of various kinds. 
 
 Decorated English Tablets. 
 
 The cornice is very regular, and though in some large build- 
 ings it has several mouldings, it principally consists of a slope 
 above, and a deep sunk hollow, with an astragal under it ; in 
 these hollows, flowers at regular distances are often placed, 
 and in some large buildings, and in tow'ers, 8cc. there are fre- 
 quently heads, and the cornice almost filled with them. The 
 dripstone is of the same description of mouldings, but smaller, 
 and this too is sometimes enriched with flowers. The small 
 tablet running under the window has nearly the same mould- 
 ings, but mostly without the astragal, and this sometimes 
 runs round the buttress also. The dripstone very seldom, if 
 ever, runs horizontally, though m a few instances a return is 
 used instead of the more common corbel head or shield. 
 
 The basement tablets are sometimes numerous, and often 
 have the reversed ogee repeated. 
 
 Decorated English Niches. 
 
 These form one of the greatest beauties of the style, and 
 are very various, but may be divided into two grand divisions, 
 which, if necessary, might be again variously divided, such 
 is their diversity, but these two may be sufiicieut. The first 
 are pannelled niches, the fronts of whose canopies are even 
 with the face of the wall or buttress they are set in. These 
 have their interiors either square with a sloping side, or are 
 regular semi-hexagons, &.c. In the first case, if not very 
 deep, the roof is a plain arch, but in the latter case the 
 roof is often most delicately groined, and sometimes a little 
 shaft is set in the angles or the ribs of the roof, supported 
 by small corbels. The pedestals are often high and much 
 ornamented. 
 
 The other division of niches have projecting canopies ; these 
 are of various shapes, some conical like a spire, some like 
 several triangular canopies joined at the edges, and some with 
 ogee heads ; and in some very rich buildings are niches w ith 
 the canopy bending forwards in a slight ogee, as well as its 
 contour being ogee ; these are generally crowned with very 
 large rich finials, and very highly enriched. There were also, 
 at the latter part of this style, some instances of the niche w ith 
 
ARCHITECTURE. 151 
 
 Decorated English ornaments — steeples. 
 
 a flat-headed canopy, which became so common in the next 
 style. These projecting niches have all some projecting base, 
 either a large corbel, or a basement pedestal carried up from 
 the next projecting face below. All these niches are occasion- 
 ally flanked by small buttresses, and their pinnacles ; those of 
 the first kind have very often beautiful shafts. 
 
 The chancel stalls of this style, are many of them uncom- 
 monly rich, their whole faces being often covered with orna- 
 mental carving. 
 
 Decorated English Ornaments. 
 
 As the word decorated is used to designate this style, and 
 particularly as the next is often called florid, as if it were 
 richer in ornament than this, it will be necessary to state, 
 that though ornament is often profusely used in this style, 
 yet these ornaments are like Grecian enrichments, and may 
 be left out without destroying the grand design of the build- 
 ing, while the ornaments of the next are more often a minute 
 division of parts of the building, as pannels, buttresses, 8cc. 
 rather than the carved ornaments used in this style. In 
 some of the more magnificent works, a variety of flowered 
 carvings are used all over, and yet the building does not 
 appear overloaded ; while some of the later perpendicular 
 buildings have much less flowered carvings, yet look over- 
 loaded with ornaments, from the fatiguing recurrence of 
 minute parts, which prevent the general design being com- 
 prehended. 
 
 The tomb of the Percys at Beverley, and one or two at York, 
 are as rich as can well be conceived in ornamental carvings, 
 yet the general design is noble, and may be clearly understood, 
 while the design of Henry the VII. 's chapel can hardly be 
 comprehended, from the constant repetition of the same orna- 
 ments, which, if worked singly, are not very rich. 
 
 The flower of four leaves in a hollow moulding, has already 
 been spoken of, and in these hollow mouldings various other 
 flowers are introduced, as well as heads and figures, some of 
 them very grotesque ; and as to capitals, there are very seldom 
 found two alike. The foliage forming the crockets and finials 
 is also extremely rich, and the pinnacle, in its various forms, 
 is almost constantly used. The spandrells of ornamental 
 arches are sometimes filled with beautiful foliage, perhaps few 
 superior to some in the church at Ely, which was the lady 
 chapel of the cathedral. 
 
 Decorated English Steeples. 
 
 Of this style are many of these beautiful ornaments of the 
 
152 AUCllITECTLRE. 
 
 Decorated EaiglisL steeples. 
 
 country; at the commencement of it, several fine spires were 
 added to towers then existing, and in after times many very 
 fine towers and spires were erected. Grantham and several 
 other Lincohishire spires are very fine, and there are many 
 good towers. These are generally flanked with buttresses, 
 many of which are diagonal, and are generally crowned with 
 fine pinnacles. Perhaps the church of St. ISIichael, at Co- 
 ventry, is as elegant a spire and tower as any of this age, and 
 is curious, from the spire standing on a lantern above the 
 tower. In Lincolnshire and some of the adjoining counties, 
 there are many village churches with fine spires, and some of 
 this style ; of these, perhaps few, if any, exceed in beauty of 
 proportion and delicacy of composition that of Norton, a vil- 
 lage in Leicestershire, a few miles to the left of the road from 
 Uppingham to Leicester. The singular crowned steeple of 
 St. Nicholas, at Newcastle upon Tyne, is either of this style 
 or early in the next. 
 
 There are many of the towers of this age whose windows, 
 or at least the mullions, seem to have been renewed in the 
 perpendicular style, and indeed, in small churches, it is not 
 always easy precisely to fix the style of the tower because of 
 these alterations. 
 
 Although they have some appearances of the windows which 
 belong to the next style, yet to this age should be referred the 
 towers of York minster, which possess imcommon beauty. 
 
 Though we have not the advantage of any one large build- 
 ing of this style in its pure state, like Salisbury, yet we have 
 the advantage of four most beautiful models, which are in the 
 highest preservation, besides many detached parts. These 
 are at Lincoln, Exeter, York, and Ely, and though differently 
 worked, are all of excellent execution. Of these, Exeter and 
 York are far the largest, and York, from the uncommon gran- 
 deur and simplicity of the design, is certainly the finest; orna- 
 ment is no where spared, yet there is a simplicity which is 
 peculiarly pleasing. Amongst the many smaller churches. 
 Trinity church at Hull deserves peculiar notice, as its deco- 
 rated part is of a character which could better than any be 
 imitated in modern work, from the great height of its piers, 
 and the smallness of their size. The remains of Melrose ab- 
 bey are extremely rich, and, though in ruins, its parts are yet 
 very distinguishable. In imitations of this style, great deli- 
 cacy is required to prevent its running into the next, which, 
 from its straight perpendicular and horizontal lines, is so 
 much easier worked ; whatever ornaments are used, should be 
 very cleanly executed, and highly finished. 
 
ARCHITECTURE. 153 
 
 Perpendicular English doors — windows. 
 
 Of the Fourth, or Perpendicular Style. 
 
 Perpendicular English Doors. 
 
 An impression from an engraving of a perpendicular door 
 having been given on the cover of several numbers of this 
 work, our readers must, by this time, be well acquainted with it. 
 A copy is annexed, for the purpose of permanent reference, 
 with the other plates. It has been drawn to convey as dis- 
 tinct an idea as possible of the character of the generality of 
 these doors, the great distinction of which, from those of the 
 last style, is the almost constant square head over the arch, 
 which is surrounded by the outer moulding of the architrave, 
 and the spandrell filled with some ornament, and over all a 
 dripstone is generally placed. This ornamented spandrell in 
 a square head, occurs in the porch to Westminster Hall, one 
 of the earliest perpendicular buildings, and is continued to the 
 latest period of good execution, and in a rough way much 
 later. In large very rich doors, a canopy is sometimes includ- 
 ed in this square head, and sometimes niches are added at the 
 sides, as at King's college chapel, Cambridge. This square 
 head is not always used interiorly, for an ogee canopy is some- 
 times used, or pannels down to the arch, as at St. George's, 
 Windsor ; and there may be some small exterior side doors, 
 without the square head, but they are not common. The shafts 
 used in these doors are small, and have plain capitals, which 
 are often octagonal, and the bases made so below the first as- 
 tragal. It is also very common for the architrave to consist of 
 oo-ee moulding-s, as well as the rounds and hollows which have 
 been before used. 
 
 Perpendicular English Windows. 
 These are easily distinguished by their mullions running in 
 perpendicular lines, and the transoms, which are now general. 
 The varieties of the last style were in the disposition of the 
 principal lines of the tracery ; in this, they are rather in the 
 disposition of the minute parts, a window of four or more 
 lights is gene-rally divided into two or three parts by stronger 
 mullions running quite up, and the portion of arch between 
 them doubled, from the centre of the side division. In large 
 windows, the centre one is again sometimes made an arch, and 
 often in windows of seven or nine lights, the arches spring 
 across, making two of four or five lights, and the centre be- 
 longing to each. The heads of windows, instead of being 
 filled with flowing ramifications, have slender mullions running 
 from the heads of the lights, between each principal mullion, 
 and these have small transoms till the window is divided into 
 7.— Vol. I. X 
 
154 ARCHITECTURE. 
 
 I'crpendicular English window: 
 
 a series of small pannels; arrd the heads being arched, are tre- 
 foiled or cinquefoiled ; sometimes these small muUions are 
 crossed over each other in small arches, leaving minute qua- 
 trefoils, and these are carried across in straight lines. Under 
 the transom is generally an arch of some kind, but in York- 
 shire, Lincolnshire, and Nottinghamshire, and perhaps in some 
 other parts, there is a different mode of foliating the straight 
 line without an arch, which has a singular appearance, (see 
 plate I.) In the later windows of this style, the transoms are 
 often ornamented with small battlements, which, when well 
 executed, have a very fine effect. Amidst so great a variety 
 of windows, (for perhaps full half the windows in English 
 edifices over the kingdom are of this style,) it is difficult which 
 to notice; but Windsor, St. George's, for four lights, and the 
 clerestory windows of Henry the VII, 's chapel for five, are some 
 of the best executed ; for a large window, the east window of 
 York has no equal, and by taking its parts, almost any sized 
 window may be formed. There are some good windows, of 
 which the heads have the mullions alternate, that is, the per- 
 pendicular line rises from the top of the arch of the pannel 
 below it. The windows of the Abbey church, at Bath, are of 
 this description. 
 
 It is necessary here to say a little of a window which may be 
 mistaken for a decorated window : this is one of three lights, 
 used in many country churches, the mullions simply cross each 
 other, and are cinquefoiled in the heads, and quatrefoiled in 
 the three upper spaces ; but to distinguish this from a decorated 
 window, it will generally be necessary to examine its arch, its 
 mullion mouldings, and its dripstone, as well as its being (as 
 it often is) accompanied by a clearly perpendicular window at 
 the end, or connected with it so as to be evidently of that time. 
 Its arch is very often four-centred, which at once decides its 
 date ; its mullion mouldings are often small, and very delicately 
 worked ; its dripstone often has some clear mark, and when the 
 decorated tracery is become familiar, it will be distinguished 
 by its being a mere foliation of a space, and not a flowing 
 quatrefoil with the mouldings carried round it. 
 
 Large circular windows do not appear to have been in use 
 in this style; but the tracery of the circles in the transepts of 
 Westminster abbey appear to have been renewed during this 
 period. At Henry the VII. 's chapel, a window is used in the 
 aisles, which seem to have led the way to that wretched sub- 
 stitute for fine tracery, the square-headed windows of queen 
 Elizabeth and king James the First's time. This window is a 
 series of small pannels forming a square head, and it is not flat, 
 but in projections, and these, with the octagonal towers used 
 
ARCHITECTURE. 155 
 
 Perpendicular English arches — piers. 
 
 for buttresses, throw the exterior of the building into fritter, ill- 
 assorting with the richness of the clerestory windows. In most 
 of the later buildings of this style, the window and its archi- 
 trave completely fills up the space between the buttresses, and 
 the east and west windows are often very large ; the west win- 
 dow of St. George's, Windsor, has fifteen lights in three divi- 
 sions, and is a grand series of pannels, from the floor to the 
 roof; the door is amongst the lower ones, and all above the 
 next to the door is pierced for the window. The east window 
 at Gloucester is also very large, but that is of three distinct 
 parts, not in the same line of plan. 
 
 When canopies are used, which is not so often as in the 
 last style, they are generally of the ogee character, beautifully 
 crocketed. 
 
 Perpendicular English Arches. 
 
 Although the four-centred arch is much used, particularly 
 in the latter part of the style, yet, as in all the other styles, we 
 have in this also arches of almost all sorts amongst the orna- 
 mental parts of niches, 8cc. and in the composition lines of 
 pannels, are arches from a very fine thin lancet to an almost 
 flat segment. Yet, with all this variety, the four-centred arch 
 is the one most used in large buildings, and the arches of other 
 characters, used in the division of the aisles, begin to have 
 what is one of the great distinctions of this style, — the almost 
 constant use of mouldings running from the base all round the 
 arch, without any stop horizontally, by way of capital, some- 
 times with one shaft and capital, and the rest of the lines run- 
 ning. The shafts in front running up without stop to the roof, 
 and from their capitals springing the groins. In window arches, 
 shafts are now very seldom used, the architrave running all 
 round, and both window arches and the arches of the interior 
 are often enclosed in squares, with ornamented spandrels, either 
 like the doors, or of pannelling. Interior arches have now sel- 
 dom any dripstone when the square is used, but at Bath there 
 is a clear dripstone distinct from the arch mouldings. 
 
 Another great distinction of these arches, in large buildings, 
 is the absence of the triforium or gallery, between the arches of 
 the nave and the clerestory windows; their place is now sup- 
 plied by pannels, as at St. George's, Windsor, or statuary 
 niches, as at Henry the VII. 's chapel ; or they are entirely 
 removed, as at Bath, and Manchester Old church, &c. 
 
 Perpendicular English Piers. 
 
 The massive Norman round pier, lessened in size and ex- 
 tended in length, with shafts set round it, became the Early Eng- 
 
156 ARCHITECTURE. 
 
 PtTpfiidicular English piers — buttresses. 
 
 lish pier; the shafts were multiplied and set into the face of 
 the pier, which became, in its plan, lozenge, and formed the 
 decorated pier; we now ftnd the pier again altering in shape, 
 becoming much thinner between the arches, and its proportion 
 the other way, from the nave to the aisle, increased by having 
 those shafts which run to the roof, to support the springings 
 of the groins, added in front, and not forming a part of the 
 mouldings of the arch, but having a bold hollow between them : 
 this is particularly apparent at King's college chapel, Cam- 
 bridge, St. George's, Windsor, and Henry the VII. 's chapel, 
 the three great models of enriched perpendicular style; but 
 it is observable in a less degree in many others. In small 
 churches, the pier mentioned in the last style, of four shafts 
 and four hollows, is still much used ; but many small churches 
 havehumbleimitationsof the magnificent arrangement of shafts 
 and mouldings spoken of above. There are still some plain 
 octagonal, 6cc. piers, in small churches, which may belong to 
 this age. 
 
 Though filleted shafts are not so much used as in the last 
 style, the exterior moulding of the architrave of interior arches 
 is sometimes a filleted round, which has a good effect ; and in 
 general the mouldings and parts of piers, architraves, &,c. are 
 much smaller than those used in the last style. 
 
 Perpendicular English Buttresses. 
 
 These differ very little from those of the last style, except 
 that triangular heads to the stages are much less used, the set- 
 ofl^s being much more often bold projections of plain slopes ; 
 yet many fine buildings have the triangular heads. In the up- 
 per story, the buttresses are often very thin, and of diagonal 
 faces. There are few large buildings of this style without tlying 
 buttresses, and these are often pierced ; at Henry VII. 's chapel 
 they are of rich tracery, and the buttresses are octagonal turrets. 
 At King's college chapel, Cambridge, which has only one height 
 within, the projection of the buttresses is so great as to allow 
 chapels between the wall of the nave and another level with the 
 front of the buttresses. At Gloucester, and perhaps at some 
 other places, an arch or half arch is pierced in the lower part 
 of the buttress. There are a few buildings of this style without 
 any buttresses. All the kinds are occasionally ornamented with 
 statuary niches, and canopies of various descriptions, and the 
 diagonal corner buttress is not so common as in the last style ; 
 but the two buttresses often leave a square, which runs up, 
 and sometimes, as at the tower of the Old church at Manches- 
 ter, is crowned with a third pinnacle. 
 
 Although pinnacles are used very freely in this style, yet 
 
ARCHITECTURE. 157 
 
 Perpendicular English tablets — niches. 
 
 there are some buildings whose buttresses run up and finish 
 square without any ; of this description is St. George's, Wind- 
 sor. The buttresses of the small eastern addition at Peter- 
 borough cathedral are curious, having statues of saints for 
 pinnacles. 
 
 In interior ornaments, the buttresses used are sometimes 
 small octagons, sometimes pannelled, sometimes plain, and 
 then, as well as the small buttresses of niches, are often band- 
 ed with a band different from the Early English, and much 
 broader. Such are the buttresses between the doors of Henry 
 the VII.'s chapel. 
 
 Perpendicular English Tablets. 
 
 The cornice is now, in large buildings, often composed of 
 many small mouldings, sometimes divided by one or two consi- 
 derable hollows, not very deep ; yet still, in plain buildings, the 
 old cornice mouldings are much adhered to ; but it is oftener 
 ornamented in the hollow with flowers, &c. and sometimes 
 with grotesque animals ; of this the churches of Gresford and 
 Mold, in Flintshire, are curious examples, being a complete 
 chase of cats, rats, mice, dogs, and a variety of imaginary 
 figures, amongst which various grotesque monkeys are very 
 conspicuous. In the latter end of the style, something very 
 analogous to an ornamented frieze is perceived, of which the 
 canopies to the niches, in various works, are examples ; and the 
 angels so profusely introduced, in the later rich works, are a 
 sort of cornice ornaments. These are very conspicuous at St. 
 George's, Windsor, and Henry the VII.'s chapel. At Bath is a 
 cornice of two hollows, and a round between with fillets, both 
 upper and under surface alike. The dripstone of this style is, 
 in the heads of doors and some windows, much the same as 
 in the last style, and it most generally finishes by a plain re- 
 turn; though corbels are sometimes used, this return is fre- 
 quently continued horizontally. Sometimes a much smaller 
 dripstone is used, of only a round and hollow. 
 
 Tablets under the windows are like this last or other drip- 
 stone, and sometimes fine bands are carried round as tablets. 
 Of these there are some fine remains at the cathedral, and at 
 the tower of St John's, Chester. 
 
 The basement mouldings ordinarily used are not materially 
 different from the last style ; reversed ogees and hollows, vari- 
 ously disposed, being the principal mouldings. 
 
 Perpendicular English Niches. 
 
 These are very numerous, as amongst them we must include 
 nearly all the stall, tabernacle, and screen work, in the English 
 
158 ARCHITECTURE. 
 
 I'erpendicular English niches — ornajnent*. 
 
 churches; for there appears little if any wood-work of an older 
 date, and it is probable that much screen work was defaced at 
 the reformation, and restored in queen JNIary's time, and not 
 attain destroyed, at least the execution of much of it would 
 lead to such a supposition, being very full of minute tracery, 
 and much attempt at stiffly ornamented friezes. Many niches 
 are simple recesses, with rich ogee canopies, and others have 
 overhanging square-headed canopies, with many minute but- 
 tresses and ninnacles, crowned with battlements; or, in the 
 latter part of the style, with what has been called the Tudor 
 flower, an ornament used instead of battlement, as an upper 
 finish, and profusely strewed over the roofs, &c. of the richer 
 later buildings. Of these niches, those in Henry the VII.'s cha- 
 pel, between the arches and clerestory windows, are perhaps 
 as good a specimen as any. Of the plain recesses, with ogee 
 canopies, there are some fine ones at Windsor. 
 
 The whole interior of the richer buildings of this style, is 
 more or less a series of pannels, and therefore, as every pan- 
 nel may, on occasion, become a niche, we find great variety 
 of shape and size ; but like those of the last style, they may 
 generally be reduced to one or other of these divisions. 
 
 Perpendicular English Ornaments. 
 
 The grand source of ornament, in this style, is pannelling ; 
 indeed, the interior of most rich buildings is only a general 
 series of it ; for example. King's college chapel, Cambridge, 
 is all pannel except the floor ; for the doors and windows are 
 nothing but pierced pannels included in the general design, 
 and the very roof is a series of them of different shapes. The 
 same may be said of the interior of St. George's, Windsor, 
 and still further, Henry the VII.'s chapel is so both within and 
 without, there being no plain wall all over the chapel, except 
 just the exterior below the base moulding, all above is orna- 
 mental pannel. All the small chapels of late erection, in this 
 style, such as those of Bishop Fox at Winchester, and several 
 in \\ indsor, are thus all pierced pannel. Exclusive of this 
 general source of ornament, there are a few peculiar to it: 
 one, the battlement to transoms of windows, has already been 
 mentioned ; this, in works of late date, is very frequent, some- 
 times extending to small transoms in the head of the window, 
 as well as the general division of the lights. Another, the 
 Tudor flower, is, in rich work, equally common, and forms a 
 most beautiful enriched battlement, and is also sometimes 
 used on the transoms of windows in small work. Another 
 peculiar ornament of this style, is the angel cornice, used at 
 Windsor and Henry the VII.'s chapel; but though according 
 
ARCHITECTURE. 159 
 
 Perpeudicular English steeples. — Miscellaneons remarks. 
 
 Avitli the character of those buildings, it is by no means fit 
 for general use. These angels have been much dilfused, as 
 supporters of shields, and as corbels to support roof beams, 
 &c. Plain as the Abbey-church at Bath is in its general 
 execution, it has a variety of angels as corbels, for different 
 ])urposes. 
 
 Flowers of various kinds continue to ornament cornices, &c. 
 and crockets were variously formed towards the end of the 
 style, those of pinnacles were often very much projected, which 
 has a disagreeable effect; there are many of these pinnacles at 
 Oxford, principally worked in the decline of the style. 
 
 Perpendicular English Steeples. 
 
 Of these there remain specimens of almost every descrip- 
 tion, from the plain short tower of a country church, to the 
 elaborate and gorgeous towers of Gloucester and Wrexham. 
 There are various fine spires of this style, which have little 
 distinction from those of the last, but their age may be gene- 
 rally known by their ornaments, or the towers supporting them. 
 Almost every conceivable variation of buttress, battlement, 
 and pinnacle, is used, and the appearance of many of the 
 towers combines, in a very eminent degree, extraordinary 
 richness of execution and grandeur of design. Few counties 
 in England are without some good examples; besides the two 
 already mentioned, Boston in Lincolnshire, All-Saints in 
 Derby, St. Mary's at Taunton, St. George's, Doncaster, are 
 celebrated; and the plain, but excellently proportioned, tower 
 of Magdalen college, Oxford, deserves much attention. 
 
 Amongst the smaller churches, there are many towers of 
 uncommon beauty, but few exceed Gresford, between Chester 
 and Wrexham; indeed, the whole of this church, both interior 
 and exterior, is worth attentive examination. Paunton, near 
 Grantham, has also a tower curious for its excellent masonry. 
 There are of this style some small churches with fine octa- 
 gonal lanterns, of which description are two in the city of 
 York. 
 
 Miscellaneous Remarks on Perpendicular Buildings. 
 
 Of this style are so many buildings in the finest preserva- 
 tion, that it is difficult to select; but, on various accounts, 
 several claim particular mention. The choir at York is one of 
 the earliest buildings ; indeed it is, in general arrangements, 
 like the nave, but its ornamental parts, the gallery under the 
 windows, the windows themselves, and much of its pannelling 
 in the interior, are completely of perpendicular character, 
 though the simple nobility of the piers is the same as the 
 
ICO ARCHITECTURE. 
 
 Battlements in general. 
 
 nave. The choir of Gloucester is also of this style, and most 
 completely so, for the whole interior is one series of open- 
 work pannels laid on the Norman work, parts of which are cut 
 awav to receive them ; it forms a very ornamental whole, but 
 bv lio means a model for imitation. 
 
 Of the later character, are three most beautiful specimens. 
 Kind's college chapel, Cambridge, Henry the VII. 's chapel, 
 and St. George's, Windsor ; in these, richness of ornament is 
 lavished on every part, and they are particularly valuable for 
 being extremely different from each other, though in many re- 
 spects alike. Of these, undoubtedly St. George's, Windsor, 
 is the most valuable, from the great variety of composition 
 arising from its plan; but the roof and single line of wall, of 
 Kincr's college chapel, Cambridge, deserves great attention, 
 and the details of Henry the VI I. 's chapel will always com- 
 mand it, from the great delicacy of their execution. 
 
 Of small churches, there are many excellent models for imi- 
 tation, so that in this style, with some care and examination, 
 nothing hardly need be executed but from absolute authority. 
 The monumental chapels of this style are peculiarly deser\"ing 
 attention, and often of the most elaborate workmanship. 
 
 Of Battlements. 
 
 Having now gone through the styles in detail, we come to 
 those sections which, as before mentioned, it is necessary to 
 give in a connected series. 
 
 From exposure to weather and various accidents, we find 
 very few roofs in their original state, and from the vicinity of 
 the battlement, &c. we find these also are very often not origi- 
 nal. It seems difficult to ascertain what the Xorman battle- 
 ment was, and there seems much reason to suppose it was 
 only a plain parapet ; many Norman structureii have either 
 battlements evidently of later date, or parapets as evidently 
 mutilated, and in the larger buildings of the early English style, 
 the parapet continues mostly to be used. Perhaps some of the 
 earliest battlement is that at the west end of Salisbury cathedral, 
 plain, of nearly equal intervals, and with a plain capping mould- 
 ing; but it may be doubted if even this is original. An orna- 
 mented parapet continued to be used through the next style, 
 bat with the very frequent use of battlements of several sorts, 
 both plain and pierced ; and as these continued with less altera- 
 tion than many other parts, through the perpendicular style, it 
 will be better to describe them all together, just observing, that 
 a considerable degree of perpendicular pannelling prevails in 
 the battlements of the later edifices. The most frequent early 
 
ARCHITECTURE. 161 
 
 Ens;lish battlements — roofs. 
 
 pierced parapet, is a series of interchanged trefoils with a fine 
 serpentine line separating them; this has a fine effect, and is 
 mostly used in Decorated English buildings ; for in the Per- 
 pendicular, the dividing line is straight, making a series of 
 interchanged triangles. Of pierced battlements there are many 
 varieties, but the early ones have frequently quatrefoils, either 
 for the lower compartments, or on the top of the pannels of 
 the lower, to form the higher; the latter have often two heights 
 of pannels, one range for the lower, and another over them 
 forming the upper ; and at Loughborough is a fine battlement 
 of rich pierced quatrefoils, in two heights, forming an indented 
 battlement. These battlements have generally a running cap 
 moulding carried round, and generally following the line of 
 battlement. There are some few later buildings, which have 
 pierced battlements, not with straight tops, but variously or- 
 namented ; such is the tomb house at Windsor, with pointed 
 upper compartments, and such is the battlement of the eastern 
 addition at Peterborough, and the great battlement of King's 
 college chapel, Cambridge, and also that most delicate battle- 
 ment over the lower side chapels ; this is perhaps the most 
 elegant of the kind. Sometimes exteriorly, and often inte- 
 riorly, the Tudor flower is used as a battlement, and there are 
 a few instances of the use of a battlement analogous to it in 
 small w^orks long before ; such is that at Waltham cross. 
 
 Of plain battlements there are several descriptions : 1st, 
 that of nearly equal intervals, with a plain round capping 
 running round with the outline. 2nd, The castellated battle- 
 ment, of nearly equal intervals, and sometimes with large bat- 
 tlements and small intervals, with the cap moulding running 
 only horizontal, and the sides cut plain ; this is perhaps the 
 best in point of effect of any. 3d, A battlement like the last, 
 with the addition of a moulding which runs round the out- 
 line, and has the horizontal capping set upon it. 4th, The 
 most common later battlement, with the cap moulding broad, 
 of several mouldings, and running round the outline, and thus 
 often narrowing the intervals, and enlarging the battlement. 
 To one or other of these varieties, most battlements may be 
 reduced ; but they are never to be depended on alone, in 
 determining the age of a building, from the very frequent 
 alteration they are liable to. 
 
 Of Roofs. 
 
 Roofs may be conveniently divided into two principal divi- 
 sions ; the first including those in which the sloped framing, 
 carrying the lead or other covering, is visible ; and the second, 
 those which Jiave an inner roof of various materials. 
 7.- Vol. I. Y 
 
162 ARCHITECTURE. 
 
 English roofs. 
 
 It is difficult to say what were the open Norman roofs, but 
 it seems most likely they exposed the rafters and other fram- 
 iniT of the roof, and probably had straight beams laid across 
 the walls of the nave over each pier. If any original roofs of 
 this kind remain, Rochester cathedral seems most likely to be 
 one. The first attempt to ornament these roofs, seems to have 
 been to make a timber arch over each pier, and to frame tim- 
 bers in squares diasonally, and these are sometimes made into 
 quatrefoils, and afterwards the arch framing became variously 
 ornamented, till it came to the gorgeous hall roof, of which 
 there are many fine specimens, but perhaps few, if any, superior 
 to that of Westminster hall. The roof of St. Stephen's cha- 
 pel was of peculiar beauty, and that of the chapter-house at 
 Exeter, is much like it. From the piers, springs an arch which 
 is pierced in the spandrells, and richly ornamented with pierced 
 featherings ; and the sloping roof is constructed in small 
 squares, beautifully ornamented with quatrefoils. 
 
 There are buildings in which, though the upper roof is 
 shown, there is a preparation for an inner roof; such is Ches- 
 ter cathedral, where only the lady chapel, and the aisles of 
 the choir, are groined, and the whole of the rest of the church 
 is open; but on the top of the shafts are the commencement 
 springing of a stone groined roof. There is a chapel in a 
 church in Cambridgeshire, Willingham, between Ely and Cam- 
 bridge, which has a very singular roof; stone ribs rise like the 
 timber ones, the intervals are pierced, and the slope of the 
 roof is of stone ; it is high pitched, and the whole appears of 
 decorated character. 
 
 The second division, or inner roofs, are very various; from 
 history it seems as if the most early inner roof was flat over 
 the beams, and these were planked and painted, as at St. Al- 
 ban's and Peterborough. The latter is, indeed, a singularly 
 beautiful relic ; it has lately been repainted, as it was origi- 
 nally, an<l now presents an appearance of rich mozaic, like a 
 carpet full of stiff lines, and its general division is into lo- 
 zenges, with flowers of Norman character, and the whole ac- 
 cording in design with the ornaments of that style. This kind 
 of roof most likely contributed much, especially when the ex- 
 terior roof was covered with shingles, to spread those de- 
 structive fires which we so frequently read of in the history of 
 the early churches. There are some roofs of this construction, 
 in country churches, but generally of a very late date, and 
 sometimes painted with wretched attempts at clouds. 
 
 There does not seem to be any wooden inner roofs, except 
 plaster groining, now remaining*, till we come to the perpendi- 
 cular ordinary style of roofing, which was rich, though easily 
 
ARCHITECTURE. 163 
 
 English roofs. 
 
 constructed ; a rib crossed above the pier, with a small flat 
 arch, and this was crossed by another in the centre of the 
 nave, and the spaces thus formed were again divided by cross 
 ribs, till reduced to squares of two or three feet; and at each 
 intersection, a flower, shield, or other ornament, was placed. 
 This roof was sometimes in the aisles made sloping, and oc- 
 casionally coved. In a few instances, the squares were filled 
 with fans, Sec of small tracery. 
 
 The next and most important mode of interior roofing, is 
 the regular groined roof; and of this description we have a 
 regular and valuable series, from the plain Norman round 
 arched roof, to the elaborate pendanted roof of Henry the 
 VII.'s chapel. 
 
 • The various Norman crypts, and some small churches, give 
 very good specimens of the Norman roof, which has simply 
 four cross springers, often without straight ribs from the op- 
 posite piers. The cross springers were ornamented in the 
 usual manner with carvinss of zio-zasc and other Norman orna- 
 ments. The first pointed roof added nothing to the Norman, 
 in ribs, except the one from pier to pier. The ribs were often 
 enriched by the toothed ornament, and generally a boss or 
 knot at the centre intersections. Canterbury, some parts of 
 Lincoln, but above all, Salisbury cathedral, are admirable spe- 
 cimens of these roofs, which were erected mostly in the time 
 of the Early English style, or attached to buildings of that 
 date. 
 
 The next advance appears in the roof of the nave at Wells 
 cathedral; in this a plain rib runs longitudinally at the top, 
 crossing the rib from the piers, and also the intersection of 
 the cross springers, and another rib runs crossways at the top 
 of the window arches, crossing the centre intersection. To 
 this soon succeeded the multiplication of the ribs, which meet 
 the longitudinal straight cross rib, and at their intersections 
 have bosses. To this is added, in the richer roofs, short ribs 
 running from one of these bosses to another, and these are in- 
 creased in the later roofs, till the whole is one series of net 
 work, of which the roof of the choir at Gloucester, is one of 
 the most complicated specimens. The later monumental 
 chapels and statuary niches, mostly present in their roofs very 
 complicated tracery. 
 
 Of the ribbed roofs, which are rich without being gorgeous, 
 perhaps York minster exhibits a specimen not inferior to any 
 other. 
 
 We now come to a new and most delicate description of 
 roof, that oi fan tracery, of which probably the earliest, and 
 certainly one of the most elegant, is that of the cloistsrs at 
 
164 AftCHITECTL'RE. 
 
 En^Ubh roof:> 
 
 Glouctsttr. Ill these roofs, from the top of the shaft springs 
 a small fan of ribs, which doubling out from the points of the 
 pannels, ramify on the roof, and a quarter or half circular rib 
 forms the fan, and the lozenge interval is formed by some of 
 the ribs of the fan running through it, and dividing it into 
 portions, which are filled with ornament. King's college 
 chapel, Cambridge, Henry the VII. 's chapel, and the Abbey 
 church at Bath, are the best specimens, after the Gloucester 
 cloisters ; and to these may be added the aisles of St. George's, 
 Windsor, and that of the eastern addition to Peterborough. 
 To some of these roofs are attached pendants, which, in Henry 
 the VII.'s chapel, come down as low as the springing line of 
 the fans. 
 
 The roof of the nave and choir of St. George's, Windsor, is 
 very singular, and perhaps unique. The ordinary proportion 
 of the arches and piers is half the breadth of the nave ; this 
 makes the roof compartments two squares, but at Windsor the 
 breadth of the nave is nearly three times that of the aisles, and 
 this makes a figure of about three squares. The two exterior 
 parts are such as, if joined, would make a very rich ribbed 
 roof; and the centre compartment, which runs as a flat arch, 
 is filled with tracery pannels, of various shapes, ornamented 
 with quatrefoils, and forming two halves of a star; in the choir 
 the centre of the star is a pendant. This roof is certainly 
 the most singular, and perhaps the richest in effect of any 
 we have ; it is profusely adorned with bosses, containing 
 shields, Sec. 
 
 There still remains one more description of roof, which is 
 used in small chapels, but not common in large buildings ; 
 this is the arch roof; in a few instances it is found plain, with a 
 simple ornament at the spring and the point, and this is gene- 
 rally a hollow moulding with flowers, &.c. but it is mostly pan- 
 nelled. Of this roof the nave of Bath is a most beautiful spe- 
 cimen. The arch is very flat, and is composed of a series of 
 small rich pannels, with a few large ones at the centre of the 
 compartments formed by the piers. Another beautiful roof of 
 this kind is the porch to Henry the VII.'s chapel ; but this is 
 60 hid, from the want of light, as to be seldom noticed. 
 
 The ribbed roofs are often formed of timber and plaster, but 
 are generally coloured to represent stone work. 
 
 There may be some roofs of different arrangements from any 
 of these ; but in general they may be referred to one or other 
 of the above heads. 
 
ARCHITECTURE. 165 
 
 Rosljn chapel. — Alterations of ecclesiastical edifices. — Locality of style. 
 
 Miscellaneous Remarks on Buildings of English Architecture. 
 
 Having now given an outline of the details of the different 
 styles, it remains to speak of a few matters which could not 
 so well be previously noticed. As one style passed gradually 
 into another, there will be here and there buildings partaking 
 of two, and there are many buildings of this description whose 
 dates are not at all authenticated. Litchfield cathedral is a 
 fine instance of the gradation from richer Early English into 
 Decorated, as are some of those delicate monuments, the 
 crosses of Edward the I. and many of the Lincolnshire spires. 
 There are also many beautiful gradations from Decorated to 
 Perpendicular. Of these, the choir of York, and the upper 
 part of the two towers at the west end, and the remains of 
 Melrose abbey, may be mentioned. 
 
 There is one building which deserves especial mention, from 
 the singularity of its character, ornaments, and plan ; this is 
 Roslyn chapel. It is certainly unclassable as a whole, being 
 unlike any other building in Great Britain of its age, (the lat- 
 ter part of the fifteenth century,) but if its details are minutely 
 examined, they will be found to accord most completely in 
 the ornamental work with the style then prevalent, though 
 debased by the clumsiness of the parts, and their want of pro- 
 portion. There seems little doubt that the designer was a fo- 
 reigner, or at least took some foreign buildings for his model. 
 
 It will now be proper to add a few words on the alterations 
 and additions which most ecclesiastical edifices have received ; 
 and some practical remarks as to judging of their age. The 
 general alteration is that of windows, which is very frequent ; 
 very few churches are without some Perpendicular windows. 
 We may therefore pretty safely conclude that a building is at 
 least as old as its windows, or at least that part is so which 
 contains the windows. But we can by no means say so with 
 respect to doors, which are often left much older than the rest 
 of the building. 
 
 A locality of style may be observed in almost every county, 
 and in the districts where flint abounds, it is sometimes almost 
 impossible to determine the date of the churches, from the 
 absence of battlements, architraves, and buttresses; but 
 wherever stone is used, there is generally enough of indica- 
 tion to assign each part to its proper style, and with due 
 regard to do the same with plates of ordinary correctness, a 
 little habitual attention would enable many persons to judge 
 at once, at the sight of a plate or drawing, of its correctness, 
 from its consistency, or the contrary, with the details of its 
 apparent style. 
 
1G6 
 
 ARCHITECTURE. 
 
 Greciao and English architectare C0Dtra;>tcJ. 
 
 In a sketch like the present, where the Author is confined 
 to a certain space, it is impossible to notice every variety; but 
 at least he now presents the world, for the first time, with a 
 rational arrangement of the details of a mode of architecture 
 on many accounts valuable, and certainly the most proper for 
 ecclesiastical edifices. Still further to enable the reader to 
 distinguish the principles o-f Grecian and English architecture, 
 he adds a few striking contrasts, which are formed by those 
 principles in buildings of real purity, and which will at once 
 convince any unprejudiced mind of the impossibility of any 
 
 thing like a good mixture. 
 
 Grecian. 
 The general running lines 
 are horizontal. 
 
 Arches not necessary. 
 
 An entablature absolutely 
 necessary, consisting always 
 of two, and mostly of three 
 distinct parts, having a close 
 relation to, and its character 
 and ornaments determined by 
 the columns. 
 
 The columns can support 
 nothing but an entablature, 
 and no arch can spring directly 
 from a column. 
 
 A flat column maybe called 
 a pilaster, which can be used 
 as a column. 
 
 The arch must spring from 
 a horizontal line. 
 
 Columns the supporters of 
 the entablature. 
 
 English. 
 The general running lines 
 are perpendicular. 
 
 Arches a really fundamental 
 principle, and no pure English 
 building or ornament can be 
 composed without them. 
 
 No such thing as an entab- 
 lature composed of parts, and 
 what is called a cornice, bears 
 no real relation to the shafts 
 which may be in the same 
 buildins. 
 
 The shafts can only sup- 
 port an arched moulding, and 
 in no case a horizontal line. 
 
 Nothing analogous to a pi- 
 laster; every flat ornamented 
 projecting surface, is either a 
 series of pannels, or a buttress. 
 
 No horizontal line necessa- 
 ry, and never any but the small 
 cap of a shaft. 
 
 Shaft bears nothing, and is 
 only ornamental, and the round 
 pier still a pier. 
 
ARCHITECTURE. 
 
 167 
 
 Grecian and English architecture contrasted. 
 
 Grecian. 
 No projections like buttres- 
 ses, and all projections stopped 
 by horizontal lines. 
 
 Arrangement of pediment 
 fixed. 
 
 Openings limited by the 
 proportions of the colmnn. 
 
 Regularity of composition 
 on each side of a centre neces- 
 sary. 
 
 E7iglish. 
 Buttresses essential parts, 
 and stop all horizontal lines. 
 
 Pediment only an ornament- 
 ed end wall, and may be of 
 almost any pitch. 
 
 Openings almost unlimited. 
 
 Regularity of composition 
 seldom found, and variety of 
 ornament universal. 
 
 Cannot form good steeples, I From its perpendicular lines, 
 because they must resemble I may be carried to any practi- 
 unconnected buildings piled I cable height, with almost in- 
 on each other. 1 creasing beauty. 
 
 In the foregoing details we have said nothing of castellated 
 or domestic architecture ; because there does not appear to be 
 any remains of domestic buildings, so old as the latest period 
 of the English style, which are unaltered; and because the cas- 
 tellated remains are so uncertain in their dates, and so much 
 dilapidated or altered, to adapt them to modern modes of life 
 or defence, that little clear arrangement could be made, and a 
 careful study of ecclesiastical architecture will lead any one, 
 desirous to form some judgment of the character of these build- 
 ings, to the most accurate conclusions on the subject which 
 can well be obtained in their present state. 
 
 Description of Plate I. 
 
 In order that the plate may be kept free from figures, which 
 would have taken orf" greatly from its good effect, a descrip- 
 tion of what it contains is annexed without letters of reference. 
 And it may be necessary to state, that in order to comprise as 
 much as possible of the different styles in a small space, there 
 has been an unavoidable departure from the relative propor- 
 tion of parts in several instances, but not to any very great 
 extent. 
 
IGS ARCHITECTURE. 
 
 Desi-ription of Plate first. 
 
 The upi)er part of the plate contains sixteen detached speci- 
 mens, which, beginning at tlie left hand at the top, are as 
 follow : 
 
 1st, The plain semi-circular arch. 
 
 2nd, The segmental arch. 
 
 3rd, The equilateral arch. 
 
 4th, The drop arch. 
 
 oth, The lancet arch. 
 
 6th, The horse-shoe arch. 
 
 7th, The ogee or contrasted arch. 
 
 8th, The four-centred or Tudor arch. 
 
 9th, A plain circular trefoil window, with ornamented 
 points. 
 
 10th, A plain trefoil window head. 
 
 11th, A triangular pannel feathered in three divisions, each 
 of which is again feathered. 
 
 12th, The straight-headed cinquefoil, so much used in 
 small tracery of windows, in the eastern part of the king- 
 dom. 
 
 13th, A plain circular quatrefoil window. 
 
 14th, The usual mode of cinquefoiling an ogee head, and its 
 insertion in a transom. 
 
 loth, A very beautiful mode of double feathering a cinque- 
 foil, from the east window at York. 
 
 16th, A plain circular cinquefoil window. 
 
 The lower design is divided by buttresses into three com- 
 partments, of which the lower part of the centre contains two 
 Norman windows, one with shafts, the other with the zigzag 
 moulding, and having between them a Norman buttress, and a 
 Norman cornice over them. The upper part of this compart- 
 ment is Early English, having three windows with shafts divid- 
 ed by bands, and resting on a tablet. The pediment has a 
 projecting parapet with a cornice moulding under it, and a 
 cap moulding above. This parapet is ornamented with sunk 
 quatrefoils. 
 
 The left hand compartment, and its two buttresses, are De- 
 corated English. The window of flowing tracery is of the style 
 of that in the west end of Newark, but much of its delicate 
 small ornaments are obliged to be omitted, on account of its 
 size. The canopy is executed at Chester cathedral. The cor- 
 nice is the ordinary Decorated cornice with flowers. The bat- 
 tlement is that of the nave of York minster. The pinnacles 
 are of the description very common in the west of England, 
 with square bases and small battlements. 
 
 The right-hand compartment contains a perpendicular win- 
 dow of five lights in a four-centred arch, with a crock eted ogee 
 
^ABCinTKCTrRE . 
 
 PiibUshrd by Henry Fishci Caxton London 18.'." 
 
Architecttrx . 
 
 FL.ir. 
 
 PuliliEled \y flenrf Esher, Caiton. londoa. l222. 
 
ARCHITECTURE. 169 
 
 Plate second. — References to delineations of English architecture. 
 
 canopy. It is divided by a transom, and has the transom 
 moulding battlemented. This window is of simple construc- 
 tion, aiid is executed with slight variations in various parts. 
 
 The cornice is that of the upper roof of the Abbey-church 
 at Bath, and the battlements are also taken from thence ; but 
 from the small size of the plate, both are obliged to be much 
 simplified as to their mouldings. The pinnacles are the ordi- 
 nary pinnacles of this style. 
 
 Plate II. affords an example of the square-headed door of 
 the Perpendicular style ; it has a shaft supporting the inner 
 moulding of the arch, and another supporting the exterior 
 mouldings, which forms also the square, and both are included 
 in a hollow moulding. The dripstone is nearly the most fre- 
 quent plain dripstone, and the cornice shews the effect of the 
 introduction of flowers. 
 
 Plates of almost all the buildings mentioned in this sketch, 
 may be found in one or other of the following works, which 
 all contain plates of good character, though of course perhaps 
 not all of equal correctness in drawing the detail 
 
 The Cathedrals, &c. published by the Society of Antiquaries, 
 
 Carter's Ancient Architecture, 
 
 King's Munimenta Antiqua, 
 
 Halfpenny's Engravings of York Minster, 
 
 Buckler's Views of the Cathedrals, 
 
 Britton's Architectuial Antiquities, 
 
 Storer and Greig's Illustrations of Pennant's London, 
 
 Antiquarian and Topographical Cabinet, 
 
 Ancient Relics, 
 
 Lyson's Magna Brittanica, 
 
 Britton and Brayley-s Beauties of England, 
 
 Archaeologia, 
 
 Chalmer's History of Oxford. 
 
 8.— Vol. I. 
 
70 AUCHITIXTUUE. 
 
 Five or(ier:> of Grecian arcliiloclurc di.N(iii;;ui:>LL-(l. 
 
 GRECIAN ARCHITECTURE. 
 
 The many valuable treatises and excellent delineations of 
 the Grecian and Roman buildings, and the details of their 
 parts, will render unnecessary that minuteness in this disser- 
 tation, which, from the total absence of a previous system, it 
 became necessary to adopt in the description of the English 
 styles. But in this sketch something of a similar plan will be 
 followed, first giving the name and grand distinctions of the 
 orders, then describing the terms and names of parts necessary 
 for those who have not j)aid attention to the subject to under- 
 stand, and a concise description of each order will follow; but 
 from the diversity of judgment ))revailing, it will be most pro- 
 per to leave the reader to select his own examples, as in this 
 country we liave not, as in the English architecture, the origi- 
 nals to study, but a variety of copies, adapted to the climate 
 and the convenience of modern times. 
 
 In dividing the Grecian and Roman architecture, the word 
 order is used, and much more properly than style; the English 
 styles regarded not a few parts, but the composition of the 
 whole building; but a Grecian building is denominated Doric 
 or Ionic, merely from its ornaments ; and the number of co- 
 lumns, windows, Sec. may be the same in either order, only 
 varied in proportion. 
 
 The orders are generally considered to be five, and are usu- 
 ally enumerated as follows : 
 
 Tuscan, of which the usual height of the col. is 7 diameters. 
 
 Doric, . . - 8 
 
 Ionic, - - - 9 
 
 Corinthian, - - - 10 
 
 Composite, - - - 10 
 
 Their origin will be treated of hereafter. Their prominent 
 distinctions are as follow : 
 
 The Tuscan is quite plain, without any ornament whatever. 
 
 The Doric is distinguished by the channels and projecting 
 intervals in the frieze, called tiig/i/p/is. 
 
 The Ionic by the ornaments of its capital, which are spiral, 
 and are called volutes. 
 
 The Corinthian by the superior height of its capital, and its 
 being ornamented with leaves, which support very small 
 volutes. 
 
 The Composite has also a tall capital with leaves, but is dis- 
 tinguished from the Corinthian by having the large volutes of 
 the Ionic capital. 
 
ARCHITECTURE. 171 
 
 Divisiou of a complete order. 
 
 A complete order is divisible into three grand divisions, 
 which are occasionally executed separately, viz. 
 
 The coliouii, including its base and capital, 
 
 The pedestal, which supports the column. 
 
 The entablature, or part above and supported by the column. 
 
 These are again each subdivided into three parts. 
 
 The pedestal into base or lower mouldings; dindo or die, the 
 plain central space; and surbase or upper mouldings. 
 
 The column into base or lower mouldings, shaft or central 
 plain space, and capital or upper mouldings. 
 
 The entablature into architrave, or part immediately above 
 the column ; frieze or central flat space ; and cornice or upper 
 projecting mouldings. 
 
 These parts may be again divided thus : the lower portions, 
 viz. the base of pedestal, base of column, and architrave, 
 divide each into two parts; the first and second into plinth 
 and mouldings, the third into face or faces, and upper mould- 
 ing or tenia. 
 
 Each central portion, as dado of pedestal, shaft of column, 
 and frieze, is undivided. 
 
 Each upper portion, as surbase of pedestal, capital of column, 
 cornice of entablature, divides into three parts : the first into 
 bedmould, or the part under the corona; corona, or plain face; 
 and cymatium, or upper moulding. 
 
 The capital, into neck, or part below the ovolo ; ovolo or 
 projecting round moulding; and abacus or tile, the flat upper 
 moulding mostly nearly square. These divisions of the 
 capital, however, are less distinct than those of the other 
 parts. 
 
 The cornice into bedmould, or part below the corona ; corona, 
 or flat projecting face; cymatium, or moulding above the 
 corona. 
 
 Besides these general divisions, it will be proper to notice 
 a few terms often made use of. The ornamental moulding 
 running round an arch, or round doors and windows, is called 
 an architrave. 
 
 An ornamental moulding for an arch to spring from, is 
 called an impost. 
 
 The stone at the top of aa arch, which often projects, is 
 called a key-stone. 
 
 The small brackets vinder the corona in the cornices, are 
 called mutules or modillions ; if they are square, or longer in 
 front than in depth, they are called mutules, and are used in 
 the Doric order. If they are less in front than their depth, 
 they are called modillions, and in the Corinthian order have 
 carved leaves spread under them. 
 
172 ARCHITECTURE. 
 
 Aicbilcclural uiuuldings. 
 
 A truss is a modillion enlarged, and placed flat against 
 a wall, often used to support the cornice of doors and 
 windows. 
 
 A console is an ornament like a truss carved on a key-stone. 
 
 Trusses, when used under modillions in the frieze, are called 
 cantalivers. 
 
 The space under the corona of the cornice, is called a soffU, 
 as is also the under side of an arch. 
 
 Dentils are ornaments used in the bedmould of cornices ; 
 they are parts of a small flat face, which is cut perpendicu- 
 larly, and small intervals left between each. 
 
 A flat column is called o. pilaster; and those which are used 
 with columns, and have a diffprent capital, are called anta. 
 
 A small height of pannelling above the cornice, is called an 
 attic, and in these pannels, and sometimes in other parts, are 
 introduced small pillars, swelling towards the bottom, called 
 balustres, and a series of them a balustrade. 
 
 If the joints of the masonry are channelled, the work is called 
 rustic, which is often used as a basement for an order. 
 
 Columns are sometimes ornamented by channels, which are 
 coWed Jiutes. These channels are sometimes partly filled by a 
 lesser round moulding ; this is called cabling the flutes. 
 
 For the better understanding the description to be given oi 
 the orders, it will be proper first to notice the different mould- 
 ings, which by different combinations form their parts. 
 
 The most simple mouldings are, 1st, the Ovolo, or quarter 
 round, see pi. III. fig. 1. 
 
 2nd, The cavetto, or hollow, fig. 2. 
 
 3rd, The torus, or round, fig. 3. 
 
 From the composition of these are formed divers others, and 
 from the arrangement of them, with plain flat spaces between, 
 are formed cornices and other ornaments. A large flat space 
 is called a corona if in the cornice ; :i fascia in the architrave ; 
 and the frieze itself is only a flat space. A small flat face is 
 called a Jillct, or listel, and is interposed between mouldings 
 to divide them. 
 
 A fillet is, in the bases of columns and some other parts, 
 joined to a face, or to the column itself by a small hollow, 
 then called apop/ij/ges, fig. 4. 
 
 The torus, when very small, becomes an astragal, which 
 projects, fig. 5; or a bead, which does not project, fig. 6, 
 No. 1, 2. 
 
 Compound mouldings are, the ci/ma recta, which has the 
 hollow uppermost and project! no-, fig. 7. 
 
 The cijnia reversa, or ogee, which has the round uppermost 
 and projecting, fig. 8. 
 
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ARCHITECTURE. 173 
 
 Tuscan order. 
 
 The scoiia ; which is formed of two hollows, one over the 
 other, and of different centres, fig. 9. 
 
 In the Roman works, the mouldings are generally worked 
 of equal projection to the height, and not bolder than the 
 above regular forms ; but tlie Grecian mouldings are often 
 bolder, and worked with a small return, technically called a 
 quirk, and these are of various proportions. 
 
 The ogee and ovolo are most generally used as represented 
 by fig. 10 and 11. 
 
 Several beads placed together, or sunk in a flat face, are 
 called reedings, see fig. 12. 
 
 All these mouldings, except the fillet, may be occasionally 
 carved, and they are then called enriched mouldings. 
 
 From these few simple forms, by adding astragals and fillets, 
 and combining differently ornamented mouldings, faces, and 
 soffits, are all the cornices, pannels, &c. formed, and the mo- 
 dern compositions in joiners' work. Sec. are very numerous, 
 and too well known to need describing. 
 
 Tuscan Order. 
 
 Though this is not, perhaps, the most ancient of the orders, 
 yet, from its plainness and simplicity, it is usually first no- 
 ticed. Its origin is evidently Italian ; for the Grecian work, 
 however plain, has still some of the distinctive marks of mas- 
 sive Doric, whilst the Tuscan always bears clear marks of its 
 analogy to the Roman Doric. 
 
 The pedestal, when used, is very plain, but it is more often 
 set on a plain square block plinth, which suits the character 
 of the order better than the higher pedestal. This block pro- 
 jects about half the height of the plinth of the base beyond 
 its face. 
 
 The column, including the base and capital, is about seven 
 diameters high. The column, in this, and indeed in all the 
 orders, is diminished the upper two-thirds of its height, so 
 that the diameter under the neck of the capital is from one- 
 sixth to one-fourth less than that just above the base. This 
 diminution is not conical, but bounded by a curved line, 
 which is variously determined, but does not differ much from 
 what an even spring would assume, if one part of it were 
 bound, in the direction of the axis of the shaft, to the cylin- 
 drical third, and then, by pressure at the top only, brought to 
 the diminishing point. 
 
 The Tuscan base is half a diameter in height, and consists 
 of a plain torus with a fillet and apophyges, which last is part 
 
174 ARCHITECTLRE. 
 
 Tascao order. 
 
 of the shaft, and not of the base, as indeed all apophygae are 
 considered to be ; and also all the astragals underneath the 
 capitals, as well as the upper fillet of the base in all the 
 richer orders, and in masonry should be executed on the 
 shaft stones. 
 
 The capital of the Tuscan order is (exclusive of the astra- 
 gal) half a diameter in height, and consists of a neck on 
 which is an ovolo and fillet, joined to the neck by an apo- 
 phyges, and over the ovolo a square tile, which is ornamented 
 by a projecting fillet. 
 
 The shaft of this order is never fluted, but some architects 
 have given to this order, and some have even added to the 
 richer orders, an ornament (if so it may be called) of large 
 square blocks as parts of the shaft, which are called rustica- 
 tions, and are sometimes roughened. 
 
 The Tuscan entablature should be quite plain, having 
 neither mutules nor modillions. The architrave has one or 
 sometimes two faces, and a fillet ; the frieze quite plain, and 
 the cornice consisting of a plain cyma recta for cymatium, and 
 the corona with a plain fillet, and small channel for drip in the 
 soffit. The bedmould should consist of an ovolo fillet and 
 cavetto. 
 
 This Tuscan is that of Palladio; some other Italian archi- 
 tects have varied in parts, and some have given a sort of block 
 modillions like those used in Covent-Garden church ; but these 
 are of wood, and ought not to be imitated in stone. 
 
 This order is little used, and will most likely, in future, be 
 still less so, as the massive Grecian Doric is an order equally 
 manageable, and far more elegant. 
 
 Having explained the parts of one order, it will be necessary 
 to make a few remarks, which could not so well be previously 
 introduced. — If pilasters and columns are used together, and 
 they are of the same character, and not antae, the pilasters 
 should be diminished like the columns; but where pilasters 
 are used alone, they may be undiminished 
 
 The moulding under the -"ymatium, which, in rich orders, 
 is often an ogee, is part of the corona, and as such is continued 
 over the corona in the horizontal line of pediments, where the 
 cymatium is omitted ; and is also continued with the corona 
 in interior work, where the cymatium is often with propriety 
 omitted. 
 
 In pediments, whose cornices contain mutules, modillions, 
 or dentils, those in the rakino; cornice must be placed perpen- 
 dicularly over those in the horizontal cornice, and their sides 
 must be perpendicular, though their under parts have the rake 
 of the cornice. 
 
AKCHITKCTTTIE. 175 
 
 Grecian Doric. 
 
 Doric Order. 
 
 The ancient Grecian Doric appears to have been an order of 
 peculiar nobility, simple and bold, its ornaments were the re- 
 mains of real utility, and perhaps originallv it was worked with 
 no moulding but the cymatium, to cover the ends of the tiles, 
 its triglyphs being the ends of the beams, and its niutules 
 those of the rafters. In after times its proportions were made 
 rather less massive, and its mouldings and ornaments, though 
 not numerous, were very beautiful. The Romans coTisiderably 
 altered this order, and by the regulations they introduced, ren- 
 dered it peculiarly difficult to execute on large buildings. i\.s 
 the examples of the two countries are very different, we shall 
 treat of them separately, and first therefore of the 
 
 Grecian Doric. 
 
 The columns of this order were, in Greece, generally placed 
 on the floor, without pedestal and without base ; the capital, 
 which occupied a height of about half a diameter, had no as- 
 tragal, but a few plain fillets, with channels between them, 
 under the ovolo, and sometimes a small channel under the 
 fillets. The ovolo is generally flat, and of great projection, 
 with a quirk or return. On this was laid the abacus, which 
 was only a plain tile, without fillet or ornament. 
 
 In the division of the entablature, the architrave and frieze 
 have each more than a third in height, and the cornice less. 
 The architrave has only a plain broad fillet, under which are 
 placed the drops or guttse, which appear to hang from the tri- 
 glyphs. 
 
 The triglyph, in Greece, appears to have been generally 
 placed at the angle, thus bringing the interior edge of the tri- 
 glyph over the centre of the angular column. The metope, or 
 space between the triglyphs, being the square of the height of 
 the frieze, and a mutule was placed not only over each tri- 
 glyph, but also over each metope, and it appears probable that 
 the triglyphs were often omitted, except over the centre of 
 each column, and at the angle, and hence the order would be 
 easily worked at any desirable intercolumniation. The cornice 
 of this order, in Greece, consisted of a plain face, under the 
 mutule, which was measured as part of the frieze, and then the 
 mutule, which projected sloping forward under the corona; so 
 that the bottom of the mutule in front was considerably lower 
 than at the back, where, in some examples, an ogee is placed 
 under the mutule. Over the corona was commonly a small 
 ovolo and fillet, and then a larger ovolo and fillet for the 
 cymatium. 
 
176 ARCHITECTURE. 
 
 Roman Doric. 
 
 The ornaments of this order, in Greece, Avere, 1st, the flut- 
 ings of the column, which are peculiar to the order, and are 
 twenty in number, shallow, and not with fillets between them, 
 but sharp edges. These flutes are much less than a semicircle, 
 and should be elliptic. 
 
 2d, At the corner, in the space formed in the soffit of the 
 corona, by the interval between the two angular mutules, was 
 sometimes placed a flower, and the cymatium of the cornice 
 had lions' heads, which appear to have been real spouts. 
 
 3d, In addition to the drops under the triglyph, the mutules 
 also have several rows of drops of the same shape and size. 
 
 This order appears in general to have been worked very 
 massive, the best examples are about six diameters high, which 
 is lower than the Italians usually worked the Tuscan; but this 
 gave peculiar nobility to the temples in which it is thus em- 
 ployed. It does not appear that a pedestal was used in 
 Greece with this order. . 
 
 Roman Doric. 
 
 This differs from the Grecian in several important particu- 
 lars, which will appear from the following rules, from the 
 strictness of which follows that extreme difficulty of execu- 
 tion which has been so often complained of in this order: 1st, 
 The triglyphs must be precisely over the centre of the co- 
 lumns. 2d, the metopes must be exact squares ; 3d, the mu- 
 tules also must be exact squares. 
 
 As, therefore, the intercolumniation must be of a certain 
 number of tiiglyphs, it will be easily conceived how difficult 
 it will be, in large buildings, where a triglyph is several feet, 
 to accommodate this order to the internal arrangements. 
 • The Roman Doric is sometimes set on a plinth, and some- 
 times on a pedestal, which should be of few and plain mould- 
 ings. The bases usually employed, are either the attic base 
 of a plinth, lower torus, scotia, and upper torus, with fillets 
 between them, or the proper base of one torus and an astragal; 
 or, in some instances, of a plinth and simple fillet. The shaft, 
 including the base and capital, each of which is half a diame- 
 ter, is generally eight diameters high, and is fluted like the 
 Grecian. The capital has an astragal and neck under the 
 ovolo, which has sometimes three small fillets, projecting over 
 each other, and sometimes another astragal and fillet. The 
 ovolo should be a true quarter round. The abacus has a 
 8mall ogee and fillet on its upper edge. 
 
 The architrave has less height than the Grecian, being only 
 two-thirds of the frieze, which is equal in height to the cornice. 
 
ARCHITECTURE. 177 
 
 RomaB Doric. — Ionic order. 
 
 In a few instances the architrave has two faces, but mostly- 
 only one. 
 
 The frieze has nothing peculiar to this mode ; if plain, its 
 metopes being, as before observed, square. 
 
 The cornice differs much from the Grecian, having its soffit 
 flat, and the mutules square, with a square interval between 
 them. The Grecian drops in the mutules generally appear in 
 front, below the mutules ; but the Roman do not, and are 
 sometimes omitted. 
 
 The cymatium is often a cavetto, with an ogee under it, 
 and sometimes a cyma recta. The mutules have a small ogee, 
 which runs round them, and also round the face they are 
 formed of; and under the mutules are an ovolo and small fil- 
 let, and the flat fillet which runs round the top of the triglyphs 
 here belongs to the cornice, and not, as in the Grecian, to the 
 frieze. 
 
 In some instances, dentils appear to have been used in the 
 jDoric cornice at Rome, but there are not many examples of 
 this practice. 
 
 The Roman Doric is susceptible of much ornament, for in 
 addition to the flutes, the guttae of the triglyphs, and the roses 
 in the soffit of the corona, the neck of the capital has some- 
 times eight flowers or husks placed round it, the ovolo carved, 
 and the metopes in the frieze filled with alternate ox skulls 
 and paterae, or other ornaments. In interior decorations some- 
 times one or two of the mouldings of the cornice are enriched; 
 but with all this ornament, the Roman Doric is far inferior, in 
 real beauty, to the Grecian. 
 
 Ionic Order. 
 
 As the Greeks and Romans differed much in their modes of 
 working the Doric order, so there was considerable difference 
 in their execution of the Ionic, though by no means so great 
 as in the former. 
 
 The distinguishing feature of this order is the capital, which 
 has four spiral projections called volutes. These in Greece 
 were placed flat on the front and back of the column, leaving 
 the two sides of a different character, and forming a balustre ; 
 but this at the external angle producing a disagreeable effect, 
 an angular volute was sometimes placed there, shewing two 
 volutes to each exterior face, and a balustre to each interior ; 
 but this not forming a good combination, a capital was in- 
 vented with four angular volutes, and the abacus with its sides 
 hollowed out. This is called the modern Ionic capital. In 
 the ancient, the list or spiral line of the volute runs along the 
 face of the abacus, straight under the ogee ; but in the 
 8.— Vol. I. 2 A 
 
178 ARCHITECTURE. 
 
 Ionic order. 
 
 modern, this list springs from behind the ovolo, and in the 
 hollow of the abacus, which is an ovolo, fillet, and cavelto, is 
 generally placed a flower. 
 
 The Ionic shaft, including the base, which is half a diameter, 
 and the capital to the bottom of the volute generally a little 
 more, is nine diameters high. 
 
 The pedestal is a little taller and more ornamented than the 
 Doric. 
 
 The bases used to this order are very various ; some of the 
 Grecian examples are of one torus and two scotia, with astra- 
 gals and fillets; others of two large tori and a scotia of small 
 projection ; but the attic base is very often used, and with an 
 astragal added above the upper torus, makes a beautiful and 
 appropriate base for the Ionic. 
 
 The cornices of this order may be divided into three divi- 
 sions ; 1st, the plain Grecian cornice ; 2nd, the dentil cornice; 
 3d, the modillion cornice. 
 
 In the first, the architrave is often of one face, the frieze 
 plain, and the cornice composed of a corona with a deep soffit, 
 and the bedmould moulding hidden by the drip of the soffit, or 
 coming very little below it, and sometimes with a plain dentil 
 set close under the corona. The cymatium generally a cyma 
 recta, and ogee under it. 
 
 The second, which was mostly used with the ancient capital 
 at Rome, has generally two faces in the architrave, and the 
 cornice, which is rather more than one-third of the height of 
 the entablature, has a corona with a cyma recta and ogee for 
 cymatium, and for bedmould a dentil face between an ovolo 
 and ogee. The soffit of the corona sometimes ornamented. 
 
 The third, or modillion entablature, has the same architrave, 
 frieze, and cymatium of its cornice, as the last, but under the 
 soffit of the corona are placed modillions, which are plain and 
 surrounded by a small ogee; one must be placed over the 
 centre of each column, and one being close to the return makes 
 a square pannel in the soffit at the corner, and between each 
 modillion, which is often filled with a tiower. The bedmould 
 below is generally an ovolo, fillet, and cavetto. 
 
 It was once the custom to work the Ionic frieze projecting 
 like a torus, thus giving an awkward weight to an order which 
 ought to be light. The introduction of good Grecian models 
 has driven out this impropriety, and much improved the 
 present execution of the order, which is very beautiful, if well 
 executed. 
 
 The Ionic shaft may be fluted in twenty-four flutes, with 
 fillets between them; these flutes are semi-circular. This 
 order may be much ornamented if necessary, by carving the 
 
ARCHITECTURE. 179 
 
 Corinthian order. 
 
 ovolo of the capital, the ogee of the abacus, and one or two 
 mouldings of both architrave and cornice ; but the ancient 
 Ionic will look extremely well without any ornament what- 
 ever. 
 
 Corinthian Urder. 
 This order originated in Greece, and the capital is said to 
 have been suggested by observing a tile placed on a basket 
 left in a garden, and round which sprung up an acanthus. All 
 the other orders have, in various countries and situations, 
 much variety ; but the Corinthian, though not without slight 
 variations even in the antique, is much more settled in its pro- 
 portions, and its greater or less enrichment is the principal 
 source of variety. , -^ x • -Ui. 
 
 The capital is the great distinction of this order, its height 
 is more than a diameter, and consists of an astragal, fillet, and 
 apophyo-es, all which are measured with the shaft, then a bell 
 and horned abacus. The bell is set round with two rows of 
 leaves, eio-ht in each row, and a third row of leaves support 
 eio-ht small open volutes, four of which are under the tour 
 horns of the abacus, and the other four, which are often inter- 
 woven, are under the central recessed part of the abacus and 
 have over them a flower or other ornament. These volutes 
 sprino- out of small twisted husks placed between the leaves ot 
 the second row, and which are called cauUcoks. The abacus 
 consists of an ovolo, fillet, and cavetto, like the modern lomc. 
 There are various modes of indenting the leaves, which are 
 called, from these variations, acanthus, olive, &:c. The column, 
 including the base of half a diameter, and the capital, is ten 
 diameters high. 
 
 If a pedestal is used, it should have several mouldings, 
 some of which may, if necessary, be enriched. The base may 
 be either an attic base, or with the addition of three astragals, 
 one over each torus, and one between the scotia and upper 
 torus ; or a base of two tori and two scotise, which are divided 
 by two astragals : one or two other varieties are sometimes, 
 but not often, used. 
 
 The entablature of this order is very fine. The architrave 
 
 has mostly two or there faces, which have generally small 
 
 osees or be-ads between them. nu ^ c 
 
 The frieze is flat, but is often joined to the upper fiUet ot 
 
 the architrave by an apophyges. , , ., , . „ 
 
 The cornice has both modillions and dentils, and is usually 
 thus composed : above the corona is a cymatium, and small 
 ogee- under it the modillions, whose disposition, hke the 
 Ionic, must be one over the centre of the column, and one 
 close to the return of the cornice. 
 
ISO ARCHITECTURE. 
 
 Composite order. 
 
 These modillions are carved with a small balustre front, and 
 a leaf under them; they are surrounded at the upper part by a 
 small 02:ee and fillet, which also runs round the face they spring 
 from. Under the modillions is placed an ovolo, and then a 
 fillet and the dentil face, which is often left uncut in exterior 
 ■work. Under the dentils are a fillet and ogee. In some cases 
 this order is properly worked with a plain cornice, omitting 
 the modillions, and leaving the dentil face uncut. 
 
 The enrichments of this order may be very considerable; 
 some of the mouldings of the pedestal and base may be en- 
 riched. The shaft may be fluted as the Ionic, in twenty-four 
 flutes, which may be filled one-third high by staves, which is 
 called cabling the flutes. The small mouldings of the archi- 
 trave, and even some of its faces, and several mouldings of the 
 cornice, may be enriched ; the squares in the sofiit of the corona 
 pannelled and flowered, and the frieze may be adorned with 
 carvings. But though the order will bear all this ornament 
 without overloading it, yet, for exteriors, it seldom looks 
 better than when the capitals and the modillions are the only 
 carvings. 
 
 The Composite Order. 
 
 The Romans formed this order by mixing the Corinthian 
 and Ionic capitals; like the Corinthian, the capital is its prin- 
 cipal distinction. This is of the same height as the Corinthian, 
 and it is formed by setting, on the two lower rows of leaves of 
 the Corinthian capital, the modern Ionic volutes, ovolo, and 
 abacus. The small space left of the bell is filled by caulicoles, 
 with flowers, and the upper list of the volute is often flowered. 
 
 The column is of the same height as the Corinthian, and 
 the pedestal and base differ very little from those of that order, 
 the pedestal being sometimes a little plainer, and the base 
 having an astragal or two less. 
 
 The entablature mostly used with this order is plainer than 
 the Corinthian, having commonly only two faces to the archi- 
 trave, the upper mouldings being rather bolder; and the cor- 
 nice is different, in having, instead of the modillion and den- 
 til, a sort of plain double modillion, consisting of two faces, 
 the upper projecting farthest, and separated from the lower by 
 a small ogee. Under this modillion is commonly a large ogee 
 astragal and fillet. 
 
 A plain cornice, nearly like that used to the Corinthian or- 
 der, is sometimes used to this order, and also a cornice with the 
 modillions bolder, and cantalivers under them in the frieze. 
 
 This order may be enriched in the same manner as the 
 Corinthian. 
 
INSERT FOLDOUT H 
 
A]R 'C IHLll TK >r TITMIE = 
 
 £ruiruved- by J'ai .Da.vi. 
 
 ?TlKll<?>lP?l "Vto- "FTptittt FlsllPr Pa-BTfvn [n-nrln-n Ifl?.^ 
 
ARCHITECTURE, 181 
 
 Composed orders. — Ernlyn's attempt to form a new order. 
 
 Having gone through the most usual forms and distinctions 
 of the orders, it is proper to say, that, even in Greece and 
 Rome, Ave meet with specimens whose proportions and compo- 
 sition do not agree with either of them. These are comprised 
 imder the general name of composed orders, and though some 
 are beautiful as small works, scarcely any of the ancient ones 
 are worthy of imitation in large buildings, and modern compo- 
 sition has run very wild, and produced scarcely any thing 
 worth prolonging by description. There was, however, one 
 attempt of a singular kind, made some years since by an archi- 
 tect at Windsor, who published a magnificent treatise, and 
 executed one portico and a few door-cases in and near Windsor. 
 This was H. Emlyn, who conducted the restoration of St. 
 George's chapel. His order, he says, Avas first brought into 
 his mind by the twin trees in Windsor forest. He makes an 
 oval shaft rise about one-fourth of its height, and then two 
 round shafts spring from it close to each other, and the dimi- 
 nution affords space for two capitals, which have volutes, and, 
 instead of leaves, feathers like the caps of the knights of the 
 garter. His entablature has triglyphs, and his cornice mutules. 
 The triglyphs are ostrich feathers, the guttae acorns, and the 
 metopes are filled with the star of the garter. 
 
 To conceal the awkward junction of the two columns to the 
 lower part, an ornament is placed there, which is a trophy with 
 the star of the garter in the centre. 
 
 It is obvious that this order must be extremely unmanage- 
 able, as it is difficult, and indeed almost impossible, to make 
 a good angle column, and if its entablature is proportioned to 
 the diameter of one column, it will be too small; if to the 
 whole diameter, it will be too heavy, and a mean will give the 
 capitals wrong-, so that in any shape some error arises. In 
 the portico above mentioned, the entablature is so light as to 
 appear preposterous. This attempt is not generally known, 
 as the book was very expensive, and the portico at a distance 
 from a public road ; but it deserves consideration, because, 
 though the idea was new, its execution seems completely to 
 have failed; and, indeed, in large designs, no composed order 
 has ever yet appeared that can come into competition with a 
 scrupulous attention to those excellent models of Greece and 
 Rome, now, through the effects of graphic art, happily so 
 familiar to almost every English architect. 
 
1 82 B U 1 1. D I \ G . 
 
 l)iiral)ilit\ of the bricks ol' tlie ancients. — Means of ei|uallin;; them. 
 
 BUILDISG. 
 
 Bricklaying is the art of building with bricks. 
 Of Bricks. 
 
 Previous to entering upon details belonging exclusively to 
 the art of bricklaying, we shall introduce an account of the 
 manufacture and dift'erent sorts of bricks. 
 
 On comparing the strength and durability of modern bricks 
 with those of the ancients, it is evident that the former are in 
 every respect inferior. The ancients appear not only to have 
 selected the best sort of clay, but to have combined it with 
 other ingredients well adapted to improve its properties. Pro- 
 fessor Pallas, on his last journey through the southern pro- 
 vinces of Russia, discovered, in the stupendous Tartar monu- 
 ments, bricks which would scarcely yield to the hammer. 
 Another circumstance contributing materially to the excellence 
 of the bricks and tiles manufactured by the ancients, arose 
 from their burning them uniformly, after they had been tho- 
 roughly dried. No doubt can be entertained, that if modern 
 brickmakers were to pay more attention to their art, by dig- 
 ging the clay at proper seasons, exposing it much longer to the 
 air than is done at present, working it sufficiently, bestowing 
 more care upon the burninp; of the bricks, and that the latter 
 operation may be done uniformly, making them much thinner 
 than is prescribed by the standard form, we should be provided 
 with bricks equal in point of strength and durability to the 
 best of former times. In a variety of instances, persons may 
 have it in their power to obviate some of these causes of de- 
 fect, and it is therefore proper to mention them ; but the state 
 of society is not favourable to any general change in the sys- 
 tem which brickmakers pursue ; the expedition with which a 
 given number of bricks can be furnished, and the cheapness of 
 the rate at which they can be manufactured, are to them the 
 primary objects of consideration; and the speculative builder 
 cares little about the real value and durability of his edifice, 
 provided the difference between the cost and sale price is suf- 
 ficiently ample to recompense him to his satisfaction. 
 
 It is an erroneous notion that bricks may be made of any 
 earth that is not stony, or even of sea ouse ; too much sand 
 
BUILDING. 183 
 
 Nature of the earth fit for bricks — method of pvepariiig it. 
 
 entering into their composition, renders them heavy and brittle, 
 and too much fat argillaceous matter causes them to crack in 
 drying: those only will burn red which contain iron particles. 
 In England they are chiefly made of a motley, yellowish, or 
 somewhat reddish fat clayey earth, commonly called loam. 
 Those of Stourbridge clay, and Windsor loam, are esteemed 
 the most proper and durable bricks, and they will stand very 
 high degrees of heat without melting. The earth for this manu- 
 facture should not contain too many calcareous and ferruginous 
 ingredients; as the former prevent the mass from becoming 
 firm in burning, and occasion the bricks to crumble when ex- 
 posed to the air ; while a superabundance of iron particles re- 
 tards the preparation of bricks so mxtch as often to occasion 
 considerable difficulty in giving them due consistence. This 
 inconvenience may, however, be remedied, by allowing the 
 clay to lie a considerable time under the influence of the atmo- 
 sphere, then soaking it in pits, and afterwards working it well 
 in the usual manner. The common potter's clay, which is also 
 employed in the manufacture of bricks, is opaque, imparts a 
 slight colour, sometimes yellowish, bluish, greenish, but more 
 frequently of diff'erent shades of light grey; when kneaded and 
 spread, it becomes smooth and glossy; it is soft, fat, and cold, 
 somewhat agreeable to the touch, slightly adheres to the 
 tongue, has a fine earthy fracture, and is very plastic. When 
 of the best quality, it is not remarkable either for its lightness, 
 or weight. By chemical examination it is found to consist of 
 thirty-seven parts of pure argillaceous or clayey earth, and 
 sixty-three parts of siliceous or flinty earth. 
 
 Of whatever description the earth intended for bricks may 
 be, it ought to be dug after midsummer, that is, between the 
 beginning of July and the latter end of October, before the 
 first frost appears ; it should be repeatedly worked with the 
 spade during the winter, and not formed into bricks till the 
 following spring. If the earth were not used till two or three 
 years after it had been dug, the quality of the bricks produced 
 would be very materially improved ; and in all cases, the 
 oftener it is turned and the more completely it is incorporated, 
 the better will be the bricks. 
 
 The clay, before it is put into pits for soaking, must be 
 broken as small as possible, and allowed to lie at least ten days ; 
 every stratum of twelve inches should be covered with water, 
 in order that it may be uniformly softened. Two pits, at least, 
 will be necessary for every brick manufactory, so that, after 
 having been suff'ered to remain for five days, the second may 
 be prepared, and thus the manufacture carried on without in- 
 terruption. The earth should as much as possible be divested 
 
184 BUILDING. 
 
 Tempering of brick earth. — Burning bricks in kilns. — Gallon's experiment. 
 
 of stony particles, and other extraneous matter, and sliould 
 have suihcient time to mellow and ferment, otherwise it will be 
 difficult to temper. On the treading and tempering, twice the 
 customary quantity of labour ouglit to be bestowed. INIuch 
 of the goodness of bricks depends upon the proper manage- 
 ment of its first preparation, for the earth itself, previous to 
 its being wrought, possesses very little tenacity ; but by long 
 exposure to the air and frost, and thorouglily working and in- 
 corporating it together, it is converted into a tough, gluey 
 substance, in which state alone it is fit for moulding. 
 
 In the vicinity of London, coal ashes, and in other parts of 
 the country, light sandy earth, is usually mixed with the clay, 
 which, by such addition, is more easily and expeditiously 
 wrought, and requiring rather less fuel, occasions some saving 
 in the expense of burning the bricks; but here the advantages 
 of it terminate ; in other respects it is injurious rather than 
 otherwise. If iu temperiv.g -the earth, too much water be 
 used, the bricks become dry and brittle; but if duly tem- 
 pered, they will be smooth, solid, hard, and durable. A 
 brick properly made, requires nearly as much earth as a 
 brick and a half made in the common way, when too great 
 a pro'.'^ortion of water has been added, which tends to render 
 the bricks spongy, light, and full of Haws. As bricks made 
 in the best manner are more solid and ponderous than the 
 common ones, they require a much longer time to dry ; 
 they ought not to be burnt till they will give a hollow 
 sound on collision. Proper attention to the drying of 
 bricks is necessary to prevent their cracking and crumbling 
 iu the kiln. 
 
 Of whatever materials the kiln be constructed, each burn- 
 ing of from six to ten thousand bricks requires the fire to be 
 kept up at least for twenty-four hours, and double that time 
 for a number of from twelve to fifty thousand. The uniform 
 increase of heat deserves particular attention; its duration 
 should be regulated according to the season : in cold weather 
 fire burns most fiercely. During the last twenty-four hours, 
 the fire should be uninterruptedly supported by means of 
 flues; but afterwards the fire should not be suddenly closed, 
 as there is always some danger of bursting the flues or melt- 
 ing the bricks. 
 
 The following experiment, by Gallon, made with a view 
 to ascertain the difference in the quality of bricks dilferently 
 manufactured, deserves to be generally known. A certain 
 quantity of the earth prepared for moulding into bricks was 
 taken for the experiment; at the end of seven hours, it was 
 moistened and beaten for the space of thirty minutes. The 
 
BUILDING. 185 
 
 Strength of bricks difterently made. — Moulding of bricks. 
 
 next morning the same operation was repeated for an equal 
 lenoth of time ; in the afternoon it was again beaten for fif- 
 teen minutes. Thus this earth had not only been worked for 
 an hour and a quarter longer than usual, but at three different 
 times; the consequence was, that its density was increased; 
 for a brick made of it weighed five pounds eleven ounces, while 
 another brick made in the same mould, of the earth that had 
 not received this preparation, weighed only five pounds seven 
 ounces. The two sorts of bricks were dried in the air, for the 
 space of thirteen days ; they were then burnt with others, with- 
 out any particular precautions, and when they were taken from 
 the kiln, it was found that the bricks made of the earth which 
 had been most worked, still weighed four ounces more than 
 the others, each having lost five ounces by the evaporation of 
 the moisture. They differed also very remarkably in strength ; 
 fer on placing them with the centre on a sharp edge, and load- 
 ing the two ends, the bricks formed with the well-tempered 
 earth were not broken with a less weight than sixty-five pounds, 
 or one hundred and thirty pounds in all; while the others were 
 broken with thirty-five pounds at each end, or seventy pounds 
 in the whole. That the quality of bricks should be improved, 
 by bestowing more labour upon the preparation of the earth, 
 will hardly excite surprise, though the degree of the improve- 
 ment, as just stated, may certainly be considered remarkable; 
 but there is another mode of strengthening these artificial stones, 
 still more extraordinary, and not so easily to be accounted for. 
 Goldham observes, that bricks which have been once burnt, 
 then steeped in water, and burnt again, become doubly strong. 
 We know not that this observation, which is repeated without 
 comment, by nearly all the writers who have occason to treat 
 of this subject, will always be verified in practice, but it de- 
 serves attention, from the number and respectability of the 
 writers who have contributed to give it currency. 
 
 When the earth is sufticiently prepared, it is lorought to the 
 bench of the moulder, who works the clay into the brickmould, 
 which he has previously dipped in dry sand lying near him for 
 the purpose; he then strikes off the superfluous soil with a flat 
 smooth stick, which is dipped in water before it is used. The 
 bricks, as they are delivered from the mould, are ranged on the 
 ground, with a small interval between each ; in piling row 
 upon row, they are laid rather crosswise, and sand is thrown 
 over them, to prevent their adhering to each other. As soon as 
 they have acquired sufficient hardness to admit of handling, 
 they are dressed with a knife, turned, and reset more open. 
 A gang, consisting of six persons, will make twenty thousand 
 bricks in the course of a week. The time required for drying 
 8.— Vol. I. 2 B 
 
186 BUILDING. 
 
 Burning of bricks in kilns — clamps. 
 
 them necessarily varies with the state of the weather; if this be 
 favourable, fourteen or sixteen days will often be sufficient. In 
 showery weather, the piles are usually protected from its bad 
 effects by some cheap covering, such as straw, or old light 
 boards. In grounds not extensive, sheds are often erected. 
 
 When the bricks have been sufficiently dried in the air, they 
 are burnt, whicli is done either in a kiln, or in a stack, techni- 
 cally called a clamp. The kilns are usually made large enough 
 to burn twenty thousand bricks, being about thirteen feet 
 long, ten feet broad, and twelve feet in height. The aperture 
 is diminished by contracting the walls towards the top, where 
 the area is about one-tenth smaller than below. The thick- 
 ness of the walls should at least be a brick and a half. Bricks 
 are burned in kilns with less fuel, and with greater uniformity 
 and expedition, than in clamps. Wiien they have been set or 
 plviced in the kiln, they are covered with pieces of bricks or 
 tiles, and dried by kindling a gentle fire, which is kept up for 
 two or three days, or till the smoke becomes light. More 
 fuel is then added, and the mouth or mouths of the kiln are 
 nearly closed with bricks and wet clay ; as soon as the arches 
 of the kiln look white, and the fire begins to appear at the 
 top, they slacken the heat for an hour, and let all cool by de- 
 grees. This they continue to do, alternately heating and 
 slacking, till the bricks are thoroughly burnt, which is usually 
 effected in forty-eight hours. 
 
 The stacks or clamps are built of the bricks themselves. 
 The foundation is commonly somewhat raised from the sur- 
 rounding ground, and of an oblong form; the sides slant in- 
 wards a little towards the top ; hence the clamp, in its figure, 
 is a truncated pyramid. Flues, about the length of a brick in 
 breadth, are made entirely through the clamp ; they are about 
 six feet apart when the burning is to be hastened, otherwise 
 they are made about nine feet from each other. The arching 
 of the flues is performed by laying the successive layers of 
 bricks a little over the edge of those below them, till they 
 nearly meet, and then a binding brick at the top finishes the 
 arch. In every direction, the bricks are separated from each 
 other by a straturn of coals and cinders. To facilitate setting 
 fire to the clamp, a quantity of wood is laid with the coal in 
 the flues. When the fire is kindled, if it burn strongly, or the 
 weather is precarious, they plaster the outsides of the clamp 
 with clay, and close the apertures of the flues. On the top of 
 the clamp, a thick layer of breese (cinders) are uniformly laid. 
 When the whole of the fuel is consumed, the manufacturer 
 concludes that the bricks are sufficiently burnt. The opera- 
 tion requires from twenty to thirty days, according to the 
 
BUILDING. 187 
 
 Different kinds of bricks. — Cartwright's bricks. 
 
 quantity of fuel, the proximity of the flues, and the state of 
 the weather. 
 
 The different kinds of bricks made in this country are, prin- 
 cipally, place bricks, grey and red stocks, marl facing bricks, 
 and cutting bricks. The place bricks and stocks are used in 
 common walling ; the marls are made in the neighbourhood of 
 London ; these are very beautiful bricks, of a fine yellow colour, 
 hard, and well burnt, and in every respect superior to the 
 stocks. The finest kind of marl and red bricks are called cut- 
 ting bricks; they are used in the arches over windows and 
 doors, being rubbed to a centre, and gauged to a height. 
 
 An acre of land, including the ashes mixed with the earth, 
 is computed to yield about one million of bricks for every foot 
 in depth. The brick mould is ten inches in length, and three 
 in breadth, and the finished bricks are about nine inches long, 
 four and a half broad, and two and a half thick. Different 
 qualities of earth, however, produce bricks of different dimen- 
 sions from the same mould ; and even the same earth, in pro- 
 portion as it is more or less wrought or burnt, exhibits similar 
 results. 
 
 It is extremely probable that bricks, properly made, would 
 prove superior in durability to almost every kind of stone. 
 In Holland, the streets are every where paved with a hard kind 
 of bricks, known by us under the name of clinkers, which are 
 often imported into this country, and used for paving stabler 
 and court yards ; and houses in Amsterdam which have stood 
 more than two centuries, so far from being decayed, appear 
 perfectly fresh, as if new. 
 
 The numerous patents which have been granted for the 
 making of bricks, appear to have had improvements in the 
 formation of the article for their principal object, without much 
 regard to the materials of which it is composed. Cartwright's 
 patent, the exclusive privilege conferred by which has now 
 expired, is perhaps one of the most important. His improve- 
 ment consists in giving bricks such a shape or form that they 
 shall mutually lock or cramp each other. The principle of his 
 invention may be understood, by supposing the two opposite 
 sides of a common brick to have a groove or rabbet down the 
 middle, a little more than half the width of the side of the 
 brick in which it is made; there will then be left a shoulder 
 on each side of the groove, each of which shoulders will be 
 nearly equal to one quarter of the width of the side of the 
 brick, or to one-half of the groove or rabbet. A course of 
 these bricks being laid shoulder to shoulder, they will form an 
 indented line of nearly equal divisions ; the grooves or rabbets 
 Ijeing somewhat wider than the two adjoining shoulders, to 
 
188 BUILDING. 
 
 Ad\antagcs of Carlnright's bricks. 
 
 allow for mortar, Sec. When the next course comes on, the 
 shoulders of the bricks which compose it, will fall into the 
 grooves of the first course ; and the shoulders of the first 
 course will fit into the grooves or rabbets of the second, and 
 so on. This configuration of the bricks is to be preferred, as 
 it is perfectly simple ; but the principle will be preserved by- 
 whatever form of indenture tiiey lock or cramp each other. 
 For the purpose of turning the angles, it may be expedient to 
 liave bricks of such a size and shape as to correspond with 
 each wall respectively ; though this is not absolutely necessary, 
 as the grooves in the bricks of each wall, where they cross or 
 meet each other, may be levelled, and the bricks lap over as 
 in the common mode. For the purpose of breaking the joints 
 in the depth of the wall, bricks will be required of difl'erent 
 lengths, though of tlie same width. Buildings constructed 
 with bricks of this principle, will require no bond timber, one 
 universal bond running through and connecting the whole 
 building together ; the walls of which can neither crack nor 
 bulge out, without breaking through the bricks themselves. 
 When bricks of this form are used for the construction of 
 arches, the sides of the grooves or rabbets, and the shoulders, 
 should be the radii of the circle of which the intended arch is 
 to be the segment. In fonning an arch, the bricks must be 
 coursed across the centre on which the arch is turned, and a 
 grooved side of the bricks must face the workman. It may 
 be expedient, though not absolutely necessary, in laying the 
 first two or three courses at least, to begin at the crown and 
 work downwards. The bricks may be either laid in mortar, or 
 dry, and the interstices afterwards filled and wedged up, by 
 pouring in lime putty, plaster of Paris, grouting, or anv other 
 convenient material, at the discretion of the workman or 
 builder. Arches on this principle, it is stated, having no la- 
 teral pressure, can neither expand at the foot, nor spring at 
 the crown, consequently they will want no abutments, requir- 
 ing only perpendicular walls to be let into, or to rest upon; 
 and they will want no incumbent weight upon the crown to 
 prevent their springing up, a circumstance often of great im- 
 portaace in the construction of bridges. Another advantage 
 attending this mode of arching is, that the centres may be 
 struck immediately ; so that the same centre (which in no case 
 need be many feet wide, whatever may be the breadth of the 
 arch,) may be regularly shifted as the work proceeds. But the 
 greatest and most striking advantage attending this invention, 
 is the absolute security it affords (and at a very reasonable 
 rate) against the possibility of fire ; for, from the peculiar pro- 
 perties of this arch, requiring no abutments, it may be laid 
 
BUILDING. 189 
 
 Fire bricks. — New method of dividing bricks. — Analysis of clay for bricks. 
 
 upon, or let into, common walls, no stronger than what are 
 required for timbers, of which, precluding the necessity, it 
 saves the expense. A more particular account of this inven- 
 tion, illustrated by two plates, may be seen in the third 
 volume of the " Repertory of Arts and Manufactures." 
 
 In 1798, Francis Farquharson, of Birmingham, obtained a 
 patent for making bricks and tiles by machinery, and indeed 
 the use of horse power, in working the clay, is now very 
 common. 
 
 Whitmore Davis, of Castle Comber, in the county of 
 Kilkenny, Ireland, observed some persons in the vicinity of a 
 colliery, to employ a mortar, for the backs of their grates, 
 which in a short time became very hard. This substance 
 he found, on inquiry, to be what miners term seat-coal, or 
 that fossil which lies between coal and the rock. It has 
 been examined by Kirvvan, who is of opinion that it will, 
 when mixed with a due proportion of clay, produce a kind of 
 bricks, capable of resisting the action of fire, and conse- 
 quently well calculated for furnaces, or similar structures. 
 The discovery of the use of this substance is considered im- 
 portant, and it is further observed, that seat-coal, properly 
 prepared, will answer every purpose of tarras, for buildings 
 beneath water. 
 
 In building, a considerable vaste of time arises from the 
 necessity of making bricks less than the common size, to suit 
 particular situations. Nor is tie waste of time the sole loss ; 
 in attempting to divide a brick, especially in the direction of 
 its length, one half of it is generally reduced to useless splin- 
 ters ; but bricks have lately been made, which in their soft 
 state were nearly cut through by pressing a wire upon them ; 
 they can then be divided by e. single blow : a proportion of 
 them along with the common sort, produces on the whole a 
 saving of some moment. 
 
 It is of considerable importance to examine clay before it is 
 made into bricks, in order to ascertain whether any addition 
 can be made to it which will improve its quality. According 
 to the observation of Bergman, the proportion of sand to be 
 used with any clay, must be greater, the more such clay is 
 found to contract in burning, but the best clays are such as 
 require no sand. This illustrious chemist recommends the 
 following mode of analysis to manufacturers : Nitric acid 
 poured upon unburned clay, detects the presence of lime, by 
 producing an effervescence. Calcareous clays, or marls, are 
 often the fittest materials for making bricks. In the next 
 place, a lump of clay, of a given weight, is to be diffused in 
 water by agitation. The sand will subside, and the clay 
 
190 BLILDING. 
 
 Diflerent kinds of liles. 
 
 remain suspended. Other washings of the residue will carry 
 off some clay, and by due management in this way, the sand, 
 or quartzose luatter, may be had separate. Nitric acid by 
 digestion will take up the lime from a part of the clay, pre- 
 viously weighed, and this may be precipitated by volatile 
 alkali. The clay, the sand, and the lime, may thus be well 
 enough ascertained by weight, so as to indicate the quan- 
 tity of sand or other material requisite to be added, in 
 order to form that compound, which, from other experiments, 
 may have been found best adapted to produce good tiles and 
 bricks. An examination with the microscope will shew 
 whetlier the sand contain feld spar or other stones of known 
 figure. 
 
 Pavino; tiles are a Ions flat kind of brick, used for lavincr 
 the floors of kitchens, dairies, cellars, &c. Two or three 
 sizes of them are generally made; the least of Ens;lish manu- 
 facture are about the same length and breadth as common 
 bricks ; the largest are about twelve inches square ; a middle 
 size, about nine inches square, is also in common use. The 
 thickness of all the sizes of saving tiles is only about an inch 
 and a half. They are made of the strongest kind of clay, and 
 well burnt. Paving tiles or oricks look the best when laid 
 diagonally. 
 
 The most common tiles for roofs, are those called pan tiles, 
 which are thirteen inches long and eight broad, and about 
 half an inch thick ; they are curved in the direction of their 
 length, so that their transverse section is a figure of contrary 
 curvature, like the letter in ; the hollow of one side serves as 
 a channel for the rain, and for ihat purpose is made of greater 
 radius than the other, which is employed to overlap the edge 
 of the adjoining tile. The tile at the upper end of its under 
 surface, has a knob, by which ii is hung to the lath. Tiles 
 constitute a very heavy coverir.g, and require the laths by 
 which they are supported to be proportionately strong. The 
 laths, for pan tiles, are about th;ee-quarters of an inch thick, 
 and an inch and a quarter broad ; they are generally made of 
 deal. The other sorts of tiles are chiefly plain tiles, hip tiles, 
 and ridge tiles ; the latter resemble half a hollow cylinder, 
 and are laid on the ridges of houses. Ridge tiles are required 
 by statute to be thirteen inches in length, and six inches and 
 a half in breadth.* 
 
 Brick-trater, or water unpregaated with the contents of bricks or tilei, is 
 possessed of properties so remarkable, and at the same lime so pernicioas in 
 their effects, wheo used for culiasrjr purposes, that we cannot refuse a place to 
 
BUILDING. 19\ 
 
 Use of tar iu preserving tiles. — Statute regulations. 
 
 In Holland, they frequently glaze their roofing tiles, which 
 increases their durability, but considerably enhances the price 
 of them. The early destruction of unglazed tiles is occasioned 
 by the moisture they imbibe; for when the water has sunk into 
 them, they break with the action of the frost. Sonini has, 
 however, discovered an excellent preventive of this eifect, 
 nearly equal to glazing, and very cheap : he directs us to 
 brush over the tiles with tar, after they have been well warmed 
 in the sun. Tiles which have been cracked by the frost, may 
 be preserved from the further influence of the same cause, by 
 the like application. 
 
 By Stat. 2, Geo, 2, cap. 15, any two, three, or more per- 
 sons, appointed by the justices of the peace, are empowered, 
 within fifteen miles of London, to go in the day time, into 
 any grounds, sheds, or places, where any clay or earth shall 
 be digged or digging, for bricks or pan tiles, or any bricks 
 or pan tiles shall be making or made for sale, and there to 
 search for, view, and inspect the same, &c. — Off'enders to 
 forfeit twenty shillings for every thousand of unstatutable 
 bricks, and ten shillings for every thousand of such tiles ; 
 one half of the fine to the prosecutor, and the other to 
 the poor of the parish in which the offence shall be com- 
 mitted. 
 
 By Stat. Geo. 2, cap, 22, there may be mixed with the brick 
 earth any quantity of sea-coal ashes, sifted or skreened through 
 a sieve or skreen half an inch wide, and not exceeding twenty 
 loads to the making of 100,000 bricks, each load not exceeding 
 thirty-six bushels. And breese may be mixed with coal in 
 the burning of bricks in clamps for sale, &c. Stock bricks 
 and place bricks may be burned in the same clamp, provided 
 that the stock bricks be set in one distinct parcel, and not 
 mixed and surrounded with place bricks. 
 
 By Stat. 10, Geo. 3, cap. 49, the earth for making bricks 
 must be turned at least once after it is dug, before it is made 
 into bricks. 
 
 the following cnrious experiment made by Dr. Percival, and stated in the first 
 volume of his Essays. He steeped two or three pieces of common brick, four 
 days in a basin full of distilled water, which he afterwards decanted off, and 
 examined by various chemical tests. It was not miscible with soap ; struck a 
 lively green with syrup of violets, became slightly lactescent by the volatile 
 alkali, but entirely milky by the fixed alkali, and by a solution of sugar of lead. 
 No change was produced on it by an infusion of tormentil root. Hence the 
 Doctor justly concluded, that the lining of wells with bricks, a practice verj 
 common in various places, is extremely improper, as it cannot fail to render 
 the water hard and unwholesome. 
 
192 BriLDING. 
 
 Duties on bricks. — BricWIaver's trowel — baintner — plumh-rnli'. 
 
 By 17, Geo. 3, cap. 42, stock and place bricks made for 
 sale, shall, wiien burnt, be not less than eight and a half 
 inches long, four inches wide, and two and a half inches 
 thick. 
 
 By 43, Geo. 3, cap. 69, (consolidating the excise duties,) 
 every thousand of bricks made in Great Britain, not exceeding 
 ten inches long, three inches thick, and five inches wide, is 
 liable to a duty of Jive s/ti/lings, and if exceeding these dimen- 
 sions, to te?i shil/ings: and every thousand of bricks made in 
 Great Britain, and smoothed or polished on one or more sides, 
 not exceeding the superficial dimensions of ten inches long, 
 and five inches wide, is subject to a duty of twelve shillings: 
 and if such bricks exceed these dimensions, to tlie duty on 
 paving tiles. The said duties are to be paid by the makers. 
 An additional duty of tcupence per thousand was imposed on 
 bricks and tiles in 1805. 
 
 Oj the Tools used in Bricklaying, 
 
 The brick troicel, which is used for taking up and spreading 
 the mortar, is also used for cutting the bricks to any required 
 size, and should therefore be made of the best steel, and well 
 tempered. 
 
 The hammer used by the bricklayer, is adapted either to 
 strike a blow, or to divide the bricks, as may be required in 
 cutting a hole through a brick wall, or other operations. One 
 end of the head of it has therefore a face similar to that of any 
 common hammer, and the shape of the other end resembles 
 that of a carpenter's axe, thougli far narrower in proportion to 
 its length. Tlie handle is inserted much nearer the face of 
 the hammer, than the other extremity or edge. Another kind 
 of hammer, often employed in taking down brick-work, ditfers 
 from the above only in havi.ig, instead of the axe part, nearly 
 the shape of an adze, but not so broad for its length; hence it 
 may be driven with facility between bricks to separate them. 
 
 The plumh-riile is similar to that used by carpenters and 
 other artizans. It consists of a well-seasoned board, the length 
 of which should at least be four feet; its thickness and breadth 
 are not very material, provided they be sufficient to prevent its 
 warping. Down the middle of one of the broad surfaces of 
 the board is drawn a straioht line, and at one extremity in this 
 line, IS attached a small cord with a weight at the lower end. 
 If the long narrow sides of the rule be perfectly straight and 
 parallel with each other, and tlie line is equidistant from the 
 arris on each side of it, the plummet being hung in this line, 
 will form a correct instrument. Either of the lonfr narrow 
 
BUILDING. 193 
 
 The level The square The bevel. — The rod. 
 
 sides of this rule is applied to the wall, so that the plummet 
 and its cord may face the workman, who, by frequently using 
 it, is enabled to carry up his wall perpendicularly. If in any 
 of these trials, he observes that the cord of the plummet does 
 not coincide with the line on the rule, he sets the bricks fur- 
 ther in or out, as may be required to rectify the error, taking 
 care to do it while the mortar they are set in is yet wet. — As 
 the plummet is not made to hang below the rule, a hole is cut 
 in the latter, to allow the cord to hang straight. 
 
 The level employed by the bricklayer, is also similar to that 
 of the carpenter. It is of various lengths from six to twelve 
 feet. If one end of the plumb-rule above-mentioned were 
 joined at right angles to the middle of the long narrow edge 
 of another board of the same thickness, but about double its 
 length, it would become a level, the lower edge or side of the 
 piece thus added to the rule, becoming the surface placed on 
 walls, particularly at window sills and wall-plates, to ascertain 
 whether they are horizontal or not. To try the correctness of 
 a level, place it vertically, that is, in the position in which it 
 is used, upon any flat surface, or merely place eacii end of the 
 bottom edge upon a block of wood, and raise or lower the sup- 
 ports till the cord of the plummet exactly coincides with the 
 line on the perpendicular rule or limb of the instrument. 
 When this is obsei'ved, reverse the ends, and if the same coin- 
 cidence then takes place, the level is true ; but if it does not, 
 the bottom must be planed, till the trial will succeed. The 
 perpendicular and horizontal parts of the level are not only 
 fastened together by mortise and tenon, but, for greater firm- 
 ness, and to prevent warping, two braces are added, which 
 extend, in a slanting direction, from the horizontal piece nearly 
 to the top of the perpendicular one. 
 
 A lai'ge square is employed in setting out the sides of build- 
 ings at right angles; and a small square for trying the bedding 
 of bricks, and squaring the soffits across their breadth. 
 
 A bevel is required for drawing the soffit line on the face of 
 bricks. 
 
 The rod, for measuring, is either five or ten feet long; the 
 feet are divided by notches, and one of those next the extre- 
 mity of the rule, is divided into inches. Dimensions may be 
 more expeditiously ascertained with the rod than with a 
 pocket rule ; but bricklayers are generally provided with a 
 measuring tape, which is coiled up by its winch into a cylin- 
 drical box, of such small dimensions, as to unite a more con- 
 venient portability than the pocket rule, with greater dispatch 
 in the general operations of measurement, than can be ob- 
 tained by the use of the rod. 
 9.— Vol. I. 2 C 
 
194 BUILDING. 
 
 JoiutiDg-rulc. — Raker. — IIoJ. — Line. — Rammer. — Crow and pick-axe. 
 
 The joinli/ig-rule, employed in running the joints of brick- 
 work, is eight or ten feet long, and four inches broad. When 
 designed for the use of two bricklayers, the latter length is 
 employed. 
 
 The iron tool used along with the jointing-rule, to mark the 
 joints of brick-work, is called ■ajoiufcr; its form is nearly that 
 of the letter cc , though its flexure is not in proportion so con 
 siderable. 
 
 The raker has its use designated by its name. It is em- 
 ployed to rake or scrape loose and decayed mortar out of the 
 joints of walls, the appearance of which is intended to be im- 
 proved by pointing them afresh. The raker is made of iron, 
 pointed with steel, and at about one-fourth of its length from 
 each extremity, it is bent to a right angle, so that it would 
 resemble a Z, if the stroke connecting the top and bottom of 
 that letter were perpendicular instead of slanting. 
 
 The hod is an angular wooden trough, closed only at one 
 end ; so that it resembles the half of a lectangular box divided 
 in such a manner as tu consist of two entire sides and one 
 end. From the middle, of the angular ridge formed by the 
 meeting of the two sides, proceeds a pole or handle about 
 four feet long ; and so much of this ridge as lies between the 
 handle and the end piece, is covered with a cushion of se- 
 veral thicknesses of leather, or leather stuU'ed with wool. In 
 this utensil, the labourer carries upon his shoulder the bricks 
 and mortar with which he supplies the bricklayer ; the cushion 
 takes off the sharpness of the ridge, and by means of the 
 handle, he can support it without difficulty, either in walking 
 on level ground, or ascending a ladder. It is customary to 
 sprinkle the inside of the hod with clean dry sand, before it 
 is filled with mortar, which is thereby prevented from adher- 
 ing to the wood. 
 
 A cord or line, to serve as a guide in laying the courses of 
 bricks exactly straight, is stretched close to the wall, and re- 
 moved at the proper intervals as the work advances. This 
 line, to aflford the means of stretching it wherever it is wanted, 
 is fastened to two pointed iron pins, called the line viiis, one 
 being attached to each end of it. 
 
 The bricklayer'^ rammer differs not from the pavier's. If 
 the ground intended for a foundation is deemed not sufficiently 
 solid, it is compressed as much as possible by this tool, a due 
 attention to the use of which will prevent fractures that may 
 endanger the buildino;. 
 
 The iroji-crow and the pick-axe, are useful assistants to tne 
 bricklayer, of obvious utility ; being sometimes employed in 
 conjunction, and sometimes alone, in digging, breaking through 
 
BUILDING. 195 
 
 The compasses and grindstone. — Chopping block. — Banker. — Camber slip, &c. 
 
 walls, raising heavy bodies out of the ground, and sinailar 
 operations. 
 
 The compasses for traversing arches, and the grinding-stone for 
 sharpening tools, scarcely, perhaps, require to be mentioned. 
 
 The cutting and management of bricks for the construction 
 of gauged arches, add the following tools, besides the small 
 square and bevel above-mentioned, to the catalogue belonging 
 this trade : 
 
 The chopping block, is made out of any piece of wood which 
 happens to be at hand, set or fixed with sufficient stability for 
 axing the bricks upon. Its height may be about thirty inches; 
 the area of its upper surface is generally inconsiderable; if six 
 or eight inches square, it is sufficiently large for one man to 
 work at. When several men are at work, it is better that they 
 should have separate chopping blocks than a single large one, 
 unless the single one be very firmly fixed, so that the vibrations 
 communicated by any of the workmen, may not inconvenience 
 the rest, and unless also its dimensions be increased in a much 
 greater proportion than the number of hands. 
 
 The banker, is the appellation which designates a large bench, 
 upon which the bricks for arch-work are rubbed and gauged. 
 It is from two to three feet in breadth, and from six to twelve 
 feet in length, according to the number of men it is intended 
 to accommodate, or the dimensions of the timber which hap- 
 pens to be at hand or most cheaply obtainable. The banker 
 is generally made thirty or thirty-two inches high: as it is not 
 necessary for it to be very thick, an old door may be converted 
 into one, supporting it by four or five posts or piers, and for 
 greater steadiness setting one edge against a wall. 
 
 The camber slip is a piece of board cambered or made con- 
 vex on one or both edges, but not confined to any particular 
 length, breadth, or thickness, thougli the latter dimension is 
 generally about half an inch. It is employed as a ruler. When 
 only one edge is curved, it rises about one inch in six feet, for 
 drawing the soffit lines of straight arches : when the opposite 
 edge is curved, it rises only about half an inch in six feet, and 
 is used to draw the upper side of straight arches. This small 
 convexity is an allowance for the settling of the areh, which 
 many disregard, and therefore make the upper side of the arches 
 in question straight. When the bricklayer has drawn his lines 
 with the camber slip, he hands it to the carpenter, who by it 
 forms the centre to the curve of the soffit. To prevent the 
 necessity of having many camber slips, one is made large 
 enough for the widest aperture likely to be arched. 
 
 The mould is used to obtain the proper form of the brick, 
 that it may be reduced to the requisite taper, one edge of the 
 
196 BUILDING. 
 
 The scribe. — The tin-saw. — The brick-axe. — The rubbing-stoue, &c. 
 
 mould beini; brought close to the bed of the brick previously 
 squared. There is a notch in the mould for every course of 
 the arch. 
 
 The scribe is any piece of iron, such as a large nail or the 
 like, ground to a point, for the purpose of marking the bricks 
 •where they are to be cut. 
 
 The tin-saw is used in cuttino; the bricks about the ei'j-hth 
 of an mch deep, at the lines marked with the scribe, so that 
 an entrance may be made for the brick-axe, and that they 
 may be reduced to the proper form for arches, without splin- 
 tering or jagging their edges. The false joints are also cut 
 with this instrument. 
 
 The brick-axe is used for reducing bricks to the saw cut- 
 tings and lines drawn by the scribe. The subsequent labour 
 of rubbing the bricks is shortened in proportion to the dex- 
 terity shown in the management of the axe. 
 
 AVhen the bricks have been cut with the requisite exactness 
 by the axe, they are taken to the rubbing-stone, upon which 
 they are rubbed smooth. The rubbing-stone is generally fixed 
 upon a bed of mortar at one end of the banker. To keep it 
 tolerably level, the bricks should be rubbed equally on every 
 part of it, and in different directions ; it should not therefore 
 be '■o large as to prevent the workman from easily reaching 
 over it, on whatever side of it he stands: about twenty inches 
 in diameter is a convenient size. If the grain of it is not sharp 
 enough to reduce the brick with the necessary expedition, sand 
 may be used. The headers and stretchers in returns, which 
 are not axed, are also dressed upon the rubbing-stone. 
 
 The bedding-stone is a marble slab, about ten inches broad, 
 and twenty inches long, with one flat surface, which is used to 
 try whether the rubbed surface of the brick be straight, so as 
 to fit upon the leading skew back, or leading end of the arch. 
 
 The stone upon which bricks cut with curved surfaces are 
 rubbed, is called a.Jloat-stone, which must itself necessarily be 
 curved in the reverse form, though of a radius equal to that 
 intended for the brick. 
 
 Of Foundations 
 
 Foundations are either natural or artificial; natural, where 
 the ground is rocky or good ; artificial, when, from its boggy, 
 sandy state, or from its having been lately dug up, piling, or 
 some other precaution, must be resorted to, for the support of 
 the building. Apj^earances are so often deceitful, that the 
 prudent builder will never depend upon them, but will examine 
 the ground intended for a foundation with the utmost attention. 
 If the ground shake on being struck with the rammer, the 
 
BUILDING. 107 
 
 Foundations. 
 
 nature of it must be ascertained by piercing it with a well- 
 digger's borer. Having found how far the firm ground is 
 beneath the surface, the loose or soft parts must be removed, 
 if not very deep : the excavations made on these occasions, 
 should widen upwards, and their sides be cut in the form of 
 steps : by which means a firmer bed will be obtained for the 
 wall, than if the sides of the trenches were simply inclined 
 planes. 
 
 Palladio directs the ground for the foundation to be pene- 
 trated to a sixth part of the whole height of the building, un- 
 less there be cellars, in which case he recommends digging 
 deeper. It is a good rule to make the foundation double the 
 breadth intended for the superincumbent wall, and the con 
 traction of it should be made alike on both sides. 
 
 If the earth is very unsolid, piles, close to each other, and 
 long enough to reach the good ground, must be driven in. 
 The thickness of these piles should not be less than one- 
 twelfth of their length ; they should be made to present as 
 even a surface as possible, and should then be covered with 
 planks. It is a curious fact, that dry, straight-grained piles, 
 may be driven much further by the same force, than if they 
 be made of wood in an opposite state. 
 
 When the infirmity of the ground is uniform, but not very 
 considerable, it may be made good by laying pieces of sound 
 oak about two feet apart, across the breadth of the trench in 
 which the wall is to stand, and when these have been firmly 
 bedded and rammed down, planks of the same timber, or of 
 pitch pine, (which is equally as durable in such situations,) 
 must be laid down and spiked upon them. The planks should 
 be half a foot wider than the base of the foundation wall. 
 Ground of this description may also be made good by ram- 
 ming large stones upon it closely together, and extending in 
 breadth about a foot on each side of the wall. Upon the first 
 course of stones, another course, rather narrower, may be laid, 
 taking care, as in walling generally, to make the joints of one 
 course fall on the middle of the stones in the other. 
 
 If the ground be found defective in one place and good in 
 another, the unequal settlement which would be the inevitable 
 consequence of building upon it in such a state, may be pre- 
 vented by a plan which is now becoming daily more common, 
 and which, if carefully executed, is always successful. It con- 
 sists in the use of arches, either inverted or suspended, ac- 
 cording to circumstances, which we shall now advert to. 
 
 When the soft parts of the ground are under the apertures 
 only, inverted arches are to be turned under such apertures, 
 in the manner represented by fig. 1, pi. 1. By this means the 
 
198 BUILDING. 
 
 I'ouiidalious. — Use of inverted and suspended arches. 
 
 advantage of one continued base is obtained, for the piers can- 
 not sink without carrying the arches, and consequently the 
 ground upon which they stand, along with them. The whole 
 building will therefore sink equally, and no fracture of the 
 Avails will ensue. So mucii is the use of inverted arches under 
 a|)ertures approved, that they are considered indispensable in 
 all buildings of considerable weight or consequence, even 
 when the ground is not found to be defective. It is suffi- 
 ciently obvious, that the walls of all buildings, the base of the 
 foundation of which is not actually laid upon the bare, solid 
 rock, will sink a little; and as the pressure of the piers is in- 
 comparably greater than that of the low piece of walling un- 
 der the apertures, the risk is extreme, that the resistance of 
 the ground against this low walling will not allow it to sink 
 with the piers, and consequently the fracture of the wall, and 
 probably the breaking of the window sills, will be the conse- 
 quence. The inverted arches prevent these bad effects ; and 
 as they have so important a service to perform, they should 
 be tlirown with the greatest care, closely jointed, and their 
 depth at least equal to half their width. 
 
 \\ hen the reverse of the preceding case occurs, that is, 
 when the solid parts of a foundation are only to be found 
 under apertures, then piers must be built in these places, and 
 arches suspended between them, as represented by fig. 2. It 
 is best to make the middle of the pier rest upon the middle of 
 the summit of the arch. If the pier does not cover the arch, 
 the narrower it is the greater should be the curvature of the 
 latter at the apex. When arches are used in this way, the 
 intrados ought to be clear, that they may have their full 
 effect. The uniform resistance of the ground upon which 
 the piers are erected, is also of greater importance than even 
 its perfect hardness ; for if it resist uniformly, the building 
 will sink in every part alike, and remain uninjured. 
 
 When wood is laid in the trench of a foundation, the first 
 course of stone or brick should be laid as close as possible, 
 without mortar, which injures the timber. If there be any 
 difference in the quality of the bricks, the strongest and 
 closest, which are least liable to imbibe moisture, should be 
 selected for foundation work. 
 
 Of Cements. 
 
 The bricklayer being provided with tools, with bricks, and 
 having prepared the trenches of his foundation for walling, 
 finds himself in immediate want of mortar or cement, a sub- 
 ject which next claims our consideration. 
 
BUILDING. 199 
 
 Cements. — Importance of Higgins's experiments — his mortar or stucco. 
 
 The nature and best metliods of preparing calcareous ce- 
 ments, have been investigated by Dr. Higgins, with great 
 ability and success. He has advanced the most satisfactory 
 proofs, founded upon analysis, that the Romans, whose mor- 
 tar or cement, after a lapse of two thousand years, instead of 
 being decayed, has become as hard as the stones it binds to- 
 gether, possessed no uncommon secret, which we are unable 
 to discover. His publication first appeared in 1780, and is 
 evidently the production of a liberal and intelligent mind. 
 He struck into a path with which we were but little acquaint- 
 ed, though the knowledge of it is of very considerable im- 
 portance to the public collectively, as well as to individuals : 
 for it is certainly lamentable to observe public or private edi- 
 fices, insecure, or prematurely mouldering away, from the 
 ignorance or disregard of a few particulars, which might not 
 only be observed with ease, but, in many instances, with a 
 diminution of the original expense. The Doctor's conclusions, 
 ■which were drawn from innumerable experiments, constitute 
 a great portion of the best part of our knowledge on this sub- 
 ject at the present time ; but though so many years have 
 elapsed since they were communicated to the public, they are 
 far from being yet generally known, and consequently are not 
 reduced into general practice. 
 
 Such is the neglect shewn on this subject, that the timbers 
 of our houses last longer than the walls, unless the moulder- 
 ing cement be frequently replaced by pointing. The follow- 
 ing directions, for preparing durable mortar or stucco, con- 
 tain the result of the Doctor's experience, and have been at- 
 tended to with remarkable success. 
 
 Sharp sand free from clay, salts, calcareous, gypseous, or 
 other grains less hard and durable than quartz, is better than 
 any other. When a coarse and a fine sand, corresponding in 
 the size of their grains, to the coarse and fine sand hereafter 
 described, cannot be easily obtained native, the following 
 method of sorting and cleansing it must be resorted to. Let 
 the sand be sifted in streaming clear water, through a sieve 
 which will allow all grains not exceeding one-sixte«nth of an 
 inch to pass through, and let the stream of water be regulated 
 so as to wash away the very fine parts of the sand, the clay, 
 and every other matter lighter than sand. The coarse rubbish 
 left on the sieve must be rejected. The sand which subsides 
 in the receptacle must then be further cleansed and sorted into 
 two parcels, by the use of a sieve which allows no grains to 
 pass but what are less than one-thirteenth of an inch in dia- 
 meter. That part which passes through this sieve, we shall 
 call fine sand, the remaining portion, coarse sand. These 
 
200 BUILDING. 
 
 Liccllcnt mode of preparing mortar. 
 
 separate portions may then be dried in the sun, or by means 
 of a fire. 
 
 That sort of lime must be chosen which heats the most in 
 slakino-, and slakes the quickest when duly watered; wliich is 
 the Ireshest made, and has been the closest kept, which dis- 
 solves in distilled vinegar with the least effervescence, and 
 leaves the smallest residue insoluble, and in this residue the 
 smallest quantity of clay, crypsum, or martial matter. Put 
 fourteen pounds of the lime chosen according to these im- 
 portant rules into a brass wire sieve still finer than the last 
 mentioned. Slake the lime by alternately plunging it into 
 and raising it out of a butt of soft water; reject all the matter 
 which does not easily pass through the sieve, and use fresh 
 portions of lime in a similar manner, until as many ounces of 
 lime have passed through the sieve as there are quarts of water 
 in the butt. This is the lime-water, which contributes materi- 
 ally to the excellence of the stucco. As soon as a sufficient 
 portion of lime has been imparted to it, it should be closely 
 covered until it becomes clear, and then be drawn off", by 
 wooden cocks, placed at different heights as the lime sub- 
 sides, without breaking the crust formed on the surface. The 
 freer the water is from saline matter, the better will this liquor 
 be. Lime-water must be kept in air-tight vessels till the mo- 
 ment it is used. 
 
 Slake fifty-six pounds of lime chosen as above directed, by 
 gradually sprinkling on it the lime-water. Sift the slaked part 
 of the lime immediately through the last mentioned fine brass- 
 wire sieve; the lime which passes must be used instantly, or 
 kept in air-tight vessels, and the rest rejected. This finer, 
 richer part of the lime, may be called purified lime. It is 
 always advisable to sift the lime immediate/i/ after the slaking, 
 otherwise much of the ill-burnt lime and heterogeneous matter 
 which it may contain, will pass through the sieve. 
 
 The materials of the cement being thus prepared, take fifty- 
 six pounds of the coarse sand, and forty-two pounds of the fine 
 sand; mix them on a large plank of hard wood placed horizon- 
 tally; then spread the sand so that it may stand to the height of 
 six inches with a flat surface on the plank, and wet it with lime- 
 water, of which so much must be allowed to flow away off" the 
 plank as the sand in the condition described cannot retain. To 
 the wetted sand add fourteen pounds of the purified lime in 
 several successive portions, mixing and beating them up toge- 
 ther with the instruments generally used in making fine mortar. 
 
 Then add fourteen pounds of bone-ashes in successive por- 
 tions, mixing and beating all together. The quicker and more 
 perfectly these materials are mixed and beaten together, and the 
 
BUItDINO. 201 
 
 Fine-grained mortar. — Coarse mortar. 
 
 sooner the cement thus formed is used, the better it will be. 
 As this cement is shorter than mortar or common stucco, and 
 dries sooner, it ought to be worked expeditiously in all cases, 
 and in stuccoing it ought to be laid on by sliding the trowel 
 upwards on it. The materials used along with it in building, 
 or the ground on which it is laid in stuccoing, ought to be 
 well wetted with the lime-water, at the instant of laying it on; 
 and when the cement requires moistening, lime-water should 
 always be used. 
 
 The proportions above given are intended for a cement mad^ 
 with sharp sand, for incrustation in exposed situations, where 
 it is necessary to guard against the effects of hot weather and 
 rain. In general, half this quantity of bone-ashes will be 
 found sufficient ; and although the incrustation in this latter 
 case will not harden deeply so soon, it will be ultimately 
 stronger, provided the weather be favourable. 
 
 When a mortar or cement of a fine texture is required, take 
 ninety-eight pounds of the fine sand ; wet it with the lime- 
 water, and mix it with the purified lime and the bone-ash in 
 the quantities and in the manner above described, with this 
 diff*erence only, that fifteen pounds or thereabouts of lime are 
 to be used instead of fourteen pounds, if the greater part of 
 the sand be very fine. This cement is suitable for the last 
 coating of any work intended to imitate the finer grained 
 stones ; but it may be applied to all the uses of the first men- 
 tioned composition. 
 
 When a mortar or cement is required, which shall be still 
 cheaper and more coarsely grained, much coarser sand than 
 the coarsest sort already spoken of may be made use of; for 
 the coarser the sand, the less the proportion of lime which will 
 be required. For example, of the coarsest sand alluded to, 
 take fifty-six pounds; of the coarse sand which passes through 
 the meshes of a sieve one-sixteenth of an inch in diameter, 
 twenty-eight pounds; and of the fine sand, fourteen pounds; 
 and after mixing these and wetting them with lime-water, in 
 the manner already described, add fourteen pounds, or some- 
 what less, of the purified lime, and then fourteen pounds, or 
 somewhat less, of the bone-ash. 
 
 When these cements are intended to be white, white sand, 
 white lime, and the whitest bone-ash, are to be chosen. Grey 
 sand, and grey bone-ash, formed of half-burnt bones, are to 
 be chosen to make the cement grey ; and any other colour 
 may be obtained either by chusing coloured sand, or by the 
 admixture of the necessary quantity of coloured talc in pow- 
 der, or of coloured vitreous or metallic powders, or other in- 
 gredients of a similar nature. 
 9.— Vol. I. 2D 
 
202 BUILDING. 
 
 Artificial stone. — Water cement. — Fluid stucco 
 
 These cements are applicable in forming artificial stone, by 
 making alternate layers of the cement and of flint, hard stone, 
 or brick, in moulds of the figure of the intended stone; the 
 stones thus formed being exposed to the open air to harden, 
 hut not exposed to rain till they are almost as strong as fresh 
 Portland stone. They may be made very hard, and beautiful, 
 by soaking them, after they are thoroughly dry, in the lime- 
 water, and repeating this process several times at distant in- 
 tervals. Incrustations, also, are greatly benefited by the ap- 
 plication of lime-water, the entrance of which is facilitated by 
 the use of bone-ashes in the cement. 
 
 When any of the above cements are intended to be used for 
 water-fences, two-thirds of the prescribed quantity of bone- 
 ashes are to be omitted, and an equal measure of powdered 
 terras used instead ; and if the sand employed be not of the 
 coarsest sort, more terras must be added, so that the terras 
 fehall be by weight one-sixth part of the weight of the sand. 
 
 When a cement is required of the finest grain, or in a fluid 
 form, so that it may be applied with a brush, for the purpose 
 of smoothing and finishing the stronger crustaceous works, or 
 for washing walls to a lively and uniform colour, the fine pow- 
 der of calcined flints, or the powder of any quartzose or hard 
 earthy substance, may be used in the place of sand ; but in a 
 quantity smaller as the flint or other powder is finer; so that 
 the powder shall not be more than six times the weight of the 
 lime, nor less than four times its weight. The greater the 
 quantity of lime within these limits, the more tlie cement will 
 be apt to crack by quick drying, and vice versa. For washing 
 walls, the cement should not be made thicker than new cream, 
 and should be laid on briskly with a brush, in dry weather. 
 Fine yellow ochre is the cheapest colouring ingredient for 
 such a wash, when it is required to imitate Bath stone, or the 
 warmer white stones. 
 
 Where sand cannot be procured, any durable stony body, 
 or baked earth grossly powdered, and sorted as if it were 
 sand, may be used, measure for measure, but not weight for 
 weight, unless the same bulk of the gross powder be the same 
 as that of sand. But all substitutes for pure siliceous sand, 
 •are imperfect in proportion as the particles of which they are 
 composed, are less hard than those of that material. The 
 scrapings of roads, which consist principally of powdered cal- 
 careous stone, the old mortar and other rulDbish from ancient 
 buildings, have often been more strongly recommended than 
 they deserve. These, and all muddy, soft, minutely divided 
 Wiaters, require a large proportion of lime, and never possess 
 the hardness and durability which belong to good mortar. 
 
BUILDING. 203 
 
 Use of lime-water — bone-ashes. — Sharp sand. 
 
 Sea sand, well washed in fresh water, is as good as any other 
 round sand ; but if used without being freed from salt, the 
 mortar made with it is extremely liable to be damp. 
 
 The proportion of lime may be increased without inconve- 
 nience, when the cement or stucco is to be applied where it is 
 not liable to dry quickly; and in the contrary circumstance, 
 this proportion may be diminished. The defect of lime in 
 quantity or quality, is best supplied, by soaking the work, at 
 distant intervals of time, with lime-water. It is proper to 
 mix hair with these cements, when employed for interior 
 work. 
 
 The powder of almost every well dried or burnt animal sub- 
 stance may be used instead of bone-ash. 
 
 The bone-ashes facilitate the operation of plastering, by 
 increasing the plasticity of the mortar into which they enter. 
 They also render the mortar less liable to crack, and cause it 
 to acquire more quickly that state in which it is not easily in- 
 jured by unexpected rain. If employed in a less proportion 
 than one-fourth of the lime, they are of little use, and if they 
 exceed the lime in quantity they are injurious to the cement. 
 Hence the use of them should be regulated according to the 
 following circumstances : when the artist is more solicitous to 
 secure an incrustation from the effect of hot weather, to finish 
 it quickly, and to guard against rain, than to make it durable 
 in the highest degree, he may use as much bone-ashes as lime; 
 but when the season, exposure, and other circumstances, per- 
 mit him to attend solely to the true excellence and duration 
 of his work, he must use, in his best calcareous cements, only 
 one part of bone-ashes for every four parts of lime. By these 
 rules he may chuse intermediate quantities adapted to his pur- 
 poses. The bone-ashes should not be in so coarse a powder 
 as they are when used for cupels, yet they should by no means 
 be levigated or ground to extreme fineness. 
 
 By sharp sand, is meant such sand as consists of grains 
 with flat surfaces ; these flat surfaces, when enveloped and 
 cemented together by the lime paste, possess a much stronger 
 cohesion than if the grains were globular. 
 
 The preceding method of making mortar oj", stucco differs, 
 it will be perceived, from the common process in several essen- 
 tial particulars; among which, the purity and sorting of the 
 sand, the use of lime-water, the newness of the lime, and the 
 large proportion of the sand to the lime, ought to be particu- 
 larly noticed. It may be useful to inquire into some of the 
 causes of differences of practice so remarkable. When the 
 sand contains much clay, or other impurities, the bricklayers 
 find that the best mortar they can make, must contain about 
 
204 BUILDIN'G. 
 
 Causes of tbe debasement of lime. 
 
 one-half lime; and in consequence pronounce, without further 
 investigation, half sand and half lime, to be the best compo- 
 sition. But with sand requiring so much lin>e, they never 
 can make durable mortar, though of this fact it may be diffi- 
 cult to convince those who are little disposed to investigate 
 causes. Too many artisans entertain an opinion that they have 
 nothing new to learn wliich is worth notice ; they are apt in 
 effect to say, that havingserved an apj)renticeship to their busi- 
 ness they ought to ]i.now something; and thus, because they ought 
 to know something, they seem to expect submission to their very 
 errors. To such characters we speak not ; to convince them is 
 impossible, and therefore the attempt is folly. But those who 
 consider the interest of their employers, and that the warmth, 
 dryness, and salubrity of a house, so far as the building is 
 concerned, is completely in their power, will not despise any 
 hint which may extend their resources. 
 
 It is a common fault to build lime-kilns so high, that at the 
 bottom of the cavity, the lime is ready perhaps eighteen hours 
 before that in the upper part, and is greatly injured by its ex- 
 posure to the draft of air passing through the fire. Lime-kilns 
 ought to be made much broader and shallower than customary, 
 with the cavity tapering upwards, and should terminate in a 
 lofty flue, in order to accelerate the combustion, when requir- 
 ed, by a quick current of air. Calcareous stones acquire in 
 the most eminent degree the properties of lime, when they are 
 slowly heated in small fragments of uniform size, until they 
 appear to glow with a white heat, and this is continued until 
 they become non-effervescent if steeped in an acid. The art 
 of preparing lime consists chiefly in attending to these parti- 
 culars. The whiteness of lime shews it to be free from metallic 
 impregnation. JNIerely to keep lime dry is not enough to pre- 
 serve it; it grows worse for mortar every day it is kept in heaps 
 or untight casks, and is soon reduced nearly to the state of chalk. 
 It may be greatly debased, without slaking sensibly, and such 
 parts or fragments as fall to powder merely by exposure to 
 the air are unfit for mortar. It has been found by experiment, 
 that lime will absorb one-fourth of its weight of water, and 
 yet remain perfectly dry. Bishop Watson found, that, upon 
 an average, every ton of limestone produced eleven hundred 
 weight, one quarter, and four pounds, of lime, weighed before 
 it was cold ; and that, when exposed to the air, it increased 
 in weight daily, at the rate of a hundred weight per ton, for 
 the first five or six days after it was drawn from the kiln. 
 Hence those who have to fetch lime from great distances, may 
 save even in point of cartage, by receiving it as it is taken out 
 of the kiln. 
 
BUILDING. ~ 205 
 
 Dryiug of mortar. 
 
 Mortar which sets without cracking, whether this be owing 
 to the due proportion of sand, or to the slow exhalation of the 
 water from mortar containing less sand, never cracks after- 
 wards, whatever its faults, in other respects, may be. As it 
 is the lime paste, and not the sand, which contracts and pro- 
 duces fissures in drying, so the more sand there is in the com- 
 position, the less the cracks will be seen. Mortar which is 
 liable to crack, becomes irreparably injured by frequent alter- 
 nations of wetting and freezing ; for the water imbibed by the 
 smallest fissures, dilating as it congeals, loosens its whole tex- 
 ture. Where it is composed of seven parts of sorted sand, to 
 one of lime, it is not disposed to crack. 
 
 That mortar may become indurated the soonest and in the 
 highest degree, and operate the most effectually as a cement, 
 it must be suffered to dry gently and set ; the exsiccation of 
 it must be effected by temperate air, and not accelerated by 
 the heat of the sun or fire. It must not be wetted till after it 
 sets; and afterwards it ought to be protected from wet as much 
 as possible, until it is completely indurated ; the absorption of 
 carbonic acid must be prevented as much as possible till the 
 mortar is finally placed and quiescent, and then it must be as 
 freely exposed to the open air as the work will admit, in order 
 to supply carbonic acid, and enable it sooner to sustain the 
 trials to which mortar is exposed in cementitious buildings 
 and incrustations. To shew more clearly how much our 
 slight buildings are weakened by the agitations and percus- 
 sions to which they are exposed, first in erecting the walls 
 and settling the timbers, and then in driving those wedges to 
 which the wainscots, mantle-pieces, and other ornaments, are 
 fastened, we must observe, that the absorption of carbonic 
 acid by mortar contributes nothing to the strength of it, if it 
 enter before it is finally fixed in a quiescent state. A little 
 experience is sufficient to teach us, that the same matter which 
 assists in the induration of mortar, never serves to repair the 
 fissures, or solution of continuity between the bricks and ce- 
 ment, which happens after it is set. When mortar is set, and 
 before it is indurated, it may be easily severed from the bricks 
 and crumbled ; and for want of softness, it cannot bend into 
 the fissures, or resume its former condition in any time. 
 Hence by heavy blows, and in wedging, our walls must be 
 greatly weakened ; and the more so, as the houses are slight, 
 quickly built, and hastily finished. 
 
 Nothing is more common than for bricklayers to keep their 
 mortar some time exposed to the air in heaps, before they con- 
 sider it proper to use ; a practice which may perhaps be ac- 
 counted for, if we consider that some portions of every kind of 
 
206 BUILDING. 
 
 Atlvantaijes of steepiog bricks in lime-water. 
 
 lime used in this country, do not slake freely, by reason of 
 their not being sufficiently burned, or of the admixture of gyp- 
 seous or argillaceous matter; which portions, like marl, slake 
 in time, though not so quickly as the purer lime. The plas- 
 terers, who use a finer kind of mortar, made of sand and lime, 
 observe that their stucco blisters, if it contain small bits of 
 unslaked lime, and as smoothness of surface is with them of 
 more consequence than excessive hardness, they take care to 
 secure the perfect slaking of their lime by allowing sufficient 
 time for the imperfect parts to be penetrated by the moisture. 
 The bricklayers, trusting, perhaps, more to the judgment of the 
 plasterers, in this respect, than to their own, and considering 
 It very convenient to slake a large quantity of lime at once, 
 follow the same practice, without caring for or apprehending tiie 
 real fact, that mortar is worse for every hour it is kept, and that 
 they are taking such measures as will prevent it from ever ac- 
 quiring that degree ofhardness in which its perfection consists. 
 
 Among the circumstances which contribute to the speedy 
 ruin of modern buildings, it may also be observed, that mortar 
 made with bad lime, and a great excess of it, is used with dry 
 bricks, and not unfrequently with warm ones. These imme- 
 diately imbibe or dissipate much of the water, and as the ce- 
 ment approaches nearer to be dry, whilst it is still liable to 
 be displaced by the percussions of the workmen, render it 
 little better than equivalent to a mixture of sand and pow- 
 dered chalk. To make strong work, the bricks ougiit to be 
 soaked in lime-water, and freed from the dust with which 
 they are commonly covered. By this means the bricks are 
 rendered closer and harder, the cement, by setting slowly, 
 admits the motion which the bricks receive when the work- 
 man dresses them, without being impaired, and it adheres and 
 indurates more perfectly. This steeping of the bricks is an 
 imitation of the practice of the plasterers, who always wet the 
 wall before they commence their work, because they know 
 the cement will not otherwise adhere. This ought to be done 
 as long as the wall is thirsty, and lime-water is the most pro- 
 per liquid thev can use. The same advantage that attends 
 the soaking: of bricks, would attend the soaking of bibulous 
 stones in lime-water. 
 
 Mortar made with sand containing one-seventh or one-eighth 
 of fat clay, moulders in winter like marl; a circumstance which 
 proves the propriety of freeing from clay the sand used in mor- 
 tar. The washing performed for this jiurpose, will be found 
 a very cheap operation, even in cities, if the water which car- 
 ries off the clay be directed into a receptacle where it may be 
 depurated by subsidence for repeated use. 
 
BUILDING. 207 
 
 Chalk lime. — Water fittest for mortar. — Causes of damps on stucco. 
 
 Chalk lime may be easily prepared, so as to be fully equal if 
 not superior to stone lime. The reason why this is not generally 
 thought to be the case, probably is, that not being of so close a 
 texture, it is sooner spoiled by the absorption of carbonic acid, 
 when exposed to the atmosphere after it is made. A cask of 
 chalk lime should therefore never be opened till the moment it 
 is to be slaked, and the greatest expedition should be used in the 
 slaking, and in the making and applying the mortar to use. In 
 the quiescent air of a room, a pound avoirdupois of chalk 
 lime, becomes two ounces and a half heavier in two days; and 
 nearly the whole of this increase of weight, consists of the car- 
 bonic acid which it has imbibed from the atmosphere. 
 
 The fittest water for making mortar, is rain water; river 
 water holds the next place; land water the next ; spring water 
 the last; sea water, and all waters noticed medicinally or 
 otherwise, for their saline contents, ought never to be used for 
 this purpose. 
 
 The compositions mostly used for stuccoing within doors, 
 are incapable of haidening considerably, and when they are 
 laid on the naked walls, soon become tarnished, unsightly, and 
 inconvenient, by the damps which workmen call sweating. 
 Sometimes these damps are occasioned by the bad construc- 
 tion of the walls, the joints of the facing bricks having become 
 hollow by the decay of the mortar, or when the copings or 
 gutters are defective: a damp by transpiration also occurs 
 when the principal walls are stuccoed before they are dried, 
 or when the materials of them are so spongy as to imbibe the 
 rain, and the circulation of air is not sufficient to waft away 
 the transuding moisture. The damp by condensation is also 
 very common, and appears most on the closest incrustations, 
 however perfect and old the walls may be. In such instances, 
 the damp is owing to the closeness of the body, and a stucco 
 pervious in a certain degree to air and moisture, will be free 
 from it, as well as from the damp before mentioned. The cus- 
 tomary mode of avoiding these damps, is to case the principal 
 walls of houses with lathwork, on which the incrustation or 
 plaster is laid at some distance from the wall. The narrowing 
 of rooms and passages very perceptibly is the consequence of 
 this method, besides its expense. Bone-ashes, each grain of 
 which is tubulated in every direction, added to the stucco in 
 half the quantity of the lime, are a preventive of these damps 
 without lathwork. 
 
 The drying, induration, and texture of incrustations made on 
 brick walls and other irregular surfaces, are always so far un- 
 equal as to exhibit visible traces which deform the work, and 
 cannot be effectually obliterated by any known method so con- 
 
20O BUILDING. 
 
 Colouring of stucco. — Effect of paiDting it. — Water ceraents. 
 
 venient as that of covering the first coarse incrustation, after 
 it has dried, with another coat which may be made finer and 
 smoother. Thus the expense of fine-grained, smooth, or co- 
 loured stucco, is rendered moderate ; because the finer, or the 
 colouring ingredients, may be reserved for the exterior coat, 
 wliich will last for ages, if the cement be good 
 
 To tinge a cement sufficiently, of any colour which is not 
 found in sand, so that the incrustation shall not be impaired, 
 and that the colour shall be as durable as the cement, the 
 most proper ingredients which can be used in lieu of sand, or 
 of part of it, are coloured glasses or coloured stones of the 
 hardest kind, beaten to coarse powder; the finer parts of which 
 are to be washed away, not merely because they are injurious 
 to the cement, but because they contribute very little to the 
 intended colour. 
 
 Stucco made with the best proportions of lime, sand, and 
 lime-water, is not bettered by painting as soon as it dries ; as 
 this covering retards the induration of it, by cutting off its 
 communication with the air. It therefore renders it liable to 
 be irreparably injured in wet weather, wherever the water can 
 get behind the paint. If paint or oil ought ever to be applied 
 on such stucco, it ought not to be laid on in less than a year 
 after the incrustation is made. The painting and sanding of 
 the common defective incrustations, contribute very little to 
 their duration, although it hardens them at the surface; for it 
 does not effectually prevent them from cracking, and it avails 
 very little to paint the cracked stucco again, because cracked 
 stucco is always hollow, as the workmen term it; that is, it 
 parts from the wall in the parts contiguous to the cracks, 
 sounds hollow on being struck w ith the knuckle, and falls off 
 in a few years, if it be so thick and large in extent, as to break 
 the adhering portions bv its weight. 
 
 Mortar made of sand, water, and lime, whatever maybe the 
 proportions of the mixture, cannot be employed in aqueducts, 
 reservoirs, and other aquatic buildings, unless sufficient time 
 be allowed for its perfect induration before the admission of 
 the water; but when mixed up with a quantity of terras, as 
 already stated, it acquires the desirable property of hardening 
 under water. A few additional remarks on this subject will 
 perhaps be acceptable. A mortar made of terras powder and 
 lime was used in water-fences by the Romans, and has been 
 generally employed in such structures ever since their time. 
 As it sets quickly, and when set is impenetrable to water, 
 some people have hastily concluded that it is the best kind of 
 mortar for any purpose. But it is found by experience, that 
 mortar made of terras powder, whether coarse or fine, will 
 
BUILDING. 209 
 
 Water cements. — Brick bond. 
 
 not grow SO hard as mortar made with lime and sand, nor 
 endure the weather so well ; on the contrary, it is apt to crack 
 and perish in the open air. Its efficacy in water-fences is ex- 
 perienced only where it is always kept wet, and seems to de- 
 pend principally upon the property which the powder of 
 terras has, in common with other argillaceous bodies, but in 
 a higher degree, of expediting the crystallization of the cal- 
 careous matter, by imbibing the water in which it is diffused 
 in the mortar, and of swelling, during this absorption, so 
 much as to render the mortar impenetrable to any more 
 water. It seems, also, that an acid of the vitriolic kind, 
 which is contained in terras, contributes to the speedy setting 
 of the cement, by reducing a part of the lime to the condition 
 of gypsum. Terras is a volcanic production, consisting chiefly 
 of clay and oxide of iron indurated together; and baked clay, 
 reduced to powder, communicates to mortar properties of a 
 similar kind. 
 
 Pozzolana is another volcanic production differing little 
 from terras, as to the effect it produces in mortar. It is 
 thrown out of volcanoes in the form of ashes, and is found in 
 many countries, but most abundantly in the kingdom of 
 Naples. The cement used by Smeaton, in the construction of 
 the Eddystone light-house, was composed of equal parts by 
 measure of lime and pozzolana ; a mixture which was deemed 
 the most suitable, as the building was exposed to the utmost 
 riolence of the sea ; but a composition exceedingly proper for 
 aquatic works in general, may be composed of two parts of 
 iime, one of pozzolana, and three of clean sand. 
 
 It has lately been discovered, that manganese is a valuable 
 ingredient in water-cements, if used in the following manner-: 
 mix together four parts of gray clay, six of the black oxide of 
 manganese, and ninety of good limestone reduced to fine 
 powder ; then calcine the whole to expel the carbonic acid. 
 When this mixture has been well calcined and cooled, it is to 
 be worked into the consistence of a soft paste with sixty parts 
 of washed sand. If a lump of this cement be thrown into 
 water, it will harden immediately. 
 
 Of Brick Bond and Walling, 
 
 When a brick is laid so that its length is in the direction of 
 the length of the wall, it is called a stretcher; and when its 
 length crosses that of the wall, it is called a header. 
 
 The term bond is applied to any disposition of the bricks, 
 by which the continuity, in a straight line, of the joints of a 
 wall is interrupted. It is obvious that a bond may be adopted, 
 9.— Vol. I. 2 E 
 
110 BUILDING. 
 
 Englisb and Flemish bond. 
 
 which will interrupt the rectilinear direction of both the hori- 
 zontal and vertical joints of a wall; but in the two kinds of 
 bond which have hitherto prevailed, the horizontal joints are 
 contnnued in the same line round the whole building, and the 
 vertical ones only interrupted. \Vhen the wall is only intended 
 to be half a brick, or four inches and a half in thickness, the 
 whole of the bricks are laid so as to form stretchers, that is, 
 their length is laid in the direction of the length of the wall, 
 and the bond is obtained simply by making the vertical joints in 
 every course exactly opposite the middle of the bricks above 
 and below. But when the wall is intended to be the length 
 of a brick or more in thickness, it would be apt to split into 
 parts if it consisted only of two or more walls separately bond- 
 ed, as in the instance just mentioned of the half brick wall. 
 The bricks, therefore, in thick walls, must be connected ia 
 their breadth as well as their length, and this is done accord- 
 ing to two principal modes, one of which is called English, 
 the other Flemish bond. 
 
 English bond consists of headers and stretchers crossing 
 each other in separate horizontal courses. In Flemish bond, 
 the headers and stretchers are placed alternately in the same 
 horizontal course. Flemish bond is now so common, that 
 hardly any other kind is to be seen; it is preferred, for its ap- 
 pearance, to the English, which is much superior in point of 
 strength, and in facility of execution. ISIany attempts have 
 been made to unite Flemish facings with complete bond, but 
 without success. Some have laid thin slips of iron occasion- 
 ally in the horizontal joints between the two courses; and 
 others have laid diagonal courses of bricks in the core of thick 
 walls, so as to cross each other at right angles in successive 
 courses. By the latter means, though the bricks in the middle 
 of the core have a strong bond, yet as they form triangular 
 interstices with the bricks en each side, the bond of the whole 
 wall is very incomplete. As the adjustment of the bricks in 
 one course must depend on the course beneath, the latter, in 
 making the Flemish bond, must be seen or recollected by the 
 workman. The view of the joints of the under course is hin- 
 dered by the mortar with which thev are covered, to bed the 
 bricks of the succeeding course upon, and it is perplexing for 
 the workman to recollect the arrangement of them, so that he 
 is in danger of making the joints frequently to correspond, 
 and thus rendering the bond imperfect. In the old English 
 bond, the outside of each course points out the proper dis- 
 position of the next, so that it is difl&cult for the workman 
 to err. 
 
 The following plans of walls in English bond, wb'^eh well 
 
BUILDING. 211 
 
 English and Flemish bond. 
 
 deserve to be revived in this country, will render this subject 
 more intelligible : 
 
 Fig. 3, pi. I. is the bond of a nine-inch wall. In order that 
 two vertical joints may not run over each other, at the end of 
 the first stretcher from the corner, after placing the return 
 corner stretcher, which becomes a header in the face that the 
 stretcher is in below, half the length of which it covers, a half 
 brick is placed on its side, so that with the return corner 
 stretcher, it extends six inches and three quarters, and thus a 
 lap of two inches and a quarter is obtained for the next header; 
 and the bond is continued by working up the wall with alter- 
 nate rows of headers and stretchers mutually crossing each 
 other. The half brick, or brick-bat, thus introduced, is called 
 a closer, and must be divided, it will be understood, through 
 its too broadest surfaces, in the direction of their length. The 
 same effect might be obtained by the introduction of a three- 
 quarter brick at the corner of the stretching course, for then 
 when the corner header is laid over it, a lap of two inches and 
 a quarter will be left at the end of the stretchers below for the 
 next header, the middle of which, when laid, will be over the 
 joint below the stretcher, and thus constitute a bond as before. 
 The brick for the three-quarter bat, or closer, must be divided 
 through its two broadest surfaces, in the direction of their 
 breadth. 
 
 Fig. 4, represents the English bond of a brick and a half or 
 fourteen-inch wall. Here the stretching course is so disposed, 
 that the middle of the breadth of the bricks in the same layer 
 or level, falls alternately upon the middle of the stretchers, 
 and upon the joints between the stretchers. 
 
 Fig. 5, represents the English bond of a two-brick wall. 
 To break the joints in the core of the wall, every alternate 
 header, in the heading course, is only half a brick thick. 
 
 Fig. 6, represents the English bond of a wall two bricks and 
 a half in thickness. The disposition of the bricks is similar 
 to that of those in the last example. 
 
 Fig. 7, part of the front of a wall in English bond, the un- 
 broken side being the corner. 
 
 Fig. 8, the Flemish bond of a nine-inch wall. Two stretchers 
 lie between two headers ; and bricks being twice as long as 
 they are broad, the breadth of the two stretchers is equal to 
 the length of the header, which is the whole thickness of the 
 wall. The dotted lines shew the disposition of the bricks in 
 the second course. 
 
 Fig. 9, the Flemish bond of a brick and a half or fourteen- 
 inch wall. On one side the bricks are laid as in the last 
 example, and on the other side, half headers are placed oppo- 
 
212 BUILDING. 
 
 Flemish bond. — -X'ertical bond. — Straight arch. 
 
 site the middle of the stretchers, and the middle of the 
 stretchers opposite the middle of the end of the headers. 
 
 Fio". 10, another example of Flemish bond, for a wall of the 
 same thickness as the last. Here the disposition of the bricks 
 is alike on both sides of the wall, the tail of the headers being 
 placed contiguous to each other, an arrangement that pro- 
 duces, in the core of the wall, square spaces, which must be 
 filled with half bricks. 
 
 Fig. 11, part of the front of a wall in Flemish bond, reach- 
 ing on one side to the corner. 
 
 Fig. 1, 2, 3, and 4, pi. II. exhibit plans of brick piers in 
 Flemish bond. ISo. 1, in each figure, shews the bottom 
 course, and No. 2 the upper course; or, which amounts to the 
 same thing, No. 2 may be considered the lower course, and 
 No. 1 will then be the upper one. 
 
 Fig. 1, is a pier, two bricks, that is, eighteen inches square; 
 for in speaking of the thickness of a wall or pier as consist- 
 ing of so many bricks, the length of a brick is always to be 
 understood. 
 
 Fig. 2. a two and a half brick pier. 
 
 Fig. 3, a pier three bricks square. 
 
 Fig. 4, a pier three bricks and a half square. 
 
 Before we take leave of the subject of bond, we must re- 
 mark, that a patent has very lately been taken out, by Moore 
 and Co. of London, for a vertical bond, which is intended 
 to supersede the use of the bond timbers introduced to 
 secure the equal settlement of the wall. In case of fire, 
 when the bond timbers of a house are consumed, the falling 
 of the wall almost necessarily follows. The patentees, there- 
 fore, instead of these timbers, place rows of hard strong 
 bricks perpendicularly in the middle of their walls, at short 
 distances from each other in height as well as horizontal 
 measurement, and they place each row of the perpendicular 
 bricks in such a manner, as to be opposite the middle of 
 the space between the row standing in the same position 
 immediately above or below it. This plan for obtaining a 
 vertical bond, seems new and ingenious, and is applicable, 
 as indeed the patentees observe, to stone walls, as well as 
 to those of brick. 
 
 Fig. 5, represents a straight arch, which is usually made 
 the height of four courses of brick ; but in considerable build- 
 ings, it may with great propriety be made the height of five 
 courses. The manner of drawing the joints of a straight arch 
 will be evident from an inspection of the figure. The joints of 
 the arch-stones must all be made to lie in a direct line to the 
 point C. The point C is easily obtained, as it is as far from 
 
BUILDING. 213 
 
 Scheme, Serai-circular, and Elliptical arches. — Tapper's mode of groining. 
 
 the points A and B, (which are separated by the whole breadth 
 of the aperture,) as A and B are from each other. Hence the 
 lines connecting A, B, and C, form an equilateral triangle. 
 To key the arch in, it is usual to have a brick, and not the 
 joint between two bricks, in the centre; and therefo"e the di- 
 vision of the arch must be managed accordingly. Though the 
 brick in the middle tapers more in the same length than the 
 extreme bricks, yet as the difference is very small, it is dis- 
 regarded, from the great convenience of drawing all the bricks 
 with the same mould. It may, however, be observed, that the 
 real taper of the mould may be a medium between that re- 
 quired for the middle and that for either extreme distance. 
 But whether this be done or not, the defect will not appear in 
 practice. 
 
 Fig. 6, a scheme arch, one brick, that is nine inches high. 
 
 Fig. 7, a semi-circular arch, one brick high. 
 
 Fig. 8, an elliptical arch, one brick high, the top of which 
 is divided into equal parts, and not the under side. It is 
 struck from three centres. A, B, and C. 
 
 The arches delineated in the last three figures are often made 
 a brick and a half, or even two bricks high; but for crowning 
 the apertures of ordinary dwellings, the height of one brick is 
 deemed sufficient, both for stability and appearance. In arches 
 of one brick high, when the walls are only half a brick thick, 
 it is evident there is no necessity for joints following the course 
 of the arch in every alternate brick. Accordingly, tliese joints, 
 in such walls, are generally false ones, being merely nicks of 
 little depth cut with the tin-saw; and are made as a kind of 
 decoration, in arches exposed to view, to give, when pointed 
 along with the other joints, a more lively appearance. But 
 when the walls are of one brick or a greater thickness, the 
 joints in question must be real, and formed of two bricks dis- 
 posed as headers, for the sake of bond. 
 
 Fig. 9, a plan of Tapper's improved method of groining. The 
 improvement consists in raising the angles from an octagonal 
 pier instead of a square one. By this means, the angles of the 
 groins are strengthened by carrying the band round the dia- 
 gonals of equal breadth, thus affording better bond to the 
 bricks, which are usually so much cut away, that instead of 
 giving support, they are themselves supported by the adjacent 
 fiUing-in arches. Square piers are very inconvenient in cel- 
 lars, by hindering the turning of goods round their angles. 
 
 In different parts of the country, it will naturally be sup- 
 posed that many differences of practice will prevail in the de- 
 tails of building. Several of these differences originate in local 
 peculiarities of materials, of one kind or another; but there ia 
 
214 BUILDING. 
 
 Exterior and interior kcallbldiug. — Precautions to obtain dry walls. 
 
 one, not belonging to this class, respecting which we might 
 have expected to find a very general uniformity of practice. 
 In London, and a wide district around, the scaflblding for the 
 workmen, in erecting the walls of a building, is external; but 
 in Liverpool, and several other parts of Lancashire, and adja- 
 cent countries, the scatt'olding is wholly within the building, 
 whatever may be its size or consequence. On the merits of 
 either plan we shall not offer a decisive opinion; yet we may 
 remark, that external scaffolding is not only the most expensive, 
 but has an air of insecurity to the workman, and of incompact- 
 ness, which is unpleasant to the observer, especially when he is 
 aware that such cumbersome appurtenances may be dispensed 
 with. In populous towns, or confined places, also, the en- 
 croachment of external scaffolding upon the street, is not a 
 triHing inconvenience, particularly as the bricks and other ma- 
 terials must be at some distance beyond its limits, to prevent 
 accidents. Interior scaffolding, on the contrary, being support- 
 ed on the joists of each floor as the work proceeds, is erected 
 with little trouble or expense, the workman marks his joints with 
 as much ease and regularity as if he were at the outside, such a 
 thing as a falling brick or splinter is hardly known, and the 
 bricks and other materials, may, if necessary, be laid close to 
 the wall, so as to occasion little inconvenience in the street. 
 
 To obtain the desirable requisite of dry walls, we have al- 
 ready observed, that the usual resort is the use of interior lath- 
 work, and have adverted to the means of preventing this ex- 
 pense, by a proper composition of the mortar or stucco. An- 
 other expedient in common use, intended to secure the dryness 
 of the walls when lathwork is deemed too expensive, consists 
 in leaving a portion of the bricks in the core or middle of the 
 walls, without mortar from the top to the bottom. The inter- 
 stices thus left, serve, in some measure, the same purpose as 
 the space between the lathwork and the bricks. Perhaps the 
 reference to a figure may make the nature of the plan more 
 easily apprehended. In the nine-inch wall, fig. 8. pi. L the 
 longitudinal joint a b, and the middle third, or from e to J', of 
 the transverse joint c d, would be left without mortar; and the 
 same thing is done to all the other joints or portions of joints 
 similarly situated. By this contrivance, the bond of the wall 
 is greatly weakened; but strength, it will be perceived, is not 
 the object of it; the walls in the inside are drier, they look as 
 well, the house will let for as high a rent as a stronger, and its 
 instability is left for the purchaser, or the next generation, to 
 discover. It has not unfrequently happened, however, that the 
 slightness of modern houses has been quickly proved, by their 
 having tumbled down before they were even finished. 
 
BUILDING. 215 
 
 Advantage ofcasbig walls. — Directions for walling. 
 
 There is one advantage of casing external walls with lath- 
 work, which is independent of damp, and therefore not the 
 object, though the consequence, of that operation. As air is 
 one of the most imperfect conductors of heat known, the co- 
 lumn of it included between the plaster or stucco and the 
 bricks, tends to prevent the temperature of apartments from 
 being affected by sudden vicissitudes in the heat of the exter- 
 nal atmosphere. — Lathwork casing should be composed of 
 well-seasoned heart laths, as the sap laths will shrink. Reeds 
 are used instead of laths in some parts of the country ; but 
 they require a greater quantity of mortar than laths, and pro- 
 duce on the whole little or no saving. 
 
 Frost is exceedingly prejudicial to new brick-work, and its 
 effects ought to be guarded against with the utmost care. 
 When it is apprehended, the wall should not be left uncovered 
 at night ; a capping of straw, or of weather boarding formed 
 like the roof of a house, to carry off the rain equally on both 
 sides, if any occur, is generally employed ; and sometimes, for 
 the more complete security, both the straw and the boarding 
 are employed at the same time, the straw being placed next 
 the wall. In winter, the mortar should be used stiffer than at 
 other seasons, and if a quantity of lime, which is quite fresh, 
 or has been kept in tight casks till it is wanted, were reduced 
 to powder without slaking, and well beat up with it, the set- 
 ting of it would be very materially hastened. Where strong 
 work is required, it is not expedient, even in winter, to relin- 
 quish the practice already recommended, of steeping the 
 bricks in lime-water; and when this is done, the method just 
 mentioned of preparing the mortar, is the more useful. The 
 bricks should not, however, be laid so dripping wet in winter 
 as in warm dry weather. When the practice of steeping the 
 bricks in lime-water is rejected as too troublesome, the sprin- 
 kling of each course with common water may be considered the 
 easiest substitute for that operation. This method of strength- 
 ening the work, was adopted in the building of the College of 
 Physicians, London, at the judicious suggestion of Dr. Hooke. 
 
 If the mortar has been suffered to lie any time, previous to 
 its being used, the labourer should beat it up again to give it 
 tenacity, and to prevent the bricklayer from losing time in 
 working it with his trowel. 
 
 In working up the wall, it is by no means advisable to work 
 more than four or five feet in height at a time; for as all walls 
 shrink a little soon after building, if the different parts of the 
 circuit of the walls or carcass be carried up at distant inter- 
 vals, one part will sink by itself, and will consequently sepa- 
 rate from the other part which has become fixed. No portion 
 
216 BUILDING. 
 
 Directions for walling. — Measurement of brick-work. — Mason's tools. 
 
 of a wall ought to be carried up more than the height of one 
 scaftold above the rest, except in some case of pressing emer- 
 gency. In carrying up any particular part, each side on the 
 right' and left should be sloped off, to receive the bond of the 
 adjoining work. 
 
 When the house the bricklayer is employed upon is intended 
 to have otlier houses parallel with it, half bricks in a straight 
 line with the front, should be left projcctino; from every alter- 
 nate course, at the corner or corners to which the addition is 
 intended to be made; or, otherwise, an excavation equal to the 
 breadth of a brick in front, and to the thickness of the wall 
 from front to back, should be left in the alternate courses. In 
 either case, the two fronts will be bonded together, and the 
 gaps, so frequently deforming contiguous houses, when the 
 fronts have been built independently of each other, will be 
 prevented. 
 
 Bricks, as a building material, have several advantages over 
 stone; from their porous texture, they unite better with the 
 cement, are much lighter, and the walls built with them are 
 very little subject to damps arising from the condensation of 
 the moisture in the atmosphere. When all materials are ready, 
 a good workman with his labourer will lay a thousand or 
 twelve hundred bricks in one day. 
 
 Brickwork is measured by the square foot, reduced to the 
 thickness of one brick and a half; thus a wall two bricks 
 thick, ten feet long, and three feet high, and therefore contain- 
 ing only thirty square feet of surface in front, would be called 
 forty feet reduced. It is valued by the rod of two hundred and 
 seventy-two feet. Facing and gauged arches are measured by 
 the superficial square foot ; and cornices by the foot running, 
 or length. 
 
 MASONRY. 
 
 Masonry is the term used to designate the art of building 
 with stone, as well as the work itself when executed. 
 
 Of the 3Iasous l^ools. 
 
 The tools required by the mason consist principally of a 
 Level, a Plumb Rule, a Square, a Bevel, a Trowel, a Hod, a 
 pair of Compasses, a Saw, a Mallet, and various sorts of Ham- 
 mers and Chisels. The whole of these tools, except the four 
 sorts last named, are similar to those bearing the same name 
 among bricklayers, and therefore most of them have already 
 been sufficiently described. 
 
BUILDING. 217 
 
 The masoa's saw — mallet — chisels. 
 
 The masons generally employ labourers to make use of their 
 saio, as they can hire them at a cheaper rate than regular 
 artists. The saw is without teeth, and is stretched in a frame, 
 so as nearly to resemble the joiner's frame-saw. It is generally 
 made from four to six feet in length, and for cutting through 
 slabs of uncommon size, it is occasionally made much larger. 
 Its progress through the stone is facilitated by the use of sharp 
 siliceous sand and water. The sand is placed upon an inclined 
 plane, and the water, exuding drop by drop from a small bar- 
 rel or other convenient vessel, runs over this plane, and carries 
 along with it a portion of sand into the kerf. The workman, 
 in the mean time, slowly drags the saw backwards and forwards 
 horizontally, taking a range of about twelve inches before he 
 makes the return stroke. By this simple process, the hardest 
 calcareous stones are cut into slabs of the required thickness, 
 with very little loss of their substance. But though the prac- 
 tice of sawing stone by hand may be considered easy, it is at 
 the same time slow and expensive. — Mills have therefore been 
 erected in various parts of the kingdom, for sawing and polish- 
 ing marble, particularly at Ashford in Derbyshire, and at Ken- 
 dal in Westmoreland. At the latter place, the machinery pro- 
 duces on stone, every moulding which the turner can produce 
 on metal, or the joiner with his plane on wood. The beautiful 
 marbles of the neighbourhood, are wrought at a cheap rate, 
 into various forms, which, from the great expense that would 
 be incurred, would never be attempted by manual labour. It 
 seems to be a desideratum in the management of such ma- 
 chinery, to produce the return of the mouldings. Artists in 
 wood do not require this return, from the facility with which 
 they can make use of mitre-joints; but to the mason, who can 
 rarely adopt joints of this nature, the return of the moulding 
 is mostly an indispensable requisite. In a chimney-piece, or 
 the architrave of a door or window, for example, the return is 
 required on the horizontal piece. 
 
 The shape of the mason's mallet differs from that of other ar- 
 tists. Its contour is not unlike that of a bell, except that a 
 small portion of the broadest part of it is cylindrical ; this part 
 is usually about eight or ten inches in diameter. The handle 
 is just long enough to be firmly grasped in the hand. The 
 chisels are struck with any part of the cylindrical surface. 
 
 The chisels used by the masons are required to be of various 
 breadths on the cutting edge, but three inches for the broadest, 
 and a quarter of an inch for the narrowest, may be considered 
 the general limits of their sizes in this respect. They are usu- 
 ally made of iron and steel welded together, and the steel ex- 
 tends no farther than thev are likely to be ground for use. 
 10.— Vol. I. ■' 2 F 
 
218 BUILDING. 
 
 Diflerent kinds of chisels — hammer. — .Marble. 
 
 When new, they are mostly about eight or nine inches long. 
 The part held in the hand, is generally about five-eighths or 
 three quarters of an inch in diameter, and is forged to an oc- 
 tao-onal shape. When the cutting edge is broader than this 
 octao-onal portion for the hand, the lower part is first expanded 
 in a dove-tail form, and then the sides for a short distance are 
 parallel to the edge. When the cutting edge is narrower than 
 the octagonal part, the lower end is sloped oft' in a pyramidal 
 form. The larger sizes of chisels obtain the name of ^00/5, the 
 act of using them is called tooling, and the stone to which they 
 have been applied is said to be tooled. The chisel with the nar- 
 rowest edge, which is seldom broader, and often not so broad, 
 as a quarter of an inch, is called a point. The point is em- 
 ployed to reduce the larger irregularities of a surface, which is 
 afterwards made tolerably regular with the broad chisels or 
 tools. In chipping stone intended to be finished smooth and 
 neat, great care should be taken to avoid splintering the arris, 
 which will almost certainly happen if the edge of the chisel be 
 directed outwards in making the blow. Even for the edge of 
 the chisel to be at right angles to the arris it is applied to form, 
 is not always a safe position ; but if directed inwards, so as to 
 form an angle of forty-five degrees with the line of the arris, 
 which it should overhang a little, the chipping may be safely 
 executed. To direct the workman in the use of the chisel, a 
 thin board planed true, is used as a straight-edge, to point out 
 cross-windings and other inequalities of surfaces, the promi- 
 nent defects of which have previously been removed. 
 
 On this occasion, we shall make an observation of which 
 perhaps very few artists are aware. A chisel made entirely of 
 steel, will produce a greater eftect with a given impulse than 
 one composed partly of iron and partly of steel, although the 
 steel of tne latter forms its edge, and is there equally as good 
 and hard as the steel of the former. But tempered steel is 
 more elastic than iron, and therefore transmits more faithfully 
 the impulse it receives. 
 
 In some parts of the country, masons are provided with such 
 hammers as they can use (particularly when they are employed 
 upon the harder kinds of calcareous stones) instead of chisels, 
 as well as for the general purpose of dividing stones. A heavy 
 pointed hammer serves instead of the point, and another which 
 18 also heavy, and has an edge like that of a chisel, serves very 
 effectually to produce those narrow marks of furrows left upon 
 hewn stone-work which is not ground on the face. 
 
 Of Stoties. 
 
 All calcareous stones, namely, those that burn to lime, if hard. 
 
BUILDING. 219 
 
 Marble. — Limestone. — Portland and Purbeck stone. — Freestone. 
 
 of a close texture, and beautiful appearance, from the variega- 
 tion, or clearness and uniformity of the colours, are called 
 marble. The names of the different kinds of marble are gene- 
 rally derived either from their colour, or the place where they 
 are obtained. The most valuable kind of milk-white marble is 
 obtained in Italy, and is too costly to be often used for any but 
 the smaller ornamental parts of buildings. This, when cut into 
 thin slices, is semi-transparent. Many parts of the united king- 
 dom abound with marble, which is mostly more or less colour- 
 ed, often close in its texture, and capable of receiving a high 
 polish. Derbyshire and Westmoreland, in England, are the 
 counties which supply the greatest quantity and variety of 
 marbles, some specimens of which are highly esteemed. 
 
 When marble that has been faced with a chisel, is intended 
 to be polished, it is ground by rubbing it with rough-grained 
 freestone, assisted by sand and water, until the chisel marks 
 are removed. Finer and fine-grained freestone, with water, 
 but no sand, is then used, till the surface becomes very 
 smooth. If the finer grit-stones be found too slow in their 
 operation, a little fine flour of emery is used. The last and 
 highest lustre is given with oxide of tin, well known under 
 the name of putty. When the surface of the marble to be 
 polished has been cut with a saw, the very rough freestone 
 and sand are not necessary. In other respects it must be 
 finished by the same process. 
 
 Limestone is a coarse kind of marble, and is cut and polished 
 in a similar manner. In many districts, it forms immense 
 strata. From its great hardness, it is only hammer-dressed 
 when used for the fronts of buildings ; but when this is done 
 in the best manner, the effect is very fine. 
 
 The stone most commonly used in London, is Portland 
 stone, which is brought from the island of Portland in Dorset- 
 shire. It is of a dull whitish colour, though the buildings con- 
 structed with it have a handsome appearance. When recently 
 dug, it is soft, and easy to work, but acquires hardness with 
 age. Although it contains silex, the hardness it acquires is 
 not so great that it will strike fire with steel. Under great 
 pressure, Portland stone is apt to splinter at the joints. 
 
 Purbeck stone is brought from the island of Purbeck, also 
 in Dorsetshire; it is mostly employed in rough work, such as 
 steps, paving, &c. 
 
 Freestone is a general name for stones of very different qua- 
 lities as to their value in building. It consists of clay and 
 silex, sometimes the silex amounts to nearly one-half its weight, 
 but generally it is considerably less, and the hardness of the 
 stone varies with the proportions of its component parts. 
 
220 BUILDING. 
 
 Freestone. — Strength of stone different in different positions. 
 
 is often called grit or sand stone. It is a very plentiful stone; 
 the strata of entire districts, under a slight covering of soil, 
 appearing, in various instances, to be composed of it. The 
 particles of some kinds of it have so little cohesion, that small 
 bits may be granulated between the fingers; this is the sort 
 commonly used for hltering-stones. All kinds of it are softer 
 when taken out of the quarry, than they afterwards become 
 when dry, or after long exposure to the atmosphere. Freestone 
 is generally about two and a iialf times the weight of water. 
 That from Hollington, near Utoxeter, is of a whitish or yellow- 
 ish gray; that from Knipersly, in Staffordshire, is of a bluish 
 gray, and so infusible as to be used as a fire stone. The co- 
 lour of freestone is often a dull red, but sometimes so nearly 
 white, that buildings constructed with it look as well as those 
 of Portland stone. Different parcels of freestone, taken from 
 the same quarry, frequently exhibit a considerable diversity of 
 colour; a circumstance which gives a motley appearance to 
 buildings in other respects perhaps faultless. When the stone 
 has been recently dug, and is damp, these differences are often 
 not perceptible. If, therefore, uniformity of appearance be 
 desired, the stones should be faced, dried, and sorted before 
 they are used. Freestone is incapable of receiving a polish, 
 and therefore, when it has been cut with a saw, it is rarely sub- 
 mitted to any subsequent operation to produce greater smooth- 
 ness; but when it has been reduced with the chisel, it is made 
 smooth with another piece of stone of the same kind, along with 
 sand and water. When the freestone has a laminated texture, 
 it is called flag-stone, and is divided into thin pieces for the 
 purpose of covering houses, and for flooring. 
 
 The position which stones have had in the quarry, is not a 
 matter of indifference to the attentive mason. Stones intended 
 to sustain great vertical pressure, as pillars, should stand in a 
 building as they stood in the quarry from which they were 
 taken; for pillars, the axis of which were horizontal in the 
 quarry, when placed perpendicularly, are apt to split under a 
 great strain. Perhaps all stones, however solid they may ap- 
 pear, ))Ossess more or less of a laminated texture, — a property 
 doubtless occasioned by separate depositions, or crystalliza- 
 tions of their peculiar matter. What kind of a pillar any stone 
 tvell known to be laminated, would produce, in different posi- 
 tions, it is not difficult to conjecture. If, for example, a block 
 of flag-stone were converted into a pillar, so as to leave each 
 lamina or flag of which it is composed posited horizontally, it 
 Vi'ould sustain any weight not capable of crushing it to atoms; 
 but if the lamina were placed in an inclined position, so as to 
 form an acute angle with the axis, an inconsiderable pressure 
 
BUILDING, 221 
 
 Jlortar. — Oil cement.^ — Plaster of Paris. 
 
 would occasion them, where the cohesion was slightest, to 
 slide over each other; lastly, if the lamina were placed parallel 
 to the axis, the pillar, under sufficient pressure, would divide 
 vertically into several parts, and though rather stronger than 
 in the last instance, would still be comparatively weak. An 
 attention therefore to the position of stones, and to veins or 
 other signs which may indicate the existence of lamina, well 
 deserves the mason's regard. 
 
 Of Cements. 
 
 The cement used by the mason, for the ordinary purposes 
 of walling, is mortar, differing not from that used by the brick- 
 layer; and as we have already treated of this subject rather at 
 length, a few remarks in this place will suffice. For the joints 
 of hewn stone, the mortar should be much finer than the brick- 
 layer requires; and sometimes a mixture of oil-putty, or very 
 thick white-lead paint, is used as the cement. These compo- 
 sitions will last longer than almost any stones, and will remain 
 prominent when the face of the softer kinds of stone has been 
 corroded by age. 
 
 The cement used in setting column stones, is mostly oil- 
 putty, or white lead mixed with chalk-putty, or fine mortar. 
 Sometimes columns are set upon milled lead ; in this case, the 
 lead should not be quite equal to the column in diameter, but 
 so as to leave a narrow ring externally, which must be filled 
 with oil-putty. 
 
 In situations not exposed to damp, plaster of Paris is employ- 
 ed as a bedding for stones or marble. When a mantle-piece is 
 composed of valuable marble, the various pieces are commonly 
 very thin. In this case, a considerable thickness of plaster of 
 Paris is laid on the back, and a slate or some ordinary stone 
 bedded in it, to give greater strength. Good plaster of Paris is 
 scarcely to be met with, nor are the causes of its imperfections 
 generally understood by workmen. We shall therefore point 
 them out, and give directions for preparing it, of a uniform and 
 excellent quality, when we treat of casting in plaster. 
 
 The Greeks and Romans constructed their works of wrought 
 stone without cement; but as they used a profusion of cramps 
 and bands of iron and bronze, and the beds of the enormous 
 stones they used were finished with almost mathematical pre- 
 cision, their edifices were substantial and durable in the 
 highest degree. Metallic bands and cramps are still used in 
 aquatic works, as well as for lofty steeples, and other slender 
 buildings much exposed to the action of the winds, also to 
 connect the different stones composing mantle-pieces, &c. 
 with the wall. 
 
222 BUILDING. 
 
 Dt-liiiilions. — Footings of walls. 
 
 Of S tone Walls. 
 
 The propriety of erecting suspended or inverted arches, 
 according to circumstances, and other general directions 
 already given respecting foundations, being as applicable to 
 stone walls as to those of brick, need not be adverted to 
 again. The explanation of a few technical terms will there- 
 fore be our first object. 
 
 Stones which run through the thickness of a wall, in order 
 to bind it, are called bo}id stojies; in some parts of the country 
 they obtain the name of through stones. 
 
 When the side or sides of a wall lean back, so that the plumb 
 would fall within the base of the wall, the inclination is called 
 battering: it is generally made about one inch in a foot. 
 
 The large stones at the base of a foundation, which project 
 beyond the vertical surface or Iront of the superincumbent 
 wall, are called J"oo////gs. 
 
 The parts of a wall between apertures, or between an aper- 
 ture and the corner, are caWad piers. 
 
 The beds of a stone are its upper and under surface, which 
 are generally in a horizontal position within the wall. 
 
 Walls built with unhewn stone, with or without mortar, are 
 called rubble walls. Rubble walls are of two kinds, the 
 coursed and the uncoursed. In the coursed, the stones are 
 hammer-dressed or axed, and adjusted by a sizing rule, so that 
 each row of stones forms a horizontal surface. In the un- 
 coursed, the stones are used in a rough state, nearly as they 
 come out of the quarry. 
 
 Walls which are faced with squared stones, hewn or rubbed, 
 and backed with rubble, stone, or brick, are called ashlar. 
 
 Wall-plates, are horizontal pieces of timber, commonly laid 
 even with the interior of walls, for the ends of the joists and 
 other timbers to rest upon. 
 
 The footmgs of walls ought to consist of the largest stones 
 which can be conveniently procured. It is better to have them 
 of a rectangular form than any other, and if not square, their 
 largest suri'aces should be laid horizontally. With this shape 
 and disposition, they will make the greatest resistance to sink- 
 ing. If the stones, intended to be employed as footings, de- 
 viate materially from a rectangular figure, when received from 
 the quarry, they ought to be hammer-dressed ; as, if they taper 
 downwards, or rest upon angular ridges, they will be apt to 
 give way under the weight of the superstructure. When the 
 footings can be obtained the full breadth of the wall in one 
 piece, they are to be preferred ; but when a sutHcient number 
 of stones of this description cannot be obtained, then every 
 
BUILDING. 223 
 
 Uucoarsed and coursed rubble walls. — Ashlar walls. 
 
 alternate stone in the course may be the whole breadth, with 
 two stones next to it, disposed like two stretchers in a nine- 
 inch wall of Flemish brick-work. When the largest stones 
 which can be conveniently obtained, are insufficient even for 
 the latter arrangement, the most suitable which can be pro- 
 cured, must be disposed so as to break in the best manner 
 circumstances will admit, the vertical joints in the same course, 
 as well as those of the different courses with respect to each 
 other. Each course, also, should be well bedded in mortar. 
 
 When bond timber will be required, the uncoursed rubble is 
 an inconvenient mode of building, as the heights on which 
 they are disposed must be levelled. The best kind, or coursed 
 rubble, admits of bond timbers without difficulty, for though 
 the different courses are not of the same height, the surface of 
 each of them is level; but as the walls in which bond timbers 
 are introduced, are apt to warp or even fall in case of fire, the 
 use of them should be avoided in strong well-built walls. 
 
 The stones of an ashlar front should have their upper and 
 under surface correctly parallel with each other, and correctly 
 at right angles to the face. If these surfaces be carelessly left 
 concave, they will be apt to splinter near the edge under great 
 pressure. On the right and left they should taper inwards, but 
 the taper should not be continued quite to the face, though 
 it may reach the face within an inch or two. The ashlar 
 stones having the form of a truncated wedge, they will, in each 
 course, present a series of angular indentations within the 
 wall, like the spaces between the teeth of a saw. The stones 
 are so selected and disposed that the vertical or upright joints, 
 and consequently the angular spaces, of one course fall on the 
 middle of the stones below. By this means the ashlar face is 
 bonded to the rubble, brick, or rough stone of the back, and 
 the strength of the wall much greater than if each stone was 
 of an equal rectangular figure. Strength is also to be pro- 
 moted by adopting a plan not commonly regarded, that of 
 sorting the stones, so that in each alternate course they will 
 extend farther into the wall than those of the course immedi- 
 ately above and below. In ashlar work, the bond stones, 
 which ought frequently to be introduced, cannot like the other 
 stones have a wedge-like form ; they must be rectangular; and 
 they produce the best effect, when so disposed in each course 
 that they will be opposite the middle of the space between 
 the two bond stones in the course immediately above and be- 
 neath them. 
 
224 BUILDING. 
 
 Working of ihe stones for culiuuns. 
 
 Of Stone Columns. 
 
 When large stone columns are made in one piece, their 
 effect, from that circumstance alone, is very strikint^; but as 
 this advantage is not always obtainable, the next object is to 
 make the joints as few and as minute as possible, as well as to 
 be very attentive in selecting the different stones to be com- 
 bined, that the joints may not be descried at a distance, by 
 the commencement of a different colour. From what has 
 been said in the section on the different kinds of stone, it will 
 be understood, that none but horizontal joints can be allowed 
 in anv shaft ; all others being inconsistent with the laws of 
 strength. 
 
 The stones proper for an intended column being procured, 
 and the order in which they are to succeed each other being 
 determined, the next consideration will be to ascertain the 
 exact diameter proper for each end of every one of them. For 
 this purpose, draw an elevation of the proposed column to the 
 full size, divide it by lines parallel to the base, into as many 
 heights as the column is intended to contain stones, taking 
 care that none of the heio;hts exceed the lengrth that the stones 
 will produce. The working of the stones to the diameters 
 thus obtained then becomes easy. The ends of each stone 
 must first be wrought so as to form exactly true and parallel 
 planes. The two beds of a stone being thus formed, find their 
 centres, and describe a circle on each of them. Divide these 
 circles into the same number of equal parts, which may, for 
 example, amount to six or eight. Draw lines across each end 
 of the stone, so that they will pass through the centre, and 
 through the opposite divisions of the same end. The extre- 
 mities of these lines are to regulate the progress of the chisel 
 along the surface of the stone, and therefore when those of 
 one end have been drawn, those of the other must be made in 
 the same plane, or opposite to them respectively. The cylin- 
 drical part of the stones must be wrought with the assistance 
 of a straight-edge ; but for the swell of the column, a dimi- 
 nishing rule, that is, one made concave to the line of the co- 
 lumn, must be employed. This diminishing rule will serve to 
 plumb the stones in setting them. If it be made the whole 
 leng-th of the column, the heiohts into which the elevation of 
 the column is divided, should be marked upon it, so that it 
 may be applied to give each stone its proper curvature. But 
 as the use of a long; diminishing- rule, when the stones are in 
 many and short lengths, would be inconvenient, rules corre- 
 sponding in length to that of the different heights, may be em- 
 ployed with advantage. 
 
BUILDING. 225 
 
 To draw the curve of a shaft. 
 
 Of the different methods which may be practised, to obtaia 
 the curvature of the rule to be used in the diminution of the 
 shaft of a column, the following may be considered the easiest 
 and best adapted for general use: Let A B, fig. 12, pi. I. re- 
 present the height of the column, efihe semi-diameter of th© 
 lower part, and g h the semi-diameter of the upper part, ac- 
 cording to the customary proportion of the diminution. A% 
 the lower one-third of the column must be cylindrical, draw a 
 line from e to i, parallel to A B. What we now want is, tp 
 obtain a curved line from i, which, will fall into g without mak- 
 ing the diameter of the upper two-thirds of the column in any 
 point greater than that of the lower or cylindrical third. With 
 the radius i k, draw the quarter of a circle C. Draw a line 
 from the narrowest part of the column, that is from g, parallel 
 to the axis or middle line A B, till it cuts the quadrant of a 
 circle C. Divide the arc contained between i and the point 
 w here g cuts C, into four equal parts, as /, m, n, o, and divide 
 the height A k into the same number of divisions as this arc, 
 as 1, 2, 3, From the points, draw a line parallel to the axis, 
 which will cut the transverse line 3 at v; from n draw another 
 line parallel to the axis, which will cut the transverse line 2 in 
 w; in the same manner draw a line from o to the transverse line 
 1, which it will cut in x. Now the curved line of the column, 
 between i and g, must pass through the points x, to, v; there- 
 fore at or near all the points from i to g inclusive, drive in two 
 pins or nails, in such a manner that the direction in which each 
 pair of nails stand shall be the same as the transverse lines of 
 the column. Between each pair of nails, also, there must be 
 just sufficient space left to admit a thin slip of wood, like a 
 lath, or some other equally flexible substance, and care must 
 be taken to fix the nails in such a position, that when this slip 
 of wood is placed between them, either its inner or its outei 
 perpendicular surface shall exactly coincide with the several 
 points of diminution i, x, w, v, g. This being done, it is easy, 
 with a pencil or any marking instrument suited to the surface 
 worked upon, to draw a line along the slip of wood, which 
 line will be the curve of the shaft. The piece intended to be 
 used as a diminishing rule may have the curve made on it, or 
 transferred to it, as may be deemed most convenient. It will 
 also be understood, that the number of the parts into which 
 the arc / i, and the height A k are divided, and which deter- 
 mine the number of diminishing points obtained, may be 
 varied at pleasure. In no case, however, can it be considered 
 advisable to make these divisions fewer than four, though for 
 lofty columns they may be made two or three times that num- 
 ber, as they constitute so many checks upon the irregular or 
 10.— Vol. I. 2 G 
 
226 BUILDING. 
 
 To draw the curve of a shaft. — To draw flutes aiid fillets on a sliaft. 
 
 imperfect flexibility of the lath or spring rule. As so much of 
 the beauty of a shaft depends upon the easy and imperceptible 
 transition from the cylindrical to the tapering part, it may be 
 advantageous to divide the arc from i to o into two parts, and 
 the division k 1, of the shaft, also into two parts; a diminish- 
 ing point will then be obtained between / and .r, which w ill be 
 of use to lessen the ])0ssibility of imperfection. 
 
 Another mode, which we shall recite, of diminishing a shaft, 
 is not essentially different from the preceding: divide the 
 shaft from D to E, fig. 17, into four parts, as 1, 2, 3; divide 
 the space EF, which is the difference between the superior 
 semi-diameter E /•, and the inferior or lower diameter D g, 
 into the like number of parts, viz. four. Draw lines from each 
 division on EF tending to the point D. The first line next to 
 F will reach to D, the point from which the diminution com- 
 mences, in a direction parallel to the axis; the next line, reck- 
 oning from the same side, will give the point a ; the third line 
 the point b; and the fourth line the point c; a line from the 
 point E, which is the limit of the diminution, need not be 
 drawn. The points E, c, b, a, D, constitute the places where 
 the nails must be driven in, to direct the path of the lath as 
 in the last example. Some artists prefer this mode of dimi- 
 nution. Here, also, as before, the number of tlie divisions is 
 optional, but a greater error may be committed by making 
 them too few than too numerous. 
 
 To draw the flutes and fillets on the shaft of a column, the 
 following will be found an easy and effectual method: let A, 
 fig. 10. pi. II. be the shaft; make a hole exactly in the centre 
 of both ends, and into each hole drive a piece of wood, so as 
 to be quite tight, and to project five or six inches. The pro- 
 jecting parts, b b, of the pieces of wood, must be well rounded 
 off, and made exactly in the middle of the ends. Being pro- 
 vided with a diminishing rule, made as above-mentioned to 
 the curve of the shaft, it may here be used as the ruler, by fix- 
 ing it in a groove in two pieces of wood, c c, so as to revolve 
 with them upon the pins bb. The edge of the rule must be 
 brought very near the shaft, and one side of it must tend 
 exactly to the centre, which is done bv giving that direction 
 to one side of the grooves in c c, as shewn by the dotted lines 
 a a. As the diminishing rule d, is commonly made rather 
 slender, and therefore will be apt to bend with the force em- 
 ployed to draw the lines, it will be proper to fix a piece of 
 wood, of sufficient thickness to keep it straight, on the back, 
 or that side of it which does not tend to the centre. As in 
 marking a long column, there may be some difl[iculty in keep- 
 ing the rule steady, while the lines are drawn, the strong piece 
 
BUILDING. 227 
 
 To draw the flutes and fillets on a diminished pilaster. — Stone bridges. 
 
 of wood thus attached to the back of the diminishing rule, may 
 have one, two, or more screws, according to its length, passing 
 through it, as//, and pressing against the shaft, by which 
 means the rule will be staid at any part of a revolution, much 
 more effectually than with the assistance of several men. The 
 screws ought to be of wood, or, if iron, their extremities should 
 be prevented from injuring the stone, by the interposition of a 
 flat piece of metal, or some other suitable substance. These 
 arrangements being made, the lines desired may be drawn with 
 any sharply-pointed steel instrument kept close to the rule, with 
 the greatest ease arid precision, from divisions previously mark- 
 ed at one end. To obtain these divisions, suppose it were 
 desired to flute the Ionic, the Corinthian, or Composite co- 
 lumns, the circumference at one end will be divided into six 
 equal parts, by taking half the diameter at that end, and apply- 
 ing it round the said circumference ; and if each of these divi- 
 sions be divided into four, the whole circumference will be di- 
 vided into twenty-four. In order to make the proportion of a 
 flute to a fillet as one to three, divide any one of these last di- 
 visions, or twenty-fourth part of the circumference, into four 
 equal parts, and one of these parts will be the breadth of a fil- 
 let; which being set off from the same side of each division, the 
 whole shaft will be properly divided for flutes and fillets, and 
 consequently prepared for the use of the rule. 
 
 To draw the flutes and fillets on a diminished pilaster, make 
 a line down the middle of the face of it, from top to bottom ; 
 divide this longitudinal line into any convenient number of 
 equal parts, and through the points of division draw transverse 
 lines crossing the breadth of the pilaster ; set off the flutes 
 and fillets on each transverse line; fix pins or nails at each cor- 
 responding point of the transverse line, and draw the lines 
 with the help of a pliable slip of wood, as in obtaining the 
 diminution of a shaft. 
 
 Stotie Bridges. 
 
 Here it will be proper, in the first place, to give an explana- 
 tion of the principal terms used in speaking of bridges. 
 
 The abutments of a bridge are the walls adjoining to the 
 land, each of them supporting the end of an extreme arch. — - 
 The walls supporting the ends of the arches between the abut- 
 ments, are called piers. 
 
 The imposts are the highest course of the stones of piers, 
 from which an arch immediately springs, and which project a 
 little from the piers. 
 
 Batterdeau, or Coffer-dam, a 'case of piling, to divert the 
 stream of a river while the piers are building. 
 
2^26 BUILDING. 
 
 DtTinitiuas. — Requisites of a good bridge. 
 
 The span of an arcli is its greatest horizontal width. 
 
 Caissons are large hollow vessels, framed of strong timbers, 
 iind made water tight, which being launched and floated to a 
 proper position in the river, where the bed has been previously 
 excavated and levelled, are there sunk. The piers of thfe 
 bridge are built within them, and when carried up above or 
 nearly to the level of the water, the sides of the caisson are 
 detached from the bottom, and removed; the bottom is left 
 remaining, and serves for a foundation to the pier. 
 
 Drift, push, shoot, thrust, terms used indiscriminately, to ex- 
 press the horizontal force of an arch, by which it endeavours 
 to spread itself out and overset the piers and abutments. 
 
 The extrados of an arch is the outside or convex curve on the 
 top of the arch stones: hence the extrados of a bridge is the 
 curve of the road-way. Extrados is the term opposed to in- 
 trados, which signifies the concave side of an arch. 
 
 Those stones, one side of which is in front, and another side 
 forms part of the intrados of an arch, are called voussoirs or 
 arch-stones; the middle voussoir, at the summit of the arch, 
 which is put in last, for wedging and closing the whole, is also, 
 and indeed most commonly, called the keystone. 
 
 Parapets are the low walls erected on the sides of the extra- 
 dos or road-way of a bridge, for the safeguard of passengers. 
 
 These explanations being premised, we may proceed with 
 the subject. Palladio observes, that bridges ought to have the 
 same qualifications which are deemed necessary in all build- 
 ings of consequence, namely, that they should be convenient, 
 beautiful, and lasting. To make a bridge lasting, the good- 
 ness of its foundation is one of the most important things to 
 be attended to ; to make it convenient, an easy ascent, and a 
 width suited to its situation, are absolutely necessary; and to 
 make it beautiful, its workmanship must be good, and its 
 several parts well desio;ned and proportioned. 
 
 Bridges ought always to be constructed at right angles to 
 the current. The piers ought to be so proportioned, as to en- 
 able them to withstand the thrust of the adjoining arches, 
 though the rest were thrown down; yet to fall short of the size 
 required for this purpose, would hardly be a greater fault than 
 materially to exceed it; for when the piers are unnecessarily 
 large, the current being contracted, increases its velocity, and 
 is apt to undermine the foundation. 
 
 A judicious regard to local circumstances, is always of great 
 importance to the durability of a bridge. Over rivers which 
 are subject to great inundations, arches that will be dry at 
 ordinary seasons will often be necessary; and when a choice 
 with respect to the situation can be made, a wide part of the 
 
BUILDING. 229 
 
 Difference of opinion respecting the proper curve for arches. 
 
 Btream should be preferred to the narrow part, because the 
 water at the narrow part has not only a greater velocity than 
 at the other, but that velocity would be increased by the 
 space which the piers would occupy. Another point to be 
 duly regarded in selecting the site of a bridge, is the solidity 
 or otherwise of the ground in different parts of the river, for 
 the foundations of piers and abutments. An immense ex- 
 pense in piling, and other artificial means of improving in- 
 adequate ground, may often be saved by proper attention in 
 this respect. 
 
 The arches in a bridge ought to be an odd number, that one 
 of them may stand in the middle, where the velocity of the 
 stream is greatest. The middle arch is also to be made the 
 largest. As the expense of piers is very considerable, besides 
 the perpetual inconvenience of them in navigable rivers, 
 bridges should always be constructed with the fewest arches 
 possible. 
 
 With regard to the most suitable curve for the arches of a 
 bridge, the subject has been much controverted. Not only 
 architects, but mathematicians have, on this point, differed 
 widely from each other. This diversity of judgment seems to 
 hav€ been perpetuated by the fact, that bridges of acknow- 
 ledged, and to all appearance of equal excellence, have been 
 constructed with semi-circular, semi-elliptical, and curves of 
 various other denominations, and consequently possessing very 
 different properties. Some have contended that semi-circular 
 arches ought in most cases to be preferred, because they press 
 more perpendicularly upon the piers than smaller portions of 
 circles, and in proportion to their number will diminish the 
 pressure on the abutments. Others have preferred the ellip- 
 tical curve, where the arches were to be large and but few in 
 number, because the extensive radius of the semi-circular arch 
 would occasion the central part of the extrados or road-way 
 to be very inconveniently elevated. This objection to the 
 semi-circular form is obviated by adopting the elliptical one, 
 and Muller asserts that the elliptical arch does not press 
 against the piers with a greater force than a circular one ; and 
 being lighter, from requiring a smaller quantity of materials, 
 will consequently be more lasting. 
 
 As the strength of a bridge depends so much on mathemati- 
 cal principles, it seems natural to look for, and to regard with 
 deference, the opinions of the most eminent mathematicians. 
 If mere theorists differ from each other, those who enjoy alike 
 the light of theoretical and practical knowledge, may discover 
 the sources of the errors which have occasioned contradictory 
 deductions. It was the opinion of the celebrated Dr. Hutton, 
 
230 BUILDING. 
 
 Arch of equilibration. 
 
 late professor of the mathematics to the royal military academy, 
 Woolwich, that the mechanical arch of equilibration is the 
 only perfect one adapted to the princi|)les of bridges. This 
 arch, it is asserted, being in exact equilibrium in all its parts, 
 can have no tendency to break in one part more than another, 
 and must therefore be the safest and strongest. A table for 
 constructing the arch of equilibration, is given in the Doctor's 
 Mathematical and Philosophical Dictionary, under the article 
 Bridge. To explain the nature of the equilibrated arch, we 
 shall advert, in the first place, to the construction of the cate- 
 narian curve. If a perfectly flexible rope, of equal thickness 
 and density in every part, were fastened by its ends to hooks, 
 or some other supports, at any distance insufficient to draw it 
 tight, the position which it would assume is called the cate- 
 narian curve. This is a curve which has had many advocates, 
 among whom is found the celebrated Emerson, who considered 
 it of all curves the best adapted to bridge building. The fact 
 is, that if the arch of a bridge were, like the rope, of uniform 
 thickness (which for the present we will suppose to imply uni- 
 form density) in every part, the catenarian curve would be the 
 most proper for it, because that curve, under such circum- 
 stances, is a true arch of equilibration. Hence, as the uni- 
 form thickness of an arch can rarely be made compatible with 
 other objects in constructing a bridge, the catenarian curve 
 can as rarely be the proper arch to use. Every particular 
 form of the extrados of an arch, requires, according to the 
 principles of equilibration, a difference in the form of the 
 intrados. To render this more clear, we will return to the 
 consideration of the rope, suspended as above stated. Sup- 
 posing it to hang along a wall, the curve it exhibits may be 
 traced. AVhen this has been done, let the substance of the 
 rope, on the convex side, be increased in some parts, for 
 example, at the haunches ; it will then no longer describe a 
 catenary, and it would be found that every difference in the 
 additions made to it will produce a difference in its curva- 
 ture. If the experiment could be made with materials really 
 possessing perfect flexibility, these differences would be very 
 perceptible, and if the points of suspension were at a dis- 
 tance from each other just equal to the diameter of a circle 
 which the rope would half encompass, it would be found 
 that the rope would require the additions made to it to be as 
 represented by jNo. '2, fig. 11. pi. II. before the concave side 
 of it would describe a semi-circle. But when these addi- 
 tions were made, tlie contour of the figure, if introverted, as 
 shewn by No. 1, would represent the section of an equili- 
 brated arch, the intrudes of which should be semi-circular. As 
 
BUILDING. 231 
 
 Arch of equilibration. 
 
 an extrados or terminating line of wall running up at both 
 ends in this manner, is far remote from any form that is ad- 
 missible in practice, semi-circular arches are never balanced 
 or equilibrated; and the advocates of the equilibrating system 
 assert, that though such arches have stood for centuries, this 
 event has only happened in consequence of the cement and 
 friction which have prevented the stones from being pushed 
 out of their places. Hence, they add, that if the materials 
 were only placed in contact, without cohesion or friction, they 
 would not stand when the road-way was straight, or a convex 
 curve throughout the length of the arch, or in any other way 
 different from the figure above referred to. 
 
 These observations will, we hope, communicate to the 
 reader, an idea of the nature of equilibrated arches. The 
 subject is interesting and important, and to dismiss it with- 
 out some explanation of this kind, would be denying it the 
 notice to which it is fairly entitled. Here, however, it may 
 be observed, that the subtle refinements of theory, whatever 
 may be their general utility, are not always found to be 
 directly applicable to practice ; and we may therefore some- 
 times act in opposition to them, not only with safety, but 
 with advantage. Bridges, which have not been equilibrated, 
 have, as before stated, endured for ages, and appear likely 
 to endure till the materials of which they are composed 
 crumble away. Hence we are afforded a reasonable en- 
 couragement to construct again such arches as they are com- 
 posed of, whenever their form suits us in other respects. 
 Though the mathematical reasoning on arches of equilibra- 
 tion be just, yet it applies with strict propriety only to homo- 
 geneous materials, and to structures acted upon by no power 
 but that of gravitation. If the form it prescribes were at- 
 tended to, it would cease to be an arch of equilibration, un- 
 der the pressure of a waggon or any other load ; and without 
 the friendly aid of cement and friction, would no more be 
 able to stand than any other kind of an arch. In the practi- 
 cal operations, also, of constructing a bridge, the proper con- 
 figuration of the stones for many kinds of curves, either can- 
 not be determined by any unexceptionable rule, or if deter- 
 minable, is apt to be mistaken, and subject to considerable 
 difficulties of execution. The aggregate of small deficiencies 
 originating in this way, is, in extensive structures, incalculably 
 subversive of strength. Semi-circular arches, or those of any 
 less portion of a circle, having all their stones of the same 
 figure, and which therefore it is easy to form with correctness, 
 and impossible to misarrange in the work, are, there can 
 hardly be a doubt, more uniformly built strong, than any other 
 
232 BUILDING. 
 
 Elliptical arches. — Piers. 
 
 kind of arches. Next to circular curves, elliptical ones admit 
 the stones of which they must be composed to be formed in 
 the most certain and satisfactory manner, and though it may 
 be much more difficult in them, than in circular arches, to 
 make the surfaces intended to be in contact, press against 
 each otlier simultaneously and uniformly in every part, un- 
 der any constant or occasional weight, yet with care it may 
 be done, or at least the imperfection may be so small as to 
 produce little ultimate disadvantage. For other curves, it 
 appears difficult to contrive an easy and unexceptionable 
 mode of forming the stones, or to determine from what point 
 or points, their joints should appear to radiate; besides the 
 constant risk, from their varied shapes, of putting them 
 in wrong places. But as semi-elliptical arches approxi- 
 mate very nearly to the form of equilibrated arches, as their 
 contour is not only graceful, but, in navigable rivers especi- 
 ally, very convenient, from the elevation of their haunches, as 
 they can be made also to so great a variety of heights with 
 the same span, and combine with these properties the greatest 
 facility of execution, next to the circular form, the opinion of 
 architects generally, is, for extensive structures, decidedly in 
 their favour. In small bridges, a semi-circular arch may be 
 adopted w ith propriety, where the extra quantity of materials 
 required by that form is no object, and when a strong arch of 
 the easiest execution is desired. When a serai-circular arch 
 would be too elevated for the situation, a segment of a circle 
 not exceeding sixty degrees, may be employed. — All arches, 
 less than a semi-circle, are called scheme-arches. 
 
 The dimensions of the piers of bridges are commonly deter- 
 mined by those of the arch. Their breadth should not exceed 
 a fouruh, nor be less than one-sixth of its span. To break the 
 force of the current, they have mostly angular ends so as to pre- 
 sent an edge in front; though sometimes their ends are semi- 
 circular, in order that such bodies as are impetuously brought 
 down the river, when they strike against them, may be deflected 
 towards the middle of the arch. To prevent the stability of the 
 piers from being endangered, by the washing away of their 
 foundations, occasioned byanincreased velocity of the current, 
 the bed of a river must be sunk or hollowed in proportion to 
 the room taken up by the piers, in order that the water may 
 gain in depth what it looses in breadth; or the current may be 
 broken, by stopping the bottom with rows of planks, stakes, or 
 piles. The celebrated Palladio gives the following proportions 
 of a bridge designed by him : the river was one hundred and 
 eighty feet wide ; making three arches in the bridge, he ga?e 
 sixty feet to the centre arch, and to the other two forty-eight feet 
 
BUILDING. 233 
 
 Seinple's method of laying the foiuidations of Essex-bridge. 
 
 each. The piers were twelve feet thick, or one-fifth part of the 
 span of the middle arch, and a fourth part of that of the smaller 
 ones. The arches were a little less than a semi-circle, and their 
 keystone one-seventeenth part of the span of the middle arch, 
 and one-fourteenth part of the other two. In an arch of twenty- 
 four feet, Palladio makes the length of the keystone about six- 
 teen inches, which is deemed a very eligible size. 
 
 Autumn is in general the most favourable time to lay the 
 foundations of a bridge, as it is commonly the driest season of 
 the year, and the consequent lowness of the waters is favour- 
 able to the work. The simplest mode of overcoming the incon- 
 venience of the water, in laying the foundations of piers, con- 
 sists in turning the river out of its course, above the place de- 
 signed for the bridge, into a new channel cut for it near where 
 it makes an elbow or turn. As this plan is, however, seldom 
 expedient, and often not practicable at a reasonable expense, 
 the use of the coffer-dam, or enclosure to keep off the water, is 
 much more common. By the cofier-dam, a part only at a time 
 of the bed of the river is enclosed from the water, which flows 
 in a free current along the unenclosed parts of its bed. An 
 account of the method employed by the ingenious Robert 
 Semple inlaying the foundations of Essex-bridge, in Dublin, 
 will illustrate this subject. Hound the place where the intended 
 pier was to be built, two rows of strong piles were driven, 
 about thirty inches from each other, and which were left at 
 low-water-mark. These piles were lined with planks,, between 
 which was rammed a quantity of clay, and thereby the wall of 
 the coffer-dam was formed. Within this wall were driven a 
 row of piles, dove-tailed at their edges, so as to receive each 
 other, and which formed the extremities of the plan of the 
 piers at the level of the bed of the river. After having dug to 
 a fine stratum of sand, about four feet lower, within these a 
 great number of other piles were driven as deep as they could 
 possibly be made to penetrate. The intervals of these piles 
 were filled up, and in order to produce a solid foundation, the 
 first course was laid with mortar made of roach-lime and sharp 
 gravel, and on this large flat stones were rammed to about a 
 foot in thickness. On this first course was laid a thick coat 
 of dry lime and gravel of the same quality, on which were 
 again laid stones and the mortar as at first ; and so on alter- 
 nately, until the pier arrived at a level with the piles. Three 
 beams, stretching the whole length of the pier, from sterling 
 to sterling, were fastened down to the ends of these piles, and 
 their intervals filled up with masonry. On this platform, 
 which was four feet six inches under low-water-mark, was 
 laid the first course of stones for the pier, cramped together, 
 10.— Vol. I. 2 H 
 
234 BUILDING. 
 
 Manner orcoDstrocting^ and using caissuiu. 
 
 and jointed with terras mortar as usual; courses of stones 
 were laid in this manner, until the piers were on a level with 
 the water at ebb-tide. 
 
 The caisson is a contrivance of still more extended utility 
 than the coffer-dam, from being better suited to very deep and 
 rapid streams. The most considerable work where caissons 
 have been employed, was in the building of Westminster-bridge, 
 of which, therefore, a particular account may be interesting. 
 Each of the caissons contained one hundred and fifty loads of 
 fir timber, and was of greater tonnage than a frigate of forty 
 guns; their size was nearly eighty feet from point to point, 
 and thirty feet in breadth; the sides, which were ten feet in 
 height, were formed of timbers laid horizontally over one an- 
 other, pinned with oak trunnels, and framed together at all the 
 corners, except the salient ancrles, where they were secured by 
 proper iron-work, which being unscrewed, would permit the 
 sides of the caisson, had it been found necessary, to divide 
 into two parts. These sides were planked across the timbers, 
 inside and outside, with three-inch planks in a vertical posi- 
 tion. The thickness of the sides was eighteen inches at the 
 bottom, and fifteen inches at the top ; and in order to strengthen 
 them more, every angle, except the two points, had three oaken 
 knee timbers properly bolted and secured. These sides, when 
 finished, were fastened to the bottom or grating, by twenty- 
 eight pieces of timber on the outside, and eighteen within, 
 called straps, about eight inches broad'and three inches thick, 
 reaching and lapping over the tops of the sides. The lower 
 part of these straps were dove-tailed to the outer curb of the 
 grating, and kept in their places by iron wedges. The pur- 
 pose to be answered by these straps and wedges was, that 
 when the pier was built up sufficiently high above low-water- 
 mark, to render the caisson no longer necessary for the masons 
 to work in, the wedges being drawn up, gave liberty to clear 
 the straps from the mortises, in consequence of which the 
 sides rose by their own buoyancy, leaving the grating under 
 the foundation of the pier. The pressure of the water upon 
 the sides of the caisson, was resisted by means of a ground 
 timber or ribbon, fourteen inches wide and seven inches thick, 
 pinned upon the upper row of timbers of the grating; and the 
 top of the sides was secured by a sufficient number of beams 
 laid across, which also served to support a floor on which the 
 labourers stood to hoist the stones out of the lighters, and to 
 lower them into the caisson. 
 
 The caisson was provided with a sluice to admit the water. 
 The method of working was as follows: a pit being dug and 
 levelled in the proper situation for the pier, of the same shape 
 
BUILDTNG. 235 
 
 Stone caissons suggested. 
 
 as the caisson, and about five feet wider all round, the caisson 
 was brought to its position, a few of the lower courses of the 
 pier were built in it, and it was sunk once or twice to prove the 
 level of the foundation; then, being finally fixed, the masons 
 worked in the usual method of tide-work. About two hours 
 before low water, the sluice of the caisson, kept open till then, 
 lest the water, flowing to the height of many more feet on the 
 outside than the inside, should float the caisson and all the 
 stone-work out of its true place, was shut down, and the water 
 pumped low enough, without waiting for the lowest ebb of the 
 tide, for the masons to set and cramp the stone-work of the 
 succeeding courses. Then, when the tide had risen to a con- 
 siderable height, the sluice was opened again, and the water 
 admitted; and as the caisson was purposely built but ten feet 
 high to save useless expense, the high tides flowed some feet 
 above the sides, but without any damage or inconvenience to 
 the works. In this manner the work proceeded, till the pier 
 rose to the surface of the caisson, when the sides were floated 
 away, to serve the same purpose at another pier. 
 
 To change altogether, for a time, the course of a river, by 
 providing it with a new channel ; or to divert a part of the 
 current at once by the erection of a coffer-dam, may be classed 
 among the simple and obvious expedients, to obtain the means 
 of laying the foundations of a bridge, which would suggest 
 themselves in the infancy of aquatic architecture. Improve- 
 ment seems to have gone no further for a long time. In early 
 periods, if the first means that suggest themselves to accom- 
 plish the end in view, possess the two grand requisites of pos- 
 sibility and efficiency, the march of invention is impeded by a 
 willing prodigality of labour. The caisson, an invention of 
 greater refinement, and bolder aspect, probably originated in 
 a much later period; but when the success which attended the 
 use of it once became familiar to architects, the transition seems 
 natural and easy, from a caisson of wood, which is floated 
 away when no longer wanted, to a caisson of stone, which 
 should permanently remain the external part of the pier. It 
 would soon appear to the mind of him who was first struck 
 with this happy idea, that by the common well-known means, 
 the stones might be cemented so perfectly as to be impervious 
 to water; that they might be cramped so firmly together, like- 
 wise, that no force to which they need be exposed would in- 
 jure the work; and that as the weight of the heaviest stones 
 employed in building is only about two and a half times greater 
 than that of water, the proportion which must subsist between 
 the solid and the hollow part of any mass of masonry intended 
 to float, could not only always be determined with facility. 
 
23<1 BIILDIXG. 
 
 Ueiirral Bentliam'a patent. — Hawkins's claims and inventions. 
 
 but was SO small as not to occasion a fear of success. Indeed, 
 so 2;reat appears to be the practicability of the plan, that the 
 praise of ingenuity exclusively belongs, not to him who shall 
 carry it into execution, but to him by whom it was first sug- 
 i^ested. We shall proceed concisely to notice a few particu- 
 Tars respecting it. General Bentham took out a patent, dated 
 April 2, 1811, for a new and economical method of laying the 
 foundations for bridges, wharfs, &c. He proposes the con 
 struction of hollow masses of masonry, brickwork, 8vc. which 
 he would afterwards float to and sink at the place desired. 
 He observes, that these masses, if filled with casks, might be 
 floated without having themselves any bottom ; and directs a 
 calculation to be made of the weight which any of them will 
 have to bear when employed as piers, or for any other purpose, 
 so that vessels properly loaded may be grounded upon them, 
 and by that means sink them, when the tide retires, as low as 
 they would otherwise have been ultimately sunk by the weight 
 thev are to sustain, and thus prevent their sinking after the 
 structure is finished. J. 1. Hawkins, in a letter to tlie Editors 
 of the Repertory of Arts, ;kc. states, that, several years pre- 
 vious to the date of the above specification, he had mentioned 
 the leading feature of the General's plan, and also of that with 
 respect to the suspension of centering, proposed by Telford, 
 in his report to the House of Commons, for throwing a bridge 
 over the straits of !Menai, to several of his friends, who highly 
 approved of them. He attributes not disingenuity to these 
 gentlemen, in borrowing his plans, being aware of the simi- 
 larity of ideas which may occur to diflerent persons, indepen- 
 dent of each other, when engaged with the same subject; but 
 lavs claim to priority of invention, and developes to the public 
 the particulars of a plan which he had long before circulated 
 among his friends. He would build his piers on shore, in some 
 situation where they might be launched like a ship; he would 
 cramp the outside stones strongly with iron, and would make 
 the walls of such a thickness that they might float in water. 
 He would have a valve in tlie wall, to admit water whenever 
 it might be proper to sink the pier, and a pipe fixed through 
 the top, communicating with the lower part of the inside, on 
 which pipe a pump must be fixed, for drawing the water out, 
 should that measure be necessary. 
 
 To form a good foundation, a space in the bed of the river, 
 rather larger than the base of the pier, must be excavated and 
 levelled by the means used in takino- up ballast, and the spot 
 must be piled, if necessary, as on other occasions of the kind. 
 He details the means of making a platform over the place in- 
 tended for erecting the pier, which, when sunk, must have 
 
BUILDING. 237 
 
 Hawkins's experiments to float brickwork. 
 
 bricks or stones let clown into it equal at least in weight to 
 the water required to sink it. The water must then be pumped 
 out, when the workmen may descend, to lay the stones or 
 bricks in mortar, and fill up the whole interior; thus he ob- 
 tains a solid pier. 
 
 In situations where, from the state of navigation, or any 
 other cause, it may be inconvenient to erect centering in the 
 customary way, he would not hesitate to adopt suspending 
 chains. However bold the idea may appear, it will not be 
 deemed impracticable, when it is considered that the weight 
 of an arch of any given dimensions is easily calculated, and 
 the suspending power of iron is known; consequently no arch 
 can be so heavy, but that a sufRcient number of chains may 
 be provided to bear more than the weight. The chains may 
 be advantageously composed of long bars of iron, merely 
 turned up at the ends, so that when done with, they may have 
 these ends cut off, and be sold as bar iron again. 
 
 That the piers of bridges may be built hollow, and rendered 
 perfectly manageable in or under water, at a considerable 
 depth, is put beyond a doubt by an experiment which J. I. 
 Hawkins conducted for the Thames Archway Company. Two 
 hollow cylinders of brickwork, upwards of eleven feet diame- 
 ter, and twenty-five long each, were sunk through thirty feet 
 of water in the river Thames, and bedded precisely at the 
 spot proposed. These cylinders were built in a barge, in 
 October and November, 1810, and launched into the water, 
 where they remained floating all winter, and were sunk in the 
 river in the spring of 1811. They were under such perfect 
 command, that from a stage erected on the bed of the river, 
 and supplied with suitable windlasses, pulleys, ropes. See. 
 they were lowered, raised, or moved, in any lateral direction 
 without difliculty. These experiments left not a doubt, that 
 masses of masonry, of large dimensions, might be fixed 
 through the water, in the bottom of a river or harbour, to the 
 depth, if requisite, of one hundred or more feet. They were 
 instituted for the purpose of ascertaining the practicability of 
 forming a tunnel under the Thames, upon a cheaper and more 
 certain plan than undermining it, as was first projected. The 
 result proves, that by this method, communications may 
 cheaply and to a certainty be formed between the opposite 
 shores of rivers, in almost all situations where the navigation, 
 or any other circumstance, renders bridges inadmissible. 
 
 The inventions recorded in the preceding details, promise 
 the dawn of a new era in the science of aquatic architecture. 
 They extend incommensurably the limits of our views, and 
 will lead to undertakings of the boldest and most novel 
 
238 BUILDING. 
 
 Brradth of bridges. — Hollow piers and abutments, &c. 
 
 character. Telford, in his report to the House of Commons, 
 on tlirowing an iron bridge of one arch over the straits of 
 Menai, to connect the Isle of Anglesea with the county of 
 Carnarvon, proposes to suspend the centering, although the 
 span of the required arch will be six hundred feet. 
 
 The common breadth of bridges is about thirty feet; but 
 when they form the avenues to large towns, the carriage 
 road is nearly this breadth, and they have besides a raised 
 foot-path from six to nine feet broad on each side. The 
 breadth of Westminster bridge over the Thames is forty-four 
 feet, and the breadth of Blackfriars' bridge, over the same 
 river, forty-two feet. In large structures, the parapets are 
 about eighteen feet in thickness; they often project from the 
 bridge with a cornice underneath. The parapets are parallel 
 for the greater part of their length, in the middle; but at 
 each end they diverge, in order that the increased width of 
 the road thus obtained may render the entrance upon the 
 bridge more free, and unite better with streets, which are 
 commonly wider than bridges. These wide parts of the ends 
 are generally supported by the solid abutments alone. 
 
 Piers and abutments are not always built solid through- 
 out. By the judicious use of arches or vaults, the structure 
 may be considerably lightened, without having its strength 
 or durability impaired. The arch-work or hollows of the 
 piers are generally made a little above the spring of the 
 arch. When the abutments are vaulted, care must be taken 
 to do it in such a manner that they will bear the push of 
 the arch as completely as before. The sides of the founda- 
 tions of abutments should be concave or dished, as this form 
 will conduce the more effectually to their resisting the pres- 
 sure upon them. 
 
 With regard to the superstructure of a bridge, it may be 
 observed, that when the first course of stones has been dis- 
 posed upon the centering, or wooden frame-work, the re- 
 maining courses may be laid with their bedding joints ho- 
 rizontal, in which case they will press upon the first course 
 as a dead weight, without contributing, except by their 
 pressure, to the strength of the arch ; or the subsequent 
 courses may be laid in the same manner as the first course, 
 so as to be like a series of arches built immediately upon one 
 another. The latter is the mode most consonant to the laws 
 of strength. 
 
 As the principal objects of this work are of a practical na- 
 ture, we confine ourselves to few and transient sketches of his- 
 torical research ; yet in this place it will be interesting to many, 
 and afford matter for deductions which will supersede the 
 
BUILDING. 239 
 
 An account of the most remarkable stone bridges. 
 
 dryness of further precept, if we notice some of the most re- 
 markable stone bndges which have been erected at different 
 periods. 
 
 Of all the bridges of antiquity, that built by the Roman 
 emperor Trajan over the Danube, is allowed to have been the 
 most magnihcent. It was demolished by his immediate suc- 
 cessor Adrian, and the ruins are still to be seen in the middle 
 of the Danube, near the city of Warhel, in Hungary. It had 
 twenty piers of stone, and each pier was one hundred and fifty 
 feet hioh above the foundation, sixty feet in breadth, and one 
 hundred and seventy feet distant from one another, which was 
 the span or width of the arches, so that the whole length of 
 the bridge, exceeding fifteen hundred and ninety yards, was 
 nearlv one mile. 
 
 In France, the Pont de Garde is a very bold structure; the 
 piers being only thirteen feet thick, yet serving to support the 
 immense weight of a triplicate arcade, and joining two moun- 
 tains. It consists of three bridges, one over another; the up- 
 permost of which is an aqueduct. 
 
 The bridge of Avignon, which was finished in the year 1188, 
 consists of eighteen arches, and measures thirteen hundred 
 and forty paces, or about one thousand yards in length. 
 
 The famous bridge at Venice, called the Rialto, has been 
 usually regarded as a master-piece of art. It consists of only 
 one very flat, bold arch, nearly one hundred feet span, and 
 only twenty-three feet high above the water. It was built in 
 1591. Yet such structures sink to insignificance when com- 
 pared with a bridge in China, built from one mountain to an- 
 other, consisting of a single arch, six hundred feet long, and 
 seven hundred and fifty feet high; whence it is called the fly- 
 ing bridge. 
 
 London bridge was built about the commencement of the 
 thirteenth century. It consisted at first of twenty arches, each 
 of them only twenty feet wide ; but the two middlemost were 
 thrown into one in 1738, and another next one side is concealed 
 or covered up. It is nine hundred feet long, sixty feet high, 
 and seventv-four feet wide. The piers are from twenty-five to 
 thirty-four feet broad, with sterlings projecting at the ends; so 
 that "the great water-way, when the tide was above the sterlings, 
 was four hundred and fifty feet, scarcely half the breadth of 
 the river; and below the sterlings, the water-way was reduced 
 to one hundred and ninety-four feet, before the opening of the 
 centre. At the time the two middle arches were thrown into 
 one, the houses which had been erected upon the bridge were 
 removed, and various other alterations and repairs undertaken 
 at an expense of £80,000. But so large a sum is still annually 
 
240 BL'ILUING. 
 
 All account i>( the most remarkable stooe bridges. 
 
 required to keep it in repair, that the preservation of it, ic- 
 stead of erecting a new one, may be considered an act of na- 
 tional improvidence. In consequence of the water-way bein^ 
 so much contracted, the tide, at its ebb and flow, rushes 
 through the arches with such prodigious violence, as to oc- 
 casion a cataract exceeding the height of four feet, and which 
 prevents or renders highly dangerous the passage of boats. 
 The fall of the water passing through Westminster bridge is 
 only one inch. 
 
 The longest bridge in England, is that over the Trent at Bur- 
 ton, built in the twelfth century, of squared free-stone. It is 
 rather low, but strong, and contains thirtv-four arches. Its 
 whole length is fifteen hundred and forty-five feet. 
 
 One of the most singular bridges in Europe, is that built in 
 the year 1756, over the Taaf, in Glamorganshire, by AViliiam 
 Edward, a poor country mason. It consists of only one stu- 
 pendous arch, which though only eight feet broad, and thirty- 
 five feet high, is no less than one hundred and forty feet span, 
 being part of a circle of one hundred and seventy-five feet in 
 diameter. It is the last of three bridges erected on ihe same 
 spot by the same person, and it deserves to be mentioned, 
 were it only to record the amazing perseverance of the builder, 
 under very discouraging circumstances. The first bridge he 
 erected consisted of two arches, and he was bound to uphold 
 it for seven years. It was swept away before the expiration 
 of the term, by a flood, that brought down a prodigious mass 
 of floating materials, which obstructed the progress of the tor- 
 rent through the arches. Edward, whose perseverance in- 
 creased, and whose genius expanded, under his misfortunes, 
 determined to prevent the recurrence of a similar event, from 
 the like cause, by making only one arch. The first bridge 
 attempted upon this plan, sprung at the crown, and fell before 
 it was finished, from the great weight of the end. This severe 
 blow again called forth the resources of his ingenuity; he dis- 
 dained to give up the project of having one arch only, but 
 guarded against his late accident, by making the ends of the 
 bridge hollow, and thus he succeeded. 
 
 Of modem bridges, those of Westminster and Blackfriars, 
 over the Thames, at London, are esteemed the two finest in 
 Europe. The former is upwards of four hundred yards long. It 
 consists of thirteen large and two small arches, all semi-circu- 
 lar, w ith fourteen intermediate piers. The arches all spring from 
 about two feet above low-water-mark. The middle arch is se- 
 venty-six feet wide, and the others on each side decrease always 
 by four feet at a time. The two middle piers are each seventeen 
 feet thick at the springing of the arches ; and the others decrease 
 
BUILDING. 241 
 
 Blackfriars' bridge. — Situation of houses. 
 
 equally on each side by one foot at a time ; every pier terminat- 
 ing with a salient right angle against either stream. This bridge 
 is built of the best materials, and in a neat and elegant taste, 
 but the arches are too small for the quantity of masonry it 
 contains. It was begun in the year 1738, and opened in 1750. 
 It was executed at an expense of £389,500. 
 
 Blackfriars' bridge, which is nearly opposite the centre of 
 the city of London, was begun in 1760, and completed within 
 eleven years from that time. It is an exceedingly light and 
 elegant structure; but the materials do not seem to have been 
 well selected, as some of the arch-stones are already decaying. 
 It consists of nine elliptical arches. The centre arch is one 
 hundred feet wide, and those on each side decrease in regular 
 gradation, to the smallest, at each extremity, which are seventy 
 feet wnde. Its length, from wharf to wharf, is about nine 
 hundred and ninety-five feet. The extrados is a portion of a 
 very large circle, of convenient passage over it. It cost 
 £150,840. This amount being so much less than the cost of 
 Westminster bridge, even after making an ample allowance 
 for the greater length of that biidge, is a proof of the advan- 
 tage of few and elliptical arches over many and semi-circu- 
 lar ones. 
 
 Miscellaneous Remarks relative to Building. 
 
 Under the present title, we shall include a variety of parti- 
 culars, either not belonging, or not exclusively belonging, to 
 Masonry or Bricklaying. Under the first head of tliis subdi- 
 vision of the chapter on Building, the earliest notice seems 
 due to a sketch of the general rules proper to be observed 
 with respect to the 
 
 Situation and Plan of Houses. 
 
 With regard to situation, when the person who is intending 
 to build enjoys the advantage of choice, the proximity of 
 marshes, fens, of a boggy soil, or stagnant water, should be 
 avoided. If a river be very near, the site of the house should 
 be on elevated ground, so as to be out of the reach of the un- 
 wholesome fogs and mists arising from it early in the morning. 
 A neighbourhood where cattle thrive, and where the inhabit- 
 ants are healthy and cheerful, or are remarkable for their lon- 
 gevity, may be regarded as possessing a salubrious air. The 
 most essential requisites of a good situation, or those which are 
 most conducive to health, being obtained, minor advantages 
 may be regarded according to their importance. Abundance 
 of water, fuel, and ways of easy access to arrive at the house, 
 11.— Vol. I. "2 I 
 
242 BUILDING. 
 
 .Situations unci plans of liuuses. 
 
 are conveniences of lasting value. The advantages of a situa- 
 tion, with regard to prospect, are of a more problematical 
 nature, as they are so differently valued by different persons. 
 A man of taste, will, however, undoubtedly j)refer a spot, the 
 pros)>ect from which is most agreeably diversified in the dis- 
 tribution of its land, wood, and water; and those who have 
 little or no relish for the charms of nature, will perhaps con- 
 sult their own comfort more than they may be aware of, by 
 making the same choice. There are very few so obstinately 
 morose, as to be uninfluenced by the opinions of others; and 
 to observe those about them, particularly visitors, warm in 
 their admiration of the surrounding scenery, may create a be- 
 neficial complacency which they would otherwise want. 
 
 Trees at a little distance from a house are better than 
 hills, as they yield during the day, in summer, a cooling, re- 
 freshing, sweet, healthy air; and, in winter, break and diminish 
 the keenness of the winds. Hills on the east and south side 
 are the most inconveniently situated. If the site of a house 
 be low, the first floor should be set so nmch the higher. Cel- 
 lars contribute to the dryness of a house, if the ceilings and 
 floors be good. 
 
 With respect to the situation of the parts of a house, the 
 studies, libraries, and chief rooms, particularly the bed-cham- 
 bers, should face the east; those offices which require heat, as 
 kitchens, brewhouses, bakehouses, and distilleries, should have 
 southern aspects; and those which require a cool fresh air, as 
 cellars, pantries, dairies, granaries, a northern one, which is 
 also proper for galleries, paintings, museums, &c. which re- 
 quire a steady light. 
 
 When a situation has been fixed on, the plan and elevation 
 of every part should be made by some person well acquainted 
 with the theory and practice of building. A skilful architect 
 will not only make the structure handsome and convenient, 
 but will save the great expenses often incurred in rectifying 
 the blunders of hasty and injudicious management. For a 
 small building, the elevation of each front, with a plan, may 
 indicate with sufficient correctness, its ultimate advantages; 
 but for a large mansion, or structure of any description con- 
 sisting of many complex parts, the most certain way to pre- 
 vent mistakes, is to have a perfect model of the whole made to 
 a regular scale. In order that such a model may not mislead 
 the judgment by pleasing the eye, it should be made of plain 
 wood, of one colour. 
 
 Lodges and small houses, standing alone, have an agreeable 
 appearance when of a cubical figure ; but large mansions of 
 this shape look rather clumsy ; an oblong is therefore commonly 
 
BUILDING. 243 
 
 Forms of rooms. 
 
 preferred, care being taken not to make the plan so long as to 
 lose much room in the passages which will be required, and 
 which will be difficult to light. When the length of a mansion 
 does not exceed the breadth more than one-third, the propor- 
 tion is good. 
 
 Of Hooms. 
 
 The principal objects to be regarded in the arrangement and 
 proportions of rooms, are doubtless those of conveniencv, and 
 their adaptation to health. Rooms, the plan of which are 
 rectangular, give the greatest facility to convenience of ar- 
 rangement, without the disadvantage of losing the space ren- 
 dered unavoidable by adopting circular or other curved forms ; 
 but with regard to health, the height of a room should at least 
 be ten or twelve feet, and this point should be regarded even 
 in apartments too small to render such an elevation proper in 
 an architectural point of view. A square is an agreeable 
 form, but is most proper for rooms net exceeding a moderate 
 size, as it cannot well, if very large, be completely lighted 
 from one wall, and the company, while ranged on each side, 
 are too far apart. Fot spacious apartments, a rectangular 
 parallelogram or oblong is a more convenient figure, and, 
 with regard to beauty, every variation in the proportion, 
 from nearly a square to a square and a half or sesquialteral, 
 may be employed. If the length of the plan be extended 
 materially beyond a sesquialteral, the apartment obtains rather 
 the appearance of a passage or gallery, and it becomes impos- 
 sible to adjust the height so as to suit both the length and 
 breadth. In square rooms of the first story, the height 
 may be from four-fifths to five-sixths of the breadth of the 
 side; and in oblong rooms, the height may be equal to the 
 width. An error in favour of height, is preferable to making 
 a room too low. 
 
 The height of rooms on the second story may be one-twelfth 
 part less than that of the chambers below : and if there is a 
 third story, divide the height of the second into twelve equal 
 parts, of which take nine for the height of such rooms. 
 
 An eligible length for galleries is five times their breadth, and 
 they should rarely exceed eight times their width in length ; 
 their height may exceed their breadth in the proportion of a 
 third or even three-fiftlis, according to their length. 
 
 As, however, in modern houses, all the rooms of the same 
 story are commonly of the same height, and convenience 
 requires them to be of different sizes, according to their use, 
 it follows that the best proportion of the height to the other 
 dimensions, cannot always be observed, without incurring some 
 
244 BLILDING. 
 
 Arraiiijemenlb for tlie comfort of rooms 
 
 extraordinary expense. Where expense is not a hinderance, 
 the height of the story may be suited to the principal rooms, 
 and the middle-sized rooms may be reduced bv coving the 
 ceilings, with a flat in the middle, or by groins or domes, 
 which will add to their beauty, independently of bettering 
 their proportions. 
 
 Precautions should be taken to prevent the effluvia from the 
 kitchen, brewhoiise, and other oflices, from penetrating to the 
 bed-chambers and dining rooms. The most difficult object to 
 attain of this description, is to prevent the effluvia of the 
 kitchen from annoying the dining-room, to which the access 
 from it should be as easy and short as circumstances will 
 allow, for the convenience of the servants waiting at table. 
 In country mansions, which admit of the greatest liberty of 
 plan, and where the kitchens are above ground, this may 
 generally be done, by such an arrangement of the doors and 
 passages of communication, that no current of air from the 
 kitchen can proceed directly towards the dining-rooms. But 
 in town houses, where the kitchen is beneath the parlour floor, 
 and therefore not only far nearer in point of situation (though 
 not perhaps of access for persons) than it is usually placed in 
 the country, but on a lower level, the lighter warm air, charged 
 with the smell of the various operations of cookery, is apt to 
 be felt above, from its disposition to ascend. It mav, how- 
 ever, be eftectually removed by a small separate funnel, carried 
 up in the stack with the rest, and next to that of the kitchen. 
 This funnel, to be used for no other purpose, must have its 
 throat or lower opening level with the ceiling of the kitchen. 
 The lighter air, charged with the vapours of the cooking, will 
 then pass off into the external atmosphere by this aperture, 
 instead of accumulating under the ceiling of the kitchen, until 
 it forms a stratum as low as the top of the kitchen door, and 
 then ascending throu^rh the house by the stairs and passages. 
 The opening of this funnel or pipe mav be closed by a hinged 
 door, when no operation is going on in the kitchen which can 
 create a disagreeable smell. 
 
 Every chamber in a house should, if possible, have a fire- 
 place, the place of which, in those employed as bed-rooms, if 
 they are not very spacious, sliould be about two feet or two 
 feet and a half out of the middle, to allow room for the bed. 
 In apartments of twenty or twentv-four feet a side, this arrange- 
 ment need not be studied, as the bed can without it be placed 
 sufficiently far from the fire. 
 
BUILDING. 245 
 
 Best form for cbiiuiieys. — Definitious 
 
 Chimneys. 
 
 That no apartment can be comfortable which is incommoded 
 with smoke, will not be contradicted ; yet we find a very general 
 disregard of the precautions which would secure a strong draft 
 up the chimney. jNIasons and bricklayers follow their own 
 fancy or judgment, which are often influenced by their conve- 
 nience, or by local customs, wuth little regard to rational prin- 
 ciple. It frequently happens that the smoking of chimneys is 
 occasioned by their being carried up narrower at the top than 
 below, or by their having one or more sharp angular turns. 
 When chimneys are constructed in a pyramidal or tapering 
 form, and are besides left rough, or unplastered, with stones 
 or bricks projecting into them, as well as the mortar pressed 
 from the joints in the wall, their smoking is almost certain. The 
 air, rarefied by the fire, passes up a chimney with the smoke; 
 but as it recedes from the impelling power, or fire, it moves 
 slower, and requires a greater portion of space to circulate 
 through; if, then, the upper part of a chimney, instead of being- 
 wider than below, be contracted, and if the roughness of the 
 walls concur at the same time to increase the obstruction, it is 
 no wonder that the smoke, taking the path of least resistance, 
 should find its way into the room, whenever assisted by a cur- 
 rent from above. It may be urged, that a chimney wider at the 
 top than below, allows the wind more liberty to blow down ; 
 but it must be considered, that the wind having no adequate 
 resistance from above to overcome,must necessarily return, and 
 thus facilitate the free egress of the smoke. On the other hand, 
 when a current of air rushes down a pyramidal chimney, it 
 becomes confined or wedged in, and if urged by a constant 
 wind, the rarefied air from the fire cannot make head against 
 it, and therefore the smoke bursts into the room. 
 
 Experiments, in endless variety, and often at great expense, 
 have been made to prevent or cure smoky chimneys : we shall 
 notice some of the most simple and efficient ; but it will be 
 proper to explain, in the first place, some of the terms which 
 are used in speaking of chimneys. The aperture from a chim- 
 ney into the room, is called the Jire-place. The projecting 
 parts of the wall, on the right and left of the fire-place, are 
 called jambs. The head of the fire-place, resting upon the 
 jambs, is called the mantle. The mantle, and the whole side 
 of the chimney above it up to the top, are called the breast. 
 The side of the chimney, called the breast, being pointed out, 
 the application of the term back, to the opposite side, explains 
 itself. The sides of the fire-place contained between the jambs 
 and the back, are called covings. The throat is that part of 
 
246 BUILDING. 
 
 Precautions to prevent smoky chimneys. 
 
 the opening immediately above the fire, and contained between 
 the mantle and the back. 
 
 When chimneys are bounded on the top by a zigzag line, 
 so as to resemble the teeth of a saw, they are found to be 
 far less liable to smoke than those in other respects under 
 tl)e same circumstances; and in a great variety of cases, 
 the cheap and easy expedient of altering the tops of smoky 
 chimneys to this form, has been attended with complete suc- 
 cess. The partitions in a stack should be indented as well as 
 the outside wall. 
 
 The fire-place is generally an exact square. Its height, in 
 large rooms, is often very properly made less than a square, 
 and in small rooms, particularly when the chimney is in a 
 corner, it is rather more. In rooms from twenty to twenty- 
 four feet square, or of equal area, it may be from four feet 
 to four feet and a half broad ; in rooms from twenty-four 
 to thirty feet square, it may be from four and a half to five 
 feet; and in rooms still larger, it may be extended in a 
 similar proportion to six feet. If much beyond six feet, 
 whatever may be the size of the room, the eftect will not be 
 good ; for if the fire be proportionate, it will excite rather the 
 idea of a furnace. Two fire-places will certainly be better 
 than one of such overgrown dimensions. As to the effect of 
 the form of this aperture on the draft, its breadth is not very 
 important, provided it be not so narrow as to prevent the 
 covings from standing with their greatest power of reflection 
 towards the room; but the height should seldom exceed two 
 feet six inches to the under side uf the mantle. The throat 
 should not be more than four or five inches wide; but should 
 be contracted by a part moveable at the back, when the chim- 
 ney requires sweeping. The nearer the throat is brought to 
 the fire, the stronger the draft will be. For fire-places about 
 three and a half feet wide in front, the flues of chimneys, above 
 the throat, are usually made equal to about twelve inches 
 square; and the general rule is, to make the area of the hori- 
 zontal section of the flue, equal to the area of the horizontal 
 section of the fire. If the flue were smooth and circular, this 
 mode of proportioning its size, would doubtless be found to 
 allow it unnecessarily large. Where bends are required in a 
 flue, they should be in a curved, and not an angular direction. 
 High chimneys always draw better than low ones, as, in pro- 
 portion to their length, the influence of the wind extends a 
 shorter way down them. An apartment made wind-tight, by 
 listing the doors and other contrivances, is liable to be incom- 
 moded with smoke, from the want of draft, even when the 
 chimney is constructed in the most proper manner : a few 
 
BUILDING. ^47 
 
 Precaution to prevent a smoky clihuney. — Smart's sweeping apparatus. 
 
 minute holes, communicating with the exterior, will, in such 
 cases, constitute an effectual remedy. 
 
 Another method of increasing the draft of a chimney, con- 
 sists in setting the grate, if a Bath stove, eleven or twelve 
 inches from the fender; and in cutting away the back of the 
 chimney, so as to leave a space of two inches between it and 
 the back of the grate. If the grate be of the common form, 
 the sides should be filled up with brickwork. By this con- 
 struction the air that passes behind the back of the grate, as- 
 sisting to impel the smoke, prevents its bursting into the room. 
 — That a chimney clogged with soot will be apt to smoke, is 
 so well known, as to require no notice here. 
 
 The evils attending the practice of cleaning chimneys by 
 means of climbing boys, are of the first magnitude. The 
 degraded situation of the children in this employment, so 
 destructive to health at any period of life, but especially in 
 early youth, has often and strongly excited the commisera- 
 tion of philanthropical men, and many schemes have been 
 proposed to render their ascending unnecessary, and thus 
 abolish a practice so offensive to humanity. Of these plans, 
 the most feasible consists in the use of brushes, which 
 are drawn up and down by men at the top and others at 
 the bottom, or are pushed up from the bottom, and drawn 
 down by persons within the apartment. The former me- 
 thod is inconvenient and expensive ; the latter, which is 
 the indention of Smart, is often practised, and has justly 
 received much approbation. The rods by which the brush 
 is pushed upwards, are made hollow and in short lengths, 
 and are connected together by a cord passing through them. 
 As soon as the lower end of the first rod is pushed about 
 the height of the mantle, another rod is slipped over the 
 cord, and the lengthening of the whole is thus continued, 
 till the brush clears the top of the chimney, where it spreads 
 itself out, and is prevented from contracting by a spring ; 
 so that the soot is brought down along with it. But as long 
 as the practice is continued, of making chimneys square, 
 crooked, and rough, it is to be feared that no sweeping appa- 
 ratus can be contrived which will be found completely suc- 
 cessful. Inventions recommended merely by their humanity, 
 are often so slow in their progress towards general adoption, 
 that an eminent service will be rendered to society, by the 
 man who makes the convenience and interest of individuals 
 conspire with their benevolence, to promote the purpose be- 
 fore us. The Author of the treatise on Architecture in this 
 work, has furnished us with the following remarks on the 
 present subject, which we have pleasure in laying before the 
 
248 BUILDING. 
 
 New a])plicatii)tis of iron. — Use of iron for cliimnt-ys proposed. 
 
 public. He commences with a few observations on the late 
 rapidlv extended use of iron: 
 
 Amongst the various improvements of modern times, per- 
 haps few have been more really beneficial than the use of 
 metals, for purposes to which, only half a century back, they 
 were hardly thought applicable. It is perhaps not generally 
 known, that the great variety of articles of fittings now con- 
 stantly kept in ironmongers' shops, in the remotest villages, 
 were hardly known in country places at the beginning of the 
 last century ; but wherever a house was built, its bolts and 
 latches were mostly of wood ; its windows, if sashes, were 
 massive, and if one sash run, the sash-cord ran over a wooden 
 pulley ; and many other minute fittings were either of wood, 
 or, if of metal, were individually made by the adjacent smith, 
 who produced a clumsy article at a great waste of time and 
 labour. We now see the use of iron and brass extensively 
 superseding timber and stone in the most advantageous 
 manner. At Eaton Hall, the magnificent mansion of Lord 
 Grosvenor, near Chester, the tracery of the windows, many 
 shields, a stair-case, and a variety of ornaments, are of cast 
 iron. A large portion of tracery pannelling, for a terrace 
 balustrade, are of the same material, and being painted and 
 sanded, have all the beauty of stone. Hollow Grecian balus- 
 trades, for a public building in Ireland, have been made of 
 iron, in Liverpool. Two patents have been taken out for 
 roof framings of iron, one of which, at least that of J. Cragg, 
 Liverpool, is particularly remarkable for its simplicity, and 
 the beauty of its execution. This gentleman also executes 
 stair-cases of iron, in so accurate a manner that they may 
 be put up at any place in a few hours. Cast iron bridges 
 have long been deservedly celebrated, and the dearness of 
 wood, and tlie supposed ineflBciency of the patent stone pipe, 
 have caused cast iron water-pipes to be used in London, and 
 many other places. Cast iron has also been advantageously 
 used for beams and their supports in a large way, the Trus- 
 tees of the Duke of Bridgewater's canal having used it for 
 beams in a new warehouse at Liverpool, of more than thirty 
 feet clear span ; and the Carron Company have used some 
 excellently arranged hollow iron stanchions in a new ware- 
 house at Liverpool. 
 
 There is yet another purpose, for which iron seems so well 
 adapted, that it deserves at least a fair trial; it is that of cast 
 pipe for chimneys. The present mode of building chimneys is 
 not only extremely unsafe, but also takes up a great deal of 
 room; and from their being square, there is a great difiiculty in 
 cleaning them thoroughly, except by climbing boys, or, as is 
 
BUILDING. 249 
 
 Advantages of iron-pipe for ordinary chimneys. 
 
 frequently practised, by purposely firing them. Smart's appa- 
 ratus does not completely answer, because the corners which 
 contain most soot do not get swept completely, and are often 
 not touched. Another objection to ordhiary brick square 
 chimneys is their smoking, or, in other words, not drawing; 
 an evil not considered, by those who suffer from it, of the 
 most trivial magnitude. All these inconveniences might be 
 remedied by using for chimneys cast iron pipes three-eighths 
 of an inch thick, and when once the utility of the plan became 
 known, the article would be as common in iron shops as fronts 
 of grates. The present openings into the room are built far 
 too large, and are now generally contracted by various means 
 when the grate is set, but while the present arrangement of 
 brick chimneys for climbing boys remains, they cannot well 
 be much altered; but if iron pipe were used, an iron fire-place 
 top would be cast for the pipe to fit into, and would of course 
 be made of various sizes and shapes like grate fronts, and 
 iron articles in general, cast by the Carron Company and 
 other founderies. 
 
 More clearly to exemplify the advantages of this invention, 
 it will be proper to detail the arrangements necessary. Pre- 
 judice will at first perhaps require the pipe to be eight inches 
 in diameter, though there is little doubt that seven, six, or 
 even five inches diameter, would be amply sufficient for the 
 largest fire. However this may be, it should be cast in lengths 
 of from three to five feet, which at three-eighths of an inch 
 thick would render the pieces of a manageable weight ; they 
 should be carried up as close as possible to each other, and 
 as four inches of brick-work is more than enough on each 
 side of them, in walls of two bricks thick, they would cause 
 no projection into the apartment. The interstices between 
 the bricks and pipe should be filled with earth, or, what 
 would be still better, rubbish and liquid cement. A bevel 
 joint, a, fig. 15, pi. I. might be made to the pipe, to fit so 
 closely that no crevice would be left to lodge soot, when the 
 cement surrounded the exterior. 
 
 If one, or indeed all the chimneys in a stack, constructed 
 upon this plan, were to get on fire, it would be of little conse- 
 quence, as the heat could never be so great as to act through 
 the pipe and such a solid mass as the stack would be. The 
 soot would not lodge in such chimneys as in the common 
 ones, for in a round chimney the draft would clear more smoke 
 out of the chimney before it was condensed; and when they 
 required cleaning, Smart's apparatus would sweep them com- 
 pletely. — When a house with ordinary chimneys is burnt, the 
 stack usually falls down, tearing the walls, and rendering it 
 11.— Vol. I. 2 K 
 
250 BUILDING. 
 
 Adtantagos of iroo-pipe for ordiaary chimneys. 
 
 necessary to rebuild them; but if a house with an iron-pipe 
 stack was burnt, it would not only in all probability stand, but 
 act as a buttress to sustain the walls; or, in case it fell, the 
 greater part of the pipe would remain as fit for use as at first. 
 Although it may be least trouble generally to leave the breadth 
 of half a brick round the pipe, yet, in small buildings, and in 
 cases where room is wanted, a flat brick would, there is no 
 doubt, be quite sufficient for all purposes of safety. In all 
 cases, too, where iron-pipe may be used, the labour of plaster- 
 ing chimneys will be saved, which, though in many places 
 omitted, is done to all well-built houses. 
 
 The saving of room which would be produced by the use of 
 iron-pipe, in a stack of four eight-inch chimneys, will be im- 
 mediately seen by the inspection of fig. 13, No. 1 and 2, pi. I ; 
 and fig. 14, Xo. 1 and 2, exemplifies the same thing with re- 
 spect to a stack of five chimneys. If nine inches be allowed 
 for the exterior diameter of the pipe (and such an allowance 
 will be enough for all contingencies,) it may be carried up, in 
 all walls exceeding two bricks thick, without any projection 
 into the apartments. In the corners of a two brick, or even a 
 one and a half brick wall, there is sufficient room, as may be 
 seen by referring to the plans of such walls in pi. I, to carry 
 up such a chimney, which would rather Increase than diminish 
 the strength of the wall. By the figures of the plate just re- 
 ferred to, it will be seen that nearly one-half of the space taken 
 up by chimneys in the common way will be saved, under cir- 
 cumstances the most unfavourable to shewing the advantages 
 of the new plan ; for the diameter of the pipes is reckoned at 
 the largest that can be required ; and the brick-work sur- 
 rounding the square chimneys at the thinnest it can with any 
 propriety be made. 
 
 The increased expense of round work, eitlier in brick or 
 stone, is probably the principal reason that round chimneys 
 are not in general use, though the advantages of them, parti- 
 cularly in being free from the currents or eddies of air occa- 
 sioned by square chimneys, are not unknown to intelligent 
 builders. 
 
 Iron pipe, it is well known, answers perfectly well for stoves, 
 and the portable furnaces of chemists, although the area of it 
 is seldom more than half that of the fire, and the quantity of 
 air required to maintain the vehement heat which can be ex- 
 cited at pleasure, occasions a much greater draft than that of 
 ordinary domestic fires of twice the size. Hence the above 
 plan only proposes the extended use of a kind of chimney, 
 the adequateness of which is already proved. 
 
BUILDING. 251 
 
 Proportions of doors. 
 
 DOOJ'S. 
 
 The ancients, according to Vitruvius, frequently made their 
 doors rather narrower in breadth at the top than at the bottom. 
 These trapezoidal doors were probably adopted from their hav- 
 ing the property of closing themselves, and in modern times 
 they are useful besides as the simplest mode of raising the 
 door, in the act of opening, above the floor, in order to keep 
 it clear of the carpet. There are examples of them in the Bank 
 of England ; but they are not often introduced by architects. 
 
 Doors are varied in their dimensions according to the height 
 of the story and the magnitude of the building in which they 
 are placed. In private houses, they can rarely with propriety 
 be made wider than four feet, and in general three feet will be 
 sufficient. For small doors, when the height is to the breadth 
 in the ratio of seven to three, the proportion may be con- 
 sidered good ; but the height of large doors need not be more 
 than double their breadth. The entrance doors of palaces and 
 tJie mansions of noblemen, where much company resort, are 
 often made from four to six feet wide ; and those of public 
 edifices may be from six to ten feet wide. Doors much ex- 
 ceeding three feet in width, should have folding leaves. In 
 modern houses, it is not uncommon to have large folding 
 doors, the opening of which serves, instead of removing a 
 whole partition, to throw two rooms into one. In such cases, 
 the width of the aperture will generally be of less height than 
 twice its breadth, as all the doors of the same story are com- 
 monly of the same height. 
 
 When the principal door of entrance is in the middle, its 
 communication with every part of the building is not only the 
 most readily effected, but it contributes so much to the sym- 
 metry of the front, that when the plan renders such a position 
 inadmissible, a blank door is frequently substituted for a real 
 one, which is then made in the most convenient place. The 
 entrance doors of stately houses, are frequently adorned with 
 porticoes, in the Grecian or Roman taste ; but the most com- 
 mon mode of adorning entrance doors, is to surround them 
 with an architrave, surmounted with a cornice, or with a frieze 
 and cornice forming a complete entablature. These decora- 
 tions are made of stone, wherever a suitable kind can be had 
 at a reasonable price. 
 
 Windows. 
 
 In determining the number and size of windows, regard 
 must be paid to the destination, local position, and elevation of 
 the building, as well as to the cubature and height of the story 
 
252 BL'ILDING. 
 
 Proportions of windows. 
 
 in the rooms to be lighted, and the thickness of the walls. 
 AVith respect to private houses, though considerable latitude 
 mav be allowed in the determination of this subject, still there 
 are limits which cannot be disregarded without losing the 
 beautv of proportion, and the convenience of a due quantity of 
 light. In general, the piers should not be of less breadth 
 tiian the apertures, nor more than twice such breadth. The 
 windows in all the stories of the same aspect, should be of 
 the same breadth, unless a variation be required from this 
 rule for the convenience of particular offices in the lowest 
 story. The laws of svmmetrv and t^trength alike require them 
 to be exactly one above another; this practice, so strangely 
 neglected by our ancestors, is now, indeed, duly attended to. 
 The apertures of windows should widen inwards on each side, 
 by which means the quantity of light admitted will be nearly 
 as much as if they were externally of the same size as the in- 
 creased internal dimensions. 
 
 To determine the aggregate area of the windows proper to 
 be made in an rtparfmpnt, extract the square root of the cuba- 
 ture of such apartment, and the quotient will be the answer. 
 For example, suppose the room to be forty feet long, thirty 
 feet broad, and sixteen feet high, then 40x30x16=19200, 
 which product is, in feet, the cubature sought, and the square 
 root of it, neglecting a small remainder, is one hundred and 
 thirty-eight feet for the aggregate area of the apertures. One 
 hundred and thirty-eight feet will make four windows of a 
 handsome size and shape, adapted to the apartment in ques- 
 tion ; and if divided accordingly into four parts, thirty-four feet 
 and a half will be the area of one of them. The area thus ob- 
 tained, when set out, for a ground floor, according to the cus- 
 tomary rule, which allows rather more than two squares in 
 heioht, each window may be about eight feet eight inches 
 high, by four feet broad. By the same rule, the dimensions 
 of the apertures of windows for rooms of any other cubature 
 may be determined. 
 
 The sills of windows have been mostlv made from three 
 feet to three feet six inches distant from the level of the tloor, 
 as at that heisht thev formed a convenient parapet to lean 
 upon ; but the French fashion having been introduced, of hav- 
 ing the window^s, at least in the principal drawing rooms, down 
 to the floor, window-sills are now, partly in imitation of it, 
 made lower than formerly, and in ordinary dwellings are fre- 
 quently not higher than two feet, and in the extreme not more 
 than two feet six inches. 
 
 It will be proper to remind those who are partial to spacious 
 and numerous windows, and who are not disposed to modify 
 
BUILDING. 253 
 
 Utility of double windows. — Requisites of a good staircase. 
 
 their choice by motives of economy, that as the aggregate area 
 of the windows is enlarged, it becomes increasingly difficult 
 in winter to keep apartments warm, the heat produced in them 
 beino- so very speedily communicated through the glass to the 
 atmosphere without. It is for this reason, that in Russia they 
 often make their windows double, and as air is a bad conduc- 
 tor of heat, the stratum of it interposed between the two win- 
 dows in the same frame, tends very materially to prevent the 
 temperature of the room from being carried otf. The cold sea- 
 son is not so severe, or of such long continuance, as to have 
 occasioned the introduction of this practice for front windows 
 into the United Kingdom, but it might be advantageously 
 acted upon with respect to the skylights employed to light 
 staircases, as such windows, when only single, contribute 
 greatly to the speedy dissipation of the warm air which as- 
 cends to the top of the house. 
 
 The number of windows on each side of the entrance door 
 should be equal, and an odd number of windows in an apart- 
 ment, when they are all on one side of it, is better than an 
 even number, as it avoids the necessity of having a pier oppo- 
 site the middle of the floor. 
 
 The windows of the principal floor are generally the most 
 enriched, the simplest mode of adorning them, is, to surround 
 them with an architrave, which sometimes has, and sometimes 
 is left without, a frieze and cornice ; frequently, the whole of 
 the windows are left plain, except the central one of the second 
 story. When the windows of the principal story have pedi- 
 ments, those of the story immediately above should have archi- 
 traves surmounted by a frieze and cornice, and those of a next 
 higher story, an architrave only. The sills of all the windows 
 in the same floor should be on the same level. 
 
 Stairs. 
 
 To unite the requisites which a good staircase requires, 
 namely, convenience in situation and form, with a sufficiency 
 of light, affords one of the strongest proofs of an architect's 
 skill. In stately mansions, the steps should not be less than 
 four, nor more than six inches high ; nor more than eighteen 
 or less than twelve inches broad; and the width should not 
 be less than six or exceed fifteen feet. In ordinary houses, 
 the steps are generally made higher, and are almost necessarily 
 narrower, both in width and length; but, while any thing like 
 handsomeness of appearance and convenience of ascent are 
 studied, they should not exceed seven inches in height, nor 
 be less than ten inches broad, and three feet long. Stairs are 
 ascended with more ease when laid somewhat sloping, or a 
 
254 BUILDING. 
 
 Pilch of roofs. 
 
 little higlier at the back. The ancients were partial to an odd 
 number of steps ; one consequence of which choice was, that 
 the same foot which was placed on the tirst step was first 
 placed at the top on the landing. 
 
 Roofs. 
 
 Architects include, under the term roof, not only the exte- 
 rior covering of a house, but all the beams and other parts 
 necessary to support that covering. 
 
 Among the ancients, in those countries where it seldom 
 rained, roofs were made quite flat; but the Greeks, perceiving 
 the inconvenience of this form, deviated from it a little, by in- 
 clining their roofs, as appears from several ancient remains, 
 about one-eighth or one-ninth part of the span. The Romans, 
 who had rather more occasion than the Greeks to provide for 
 the speedy discharge of rain from their houses, made the 
 height of the inclination of their roofs from one-fifth to two- 
 ninths of the span. Among the northern nations of Europe, 
 after the decline of the Roman empire, high-pitched roofs 
 began to be in general request, and that was considered the 
 standard form, the vertical section of which was an equilateral 
 triangle. No part of the art of building has been more sub- 
 ject to caprice, than the height of the inclination, or, as it is 
 usually expressed, the pitch of roofs. In the present day, a 
 great variety of pitch is employed, but the equilateral one is 
 seldom seen. In ordinary dwellings, the pitch of the roof is 
 from one-third to one-fourth part of the span; but mansions 
 and public edifices are still executed in every diversity of style 
 that fancy or particular views can dictate. 
 
 High-pitched roofs discharge rain and snow more quickly 
 than others; they are also not so easily stripped by the wind; 
 the rain is not so easily blown between the slates, and from 
 their approach towards perpendicularity of pressure, they are 
 not so great a burden to the walls. They are, however, more 
 expensive than low roofs, as they require longer and stronger 
 timbers; and from their greater surface, they require a larger 
 quantity of the covering material : but though low roofs have 
 the advantage in point of cheapness, they require large slates, 
 and great care of execution. 
 
 The roof is one of the principal ties of a building, when skil- 
 fully executed, in connecting the exterior walls. Some idea may 
 be formed of the success with which scientific knowledge and 
 experience may be employed in the construction of roofs, when 
 it is observed, that roofs have been constructed sixty feet wide, 
 although they have not contained a single piece of timber more 
 than ten feet long and four inches square. 
 
BTTILBING. 
 
 PL.I. 
 
 Fiff.I. 
 
 Fiq.3. 
 
 Fig. 8. 
 
 mm 
 
 -Ti 
 
 ,:^i ki 
 
 _LX! 
 
 f^9 9- 
 
 LW^MMMM 
 
 t—t- 
 
 Fio. J. 
 
 Fu? JO. 
 
 . ^/y.h/Ac'/.&/<h/.y}M.A I 
 
 -J L 
 
 i,. i 
 
 
 F^JJ 
 
 FiffS 
 
 
 1 '~\ ~ ^' 
 
 1- rcrizcirr: 
 
 -p^ 
 
 :4 
 
 FlO 7 
 
 
 ..CXDQ. 
 
 ^t^jKCticn JtC£ theR^c 
 
 ^ I f Mifllf^ '''""" 
 
 Fiff.12. Fi^.ij. 
 
 .0 
 
 Ft^ 25. 
 
 Frajeodcn. ^■•. vTcftAf 
 iTii^ brick »aK- 
 
 Fublisliea \y Henry Fis"her, Cazton.Londnn. 1822. 
 
BTiriiLiD3:^(&. 
 
 FL.TI. 
 
 MrjMn IruJSuLith 
 
 Erttfravfd btfXafJtari*. 
 
 -Fnllis^ei "by Henry Rsher . CaxtoTt.XcmioiL . 1823. 
 
BUILDING. 255 
 
 Pitch of the rafters, for lead, pantiles, plain tiles. — Experiment on tiles and slate. 
 
 In determining the pitch of rafters, when mere fancy is not to 
 be our guide, the nature of the intended covering should be 
 taken into consideration, and the inclination proportioned ac- 
 cordingly. The following rules may be observed with propriety : 
 
 For Lead. — Divide the width first into two parts, see fig. 16, 
 pi. I. and one of these parts into four, as 1, 2, 3, 4; with two 
 of these parts, describe a quarter circle, 2, which gives a pro- 
 per pitch or slope to be covered with lead, and is called a pedi- 
 ment pitch. 
 
 For Pantiles. — Divide the width as before into two parts, and 
 one of these two parts into four, as 1, 2, 3, 4; with three parts, 
 describe a quarter circle, 3, which gives a proper pitch for use. 
 
 For Plain Tiles. — Divide the width into two parts; with one 
 of them make a quarter circle, which gives a pitch or slope 
 proper for the roof. — The lighter the covering material, the 
 lower the roof may be, and therefore the pitch for slates, may 
 be the same as that for the covering which the particular 
 quality used most nearly approaches to in weight. 
 
 Tiles, though extensively used in many parts of the country, 
 constitute a very heavy covering for houses, and, what is still 
 worse, they injure the timber upon which they are laid, and 
 tend to make a house damp, from the facility with which they 
 are penetrated with moisture. The following experiment, by 
 the bishop of Llandaff, decisively proves their great porosity, 
 and their inferiority to slate. The Bishop observes, that sort 
 of slate, other circumstances being the same, is esteemed the 
 best which imbibes the least water; for the imbibed water not 
 only increases the weight of the covering, but, in frosty weather, 
 being converted into ice, it swells and shivers the elate. This 
 effect of frost is very sensible in tiled houses, but is scarcely 
 felt in those which are slated ; for good slate imbibes but little 
 water; and when tiles are well glazed, they are rendered in some 
 measure, with respect to this point, similar to slate. He took 
 a piece of Westmoreland slate, and a piece of common tile, and 
 weighed each of them carefully ; the surface of each was about 
 thirty square inches. Both the pieces were immersed in water 
 for about ten minutes, and then taken out and weighed as soon 
 as they had ceased to drip. The tile had imbibed above a 
 seventh part of its weight of water; and the slate had not im- 
 bibed above a two-hundredth part of its weight; indeed the 
 wetting of the slate was merely superficial. He placed both 
 the wet pieces before the fire; in a quarter of an hour the slate 
 was become quite dry, and of the same weight it had before it 
 was put into the water; but the tile had lost only about twelve 
 grains of the water it had imbibed, which was, as nearly as could 
 be expected, the very same quantity that had been spread over 
 
25G BLILDING. 
 
 Prices of slate. — Statemeot of Uie weight of ililTerent corerings. 
 
 its surface ; for it was tlie quantity which had been imbibed by 
 the slate, the surface of which w as equal to that of the tile. The 
 tile was left to dry in a room heated to sixty degrees, and it 
 did not loose all the water it had imbibed in less than six days- 
 If, then, tiles imbibe a seventh part of their weight of water in 
 ten minutes, and cannot be deprived of this water without a 
 degree of heat equal to sixty degrees continued for six days, it 
 must be obvious that a roof covered with them, can, in this 
 country, seldom be dry. The timbers also of the roof must be 
 calculated to support their weight in their wet state. 
 
 The finest sort of blue slate, w hich is obtained in the neigh- 
 bourhood of Kendal, is sold there for 3s. 6d. per load, which 
 comes to £1. los. per ton, the load weighing two hundred 
 weight. The coarsest sort may be had for 2s. 4d. per load, or 
 £1. 3s. 4d. per ton. Thirteen loads of the finest sort will cover 
 forty-two square yards of roof, and eighteen loads of the 
 coarsest sort will cover the same extent. Hence there is half 
 a ton less weight upon forty-two square yards of roof when 
 the finest slate is used, than when it is covered with the coarsest 
 kind, and the difference in the expense of the material is only 
 3s. 6d.; yet as fine slate owes its lightness, not so much to a 
 ditJerence in the quality of the stone from which it is split, as 
 to the thinness to which it is reduced, it is inferior to the 
 heavier kind in point of durability. 
 
 The following statement shews the average weight of the 
 covering laid upon fortv-two square yards of building, accord- 
 ing to the material employed : 
 
 Copper 4cu:t. 
 
 Fine Slate 26 
 
 Lead 27 
 
 Coarse Slate 36 
 
 Tiles 54 
 
 Such are the advantages of slate as a covering, that 
 wherever it can be procured without much land carriage, it 
 obtains the preference of all other materials. Its durability is 
 so great, that it has been known to continue sound and good for 
 centuries ; but all kinds of it are not equally excellent in this 
 respect. Its most usual colours are white, brown, and blue, 
 and the colour affords some index to the quality of the slate. 
 The light blue sort is always the least penetrable to water, 
 which the deep black blue is apt to imbibe rather freely. 
 Several methods may be practised to ascertain the goodness of 
 slate, when not brought from a quarry of well known character. 
 If a slate, when struck sharply against a large stone, produce a 
 complete sound, it is a mark of goodness : and if it does not 
 
BUILDING. 257 
 
 Test* of the quality of slates. — Patent roofing. 
 
 shatter before the edge of the zax, or instrument used for hew- 
 ing it, the criterion is decisive. Another method is, to place 
 the slate lengthwise and perpendicularly in a tub of water, 
 about half a foot deep, care being taken that the unimmersed 
 part of the slate be not in any way accidentally wetted. Let 
 it remain in this state twenty-four hours : at the end of that 
 time, if the slate be good, it will not have drawn water more 
 than half an inch above the surface of that fluid, and that per- 
 haps at the edges only, where the texture has been a little 
 loosened in the hewing ; but a spongy defective stone will 
 draw water to the very top. Another mode of trial, on the 
 result of which full reliance may be placed, is to weigh two or 
 three of the most suspected slates, and note their precise 
 weight; then immerse them entirely in water for twelve hours: 
 take them out, wipe them as clean as possible with a linen 
 cloth, and if their weight differs not, or differs but very little 
 from Avliat it was at tirst, they may be considered good ; a 
 drachm in a dozen pounds is allowable, but not more. The 
 principal reason of the inferiority of the slates which imbibe 
 much moisture, being that they are shivered by frost ; when 
 none but a porous kind can be easily obtained, they might 
 doubtless be improved by the application of tar, as already 
 mentioned for tiles, when treating of the manufacture of the 
 latter article. 
 
 John's tessera is perhaps the best of those artificial compo- 
 sitions which are designed for roofing, and it certainly has 
 advantages of the first class. It is cheaper than any covering 
 hitherto known; it is superior in lightness and evenness of sur- 
 face to lead, and is said to equal that metal in point of dura- 
 bility. It is equally adapted to the flat roof intended to be 
 Avaiked on, and to angular roofs. The roofs intended to be 
 covered with it must in all cases be boarded as for lead, ex- 
 cept that a less thickness of timber will sufiice. For a flat 
 roof, it may be laid on three-quarter inch deal ; but for an an- 
 gular roof, half-inch will be sufficient. This roofing, for which 
 a patent was taken out, dated the 22d December, 1806, con- 
 sists of calcareous stone and tar, with a little of the powder of 
 burnt bones. The sheets are made of an equal thickness by 
 rolling. From the quantity of tar entering into the composi- 
 tion, it may be considered very combustible; but the contrary 
 has been proved by experiment. It is found to resist the 
 effects of heat two or three times as long as lead. Tessera is 
 made in sheets about four feet long, by two feet wide ; and the 
 sheets are united by solder of the same composition. 
 
 11.— Vol. I. 2 L 
 
25S BUILDING. 
 
 Qualities auil ue(bu«ls of jointing fluoriui; tjoards 
 
 Flooring boards are mostly made of fir. The first class are 
 selected free from knots, shakes, sap-wood, or cross-grained 
 stufl'; the second class consists of boards also free from shakes 
 and sap-wood, but not from small sound knots; the third class 
 contains the residue of any parcel, or such boards as cannot be 
 included in either of the preceding: classes. AV'hen an agree- 
 ment is entered into for the erection of a building, the quality 
 of the boards should be specified, to prevent subsequent dis- 
 putes. As all boards shrink in the course of time, and as the 
 quantity of their contraction increases with their dimensions, 
 floors which are laid with very broad boards, soon exhibit, at 
 the joints, wide fissures that have an unpleasant appearance. 
 It is therefore the practice in good houses, not only to select 
 the best part of the wood, but to cut the boards into narrow 
 scantlings ; so that, if properly seasoned, and laid close at first, 
 their shrinking afterwards is so small as to make no openings of 
 consequence. Boards about five inches broad may be reckoned 
 narrow, but when they measure nine inches or more in the same 
 direction, they must be considered broad. 
 
 The manner of jointino" flooring boards, and fastening them 
 down upon the joists, is performed in a variety of ways, the 
 most usual of which is, to plane the edges of the board quite 
 square, that is, at right angles to the upper and under surface, 
 and then, placing them as closely to each other as possible, 
 to nail them down from the upper surface. Sometimes, par- 
 ticularly when the wood is known to be insufficiently sea- 
 soned, after the first board has been fastened down, the 
 fourth board is secured in like manner, the two intermediate 
 boards are then made somewhat wider than the space to re- 
 ceive them, and forced into their places by j umping upon them. 
 To do this with the most ease and advantage, the intermediate 
 boards are laid aslant, so as to be highest in the middle, and 
 those edges which are placed together being sloped a little, 
 so as to form rather less than a right angle with their respec- 
 tive upper surfaces, they are, by an adequate weight, at once 
 compressed and levelled. The fourth board of the last series 
 becomes the first of the next, and the operation, which is 
 called folding the boards, is repeated till the floor is finished. 
 The nails are driven in a little below the surface of these 
 boards, and the cavity is filled with glazier's putty. But in 
 rooms not intended to be carpeted, and yet where a neat and 
 clean appearance is indispensable, the use of putty must be 
 avoided, and the nails must not be driven in from the top. 
 This object is obtained by doweling the joints, that is, driving 
 
BUILDING. 259 
 
 Jointinor of Qooring boards. — Mortar floors. 
 
 wooden pins into them in the middle of their thickness, and 
 parallel to the surface, in the same manner as the coopers joint 
 the boards forming the ends of their casks. In this case, one- 
 half of each pin entering the edges placed together, the boards, 
 if the dowels be sufficiently numerous and properly placed, 
 cannot rise or sink but in conjunction. The best place for the 
 dowels is in the middle of the space between the joists. In the 
 best doweled work, the nails are concealed when the floor is 
 finished, for they are driven in slantwise through the outer 
 edge only of each board. Sometimes the joints of flooring 
 boards are rabbeted, that they may lap over each other a little 
 way, and sometimes toothed into each other, or, as it is tech- 
 nically expressed, ploughed and tongued. When either of 
 these methods is adopted, the boards are not separated on their 
 contraction so as to leave an aperture between each pair, 
 through which any thing can drop; but such floors are more 
 costly than others, not only on account of the extra labour, 
 but the greater quantity of wood which they require. 
 
 It is always desirable to cover a floor with boards in one 
 length ; but as this may not always be convenient, when it is not 
 done, the ends of the two boards that meet are called headings. 
 The headings should invariably be upon a joist, and two of 
 them should never be together in the same line. 
 
 Before the boards are laid, it is necessary to examine whe- 
 the upper sides of the joists all lie in the same plane. The 
 defect they are most liable to, is that of being depressed in 
 the middle; in which case they must be raised by the addi- 
 tion of suitable pieces, but if found too protuberant, they must 
 be reduced by the adze. 
 
 Yellow deal, well seasoned, is one of the best woods that 
 can be selected for floors, and retains its colour for a long 
 time ; whereas the white sort, by frequent washing, becomes 
 blackish and disagreeable in its appearance. 
 
 In the habitations of the labouring classes of society, the 
 ground floors are often made of a kind of mortar. The best 
 materials for this purpose are two-thirds of lime, one of coal- 
 ashes, and a small portion of clay. These ingredients are to be 
 well tempered with water, left to subside for a week or ten days, 
 and then well worked up again. This operation should be re- 
 peated in the course of three or four days, till the mixture be- 
 comes smooth and glutinous, when it is fit for use. After the 
 ground is made perfectly level, the composition is to be laid 
 on to the depth of two and a half or three inches, and carefully 
 smoothed with a trowel. The hottest season of the year is the 
 most proper for applying this composition, which, when com- 
 pletely dried, will make a most durable floor. 
 
260 
 
 BUILDING. 
 
 Proportions of tiiubt^rs. 
 
 Proportions of Timbers, S<c. 
 
 In the treatise entitled the " British Carpenter," already re- 
 ferred to, are given the followinf^ Tables to shew the propor- 
 tions of timbers for small and larsje buildings : 
 
 PROPORTIONS OF TIMBERS FOR SMALL BUILDrS'GS. 
 
 Hearing Posts of Fir. 
 
 Bearing Posts of Oak. \ 
 
 Height 
 
 Scantling 
 
 Height 
 
 Scantling 
 
 if 8 feet 
 
 4 inc. square 
 
 if 10 feet 
 
 G inc. square 
 
 10 
 
 5 
 
 12 
 
 8 
 
 12 
 
 6 
 
 14 
 
 10 
 
 Girders of Fir. 
 
 Girders of Oak. \ 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling 
 
 if 16 feet 
 
 8 inc. by 11 
 
 if 16 feet 
 
 10 inc. by 13 
 
 20 
 
 10 12i 
 
 20 
 
 12 14 
 
 24 
 
 12 14 
 
 24 
 
 14 15 
 
 Joists of Fir. 
 
 Joists of Oak. 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling 
 
 if 6 feet 
 
 5 inc. by 2J 
 
 if 6 feet 
 
 5 inc. by 3 
 
 9 
 
 6i 2^ 
 
 9 
 
 7'- 3 
 
 12 
 
 8 2i 
 
 12 
 
 10 3 
 
 Bridgings of Fir. 
 
 Bridgings of Oak. 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling 
 
 if G feet 
 
 4 inc. by 2| 
 
 if 6 feet 
 
 4 inc. by 3 
 
 8 
 
 5 2| 
 
 8 
 
 5i 3 
 
 10 
 
 6 3 
 
 10 
 
 7 3 
 
 Small Rafters of Fir. 
 
 Small Rafters of Oak. 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling i 
 
 if 8 feet 
 
 3i inc. by 2' 
 
 if 8 feet 
 
 4' inc. by 3 
 
 10 
 
 4i 2 
 
 10 
 
 oj 3 
 
 12 
 
 b^ 2\ 
 
 12 
 
 6^ 3 
 
 Beams of Fir, or Ties. 
 
 Beams of Oak, or Ties. 
 
 Length 
 
 Scantling 
 
 Length 
 
 Scantling 
 
 if 30 feet 
 
 6 inc. by 7 
 
 if 30 feet 
 
 7 inc. by 8 
 
 45 
 
 9 8i 
 
 45 
 
 10 Hi 
 
 60 
 
 12 11 
 
 60 
 
 13 15 
 
 Principal Rafters of Fir. 
 
 Principal Rafters of Oak. 
 
 Scantling 
 
 Scantling 
 
 Length 
 
 Top Bottom 
 
 Length 
 
 Top ; Bottom 
 
 if 24 feet 
 
 5 ins. & 6 6 ins. & 7 
 
 if 24 feet 
 
 7ins. &:8| 8 ins. & 9 
 
 36 
 
 6i 8 8 10 
 
 36 
 
 8 9i 9 lOi 
 
 48 8 10 10 1-2 
 
 48 9 10 10 12 1 
 
BUILDING. 
 
 261 
 
 Proportions of timbers. 
 
 PROPORTIONS OF TIMBERS FOR LARGE BUILDINGS. 
 
 Bearinrf Posts of Fir. \ 
 
 Bearing Posts of Oak. | 
 
 Height 
 
 Scantling 
 
 Height 
 
 Scantling 
 
 if 8 feet 
 
 5 inc. square 
 
 if 8 feet 
 
 8 inc. square 
 
 12 
 
 8 
 
 12 
 
 12 
 
 16 
 
 10 
 
 16 
 
 16 
 
 Girders of Fir. 
 
 Girders of Oak. | 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling 
 
 if 16 feet 
 
 9\ inc. by 13 
 
 if 16 feet 
 
 12 inc. by 14 
 
 20 
 
 12 14 
 
 20 
 
 15 15 
 
 24 
 
 13i 15 
 
 24 
 
 18 16 
 
 Joists of Fir. 
 
 Joists of Oak. 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling 
 
 if 6 feet 
 
 5 inc. by 3 
 
 if 6 feet 
 
 6 inc. by 3 
 
 9 
 
 n 3 
 
 9 
 
 9 3 
 
 12 
 
 10 3 
 
 12 
 
 12 3 
 
 Bridyinqs of Fir. 
 
 Bridgings of Oak. j 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling 
 
 if 6 feet 
 
 4 inc. by 3 
 
 if 6 feet 
 
 5 inc. by 3^ 
 
 8 
 
 51 
 
 8 
 
 6^ H 
 
 10 
 
 7 3 
 
 10 
 
 8 3i 
 
 Small Rafters of Fir. 
 
 Small Rafters of Oak. \ 
 
 Bearing 
 
 Scantling 
 
 Bearing 
 
 Scantling 
 
 if 8 feet 
 
 4^ inc. by 3 
 
 if 8 feet 
 
 bl inc. by 3 
 
 10 
 
 5^ 3 
 
 10 
 
 7 3 
 
 12 
 
 6i 3 
 
 12 
 
 9 3 
 
 Beams of Fir, or Ties. 
 
 Beams of Oak, or Ties. 
 
 Length 
 
 Scantling 
 
 Length 
 
 Scantling 
 
 if 30 feet 
 
 7 inc. by 8 
 
 if 30 feet 
 
 8 inc. by 9 
 
 45 
 
 10 lU 
 
 45 
 
 11 12^ 
 
 60 
 
 13 15 
 
 60 
 
 14 16 
 
 Principal Rafters of Fir. 
 
 Principal Rafters of Oak. 
 
 Scantling 
 
 Scantling 
 
 Length 
 
 Top 
 
 Bottom 
 
 Length 
 
 Top 
 
 Bottom 
 
 if 24 feet 
 
 Tins. & 9 
 
 8 ins. & 9 
 
 if 24 feet 
 
 8 ins. & 9 
 
 9 ins. & 10 
 
 36 
 
 8 9 
 
 9 lOi 
 
 36 
 
 9 10 
 
 10 12 
 
 48 
 
 9 10 
 
 10 12 
 
 48 
 
 10 13 
 
 12 14 
 
 The Author of the preceding Tables observes, that though 
 they seem so plain as not to exceed explanation, yet a few re- 
 marks might be subj oined with propriety. All binding or strong 
 joists, he then adds, ought to be half as thick again as common 
 joists i that is, if a common joist be given three inches thick, 
 
262 BUILDING. 
 
 Proportion* of timWrs. — Camber of limber-. 
 
 a binding joist should be four inches and a half thick, althouf^h 
 of tlie same depth. 
 
 If it be not convenient to allow the posts in partitions to be 
 square, which is the best form, in such cases, multiply the 
 square of the side of the posts, as here given, by itself: for in- 
 stance, if it be six inches square, then as six times six is thirty- 
 six, to keep this post nearly to the same strength, find two 
 numbers producing the same amount; as suppose the partition 
 to be four inches thick, then let the post be nine inches the 
 other way, so that nine times four being thirty-six, the area of 
 its horizontal section is the same, and its strength nearly equal 
 to the square post. 
 
 Posts that go to the height of two or three stories, need not 
 hold the proportions given in the table, because at every floor 
 they meet with a tie. Admit a post to be thirty feet high, and 
 that in this height there are three stories, two often feet and one 
 of eight feet; look for posts of fir ten feet high, their scantling 
 is five inches square, that is, twenty-five square inches, which 
 double for the two stories: and also take that of eio;ht feet hish, 
 being tour inches, that is, sixteen inches square, all which being 
 added together, make sixty-six inches ; so that such a post 
 would be rather more than eight inches square. On occasion 
 it may be lessened in each story as it rises. 
 
 All beams, ties, and principal raftore, ought to be cut or forced 
 in framing to a camber, or roundness, on the upper side, and the 
 convexity may be about one inch in eighteen or twenty feet. 
 The reason is, that all timber, partly from its own weight, but 
 principally from the weight of the covering or other burden it 
 has to bear, will swag ; and unless prepared in this manner, 
 that it may never become concave, a degree of unsightliness, 
 and often of inconvenience, will be produced. 
 
 The joists in floors, the purlines (or timbers into which the 
 small rafters are tenoned in roofs,) 8cc. should not exceed twelve 
 feet in the length of their bearing, or from support to support. 
 The strong joists of floors should not be at a greater distance 
 than five feet, nor common joists more than ten or twelve 
 inches apart. 
 
 According to the experiments of Muschenbroek, fir is able to 
 bear compression in the direction of the length of its fibres, or to 
 sustain as a post, a much greater weight than oak, but is far infe - 
 rir to oak when the weight is suspended. In the preceding ta- 
 bles, therefore, the scantlings of fir bearing posts and principal 
 rafters are properly made kss than those of oak; but for other 
 timbers, particularly for ties, many are of opinion that the pro- 
 portions of the Author's tables should be reversed, and the 
 scantling which he has assigned to fir should be given to oak. 
 
BUILDING. 263 
 
 Provisions of the Building Act affecting the Carpenter. 
 
 Building Act. 
 
 All the buildings erected in London and the several Parishes 
 within the Bills of Mortality, are subjected to the regulations 
 of an Act of Parliament, of the 14. Geo. III. the main object 
 of which is to lessen the danger to be apprehended from fire. 
 As many of the provisions of this act are of great impor- 
 tance, and deserve to be universally known and acted upon, 
 we shall conclude the subject of Building by an abstract 
 of them. Those which relate to the Carpenter are the fol- 
 lowing : 
 
 Timber partitions between building and building, erected or 
 erecting before the passing of the act, may remain till one of 
 the adjoining houses is rebuilt, or till one of the fronts, or 
 two-thirds of the fronts which abut on such timber partition, 
 is taken down to the bressummer, or one pair of stairs floor, 
 and rebuilt. 
 
 Three months' notice of the pulling down of such wooden 
 partition, when decayed or of insufficient thickness, to be 
 given by the proprietor to the owner or occupier of such a 
 house, and if the house be empty, such notice to be stuck up 
 on the front or front door of it. 
 
 No timber hereafter to be laid in any party arch, or party 
 wall except for bond to the same; nor any bond timber within 
 nine inches of the opening of a chimney, nor within five inches 
 of the flue; nor any timber within two feet of any oven, stove, 
 copper, still, boiler, or furnace. 
 
 The wood work of chimney breasts to be fastened to the 
 said breast with iron wall hooks, spikes, nails, or holdfasts, 
 which must not be driven more than three inches into the wall, 
 or nearer than four inches to the inside of the opening of the 
 chimney. 
 
 No timber bearer to wooden stairs let into an old party wall, 
 must come nearer than eight inches and a half to the flue, nor 
 nearer than four inches to the internal finishing of the adjoin- 
 ing building. 
 
 No timber to be laid under any hearth to a chimney, nearer 
 than eighteen inches to the upper surface of such hearth. 
 
 No timber must be laid nearer than eighteen inches to any 
 door of communication through the party walls of warehouses 
 and stables. 
 
 Bressummers, story posts, and plates thereto, are only 
 permitted in the ground story, and may stand even with the 
 outside of the wall, but must go no deeper than two inches 
 into a party wall, nor nearer than seven inches to the centre of 
 a party wall, when it ia two bricks thick, nor nearer than four 
 
264 BUILDING. 
 
 I'roviiiuus of tbe Building Act aHecting the Bricklayer. 
 
 inches and a half, provided the party wall does not exceed one 
 brick and a half in thickness. 
 
 Every corner story post must be of oak, and at least twelve 
 inches square, w hen employed for the support of two fronts. 
 
 ^Vindow frames and door frames to the first, second, third, 
 and fourth rate classes, are to be recessed in reveals, four 
 inches at least. 
 
 Door-cases and doors to warehouses only of the first, second, 
 third, or fourth rate classes, may be even with the outward 
 face of the wall. 
 
 No external decoration to be of wood, except cornices or 
 dressings to shop windows; frontispieces to door-ways of the 
 second, third, and fourth rate classes, and covered ways or por- 
 ticoes to buildings; but not to project beyond the original line 
 of the house in any street or way. Such covered way or por- 
 tico not to be covered with wood ; nor such rnrnice, covered 
 way, or roof of the portico, to be higher than the under side of 
 the sill to the windows of the one pair of stairs floor. No flat 
 gutter or roof, nor any turret, dormer, or lantern light, or other 
 erection placed on the flat of the roof belonging to the first, 
 second, third, fourth, and fifth rate classes, to be of wood. 
 
 No wooden water tanks must be hicrher from the "lound than 
 the tops of the windows of the ground story. 
 
 Those provisions of tlie Act which relate to the Bricklayer 
 are the most numerous. Every master bricklayer must give 
 twenty-four hours' notice to the Surveyor of the district, con- 
 cerning the building to be altered or erected ; but if the build- 
 ing is to be piled or planked, or begun with wood, it becomes 
 the business of the carpenter to give such notice. 
 
 The footings of the walls are to have equal projections on 
 each side ; but where any adjoining building will not admit 
 of such projections to be made on the side adjoining to such 
 building, this direction to be complied with as nearly as pos- 
 sible. 
 
 The timbers in each rate, as girders, beams, trimming joists, 
 &c. may have as much bearing as the nature of the wall will 
 admit, provided four inches be left between the ends of such 
 timber and the external surface of the wall. 
 
 External Walls. 
 
 Every front, side, or end wall, not being a party wall, is 
 called an external wall. 
 
 External walls, and other external inclosures to the first, 
 second, third, fourth, and fifth rates of buildings, must be of 
 brick, stone, artificial stone, lead, copper, tin, slate, tile, or 
 
BUILDING. 265 
 
 Provisions of the Building Act aflecting the Bricklayer. 
 
 iron; or of some or all of these materials in conjunction, ex- 
 cept the planking, oiling, 8cc. for the foundation, which may 
 be of wood. 
 
 If any part to an external wall of the first and second rate is 
 built wholly of stone, it is not to be less in thickness than as 
 follows: first rate, fourteen inches below the ground floor, nine 
 inches above the ground floor; second rate, nine inches above 
 the ground floor. 
 
 Where a recess is meant to be made in an external wall, it 
 must be arched over, in such a manner, that the arch and the 
 back of such recess, shall respectively be of the thickness of 
 one brick in length ; hence no walls are allowed to be recessed 
 which are not more than one brick in thickness. 
 
 No external wall to the first, second, third, and fourth rate, 
 is ever to become a party-wall, unless the same shall be of the 
 height and thickness above the footing, as is required for each 
 party-wall to its respective rate. 
 
 Party Walls. 
 
 Buildings of the first, second, third, and fourth rate, which 
 are not yet designed by the owner thereof to have separate 
 and distinct side walls, on such parts as may be contiguous to 
 other buildings, must have party walls; and they are to be 
 placed half and half on the ground of each owner, or of each 
 building respectively, and may be built thereon, without any 
 notice being given to the owner of the other part, the first 
 builder having a right so to do, when building against vacant 
 ground. 
 
 Party-walls, chimneys, and chimney shafts hereafter to be 
 built, must be of good sound brick or stone, or of sound 
 bricks and stone together, and must be coped with stone, tile, 
 or brick. 
 
 Party-walls, or additions thereto, must be carried up thirteen 
 inches above the roof, measuring at right angles with the back 
 of the rafter, and twelve inches above the gutter of the highest 
 building which gables against it; but where the height of a 
 party-wall so carried up, exceeds the height of the blocking 
 course or parapet, it may be macie less than one foot above the 
 gutter, for the distance of two feet six inches from the front 
 of the blocking course or parapet. 
 
 Where dormers (the term for windows in roofs, differing 
 from sky-lights by their being vertical,) or other erections are 
 fixed in any flat or roof, within four feet of any party-wall, 
 such party-wall is to be carried up against such dormer, and 
 must extend at least two feet wider, and to the full height of 
 every such dormer or erection. 
 12.— Vol.. I. 2 M 
 
266 BUILDING. 
 
 Provisions uf the Builidog Act atVecting tbe Bricklayer. 
 
 No recess is tobe hereafter made in any party-wall of the first, 
 second, third, and fourth rate, except for chimney flues, girders, 
 &c. and for the ends of walls or piers, so as to reduce such wall 
 in anv part of it to a less thickness than is required by the act, 
 for the highest rate of building to which such wall belongs. 
 
 No opening is to be made in any party-wall, except for com- 
 munication from one stack of warehouses to another, and from 
 one stable building to another, and the communications al- 
 lowed must have wrought iron doors, and the pannels thereof 
 are not to be less than a quarter of an inch thick, and must 
 be fixed in stone door-cases and sills. But there may be 
 openings for passages or ways on the ground, for foot pas- 
 sengers, cattle, or carriages, which must be arched over 
 throughout with brick or stone, or brick and stone together, 
 of the thickness of a brick and a half at the least, to the first 
 and second rate, and one brick to the third and fourth rate. 
 And if there is any cellar or vacuity under such passage, it is 
 to be arched over throughout in the same manner as the pas- 
 sage over it. 
 
 No party-wall, or party-arch, or shaft of any chimney, new 
 or old, must be cut into, except for the following purposes: if 
 the fronts of buildings are in a line with each other, a recess 
 may be cut, both in the fore and back front of such buildings, 
 (as may be already erected,) for the purpose of inserting the 
 end of such other external wall, which is to adjoin thereto. 
 This recess must not be more than nine inches deep from the 
 outward faces of such external walls, and not be cut beyond 
 the centre of the party-wall. And for the purpose of inserting 
 bressummers and story-posts, that are to be fixed on the ground 
 floor, either in the front or back wall, the recess may be cut 
 from the foundation of such new wall to the top of such bres- 
 summer, fourteen inches deep from the outward face of such 
 wall, and four inches wide m the cellar story, and two inches 
 wide on the ground story. The same may also be done for the 
 purpose of tailing-in stone steps, or stone landings, as for 
 bearers to wood stairs, or for laying-in stone corbels for the 
 support of chimney jambs, girders, beams, purlines, binding 
 or trimming joists, or other principal timbers. 
 
 Perpendicular recesses may also be cut in any party-wall, 
 whose thickness is not less than thirteen inches, for the pur- 
 pose of inserting walls and piers therein ; but they must not be 
 wider than fifteen inches, or more than four inches deep ; and 
 no such recess is to be nearer than ten feet to any other recess. 
 All such cuttings or recesses must be immediately made good, 
 and effectually pinned up, with brick, stone, slate, tile, shell, 
 or iron, bedded in mortar. 
 
BUILDING. 267 
 
 Provisions of the Building Act affecting the Bricklayer. 
 
 No party-wall must be cut for any of the above purposes, if 
 the same will injure, displace, or endanger the timbers, chim- 
 neys, flues, or internal finishings of the adjoining buildings. 
 
 The footing may be cut off on the side of any party-wall, 
 where an independent side wall is intended to be built against 
 such party-wall. 
 
 When any buildings (inns of court excepted) that are erected 
 over gate-ways, or public passages, or have different rooms and 
 floors, the property of diti'erent owners, are to be rebuilt, they 
 must have a party-wail, with a party-arch or arches of the 
 thickness of a brick and a half at the least, to the first and 
 second rate, and of one brick to the third and fourth rate, be- 
 tween building and building, or between the different rooms 
 and floors that are the property of different owners. 
 
 Inns of court are required only to have party-walls where 
 any room or chamber communicates to each separate and dis- 
 tinct staircase, and which are also subject to the same regula- 
 tions as respect other party-walls. 
 
 If buildings of different rates adjoin each other, and any ad- 
 dition is intended to be made to the lower rate, the party-wall 
 of such building must be such as is required for that of the 
 higher rate adjoining. 
 
 When any party-wall is raised, it is to be made of the same 
 thickness as the wall in the story next below the roof of the 
 highest building adjoining, but it must not be raised at all, 
 unless it can be done with safety to such wall, and the build- 
 ing adjoining thereto. 
 
 Every dwelling-house built four stories high from the foun- 
 dation, exclusive of rooms in the roof, must have its party-wall 
 built according to the third rate, although such dwelling-house 
 may be of the fourth rate. Every dwelling-house, also, ex- 
 ceeding four stories in height from the foundations, exclusive 
 of the rooms in the roof, must have its party-wall built accord- 
 ing to the first rate, although such house may not be of the 
 first rate. 
 
 Chimneys. 
 
 No chimney is to be erected on timber, except on the piling, 
 planking, &c. of the foundations of the building. 
 
 Chimneys may be built back to back in party-walls; but when 
 this is done, they must not be less in thickness from the centre 
 of such party-wall than as follows: first rate, or adjoining 
 thereto, must be one brick thick in the cellar story, and half 
 a brick in all the upper stories. Second, third, and fourth 
 rate, or adjoining thereto, must be three-quarters of a brick in 
 the cellar story ; and half a brick in all the upper stories. Such 
 
268 BUILDING. 
 
 Provisions of the Duiliiin^ Act ad'cctin;; the BricklaM-r. 
 
 chimneys in party-walls of any of the four rates, as do not 
 Bland back to back, may be built as follows; from the external 
 face of the party-wall to the inward face of the back of the 
 chimney in the cellar story, one brick and a half thick, and in 
 the upper stories, one brick thick from the hearth to twelve 
 inches abovp the mantle. If such chimney is built against 
 any other wall, the back may be half a brick thinner than 
 above stated. 
 
 Those backs of chimneys which are not in party-walls of the 
 second, third, and fourth rate, must be in every story one brick 
 thick at least, from the hearth to twelve inches above the mantle. 
 These backs may also be half a brick thinner, if such chimney 
 be built against any other wall. 
 
 The breasts of chimneys, whether in party-walls or not, are 
 not to be less than one brick thick in the cellar story, and half 
 a brick thick in every other story. 
 
 All partitions between flues must not be less than half a 
 brick thick. 
 
 Flues may be built opposite to each other in party-walls, 
 but they must not approach nearer to the centre of such wall 
 than two inches. 
 
 All chimney breasts next to the rooms, and chimney backs, 
 and all flues, are to be rendered or pargeted. 
 
 Backs of chimneys, and flues in party-walls against vacant 
 ground, must be lime whited, or marked in some durable man- 
 ner, but must be rendered or pargeted as soon as any other 
 building is erected to adjoin them. 
 
 No timber must be over the opening of any chimney for 
 supporting the breast ; but all chimneys must have a brick or 
 stone arch, or iron bar or bars. 
 
 All chimneys must have slabs or foot paces of stone, marble, 
 tile, or iron, at least eighteen inches broad, and at least one 
 foot longer than the opening of the chimney when finished; 
 and such slabs or foot paces must be laid on brick or stone 
 trimmers at least eighteen inches broad from the face of the 
 
 ... 
 
 chimney breast, except there be no room or vacuity beneath, 
 in which case they may be bedded on the ground. 
 
 Brick funnels must not be made on the outside of the first, 
 second, third, or fourth rate, next to any street, square, court, 
 road, or way, so as to extend beyond the general line of the 
 buildings in such situations. 
 
 No metallic funnel or other pipe, for conveying smoke or 
 steam, is allowed to be fixed near any public street, square, court, 
 or way, to the first, second, third, or fourth rate, and no such 
 pipe is to be fixed on the inside ofany building nearer than four- 
 teen inches to any timber, or other combustible material. -_ 
 
MECHANICS. 2G9 
 
 Delinitioiis. 
 
 MECHANICS, 
 
 The science of Mechanics has been very concisely defined, 
 the geometry of motion. It is divided by Sir Isaac Newton 
 into the two branches of practical and rational mechanics. 
 Practical mechanics treats of the six mechanical powers, of 
 one or more of which every machine is composed ; and rational 
 mechanics comprehends the whole theory of motion, shews 
 how to determine the motions produced by given powers or 
 forces : and, conversely, when the phenomena of the motions 
 are given, how to trace the powers or forces from which they 
 arise. 
 
 Of Matter. 
 
 Every branch of natural philosophy acquaints us with some 
 new properties of matter, the general nature of which, and 
 those properties of it which respect mechanical science, it will 
 here be requisite for the reader to consider. 
 
 The terms matter, substance, and bodj/, though so nearly al- 
 lied that they are occasionally employed in the same sense, 
 without creating much confusion of ideas, have different sig- 
 nifications, which ought to be understood. 
 
 Matter is the most general term of the three, and compre- 
 hends whatever is possessed of extension, and capable of mak- 
 ing resistance, without regard to figure or quantity. 
 
 The word substance is compounded of the Latin preposition 
 sub (under,) and the verb stare (to stand,) and approaches very 
 nearly to the signification of the word matter, as it implies that 
 which supports or stands under the different forms and appear- 
 ances which are presented to our senses. Its meaning is, 
 however, more restricted, and it is generally accompanied by 
 the article, to denote a particular portion of matter. 
 
 The term bodi/ comes from the Saxon, and originally signi- 
 fied the person or form of a man, or other creature ; hence it 
 ought to be applied only to a substance possessing a definite 
 form. 
 
 An examination of the general properties of matter will afford 
 us a better idea of its nature, than can possibly be given by a 
 definition. Some kinds of matter, as metals, wood, stone, &c. 
 are visible, a property dependent upon their opacity, or power 
 of reflecting to our eyes, some or all of the rays of light which 
 
270 MECHANICS. 
 
 I'loperlies cif matter. — Exleiisioii. — Divisibility. 
 
 full upon tliLin. Other kinds of matter are invisible, on ac- 
 count of their perfect transparency, and their existence is as- 
 certained only by their eflects. Of this class are the various 
 kinds of gases; for example, the atmosphere or air that 
 we breathe, which, though totally invisible when dry and 
 pure, yet is matter, as much as iron or the hardest body in 
 nature. 
 
 Among the properties attributed to all matter, tlie following 
 are too important to be passed over without particular notice, 
 
 viz. SOLIDITY, EXTENSION, DIVISIBILITY, MOUILITY, INER- 
 TIA, ATTRACTION, and REPULSION. 
 
 The soUditif of matter here meant is not opposed to fluidity, 
 but expresses that property which every body possesses, of not 
 permitting any other body to occupy the same place with it at 
 the same time. This fact is an axiom in philosophy of the 
 most incontestable kind. If a piece of wood or stone occupy 
 a certain space, it must be removed before another body can 
 be put into that space, and though the tyro may suggest that 
 fluids do not oppose such resistance, it is only the facility with 
 which they escape that induces the supposition of their being 
 an exception to this universal property of matter. Under pro- 
 per circumstances, their solidity is as obvious as that of the 
 most solid substance: the piston of a syringe drawn full of 
 water, cannot be thrust down if the aperture for the jet be 
 stopped; and a pair of bellows filled with air, resists compres- 
 sion if the pipe be closed. The solidity of matter thus under- 
 stood, is the same with what some writers call its impenetra- 
 bility. These words, in common language, denote the property 
 of not being easily separated into parts; a meaning very dif- 
 ferent from that attached to them in the sense just explained, 
 and which should therefore be carefully remembered. 
 
 Extension is another property of matter inseparable from its 
 existence. The idea which we obtain of solidity by the resist- 
 ance of bodies, and the impossibility of two bodies co-existing 
 in the same identical place, immediately suggest and prove to 
 us that matter is extended, or occupies a certain portion of 
 space. 
 
 Div'mbiUli/ is that property by which matter is capable of 
 being separated into parts removeable from each other. We 
 cannot conceive a particle of matter to be so small, as not to 
 consist of two halves; this being the case, we are directly led 
 to the conclusion that matter is capable of being divided to 
 infinity. But, however natural this mode of reasoning appears, 
 it has had many opponents, and those who suppose it just, 
 have been considered as involving themselves in a cloud of 
 palpable contradictions. To assume, it has been said, as a 
 
MECHANICS. 271 
 
 Properties of matter — Divisibility. 
 
 first principle in philosophy, that matter is infinitely divisible, 
 is to assert that it has no beginning of substance; that there 
 are no limits between matter and nothing; and that a finite 
 thing has infinite properties. Such being the difficulties at- 
 tending the assumption of the infinite divisibility of matter. 
 Sir Isaac Newton closes an admirable disquisition on the 
 nature, laws, and constitution, of matter, by stating the great 
 probability that God in the beginning formed matter into solid, 
 massive, impenetrable, moveable particles or atoms, of such 
 sizes and figures, and with such other properties, and in such 
 proportion to space, as most conduced to the end for which he 
 formed them; and that these primitive particles being absolute 
 solids, are incomparably harder than any of the bodies com- 
 pounded of them, even so hard as to be incapable of wearing 
 or breaking in pieces, nothing but Infinite Power being able 
 to destroy what Infinite Power made one in the first creation. 
 That nature may be lasting, the changes of corporeal things 
 are to be attributed only to the various separations and new 
 associations of these permanent particles, and when compound 
 bodies break, it is not in the midst of solid particles, but where 
 these are laid together and touch only in a few points. By 
 adopting this theory of ultimate atoms, we avoid the toils of 
 metaphysics, although, at the same time, we take for granted 
 what we cannot directly prove. But those propositions which 
 are proved by the absurdity of supposing the contrary, are 
 often as important, and nearly as well entitled to be received, 
 as those which admit of strict demonstration; and although 
 Sir Isaac Newton's conjecture respecting solid, indestructable 
 atoms, has been buff"eted among men of science for about one 
 hundred years, it remains to be the prevalent opinion at this 
 day; not because much new light has been thrown upon the 
 subject, since the time of that justly renowned philosopher, 
 but because it comports so well with the phenomena of nature, 
 that an assent to it can scarcely be denied. The extreme te- 
 nuity of certain substances to our general perception, is no 
 proof against their being composed of particles perfectly solid; 
 if a wet bladder be tied over the mouth of a pneumatic jar 
 (that is, a jar open at the bottom,) and then gently dried, so as 
 to remain well stretched, and the jar be then placed upon the 
 air-pump, as soon as it is exhausted of air, the atmosphere pres- 
 sing upon the exterior, will burst the bladder, and falling upon 
 the pump-plate, will produce a loud report, like a gun. This 
 effect could not be produced without the intervention of solid 
 particles in air. Further, it seems impossible to account for 
 the power of the most subtile agents of nature, if their ultimate 
 atoms, however few they may be in a given compass, were not 
 
272 MECHANICS. 
 
 Properties of matter. — Divisibility. 
 
 ■equal in solidity to those which appear to us the hardest. 
 Thougli the velocity of the electric fluid is immeasurably great, 
 that velocity alone would be insufficient to produce its well- 
 known effects on bodies of the closest te.xture, if it contained 
 no principle of hardness within itself. 
 
 Having premised these considerations, we shall now take a 
 view of parts actually separate, and if we shall find that these 
 are so small and so numerous as to surpass imagination, we 
 shall have approximated as nearly as the human understanding 
 can do, to the attainment of an idea of the inconceivable 
 minuteness of those solid, ultimate, indivisible atoms, which 
 constitute matter. A pound of so gross a substance as cotton, 
 may be spun into a thread exceeding one hundred miles in 
 length ; and the celebrated Boyle speaks of a thread of silk 
 three hundred yards in length, which weighed no more than 
 three grains and a half. But the ductility of gold is still more 
 astonishing; a grain of gold can be hammered by the gold- 
 beaters, until it will cover fifty square inches, and may be 
 divided into two millions of visible parts, the gold which 
 covers the silver wire used in making silver lace, is spread over 
 a surface twelve times greater than in the last mentioned in- 
 stance. In making this wire, a cylindrical bar of silver is 
 strongly gilt, and afterwards converted into wire by drawing it 
 successively through holes diminishing in magnitude, formed 
 in plates of steel. By this means the surface is prodigiously 
 augmented; but the wire still remains gilt, and preserves a 
 uniform appearance, even when examined by the microscope. 
 Sixteen ounces of gold, which would not occupy more space 
 than one cubical inch and a quarter, will completely gild a 
 wire sufficient to encompass the whole globe of the earth. 
 The metallic particles in acid solutions are still more minutely 
 divided. A single grain of copper dissolved in an ounce of 
 diluted nitrous acid, will impart a green colour to a gallon of 
 water, or cover one thousand square inches of bright iron with 
 a coat of copper. 
 
 The odour of all bodies that excite the sensation of smell 
 cannot be given out without a waste of their substance ; yet 
 this waste is so very small, that is, the fragrant parts of bodies 
 are endued with such prodigious divisibility, as in general, 
 for long periods, to occasion no perceptible diminution of the 
 substance by which it is sustained. The odour of a grain of 
 musk will continue for twenty years in an apartment where 
 fresh air is admitted every day. Instances of the wonderful 
 divisibility of matter are very abundant. Gunpowder, when 
 exploded, expands to two hundred and forty-four times the 
 bulk it occupied in a solid state; and water, when converted 
 
MECHANICS. 273 
 
 Properties of matter — Divisibility, — Mobility. 
 
 into vapour or steam, fills a space eighteen hundred times 
 greater than in its fluid form. 
 
 The wonders of the organized creation, as laid open to us 
 by the microscope, are not less extraordinary than any thing 
 we have yet contemplated, as exemplifying the divisibility oi 
 matter. Lewenhoeck discovered in the melt of a single cod- 
 fish a greater number of animalculae than there are inhabitants 
 upon the face of the earth ; and he calculates that one thou- 
 sand millions of such animalculse as are discovered in com- 
 mon water would not equal in magnitude a grain of common 
 sand. Thousands of these minute beings might be contained 
 on the point of a fine needle; yet, if we suppose them to be 
 furnished with blood, like other animals, and if the globules 
 of their blood bear the same proportion to their bulk as 
 those of a man bear to his body, it may be proved that the 
 smallest visible grain of sand would contain more of these 
 globules than ten thousand of the largest mountains in the 
 world would contain grains of sand. When we have well 
 considered the minuteness of the smallest object in the mi- 
 croscopic world, and are apprized that innumerable particles 
 of light must proceed to our eye from every part of that 
 object, or it would not be visible, we may perhaps be led to 
 institute an inquiry respecting the size of the particles of 
 light itself. The result of such an investigation will mock 
 our conception, even if nothing we have previously learned 
 has had this effect. By a calculation apparently well con- 
 ducted, a particle of light has been estimated at ao bn Jo ui l ^h 
 
 part of a grain. 
 
 From the preceding statement it is clear, that matter is 
 actually divisible to a greater extent than we can conceive. 
 On the other hand, it is next to an absolute certainty, that 
 the hardest and most compact bodies are full of pores or in- 
 terstices, their particles being either no-where in actual con- 
 tact, or touching only in a few points. One proof of this is, 
 that the hardest bodies are known to contract by cold, which 
 contraction would be impossible, if their particles were in- 
 capable of a nearer approach to each other. 
 
 Mobility expresses tlae capacity of matter to be moved from 
 one position or part of space to another. 
 
 Space is an abstract idea, and must be described princi- 
 pally by its want of properties. Its extension or capacity is 
 without limits, and it does not consist of parts capable of actual 
 separation from each other; the division of it therefore is 
 always merely hypothetical. It is incapable of resisting in 
 any degree the passage of bodies through it ; and being per- 
 12.— Vol. I. 2 N 
 
274 MECHANICS. 
 
 Properlies of matter. — Inertia. — Attractiua. 
 
 fectly uniform in all its parts, these cannot be distinguished 
 from each other except by the bodies placed in them. When 
 any given length, as a yard or a mile, has become familiar to 
 us, we can reduplicate these measures as often as we please, 
 without joining them to the idea of body, and we thus obtain 
 our ideas of immensity. 
 
 Inertia is the term which designates the passiveness of 
 matter, which, if at rest, will for ever remain in that state 
 until compelled by some cause to move ; and, on the con- 
 trary, if in motion, that motion will not cease, or abate, or 
 change its direction, unless the body be resisted. That a 
 body at rest will not move of itself, will be readily admitted ; 
 but its tendency to continue the motion once communicated 
 to it, contradicting our ordinary experience, requires a little 
 explanation. We can indeed produce no species of motion 
 which will fully illustrate the proposition by experiment ; 
 but the conclusion seems undeniable, when we consider the 
 effect produced by diminishing the obstructions to a body 
 in motion. These obstructions are, gravitation, the resist- 
 ance of the air, and friction. Gravitation, as will be under- 
 stood when we come to treat of it, operates according to 
 established laws, unsusceptible of change or modification by 
 human art ; most of the resistance of the air may be removed 
 by means of the air-pump, but experiments with this machine 
 can only be of small extent ; the last-named obstruction to 
 motion, viz. friction, is therefore the only one we have in 
 general the power of diminishing ; and yet in proportion as 
 this one is diminished, we find the motion communicated to 
 a body by a given impulse, so much increased that we can- 
 not hesitate to consider the action of gravitation and the re- 
 sistance of the air, combined with the friction yet unde- 
 stroyed, as the sole causes of its ever ceasing. If a ball be 
 projected along a rough pavement, it will soon stop; if pro- 
 jected on a level floor, the same force will send it much 
 further; and on a surface perfectly plane, hard, and smooth, 
 a ball also perfectly hard and smooth, as well as globular, 
 would be carried perhaps five hundred yards, by the same 
 force that would scarcely carry it twenty yards upon the 
 rough pavement. So far, also, as reasoning confirms the ex- 
 planation given of the inertia of matter, it seems as absurd 
 to suppose that matter once put in motion can stop with- 
 out a cause, as that when at rest it can move without a 
 cause. 
 
 Attraction denotes the property which bodies have to 
 approach each other. Philosophers enumerate five kinds 
 of attraction, viz. the attraction of cohesion, of gravitation, 
 
MECHANICS. 275 
 
 Properties of matter. — Attraction of cohesion— gravitation. 
 
 of electricity, of magnetism, and chemical attraction. Only the 
 two first kinds of attraction belong to our present subject. 
 
 The attraction of cohesion takes place between bodies only 
 when they are at very short distances from each other, and 
 may be exemplified in a variety of ways. If two pieces of lead 
 be scraped clean and smooth, and then strongly compressed, 
 they will cohere almost as firmly as if united by solder. Planes 
 of glass, marble, and other substances, exhibit similar pheno- 
 mena. That this cohesion is not owing merely to the exclu- 
 sion of the atmoshere, from the evenness of the surfaces em- 
 ployed, is proved by its taking place in vacuo. 
 
 The strength of the attraction of cohesion differing in 
 different kinds of matter, is supposed to be the cause of the 
 relative degrees of hardness in different bodies. It is therefore 
 weakest in fluids ; yet substances of this description indicate 
 a disposition to unite. A fluid may be poured into a vessel 
 till it rises above the brim, because the attraction between its 
 particles resists, to a certain extent, its overflowing. From 
 the same cause, the drops of dew upon the leaves of plants, 
 and water thrown upon a dusty floor, where it is prevented 
 from spreading, assume a globular form ; small portions of 
 quicksilver, also, when brought near each other, coalesce, and 
 assume the same globular appearance. 
 
 A fluid contained in a vessel not full to the brim, assumes a 
 concave form, being highest at the edge. If a plate of glass 
 be immersed in water, the water immediately surrounding it 
 will rise above the general surface. If a second plate of glass 
 be immersed in the water, parallel to the former, and at a very 
 short distance from it, the fluid will rise above its level be- 
 tween the two plates, and the nearer they are brought to 
 each other, so as not to touch, the higher will the water 
 rise. If the water be darkened, as by the addition of a little 
 ink, and a glass tube with an excedingly small bore be placed 
 perpendicularly in it, the rise may be distinctly detected to 
 be very considerable. All effects of this description, to 
 which may be referred the rising of water in a sponge, or 
 other porous bodies, are usually attributed to what is called 
 capillary attraction ; but capillary attraction is only a particu- 
 lar modification or branch of the attraction of cohesion. 
 
 The attraction of gravitation, differs from the attraction of 
 cohesion in this respect, that it is exerted at all distances, 
 and by every particle of matter upon every other particle. 
 This principle is the basis of the Newtonian philosophy. 
 The planets and comets all gravitate towards the sun, and 
 towards each other, as well as the sun towards them. The 
 gravitating power of a body, is always proportionate to it* 
 
276 MECHANICS. 
 
 Properties of matter. — AttracUon of gravitatian. 
 
 quantity of matter; and all the heavenly bodies are retained 
 in their places by the due balance of their action on each other. 
 An eftect of gravity, or gravitation, familiar to all mankind, is 
 the tendency of bodies to fall to the earth. This tendency is 
 always towards a point, which is either accurately or very 
 nearly in the centre of the earth ; consequently bodies fall 
 every-where perpendicularly to the surface, and on opposite 
 sides of the globe, they fall in opposite directions or towards 
 each other. It is not meant that there is any peculiar virtue 
 or charm in the point called the centre ; but because such is 
 the result of the gravitation of bodies towards all parts of which 
 the earth consists. The pressure of bodies to attain, in all 
 cases, the lowest situation possible, or that nearest the centre 
 of the earth, is what constitutes their weight. All substances 
 having a certain degree of gravity ; they have consequently all 
 weight. Even smoke and vapours are possessed of it, the rea- 
 son of their rising from the earth being the same as tiiat which 
 causes a piece of wood to swim in water, viz. they are lighter 
 than an equal bulk of the atmosphere or fluid in which they are 
 disengaged, and therefore their falling to the ground is as effec- 
 tually resisted as the falling of a stone supported by the hand. 
 
 As the gravitating force is always proportionate to the quan- 
 tity of matter, the most compact and the most loose, the 
 greatest and the smallest bodies, descend through equal spaces 
 in equal times, unless they fall through a resisting medium, 
 which operates most upon those which have the greatest ex- 
 tension for their weight. If a guinea and a feather were dropt 
 at the same instant from the top of a house, no one will be at 
 a loss to say, which would soonest reach the ground ; but in 
 the exhausted receiver of an air-pump, these two bodies fall 
 together. The gninea containing more solid matter than the 
 feather, requires more force to put it in motion; but the attrac- 
 tive power being proportioned to the quantity of matter, its 
 velocity is not greater than that of a body which requires less 
 force to put it in motion. Another proof that the gravity of 
 bodies is proportionate to their quantity of matter, is derived 
 from experiments on the motion of pendulums. When the 
 lengths of pendulums are equal, and they vibrate in equal 
 arcs, they always acquire equal velocities at the correspond- 
 ing points of those arcs, and their vibrations are consequently 
 performed in times exactly equal, however different the bulk 
 and texture of the material of which they are composed. 
 The resistance of the air must be understood to be excluded 
 in this experiment, because it acts unequally on different 
 bodies, as already exemplified in the guinea and feather 
 experiment. 
 
MECHANICS. 277 
 
 Properties of matter — Attraction of gravitation. 
 
 In all places equally distant from the centre of the earth, 
 the force of gravity is nearly equal. The earth is, however, 
 not a perfect globe, but a little depressed on two opposite 
 sides, partly like an orange. These depressed parts are at the 
 poles, and the polar diameter of the earth has been found to 
 be about thirty-four miles shorter than the equatorial one. 
 The surface of the earth at the equator being therefore seven- 
 teen miles further from the centre than at the poles, the force 
 of gravity there is less than at the poles. It is for this reason 
 that a pendulum calculated to swing seconds in the polar re- 
 gions, must be shortened before it will swing seconds at the 
 equator; and that bodies at the equator loose jioP^^^ ^^ ^^^^ 
 weight which they would have at the poles. 
 
 The power of gravity, at any given place, is greatest at 
 the earth's surface, from whence it decreases both upwards 
 and downwards, but not both ways in the same proportion. 
 The force of gravity upwards decreases as the square of the 
 distance from the centre increases; so that at a double dis- 
 tance from the centre above the surface, the force would only 
 be one-fourth of what it is at the surface. The surface of the 
 earth is, in round numbers, four thousand miles from the 
 centre ; if then a body at the surface weighs four pounds, 
 and falls through sixteen feet in a second of time, it will at 
 double this distance from the centre weigh but one pound, 
 and will fall through but four feet in a second of time. Below 
 the surface of the earth, the power of gravity diminishes in 
 such a manner that its intensity is in the direct ratio of the 
 distance from the centre, and not as the square of the distance; 
 so that at the distance of two thousand miles, which is half a 
 semi-diameter from the centre, the force would be but half 
 what it is at the surface; at one-third of a semi-diameter, the 
 force would be one-third, and the same ratio is applicable to 
 all other distances. But altliough the force of gravity, strictly 
 speaking, varies in the manner just stated, in receding from 
 the surface, its operation at short distances is considered uni- 
 form, a quarter or even half a mile bearing so small a propor- 
 tion to the earth's radius, that the difierence is too insignifi- 
 cant to be noticed in calculations. 
 
 As the power of gravity appertains to every particle of 
 matter, and the gravitating power of entire bodies consists of 
 that of all their parts, under certain circumstances the gravity 
 of a part of the earth somewhat counteracts that of the whole 
 earth. Thus, the attraction of a lofty mountain is found to 
 draw a plumb-line at the foot of it a little out of the perpen- 
 dicular, so that in such a situation it does not tend to the 
 centre of the earth. 
 
278 MECHANICS. 
 
 Properties of matter. — Attraction of gravitation. 
 
 The space which borlies actually fall through is sixteen feet 
 and one-twelfth in the first second of time, in the latitude of 
 London ; and for other times either greater or less than that, 
 the spaces descended from rest, are directly proportional ta 
 the squares of the times, while the falling body is not far from 
 the earth's surface. 
 
 If two bodies, which contain equal quantities of matter, were 
 placed at ever so great a distance from one another, and then 
 left at liberty in space, and if there were no other bodies in the 
 universe to affect them, they would fall equally swift towards 
 one another, with a velocity continually accelerated, and would 
 meet in a point which was at first exactly half way between 
 them. But if two bodies, containing unequal quantities of 
 matter, were placed at any distance, and left in the same man- 
 ner at liberty, they would fall towards one another, with velo- 
 cities which would be in an inverse proportion to their respec- 
 tive quantities of matter; moving as in the former case with an 
 accelerated motion, they would meet in a point as much nearer 
 to the place from which the heavier body began to fall, than 
 to the place from which the lighter began to fall, as the quan- 
 tity of matter in the former exceeded that in the latter. 
 
 That gravity should accelerate the descent of falling bodies, 
 is an effect of its uniform action under all circumstances. Let 
 us suppose that it causes a body to descend through the space 
 of one mile in a minute; at the end of this time, the body will 
 have acquired a velocity sufficient to carry it through two 
 miles the next minute, although it received no new impulse 
 from gravity; but as this accelerating cause remains, it adds 
 anotiier mile to its effect in the first minute, and therefore, at 
 the expiration of two minutes, the body will have descended 
 through four miles. 
 
 The spaces described by a uniformly accelerated motion, are 
 always as the odd numbers, 1, 3, 6, 7, &,c. and consequently 
 the whole spaces are as the squares of the times, or of the last 
 acquired velocities; for the continued addition of the odd 
 numbers yields the squares of all numbers from unity upwards. 
 Thus 1 is the first odd number, and 1 is the square of one ; 3, 
 the next number, added to 1, makes 4, which is the square of 
 two ; 5, added to four, makes 9, the square of 3, and so on. 
 Tile times and velocities proceeding evenly and constantly, as 
 1, 2, 3, 4, &.C. and the spaces described as 1, 3, 5, 7, &,c. it fol- 
 lows that the spaces described 
 
 In 1 minute will be 1, which is the square of 1 
 
 In 2 minutes will be 1+3=4, — — 2 
 
 In 3 minutes will be 1+3 + 5=9, — — 3 
 
 In 4 minutes will be 1+3 + 5 + 7=16, — — 4 
 
MECHANICS. 279 
 
 Distiaction between gravity and weight. 
 
 Hence it is apparent that the spaces described in different 
 times by a falHng body, are to each other as the squares of the 
 times from the beginning of the descent, or, which produces 
 the same result, they are as the squares of the velocities ac- 
 quired at the ends of those times. 
 
 The motion of a falling body being uniformly accelerated 
 by gravity, the same cause uniformly retards the motion of a 
 body thrown directly upwards. A body projected perpendicu- 
 larly, with a velocity equal to that which it would have ac- 
 quired by falling from any height, will ascend to the same 
 height before it loses all its velocity. 
 
 Gravity/ and tceight, it ought to be understood, are not in- 
 terchangeable terms. Gravity is a power of which weight is 
 the effect. Gravity has a constant tendency to impress, on 
 every particle of bodies, a certain velocity, which would 
 cause them to fall if they were not supported ; weight is the 
 resistance necessary to destroy this velocity, or produce this 
 support. 
 
 When the many and wonderful discoveries which signalize 
 the present age are considered, it seems presumptuous to mark 
 the boundaries of success to human inquiry; but we may very 
 safely assert, that no researches have ever yet been made, which 
 enable us to discover, in the essential properties of matter, the 
 cause of gravity. Of the existence of such a power, we are 
 continually surrounded by the most .indubitable proofs ; and 
 that its influence extends over and governs the solar system, 
 and if the solar system, the whole material universe, there 
 seems as little reason to deny. Sir Isaac Newton has conjec- 
 tured that matter is composed of indivisible, perfectly solid 
 particles or atoms, a theory that has been explained in the 
 former part of this subject; but if the ultimate particles of 
 matter be perfectly solid, they cannot be pervaded; if they be 
 incapable of wearing or separation, they can throw nothing 
 off; and if no single atom can receive or part with any thing, 
 how can it act at all distances upon every other portion of 
 matter in the universe? or how can any aggregation of atoms 
 possess a power incompatible with the nature of its component 
 parts? Such is a slight view of the argument on this subject; 
 it is not our object to involve the reader in the mazes of useless 
 theories, though we may, for his amusement, and the exercise 
 of his judgment, occasionally glance at them and pass on. 
 In various branches of knowledge, we discover abundant 
 proofs that the effects we observe are produced by secondary 
 causes, that is, these effects spring from the inherent proper- 
 ties, the properties originally impressed upon things by the 
 Creator; but in the absence of the secondary cause of gravity . 
 
280 MECHAXICS. 
 
 Properties of matter. — RepaJ:>ioa. 
 
 or until we can prove it to be dependent upon the essential 
 properties of matter, gravity should be considered as a term 
 expressing merely a fact or phenomenon; and it is wise to 
 refer the cause of it at once to the Final Cause of all things, 
 and to regard it as the "Finger of God, the constant impres- 
 sion of Divine power." 
 
 Repulsion is the last property of matter which we have 
 enumerated. A variety of considerations induce philosophers 
 to admit that there is a sphere of repulsion which extends to 
 a small distance round bodies, and prevents them from coming 
 into actual contact with each other, except some force is 
 exerted to overcome this resistance, and then the attraction 
 of cohesion takes place. Dr. Knight defines repulsion to be 
 that cause which makes bodies mutually endeavour to recede 
 from each other, with different forces at different times; 
 and that such a cause exists in nature, he thinks evident for 
 the following reasons: 1. Because all bodies are electrical, 
 or capable of being made so ; and it is well known that elec- 
 trical bodies both attract and repel. 2. Both attraction and 
 repulsion are very conspicuous in all magnetical bodies. 3. 
 Sir Isaac Xewton has shewn from experiments, that the sur- 
 faces of two convex glasses repel each other. 4. The same 
 great philosopher has explained the elasticity of the air by 
 supposing its particles mutually to repel each other. 5. The 
 particles of light are, in part at least, repelled from the sur- 
 faces of all bodies. 6. Lastly, it seems highly probable that 
 the particles of light mutually repel each other as well as the 
 particles of air. The Doctor ascribes the cause of repulsion, 
 as well as that of attraction, to the immediate effect of the 
 w ill of God ; and as attraction and repulsion are contraries, 
 and consequently cannot at the same time belong to the same 
 substance, he supposes there are in nature two kinds of mat- 
 ter, one attracting, the other repelling ; and that those par- 
 ticles of matter which repel each other, are subject to the 
 general law of attraction in respect of other matter. A repel- 
 lent matter beinsr thus supposed equally dispersed through 
 the whole universe, the Doctor refers to its operation a variety 
 of natural phenomena; but whether, on his hypothesis, all the 
 particular effects of repulsion can be accounted for, time and 
 experience alone must determine. 
 
 in the instance which has already been adduced of the round 
 drops of dew upon the leaves of plants, it is supposed not 
 only that there exists an attractive force between the particles 
 of the fluid, but a repulsive force between them and the leaf 
 upon which they are suspended. That the drops are not in 
 actual contact with the leaf, is evident from their rolling off in 
 
MECHANICS. 281 
 
 Motion either absolute or relative. 
 
 a compact body with the greatest ease ; as well as from their 
 white or pearly appearance, which is an effect of the copious 
 reflection of white light from the flattened part of the surface 
 contiguous to the plant, and could not take place unless there 
 was a real interval between the under side of the drop and the 
 surface of the leaf. The power of repulsion will, in certain 
 cases, cause metals to swim in fluids much lighter than them- 
 selves. A fine needle, if gently laid on the surface of water, 
 will swim, and may be drawn off" again by a magnet, without 
 having in reality touched the fluid. In this instance, the needle 
 is not heavy enough to overcome the power of repulsion be- 
 tween itself and the water, the attraction of cohesion cannot 
 therefore operate, and though as much heavier than its own 
 bulk of water as the largest piece of steel, it will float till pres- 
 sed down by a greater force than its own weight. It would 
 appear that, from the same cause, flies walk upon water, and 
 oil refuses to mix with that fluid. Hence the feathers of wa- 
 ter-fowl, which are covered with a thin coating of subtile oil, 
 actually repel the surrounding water. 
 
 Of Motion. 
 
 No definition can be given of the term Motion which will 
 satisfy the casuist. It expresses a simple idea, and cannot 
 be explained by words more simple than itself. It has been 
 called " a change of place," or the act by which a body cor- 
 responds with different parts of space at different times; but it 
 would require great ingenuity to prove that these definitions 
 amount to more than the assertion that " motion is motion." 
 Perhaps that philosopher answered the question of " what is 
 motion?" with as much pei'spicuity at least as any other, 
 who began to walk, and observed to his inquirer that " that 
 was motion." 
 
 It is by motion alone that we know the existence of bodies, 
 and that a relation is established between them and our senses. 
 Nothing can be produced or destroyed without motion, and 
 every thing that happens depends upon it. 
 
 Space being nothing but an absolute and infinite void, the 
 place of a body is that part of the immense void which it takes 
 up or possesses ; and this place may be considered absolutely or 
 in itself, in which case it is called the absolute place of the 
 body; or else with regard to the place of some other body, and 
 then it is called the relative or apparent place of the body. As 
 the place of a body may be considered absolutely or relatively, 
 so may the motion of a body be distinguished in like manner. 
 All motion is in itself absolute, or the change of absolute space ; 
 but when the motions of bodies are considered and compared 
 12.— Vol. I. 2 
 
282 MECHANICS. 
 
 l)iHicult> of judging correctly of motion. 
 
 witli eacli other, then are they usually denominated relative 
 and apparent only: they are relative, as they are compared to 
 each other; and they are apparent only, insomuch that not their 
 true or absolute motion, but the sum or difference of the mo- 
 tions only is perceivable to us. Hence the absolute and rela 
 tive motions of bodies may be different and even contrary. 
 
 If two ships set out and sail tooether in the same direction 
 and with the same velocity, neither of them will appear to the 
 other to move. Hence it is, that though the earth is continu- 
 ally revolving about its axis, and advancing in its orbit, yet, as 
 all objects on its surface partake of the same common motion, 
 they appear not to move at all, but are relatively at rest. 
 
 If two ships set sail at the same moment, in the same direc- 
 tion, but one of them sails only three miles while the other 
 sails five miles an hour, the difference of their velocities, viz. 
 two miles per hour, will alone be perceptible to a spectator in 
 either of them, looking at the other. 
 
 But if two ships pass each other, the one will appear to the 
 other to move with the sum of both velocities ; so that in this 
 case the apparent motion exceeds the true, as in the other in- 
 stances it fell short of it. The reason of these phenomena of 
 motion will be evident, if we consider that we must be abso- 
 lutely at rest if we would discern at once the true or real mo- 
 tions of bodies about us. But as at absolute rest we can never 
 be, from the motion of the earth, we must detect the real and 
 absolute motions of bodies in general by means of observa- 
 tions made on their relative motions. 
 
 We are best acquainted with that kind of relative motion 
 which consists in the transfer, from one place to another, of 
 entire bodies, as the fallino- of a stone, or the fli£:ht of an ar- 
 row. But, besides this, there is another kind of relative mo- 
 tion, which, though not so obvious, is not less common or 
 important. We allude to the motion of the parts of bodies 
 among themselves, which though sometimes the object of 
 our senses, yet, in other cases, we require the aid of reflec- 
 tion to be convinced of its existence. It is by this impercep- 
 tible motion that plants and animals grow, and by which the 
 greatest number of the compositions and decompositions of 
 the globe take place. We may form some idea of this, by 
 observing the continual motion of the light particles which 
 sometimes float about in water, when it is held in the rays 
 of the sun ; which proves that the parts of the water are in 
 constant motion among themselves. But if we reflect a little, 
 we shall discover that the particles of the most solid sub- 
 stances are also continually changing their situations. Heat 
 expands and cold contracts the size of all bodies ; and as we 
 
MECHANICS. 283 
 
 The various circumstances attending motion. 
 
 know from experience, that the temperature of bodies is con- 
 stantly varying, their particles must consequently be in con- 
 tinual agitation, in order to adapt themselves to the ever- 
 changing size of the body. This is one of the causes of per- 
 petual motion of the particles of matter ; but there are no 
 doubt an infinite number of other causes which escape our 
 observation, and which we are perhaps incapable of discover- 
 ing. The gradual changes that take place in all bodies during 
 a series of years, sufficiently prove that they are constantly 
 acting on each other; and from a view of all that we know on 
 this subject, we are compelled to conclude that no particle of 
 matter is in a state of absolute rest. 
 
 In considering motion, the several circumstances attending 
 the communication of it from one body to another may be 
 classed as follows : 
 
 1. The force which impresses the motion. 
 
 2. The quantity of matter in the moving body. 
 
 3. The velocity and direction of the motion. 
 
 4. The space passed over by the moving body. 
 
 5. The time employed in going over this space. 
 
 6. The force with which the moving body strikes another 
 which is opposed to it. 
 
 In a mechanical sense, the inertia of every body causes it to 
 resist all change of state. If at rest, it will not begin to move 
 of itself ; and if motion is communicated to it by another body, 
 it will continue to move for ever uniformly, except it be stop- 
 ped by an external agent. This has been shewn in treating of 
 inertia. The causes by which bodies are put in motion, are 
 called motive powers, of which the following are those gene- 
 rally used in mechanics : the action of men and other animals, 
 wind, water, gravity, the pressure of the atmosphere, and the 
 elasticity of fluids and other bodies. 
 
 The velocity of motion is estimated by the time employed 
 in moving over a certain space, or by the space moved over 
 in a certain time. The less the time, and the greater the 
 space moved over, the greater of course is the velocity ; and, 
 conversely, the greater the time, and the less the space moved 
 over, the less is the velocity. As no motion can be instan- 
 taneous, every body in motion must have a determinate velo- 
 city. To ascertain the degree of this swiftness or velocity, 
 the space run over must be divided by the time. For ex- 
 ample, suppose a body moves over 1000 yards in ten minutes, 
 its velocity is 100 yards per minute, because 100 is the quo- 
 tient of 1000 divided by 10. If we would compare the velo- 
 cities of two bodies A and B, of which A moves 54 yards in 
 9 minutes, and B 96 yards in 6 minutes, the velocity of A will 
 
284 MECHANICS. 
 
 Compound motioa. 
 
 be to that of B in the proportion of 6 to 16, because the quo- 
 tient of 54 divided by 9, is 6 ; and the quotient of 96 divided 
 by 6, is 16. 
 
 To know the space run over, the velocity must be multi- 
 plied by the time; for it is evident, that if either the velocitv 
 or the time be increased, the space run over will also be 
 increased. If the velocity be doubled, then the body will 
 move over twice the space in the same time ; or if the time be 
 twice as great, then the space will be doubled; but if the 
 velocity and time be both doubled, then will the space be four 
 times as great. Hence when two bodies move over unequal 
 spaces in unequal times, their velocities are to each other as 
 the quotients arising from dividing the spaces run over by the 
 times. If two bodies move over unequal spaces in the same 
 time, their velocities will be in pruportiun to the spaces passed 
 over. Again, if two bodies move over equal spaces in unequal 
 times, then their respective velocities will be inversely as the 
 time employed; that is, if A in one minute, and B in two mi- 
 nutes, run over one hundred yards, the velocity of A will be 
 to that of B as two to one. 
 
 A body in motion must every instant tend to some parti- 
 cular point. It may either alwavs be to the same point, in 
 which case the motion will be rectilinear; or it may be con- 
 tinually changing the point to which its motion is directed, 
 and this will produce a curvilinear motion. 
 
 ^\ hen a body is acted upon by one force, or by several 
 forces in the same direction, its motion will be in the same 
 direction as that in which the moving force acts; as the mo- 
 tion of a boat which a man draws to him with a rope. But if 
 several powers differently directed, act upon a body at the 
 same time, it will not exactly obey anv of them, but will move 
 in a direction somewhere between them. This subject may 
 be rendered more plain by a diagram, and the illustration of 
 it ought to be well considered by the young mechanic. Let 
 a body A, fig. 1. pi. I. be impelled by a force acting on it in 
 the direction AC. At the same instant, let it be impelled to- 
 wards B, by another force that will carry it direct from A to 
 B in the same time that the former force would carry it from 
 A to C. Complete the parallelogram, ACBD, and draw the 
 diagonal AD, and this line will represent the direction and 
 distance the body will move in the same time when acted 
 upon by both forces conjointly; for let us suppose a tube, 
 equal to AB in length, in which a ball. A, can move freely; 
 and that in the same time that the ball is moving uniformly 
 from A to B, the tube is also moving uniformly from A to C, 
 but so as to be always parallel to AB, and its extremities 
 
MECHANICS. 285 
 
 Compouud motion. 
 
 describing- the lines AC to BD. The ball has moved from A 
 to B in the tube, in the same time that the tube has descended 
 to CD, and therefore when the tube coincides with the line 
 CD, the ball will be at the extremity, D, of that line, where 
 it has arrived in the same time that it would have taken to 
 describe either side. It is obvious also, that the ball thus 
 subjected to the impulse of different forces, can have de- 
 scribed no other line than the diagonal one; for by assuming 
 smaller forces, and forming the parallelograms A efg, A h i k, 
 &c. it will be found at every interval, in the diagonal of the 
 parallelogram. The motions along AB, AC, may be called 
 the simple or constituent motions; the motion along AD is 
 called the compound or resulting motion. Hence if we know 
 the effect which the joint action of two forces have upon a 
 body, and the force and direction of one of them, it is easy to 
 find that of the other : for suppose AD to be the direction and 
 force with which the body moves, and AB to be one of the 
 impelling forces, then, by completing the parallelogram, the 
 other power is found. 
 
 The practice of reducing compound forces to simple, and 
 that of finding two or more forces equivalent to one, is called 
 the composition and resolution of forces, the whole theory of 
 which is comprised in and may be deduced from the following 
 principle: two forces acting at the same time on a body, in 
 directions which are oblique to each other, do not move the 
 body by that part of their force, which, on acconnt of their 
 obliquity, is opposite and contrary, but by what remains after 
 the opposite forces are deducted. 
 
 Instances of motion produced by several powers acting at 
 the same time are innumerable, and the application of this 
 useful principle, by which they are governed, is therefore very 
 extensive. A ship impelled by the wind and tide is one well 
 known, A kite, acted upon by the wind and the string ; the 
 rain and snow that fall more or less obliquely according to 
 the action of the wind ; are other instances not less familiar. 
 A fish, by striking the water with its tail, advances for- 
 ward in a mean direction between the two impulses. — In 
 jumping out of a carriage in motion, accidents frequently 
 occur, and the adventurer falls short of the spot he aims at, 
 for want of duly considering, or not knowing, that the lateral 
 impulse he gives himself must, in a ratio proportionate to the 
 swiftness of the carriage, be greater than if he sprung from a 
 state of rest. 
 
 Motion is said to be accelerated if its velocity continually 
 increases ; and it is said to be uniformly accelerated, if its velo- 
 city increases equally in equal times. 
 
286 MECHANICS. 
 
 Jllustiatiuii of accelerated mutiou. 
 
 Motion is said to be retarded, if its velocity continually de- 
 creases ; and to be uniformly retarded, if its velocity t/ecrease 
 equally in equal times. 
 
 If a body be put in motion by a single impulse, and, 
 niovin<T' uniformlv, receive a new impulse in the same direc- 
 tion, its velocity will be augmented, and with that augmenta- 
 tion of velocity it will again proceed uniformly. But if at each 
 instant of its motion it receives a new impulse, its velocity 
 will be continually increasing ; and if this impulse is always 
 equal, and acts in equal times, the velocity will be uniformly 
 accelerated. 
 
 On the contrary, if a certain velocity be given to a body, 
 and it loses equal portions of that velocity, at each equal in- 
 stant, by new impulses acting in a direction exactly opposite 
 to its motion, it will be uniformly retarded. 
 
 The effect of gravitiition in uniformly accelerating the de- 
 scent of a body, and uniformly retarding one thrown directly 
 upwards, has been shewn in the last section ; but as a right 
 notion of this doctrine is very im])ortant, in adverting to it 
 again, we shall endeavour to exhibit it to the eye as well as 
 the understanding. Let the perpendicular line AB, of the 
 right-angled triangle ABC, fig. 2, pi. I. be considered as ex- 
 pressing the time which a body takes in falling, under the in- 
 fluence of gravity or any accelerating force, and the base line 
 BC, as expressing the velocity acquired at the end of the fall. 
 The time expressed by the line AB is divided into four equal 
 parts or moments, A r, r s,st,t B. The close parallel lines in 
 the triangle A r k, repeated at equal intervals, and from the 
 nature of the triangle, regularly increasing in length as they 
 recede from the point A, denote equal accelerations of the 
 velocity from the instant in which the body begins to fall. 
 The line r k, therefore, will represent the velocity acquired by 
 a falling body in the first moment of time ; s I, the velocity 
 acquired at the end of the second moment of time; to, the 
 velocitv at the end of the third moment, and BG the velocity 
 at the expiration of the fourth moment, or termination of the 
 fall. 
 
 The bodv, during the second moment of time, if retaining 
 only the velocity r k, which it had acquired at the end of the 
 first, will describe the square surface ;• k s m; for this surface is 
 generated by a continual repetition or motion of the line r k, 
 during the time expressed by r s ; as the area of the triangle 
 A r k, is described by a uniformly increasing velocity during 
 the time A r. But the area of the square is manifestly double 
 the area of the triangle above it; whence it appears, that a 
 body moving on, during the second moment, with the velocity 
 
MECHANICS. 287 
 
 Illustration of accelerated motion. 
 
 acquired at the end of the first, will fall twice as far in the 
 second moment as in the first; and the rule deducible from 
 this instance will universally hold, that is, the velocity ac- 
 quired at the end of any given time, will carry the body twice 
 as far in the same time. In pursuing the illustration of the 
 figure, this will still further appear. 
 
 If the velocity continue to increase uniformly during the se- 
 cond moment, then the space will be as expressed by the area 
 rslk, and will be equal to three times the triangle Ark. 
 
 The tvhole space described by the body in the two first mo- 
 ments will be as the area As/, which is four times greater than 
 that of A r k; rendering it apparent that the space described 
 by a body in its fall, is as the square of the time in which it 
 falls ; for here the time is 2, (because A s expresses two mo- 
 ments of the descent,) and the square of two is 4. 
 
 In the third moment, were the body to fall with the velocity 
 s /, during the time s t, the space described will be as the rect- 
 angle under the time and velocity, that is, as the rectangular 
 space sit n, on which rectangle may be described four tri- 
 angles, each equal to A r k; but as the velocity is still uni- 
 formly accelerated by the continued action of gravity, the 
 space fallen through in the time s t, or third moment, will be 
 as the area s t o I, or five times as great as A ;• k. 
 
 As the triangles Ark, A s I, At o, ABC, are all similar ; 
 as A s is twice as much as A r, s Z will be twice as much as r k; 
 and as A s expresses the time, and s I the velocity, where the 
 time is double the velocity is double. This rule applies to 
 every part of the descent, and proves that the velocity is as 
 the time. 
 
 If the spaces described in each moment be considered sepa- 
 rately, the space in the first moment being 1, the space in the 
 second moment, it will be obvious to inspection, is 3, in the 
 third 5, in the fourth 7, the difference each time being 2. 
 
 The motion of a body ascending from B to A, and therefore 
 uniformly retarded by the action of gravity, may be illustrated 
 by the same figure, if we change only a few of the terms of 
 explanation; thus BA will express the time which the body 
 takes to rise to A, and any horizontal line compared with the 
 base, as ^ 0, s /, r k, will shew the velocity lost at the height at 
 which it is drawn. 
 
 It is to be understood that the velocity above assigned to 
 falling bodies, is that which they would acquire if they passed 
 through a space where there was no air; but in fact the resist- 
 ance of that fluid considerably diminishes the velocity acquir- 
 ed in falling, even when the body, from its density, is of a kind 
 least afiected by its action. A leaden bullet dropt from an 
 
288 MECHANICS. 
 
 Motion of projectiles. 
 
 altitude of 272 feet, was found by Dr. Desaguliers to reach 
 the ground in four seconds and a half; in which time, from 
 theory, it should have descended through 325. (i feet, which 
 makes a difference of about one-fifth of the actual descent 
 between the experiment and the theory. 
 
 It has already been shewn, that if two forces act uniformly 
 upon a body, they will cause it to move in a straight line; but 
 if one of the forces is not uniform, but either accelerating or 
 retarding, the moving body will describe a curve. A ball pro- 
 jected from a cannon would always proceed in a right line, if 
 it were acted upon by no force except the impulse it received 
 from the powder; but as soon as it leaves the mouth of the can- 
 non, gravity acts upon it, and changes its direction. The ball, 
 acted upon only by gravity and the original impulse, would 
 describe a peculiar curve, called a parabola; but as the resist- 
 ance of the air also contributes to the variation of the line de- 
 scribed, and this resistance differs with the velocity of the ball, 
 its path is not exactly determinable. In some cases, the resist- 
 ance amounts to more than twenty times the weight of the 
 ball ; and when a ball moves with a velocity of two thousand 
 feet per second, the amount of this resistance has been found 
 to be one hundred times its weight. Hence the parabolic 
 theory of projectiles is inapplicable to practice, oir Isaac 
 Newton has, indeed, shewn that the curve described by a pro- 
 jectile approaches more nearly to an hyperbola than a para- 
 bola : and that the resistance to the body is not proportional to 
 the velocity itself, but to the square of the velocity. About two 
 hundred years ago, philosophers took the line described by a 
 body projected horizontally, such as a ball out of a gun, while 
 the force of the powder greatly exceeded the weight of the 
 bullet, to be a right line, after which they allowed it became 
 a curve. Nicholas Tartaglia was the first who maintained that 
 its path was a curve through the whole of its extent; but it 
 was Galileo who determined the curve to be a parabola in a 
 non-resisting medium. 
 
 The force with which a body moves, or which it would exert 
 upon another body opposed to it, (force being constantly mea- 
 sured by its effects,) is always in proportion to its velocity 
 multiplied by its weight or quantity of matter. This force is 
 called the monienlum of the body. If two equal bodies move 
 with different velocities, their forces or momenta are as their 
 velocities; and if two bodies move with the same velocity, 
 their momenta are as their quantities of matter ; therefore, in 
 all cases, their momenta must be as the products of their 
 quantities of matter and their velocities. This rule is the 
 foundation of mechanics. 
 
MECHANICS. 289 
 
 Proofs that action and re-action are always equal. 
 
 Laics of Motion. 
 
 A summary of the doctrine of motion has been reduced to 
 three axioms, which are usually called the laws of motion, and 
 are as follow : 
 
 I. All bodies are perfectly indifferent to motion and rest, that 
 is, they are incapable when at rest of moving, and when in mo- 
 tion of stopping, without the action of an external cause. 
 
 II. The alteration of the state of any body, whether from 
 rest to motion, or from one degree of motion to another, is 
 always proportionate to the force which is impressed, and in 
 the direction of that force. 
 
 III. Re-action is always equal to action; or, in other words, 
 the actions of two bodies on each other are always equal, 
 and exerted in opposite directions. 
 
 The proofs of the first and second of these laws have been 
 shewn in the preceding sections, from which they are ob- 
 viously deducible. The last is not so clearly implied ; though 
 as easily admitted to be true on a little consideration. That 
 any body acting upon another loses as much force as it com- 
 municates, will be evident, if with a bullet suspended from a 
 string we strike another bullet which is at rest, in which case 
 the striking body v/ill lose half its quantity of motion, and 
 what it loses will be communicated to the other body. If the 
 finger be pressed upon one scale of a balance, to counterpoise 
 a weight in the other scale, the scale pressed by the finger 
 acts against the finger with a force equal to that with which 
 the other scale endeavours to descend. A great variety of 
 facts might be adduced to the same purpose. If a man in a 
 boat draws another boat to him, by means of a rope, the two 
 boats will approach each other with equal quantities of mo- 
 tion. When a load is drawn by a horse, the load re-acts 
 against the motion of the horse, and the progression of the 
 animal is as much impeded by the load, as the motion of the 
 load is promoted by the efforts of the horse; and suppose the 
 animal to possess a force equal to one hundred, and the force 
 necessary to keep the traces tight be equal to fifty, it will only 
 be able to draw with the remaining force of fifty. When a can- 
 non is discharged, the rarefied powder presses it backwards and 
 the ball forwards with equal force, though the velocities are very 
 different. If the ball weighs lOlbs. and the cannon and carriage 
 lO.OOOlbs. the velocity of the ball will be a thousand times 
 greater than that of the cannon, but the quantity of motion 
 equal. The water by its re-action communicates to an oar as 
 much motion as it receives, and therefore impels a boat: fishes 
 swim and birds fly upon the same principle. 
 I3.-V0L. I, 2P 
 
290 MECHANICS. 
 
 C'cQtral forces. — Centre of gra\itN 
 
 Of Central Forces. 
 
 All moving bodies endeavour to obtain a rectilinear motion, 
 because it is the shortest and most simple ; whenever, therefore, 
 thev move in a curve, there must be something that draws 
 them from their rectilinear motion, consequently we may be 
 certain they are acted upon by two powers at the least, and 
 •were the detaining power to cease, the moving body would 
 instantly fly off in a straight line. 
 
 That force by which a body describing a curve endeavours 
 to fly off in a straight line, is called the centrifugal force ; and 
 the opposite force, or that by which a body is every-where 
 impelled, or in any manner tends towards some point as a 
 centre, is called the centripetal force. If a bullet fastened to a 
 string held in the hand be whirled round, and the string be 
 broken or let loose, the bullet immediately flies off in a tangent 
 to the circle it was previously describing ; in this case, the 
 string represents the centripetal force, and the power it has ac- 
 quired to fly off, is the centrifugal one. 
 
 The centrifugal and centripetal forces are called together 
 central forces. 
 
 Of the Centre of Gravitj/. 
 
 In every body, there is a certain point called the centre of 
 gravity/, the nature and properties of which require our atten- 
 tion, before we begin to treat of the mechanical powers. 
 
 The centre of gravity is that point in a body, about which all 
 its parts exactly balance each other in every position. 
 
 If a body be suspended or supported by the centre of gra- 
 vity, it will remain at rest in any position indifferently ; and 
 whatever supports this point bears the whole weight of the 
 body, which, while so supported, cannot fall. The whole 
 weight of a body may therefore be considered as centred in 
 this point; and mathematicians, by the place of a body, often 
 mean that point where the centre of gravity is situated. A 
 body suspended by any other point, can rest only in two posi- 
 tions, viz. when the centre in question is either exactly above 
 or below the point of suspension. 
 
 To balance a stick, which is of equal thickness throughout, 
 across a finger, or any other narrow support, every child knows 
 that he has only to find the middle of it ; to balance one which 
 is thicker at one end than the other, he knows that he must 
 place the support nearer the extremitv of the heavy end than 
 the other, and that the difference of length on each side of 
 the balancing point must be proportionate to the difference 
 
MECHANICS. 291 
 
 Centre of gravity. — Line of direction. 
 
 in the weight of the two ends. On the same principle, the 
 common centre of gravity of two equal bodies, is just mid- 
 way between them ; but when the two bodies are unequal, 
 it is nearer the greater body in proportion as it is heavier 
 than the other; or the distances from the centre are inversely 
 as the weights of the bodies. Let A, fig. 3, pi. I. be greater 
 than B ; join AB, upon which take the point C, so that CA : 
 CB : : B : A, that is, if the weight of A be multiplied by the 
 distance AC, and the product be the same as that of the 
 weight of B multiplied by the distance BC, then C is the 
 centre of gravity of the two bodies A and B. If the common 
 centre of gravity of three bodies be required, first find C, the 
 centre of gravity of A and B ; and supposing a body to be 
 placed there equal to the sum of A and B, find G, tne com- 
 mon centre of gravity of it and D ; then will G be the com- 
 mon centre of gravity of the three bodies A, B, and D. In a 
 similar manner the centre of gravity of any number of bodies 
 may be determined. 
 
 As gravity always acts in a direction perpendicular to the 
 horizon, and as if the whole weight of a body were collected 
 in the centre of gravity, this point always endeavours to 
 descend in a vertical line, and with a force equal to the 
 body's weight. Hence a vertical line passing through the 
 body's centre of gravity, is called the line of direction. While 
 the line of direction falls within the base upon which a body 
 stands, the body cannot descend, but if it fall without the 
 base, the body will fall. Thus the inclining body ABCD, 
 fig. 4, pi, I. whose centre of gravity is E, stands firmly on 
 its base CD, because the line of direction, EF, falls within 
 the base. But if we attempt to set up a longer body, GHIK, 
 fig. 5, with the same inclination, the centre of gravity L will 
 be more elevated, and the line of direction LM falling with- 
 out the base, it will drop the instant it is left to itself. The 
 same effect would be produced by placing a sufficient weight 
 on the top of the former body, fig. 4, for every such addition 
 of weight would raise the centre of gravity, and therefore the 
 weight would be sufficient to cause the fall of the body, the 
 moment it was high enough to cause the line of direction to 
 fall without the base, as shewn by fig. 5. 
 
 From the preceding observations, we may deduce the 
 danger and absurdity of people's rising in a boat or other 
 vehicle which is likely to be overset ; for by that means they 
 raise the centre of gravity, and a swing which would not have 
 been attended with the least hazard while they were sitting, 
 will then throw the line of direction beyond the base, and 
 thus render their being overset inevitable. If, instead ef 
 
292 MECHANICS. 
 
 Line of direction. — Experiment nilli a duuble cone. 
 
 rising, the people had crouched as low as possible, many a 
 boat's party would have saved themselves. 
 
 As the broader the base, and the nearer the line of direction 
 is to the middle of it, the more firmly does the body stand ; so, 
 on the contrary, the narrower the base, and the nearer the line 
 of direction is to the side of it, the more easily may the body 
 be overthrown, because a less change of position is sufficient 
 to remove the line of direction out of the base, an-d a less force 
 is required to effect that change of position. It is for this 
 reason that a sphere is so easily rolled upon a horizontal plane, 
 and that it is so difficult to make any b dy stand upright on 
 a point. 
 
 Various contrivances have been executed, taking their rise 
 from the principle, that the centre of gravity always tends to 
 the lowest place possible, as the manner of suspending the 
 marine barometer, compasses, &c. An experiment frequently 
 shewn by lecturers on natural philosophy, in illustration of the 
 principle, is that made with a double cone, which appears to 
 roll up two inclined planes, forming an angle with each other, 
 and lying in the same plane. In this case, the double cone 
 actually sinks as it advances, and by that means the centre of 
 gravity keeps continually descending. It is necessary to this 
 effect, that the height of the planes be less than the radius of 
 the base of the cone. If the height be equal to the radius, the 
 body will rest in any part of the plane ; if the height be greater 
 than the radius, it will descend. The two rules AB, CD, 
 fig. 6, pi. I. are united by a hinge CA. The lower sides are 
 straight ; on the upper sides, one end is wider than the other, 
 so that when opened they form two inclined planes. If a 
 double cone EF, be placed near the hinge, it will roll to- 
 v.'ards the upper end of the planes, and thus apparently as- 
 cend ; but in reality it is let down, because as the rules widen, 
 the cone touches them in parts nearer and nearer the apex on 
 each side. 
 
 ^\ hen the line of direction of a body upon an inclined plane 
 falls within the base, the body will slide down the plane; but 
 it will roll down, if the friction of the surfaces be sufficient, 
 when that line falls without the base. Thus the body C, fig. 7, 
 will only slide down the inclined plane AB; but tlie body D 
 will roll down the same surface. A sphere would descend upon 
 an inclined plane without rolling, if there were no friction, 
 which is the only cause of its rotation. 
 
 When a man is standing, the line of direction passes 
 between his feet; when he walks, most of the motion is to 
 preserve this line in the same position. The various methods 
 and postures which we instinctively use to retain or to recover 
 
MECHANICS. 293 
 
 Mechanical rule for liuding the centre of gravity. — Mechanical powers. 
 
 that position of the line of direction which ensures our stability, 
 might afford matter for curious reflection. We bend our body 
 forward when we rise from a chair, or go up stairs ; and in 
 carrying a load, we always lean from it. Thus a man leans 
 forward when he carries a burden on his back ; backward, when 
 he carries a burden on his breast ; and to the right or left 
 according to the situation of his load. The purpose of all 
 these changes, is to make the line of direction fall between 
 his feet. 
 
 If a body be suspended freely from different centres, its 
 centre of gravity w 1 be in the intersection formed by lines 
 drawn from those centres perpendicular to the horizon. Hence 
 we obtain an easy practical method of finding the centre of 
 gravity of any irregular plane figure : suspend it by any point 
 with the plane perpendicular to the horizon ; from the point of 
 suspension hang a plumb-line, and draw a line upon the body 
 where the string passes over. Do the same for any other point 
 of suspension, and where the two lines meet must be the centre 
 of gravity. For example, supposing AB, fig. 8, pi. I. to be 
 the body of which the centre of gravity is to be found. Sus- 
 pend it in the first instance from any point, as D, so that it 
 may move freely on that part: let a plumb-line hang from the 
 pin on which it is suspended, and mark correctly tlie direction 
 of the plumb-line on the body. Then suspend the body by 
 another part, as F, and use the plumb-line as before. The 
 line last drawn will intersect the first line DE in C, which is 
 the centre of gravity. 
 
 Of the Mechanical Powers 
 
 The simple machines, of a combination of two or more of 
 which all complex engines must consist, are called, by way of 
 distinction, the Mechanical Powers. They are six in number, 
 viz. the LEVER, the pulley, the wheel and axle, the in- 
 clined PLANE, the wedge, and the screw. Some authors 
 are of opinion that we ought only to reckon two simple ma- 
 chines, the lever and the inclined plane ; for the pulley and the 
 wheel and axle may be considered as compound levers, and 
 the wedge and the screw are only modifications of the inclin- 
 ed plane; but as this enumeration is not in general use, and 
 as it is in fact calculated rather to confuse than to simplify 
 the subject, we shall pass it by. 
 
 If the abilities of man were limited by the extent of his 
 natural strength, small indeed would be his knowledge of the 
 works of nature, and few the refinements and comforts of civi- 
 lized society. We can hardly look upon any production of art 
 
294 ^MECHANICS. 
 
 Postulala. 
 
 which could have been obtained without the aid of mechani- 
 cal contrivances. Hence we may conclude, that the construc- 
 tion of machines must have been long antecedent to a know- 
 ledge of the theory upon wliich their principles depend. The 
 remains of Egyptian architecture exhibit the most surprisinf 
 marks of mechanical genius. The stones laid upon the tops 
 of the pyramids of Egypt, are eacii of them equal in size to 
 a small liouse. The elevation of such immense and ponderous 
 masses, to the tops of these and other stupendous fabrics, 
 must have required an accumulation of mechanical power, 
 Avhich the architect of the present day cannot regard without 
 astonishment. 
 
 In establishing the theory of the science of mechanics, some 
 assumptions are made and taken for granted, though not strictly 
 true, and when the theory has been explained, the proper allow- 
 ances are made. The assumptions alluded to, are commonly 
 called postulata, and are principally the four following : 
 
 1. That a small portion of the surface of the earth, although 
 in reality convex, may be considered as a plane. 
 
 2. That heavy bodies descend in lines parallel to each other; 
 for though all bodies tend towards the centre of the earth, yet 
 the distance from which they fall is so inconsiderable, when 
 compared with their distance from the centre of the earth, that 
 their inclination is very trifling, 
 
 3. That the eftort of any given power or weight, is the same 
 in all points of its direction ; or if a l)ody be acted upon by 
 any power in a given direction, the action will be the same, in 
 whatever part of that direction it be applied. 
 
 4. That though all surfaces are more or less rough, and all 
 machines imperfect, yet we must supjjose all planes to be per- 
 fectly even; all surfaces quite smooth ; all levers straight and 
 inflexible, and without thickness and weight; all cords perfectly 
 pliable, and all machines without friction and inertia. 
 
 Three things are always to be considered in treating of me- 
 chanical engines ; a tceight to be raised ; the powo- by which it 
 is to be raised ; and the instrument or engine by which that 
 power acts upon the weight. 
 
 The artifice in all mechanical contrivances is, to distribute 
 Ihe weight among such a number of agents, that the part sus- 
 ained by the power may bear only a small proportion of the 
 ,vhole. 
 
 In calculating the power of a machine, it is usually con- 
 sidered in a state of equilibrium; that is, in the state when the 
 power which has to overcome the resistance, just balances it. 
 Having discovered liow much power will be requisite for this 
 purpose, it will tlien be necessary to add so much more as will 
 
MECHANICS. 295 
 
 Three kinds of levers. 
 
 overcome the friction and weight of the machine itself, and 
 give the necessary velocity. 
 
 Of the Lever. 
 
 The lever is of all machines the most simple; it is merely a 
 bar of iron, of wood, or any solid material, by means of 
 which a certain force, when one part of it rests against a ful- 
 crum or prop, is capable of overcoming or resisting a greater 
 force. 
 
 In the lever there are three circumstances to be particularly 
 noticed: 1. The fulcrum or prop by which it is supported, or 
 on which it turns as an axis or centre of motion. 2. The power 
 to raise and support the weight. 3. The resistance or weight 
 to be raised or sustained. 
 
 The points of suspension are those points where the weights 
 really are, or from which they hang freely. 
 
 The power and the weight are always supposed to act at 
 right angles to the lever, unless it be otherwise expressed. 
 
 The lever is distinguished into three sorts, according to the 
 different situations of the fulcrum or prop, and the power, with 
 respect to each other. 
 
 The lever is of the Jirst order, when the fulcrum or prop is 
 placed between the power and the weight. 
 
 The lever is of the second order, when the prop is at one end, 
 the power at the other, and the weight between them. 
 
 The lever is of the third order, when the power is applied 
 between the weight or resistance and the fulcrum. 
 
 The greater number of instruments in general use are levers 
 of one kind or other. A poker, in stirring a fire, is a lever of 
 the first order; the bar of the grate upon which it rests, is the 
 fulcrum; the fire, the weight or resistance to be overcome; and 
 the hand, is the power. Of this kind of lever, are constructed 
 balances, steelyards, scissars, pincers, snuffers, 8vC. The 
 instrument commonly called the iron-crow, by which large 
 stones are loosened, and great weights raised to small heights, 
 in order to get ropes under them, for raising them still higher, 
 is also a lever of the first kind. AB, fig. 9. pi. I. represents 
 this lever, in which C is the fulcrum, A the end at which the 
 power is applied, and B the end where the weight acts. The 
 parts AC and CB, on the right and left of the fulcrum, are 
 called the arms of the lever. To find when an equilibrium 
 will take place between the power and the weight, we must 
 recur to what was formerly premised respecting the momenta 
 of bodies, viz. that their momenta are always as the products 
 
296 MECHANICS. 
 
 Lever of the first order. 
 
 of their quantities of matter multiplied by tlieir velocities ; and 
 therefore that the momentum of a small body will be equal to 
 that of a large one, if its velocity, or the space it passes 
 through, be such as to make their respective products equal. 
 Now let us consider when this equilibrium will take place in 
 tiie lever. Suppose the lever, AB, tig. 10. to be turned on its 
 axis or fulcrum, so as to come into the situation DC; as the 
 end D is at the greatest distance from the centre of motion, 
 and as it has moved through the arch AD in the same time 
 that the end B moved through the arch BC, it is evident that 
 the velocity of the end A must have been greater than that of 
 B, and for this reason it requires less weight or quantity of 
 matter to produce an eqilibrium than B. 
 
 Let us now ascertain how much more weight B will require 
 than A to balance. As the radii* of circles are in proportion 
 to their circumferences, they are also proportionate to similar 
 parts of them ; therefore as the arches AD, CB are similar, 
 that is, are both equal portions of the circles to which they 
 respectively belong, the radius or arm DE, bears the same pro- 
 portion to EC, that the arch AD bears to CB. But the arches 
 AD and CB represent the velocities of the end of the lever, 
 because they are the spaces which they moved over in the 
 same time; therefore the arms DE and EC may also represent 
 these velocities. 
 
 It is evident, then, that an equilibrium will take place, when 
 the length of the arm AE multiplied into the power at A, shall 
 equal EB multiplied into the weight at B; and, consequently, 
 that the shorter EB is, the greater must be the weifrht at B 
 to produce the balance ; that is, the power and the weight 
 must be to each other inversely as their distances from the 
 fulcrum. Thus, supposing AE, the distance of the power 
 from the fulci'urn, to be twenty inches ; and EB, the distance 
 of the weight from the fulcrum, to be eight inches ; and the 
 weight to be raised at B to be five pounds ; then the power 
 to be applied at A must be two pounds ; because the dis- 
 tance of the weight from the fulcrum, viz. eight, when 
 multiplied into the weight five, makes fortv ; therefore 
 twenty, the distance of the power from the prop, must be 
 multiplied by two to get an equal product, which will pro- 
 duce an equilibrium. 
 
 It is obvious, that while the distance of the power from the 
 ])rop exceeds that of the weight from the prop, a power less 
 than the weight will raise it; so that then the lever affords a 
 
 • The ra('.iu> u( a circle is a line procesding directly from tLe centre to the circnm- 
 ferenoe. 
 
MECHANICS. 297 
 
 Hammer lever. — Second kind of lever. 
 
 mechanical advantage. But when the distance of the power 
 is less than that of the weight from the prop, the power must 
 be greater than the weight, or it will not raise the latter; when 
 both the arms are equal, the power and the weight must be 
 equal, to be in equilibrium. 
 
 The hammer lever differs in nothing but its form from a 
 lever of the first kind. Suppose the handle of the hammer to 
 be ten times the length of the iron part that draws the nail, 
 then pulling backwards the handle, while the lower end rests 
 upon the board as a fulcrum, the nail may be drawn with one- 
 tenth part of the power which would be required to pull it out 
 with pincers ; because in using the pincers, the nail would move 
 as fast as the hand, but in using the hammer the hand moves 
 over ten times the space of the nail. A pair of scissars is com - 
 posed of two levers of the first kind, the centre of motion being 
 the rivet. If the hand or power be applied three times as far 
 from the rivet as the material to be cut, each lever acting with a 
 force of three, the scissars will act on the material six times more 
 strongly than if the same power were applied directly to it. 
 
 The second kind of lever, that is, a lever employed so as to 
 have the weight between the fulcrum and the power, is repre- 
 sented by fig. 11, pi. I.; A is the fulcrum, B the weight, and C 
 the power. The advantage gained by this lever, as in the first, 
 is as great as the distance of the power from the prop exceeds 
 the distance of the weight from the prop. Thus if the point 
 a, on which the power acts, be seven times as far from A as 
 the point h, on which the weight acts, then one pound applied 
 at C will raise seven pounds at B, 
 
 From the properties of this lever it is evident, that if two 
 men carry a burden upon a stick between them, the shares of 
 the weight which they bear, are to one another in the inverse 
 proportion of their distances from it. The fact, indeed, is 
 well known, that the nearer either of them is to the burden, 
 the greater share he bears of it; and if one of them go directly 
 under it, he bears the whole. If one man be at A and the 
 other at a, with the pole or stick resting on their shoulders, 
 and the burden B is placed five times as near the man at A, 
 as it is to the man at a, the former will bear five times as 
 much weight as the latter. Upon the same principle, two 
 horses of unequal strength, may be yoked so that each horse 
 may draw a part proportionable to his strength ; for the 
 beam they pull may be divided in such a manner, that the 
 point of traction may be as much nearer to the stronger 
 horse, as will be required to balance the different effects of 
 their strength. 
 
 The oars and rudders of vessels are levers of the second kind; 
 13.— Vol. I. 2 Q 
 
298 MECHANICS. 
 
 Third kind of lever. 
 
 the vessel is the weight or resistance, the water is the fulcrum, 
 and the man who governs their motions is the power. A door 
 is a lever of the second kind ; the hinges are the centre of mo- 
 tion, the body of the door is the weight, and the hand by which 
 it is moved is the power. A pair of bellows, nut-crackers, Slc. 
 are composed of two levers of the second kind. 
 
 In the third kind of lever, that is, when the power is between 
 the weight and the prop, the power and the weight are in equi- 
 librium, when the intensity of the power exceeds the intensity 
 of the weight just as much as the distance of the weight from 
 the prop exceeds the distance of the power. Thus, let E, fig. 
 12, be the prop of the lever EF, and \V a weight of one pound, 
 placed five times as far from the prop as the point at which 
 the power P, acts, by the cord going over the fixed pulley I); 
 in this case, the power must be equal to five pounds, in order 
 to support the weight of one pound. 
 
 The third kind of lever is used as little as possible, on 
 account of the disadvantage to the moving power; but it can- 
 not always be avoided; an example of it is seen in rearing a 
 tall ladder against a wall, where the power or strength of a 
 man is exerted at a short distance from one end, and the task 
 cannot be performed without more effort than would be ne- 
 cessary to bear the ladder. 
 
 The bones of a man's arm, and the limbs of animals gene- 
 rally, are levers of the third order; for when we lift a weight 
 by the hand, the muscle that exerts its force to raise that 
 weight, is fixed to the bone about one-tenth part as far below 
 the elbow as the hand is, and the elbow being the centre 
 round w hich the lower part of the arm turns, the muscle must 
 therefore exert a force ten times as great as would be required 
 simply for the suspension of the weight that is raised. The 
 principle of vitality has an influence uj)on tlie strength of the 
 muscles for which we are wholly unable to account ; and a 
 weight which would instantly break a muscle the moment it 
 was dead, can be raised by that muscle while alive, without 
 the smallest pain or difficulty. Vitality, therefore, being or- 
 dained to communicate so much enero:y to such flexible mate- 
 rials as flesh and blood, a lever of the third sort became most 
 admirably adapted to the animal frame, because, if the power 
 be sufficient, its operations are quick, and it is exerted or re- 
 sides in a small compass. 
 
 In every species of lever there will be an equilibrium, when 
 the power is to the weight as the distance of the weight from 
 the fulcrum is to the distance of the power from the fulcrum. 
 
 In making experiments to prove the truth of the theory of 
 the mechanical powers, as it is impossible to obtain materials 
 
MECHANICS. 299 
 
 Eflects of changing the position of the weight supported by a lever. 
 
 void of weight, the young mechanic ought to be apprized of 
 the necessity of taking the only precaution in his power, that 
 of perfectly balancing the levers, 8cc, before the weights and 
 powers are applied, otherwise his results will not be satisfac- 
 tory. Thus a lever made to exemplify the theory explained 
 by fig. 9, pi. I. should from B to C be so much thicker than 
 from C to A, that BC will balance CA when the fulcrum is at 
 C. In like manner, for every other position of the fulcrum, 
 the shorter arm of the lever should be made of the same weight 
 as the longer. 
 
 If the weight to be raised be of considerable bulk, and if it 
 be fixed either above or below the end of the lever, it will vary 
 in its intensity according to the position of the lever. Let 
 AB, fig. 13, represent a lever having a weight fixed above it, 
 as A, of which the centre of gravity is a, and the line of di- 
 rection a b ; then a 6 is the point in the lever upon which the 
 weight acts ; but if the lever is moved into the position CD, 
 the line of direction of the weight will fall nearer to the ful- 
 crum of the lever, and consequently act with less force upon 
 it; but if the lever is placed in the direction EF, the line of 
 direction will fall further from the fulcrum, and therefore its 
 action on the lever will be increased. On the contrary, op- 
 posite effects will take place when the weight is below the 
 lever, as represented by fig. 14. 
 
 When the weight is suspended from the lever by a rope, or 
 any flexible material, no alteration, as exemplified by fig. 13, 
 can take place, because the point of suspension or point of 
 action is not altered. When, therefore, two draymen carry a 
 barrel on a coulstaff", to which it is suspended by a chain, the 
 point on which the weight acts not being altered by inclining 
 the staff' in going up or down hill, each man will sustain the 
 same weight as if on level ground. But if they carry the 
 barrel upon two dogs, then the weight does not swing, and 
 the centre of gravity is below the lever ; therefore the point on 
 which the weight acts, will, by inclining the lever, be made 
 to approach the highest end ; and the first man, in going- 
 down hill, by having this point removed from him, will be 
 eased in part of his burden, and the last man will have his 
 equally increased. 
 
 If several levers be combined together, so that a weight 
 appended to the first lever, may be supported by a power 
 applied to the last, as in fig. 13, pi. I. where three levers of 
 the first kind are so disposed that a power applied to the point 
 L of the lever C, may sustain a weight at the point S of the 
 lever A, the power must be to the weight, in a ratio, or pro- 
 portion compounded of the several ratios which those powers 
 
300 MECHANICS. 
 
 Power of levers in coinbiiiatinn. — Tbe balance. 
 
 that can sustain the weight by the lielp of eacli lever, when 
 used singly and apart from the rest, have to the weight. For 
 instance, if the power which can sustain the weight W by 
 the help of the lever A, be to the weight as 1 to 5 ; and if the 
 power which can sustain the same weight by the lever B alone, 
 be to the weight as 1 to 4 ; and if the power which could sus- 
 tain the same weight by the lever C, be to the weight as 1 to 
 6 ; then the power which will sustain them by the help of the 
 three levers joined together, will be to the weight in a pro- 
 portion consisting of the several proportions multiplied to- 
 gether, of 1 to 5, 1 to 4, and 1 to 5; that is, as 5x4x5, or of 
 1 to 100. For since, in the lever A, a power equal to one- 
 fifth of the weight W, pressing down the lever at L, is suf- 
 ficient to balance the weight; and since it is the same thing 
 whether that power be applied to the lever A at L, or the 
 lever B at S, the point S bearing on the point L, a power 
 equal to one-fifth of the weight W, being applied to the 
 point S of the lever B, will support the weight ; but one-fourth 
 of the same power being applied to the point L of the lever 
 B, and pushing the same upward, will as effectually depress 
 the point S of the same lever, as if the whole power were 
 applied at S ; consequently a power equal to one-fourth of 
 one-fifth, that is, one-twentieth of the weight W, being applied 
 to the point L of the lever B, and pushing up the same, will 
 support the weight. In like manner, it matters not w^hether 
 that force be applied to the point L of the lever B, or to the 
 point S of the lever C ; since if S be raised, L, which rests on 
 it, must also be raised ; but one-fifth of the power applied at 
 the point L of the lever C, and pressing it downwards, will as 
 effectually raise the point S of the same lever, as if the whole 
 power were applied at S, and pushed that lever up ; conse- 
 quently a power equal to one-fifth of one-twentieth, that is, 
 one-hundredth part of the weight W, being applied to the 
 point L of the lever C, will balance the weight at the point S 
 of the lever A. This method of combining levers is frequently 
 used in machines and instruments, and is of great service, 
 either in obtaining a greater power, or applying it with more 
 convenience. 
 
 Of the Balance. 
 
 The common balance, the extensive utility of which, in com- 
 
 fiaring the weights of bodies, is so well known, consists of a 
 ever of the first kind, the arms of which are equal in length. 
 The points, therefore, from which the weights are suspended, 
 
MECHANICS. 301 
 
 The balance. 
 
 being equally distant from the centre of motion, will move 
 with equal velocity, consequently, if equal weights be applied, 
 their momenta will be equal, and the balance remain in equi- 
 librium. 
 
 In order to have a balance as perfect as possible, it is ne- 
 cessary to attend to the following circumstances : 
 
 1. The arms of the beam ought to be exactly equal both as 
 to weight and length, and should at the same time be as long 
 as possible, relatively to their thickness, and the weight they 
 are intended to support; because the further the points of sus- 
 pension are from the centre of motion, the more the momen- 
 tum of the weights is increased, and the more sensible will be 
 the instrument. 
 
 2. The points from which the scales are suspended, should 
 be in a right line, passing through the centre of gravity of the 
 beam ; for by this means the weights will act directly against 
 each other, and no part of either will be lost, on account of 
 any oblique direction. 
 
 3. If the fulcrum, or axis of motion, passes through the 
 centre of gravity of the beam, and if the fulcrum and the 
 points of suspension be in the same right line, the balance will 
 have no tendency to one position more than another; but will 
 rest in any position it may be placed in, whether the scales be 
 on or off, empty or loaded, provided the weight in each scale 
 be the same. The equality of two weights suspended from a 
 beam, where the centres of gravity and of motion are thus coin- 
 cident, being shewn by their quiescence, and not by any par- 
 ticular position, such a beam is evidently inapplicable to com- 
 mon use, for which but one position ought to denote the equa- 
 lity of the weights ; and for that one, the horizontal position 
 is the most convenient. 
 
 If the centre of gravity of the beam, when level, be imme- 
 diately above the fulcrum, it will overset with the smallest ac- 
 tion ; that is, the end which is the lowest will descend and 
 not rise again, and it will descend thus with more swiftness, 
 the higher the centre of gravity, and the less the points of 
 suspension are loaded. Hence such a beam will make equal 
 weights appear unequal. If the centre of gravity of the beam 
 be below the fulcrum, the beam will not rest in any position 
 but when level ; but if disturbed from that position, and then 
 left at liberty, it will vibrate, and at last come to rest on the 
 level. In a balance, therefore, the fulcrum ought always to 
 be placed a little above the centre of gravity. Its vibrations 
 will be quicker, and its horizontal tendency stronger, the lower 
 the centre of gravity, and the less the weight upon the points 
 of suspension. 
 
302 MECHANICS. 
 
 Tbe balance. 
 
 4. The friction of the beam upon the axis ought to be as 
 little as possible ; because, should the friction be considerable, 
 the force required to overcome it will much injure the sensi- 
 bility of the instrument; upon which accounl, though one 
 weight should a little exceed the other, the greater will not 
 preponderate, if the excess be insufficient to overcome the 
 friction and bear down the beam. The axis of motion should 
 be formed with an edge like a knife, and made very hard. 
 These edges, in small balances, are at first made sharp, and 
 then rounded upon a fine hone, or piece of butt' leather, covered 
 with some impalpable cutting powder. This operation causes 
 a sufficient bluntness or rolling edge. The excellence of the 
 instrument depends in a great measure upon the regular form 
 of this rounded part. The scales should be hung upon an 
 edge of the same kind. 
 
 5. The pivots, which form the axis of motion, should be in 
 a straight line, and at right angles to the beam. 
 
 6. The rings, or pieces on which the axis bears, should be 
 very hard and well polished, parallel to each other, and of an 
 oval figure, that the axis may keep its proper bearing, or al- 
 ways remain at the lowest point 
 
 7. The beam should always be made so strong, as to be 
 inflexible by the greatest weight it is likely to sustain; as if 
 it bend it will be rendered less sensible, and as the arms 
 will probably bend unequally, the balance will cease to 
 give a correct result. That the beam may not be thick 
 or clumsy without utility, it should be strongest at the 
 middle, from which part to the points of suspension it 
 should be gradually diminished in thickness, because the 
 strain upon it is likewise so diminished. In small balances, 
 the beam is mostly round, but in large ones it is generally 
 rectangular, its transverse section being a parallelogram, of 
 which the long sides are vertical. A square, a round, or any 
 other form, would require a greater quantity of metal to pos- 
 sess the same strength. 
 
 8. Very delicate balances are not only useful in nice expe- 
 riments, but are likewise much more expeditious than others 
 in common weighing. If a pair of scales, with a certain load, 
 be barely sensible to one-tenth of a grain, it will require a 
 considerable time to ascertain the weight to that degree of 
 accuracy, because the turn, being very small, must be ob- 
 served several times over. But if no greater accuracy were 
 required, and a balance were used which would turn with the 
 one-hundredth part of a grain, a tenth of a grain more or less 
 would make so great a difference in the turn that it would be 
 seen immediately. 
 
MECHANICS. 303 
 
 The balance. 
 
 9. A curious effect caused by exciting a tremulous motion in 
 the beam, deserves to be noted. If a balance be found to turn 
 with a certain addition, and is not moved with any smaller 
 weight, a greater sensibility may be given to that balance by 
 drawing a file, a saw, or any similar instrument, along any 
 part of the beam o.r its support ; for the jar produced by this 
 operation will diminish the friction on the moving parts so 
 much, that the turn will be evident with one-third or one- 
 fourth of the addition that would else have been required. 
 In this way, a beam which would hardly turn with the addi- 
 tion of one-tenth of a grain, will turn with one-thirtieth or 
 fortieth of a grain. 
 
 10. When the arms of a balance are unequal, the balance is 
 said to he false, because it does not give the true weight of the 
 body, whether it be suspended from the shorter arm or the 
 longer one. There are, however, several properties of the 
 false balance, which are extremely useful in the estimation of 
 weights, as well as in correcting errors which may have arisen 
 in the adjustment of the true balance. 
 
 A balance with unequal arms will weigh as accurately as 
 another of the same workmanship, provided the standard 
 weight be first counterpoised, then taken out of the scale, and 
 the thing to be weighed be put into the scale, and adjusted 
 against the counterpoise. Or when proportional quantities 
 only are considered, as in chemical and other philosophical 
 experiments, the bodies and products under examination 
 may be weighed against the weights, taking care always to 
 put the weights into the same scale; for then, though the 
 bodies may not be equal to the weights, yet their proportions 
 among each other will be the same as if they had been accu- 
 rately so. 
 
 A weight which counterpoises an ounce when suspended from- 
 the longer arm of a false balance, being added to the weight 
 which counterpoises an ounce suspended from the shorter arm, 
 will always be greater than two ounces. The excess is that 
 part of an ounce which is expressed by a fraction, of which 
 the numerator is the square of the difference of the arms, and 
 the denominator the product of the arms. If any substance 
 be successively weighed from the longer and shorter arms of 
 a false balance, the true weight will be a geometrical mean- 
 between the false weights. 
 
 11. But though the equality of the arms may not always 
 be essential, yet it is indispensably necessary that their rela- 
 tive lengths, whatever they may be, should continue invariable^ 
 For this purpose, it is requisite, either that the three edges be 
 all truly parallel, or that the points of suspension and support 
 
304 MECUAXICS. 
 
 Accuracr obuioed in coasimcUng balances. 
 
 should be always in the same part of the edge. The last point 
 is the most easily obtained. 
 
 The index of a balance is that slender rod rising perpendi- 
 cularly from the middle of the beam, of which it shews the 
 inclination from the horizontal position. A sliding weight is 
 sometimes fitted upon the index, in order to raise or depress 
 the centre of gravity of the balance. This contrivance is use- 
 ful in adjusting the best distance between the centre of gravity 
 and the fulcrum. 
 
 The lever, we have observed, is of all machines the most 
 simple; the balance is merely a lever, yet, in applying it to 
 practice, it appears, from the preceding considerations, that 
 many difficulties are to be encountered before it becomes fit- 
 ted to afford any thing like accurate results. The workman 
 finds it a severe check upon him, to keep the arms equal, 
 while he is making the other adjustments, and this point, 
 though certainly not the easiest to manage, is but one of many 
 in which he ought perfectly to succeed. If then the adjust- 
 ment of a simple lever be so difficult a matter, the sources of 
 imperfection in complex machines may well be supposed to 
 be very considerable. Yet when we regard what has been 
 actually executed, we find no reason to conclude that the in- 
 genuity of man is too limited for his happiness; or that most 
 of the obstacles which, at present baffling us, form the deside- 
 rata of human knowledge, will not in time be removed. In 
 this place, it may gratify the reader to notice the neat approxi- 
 mation to perfection which has been shewn in the construc- 
 tion of the balance. 
 
 Muschenbroek mentions his being possessed of a balance 
 which turned with one-fortieth of a grain. The substances 
 he weighed were between 200 and 300 grains. The balance 
 therefore weighed to -r^^xro part of the w hole. 
 
 Two accurate balances of Bolton's are mentioned in the 
 Philosophical Transactions, vol. 66. One of them, it is 
 said, would weigh a pound, and turn with one-tenth of a 
 grain. This, supposing the pound to be avoirdupoise, isyx?W(y 
 of the weight, and shews that the balance could be depended 
 on to four places of decimals, and probably to five. The 
 other balance was adaptea to weigh half an ounce or under, 
 and turned with j^i^ of a grain, which is about ^-c^^j of the 
 weight. 
 
 In the same volume of the Philosophical Transactions, two 
 other balances, one of them Read's, and the other Whitehurst's, 
 are noted. Read's, when loaded with 55 pounds, readily 
 turned with one pennyweight; but very distinctly turned with 
 four grains when tried more patiently. This being about -j^g^^j 
 
MECHANICS. 305 
 
 Accuracy obtained in constructing balances. — Steel-yard. 
 
 part of the weight, the balance may be well depended on to 
 five places of figures. — Whitehurst's balance weighs one penny- 
 weight, and is sensibly affected with 2-0V1T of ^ grain, which is 
 -4-8^0 (J part of the weight. 
 
 W. Nicholson, so well known as the intelligent author of 
 many scientific works, observes, that he has a balance made 
 expressly for him by a skilful artist in London. With 1200 
 grains in each scale, it turns with one-seventieth of a grain. 
 This isg^^ijof the whole; and therefore about this weight 
 may be known to five places of figures. The proportional 
 delicacy is less in greater weights. The beam will weigh 
 nearly one pound troy, and when the scales are empty, it is 
 affected by tijoit of a grain. On the whole, it may be usefully 
 applied to determine all weights between 100 and 4000 grains 
 to four places of figures. 
 
 A balance in the possession of Dr. George Fordyce, is men- 
 tioned in the Philosophical Transactions, vol. 75. It was made 
 by Ramsden, and turns upon points instead of edges. With a 
 load of four or five ounces, a difference of one division in the in- 
 dex was made by yg^jj^y of a grain. This is 3 g'/oijg part of the 
 weight, and consequently this beam will ascertain such weights 
 to five places of figures, besides an estimate figure. 
 
 The Royal Society's balance, which was also made by Rams- 
 den, turns on steel edges, upon planes of polished chrystal. Ni- 
 cholson, who speaks of it in his Chemical Dictionary, says he 
 was assured, that it ascertained a weight to the seven-millionth 
 part. He was not present at this trial, which must have requir- 
 ed great care and patience, as the points of suspension could 
 not have moved over much more than one-fiftieth of an inch in 
 the first half minute : but from some trials which he saw, he 
 considered it probable, that it might be used in general prac- 
 tice to determine weights to five places and better. 
 
 Tables of specific gravities, are sometimes carried to five, six, 
 and even seven places of decimals ; but from the preceding 
 account of balances it is obvious, that experience does not au- 
 thorise the practice; and that deductions founded on such a 
 supposed accuracy in weighing, are of a very questionable 
 nature. In general, where weights are given to five places of 
 figures, the last figure is hypothetical ; and where they are 
 carried further, we have reason to doubt the veracity or com- 
 petence of the experimentist. 
 
 Of the Steel-yard. 
 
 The statera, or Roman steel-yard, is a lever of the first kind, 
 and is used for finding the weights of diff'erent bodies, by one 
 single weight, placed at different distances from the prop or 
 13.— Vol. I. 2 R 
 
306 MtCHAMCS. 
 
 Danish and Swedish steel-jard. — Pulley. 
 
 centre of motion U, fifij. 16, pi. I. The shorter arm UM, is of 
 such a weight as exactly to counterpoise the longer arm UN. 
 If the longer arm be divided into as many equal parts as it will 
 contain, each equal to UO, the single weight Q (which we 
 mav suppose to be one pound) will serve for any thing as heavy 
 as itself, or as many times heavier as there are divisions in the 
 arm UN equal to the distance UO ; or any quantity between its 
 own weight and that quantity. For example, if Q be one pound, 
 and be placed at the first division 1, in the arm V}^, it will 
 balance one pound in a scale or on a hook at P; if it be remov- 
 ed to the second division at 2, it will balance two pounds at 
 P; if to the third, three pounds, and so on to the end of the arm 
 UN. If these integral divisions be subdivided into as many 
 equal parts as a pound contains ounces, and the weight Q be 
 placed at any of these subdivisions, so as to counterpoise the 
 goods in the scale, the pounds and odd ounces of those goods 
 will by that means be ascertained. 
 
 In the Danish and Swedish steel-yard, the body to be weigh- 
 ed, and the constant weight, are fixed at the extremities of the 
 steel-yard, but the point of suspension or centre of motion, 
 moves along the lever till the equilibrium takes place. The 
 centre of motion therefore shews the weight of the body. 
 
 Of the Pulley. 
 
 The pulley is a small wheel turning on an axis, with a draw- 
 ing rope passing over it. The circumference of the pulley is 
 generally hollowed to receive the rope, which is attached on 
 the one hand to the moving power, and on the other to the 
 resisting force. 
 
 The pulley is usually called a sheave, and is so fixed in a 
 frame or block, as to be moveable on a pin passing through its 
 centre. When pulleys are made of wood, a ring of iron or 
 brass is generally let into the middle of them, to work upon 
 the pin, as thev would otherwise wear unequally, and their 
 motion would then be impeded by an increased degree of 
 friction. 
 
 AJixed pullev is one which has no motion except upon its 
 axis ; a moveable pulley is one which rises and falls with the 
 weight. The ex.ijression " moveable pulley" is clear enough, but 
 the epithet //j£ti being rather calculated to exclude the idea of 
 motion entirely, the expression "fixed pullev," requires more 
 particular notice from those to whom the subject is new. 
 
 The gorge or groove of a pulley, is the hollow part of the cir- 
 cumference which receives the rope or cord ; it is frequently hoi- 
 
MECHANICS. 307 
 
 The pulley. 
 
 lowed out angularly, so that the rope is, by the pressure, 
 so wedged in the angle, that it cannot glide or slip in its 
 motion. 
 
 A pair of blocks, with the rope fastened round it, is com- 
 monly called a tackle. 
 
 Two equal weights attached to the ends of a rope going over 
 a fixed pulley, as fig. 1, pi. II. will balance each other, for 
 they stretch the rope equally, and if either of them be pulled 
 down through any given space, the other will rise through an 
 equal space in the same time, and consequently as their velo- 
 cities are equal, they must balance each other. This kind of 
 pulley, therefore, gives no mechanical advantage, but the use 
 of it is a source of great convenience. It serves to change the 
 direction of draught; it gives a man an opportunity of apply- 
 ing his weight instead of his muscular strength, but not of lift- 
 ing more than his weight ; it also enables a man to raise a 
 weight to any point, without moving from the place he is in, 
 whereas he would otherwise have been obliged to ascend with 
 the weight; and, lastly, by it several men may apply their 
 strength to the weight by means of the rope, with as much 
 facility, under the same circumstances, as one person only. 
 
 In treating of the lever, it might have been observed, that 
 the prop may be regarded as a third power, which keeps in 
 equilibrium the motive force and the resistance, or which con- 
 curs with the one, to enable it to sustain the effort of the other. 
 If the lever of the second order, AB, fig. 3, have its fulcrum 
 at B, the weight in the middle at C, and the power at A, half 
 the weight being supported by the fulcrum, a power equal to 
 the other half will keep it in equilibrium. This will apply 
 to the illustration of the action of pulleys, which, when the 
 weight is appended to the circumference, may be considered 
 as levers of the first kind, and when the weight is appended 
 to the centre, they may be considered as levers of the second 
 kind : hence the ropes a b, fig. 1, hanging at equal distances 
 from the centre, c, (which must be regarded as the fulcrum,) 
 equal weights must be in equilibrium, exactly as they would 
 be if placed in the scales of a common balance. But if one 
 weight be further from the centre or fulcrum than the other, 
 they will balance each other only as they would in a steel- 
 yard, and, therefore, though still a lever of the first kind, a 
 less weight will suspend a greater. Thus, if the pulley, as in 
 fig. 2, have different gorges, and the weight R. of six ounces, 
 be hung at the distance of one inch from the fulcrum, c, and 
 the weight S of three ounces be hung at the distance of two 
 inches from the same centre; the two weights R and S, 
 though in the proportion of 2 to I, will balance each otlier. 
 
308 MECHANICS. 
 
 The pullej. 
 
 If the weight S were only two ounces, it would produce the 
 same effect upon R, provided its distance from the fulcrum 
 were proportioned to the diminution of its weigiit ; that is, if 
 it were three times as far from the centre c, as R. 
 
 We have now to shew that the moveable pulley acts like a 
 lever of the second order. Let the moveable pulley A, fig. 4, 
 pi. II, be fixed to the weight W, with which it rises and falls. 
 In comparing it with the lever alluded to, the fulcrum must be 
 considered as at F ; the weight acts upon the centre c, by 
 means of the neck ch ; the power is applied at D; and the 
 line DF will represent the lever. The power, therefore, as 
 in fig. 3, is twice as far from the fulcrum as the weight, 
 and the effect in both cases is alike, viz. the proportion be- 
 tween the power and the weight, in order to balance each 
 other, must be as 1 to 2. It is evident, therefore, that the 
 use of this pulley doubles the power, and that it will enable a 
 man to raise twice as much as by his strength alone. Or, as 
 variety in illustration will sometimes catch the attention, and 
 familiarize a subject to some whose ideas of it would not 
 otherwise be distinct, the action of this pulley may be viewed 
 in a light somewhat different from the above. Every move- 
 able pulley may be considered as hanging by two ropes 
 equally stretched, and which must consequently bear equal 
 parts of the weight ; the rope FG being made fast at G, 
 half the weight is sustained by it, and the other part of the 
 rope, to which the power is applied, has only the other half 
 of the weight to support; consequently the advantage gained 
 is as 2 to 1. 
 
 When, as in fig. 5, the upper and fixed block, or pulley- 
 frame, contains two pulleys, which only turn upon their axes, 
 and the lower moveable block contains also two, which not 
 only turn on their axes, but rise with the weight W, the ad- 
 vantage gained is as 4 to 1 ; for each lower pulley will be 
 acted upon by an equal part of the weight ; and because each 
 pulley that moves with the weight, diminishes one-half the 
 power necessary to keep the weight in equilibrium, the power 
 by which W may be sustained will be equal to half the 
 weight divided by the number of lower pulleys ; that is, as 
 twice the number of the lower or moveable pulleys is to 1, 
 so is the weight suspended to the power. But if the ex- 
 tremity A, fig. 6, be fixed to the lower block, it will sustain 
 half as much as a pulley ; consequently here the rule will be, 
 as twice the number of moveable pulleys, adding unity, is to 
 1, so is the weight to the power. To prevent the ropes a and 
 b from rubbing against each other, the upper fixed pulley 
 may have a double gorge. The pulley d belongs not to the 
 
MKCH:A>fIC s 
 
 PL.I. 
 
 TuibUslied Vy H«-nT_j- fisher. CsxTon. London. lO'/i'. 
 
MECHANICS. 309 
 
 The puHey. 
 
 system of pulleys, it is merely used in the plate, to separate 
 from the ropes, and shew more distinctly the power, P. 
 
 If instead of one rope going round all the moveable pulleys, 
 the rope belonging to each of them be made fast at the top, as 
 in fig. 7, a different proportion between the power and the 
 weight will take place. Here it is evident, that each moveable 
 pulley doubles the power; thus, if there are two moveable pul- 
 leys, the power will sustain four times its own force or weight; 
 if three pulleys, eight times its own weight; if four pulleys, 
 sixteen times its own weight. In the figure where three move- 
 able pulleys are shewn, the weight W, of sixteen ounces, is 
 supported by the power P, of two ounces. This arrangement 
 of pulleys takes up much room, raises the weight very slowly, 
 and is not convenient to fit up. It is therefore seldom used, 
 notwithstanding the great power gained. 
 
 These rules are applicable, whatever may be the number of 
 pulleys employed. 
 
 The large space occupied by pulleys, when arranged under 
 each other, as in fig. 5 and 6, is an inconvenience that would 
 often render them useless, and such an arrangement would 
 increase the liability to entanglement, particularly on ship- 
 board; it is therefore common to place all the pulleys in each 
 block on the same pin, by the side of each other, as in fig. 8. 
 The advantage, and the rule for the power, are the same 
 here as in fig. 5. In this kind of tackle, the ropes are 
 not exactly parallel, a direction which should be preserved 
 as much as possible ; but the defect is not very consider- 
 able. 
 
 The reason of the parallel direction of the ropes being better 
 than an oblique one, is that less power is required to sustain 
 the same weight; and in proportion to the obliquity of the 
 ropes must be the increase of the power. When there are 
 many pulleys in the same block, and the end of the rope to 
 which the power is applied terminates over one of the outside 
 pulleys, that pulley always endeavours to get into a line with 
 the centre of suspension or middle of the moveable pulleys, 
 from which the weight hangs. In consequence of this, the 
 friction of the pulleys against the sides of the block is so great 
 as sometimes to equal the power. Hence the multiplication 
 of pulleys thus used, soon ceases to be advantageous ; they 
 are seldom effective, if their number exceeds three or four. 
 Smeaton, the eminent engineer, was the first who disencum- 
 bered himself of the difficulty here stated, by making the rope 
 terminate over the middle pulley or sheave in the fixed block, 
 which is thereby kept perpendicularly under t!ie other, and the 
 friction of the sheaves is on their centres of motion only. The 
 
310 MECHANICS. 
 
 The pulley. 
 
 number of sheaves must always be uneven, or this imj>rove- 
 ment cannot be adopted. 
 
 To avoid as much as possible the friction and shaking 
 motion of a combination of pulleys, James White, a very able 
 mechanic, invented and obtained a patent for the concentric 
 pulley, fig. 9. INI and N are two of these pulleys, one of them 
 being fixed, the other moveable. They are usually made of 
 brass, and answer the purpose of as many distinct pulleys as 
 there are grooves. In this case, as in tijr. 5, the weight beinc 
 divided among the number of ropes, a power of 1 will support 
 a weight of 12. 
 
 In speaking simply of a system of pulleys, the common ar- 
 rangement of them is meant, viz. that where the number of 
 ropes is just twice the number of the moveable pnlleys. Fig. 
 4, 5, and 8, are all systems of tliis kind. The ropes are 
 spoken of as if they were in different lengths, but it can 
 hardly require an observation, that the expression is used 
 merely because it is convenient, and that there is in fact but 
 one rope, the parts of which are alluded to as if they were 
 separate. 
 
 It has been shewn, in illustrating fig. 2. pi. II, that by 
 means of a pulley of several grooves, the actions of two 
 unequal powers may be made to balance each other. In 
 like manner, a constant equilibrium or relation may be pre- 
 served between two powers, the relative forces of which 
 continually change. Watchmakers derive great advantage 
 from the application of this principle to their work. The 
 spring of a watch always acts with the greatest power imme- 
 diately after it has been wound up, and its power is continu- 
 ally but gradually diminishing, till the watch stops. If this 
 inequality of the maintaining power operated upon the wheels, 
 the watch would not go two successive hours at the same 
 rate ; but the effects of it are completely avoided by the pe- 
 culiar conformation of the pulley off' -which the spring draws 
 the chain. Instead of many concentric gorges upon the 
 fusee, they make only one, but that one is in a spiral form 
 upon a truncated cone, see fig. 10. When the watch is wound 
 up, the chain extends from the upper or narrowest part e, to 
 the spring-box. The spring is then at the strongest ; but it 
 acts on a part so near the centre of motion, or axis F G, that 
 its power on the wheels is the same as just before it stops, 
 when its absolute strength is much diminished ; and its weak- 
 ness is favoured by pulling at a longer lever, or at a greater 
 distance from the centre of motion, that is, at f. Now as 
 the alteration in the power of the spring from its greatest to 
 its least strength is gradual, so is the extension of tlie lever, or 
 
MECHANICS. 311 
 
 Convenience, and not absolute increase of power, gained by machines. 
 
 increase of the distance from the centre of motion, FG, also 
 gradual between the extremes ef; therefore the spring and the 
 fusee may be so adjusted to each other, that the power operat- 
 ing upon the wheels will always be the same. 
 
 We have postponed till now one remark which the reader 
 has probably anticipated, viz. that it is convenience alone, and 
 not any actual increase of power, which we gain by machines ; 
 for in all contrivances by which power is gained, a propor- 
 tional loss of time is suffered. This is evident from a consi- 
 deration of the properties of the lever ; and is still more evi- 
 dent, if possible, in considering the action of pulleys. If one 
 man, by means of a tackle, can raise as much weight as ten 
 men could by their unassisted strength, he will be ten times 
 as long in performing his task. Suppose a man at the top of 
 a house draws up ten weights, one at a time, by a single rope, 
 in ten minutes, let him have a tackle of five moveable pulleys, 
 and he will draw up the whole ten at once with the same ease 
 as he before raised up one; but in ten times the time, that is, 
 in ten minutes. Thus we see the same work is performed in 
 the same time, whether the tackle be used or not; but the con- 
 venience is, that if the whole of the ten weights were in one 
 mass, they may be raised with the tackle, though it would be 
 impossible to move them by the unassisted strength of one 
 man. 
 
 Or suppose, instead often weights, a man draws ten buckets 
 of water from the hold of a ship in ten minutes, and that the 
 ship being leaky, admits an equal quantity in the same time. 
 It is proposed, that by means of a tackle he shall raise a 
 bucket ten times as capacious with the same exertion of 
 strength. With this assistance, he draws up the large bucket, 
 but in as long a time as he employed to draw up the ten, and 
 therefore he is as far from gaining on the water in the latter 
 case as in the former. 
 
 The remark, that whatever is gained in power is lost in time, 
 would be perfectly correct, even if machines were destitute of 
 friction and of inertia, but as these hinderances are always 
 present, the truth is, as we shall afterwards see, that power, 
 instead of being increased by the use of machines, is actually 
 diminished. The convenience, however, which is obtained by 
 a well constructed machine, is an ample recompense for the 
 actual loss of power. 
 
 Of the Wheel and Axle. 
 
 The wheel and axle, sometimes called the axis in peritrochio, 
 is a machine of the most extensive utility, and, to suit different 
 purposes, is greatly diversified in form. It is nearly allied to 
 
312 MECHANICS. 
 
 Wheel and axle. 
 
 the pulley ; the same illustrations will frequently serve for 
 them both ; and, like the pulley, it may be considered as an 
 assemblage of levers, or a perpetual lever, because from its 
 constant renewal of the points of suspension or resistance, it 
 frees us from the great defect of the simple lever, which can 
 only be used to raise weights to small elevations. 
 
 This mechanical power consists generally of a wheel with 
 an axis fixed to it, so as to turn round Avitli it ; the power beinor 
 applied at the circumference of the wheel, the weight to be 
 raised is fastened to a rope which coils round the axis. But a 
 machine which has in reality no wheel, comes under the deno- 
 mination of the wheel and axle ; such is a windlass, where an 
 axle is turned by a winch or handle : here the handle is virtually 
 the wheel, its revolution renders it a perpetual lever, its power 
 is the same as that of a wheel whose circumference is equal to 
 the circumference of the circle the handle describes, and there- 
 fore the machine is identified with the wheel and axle. 
 
 AB, fig. 11, is a wheel, and CD an axle fixed to and mov- 
 ing round with it. If the rope which goes round the wheel 
 be pulled, and the wheel be turned once round, it is evident 
 that so much rope will be drawn off as would encompass the 
 wheel; but while the wheel turns once round, the axle turns 
 once round, and consequently the rope by which the weight 
 is suspended will wind once round the axle, and the weight 
 will be raised through a space equal to the circumference of 
 the axle. Hence the velocity of the power will be to that of 
 the weight, as the circumference of the wheel is to the cir- 
 cumference of the axle. This being the case, the weight and 
 the power will be in equilibrium, when the one is to the other 
 as the circumference of the wheel is to the circumference of 
 the axle. 
 
 Mathematicians demonstrate, that the circumferences of dif- 
 ferent circles bear the same proportion to each other as their 
 respective diameters; consequently, the power and the weight 
 will balance each other, when the power bears the same pro- 
 portion to the weight that the diameter oHhe axis bears to the dia- 
 meter of the wheel. Thus, let fig. 2, pi. II, which in a preced- 
 ing page was considered as a pulley of two concentric grooves, 
 be now considered as a wheel and axle ; d e representing the dia- 
 meter of the axle, which we will suppose to be one inch; andy'g 
 the diameter of the wheel, which we will suppose to be six 
 inches; then one ounce acting as the power, S, will balance a 
 weight or resistance of six ounces acting as a weight, R. In 
 whatever form the machine presents itself, this proportion be- 
 tween the power and the weight will subsist, when the power is 
 applied to the circumference of the wheels or to a winch, as at 
 

MECHANICS. 313 
 
 Wlieel and axle. 
 
 E, fig. 11, and the weight to the axle. Therefore if W be 100 
 lbs. and the power, P, or a force acting at E, be equal to lOlbs. 
 supposing the diameter of the wheel, or the diameter of the 
 circle described by the winch, to be ten times greater than the 
 diameter of the axle, they will be in equilibrium, and a small 
 additional force will cause the wheel to turn with its axle, and 
 raise the weight, and for every inch which the weight rises, 
 the power will fall ten inches 
 
 When the wheel and axle are considered as a perpetual le- 
 ver, the fulcrum is the centre of the axle, the longer arm is 
 the radius of the wheel, and the shorter arm the radius of the 
 axle. From this again it is evident that the longer the wheel, 
 and the smaller the axle, the stronger is the power of the ma- 
 chine ; but then, as in other instances, the drawback on the 
 j>ower gained is the time lost, which is always proportionate 
 to the disparity between them. 
 
 A capstan is an axle or cylinder of wood, with holes in it, 
 in which levers are inserted, to turn it round ; these are like 
 the spokes of a wheel without the rim. 
 
 In some cases, the weight is not connected with the axle by 
 a rope, but is immediately affixed to the axle itself. A bell, 
 which is moved in ringing by a wheel and axle, is an example 
 of this : here, in once turning the bell round, the velocity 
 of the bell is as the circumference described by its centre of 
 gravity. 
 
 On the other hand, in what is called the circular crane, the 
 power is not applied to the wheel by means of a rope, nor 
 does it act upon any handle or spokes, but by a man walking 
 within the wheel ; as the man steps forwaid, the part upon 
 which he treads becomes the heaviest part of the wheel, and 
 descends till it is the lowest. Thus he keeps going on, and 
 by treading upon every part of the wheel's circumference in 
 its turn, the revolution is effected. This mechanical contriv- 
 ance is unwieldy, from the necessity of employing a large 
 wheel, and its power insignificant, because the man can ascend 
 so little from the bottom of the wheel. It is also unsafe for 
 the man, for if the rope suspending the weight give way, or 
 his feet slip, he is exposed to the greatest danger. 
 
 In considering the theory of the wheel and axle, the rope is 
 supposed to have no sensible thickness ; but if the rope is 
 thick, or if there is several folds of it round the axle, in ob- 
 taining the radius of the axle, the measurement must be made 
 to the middle of the outside rope; for it is plain, that the dis- 
 tance of the weight is as effectually increased by the coiling 
 of the rope as by any other means. 
 
 If teeth are cut in the circumference of a wheel, and if 
 14.— Vol. I. 2 S 
 
314 MECHANICS, 
 
 Wbeel and axle. 
 
 they work in the teeth of another wheel of the same size, as 
 fig. 1, pi. Ill, it is evident that both the wheels w ill revolve in 
 the same time ; and the weight appended to the axle of the 
 wheel B, will be raised in the same time as if the axle had 
 been fixed to the wheel A. But if the teeth of the second 
 wheel be made to work in teeth made in the axle of the first, 
 as fig. 2, every part of the circumference of the wheel D, is 
 applied successively to the circumference of the axle E, of the 
 first wheel C; and as E is much less than D, it is evident that 
 it must go round as many times oftener than D, as the circum- 
 ference of D exceeds its own circumference ; or, which amounts 
 to the same thing, if the number of teeth in the axle E, be 
 divided by the number of teeth in the wheel D, the product 
 will shew the number of revolutions which E will make for one 
 revolution of D. In order to obtain a balance here, between 
 the power P, and the weight W, the power must be to the 
 weight, as the product of the circumferences or radii of the 
 two axles multiplied together, is to the circumferences or 
 radii of the two wheels. This will become sufficiently clear, 
 if the whole be considered as a compound lever, the explana- 
 tion of which, as given in its proper place, will shew that fig. 
 2, requires the same proportion between the weight and the 
 power, and therefore is represented by the compound lever, 
 fig. 3. The dotted lines shew those halves of the wheels and 
 axle, of which the different parts of the lever are equal to 
 the radii. 
 
 Instead of a combination of two wheels, three or four, or 
 any number of wheels, may work in each other, and by thus 
 increasing the number of wheels, or by proportioning the 
 wheels to the axles, any degree of power may be acquired. 
 By increasing the length of the axle, by varying the sizes of 
 the wheels, and placing their teeth sometimes on the circum- 
 ference, and sometimes on the side of the rim, the action of 
 the power may be transmitted to a distance, the direction of 
 the movement changed, and any given velocity assigned to 
 particular parts. 
 
 What we have uniformly called the teeth of wheels, are not 
 in all cases distinguished by that name, though they always 
 are so when the work is small, as clockwork, and generally 
 when the wheels are made of metal, whatever be the size of 
 the work ; but in large works, where the wheels are of wood, 
 and the teeth are separate pieces mortised into the rim, they 
 are called cogs. 
 
 It may also be observed, that we have called the small 
 wheel E, fig. 2, pi. Ill, an axle, because in point of reasoning 
 as to the effect, it is the same thing whether it is really a 
 
MECHANICS. 315 
 
 Wheel and axle. — Application of the wheel and axle to the crane. 
 
 toothed axle, or a wheel of that, size upon a small axle. For 
 the sake of distinction, the small wheel E is called by me- 
 chanics a pinion, and sometimes a nut. Its teeth are also called 
 leaves. In large machines, trundles are frequently substituted 
 for pinions or nuts, being of easier manufacture, and per- 
 forming the same office. These trundles are cylinders or 
 spindles, parallel to each other, and placed circularly in two 
 plain pieces of wood at the top and bottom. The teeth of the 
 wheel then catch the spindles of the trundle, as they would 
 do the teeth of a nut or pinion. 
 
 That useful machine, the crane, so much employed in rais- 
 ing or lowering goods, is indebted for its value principally to 
 the wheel and axle. Cranes are very variously constructed, 
 but the object in all of them is to manage a great weight with 
 an inconsiderable power. An elevation of a crane, the gene- 
 ral construction of which is now becoming very prevalent in 
 London, is shewn in fig. 4, pi. III. It is approved, from its 
 requiring no very expensive frame-work, and because it can 
 be turned all round. AB is a stout beam, turning in a cast- 
 iron collar at B, affixed to the beams in the floor of the 
 wharf; it goes down about twelve feet below this, and has a 
 steel pivot at the lower end, which works in a brass socket 
 or collar, so that the beam AB can turn freely round with- 
 out shaking. CD are the two beams of the jib, with a fixed 
 pulley at E, over which the chain for twisting the goods 
 works. The other end of this chain winds round the axle e, 
 of the great wheel F, of 98 teeth ; this wheel works in a 
 pinion of seven leaves, on the same axle with the wheel G 
 of 35 teeth. The wheel G, works in a pinion, H, of 14 teeth. 
 When a great power is required, the winch handle is ap- 
 plied to a square on the end of the axis or spindle of this 
 pinion ; but for a less weight, the winch is put on the axle of 
 the wheel G. In this case, to lessen the friction, the pinion 
 may be disengaged, or, as it is usually called, thrown out of 
 gear, by sliding its axle lengthways. For this purpose, it 
 must be provided with two grooves, and must have a clip or 
 catch to fall into one of these in either of its situations. The 
 frame containing the wheels is formed by two cast-iron 
 crosses, bolted to the main beam, AB, by their vertical arms, 
 of which IK are the two in front. 
 
 Machines of this description should be furnished with a 
 ratchet wheel, as m, with a catch to fall into its teeth. This 
 wheel will at any time support the weight, and keep it from 
 descending, if the person who turns the handle should, through 
 ^ inadvertence or carelessness, quit his hold while the weight is 
 suspended. This very easy mode of preventing the danger 
 
3\C) MECHANICS 
 
 Tbe inclined plane. 
 
 uhicli w ouki result IVom the running down of the weight, if 
 left at liberty, should never be omitted. 
 
 Or THE Inclined Plane. 
 
 The inclined plane is any flat surface which forms an angle 
 less than a right angle, with the plane of the horizon. When 
 hogsheads, or pipes of wine, SvC. are to be let down into a cel- 
 lar, or brought up out of it, a plank is laid along the stairs, 
 and thus forms an inclined plane. 
 
 The time which a rolling body takes to descend upon an in- 
 clined plane, is to the time in which it would descend vertically 
 by its absolute gravity, in free space, from the highest part of 
 the plane, in the ratio or proportion which the length of the 
 plane bears to its perpendicular height. The whole theory of 
 the inclined plane rests upon this principle. 
 
 Suppose the plane AB, fig. 5, pi. Ill, to be parallel to the 
 horizon, the cylinder, C, will remain at rest on whatever part of 
 the plane it is laid. If the plane be placed perpendicularly, as 
 AB, fig. 6, the plane will contribute nothing to the support of 
 the cylinder, C, which will therefore descend with its whole 
 force of gravity, or require a power equal to its whole weight 
 to keep it from descending. But suppose AB, fig. 7, to be a 
 jplane parallel to the horizon, and AD a plane inclined to it; 
 if the whole length AD be three times as great as the perpen- 
 dicular DB, the cylinder C will be supported upon the plane, 
 or kept from rolling down, by a power equal to a third part of 
 the weight of the cylinder; it is clear, therefore, that a weight 
 may be rolled up this inclined plane by a third part of the 
 power which would be required to draw it up by the side of 
 an upright plane, as AB, fig. 6, -where the force required must 
 be equal to its whole weight ; but it is to be noticed, that upon 
 the inclined plane the weight goes over three times the space; 
 which proves that, as in all other cases, we only substitute 
 time for power. 
 
 As the horizontal plane, fig. 5, supports the whole weight of 
 the cylinder, C, and the vertical plane, fig. 6, or plane perpen- 
 dicular to the horizon, supports none of it; so, in all cases, the 
 less the angle of elevation, or the gentler the ascent is, the 
 greater will be the weight which a given power can draw up ; 
 for the steeper the inclined plane, the less does it support of 
 the weight, and of course the greater the tendency the weight 
 has to roll. 
 
 The w^eight is always most easily either drawn or pushed in 
 a line g r, parallel to the plane, and passing through the centre 
 of the weight ; for if one end of the line be fixed at g, and the 
 other end inclined towards D, the cylinder C would be drawn 
 
MECHANICS. 317 
 
 The wedge. 
 
 ao-ainst the plane, and tlie power must be increased in propor- 
 tion to the greater difficulty of the line of traction; also if the 
 line be carried above r, the power must be increased, but in 
 that case only in proportion as it endeavours to lift the body 
 off" the plane. In stating the power necessary to support the 
 cylinder C, on the inclined plane AD, the parallel direction of 
 the line of traction, as it is the most favourable one, is taken 
 for granted. 
 
 In estimating the draught of a waggon or other vehicle up- 
 hill, the draught on the level must be added. Suppose the 
 hill rises one foot in four, then one-fourth part of the weight 
 must be added to the draught on level ground. If the weight 
 were 12 cwt. and its draught on the level wtis U cwt. then one- 
 fourth of 12 cwt. or 3 cwt. added to U cwt. would give 4i cwt. 
 for the real draught necessary to draw 12 cwt. up a hill rising 
 one foot in four. Hence may be discerned the great disad- 
 vantage of hilly roads, and the propriety of loosing a little time 
 in winding about hills where it is possible, rather than exhaust 
 the power of the horses in passing over them. 
 
 Of the Wedge. 
 
 The fifth mechanical power or simple machine, is the wedge, 
 which is a piece of wood, metal, or any firm material, thin at 
 one end and thick at the other. The thin end is called the 
 point or edge, and the thick end is called the head or base of 
 the wedge. 
 
 The action of the wedge resembles most that of the inclined 
 plane, but the theory of it is by no means complete. The 
 wedge, when driven, as it mostly is, by percussion, for example, 
 by the blow^ of a hammer, produces an effect differing consi- 
 derably from that of pressure, so as not to admit of exact 
 calculation. A weight of five hundred pounds, pressing on 
 the back of a wedge, will often make very little impression 
 upon a body, which a hammer weighing only two pounds, 
 if, when the force of the blow is extinguished, it have ac- 
 quired a velocity rendering its momentum equal to five hun- 
 dred jjounds, would instantly sever. This great difference of 
 effect between percussion and pressure, when the momenta 
 are equal, is perhaps owing to the tremulous motion or vibra- 
 tion producedby percussion amongthe parts of thebody, which, 
 when so agitated, and the wedge has once entered, has the fric- 
 tion between its sides and that of the wedge lessened, as well as 
 the cohesion of its parts diminished. Until therefore we are 
 thoroughly acquainted with the nature and force of the tenacity 
 of bodies, and the activity with which they vibrate under a 
 
318 MECHANICS. 
 
 The wedge. 
 
 given impulse, (and these are })ranches of knowledge to which 
 pliilosopliers are as yet nearly strangers,) the theory of the 
 wedii-e will not be susceptible of much precision. We are not, 
 however, wholly in the dark, and the following mode of re- 
 garding the subject is generally approved. 
 
 AB, fig. 8, pi. HI, is a wedge driven into the cleft CDE, of 
 the wood FG. When the wood does not cleave at any dis- 
 tance before the wedge, there will be an equilibrium between 
 the power impelling the wedge downward, and the resistance 
 of the wood acting against the two sides of the wedge, when 
 the power is to the resistance as half the thickness of the wedge 
 at its back, that is, from A to B, is to the length of one of its 
 sides ; because the resistance then acts perpendicularly to the 
 sides of the wedge. 
 
 When the wood cleaves at some distance oefore the wedge, 
 which is generally the case, the power impelling the wedge will 
 not be to the resistance of the wood as the thickness of the back 
 of the wedge is to the length of both its sides, but as half the 
 thickness or length of the back is to the length of either side 
 of the cleft, estimated from the top or acting part of the wedge. 
 The reason of this is, that if we suppose the Avedge to be 
 lengthened, so as to reach the bottom of the cleft at D, the 
 same proportion will hold ; namely, the power will be to the 
 resistance as half the length of the back of the wedge is to 
 the length of either of its sides; or, which amounts to the 
 same thing, as the whole length of the back is to the length 
 of both the sides. 
 
 The thinner a wedge is at the back, that is, the more vicute 
 the angle of its longitudinal section, the more powerful is its 
 action, or the greater the effects which may be produced by 
 the same force. 
 
 When this instrument is employed to cleave a hard body, 
 the parts of which strongly adhere together, its advantage is 
 augmented in proportion as the wedge is sunk or driven 
 deeper between these parts. For example, if the piece of 
 wood, FG, have three bandages, /•, s, 4, all of equal strength, 
 and which may represent the strength with which the parts of 
 the wood cohere, the wedge may be considered as acting by 
 the anns DE, DC, of two angular levers. If then the force of 
 the wedge, exceeds a little that of the first bandage r, this 
 bandage will be severed. The second bandage, s, though as 
 strong as the first, will be more easily broken by the action 
 of the same wedge, because the arms of the lever by which 
 it then acts are lengthened by the quantity ;• s; and the 
 increased facility with which the last bandage will be 
 broken, will, in like manner, be proportionate to the in- 
 
MECHANICS. 319 
 
 The screw. 
 
 creased length of the arms of the lever formed by the sides of 
 the cleft. 
 
 The wedge is a mechanical power of singular efficacy, and 
 the percussion by which its action is obtained, is precisely that 
 force which we can, with the greatest convenience, almost in- 
 definitely increase. By means of the wedge, the walls of houses 
 may be propped, rocks split, and the heaviest ships raised up; 
 — operations to which the lever, the wheel and axle, and the 
 pulley, are either ill-adapted, or entirely incompetent. 
 
 To the wedge are referred the axe, the spade, chisels, 
 needles, knives, punches, and, in short, all instruments which, 
 beginning with edges or points, grow gradually thicker. A 
 saw is a number of chisels fixed in a line ; and a knife, if its 
 edge be examined with a microscope, will be found to be only 
 a fine saw. 
 
 Of the Screw. 
 
 The sixth and last mechanical power which we have to 
 notice, is the screw. 
 
 The screw, strictly speaking, consists of two parts, which 
 work within each other. One of these parts, and which is 
 always meant when the word screw is used alone, is a solid 
 cylinder, on the circumference of which is cut a spiral groove; 
 it is, as we have formerly observed, when specifically named, 
 called an outside or convex screw. The other part, is a JioUoxo 
 cylinder, or, at least, whatever its external form may be, it 
 contains a cylindrical hole, within which is cut a spiral 
 groove corresponding to that of the convex screw, which can 
 be turned within it, and the spiral projections of the one lock 
 into the spiral hollows of the other. For the sake of neces- 
 sary contradistinction, this latter part is called an inside, a 
 concave, or socket screw, when spoken of generally, without 
 reference to any other use than its principal one, of an indis- 
 pensable companion to the convex screw ; but when it con- 
 sists of a small piece of metal, as for drawing tight bolts of 
 any description, it is most commonly called a tiut ; and when 
 it is of considerable size, as for a large press or vice, it is 
 usually called a box. 
 
 The thread of a screw is its spiral projection; the pace or 
 step of a screw is the distance between the threads ; and the 
 groove or gorge is the hollow between the threads. 
 
 To obtain an idea of the nature of the screw, and of its 
 affinity to the inclined plane, cut a piece of paper in the form 
 of an inclined plane, or half wedge, as LMN, fig. 9, pi. Ill, 
 and then wrap it round a cylinder, fig. 10 ; the edge of this 
 plane or paper, LMN, will form a spiral round the cylinder, 
 
320 MECHANICS. 
 
 The screw. 
 
 which will give the thread of the screw. The height of the 
 plane is the pace of the screw, or distance of one thread from 
 another; its base is the circumference of the screw, and its 
 length is estimated by this circumstance and the height of 
 the pace. 
 
 A screw is seldom used without the application of a lever 
 to assist in turning it; it then becomes a compound machine 
 of great force, either in compressing the parts of bodies to- 
 gether, or in raising great weights. As the lever or winch 
 must turn the cylinder once round, before the weight or re- 
 sistance can be moved from one spiral winding to another, or 
 before the screw working in its box can rise or sink the dis- 
 tance between the threads as from a to b, therefore as much 
 as the circumference of the circle described by the lever is 
 greater tlian the pace of the screw, or distance between the 
 threads, so much does the force of the screw exceed the motive 
 force. For example, suppose the pace or distance of the 
 threads to be half an inch, and the length of the lever 12 
 inches, the circle described by the extremity of the lever 
 where the power acts, will be about 76 inches, or 152 half 
 inches, consequently 152 times as great as the distance be- 
 tween two contiguous threads ; therefore, if the intensity of 
 the power at the end of the lever, be equal to one pound, that 
 single pound will balance 152 pounds acting against the screw. 
 If as much additional force be exerted as is sufficient to over- 
 come the friction, the 152 pounds may be raised; and the ve- 
 locity of the power will be to the velocity of the weight as 152 
 to 1. Hence we may clearly perceive, that the longer the 
 lever, and the nearer the threads to one another, so much the 
 greater is the force of the screw. 
 
 The friction of the screw is very great, but we are indebted 
 to this circumstance for a peculiar advantage in the use of this 
 machine, which will sustain a weight, or press upon a body 
 against which it is driven, after the power is removed or ceases 
 to act. To enumerate all the uses of the screw would be im- 
 possible. Among other purposes, it is a])plied to great ad- 
 vantage for measuring or subdividing small spaces ; when thus 
 applied it is called a micromcler, which may be made to indi- 
 cate on an index plate, a portion of a turn, advancing the screw 
 less than the hfty thousandth part of an inch. 
 
 The threads of screws are differently formed, according to 
 the materials of which they are made, or the use for which they 
 are intended. The threads of wooden screws are generally an- 
 gular, that they may rest upon a broad base, and thereby have 
 their strength increased to the utmost. Small screws, whatever 
 material they are made of, are generally angular also, not only 
 
MECHANICS. 321 
 
 The endless crew. 
 
 for the same reason as the wooden ones, but because the an- 
 glnar thread is the most easily made. The metal screws which 
 are used for large presses, vices, Jcc. generally have a square 
 thread, a form which gives great steadiness of motion. A 
 thread, of which the sides are parallel, and the top and bottom 
 a little rounded, is perhaps the most perfect of all forms. 
 
 In the common screw, to which the preceding observations 
 are exclusively applicable, the threads are one continued spiral, 
 from one end to the other; but where there are two or more 
 separate spirals running up together, as in the worm of a jack, 
 or the principal screw of a common printing-press, the descent 
 of the screw in a revolution will be proportionately increased ; 
 and therefore whatever be the number of the spirals, they must, 
 in calculating the power, be naeasured and reckoned as one 
 thread. 
 
 Of the Endless Screw. 
 
 A screw is sometimes cut on an axle, to serve as a pinion, 
 and by working in the circumference of a wheel ; it is then 
 called an end/ess screw, because it may be turned perpetually 
 without advancing or receding, that is, without any other mo- 
 tion than a rotary one. The threads of this screw are of the 
 square form, and fit exactly into the spaces between the teeth 
 of a wheel, which teeth are cut obliquely to answer to the 
 threads. When the endless screw has been turned once round, 
 the wheel has only made a portion of a turn equal to the dis- 
 tance between one of its threads, that is, the wheel has moved 
 one tooth, and therefore the number of its teeth is always the 
 same as the number of the revolutions made by the screw 
 before it is once turned round. 
 
 This construction and mechanical advantage gained by this 
 screw may be best illustrated by a figure; let the wheel C, fig. 
 11, pi. Ill, have an endless screw B, on its axis, working in a 
 wheel D, of 48 teeth. The screw B, and the wheel C, being 
 on the same axis, every time they are turned round by the 
 winch, the wheel D will be moved one tooth forward by the 
 screw, and therefore 48 revolutions of the winch will be re- 
 quired to turn the wheel D once round. Then, if the circum- 
 ference of the circle described by the handle of the winch A, 
 be equal to the circumference of a groove round the wheel D, 
 the velocity of the handle will be 48 times as great as the ve- 
 locity of any given point in the groove ; consequently, if a line 
 G, goes round the groove, and has a weight of 48 pounds hung 
 to it, a power equal to one pound at the handle will balance 
 and support that weight. If any apparatus were constructed 
 for the purpose, this might be proved by making the circum- 
 14.— Vol. I. 2 T 
 
32*2 MECH ANTICS. 
 
 Tbe endless screw 
 
 ferences of the wheel C and D equal to one another; and tlien 
 if a weiirht H, of one pound, were suspended by a line going 
 round the groove of the wheel C, it would balance a weight of 
 48 pounds hanging by the line G, and a small addition to the 
 weight H will cause it to move the weight G as in every other 
 case after the equilibrium takes place. 
 
 If a line G, instead of going round the groove of the wheel 
 T>, goes round its axle I, the power of the machine will be as 
 much increased as the circumference of the groove exceeds the 
 circumference of the axle. If then the circumference of the 
 groove, be six times greater than the circumference of the axle, 
 one pound at H will balance six times 48, or 288 pounds, hung 
 to the line on the axle ; the power gained will therefore be as 
 288 to 1 ; and a man whose natural strength would enable him 
 to lift one hundred weight, will be able to raise 288 hundred 
 weight by this engine. 
 
 The use of the endless screw affords a very ready means of 
 greatly diminishing a rotary motion, and accomplishes at once 
 what would otherwise require the intervention of two or three 
 wheels; and although it operates wholly by a sliding motion, 
 it has not probably more friction than any of the less simple 
 combinations which might be employed to effect the same ob- 
 ject. It possesses the advantage, too, of moving a wheel with 
 much more steadiness than a pinion, when the workmanship 
 of both is of equal quality. This circumstance is not so much 
 regarded by mechanics as perhaps it ought, and therefore the 
 endless screw is often not used when it would be advantageous. 
 Cast iron wheels are becoming increasingly general for all 
 kinds of large machinery requiring wheel-work. Their dura- 
 bility, and the little room they take up, when compared with 
 the wooden ones, to answer the same purpose, entitle them to 
 a decided preference; but still there are many great impedi- 
 ments to their being made true. They must necessarily par- 
 take of the defects of the pattern made for casting them ; very 
 few millwrights make good patterns, and even their best efforts 
 are sometimes rendered, in some measure, abortive by the 
 warping or shrinking of the wood they have used. But sup- 
 posing all diificulties with respect to the pattern to be con- 
 quered, the liability to incorrect work from the imperfections 
 of the niould, the presence of stagnated air, and the unequal 
 contraction of different j)ortions of the metal, are very consi- 
 derable, and never absolutely overcome by the most careful 
 workmen. It is true that cast iron wheels are better than any 
 other, and that they may be rectified by hand ; but this ope- 
 ration, if carried beyond a certain point, would prove very ex- 
 pensive; it is besides for other reasons indesirable, and seldom 
 
MECHANICS. 323 
 
 The nature of the advantages of machines. 
 
 attempted in any great degree, because, as the surface of cast 
 iron is far harder than the interior, it would remove that por- 
 tion of the metal which always wears the best. Whoever, 
 therefore, attends to the motions of machinery, will frequently 
 observe some parts to move by jolts or starts; the cause of 
 which is, the inaccuracy of the teeth or cogs. In consequence 
 of this, engineers, when they want a steady motion, as for a 
 lathe, resort to the expedient of employing a large train of 
 wheels. They thus commonly attain their prime object, for 
 the probability is, that opposite imperfections will balance each 
 other; but they have introduced a great increase of friction, and 
 a consequent necessity for a greater motive force, not to men- 
 tion the expense of the additional parts of the machinery. To 
 avoid in part these disadvantages, they might more frequently 
 employ the endless screw, for the reasons already alleged. 
 The principal restriction to the use of this screw is, its being 
 so liable to wear when its motion is very rapid; a rapid motion, 
 therefore, should not be assigned to it unless it be made of 
 hardened steel, when the objection will not apply. 
 
 Of Compound Machines. 
 
 When two or more of the simple mechanical powers are 
 made to act in conjunction, to produce a given effect, the con- 
 trivance resulting from the union is called a compound machine 
 or engine. 
 
 Though any one of the mechanical powers is capable of 
 overcoming the greatest possible resistance in theory ; yet, in 
 practice, if used singly, they would frequently be so unma- 
 nageable as to render their properties nugatory. It is there- 
 fore generally found best to combine them together; by which 
 means the power is more easily applied, and many other ad- 
 vantages obtained. 
 
 As the mechanical powers, in whatever manner they are com- 
 bined, still preserve their properties ; so, in compound as in 
 simple engines, whatever is gained in power is lost in time ; 
 consequently, if a given power will raise one pound with a 
 given velocity, it will be impossible for that power, by the help 
 of any machine, to raise two pounds with thesame velocity ; yet, 
 by the assistance of a machine, two pounds may be raised with 
 half that velocity, or one thousand pounds with a thousandth 
 part of it; but still there is no greater quantity of motion pro- 
 duced when a thousand pounds are moved, than when only 
 one pound is moved ; because the greater weight moves pro- 
 portionately slower than the lighter. The power, then, of ma- 
 chines, consists only in this, that by their means the velocity 
 of the weight may be diminished at pleasure, so that with a 
 
324 MECHANICS. 
 
 Simplicitj of machines to be studied. — Models not alivajs to be depended on. 
 
 pjiven force any given resistance may be overcome; and the 
 advantages they aHbrd us are confined to convenience; for in- 
 stance, by machines, we are enabled to assign a convenient 
 direction to the moving power, and to apply its action at some 
 distance from the body to be moved, which are circumstances 
 of the highest importance. By machines, also, we can so 
 modify tlie energy of the moving power, as to produce eti'ects 
 not otherwise obtainable. 
 
 In contriving machines, simplicity of parts should always 
 be studied; for in general the more complex they are, the 
 more frequently they are out of order, and the more difficult 
 to repair; the more expensive also at first, and the greater 
 their friction, from the number or extent of their rubbing 
 parts. A very complex engine may indeed be a proof of the 
 ingenuity of the inventor ; but if that be all, as soon as a more 
 simple mode of effecting the same purpose shall be known, 
 the precarious tenure of his celebrity will soon be evident, 
 and he will be convinced that ingenuity without science is 
 exerted to very little purpose ; and the display of science is 
 always the more complete, the fewer the parts of a machine, 
 or the more simple the means by which a given purpose is 
 attained. 
 
 To those who have a mechanical invention in view, and 
 who are as yet possessed of little practical knowledge, it may 
 prevent some disappointment to observe, that the performance 
 of the models, or small machines made for the sake of trial, 
 may equal their expectations, yet the action of such a machine 
 upon a large scale, may not prove permanently advantageous, 
 nor perhaps capable even for a short time of supporting an 
 analogous action. A large machine has not the same relative 
 strength as a small one; it frequently admits not of the same 
 excellence of workmanship, and in general it cannot be made 
 of materials so durable or possessed of so little friction. On 
 the contrary, it will sometimes happen, that a model will per- 
 form indifferently, though a large machine on the same plan 
 will give satisfaction; this may occur, when some of its parts 
 are so minute, and of such a description, that a common de- 
 gree of manual skill is insufficient to form them correctly. 
 Experience alone can effectually teach the art of making the 
 proper allowances in these different cases; and the mere study 
 of the theory of the mechanical powers, will therefore supply 
 the mechanic with but a small portion of the knowledge which 
 he ought to possess. 
 
 To discover the mechanical power of any engine, it will be 
 sufficient to measure the space described in the same time by 
 the power and the resistance, or weight; for the power always 
 
MECHANICS. 32S 
 
 Wbeel-work. 
 
 balances the weight, when it is in the same proportion as the 
 velocity of the weight to the velocity of the power. Or, divide 
 the machine into all the simple ones of which it is formed ; 
 then begin at the power and call it one, and by the properties 
 of the mechanical powers find the forces in numbers which the 
 first simple machine exercises upon the second. Call this force 
 one, and find the force in numbers with which it acts upon the 
 third ; and putting this force as one, ascertain its action on the 
 fourth in numbers, and so on to the last. Then multiply all 
 these numbers or individual ratios of the power to the weight 
 together, and the product will be the force of the machine, 
 supposing the first power to be one. This has been exem- 
 plified by the method of calculating the power of a compound 
 lever. 
 
 In wheel-work, it is evident from the principles already laid 
 down, that the velocity of a wheel is to that of a pinion, or 
 smaller wheel which it drives, or by which it is driven, in pro- 
 portion to the diameter, circumference, or number of teeth in 
 the pinion to that of the wheel. If then the teeth in a wheel 
 amount to 80, and the leaves in a pinion to ten, the pinion will 
 go 8 times round for one revolution of the wheel, because 80 
 divided by 10, gives 8 for the quotient. 
 
 If the product of the teeth in any number of wheels acting 
 on so many pinions, be divided by the product of the teeth or 
 leaves in the pinions, the quotient will give the number of 
 turns of the last pinion in one turn of the first wheel. Thus, 
 if a wheel A, (fig. 12, pi. Ill,) of 48, acts on a pinion B of 8, 
 on the axis of which pinion there is a wheel C of 40, driving 
 a pinion D of 6, carrying a wheel E of 36, which moves a 
 pinion F of 6, carrying an index ; then the number of turns 
 made by the axis of the last pinion, and consequently by the 
 index, will be found in this manner: *^ x -^^ x 3/=:^f^°=:240, 
 which are the number of turns made by the index, for one turn 
 of the wheel A. 
 
 It will be evident on a little consideration, that whatever 
 may be the number of teeth in the wheels and pinions, if they 
 bear the same ratio, they will give the same number of revo- 
 lutions to an axis. Thus, |*x ^-£ x •■'/ = i l^-^o = 240, as before. 
 The numbers, therefore, may be varied at the discretion of the 
 engineer, whose design must of course regulate his choice. 
 One wheel and pinion, it is also evident, will give the same 
 motion as many wheels and pinions, if the number of teeth 
 contained respectively in the single wheel and pinion, bear to 
 each other the same proportion as the product of the teeth in 
 a train of wheels bears to the product of the teeth in the 
 pinions belonging to that train. 
 
320 MECHANICS. 
 
 Example of the luode of calculating tbe power of a componntl machine. 
 
 When a wheel is moved immediately by tlie power, it is called 
 a leader : and if there be another wheel or pinion on the same 
 axis, it is called aj'u//ower. The leader receives, and the fol- 
 lower imparts the motion. 
 
 As an example of the methoci of calculating the power of a 
 compound machine, we shall refer to the crane, fig, 4, pi. III. 
 When the handle is affixed to the axis of the wheel H of four- 
 teen teeth; if the length from / to the centre of the square end 
 of the axis upon which it is fastened, be four times the semi- 
 diameter of the pinion II, then is the acting part of the lever 
 four times the length of the resisting part; and it will act upon 
 the wheel G with four times the eli'ect of the same power ap- 
 plied directly to G, as for example in pulling it round by its 
 teeth ; therefore, if the man at the winch exert a power equal 
 to thirty pounds, the effect of it on the wheel G will be equal 
 to one hundred and twenty pounds. If then G be turned by 
 H with a force of 120, and having two and a half times as 
 many teeth, it only makes one revolution while H makes two 
 and a half, its pinion will exert a force of twice 120 added 
 one half, or 300 on the wheel F. But as the wheel F has 
 14 times as many teeth as the pinion by which it is driven, 
 its revolutions will be 14 times fewer than that pinion, and 
 the power will be proportionately increased ; therefore 14 
 times 300, which amounts to 4,200, expresses the weight 
 which, if appended to the circumference of the wheel F, 
 would keep a power of 30 at the winch in equilibrium. The 
 weight, however, is not appended to the circumference of 
 the wheel F, but to the circumference of its axle, which 
 being four times less, its velocity is in the same proportion 
 diminished, and its intensity must be quadrupled to have the 
 same effect on the power; so that 4 times 4,200, or 16,800 
 pounds will express the whole weight which a power of 30 
 pounds applied to the wincli when on the axis of the wheel H, 
 will be able to keep in equilibrium; and when the equilibrium 
 is produced, so small an addition to the power gives motion to 
 the weight, that it is seldom mentioned, the power which 
 effects the equilibrium being generally spoken of as if it were 
 actually sufficient to raise the weight. We have supposed the 
 crane to have but one winch, but such machines are generally 
 provided with two, one at each extremity of the same axle ; 
 and it follows, that a double intensity of power will overcome 
 a double resistance. We have also supposed the power to 
 be 30 pounds, because that is about the intensity of the power 
 which one man could exert on the winch ; but as, in the cus- 
 tomary way of expressing the advantage gained by a ma- 
 chine, the power is considered to be 1, to suit this mode of 
 
MECHANICS. 327 
 
 Importance of machinery to Great Britain. 
 
 expression, 16,800 must be divided by 30, and we shall then 
 have a quotient of 560, which number is to i, as 16,800 is to 
 30, and it shews how much the velocity of the power exceeds 
 that of the weight, when the velocity of the latter is 1, — the 
 difference of these velocities denoting the power of the ma- 
 chine, and being synonimous with the expression, that a power 
 of one pound would raise 560 pounds. 
 
 Another mode of estimating the power of the crane, fig. 4, 
 pi. Ill, will be, instead of proceeding from wheel to wheel, to 
 divide 3420, the product of the teeth in the wheels F and G, 
 by 98, the product of the teeth in the wheel H, and the pinion 
 on the axis of G; the quotient of this division will be 35, 
 which, if multiplied by 4, the difference between the radius of 
 the circle described by the winch and the radius of the wheel 
 H; and again by 4, or the difference between the radius of the 
 axle round which the chain coils, and the radius of the wheel 
 F ; — the product will as before be 560 for the power gained. 
 
 The practical application of mechanics to the construction 
 of machinery, is a subject of the utmost importance to our 
 country ; the prosperity of which materially depends upon its 
 commerce ; its commerce is derived chiefly from its manufac- 
 tures ; and some of its most important manufactures are 
 indebted to the general introduction of machinery for the pre- 
 ference they every-where receive. It is to this source we must 
 attribute the increase of property of every description, as the 
 introduction of a machine is a virtual creation of all the work 
 it will perform without farther increasing human labour. 
 Among many, it is a prevalent opinion, that machinery is pre- 
 judicial to the interests of mankind, from its supposed tendency 
 to diminish the amount of that labour by which the lower 
 classes of society can alone purchase the means of subsistence. 
 This opinion is, however, erroneous, as applied to society in 
 general ; though individuals, whose labours are superseded by 
 machines, may suffer inconvenience for a time, yet it is only 
 for a time, and until they, or others more intelligent, discover 
 a new channel for the exertion of their industry. If improve- 
 ments in machinery enable our manufacturers to offer their 
 goods at lower prices than before, these goods will command 
 a sale in foreign markets where it was previously useless to 
 carry them. The result of their success will give them, and 
 consequently their country, an accession of capital ; and the in- 
 evitable operation of an accession of capital, will be for the 
 advantage of the labouring classes, because it can no way be 
 expended, without increasing the general amount of labour, 
 and therefore making them in the end full participators in its 
 benefits. Thus are the real interests of all ranks inseparately 
 
328 MECHANICS. 
 
 ^ alue of raacbinery. — Importance of aniformitv of motion. 
 
 connected ; and a retrospect of the last forty years will shew, 
 that though so many machines have been employed in all 
 trades and manufactures, as probably to do more work, than 
 the whole population could do previous to that period, yet the 
 value of human labour has increased in the same proportion 
 as other articles have advanced in price, except so far as this 
 natural tendency of things has been checked by the regula- 
 tions of war. As machines tend to increase the quantities 
 of those luxuries and necessaries of life which mankind are 
 so anxious to obtain, it only requires that an equitable divi- 
 sion of these benefits should be effected, and then every ob- 
 jection to them will be obviated. But such a division is not 
 to be obtained by the infatuated violence of individuals, nor 
 by legislative or municipal constraints. On the contrary, this 
 benefit can only spring: with full vigour from the ashes of all 
 monopolies, combinations, or exclusive immunities in trade. 
 Let every individual be at all times free from the least restric- 
 tion or check in the pursuit of any business consistent with 
 the common weal, every one would be a gainer by the change; 
 if one source of employment failed, abundance of other sources 
 would still be open ; reward would be proportioned to inge- 
 nuity; every branch of art would be occupied by those who 
 were led to it more by choice than accident, and who were 
 anxious to excel as much for their credit as from necessity. 
 The aspect of the country would then be improved ; more 
 cheerful industry would enliven it ; that laxity of moral con- 
 duct which is produced by insubordination, and fostered by 
 the protecting arm of injudicious immunities, would cease to 
 be so flagrant, if it did not disappear; and the advancement 
 of machinery, far from being viewed by any class with malig- 
 nant eves, would be regarded as an increasing antl inexhaus- 
 tible source of national prosperity. 
 
 If we supposed the preceding observations on the utility 
 of machinery were not radically true, it would be an invidious 
 task to pursue the present subject ; but unassailed by any 
 apprehension of this kind, we shall make them the prelude to 
 some general remarks which may be useful to the young 
 mechanic. 
 
 In contriving machinery, it should always be remembered, 
 that nothing will contribute more to its perfection, especially 
 if it be massive and ponderous, than great uniformity of mo- 
 tion. Every irregularity of motion wastes some of the impel- 
 ling power; strains, jolts, and whatever occasions a vibratory 
 motion of the parts within themselves, weaken the coh«»sion 
 of the most solid substances, and are particularly injurious to 
 cast iron, and the pressures at the communicating points are 
 
MECHAXI C S 
 
 PL.m. 
 
 i..>wry. 
 
 I^U'Ushcd du f/cn/y /is/ur. Carton, /.on don. 
 
MECHANICS. 329 
 
 Communication of motion by levers — cranks — toothed wheels. 
 
 inconstant and unequal. A great engine, constructed without 
 due regard to the uniformity of its motion, will shake the 
 firmest building; but when uniform motion pervades the whole, 
 the inertia of each part tends to preserve this uniformity, and 
 all goes smoothly. 
 
 In modifying the motion of the first mover, and communi- 
 cating it in a proper manner to tlie subject to be operated 
 upon, the slow rotative motion of a water-wheel is, by the 
 machinery of cranks, levers, and toothed wheels, converted 
 into a rapid reciprocating motion for working saws; and the 
 velocity of the motion is increased or diminished, as the oc- 
 casion requires either great power or great speed. In like 
 manner, the rectilinear motion of the piston rod of a steam 
 engine is, by. the machinery of parallel levers, working beam, 
 connecting rod, crank and fly-wheel, converted into a rotative 
 motion ; and this motion is again, by the machinery of wheel- 
 work, adapted to work grinding stones, circular saws, thresh- 
 ing mills, and other similar machines which require great ve- 
 locity ; or flatting mills, boring machines, machines for rasp- 
 ing dyewoods, drawing lead pipe, &c. which require great 
 power to give them motion, and are therefore performed with 
 less velocity. 
 
 In modifying a rotary motion, toothed wheels are most gene- 
 rally employed; and if the teeth are properly formed, wheels, 
 perhaps, consume less force in friction than any other mode 
 of transmitting motion. In forming the teeth of wheels, a de- 
 viation from the perfect form is of most importance where a 
 very large wheel drives a very small one, a case the judicious 
 engineer should always endeavour to avoid. It is of great im- 
 portance to make all the teeth of a wheel precisely equal, and 
 to make as great a number of them as the necessary strength 
 will allow. The greater the number of the teeth, the less will 
 be the time that any one of them will act upon its fellow, and 
 several teeth being in action at once, will cause the communi- 
 cation of the motion to be extremely smooth and uniform. To 
 obtain strength, when the cogs are made fine, the width or 
 thickness of the wheel must be increased ; and this is one of the 
 greatest practical improvements which have been made in ma- 
 chinery for twenty years past. When the teeth are far apart, 
 three, four, or five inches, for example, they always act un- 
 equally upon each other, in consequence of the point of con- 
 tact altering its position, becoming alternately nearer or far- 
 ther from the centre of one or other of the wheels; the acting 
 radius of one is thus increased, while that of the other is di- 
 minished, and their velocity and powers varying in consequence 
 with every cog that passes by, the machine works by starts 
 14.— Vol. I. 2 U 
 
330 MECHANICS. 
 
 ('umniiiiiirutlon uf motion bv endless strups and hands. 
 
 and jerks. Wheels are found to work most smoothly, when 
 the teeth of the large wheel are made of hard wood, and the 
 teeth of the small one of cast iron. 
 
 A rotary motion is very frequently transmitted by means of 
 an endless strap, or belt, particularly when a very quick mo- 
 tion is to be created, and the re-action to be overcome is nearly 
 equable. In such cases, it has the advantage of wheel-work 
 from its simplicity, tiie ease of its motion, and the distance to 
 which it may be convoyed. A strap should always work on 
 a pulley which is highfjst in the middle of its circumference, 
 otherwise it will be exceedingly apt to slip oft'. If one of the 
 pulleys is stopped while the strap is moved round by the mo- 
 tion of the other, the strap instantly flies oft' its pulley, unless 
 the breadth of the circumference of the pulley greatly exceeds 
 that of the strap. This property is a great recommendation 
 of it on many occasions, where the machines might be much 
 injured or destroyed if driven by wheel-work and accidentally 
 stopped. Straps shoidd be of an equal thickness and breadth 
 throughout. They are mostly joined by sewing; but the 
 best method is by gluing them together, with a glue com- 
 pounded of Irish glue, isinglass, ale grounds, and boiled lin- 
 seed oil. The ends that overlap should be pared thin, taper- 
 ing to the edge in the form of an inclined plane, so that the 
 juncture, when they are placed upon each other, shall be no 
 thicker than the rest of the strap. 
 
 The wheels which are turned by straps never make quite so 
 many revolutions as they ought to do from a calcidation of the 
 diameters of the pulleys over which the straps pass. Some- 
 times the straps may slip a little, but the principal source of 
 the error has been ingeniously attributed to their elasticity, 
 which permits them to stretch on that side which bears the 
 strain (called the leading side) and to collapse on the return- 
 ing side. This error, if elasticity be the cause of it, will aug- 
 ment with the intensity of the strain, and the distance between 
 the pulleys ; but its utmost amount is so small as to be rarely 
 of any great consequence. 
 
 Bands of rope or catgut are frequently employed to com- 
 municate motion, particularly to the mandrels of lathes. 
 Catgut is the best material known for a band, and always to 
 be preferred when it can be had of sufficient strength for the 
 purpose: the ends are united by a small steel hook and eye, 
 each of these has a socket screw to receive the band, which 
 is tapered a little, and screwed into it with a little rosin. If 
 the hook and eye be made warm enough to keep the rosin 
 fluid during the fastening, the band will be very firm, though 
 it may, for further security, be seared with a hot wire on the 
 
MECHANICS. 331 
 
 ConiniuDication of motiou hy endless chains and by I'riclion. 
 
 extremity projecting through the socket. The groove of pulleys 
 intended to receive a band, should be of a sharp angular form, 
 that the band may not touch the bottom of it, in which case it 
 would be liable to slip. Wood is the best material for pul- 
 leys, because the polish soon acquired by metals prevents the 
 band from holding firmly. The wood for a pulley should be 
 cut endways, that is, with its grain in the direction of its axis, 
 so that every part of the circumference being of a similar tex- 
 ture, it will wear equally. Two methods of applying a band 
 present themselves : in one of them it is carried over parts of 
 the wheel and pulley corresponding in position, the upper part 
 of the band passing directly to the upper part of the pulley, 
 and the lower part to the lower part; but in the other method, 
 the band is crossed, and therefore it passes from the upper part 
 of the wheel to the under part of the pulley. A crossed band 
 answers the best end ; for as it envelopes a larger portion of 
 the pulley, it produces a better effect than the other, even 
 when not so tight. 
 
 Endless chains are sometimes used to communicate motion, 
 and where their slipping would be injurious, cogs are frequently 
 formed on the wheels, to be received into the links of the chain. 
 The links should be formed with great exactness. 
 
 For very light machinery, a very neat and even elegant mode 
 of communicating motion has long been partially in use ; it con- 
 sists in covering the circumferences of the wheels with buflf 
 leather, which creates sufficient friction to make them turn 
 each other freely, although not pressed very hard together. 
 The same principle has been adopted upon a large scale for a 
 saw-mill, in which the wheels acted upon each other by the con- 
 tact of the end grain of wood instead of cogs. The machinery 
 wore well, made little noise, and was in use twenty years. 
 When this mode of transmitting power is adopted, a contrivance 
 to make the wheels bear firmly against one another, either by 
 wedges at the socket, or by levers, must be included. 
 
 It requires all the art of the engineer, when reciprocating 
 and desultory motions are required, to introduce tliem in the 
 most advantageous manner. Eccentric wheels are frequently 
 employed, but the common crank is perhaps the most lastingly 
 useful (principally on account of its being the most simple) 
 contrivance, for converting a reciprocating motion into a cir- 
 cular one, or the contrary. Attempts have been made to move 
 the pistons of pumps by means of a double rack on the piston 
 rod ; a half wheel takes hold of one rack, and raises it to the 
 required height; the moment the half wheel has quitted that 
 side of the rack, it lays hold of the other side, and forces the 
 piston down again. This has been proposed as a great im- 
 
332 MECHANICS. 
 
 Metijods of raising stampers. 
 
 provement, by correcting the unequal motion of the piston, 
 moved in the common way by a crank motion ; but it occasions 
 such abrupt changes of motion, as to be wholly inadmissible in 
 practice; the more ponderous the machine, and the more cor- 
 rect its workmanship, the sooner it would shake it to pieces. 
 
 When heavy stampers are to be raised, in order to drop on 
 the matter to be pounded, the wipers by which they are lifted, 
 should be made of such a form, that the stamper may be raised 
 by a uniform pressure, or with a motion almost imperceptible 
 at first. If this is neglected, and the wiper is only a pin stick- 
 ing out from the axis, the stamper is forced into motion at once. 
 This occasions a violent jolt to the machine, and great strains 
 on its moving parts and their points of support ; whereas, when 
 they are gradually lifted at first, the inequality of desultory 
 motion is never felt at the impelled point of the machine. The 
 principle, however, of communicating the motion gradually may 
 be carried too far. In order to avoid the great inconvenience 
 arising from the abrupt motion given to a great sledge hammer 
 of seven hundred weight, resisting with a tive-fold momentum, 
 an engineer formed the wipers for lifting it into spirals, which 
 communicated motion to the hammer with scarcely any jolts 
 ■whatever ; but the result was, that the hammer rose no higher 
 than it had been raised in contact with the wiper, and then fell 
 on the iron with very little effect. Wipers of the common form 
 were therefore of necessity substituted for the spirals ; for in 
 this operation the rapid motion of the hammer, during the 
 greater part of its progress, is absolutely necessary ; it is not 
 enough to lift it up ; it must be flung up so as to rise higher 
 than the wiper lifts it, and to strike with great force the strong 
 oaken spring which is placed in its way. It compresses this 
 spring, and is reflected by it with a considerable velocity, so as 
 to hit the iron as if it had fallen from a great height; had it been 
 allowed to fly to tiiat height, it would have fallen upon the iron 
 with somewhat more force, (because no spring is perfectly elas- 
 tic) but twice the time would have been required. 
 
 All ponderous movements should be supported by a frame- 
 work of wood, or of iron upon wood, independent of the build- 
 ing of masonry or brick-work containing them. Tlie want of 
 attention in this respect, has not unfrequently occasioned 
 buildings to be shaken to pieces. If the gudgeons of a water- 
 wheel, for example, rest upon the wall of a building recently 
 erected, it can scarcely fail to prevent the perfect induration 
 of the mortar, and the strength of the wall will thus be com- 
 pletely crippled. If such a situation must be selected, the 
 gudgeons should be supported upon a block of oak laid a little 
 hollow. This will soften all tremors, like the springs of a 
 
MECHANICS. 333 
 
 Detaching and reversing motions. 
 
 carriage, and a prudent extension of the principle would be 
 very serviceable in many parts of the construction. 
 
 To avoid the injurious effects occasioned by urging sud- 
 denly or by jerks the parts of a ponderous machine from a 
 state of rest into a state of motion, an ingenious contrivance 
 has lately been introduced, which deserves to be generally 
 known. The arm which gives motion to the machine, when 
 the clutch or connecting part of the running spindle is engaged 
 with it, is not fixed fast upon the spindle, but is made in two 
 halves screwed together upon a cylindrical part of the spindle, 
 and so closely pinched upon it by screws, that it will have suf- 
 ficient friction to turn the machine round in the ordinary 
 course of its work, but slips round upon the spindle, if the re- 
 sistance is greater than this friction, which thus becomes the 
 measure of the power exerted upon the machine. 
 
 Contrivances for uniting or detaching motions are very vari- 
 ous. The supports of the gudgeons of toothed wheels are 
 sometimes fitted up so as to be moveable, so that the wheels 
 can be separated so far as to relieve each other's teeth. At 
 other times one of the wheels is fitted on a round part of its 
 axis, and united with it at pleasure, by what is called a clutch- 
 box. Thus the wheels are always in motion, but one of them 
 can be detached at pleasure from its axis, on which it slips 
 freely. Bevelled cog-wheels are easily disengaged, by mov- 
 ing the axis of one of them a little endways. For disengaging 
 the motion of a strap, the contrivance called the live and dead 
 pulley is very ingenious and effectual ; it consists of two pul- 
 leys placed close together upon any axis which is to receive a 
 circular motion. One of them is fast upon the spindle, and 
 the other loose, so as to slip round. It is necessary that the 
 wheel by which these pulleys are turned, should have its rim 
 at least equal in breadth to that of both the pulleys. This 
 contrivance is extensively used, and, as applied to a lathe, has 
 been described in the section on Turning. 
 
 It is frequently necessary in machinery to have the power 
 of reversing the motion of a wheel or axis at any required in- 
 terval. Various means are used for effecting this object. The 
 most common is by two equal and similarly bevelled or con- 
 trate wheels, situated on the same axis, with their teeth to- 
 wards each other. A third bevelled wheel is applied with its 
 axis perpendicular to these ; and as its teeth, by simply mov- 
 ing a lever, can be made to engage either of them at pleasure, 
 they will, as they act on contrary sides of this third wheel, 
 communicate to it opposite motions. Smeaton applied this 
 movement to draw coals from coal-pits. 
 
334 MECHANICS. 
 
 I'lilitjf of fl^-wheels. 
 
 Of Fly Wheels. 
 
 In all machines, the moving power and the resistance are 
 subject to fluctuations of intensity. It becomes, therefore, an 
 object of great moment, to have, in most compound machines, 
 some means of accumulating the excess of the motive power, 
 and of expending this excess, when the motive power operates 
 too feebly. This equalization of motion is usually obtained 
 by what is called a //y, which is generally made in the form of 
 a wheel, though sometimes it is merely two bars crossing one 
 another at right angles in the middle, with weights at the four 
 extremities. 
 
 A fly being made to revolve about its axis, keeps up the 
 force of the power, and distributes it equally in all parts of its 
 revolution. On account of its weight, a small variation in 
 force does not sensibly alter its motion ; whilst its friction, 
 and the resistance of the machine, prevents it from accelerat- 
 ing. If the motive power slackens, it impels the machine for- 
 ward, and if the power tends to move the machine too fast, it 
 keeps it back. 
 
 In all machines in which flies are used, either a considerably 
 greater force must be applied at first than what is necessary 
 to give motion to the machine without it, or the fly must be 
 set in motion some time before the force is applied to the ma- 
 chine. This superfluous power is collected in the fly, which 
 is in fact a reservoir of motion. A man, working at a common 
 windlass, exerts a very irregular pressure on the winch. In 
 two of his positions, during each turn, he can exert a force of 
 nearly seventy pounds without fatigue, but in otiier positions 
 he exerts a force of little more tlian twenty-five pounds; nor 
 must he in general have to oppose much above this ; but if a 
 large fly be properly connected with the windlass, he will act 
 with equal ease and speed against thirty or even forty pounds. 
 
 The motion communicated to a fly-wheel by means of a small 
 force, may be accumulated to such a degree as to produce ef- 
 fects which the original force would never have accomplished. 
 Atwood has demonstrated in his "Treatise on Rectilineal and 
 Rotary Motion," that a force equivalent to 20 pounds, applied 
 for the space of 37 seconds to the circumference of a cylinder 
 20 feet in diameter, which weighs 4713 pounds, would, at the 
 distance of one foot from the centre, give an impulse to a mus- 
 ket-ball equal to that which it receives from a full charge of 
 powder. In the space of six minutes and ten seconds, the 
 same effect would be produced, if the wheel were driven by a 
 man, who constantly exerted a force of 20 pounds at a winch 
 one foot long. This accumulating power of a fly, induces 
 
MECHANICS. 335 
 
 Power accumulated, not increased l)y a fl^i. 
 
 among many a supposition, that a fly really adds power or me- 
 chanical force to an engine ; accordingly, from not understand- 
 ing on what its efficacy depends, nor considering, that if it 
 communicated a power which it did not receive, it must, con- 
 trary to the nature of matter, possess a principle of motion 
 within itself, they often place the fly in a situation where it 
 only adds a useless burden to the machine. If intended for a 
 mere regulator, it should be near the first mover ; if intended 
 to accumulate force in the working point, it should not be far 
 separated from it. 
 
 It is certain, that a fly does not communicate any absolute 
 increase of motion to the machine ; for if a man, or any animal, 
 is not able to set an engine in motion without a fly, he will 
 not be able to do it though a fly be applied ; nor will he be 
 able to keep it in motion, though set to work with a fly by 
 means of a greater power. The apparent creation of power by 
 a fly consists in its accumulating into one moment, the exer- 
 tions of many. A man, caught by some of the movements of 
 a country mill, may be instantly deprived of a limb, or of life. 
 In this case, the power of the stream is conceived to be pro- 
 digious, and yet we are certain, upon examination, that it 
 amounts to the pressure of no more than fifty or sixty pounds; 
 but this force has been acting for some time, and there is a 
 millstone of a ton weight whirling twice round in a second ; 
 the effect, therefore, not of any self-derived but of the accu- 
 mulated power, is enormous. Contrivances to prevent acci- 
 dents from the force of machinery, deserve every encourage- 
 ment ; and perhaps, among the improvements yet to be made 
 in practical mechanics, they will be more conspicuous than 
 they have hitherto been. It has been asserted, that in the 
 neighbourhood of Elbingroda, in Hanover, there was a con- 
 trivance which disengaged the millstone when any thing got 
 entangled in the teeth of the wheels. On being tried with 
 the head of a cabbage, it crushed it, but not violently, and 
 would by no means have broken a man's arm. 
 
 The resistance which the air opposes to any body in mo- 
 tion, and the friction of the pivots which support the axis of 
 a fly, are considerable deductions from the power communi- 
 cated to this appendage of machinery, so that instead of 
 really gaining power, a fly-wheel requires a constant exertion 
 to keep it in motion, even when no further resistance is ap- 
 plied to prevent it. For this reason, a fly-wheel should 
 never be introduced into a machine unless the advantages to 
 be derived from its action are greater than the actual loss of 
 power it occasions. In general, where the power is tolerably 
 uniform in its action, if the resistance can be made so too. 
 
33G MECHANICS. 
 
 Best form for a fly. — Method of calculating the effect of a (It. 
 
 a fly is not necessary. If two hammers are raised at the 
 same moment by a water-wheel, during the interval of their 
 descent much power would be lost, unless it were collected 
 by a fly ; but if the two hammers, or any other number, 
 were raised in succession, tiie resistance would be rendered 
 nearly as uniform as with a fly-wheel, without its inconve 
 niences. 
 
 A fly for the accumulation of power, sliould be so constructed 
 as to present the smallest resistance to the air. A wheel is the 
 best form; it should be made of metal, that it may have a great 
 weight under a small surface, and it should be smooth and 
 truly circular, without any projecting nuts. If the transverse 
 section of the rim be a circle, and the transvere sections of 
 the arms connecting the rim with the centre, be ellipses, pre- 
 senting their thin edge to divide the air, the fly will be less 
 resisted by the air than under any other form. This configu- 
 ration of a fly is included in a patent taken out by Murray 
 and Wood, of Leeds. 
 
 Fly-wheels are usually made of iron ; when they are too 
 large to be cast in one piece, it is very important to unite the 
 parts in the most substantial manner; for the centrifugal force 
 of a great wheel in rapid motion is prodiu;ious, and if the bolts 
 be insufiicient to withstand the strain upon them, the parts 
 broken ofl' will be projected with a velocity giving them the 
 destructive power of a cannon-shot. 
 
 Suppose a fly be employed in a machine required to raise a 
 pestle of thirty pounds weight to the height of one foot sixty 
 times in a minute; here the weight of a fly is a principal 
 object, and its eff'ect is calculated by a comparison with the 
 weight to be raised. Let the diameter of the fly be seven feet, 
 and supposing the pestle to be raised once by every revolu- 
 tion, we must then consider what weight, passing in one 
 second, through a space equal to the circumference of the fly, 
 which is about 22 feet, will be equivalent to 30 pounds pass- 
 ing through one foot in a second. Tiiis will be 30 divided 
 by 22, or lA. Were a fly of tins kind applied, and the ma- 
 chine set in motion, it would be able to lift the pestle once 
 after the moving power was withdrawn ; but by increasing the 
 weight of the fly to ten, twelve, or twenty pounds, the machine, 
 when left to itself, would make a considerable number of 
 strokes, and after the incumbrance of the fly at the outset was 
 overcome, it would be worked with much less labour than if 
 no fly had been used. The mode of calculation here adopted, 
 is equally applicable to the motion of pumps; but the weight 
 which can be most advantageously given to a fly has never 
 been satisfactorily determined. 
 
MECHANICS. 337 
 
 Causes obstructing the motions of machines. 
 
 Of Friction. 
 
 With the exception of a few incidental remarks, we have 
 hitherto paid no attention to the physical properties of the ma- 
 terials of which machines are composed, and of the alterations 
 those properties occasion in the effects established by theory 
 alone. This was necessary, to disentangle the theory from the 
 constant recurrence of provisoes and limitations, which would 
 be best understood if distinctly considered ; but we must now 
 proceed to point out the impediments to the perfect action of 
 machines, and the allowances to be made for them. 
 
 Among the various physical causes which occasion a dif- 
 ference between theory and practice, with respect to machines, 
 the two following may be considered the most important and 
 most general : 1. The weight of the parts composing the ma- 
 chines. 2. Friction, a terra which designates the obstruction 
 to the motion of a machine, produced by the resistance of the 
 air, as well as that produced by the rubbing of one part 
 against another. In trying experiments to exemplify the 
 theory of the lever, it has been shewn that the shorter arm 
 must be made as heavy as the longer arm, otherwise they will 
 not perfectly succeed. This shews the principle on which al- 
 lowances must be made for the weight of machinery, and how 
 this cause of obstruction may be obviated. Friction is, how- 
 ever, a cause of obstruction always present; it may be dimi- 
 nished by various artifices, but never entirely removed. 
 
 Leslie, in his valuable work on the nature and propagation 
 of heat, adverts to the cause of friction in a very able manner: 
 " If the two surfaces," says he, " which rub against each other, 
 are rough and uneven, there is a necessary waste of force, oc- 
 casioned by the grinding and abrasion of their prominences. 
 But friction subsists after the contiguous surfaces are worked 
 down as regular and smooth as possible. In fact, the most 
 elaborate polish can operate no other change than to diminish 
 the size of the natural asperities. The surface of a body being- 
 moulded by its internal structure, must evidently be furrowed, 
 toothed, or serrated. Friction is, therefore, commonly ex- 
 plained on the principle of the inclined plane, from the effort 
 required to make the incumbent weight mount over a succes- 
 sion of eminences. But this explication, however currently 
 repeated, is quite insufficient. The mass which is drawn 
 along is not continually ascending ; it must alternately rise 
 and fall : for each superficial prominence will have a corre- 
 sponding cavity ; and since the boundary of contact is supposed 
 to be horizontal, the total elevations will be equalled by their 
 collateral depressions ; consequently, if the lateral force might 
 15.— Vol. I. 2 X 
 
338 MECHANICS. 
 
 Leslie's investigatiou of the naiure of friction. 
 
 suffer a perpetual diminution in lifting up the weight, it would, 
 the next moment, receive an equal increase by letting it down 
 again ; and those opposite effects, destroying each other, could 
 have no influence whatever on the general motion. 
 
 " Adhesion seems still less capable of accounting for the 
 origin of friction. A perpendicular force acting on a solid, 
 can evidently have no effect to impede its progress ; and though 
 this lateral force, owing to the unavoidable inequalities of 
 contact, may be subject to a certain irregular obliquity, the 
 balance of chances must, on the whole, have the same tendency 
 to accelerate, as to retard, the motion. If the conterminous 
 surfaces were, therefore, to remain absolutely passive, no fric- 
 tion could ever arise. Its e.\istence demonstrates an unceas- 
 ing mutual change of hgure, the opposite planes, during the 
 passage, continually seeking to accommodate themselves to all 
 the minute and accidental varieties of contact. The one sur- 
 face being pressed against the other, becomes, as it were, com- 
 pactly indented, by protruding some points and retracting 
 others. This adaptation is not accomplished instantaneously, 
 but requires very different periods to attain its maximum, ac- 
 cording to the nature and relation of the substances concerned. 
 In some cases a few seconds are sufficient; in others, the full 
 effect is not produced till after the lapse of several days. 
 While the incumbent mass is drawn along, at every stage of 
 its advance, it changes its external configuration, and ap- 
 proaches more or less towards a strict contiguity w ith the un- 
 der surface. Hence the effort required to put it first in mo- 
 tion ; and hence, too, the decreased measure of friction, which, 
 if not deranged by adventitious causes, attends generally an 
 augmented rapidity. This a])pears clearly established by the 
 curious experiments of Coulomb, tlie most original and valu- 
 able which have bepn made on that interesting subject. Fric- 
 tion cuHsists ill the force expended to raise contitnially the surface 
 of pressure by an oblique action. The upper surface travels over 
 a perpetual system of inclined planes ; but that system is ever 
 changing with alternate inversion. In this act, the incumbent 
 weight makes incessant, yet unavailing, efforts to ascend : for 
 the moment it has gained the summits of the superficial pro- 
 minences, these sink down beneath it, and the adjoining cavi- 
 ties start up into elevations, presenting a new series of ob- 
 stacles which are again to be surmounted ; and thus the la- 
 bours of Sisiphus are realized in the phenomena of friction. 
 
 " The degree of friction must evidently depend on the 
 angles of the natural protuberances, and which are determined 
 by the elementary structure or the mutual relation of the 
 two approximate substances. The effect of polishing is- 
 
MECHANICS. 339 
 
 Quantity of friction in various instances. 
 
 only to abridge those asperities and increase their number, 
 without altering in any respect their curvature or inflexions. 
 The constant or successive acclivity produced by the ever- 
 varying adaptation of the contiguous surfaces, remains, there- 
 fore, the same, and consequently the expense of force will 
 still amount to the same proportion of the pressure. The 
 intervention of a coat of oil, soap, or tallow, by readily 
 accommodating itself to the variations of contact, must tend 
 to equalize it, and therefore must lessen the angles, or soften 
 the contour, of the successively emerging prominences, and 
 thus diminish likewise the friction which thence results." 
 
 The friction of a single lever is very trifling. The friction 
 of the wheel and axle is in proportion to the weight, velocity, 
 and the diameter of the axle ; the smaller the diameter of the 
 axle, the less will be the friction. 
 
 Pulleys have very great friction, on account of the smallness 
 of their diameters in proportion to that of their axles, and 
 their friction is greatly increased when they bear, as they are 
 very apt to do, against their blocks, and when their centres 
 and axles are worn untrue. 
 
 The friction of bodies is in general proportionate to their 
 weight, or the force with which their rubbing surfaces are 
 pressed together; and is for the most part equal to between 
 one-half and one-fourth of that force. Although friction in- 
 creases with an increase of surface, yet this does not take 
 place in direct proportion to that increase. It also increases, 
 with some exceptions, in proportion to the velocity of bodies, 
 particularly when very different substances are employed with- 
 out an unguent. 
 
 According to Emerson, when a cubical piece of soft wood, 
 of eight pounds weight, moves upon a smooth plane of soft 
 wood, at the rate of three feet per second, its friction is about 
 one-third of its weight ; but if it be rough, the friction is 
 little less than half the weight : on the same supposition, 
 when both the pieces of wood are very smooth, the friction 
 is about one-fourth of the weight; the friction of soft wood 
 on hard, or of hard wood on soft, is one-fifth or one-sixth 
 of the weight; of hard wood upon hard wood, one-seventh 
 or one-eighth ; of polished steel moving on steel or pew- 
 ter, one-fourth ; moving on copper or lead, one-fifth of the 
 weight. 
 
 It was generally supposed, that in the case of wood, the 
 friction is greatest when the bodies are dragged contrary to 
 the course of their fibres ; but the experiments of Coulomb 
 demonstrate the contrary. 
 
 The longer the rubbing surfaces remain in contact, the 
 
340 MECHANICS. 
 
 yQiiitity of friction in \arioos inslanoe* 
 
 greater is their friction. When wood was moved upon wood 
 according to the direction of the fibres, the friction was in- 
 creased by keeping the surfaces in contact for a few seconds; 
 and when the time was prolonged to a minute, the friction 
 seemed to have reached its farthest limit. But when tlie 
 motion was performed contrary to the course of the fibres, 
 a greater time was necessary before the friction anived at its 
 maximum. When wood was moved upon metal, the friction 
 did not attain its maximum till the surfaces continued in con- 
 tact for four or five days ; and it is very remarkable, that 
 when wooden surfaces were anointed uith tallow, the time re- 
 quisite for producing the greatest quantity of friction was in- 
 creased. The increase of friction which is generated by pro- 
 longing the time of contact is so great, that a body weighing 
 1650 pounds was moved with a force of 64 pounds when first 
 laid upon its corresponding surface. After having remained 
 in contact for the space of three seconds, it required 160 
 pounds to put it in motion, and when the time was prolonged 
 to six days, it could scarcely be moved with a force of 622 
 pounds. When the surfaces of metallic bodies were moved 
 upon one another, the time of producing the greatest effect or 
 maximum of friction was not changed by the interposition of 
 olive oil ; more time, however, was required to produce the 
 maximum when swine's grease was employed as an unguent ; 
 and it was prolonged to five or six days when the surfaces 
 had been besmeared with tallow. 
 
 In wood rubbing upon wood, oil, grease, or black-lead, pro- 
 perly applied, makes the friction two-thirds less. Wheel 
 naves, when greased, have not more than one-fourth of the 
 friction they would have if only wetted. 
 
 When polished steel moves on steel or pewter, properly 
 oiled, the friction is about one-fourth of the weight ; on cop- 
 per or lead, one-fifth of the weight; on brass, one-sixth; 
 and metals have more friction when they move on metals 
 of the same kind, than when they move on different metals. 
 This seems principally owing to the superior strength of 
 the attraction of cohesion between similar metals. It is 
 always desirable, therefore, to make the parts of machines 
 opposed to or working in each other, of different materials ; 
 thus, in clocks and watches, the wheels are brass, and the 
 pinions steel, and iron pivots are made to work in brass 
 or bell-metal collars. The axes of wheels should also be 
 made as small as the weioht they have to bear will allow ; 
 hecause the diminution of the surfaces rubbing against each 
 other, will be attended with a diminution of the friction. 
 
 According to V'ince's experiments, which were made with 
 
MECHANICS. 341 
 
 Vince's experiments on friction. — Coulomb's experiments. 
 
 great care, and frequently repeated, the friction of hard 
 bodies in motion is a uniformly retarding force. Experiments 
 were instituted to determine whether the same law obtained 
 when the bodies were covered with cloth, woollen, &c. and it 
 was found, in all cases, that the retarding force increased with 
 the velocity; but on covering the bodies with paper, the re- 
 sults agreed with those before stated. By other experiments, 
 the same philosopher found, that the quantity of friction, 
 contrary to the prevailing opinion, increased in a less ratio 
 than the quantity or weight of the body; also, that the smallest 
 surface has the least friction. It may be proper to describe 
 the apparatus by which these results were obtained : a plane 
 was adjusted parallel to the horizon; at the extremity was 
 placed a pulley, which could be elevated or depressed, so as 
 to render the string which connected the body and the moving 
 force, parallel to the plane. A divided scale was placed near 
 the pulley, perpendicular to the horizon; and the moving force 
 descended by the side of this scale. A moveable stage was 
 placed upon the scale, which could be adjusted to the space 
 through which the moving force descended in any given time, 
 which time was measured by a well regulated pendulum, vi- 
 brating seconds. 
 
 According to Coulomb's experiments, which were con- 
 ducted on a large scale, and are therefore much relied on, 
 the friction of lignum-vitee cylinders, two inches in diameter, 
 and loaded with one thousand pounds, was 18 pounds, or 
 nearly 5*g^ of the weight or force of pression. In cylinders of 
 elm, the friction was greater by f , and was scarcely diminished 
 by the interposition of tallow. From a variety of experiments 
 on the friction of the axes of pulleys, the following results 
 were obtained ; when an iron axle moved in a brass bush or 
 bed, the friction was I of the pression ; but" when the bush 
 was besmeared with very clean tallow, the friction was only 
 T^T ; when swine's grease was interposed, the friction was 
 about ^, and when olive oil was employed, it was about ^. 
 When the axle was of green oak, and the bush of lignum- 
 vitee, the friction was 2V when tallow was interposed ; but 
 when the tallow was removed, so that a small quantity of 
 grease only covered the surface, the friction was increased 
 to -rV' When the bush was made of elm, the friction was, in 
 similar circumstances, -^ and -^^, which is the leas-t of all. 
 When the axle was made of box, and the bush of lignum-vitae, 
 the friction was 2V ^nd -jV, circumstances being the same as 
 before. If the axle be of boxwood, and the bush of elm, 
 the friction will be -^V and -oV ; and if the axle be of iron, and 
 the bush of elm, the friction will be Vo of the force of pression. 
 
342 MECHANICS. 
 
 Use of friclioD rollers. 
 
 In calculating the force of an engine, friction should never 
 be overlooked. Though it varies so much with circumstances, 
 that it is not yet reduced to certain rules, still the specific 
 details we have given will enable the young mechanic to come 
 tolerably near the truth, in ascertaining its amount, at each 
 part of a machine, according to the pressure, surface, and ma- 
 terials ; and as he goes along from the power to the resistance, 
 he must consider these amounts as actual deductions from the 
 advantage of the machine. It must be understood, that the 
 amount of friction stated in this section, will apply only to 
 machines that are well made ; the loss of power that may be 
 occasioned by bad workmanship is incalculable, and as bad 
 workmanship may exist when it is not perceived, no conjec- 
 tural calculation should be relied on, when the real loss of 
 power can be obtained by experiment. 
 
 One general rule of preventing friction is to substitute, 
 whenever it is possible, the rolling for the sliding motion. 
 Every one knows that a weight which the application of a 
 given force cannot drag, may be easily drawn along by the 
 same force, if mounted upon wheels turning on their axes. 
 On this principle depends the utility of what are called fric- 
 tion rollers, which are small cylinders or spheres interposed 
 so as to revolve between surfaces that would otherwise rub 
 upon each other ; or small wheels so disposed that the pivots 
 of larger wheels revolve upon their circumferences, instead of 
 turning in a bush or socket. Three wheels, it is obvious, if 
 they touch the different sides of the pivot in three points equi- 
 distant from each other, will support it as effectually as a 
 cylindrical socket, which would have far greater friction. 
 When the motion to which these rollers or wheels are sub- 
 jected, is equal and steady, the use of them will often be pro- 
 ductive of permanent advantage ; but when, as for the axle- 
 trees of carriages, they are subject to violent strains, jolts, 
 and, occasionally at least, to enormous pressure, they are, 
 however excellent in principle, seldom found of much practi- 
 cal utility, and are often positively injurious ; for they and the 
 parts bearing upon them, either do not receive from the work- 
 man the precise figure they ought to have, or if well made, 
 they wear so unequally as soon to lose that correctness of 
 figure which is inseparable from their value. 
 
MECHANICS. 343 
 
 Variableness of the force exerted bj a man at a winch. 
 
 Of the Application of Men and Horses, as moving Poivers in 
 Machinery, S^c. 
 
 A man turning a horizontal windlass by a handle or winch, 
 should not have to exert a greater force than 30 pounds, if he 
 have to work ten hours a day ; therefore the power exhausted 
 by friction, the stiffness of ropes, and the intensity of the re- 
 sistance, should not, altogether, require more than a force of 
 30 pounds to overcome them. 
 
 In the operation of turning a winch, the effect of a man's 
 force varies in every part of the circle described by the handle. 
 The greatest force is when he pulls the handle iipwards from 
 about the height of his knees ; and the least force, when, the 
 handle being at the top, he thrusts from him horizontally; then 
 again the effect is increased as he lays on his weight in push- 
 ing downwards ; but that action is not so great as when he 
 pulls up, because it is merely produced by the weight of his 
 body, whereas, in pulling up, he can exert his whole strength. 
 In pulling the handle horizontally, when at its lowest, the force 
 exerted is very small. 
 
 The weight of a man of moderate strength, may be stated 
 at 140 pounds, and such a person may be considered capable 
 of exerting the following forces, viz. in the strongest point, or 
 that position of the winch which is most favourable to him, a 
 force equal to 160 pounds ; in the weakest, a force equal to 27 
 pounds ; in the next strong point, 130 pounds; and in the last, 
 or second weak point, 30 pounds. The sum of these forces is 
 347, which, divided by 4, gives 84^ pounds for the weight that 
 a man might lift by a winch, if he could exert his whole 
 strength continually, without stopping to take breath ; but this 
 being impossible, the weight must return, and overpower him 
 at the first weak point, especially when the handle moves 
 slowly, as it must if he would exert his utmost strength all 
 round. Besides, in overcoming such a resistance, the man is- 
 in theory supposed to act always along the tangent of the cir- 
 cle of motion, which application of his force is not practicable; 
 and there must also be such a velocity given, that the force 
 applied at the strong points may not be spent before the hand 
 comes to the weak ones, which is a regulation of exertion also 
 unattainable ; hence, when no adventitious advantages are 
 superadded, the resistance ought to be no more than 30 
 pounds. If a fly be added to the windlass, when the motion 
 is pretty quick, as about four or five feet per second, a man 
 may exert for a short time a force of 80 pounds, and work a 
 whole dav against a resistance of 40 pounds. 
 
344 MECHANICP. 
 
 Best position for the haodles of a >vindJass. — Powers of men and tior>cs. 
 
 If the windlass be provided with two handles, one at each 
 extremity, and the elbows of these handles are at right angles 
 to each other, two men will more easily, for the same length 
 of time, act against a constant resistance of 70 pounds, than 
 a single man against 30 pounds ; for one man will act at the 
 strongest point while the other acts at the weakest point of 
 the revolution, and thus they will mutually and successively 
 help one another. The utility of this disposition of the 
 handles is now generally known and attended to, but it was 
 formerlv little tliought of. 
 
 The whole art of carrying large burdens consists in keeping 
 the column of the body as directly under the weight and as 
 upright as possible. Standing in his natural posture, a man 
 can support a weight which would break the back of the 
 strongest horse. The reason is evident : the column of the 
 man's bones support the weight directly ; but the weight is 
 laid across the column of the horse. The more a man bends 
 his body, the less weight he can support ; hence two men car- 
 rying a load, can sustain much more than double the weight 
 which either of them could carry separately, because they can 
 move more uprifjht, and with the column of their bones more 
 opposed to it. Chairmen having straps from their shoulders 
 to the poles of the chair, will walk with 300 pounds (that is 
 150 pounds each) at the rate of four miles an hour. A porter 
 will carry upon his shoulders a load of 180 pounds, and walk 
 at the rate of three miles an hour ; a coal-heaver v. ill carrv 250 
 pounds, but he only goes to a short distance with his load. 
 In rowing, men exert their strength with great etlect ; and they 
 more usually draw the oar to them than push it from them, 
 because, in the former case, they can bring into action a 
 greater number of muscles, and experience quickly convinces 
 them of the best mode. 
 
 A horse draws with the greatest advantage when the line of 
 drau'^ht is not level with his breast, but inclines upwards, 
 making a small angle with the horizontal plane. 
 
 A horse drawing a weight over a single pulley can exert 
 a force of 200 pounds, while walking at the rate of two 
 miles and a half per hour, or about three feet and a half 
 per second. If the same horse have to draw 240 pounds, 
 he can work but six hours a day, and cannot go quite so 
 fast. To this mode of exertion may be referred the work- 
 ing of horses, in all sorts of mills, in calculating the pro- 
 bable effect of which, previous to their being erected, after 
 making the necessary allowances for all frictions and hin- 
 derances, the task assigned to the horse should be carefully 
 determined. 
 
MECHANICS. 345 
 
 Power of horses. — Different kinds of mills. 
 
 The force with which a horse acts is compounded of his 
 weight and muscular strength. If then the weight of one horse 
 exceed that of another to which it is inferior with respect to 
 strength, the weaker horse will overcome a resistance which 
 the stronger cannot, provided the excess of his weight in the 
 smallest degree exceeds his deficiency in strength. 
 
 When a horse draws in a mill or gin of any kind, great care 
 should be taken that the horse-walk, or circle in which he 
 moves, be large enough in diameter, otherwise he cannot exert 
 all his strength : for in a small circle, the tangent in which he 
 draws, deviates more from the circle in which he is obliged to 
 go than in a larger circle. The diameter of the horse-walk 
 should never, if possible, be less than forty feet. In a walk of 
 nineteen feet, it has been calculated that a horse loses two- 
 fifths of his strength. 
 
 A horse exerts his force to the greatest disadvantage in draw- 
 ing or carrying up a hill. The human form is so much better 
 adapted for climbing than that of a horse, that if the hill be 
 steep, three men will do more than a horse ; each man, loaded 
 with 100 pounds, will move up faster than a horse that is 
 Joaded with 300 pounds. But one horse can give motion to 
 the horizontal beam in a walk of forty feet, with as much ease 
 as five men ; in a walk of nineteen feet, three men would exert 
 themselves with as much effect as a horse. 
 
 Of Mill Work. 
 
 The term mill, originally signified a machine for grinding 
 corn^ but at present the expression mill-work is frequently 
 applied to all kinds of machinery where large wheels are em- 
 ployed. 
 
 Mills are distinguished into various kinds, eithsr according 
 to the powers by which they are moved, or the uses to which 
 they are applied; hence we have xoater-milh, horse-mills, and 
 wind-mills; corn-mills, fulling-mills, powder-mills, boring-mills, 
 &c. These appellations, indefinite as they are, answer the 
 purpose of common conversation; but it is evident, that a mill 
 is not completely named, unless its use as well as its motive 
 force, is designated. 
 
 In ancient times, corn was ground only by hand-mills, con- 
 sisting of two stones, similar to those used in water-mills, but 
 much smaller, the lower one being fixed, and the upper one 
 having a piece of wood fastened into it to move it by. Ma- 
 chines of this description are still used in India, and' also in 
 some sequestered parts of Scotland; in the latter country, they 
 are called querns; but in general, wherever large quantities of 
 15.— Vol. I. 2 1- 
 
346 MECHANICS. 
 
 Different kinds of wHler-wbeels. — Directions to obtaiu the velocity of a stream. 
 
 grain are to be ground, they have been entirely superseded by 
 mills not moved by manual labour. 
 
 In treating of water-mills, which will here be our principal 
 object, we shall have occasion to advert to the most eligible 
 mode of forming the cogs of wheels, and other particulars ap- 
 plicable to machinery in general, and shall conclude with a 
 description of the most simple form of the water corn-mill. 
 
 Water-mills are of three kinds, viz. breast-mills, umlcrshot- 
 mills, and otershot-mills, according to the manner in which the 
 water is applied to the great-wheel. In the first, the water 
 falls down upon the wheel at right angles to the float-boards or 
 buckets placed all round the wheel to receive it. In the second, 
 which is used where there is no fall but a considerable body of 
 water, the stream strikes the float-boards at the lower part of 
 the wheel. In the third, the water is poured over the top, and 
 is received in buckets formed all round the wheel. 
 
 It was the opinion of Smeaton, that the powers necessary 
 to produce the same effect on the undershot-wheel, a breast- 
 ivheel, and an overshot-wheel, must be to each other as the 
 numbers 2.4, 1.75, and 1. 
 
 The effect or momentum of water depending jointly upon 
 its velocity and its quantity, it is of importance to ascertain 
 these particulars ; Dr. Desaguliers has given the following 
 easy directions for the purpose : Observe a place where the 
 banks of the river are steep, and nearly parallel, so as to make 
 a kind of trough for the water to run through ; then by taking 
 the depth in various parts of the stream's breadtli, obtain a 
 correct section of the river. Stretch one line over it at right 
 angles, and another at a small distance above or below, but 
 perfectly parallel. Now throw in some buoyant body (such 
 as an apple, which will not float so high as to be aftected 
 by the wind) immediately above the upper line : observe the 
 time it occupies in passing from one to the other string. 
 Thus you ascertain how many feet the current runs in a se- 
 cond, or in a minute. Then having the two sections, that is, 
 one at each line, reduce them to a mean or average depth, 
 and compute the area of the mean section, which being multi- 
 plied by the distance between the lines, w'U give the solid 
 contents of the intermediate volume of f^uid, which in the 
 noted time passed from one string to the other. Now this 
 v/ay, by the rule of three, is adapted to any portion of time j 
 the question being merely, if the velocity be such in such an 
 area, or trough, w hat would be the velocity in another of less 
 $ize. It is obvious, that if the area give twelve solid feet, and 
 that the water passed at the rate of four feet in a second, 
 through a conduit of one foot square, if the conduit were 
 
MECHANICS. 347 
 
 Rules for the construction of mills. 
 
 only six inches square, the velocity would be as sixteen to 
 four ; or in other words, quadrupled. 
 
 The arch of a bridge is often an excellent station for ob- 
 serving the force of a stream ; because the sides are there re- 
 gular, and the intermediate space may be correctly measured. 
 But the depth is not always to be ascertained in such places 
 without the aid of a boat, or of two intelligent assistants, who 
 should be very correct in their observations. The arch of a 
 bridge is not a proper station, when the velocity of the cur- 
 rent is accelerated for want of sufficient water-way. — For some 
 further remarks on this subject, see page 110, vol. 2. 
 
 Practical Rules and Observations relative to the Construction of 
 
 Water-Mills. 
 
 1. Measure the perpendicular height of the fall of water, in 
 feet, above that part of the wheel on which the water begins 
 to act, and call that the height of the fall. 
 
 2. Multiply this constant number 64.2822 by the height of 
 the fall in feet, and the square root of the product will be the 
 velocity of the water at the bottom of the fall, or the number 
 of feet that the water there moves per second. 
 
 3. Divide the velocity of the water by three, and the quo- 
 tient will be the velocity of the float-boards of the wheel, or 
 the number of feet they must each go through in a second, 
 when the water acts upon them so as to have the greatest 
 power to turn the mill. 
 
 4. Divide the circumference of the wheel in feet by the velo- 
 city of its floats in feet per second, and the quotient will be 
 the number of seconds in which the wheel turns round. 
 
 5. By this last number of seconds divide 60, and the quo- 
 tient will be the number of turns of the wheel in a minute. 
 
 6. Divide 120 (the number of revolutions a mill-stone four 
 feet and a half in diameter ought to have in a minute) by the 
 number of turns of the wheel in a minute, and the quotient 
 will be the number of turns the mill-stone ought to have for 
 one turn of the wheel. 
 
 7. Then, as the number of turns of the wheel in a minute is 
 to the number of turns of the mill-stone in a minute, so must 
 the number of staves in the trundle be to the number of cogs 
 in the wheel, in the nearest whole numbers that can be 
 found. 
 
 By these rules the following table is calculated to a water- 
 wheel eighteen feet in diameter, which size has been found by 
 experience to be the most eligible for general use. 
 
348 
 
 MECHANICS. 
 
 CalculatiuD!) of mill-work. 
 
 The Millwright's Table. 
 
 
 Velocity 
 
 Velocity 
 
 Revolutions 
 
 Ue\ulation 
 
 Cogs in the 
 
 Revolutions 
 
 o o 
 
 of the fall 
 
 of the 
 
 of the 
 
 wheel 
 
 of the iiiill- 
 
 wheel, 
 
 and 
 
 of the mill- 
 
 ■^5 « 
 
 of water 
 
 wheel per 
 
 per ininate. 
 
 stone for 
 
 staves 
 
 in the 
 
 stone per mi- 
 
 *5 9) a 
 
 per se- 
 
 second. 
 
 
 
 one of the 
 
 trundle. 
 
 
 nute bT these 
 
 S-S f 
 
 cond. 
 
 
 
 
 wheels. 
 
 
 
 stave*6ccogR 
 
 
 ^ 
 
 « 
 
 
 s 
 
 er. 
 
 
 
 « 
 
 . 
 
 
 J So 
 
 3 ^ 
 
 a > 
 
 3 « *• 
 
 i 
 
 S 
 
 3 . ** ? 
 
 V 
 
 
 0) ^ 
 
 C •" 
 
 
 
 to 
 
 
 
 £ 
 
 o o 
 
 ^ i^ 
 
 > o 
 
 J a 
 
 £.2 £ ■* 
 
 O 
 
 
 5 ' .= « 
 
 
 "" 
 
 ^^ 
 
 
 
 
 
 
 
 1 
 
 8 .02 
 
 2 .67 
 
 2 
 
 .83 
 
 42 . 40 
 
 254 
 
 6 
 
 119-84 
 
 2 
 
 11 .34 
 
 3 .78 
 
 4 
 
 .00 
 
 30 . 00 
 
 210 
 
 7 
 
 120-00 
 
 3 
 
 13 .89 
 
 4 .63 
 
 4 
 
 .91 
 
 24 .44 
 
 196 
 
 8 
 
 120-28 
 
 4 
 
 16.04 
 
 5 .35 
 
 5 
 
 .67 
 
 21 . 16 
 
 190 
 
 9 
 
 119-74 
 
 5 
 
 17 .93 
 
 5.98 
 
 6 
 
 .34 
 
 18 . 92 
 
 170 
 
 9 
 
 119-68 
 
 6 
 
 19 .64 
 
 6 . 55 
 
 6 
 
 • 94 
 
 17 .28 
 
 I 156 
 
 9 
 
 120-20 
 
 7 
 
 21 .21 
 
 7 .07 
 
 7 
 
 .50 
 
 16.00 
 
 144 
 
 9 
 
 120-00 
 
 8 
 
 22.68 
 
 7 .56 
 
 8 
 
 .02 
 
 14 .96 
 
 134 
 
 9 
 
 119-34 
 
 9 
 
 24.05 
 
 8 .02 
 
 8 
 
 . 51 
 
 14 . 10 
 
 140 
 
 10 
 
 119-14 
 
 10 
 
 25 . 35 
 
 8 .45 
 
 8 
 
 .97 
 
 13 -38 
 
 134 
 
 10 
 
 120-14^ 
 
 11 
 
 26 . 59 
 
 8 .86 
 
 9 
 
 .40 
 
 12 . 76 
 
 128 
 
 10 
 
 120-32 
 
 12 
 
 27 .77 
 
 9 .26 
 
 9 
 
 .82 
 
 12 .22 
 
 122 
 
 10 
 
 119-80 
 
 13 
 
 28 . 91 
 
 9 .64 
 
 10 
 
 . 22 
 
 11 . 74 
 
 118 
 
 10 
 
 120-36 
 
 14 
 
 30 • 00 
 
 10 . 00 
 
 10 
 
 .60 
 
 11 .32 
 
 112 
 
 10 
 
 118.72 
 
 15 
 
 31 -05 
 
 10 .35 
 
 10 
 
 • 99 
 
 10 .98 
 
 110 
 
 10 
 
 120.96 
 
 16 
 
 32 .07 
 
 10 .09 
 
 11 
 
 .34 
 
 10 .58 
 
 106 
 
 10 
 
 120.20 
 
 17 
 
 33 .06 
 
 11 .02 
 
 11 
 
 . 70 
 
 10 .26 
 
 102 
 
 10 
 
 119.34 
 
 18 
 
 34 • 02 
 
 jll .34 
 
 12 
 
 .02 
 
 9 .98 
 
 100 
 
 10 
 
 120.20 
 
 19 
 
 34 .95 
 
 11 .65 
 
 12 
 
 .37 
 
 9 • 70 
 
 98 
 
 10 
 
 121 .22 
 
 20 
 
 35 . 86 
 
 11 .95 
 
 1 
 
 12 
 
 .68 
 
 9 .46 
 
 94 
 
 10 
 
 119.18 
 
 1 
 
 2 
 
 1 
 3 
 
 4 
 
 ' 5 
 
 i 
 
 
 7 
 
 To construct a mill by this table, find the height of the fall 
 of water in the first column, and against that height in the 
 sixth column, is given the number of cogs in the wheel and 
 staves in the trundle, for causing a mill-stone four feet six 
 inches in diameter, to make 120 revolutions in a minute as 
 nearly as possible, when the circumference of the wheel mores 
 with one-third part of the velocity of the water. And it ap- 
 pears by the seventh column, that the number of cogs in the 
 wheel, and staves in the trundle, are so nearly adapted to the 
 required purpose, that the least number of revolutions of the 
 
MECHANICS. 349 
 
 Computations of the efl'ects of water-wbeels. 
 
 millstone in a minute is 118, and the greatest number ex- 
 ceeds not 121, which is according to the speed of some of the 
 best mills. 
 
 It should be observed, that the breadth of the water-wheel 
 ought to correspond with the power necessary on the occasion, 
 supposing that a proportionate volume of water is at command ; 
 for a wheel of two feet in breadth will be more than doubly as 
 powerful as one only a foot broad, there being a double volume 
 of water acting upon it, while the friction of the axis is by no 
 means doubled with this augmentation of breadth. 
 
 To compute the effects of water-wheels with precision, it is 
 necessary to ascertain, 1. The real velocity of the water which 
 acts upon the wheel; 2. the quantity of water expended in a 
 given time ; and 3. how much of the power is lost by friction. 
 After a variety of experiments, Smeaton found that the mean 
 power of a volume of water 15 inches in height gave 8.96 feet 
 of velocity in each minute to a wheel on which it impinged. 
 The computation of the power to produce such an effect, al- 
 lowing the head of water to be 103.8 inches, gave 264.7 pounds 
 of water descending in one minute through the space of 15 
 inches ; therefore 264.7, multiplied by 15, was equal to 3.970. 
 But as that power will raise no more than 9.375 pounds to the 
 height of 135 inches, it was manifest that the major part of the 
 power was lost; for the multiplication of these two sums only 
 amounted to 1,266; of course the friction was equal to three- 
 fourths of the power. The distinguised Engineer above-men- 
 tioned, considers this the maximum single effect of water upon 
 an undershot-wheel, where the fall is fifteen inches. The re- 
 mainder of power, it is plain, must equal that of the velocity 
 of the wheel itself, multiplied into the weight of the water, 
 which in this case brings the true proportion between the power* 
 and the effect to be as 3,849 to 1,266, or as 11 to 4. 
 
 Care should be taken to make the float-boards rather nu- 
 merous than few. Smeaton found, that in undershot-mills, 
 when he reduced the number of floats from twenty-four to 
 twelve, the effect was reduced one-half, because the water 
 escaped between the floats without touching them ; but when 
 he added a circular sweep of such length, that before one 
 float-board quitted it, another had entered it, he found the 
 former effect nearly restored. This mode more particularly 
 applies to breast-wheels, or such as receive the water imme- 
 diately below the level of the axis. In such the circular 
 trough is necessary, to make the water communicate the full 
 effect desirable from the joint operation of velocity and weight. 
 In wheels of this kind, the float-boards should be con6ned 
 both at their sides and at their extremities, so that the water 
 
350 MECHANICS. 
 
 The overshot-wheel the most powerful. 
 
 mav accompany them all the way from the head down to the 
 lowest part of the wheel, whence it should pass off with suf- 
 ficient readiness to allow the succeeding fall to supply its 
 place, without being in the least retarded. Any quantity of 
 \\ ater remaining in the trough, at the bottom of a breast-wheel 
 in particular, must tend to oppose its motion, in the exact 
 ratio with the disposition of the fluid to become stagnant or 
 stationary. It has been ascertained that a very sensible ad- 
 vantage is gained by inclining the float-boards to the radius 
 of the wheel, so that each float-board, when lowest, shall not 
 be vertical, but have its edge turned up the stream about 
 twenty degrees. 
 
 The overshot-wheel is by far the most powerful; both be- 
 cause it receives the water at the very commencement of its 
 descent, and because the buckets with which it is ordinarily 
 furnished retain the power so long, the water being gradually 
 discharged, as these buckets successively become inferior 
 parts of the circumference. It may be proper to state, in this 
 place, that much may be effected by allowing the water merely 
 to flow upon the upper part of the wheel, into the superior 
 buckets, whereby an immense auxiliary force is erected as 
 they successively become filled. Add to this, Smeaton's dis- 
 covery, that " the more slowly any body descends by the force 
 of gravity while acting upon any piece of machinery, the more 
 of that force will be spent upon it, and consequently the eflect 
 will be the greater." That eflect is by no means increased in 
 proportion to the velocity of the wheel's motion ; on the con- 
 trary, Smeaton found, that when the wheel with which he ex- 
 perimented, and which was two feet in diameter, revolved 20 
 times in a minute, its effect was greatest: when it made only 
 18^ turns, the eflect was irregular : and when so laden as not 
 to make 18 turns, the wheel was overpowered by the load. 
 He found that 30 turns in the minute occasioned a loss of 
 about one-twentieth, and that when turned above 30 times in 
 a minute, the diminution of effect was nearly one-fourth of its 
 powers. This proportion may be easily estimated on any 
 wheel of greater extent, by computing the proportion of ac- 
 cumulated power lost by greater velocity than may be sufficient 
 to load the wheel by means of the buckets being filled ; ob- 
 serving that the progress of a machine may be so much re- 
 tarded as to cause the effect to be irrelevant of the purpose, 
 although the machine may be kept in motion. Some machines 
 do their work well, simply in consequence of a certain celerity, 
 as is generally the case in a grinding apparatus : and every 
 person conversant in the practice of agriculture is aware, that 
 when a plough is drawn at a certain pace, it will cut the soil 
 
MECHANICS. 351 
 
 Power of the breast-wbeel. — Management of the supply of water. 
 
 regularly and freely, while, on the other hand, the same cattle 
 proceeding at a very slow pace will be more fatigued though 
 they do less work, and that diminished quantity of work by 
 no means so neatly executed. 
 
 The breast-wheel, when well constructed, will carry an effect 
 equal to half or even three-fifths of the power, while the over- 
 shot-wheel will work with a result equal to four-fifths of the 
 power; yet in general, from inattention to the lessening of 
 friction, and other imperfections of construction, the overshot- 
 wheel does not perhaps perform work beyond half the power, 
 and the effect of the breast-wheel, from similar causes, is pro- 
 portionately reduced. 
 
 When the stream would supply too much water, the redun- 
 dancy can in general be easily carried ofi' by sluices or over- 
 flows, constructed for the purpose ; and when it is desirable 
 to increase the velocity of the usual current, much may be 
 done towards the complete attainment of this object, by con- 
 tracting the banks. It is also very obvious, that by giving 
 additional height to the fall, or head, whence the water flows 
 upon the wheel, velocity, or at least power, may be greatly 
 augmented. 
 
 In situations where the supply of water, though often super- 
 abundant, is at other times liable to be greatly deficient, the 
 propriety of forming a suitable reservoir, from which the sup- 
 ply may be derived in seasons of drought, deserves to be con- 
 sidered. In some cases, the expense of carrying such a plan 
 into execution, would doubtless exceed any advantage it 
 would produce : but in others it might be adopted with the 
 happiest success ; for it will certainly be understood, that 
 such a reservoir is not necessarily required to be near the mill, 
 but at any part of the course of the stream where the cheap- 
 ness of the land combines with its suitableness in other re- 
 spects for the purpose. 
 
 Attempts have been made to construct water-wheels which 
 receive the impulse obliquely, like the sails of a common 
 windmill. By this means a slow but deep river could be made 
 to drive our mills; though much power would be lost by the 
 obliquity. Dr. Robinson describes one that was very power- 
 ful ; it was a long cylindrical frame, having a plate standing 
 out from it about a foot broad, and surrounding it with a very 
 oblique spiral like a cork-screw. This was immersed nearly 
 a quarter of its diameter, (which was twelve feet,) having its 
 axis in the direction of the stream. By the work performed, 
 it seemed more powerful than a common wheel that occupied 
 the same breadth of the river. Its length was not less than 
 twenty feet ; had it been twice as long, it would have nearly 
 
352 MECHANICS. 
 
 TuoUied wheel-worlt. 
 
 doubled its power, without occupying more of the water-way. 
 Perhaps such a spiral continued quite to the axis, and moving 
 in a suitable canal, wholly filled by the stream, might be an 
 advantageous way of employing a deep and sluggish cur- 
 rent. 
 
 Emerson observes, that the teeth of wheels ought not to act 
 upon each other before they arrive at the line which joins their 
 centres; and though the inner or under sides of the teeth may 
 be of any form, vet it is better to make both sides alike, that 
 the wheels may admit of being revolved either way w ith equal 
 facility. The utility of making the teeth as fine as the case 
 admits, so that the greatest number possible may be in con- 
 tact at once, has already been insisted on; and the utmost 
 care should be taken to have them so regularly disposed that 
 they may not interfere with each other before they begin to 
 vork. 
 
 It is of the greatest consequence to have the teeth so form- 
 ed, that the pressure by which one of them urges the other 
 round its axis, may be constantly the same. This is by no 
 means the case, when the common construction of a spur- 
 wheel, acting in the cylindrical staves of a lantern or trundle, 
 is used. The ends of teeth should never be formed of parts 
 of circles, unless working with other teeth specifically adapted 
 to them, as will be more fully explained hereafter. 
 
 The wheels and pinions of the best clock and watch-work, 
 are made true with almost mathematical precision ; but in 
 treating of the endless screw, we have had occasion to notice 
 that many great impediments combine to prevent very large 
 wheels, either in wood or metal, from possessing that ab- 
 solute correctness of form which is so much to be desired. 
 In consequenc-e, the trundle seldom divides the wheel so ex- 
 actly, as to make a given number of revolutions for one of the 
 wheel without a fraction ; but as any exact number is not 
 necessary in mill-work, and the cogs and rounds cannot be 
 set in so truly as to make all the intervals between them pre- 
 cisely equal, it is a useful precaution, which skilful mill- 
 wrights seldom fail to adopt, to give the wlieel what is called 
 a huntinn-coz ; that is, one cog more than what will answer to 
 an exact division of the wheel by the trundle. This being 
 done, every cog, as it comes to the trundle, will take the next 
 staff or round behind the one which it took in the former re- 
 volution ; and by this means, the parts of the cogs and rounds 
 which work together, will, in a little time, be worn equally, 
 and to equal distances from one another. 
 
MECHANICS. 353 
 
 Setting out a spur-wheel — wallower — face-wbeel — spur-nuts. 
 
 The Method of setting out Wheels. 
 
 For a spur-wheel and wallower, draw the pitch lines A 1, 
 B 1, A 2, B2, (fig. 1, pi. IV.) then divide them into the num- 
 ber of teeth or cogs required, as a 6 c. Divide one of these dis- 
 tances, as b c, into seven equal parts, as, 1, 2, 3, 4, 5, 6, 7 : allow 
 three parts for the thickness of the cogs, as 1,2, 3, in the cog 
 a; and four for the diameter of the stave of the wallower, as, 1, 
 2, 3, 4, in the stave m, fig. 2. Three parts are allowed for the 
 cog, and four for the stave, because the wallower is supposed to 
 be of less diameter than the wheel, therefore subject to more 
 wear, in proportion as the number of cogs exceed the number of 
 staves ; but if in any case the number of staves and cogs be the 
 same, they may be of equal thickness. The height of the cog 
 is equal to four parts ; then divide its height into five equal parts, 
 as 1,2, 3, 4, 5, in the cog c; allow three for the bottom to the 
 pitch line of the cog ; the other two parts for the curve which 
 must be given it to make it fit and bear on the stave equally. 
 
 In common practice, the millwrights are accastomed to put 
 the point of a pair of compasses in the dot 3 of the cog a, and 
 strike the line d e; then they remove the point of the com- 
 passes to the point d, and strike the curve 3/, by which means 
 they obtain a curve which they consider sufficiently correct 
 for their purpose. 
 
 For a face-wheel, tJie following method is adopted : divide 
 the pitch line AB, fig. 2, into the number of cogs intended, as 
 a b c; divide the distance b c, into seven equal parts; three of 
 those parts allow for the thickness of the cogs, as 1, 2, 3, in 
 the cog a, four for the height, and four for the width, as d e, 
 and four for the thichness of the stave m. Draw a line through 
 the centre of the cog, as the line A 1, at S ; and on the point 5 
 describe the line d e; remove the compasses to the point A, 
 and draw the line f g, by which the shape of the cog will be 
 determined. 
 
 For common spur-nuts, divide the pitch-line. A, fig. 3, into 
 twice as many equal parts as there are intended to be teeth, 
 as a, b, c, d, e; with a pair of compasses opened to half the 
 distance of any of these divisions, from the points a 1, c 3, e 5, 
 draw the semi-circles a, c, and e, which will form the ends of 
 the teeth. From the points 2, 4, and 6, draw the semi-circles 
 g h i, which will form the lower parts of the spaces. Though 
 spur-nuts are usually set out in this manner, yet it should be 
 remembered, that in all good work, the ends of the teeth must 
 not be semi-circles unless working with other teeth adapted 
 to them, as will be aftei'wards noticed. 
 15.— Vol. I. 2 Z 
 
354 MECHANICS. 
 
 Nature of bevel-peer. 
 
 Of Bevel-Geer. 
 
 Instead of spur-wheels and trundles, bevelled wheels, more 
 commonly called hevel-geer, are now generally used. Wheels 
 of this class, it may be shewn, are, in effect, truncated cones, 
 rolling on the surface of each other. Suppose the cones A 
 and B, revolving on their centres a p, a c, fig. 4, pi. IV ; if 
 their bases are equal, they will perform their revolutions in 
 equal times, and consequently any two points equally distant 
 from the centre a of A, b.s a b, a c, a d, a e, will revolve in the 
 same time as af, a g, a h, a i. In like manner, if one of the 
 cones, as in fig. 5, be twice the diameter of the other at the 
 base, and they are turned upon their centres, the base of the 
 larger will only have made one revolution, while that of the 
 smaller will have made two revolutions ; and all the corre- 
 sponding parts of the conical surfaces will observe the same 
 proportion; that is, a b, a c, a d, a e, will turn only once round, 
 while af, a g, u h, a i, turn twice round. Hence it is obvious, 
 that the number of the revolutions of all cones revolving in 
 this manner, must be to each other as their respective dia- 
 meters. IVow let two cones have teeth cut in them; as repre- 
 sented by fig. 6 ; they will then become bevel-geer. The teeth 
 in an entire cone would be broadest at the base, from whence 
 they would gradually taper with the lessening circumference 
 of the cone, till they terminated at the apex or centre a in a 
 point ; but as such an extent of teeth would be unnecessary, and 
 if the cones were entire the axes at a would incommode each 
 other, the slender useless part of the teeth are cut oft", 
 as at E and F ; or rather, bevel-geer is composed of wheels 
 made in the form of truncated cones, as shewn by fig. 7, 
 where the upright shaft or axle, AB, with the bevel- 
 wheel CD, turns the bevel-wheel EF, with its shaft GH, 
 and the teeth work, freely in each other. The teeth may be 
 made of any dimensions, according to the strength re- 
 quired ; and this method will enable them to overcome a 
 greater resistance, and work much more smoothly than a com- 
 mon face-wheel and trundle ; besides, the facility with which 
 it enables us to change the direction of a motion, is of great 
 importance. 
 
 The method of conveying motion in any direction, and of 
 
 f»roportioning or shaping the wheels accordingly, is as follows : 
 et the line a b, fig. 8, represent a shaft coming from a wheel; 
 draw the line c d to intersect the line a b, in the direction 
 intended for the motion to be conveyed, and this line c d will 
 represent the shaft of the bevel-wheel which is to receive the 
 
MECHANICS. 355 
 
 Universal joint — Cycloid and epicycloid. 
 
 motion. Suppose then the shaft c J is to revolve three times, 
 whilst the shaft a b revolves once; draw the parallel line i i at 
 any moderate distance (suppose one foot by a scale,) then 
 draw the parallel line k k, at three feet distance, after which 
 draw the dotted line xo x, through the intersection of the shafts 
 a b and c d, and likewise through the intersection of the pa- 
 rallel lines i i and k k, in the points x and y, which will be the 
 pitch-line of the two bevel-wheels, or the line where the teeth 
 of the two wheels agt on each othex", as may be seen by fig. 9, 
 where it is obvious to inspection that the motion may be con- 
 veyed in any direction. 
 
 Of Hooke's Universal Joint. 
 
 The contrivance called the universal joint, which was invented 
 by Dr. Hooke, may be applied to communicate motion instead 
 of bevel-geer, where the velocity is not to be changed, and where 
 the angle does not exceed 30 or 40 degrees. This joint may 
 be constructed as represented by fig. 10; or with four pins fast- 
 ened at I'ight angles upon the circumference of a hoop, or solid 
 ball. It is useful in cotton-mills, where the tumbling shafts 
 are continued to a great distance from the moving power, as 
 the use of it allows the convenience of cutting them into con- 
 venient lengths. It is most proper, when the irregularity of its 
 motion, as it recedes from a right line, is not disadvantageous. 
 
 Of the Cycloid and £,picycloid, and the formation of the Teeth 
 
 of Wheels. 
 
 If on the plane CD, fig. 11, a circle B, proceeds in a 
 right line, and at the same time revolves round its centre, till 
 every part of the circumference has touched the plane, a point 
 or pencil, at a, which was lowest at the commencement of the 
 motion, will have described the curve CED, which is called a 
 cycloid, and is evidently compounded of a rectilinear and cir- 
 cular motion. 
 
 If a circle A, fig. 12, roll from o to q, on the convex circum- 
 ference of another circle B, the point o will describe the curve 
 p q, which is called an exterior epicycloid; and if the circle A 
 were to roll on the concave circumference of the circle B, as 
 from r to s, the point r would describe an interior epicycloid. — 
 In all these cases, the circle by which the curve is obtained, is 
 called the generating circle. 
 
 The teeth of wheels and leaves of pinions require great 
 care and judgment in their formation, that they may neither 
 clog the machinery by unnecessary friction, nor act so irregu- 
 
356 MECHANICS. 
 
 Modes in Nvhicli teeth act upou eacli tither. 
 
 larly as to produce any inequalities in the motion, and the 
 wearing of one part before another. It has long been known 
 that one wheel will not drive another with uniform velocity, 
 unless the teeth of one or more of the wheels have their act- 
 ing surfaces formed into a curve generated after the manner 
 of an epicycloid. But in order to ensure a uniformity of pres- 
 sure and velocity in the action of one wheel upon another, it 
 is not absolutely necessary that the teeth of one or both 
 wheels be exactly epicycloids ; for if the teeth of one of 
 them be either circular or triangular, with plain sides, or like 
 a triangle with its sides converging to the centre of the wheel, 
 or of any other form, this miiformity of force and motion 
 will be attained, ])rovided that the teeth of the other wheel 
 have a figure which is compounded of that of an epicycloid, 
 and the figure of the teeth of the first wheel. De la Hire 
 has shewn, in a variety of cases, how to find this compound 
 curve ; but as it is often difficult to describe, or even to 
 discover its nature, we shall select such forms for the teeth, 
 as are better adapted to practice. There are three different 
 ways in which the teeth of wheels may act upon one 
 another ; and each mode of action requires a different form 
 for the teeth : 
 
 1st. When the teeth of the wheel begin to act upon the 
 leaves of the pinion just as they arrive at the line of centres ; 
 and their mutual action is carried on after they have passed 
 this line. 
 
 2nd. When the teeth of the wheel begin to act upon the 
 leaves of the pinion, before they arrive at the line of centres, 
 and conduct them either to this line or a very little beyond it. 
 
 3rd. When the teeth of the wheel begin to act upon the 
 leaves of the pinion, before they arrive at the line of centres, 
 and continue to act after they have passed that Ime. 
 
 When the first mode of action is adopted, the acting faces 
 of the leaves of the pinion should be parts of an interior epicy- 
 cloid, generated by a circle of any diameter rolling upon the 
 concave superficies of the pinion ; and the acting surfaces of the 
 teeth of the wheel should be portions of an exterior epici/cloid, 
 formed by the same generating circle rolling upon the convex 
 superficies of the wheel. Now it is demonstrable, that when 
 one circle rolls within another whose diameter is double that 
 of the rolling circle, the line generated by any point of the 
 latter will be a straight line tending to the centre of the larger 
 circle. If the generating circle, therefore, mentioned above, 
 should be taken with its diameter equal to the radius of the 
 
MECHANICS. 357 
 
 Formatian of the teeth of wheels. 
 
 pinion, and be made to roll upon the concave superficies of 
 the pinion, it will generate a straight line tending to the pi- 
 nion's centre, which will be the form of the acting faces of its 
 leaves ; and the teeth of the wheel will in this case be exterior 
 epicycloids, formed by a generating circle, whose diameter is 
 equal to the radius of the pinion, rolling upon the convex 
 superficies of the wheel. This construction of the teeth of 
 the wheel, and leaves of the pinion, is represented by fig. 13, 
 pi. IV; it is strongly recommended by De la Hire and Camus, 
 and is perhaps the most advantageous, as it requires less trou- 
 ble, and may be executed with greater accuracy than if the 
 leaves of the pinion had been curved as well as the teeth of 
 the wheel. 
 
 Lanterns or trundles, which consist of cylindrical staves 
 fixed by both ends nearly at the circumferences of two equal 
 circular boards, and which are so frequently substituted by 
 millwrights for pinions, may often be adopted with great pro- 
 priety, provided the teeth of the wheels working in them, have 
 a proper form. The construction pointed out by Dr. Brewster, 
 which we shall here present to the reader, possesses the merit 
 of greatly diminishing the friction arising from the mutual ac- 
 tion of the staves and the teeth, and of being easily reduced 
 to practice. 
 
 Let A, fig. 14, pi. IV, be the centre of the small wheel or 
 trundle, TCHQ, whose teeth are circular like ICR, having their 
 centres in the circle PDEY. Upon B, the centre of the large 
 wheel, at the distances BC, BD, describe the circles FCK, 
 GDO ; and with PDEY, as a generating circle, form the ex- 
 terior epicycloid DNM, by rolling it upon the convex super- 
 ficies of the circle GDO. The epicycloid DNM thus formed, 
 would have been the proper form for the teeth of the large 
 wheel GDO, had the circular teeth of the small wheel been 
 infinitely small; but as their diameter must be considerable, the 
 teeth of the wheel should have another form. In order to 
 determine their proper figure, divide the epicycloid DNM into 
 a number of equal parts, 1, 2, 3, 4, &.c. as shewn in the figure, 
 and let these divisions be as numerous as possible. Then, upon 
 the points 1, 2, 3, &c. as centres, with the distance DC equal 
 to the radius of the circular tooth, describe portions of circles 
 similar to those in the figure ; and the curve OPT, which 
 touches these circles, and is parallel to the epicycloid DNM, 
 will be the proper form for the teeth of the large wheel. 
 
 In order that the teeth may not act upon each other till they 
 reach the line of centres AB, the curve OP should not touch 
 the circular tooth ICR till the point O has arrived at D. The 
 tooth OP, therefore will commence its action upon the circu- 
 
358 MECHANICS. 
 
 FurmatioD of the teeth of wheels. 
 
 lar tooth at the point I, wliere it is cut by the circle DRE. 
 On this account, the part ICR of the cylindrical pin being 
 superfluous, may be cut off, and the staves of the trundle will 
 then be segments of circles similar to the shaded part of 
 the figure. 
 
 l( the teeth of wheels and the leaves of pinions consisted of 
 materials perfectly hard, and were accurately formed according 
 to these directions, they would act on each other not only with 
 uniform force, but also without friction ; because the surfaces 
 in contact would roll upon each other, and neither slide nor 
 rub so as to occasion any imperfection in the performance. 
 But as it is impossible in practice to attain the perfection 
 which theory requires, a certain quantity of friction will re- 
 main after everv precaution has been taken in the formation of 
 the communicating parts. This friction may be removed, or 
 at least greatly diminished, with respect to a trundle, in the 
 following manner. 
 
 If instead of fixing the staves as in the customary manner, 
 at the top and bottom, they are made capable of receiving a 
 rotary motion in their frame, all the friction will be taken 
 away except that which arises from the motion of the cylin- 
 drical tooth upon its axis. The advantages attending this 
 mode of construction are very important. The cylindrical 
 staves or teeth may be formed in the lathe with the greatest 
 accuracy; the curve required for the teeth of the large wheel is 
 easily traced ; the pressure and motion of the wheels will be 
 uniform : and the teeth are very little subject to wear, because 
 whatever friction remains is almost wholly removed by the revo- 
 lution of the cylindrical spokes about their axis. This improve- 
 ment, however, can only be adopted where the machinery is 
 large ; for small works, the acting faces of the leaves of the 
 pinion or small wheel should be rectilinear, and those of the 
 large wheel epicycloidal, as exemplified by fig. 13. 
 
 We have now to consider the second mode of the mutual 
 action of wheels and pinions, viz. " when the teeth of the wheel 
 begin to act upon the leaves of the pinion, before they arrive at 
 the line of centres, and conduct them either to this line or a 
 very little beyond it." This mode of action is by no means so 
 advantageous as the former, and therefore should, if possible, 
 be always avoided. It is evident, that when the tooth of the 
 wheel acts upon the leaf of the pinion before they arrive at the 
 line of centres, and quits the leaf when they reach this line, 
 that the tooth works deeper and deeper between the leaves of 
 the pinion the neartr it comes to the line of centres ; hence a 
 considerable quantity of friction arises, because the tooth does 
 not, as before, roll upon the leaf, but slides upon it; and from 
 
MECHANICS. 359 
 
 Formation of the teeth of wheels. 
 
 the same cause, the pinion soon becomes foul, as the dust 
 which lies upon the acting faces of tl^ie wheels is pushed into 
 the hollows between them. One advantage, however, attends 
 this mode of action, for it allows us to make the teeth of the 
 large wheel rectilineal, and thus renders the labour of the 
 mechanic less, and the accuracy of his work greater, than if 
 they had been of a curvilineal form. If the teeth therefore of 
 the wheel are made rectilineal, having their surfaces directed 
 to the wheel's centre, the acting surfaces of the leaves must 
 be epicycloids formed by a generating circle, whose diameter 
 is equal to the sum of the radius of the wheel, added to the 
 depth of one of its teeth, rolling upon the circumference of 
 the pinion. But if the teeth of the wheel and the leaves of 
 the pinion are made curvilineal, the acting surfaces of the teeth 
 of the wheel must be portions of an interior epicycloid formed 
 by any generating circle rolling within the concave superficies 
 of the large circle, and the acting surfaces of the pinion's leaves 
 must be portions of an exterior epicycloid, produced by roll- 
 ing the same generating circle upon the convex circumference 
 of the pinion. 
 
 When the teeth of the large wheel are cylindrical spindles, 
 either fixed or moveable upon their axis, an exterior epicycloid 
 must be formed like DNM, in fig. 14, pi. IV, by a generating 
 circle whose radius is AC, rolling upon the convex circumfe- 
 rence FCK ; AC being in this case the diameter of the wheel, 
 and FCK the circumference of the pinion. By means of this 
 epicycloid, a curve OPT must be formed as before described, 
 which will be the proper curvature for the acting surfaces of the 
 leaves of the pinion, when the teeth of the wheel are cylin- 
 drical. In determining the relative diameter of the wheel 
 and pinion for this mode of action, the radius of the wheel is 
 reckoned from its centre to the extremity of its teeth, and the 
 radius of the pinion from its centre to the bottom of its leaves. 
 
 The third mode in which one wheel may drive another, viz. 
 " when the teeth of the wheel begin to act upon the leaves of 
 the pinion before they arrive at the line of centres, and continue 
 to act after they have passed that line," remains to be con- 
 sidered. It is represented by fig. 1, pi. V, and as it is a 
 combination of the two first modes, it partakes both of their 
 advantages and disadvantages. It is evident from the figure, 
 that the portion e h of the tooth acts upon the part b c of the 
 leaf till they reach the line of centres AB, and that the part 
 e d oi the tooth acts upon the portion 6 a of the leaf after they 
 have passed that line. It follows, therefore, that the acting 
 parts e h and b c must be formed according to the directions 
 given for the first mode of action, and that the remaining 
 
3G0 MECHANICS. 
 
 Formation of the teeth of wheels. 
 
 parts e d, b a, must have that curvature which the second mode 
 of action requiiies ; consequently e h should be part of an inte- 
 rior epicycloid formed by any generating circle rolling on the 
 concave circumference of the wheel, and the corresponding 
 part 6 c of the leaf should be part of an exterior epicycloid 
 formed by the same generating circle rolling upon 6 E o, the 
 convex circumference of the pinion ; the remaining part e d of 
 the tooth should be a portion of an exterior epicycloid, formed 
 by any generating circle rolling upon e L, the convex super- 
 ficies of the wheel ; and the corresponding part b a o( the leaf 
 should be part of an interior epicycloid described by the same 
 generating circle rolling along the concave side 6 E o of the 
 pinion. But, as in practice, the production of this double 
 curvature of the acting surfaces of the teeth would be ex- 
 ceedingly troublesome to the workman, who would probably 
 never correctly accomplish his object, his labour may be 
 abridged by making e h and b a radial lines, that is, e A a 
 straight line tending to the centre of the wheel B, and b a like- 
 wise a straight line tending to the centre A, of the pinion. 
 
 In the preceding remarks, the form assigned to the teeth has 
 been stated on the supposition that the wheel drives the pinion ; 
 but when, on the contrar\% the pinion drives the wheel, the 
 form assigned to the teeth of the wheel must be given to the 
 leaves of the pinion, and the shape assigned to the leaves of 
 the pinion must be transferred to the teeth of the wheel. 
 
 A still different mode of forming the teeth of wheels has 
 had many advocates, who have considered it well calculated 
 to ensure the uniformity of action so much desired. It con- 
 sists in making the acting faces of the teeth involutes of 
 the wheel's circumference. Thus, let AB, fig. 2, pi. V, be a 
 portion of the wheel on which the tooth is to be fixed, and let 
 A ^ o be a thread wrapped round its circumference, having a 
 loop-hole at its extremity, a. In this loop-hole fix a pin a, 
 with which describe the curve or involute, a b c d e h, by un- 
 wrapping the thread gradually from the circumference A p m. 
 The curve thus obtained will be the proper form for the teeth 
 of a wheel whose diameter is AB. It is a form which admits 
 of several teeth acting together, a circumstance attended with 
 the advantage of diminishing the pressure upon any one tooth 
 so much as to make the wheels wear longer and more equally; 
 and it possesses the merit of being; more easily understood 
 than the other methods directed to be observed. 
 
 This last mode of forming the teeth of wheels, is, however, 
 only a modification of the general principle, and indeed an 
 involute is sometimes reckoned among the exterior epicycloids. 
 The propriety of this will be allowed, when it is considered 
 
INSERT FOLDOUT HERE 
 
MECHANICS. 361 
 
 The drawing of epicycloids. 
 
 that the involute abed, Sec. may be produced by an epicy- 
 cloidal motion. Thus, let o /i be a straight ruler, at whose ex- 
 tremity is fixed the pin ;/, and let the point of the pin be placed 
 upon the point m of the circle, then by rolling the straight 
 ruler upon the circular base, so that the point in which it 
 touches the circle may move gradually from 7u towards B, the 
 curve m n will be generated exactly similar to the involute 
 a be, See. obtained by the string. 
 
 The practical mechanic may wish to have more particular 
 directions for drawing epicycloids, than he can derive from the 
 explanation of these curves at the commencement of the pre- 
 sent section. For this purpose then, let him take a piece of 
 plain wood G H, fig. 3, pi. V, and fix upon it another piece of 
 wood E, having its circumference m b of the same curvature 
 as the circular base upon which the generating circle AB is to 
 roll. When the generating circle is large, the shaded segment 
 B will be sufficient. In any part of the circumference of this 
 segment, fix a sharp-pointed steel pin a, which ought to be 
 tempered, that it may easily make a distinct mark ; and it must 
 be driven in sloping, so that the distance of its point from the 
 centre of the circle may be equally exact to its radius. Fasten 
 to the board GH, a piece of thin brass, or copper, or tin-plate, 
 a b. Place the segment B in such a position, that the point 
 of the steel pin a may be upon the point b, and roll the seg- 
 ment towards G, so that the nail a may rise gradually, and the 
 point of contact between the two circular segments may ad- 
 vance towards m ; the curve a b, described upon the brass 
 plate, will be an accurate exterior epicycloid. Remove, with 
 a file, the part of the brass on the left hand of the epicycloid, 
 and the remaining concave arch a b will be a pattern tooth, by 
 means of which all the rest may easily be formed. When an 
 interior epicycloid is required, the generating circle must re- 
 volve upon a eoiicave instead of a eonuex base, as in the present 
 instance. The ei/eloid, which is useful in forming the teeth of 
 r^ek-work, is generated in precisely the same manner, with 
 this difference only, that the base on which the generating 
 circle rolls must be a straight line. 
 
 Perhaps no part of the mechanism of mill-work is executed 
 with so little attention to theory as the teeth of wheels. Al- 
 most every celebrated millwright has his favourite construc- 
 tion, and it is seldom indeed that the best methods are adopt- 
 ed. Gregory describes one of the many plans in ordinary use, 
 and we shall here recite it ; from its being, as he observes, of 
 tolerably easy application, and allowing much strength to the 
 teeth, while it is tolerably free from friction in comparison with 
 other practical methods. Let AB, fig. 4, pi. V, be two spur- 
 16.— Vol. I. 3 A 
 
362 MECHANICS. 
 
 loitance of the common mode of furniing cogs. — Forms of wipers. 
 
 wheels of different diameters, of which the cogs are intended 
 to work into each other at half pitch. The dotted circular arcs 
 GH, EF, touching each other between s and d, are the centre 
 or pitch-lines, from which the teeth are formed. If the teeth 
 of both wheels are iron, as is generally the case in the first 
 motions of works, those teeth are then made nearly both of a 
 size at the pitch-line ; but if the teeth of one be wood and the 
 other iron, then the iron ones are made to have less pitch than 
 the wooden ones, because they are then found to wear better. 
 In the figure, both are supposed to be of iron. Suppose the 
 wheels to move from G towards H, and from E towards F, and 
 that the sides of the teeth at b c, and d e, are in contact; from 
 6 as a centre, with a radius equal to b p, describe the arcs p d, 
 1 711 : from c? as a centre, with the same radius, describe the 
 arcs h i, fg, c k. Thus the same opening of the compasses, 
 and a centre chosen where the wheels are in contact on the 
 pitch-lines, will mark the contour of the upper part of a tooth 
 of one wheel, and the lower part of a corresponding tooth of 
 the other wheel ; and by taking several centres on the two 
 pitch-lines, the various teeth may be formed. To prevent the 
 cogs from bottoming, as the workmen call it, let the lower part, 
 r e, of one tooth be made rather longer than the upper part, 
 p d, of the other which is to play into it. The way in which 
 cogs thus constructed will work into one another, may be un- 
 derstood by considering the motion of two of them, n and o 
 for example : when they first come into contact, they will ap- 
 pear as the curve x P 2; when they arrive at Q, the same sides 
 will appear as in the dotted lines there represented; and when 
 the same arrive at R S, they are in contact on their middle 
 points. 
 
 Of Wipers for raising Stampers and Hammers. 
 
 The notches which project from ths circumference of a 
 wheel or an axle, for the purpose of raising stampers, poun- 
 ders, or hammers in a vertical direction, and then leaving 
 them to fall by their own weight, are usually called wipers, 
 though sometimes denominated lifting cogs. 
 
 When the wipers are only small cylinders or pins projecting 
 perpendicularly from the surface of a horizontal arbor, the 
 force with which they elevate the stampers, &.c. will not act 
 uniformly during the whole time in which they are rising; 
 yet a uniformity of force and velocity is generally desirable, 
 and may always be obtained by assigning a proper form to the 
 communicating parts. On this subject, a few directions for 
 the use of the practical mechanic will take up little room. 
 
 Fig. 5 pi. V, represents portions of a stamper for bruising 
 
MECHANICS. 363 
 
 Forms of wipers. — Description of a corn-mill. 
 
 ore, beating hemp. Sec. and of its shaft with lifting cogs. G 
 is the vertical arm of the stamper, sliding, when in actual work, 
 between rollers or in a groove, to keep it steadily in its proper 
 position; a is the horizontal arm of the stamper; H part of the 
 axle, on which the wipers or lifting cogs EF are fixed; the 
 dotted lines at A, shew the height to which the horizontal arm 
 a, of the stamper, is elevated by each wiper. AB is a line 
 corresponding with the arm of the stamper upon which the 
 wipers first act ; CD is the pitch-line of the axis, or the bot- 
 tom of the curves of the wipers. The curved or acting faces 
 of the wipers are involutes of a circle equal in radius to the 
 axis CD, and obtained as already described in noting its ap- 
 plication to the formation of the teeth of wheels, viz. by un- 
 wrapping from the circumference of the circle alluded to, a 
 thread or cord b, in the loop-hole at the extremity of which is 
 a pencil or marking point, describing the curve as it ap- 
 proaches towards c. The arm of the stamper is flat at the part 
 where the wiper acts upon it, and should be placed in a line 
 with the centre of the shaft or axis, at the time the first wiper 
 comes into contact with it. 
 
 Fig. 6, pi. V, exhibits the form of the wipers for a forge 
 hammer. The centre b, of the cylinder AB, in which the 
 wipers are fixed, the flat part or tail end of the hammer, where 
 acted upon by the wipers, and the centre of the axis a, of the 
 hammer, must be in the same right line. The proper curve 
 for the wipers is an exterior epicycloid ; formed by rolling 
 upon the circumference of the circle at B, a circle of which 
 the radius is equal to the distance from the centre of the axis 
 a, to the extremity of the tail of the hammer. 
 
 Description of a Corn-mill, 
 
 The following is a description of a corn-mill of the most 
 common sort. AB, fig. 7, pi. V, is the water-wheel, which is 
 generally from eighteen to twenty-four feet in diameter, reck- 
 oning from the outermost edge of any float-board at A, to that 
 of the opposite one at B. The water striking on the floats of 
 this wheel, drives it round, and gives motion to the mill. The 
 wheel is fixed upon a very stong axis or shaft C, one end of 
 which rests on D, and the other on E within the mill-house. 
 
 On the shaft or axis C, and within the mill-house, is awheel 
 F, about eight or nine feet in diameter, having cogs all round, 
 which work in the upright staves or rounds of a trundle G. 
 This trundle is fixed upon a strong iron axis, called the spin- 
 dle, the lower end of which turns in a brass foot fixed at H, 
 in a horizontal beam H, called the bridge-tree ; and the upper 
 end of the spindle turns in a wooden bush fixed into the nether 
 
364 MECHANICS. 
 
 Description of a corn-mill. 
 
 mill-stone, which lies upon beams in the tloor I. The top ot" 
 the spindle above the bush is square, and goes into a square 
 hole in a strong iron cross, a b c d, fig. 8, called the rynd; un- 
 der which, and close to the bush, is a round piece of thick 
 leather upon the spindle, which it turns round at the same 
 time it does the rynd. 
 
 The rynd is let into grooves in the under surface of the up- 
 per or running mill-stone, which it turns round in the same time 
 that the trundle G is turned round by the cog-wheel F. This 
 mill-stone has a large hole quite through its middle, called the 
 eye of the stone, through which the middle part of the rynd 
 and upper end of the spindle may be seen ; w hilst the four ends 
 of the rynd lie below the stone in their grooves. 
 
 One end of the bridge-tree H, which supports the spindle, 
 rests upon the wall, and the other end is let into a beam, called 
 the brayer, LM. The brayer rests in a mortice at L, and the 
 other end M, hangs by a strong iron rod X, which goes through 
 the floor I, and has a screw and nut on its top at O ; by the 
 turning of this nut, the end M of the brayer is raised or de- 
 pressed at pleasure, and consequently the bridge-tree and the 
 upper mill-stone. By this means the upper mill-stone may be 
 set as close to the under one, or raised as much above it as 
 may be necessary. It will of course be understood, that the 
 nearer the mill-stones are to each other, the finer the corn will 
 be ground; and that, on the contrary, the further they are se- 
 parated, the coarser it will be. 
 
 The upper mill-stone is inclosed in a round box, which no- 
 where touches it, and is about an inch distant from its edge 
 all round. On the top of this box stands a frame for holding 
 the hopper P, to which is hung the shoe Q, by two lines fas- 
 tened to the hinder part of it, fixed upon hooks in the hopper, 
 and by one end of the string R fastened to the fore part of it, 
 the other end being twisted round the pin S. By turning this 
 pin one way, the string draws up the shoe closer to the hopper, 
 and so lessens the aperture between them ; and as the pin is 
 turned the other way, it lets down the shoe, and enlarges the 
 aperture. If the shoe be drawn up quite to the hopper, no corn 
 can fall from the hopper into the mill; if it be let down a little, 
 some will fall ; and the quantity will be more or less, according 
 as the shoe is more or less let down; for the hopper is open at 
 the bottom, and there is a hole at the bottom of the shoe, not 
 directly under the bottom of the hopper, but nearer to the 
 lowest end of the shoe, over the middle or eye of the stone. 
 
 In a square hole at the top of the spindle, is put the feeder 
 E, fig. 8. This feeder, as the spindle turns round, jogs the 
 shoe three times in each revolution, and so causes the corn to 
 
MECHANICS. 365 
 
 Description of a cora-mill. 
 
 run constantly down from the hopper through the shoe, into 
 the eye of the mill-stone, where it falls upon the top of the 
 rynd, and is, by the motion of the rynd, and the leather under 
 it, thrown below the upper stone, and ground between it and 
 the lower one. The rapid motion of the stone, creates a cen- 
 trifugal tendency in the corn going round with it, by which 
 means it gets farther and farther from the centre, as in a spi- 
 ral, in every revolution, until it is quite thrown out; and being 
 then ground, it falls through a spout, called the mill-eye, into 
 a trough placed for its reception. 
 
 When the mill is fed too fast, the corn bears up the stone, 
 and it is ground too coarse; besides, the mill is apt to get 
 clogged, and to go too slowly. When the corn is scantily 
 supplied, the mill goes too fast, and the stones, by their col- 
 lision, are apt to strike fire. Both these inconveniences are 
 avoided, by turning the regulating pin S backward or forward, 
 in order to draw up or let down the shoe, as the case is ob- 
 served by the miller to require. 
 
 The heavier the runnino- mill-stone, and the greater the 
 quantity of water falling upon the wheel, the faster will the 
 mill bear to be fed, and consequently the greater the perform- 
 ance of the mill ; and, on the contrary, the lighter the stone, 
 and the less the quantity of water, the slower must be the 
 feeding. When the stone is considerably worn, and become 
 light, its weight must either be increased by some artificial 
 addition, or the mill must necessarily be fed slowly ; other- 
 wise the stone will be too much borne up by the corn under 
 it, to grind the meal sufiiciently fine. 
 
 The power necessary to turn a heavy mill-stone, is but very 
 little more than what is necessary to turn a light one ; for as 
 the stone is supported upon the spindle of the bridge-tree, and 
 the end of the spindle that turns in the brass foot is but small, 
 the difference arising from the weight produces only an incon- 
 siderable action against the power or force of the water. Be- 
 sides, a heavy stone affords the same advantage as a heavy fly, 
 that is, it regulates the motion much better than a light one, 
 from its not being liable to such great fluctuatio-ns of velocity. 
 
 The centrifugal force carrying the corn towards t*he circum- 
 ference of the stones, it is obvious that it will be crushed when 
 it comes to a place where the interval between the two mill- 
 stones is less than its thickness; yet, as the upper mi'11-sto.ne is 
 supported on a point which it can never quit, it may not be 
 considered equally obvious why it should produce a greater 
 effect when it is heavy than when it is light; since, if it were 
 equally distant from the nether miH-stone, it could only be 
 capable of a limited impression. But as experience proves 
 
366 MECHANICS. 
 
 Fomi uf the acting faces of niill-stODes. 
 
 that the difference actually occurs, it may be proper to state 
 the cause. The spindle of the mill-stone being supported by 
 a horizontal piece of timber, about nine or ten feet lon'ir, resting 
 only on both its ends, the upper mill-stone, by the elasticity 
 of this piece, is allowed a vertical motion, and plays up and 
 down ; by which movement, the heavier the stones are, the 
 more forcibly is the corn wedged in between them. 
 
 In order to cut and grind the corn, both the upper and un- 
 der mill-stones have channels or furrows cut in them, proceed- 
 ing obliquely from the centre to the circumference. These 
 furrows, in the direction of their length, are cut slantwise on 
 one side, and perpendicularly on the other, so that each of the 
 ridges which they form has a sharp edge ; and in the two 
 stones, these edges pass one another like the edges of a pair of 
 scissars, and so cut the corn, to make it grind the more easily, 
 when it falls upon the furrows. The furrows are cut the same 
 way in both stones, when they lie upon their backs, which 
 makes them run crosswise to each other when the upper stone 
 is inverted bv turning its furrowed surface towards that of the 
 lower; for, if the furrows of both stones laid the same way, 
 part of the corn would be driven onward in the lower furrows, 
 and come out from between the stones without being either 
 ground or bruised. 
 
 The grinding surface of the under stone is a little convex 
 from the edge to the centre, and that of the upper stone a little 
 concave; and they are farthest from one another in the middle, 
 but approach gradually nearer towards the edges. By this 
 means the corn, at its first entrance between the stones, is only 
 bruised; but as it goes farther on towards the circumference 
 or edge, it is cut smaller and smaller, and at last finely ground, 
 just before it comes out from between them. 
 
 When the ridges become blunt and the furrows shallow by 
 wearing, the running stone must be taken up, and both of them 
 may then be drest anew with a chisel and mallet. Every time 
 the stone is taken up, there must be some tallow put round the 
 spindle and upon the bush; this unguent will soon be melted 
 by the heat the spindle acquires from its turning and rubbing 
 against the bush, which it will prevent from taking tire. 
 
 The bush must embrace the spindle quite close, to prevent 
 any shake in the motion, which would cause some parts of the 
 stones to grate against each other, whilst the other parts of 
 them would be too far asunder, and bv that means spoil the 
 meal. Hence, whenever the spindle has worn the bush, so as 
 to begin to shake in it, the stone must be taken up, and a 
 chisel driven into several parts of the bush ; and when it is 
 taken out, wooden wedges must be forced into the holes; by 
 
MECHANICS. 367 
 
 Aniericau mode of raising ground corn to the top of tlie mill. 
 
 which means the bush will be made closely to embrace the 
 spindle ag;ain all round. In doing this, great care must be 
 taken to drive equal wedges into the bush on opposite sides of 
 the spindle; otherwise it will be thrown out of the perpendi- 
 cular, and so hinder the upper stone from being set parallel to 
 the under one, which is absolutely necessary for makino- good, 
 work. When any accident of this kind occurs, the perpendi- 
 cular position of the spindle must be restored, by adjusting 
 the bridge-tree with proper wedges put between it and the 
 brayer. 
 
 It often happens, that the rynd is a little wrenched in lay- 
 ing down the upper stone upon it, or is made to sink a little 
 lower on one side of the spindle than on the other; and this 
 will cause one edge of the upper stone to drag all round upon 
 the lower, while the opposite edge will not touch. This is 
 easily rectified, by raising the stone a little with a lever, and 
 putting bits of paper, card, or thin chips, between the rynd 
 and the stone. 
 
 We shall mention in this place a very useful and ingenious 
 contrivance, adopted by the American millwrights for raising 
 the ground corn to the cooling boxes, or place from which it 
 is conveyed into the bolting machine. They place a large 
 screw horizontally in the box which receives the flour from 
 the mill-stones. The thread or spiral line of the screw is com- 
 posed of pieces of wood about two inches broad and three 
 long, fixed into a wooden cylinder seven or eight feet in length, 
 which forms the axis of the screw. When the screw is turned 
 round this axis, it forces tlie meal from one end of the trouo-h 
 to the other, where it falls into another trough, from which it 
 is raised to the top of the mill-house by means of elevators, a 
 piece of machinery similar to the chain pump. These ele- 
 vators consist of a chain of buckets, or concave vessels like 
 large tea-cups, fixed at proper distances upon a leathern band,- 
 going round two wheels, one of which is placed at the top of 
 the mill-house, and the other at the bottom in the meal-trough. 
 W^hen the wheels are put in motion, the band revolves, and 
 the buckets, dipping into the meal-trough, convey the meal ta 
 the upper story, where they discharge their contents. The 
 band of buckets is inclosed in two square boxes, in order to 
 keep them clean, and preserve them from injury. It is ob- 
 vious how much more complete this contrivance is, than the 
 mode adopted in this country, of putting the meal into sacks, 
 and then raising it up by the common machinery for that 
 purpose. 
 
368 MECHANIC'S. 
 
 The inecbanism of a horse witli respect to draught. 
 
 Of Wheel-carriages. 
 
 In considering this subject, it will be proper to advert to the 
 formation of the animal by which wheel-carriages are put in 
 motion. The horse is admirably calculated for draught, and 
 the circumstances enablintj him to draw to the greatest ad- 
 vantage are, to a certain extent, so well known to every on* at 
 all conversant with mechanics, that it is not less a cause of 
 surprise than of regret, that his valuable properties should still 
 continue to be so much abused as we find them to be. But 
 information spreads slowly among the mass of the people, and 
 it is long in reaching provincial wheelwrights, among whom, 
 as amongst other classes of men, there is a disposition to fol- 
 low the practice of their forefathers, without inquiring whether 
 they are right or wrong. Much as men are attached to their 
 interest too, the arrogance of dominion, even over a brute, 
 often renders them cruel in opposition to it; and cruelty can 
 never bear the light of reason in her path. When the broad- 
 wheel act was passed, a great outcry was raised against it by 
 those whom it would have benefited, and who, instead of com- 
 plying vviih it, perversely rendered its provisions worse than 
 nugatory, by bevelling their wheels. Thus, instead of reliev- 
 ing their horses, they adopted a practice tending to oppress 
 them more than ever. With respect to the position of the line 
 of traction, errors of equal moment are frequently committed, 
 as a little attention will enable any one to perceive, who shall 
 consider for a moment the form of the shoulders of a horse. 
 It is evident (see fig. 9, pi. V,) that, at the place where the 
 neck rises from the chest of the animal, the shoulder-blades 
 form the resting place of his collar or harness into a slope, up. 
 This slope or inclination forms an angle with a perpendicular 
 to the horizon, of about fourteen or fifteen degrees; and there- 
 fore tlie line of traction or draught should form the same angle 
 with the horizon, because he will then pull perpendicularly to 
 the shape of his shoulder, and all parts of that shoulder will 
 be equally pressed by the collar. Besides, in overcoming 
 obstacles, the advantage of this inclined direction is mec/iuni- 
 calli/ great; the following demonstration of it, taken from 
 Walker's "System of Familiar Philosophy," has not perhaps 
 been improved upon. Call a, fig. 10, a wheel, b an obstacle, c 
 the axle of the wheel, d the spoke which at present sustains the 
 weight. A line drawn from the nearest part of the horizontal 
 line of draught c k to the fulcrum or obstacle at e, will form the 
 acting part of a leverage'; and another line e t/ being drawn 
 from the fulcrum e to the nearest part of the spoke d, will form 
 the resisting part of the same lever. Now as the acting and re- 
 
MECHANICS. 369 
 
 Advantages of an inclined liive of draught. 
 
 sisting arms of the lever are of equal lengths, the lever becomes 
 a scale-beam, and a draught in the line g fc must be equal to the 
 weight of the wheel and all that it sustains, besides the fric- 
 tion ; for if g e d he 2i crooked lever, a pull at g must be equal 
 to all the weight supported by d. But when a horse draws 
 a.greeably to the shape of his shoulder, in the line i h, the act- 
 ing part of the lever A e is lengthened nearly one-fourth ; so that 
 if it would require a pull at g equal to four hundred weight, a 
 power applied at h will draw the wheel over the obstacle b with 
 three hundred weight. To those unacquainted with the prin- 
 ciples of mechanics, this truth may be easily proved by an 
 ordinary scale-beam. The horse himself, considered as a lever, 
 has in this inclined draught a manifest advantage over his ob- 
 stacles, in comparison of a horizontal draught, as may be seen 
 by fig. 9. When the horse is yoked to a post, or has any great 
 obstacle to overcome, he converts himself into a lever, making 
 his hind feet the fulcrum, and the centre of gravity of his body 
 to lean over it, at as great a distance as possible, by thrusting 
 out his hind feet; by this means, acting both by his weight 
 and muscular strength, and lengthening the acting part of the 
 lever a b, he overcomes the difficulty more by his weight than 
 by his muscular strength ; for the muscles of the fore legs act 
 upon the bones to so great a mechanical disadvantage, that 
 though he exerts them with all his might, they serve, in great 
 efforts, for little more than props to the fore-part of his body. 
 Hence we see the great use of heavy horses for draught. But 
 the great mechanical use and advantage of the inclined line of 
 draught may be more particularly seen, by calling the line a b, 
 fig. 9, the acting part of the lever, and the nearest approach 
 from the fulcrum b to the inclined line of draught (that is, b c) 
 the resisting part of the lever: compare this with the resisting 
 part of a lever touching the horizontal line of draught, (that 
 is, b d) and it will be found nearly double; in consequence, 
 agi'eeably to the known properties of the lever, a weight at g 
 would require double the exertion in the horse to remove it, 
 that the same weight would require were it placed at e. 
 
 From the above data, several important practical conclusions 
 may be drawn; — one is particularly important, that single- 
 horse carts are preferable to teams, because in a team, all but 
 the shaft horse must draw horizontally, and consequently in a 
 manner inconsistent with their structure, and the established 
 laws of mechanics. The small horses of the north of England 
 draw more weight of actual goods than our largest waggon 
 Itorses, and go longer stages. The small horses of Ireland, as 
 u common load, draw fifteen hundred weight of goods, and 
 travel farther in a day than our waggo.ns, and over worse roads 
 16.— Vol. I. 3 B 
 
370 MECHANICS. 
 
 Farms of wheels. — Source of the advantage of wheels. 
 
 than ours are in general ; ten or twelve hundred weight of real 
 goods is as much as falls to the share of one waggon horse, 
 whose superior strength is wasted upon a cumbrous vehicle, 
 and by the mechanical disadvantages of his draught 
 
 Waggon-wheels are generally made with the extremities of 
 the axle inclined downwards; thus is forfeited the advantage 
 of their being formed in a lathe, without more trouble than tiie 
 makers are inclined to bestow for ordinary purposes ; and the 
 ends are seldom, perhaps never, inclined in the same angle or 
 exactly opposite each other, consequently the tendency of the 
 motions of the wheels is in different directions, and the 
 draught of the horses is constantly exerted in twisting them 
 out of their natural course. This mischief is increased by 
 bevelling the wheels, and the horses are harassed to no pur- 
 pose except that of grinding the roads. Let a bevelled wheel 
 be rolled by itself, it will soon be seen that it will not proceed 
 in a straight line, but in a curved line, like a cone. Sir George 
 Saville, a philosophic patriot, whose memory is illustrious, 
 calculated how far a waggon was rendered a sledge in a 
 journey from London to York. The distance is two hundred 
 miles; thirty of which he found the waggon is drawn as if it 
 were a sledge without wheels. Another disadvantage of a 
 waggon arises from the sluggishness of its motion. This will 
 be readily understood and allowed, when it is considered how 
 small a force will continue the motion of a heavy body, mov- 
 ing with a certain degree of rapidity, in comparison with what 
 is required to impel it from a state of rest ; but if the motion 
 of the body be extremely slow, the force necessary to keep 
 it up, must be nearly equal to that which moved it at first. 
 The latter case is precisely that of waggon horses, which have, 
 every instant of their draught, to overcome nearly the whole 
 inertia of their load. 
 
 A sledge, in sliding over a plane, suffers a friction equiva- 
 lent to the distance through which it moves; but if we apply 
 wheels, the circumference of wliich is eighteen feet, and they 
 turn upon axles, the circumference of which is only six inches, 
 it is plain, that while the carriage moves eighteen feet over 
 the plane, the wheels make but one revolution ; and as there 
 is no sliding of parts between the plane and the wheels, but 
 only a mere change of surface, no IViction takes place there, 
 the whole being transferred to the nave acting on the axle; 
 so that the only sliding of parts has been betwixt the inside 
 of the nave and the axle, which, if they fit one another exactly, 
 is no more than six inches; hence the friction is reduced in 
 the proportion of six inches to eighteen feet, that is, as thirty- 
 six to ni^-^. In all casp^ bv applying wheels, the friction »« 
 
MECHANICS. 371 
 
 Reason of low fore-wheels, — Lord Somerville's cart. 
 
 thus lessened, in the proportion of the diameter of the axles 
 to that of the wheels. Another advantage is also gained, by 
 having the surfaces of friction confined to so small an extent, 
 arising from the circumstance of their being more easily made 
 true, kept smooth, and fitted to each other. The only incon- 
 venience is the height of the wheels, which must in most 
 cases be added to that of the carriage itself. 
 
 A four-wheeled carriage may be drawn with five times as 
 much ease as one tliat slides upon the same surface in the 
 condition of a sledge. In four-wheeled carriages, the fore- 
 wheels are made of a less size than the hind ones, in order to 
 enable them to turn in less room ; and not for the purpose of 
 bringing into action any supposed pushing quality in high 
 back-wheels. 
 
 Large wheels have the advantage over small ones in over- 
 coming obstacles, because wheels act as levers in proportion 
 to their various sizes ; but when they are so high as not to 
 allow the line of draught to have the inclination before stated, 
 their advantage as longer levers is counterbalanced by their 
 lessening the intensity of the moving power ; therefore the 
 total advantages of wheels drawn horizontally do not increase 
 proportionally to their height. 
 
 In ascending, high wheels will be found to facilitate the 
 draught in exact ratio with the squares of their diameters ; 
 but in descending, they are liable to press in the same pro- 
 portion. An admirable device was produced by Lord So- 
 merville, to remedy the latter evil ; it consisted in throwing 
 the weight behind the centre in going down hill, by raising 
 the fore part of the body of the cart ; so that while the shaft 
 may incline downwards, in proportion to the line of declivity, 
 the bottom of the cart's body should remain horizontal. This 
 construction is now common in Devonshire, and some other 
 counties. 
 
 As small wheels turn as much oftener round than large 
 ones as their circumferences are less, so when the carriage is 
 loaded with an equal weight on both axles, the fore axle must 
 sustain as much more friction, and consequently wear out as 
 much sooner than the hind axle, as the fore-wheels are less 
 than the hind ones. This points out that the greatest weight 
 should be laid upon the large wheels ; yet it is generally the 
 practice to put the greatest load over the small wheels, which 
 not only makes the friction greatest where it ought to be the 
 least, but also presses the fore-wheels deeper into the ground 
 than the hind-wheels, notwithstanding the former are with 
 more difl&culty drawn out of it than the latter. The limita- 
 tion to loading the hind-wheels with the greatest part of the 
 
372 MECHANICS 
 
 Dished wheels. 
 
 weight, will consist in not carrying it to such an excess as to 
 endanger the tilting of the vehicle, in going up-hill 
 
 Wheels are commonly made with what is called a dish, that 
 is, the spokes are inserted not at right angles, but with an in- 
 clination towards the axis of the nave or centre-piece; so that, 
 if the interior end of the nave were placed on the ground, the 
 spokes being higher at the outside than at their termination 
 in the nave, the wheel appears dished or hollow. Wheels are 
 usually dished about four inches in a diameter of five feet. If 
 the wheels were always to go on smooth and level ground, the 
 best way would certainly be to make the spokes perpendicular 
 to the naves and axles; because they would then bear the load 
 perpendicularly, in which position wood supports the greatest 
 weight. But because roads are generally uneven, one wheel 
 often falls into a cavity, or rut, when the otiier does not, and 
 then it bears much more than an equal share of the load. 
 Hence the utility of dishing the wheels, because when a dished 
 wheel falls into a rut, the spokes become perpendicular in the 
 rut, and therefore have the greatest strength when the ob- 
 liquity of the load throws most of its weight upon them; whilst 
 those on the high ground, having less weight to bear, have no 
 occasion, at that time, for their utmost strength. Dished 
 Avheels, when on straight or horizontal axles (and no otiier 
 axles should be used,) have many other excellencies ; they 
 make carriages to stand on a broader base, and therefore ren- 
 der them less liable to be overturned ; they give more room to 
 the body of the carriage than if the spokes were perpendicu- 
 lar ; they stand against side-jolts like an arch, and when the 
 carriage is going along the inclined side of a road, they ren- 
 der it less liable to be overthrown. 
 
 If the spokes be set so far from the outer end of the nave, 
 that a perpendicular from the sole to the under side of the 
 axle may fall between an inch and two inches between the 
 bushes, the pressure will be somewhat greater on the outer 
 than on the inward bush, when the wheels are on a level This 
 will be an advantage, particularly when the inner part of the 
 axle-arm is much larger than the outer ; as it has then more 
 friction, therefore the pressure should be diminished; besides, 
 every sinking of one Asheel more than the other, causes it to 
 pinch the inner bush. It has been proposed, as the best mode 
 of placing the spokes in the naves, to mortise them in two 
 rows, alternately ; this does not weaken the centre so much 
 as when all the spokes are in one row or band, and gives a 
 greater power of resistance outwards. 
 
 The question whether broad or narrow w.ieels are best, has 
 been much contested. The popular opinion has always been 
 
MECHANICS. 373 
 
 Breadth of wheels. — Form of axle-arms. 
 
 in favour of narrow wheels; and accordingly, the carriers 
 thought themselves injured by the broad-wheel act, though it 
 allowed them to draw with more horses, and carry greater 
 loads than usual. This matter deserves a moment's attention. 
 — We have observed on a former occasion, that friction in- 
 creases somewhat with an increase of surface : so far then, the 
 opinion of the carriers tallies with a general truth ; but it is at 
 the same time true, that if the moving body have so thin a 
 resting edge as to cut into the surface over which it passes, 
 the friction will be actually increased by the diminution 
 of the surfaces in contact. It is the want of duly consi- 
 dering this, that supports the prejudice in favour of narrow 
 wheels, which cut and sink into the roads, and may be con- 
 sidered, except on the very few roads that are impervious to 
 them, as constantly going up-hill, even upon level ground. 
 But experience testifies, that instead of thus ploughing the 
 roads, broad cylindrical wheels smooth and harden them, and 
 move with greater facility. 
 
 If the tire or iron binding of a wheel be in separate parts, 
 and not in one single hoop, these parts should not be made 
 quite to meet each other at the first; because when the wheel 
 has been some time in use, they will settle more closely to the 
 wheel than they can be laid, and the vacancies will then be 
 filled up. The axle-arm should be a perfect cylinder, or if 
 tapered towards the extremity, the difterence of its two dia- 
 meters should be very trifling; a small degree of taper is pre- 
 ferred by many, because it gives the. wheel rather a disposi- 
 tion to slide off, thus preventing it from being apt to close in- 
 wardly, and creating excessive friction ; but it increases the 
 necessity for good iron washers exteriorly, and of substantial 
 linch-pins. It is not an uncommon practice to set the wheels, 
 that is, to give them a slight inclination towards each other, 
 whereby they are, perhaps, an inch nearer at the front thaa 
 at the back ; this is chiefly done to wheels that are bevelled, 
 with a view to make them run more evenly on their sole or 
 bearing part, and to prevent their gaping forward ; but it is 
 evidently a distortion, an attempt to rectify one bad thing by 
 another of the same stamp, as if the multiulication of mischief 
 would produce good 
 
 The nave of a heavy wheel, as for an ordinary cart for field 
 purposes, need not be more than twelve or fourteen inches in 
 length; if too short, the wheel will wabble, unless fitted very 
 tightly on the axle ; while too long a nave is apt to catch the 
 dirt from the upper part, and to project too much beyond the 
 outer face of the fellies; the length just stated is exclusive of 
 the pan at the outer end. 
 
374 MECHANICS. 
 
 Ht-i.'lit ol" csrria;^e-» heels. — Clork-work. 
 
 The proportions of wheels are often regulated as muclf by 
 the purposes to which the vehicles are applied, as by the fa- 
 cilities they afford to motion ; thus waggons have in general 
 large hind-wheels, w hile in timber-carriages the four are nearly 
 of the same heicrht; the London common stage-carts have larjje 
 wheels, while the drays used by brewers have very low ones. 
 The reason is obvious: wa<jgons and carts load behind; but 
 drays, and the timber-carriages alluded to, load at the sides; 
 and therefore, for them, large wheels, however much they 
 might favour the draught, would be extremely inconvenient, 
 indeed incompatible with their use. The wheels of single- 
 horse carts for ordinary purposes, where there is no particular 
 necessity for having them low, may be from four feet to four 
 feet six inches in diameter, for a horse of about sixteen hands 
 high. For four-wheeled carriages, suppose four feet to be the 
 height of the fore-wheels, and the line of traction to be draw a 
 at an elevation of fourteen deirrees from the centre of its axle, 
 the point where that line cuts the circumference of the wheel in 
 its front, gives that height from the plane on which the carriage 
 stands, that will determine the radius of the hind-wheels. 
 
 Wheels, whatever their size, should be made of well-season- 
 ed, tough wood, perfectly free from blemish ; the naves are 
 generally of elm, the spokes of oak, and the fellies of elm or 
 of ash. The bent fellies, when the wood has not been hurt 
 by too much heat, have greater strength with less wood, than 
 those whicli are cut by the saw in a curved direction. 
 
 It is common in many places, and deserves to be so every- 
 where, to have a contrivance for relievino; the horse of a loaded 
 cart from the weight pressinsr on his shoulders, when it is ne- 
 cessary for any purpose to stop awhile ; this useful object is 
 attained by an appendage so simple as that of a pole or staff, 
 which, turnins on a hook-and-eve hinge, is let down from one 
 of the shafts when the occasion requires. 
 Clock-Work. 
 
 The terra clock-tvork, originally imported those wheels, pi- 
 nions, and other mechanism, which constituted the striking 
 part, or what was formerly called the clock part of a movement 
 for measuring time ; and that part of the machine which gave 
 motion to the hands for shewing the parts of time, was called 
 the watch part. But at present the term watch is appropriated 
 to such movements for measuring time as are carried in the 
 pocket; and the larger movements, whether they strike or not, 
 are called clocks. 
 
 Watches which i-egularly strike the hour, are called pocket- 
 clocks: if they only strike the hour, when some particular part 
 for that purpose is touched, tliey are called repeating tcatches. 
 
INSERT FOLDOUT HERE 
 
MECHANICS. 37C} 
 
 Definitions. — -Historical remarks. 
 
 The term chronomeier, is chiefly used by workmen and navi- 
 gators, to denote a watch or portable machine, constructed 
 with so much care and skill, as to be fit for determining the 
 longitude at sea. The word time-keeper is a fit appellation for 
 an astronomical clock, but it is often employed with the same 
 latitude as time-piece, which is of more extensive signification, 
 as a general name, or a varied or compendious mode of de- 
 noting "any instrument for measuring time." 
 
 In treatises, it is usual, for the sake of distinction, to use 
 the terms clock and watch in their ancient sense, generally 
 with the word part after them ; thus, by the expression clock 
 part, is meant the striking portion only, and by that of watch 
 part, the going portion only. 
 
 There are many documents to prove the existence of clocks, 
 with wheels and weights, in the middle of the fourteenth cen- 
 tury ; but the invention cannot be traced to an earlier date with 
 any certainty. The sphere of Archimedes, indeed, has been 
 considered as the first attempt towards the formation of a clock; 
 it had a maintaining power, but being without any kind of 
 regulator, could only measure time, as a planetarium exhibits 
 the motion of the stars, with relative, but not positive precision. 
 The opinion of Berthoud, who has investigated the subject 
 with attention, is evidently just, when he asserts that the clock 
 is not the invention of any one man, but an assemblage of suc- 
 cessive inventions, each of which is worthy of a separate con- 
 triver. 1. Wheelwork, which was known in the time of Archi- 
 medes; 2. the application ef the weight as a maintaining 
 power; 3. the use of the fly as a regulator; 4. the ratchet 
 wheel and click; 5. the substitution of the balance for the fly, 
 and the escapement which was necessarily introduced at the 
 same time; 6. the application of the dial and hands ; and 7. the 
 addition of the striking part. 
 
 The introduction of the spiral spring, as a first mover, in- 
 stead of a weight, took place about the beginning of the six- 
 teenth century. About the year 1650, a new era in the art of 
 clock-work commenced, by the application of the pendulum 
 as a regulator; in the year 1715, a compensation for the effects 
 of change of temperature was applied to time-pieces, which, 
 not long after, were contrived so as to go while they were 
 wound up. From this time, improvements succeeded each 
 other with great rapidity. These improvements relate partly 
 to more accurate workmanship, but principally to the escape- 
 ment, or diff'erent modes of connecting the wheel-work with 
 the pendulum or balance. Volumes would be required to do 
 justice to the persons who have distinguished themselves in 
 this line of art ; but we must go little beyond the general 
 
376 MECHANICS. 
 
 Dcsiriptinn of a duck. 
 
 principles of this sort of mechanism, and these will perhaps 
 be best understood, if, in the first place, we describe the parts 
 of a clock. 
 
 The profile of the watch or going part of a clock is shewn by 
 fig, l,pl.VI, P is a weight which keeps the clock going; it is 
 suspended by a catgut band that winds about the cylinder or 
 barrel C, which is fixed upon the axis a a; the pivots b b go 
 into holes made in the brass plates SS, TT, in which they turn 
 freely. These plates are connected by means of four pillars, 
 only two of which, ZZ, can be seen in the profile, and the whole 
 together is called the frame. On the circumference of barrel 
 C is a spiral groove, in which the catgut lies, and is thereby 
 caused to wind round in a regular manner. 
 
 The weight P, if not restrained, would necessarily turn the 
 barrel C, with a uniformly accelerated motion, in the same 
 manner as if the weight was falling freely from a height ; but 
 the barrel is furnished with a ratchet-wheel, KK, the right sides 
 of the teeth of which strike against the click, which is fixed 
 ■with a screw to the wheel DD, so that the action of the weight 
 is communicated to the wheel DD, the teeth of which act 
 upon the leaves of the pinion which turns upon the pivots cf. 
 This communication of the teeth of one wheel with another is 
 called pitching. Several things are requisite to form a good 
 pitching, which is very important in all machinery where 
 wheels and pinions are employed ; the teeth and pinion leaves 
 should be of a proper shape, and perfectly equal among them- 
 selves; the size also of the pinion sliould be of a just propor- 
 tion to the wheel. 
 
 The wheel EE is fixed upon the axis of the pinion d ; 
 and the motion communicated to the wheel DD by the 
 weight, is transmitted to the pinion </, consequently to the 
 wheel EE, as likewise to the pinion e, and wheel FF, which 
 moves the pinion j', upon the axis of which, the last or 
 swing wheel GH is fixed. In a word, the motion begun by 
 the weight is transmitted from the wheel GH to the pallets 
 IR of the escapement (fig. 2,) fixed on an arbor going 
 through the back plate of the frame, and carrying a lever 
 XU, (fig. 1,) which is forked at the lower part to receive the 
 pendulum. The pendulum consists of a metallic rod, sus- 
 pended by a very slender piece of steel spring j/, from a brass 
 bar A, screwed to the frame of the clock, and has a heavyweight 
 or bob at its lower end. The pendulum y B, if once put in 
 motion, will describe round the point of suspension y, an arc 
 of a circle, and will continue to go alternately backward and 
 forward till the force impressed upon it is wasted, by the usual 
 causes which tend to destroy all other artificial motions, viz. 
 
MECHANICS 377 
 
 Description of a clock. 
 
 friction and gravitation. The pendulum is the time-measurer: 
 at every vibration which it makes, the teeth of the swing-wheel 
 GH act upon the pallets IR, (fig. 2,) in such a manner, that 
 after one tooth H has communicated motion to the pallet R, 
 that tooth escapes ; then the opposite tooth G acts upon the 
 pallet I, and escapes in the same manner : and thus each tooth 
 of the wheel escapes the pallets IR, after having communicated 
 their motion to the pallets; so that the pendulum, instead of 
 stopping, continues to move. 
 
 The wheel EE, (fig. 1,) revolves in an hour ; the pivot c of 
 this wheel passes through the plate, and is continued to r : 
 upon this pivot is a wheel NN, with a long socket fastened in 
 its centre ; upon the extremity of this socket r the minute- 
 hand is fixed. The wheel NN acts upon the wheel O ; the 
 pinion of which, p, acts upon the wheel gg, fixed upon a socket 
 which turns along with the wheel N. This wheel gg, makes 
 its revolution in twelve hours, and upon the barrel of it is 
 fixed the hour-hand. 
 
 From the above description it is easy to see, I. that the 
 weight P turns all the wheels, and at the same time continues 
 the motion of the pendulum; 2. that the quickness of the mo- 
 tion of the wheels is determined by that of the pendulum; and, 
 3. that the wheels point out the parts of time divided by the 
 uniform motion of the pendulum. When the catgut upon 
 which the weight is suspended is entirely run down from off 
 the barrel, it is wound up again by means of a key, which goes 
 on the square end of the arbor or axis Q, by turning it in a 
 contrary direction from that in which the weight descends. 
 For this purpose, the inclined side of the teeth of the ratchet- 
 wheel K, (fig. 2,) removes the click C, so that the wheel K 
 turns with the barrel, while the wheel D is at rest; but as soon 
 as the band is wound up, the click falls in between the teeth 
 of the wheel K, and the right side of the teeth again act upon 
 the end of the click, which oblisies the wheel D to run along 
 with the barrel, and the spring A keeps the click between the 
 teeth of the ratchet-wheel K. Supposing the wheel DD to 
 turn once round in twelve hours, which it is usually calculated 
 to do in the best time-pieces, then will sixteen turns of the 
 catgut on the barrel C, (fig. 1,) suffice to keep the clock going 
 eight days. 
 
 It will now be proper to explain how time is measured by 
 the motion of the pendulum; and how the wheel E, upon the 
 axis of which the minute-hand is fixed, makes but one precise 
 revolution in an hour. The vibrations of a pendulum are per- 
 formed in a shorter or longer time in proportion to the length 
 of the pendulum itself. A pendulum of 39| inches in length 
 16.— Vol. I. 3 C 
 
378 MECHANICS. 
 
 Description of a clock. 
 
 makes 3600 vibrations in an hour; that is, each vibration is 
 performed in a second of time, and for that reason it is called 
 a second pendulum. But a pendulum of 9jf inches makes 
 7,200 vibrations in an hour, or two vibrations in a second of 
 time, and is called a half second pendulum. Hence, in con- 
 structing a wheel whose revolution must be performed in a 
 given timj, the time of the vibrations of the pendulum which 
 regulate its motion must be considered. Supposing then, that 
 the pendulum y B makes 7,200 vibrations in an hour, let us 
 consider how the wheel E shall take up an hour in making 
 one revolution. This entirely depends on the number of teeth 
 in the wheels and pinions. If the swing wheel contains 30 
 teeth, it will turn once round in the time that the pendulum 
 makes 60 vibrations ; for at every turn of the wheel, the same 
 tooth acts once on the pallet I, and once on the pallet K, 
 and at each stroke the pendulum makes a vibration ; there- 
 fore, as the wheel has 30 teeth, it occasions twice 30 vibra- 
 tions ; consequently this wheel must perform 180 revolutions 
 in an hour ; because 60 vibrations, which it occasions at 
 every revolution, are contained 120 times in 7,200, the num- 
 ber of vibrations performed by the pendulum in an hour. 
 Now in order to determine the number of teeth for the wheels 
 EF, and their pinions ef, it must be remarked, that one revo 
 lution of the wheel E must turn the pinion e as many times as 
 the number of teeth in the pinion is contained in the number 
 of teeth in the wheel. Thus, if the wheel E contains 72 teeth, 
 and the pinion e contains 6, the pinion will make 12 revolu- 
 tions in the time that the wheel makes one ; for each tooth of 
 the wheel drives forward a tooth of the pinion; and when the 
 6 teeth of the pinion are moved, a complete revolution is per- 
 formed ; but the wheel E has by that time only advanced 6 
 teeth, and has still 66 to advance before its revolution is com- 
 pleted, which will occasion 11 more revolutions of the pinion. 
 For the same reason, the wheel F having GO teeth, and the 
 pinion /'only 6, the pinion will make the 10 revolutions while 
 the wheel performs one. Now the wheel F being turned by 
 the pinion e, makes 12 revolutions for one of the wheel E; 
 and the pinion y'makes 10 revolutions for one of the wheel 
 F; consequently, the pinion y" performs 10 times 12, or 120 
 revolutions in the time the wheel E performs one. But 
 the wheel G, which is turned by the pinion /", occasions 60 
 vibrations in the pendulum each time it turns round ; con- 
 sequently, the wheel G occasions 60 times 120, or 7,200 
 vibrations of the pendulum while the wheel E performs 
 one revolution ; but 7,200 is the number of vibrations made 
 by the pendulum in an hour, and consequently the wheel 
 
MECHANICS. 379 
 
 Description of a clock. 
 
 E performs but one revolution in an hour; and so of the 
 rest 
 
 From this reasoning it is easy to discover how a clock may 
 be made to go for any length of time without being wound 
 up: I, by increasing the number of teeth in the wheels; 2, 
 by diminishing the number of teeth in the pinions ; 3, by in- 
 creasing the length of the cord that suspends the weight ; and 
 lastly, by adding to the number of wheels and pinions: but, 
 in proportion as the time is augmented, the weight must be 
 increased, or the force which it communicates to the last 
 wheel GH will be diminished. 
 
 With respect to the watch part of the movement we are 
 considering, it only remains to take notice of the number of 
 teeth in the wheels which turn the hour and minute hands. — 
 The wheel E performs one revolution in an hour ; the wheel 
 NN, which is turned by the axis of the wheel E, must like- 
 wise make only one revolution in the same time ; and the 
 minute hand is fixed to the socket or tube of this wheel, 
 which is fitted pretty tight upon the axis c of the wheel E, and 
 being thus carried round only by friction, the hand may be 
 moved round without affecting the wheels between the brass 
 plates. The wheel N has 30 teeth, and acts upon the wheel 
 O, which has likewise 30 teeth, and the same diameter ; con- 
 sequently the wheel O takes an hour to a revolution ; now 
 the wheel O carries the pinion p, which has six leaves, and 
 which acts upon the wheel gg of 72 teeth ; consequently 
 the pinion p makes 12 revolutions while the wheel gg makes 
 one ; of course the wheel gg takes twelve hours to one 
 revolution, and upon the barrel of this wheel the hour-hand 
 is fixed. 
 
 Most of the wheels belonging to the striking part, as well 
 as most of those belonging to the going part, are included 
 between the brass plates SS, TT, seen edgeways in fig. 1 ; the 
 reason of their being omitted in that figure, is to avoid con- 
 fusion ; but a projection of the whole of the wheels between 
 the plates, as they would be seen if the brass plate SS was 
 removed, is shewn by fig. 2. 
 
 Fig. 3. shews so much of the mechanism of the striking 
 part as is contained between the brass plate SS and the dial- 
 plate. The numbers annexed to the different wheels denote 
 the number of teeth they contain. 
 
 The striking part now more particularly requires our atten- 
 tion. In fig. 2, h is the great wheel of this part, with a bar- 
 rel and click the same as D ; it turns a pinion of eight, on 
 the same arbor with which pinion is the wheel i, turning a 
 pinion of eight on the arbor of the wheel k of 48 teeth. The 
 
380 MECHANICS. 
 
 Description of a clock. 
 
 wheel k turns another pinion of eight, on the same arbor with 
 the wheel t of 48, and this last wheel turns a pinion of 6, 
 on the axis of which is a broad flat piece of metal, called the 
 fly, seen edgeways at s. This fly strikes the air with so large 
 a surface, that the resistance it experiences prevents the train 
 of wheels from going too fast. The wheel i has eight pins 
 projecting from it ; these pins raise the tail of the hammer in 
 succession, as the rotation of the wheel brings them to it; 
 the hammer is returned violently when the pins leave its tail, 
 by a spring z, pressing on the end of a pin through its aibor, 
 and strikes the bell x: u is a short spring which t'le end of a 
 pin through the arbor touches, just before the hanmier strikes 
 the bell, and its use is to lift the hammer off" the bell the in- 
 stant it has struck, that it may not stop the sound. The 
 eighth pin in the wheel i, which is called the pin-wheel, must 
 pass by the hammer-tail 78 times in striking the 12 hours; 78 
 iDeing the number of strokes the bell receives during that time. 
 As the pinion of the wheel /, has eight leaves, each leaf of the 
 pinion answers to one of the pins; and as the wheel h has 78 
 teeth, it will turn once in 12 hours, like the great wheel D of 
 the watch part. In the pin-wheel /, eight teeth correspond to 
 one of the pins for the hammer, and as the pinion of the wheel 
 k has eight teeth, the wheel k will turn once for each stroke of 
 the hammer. Then, as the pinion of the wheel t ti.rns 6 times 
 for the wheel k once, and the pinion of the fly turns 8 times 
 for the wheel t once, 6x 8=48, the number of turns made by 
 the fly for one stroke of the hammer. 
 
 In fig. 3, r is a small pinion of one tooth called the gather- 
 ing pallet ; it is fixed on the arbor of the wheel A-, (fig. 2,) 
 which arbor comes through the brass plate SS (fig. 1,) for the 
 purpose ; and consequently, like the wheel /:, the gathering 
 pallet turns round once for each stroke of the hammer; 5 is 
 a segment of a large wheel, turned bv the gatherintr pallet, 
 and called the rack. The arm a is attached to the rack, and 
 the end of it rests against a spiral plate, v, called, from its 
 form, the snail. The ^nail is fixed on the same tubular arbor 
 as the hour-hand and wheel 72, and turns round with it once 
 in 12 hours. Each of the 12 divisions or steps, as they are 
 called, of the snail, answers to an hour ; the circular arcs 
 forming their circumference are struck from the centre of 
 the arbor with a different radius, decreasing a certain quan- 
 tity each time, in the order of the hours ; and each step is an 
 arc equal to the twelfth part of the circumference of which it 
 is a part. 
 
 The circular part of the rack 5 is cut into teeth, each of 
 which is of such a length that every step upon the snail 
 
MECHANICS. 381 
 
 Description of a clock. 
 
 answers to one Oi them. A spring w presses against the tail 
 of the rack, and its use is to throw the arm a of the rack 
 against the snail. The click g, is called the hawk's bill ; it 
 takes into the teeth of the rack, and holds it up in opposition 
 to the spring iv. The three-armed detent, b k, is called the 
 warning piece ; the arm k is bent at its end, and passes through 
 a hole in the plate SS of the frame, so as to catch a pin fixed 
 in one of the arms of the wheel t, fig. 2, and which pin de- 
 scribes the dotted circle in fig. 3 ; the other arm b, stands so 
 as to fall in the way of a pin in the wheel O, of 30 teeth. In 
 the present position of the figure, the wheels of the striking 
 train are in motion, and would continue turning until the ga- 
 thering pallet r, which, as before observed, turns once at each 
 stroke of the hammer, lifts the rack s, in opposition to the 
 spring w, one tooth each turn, and the hawk's bill g retains 
 the rack, until a pin in the end of the rack is brought in the 
 way of the lever of the gathering pallet r, and stops the wheels 
 from turning any farther : it is in this position, with the rack 
 wound up, that we shall begin to describe the operation of the 
 striking of the clock. — The wheel O, as just observed, turns 
 once in an hour, and consequently at the expiration of every 
 hour, the pin in it takes the end b, and moves it towards the 
 spring near it; this depresses the end k, until it falls into the 
 circle of the motion of the pin in the wheel t, fig. 2, at the 
 same time the short tail depresses one end of the hawk's bill, 
 and raises the other g, so as to clear the teeth of the rack s ; 
 immediately the spring to throws the rack back, until the end 
 of its arm a rests against the snail. When the rack falls back, 
 the pin in it is moved clear of the gathering pallet r, and the 
 wheels are set at liberty; the maintaining power or weight puts 
 them in motion, but in a very short time before the hammer 
 has struck, the pin in the wheel t falls against the end k, and 
 stops the whole: this operation happens a few minutes before 
 the clock strikes, and the noise of the wheels turning is called 
 the warning. When the hour is expired, the wheel O has 
 turned so far as to allow the end of the arm b to slip over its 
 pin, as in the figure ; the small spring pressing against it raises 
 the end k so as to be within the circle of the pin in the wheel 
 t, fig. 2 : every obstacle is now removed ; the pin-wheel i, fig. 2, 
 raises the hammer p, and it strikes the bell; the gathering pal- 
 let r takes up the rack, a tooth at each turn ; the hawk's bill g 
 retaining it until the pin in the rack comes under the gather- 
 ing pallet r, and stops the motion of the train, till the pin in 
 the wheel O, at the next hour, takes the warning piece b k, and 
 the whole operation is repeated. 
 
 As the gathering pallet turns once for each blow of the 
 
382 BIECHANICS. 
 
 Description of a clock. 
 
 hammer, and it gathers up one tooth of the rack at eacli turn, 
 it is evident that the number of teeth the rack can fall back, 
 is the number of strokes the hammer will make. It is obvious 
 also, from the form of the snail, a fresh step of which is turned 
 to the end of the arm a of the rack every hour, that the 
 rack must fall back differently each time at the end of that 
 period, and as each step of the snail answers to one tooth of 
 the rack, and each tooth of the rack to one stroke of the ham- 
 mer, the number of strokes is increased, one at a time, from 
 one to twelve. 
 
 As nothing affects the position of the snail but the motion 
 of the wheel g, upon the axis of which it is affixed, and 
 as the step upon which the arm a, fig. 3, of the rack rested, 
 while any given hour was struck, still remains to be the step 
 upon which it rests till the next has arrived; so, if in any 
 part of the interval between the striking of the given hour, 
 and the warning of the next, any contrivance be adopted to 
 move the arm b of the detent, as much as it is moved in the 
 regular period by the pin in the wheel O, the hour which was 
 last struck will be struck over again, or, according to the 
 usual expression, the hour will be repeated ; and yet all the 
 subsequent hours will be struck with the same precision, as 
 if the mechanism had not been touched. A slender cord, for 
 instance, attached to the arm 0, will be quite sufficient to 
 effect the purpose, and might be conveyed through the side 
 of the clock to the bedside, by which means a person may 
 easily ascertain, during the night, the last hour which the 
 clock has struck. 
 
 It will also probably be understood, that as the snail accom- 
 panies the wheel g, fig. 1, on the end of the hollow axis of 
 which the hour hand is fixed, and as g, through the medium 
 of the pinion p, and the wheel O, derives its motion from the 
 wheel X, the hollow axis of which, carrying at /• the minute 
 hand, fits tightly, but is not imraoveably fixed, upon the arbor 
 c; so, if the time of the clock be rectified by pushing forward 
 the hands, the clock part will undergo corresponding changes, 
 and strike each hour passed over, although the wheels of the 
 watch part, between the brass plates, are not aft'ected by the 
 operation. 
 
 Clocks intended to keep time with the greatest nicety, are 
 generally contrived so as to go while they are wound up. For 
 this purpose, a second larger ratchet-wheel is added on the same 
 arbor with that which admits the clock to be wound up, but 
 with teeth pointing the contrary way; a strong spring, usually 
 the greatest portion of a circle, connects this large ratchet-wheel 
 with the great wheel of the clock whicli is on the same axis 
 
MECHANICS. 383 
 
 Division of circles into odd numbers. 
 
 with it; one end of this spring being attached to the great 
 wheel, and the other end to the large ratchet ; and a catch 
 proceeds from the inner face of the back-plate to the teeth of 
 this ratchet, which prevents its moving back when the clock 
 is winding up, and serves as a support for the re-action of the 
 maintaining spring. When the clock is left to the operation 
 of the weight, the small ratchet turns round the large one, and 
 contracts or coils up the spring till it has strength sufficient to 
 impel the great wheel and train; and when the action of the 
 weight is suspended as in winding up, the spring, freed from 
 the contracting power of the weight, expands itself, and forces 
 round the great wheel; its action on the contrary direction 
 on the great ratchet being prevented by the catch before 
 mentioned. 
 
 Clocks which have pendulums vibrating half seconds, fre- 
 quently have a spring instead of a weight for the maintaining 
 power. This spring consists of a long flat piece of steel, 
 coiled up in a spiral form ; it is inclosed in a cylindrical box, 
 to which its external extremity is attached, while its internal 
 end is. connected with a fixed axis, round which the spring 
 box revolves. As the strength of the spring is greater the 
 more it is coiled up by turning round the box, its action would 
 be unequal in impelling the work of the clock ; and to remedy 
 this inconvenience, the fusee wheel, of the same construction 
 as in watches, is adopted. The manner in which the fusee 
 regulates the action of the spring, has been explained in page 
 310, and illustrated by a figure. If, instead of the barrel C, 
 fig. 1, a fusee wheel or pulley be supposed to be substituted, 
 and the spiral spring, inclosed in its cylindrical barrel, be 
 added immediately below it, with a catgut to connect the fusee 
 and barrel of the spring, a good idea will be obtained of a 
 spring clock, for in other respects the work is the same. 
 
 Mode of dividing the Circumferences of Circles. 
 
 Very uncommon and odd numbers of teeth are sometimes 
 required for the wheels of astronomical clocks, orreries, &c. 
 such as the plate of no common engine used by clockmakers 
 for cutting the teeth of their wheels, in the common routine 
 of their business, is calculated for : the following directions 
 are given, for the purpose of shewing how to divide a circle 
 into any given odd number of equal parts, so that the num- 
 ber may be laid down upon the dividing plate of a cutting 
 engine. 
 
 With respect to the division of a circle into any even num- 
 ber of equal parts, no difficulty can arise ; and if this be easy. 
 
384 MECHAXICS, 
 
 Divi.sioD of circles into odd numbers. 
 
 the division of any given portion of a circle into any number 
 of equal parts is not less so. A little consideration will shew 
 that tliis leads us to a solution of our difficulty. There is 
 no odd number, but, if a certain number be subtracted from 
 it, an even number, of easy subdivision, will remain. Sup- 
 posing the number of equal divisions wanted to be 69, sub- 
 tract 9, and 60 will remain ; then, as every circle is supposed 
 to contain 360 degrees, say, as the desired number of equal 
 parts in the circle, which is 69, is to 360 degrees, so are 9 
 parts to the corresponding arc of the circle that will contain 
 them; which arc, by the rule of three, will be found to be 
 46.95. Therefore, by the line of chords on a common scale, 
 or rather with a sector, set off 46.95 (or 46.9) degrees with a 
 pair of compasses, on the circumference of the circle, and 
 divide that arc, or portion of a circle into 9 equal parts, and 
 the rest of the circle into 60; so will the whole be divided into 
 69 equal parts. 
 
 It is obviously not necessary to take off so many parts as 
 nine, in order to leave a convenient number for subdivision; 
 but this advantage attends taking off a considerable number 
 of degrees from a scale, particularly when they are taken from 
 the scale of chords of a common rule, that they are in general 
 less liable to be taken off inaccurately than a few degrees. 
 The following example is proper when the scale intended to 
 be used is a good one : suppose it is required to divide the cir- 
 cumference of a circle into 83 equal parts, subtract 3, and 80 
 will remain; then, as 83 are to 360 degrees, so, by the rule of 
 propoitiun, aie three parts to 13.01 degrees. The small fraction 
 here brought out may be neglected ; therefore, by the line of 
 chords or sector, as before, with a pair of finely pointed com- 
 passes, set off 13 degrees on the circumference of the circle ; 
 divide the arc thus set off into three parts, and the remainder 
 of the circle into 80, and the work will be done. 
 
 Again, suppose it is required to divide a given circle into 
 365 equal qarts, subtract 5, and 360 will remain ; then, as 365 
 parts are to 360 degrees, so are 5 parts to 4.93 degrees; there- 
 fore set off 4.93 (or 4.9) degrees by the scale ; divide that 
 space into 5 equal parts, and the rest of the circle into 360 ; 
 the whole will then be divided into 365, the desired number 
 of equal parts. 
 
 Any person accustomed to the use of a pair of compasses, 
 and to the scale or sector, may very easily, by a little practice, 
 take off degrees, and fractional parts of a degree, with great 
 facility by the accuracy of his eye. 
 
 A few remarks with respect to the sector, with which a 
 case of mathematical instruments is always furnished, may be 
 
MECHANICS. 385 
 
 Use of the sector in dividing circles. — Sizing of wheels and pinions. 
 
 useful to some. The sector is made to fold in the middle, not 
 only that it may lie in a smaller compass, but to solve many 
 problems by means of the references given to various tables 
 and scales that are engraved on both sides of each limb. When 
 opened to its full length, it commonly measures one foot, each 
 inch being numbered and divided into tenths. At the edge is 
 another scale, which divides the foot into ten equal parts, and 
 each tenth part of the foot is again subdivided into ten ; thus 
 giving a division of the 12 inches into lUO equal parts. But 
 the first scale which we have to notice as useful in dividino- 
 circles, is that next to the inner edges, marked Pol. for poly- 
 gon. By opening the sector to such a width as may admit 
 the radius of any circle to measure exactly from the noure 6 
 on one limb, to the figure 6 on the other, we at once ascertain 
 the division of that circle's circumference into any number 
 of equal parts, from four to twelve; because from the figure 4 
 to the opposite figure 4 will give a chord subtending a qua- 
 drant of the circle; from 5 to 5 will give the side of a regular 
 pentagon, or divide the circumference of the circle into five 
 parts; from 6 to 6 into six parts; and so on. Two equal lines 
 of chords marked with the letter C, are inclined to each other, 
 and meet at the centre of the joint of the instrument. To set 
 ofl" any number of degrees on the circumference of any circle, 
 the radius of which does not exceed the length of both these 
 lines together, open the sector till the distance between the GOth 
 degree on each scale of chords, is exactly equal to the radius 
 of the circle to be divided ; then from 50 to 50 will give an 
 extent of 50 degrees for the same circle ; from 30 to 30 an 
 extent of 30 degrees ; and so on for any other number. 
 
 Ofproportioiiiug the Diameters of JVheels and Pinions. 
 
 The due proportioning of wheels and pinions is an important 
 object in all kinds of wheel-work, but especially in clock- 
 making, where, unless the respective sizes be properly adjust- 
 ed, the transmission of the maintaining power, and communi- 
 cation of motion, will be unequable, and the mechanism liable 
 to rapid destruction. The subject has on these accounts en- 
 gaged much of the attention of writers on clock-work ; but 
 practical men are not yet agreed in the observance of any in- 
 variable rule. The usual mode of proportioning wheels and 
 pinions, is, first to make both a little too big for the proposed 
 calliper, and then, having rounded all the teeth of the pinion, 
 and a few of the teeth of the wheel, they gradually diminish 
 the latter in the lathe, until, by successive trials in the clock- 
 frame, they are found to act at a proper depth, when placed 
 in the pivot holes previously made. This practice is extremely 
 17.— Vol. I. 3D 
 
386 MECHANICS. 
 
 Sizing of wheels and pinions 
 
 objectionable, as it loses much time, and leaves to the discre- 
 tion of the workman the determination, of the very matter in 
 ■^hich he is most apt to err; accordingly, he will at different 
 .imes differ from himself, and almost to a certainty from other 
 workmen. We shall therefore advert to the directions of the 
 best authorities on this subject, any of which may be followed 
 in preference to the common practice, which has so intimate 
 a connection with caprice. 
 
 If the teeth were intended to be rounded in a circular shape 
 which is, however, by no means to be recommended, the pitch- 
 line would be considered as at one-half of the breadth of the 
 tooth from the extreme edge ; but if they be rounded in an 
 epicycloidal, or, as the workmen call it, the bay-leaf form, 
 Hatton found, by numerous experiments, that the depth, or 
 distance of the pitch-line from the circumference, will gene- 
 rally be three-quarters of the breadth of the tooth in any wheel 
 or pinion ; and as the epicycloidal shape is the best for the 
 regular transmission of force and velocity, it is well entitled 
 to be generally adopted in practice. If then we suppose the 
 teeth, and the spaces between them, to be reciprocally equal, 
 which they usually are in clock-work, we shall have the true 
 acting diameter of any wheel or pinion greater than the dia- 
 meter to the pitch-line, (which is sometimes called the geo- 
 metrical, and sometimes the primitive diameter,) by -^ of a tooth 
 or space on each side of the centre, or 1^ in the whole diame- 
 ter. A tooth or space may be called a measure, and it is ob- 
 vious that there must in any wheel be twice as many measures 
 as teeth. These measures of the circumference may be re- 
 duced into measures of the diameter by the usual ratio, of 
 3.1416 : 1, and then, if H be added to such geometrical mea- 
 sures of the diameter, we shall have the proper acting diame- 
 ter, which may be expressed in inches and parts, when the 
 number of measures in the inch are known. For instance, 
 let a wheel and its pinion, tT^ , be taken at 12 teeth per inch at 
 the pitch-line ; the number of measures of the wheel is twice 
 96, or 192, each measuring ^^th of an inch; then, as 3.1416: 
 1 :: 192:61.1; therefore, if to the geometrical diameter ex- 
 pressed by 61.1 measures, there be added 1.5, the sum 62.6, 
 or 62-t*'ff, will be the acting diameter in the same denomination, 
 which are so many 24th parts of an inch ; but 62.6-f-24, gives 
 2.6 inches for the full acting diameter of the wheel in ques- 
 tion. With respect to the pinion of 8, which has of course 
 16 similar measures in its circumference, by the same propor- 
 tion the diameter will be 5.09 measures; to which if 1.5 be 
 added, the acting diameter will be 5.09-f 1.5=6.59, or with 
 sufficient accuracy G-f^, which divided by 24 as before, will 
 
MECHANICS. 387 
 
 Sizing of wheels and pioions. 
 
 give -f-^Q of an inch or somewhat more than a quarter for the 
 acting diameter of the pinion. 
 
 The following table, which may be considered sufficiently- 
 accurate for practice, agrees very nearly with the experiments 
 for determining the proper sizes of wheels and pinions, by 
 Berthoud, an author of the first estimation on these subjects. 
 It is calculated on the supposition that the teeth are epicy- 
 cloidal, and that the circumference is to the diameter as 3: 1, 
 instead of 3.1416: 1. 
 
 Table of the Practical Sizes of Pinions. 
 
 Teeth in the Measures of the Wheel for a 
 
 Pinions Diameter of the Pinion. 
 
 3 3.5 
 
 4 4.1 
 
 5 4.8 
 
 6 5.5 
 
 7 6.1 
 
 8 6.8 
 
 9 7.5 
 
 10 8.1 
 
 11 8.8 
 
 12 9.5 
 
 13 10.1 
 
 14 10.8 
 
 15 11.5 
 
 16 12.1 
 
 To state the manner in which this table is constructed, will 
 enable any person to continue it as far as he pleases. It is 
 simply this : multiply the number of the leaves in the pinion 
 by 2, for the measures in the circumference, divide by 3 for 
 the diameter, and add thereto li for the acting size. Thus, 
 suppose the diameter of a pinion of 9 leaves be required: 
 9x2=18, and 18-1-3=6, and 6 + 1.5=7.5, or 7i, which last 
 quantity, taken by the callipers across the extreme edge of 
 the wheel, (the teeth of which are supposed to be cut, but 
 not rounded) will be 3J teeth and 4 spaces, or 4 teeth and 
 3^ spaces. 
 
 Perhaps some mechanists, not acquainted with decimal 
 arithmetic, may wish for still plainer directions. On this ac- 
 count, and to shew the agreement of Berthoud's practical direc 
 tions with the rule just laid down, we shall recite the method 
 of sizing pinions practised and recommended by him, viz 
 
388 MECHANICS. 
 
 Sizing of wheels an>1 pinion* 
 
 ^"- "'^ The full or actiitg Dinmeter of the Pimon. 
 
 Lc-;Mcs. 
 
 4= two full teetli of the wheel, uniounded, and the 
 space between them. 
 
 5= three teeth rounded from point to point. 
 
 6r:::three full teeth, unrounded. 
 
 7=:three full teeth, and a quarter of a space beyond. 
 
 8= four teeth, rounded from point to point. 
 
 9= somewhat less than four full teeth. 
 10= four full teeth. 
 11, no measure given. 
 12= five full teeth. 
 13, no measure given. 
 14=:six teeth, rounded from point to point. 
 15=: six full teeth. 
 
 It may be proper to observe, that the relative size of a 
 well-proportioned pinion must be somewhat less for a small 
 wheel than for a large one, and also smaller when driven than 
 when it is the driver. Pennington, of Camberwell, the inge- 
 nious artist who constructed kludge's time-piece, adopts the 
 practice of adding 23 measures of the geometrical diameter 
 to the wheel, and 1.^ to the pinion, in watch-work, when 
 the wheel is the driver; and \-{^ to each when the pinion is 
 the driver. 
 
 When the distance is given between the centres of two 
 wheels, unequal in the number of their teeth, but intended to 
 turn each other, their respective diameters may be determined 
 by the following rule: as the distance between the centres of 
 the wheels is equal to the sum of both their geometrical radii, 
 (that is, their radii to the pitch-lines, or the radii which they 
 Avould have if they were merely two cylindrical rollers, one of 
 which turned the other,) therefore say, as the sum of the num- 
 ber of teeth in both wheels is to the distance between their 
 centres, taken in any kind of measure, as feet, inches, or parts 
 of an inch, so is the number of teeth in either of the wheels to 
 the radius or semi-diameter of that wheel, taken in the like 
 measure, from its centre to the pitch-line. Thus, suppose we 
 require two wheels, of such a size that the distance between 
 their centres shall be five inches, and that one of them is to 
 have 73 teeth, and the other 33 ; the sum of the teeth in them 
 both is 108; therefore, as 108 teeth are to five inches, so are 
 75 teeth to 3.47 inches; and as 108 are to 5, so is 33 to 1.52 
 inches ; so that from the centre of the wheel of 75 teeth to its 
 pitch-line is 3.47 inches; and from the centre of ihe wheel of 
 33 teeth to its pitch-line is 1.52 inches. 
 
MECHANICS. 389 
 
 Natnre of the pendulum. 
 
 With respect to the best forms for the teeth of clock-work, 
 the subject has been discussed at length in treating of Mill- 
 work, and a reference to that article will render any further 
 notice of it here unnecessary, as the same principles are appli- 
 cable to both large and small machinery. We may also ob- 
 serve, that the sizing of wheels, which was hardly noticed 
 under Mill-work, will be completed by what has now been 
 said with regard to that particular. 
 
 Of the Penduhim. 
 
 A pendulum is any heavy body so suspended that it may swing 
 backwards and forwards, about some fixed point, by the force 
 of gravity. A body thus suspended necessarily describes an arc, 
 in one half of which it descends, and ascends in the other. 
 
 Each swing which a pendulum makes is called a vibration, 
 or oscillation. 
 
 PC, fig. 4, pi. VI, is a pendulum consisting of a body P, 
 attached by a cord PC, which is fastened to and moveable 
 about the point C. If the body P was not retained by the 
 cord, it would descend in the vertical line PL, but as the cord 
 prevents its falling in this manner, it describes the arc PA, 
 which is the segment of a circle of which PC is the radius. 
 The velocity acquired by the body P in falling through the 
 arc PA, has a tendency, when it arrives at the point A, to 
 carry it otf in the tangent AD, but the cord continually draw- 
 ing it towards the centre, it rises and describes the arc AE. 
 Having arrived at E, it will fall back again, and the velocity 
 acquired in thus falling back will carry it towards P; and this 
 backward and forward motion will be continued till it is over- 
 come by the joint effects of the resistance of the air, the fric- 
 tion at the point of suspension, and the force of gravitation, 
 by which the body P is attracted to the centre of the earth, 
 in a direction perpendicular to the horizon. These causes of 
 obstruction lessen the range of the pendulum at each vibra- 
 tion, and therefore inevitably cause it to stop in a longer or 
 shorter time according to their intensity. 
 
 The nature of a pendulum consists in the following particu- 
 lars: 1, The times of the vibrations of a pendulum in very 
 small arches, are all equal. 2. The velocity of the ball or 
 bob, in the lowest point, will be nearly as the length of the 
 chord of the arch which it describes in the descent. 3. The 
 times of vibration in different pendulums, at the same part of 
 the earth, are as the square roots of the times of their vibra- 
 tions. 4. The time of one vibration is to the time of the 
 descent, through half the length of the pendulum, as the cir- 
 cumference of a circle to its diameter. 5. Whence the length 
 
390 MECHANICS. 
 
 Cvcluidal vibrations. — Long antl sliort xibratiuns. 
 
 of a pendulum vibrating seconds, is found to be 39.2 inches 
 nearly ; that of a half-second pendulum 9.8 inches ; and one 
 for quarter-seconds 2.45 inches. 6. A uniform homogeneous 
 body, BG, fig. 5, as a rod, staff, &c. which is one-third longer 
 than a pendulum, CP, fig. 4, \vill vibrate in the same time 
 with it. From these properties of the pendulum may be de- 
 duced its utility as a regulator of time, for which purpose it 
 far exceeds every contrivance yet discovered. 
 
 When pendulums were first applied to clocks, they were 
 made very short ; and the arches of the circle being large, the 
 time of vibration through different arches was therefore un- 
 equal. To remedy this defect, the pendulum was contrived to 
 vibrate in a cycloid, by suspending it between two cycloidal 
 cheeks. A thread, or some very pliable material, formed the 
 upper part of the pendulum, and folded alternately upon these 
 cheeks. The property of the cycloidal curve is, that a body 
 vibrating in it will describe all its arches, whether great or 
 small, in equal times ; the theory is therefore good, but the 
 practical application of it to the motion of the pendulum, is 
 so imperfect, that cycloidal cheeks are entirely disused. In 
 clocks for astronomical purposes, the arc of vibration must be 
 accurately ascertained, and if it be different from that described 
 by the pendulum when the clock keeps time, a correction must 
 be applied to the time shewn by the clock. This correction, 
 expressed in seconds of time, will be equal to the half of three 
 times the diflerence of the square of the given arc, and of that 
 of the arc described by the pendulum when the clock keeps 
 time, these arcs being expressed in degrees; and so much will 
 the clock gain or lose, according as the first of these arches is 
 less or greater than the second. Thus, if a clock keeps true 
 time when the pendulum vibrates in an arch of 3 degrees, it 
 will lose 10.} seconds daily in an arch of 4 degrees, and 24 
 seconds in an arch of 5 degrees. 
 
 In all that has hitherto been said, the power of gravity has 
 been supposed constantly the same. But this power is not the 
 same in different latitudes; and the length assigned above to 
 the second, half-second, and quarter-second pendulum, is accu- 
 rately adapted only to the latitude of London. A pendulum, to 
 vibrate seconds at the equator, must be somewhat shorter; for 
 the semi-diameter of the earth's equator is about seventeen 
 miles longer than the polar semi-diameter, consequently the 
 force of gravity is on this account less at the equator than at 
 or towards the poles ; and it is further greatly diminished by 
 the centrifugal force, arising from the diurnal motion of the 
 earth being greatest at the equator. The following Table 
 shews the amount of the variations thus arising : 
 
MECHANICS. 
 
 391 
 
 Centre of oscillation. 
 
 Length of Pendulums to vibrate Seconds at every Fifth Degree 
 
 of Latitude. 
 
 
 s 
 
 ° ^ 
 
 "So-* 
 
 c 
 
 as 
 
 o 
 
 8 
 
 o 5 
 — "a 
 
 >3 
 
 a, .^ 
 
 a 
 
 Length of Pen- 
 dulum. 
 
 
 Inches. 
 
 
 Inches. 
 
 
 Inches. 
 
 
 
 39-027 
 
 35 
 
 39-084 
 
 65 
 
 39-168 
 
 3 
 
 39-029 
 
 40 
 
 39-097 
 
 70 
 
 39-177 
 
 10 
 
 39-032 
 
 45 
 
 39-111 
 
 75 
 
 39-185 
 
 15 
 
 39-036 
 
 50 
 
 39-126 
 
 80 
 
 39-191 
 
 20 
 
 39-044 
 
 55 
 
 39-142 
 
 85 
 
 39-195 
 
 25 
 
 39-057 
 
 60 
 
 39-158 
 
 90 
 
 39-197 
 
 30 
 
 39-070 , 
 
 
 
 
 
 To find the length of a pencUilum to make any number of 
 vibrations, and vice versa : Call the pendulum making 60 vi- 
 brations in a minute, the standard length ; then say, as the 
 square of the given number of vibrations is to the square of 
 60, so is the length of the standard to the length sought. If 
 the length of the pendulum be given, and the number of vi- 
 brations it makes in a minute be required ; say, as the given 
 length is to the standard length, so is the square of 60, its 
 vibrations in a minute, to the square of the number required ; 
 the square root of which will be the number of vibrations made 
 in a minute. 
 
 In considering a simple pendulum, or a ball suspended by 
 a string having no sensible weight, the whole weight of the 
 ball is supposed to be collected in its centre of gravity, and 
 the length of the pendulum is measured from the centre of 
 gravity to the point of suspension. But when a pendulum 
 consists of a ball, or any other solid, suspended by a metallic 
 or wooden rod, the length of the pendulum is the distance 
 from the point of suspension to a point in the pendulum called 
 the centre of oscillation, which does not exactly coincide with 
 the centre, of gravity of the ball. If a rod of iron, or any other 
 substance, were suspended, and made to vibrate, that point in 
 which all its force was collected, and to which, if an obstacle 
 were applied, all its motion would cease, and be received by 
 the obstacle, is tailed the centre of oscillation. We have 
 shewn that a homogeneous rod or bar, will perform its oscil- 
 
392 MECHANICS. 
 
 Mode of obviating tiie effects of change of temperature. 
 
 lations in the same time as a pendulum consisting of a ball 
 and a tiiread, though only two-thirds of its length. Hence a 
 point taken one-third of the length of such a bar from the 
 lower end, is its centre of oscillation. 
 
 The greatest inconvenience attending the use of the pendu- 
 lum, is its being constantly liable to an alteration of its length, 
 from the effects of heat which expands, and of cold which con- 
 tracts every material of which it can be made. To remedy 
 this inconvenience, the common method is, to continue the 
 rod through and a little below the bob, which will slide up- 
 wards or downwards upon it. The lower part of the rod is 
 screwed, and furnished with a nut, upon which the bob rests; 
 consequently, as the nut is screwed upwards or dow-nwards, the 
 bob, and with it the centre of oscillation, is raised or lowered. 
 This contrivance is a very unscientific one, and of little use; 
 accordingly^ the title of a compensation for temperature, is 
 given to those inventions alone in which the efficient prin- 
 ciple of regulation is contained within themselves, and con- 
 stantly acting. Of this character, we shall notice two or three; 
 to enlarge further, would exceed our limits. 
 
 In the year 1721, Graham produced a compensation pendu- 
 lum, and applied it to a clock, the going of which he com- 
 pared with one of the best of the common sort, for three 
 years together, and found its errors to be but one-eighth part 
 of those of the latter. The following considerations will 
 explain the principle of this pendulum. If a glass or metallic 
 tube, uniform throughout, filled with quicksilver, and 58.8 
 inches long, were applied to a clock, it would vibrate seconds, 
 for 39.2=*- of 58.8, and such a pendulum admits of a twofold 
 expansion and contraction, viz. one of the tube and the other 
 of the mercury, and these w ill be at the same time contrary, 
 and therefore will correct each other. The tube will extend 
 in length with heat, and so the pendulum will vibrate slower 
 on that account. The mercury also will expand with heat, 
 and since by this expansion it must extend the length of 
 the column upward, and consequently raise the centre ot 
 oscillation, so will its distance from the point of suspension 
 be shortened, and therefore the pendulum on this account 
 will vibrate quicker : hence, if the tube and the mercury be 
 skilfully adjusted, the time of the clock will, by this means, 
 for a long course of time, continue the same, without any sen- 
 sible gain or loss. 
 
 The gridiron pendulum, which has justly obtained a high 
 degree of celebrity, is a contrivance for the same purpose. 
 Instead of one rod, this pendulum is composed of any conve- 
 nient odd number of rods, as five, seven, or nine, being so con- 
 
MECHANICS. 393 
 
 Giidiiou pendulum. 
 
 nected, that the effect of one set of them counteracts tliat of 
 the other set; and therefore, if they are properly adjusted to 
 each other, the centres of suspension and oscillation will al- 
 ways be equidistant. Fig. 6, pi. VI, represents a gridiron pen- 
 dulum composed of nine rods, steel and brass alternately. The 
 two outer rods, AB, CD, which are of steel, are fastened to the 
 cross pieces, AC, BD, by means of pins. The next two rods, 
 EF, GH, are of brass, and are fastened to the lower bar BD, 
 and to the second upper bar EG. The two following rods are 
 of steel, and are fastened to the cross bars EG and IK. The 
 two rods adjacent to the central rod being of brass, are fast- 
 ened to the cross pieces IK and LM ; and the central rod, to 
 which the ball of the pendulum is attached, is suspended from 
 the cross piece LM, and passes freely through a perforation in 
 each of the cross bars IK, BD. From this disposition of the 
 rods, it is evident that, by the expansion of the extreme rods, 
 the cross piece, BD, and the rods attached to it, will descend; 
 but since these rods are expanded by the same heat, the cross 
 piece, EG, will consequently be raised, and therefore also the 
 two next rods; but because these rods are also expanded, the 
 cross piece IK will descend ; and by the expansion of the two 
 next rods, the piece LM will be raised a quantity sufficient to 
 counteract the expansion of the central rod : whence it is ob- 
 vious, that the effect of the steel rods is to increase the length 
 of the pendulum in hot weather, and to diminish it in cold 
 weather, and that the brass rods have at the same time a con- 
 trary effect. The effect of the brass rods must, however, be 
 equivalent not only to that of the steel rods, but also to the 
 part above the frame and spring which connects it with the 
 cock, and to that part between the lower part of the frame and 
 the centre of the ball. 
 
 Another useful method of constructing the pendulum is the 
 following : a bar of the same metal w ith the rod of the pendu- 
 lum, and of the same dimensions, is placed against the back 
 part of the clock-case ; from the top of this a part projects, to 
 which the upper part of the pendulum is connected by two fine 
 pliable chains or silken strings, which just below pass between 
 two plates of brass, whose lower edges will always terminate 
 the length of the pendulum at the upper end. These plates 
 are supported on a pedestal fixed to the back of the case. The 
 bar rests upon an immoveable base at the lower part of the 
 case, and is inserted into a groove, by which means it is always 
 retained in the same position. By this construction, it is in- 
 tended that the extension or contraction of the bar, and of the 
 rod of the pendulum, shall be equal, and in contrary directions. 
 Thus, suppose the rod of the pendulum to be expanded by any 
 17.— Vol. I. 3 E 
 
394 MECHANICS. 
 
 Coni]>ensatiun of pendiiltims. 
 
 given quantity by heat, and its centre of oscillation conse- 
 quently tending to a lower point, the bar being at the same time 
 expanded upwards, will, it is presumed, raise the upper end of 
 the pendulum just as much as its length is increased; and hence 
 its length below the plates will be the same as before. Of a pen- 
 dulum of this sort, somewhat improved by Crosthwaite, a clock 
 and watch-maker of Dublin, we shall insert a further descrip- 
 tion. AB, tig. 7, pi. VI, are two rods of steel, forged out of 
 the same bar, at the same time, of the same temper, and in 
 every respect similar. On the top of B is formed a gibbet C; 
 this rod is firmly supported by a steel bracket D, fixed on a 
 large piece of marble F, firmly set into a wall, and it has liberty 
 to move freely upwards between cross staples of brass, 1,2,3,4, 
 which touch only in a point in front and rear, (the staples hav- 
 ing been carefully formed for that purpose;) to the other rod 
 is firmly fixed by its centre, the lens G, of twenty-four pounds 
 weight, although it should in strictness be a little below it. 
 This pendulum is suspended by a short steel spring on the gib- 
 bet at C; all which is entirely independent of the clock. To 
 the back of the clock-plate, I, are firmly screwed tw o cheeks 
 nearly cycloidal at K, exactly in a line with the centre of the 
 verge L. The maintaining power is applied by a cylindrical 
 steel-stud, in the usual way of regulators, at JNI. Here the ex- 
 pansion or contraction in either of these exactly similar rods, is 
 counteracted by the other in the manner above mentioned ; and 
 this, which has been usually called a compensation pendulum, 
 has been supposed to have an advantage over other compensa- 
 tion pendulums; because the latter being composed of differ- 
 ent materials, however just the calculation may seem to be, 
 they will be defective, as not only different metals, but also 
 different bars of the same metal, not manufactured at the same 
 time, and exactly in the same manner, are found by a good 
 pyrometer to differ materially in their degrees of expansion and 
 contraction, a very small change affecting one and not the other. 
 No kind of steel will be so likely to answer well for the two 
 bars in question, as the best cast steel, no other kind possess- 
 ing nearly so much uniformity of texture. When it is observed 
 that the bars should be of the same temper, it is not to be un- 
 derstood, according to the usual import of the term temper 
 among artists, that they should undergo the regular process of 
 hardening, and then be reduced to a blue or any other colour 
 at discretion ; on the contrary, it seems reasonable to suppose, 
 that they can be submitted to no process of preparation so suit- 
 able as that of annealing, which will leave them in the softest, 
 but, at the same time, the most uniform state the steel is ca- 
 pable of receiving. They should be annealed alongside of 
 
MECHANICS. 
 
 TLJT 
 
 PutUsied "by He-ary Eisier, Caxton, London, l.izs 
 
MECHANICS. 395 
 
 Rittenhouse's Pendnluni. 
 
 each other, and after undergoing this operation, they should 
 not be hammered. It is, however, to be observed, of the pen- 
 dulum constructed with these two similar bars, that the bracket 
 D is supposed to be absolutely fixed, whereas it is itself 
 moveable by the expansion and contraction of the block in 
 which it is placed. The pendulum cannot therefore strictly be 
 called a compensation pendulum ; but its performance is found 
 to be much superior to that of the common construction. 
 
 The most approved mode of diminishing, in a simple pendu- 
 lum, the errors arising from change of temperature, is to make 
 the rod of straight-grained, well seasoned fir wood, of which 
 the expansion or contraction lengthwise is very small. This 
 wood is best used in its natural state, without baking, varnish- 
 ing, painting, gilding, or any other preparation, excepting, per- 
 haps, the mere rubbing of it with a waxed cloth. A pendu- 
 lum also performs better when the ball is heavy, and the arc 
 of vibration small. 
 
 In the construction of clocks intended to measure time with 
 the utmost possible exactness, a compensation should also be 
 established for the resistance of the air, which, by its unequal 
 density, varying the weight of the pendulum, must in a small 
 degree accelerate or retard its motion. The celebrated David 
 Rittenhouse, who paid particular attention to this subject, esti- 
 mates the extreme differenceof velocity, arising from this cause, 
 at half a second per day; and he observes, that a remedy de- 
 pendent on the barometer will not be strictly accurate, as the 
 weight of the entire column of air does not precisely correspond 
 with the density of its base. He proposes, therefore, as a very 
 simple and easy remedy, that the pendulum shall, as usual, con- 
 sist of an inflexible rod carrying the ball beneath, and continu- 
 ed above the centre of suspension to an equal, (or an unequal) 
 distance upwards. At this extremity is to be fixed another 
 ball of the same dimensions (or greater or less, according as 
 the continuation is shorter or longer,) but made as light as pos- 
 sible. The buoyancy of this upper ball will accelerate its oscil- 
 lations by the same quantity as those of the lower would be re- 
 tarded ; and thus, by a proper adjustment, the two effects might 
 be made to balance and correct each other. The inventor made 
 a compound pendulum on these principles, of about one foot 
 in its whole length. This pendulum, on many trials, made in the 
 air 57 vibrations in a minute. On immersing the whole in water, 
 it made 59 vibrations in the same time ; shewing evidently, that, 
 contrary to what takes place with the common pendulum, its re- 
 turns were quicker in so dense a medium as water than in air. 
 When the lower bob or pendulum only was plunged in water, 
 it made no more than 44 vibrations in a minute. 
 
396 ABSTRACT OF MECHANICS. 
 
 IM.ilter. — Space. 
 
 ABSTRACT OF MECHANICS; 
 
 OR, 
 
 A Revieiv of the moat important Dejinitions and Principles of 
 (/lis Division. 
 
 The ingenious student will readily perceive the advantage he 
 may derive from being furnished with an epitome like the 
 following, of the different branches of Natural Philosophy, 
 successively treated of. After he has perused the respective 
 dissertations at length, the condensation of the subjects of 
 them thus presented to hmi, is calculated to save him the 
 trouble of repeated perusal, as a view of the leading prin- 
 ciples will often be all that he wants to remind him of the 
 reasoning and facts by which they have been supported and 
 explained. 
 
 Of Matter. 
 
 1. Every portion of matter is possessed of the following 
 properties, viz. solidity, extension, divisibility, mobility, iner- 
 tia, attraction, and repulsion. 
 
 2. Solidity is that property by which two bodies cannot oc- 
 cupy the same place at the same time. It is sometimes called 
 the impenetrability of matter. 
 
 3. The extension, like the solidity of matter, is proved by the 
 impossibility of two bodies co-existing in the same place. 
 
 4. Divisiljiliti/ is that property by which bodies are capable 
 of being divided into parts removeable from each other. 
 
 5. Mohiliti/ expresses the capacity of matter to be moved 
 from one position or part of space to another. 
 
 6. Inertia is the term which designates the passiveness of 
 matter, which, if at rest, will for ever remain in that state until 
 compelled by some cause to move; and on the contrary, if in 
 motion, that motion will not cease, or abate, or change its 
 direction unless the body be resisted. 
 
 Space. 
 
 1. Space is either absolute or relative : 
 
 2. Absolute space is merely extension, illimitable, immoveable. 
 
ABSTRACT OF MECHANICS. 397 
 
 Attraction. — Repalsion. — Motion. 
 
 and without parts ; yet for the convenience of language it is 
 usually spoken of as if it had parts. Hence the expression, 
 
 3. lielative space, which signifies that part of absolute space 
 which is occupied by any body, as compared with any part 
 occupied bv another body. 
 
 Attraction. 
 
 1. Attraction denotes the property which bodies have to 
 approach each other. 
 
 2. There are five kinds of attraction, — the attraction of co~ 
 hesion, of gravitation, of eUctricity, of magnetis/n, and chemical 
 attraction. 
 
 3. The attraction of cohesion is exerted only at veiy small 
 distances. 
 
 4. The strength of the attraction of cohesion being dilFerent 
 in different kinds of matter, is supposed to be the cause of the 
 »'elative degrees of hardness of different bodies. 
 
 5. Capillary attraction is only a particular modification or 
 branch of the attraction of cohesion. 
 
 6. The attraction of gravitation is exerted by every particle 
 of matter on every other particle at all distances, but by no 
 means with equal intensity at all distances. 
 
 7. Gravitation decreases from the surface of the earth up- 
 wards as the square of the distance increases ; but from the 
 surface of the earth doicmcards , it decreases only in a direct 
 ratio to the distance from the centre- 
 
 Kepuhion. 
 
 1. Repulsion is that property in bodies, whereby, if they are 
 placed just beyond the sphere of each other's attraction of 
 cohesion, they mutually fly from each other. 
 
 2. Oil refuses to mix with water, from the repulsion between 
 the particles of the two substances; and from the same cause, 
 a needle gently laid upon water will swim. 
 
 Motion. 
 
 1. Absolute motion is the actual motion that bodies have, 
 considered independently of each other, and only with regard 
 to the parts of space. 
 
 2. lielative motion is the degree and direction of the motion 
 of one body, when compared with that of another. 
 
 3. Accelerated motion is when the velocity continually in- 
 creases. 
 
398 ABSTRACT OF MECHANICS. 
 
 Motion. — Central forces. 
 
 4. Retarded motion is when the velocity continually de- 
 creases ; and the motion is said to be uniformly retarded, when 
 it decreases equally in equal times. 
 
 5. The velocity of uniform motion is estimated by the time 
 employed in moving over a certain space ; or, which amounts to 
 the same thing, by the space moved over in a certain time. 
 
 6. To ascertain the velocity, divide the space run over by 
 the time. 
 
 7. To ascertain the space run over, multiply the velocity by 
 the time. 
 
 8. In accelerated motion, the space run over is as the square 
 of the time, instead of being directly as the time, as in uniform 
 motion. 
 
 9. A body acted upon by only one force, will always move 
 in a straight line. 
 
 10. Bodies acted upon by two single impulses, whether 
 equal or unequal, will also describe a right line. 
 
 11. But when a body is acted upon by one uniform force, or 
 single impulse, and another accelerating or retarding force, the 
 two forces will cause it to describe a curve. 
 
 12. The curve described by a body projected from the earth, 
 and drawn down by the action of gravity, would, in an unre- 
 sisting medium, be that of a parabola ; but from the resistance 
 of the air, which, when the velocity is very great, will often 
 amount to one hundred times the weight of the projectile, the 
 curve really described approaches more nearly to that of an 
 hyperbola. 
 
 13. The momentum of a body is the force with which it 
 moves, and is in proportion to the weight, or quantity of mat- 
 ter, multiplied into its velocity. 
 
 14. The actions of bodies on each other are always equal, 
 and exerted in opposite directions ; so that any body acting 
 upon another, loses as much force as it communicates. 
 
 Central Forces. 
 
 1. The central forces are the centrifugal and the centripetal 
 forces. 
 
 2. The centrifugal force is the tendency which bodies that 
 revolve round a centre, have to fly from it in a tangent to the 
 curve they move in, as a stone from a sling. 
 
 3. The centripetal force is that which prevents a body from 
 flying oflf, by impelling it towards the centre, as the attraction 
 of gravitation. 
 
ABSTRACT OF MECHANICS. 399 
 
 Centre of gravity. — The lever. 
 
 Centre of Gravity. 
 
 1. The centre of gravity is that point in a body, about which 
 all its parts exactly balance each other in every position. 
 
 2. A vertical line passing through the centre of gravity of a 
 body, is called the line of direction. 
 
 3. ^yhen the line of direction falls within the base of a 
 body, that body cannot descend ; but if it fall without the 
 base, the body will fall. 
 
 The Lever. 
 
 1. There are three kinds of levers, the difference between 
 which is constituted by the difference in the situation of the 
 fulcrum and the power with respect to each other. In ihejirst 
 kind of lever, the fulcrum is placed between the power and the 
 weight. In the second kind of lever, the fulcrum is at one end, 
 the power at the other, and the weight between them. In the 
 third kind of lever, the power is applied between the fulcrum 
 and the weight. 
 
 2. In all these levers, the power is to the weight, as the dis- 
 tance of the weight from the fulcrum is to that of the power 
 from the fulcrum. 
 
 3. A bent, or hammer lever, difl'ers only in form from a lever 
 of the first kind. 
 
 4. Scissors, pincers, siniifers, and the common iron-crow, are 
 all levers of the first kind. 
 
 5. The statera or Roman steeJ-yard is a lever of the first kind, 
 with a moveable weight. 
 
 6. A balance is also a lever of the first kind wnth equal arms; 
 a perfect balance should combine the following requisites : 1. 
 The arms of the beam should be exactly equal, both as to 
 weight and length, and should at the same time be as long as 
 possible, relatively to their thickness. 2. The points from 
 which the scales are suspended, should be in a right line, 
 passing through the centre of gravity of the beam. 3. The 
 fulcrum ought to be a little higher than the centre of gravity. 
 4. The axis of motion should be formed with an edge like a 
 knife, and with tlie rings and other bearing parts, should be 
 very hard and smooth. 5. The pivots, which form the axis 
 of motion, should be in a straight line, and at right angles to 
 
 11 3 ■' BO 
 
 the beam. 
 
 7. The best balances are not calculated to determine weights 
 with certainty to more than five places of figures. 
 
 8. The oars and rudders of vessels are levers of the second 
 order; a paix of bellows, nut-crackers, Sec. are composed of 
 two levers of the same kind 
 
400 ABSTRACT OF MECHAVICS. 
 
 The ptillty. — ^Wheel and axle 
 
 9. The third kind of lever is used as little as possible, on 
 account of the disadvantage to the moving power, the in- 
 tensity of which must always exceed the resistance ; yet in 
 some cases this disadvantage is overbalanced by the quick- 
 ness of its operations, and the small compass in which it is 
 exerted; hence its fitness for the bones of the arm, and the 
 limbs of animals generally. 
 
 10. In compound levers, the power is to the weight, in a 
 ratio compounded of fhe several ratios which those powere 
 that can sustain the weight by the help of each lever, when 
 used singly and apart from the rest, have to the weight. 
 
 The Pitlkj/. 
 
 1. Pulleys are of two kinds, Jiied and moveable. 
 
 2. The fixed pulley only turns upon its axis, and affords no 
 mechanical advantage; therefore when the power and the 
 weight are equal, they balance each other. It is used for the 
 convenience of changing the direction of a motion. 
 
 3. The moveable pulley not only turns upon its axis, but rises 
 and falls with the weight. 
 
 4. Every moveable pulley may be considered as hanging by 
 two ropes equally stretched, and which consequently bear 
 equal portions of the weight; therefore each pulley of this sort 
 doubles the power. 
 
 5. A pulley of one spiral groove upon a truncated cone, as 
 the fusee of a watch, is calculated to maintain a constant equi- 
 librium or relation between two powers, tlie relative forces of 
 which are continually changing. 
 
 Wheel and Axle. 
 
 1. The power must be to the weight, in order to produce an 
 equilibrium, as the circu/fijeretice of the wheel is to the circum- 
 ference of the axle. 
 
 2. As the diameters of different circles Tjear the same pro- 
 portion to each other that their respective circumferences do, 
 the power is also to the weight as the diameter of the wheel to 
 the diameter of the axle. 
 
 3. If one wheel move another of equal circumference, no 
 power will be gained, as they will both move equally fast. 
 
 4. But if one wheel move another of diflferent diameter, 
 whether larger or smaller, the velocities with which they move 
 will be inversely as their diameters, circumferences, or number 
 of teeth. 
 
 5. The wheel and axle may be considered as a perpetual 
 lever, from the constant renewal of the points of suspension 
 
ABSTRACT OF MECHANICS. 4Ul 
 
 Tire inclined plane — The wedge. — The screw. 
 
 and resistance. The fulcrum is the centre of the axis, the 
 longer arm is the radius of the wheel, and the shorter arm the 
 radius of the axis. 
 
 6. The crane, and many other machines of the first cense 
 quence, are composed principally of the wheel and axle. 
 
 The Inclined Plane. 
 
 1. The power and the weight balance each other, when 
 the former is to the latter as the height of the plane to its 
 length. 
 
 2. In estimating the draught of a waggon or other vehicle 
 up-hill, the draught on the level must be added ; so that if the 
 hill rises one foot in four, one-fourth part of the weight must 
 be added to the draught on level ground. 
 
 The Wedge. 
 
 1. When the resistance acts perpendicularly to the sides, 
 that is, when the wedge does not cleave at any distance, there 
 is an equilibrium between the resistance and the power, when 
 the latter is to the former as half the thickness of the back of 
 the wedge is to the length of one of its sides. 
 
 2. When the resistance on each side acts parallel to the 
 back, that is, when the wedge cleaves at some distance, the 
 power is to the resistance as the whole length of the back to 
 double its perpendicular height. 
 
 3. The thinner the wedge, the greater its power. 
 
 4. The further a wedge is driven into any material, the 
 greater also is its power, the sides of the cleft affording it the 
 advantage of operating on two levers. 
 
 5. Axes, spades, chisels, needles, knives, and all instruments 
 which begin with edges or points, and grow gradually thicker, 
 act on the principle of the wedge. 
 
 The Screic. 
 
 1. The screw is an inclined plane encompassing a cylinder. 
 
 2. It is generally used with a lever ; and the po>\er is to the 
 weight, as the distance from one thread or spiral to another is 
 to the circumference of the circle described by the power. 
 
 3. The friction of the screw is very great, a circumstance that 
 occasions this machine to sustain aweight or press upon a body, 
 after the power by which it was impelled is removed. 
 
 4. A screw cut on an axle to serve as a pinion, is called an 
 endless screw. 
 
 5. The endless screw is very useful, either in converting a 
 very rapid motion into a slow one, or vice versa, as for each of 
 its revolutions the wheel moves but one tooth. 
 
 17.— Vol. I. 3 F 
 
402 ABSTRACT OF MECHANICS. 
 
 Compound machines. — Fir wheels. — Friction. 
 
 Cviitpoinid Machines. 
 
 1. Ill all machines, simple as well as compound, what is 
 gained in power is lost in time ; but the loss of time is com- 
 pensated by convenience. 
 
 2. The mechanical power of an engine may be known by 
 measuring the space described in the same time by the power 
 and the resistance or weight ; or by multiplying into each other 
 the several proportions subsisting between the power and the 
 weight, in every simple mechanical power of which it is com- 
 posed. 
 
 3. The power of a machine is not altered by varj'ing the 
 sizes of the wheels, provided this proportion produced by the 
 multiplication of the power of the several parts remains the 
 same. 
 
 4. In constructing machines, simplicity of parts and unifor- 
 mity of motion should be particularly studied. 
 
 5. The teeth of wheels should always be made as numerous 
 as possible; and when great strength is required, it should be 
 obtained by increasing the width or thickness of the wheel. 
 
 G. The use of the crank is one of the best modes of convert- 
 ing a reciprocating into a rotaY^ory motion, and vice versa. 
 
 Fli/ Wheels. 
 
 1. A fly wheel is a resei-voir of power, and is employed to 
 equalize the motion of a machine. 
 
 2. This equalization of the motion is the only source of the 
 advantage of a tly, which can impart no power it has not 
 received. 
 
 3. When a fly is used merely as a regulator, it should be near 
 the first mover; if intended to accumulate force in the working 
 point, it should not be far separated from that point. 
 
 Friction. 
 
 1. Friction is occasioned by the roughness and cohesion of 
 bodies. 
 
 2. It is in general equal to between one-half and one-fourth of 
 the weight or force with which bodies are pressed together. 
 
 3. It is increased in a small degree by an increase of the 
 surfaces in contact. 
 
 4. It is increased to an extraordinary degree, by prolonging 
 the time of contact. 
 
 5. Two metals of the same kind have more friction than two 
 different metals. 
 
 6. Steel and brass are the two metals which have the least 
 friction upon each other. 
 
ABSTRACT OF MECHANICS. 403 
 
 First movers. — Mill-work. 
 
 7. The general rule for lessening friction consists in substi- 
 tuting the rolling for the sliding motion. 
 
 Men and Horses, considered as first Movers. 
 
 1. In turning a winch, a man exerts his strength in different 
 proportions at different parts of the circle. The greatest force 
 is when he pulls the handle up from the height of his knee ; 
 and the least when he thrusts from him horizontally. 
 
 2. When two handles are used to an axle, one at each extre- 
 mity, they should be fixed at right angles to each other. 
 
 3. The art of carrying large burdens, consists in keeping the 
 column of the body as directly under the weight and as up- 
 right as possible. 
 
 4. The horse exerts his force to the greatest disadvantage in 
 drawing or carrying up a hill. 
 
 5. The force with which a horse acts is compounded of his 
 weight and muscular strength. 
 
 6. The walk of a horse working in a mill, should never be 
 less than forty feet in diameter. 
 
 7. A horse exerts most strength when drawing upon a plane. 
 
 Mill Work. 
 
 1. Water-wheels are of three kinds, viz. nyidershot-wheels, 
 breast-wheels, and overshot-tvheels. The powers necessary to 
 produce the same effect on each of these, must be to each 
 other as the numbers 2.4, 1.75, and 1. 
 
 2. The iindershot-y}\\et\ is used only when a fall of water can- 
 not be obtained. 
 
 3. A water-wheel twice as broad as another, has more than 
 double the power. 
 
 4. An axis furnished with a very oblique spiral, and placed 
 in the direction of a stream, may be rendered a powerful first 
 mover, adapted to a deep and slow current. 
 
 5. A mill-stone should make 120 revolutions in a minute. 
 
 6. Bevel led-ivheels are much used for changing the direction 
 of a motion in wheel-work. 
 
 7. Hooke's indmr sal joint is sometimes used with advantage 
 for the same purpose. 
 
 8. The teeth of wheels should never, if it can be avoided, 
 act upon each other before they arrive at the line joining their 
 centres. 
 
 9. To ensure a imiformity of pressure and velocity in the 
 action of one wheel upon another, the teeth should be formed 
 into epicycloids 5 or into involutes of the circumferences of the 
 
404 ABSTRACT OF MECHANICS. 
 
 Mill-work. — \\ lurtl carriage*. 
 
 respective wheels; or if the teeth of one of the wheels be either 
 circular or triangular, the teeth of the other wheel should have 
 a figure compounded of an epicycloid and that of tke figure of 
 the first wheel. 
 
 10. The object of thus forming the teeth, is, that they may 
 not slide but rull upon each other, by which means the friction 
 is almost annihilated. 
 
 11. It is a great improvement in machinery, where trundles 
 are employed with cylindrical staves, to make these staves 
 moveable on their axis. 
 
 \'2. A heavy mill-stone requires very little more power than 
 a light one ; but it performs much more work, and more eflfec- 
 tually equalizes the motion, like a heavy fly. 
 
 13. The corn, as it is ground, is thrown out from between 
 the mill-stones by the centrifugal force it has acquired. 
 
 14. The manual labour of putting the ground corn into 
 sacks, in order to raise it to the top of the mill-house, may 
 be obviated by the use of a chain of buckets wrought by the 
 machinery. 
 
 Wheel Carriages. 
 
 1. A horse draws with the greatest advantage, when the 
 line of traction or draught is inclined upwards so as to make 
 an angle of about 15 degrees with the horizontal plane. 
 
 2. By this inclination, the line of traction is at right angles 
 to the shape of the horse's shouldc-rs, all parts of which are 
 therefore equally pressed by the collar. 
 
 3. Single horses are preferable to teams, because in a team, 
 all but the shaft horse draws horizontally, and consequently 
 to disadvantage. 
 
 4. A horse, when part of the weight presses on his back, 
 will draw a weight to which he would otherwise be incom- 
 petent. 
 
 5. The fore-wheels of carriages are less than the hind-wheels, 
 for the convenience of turning in a smaller compass. 
 
 6. In ascending, high wheels facilitate the draught,, in pro- 
 portion to the squares of their diameters; but in descending, 
 they press in the same proportion. 
 
 7. In descending, the body of a cart may be advantageously 
 thrown backwards, so that the bottom of it will be horizontal, 
 while the shafts incline downwards. 
 
 8. In loading four-wheeled carriages, the greatest weight 
 should be laid upon the large wheels. 
 
 9. Dished wheels are better calculated than any other to 
 sustain the jolts and unavoidable inequalities of pressure aris- 
 ing from the roughness of the roads. 
 
ABSTRACT OF MECHANICS. 405 
 
 Clock-work. 
 
 10. The extremities of the axles should be in the same hori- 
 zontal plane, and the wheels should be placed on them at right 
 angles. 
 
 11. Broad cylindrical wheels smooth and harden a road, 
 while narrow ones cut it into furrows, and conical ones grind 
 the hardest stones to powder. 
 
 Clock-work. 
 
 1. To ascertain the number of revolutions which a pinion 
 makes, for one of the wheel working in it, divide the number 
 of its leaves by the number of the teeth of the wheel, and the 
 answer is obtained. 
 
 2. By increasing the number of teeth in the wheels ; by 
 diminishing the number of leaves in the pinions ; by increas- 
 ing the length of the cord that suspends the weight ; and 
 lastly, by adding to the number of wheels and pinions, a clock 
 may be made to go any length of time, as a month, or a year, 
 without winding up. 
 
 3. The inconvenience of taking up more room, but princi- 
 pally the increase of friction which would be introduced, are 
 the causes of its being inexpedient to make a clock go much 
 beyond eight days. 
 
 4. Clocks intended to keep exact time, are contrived to go 
 whilst winding up. 
 
 5. Clocks which have pendulums vibrating half seconds, are 
 frequently moved by a spring instead of a weight. 
 
 6. A spring is strongest when it is first wound up, and gra- 
 dually decreases in strength till the movement stops ; it is 
 therefore contrived to draw the chain ofF a conical barrel, so 
 that the lever at which it pulls is lengthened as it grows 
 weaker, by which means its etiects are equalized. 
 
 7. The plates of clock-makers' engines, may quickly be di- 
 vided into odd numbers, by subtracting from the odd number 
 so much as will leave an even number of easy subdivision ; 
 then calculating the number of degrees contained in the parts 
 subtracted, and setting them off on the circumference of the 
 circle from a sector. 
 
 8. The geometrical radius of wheels, when the teeth are epi- 
 cycloidal, is less than the acting diameter, by about ^ths of 
 the breadth of a tooth or measure. 
 
 9. The relative size of a pinion must be less for a small 
 wheel than for a large one, and also smaller when driven than 
 when it is the driver. 
 
406 ABSTRACT OF MECHANICS. 
 
 I'euduluras. 
 
 Pendulums. 
 
 1. All the vibrations of the same pendulum, whether great 
 or small, if cycloidal, are performed in equal times. 
 
 2. The longer a pendulum, the slower are its vibrations. 
 
 3. A pendulum to vibrate seconds, must be shorter at the 
 equator than at the poles. 
 
 4. Heat lengthens and cold shortens pendulums. 
 
 5. The quicksilver pendulum, the gridiron pendulum, and 
 many others, have been contrived to obviate these effects of 
 change of temperature. 
 
 6. The vibrations of pendulums are affected by differences 
 in the density of the medium in which they are performed. 
 
 7. The merit of the only contrivance to remedy this defect 
 is due to Rittenhouse. It consists in the use of two pendu- 
 lums, one of which is very light, and placed in an inverted 
 position, extending above the point of suspension of the 
 other. 
 
 8. This compound pendulum may be made to vibrate quicker 
 in so dense a medium as water than in the open air. 
 
OPTICS. 407 
 
 Historical remarks. 
 
 OPTICS. 
 
 THIS branch of Natural Philosophy treats of the mechanical 
 properties of light, and the phenomena of vision. 
 
 Many optical appearances are of such frequent recurrence, 
 that they could not escape the notice of the earliest observers; 
 but ages appear to have elapsed, before any progress was made 
 towards an explanation of them. Empedocles was the first 
 person on record, "who attempted to write systematically on 
 light. A treatise on optics, attributed to Euclid, who flourished 
 about 400 years before the Christian era, shews the state of 
 knowledge on the subject about that time. It adverts to the 
 effect of bringing into view, by refraction, an object at the 
 bottom of a vessel, by pouring water upon it; but chiefly treats 
 of reflected rays, explaining the effect of different kinds of 
 mirrors, and demonstrating the equality of the angles of inci- 
 dence and reflection. It appears, also, that the ancients were 
 acquainted with the magnifying power of glass globes filled 
 with water, though they probably knew nothing of the reason 
 of this power ; and it is supposed that the ancient engravers 
 used glass globes to magnify their figures, that they might 
 work to more advantage. 
 
 Ptolemy, about the middle of the second century, wrote a 
 considerable treatise on optics. The work is now lost, but 
 from the accounts of others, it appears that his observations 
 had even enabled him to treat of astronomical refractions. Al- 
 hazen, an Arabian writer, was the next author of consequence; 
 he wrote about the year 1100, and gave the first account of 
 the magnifying power of glasses. 
 
 In 1270, Vitellio, a Polander, published a treatise on optics, 
 containing all that was valuable in Alhazen, digested in a bet- 
 ter manner, and wuth clearer explications of various phenomena. 
 He observes, that light is always lost by refraction, which 
 makes objects appear less luminous. He gave a table of the 
 results of his experiments on the refractive powers of air, wa- 
 ter, and glass, corresponding to different angles of incidence. 
 He ascribes the twinkling of the stars to the motion of the air 
 in which the light is refracted ; and illustrates this hypothesis 
 by observing, that they twinkle still more when viewed in wa- 
 ter put in motion. He also asserted, that refraction is neces- 
 sary as well as reflection, to form the rainbow ; because the 
 body which the rays fall upon is a transparent substance, at 
 the surface of which one part of the light is always reflected, 
 and another refracted. He makes some ingenious attempts to 
 
408 OPTICS. 
 
 Historical remarks 
 
 explain refraction, or to ascertain the law of it ; and considers 
 the foci of glass spheres, and the apparent size of objects seen 
 tiirough them, though with but little accuracy. 
 
 The celebrated Roger Bacon was contemporary with Vitellio, 
 with whose writings, if not also with those of Alhazen, he was 
 probably acquainted. He seems to have acquired the know- 
 ledge of some facts, which were unknown to them ; yet, witli 
 several important truths, he blended, on this subject, much 
 that was wild and fanciful. Tiie invention of the magic lan- 
 tern is attributed to him ; and he demonstrated, by actual ex- 
 periment, that a small segment of a glass globe would greatly 
 assist the sight of old persons; but concerning the actual in- 
 ventor of spectacles we have no correct information ; this only 
 is certain, that the use of them was generally known about the 
 beginning of the fourteenth century. 
 
 In the year 1575, Maurolycus, a teacher of mathematics at 
 Messina, published a treatise on optics, in which he demon- 
 strates that the crystalline humour of the eye is a lens, which 
 collects the rays of light from external objects, and throws 
 them on the retina or back of the eye. From this principle 
 he assigned the reason why some people are short-sighted, and 
 others long-sighted, and why the former are relieved by con- 
 cave, and the others by convex glasses. 
 
 John Baptista Porta, of Naples, was contemporary with Mau- 
 rolycus. He invented the camera obscura, and his experi- 
 ments with that instrument convinced him that light is a sub- 
 stance, by the reception of which into the eye, vision is ac- 
 complished. This was a great discovery, and corresponds very 
 nearly with the experiments and reasoning of Maurolycus; but 
 it must be remarked, that neither of these two philosophers 
 had any knowledge of what the other had done. The impor- 
 tance of Porta's discovery will be evident, when it is observed, 
 that previous to his time, vision was supposed to be dependent 
 upon what were termed visual rays, proceeding from the eye. 
 He justly considered the eye itself as a camera obscura, the 
 pupil perfor.ming the office of the hole in the window-shutter; 
 he remarked, also, that a defect of light is remedied by the 
 dilatation of the pupil, which contracts involuntarily when ex- 
 posed to a strong light, and expands when the light is too 
 faint for distinct vision. 
 
 Antonia de Dominis, whose work was published in IGll, 
 was the first who came near the true theory of the rainbow. 
 He describes the progress of the ray of light through each drop 
 of the falling rain; he shews that it enters the upper part of the 
 drop, where it suffers one refraction; that it is reflected once, and 
 then refracted again, so as to come directly to the eye of the 
 
OPTICS. 409 
 
 Historical remarks. 
 
 spectator; why this refraction should produce the different co- 
 lours, was reserved for Sir Isaac Xewton to explain. 
 
 Telescopes were invented towards the latter end of the six- 
 teenth century. Of this, as of many other memorable disco- 
 veries, accident furnished the first hint: Zacharias Jansen, a 
 spectacle maker of Middleburg, trying, it is said, the effects of 
 a concave and convex glass united, found that, if they were 
 placed at a certain distance from each other, they caused dis- 
 tant objects to appear nearer the eye. Other accounts transfer 
 the merit of the first discovery from Jansen himself to his 
 children, who, while playing with spectacle-glasses in his shop, 
 perceived, that when they held two of these glasses between 
 their fingers, at a certain distance from each other, the dial of 
 the clock appeared greatly magnified, hut in an inverted posi- 
 tion. This incident suggested to their father the idea of ad- 
 justing two of these glasses on a board, so as to move them at 
 pleasure. To the Jansens we are also indebted for the dis- 
 covery of the microscope, an instrument depending upon ex- 
 actly the same principles as the telescope. Galileo greatly 
 improved the telescope, and constructed one that magnified 
 thirty-three times; with which he made the astronomical dis- 
 coveries that have immortalized his name. 
 
 Kepler paid great attention to the phenomena of light and 
 vision ; he was the first who demonstrated that the degree of 
 refraction sufiered by light in passing through lenses, corre- 
 sponds to the diameter of the circle of which the convexity or 
 concavity is the portion of an arch. He very successfully pur- 
 sued the discoveries of r>Iaurolycus and Porta. The images of 
 external objects, he asserted, were formed upon the optic nerve 
 by the foci of rays coming from every part of the object; and 
 he attributed to habit the power of enabling us to see objects 
 in their right position, though their images upon the retina are 
 inverted. He accounted for the apparent diminution of the 
 moon's disk in solar eclipses by observing, that the disk of the 
 moon does not appear less in consequence of being unenlight- 
 ened, but that at other times it appears larger than it really is, 
 in consequence of its being enlightened: for pencils of rays 
 from such distant objects generally come to their foci before 
 they reach the retina, which they consequently reach in a state 
 of divergence. Hence he concluded, that different persons 
 may imagine the lunar disk to be of different magnitudes, ac- 
 cording to the relative goodness of their sight. 
 
 In 1625, the curious discovery of Scheiner was published at 
 
 Rome, which placed beyond the reach of contradiction, the fact 
 
 that vision depends upon the images of external objects being 
 
 depicted upon the retina, and that these images are inverted; 
 
 18.— Vol. I. 3 G 
 
410 OPTICS. 
 
 Historical remarks. 
 
 for taking the eye of an animal, and cutting away the coats of 
 the back part, and presenting ditierent objects before it, he 
 displayed their images distinctly painted on the naked retina 
 or optic nerve. 
 
 About the middle of the seventeenth century, the velocity 
 of light was discovered by Roemer ; and towards the close of 
 it published by James Gregory, the first proposal for a reflect- 
 ing telescope. 
 
 At length arose Sir Isaac Newton, who discovered the cause 
 of colours, an investigation which had eluded the abilities of all 
 tile philosophers who had gone before him. He applied his 
 principles to the satisfactory explanation of most of the phe- 
 nomena of nature where light and colour are concerned ; and 
 almost all that we know upon these subjects was laid open by 
 his experiments. He found that the refractive powers of dif- 
 ferent substances were in general proportionate to their densi- 
 ties, except when they contained inflammable or oily particles. 
 Having proved that the refractive power of diamond was much 
 greater than that of other substances of equal density, he sup- 
 posed it to contain an inflammable principle, — a happy con- 
 jecture, which the modern discoveries in chemistry have irre- 
 fragably proved, and which has always been considered a proof 
 of his wonderful sagacity, because it was at complete variance 
 with all that was known in his day respecting the nature of 
 the diamond. 
 
 The splendour of Sir Isaac Newton's discoveries obscures in 
 some degree the merit of both earlier and subsequent writers; 
 yet a great variety of interesting facts and improvements have 
 appeared since his time, though it must be admitted that the 
 light by which many of them have been discovered was fur- 
 nished by his labours. The present admirable mode of con- 
 structing reflecting telescopes, to which, after him, many in- 
 genious philosophers and artists have contributed, has led to 
 the most brilliant discoveries ; and DoUond's improvement of 
 the refracting telescope has added not a little to the conveni- 
 ence and the value of optical instruments of that description. 
 This artist, by using three glasses, of diflerent refractive powers, 
 was enabled to enlarge the diameters of his object-glasses, and 
 thus to admit more light, and enlarge his field of view, far be- 
 yond what the common telescope admitted. 
 
 After explaining the principal terms made use of in treating 
 of optics, we shall take a view of the properties of light in 
 general, and then proceed with the particular consideration of 
 refraction, reflection, inflexion, and the phenomena depending 
 upon them. 
 
OPTICS. 411 
 
 Definitions. 
 
 Definitions. 
 
 1. By a ray of light is meant the least particle of light that 
 can be either intercepted or separated from the rest. In the 
 diagrams by which the science is illustrated, rays of light are 
 represented by right lines. 
 
 2. Any small parcel of rays proceeding from or to a point, 
 considered apart from the rest, is called a pencil of rays. 
 
 3. A beam of light is used to denote any considerable aggre- 
 gate of light, or parcel of rays. 
 
 4. Parallel rays are such as move always at the same dis- 
 tance from each other, as represented by fig. 3, pi. I. 
 
 5. Converging rays are such as approach nearer and nearer 
 to each other, and tend to unite in a point; they are repre- 
 sented by fig. 4, forming a cone, the base of which is at the 
 place where the rays began to converge. 
 
 6. Diverging rays are those which continue to recede further 
 and further from each other through their whole progress, and 
 if proceeding from a point, form, as shewn by fig. 5, a cone, 
 the base of which is the termination of their course. 
 
 7. The point at which converging rays meet, is called the 
 focus. 
 
 8. When converging rays are prevented from meeting by 
 some obstacle, the point towards which they tend, and where 
 they would have united, if not intercepted, is called the virtual 
 or imaginary focus. 
 
 9. Void space, and whatever substance the rays of light pass 
 through, is called by opticians, a medium. 
 
 10. Media are either dense or rare: one medium is said to be 
 more dense than another, when it is heavier, or contains more 
 matter under the same bulk ; and vice versa, it is called more 
 rare than another, when it is lighter, or contains less matter 
 under the same bulk. Glass is more dense than water; water 
 is more dense than air. 
 
 11. By a lens is meant a transparent body of a different den- 
 sity from the surrounding medium, and terminated by two sur- 
 faces, either both curved, or one plane and the other curved. 
 As lenses are commonly made of glass, it is usual to call them 
 glasses, with the addition of an epithet, designating their use, 
 or the nature of their curve, or both; as a magnifying-glass, an 
 object or eye-glass of a telescope, a concave spectacle-glass, 
 a convex spectacle-glass. 
 
 12. An incident ray is that which comes from any body to 
 the reflecting surface ; the refected ray is that which is sent 
 back from the reflecting surface. Children sometimes amuse 
 
412 OPTICS. 
 
 Definitions. — Opinions of the aocitrots respecting li;;bt. 
 
 themselves with piesentinc: a jjiece of lookiiif^-glass to the sun, 
 and castino; a vivid spot of light on any object thev please. In 
 this case, the rays received directly from the sun are called 
 vicidcnt, and the lucid spot is made by the reflected rays. 
 
 13. The angle of incidence is the angle which is formed by 
 the incident ray with a perpendicular to the reflecting surface 
 at the point of incidence; and \.\\e angle oi reflection is the 
 angle formed by the same perpendicular and the reflected ray. 
 Thus, in fig. 6, a ray of light, a, falls in the direction a d, upon 
 the surface ef, and is reflected in the direction d l>; c d is per- 
 pendicular to ej\ therefore a d c is the angle of incidence, and 
 c d b is the angle of reflection. 
 
 14. A mirror, or speculum, is an opaque body, the surface of 
 which is very smooth and finely polished, so that it will reflect 
 the rays of light which fall upon it, and by this means repre- 
 sent the images of objects opposed to it. 
 
 15. Plane mirrors are those reflecting bodies, the surfaces of 
 which are perfectly plane, such as our common looking-glasses. 
 
 16. Concave and convex mirrors are those the surfaces of 
 which are curved. If a watch-glass were silvered on the round 
 side, it would be a concave mirror; if it were silvered on the 
 hollow side, it would be a convex mirror. 
 
 Of Light. 
 
 The nature of light has been a subject of speculation from 
 the first dawnings of philosophy. Several of the earliest phi- 
 losophers thought, that objects became visible by means of 
 something proceeding from the eye; while some maintained, 
 that vision was occasioned by particles continually flying off 
 from the surfaces of bodies, which met with others proceeding 
 from the eye; but Pythagoras is said to have ascribed the 
 effect solely to the particles proceeding from external objects, 
 and entering the pupil of the eye. The ancients possessed 
 much greater ingenuity to invent theories, than inclination (or 
 perhaps opportunity) to determine the truth of them by expe- 
 riment ; and therefore, if Pythagoras actuallv promulgated this 
 opinion, it seems to have been supported by no facts which 
 placed it on an immutable basis. Hence it gained for many 
 ages no greater credit than other opinions, the most incon- 
 sruous and vague. It was not till about a thousand years after 
 the time of Pythagoras, that J. Baptista Porta fully satisfied 
 himself and others of vision being performed entirely by the 
 intromission of light into the eye. 
 
 Several eminent philosophers have imagined, that the sen- 
 sation which we receive from li2;ht is to be attributed entirely to 
 
OPTICS. 413 
 
 Opinions respecting light. 
 
 the vibrations of a subtile medium or fluid, diff'used throughout 
 the universe, and put in action by the impulse of the sun. 
 According to this hypothesis, light may be considered as ana- 
 logous to sound, which is known to depend entirely on the 
 pulsations of the air striking upon the ear. But this theory is 
 encumbered with many difhculties. If light depended alto- 
 gether upon the vibrations of a fluid, a quick motion of the 
 hand, or of a machine contrived for the purpose, would pro- 
 duce light at any time; or rather, as no solid reason can be 
 assigned why the fluid sliould cease to vibrate in the night, 
 since the sun must always affect some part of it, we ought to 
 have perpetual day. Again, the texture of certain bodies is 
 actually changed by exposure to light, even though they be 
 inclosed in glass ; but if covered with the thinnest plate of 
 metal, which excludes the light, no alteration takes place. 
 Many other objections which militate with equal force against 
 this theory, might be adduced, but these have never received 
 answers which can be deemed satisfactory. 
 
 A late writer is decidedly of opinion that light is diluted fire, 
 that is, fire weakened and diffused, as ardent spirit when 
 mingled with water. It appears at first view favourable to 
 this opinion, that light may be collected and condensed by 
 the burning-glass, so as to burn like the fiercest flame ; ou 
 the contrary, flame itself may be attenuated, even by artificial 
 means, to such a degree as to be perfectly innoxious. "The 
 flame," says Dr. Goldsmith, " which hangs over burning 
 spirit of wine, we all know to scorch with great power, yet 
 these flames may be made to shine as bright as ever, yet be 
 perfectly harmless. This is done by placing them over a 
 gentle fire, and leaving them thus to evaporate in a close 
 room without a chimney ; if a person should soon after enter 
 with a candle, he will find the whole room filled with in- 
 noxious flames. The parts have been too minutely separated, 
 and the fluid, perhaps, has not strength enough to send forth 
 its burning rays with sufficient efl'ect." But when we con- 
 sider the remarkable discovery of Dr. Herschel, that light 
 may be separated from the caloric* which accompanies it, the 
 identity of the two substances, if not fully disproved, becomes 
 very doubtful. The Doctor, when employed in making obser- 
 vations on the sun by means of telescopes, found that the co- 
 loured glasses used to prevent the inconvenience arising from 
 the heat, very soon cracked and broke in pieces when their 
 colour was deep enough to intercept the light. On prosecuting 
 
 * Caloric is the term by which modern philosophers have agreed to distinguish that 
 pocnliar substance which produces beat. 
 
414 OPTICS. 
 
 .Separatiou of lii;lit and lieat. — \ clocily of lit;lit. 
 
 the inquiry, in a course of experiments devised for the pur- 
 pose, he found that the solar beam contained rays which gave 
 no light, and yet produced a greater degree of heat than even 
 those rays which produced the strongest light. The two spe- 
 cies of rays may be separated from each other by a very easy 
 experiment. If a glass mirror be held before the fire, it strongly 
 reflects the rays of light, but the rays of caloric which it sends 
 forth are very few ; a metallic mirror, on the other hand, made 
 of planished tin-plate, for example, reflects the rays of light 
 indifferently, but it supplies caloric rays in great abundance. 
 The glass mirror becomes hot; the metallic mirror does not 
 alter its temperature. If a plate of glass be suddenly inter- 
 posed between a glowing fire and the face, it intercepts com- 
 pletely the warming power of the fire, without causing any 
 sensible diminution of its brilliancy ; consequently it inter- 
 cepts the rays of caloric, but allows the rays of light to pass 
 If the glass be allowed to remain in its station till it becomes 
 as hot as its distance from the fire will allow, it ceases to in- 
 tercept the rays of caloric, which then pass through it as freely 
 as the rays of light. These data lead us inevitably to the con- 
 clusion, that light is not diluted fire or caloric. 
 
 We shall new proceed to notice the most important proper- 
 ties of light in an optical point of view; — a task much more 
 delightful than the investigation of its nature and essence; 
 and eminently calculated to entrance the mind with astonish- 
 ment. 
 
 In the very short space of one second, a ray of light tra- 
 verses the prodigious extent of nearly tioo hundred thousand 
 miles. The manner in which the velocity of light is calcu- 
 lated, is not less ingenious than the discovery is surprising. 
 It was by observing the eclipses of Jupiter's satellites, which 
 appeared to be eclipsed sooner or later than the times given 
 by the tables of them, and the observation was always before 
 or after the computed time, according as the earth was nearer 
 to or further from the planet Jupiter than the mean distance. 
 To understand this fact more fully, suppose the earth in going 
 its annual circuit round the sun, is at C, (fig. 1, pi. I,) and 
 that an eclipse is there observed of a satellite of Jupiter, 
 which regularly sufl'ers an eclipse in forty-two hours and a 
 half. If the earth never left C, but continued there immove- 
 able, and Jupiter remained at the same distance, the eclij)se 
 of the satellite would always be observable at the expected 
 interval of forty-two hours and a half; and consequently, in 
 thirty times forty-two hours and a half, the spectator would 
 see thirty eclipses. But the earth travelling onward to D, the 
 spectator does not see thirty eclipses till some time after the 
 
OPTICS. 415 
 
 Velocity and minuteness of light. 
 
 Stated period, for the further off the earth removes, the longer 
 the time required for the light to reach the spectator. In 
 traversing from C to D, light takes up about sixteen minutes 
 and a half, therefore, as the sun is halfway between C and D, 
 it must perform its journey from him in half that time, that 
 is, in eight minutes and a quarter ; or, according to the most 
 exact calculation, in eight minutes and seven seconds. No 
 difference in the velocity of light has ever been discovered, 
 whether it is original, as from the stars, or reflected only, as 
 from the planets. 
 
 Such being the rapidity \Aith which the rays of light dart 
 themselves forward, it becomes a matter of easy calculation, 
 and may a little assist our conception to observe, that a 
 journey which they perform in eight minutes, could not be 
 performed by a cannon ball at its ordinary speed in less 
 than thirtv-two years. That the motion of lioht is inex- 
 pressibly rapid, we may easily convince ourselves, if we 
 notice the firing of a cannon or a musket at a considerable 
 distance, and observe the time which elapses between see- 
 ing the flash and hearing the report. Sound, it has been 
 calculated, travels at the rate of 1142 feet, or 380 yards, in 
 a second ; yet the time which intervenes between seeing the 
 flash and hearing the report of the fire-arm, is a satisfactory 
 proof of the prodigious disparity between the velocit}'- of light 
 and sound. 
 
 If the velocity of light, then, be so very great, it may be 
 inquired why it does not strike against objects with a pro- 
 portionate force? If the finest sand were thrown against our 
 bodies with a hundredth part of this velocity, each grain 
 would be as fatal as the stab of a dagger; yet our eves, the 
 most exquisitely sensible of all our organs of perception, re- 
 ceive its impressions without the smallest pain. But we have 
 sufficient evidence to convince us that the minuteness of the 
 particles of light is still more extraordinary than their velo- 
 city, and that their minuteness, therefore, is the cause of their 
 being harmless. A lighted candle will fill a sphere of four 
 miles in diameter, and may be extinguished without having 
 lost any sensible portion of its weight ; yet it must have dif- 
 fused several hundreds of millions more particles of light than 
 there could be grains in the whole earth, if it were entirely 
 composed of the finest sand. 
 
 It is a principle in mechanics, that the momentum of 
 moving bodies, or the force with which they strike, is propor- 
 tionate to the quantity of matter they contain multiplied by 
 their velocity, consequently, if the particles of light were not 
 infinitely smaller than we can conceive, nothing could with- 
 
41G OPTICS. 
 
 KlTect of light on the eye m>t instantaneous. 
 
 Stand their impulse. Light moves 2,000,000 of times faster 
 tlian a cannon ball; consequently, if its particles were only- 
 equal in size to the 2,000,000th part of a grain of sand, they 
 would produce an effect equal to that of sand from the mouth 
 of a cannon. If, keeping this in mind, we further consider the 
 facility with which light penetrates the hardest bodies, as glass, 
 crystal, and even the diamond itself, through all which it finds 
 an instant passage, and that it does not displace the smallest 
 atom of dust which it encounters in its progress; we shall not 
 hesitate to admit, that the particles of light cannot be equal 
 in size to the 4,000,000th part of a grain of sand. 
 
 As the particles of light are continually passing from every 
 luminous body, in all directions, it may be inquired why they 
 do not interfere with each other in such a manner as to con- 
 found all distinct perception of objects, if not quite destroy the 
 sense of seeing? Their velocity, however, enables us to answer 
 these questions, by convincing us that they may be separated 
 at least a thousand miles, and yet be perfectly efficient to 
 the purposes of vision. It is an undoubted fact, that the 
 effect of li^lit upon the eye is not instantaneous, but that 
 the impression remains after the light has been withdrawn. 
 Of this any one may satisfy himself, by shutting his eye, 
 after having looked for some time on a candle, a star, or any 
 other luminous object, when a faint momentary picture of the 
 object will remain. The same thnig may be proved by whirl- 
 ing round a stick, the extremity of which is on fire ; if the 
 motion be quick enough, the perception of a complete circle 
 of flame will be impressed on the eye. The actual duration, 
 for a certain time, of the impression of light being thus 
 proved, let it be supposed to continue distinct only for the 
 150th part of a second ; then if one lucid point of the sun's 
 surface emit 150 particles of light in a second, these will be 
 amply sufficient to afford light to the eye without any inter- 
 mission ; and yet the particles emitted will be more than 1000 
 miles apart. 
 
 Notwithstanding the extreme tenuity of light, the task of 
 actually measuring its momentum has not proved either too 
 bold to be conceived, or too difficult to be executed. Boerhaave 
 gave motion to the needle of a compass, by concentrating the 
 rays of the sun upon it with a powerful burning-glass ; and 
 Mitchell, in later times, tried an experiment to the same purpose 
 in a still more satisfactory manner. He constructed an instru- 
 ment, in the form of a small vane or weathercock. It consisted 
 of a very thin plate of copper, of about one inch square, 
 which was attached to one of the finest harpsichord wires, 
 about tea inches long. To the middle of the wire was fixed an 
 
OPTICS, 417 
 
 Momentum of liglit. — DiminuUoii ol' the sun by (lie einisMun of liglit. 
 
 agate cap, such as is used for the smallest mariners' compasses, 
 alter the manner of which it was intended to turn; and the 
 thin plate was balanced on the other side by a grain of small 
 shot The instrument weighed ten grains ; and to prevent its 
 being affected by the vibrations of the air, it was inclosed in a 
 gl iss box. The rays of the sun were thrown upon the plate of 
 copper from a concave mirror of two feet in diameter ; in con- 
 sequence of which, the vane or copperplate moved, on repeated 
 trials, with a gradual motion of about one inch in a second of 
 time. The instrument weighing ten grains, and the velocity 
 with which it moved being one inch in a second, the quantity 
 of matter contained in the rays which fell upon the instrument 
 in that time, was equal to the twelve hundred millionth part 
 of a grain, the velocity of light exceeding the velocity with 
 which the instrument moved in that proportion. The mirror 
 containing about three square feet of surface, and mirrors in 
 general reflecting only half the rays which fall upon them, the 
 quantity of matter contained in the rays of the sun incident 
 upon a square foot and a half of surface, is no more than one 
 twelve hundred millionth part of a grain. But the density of 
 the rays of light at the surface of the sun is greater than at 
 the earth, in the proportion of 45,000 to 1. If in one second, 
 therefore, one square foot of the sun's surface emit one forty- 
 five thousandth part of a grain of matter, the supply will be 
 adequate to the consumption of light ; yet it will be little 
 more than two grains a day, or about 4,380,000 grains, or 626 
 pounds, in 6,000 years ; a loss which would have shortened the 
 sun's diameter about ten feet, if it were formed of matter the 
 density of water only. 
 
 By those who adopt the opinion that light is constituted by 
 the vibrations of a subtile fluid, it has been urged, that if a 
 flood of particles were incessantly streaming from the sun, the 
 diminution of the bulk of that luminary would have become 
 very perceptible from the diminution of his beneficial effects. 
 But even if the preceding calculation of the emission of light 
 be estimated far too low, when the immensity of the sun's 
 diameter, which amounts to 883,246 English miles, is consi- 
 dered, we shall see little reason to justify the apprehensions 
 of its being exhausted, however extravagant our ideas of the 
 duration of the ])resent system of nature, and although we 
 reject the great probability that, like our atmosphere, it is pro- 
 vided with the means of its own perpetual replenishment. 
 
 It has been objected to Mitchell's experiment, that the air, 
 rarefied by his mirror, was sufficient to give motion to the 
 vane, although the light which fell on it had no momentum 
 whatever*; but it has been found that precisely the same 
 
 18.— Vol. I. 3 H 
 
418 OPTICS. 
 
 Media not wanned bj the passage of fight. — Direction of light. 
 
 motion is produced in vacuo as when the glass case is filled 
 with air. The fact that lii;ht has momentum may therefore be 
 considered as proved, though the deductions drawn from the 
 experiment relative to the quantity of it, are liable to much 
 uncertainty. 
 
 Another arscument in favour of the extreme tenuity of light, 
 and of its particles following each other at an immense dis- 
 tance,' is the well known fact, that the rays collected by the 
 strono-est burning-glass, will not inilume the most combustible 
 matter while they merely pass through it. A phial contain- 
 ing spirit of wine will not be set on fire ; but if the spirit is 
 poured into a spoon or any opake vessel, which stops the jiro- 
 gress of the concentrated rays, inflammation instantly results. 
 This familiar experiment enables us to account for the cold 
 experienced at the summits of high mountains. The atmo- 
 sphere is not warmed by the mere passage of the rays through 
 it ; these rays first heat the earth, where they are first stopped, 
 and the earth then communicates its own temperature to the 
 air, which being a very bad conductor of heat, the lower, 
 denser stratum of it, becomes the chief residence of its 
 warmth. Such parts, therefore, as are elevated above the 
 general surface of the earth, derive, in proportion to their 
 heio-ht, a diminished advantage from this circumstance, and 
 though they may receive as many rays as an equal extent of 
 burning desert, yet the surface they contain is so trifling, when 
 compared with the stratum of air on a level with them, that 
 the heat produced is abstracted as quickly as it is formed. 
 
 The next property of light which demands our attention, is, 
 that it is propelled from every luminous body in right lines. 
 This is evident from an experiment which any person may 
 make with a bent tube, through which nothing can be dis- 
 cerned, provided it be so much bent that nothing can pass 
 through it in a right line. Another proof of the same fact, is, 
 that shadows are bounded by right lines drawn from the 
 luminous body past the contour of the body casting the 
 shadow. 
 
 It is generally supposed, according to this principle, that 
 those bodies only are transparent whose pores are such as to 
 permit the rays of light to pervade them in a rigiht line ; and 
 that those bodies are opake, which intercept the rays like the 
 bent tube. 
 
 Every ray of light carries with it the image of the point 
 from which it was emitted. If, therefore, the pencils of rays 
 from every point of an object, are united in the same order in 
 which they proceeded when first emitted, they will form a 
 perfect image or representation of that object, at the place 
 
OPTICS. 419 
 
 Means bv which objecbs become Tisible. — Reflections. 
 
 where they are thus united. The rays of light proceeding in 
 straight lines, it is obvious, that to make an object visible, at 
 any place to which a straight line can be drawn from it to the 
 eye, they must be detached from every physical point of it ia 
 all directions ; but only those rays which enter our eyes can 
 render them visible to us. Thusthe object ACB, fis;. 2, pi. I, 
 is rendered visible to an eye in any part where the rays Aa, 
 Ab, Ac, Ad, Ba, Bb, Be, Bd, Ca, Cb, Cc, Cd, can come ; and 
 these rays affect our sight with the sense of different colours 
 and shades, according to the properties of the body from which 
 the light is reflected. However confused the figure may 
 appear, those, to whom the subject is new, will observe that 
 the rays are only made to proceed from three points ; and 
 hence they may be enabled to form some slight conception of 
 the numberless crossings of the rays, in order that every part 
 of a body may be visible, in whatever direction it is looked 
 upon. 
 
 Here perhaps we may allow the young reader, before we 
 pass on with didactic philosophy, to contemplate, for a mo- 
 ment, that luminary from which all light proceeds. The bar- 
 barous and the civilized, the ignorant and the wise, have in 
 all ages regarded the sun as the grandest object of the whole 
 visible creation. With what delight does the lover of nature 
 view, in what glowing colours has the enraptured poet painted, 
 his rising, his meridian, and his settino- glories. Allusions to 
 the sun, and to light, constitute the most beautiful expressions 
 and figures of speech in all languages ; and such are the ideas 
 of association connected with them, that frequency of repeti- 
 tion, or even abuse, can scarcely render any of them con- 
 temptible. A few might be adduced, which accompany only 
 triteness of sentiment, and feebleness of diction ; but genius 
 has still the power of eliciting new descriptions and combina- 
 tions of allusion, as poetically beautiful, as philosophically 
 correct. Thomson's apostrophe to the sun is distinguished 
 for grandeur of idea, and simplicity of expression •, 
 
 " Great source of day, best image here below 
 Of thj Creator, ever poaring wide, 
 From world to world, the vital ocean round." 
 
420 OPTICS. 
 
 Oblique ravs only refract d. 
 
 Refrongibifili/ of light. 
 
 The natural progress of the rays of light, we have already 
 shewn, is in straight lines ; yet, like all other matter, light is 
 influenced by attraction, which sometimes turns it out of its 
 direct course. This happens when it passes out of one me- 
 dium into another of different density, as from air into 
 water or glass, or from water or glass into air. This disposi- 
 tion or capability of light to be bent, is called its rejrutigi- 
 bilitj/, and the change of direction actually assumed when the 
 ravs enter another medium, is called refraction. 
 
 A very easy experiment will convince any one that light is 
 influenced by some power or other when coming out of a difl^e- 
 rent medium : put one end of a stick into water, and it will 
 appear at the surface as if it were broken. This eflect is owing 
 to the rays of light being attracted or drawn out of their direct 
 course, as may be proved in a variety of ways. Place a shil- 
 ling, or any other conspicuous but small object, at the bottom 
 of a basin, and then retireto such adistance, that the edge of the 
 vessel just prevents its being seen. Let the vessel be then tilled 
 with water, and the shilling will be perfectly visible, though 
 neither it nor tlie spectator have changed places in the slightest 
 degree. Another expermient of the same nature, and more 
 easily managed by one person, is the following: take a basin, 
 or any convenient vessel, and place it in such a situation that 
 the shadow of a lighted candle will fill one half of it ; hold the 
 end of a ruler or stick exactly on the place where the shadow 
 terminates ; then pour water into the basin, and the shadow 
 V, ill be seen immediately to withdraw from the ruler. Increase 
 the quantity of water, and the distance to which the shado\*' 
 will retreat, will also be increased. 
 
 It is necessary to observe, that only those rays which enter 
 another medium obtitjiie/i/, suffer refraction, for a ray which 
 falls perpendicularly is equally attracted on all sides, and 
 therefore has no tendency to deviate in any direction. In the 
 experiment with the shillinix, the spectator looks at it in an 
 oblique direction ; and the rays proceeding from it, by which 
 it is rendered visible after the water has been poured in, are 
 bent towards him, on entering the air. A ray of light, AC, 
 fig. 7, pi. I, which passes obliquely from the air into water at 
 C, instead of continuing its course to B, takes the direction 
 CH ; and the reverse is equally true, a ray of light from H, 
 reflected in the direction IIC, instead of continuing its rectili- 
 near couise, proceeds in the direction CA; therefore an object 
 at H is seen by an eye at A, as if it were actually at B, every 
 
OPTICS. 421 
 
 Experiment to prove refraction. 
 
 object appearing to be in the direction of those rays which last 
 approached the eye. 
 
 That the rays of light actually proceed in the direction 
 ascribed to them, is not merely a matter of speculation ; we 
 may have ocular demonstration of the fact in a variety of ways, 
 of which the following is one : Take an empty basin, and on 
 the bottom of it fix marks at a small distance from each other, 
 then take it into a dark room, and let in a ray of light, and 
 where the rays fall upon the floor, place the basin, so that its 
 marked diameter may point tow^ards the window, and that the 
 light may fall upon the mark most distant from the window. 
 Pour water into the basin, and it will be found that the ray 
 which before fell upon the most distant mark, will, by the 
 refractive power of the water, be turned out of its straight 
 course, and fall two or three marks nearer the centre of the 
 basin. The water may now be rendered rather turl^id without 
 destroying its transparency, which may easily be done by the 
 addition of a few drops of milk, then on filling the room with 
 dust, the light will be completely visible, both in its passage 
 through the air and the water. Three directions of the ray 
 will be distinctly perceived ; the direction of incidence, or 
 that in which, from the aperture, it falls obliquely on the 
 water ; that of reflection, equal to that of incidence ; and 
 that of refraction, which commences at the surface of the 
 water, and is continued in a direct line to the bottom of 
 the basin. All things remaining the same, place a small 
 piece of looking-glass at the bottom of the basin, where 
 the refracted beam falls, and it will be reflected back 
 again through the water, and in passing out of the water 
 into the air, will be again refracted or turned out of its 
 course. 
 
 The greater the density of any medium, the greater is its 
 refractive power ; and of two refracting media, that which 
 is of an oily or inflammable nature, will have a greater refract- 
 ing power than the other. 
 
 The incident angle is the angle made by a ray of light and a 
 line drawn perpendicular to the refracting surface at the point 
 where the ray enters the surface ; and the refracted angle, is the 
 angle made by the ray in the refracting medium with the same 
 perpendicular continued. The sine of the angle is a line which 
 serves to measure the angle, being drawn from a point in one 
 leg perpendicular to the other. In fig. 7, pi. I, ACD is the in- 
 cident angle; HCE is the refracted angle; BCH is the angle 
 of deviation ; AF is the sine of the angle of incidence ; HG is 
 the sine of the angle of refraction. 
 
422 OPTICS. 
 
 Refrangibilitj- of light. 
 
 If the incident ray, AC, fell in a more oblique direction 
 than is represented by fig. 7, tlie refraction would be still 
 e;reater ; but in all cases of similar media, the angle of refrac- 
 tion will always be found to bear a regular and constant pro- 
 portion to the angle of incidence ; or in the language of 
 opticians, the sine of incidence is to the sine of refraction 
 in a given ratio or proportion, and this ratio is discovered 
 by experience. In the present instance, if with a pair of 
 compasses we divide the sine of incidence into four parts, 
 the sine of refraction will be exactly three of those parts. 
 Hence, when a ray passes out of air into water, the ratio is 
 said to be as 4 to 3 ; when out of water into air, the ratio is 
 reversed, and therefore becomes as 3 to 4. The ratio out cf 
 glass into air is as 2 to 3 ; consequently out of air into glass, 
 it is as 3 to 2. 
 
 It will be observed, from what has been just said, that in 
 passing into a denser medium, light is refracted towards the 
 perpendicular, that is, the angle of refraction is less than the 
 angle of incidence ; on the contrarj^ when passing into a rarer 
 medium, it is refracted /'row the perpendicular. The inspection 
 of fig. 7, will render this evident; the angle of incidence, ACD, 
 being so much larger than the angle of refraction, ECH, the 
 refracted ray CH, is nearer the perpendicular than if, proceed- 
 ing in a right line, it had gone to B. 
 
 It may seem, at first view, a little extraordinary, that light 
 should pass more directly through a dense than through a rare 
 medium ; but we have already seen that light is subject to 
 attraction, and Sir Isaac Newton discovered and demonstrated, 
 that this power is the cause of refraction. A circumstance that 
 confirms the truth of this theory, is the known fact, that the 
 change in the direction of the ray commences, not, as might 
 be supposed, when it comes in contact with the refracting 
 medium, but a little before it reaches the surface, and the in- 
 curvation augments in proportion as it approaches the medium. 
 Then as the attractive power of different substances are propor 
 tionate to their densities, we discover at once the reason of the 
 ray being bent towards the perpendicular on entering another 
 medium of greater density, and from the perpendicular, on 
 entering a medium of less density. 
 
 In passing from a dense into a rare medium, however, 
 there is a certain degree of obliquity at which the refraction is 
 changed into reflection. In other words, a ray of light will 
 not pass out of a dense into a rare medium, if the angle of 
 incidence exceeds a certain limit, but will be reflected back. 
 Thus a ray of light will not pass out of glass into air, if the 
 
OPTICS. 423 
 
 Efiects of refraction. 
 
 angle of incidence exceeds 40° 11', or out of glass into water* 
 if the angle of incidence exceeds 59° 23'. 
 
 A knowledge of refraction will enable us to explain a variety 
 of curious phenomena, and on some occasions will usefully- 
 direct our judgment. When a spectator stands on the bank 
 of a river, just above the level of the water, the bottom will be 
 one-third deeper than it appears. This is a necessary conse- 
 quence of tlie rays from the bottom being bent from the 
 perpendicular, and consequently inclined towards the specta- 
 tor, on passing out of the water to his eye. Those who shoot 
 fish in the water, soon obtain ^ practical knowledge of this 
 deception, for they find that they cannot hit their mark, 
 unless they aim considerably below the place which it seems 
 to occupy. A proof of the fact may very easily be obtained : 
 immerse a stick or straight rod of any kind, perpendicularly in 
 water, until the part which is immersed appears of equal length 
 with the part above ; upon taking it out and measuring the 
 parts, it will be found that they are to one another as 4 to 3. 
 We can now have no difiiculty in accounting for the broken 
 appearance of an oar, when partially immersed in water. Let 
 FGH, fig. 8, pi. II, represent an oar, the part GH being in the 
 w^ater, and the part OF out of it. The rays sent to the eye 
 from H, will appear to come from I, nearer to the surface of 
 the water, and as every part of GH will appear proportionately 
 nearer the surface, the part GH will appear to make an angle 
 with the part GF. 
 
 It is not merely objects that are partly out of water, which 
 lose their natural appearance from refraction; those which are 
 entirely immersed, especially when they are large, and lie at a 
 considerable depth, appear distorted in a variety of ways. A 
 flat-bottomed basin seems deeper in the middle than at the 
 sides, and a long straight leaden pipe appears to be curved. The 
 reason is, that the rays from the more distant extremities come 
 in a more oblique direction on their emergence into the air, 
 and consequently suffer a greater refraction than the rest. 
 
 The distortion of objects seen through a wrinkled or crooked 
 pane of a window, can have escaped the observation of 
 few ; it arises from the unequal refraction of the rays which 
 pass through the glass. In looking through an even pane of 
 glass, about the tenth of an inch thick, objects appear nearly 
 one-thirtieth part of an inch out of their true place; but as 
 they all maintain the same relative situation, the error is not 
 perceived. 
 
 A stranger to this subject would scarcely suspect, that we 
 never see the stars in the place where they really are, and that 
 we actually see the sun, and other heavenly bodies, before they 
 
424 OPTICS. 
 
 l-lVer(s rif refraction. 
 
 are risen above the liorizon. Our atmosphere decreases in 
 densilv as it increases in heii^ht, and though, at the tops of 
 the lii<j^hest mountains, its jjressure is so diminished, tliat 
 breathing- is ditHcult, and tlie blood often gushes through the 
 skin, yet, at the elevation of furty-hve miles, it is still capable 
 of refracting a ray of light. When, therefore, the sun is sunk 
 so far below the horizon, that liis rays can only strike the 
 upper part of the atmosphere, lie still remains visible, because, 
 instead of passing directly onward, the rays are attracted 
 towards the perpendicular, and cousecpiently bent towards the 
 spectator on the earlh. To exhibit this eflect to the eye, let 
 fig. 9, represesent the earth surrounded by its atmosphere. 
 110 is the visible horizon of a spectator standing at P. S re- 
 presents the sun as yet really below the horizon, from which 
 a ray of light ascends, and falling on the upper part of the 
 atmosj)here, is, by the attraction it there experiences, bent out 
 of its direct course towards D, into the oblique one IP, by 
 which means it falls upon the eye of the spectator, v\ho, per- 
 ceiving the sun in the direction of the refracted ray, will 
 suppose it to be at R, which is higher than its true place by 
 more than a diameter. The length of time that the sun 
 appears above the horizon, while he is actually below it, varies 
 a little with a number of circumstances, but chiefly with a 
 difference of latitude. At the approach of summer, near the 
 poles, the sun becomes constantly visible for two or three weeks 
 before he actually rises above the horizon ; and at the approach 
 of winter, when he has sunk below the horizon, his presence, 
 from refraction, continues to cheer these dreary ret^ions for 
 a similar space of time. The heavenly bodies, when in the 
 zenith, are not subject to any refraction ; but when in the 
 horizon, they have the greatest of all : from the horizon to the 
 zenith, the refraction continually decreases. 
 
 Mankind have availed themselves of the principle of refrac- 
 tion, to a most excellent purpose, in the construction of lenses ; 
 for by grinding the glass thinner at the edges than in the mid- 
 dle, those rays of light which would strike upon it in a straight 
 line, or perpendicularly, if it were plain, strike upon it ob- 
 liquely, and the refraction they suffer causes them to converge; 
 on the contrary, by making the glass thinner in the middle 
 than at the sides, the rays are refracted the contrary way, and 
 therefore become divergent. 
 
 The consideration of refraction through lenses may perhaps 
 be rendered more clear, if we reflect, that all curved surfaces are 
 composed of a number of straight lines, or points, infinitely 
 short, and inclining to each other like the stones in the arch of 
 a bridge. In fig. 10, pi. II, parallel rays are represented as 
 
OPTICS. 425 
 
 Dirterent kinds of lenses. 
 
 falling upon a surface of this sort ; and it is evident that those 
 only which enter the middle part will go on in a straight direc- 
 tion ; those which strike the sides will strike them obliquely, 
 and will consequently be made to converge. If the surface, 
 then, was a perfect curve, as in fig. 11, it is plain that only the 
 ray which strikes the centre part of the curve will enter it in a 
 straight direction ; all the rest will be more or less refracted, 
 according to the degree of obliquity with which they strike 
 the surface, and the whole of the refracted rays will converge 
 to a point called the focus. 
 
 Glasses are usually ground for optical purposes into eight 
 different forms. 1. The glass may be flat on both sides, like 
 the pane of a window. 2. It may be flat on one side, and 
 convex on the other. 3. It may be convex on both sides. 
 4. It may be flat on one side, and concave on the other. 5. 
 It may be concave on both sides. 6. It may be convex on 
 one side, and concave on the other, like the crystal of a watch. 
 
 7. It may have one side, which must be convex, ground into 
 little facets, while the other side is plane. 8. It may have 
 some considerable length, in a triangular form. — The sections 
 of these various forms of glasses are shewn at fig 12, pi. II. 
 and they are distinguished by the following names : the glass 
 No 1, is called q. plane glass, as its sides are parallel; No. 2, 
 is called a plano-convex glass ; No. 3, a double-convex glass ; 
 No. 4, a plano-concave glass ; No. 5, a double-concave glass ; 
 No. 6, a meniscus glass ; No. 7, a nmUiplying glass ; and No. 
 
 8, a prism. The term lens is given to such glasses as either 
 magnify or diminish the apparent size of objects viewed 
 through them; Nos. 2, 3, 4, and 5, are therefore lenses ; No. 6 
 is also a lens, when its surfaces are portions of different 
 spheres ; but when they are of equal radii, it has only the 
 effect of a plane glass. 
 
 From the view we have already given of refraction, the 
 effects of the first seven of these glasses will be easily under- 
 stood. — The prism will receive a distinctive consideration. A 
 ray entering the plane glass. No. 1, will indeed be refracted, 
 but it will suffer another refraction on its emergence, which 
 will rectify the former; the place of the object will therefore 
 be a little altered, but its figure will remain the same. Sup- 
 pose AB, fig. 13. to represent a solid piece of glass with two 
 parallel surfaces, an incident ray EF, will be refracted into 
 FG, and FG will be refracted on passing from the second sur- 
 face into GH, parallel to the original direction EF. 
 
 If parallel rays enter the plano-convex glass, as shewn by 
 fig. ll, the ray E will be refracted upwards to F, the ray K 
 will be refracted downwards to the same point ; there they 
 18.— Vol. I. 3 I 
 
426 OPTICA. 
 
 Kefraction through lenses. 
 
 will cross, and then go onward in a straight line, and continue 
 to diverge, till intercepted by some obstacle. 
 
 When parallel rays fall upon a double-convex glass, KG, 
 they will be refracted still more abruptly, and meet sooner in 
 a point or principal focus at F. The distance of this focus is 
 equal to the semi-diameter of the circle which the convexity 
 of the glass continued would produce. Either this glass or 
 the former, as they collect the rays of the sun into a point, 
 will burn at that point, the whole force of the rays that pass 
 through them being concentrated there. 
 
 From all luminous objects, the rays of light proceed in a 
 state of divergence ; but when the distance from which they 
 come is very great, the quantity of divergence is too small to 
 require notice. The sun, for example, is so immensely dis- 
 tant, that his rays are always considered as parallel ; and it is 
 only parallel rays which are converged to a focus, in the man- 
 ner shewn above. Diveroent rays, proceeding from a point, 
 as the flame of a candle, will be difterently affected. If, there- 
 fore, we place a candle exactly at the focal distance of a single 
 or double-convex lens, as at F, figs. 11 and 14, the rays will 
 emerge parallel to each other. If the candle is placed nearer 
 to the glass than its focal distance, the rays, after passing 
 through the glass, will no longer be parallel, but separate or 
 diverge ; if the candle be placed still further off, the rays will 
 then strike the "lass more nearly parallel, and will, therefore, 
 upon passing through, converge or unite at a distance behind 
 the glass, more nearly approaching the distance at which pa- 
 rallel rays would be converged. 
 
 After the rays have united or converged to a focus, they will 
 cross each other, and form an inverted picture of the flame of 
 a candle, which may be received on a piece of paper placed at 
 the meeting of the rays behind. The cause of the inversion 
 of the image is therefore very evident ; for the upper rays, after 
 refraction, were such as came from the under part of the lumi- 
 nous body ; and the under rays, on the contrary, came from 
 the upper part. 
 
 The foregoing theory may be demonstrated with a common 
 reading glass. If a candle be held so near a glass of this sort, 
 that the rays passing through, fall upon a screen or wall with 
 a bright spot just as large as the glass itself, the candle is 
 then at the focal distance, and it is clear that the rays which 
 struck the glass divergently, are refracted parallel, or the spot 
 of light could not be just the size of the glass. If the candle 
 is held nearer than the focal distance, the rays will then fall 
 more divergently upon the glass, and will consequently, after 
 refraction, still have so much divergence as to form a very 
 
OPTICS. 427 
 
 Refraclion throagh lenses. 
 
 broad spot of light upon the screen. If the candle be held at 
 a much greater distance than the focus, the rays will fall upon 
 the glass more nearly parallel ; and therefore, when they are 
 refracted, they tend to unite and converge behind the glass, 
 and will form upon the screen a small speck of vivid light, 
 which, if closely examined, will appear a perfect image or 
 picture of the candle. 
 
 In looking through a plano-convex or double-convex lens, 
 the objects we examine will appear magnified ; consonantly 
 with the general rule, that we see every thing in the direction 
 of that line in which the rays last approached the eye ; and it 
 will be understood when we come to treat of the eye, that the 
 larger the angle under which any object is seen, the larger 
 that object will appear. That the convergence of the rays 
 of the convex-lens, enlarges greatly the angle of vision, 
 and consequently enlarges the apparent size of objects seen 
 tlirough it, will be evident, if we continue the lines in the 
 direction of their convergence, thus, FK to L, and FG to R, 
 fig. 14. 
 
 From lenses the reverse in form to those we have hitherto 
 been considering, we shall naturally expect opposite effects. 
 Accordingly, the attractive and refractive powers of the lenses, 
 Nos. 4 and 5, fig. 12, are not towards the centre, but towards 
 the circumference. Parallel rays falling upon them are made 
 to diverge, or are dispersed. Rays already divergent are ren- 
 dered more so, and convergent rays are made less convergent. 
 Hence objects seen through one of these glasses appear 
 smaller than to the naked eye. Thus, let a b, fig. 15, pi. II, 
 represent an arrow, which woidd be seen by the eye, if no 
 lens were before it, by the convergent rays c a, z 6; but if 
 the double-concave glass DH is interposed between the ob- 
 ject and the eye, the ray a c will be bent towards g, and the 
 ray b i will be bent towards k, and consequently both will 
 be useless, as they do not enter the eye. The object then 
 will be seen by the rays a o, b r, which, on entering the glass, 
 will be refracted into the lines o c and r i ; and according to 
 the rule just laid down, the object will be seen in the last 
 direction of these rays ; therefore, as the angle o c r, is so 
 much smaller than tlie angle a c b, the arrow necessarily ap- 
 pears diminished, and as with the diminution of its apparent 
 size, we connect the idea of its being further off, it seems to 
 be at the distance n m. 
 
 The meniscus acts like a convex glass, when it is thickest 
 in the middle, that is, when its convex surface is a portion of 
 a less sphere than its concave one ; on the contrary, when it 
 is thinnest in the middle, or has its concave surface a portion 
 
428 OPTICS. 
 
 Refraction ibrou^h lenses. 
 
 of a less sphere than the other, it has the effect of a concave 
 lens. Sometimes, to distinguish these ditierent forms from 
 each other, when the glass magnifies, it is called a conve.xo- 
 concave lens; when it diminishes the apparent size of objects, 
 it is called a concavo-convex lens ; and the term meniscus is 
 restricted to such as have both their surfaces equally arched, 
 and, therefore, like the crystal of a watch, neither magnify 
 nor diminish. 
 
 The axis of a lens, is a line supposed to be drawn through 
 the centre of its spherical surface or surfaces. When one side 
 of the lens is plane, the axis of course falls perpendicularly 
 upon that side. The axis of a lens continued, would pass 
 exactly through the centre of that sphere, of which the lens is 
 the segment. This will be apparent from an inspection of figs. 
 11 and 14, and in figr. 12, aline is drawn representing the axis 
 of the lenses. When opticians mention the focal distance of 
 a convex lens, they mean the focus of parallel rays ; but when 
 they wish to be more precise, the focus of parallel rays is dis- 
 tincrnished by the term principal focus. INIathematicians de- 
 monstrate, that the principal focus of a plano-convex lens is 
 at a distance from the convex surface equal to the diameter of 
 the sphere of which it is a part; and that the principal focus of 
 a double and equally convex lens is at half the same distance. 
 The principal focus of these lenses may therefore be found, 
 with sufficient accuracy for common purposes, by holding a 
 sheet of paper behind the glass, when exposed to the rays of 
 the sun, and observing when the spot is smallest, and when 
 the paper most readily burns. Or when the focal distance 
 does not exceed two or three feet, it may be found by holding 
 the glass at such a distance from the wall opposite a window, 
 that the imase of the sash may appear distinct upon the wall. 
 When accuracy is desirable, and the focal distance exceeds 
 two or three feet, the following method may be pursued : cover 
 the lens with a piece of paper or pasteboard, in which two 
 round holes have been made; each hole being at an equal dis- 
 tance from the edge of the lens, and in one of its diameters. 
 Direct the axis of the lens thus covered to the sun, and receive 
 upon a piece of paper the circles or white spots produced by 
 the two holes, which will be observed gradually to approach 
 each other as the paper is moved farther off; at last they will 
 coincide ; and if the distance of the lens from the paper be in- 
 creased, they will again separate. The distance of the paper 
 from the glass, when the circles coincide, is the focal distance 
 required. 
 
 When a lens is more convex on one side than the other, 
 and both radii are known, the following rule will determine 
 
OPTICS. 429 
 
 Refraction through lenses. 
 
 the focus : as the sum of the radii of both convexities, is to 
 tlie radius of either, so is double the semi-diameter of the 
 other to the distance of the focus ; or divide the double pro- 
 duct of the radii by their sums, and the quotient will be the 
 distance sought. 
 
 As convex lenses cause many rays to enter the eye which 
 would otherwise have been scattered and dispersed, the objects 
 seen through them appear clearer and more splendid than when 
 viewed by the naked eye. They should always be made as thin 
 as their curvature will admit, otherwise the brilliancy of the 
 image will be lessened without necessity, by the rays which, 
 though they enter the glass, are reflected or sent back. 
 
 A large object, seen through a lens which is very convex, 
 appears more or less distorted ; this proceeds from the refrac- 
 tion not being equal at all points, and the degree of it is pro- 
 portionate to the size of the object compared with the focus 
 of the glass. When the object is not much too large, its ex- 
 tremities only appear confused. 
 
 To ascertain the focal distance of a globular glass vessel 
 containing water, such as a decanter, make a round hole, 
 about an inch diameter, in a piece of brown paper, which must 
 then be pasted on one side of the body of the decanter ; hold 
 the covered side of the decanter to the sun, that the perpen- 
 dicular rays may pass through the middle of the water, and the 
 emergent rays will be collected to a focus, whose nearest dis- 
 tance from the decanter will be nearly equal to the radius of 
 the body of it, as will appear by receiving the rays upon a 
 paper held at that distance. That this effect is owing to the 
 water, and not to the glass, may be proved by emptying the 
 decanter; for the light that then passes through the hole, will 
 be as broad as the hole itself, at all the distances of the paper 
 from the decanter, a circumstance of which we might also be 
 assured, from considering, that the sides of the empty decan- 
 ter can only act like two meniscus glasses placed at a little 
 distance from each other. If a solid globe or ball of glass, be 
 covered and tried like the decanter, the distance of the focus 
 from the nearest part of the ball, will be one quarter of its 
 diameter. 
 
 Parallel rays, on passing through a single or double-con- 
 cave lens, immediately diverge ; hence the term focus cannot 
 be applied to a concave lens, in the same sense as to a convex 
 lens. A concave lens has only what is termed a virtual focus, 
 the distance of which from its surface, is the same as that of 
 a convex lens of equal radius, but it is before the glass, be- 
 cause it denotes that point from which the rays, after their 
 last refraction, appear to diverge. To find the focal distance 
 
430 OPTICS. 
 
 Kfrructioii tliroiigli lenses. — Nature of llie iiiiiltiiilvin;^ glass. 
 
 of a concave lens, let the lens be covered with a piece of paper, 
 containing two small circular holes; and on the paper for re- 
 ccivini; the light, describe also two small circles, but with their 
 centres twice as far apart as the centres of the circular holes. 
 iMove the paper backwards and forward.s, till the middle of the 
 sun's light, coming through the holes, falls exactly on the 
 middle of the circles, and the distance of the paper from the 
 lens will then be the focal length required. 
 
 To find the vertex or centre of a lens, hold it at some dis- 
 tance from the eye, and observe the two reflected images of 
 a candle made by the two surfaces. Move the lens till these 
 images coincide, and the point of coincidence is the vertex, 
 which, if in the middle of the surface, the glass is truly 
 centred. 
 
 To understand the effect of the multiplying glass. No. 7, 
 fig. 12, pi. II, it is only necessary again to revert to the gene- 
 ral principle, that objects aj^pear in the direction of the line 
 last described by the rays that render them visible. Hence, 
 if the object B, fig. 1, pi. HI, is seen through the glass EH, 
 by the ray BA, that passes through the surface FG, the ob- 
 ject, by the eye at A, will be seen at B ; the ray BG passes 
 through the surface GH, and after refraction, comes to the eye 
 in the direction AD, as if it proceeded from D, and therefore 
 the object appears at D; and for the same reason, through the 
 surface FE, it appears at C ; consequently, there will be the 
 appearance of as many objects as there are tlat-surfaces on the 
 glass, for each of them shews the same object in a different 
 place. 
 
 To become familiar with the laws of refraction, and the pro- 
 perties of lenses, the student should make experiments with 
 lenses of different foci, diameters, and colours. The room 
 should be darkened, and the sun's rays admitted through an 
 aperture cut in the shutter, or through a tube placed in the 
 shutter, and moveable in any direction. The lenses should be 
 fitted into cells connected with a sliding board, or adapted to 
 convenient frames. By these means, the lenses may be com- 
 bined in any way required. It would be proper, also, to have 
 lenses ground to the figure of a meniscus, or watch-glass, and 
 fitted up so that two or more of them might be connected in 
 one frame, with a view to include Huids between them, and 
 thus exemplify the refractive powers of tluid lenses. The dust 
 usually in motion, will, in the darkened room, give the various 
 and natural figures of the converging and diverging pencils 
 of rays. 
 
OPTICS. 431 
 
 Reflection the effect of repulsion. 
 
 Of the Refexibility of Light. 
 
 The disposition or capability of the rays of light to be turned 
 back into the medium from whence they came, is called their 
 refexibiUty : the change of direction produced by their being 
 actually turned back, is called refection. 
 
 The property which bodies possess of reflecting light, is attri- 
 buted by Sir Isaac Xewton to the principle of repulsion. In 
 connrmr.tion of this theory, it is justly remarked by him, that 
 those surfaces, which to our senses appear smooth and po- 
 lished, are found, when viewed through a microscope, to 
 abound with inequalities; if, therefore, the power which pro- 
 duces reflection, did not act at some distance from the reflect- 
 ing surface, these inequalities would prevent the rays from 
 being reflected with so much regularity as we find thev are. 
 Other theories have been proposed, to account for reflection, 
 but none of them appears more probable in itself, or so recon- 
 cileable with the facts connected with this property of light. 
 But however well founded the opinion, that light is reflected 
 by the power of repulsion, and consequently before it comes 
 into absolute contact with the reflecting surface; it will not be 
 necessary to introduce the circumlocution required to express 
 this idea, in treating of the general phenomena; the incident 
 rays will always be spoken of as actually impinging on the 
 reflecting body, from which they rebound, as a perfectly elastic 
 ball would rebound from a marble slab. 
 
 All objects which are not themselves luminous, are rendered 
 visible by reflection. Even glass, crystal, water, and the most 
 pellucid media, reflect a part of the rays of light which fall on 
 them, or their forms and substance could not be distinguished ; 
 on the contrary, the whole of the incident light is not reflected 
 from any surface, however bright, smooth, and opaque. It is 
 calculated that the best mirrors reflect little more than half 
 the light they receive. 
 
 From the account we have given of refraction and other 
 properties of light, the student is already fully apprized, that 
 it is the rays emanating from any given point, which assure 
 our sight of the existence of a body at that point ; and that, 
 as in the instance of a fish in the water, if the direction of the 
 rays be changed in any manner, the sense of sight, always re- 
 ferring the place of an object to the point in the direction of 
 the last course of the rays, is easily deceived, and will sup- 
 pose an object to be where it is not. This it will be necessarj* 
 to bear constantly in mind while we pursue our present sub- 
 ject ; for in connection with another law of nature, it forms the 
 
A'.iJ. OPTICS. 
 
 I ni»er4al l;i» of rcdectioD. — Kcflection from plane mirrors. 
 
 key by which all the phenomena of reflection are explained. 
 The other law to which we allude, is, that the angle of rejiec- 
 tioH is always equal to the angle of incidence. In fig. 6, pi. I, 
 a d'\s ^n incident ray, falling upon the surface e f, from which 
 it is reflected in the direction d h ; hence the angle of inci- 
 dence, a d c,'\s exactly equal to the angle of reflection b d c, 
 from on either side of the perpendicular c d, the obliquity of 
 the ray is the same, and consequently when the reflecting sur- 
 face is a plane, the obliquity of the line of incidence and re- 
 flection are also equal, when measured from it. It is only the 
 rays which fall obliquely upon a reflecting surface, that are 
 reflected in an angle to their direction of incidence ; those 
 rays which fall perpendicularly, return in precisely the same 
 direction. 
 
 To pursue the consequences deducible from these principles, 
 or their application to particular efi'ects, will soon shew how 
 much depends upon them. Let NO, fig. 2, pi. Ill, be con- 
 sidered as a ray of lisht striking perpendicularly on the sur- 
 face of the mirror AB, and it is reflected back in tlie same 
 line. The ray DO, coming from the luminous body D, 
 strikes the mirror obliquely, and is reflected to the eye in 
 the line OE, thus making the angcle of reflection, XOE, 
 equal to the angle of incidence DON, and the object is seen 
 at S, in the direction of the reflected ray. To be a little 
 more explicit, with regard to the last particular, it must be 
 recollected, that an object is rendered visible, not by single 
 rays proceeding from every point of its surface, but by pen- 
 cils of rays, or collections of divergent rays issuing from 
 every point. These pencils of rays are afterwards, by the 
 refractive powers of the eye, converged again to points 
 upon the bottom of the eye or retina, and these points of 
 convergent rays in the eye, are correspondent to the points 
 of the objects, from which the rays diverged. Hence, as the 
 pencils of rays strike the mirror while they are in their diver- 
 gent state, from the equality of the angles of incidence and 
 reflection, they are reflected in the same state, and converge 
 exactly as they would have done if they had not been inter- 
 cepted by the mirror, but had gone on to G, a distance equal 
 to the incident and reflected course taken together. In con- 
 sequence of this identity in the convergence of the rays, and 
 the general rule that the place we assign to objects is in the 
 last direction of the rays, the two lines, namely, that from the 
 object to the mirror, and that from the mirror to the eye. are 
 united in the mind of the spectator, and the object is conse- 
 quently seen at S, just as far behind the mirror as the object 
 is before it. Another figure will, perhaps, render this more 
 
or 'IK'S 
 
 I'll. 
 
 Publislwi b— Ef arv flshir: . Canaa. . Lor dor. -.azz . 
 
OTTICS. 
 
 rLJi. 
 
 Biblubcd V'' 'Sr^jr- rishcr Cucnn. I/jccoa. :&i. 
 
OPTIC? 433 
 
 Reflection of light. 
 
 evident: the lines DC, fig. 3, pi. Ill, are the lines of incidence, 
 CB are the lines of reflection, and these form equal angles on 
 the surface of the mirror, so that all the rays coming from the 
 object, and falling upon the mirror at C, will strike the eye 
 •at B, and the reflected image will thus become visible. Now 
 every object appears to be in a straight line from the eye, and 
 as no alteration is made in the relative position of the rays, 
 but only in their direction, the body DD, when it comes re- 
 flected to the eye, will appear to be at AA, because rays from 
 thence would, at the mirror, exactly coincide with the rays 
 DC, DC, and would, by the time they arrived at BB, be con- 
 verged to an equal focus. 
 
 Hence we shall not find it difficult to understand why a man 
 may see his whole figure in a plane looking-glass only half as 
 long and half as broad as himself; for the image is seen under 
 an angle as large as the life : the mirror is exactly half way 
 between the image and the eye, and therefore the divergent 
 rays, by which the image is seen, only cover at the glass, a 
 space equal to half the size of the man himself. 
 
 When light is reflected from convex or concave mirrors, it 
 obeys precisely the same law as from plane mirrors, however 
 extraordinary the effects they produce may at first sight appear. 
 To understand the manner in which they act, it will be proper 
 to recur to an observation formerly made respecting spherical 
 surfaces : all curves or arches are to be considered as com- 
 posed of a multitude of infinitely short lines or points, lying 
 obliquely to one another. Parallel rays, therefore, fall upon 
 them more or less obliquely at every point of incidence except 
 the centre. This part of the subject may perhaps be best eluci- 
 dated, if we consider, in the first place, the direction given to 
 rays which fall upon two plane mirrors inclined to each other 
 in opposite directions. Thus, in fig. 4, p). Ill, the rays AB, 
 CD, which would fall perpendicularly on a flat surface in the 
 direction IK, strike obliquely upon that which is opposed to 
 them, and instead of being reflected parallel, are reflected di- 
 vergently. For the same reason, convergent rays would be 
 reflected with less convergence by such a surface as this, and 
 divergent rays would be rendered still more divergent towards 
 E and H. 
 
 Fig. 5, which is the reverse of the preceding, will serve to 
 shew us the nature of reflection from concave surfaces. Here 
 the parallel rays AB, CD, which would have been reflected 
 parallel by a plain mirror, are made to converge, and meet at 
 L, because, instead of striking this mirror in a direct line, they 
 strike it obliquely, and the planes of inclination are inclined to 
 each other, and therefore convergent rays will be reflected with 
 19.— Vol. I. 3 K 
 
434 OPTICS. 
 
 Kcllectioii ot' lielit. 
 
 greater convergence, and divergent rays will have their diver- 
 gency lessened. 
 
 As a convex mirror diminishes the convergency of the rays 
 
 which fall upon it, the etlect it produces is to diminish the 
 apparent size of objects ; because the visual angle is diminish- 
 ed, that is, there is not so large a picture formed in the eye, 
 as when the object itself is seen without the mirror. But a 
 concave mirror, on the contrary, enlarges the visual angle, or 
 picture of an object on the eye, and consequently its effects 
 are the reverse of the former. Reflection, then, from a cotitex 
 mirror, corresponds to refraction throucrh a concave lens ; and 
 upon the same principle, reflection from a concave surface 
 corresponds to refraction through a convex lens. 
 
 More particularly to exemplify this theory, let AB, fig. 6, 
 pi. Ill, represent a dart, which is seen in the convex mirror 
 CD. Though rays issue from the object AB in all directions, 
 yet it is seen only by means of those which are included within 
 the space OX, because no other can be reflected to the eye at 
 R. If the rays had gone forward in their original direction, 
 they would have united at P, and the object would have been 
 seen of its full size; but as, from the nature of a convex curve, 
 the angle of reflection cannot be equal to the angle of inci- 
 dence without diminishing their convergence, the angle SRT 
 is less than the angle APB, and the image at L appears smaller 
 than the object, and nearer to the surface of the mirror. The 
 reason of this last ettect has been already explained, when it 
 was observed, that objects are rendered visible, not by a single 
 ray, but by pencils of divergent rays proceeding from every 
 point of them; and that the eye transfers the place of the ob- 
 ject to the place where these pencils of rays unite in points. 
 Suppose, then, a radiant point G, fig. 7, sends forth a pencil of 
 divergent ravs which falls on the convex mirror AB ; the rays 
 of this pencil will diverge still more after reflection, and there- 
 fore they will appear to come from a point nearer the eye than 
 if their divergency had not suflered any change. 
 
 For these reasons, a person looking at his face in a convex 
 mirror will see it diminished. This is shewn by fig. 8, pi. Ill ; 
 though rays proceed from every part of the face, it is only the 
 rays that fall on the mirror between C and R, that can be re- 
 flected to the eye ; the rays CR beinc^ therefore rendered less 
 convergent, he will see his chin along the line OS, and the 
 forehead along the line OX ; the angle of vision being thus 
 reduced, all the rest of the features will be proportionately 
 reduced. 
 
 When large objects are seen in a convex mirror, they not 
 only appear, according to its curvature, smaller than they really 
 
OPTICS. 435 
 
 Reflection of lisrbt. 
 
 are, but somewhat distorted. The reason of this is, that the 
 different points of the object are not at an equal distance from 
 the mirror, and consequently they are reflected to the eye un- 
 der different angles. Glass globes, lined, like looking-glasses, 
 with an amalgam that converts them into mirrors, are some- 
 times suspended in apartments as an ornamental piece of fur- 
 niture. In these, the company seated in a room, or round a 
 table, are represented by very minute images, which appear very 
 near their surface, and are always, though beautiful, in some 
 degree distorted. Latterly, convex mirrors, fitted in a frame, 
 have almost superseded the globes, over which they have seve- 
 ral advantages : they can be placed against a wall as conveni- 
 ently as a picture; they can be made the segment of a larger 
 sphere than it would be convenient to have entire ; and in par- 
 ticular, they can be ground to a regular figure, whereas the 
 globes are only blown glass, and therefore can never be true. 
 
 The effects of concave mirrors may be gathered from the con- 
 siderations already premised; but though it may introduce 
 some repetition, it will be proper to consider them separately. 
 Their general effect is to render convergent rays more conver- 
 gent, and consequently they have the power of magnifying. 
 By fig. 9, pi. Ill, is represented a face looking at itself in the 
 concave mirror IK, and the exti'eme of those rays which can 
 be reflected to the eye are exhibited, one from the forehead, 
 and one from the chin. These lines, a c and w «, are reflected 
 to the eye at O, which sees the image in the line of reflection, 
 and in the angle DOQ, and therefore evidently magnified, and 
 at a small distance behind the mirror. 
 
 The magnifying effect of a concave mirror, is, however, only 
 perceived when the object is nearer to it than its centre of con- 
 cavity, or centre of the sphere to which the curve of the mir- 
 rci belongs. When we come to the description of the tele- 
 scope, the reader will better understand how the image is formed 
 by the large concave mirror of that instrument, if we here refer 
 to another diagram. Let A c B, fig. 10, pi. Ill, be the reflect- 
 ing surface of a mirror, whose centre of concavity is at C; and 
 let the upright object DE, be placed beyond the centre C, and 
 send out a conical pencil of divergent rays from its upper ex- 
 tremity D, to every point of the concave surface of the mirror, 
 A c B. But to avoid confusion, we only draw three rays of 
 that pencil, as DA, Dc, DB. From the centre of concavity, 
 C, draw the three right lines, CA, Cc, CB, touching the mir 
 ror in the same points as the three rays of the pencil from D, 
 and all these lines will be perpendicular to the surface of the 
 mirror. Make the angle CM, equal to the angle DAC, and 
 draw the right line Ac?, for the course of the reflected ray 
 
436 OPTICS. 
 
 Kellectiuu ol' lijlit. 
 
 DA : make the angle C c d, equal to the angle D c C, and draw 
 
 the rii^ht line c d, tor the course of the reflected ray Dc; make 
 also the an^le CBd, equal to the angle DBC, and draw the 
 rii^ht line I3J, for the course of the reflected ray DB. All 
 these reflected rays will meet in the point d, where they will 
 form the extremity, d, of the inverted image, e d, similar to 
 the extremity, D, of the upright object DE. If the pencil of 
 rays, E /', E g, E A, be also continued to the mirror, and their 
 angles of reflection from it be made equal to their angles of 
 incidence upon it, as in the former pencil from D, they will 
 meet at the point e, by reflection, and form the extremity e, of 
 the image, e d, similar to the extremity, E, of the object, DE. 
 As each intermediate point of the object between D and E, 
 sends out a pencil of rays in like manner to every part of the 
 mirror, the rays of each pencil will be reflected back from it, 
 and meet in all the intermediate points between the extre- 
 mities, e and d, of the image; and hence the whole image will 
 not be at /, that i?, at half the distance of the mirror from its 
 centre of concavity C, but at a greater distance between i and 
 the object DE; and the image will be inverted with respect 
 to the object. Thus it is, that when the object is more re- 
 mote from the mirror than its centre of concavity C, the 
 image appears less than the object, and between the object 
 and the mirror; contrary to wliat is observed when the object 
 is nearer than the centre of concavity, as in tlie example of 
 the face looking at itself, fig. 9, pi. III. If DE, fig. 10, be 
 the object, e r/ will be its image; for as the object recedes 
 from tiie mirror, the image approaches nearer to it, and as 
 the object approaches nearer to the mirror, the image re- 
 cedes further from it, on account of the less or greater diver- 
 gency of the pencils of the rays which proceed from the ob- 
 ject ; for the less they diverge, the sooner they are converged 
 to points by reflection ; and the reverse necessarily follows, 
 that the more they diverge, the further they must be re- 
 flected before they meet. If the radius of the mirror's con- 
 cavity and the distance of the object be known, the dis- 
 tance of the image from the mirror is found by this rule : 
 Divide the product of the distance and radius by double the 
 distance made less by the radius, and the quotient is the dis- 
 tance required. If the object be in the centre of the mirror's 
 concavity, the image and object will be coincident and equal 
 in bulk. 
 
 If a man place himself directly before a large concave mir- 
 ror, but further from it than its centre of concavity, he will 
 see an inverted image of himself in the air, between him and 
 the mirror, of a less size than his own person. If he hold out 
 
OPTICS. 437 
 
 Experiments with concave mirrors. 
 
 his hand towards the mirror, the hand of the image will come 
 out towards his hand, and coincide with it, of an equal bulk 
 when his hand is in the centre of concavity; and he will 
 imagine he may shake hands with his image. If he reach his 
 hand further, the hand of the image will pass by his hand, and 
 come between it and his body ; and if he move his hand to- 
 wards either side, the hand of the image will move towards 
 the other; so that whatever way the object moves, the image 
 will move the contrary way. A by-stander will see nothing 
 of the image, because none of the reflected rays that form it 
 enter his eyes. 
 
 From this remarkable property of a concave mirror to form 
 an image in the air, mirrors of this sort are used to produce a 
 variety of singular appearances, to amuse the curious, or to 
 impose upon the ignorant and superstitious. To a few we 
 shall give a place. If a fire be made in a large room, and a 
 smooth mahogany table be placed at a considerable distance 
 near the wall, before a large concave mirror, so situated that 
 the light of the fire may be reflected from the mirror to its 
 focus upon the table ; if a person stand by the table, he will 
 see nothing upon it but a longish beam of light ; but if he 
 stand at a distance towards the fire, not directly between the 
 fire and the mirror, he will see an image of the fire upon the 
 table, large and erect. If another person, who knows notliing 
 of the experiment beforehand, should chance to come into 
 the room, and should look from the fire towards the table, he 
 would be startled at the appearance ; for the table would seem 
 to be on fire. In this experiment, there should be no light in 
 the room, but what proceeds from the fire ; and the mirror 
 ought to be at least fifteen inches in diameter. 
 
 If the fire used in the last experiment be extinguished or 
 covered by a screen, and a large candle be placed in a similar 
 position, a person standing by the candle will see the appear- 
 ance of a star, or rather planet, upon the table, as brilliant as 
 Venus or Jupiter in a cloudless sky. If a slender wax taper 
 be placed near the caudle, a satellite to the planet will appear 
 on the table ; and if the taper be moved round the candle, the 
 mimic satellite will go round the planet. 
 
 Another experiment is the following : Take a glass bottle, 
 partly fill it with water, and cork it in the common manner; 
 place this bottle opposite a concave mirror, and beyond its 
 cer^);e of concavity, that it may appear reversed ; let the spec- 
 tator retire to a still greater distance than the bottle, which 
 he will then see in the air inverted, and the water which is 
 actually in the lower part of the bottle, will in the image 
 appear uppermost. Invert the bottle whilst before the mirror. 
 
438 OPTICS. 
 
 Exj)eriracnL» willi concave nurrors. — Foci of spLcrical mirrors. 
 
 and in the image the water will appear in the lower part of 
 the bottle : when it is in this inverted state, uncork the 
 bottle, and whilst the water is running out, the image will 
 appear to be filling; but as soon as the bottle is empty, the 
 illusion ceases. 
 
 As the image formed by a concave mirror may be thrown 
 through a hole into an adjoining room, where a spectator will 
 in certain situations perceive the image of anv object the con- 
 cealed manager may chuse, it is evident how much the credu- 
 lous may be in posed upon. Birds, anjxels. Sec. may be repre- 
 sented flying, and n,ay be instantly changed for other objects. 
 A nosegay may be shewn, and when the beholder attempts to 
 grasp it, a dagger, with all the appearance of reality, may be 
 presented to his breast, or a death's head snap at his hand. 
 
 To find the focal distance of a convex mirror or speculum, 
 cover it with paper containing two pin-holes, one near each 
 edge of the mirror; expose it to the sun, holding another paper 
 before it that has a hole large enough to let the solar rays pass 
 throuf^h to the two pin-holes. Two white spots of reflected 
 light will be observed on each side of the hole; move the paper 
 backwards and forwards, till the distance of the spots be twice 
 the distance of the holes in the cover; and that distance of the 
 paper from the cover is its focus. 
 
 To find the focal distance of a concave speculum, place it 
 so that its axis may be directed to the centre of the sun. 
 Find the burning point, or receive the image upon a white 
 piece of paper, and the distance thus found between the paper 
 and the centre of the speculum, is the focal length. Or, cover 
 the mirror with paper, in which make two or more holes, and 
 observe where the beams of light reflected from these holes 
 unite, and this will be the focal distance. Or, lastly, place 
 the speculum at the end of a long table, in a vertical position ; 
 place a candle at the opposite end of the table, so that its 
 flame may be opposite to the axis of the speculum; then take 
 a piece of white paper, and having fixed it to a stick, place 
 the stick in the socket of a candlestick, so that the paper may 
 be supported at about the same height with the candle ; then 
 move the paper or the candle forwards or backwards, till the 
 image of the candle on the paper is exactly over the candle 
 itself, and the point of coincidence is the centre of the spe- 
 culum. 
 
 For purposes of amusement, mirrors are frequently made in 
 the form of entire or half cylinders or cones. These are called 
 mixed mirrors, as they produce, at the same instant, the effects 
 of plain and spherical mirrors. The properties of cylindrical 
 mirrors are, l.The dimensions of objects corresponding length- 
 
OPTICS. 
 
 FL.m 
 
 ^.y/ 
 
 Ftff^ 
 
 P,p 10 
 
 
 H Cooper dtl 
 
 INiblished by Henrj^ Hsher, Cascton, London.. \Z22 . 
 
OPTICS. 439 
 
 Cvlindrical and conical mirrors. — Prismatic dis-section of light. 
 
 wise to the mirror, are not much changed ; but those corre- 
 sponding breadtliwise have their figure altered, and their di- 
 mensions lessened, the further they are from the mirror, whence 
 arises a very great distortion. 2. If the plane of the reflection 
 cut the cylindrical mirror through the axis, the reflection is 
 performed in the same manner as in a plane mirror; and if pa- 
 rallel to the base, the reflection is the same as in a spherical 
 niirror; if it cut it obliquely, the reflection ts the same as in 
 an elliptic mirror. Hence, as the plane of reflection never 
 passes through the axis of the mirror, except when the eye 
 and the object are in the same plane ; nor parallel to the base, 
 except when the radiant point and the eye are at the same 
 height ; the reflection is therefore usually the same as in an 
 elliptic one. 3. If a hollow cylindric mirror be directly opposed 
 to the sun, instead of a focus of a point, the rays will be re- 
 flected into a lucid line parallel to its axis, at the distance of 
 about one-fourth of its diameter. 
 
 Conical mirrors produce a still more extraordinary distortion 
 of the figures seen in them than cylindrical ones, from the 
 breadth of the image being gradually reduced as it approaches 
 the apex, where it becomes a mere point. The eff'ect of a cy- 
 lindrical mirror in diminishing the breadth of objects facing 
 it, being the same as that of a convex mirror of the same radius, 
 and the etfect of the conical mirror, at any given height, being 
 regulated by the same principle, it becomes easy to draw ana- 
 morphoses for a cylindrical or conical mirror of any size. The 
 pictures called by this name, and which are sold with mixed 
 mirrors by the opticians, appear excessively wild and deformed 
 to look at, but when placed before the mixed mirror for which 
 they are intended, their images are well proportioned and 
 beautiful. 
 
 Of the different Refrangibili:y of the Rays of Light. 
 
 The difl'erent refrangibility of the rays of lights, which we 
 have now to consider, is demonstrated by means of the prism, 
 a name which opticians give to a solid piece of glass, of any 
 length at pleasure, but of a wedge-like or triangular form. 
 As any section of a prism, passing through its axis, is a tri- 
 angle, it is always represented by a triangle, which may be 
 considered as shewing correctly one of its ends, see fig. 12, 
 ISO. 8, pi. II. 
 
 If a beam of light from the sun be let into a darkened room, 
 and be received upon a white screen or opposite wall, it will 
 form a circular image, and will be of one uniform whiteness. 
 If a prism be interposed, so that the light must pass through it 
 before it reaches the wall, the image is no longer circular and 
 
440 OPTICS. 
 
 Pi isniiitic <IL>»t:ction of ii^bu 
 
 no longer wliite. It assumes an oblong shape, terminated by 
 semicircular arches, and exhibits seven different colours. This 
 oblong image is called the spectrum, and from its being pro- 
 duced by the prism, the prismatic spectrum. In the compass 
 of philosophical experiment, a more beautiful appearance can- 
 not be presented to the eye ; and its instructive nature will ap- 
 pear not less extraordinary than its beautv, when it is consi- 
 dered, that the investigation of the cause of it, led Sir Isaac 
 Xewton to form the first rational theory of the cause of colours. 
 The manner in which this illustrious philosopher tried the ex- 
 periment of the spectrum, is described by himself in the fol- 
 lowing words, which remain to this day, perhaps, the most 
 suitable introduction to this subject : 
 
 " In a very dark chamber, at a round hole, F, fig. 1, pi. IV, 
 about one-third of an inch broad, made in the shutter of a 
 window, I placed a glass prism, ABC, whereby the beam of the 
 sun's light, SF, which came in at that hole, might be refracted 
 upwards, toward the opposite wall of the chamber, and there 
 form a coloured image of the sun, represented at PT. The 
 axis of the prism (that is, the line passing through the middle 
 of the prism, from one end of it to the other end, parallel to 
 the edge of the refracting angle) was in this and the follow- 
 ing experiments, perpendicular tj the incident ravs. About 
 this axis I turned the prism slowly, and saw the refracted 
 light on the wall, or coloured image of the sun, first to de- 
 scend, and then to ascend. Between the descent and ascent, 
 when thfe image seemed stationary, I stopped the prism, and 
 fixed it in that posture. Then I let the refracted light fall 
 perpendicularly upon a sheet of white paper, MX, placed 
 at the opposite wall of the chamber, and observed the figure 
 and dimensions of the solar image, PT, formed on the 
 paper by that light. This image was oblong, and not oval, 
 but terminated by two rectilinear and parallel sides and 
 two semi-circular ends. On its sides it was bounded pretty 
 distinctly, but on its ends very confusedly and indistinctly, 
 the light there decaying and vanishing by degrees. At the 
 distance of 18i feet from the prism, the breadth of the image 
 was about 2^ inches, but its lencrth was about 10^ inches, 
 and the length of its rectilinear sides about eight inches; and 
 ACB, the refracting angle of the prism, whereby so great a 
 length was made, was 64 degrees. With a less angle, the 
 length of the image was less, the breadth remaining the same. 
 It is farther to be observed, that the ravs went on in straight 
 lines from the prism to the image, and therefore, at their going 
 out of the prism, had all that inclination to one another from 
 vhich the length of tJie image proceeded. This image PT wa» 
 
OPTICS. 441 
 
 Prismatic dissection of light. 
 
 coloured, and the more eminent colours lay in this order from 
 tlie bottom at T to the top at P ; red, orange, yellow, green, 
 blue, indigo, violet, together with all their intermediate degrees, 
 in a continual succession, perpetually varying." 
 
 If the spectrum be divided into 100 parts, the red part of it 
 is found to occupy 11 of these parts, the orange 8, the yellow 
 14, the green 17, the blue 17, the indigo 11, and the violet 22. 
 The red part of the spectrum, it will be observed, is nearest the 
 prism, and the violet at the greatest distance. It is clear, 
 therefore, that light is not homogeneous, because the attractive 
 power of the prism has been greater upon some parts of it than 
 upon other parts. Accordingly, it is universally concluded, 
 that the solar beam, or white light, is composed of particles 
 differing in size or density; that this difference of their size or 
 density, is the cause of their being differently refrangible, and 
 that the separation of the rays of one or more sizes from the 
 rest, by various means, produces all the diversity of colours 
 which affect our sight. 
 
 We find the red part of light capable of struggling through 
 thick and resisting mediums, when all the other colours are 
 stopped. Thus the sun appears red when seen through a 
 fog ; the light of distant lamps seen through the smoke of 
 a long street, appears red, while the light of those at hand 
 is white. Dr. Hallev's hand appeared red in the water, when 
 he was in a diving-bell at the bottom of ths sea, and red light 
 always makes the strongest impression on the eye. These are 
 all proofs, that red light consists of particles which are larger 
 tlian those of other colours, and which having therefore the 
 greatest momentum, are the least disturbed by the action 
 of a given force, so that they necessarily take a shorter 
 course than other rays in passing through the prism. The 
 particles which compose orange light, are next in size and 
 refrangibilitv to the red, and so on to the violet, which con- 
 sists of the smallest sized particles, and they are therefore the 
 most turned out of their course. 
 
 The seven colours of the spectrum, are called the original 
 or primary colours. White is composed of them all mixed 
 together in their due proportions ; for if a solar ray, separated 
 by the prism into its component parts, be collected and con- 
 verged into a focus by a lens or a concave mirror, a paper 
 placed perpendicularly in this point will exhibit a white spot. 
 The same conclusion may be drawn from the experiment of 
 mixing together paints of the same colours as the parts of the 
 spectrum, and in the same proportion ; the mixture will be 
 white, though not a resplendent whiteness, because the colours 
 mixed are less bright than the primary ones, and because they 
 19.— Vol. I. 3 L 
 
442 OPTICS. 
 
 Permanencjr al" the primary colour*. 
 
 cannot be intimately blended without exercising a chemical 
 action on each other, which in part changes their properties. 
 When this action is prevented, the result of the experiment is 
 much more satisfactory : the rim of a wheel may either be 
 painted or covered with pieces of cloth. Sec. of all the seven 
 colours in due proportion, and if it be then swiftly turned upon 
 its axis, it will appear to be white. Supposing the wheel to 
 be divided into 360 equal parts, the red should occupy 45 of 
 these parts, the orange 27, the yellow 48, the green 60, the 
 blue 60, the indigo 40, and the violet 80. 
 
 It may be supposed that the seven primary colours may 
 be reduced to three, viz. red, blue, and yellow, because of 
 these three all the rest might be made. It is necessary 
 therefore to explain the sense in which it is understood that 
 there are seven primary colours. None of the colours thus 
 named can be changed by any art, or whatever number of 
 refractions or reflections they undergo. Let a hole be made 
 in the paper or screen receiving the prismatic spectrum, at 
 the place covered by the green rays ; if these rays be refracted 
 through fifty prisms, and as often reflected from mirror to 
 mirror, they still retain their greenness : hence we are com- 
 pelled to consider the green rays as composed of simple homo- 
 geneous particles, consequently one of the primary colours ; 
 and the same may be said of any other coloui', which might be 
 supposed to be compound. 
 
 When any coloured body is placed in homogeneous light, 
 it always appears of the colour of the light in which it is 
 placed, whatever may be its natural hue; thus, if prussian blue 
 and vermilion he placed in a red light, both will appear 
 red ; in a green light, green ; in a blue light, blue ; Sec. It is, 
 however, to be allowed, that a body appears brighter when in 
 a light of its own colour than in another; and from this we 
 may see that the colours of natural bodies arise from an aptitude 
 in them to reflect some rays more copiously and strongly than 
 others. But lest this phenomenon should produce a doubt of 
 the constancy of the primary colours, it is proper to assign the 
 reason of it, which is this : that when placed in its own 
 coloured light, the body reflects the rays of the predominant 
 colour more strongly than any of those intermixed with it ; 
 therefore the proportion of the rays of the predominant colour 
 to those of the others, in the reflected light, will be greater 
 than in the incident light ; but when the body is placed in a 
 light of a different colour from its own, for a similar reason, the 
 contrary eftect will follow, that is, the proportion of the predo- 
 minant colour to the others will be less in the reflected than in 
 the iDcident light ; and therefore as its splendour would be 
 
OPTICS. 443 
 
 Rays of light not actually coloured. — Cause of the rainbow. 
 
 greater in the former case, and would be less in the latter, than 
 if all the rays were equally reflected, the splendour of the pre- 
 dominant colour will be much greater in the former case than 
 in the latter. 
 
 When bodies either reflect all the rays of light which they 
 receive, or reflect them in the proportion in which they exist 
 in the solar beam, they appear white ; when they reflect none 
 of the rays, they appear black. Between perfect white and 
 perfect black, innumerable species of colours may be formed 
 by different combinations of the rays, under different influences 
 of reflection or transmission. 
 
 When the expressions, red rays, yellow rays, &c. are used, 
 it is not to be understood that the rays themselves possess 
 any property analogous to what we call colour, but merely 
 that they have the power of exciting in us the sensations 
 which we call redness, yellowness, &c. They would, there- 
 fore, with more correctness, be called red-mahiug or red-causing 
 rays, yeUow-making or yeUoiv-causing rays. Sec. but the fre- 
 quent recurrence of this precision would be as awkward as 
 it is unnecessary. 
 
 Of the Rainhoio and other natural Phenomena dependent upon 
 the Refrangihility of Light. 
 
 Of the natural phenomena produced occasionally by the 
 separation of the primary colours, the rainbow is one of the 
 most beautiful. This meteor, which in poetical language is 
 called the iris, never makes its appearance, except when the 
 spectator is situated between the sun and a shower of rain ; 
 hence we conclude that it is occasioned by the sun and the 
 drops of rain ; and that this conclusion is just, any one may 
 satisfy himself by the following experiment : Fill a hollow 
 glass globe with water, and suspend it in the sun, in such a 
 manner that it maybe raised or lowered at pleasure ; at a cer- 
 tain height above the eye of the spectator, who looks at it with 
 his back to the sun, the globe will appear to be red ; let it then 
 be slowly lowered, and it will appear to be orange, and after- 
 wards in succession, as it descends, it will appear yellow, green, 
 blue, indigo, and violet. Hence the same drop of rain, which 
 must be considered as a little globe, supplies all the seven co- 
 lours to the eye, just as they would be supplied if, in trying the 
 experiment of the spectrum, the operator were to remove the 
 paper MN, fig. 1, pi. IV, and to place his eye so that it would 
 receive the red rays, when, if he gradually raise himself, or 
 lower the aperture for the light and the prism together, he 
 would perceive all the other colours in succession. 
 
 There are sometimes two rainbows seen at the same time. 
 
444 OPTICS. 
 
 DilTcrence between the exterior ami interior rainbow. 
 
 one within the other, and, what may seem remarkable, the 
 order of the colours of the exterior bow is the reverse of that 
 of the interior one. When two bows are seen, the exterior 
 one is comparatively faint, and is therefore sometimes called 
 the false or secondary bow, while the greater distinctness of 
 the interior one has obtained for it the appellation of the 
 primary bow. To trace the progress of a ray of light through 
 a drop of rain in each of these bows, will explain the cause of 
 their diftering in brightness. 
 
 In the true or primary bow, the rays of light arrive at the 
 spectator's eye after two refractions and one reflection. Thus, 
 let A, fig. 2, ])1. IV, be a drop of rain, and Sd a ray from 
 the sun falling on the upper part of the drop at d. It will 
 suiier a refracl;ion, and instead of going forward to c, it will 
 be bent to «; at n, part of it will emerge, but the remainder 
 will be reflected tog ; at g it will be again refracted on passing 
 into the air towards the eye at// ; by being thus twice retracted 
 and once reflected, the ray is separated into its primitive 
 colours ; the red part, which is least thrown out of its course, 
 makes an ancrle, at its emero-ence, with the incident solar ray 
 of forty degrees two minutes, as S/'g; and the violet, being 
 the most easily thrown out of its course, makes with the solar 
 light an anole of 40° 17', as S c ff. The different colours, there- 
 fore, at the distance of the spectator, are considerably separated, 
 and affect the eye in succession with the seven colours ; but the 
 succession is so quick, and so many drops fall through the 
 same circuit in the same time, that the mind loses the idea of 
 succession, and the bow seems permanent as long as the 
 shower continues in a proper direction from the eye. 
 
 The exterior or secondary bow is formed by two reflections 
 and two refractions. Let B represent one of the drops of rain 
 forming this bow ; a ray, T, from the sun, falling upon it at r, 
 is refracted, and falls upon tlie back of the drop at s ; ats, from 
 the transparency of the drop, a portion of it passes through 
 towards w, but the remainder of it is reflected towards t ; here 
 again, for the same reason as before, part of it emerges from 
 the drop, in the direction .r, but the portion still left is reflected 
 to u, where it is refracted towards the spectator, with the red 
 rays uppermost. The great quantity of light lost at each reflec- 
 tion, is tlie cause of the indistinctness of this bow, and there- 
 fore we cannot be surprised that we rarely, if ever, see bows 
 formed by a still greater number of reflections within the drops; 
 for though they may exist, they are too faint to be seen. Ihe 
 secondary bow cannot be seen when the elevation of the sun is 
 above 54° 7'; it is broader than the interior bow, because the 
 rays are more dispersed before they reach the eye. 
 
OPTICS. 445 
 
 Rainbow. — Colours of the sky. 
 
 When the spectator is upon a plain, and the sun is close to 
 the horizon, the rainbow is a semi-circle ; but according as the 
 sun is higher above the horizon, so the rainbow is a smaller part 
 of a circle. When the spectator is upon an eminence, and the 
 sun is near the horizon, then the rainbow may exceed a semi- 
 circle ; and from a great elevation, a complete circle may be per- 
 ceived. In all cases, the centre of the bow, the spectator, and 
 the sun, must be in the same straight line, which is called the 
 line of aspect. The sun may therefore be considered as the ver- 
 tex of a cone, whose base is the rainbow, and whose axis con- 
 stitutes the line of aspect. If several people are standing toge- 
 ther, they each see a different rainbow, because they are each 
 in the axis of a different cone. Hence the bow moves as the 
 spectator moves, but other drops in another circle producing 
 the same eliect, the change is not perceived. The bow assumes 
 a circular form, because it can only be seen by rays which form 
 the same angle with the line of aspect. The primary bow can 
 never be seen, if the sun be elevated more than 42° 2' above 
 the horizon. 
 
 Lunar I'ainbows are sometimes observed, but the faintness 
 of the moon's light, compared with that of the sun, is the 
 reason that their colours can very rarely be distinguished. 
 
 When the sun and the eye of the spectator are properly situ- 
 ated, drops of water produce the phenomenon under a variety 
 of other circumstances. A bow with its convex side to- 
 wards the spectator, is sometimes seen on the ground, when 
 the sun shines on a very thick dew ; also at waterfalls, and at 
 sea, when part of the agitated water is dispersed in drops, the 
 bow is seen in its usual form. Water blown violently from the 
 mouth of an observer, will also produce a miniature iris, and 
 by means of a small fountain, an artificial one may be shewn 
 by candle-light. 
 
 The diversified colours of the sky vmder different circum- 
 stances, are also accounted for on the principle of the different 
 refrangibility of the rays of light. When the sun is near the 
 horizon, as in the morning and evening, the vast tract of dense 
 and vaporous air in a direct line from him to the earth, pro- 
 duces the absorption of the weaker rays, and therefore the 
 clouds reflect only the vivid yellow, the orange, and the red. 
 As the quantity of the vapours, and even their nature, are not 
 similar for two days together, this irregularity produces cor- 
 responding differences in their effects. Terrestrial objects, and 
 even the sun himself, are necessarily tinged like the light by 
 which they are seen. When the light at these times is received 
 by a prism, it is found that the rays actually absorbed are such 
 as we should expect to be so from the coloration of the 
 
44G OPTICS. 
 
 Halu. — Parhelia. — Anecdote. 
 
 moment. In consequence of the successive increase and den- 
 sity of the vapours traversed by the light, clouds differently 
 placed, are at the same instant tinged of different colours. 
 The highest may even be ^vhite, while the rest, at a less ele- 
 vation, will be yellow, and others still lower will be red, or 
 approach nearly to red. At an equal elevation, the most 
 distant from the point where the sun sets, will incline to red, 
 and the nearest to yellow. 
 
 A luminous circle, called a halo or corona, is frequently ob- 
 served to surround the sun, moon, and even the planets and 
 stars. It is sometimes coloured, but generally white. It is 
 attributed to the effects produced on the light of these 
 bodies, in passing throuc;:h dense clouds, or a frozen medium 
 of hail or snow, and it is white or coloured, according as the 
 rays have undergone a prismatic separation or not. The par- 
 helia, or mock suns, which are sometimes observed, are re- 
 ferable to a similar origin. 
 
 To the thinkingmind, it cannot but be interesting to observe, 
 in how many instances the causes of phenomena which were 
 formerly con.sidered as altogether inexplicable, and regarded 
 with superstitious dread, are now familiar to children, and allu- 
 sions to them assimilated with the language of poetry. They 
 are no longer considered as the ominous harbingers of evil; they 
 are produced for amusement by e.xperiment, and when they 
 occur under natural circumstances, are contemplated w ith de- 
 light, because they are known to be the result of those immu- 
 table laws which the Author of nature has assigned for the 
 government of his works. Optical science furnishes several 
 illustrations of this subject, and we shall mention one which is 
 not the least remarkable, from its connection with historical 
 events. A day or two before the massacre of St. Bartholomew, 
 Henry IV. of France (then a prmce at the court of Charles IX, 
 and not implicated in the guilt of that massacre,) sat down to 
 dice with the Duke of Guise, and just as thev were s^oing to 
 play, he perceived what he supposed to be drops of blood. 
 Attempts were made to wipe these drops away, but they still 
 returned, or new ones were seen in their places. The strictest 
 examination was made among the spectators, yet it could not 
 be discovered whence the drops proceeded. This event made 
 so strong an impression upon the mind of Henry, that he 
 alluded to it in tlie memorable speech he delivered to his par- 
 liament, not less than 27 years afterwards: "I," sayshe, "look- 
 ed upon it as a dismal omen, and rising up from play. I turned 
 aside, and said to some of mv particular friends, tliat I fore- 
 saw much blood would be shed." From the best evidence that 
 history affords us of this event, we can collect but little iu- 
 
OPTICS. 447 
 
 Black changed into red. — Inflection. — Deflection. 
 
 formation of the circumstances of the moment; but black, as the 
 black spots of the dice, may be so oompletely changed into scar- 
 let, that we cannot at this day consider the appearance of the 
 blood-spots as any thing more than the accidental occurrence 
 of a natural phenomenon. Let any person take a book, and 
 standing in the strong light of the sun, which must fall upon 
 his eyelids, but not upon the book, let him look steadily at 
 the printing, and he will soon see the blackness of the letters 
 changed to the hue of vermillion. 
 
 This appearance is of easy explanation ; it is well known, that 
 when we turn our eyes to the sun, with our eyelids closed, we 
 have the sensation of a lively red. Now the eyes, which are 
 covered with respect to the sun by the eyelids, are open to the 
 book, and consequently the white of the paper, which reflects 
 so much light, makes upon the retiiia an impression sufficient to 
 erase the impression of the red ravs in all the points of the 
 retina upon which the white rays fall. The black letters, on the 
 contrary, which are printed on the white paper, send back few 
 or no rays to the bottom of the eye, and consequently all the 
 points of the retina, on which, in other cases, they are depicted 
 black, preserve in all their vigour, the first impression of red 
 which had been made by the sun upon the whole retina. The 
 circumstances, essential to the production of this illusion, are, 
 1. the sun's shining upon the eyelids; 2. the rays of the sun 
 not falling upon any of the black spots ; 3. the sun's having 
 shone for at least two minutes upon the eyelids. 
 
 Of the hijiection and Deflection of Light. 
 
 The refraction of light, we have seen, is attributed to a power 
 of attraction appertaining to all bodies, and exerted at a little 
 distance from their surfaces ; reflection, on the contrary, is 
 produced by a power of repulsion, and also takes eflect before 
 the light actually strikes the reflecting surface. If these attrac- 
 tive and repulsive energies have any real existence, the rays of 
 light, under certain circumstances, will be bent, in a manner 
 that cannot be classed under either I'efraction or reflection. 
 Accordingly, we find, that if a ray of light pass very near a 
 body, without impinging upon it, it is bent inwards, or towards 
 the body; this chanoe in the direction of the ray, is called 
 inflection. If the ray pass at a greater distance, it is bent out 
 wards, or from the body; this change in the direction of the 
 ray, is called defection. 
 
 W hen a beam of the sun's light passes through a hole in a 
 window shutter, the image thrown upon a screen or an opposite 
 wall, is larger than it would be if the rays, crossing at the aper- 
 
44B OPTICS. 
 
 Ell'ects of inflrction. 
 
 ture, proceeded in straight lines from the circumference of the 
 sun's disk to the wall. It becomes therefore an object of 
 inquiry, to determine the cause by which it has been expand- 
 ed; and we tind, on close examination, that the side of the 
 aperture has inflected or caused to diverge from the axis of the 
 beam, the rays which have passed near it all around. 
 
 Inflection was first discovered by Grimaldi, who made many 
 curious experiments and observations relative to it ; but the fol- 
 lowino- experiments of Sir Isaac Newton, will be better adapted 
 than Grimaldi's to explain the nature of this property of light. 
 At the distance of two or three feet from the window of a 
 darkened room, in which there was a hole three-fourths of an 
 inch broad, to admit the light, he placed a black sheet of paste- 
 board, having; in the middle a hole about a quarter of an inch 
 square, and behind the hole the blade of a sharp knife, to inter- 
 cept a small part of the light which would otherwise have passed 
 through the hole. The planes of the pasteboard and blade were 
 parallel to each other, and when the pasteboard was removed 
 to such a distance from the wiiiduw, that all the light coming 
 into the room must pass through this hole in the pasteboard, 
 he received what came through the hole on a piece of paper 
 two or three feet beyond the knife, and perceived two streams 
 of faint lioht shooting out both ways from the beam of light 
 into the shadow. As the brightness of the direct rays obscured 
 the fainter li2."ht, bv making a hole in his paper, he let them 
 pass through, and had thus an opportunity of attending closely 
 to the two streams, which were nearly equal in length, breadth, 
 and quantity of light. That part which was nearest to the sun's 
 direct light, was pretty strong for the space of about a quarter 
 of an inch, decreasing gradually till it became imperceptible, 
 and at the edge of the knife it subtended an angle of about 
 twelve or at most of fourteen degrees. 
 
 Another knife was then placed opposite to the former, and 
 he observed, that when the distance of their edges was about 
 the four-hundredth part of an inch, the stream divided in the 
 middle, and left a shadow between the two parts which was so 
 dark, that all the light passing between the knives seemed to 
 be bent aside to one knife or the other ; as the knives were 
 brought nearer to each other, this shadow grew broader, till 
 upon the contact of the knives the whole light disappeared. 
 He observed also, fringes of ditlerent coloured light, three 
 made on one side by the edge of one knife, and three on the 
 other side by the edge of the other. 
 
 Grimaldi, Dr. Hooke, and all the philosophers who made expe- 
 riments on inflection before the time of Newton, ascribed the 
 broad shadows, and even the fringes which he mentions, to the 
 
OPTICS. 449 
 
 Brougham's experiments on inflection. 
 
 ordinary refraction of the air; but the investigation upon 
 ^vhich he entered to discover their cause, afforded him satis- 
 factory evidence to conclude that bodies have the power of 
 acting upon lioht at a distance. 
 
 The Philosophical Transactions for 1796, contain a paper by 
 Henry Brougham, F. R. S., detailing one of the most valuable 
 series of experiments on the mechanical properties of light, 
 %vhich has appeared since the time of Newton. It had been 
 generally supposed that the parts of which li£:ht consists, have 
 till the same disposition to be acted upon by bodies which iu- 
 ilect, deflect, and reflect them ; but he soon proved that this 
 opinion was erroneous. Having admitted into a darkened 
 chamber, a beam of the sun's light through a hole in a metal 
 plate (fixed in the window shutter) of ^Vfth of an inch in diame- 
 ter; and all other light being absorbed by black cloth hung 
 before the window and in the room, at the hole he placed a 
 prism of glass, whose refracting angle was 45 degrees, and 
 which was covered all over with black paper, except a small 
 part on each side, which was free from impurities, and through 
 which the Hght was refracted, so as to form a distinct and 
 tolerably homogeneous spectrum on a chart at six feet from 
 the window. In the rays, at two feet from the prism, he placed 
 a black unpolished pin, one-tenth of an inch in diameter, pa- 
 rallel to the chart, and in a vertical position. Its shadow was 
 formed in the spectrum on the chart, and had a considerable 
 penumbra, especially in the brightest red. It was by no means 
 of the same thickness in all its parts ; in the violet it was 
 broadest and most distinct; in the red it was narrowest and 
 most confused, and in the intermediate colours it was of an 
 intermediate thickness and degree of distinctness. It was not 
 bounded by straight, but by curvilinear sides, which were con- 
 cave outwardly. This figure of the shadow was not owing to 
 any irregularity in the pin, for the same thing happened with 
 all sorts of bodies that were used ; and also if the prism was 
 moved on its axis, so that the colours might ascend and de- 
 scend on these bodies, still, wherever the red fell it made the 
 least, and the violet the greatest shadow. In the next place, 
 he fixed a screen, having in it a large hole on which was a 
 brass plate, pierced with a small hole of ^V of an inch in dia- 
 meter ; then causing an assistant to move the prism slowly on 
 its axis, he observed the round image made by the different 
 rays passing through the hole to the chart ; that made by the 
 red was greatest, by the violet least, and bv the intermediate 
 rays of an intermediate size. Also, w^hen at the back of the 
 hole he held a sharp blade of a knife, so as to produce the 
 iringes mentioned by Grimaldi and Newton, those fringes in 
 19.— Vol. I. 3 M 
 
450 OPTICS. 
 
 Brougham's experiments on inflecUoD. 
 
 the red were broadest, and most moved inwards to the shadow, 
 and most dihited when the knife was moved over the hole; 
 and the hole itself on the chart was more dilated during the 
 motion when illuminated by the red, than when illuminated by 
 any other of the rays, and least of all when illuminated by the 
 violet. From these, and a great variety of other experiments, 
 well devised and often repeated with rigorous care, he infers, 
 that the rays of light are separable into their primitive colours, 
 by inflection, deflection, and reflection, as well as by refrac- 
 tion ; that the flexibilities and reflexibilities of the rays are in- 
 versely as their refrangibilities ; that is, those which deviate 
 the least by refraction, deviate the most by flection, and are 
 reflected the furthest from the perpendicular; and that the dif- 
 ferent flexibilities, reflexibilities, and refrangibilities of the 
 rays, are all produced by the differences in the magnitude of 
 the particles. He calculates that the size of the red particles 
 are to those of the violet as 1275 to 12.33. This he extends to 
 all the other colours by similar calculations, their sizes lying 
 between 1275 and 1253; which are the extreme red and ex- 
 treme violet; the red therefore is from 1275 to 1272.2 ; the orange 
 from 1272^ to 1270; the yellow from 1270 to 1267; the green 
 from 1267 to 1264; the blue from 1264 to 1260; the indi<jo from 
 1260 to 1258, and the violet from 1258 to 1253. The whole pa- 
 per is highly curious and deserving of attention, but it would 
 be impossible to do it justice by analysis in our limits. 
 
 From the separation of light into its component parts, by 
 inflection, we readily perceive how the coloured fringes of 
 light thus influenced are produced ; but other appearances de- 
 rive their origin from inflection, where colour is not always 
 observable. On looking at the printing of a book through a 
 very small hole, such as the hole made by a needle in a piece 
 of paper, the letters appear considerably magnified. In this 
 case, the rays which form the image pass so near the circum- 
 ference of the hole, as to be inflected by it, and they therefore 
 form a larger picture on the retina than the direct rays would 
 have done. \Vheu we look at a distant steeple, or any similar 
 object, and intercept the direct rays from it by crossing the 
 line of sight with a wire of rather less diameter than the pupil, 
 held very near the eye, the object will be seen by rays bent 
 inwards or inflected, which would otherwise not have entered 
 the eye, and as they make a much larger angle, the object 
 appears magnified. 
 
 When we look at a candle, or any other luminous body, with 
 our eyes alfiiost closed, streaks of light appear to dart upwards 
 and downwards; if the head be reclined, the chuncje in the 
 direction of the streaks is correspondent. This appearance has 
 
OPTICS. 451 
 
 Description of the eve. 
 
 been most commonly attributed to the inflection of light pass- 
 ing between the eye-lashes ; but it is most probable that inflec- 
 tion only occasions a very small part of it; and that the rest is 
 produced by the refraction of light through the humours ad- 
 hering to the eye-lids, because, as Brougham observes, the 
 streaks which dart from the top of the luminous body are 
 formed by the under eye-lid ; or at least by the moisture ad- 
 hering to the under ciliary process, and those which appear 
 from the bottom of the body, by the upper eye-lid ; which 
 could not be, either if they were formed, as some have sup- 
 posed, by reflection from the processes, or by inflection 
 through the lashes. 
 
 Of the Eye and the general Phenomena of Vision. 
 
 The eye is an organ, not less admirable in its mechanical 
 properties and structure, than invaluable for its use. To ex- 
 patiate on its utility would be idle, to treat of its expression 
 belongs to the physiognomist; it is our duty to explain merely 
 the manner in which it performs its office. To this end we 
 must first describe its several parts : 
 
 The eye is of a globular figure, rather protuberant in front, 
 and is composed of three membranes called coats, and three 
 pellucid substances called humours. Each coat and each 
 humour has a different name. By fig. 3, pi. IV, is represented 
 a section of the human eye. The part DHHG, of the outer 
 coat, is called the sclerotica, the exterior part or continuation 
 of it, DEFG, is called the cornea, from its resembling" horn. 
 The sclerotica is strong and unelastic, and the muscles which 
 move the eye are attached to it. What is called the white of 
 the eye, is a part of this coat. The cornea bulges out a little 
 from the eye-ball ; it is circular, and exceedingly transparent. 
 
 The next coat to the sclerotica is called the choroides, which 
 serves as a lining to it. It is of a dark colour in the human 
 eye, but white in cats and owls, and green in animals that live 
 on grass and vegetables. Its texture is soft and pulpy, and 
 too weak to be susceptible of muscular motion, except at its 
 extremities towards the front of the eye. Like the sclerotica, 
 it is distinguished into two parts; the fore-part being called 
 the iris, while the hinder part retains the name of the choroides. 
 The iris commences immediately under the commencement 
 of the cornea. It here attaches itself more strongly to the 
 sclerotica by a cellular substance, forming a kind of white, 
 narrow, circular rim, called the ciliary circle. The iris is 
 that remarkable circle, which gives the eye its character as to 
 colour ; it is composed of two sets of muscular fibres ; the 
 
452 OPTICS. 
 
 Description n( the eve. 
 
 one tending, like radii, towards the centre of the circle, and 
 the other forming a number of concentric circles round the 
 same centre. The central part of the iris is perforated, and the 
 aperture, which is called the pupil, is always round, but varied 
 in diameter by the action of the two sets of muscular fibres 
 composing the iris. When a very luminous object is viewed, 
 the circular fibres contract, the radial are relaxed, and thus the 
 size of the pupil is diminished ; on the other hand, when the 
 objects are dark and obscure, the radial fibres of the iris con- 
 tract, the circular are relaxed, and the pupil is enlarged, so 
 that it admits a greater quantity of light. By candle-li^ht, the 
 contraction and dilation of the pupil may be very distinctly 
 observed, with the assistance of a looking-glass. If, with our 
 eyes directed to the mirror, we bring the candle close to our 
 face, we shall find the pupil become very small; if the candle 
 be removed and completely shaded for about a minute, and 
 then brought to its former place, it w ill be found that the pu- 
 pil has greatly dilated, and that it again contracts as the light 
 draws nearer; if the light shine much more strongly on one 
 eye than the other, the pupil of the shaded eye will not con- 
 tract so much as the other. 
 
 The whole of the choroide membrane is opaque, by which 
 means no light can enter the eye but what passes through the 
 pupil, but to render the chamber of the eye still darker, the 
 posterior surface of this membrane is covered with a black 
 mucus called the pigmentum nigrum. 
 
 Under the iris, there is a prolongation, ?i u, of the choroides, 
 whicii forms a circular band of radial fibres, turning inwards 
 towards the centre of the eye, and filled up between with a 
 black mucus, giving it the appearance of a membrane. This 
 circular band is called the tigameuium ciliare, or ciliorj/ iiga- 
 meiit. 
 
 The third and last coat of the eye is called the retina. This 
 is a fine and delicate membrane, being an expansion of the 
 optic nerve L, which proceeds from the brain. Its course is 
 represented by dots in the figure; it is spread like a net of ex- 
 quisite delicacy, all over the concave surface of the choroides, 
 and terminates at the ciliary ligament ii n. It receives the 
 images of objects, which are depicted upon it by the rays of 
 light that enter at the pupil. It is of itself transparent, and of 
 an ash-coloured white, but appears black on account of the 
 pigmentum nigrum behind it. The optic nerve, L, which 
 passes through a small hole in the bony cavity containing the 
 eye, and conveys to the sensorium the impressions made on 
 the retina, is not in the centre of the eye, but a little on one 
 side, inclining towards the uose. 
 
OPTICS. 453 
 
 Description of the eve. 
 
 The three transparent substances enclosed by the coats of 
 the eye are called the aqueous, crj/sialiue, and vitreous humours. 
 The first of these, or aqueous humour, resembles water, whence 
 its name. It gives a protuberant figure to the cornea, fills the 
 two cavities )n m, 71 n, between the cornea and ciliary ligament, 
 which cavities communicate by the pupil P. The part m m, 
 is called the anterior, and n n the posterior portion of the 
 aqueous humour, the iris, which swims in it, constituting the 
 division. The refractive power of the aqueous humour is like 
 that of water. 
 
 The second, or crystalline humour, R, is, like the former, 
 eminent for its transparency, which exceeds that of the purest 
 crystal; it has the consistence of a hard jelly, growing some- 
 what softer from the middle towards the edges. Its form is 
 that of a double convex lens, but more convex on the interior 
 than the exterior surface. Resembling a lens in its form, it 
 also resembles one in its use : it converges the rays which 
 pass through it from every visible object to its focus on the 
 retina. It is inclosed in a fine transparent cover, or mem- 
 brane, called the arachnoides, which is attached to the ligamen- 
 tum ciliare before mentioned ; and by that means it is sus- 
 pended. The radial fibres of the ligamentum ciliare, have the 
 power of contracting and dilating occasionally, bv which means 
 they alter the shape or convexity of this natural lens, and shift 
 it a little backwards and forwards in the pye, so as to adapt 
 its focal distance from the retina to the different distances of 
 objects. Without this or some equivalent arrangements, we 
 should only see those objects distinctly that were at one dis- 
 tance from the eye. 
 
 At the back of the crystalline, lies the third or vitreous hu- 
 mour KK. It receives its name from its supposed resemblance 
 to melted glass. It is not so hard as the crystalline, nor so 
 liquid as the aqueous humour. It is by far the largest of all 
 the humours in quantity, and gives the eye its globular shape. 
 In its refractive power, it very little exceeds water; in con- 
 sistence it is like the white of an egg. 
 
 As every point of an object, ZVX, sends out pencils of rays 
 in all directions, some rays from every point on the side next 
 the eye, will fall upon the cornea between EF, and by passing 
 on through the pupil and humours of the eye, they will be 
 converged to as many points on the retina, and will there form 
 a distinct inverted picture BYA, of the object : for the pencil 
 of rays flowing from the point Z of the object, will be con- 
 verged to the point A of the retina; those from the point V 
 will be converged to the point Y; those from the point X will 
 be converged to the point B, and rays from all the interme- 
 
454 OPTICS, 
 
 I'lienomeoa of vision. 
 
 diate points being converged in like manner, the whole image 
 BYA IS formed, and the object made visible. 
 
 With two eyes we not only see objects about one-thirteenth 
 part brighter than when we use only one, but we see them in 
 a different situation. Those who have lost the use of one eye 
 are apt to mistake the distances of objects even within arm's 
 length ; and in such actions as the threading of a needle, or 
 the snuffing of a candle, they do not succeed without consi- 
 derable experience. 
 
 It has often been a subject of inquiry, why we see objects 
 in their true position, though the image on the retina is in- 
 verted, but no satisfactory solution of the difficulty has ever 
 been given. And we should be as likely to receive an 
 answer, if we were to ask, why we do not perceive every 
 object bent, because the image of it is depicted upon a con- 
 cave surface. It is certain that unless distinct images are 
 painted on the retina, objects cannot be clearly perceived. 
 If from too little light, remoteness, or any other cause, a 
 picture is indistinctly painted on the retina, an obscure or 
 indistinct idea of the object is conveyed to the mind. The 
 picture on the retina is therefore so far the cause of vision, 
 that our ideas of visible objects vary as it varies, and when 
 it is not formed, nothing is seen. Yet we may fairly con- 
 clude, that the mind does not look upon the image on the 
 retina; for in cases of the gutta serena, a disorder which 
 affects only the optic ner\-e, the pictures on the retina are as 
 perfectly formed as in the best eyes, although the patient is 
 afflicted with incurable blindness. It is the optic nerve, 
 therefore, which conveys the impressions made on the retina 
 to the brain, but how they are there communicated to the 
 mind is screened from the view of man. It has been sup- 
 posed that we acquire by experience the habit of seeing 
 objects erect, but there are many striking facts to prove the 
 contrary; persons who have been blind from infancy, and who 
 have been suddenly restored to sight by a surgical operation, 
 have not been led into the smallest mistake. In fact, no rea- 
 son can be given why the mind should not perceive as accu- 
 rately the position of bodies, when the rays reflected from the 
 upper parts of those bodies fall upon the lower parts of the 
 eye, as if the contrary took place. 
 
 To see an object distinctly, besides a sufficient quantity of 
 light, it is necessary that the pencils of divergent rays which 
 enter the eye, should converge to points on the retina. If 
 the pencils fall in an uneonverged state on the retina, the 
 scattered state of the light causes it to make an indistinct 
 impression, which may arise from two causes: 1. The rays 
 
OPTICS. 455 
 
 PhenoraeDa of vision. 
 
 may require a greater distance than the size of the eye admits 
 of, before they can be converged ; and, 2. they may converge 
 before they reach the retina, in consequence of which they will 
 fall upon it in a dispersed state, because they have crossed and 
 begun to diverge. Persons having the former defect, are called 
 long-sighted, as they can frequently see objects at a great dis- 
 tance better than those near at hand. The reason is, that 
 from age and infirmity, the cornea and crystalline lens become 
 flatter, and consequently incapable of converging the rays so 
 soon to points ; and the defect is further increased, and may 
 sometimes perhaps be wholly occasioned, by the ciliary liga- 
 ment becoming either too flaccid or too rigid, to produce the 
 various adjustments necessary to view objects at diff'erent dis- 
 tances. When the pencils of rays are converged to points 
 before they reach the retina, the persons subject to the visual 
 defect thence arising are said to be short-sighted, because 
 they can only see those objects distinctly which are very near 
 their eyes. The subject of long and short sightedness will be 
 resumed, when we treat of spectacles; here we shall only fur- 
 ther observe, that the eye is as much under the dominion of 
 habit as any of our senses, and when long inured to one class 
 of objects, becomes less fit for all others. The engraver, who 
 is constantly employed in viewing objects near at hand with 
 great attention, loses much of the facility of discerning objects 
 at a distance ; but the sailor, who never, perhaps, strains his 
 eyes, except at distant objects, becomes less capable of seeing 
 distinctly those which are near at hand. 
 
 The advantages of having two eyes, even so far as we are 
 acquainted with them, are not confined merely to improving 
 the brightness of objects, and shewing them in their true 
 places. In each eye there is a spot where no vision takes 
 place, and this spot, which is about the fortieth of an inch in 
 diameter, lies exactly upon the insertion of the optic nerve, so 
 that we cannot perceive the image of any object that falls 
 upon it at the hinder part of the eye, provided the other eye 
 be shut ; but as the insensible parts of the two eyes are on 
 the sides next each other, that part which is invisible to one 
 eye, is visible to the other, and therefore the whole is seen. 
 To be satisfied of the existence of such a spot, the following 
 experiment may be resorted to. Let three pieces of paper be 
 fastened upon the side of a room, about two feet asunder; 
 and let the person place himself opposite to the middle paper, 
 and beginning near to it, retire gradually backwards, all the 
 while keeping one of his eyes shut, and the other turned 
 obliquely towards that outside paper which is towards the 
 covered eye, and he will find a situation, (which is generally 
 
450 OPTICS. 
 
 LL\pt.Ti;Ln .lis ii:i \1»J i;.. 
 
 about five limes the distance at wliicli tlit papers are placed 
 from one anollier.) where the middle paper will entirely dis- 
 appear, wliile the two outermost continue plainly visible ; 
 because the rays which come from the middle paper will fall 
 upon that part of the retina where the optic nerve enters. 
 Hence it is evident, that if the optic nerve had not been in- 
 serted on one side, the centre of our field of view would have 
 been invisible. 
 
 We have previously had occasion to mention the visual 
 angle, and to intimate that the larger it was, the greater the 
 apparent magnitude of any object beheld. The meaning of 
 this expression will now be apparent, from the description 
 we have given of the eye; but to render it still more evident, 
 we shall refer to a fiofure. Let AB, fig. 4, pi. IV, be an object 
 viewed by the eye QR. From each extremity draw the lines 
 AX and B^I, intersecting each other in the crystalline hu- 
 fuour at I. Then draw the line IK, in the direction in which 
 the eye is supposed to look at the object. The angle WB 
 is then the optical or visual angle, and the line IK is called 
 the optical axis, because it is the axis of the lens or crystalline 
 lumour continued to the object. The apparent magnitude 
 if objects, then, depending thus on the angle under which 
 Jiey are seen, must evidently vary according to their dis- 
 tances. Thus ditrerent objects, as AB, CD, EF, the real 
 magnitudes of which are very unequal, may be situated at 
 such distances from the eye as to have their apparent mag- 
 nitudes all equal ; for if they are situated at such distance?, 
 that the rays AX, BM, shall touch the extremities of each, 
 they will then all appear under the same optical angle, and 
 the diameter MN, of each image on the retina will conse- 
 quently be equal. In the same manner, objects of equal mag- 
 nitude, situated at unequal distances, will appear unequal. 
 For let AB and GH, be two objects of equal size, placed 
 before the eye at diflTerent distances, IK and IS ; draw the 
 lines GP and HO, crossing each other in I ; then OP, the 
 image formed by the object GH on the retina, is evidently of 
 a greater diameter than the ima^e MX, which represents the 
 object AB ; in other words, the object GH will appear as large 
 as an object of the diameter TV, situated at the same place as 
 the object AB. 
 
 When we look from one end towards the other of a long 
 and straight row of houses or trees, they appear gradually to 
 diminish as they are further removed from the eye, though 
 upon a nearer inspection they are all found to be of equal size. 
 It vill be evident, from tlie observations we have just made 
 respecting the visual angle, that tliis must be the case; for the 
 
OPTICS. 457 
 
 Phenomena of vision. 
 
 angle under which similar objects are seen, and consequently 
 the evidence which sight affords us of their magnitude, is in an 
 inverse proportion to the distance of those objects. The appa- 
 rent exceptions to this rule apply to objects where the evidence 
 of sight is corrected by the judgment. When objects are near, 
 we do not judge of their magnitude according to the visual 
 angle. Though a man six feet high is seen, at the distance of 
 six feet, under the very same angle as a dwarf only two feet 
 hi^h, at the distance of two feet, still the dwarf does not 
 appear as large as the man, because we are instantly able to 
 make the requisite allowance for the difference of distance. 
 
 But when the distance is considerable, and we have no op- 
 portunity of comparing one object with another, we soon per- 
 ceive that the rule just laid down has its foundation in nature. 
 When Denon first drew near the gigantic pyramids of Gizeh, 
 he was not particularly struck with their magnitude, princi- 
 pally because there were no objects in the vicinity by which a 
 comparison could be made ; but this impression was speedily 
 effaced, when he had observed a hundred people who had pre- 
 ceded him assembled at the base of one of them, the deception 
 instantly vanished, a comparison was formed, and the stupen- 
 dous pile assumed all its appropriate majesty. 
 
 As an image of every visible object is painted on the retina 
 of each of our eyes, we may be inclined to inquire, why we do 
 not see every object doiibled? Of the various opinions which 
 have been advanced in explanation of this difllculty, the most 
 satisfactory is, that in the two eyes there are corresponding 
 parts of the retinas, which are probably susceptible of the same 
 impression in equal degree, and convey it to the sensorium in 
 that equal degree ; hence, as long as similar points of the images 
 fall upon the corresponding points of the retinas, the percep- 
 tion of the same object is single, otherwise it is double. It is 
 a confirmation of this theory, that when a person, whose sight 
 is perfect, looks with both eyes at an object straight before him, 
 the axes of both eyes are inclined towards each other in equal 
 degrees, and directed to the same point. In this case the 
 images are formed upon the corresponding parts of the retinas, 
 and in all other cases, the eyes move in unison, to produce the 
 same efl'ect; but while the same object still continues to be re- 
 garded, let the position of one eye be vai'ied a little by a slight 
 pressure of the finger, and the object will instantly appear 
 double. Now the aberration of the axis of one eye, which is 
 thus effected for a moment by design, is often produced by 
 disease or habit, so that the power of directing both eyes to 
 the same point is permanently lost : when this is the case, the 
 person is said to squint, and the squint-eyed always see objects 
 
 20.— Vol. I. 3 N 
 
458 OPTICS. 
 
 double, unless they have acquired the habit of entirely dis- 
 using: the eye of whose motion they have lost the natural and 
 perfect command. No method of curing this defect, when it 
 has not been absolutely irremediable, has been more successful 
 than that of binding; up for a time the sound eye, by which 
 means the other is obliged to perform its office. 
 
 The nearer any object is, when viewed with two eves, the 
 greater the inclination of the axes of the eyes to each other, 
 and the converse necessarily follows, when we take a distant 
 view. It is by this adjustment that we are materially assisted 
 in judging of the distances of objects not very remote, and that 
 we know whether a person is looking at us or not. 
 
 We acquire also the habit of almost involuntarily taking into 
 consideration a variety of other circumstances, in judging of 
 distances. WTien objects appear obscure or confused, we 
 judge them to be remote ; and when thev appear distinct, we 
 form a contrary opinion: of these principles painters sedulously 
 study to avail themselves. Rooms, the walls of which are 
 whitened, appear smaller than when of a dark colour ; fields 
 covered with snow, or white flowers, appear less than when 
 clothed with grass ; mountains covered with snow appear nearer 
 than at other times; and in the evening, when it is nearly dark, 
 an upright post half white and half black, may be taken for a 
 body of considerable size in horizontal extension. 
 
 An Englishman, when he first views an Italian landscape, 
 makes the most egregious mistakes, in estimating the distances 
 of places and objects by the eye. He has no conception of 
 the clearness of the air, in that delightful climate, by which he 
 is enabled to perceive objects at the distance of twenty miles, 
 with so much distinctness, that he supposes himself to be 
 within half an hour's walk of them. Italian painters, true to 
 the characteristics of their country, have made their most dis- 
 tant mountains well defined at their summits, and all other 
 objects proportionately distinct; and we are apt to think they 
 have deviated from nature, because the scenery of our own 
 country is never clothed with such fascinating splendour. 
 
 The eye can only see a very small part of an object distinctly 
 at once; for the lateral parts of an object are not represented 
 distinctly in the eye ; and therefore the eve is obliged to turn 
 itself successively to the several parts of the object it wants to 
 view, that they may fall on or near the axis of the eye, where 
 alone distinct vision is performed. 
 
 When the eye is placed above a horizontal plane, the diffe- 
 rent parts of this plane will appear elevated in proportion to 
 their distance, till at length they will appear on a level with it. 
 For in proportion as the ditFereut parts are more distant, the 
 
OPTICS. 459 
 
 Phenomena of vision. 
 
 rays which proceed from them, form angles with the optical 
 axis, IK, fig. 4, pi. IV, more and more acute, and at length 
 become almost parallel. This is the reason why, if we stand 
 on the sea-shore, those parts of the ocean which are at a great 
 distance appear elevated; for the globular form of the earth is 
 not perceptible to the eye; and ir it was, the apparent eleva- 
 tion of the sea is far greater than the arch which a segment of 
 the globe would form, within any distance that our eyes are 
 capable of reaching. 
 
 The best eye can hardly distinguish any object that subtends 
 at the eye an angle less than half a minute of a degree ; and 
 very few can distinguish it when it subtends a minute. If the 
 distance of two stars be not greater than this, they will appear 
 as one. 
 
 Though men see distinctly at different distances, by the altera- 
 tions of the position and figure of the crystalline lens, yet they 
 can only see distinctly beyond a certain extent. This extent 
 is not the same in different people, but in general it is between 
 six and ten inches. A good eye can see distinctly when the 
 rays fall parallel upon it; and then the principal focus is at the 
 bottom of the eye. 
 
 The following is a summary of the laws of vision, with regard 
 to the figure of visible objects : 1. If the centre of the eye be 
 exactly in the direction of a right line, the line will appear only 
 as a point. 2. If the eye be placed in the direction of a sur- 
 face, it will appear only as a line. 3. If a body be opposed 
 directly towards the eye, so that only one plane of the surface 
 can radiate on it, the body will appear as a surface. 4. A 
 remote arch, viewed by an eye in the same plane with it, will 
 appear as a right line. 5. A sphere, viewed at a distance, 
 appears a circle. 6. Angular figures, at a distance, appear 
 round. 7. If the eye look obliquely on the centre of a regular 
 figure, or a circle, the true figure will not be seen, but the 
 circle will appear an ellipse, 8cc. 
 
460 OPTICS. 
 
 Utility of convex and concave glasses to improve vision. 
 
 Of Optical Insteuments and Machines. 
 Of Spectacles. 
 
 By the observations illustrating fig. 4, pi. IV, we hope it is 
 clearly understood, that the nearer any object can be brought 
 to the eye, tlie larger will be the angle under which it appears, 
 and the more it will be magnified. But objects, we find by 
 experience, may be brought so near the eye, that the advantage 
 of their forming a large visual angle, is more than counter- 
 balanced by the indistinctness that ensues. Tlie generality of 
 people see small objects best, at the distance of eight inches ; 
 when such objects are brought nearer than eight inches, they 
 become indistinct, and if to four, or three, they will scarcely 
 be seen at all. The reason of this indistinctness is, that the 
 pencils of rays from objects brought within the limits where 
 distinct vision commences to the naked eye, are in so divergent 
 a state, that they are not converged to points exactly upon the 
 retina. But we have already seen, that a convex glass will 
 lessen the divergence of the most divergent rays passing through 
 it, and that if its curvature be sufficient, it will refract them 
 parallel or even convergent. If, then, an object be viewed at 
 the distance of two or three inches, with a glass of suitable 
 convexity interposed, it immediately becomes distinctly visible 
 at that distance, with the advantage of appearing larger and 
 more enlightened than it would under any circumstances ap- 
 pear to the naked eye. By an obvious parity of reasoning it is 
 evident, that if an object can be rendered distinctly visible by 
 a convex glass, at a less distance than that at which common 
 vision is effected, a person who does not see objects but at an 
 unusual and inconvenient distance, maybe made to see them at 
 the common and most eligible distance. Hence the use of 
 convex glasses, which are an invaluable remedy for the visual 
 defect of the long-sighted. Concave glasses, on the contrary, 
 which are directly opposite in their effects to the convex, prove 
 a remedy of equal value to the short-sighted, or those who 
 cannot see distant objects distinctly, and are obliged to bring 
 near objects almost close to their eyes. 
 
 By fig. 3, pi. IV, is represented an eye in its perfect state, 
 the image of the object being exactly upon the retina, or as 
 nearly so as could be shewn by an engraving ; for we need 
 not inform the most ignorant reader, that of the picture formed 
 by light, no part projects, in the manner shewn by the plate. 
 Fig. 1, pi. V, is the eye of a long-sighted person, where, from 
 the flatness of the crystalline humour, and of the cornea, the 
 
OPTICS. 461 
 
 Spectacles. 
 
 foci of the pencils of rays from objects are not at d, where they 
 ought to be, in order to render vision distinct, but tend to a 
 point at F beyond the eye. Hence the rays which flow from the 
 object C, and pass through the humours of the eye, instead of 
 forming an image of a point, from a large speck of light, and when 
 the rays from all parts of an object do the same, the whole image 
 is confused. But if a convex glass, AB, of a proper focus, be 
 interposed, the rays converge sooner, they meet in a point at d, 
 on the retina, and distinct vision is obtained. 
 
 In fig. 2, from the great convexity of the cornea and crystal- 
 line humour, the rays that enter from the object C, converge to 
 a focus in the vitreous humour, as at F, and by diverging from 
 thence, fall as in the example of the long-sighted, obtusely on 
 the retina, and vision is as indistinct as if the eye had been too 
 flat. But by placing a concave glass AB, before the eye, the rays 
 are spread out a little, or fall with greater divergency upon the 
 eye than before, and they do not unite till they fall on the re- 
 tina, consequently the defect disappears. 
 
 When glasses are put in frames for spectacles, their frames 
 ought not to be straight, but bent a little in the middle, so that 
 the axes of both glasses may be directed to one point, at the 
 distance most proper for reading or examining objects in 
 general. By this means the axes of the eyes will fall per- 
 pendicularly upon the glasses, and vision will be more dis- 
 tinct. 
 
 Spectacles are much better for the eyes than those glasses 
 wliich are held in the hand. By hand-glasses, indeed, more 
 harm than benefit may eventually result, for the distance at 
 which they are held from the eye is perpetually varied, and the 
 eye is thus perpetually strained, that it may accommodate 
 itself to such changes. 
 
 When the eyes of persons first begin to be affected by age, 
 the opticians furnish them with spectacles, the glasses of which 
 are about 40 inches focus, which are therefore called No. 1, or 
 glasses of the first sight. When the focal length is about 16 
 inches, the lenses are called No. 2. About 12 inches is the 
 focal length of No. 3. Ten inches is what they call No. 4. 
 Nine inches is that of No. 5. Eight inches is the focal length 
 of No. 6. Seven inches is the focal length of No. 7. Six 
 inches is the focal length of No. 8, and sometimes they make 
 spectacles of a focus shorter still. Concave spectacles are dis- 
 tinguished by numbers in the same way. But there is a consi- 
 derable difference in the focal distances of glasses made by dif- 
 ferent opticians, though they give them the same number. In 
 chusing spectacles, therefore, actual trial alone can be depended 
 on ; and when an actual trial is made, perhaps the best direc- 
 
462 OPTICS. 
 
 !«pectacles. — Presenatioo of the siijbt. 
 
 tion that can be given to make a proper choice, is to prefer 
 those spectacles which, when near the eye, shew objects nearest 
 their natural state, neither enlarged nor diminished, and that 
 give a blackness and distinctness to the letters of a book, with- 
 out straining the eye, or causing any extraordinary exertion of 
 the pupil. No spectacles can be relied on as properly accom- 
 modated to the eyes, which do not procure thera ease and rest ; 
 if thev fatiiTue the eves, we may safely conclude, either that 
 we have no occasion for them, or that they are ill-made, or not 
 proportioned to our sight. 
 
 In the choice of glasses for the short-sighted, no rules can be 
 laid down; it is a state of the eye which has no connection with 
 age ; no stated progression that can be a guide to the optician, 
 in order that he may recommend one glass in preference 
 to another ; but the short-sighted themselves, by trying dif- 
 ferent glasses, will soon discover which are most advan- 
 tageous. 
 
 We shall probably not be charsfed with taking up too irre- 
 levant a subject, if we introduce a few observations on the 
 means which may be employed to render spectacles unne- 
 cessary, or at least to lengthen the period of visual enjoy- 
 ment without them. The long-sighted should accustom them- 
 selves to read and examine objects at a less distance than 
 is entirely agreeable to them ; while the short-sighted should 
 attentively pursue a contrar\- practice. Nothing is more 
 conducive to the preservation of the sight, than the constant 
 use, both in reading and writing, of a moderate degree of 
 licrht. If in the apartment we commonly use, there are two 
 windows, it is better to sit at an equal distance from both, than 
 to let the principal light come from one side only. In all 
 cases, it should be our object to aflbrd each eye an equal quan- 
 tity of light. 
 
 To flaming colours and white objects, the eyes should not 
 be often or long exposed. The " poor, untutored Indian," 
 when he traverses his native wilderness, while it is every-where 
 covered with snow, fixes before his eyes a wooden frame, which 
 only permits the rays of light to pass through a very minute 
 aperture. His view is thus confined, and his light is small, 
 but he preserves his sight from certain injun,-. Long or frequent 
 excursions over snow, especially when the sun shines, it will 
 easily perhaps be admitted, must have an injurious tendency to 
 the eyes ; but it may be asserted in return, that none of the 
 circumstances attendincr conmion life in this country, can 
 occasion an efl'ect so much to be deprecated. The following 
 relation will eWnce the fallacy of this supposition. A student 
 at Cambridge, who sat daily for several hours in an apartment. 
 
OPTICS. 4G3 
 
 Preservation of the sight. — Burning lenses and mirrors. 
 
 the walls of which were white-washed, felt himself in a short 
 time aftected with dimness of sight. A fellow student had 
 the like occasion for complaint. Suspecting- the strong- 
 light reflected from the walls to be the cause, they had their 
 apartments coloured green ; and their eyes then gradually 
 regained their former strength. 
 
 After the long interval from exertion occasioned by a night's 
 rest, the sudden exposure of the eyes to a strong light, or the 
 intense exertion of them, has an unfavourable effect. Scarlet 
 window and bed-curtains are, for this reason, the worst that 
 can be chosen. The cataract, a complaint AvhicJi frequently 
 follows an inflammation of the eyes, and is often irremediable, 
 may be occasioned by looking very frequently at a fire, or any 
 very glaring object. 
 
 Green is of all colours the most agreeable to the eyes, and 
 scarlet the most offensive, or endurable for the shortest space 
 of time. White is the next to scarlet. Hats and other cover- 
 ings for the head, the undersides of which are white, certainly 
 tend to impair the sight. 
 
 The affusion of the eyes in clean, soft, cold water, contri- 
 butes greatly to strengthen them, and of all applications is the 
 most strongly to be recommended. But with respect to the 
 most proper time of performing it, an erroneous opinion is 
 prevalent. Morning is commonly thought to be the most 
 proper time. The middle of the day is certainly to be pre- 
 ferred. Morning is an ineligible time, because the eyes 
 are then well refreshed, and amply replenished with the 
 moisture which is most suitable for them ; but when the 
 affusion is postponed till mid-day, it becomes seasonable and 
 beneficial. 
 
 Of Burning Lenses mid 3Ii}rors. 
 
 A burning lens must be convex, a burning mirror must be 
 concave ; because both produce their effect by concentrating, 
 into a very small compass, the rays of light and heat, incident 
 upon a large surface. 
 
 As the rays which pass through a convex lens, or are 
 reflected from a concave mirror, are united at its focus, their 
 effect is so much the greater, as the surface of the lens or 
 mirror exceeds that of the focus. Thus, if a lens four inches 
 broad collect the sun's rays into a focus at the distance of one 
 foot, the image will not be more than one-tenth of an inch 
 broad. The surface of this little circle is 1600 times less than 
 the surface of the lens, and consequently the density of the 
 sun's rays within it is proportionately increased. It is not 
 
4G4 OPTICA. 
 
 Burning lenses and mirrors. 
 
 therefore surprising, that large lenses and mirrors burn with 
 irresistible intensity. 
 
 The most remarkable burning lens which has ever been 
 constructed, was made by Parker, of Fleet-street, London, 
 at an expense of upwards of £700. It was undertaken with 
 a view to fuse and vitrify such substances as resist the fires of 
 ordinary furnaces, and more especially of applying heat in 
 vacuo, and in other circumstances in which it cannot be 
 applied by any other means. After directing his attention to 
 this object for several years, and performing a great variety 
 of experiments in the jirosecution of it, he at last suc- 
 ceeded. His leiis was of flint-glass, three feet in diameter, 
 and when tixed in its frame, exposed a surface of two feet 
 eight inches and a half in diameter, without any other ma- 
 terial imperfection besides a disfigurement of one of the 
 edges, occasioned by a piece of scoria, which had found its 
 way into its substance. Its weight was 212 pounds ; its focal 
 length six feet eight inches ; and the diameter of the focus, 
 one inch. To concentrate the rays still further, a second 
 lens was used, and reduced the diameter of the focus to 
 half an inch. Some of the principal efi'ects of this lens are the 
 following : 
 
 1. Every kind of wood took fire in an instant, whether hard 
 or green, or soaked in water. 
 
 2. Thin iron plates grew hot in a moment, and then melted. 
 
 3. Tiles, slates, and all kinds of earth, were almost instantly 
 converted into glass. 
 
 4. Sulphur, pitch, and all resinous bodies, melted under 
 water. 
 
 5. Fir-wood, exposed to the focus under water, did not 
 seem changed, but when broken, the inside was burnt to a coal. 
 
 6. If a cavity were made in a piece of charcoal, and the 
 substances to be acted upon were put in it, the effect of the 
 lens was much increased. 
 
 7. Any metal whatever, thus inclosed in the charcoal, melted 
 in a moment, the fire sparkling like that of a forge. 
 
 8. The ashes of wood, paper, linen, and all vegetable sub- 
 stances, were instantlv turned into a transparent glass. 
 
 9. The substances most difficult to be wrought upon, were 
 those of a white colour. 
 
 10. All metals vitrified on a China plate, when it was so 
 thick as not to melt, and the heat was gradually communi- 
 cated. 
 
 11. When copper was thus melted, and thrown quickly 
 into cold water, it produced so violent a shock as to break the 
 strongest earthen vessels, and the copper was entirely dissipated 
 
OPTICS. 
 
 PL IT 
 
 ECccpti-JU 
 
 Publisied "by Henry Tislier, Caxton. London, lS22 . 
 
OPTICS. 465 
 
 Burning lenses. 
 
 Though the heat of the focus was so intense as to melt gold 
 in a few seconds; yet there was so little heat at a short dis- 
 tance from the focus, that the finger might be placed about an 
 inch from it without injury. The proprietor had the curiosity 
 to try what the sensation was at the focus, and having put his 
 finger there for that purpose, he described the sensation, not 
 as resembling that produced by a fire or lighted candle, but 
 like that of a sharp cut with a lancet. 
 
 If a piece of wood be placed in a decanter of water, and the 
 focus of a large burning glass be thrown upon it, the wood will 
 be completely charred, though the sides of the decanter through 
 which the rays pass will not be cracked, nor any way affected, 
 nor the water perceptibly warmed. If the wood be taken out, 
 and the rays be thrown on the water, neither the vessel or its 
 contents will be in the least affected ; but if a piece of metal 
 be put into the water, it soon becomes too hot to be touched, 
 and the water will presently boil. Though pure water alone, 
 contained in a transparent vessel, cannot be heated, yet if by 
 -a little ink it be made of a dark colour, or the vessel itself be 
 blackened, the effect speedily takes place. 
 
 As all transparent substances, denser than air, when they 
 are spherically convex, or approaching to that form, will con- 
 verge the rays of light, they may, under particular circum- 
 stances, produce effects which few would suspect. A very glo- 
 bular decanter full of water, standing in an apartment where it 
 is exposed to the fervid action of a summer's noontide sun, 
 will converge the incident light with sufficient regularity and 
 intensity to inflame any very combustible body that may hap- 
 pen to be at the proper distance. Wc have the recollection of 
 a serious fire having arisen from such a cause. 
 
 Upon the same principle, we may explain another appearance 
 sometimes observable. If in summer, after much dry weather, 
 a shower of rain falls, and the sun quickly after shines with full 
 splendour, an accurate observer will detect a very curious phe- 
 nomenon. Many of the leaves and flowers of plants which were 
 entire before the shower, are found to be perforated with small 
 holes. It might be supposed that the caterpillar has renewed 
 his depredations with new vigour, but it is found upon closer 
 examination, that the leaves he never touches are no more ex- 
 empt from these perforations than others. Perhaps the follow- 
 ing considerations will point out to us the truth. If water be 
 thrown upon a dusty floor, it is well known, that it collects itself 
 into small drops or globules. If then heavy drops of rain be 
 thus <3ollected on the dusty leaves, they will in effect be little 
 burning-lenses, and when steadily exposed to the direct rays of 
 a hot sun, they may produce the perforations in question. 
 20.— Vol. I. 3 
 
466 OPTICS. 
 
 Burning mirrors. — Camera obscora. 
 
 Burniiiir mirrors were made in very remote times; the most 
 famous were those of Archimedes and Proclus; by the former 
 of which the Roman ships, besieging Syracuse, according to 
 the testimony of several writers, and by the latter, the navy of 
 Vitalian, besieging Byzantium, were reduced to ashes. Among 
 the moderns, the burning mirror contrived by Buffon is the 
 most remarkable. It is a polyhedron, six feet broad, and as 
 many high, consisting of 160 small mirrors, or Hat pieces of 
 looking-glass, each six inches square; by means of this com- 
 pound mirror, with the faint rays of the sun in the month of 
 March, he set on fire boards of beech-wood at the distance of 
 150 feet. It may be used to burn downwards, or horizontally, 
 at pleasure ; for each of the pieces that compose it is move- 
 able by three screws, so that it may be set to a proper incli- 
 nation for directing the rays towards any given point ; and it 
 turns either in its greater focus, or in any nearer interval. 
 Buffon, at another time, burnt wood at the distance of above 
 200 feet, and melted silver at 50. 
 
 A metallic mirror, of the same size as one of glass, is much 
 superior to the latter in its power of burning: and those metals 
 answer perfectly well which reflect images but inditferently ; 
 a piece of tin-plate hammered into the form of a parabola, and 
 well polished, makes an excellent burning mirror. 
 
 Of the Camera Obscura. 
 
 If a hole be made in a window-shutter or side of a darkened 
 room, the inverted images of all external objects, from which 
 rays of light can enter at the hole, may be observed upon the 
 opposite wall of the room. This is the camera obscura in its 
 most imperfect state. Whether the hole be small or large, the 
 rays are so much scattered, and partly too inflected by the 
 sides of the a.perture, that the picture is indistinct, and little 
 interesting. But if a convex glass be applied to the hole, the 
 pencils of divergent rays proceeding from the illuminated 
 objects without, are converged to their proper foci, and if a 
 screen be placed there to receive them, a picture is formed by 
 them, incomparably superior to the happiest efforts of the 
 painter's skill. A large lens, with a considerable focal distance, 
 forms the best image, which is also the most beautiful, when 
 the external objects are all nearly at the same distance. When 
 they are very unequally distant, some confusion arises, because 
 the foci of the pencils of rays proceeding from them require the 
 glass to be at different distances from the screen, and no ar- 
 rangement can be made to admit of this adjustment. The eye 
 is a natural camera obscura, and we find that we cannot see a 
 near and a distant object distinctly at the same moment; if w« 
 
OPTICS. 467 
 
 Camera obscura. 
 
 look at the near one, the crystalline lens adjusts itself so that 
 the rays from it are duly converged upon the retina; if we look 
 at the further, another adjustment ensues, or the rays, from 
 their less divergence, would not form a distinct picture on the 
 retina. Hence, as we cannot hope to exceed the works of 
 nature, we must be satisfied with altering the position of the 
 glass or the screen, to suit the objects we most wish to have 
 clear and well defined. 
 
 It is necessary, in this experiment of the camera obscura, 
 that the window should not be opposite the sun, otherwise we 
 shall have no image but that of his brightness ; and yet it is 
 necessary also, that the sun should illuminate strongly the 
 objects which are to be depicted within, or the rays will be 
 sent so feebly from every part, that the picture will not be 
 brilliant. The inverted position of the image shewn by this 
 camera obscura, is rather an imperfection 5 but if you take a 
 looking-glass, and hold it before you, with the face towards 
 the picture, and inclining downwards, the image will be seen 
 erect in the glass, and appear with greater brilliancy than upon 
 the screen. The colouring of the picture is exquisitely soft 
 and delicate, every part is in due proportion, the light and 
 shade are distributed with the most accurate propriety, and 
 the motions of all objects perfectly expressed. Thus, in faith- 
 ful miniature, 
 
 " Hills, dales, and woods appear, 
 
 Flocks graze the fields, birds wing the silent air, 
 In darkened rnnms, where light can onl^ pass 
 Through the small surface of a convex glass : 
 On the white sheet the moving figures rise. 
 The forest waves, clouds Hoat along the skies." 
 
 The camera obscura is frequently made of a portable size, 
 and sometimes so small that it may be carried in the pocket. 
 The construction of them is sometimes a little varied, but they 
 are all essentially the same in principle. The section of the 
 one we shall describe, is shewn by fig. ^. pi. V. It consists 
 of a rectangular box, ABCD, in the front oi wnich slides a 
 tube F. At the extremity of this tube is a double convex lens, 
 EH. A plane mipror, ST, is contained within the box, and 
 set at an angle of 45 degrees. The pencils of rays flowing 
 from external objects through the convex glass, are reflected 
 upwards by this mirror, and meet in points on IK, which is 
 either a piece of oiled paper, or what is still better, a piece of 
 glass, of which the polish has been removed from one side, by 
 rubbing it with sandstone and water. To an eye looking from 
 the top downward upon the glass IK, a distinct pictiire of ex- 
 ternal objects in their proper colours will be perceived, and 
 
468 OPTICS. 
 
 Camera obscara. 
 
 with a black-lead pencil their outlines may be traced upon the 
 semi-transparent glass, the rough side of which should always 
 be uppermost. The view will be inverted with respect to right 
 and left, but when the outline has been finished, if the glass be 
 taken and turned down upon a piece of damp paper, and sub- 
 jected to a gentle pressure for a short time, the pencil-marks 
 on the glass will be transferred to the paper, and the right and 
 left side of the drawing will correspond to those of the objects 
 they are meant to represent. The picture formed upon the 
 glass IK, should be freed as much as possible from all extra- 
 neous light : it should therefore have a thin board in the posi- 
 tion BY, and from Y should be suspended a curtain of black 
 silk, which should extend over the ends. The inner surface 
 of the board YB, and all the internal parts of the instrument, 
 should likewise be blackened. 
 
 The tube in which the convex glass EH is fixed, slides in 
 the forepart of the box, in order that the distance of the glass 
 may be adjusted to the distance of outward objects, and the 
 proper distance is soon discovered, because the picture is then 
 the most distinct. This portable camera obscura may be made 
 in the form of a book, which will contain within itself every 
 part of the instrument except the tube. 
 
 The mirror of a camera obscura is commonly made of glass, 
 ■which contributes not a little to render the instrument imper- 
 fect, for the first surface of the glass reflects a part of the in- 
 cident light, and forms an image which the reflection from 
 the silvered side does not wholly efface, because the rays from 
 the two surfaces do not coincide ; and though the whole pic- 
 ture appears exquisitely beautiful to the eye, at a little dis- 
 tance, yet it is found, upon trial, that the double reflection de- 
 feats the attempt to make a correct copy. The occasional 
 use of the instrument will, however, furnish the young stu- 
 dent of perspective with many useful hints. A metallic mir- 
 ror, of a composition similar to that employed for telescopes, 
 is a remedy for the defect in question, but adds exorbitantly 
 to the expense. 
 
 Dr. Wollaston endeavours to prove that the image formed 
 by the camera obscura is much improved by the use of a con- 
 vexo-concave lens, instead of the double convex one usually 
 employed. The tendency of the improvement is to make the 
 6ides of the picture as distinct as the central part of it. For 
 the Doctor's paper, see the Philos. Trans, for 181l2, part II; 
 it contains the necessary data for deriving the greatest advan- 
 tage from his discovery. 
 
OPTICS. 469 
 
 Magic lanterD. 
 
 The Magic Lantern and Phantasmagoria. 
 
 The magic lantern is a machine employed to throw a mag- 
 nified image of paintings upon glass or any transparent sub- 
 stance, on a white screen in a darkened chamber. It has gene- 
 rally been devoted to the amusement of children, paintings of 
 a ludicrous description being its usual accompaniments ; but 
 it may be employed with propriety to illustrate the principles 
 of the sciences, by a selection of suitable diagrams. 
 
 A section of this machine is shewn by fig. 4, pi. V, where 
 ABCD is a tin lantern, from the side of which proceeds a 
 square tube, b nkl m c, consisting of two parts; the outermost 
 of which n k I m, slides over the other, so that the whole tube 
 may readily be lengthened or shortened. In the end of the 
 arm, Ji k I m, is fixed a convex glass k I; about d e there is a 
 contrivance for admitting and placing an object d e, painted in 
 transparent colours, on a plane thin piece of glass. A single 
 or double convex glass, b h c, is employed to cast the light 
 from the flame of the candle, a, strongly on the picture d e, 
 painted on the plane thin glass. From the shortness of its 
 focus, and consequently great convexity, it is usually called 
 the bull's eye. If the object d e, be placed further from the 
 glass k I than its focus, a distinct image of the object will be 
 projected by the glass k I, upon the opposite white wall or 
 screen FH, at/g, and it will be in an erect posture, provided 
 care be taken to slide the transparent painting invertedly into 
 its receptacle. If the tube b n k I m c, be contracted, and 
 thereby the glass, k I, brought nearer the object, d e, the re- 
 presentation, /g, will be projected so much the larger, and so 
 much the more distant from the glass k I ; the image may also 
 be enlarged by drawing back the lantern to a greater distance 
 from thescreen ; but as the image is enlarged, the same quan- 
 tity of light is spread over a greater surface, and consequently 
 diminishes in distinctness. 
 
 The apartment in which the exhibition is made, should be 
 completely darkened, and no light should escape from the lan- 
 tern except what passes through the glass b h c. To increase 
 the light, a concave reflector, ST, is frequently used, of such a 
 curvature, that the candle is in its focus, so that the rays pro 
 ceeding from it fall parallel upon the glass b h c. The glass 
 upon which the pictures are made, is generally of suflicien' 
 length to contain several sets of figures ; so that when th( 
 spectators are satisfied with the first set, by sliding the same 
 glass a little further on, another figure is exhibited. 
 
 The exhibition called the Phantasmagoria, which has been 
 
470 OPTICS. 
 
 PLantasmagoria. — Microscopes. 
 
 SO much admired, is performed by means of a magic lantern, 
 generally one of extraordinary dimensions, but in other respects 
 not much varied in its construction. In the common lantern, 
 tiie figures are painted on the glass, and all the rest of the 
 glass is left transparent, consequently the image on the screen 
 is a circle of light with the figures in the midst of it; but ia 
 the Phantasmagoria, the whole of the glass is made opaque, 
 except the space taken up by the figures painted with the 
 transparent colours, hence this ditterence in the effect is pro- 
 duced, that no light falls upon the screen but what passes 
 through the figures themselves, consequently there is no circle 
 of light, or any thing but the figures upon the screen. To 
 complete the deception which this change may be made to 
 produce, the following, or some equivalent arrangement, must 
 be resorted to. Let the door of a darkened room, in which 
 the exhibition is to be seen, be set wide open, and let its place 
 be supplied with a screen of thin silk, or fine linen, or of paper 
 rendered transparent. From the outside of the room, let the 
 pictures, painted as above described, be thrown upon the 
 screen, of a very minute size. They will immediately be seen 
 within the room, and though remarkably brilliant, they will be 
 supposed to be distant by the spectators, because they see 
 nothing but the light which comes from them. Let the lan- 
 tern be drawn back to a greater distance from the screen, and 
 as the images are gradually enlarged, the spectators will sup- 
 pose them to be actually approaching towards them, and pen- 
 dent in the air. The chief defect in this exhibition, is, that 
 the images decrease in distinctness as they increase in size; 
 but this might be remedied, if a contrivance were added to the 
 machine, so that the mirror should be wholly covered, and the 
 bull's eye covered in part, at the commencement, and gradu- 
 ally uncovered, as more light was required to keep the en- 
 larged figure as bright or somewhat brighter, than when it 
 was small. 
 
 Of Microscopes. 
 
 The microscope is an instrument for magnifying small objects. 
 This effect is produced by means of convex lenses. When only 
 one convex glass or lens is used, the instrument is called a 
 single microscope; but if two or more are employed coniointly, 
 it is called a double or compound microscope. 
 
 The Single Microscope. 
 
 The apparent magnitude of objects is measured by the angle 
 under which they are seen by the eye ; and those angles are 
 -eciproc'dUy as the distances from the eye. If eight inches be 
 
OPTICS. 471 
 
 Spherule microscopes. 
 
 the nearest limit of distinct vision to the naked eye, and by in- 
 terposing a lens or other body, we can see with equal distinct- 
 ness at a nearer distance, the object will appear to be as much 
 larger through the lens than to the naked eye, as its distance 
 from the eye is less than the distance of unassisted vision. If 
 the focal distance of a convex lens be one quarter of an inch, 
 then will that be but one thirty-second part of the common 
 limit of vision or eight inches, so that the lineal dimensions of 
 an object examined with it will be magnified 32 times; the sur- 
 face 1024 times; and the solidity 1,048,576 times; for these 
 two last numbers are the square and cube of 32. For a lens 
 or spherule of any other focus, the magnifying power is easily 
 found by the same rule. 
 
 The simplest microscope which can be employed to any use- 
 ful purpose, is perhaps that which is made with a drop of water, 
 suspended in a very small hole in a thin slip of brass, or any 
 similar material. This may easily be constructed where no 
 other microscope can be obtained, and its performance will 
 afford not a little pleasure. A spherule of water, it must be 
 observed, of the same size as one of glass, will not magnify so 
 much as the latter, because, as its density is not so great, it 
 has a longer focus. A small drop of water placed on the end 
 of a slender piece of brass wire, and held to the eye by candle- 
 light or moonlight, will, without any other apparatus, magnify 
 in a very surprising manner the animalcnla contained in it. 
 The reason is, that the rays coming from the interior surface 
 of the first hemisphere, are reflected so as to fall under the 
 same angle on the surface of the hind hemisphere, to which the 
 eye is applied, as if they came from the focus of the spherule ; 
 whence they are propagated to the eye in the same manner 
 as if the objects were placed without the spherule in its 
 focus. 
 
 These water microscopes have given rise to the use of other 
 fluids, with several varieties of construction. Brewster de- 
 scribes one in the Appendix to his edition of Ferguson's Lec- 
 tures. Instead of water, he makes use of very pure and viscid 
 turpentine, which he takes up by the point of a piece of wood, 
 and drops successively upon a thin and well-polished glass: 
 different quantities being thus taken up and dropped in a simi- 
 lar manner, form four or more plano-convex lenses of turpen- 
 tine varnish, which may be made of any focal length, by tak- 
 ing up a greater or less quantity of the fluid. The lower sur- 
 face of the glass having been first smoked with a candle, the 
 black pigment below the lenses is then to be removed, so that 
 no light may pass by their circumference. The piece of glass 
 is then to be perforated, and surrounded with a toothed wheels 
 
472 OPTICS. 
 
 Splierale microscopes 
 
 which can be moved round the hole as a centre by an endless 
 screw. The apparatus is then placed in a circular case, and 
 this case fixed to a horizontal ann by means of a brass pin, 
 which passes through its upper and under surfaces, and 
 through the hole already mentioned, which does not embrace 
 the pin very tightly, in order that the toothed wheel may 
 revolve with facility. On the upper surface of the circular 
 case is an aperture directly above the line described by the 
 centres of the fluid lenses, when moving round the central 
 hole; and in this aperture is inserted a small cap, with a little 
 hole at its top, to which the eye is applied. A moveable stage 
 carries the slider, on which microscopic objects are laid, and 
 is brought nearer or removed from the lenses by a vertical 
 screw. The objects on the slider are illuminated by a plane 
 mirror, which has both a vertical and horizontal motion for 
 this purpose. When the microscope is thus constructed, the 
 object to be viewed is placed upon the slider, and the endless 
 screw is turned till one of the lenses be directly under the 
 aperture ; and the slider is thus raised or depressed by the 
 vertical screw, till the object be brought into the focus of the 
 lens. In this manner, by turning the endless screw, and 
 bringing all the lenses, one after another, directly below the 
 aperture, the object may be successively examined with a 
 variety of magnifying powers. These fluid lenses have been 
 employed as the object-glasses of compound microscopes. 
 
 Minute glass spherules make very excellent microscopes, 
 for which we are indebted to Dr. Hooke ; but the foci of 
 the smallest sort are so short, that it requires considerable 
 attention and patience to employ them well. F. Di Torre, of 
 Naples, in 1765, sent several glass globules to the Royal So- 
 ciety. The largest of them was only two Paris points in 
 diameter, and is said to magnify the diameter of an object 
 640 times ; another was the size of one Paris point, magnify- 
 ing the diameter 1280 times ; and the smallest no more than 
 one-half of a Paris point, or the 144th part of an inch in 
 diameter, and is said to magnify the diameter of an object 
 2560 times, and consequently the square of that diameter 
 6,553,600 times. Globules so exceedingly minute as these, 
 were at one time highly prized, but spherule microscopes are 
 not now made so small, to avoid straining the eyes. The 
 third or smallest globule above mentioned, could only be the 
 576th part of an inch distant from the object, because the 
 focus of a glass globe is at the distance of one-fourth of its 
 diameter; it is obvious therefore that it would admit veiy 
 little light, and could not be used without pain and difficulty 
 even by practiced observers. 
 
OPTICS. 473 
 
 Mode of making glass spherules. 
 
 Of the various methods wliich have been recommended for 
 making glass spherules, the following by Nicholson is perhaps 
 the best. It is observed, by this valuable practical writer, 
 that the usual method has been to draw out a fine thread of the 
 soft white glass called crystal, and to convert the extremity of 
 this into a spherule by melting it at the flame of a candle. But 
 this glass contains lead, which is disposed to become opaque 
 by partial reduction, unless the management be very carefully 
 attended to. He found that the hard glass used for windows 
 •eldom fails to afford excellent spherules. This glass is of a 
 clear bright green when seen edgeways. A thin piece, less than 
 one-tenth of an inch broad, was cut from the edge of a pane of 
 glass. This was held perpendicularly by the upper end, and the 
 flame of a candle was directed upon it by the blow-pipe, at 
 the distance of about an inch from the lower end. The glass 
 became soft, and the lower piece descended by its own weight 
 to the distance of about two feet, where it remained suspended, 
 by a thin thread of glass, about ^^^o^^ of an inch in diameter. 
 A part of this thread was applied endways to the lower blue 
 flame of the candle, w^ithout the use of the blow-pipe. The 
 extremity immediately became white, and formed a globule. 
 The glass was then gradually and regularly thrust towards the 
 flame, but never into it, until the globule was sufficiently large. 
 A number of these were made, and being afterwards examined 
 "by viewing their focal images with a deep magnifier, proved 
 rery bright, round, and perfect. 
 
 Spherules are mounted for use, by placing them between 
 two very thin plates of'brass or silver, each containing a small 
 hole rather less than themselves. If any imperfection in the 
 globule is discoverable, it is placed on one side, so that it may 
 be covered by the plates. The objects may be placed on the 
 point of a needle, the direction of which should be at right 
 angles to the axis of the eye, to prevent accidents. 
 
 In using these spherule microscopes, the objects are to be 
 placed in one focus, and the eye in the other ; and from the 
 shortness of the focus, it becomes nearly impossible to view 
 any but pellucid objects, because the nearness of the eye 
 obscures the light. 
 
 At page 273, we have adverted to the discoveries of Lewen- 
 hoeck ; these were all made with single microscopes, which, 
 though spherules in appearance, were in reality double convex 
 lenses. He bequeathed to the Royal Society a cabinet of micro- 
 scopes made with his own hands. The focus of the greatest 
 magnifier was at the distance of t-V of an inch from its centre, 
 consequently it magnifies the diameter of an object 160 times, 
 the superficies 25,600, and the solidity 4,096,000, on the su-i- 
 
 20.— Vol. L 3 P 
 
474 OPTICS. 
 
 Single microscope. — ^ ollaslon'> improvement of the microscope. 
 
 position that distinct vision is not naturally effected nearer 
 than eight inches, which is an average distance. 
 
 In all microscopes, it is desirable to have the means of 
 viewing objects with ease and steadiness. The t'ollowing form 
 of a single microscope is very convenient. AD, fig. 5, plate V. 
 IS a ta})let of wood, ivorj-, &c. to which is fitted a small handle 
 of the same material. Upon the top of it is fixed a small cylin- 
 drical stem, G, and this stem receives the frame containing a 
 lens C, at the focal distance of which a small pair of pliers 
 holding the object, may be placed by means of a slide and 
 adjusting screw L. The pliers are opened by means of two 
 little studs, a e. The eye is at the other focus of the lens, and 
 the object is of course seen magnified according to the power 
 of the lens employed. This instrument, enclosed in a case, 
 may be carried in the pocket without encumbrance. Lenses, 
 the focal lengths of which are from three to five-tenths of an 
 inch, are the most suitable for ordinary use. 
 
 Dr. WoUaston has proposed an improvement of micro- 
 scopes, which he thinks highly advantageous. The great desi- 
 deratum, he observes, in employing high majjuifiers, is suffi- 
 ciency of light ; and it is accordingly expedient to make the 
 aperture cf the little lens as large as is consistent with distinct 
 vision. But if the object to be viewed is of such magnitude 
 as to appear under an angle of several degrees on each side of 
 the centre, the requisite distinctness cannot be given to the 
 whole surface bv a common lens, in consequence of the con- 
 fusion occasioned by the oblique incidence of the lateral rays, 
 excepting by means of a very small aj^erture.and proportionable 
 diminution of light. In order to remedy this inconvenience, 
 he used two plano-convex lenses ground to the same radius, 
 and applied their plane surfaces on opposite sides of the same 
 aperture in a thin piece of metal. Thus he virtually obtained 
 a double convex lens, with this advantage, that the passage of 
 oblique rays was at right angles with the surfaces, as well as 
 the central pencil. With a lens so constructed, the perfora- 
 tion that appeared to give the most perfect distinctness, was 
 about one-fifth part of the focal length in diameter, and when 
 such an aperture is well centred, the visible field is at least as 
 much as twenty degrees in diameter. It is true, that a portion 
 of light is lost by doubling the number of surfaces, but this is 
 more than compensated by the greater aperture which, under 
 these circumstances, is compatible with distinct vision. 
 
OPTICS. 475 
 
 Ellis's microscope. 
 
 Ellis's single aquatic Microscope. 
 
 This instrument takes its name from John Ellis, the author 
 of an essay towards a natural history of corallines, and of many 
 curious and uncommon zoophytes. It enabled him to pursue 
 his investigations, relative to the structure and economy of 
 these wonderful productions of Nature, with considerable suc- 
 cess ; and it is well adapted to shew botanical subjects. It is 
 simple in its construction, very portable, and commodious in 
 use. It is represented by fig. 6, pi. V. 
 
 The whole apparatus is contained in a small box or case, K, 
 which is generally covered with fish-skin. On the top of the 
 box there is a socket, for receivmg the screw which is at the 
 bottom of the brass pillar. A, when the instrument is prepared 
 for use. D is a cyluidrical brass pin, which exactly fills and 
 slides up and down in a hole drilled in the middle of the pillar 
 A. The pin D is moveable, in order to adjust the lenses to 
 their focal or proper distances from the object. It maybe fast- 
 ened at any height by the screw Z, which presses against it; at 
 the top of it is a socket, to receive the ann or bar E, which car- 
 ries the magnifiers. The arm E maybe moved backwards and 
 forwards in the socket X, and sidevrays by turning it with the 
 pin D ; so that the magnifier, which is screwed into the ring 
 at the end of the bar E, may be easily made to traverse over 
 any part of the object lying on the stage or plate B. 
 
 F is a polished silver speculum, w^th a magnifying lens 
 placed at the centre of it, which is perforated for the purpose. 
 The silver speculum screws into the arm E, as at F. G, 
 another speculum, with its lens, which is of a different magni- 
 fying power to the former. H is a brass semi-circle, which 
 supports the mirror I ; the pin R of this semi-circle passes 
 through a hole tov.ards the bottom of the pillar A. B, the 
 stage on which the objects are to be placed; it fits into a small 
 dove-tailed arm, which is at the upper end of the pillar A. C, 
 a plane round glass, with a small piece of black silk stuck on 
 it, is used to lie in a circular groove made in the stage B. A 
 hollow-glass, like a watch-glass, is occasionally laid on the 
 stage instead of the plane glass. L, a pair of nippers. These 
 are fixed to the stage by the pin at the bottom ; the steel wire 
 of these nippers slides backward and forward in the socket, and 
 this socket is moveable upwards and downwards by means of 
 the joint, so that the position of the object may be varied at 
 pleasure. The object may be fixed in the nippers, stuck on 
 the point, or affixed by a little gum- water, &;c. to the ivory 
 
47G OPTICS. 
 
 Ellis's microscope. — Hand meajalscopc. 
 
 cylinder N, which screws, when occasion requires, to the poiat 
 of tiie nippers. 
 
 To use this microscope, after taking all the parts of the 
 apparatus out of the box, bei^in by screwing the pillar A to the 
 cover ; pass the pin R, of the semi-circle which carries the 
 mirror, through the hole which is near the bottomof the pillar 
 A ; push the stage into the dove-tail at B, slide the pin D into 
 the pillar ; then puss the bar E through the socket X, which is 
 at the top of the pin D, and screw one of the magnifying lenses 
 into the ring at F. These operations, which are performed in 
 a very short time, in a less time, indeed, than is necessary to 
 describe them, prepare the microscope for use. Let the object 
 be now placed either upon one of the glasses of the stage, or 
 in the nippers L, and in such a manner that it may be as nearly 
 as possible over the centre of the stage. Bring the speculum 
 F over the part you mean to observe ; then throw as much light 
 as possible on the speculum, by means of the minor I, which, 
 it will be perceived, admits of a double motion — one horizon- 
 tally, the other to set it in any required angle : the light 
 received by the speculum is reflected by it on the object. The 
 distance of the lens F from the object, is regulated bv moving 
 the pin D up and down, until a distinct view of it is obtained^ 
 The best rule is, to place the lens beyond its focal distance 
 from the object, and then gradually to slide it down till the 
 object appears clear and well defined. The adjustment of the 
 lenses to their foci, and the distribution of the light on the 
 object, are what require the most attention ; but the manage- 
 ment of the instrument is easily acquired by a little practice. 
 This microscope is sometimes furnished with a rack and pinion, 
 for more steadily elevating or depressing the piu D,in adjust- 
 ing the lens to its focus. The mirror I should be tried in posi- 
 tions variously inclined to the object, in order to ascertain that 
 in which it reflects the light best adapted to observation. 
 
 The Hand Megalscope. 
 
 This instrument was contrived by ]Martin,and is well adapted 
 for viewing the larger sort of small objects expeditiously. A, 
 fig, 7, plate V, is the case of brass, tortoise shell, &.c. with its 
 three lenses D, E, F, each surrounded by a rim or frame of the 
 same material as the case. H is the handle. The lenses are 
 lommonly of 1, 1.^, and 2 inches focus ; as thev all move on 
 the same pin, they turn over each other, and can be used either 
 conjointly or separately. The three lenses singly, aiford, of 
 course, three magnifying powers, and by combinino; two and 
 two, we obtain three more: for D with E makes one, D with F 
 another, and E with F a third j and all three together make 
 
OPTICS. 
 
 tz.t: 
 
 KCtcftrM 
 
 TH'barlieH_ DT Hen.rv PishT Caxron . London. l823 
 
OPTICS. 477 
 
 Compound microscope. 
 
 another; so that by this simple apparatus we have seen diflf&r- 
 ent magnifying powers. When the three lenses are combined, 
 it is better to turn them in, and look through them by the small 
 aperture in the sides of the case, the eye will not then be 
 incommoded by external light; the distortion of objects by the 
 sides of the glasses will be prevented, and the eye will coincide 
 more exactly with the common axes of the lenses. 
 
 Double or Compound Microscope. 
 
 Compound microscopes present to the eye, not the object 
 itself, like the single microscope, but its image. 
 
 In treating of these instruments and of telescopes, the lens 
 which is next to the object is called the object-glass, and all 
 the other lenses are called eye-glasses. 
 
 Fig. 1, pi. VI. represents the two lenses of a compound 
 microscope; a 6c is a small object, placed at a little greater 
 distance from the object glass def, than its principal focus : 
 pencils of rays proceeding from the object, pass through the 
 glass, by which they are converged, and united in points at 
 ABC, where an image is formed, which is larger than the 
 object, in proportion as the distance B e exceeds the distance 
 ec. The eye-glass, DEF, is so placed, that its focus is at B, 
 and the eye, to view the image, must be about the same dis- 
 tance on the other side. The rays of each pencil will be 
 parallel after going out of the eye-glass, but they will be again 
 converged by the refractive powers of the eye, and will form 
 on the retina, a large inverted image of the object; for it is 
 evident that it is seen, by the interposition of the glasses, 
 under the angle IFD, instead of the angle which to the naked 
 eye would be contained between bl a. 
 
 The magnifying power of this microscope is easily computed. 
 In the first place, the image AC is to the object, as the dis- 
 tance B e is to the distance e c ; and secondly, the image AC 
 will be seen by the eye at I, under the angle DIF, which 
 is equal to the angle AEC ; and therefore the image will 
 appear as much longer than to the naked eye, as the dis- 
 tance BE is shorter than eight inches, (or the limit of dis- 
 tinct vision to the naked eye ;) so that, if the distance e c 
 be one inch, cB six inches and EB two inches, then the image 
 AC is six times longer than the object ab, and that image 
 is magnified four times by the lens DF. Hence if four be 
 multiplied by six, we shall have the total power of the micro- 
 scope, which magnifies the diameter of objects 24 times, 
 their superficies 24 times 24, or 576 times, and their solidity 
 13,824 times. 
 
47f! OPTICS. 
 
 t'oinpounii uiicroscope. — Solar niiirosco()e. 
 
 This microscope has a larger field of view than a simple 
 microscope of the same power, and its field of view may be 
 further enlarged by the addition of one or two more lenses 
 instead of the single lens DF. The magnifying power of the 
 instrument witii more than two lenses, must be computed 
 from the efl'ect of all the lenses ; or it may be ascertained 
 experimentally in the ffjllowing manner : Place part of a 
 divided ruler before the microscope, so that, looking through 
 the instrument, you may see one of its divisions magnified ; 
 tl>pn open the other eye also, and, looking with it at the 
 ruler out of the microscope, the image of the magnified 
 division will seem to be projected on the ruler ; and you 
 may easily see how^ many divisions, of the unmagnified ruler, 
 measure or are equal to the single magnified division, and 
 that number is the magnifying power of the microscope. 
 Thus, if the ruler be divided after the common way, into 
 inches and tenths, and one-tenth is magnified, so as to 
 appear equal to three inches, you may conclude that the 
 microscope magnifies the diameter or length of objects thirty 
 times. 
 
 The Solar Microscope. 
 
 This microscope is sometimes called the camera obscura mi- 
 croscope, but it still more nearly resembles the magic lantern 
 in its effect. The exhibition it affords is made in a darkened 
 room, and it can only be used when the sun shines. 
 
 This instrument consists of one plane mirror, and two lenses ; 
 the mirror, s o, fig. 2, pi. VI. must be without the window shut- 
 ter d u : the lens a bin the shutter ; and the lens n within the 
 dark room. The lens a 6 is inclosed in a brass tube, and the 
 lens n in anotlier smaller tube, which slides in the former, for 
 the purpose of adjusting it to the proper distance from the 
 object. The mirror can be so turned by adjusting screws, that 
 however obliquely the incident rays EF fall upon it, they can 
 be reflected horizontally into the dark room, through the illu- 
 minati-ng lens a b. This lens collects those rays into a focus 
 near the object eg; and passing on through the object, they 
 are met by the magnifier n: here the rays cross, and proceed 
 divergently to a vertical white screen prepared to receive them, 
 on which screen the image or shadow, q r, of the object will 
 appear. The magnifying power of this instrument depends on 
 the distance of the white screen ; and in general bears a pro- 
 portion to the distance of the object eg from the magnifier «; 
 that is, if the screen be ten times that distance from the lens n, 
 the image will be ten times as long, and ten times as broad, as 
 the object. About tf n or twelve feet is the best distance, for. 
 
OPTICS. 479 
 
 Opaqae microscope. — Management o-f microscopic objects. 
 
 if further off, the image, though larger, will be obscure, and 
 ill-defined. 
 
 A telescope may be converted into a microscope, by remov- 
 ing the object-glass to a greater distance from the eye-glass. 
 And since the distance of the image is various, according to 
 the distance of the object from the focus, (and it is magnified 
 the more, as its distance from the object is greater,) the same 
 telescope may be successively converted into microscopes, 
 which magnify in different degrees. 
 
 Opaque Microscope. 
 
 The solav microscope above described, is calculated only to 
 exhibit transparent objects, or at least such as permit apart of 
 the incident light to pass through them; to view opaque objects, 
 another mirror must be used, in order that they may be seen by 
 the light they reflect. In fig. 3, pi. VI, the mirror a, and the 
 lens c, are the same as in the common solar microscope ; but 
 the converging rays from c are met by a mirror e //, placed 
 diagonally, and which throws up the rays much condensed upon 
 the opaque object SR ; from the object they are reflected to the 
 magnifier O, from which they proceed diverging to the screen 
 p q, where the object will be painted and greatly magnified. The 
 objects are generally stuck by a wafer, or gum-water, to a thin 
 slider of wood, and not placed in the focus of the lens C, to 
 prevent their being burnt. The feathers of birds, seeds of 
 plants, sections of wood, together with .sponges, ferns, and 
 similar objects, exhibit very interesting appearances in this 
 microscope. 
 
 Of Microscopic Objects. 
 
 Whatever object offers itself as the subject of our examina- 
 tion with the microscope, the size, contexture, and nature of it, 
 are first to be considered, in order to examine it with such 
 glasses, and in such a manner, as may shew it best. The first 
 step should always be to view the whole of it together, with a 
 magnifier that will take the whole of it in at once ; and after 
 this, the several parts of it may the more fitly be examined, 
 whether remaining on the object or separated from it. The 
 smaller the parts are, the more powerful ought the magnifiers 
 to be which are employed. The transparency or opacity of the 
 object must also be considered, and the glasses employed must 
 be selected accordingly; for a transparent object will bear a 
 much greater magnifier than one which is opaque, since the 
 nearness of the eye, when a high magnifier is used, unavoidably 
 darkens an object, in its own nature opaque, and renders it 
 very difficult to be seen, unless by the help of an apparatus 
 
4S0 OPTICS. 
 
 Manageraeut of uiicroscnplc objects. 
 
 contrived for that purpose, like Ellis's microscope, with a silver 
 speculum. iNIost objects, however, become transparent by 
 beins^ divided into extremely thin parts. 
 
 The nature of the object also, w hether it be alive or dead, a 
 solid or a fluid, an animal, a vegetable, or a mineral substance, 
 must likewise be considered, and all the circumstances of it 
 attended to, that we may proceed in the most advantageous 
 manner. If it be a living object, care mnst be taken not to 
 squeeze or injure it, that we may see it in its natural state and 
 full perfection. If it be a fluid, and that too thick, it must be 
 diluted with water ; and if too thin, we should let some of its 
 watery parts evaporate. Some substances are fittest for obser- 
 vation when dry, others when moistened, and others after they 
 have been kept for some time. 
 
 Light is the next thing to be taken care of, for on this the 
 truth of all our observations depends ; a little experience will 
 shew how very differently objects appear in one degree of it to 
 what they do in another. Hence, every new object should be 
 viewed in all degrees of light, from the greatest glare of bright- 
 ness to perfect obscurity ; and in all positions to each degree, 
 till we obtain all the evidence we can of its form and figure. 
 In many objects it is very ditficult to distinguish between a 
 prominency and a depression, a black shadow and a black 
 stain ; and in colour, between a bright reflection and whiteness. 
 The eye of a fly in one kind of light appears like a lattice 
 drilled full of holes, in the sun-shine like a solid substance 
 covered with golden nails ; in one position like a surface 
 coveied with pyramids ; in another with cones, and in others 
 with still different shapes. 
 
 The degree of light must always be suited to the object. If 
 that be dark, it must be seen in a full and strong light ; but if 
 transparent, the light should be proportionably weak ; for 
 which reason, there is a contrivance, both in the single and 
 double microsco])e, to cut off' the superabundant rays, when 
 such transparent objects are to be examined with the largest 
 magnifiers. Light admitted into a darkened room, through a 
 hole in a window-shutter, suits microscopic observations ex- 
 tremely well. The li^ht of a candle, is, for many objects, 
 and especially for such as are very bright, transparent, and 
 minute, preferable to day-light ; for others, a serene day- 
 light is best : but sun-shine is the worst light of all. This 
 remark does not apply to the solar microscope, for which 
 nothing but sun-shine will answer, and the brighter it is, the 
 better; with this instrument, however, we do not see the object 
 its£lf on which the sun-shine is cast, but only the image or 
 shadow of it greatly extended on a screen ; and therefore no 
 
_______ OPTICS. 4^ 
 
 Management of microscopic objects. 
 
 confusion can arlsfe from the glaring reflection of tlie sunV 
 rays from the object to the eye, as with other microscopes. 
 And with the solar microscope, we must rest contented with 
 viewing the true figure of an object, without expecting to find 
 its natural colour; since no shadow can possibly exhibit the 
 colour of the body which it represents. 
 
 Most objects require also some management, in order to 
 bring them properly before the glasses. If they are flat and 
 transparent, and such as will not be injured by pressure, the 
 best way is to inclose them between two thin pieces of Mus- 
 covy talc, fastened by brass rings in a hole made through a 
 slip of metal or ivory, called a slider. By this means we may 
 very conveniently preserve the feathers of butterflies, the 
 scales of fishes, and the farina of flowers, as also the parts of 
 insects, the whole bodies of minute ones, and a great number 
 of other objects. Each slider should contain three, four, or 
 more holes, which should not be filled promiscuously; but all 
 the objects preserved in one slider should be such as require 
 the same magnifying power to view them, that there may not 
 be a necessity of changing the glasses for every object; and 
 the sliders should be marked with the number of the magnifier 
 to be used in viewing what it contains. In placing the ob- 
 jects in the sliders, it is proper to have at hand a small mag- 
 nifi.er, of about an inch focus, to examine and adjust them by, 
 before they are fixed down with the rings. 
 
 Small living objects, such as lice, fleas, bugs, mites, minute 
 spiders. Sec. may be placed between the talcs without injuring 
 them, if care be taken to lay on the brass rings without press- 
 ing them down; but if they are too large to be treated thus, 
 they should be either preserved between two concave glasses, 
 or else viewed immediately, by holding them in the pliers, or 
 sticking them on the point, with which the other end of that 
 instrument is usually furnished. 
 
 The circulation of the blood may be most easily seen in the 
 tails and fins of fishes, in the fine membranes between the 
 toes of a frog, or best of all in the tail of a water-newt. Such 
 parts of objects of this description as are intended to be par- 
 ticularly viewed, must be expanded within a glass tube ; seve- 
 ral sizes of glass tubes usually accompany a microscope, and 
 such a one should be selected as will just admit the object, 
 which will then remain more quiet to be examined. 
 
 If fluids come under examination, to discover the animal- 
 cula they contain, a small drop is to be taken with a hair 
 pencil, or on the nib of a clean pen, and placed on a plate of 
 glass. If they are too numerous to be thus seen distinctly, 
 some lukewarm water must be added to the drop ; they will 
 21.— Vol. I. 3 Q 
 
482 OPTICS. 
 
 Mauageiiieut of microscopic objects. 
 
 then separate, and may be seen extremely well. But if we 
 are to see the salts in a fluid, the contrary method must be 
 observed, and the plate of glass must be held gently over the 
 fire, till part of the liquor is evaporated. 
 
 The dissection of minute animals requires considerable pa- 
 tience and care ; but may be done very accurately by means 
 of a needle and a fine lancet, placing the subject in a drop of 
 water, for then the parts will readily unfold themselves, and 
 will be very distinctly seen. 
 
 Such may be the methods resorted to for preserving trans- 
 parent objects ; but the opaque ones, such as seeds, woods, 
 &c. require a very different treatment, and are best preserved 
 and viewed in the following manner : 
 
 Cut card-paper into small slips, about half an inch long, 
 and the tenth of an inch broad ; wet these half way of their 
 length in gum-water, and with that fasten on several parcels 
 of the object. These are very convenient for viewing by the 
 microscope made for opaque objects with the silver speculum; 
 but they are proper for any microscope adapted for opaque 
 bodies. 
 
 A small box should be contrived for these slips, with little 
 shallow holes for the reception of each : and this is conveni- 
 ently done, by cutting pieces of pasteboard, such as the covers 
 of books are made of, to the size of the box, so that they will 
 just go into it; and then cutting holes through them with a 
 small chisel, of the shape of the slips of card; these paste- 
 boards having then a paper pasted over their bottom, are cells 
 very proper for the reception of these slips, which may easily 
 be taken out by means of a pair of pliers, and will be always 
 ready for use. 
 
 Great caution is necessary, in forming a judgment on what 
 is seen by the microscope, if the objects are extended or con- 
 tracted by force or dryness. Notliing can be determined re- 
 specting them, witiiout making the proper allowances, and dif- 
 ferent liglits and positions will often shew the same object as 
 very different from itself. There is no advantage in any greater 
 magnifier than such as is capable of shewing distinctly the 
 object viewed ; and the ]ess the glass magnifies, the more 
 pleasantly the object is seen. 
 
 In general, the colours of objects are less to be depended 
 on in proportion as the magnifying power by which they are 
 viewed is greater ; for their several component particles 
 being by this means removed to greater distances from one 
 another, may give reflections very diff^erent from what they 
 would if seen by the naked eye. 
 
 The nifitious of living creatures, also, or of the fluids con- 
 
OPTICS. 483 
 
 Nature of telescopes. 
 
 tained in their bodies, are by no means to be hastily judged 
 of from what we see by the microscope, without due considera- 
 tion ; for as the moving body, and the space in which it moves, 
 are both magnified, the motion must be so too ; and therefore 
 that rapidity with which the blood seems to pass through the 
 vessels of small animals must be judged of accordingly : sup- 
 pose, for instan.ce, that a horse and a mouse move their limbs 
 exactly at the same time, if the horse runs a mile while the 
 mouse runs fifty yards ; though the number of steps are the 
 same in both, the motion of the horse must notwithstanding be 
 allowed to be the swiftest, and the motion of a mite, as viewed 
 by the naked eye, or through the microscope, is perhaps not 
 less different. 
 
 Of Telescopes. 
 
 Telescopes are instruments employed for discovering and 
 viewing distant objects. They are of two kinds — refracting 
 and refecting ; the former consist of different lenses, through 
 which the objects are seen by rays refracted by them to the 
 eye ; but the latter consist of specula, by which the rays 
 are reflected, with lenses to magnify the image formed by the 
 specula. 
 
 The principal effects of telescopes depend upon this maxim, 
 that objects appear larger in proportion to the angles which 
 they subtend at the eye ; and the effect is the same, whether 
 the pencils of rays, by which objects are visible to us, come 
 directly from the objects themselves, or from any place nearer 
 to the eye, where they may have been united so as to form 
 an image of the object, because they issue again, in certain 
 directions, from those points where there is nothing to in- 
 tercept them, in the same manner as they did from the corre- 
 sponding points in the objects themselves. In fact, there- 
 fore, all that is effected by a telescope, is, first to make 
 such an image of a distant object, by means of a lens or 
 mirror ; and then to give the eye some assistance for view- 
 ing that image as near as possible ; so that the angle which 
 it shall subtend at the eye, may be very large, compared with 
 the angle which the object itself would subtend in the same 
 situation. This is accomplished by means of an eye-glass, 
 which so refracts the pencils of rays, that they may afterwards 
 be brought to their several foci by the humours of the eye. 
 But if the eye was so formed as to be able to see the image 
 with sufficient distinctness at the same distance without an 
 eye-glass, it would appear as much magnified as it does when 
 
484 OPTICS. 
 
 Astronomical telescope. 
 
 II glass is used for that purpose, thougli it would not in all 
 cases have so large a field af view. 
 
 Although no image be actually formed by the foci of the 
 pencils without the eye, yet if, by the help of any eye-glass, 
 the pencils of rays shall enter the pupil, just as they would 
 have done from any place without the eye-glass, the visual 
 angle will be the same as if an image had been actually 
 formed in that place. 
 
 The forms of refracting and reflecting telescopes have been 
 frequently varied, and sometimes they are distinguished by the 
 name of their inventor, as the Galilean and Newtonian tele- 
 scopes ; sometimes by the particular use for which they are 
 most proper, as the "land telescope," "the night-glass, or as- 
 tronomical telescope;" and sometimes by one of their remark- 
 able properties, as the " achromatic telescope." We shall in 
 the first place notice refracting telescopes. 
 
 The Astronomical Telescope. 
 
 This telescope consists of two convex lenses, AB, KM, fig. 4, 
 pi. VI. each fixed at the extremity of a different tube. One of 
 the tubes is very short, as its use is merely to adjust the focus in 
 proportion to the distance of the object viewed ; and it slides 
 within the other. The tubes are not represented in the figure, 
 as the external form of telescopes are familiar almost to all. 
 Contrary to the arrangement which takes place in the micro 
 scope, the glass which has the longest focus is presented to 
 the object, and therefore constitutes the object-glass. 
 
 PR represents a very distant object, from every point of 
 which rays come so very little diverging to the object lens KM 
 of the telescope, as to be nearly parallel : ^ r is the picture of 
 the object PR, which would be formed upon a screen situated 
 at that place. Beyond that place, the rays of every single 
 radiant point proceed divergently to the eye-glass, AB, oi 
 greater convexity, and which causes the rays of each pencil 
 to become parallel, in which direction they enter the eye or 
 the observer at O. 
 
 The axes of the two lenses are coincident, in the direction 
 QLO ; L (/ is the focal distance of the object-glass, and E y is 
 the focal distance of the eye-glass ; consequently the distance 
 between the two glasses is equal to the sum of their focal dis- 
 tances. An object viewed through this telescope, by an eye 
 situated at O, will appear magnified and distinct, but inverted. 
 The object seen without the telescope, will be, to its appear- 
 ance through the telescope, as ^ E to ^ L ; that is, as the focal 
 distance of the eye-glass to the focal distance of the pbjept- 
 
OPTICS. 485 
 
 Astronomical telescope. 
 
 glass. For the pencils of rays, which, after their crossing at 
 rqp proceed divergently, fall upon the lens AB, in the same 
 manner as if a real object were situated at r q p, and conse- 
 quently, after passing through that lens, the rays of each pencil 
 proceed parallel. Now to the eye at O, the apparent magni- 
 tude of the object PR, is measured by the angle BOA, or by 
 its equal p E "; but to the naked eye at L, when the glass is re- 
 moved, the apparent magnitude of the object is measured by 
 the angle PLR, or by its equal r L p ; therefore the apparent 
 magnitude to the naiced eye, is to the apparent magnitude 
 through the telescope, as the angle rhp is to the angle j9 E r; 
 or as the distance ^ E is to the distance g L. 
 
 If the angles r L p and jo E r were equal to each other, the 
 telescope would not magnify, and they would be equal, if the 
 lenses were of equal focal distance. Hence, as the magnifying 
 power of the telescope is produced by making the focal dis- 
 tance of the eye-glass less than that of the object-glass, it will 
 easily be perceived, that the greater the difference of the focal 
 lengths, the greater will be the magnifying power. In prac- 
 tice, however, it is found that they may be so disproportionate, 
 that the increased magnifying power is overbalanced by the 
 indistinctness which ensues. In order, therefore, to obtain a 
 very great magnifying power, with the preservation of just pro- 
 portion, these telescopes have sometimes been made one hun- 
 dred feet or upwards in length ; and as they were used for 
 astronomical purposes, or mostly in the night time, they were 
 frequently used without a tube; viz. the object lens was fixed 
 on the top of a pole, in a frame capable of being moved by a 
 cord or wire in any required direction, and the eye-glass, fixed 
 in a short tube, was held in the hand, or fitted to another frame 
 about the height of the observer, so as to be susceptible of a 
 simultaneous movement. A telescope of this description was 
 called an atrial telescope. Its use is evidently incommodious; 
 but such was the incredible pains taken by philosophers in 
 exploring the wonders which even the imperfect telescopes at 
 first constructed promised to lay open, that with such an in- 
 strument the five satellites of Saturn, and many other remark- 
 able objects, were discovered. 
 
 The length of common refracting telescopes must be increase 
 ed in no less a proportion than the square of the increase of 
 their magnifying power ; so tliat in order to magnify twice as 
 much as before, with the same light and distinctness, the tele- 
 scope must be lengthened four times, and to magnify three 
 times as much, nine times. On this account, their unwieldy 
 length, when great powers are desired^ is unavoidable. 
 
 The breadth of an object-glass adds nothing to the magnify- 
 
4SG OPTICS. 
 
 AstruDomical telescope. — Night-glass. 
 
 ino- power; for v hatever it may be, the image will be equally 
 formed at the distance of its focal length ; but the brilliancy of 
 the ima'j;e will be increased by the breadth, as a greater num- 
 ber of rays will then diverge from every point of the image. 
 
 The magnifying power, and the tield of view, in tliis tele- 
 scope, may be increased by using two plano-convex lenses, 
 combined so as as to act like one glass, and such a combina- 
 tion is now generally employed. The disproportion above al- 
 luded to, which may subsist between an object-glass and an 
 eye-glass, arises from this circumstance, that a lens does not 
 converge the rays of the same radiant point, exactly to the same 
 refracted focus, and the indistinctness, which is thus occasioned 
 in the image, increases with the thickness of the lens and the 
 increase of its curvature. Hence arises the limitation to the 
 use of highly magnifying eye-glasses. The tield of view is en- 
 larged by enlarging the eye-glass : but if a lens of very short 
 focus were made large, the irregularity of its refraction would 
 throw the view into confusion ; if it were made sufficiently 
 small to prevent this, it would be equally useless from its con- 
 tracted field of view, and the want of light. But if two plano- 
 convex lenses be used, the curvature of both conjointly will 
 be less than the curvature of a single lens of equal magnifying 
 power; such a combination therefore will improve the eye-glass 
 of a telescope, because the aberration of the rays passing 
 through it will be less than through a single lens of the same 
 focus. Let IK, fig. 8, plate V, be a plano-convex lens, of which 
 the focus is at F, so that an object placed at F would be seen 
 magnified through it. If another lens LM be placed between 
 the first lens and its focus, the focus of the rays passing through 
 both will be shortened, and will fall at about the distance^", so 
 that, when thus combined, they will act like a single lens of 
 much greater curvature. 
 
 The Night Glass. 
 
 The telescope called a night-glass is nothing more than the 
 common astronomical telescope, with tubes, and made of a 
 short length, with a small magnifying power. Its length is 
 usually about two feet, and it is generally made to magnify 
 from six to ten times. It is much used by navigators at night, 
 for the purpose of discovering objects that are not very dis- 
 tant, but which cannot otherwise be seen for want of sufficient 
 light, such as vessels, coasts, rocks, &c. From the smallness 
 of its magnifying power, and the obscurity of the objects upon 
 which it is employed, it admits of large glasses being used, 
 and consequently has an extensive and well-enlightened field 
 of view. 
 
OPTICS. 487 
 
 Terrestrial telescopes. — Galilean telescope. 
 
 The Terrestrial Telescope, or Perspective Glass. 
 
 The astronomical telescope, by the use of two additional 
 eye-glasses, shews objects in their right position, and becomes 
 a terrestrial telescope, that is, adapted for looking at objects 
 on the earth. — It is still more frequently called, a perspective 
 glass. This telescope is shewn by fig. 5, plate VI. The rays 
 of each pencil coming from the image LM of the object IK, 
 emerge parallel from the lens AB, and having crossed at its 
 focus, O, they continue in that direction to the lens EF, in 
 consequence of which they form an image ST at the focus of 
 the second lens, and again diverging, they fall upon the third 
 lens, CD, in the same manner as they did upon the lens AB ; 
 therefore, after their emergence from this last lens, they fall 
 parallel upon the eye at G. But as the last image ST, is not 
 inverted as at LM, but in the same position as the object IK, 
 the eye sees a true or upright picture, as if the rays had come 
 directly from the object. 
 
 The last lens, or the one nearest to the eye, in this telescope, 
 is mostly now made double, that is, two plano-convex lenses are 
 substituted for the double convex one, for the purpose of enlarg- 
 ing the field of view, as pointed out for the eye-piece of the as- 
 tronomical telescope. By this means all the best terrestrial 
 telescopes contain four lenses in the tube next the eye. 
 
 The Galilean Telescope. 
 
 A report having reached the illustrious Galileo, that an op- 
 tical instrument had been constructed, by means of which dis- 
 tant objects appeared as if they were near; that philosopher, 
 forcibly struck, with the advantage which might be derived 
 from the contrivance, instantly directed his attention to the 
 discovery of its nature. The night after he heard the account, 
 the construction of such an instrument occurred to him ; the 
 day following he put the several parts of it together, and the 
 telescope which he thus invented is still called by his name. 
 With that noble frankness for which he was remarkable, and 
 which ought always to distinguish the true philosopher from 
 the empiric, he forthwith explained publicly the structure 
 and wonderful uses that might be made of the instrument; 
 and shortly afterwards, as an acknowledgment of his merit, 
 the Republic of Venice more than tripled the salary of his pro- 
 fessorship. 
 
 Galileo's telescope consists of a convex object-glass, and a 
 concave eye-glass, as represented by fig. 6, pi- VI. The dis- 
 tance between the two lenses is less than the focal distance of 
 the object-glass; but the concave glass is situated so as to 
 
48S OPTICA. 
 
 Gnlilean leirtcope. — Achromatic objifct-gla-^. 
 
 make the rays of each pencil fall parallel upon the eye, as is 
 evident by conceiving the rays to go back again through the 
 eye-glass, towards O; EO being the focal length of the eye- 
 glass. 
 
 \Vhen the sphere of concavity in the eye-glass of a Galilean 
 telescope is equal to the sphere of convexity in the eve-glass 
 of another telescope, their magnifying power is the same ; but 
 the concave glass being placed between the object-glass and 
 its focus, the Galilean telescope will be shorter than the other, 
 by twice the focal length of the eye-glass. Hence, if the 
 lengths of the telescopes be the same, the Galilean will have 
 the greatest magnifying power. 
 
 The field of view, or quantity of objects taken in by the Gali- 
 lean telescope, does not depend, as in those with convex glasses, 
 upon the size of the eye-glass, but upon the breadth of the pupil 
 of the eye; because the lateral pencils of rays diverge from the 
 axis of the eye-glass at their emergence from it. Upon t'^is ac- 
 count, the eye should be placed as near to the eye-glass as pos- 
 sible, in order that it may receive the greatest number of pencils. 
 Yet no nearness of the eve will whollv prevent the field of view 
 from being more confined than with convex eye-glasses of 
 equal curvature ; but this disadvantage is counterbalanced by 
 the valuable property of superior distinctness. 
 
 The Achromatic and Aplanatic Object-glasses. 
 
 The ordinary telescopes which we have hitherto described, 
 will only bear a small aperture, without exhibiting circular pris- 
 matic rings of colours, which are subversive of their utilitv. 
 Two causes contribute to this effect, 1, Spherical surfaces do 
 not refract the rays of light accurately to a point ; and 2, The 
 rays of compounded light, being differently refrangible, come 
 to their respective foci at different distances from the glass, the 
 more refrangible ravs converging, of course, sooner than the 
 less refrangible. This may be proved by an easy experiment; 
 for if the image of a paper painted entirely red, and properly 
 illuminated, be cast by means of a lens upon a screen, it will 
 be formed at a greater distance than the image of a blue paper 
 cast by the same lens. Hence the image of a white object is 
 composed of an indefinite number of coloured images, the 
 violet being nearest, and the red furthest from the lens, and th'e 
 images of intermediate colours at intermediate distances. The 
 whole image is therefore in some degree confused, though its 
 extremities are most perceptibly so ; and this confusion being 
 very much increased, not only by the magnifying power of 
 the eye-glass, but also by the dispersive power which it has in 
 common with the object-glass, the necessity for that limitation 
 
OPTICS. 
 
 Fzyi. 
 
 SCccpcrdai- 
 
 Published by Hetay Fisher. Caaaoa. London , iS7,3. 
 
OPTICS. 489 
 
 Achromatic object-glass. 
 
 vhich has been already intimated, for a certain moderate pro- 
 portion between the object and eye-glass, becomes indispen- 
 sable. Under these circumstances, to increase the length of 
 refracting telescopes, seemed to constitute the only means 
 of obtaining great magnifying powers ; and though a method 
 of converging to the same point all the rays of light which is 
 composed, still remained an object of speculation, the attain- 
 ment of it seemed, in the days of Newton, to be attended with 
 insuperable difficulties. But however great they were, they 
 have been nearly overcome, and tlie signal merit of this success 
 is due to our countryman John Dollond. By making a com- 
 pound lens of three different substances, of difterent refrangible 
 powers, the rays of light, which were too much dispersed 
 by one convex lens, are brought nearer to a union Avith 
 each other ; the telescopes made with an object-glass of 
 this kind are now commonly used, and are distinguished 
 by the name oi achromatic telescopes, a prefix which signifies 
 colourless. 
 
 The object-glasses of Dollond's telescopes are composed of 
 three distinct lenses, two of which are convex and the other 
 concave. The concave one is, by British Artists, placed in the 
 middle, as represented in fig. 1, pi. VII, where a and c shew 
 the two convex lenses, and b h the concave one. The two 
 convex ones are made of London crown glass, and the middle 
 one of white flint-glass, and they are all ground to spheres of 
 different radii, according to the refractive powers of the differ- 
 ent kinds of glass, and the intended focal distance of the 
 object-glass of the telescope. According to Boscovich, the 
 focal distance of the parallel rays for the concave glass is one- 
 half, and for the convex glass one-third of the combined focus. 
 When put together, they refract the rays in the following 
 manner : Let a b, a b, fig. 2, pi. VII, be two red rays of the 
 sun's light falling parallel on the first convex lens c. Suppos- 
 ing there was no other lens present but one, they would then 
 be converged into the lines be, be, and at last meet in the 
 focus q. Let the lines g h, g h, represent two violet rays 
 falling on the surface of the lens. These are also refracted, 
 and will meet in a focus ; but as they have a greater degree of 
 refrangibility than the red rays, they must of consequence con- 
 verge more by the same power of refraction in the glass, and 
 meet sooner in a focus, suppose at r. — Let now the concave 
 lens, d d, be placed in such a manner as to intercept all the 
 rays before they come to their focus. If this lens was made of 
 the same materials, and ground to the same radius as the con- 
 vex one, it would have the same power to cause the rays to 
 diverge, that the former had to make them converge. In this 
 
 21.— Vol I. 3 R 
 
490 OPTICS. 
 
 Achromatic object-glass. 
 
 case, the red rays would become parallel, and move on in the 
 lines 0, oo; but the concave lens being made of flint-glass, 
 and upon a shorter radius, has a greater refractive power, and 
 therefore they diverge a little after they come out of it; and 
 if no third lens was interposed, they would proceed diverging 
 in the lines opt, opt ; but, by the interposition of the third 
 lens V 0, they are again made to converge, and meet in a focus 
 somewhat more distant than the former, as at r. By the con- 
 cave lens, the violet rays are also refracted, and made to diverge; 
 but having a greater degree of refrangibility, the same power of 
 refraction makes them diverge somewhat more than the red 
 ones ; and thus, if no third lens was interposed, they would 
 proceed in such lines as / m n, Im n. Now as the differently 
 coloured rays fall upon the third lens with diflerent degrees of 
 divergence, it is plain, that the same power of refraction in 
 that lens will operate upon them in such a manner as to bring 
 them all together to a focus very nearly at the same point. The 
 red rays, it is true, require the greatest power of refraction to 
 bring them to a focus, but they fall upon the lens with the 
 least degree of divergence. The violet rays, though they 
 require the least power of refraction, yet have the greatest 
 decrree of diverrrence, and thus all meet together at the point r, 
 or very nearly so. 
 
 The achromatic effect may be produced by the union of one 
 convex and one concave lens, but not so perfectly as with 
 three lenses. 
 
 To proportion accurately the densities of the glasses to 
 each other, requires much professional practice and attention. 
 Experiments with the lenses after they are perfectly finished 
 can alone be depended on. The essential parts of telescopes 
 being few and cheap, the manirfacture of them is frequently 
 attempted by individuals for the purpose of amusement ; and 
 in every kind of telescope, but the achromatic, they have, with 
 a due degree of perseverance, a fair ho|>€ of success. But in. 
 attempting to form an achromatic object-glass, however well 
 they may think they have selected th-eir glass, and propor- 
 tioned the curvatures of the surfaces, they will be almost cer- 
 tain to find that a single set of lenses, when combined, produce 
 an effect that disajipoints their expectations. To succeed 
 perfectly, they must therefore make, from different parcels of 
 glass, a considerable number of lenses, with slight dilFerences 
 of curvature, and those must be selected which will bear the 
 largest aperture and magnifyino; power. But this would render 
 the undertaking, for the sake of one or two instruments, an 
 Herculean labour, which would not bring to any jjrivate indi- 
 vidual an adequate recompense, and which, from the number 
 
OPTICS. 491 
 
 Aplanatic object-glass. 
 
 of imperfect telescopes which are manufactured by those in 
 the most extensive line of business, it would appear that 
 opticians themselves do not fully enter into the spirit of. It 
 has been very generally said and believed, that Dollond made 
 his original experiments, and constructed those excellent 
 three-foot glasses, (which at present bear so high a price, and 
 are considered as inimitable,) with one single parcel of glass, 
 which accidentally proved superior to any that has since been 
 produced. — Nicholson has rendered it extremely probable 
 that this is a vulgar error ; the proprietor of the glass-house 
 having assured him, that the original receipts and practice are 
 still followed in the making of optical glass : that the principal 
 opticians always complain of the bad quality of the glass, 
 but never fail to take the whole quantity he malves at their 
 request ; and that, when they renew their orders, they always 
 desire it may be exactly the same as the last. It seems there- 
 fore reasonable to conclude, that though diiferent parcels of 
 glass made according to the same formula, may differ a little, 
 yet that as good glass for optical uses may be obtained now 
 as formerly, and consequently as good telescopes, if the same 
 great skill and disregard of expense which Dollond evinced, in 
 adapting the curvatures of his lenses to each other and to the 
 glass, were again brought into action. 
 
 The impossibility of obtaining perfectly homogeneous glass, 
 and the consequent failure of producing that complete correc- 
 tion of the aberration of the rays of light in the telescopes 
 called achromatic, induced Dr. Robert Blair to try the effects 
 of fluid mediums ; and his success was such as to induce him 
 to give the term aplanatic, or, '' free from error," to the object- 
 glasses he thus constructed. He made a compound lens, con- 
 sisting of a plano-convex of crown-glass, with its flat side 
 towards the object, and a meniscus of the same materials, 
 with its convex side in the same direction, and its flatter con- 
 cave next the eye, and the interval between these lenses be 
 filled with a solution of antimony in a certain proportion of 
 muriatic acid. The lens thus constructed, did not exhibit the 
 slightest vestige of any extraneous colour; still, the invention, 
 after a lapse of more than twenty years, has not come into 
 general use, probably from the difficulty of preserving any fluid 
 from growing turbid in the course of time. 
 
 The Reflecting Telescope. 
 
 Mathematicians had demonstrated, that a pencil of rays 
 could not be collected in a single point by a spherical lens ; and 
 also that the image transmitted by such a 'eo^, would h^ '" 
 
49i OPTICS. 
 
 ReOecting telescope. 
 
 some dec^ree incurvated. Gregory, a young man of great abili- 
 ties, was induced to believe that these inconveniences, and 
 also the great length of refracting- telescopes, would be obviated 
 by substituting for the object-glass of the common telescope, 
 a metallic speculum, of a parabolic figure, to receive the image, 
 and to reflect it towards a small speculum of the same metal. 
 This asrain was to return the image to an eye-glass placed 
 behind the great speculum, which for that purpose was to be 
 perforated in its centre. These ideas were published by 
 Gregory- in 1663, but as he possessed no mechanical dexterity 
 himself, and could find no workman capable of realizing his 
 invention, it might have sunk into oblivion, had not Sir Isaac 
 Kewton, in the course of his discoveries, found that the errors 
 arising from the different refrancribility of light, were incom- 
 parably greater than those arising from reflection, and being 
 himself as remarkable for manual skill, as geometrical know- 
 ledge, he was independent of others for the execution of his 
 design. He therefore determined to tr\- what could be done, 
 and executed two reflectinnr telescopes, in 1672, on a plan 
 somewhat different to what Gregorys had proposed. Although 
 a good metal for the speculum was not then known, although 
 he had to contrive for himself the method of polishing it, and 
 althoufrh his telescopes were but six inches long, vet in power, 
 they were comparable to a six-feet refractor, and were capable 
 of shewino; Jupiter's satellites. 
 
 One great advantage of the reflecting telescope is, that it will 
 admit of an cyc-glass of a much shorter focal distance than a 
 refracting telescope of the same length, consequently it will 
 magnify so much the more. This advantage arises principally 
 from the rays of light not being dispersed bv reflection as thev 
 are by refraction, and partly from the practicabilitv of giving 
 the large reflectors a form either parabolical, or at least such 
 as answers better than the spherical figure. 
 
 Gregorian Telescope. 
 
 This construction of the refle n^ telescope, which is more 
 common than any other, is represented by fio-. 3, pi. VII. At 
 the bottom of the tube ABCD, is placed the laro;e concave 
 reflector k I, with a circular hole through the middle of it in the 
 direction of its axis. Within the tube of the telescope, and 
 directly facing the perforation, is placed the small concave 
 speculum, g A, supported by the arm?. Two lenses, f ^ and 
 ^q, are contained in the eye-tube LMNO, and the observer 
 applies his eye to a small hole at /".in order to view the magni- 
 fied image of the distant object YZ. The large reflector k I, 
 
OPTICS. 493 
 
 Gregorian telescope. — Newtonian telescope. 
 
 receives the rays from the distant object, and reflects them to 
 its focus, where they form an inverted image, EF, in the man- 
 ner already mentioned as one of the peculiar properties of a 
 concave mirror. Diverging from the points of union, the 
 pencils of rays proceed onwards, and cross each other a little 
 before they reach the small mirror g h, the focus of which 
 is at n, or a little further from the large speculum than its 
 principal focus. From the small mirror, the rays are re- 
 flected somewhat convergently, and in that state are received 
 before they meet by a plano-convex lens t t. By the action 
 of this glass, their convergency is increased, and they form 
 a second image, a h, which is erect like the object. This 
 second image is magnified by the lens q q, through which 
 the rays of each pencil pass nearly in a parallel direction 
 to the eye ; to exclude all extraneous light, the eye is ap- 
 plied to a very small hole, and sees the image under the angle 
 cfd. 
 
 If the lens f f,\vere removed, the image would be formed some- 
 what larger at r, but the area or field of view would be smaller 
 and less pleasant; for which reason it is not usual to omit the 
 second lens. Sometimes the lens q q, is made double, for the 
 reason assigned, in p. 486, in illustrating fig. 8, pi. V. 
 
 In this and other reflecting telescopes, containing two curved 
 reflectors, it is necessary to have the power of altering the dis- 
 tance between the two mirrors. This is usually done by a wire, 
 es, passing along the outside of the tube, and with a screw at 
 the end of it, which works in an external projection iv, of the 
 arm /, within the tube. The other end of the wire passes 
 throuoh a small stud aflixed to the tube of the telescope at m ; 
 and the observer, while looking through the hole at /, turns 
 the milled head p, of the wire, which is very near him, and 
 thus regulates the distance of the small speculum as he finds 
 requisite. 
 
 Neictonian Telescope. 
 
 A section of this telescope, is shewn by fig. 4, pi. VII. 
 ABCD is the tube, which is open at the end AB, opposite the 
 large speculum, 7i o. The large concave speculum, n o, is not 
 perforated as in the last-mentioned instrument, but the small 
 speculum, q, is set aslant, so as to direct the rays received from 
 the large speculum through an aperture, g, at the side of the 
 tube, where they are received and refracted to the eye by a lens 
 or lenses in a tube h. The speculum q, is suspended within the 
 tube, by an arm, p, with its centre opposite the centre of the 
 speculum n o; it is not curved, but plane, and has therefore no 
 other effect than that of chansins: the direction of the rays. 
 
494 oi'Tics. 
 
 NuntODian telescope. — Uerschel's telescope. 
 
 Without the small reflector, the rays from the large speculum 
 would be converged at R, and the observer might nave an eye- 
 glass placed to view the image furnied there, with his face to- 
 wards the speculum no; but in this case his head would 
 intercept the greatest part of the rays, unless the instrument 
 was very large. 
 
 The Newtonian telescope, as first described, is very con- 
 venient for viewing objects in the zenith ; as they may be seen 
 while the observer retains his ordinary position of looking 
 forward horizontally. 
 
 HerscheVs Telescope. 
 
 The best and most powerful telescopes ever constructed, 
 have been made by Dr. Herschel, who is so well known by his 
 labours, as one of the most eminent astronomers of the present 
 day. The astronomers of the Continent have adopted a phra- 
 seology, which contains a very extraordinary acknowledgment 
 of the value of his telescopes; when they wish to convey the 
 highest possible idea of the excellence of a telescope, they 
 assert that it is equal to one of Herschel's. 
 
 The largest reflecting telescope now existing was made by 
 Dr. Herschel. It is forty feet in length, and the polished sur- 
 face of the large speculum is four feet in diameter. It has no 
 second reflector, a circumstance that adds much to the bright- 
 ness of the objects viewed in it. The observer, who looks 
 through an eye-glass, as in other telescopes, has of course his 
 back to the objects; but it is so contrived that little or no light 
 is intercepted by this means. We may use the diagram of the 
 Newtonian telescope, fig. 4, pi. VII, to illustrate the position 
 of the observer more particularly. Supposing the speculum q, 
 and its support to be removed, the rays n o, as before observed, 
 would be converged to II ; but if the observer were placed 
 there, he would intercept a large portion of the light, even when 
 facing this gigantic telescope ; suppose then the upper part n, 
 of the speculum, to incline downwards, that is, to be set at an 
 angle to the axis of the tube, the rays may be directed to S, or 
 any other point nearer the tube, where the spectator may be 
 placed, and will occasion no sensible dimness of the image 
 tiirown by a large mirror. In Herschel's forty-feet telescope, 
 the converging rays reflected by the mirror, pass the extremity 
 of the tube at the distance of four inches, and come into the 
 air; so that the observer scarcely at all interferes with the inci- 
 dent light, as the diameter of the tube exceeds that of tiie 
 mirror about ten inches. It will bear a magnifying power ol 
 six thousand times the diameter of an object. 
 
OPTICS. 495 
 
 Cassegiainian telescope. — Magnifying power of telescopes. 
 
 Cassegrainian Telescope. 
 
 This telescope differs from those of the Gregorian and New- 
 tonian construction, in having the small reflector convex instead 
 of concave ; in consequence of which the small reflector must 
 be placed nearer to the large reflector than the focus of the lat- 
 ter. It shews objects inverted, because the rays do not cross, 
 and no image is formed between the two reflectors ; but with 
 the same magnifying power, it is shorter than the Gregorian, 
 by twice the focal length of the small mirror. 
 
 Of the Magnifying Power of Telescopes. 
 
 The magnifying power of a telescope is constituted by the 
 difference in size between the object viewed by the naked eye, 
 and the image seen in the field of view of the instrument ; and 
 this difference may be expressed in three different ways: 1, we 
 express the effect of the telescope on the apparent increase of 
 the diameter of objects ; 2, on the superficies ; and 3, on the soli- 
 ditij or cube of the objects. But it has been generally agreed 
 to express the magnifying power of telescopes by the effect 
 they produce on the diameter only of objects; this mode of 
 expression conveys clearer ideas of the power of the instru- 
 ment than any other, and is always understood to be meant 
 when the contrary is not expressed. Hence, if a telescope 
 shews an object, at the distance of 100 yards, as large as that 
 object would appear to the naked eye at the distance of one 
 yard, it is imderstood to magnify one hundred times. 
 
 The magnifying power of the astronomical telescope, or the 
 night-glass, is as the focal distance of the eye-glass to the focal 
 distance of the object-glass; therefore, if the latter be divided by 
 the former, the quotientwill express the magnifying power. The 
 power of the perspective-glass may be estimated in the same 
 way; for the two additional eye-glasses are not employed to 
 change the magnifying power, being equal in focal distance to the 
 third, but only to produce the erect position of objects; by 
 dividing the focal distance of one eye-glass by the focal distance 
 of the object-glass, the true power is therefore obtained. 
 
 In the Galilean telescope, the virtual focus of the concave 
 or eye-glass, must be divided by the real focus of the object- 
 glass. 
 
 To find the magnifying power of the Gregorian telescope, 
 multiply the focal distance of the great mirror by the distance 
 of the small mirror from the image next the eye ; and multiply 
 the focal distance of the small niirror by the focal distance of 
 
49G OPTICS. 
 
 M :i_':j!iMiig jiower ol" telescopes. — Binornlar telescope. 
 
 the eve-glass ; then divide the product of the former multipli- 
 cation by the product of the latter, and the quotient will e.\- 
 press the magnifying power. 
 
 The magnifyinsr power of the Newtonian telescope, or of 
 llerschel's, is found by dividing the focal distance of the large 
 speculum by the focal distance of the eye-glass. 
 
 The magnifying power of the Cassegrainian reflector is the 
 Eame as that of the Gregorian, when their respective specula 
 are of equal curvature. 
 
 As it is often inconvenient, and sometimes impossible without 
 injuring the instrument, to ascertain with precision the foci of 
 the lenses and mirrors, it is advantageous to possess the means 
 of ascertaining the magnifying power of a telescope experi- 
 mentally. This may be done in several ways, of which the 
 following is the one recommended by Edwards : 
 
 At the distance of 100 or 200 yards from the telescope, put 
 up a small circle of paper, of any determinate diameter, an inch 
 for instance. Upon a card, or any piece of strong paper, 
 through which the light cannot be easily transmitted, draw two 
 black parallel lines, the distance of which from each other is 
 exactly equal to the diameter of the small circle. Adjust the 
 telescope to distinct vision, and through it view the small circle 
 w ith one eye, and with the other eye, open also, view at the 
 same time the two parallel lines. Let the parallel lines be then 
 moved nearer to, or farther from the eye, till you see them 
 exactly to cover the small circle viewed in the telescope. 
 Pleasure now the distance of the small circle, and also of the 
 parallel lines, from your eye ; then divide the distance of the 
 former by that of the latter, and you will have the magnifying 
 power of the telescope. 
 
 ^liscellaneous Remarks relative to Telescopes. 
 
 In treating of vision, we observed, that objects seen with 
 both eyes at once, appear more vivid than thev do to either eye 
 singly. This fact gave rise to what is called the binocular or 
 double telescope, in the use of which the phenomenon is very- 
 obvious. This instrument sonsists of two equal telescopes, 
 fastened together, and made to pointto the same object. They 
 must be at the same distance from each other as the pupils of 
 the eyes of the observer, who must look through both the teles- 
 copes at once, by which means objects will appear brighter, and 
 somewhat larger, than when only one telescope is used, though 
 the magnifying power is the same in both cases. The incon- 
 venience of managing the binocular telescope, seems, however, 
 to be the cause of its being disused. 
 
OPTICS. 497 
 
 Miscellaneous remarks relative to telescopes. 
 
 Valuable telescopes are usually furnished with what is called 
 Ajinder, which is nothing more than a short telescope, and is 
 generally affixed to the tube of the large telescope, as at G, fig. 
 3, pi. VII. It is used for the purpose of bringing an object ex- 
 peditiously into the field of the large telescope. It magnifies 
 but little, seldom more than six or eight times ; but it has a very 
 extensive field of view, so that any given object is easily com- 
 prehended within it. In the inside of its tube, and exactly at 
 the focus of the eye-glass, there are two slender wires, which 
 cross each other in the axis of the telescope. Now the finder 
 is so adjusted, that when an object seen through it appears to 
 be near the crossing of its wires, it is at the same time visible 
 through the great telescope, and by this means much time is 
 saved, especially to unpractised observers. 
 
 Both refracting and reflecting telescopes, of the best sort, 
 are furnished with two or more different eye-pieces, in order 
 that a greater or less power may be employed, as the occasion 
 appears to require. 
 
 In every telescope, the distance between the eye and the 
 object-glass must be alterable, in order to suit different eyes, 
 or render objects, at different distances, distinctly visible to 
 the same eye. 
 
 The following observations on another adjustment, of which 
 telescopes ought to be susceptible, are from the first volume of 
 Nicholson's Journal, 4to series: "Every attentive observer 
 must have taken notice, that light is of as much consequence 
 to artificial vision as magnifying power. It may therefore 
 afford matter of surprise, that the most variable of all adjust- 
 ments of the eye, viz. that of aperture, should never be intro- 
 duced into our artificial combinations. Distant woods, and 
 other land objects, are invisible to a high magnifying power, 
 for want of light, when the same objects may be distinctly seen 
 with a lower. By means of an artificial iris, which an ingenious 
 artist will find little difficulty in contriving, this disadvantage 
 in telescopes might be obviated. Suppose a brass ring to sur- 
 round the object end of the telescope, and upon this let eight 
 or more triangular slips of brass be fixed, so as to revolve on 
 equidistant pins passing through each triangle near one of 
 its corners. If the triangles be slidden in upon each other, it 
 may readily be apprehended that they will close the aperture ; 
 and if they be all made to revolve or slide backwards alike, it 
 is clear that their edge will leave an octagonal aperture, greater 
 or less, according to circumstances. The equable motion of 
 all the triangles may be produced either by pinions and one 
 toothed wheel, or by what is called snail-work. Another kind 
 of iris, more compact, may be made by causing thin elastic 
 21.— Vol. I. 3 S 
 
498 OPTICS. 
 
 MJscfllaiieous remarks relative to telescopes. 
 
 slips of brass to slide along parallel to the tube, and be con- 
 ducted through a slit in a brass cap, which will lead them 
 across the aperture in a radial direction." 
 
 To stop the wandering rays which would tend to confuse 
 the image of a telescope, a plate of metal, which has a small 
 round hole in the middle of it, is placed in the tube containing 
 the eye-glasses, parallel with them. It is called the diaphragm, 
 and is generally fixed within the telescope, at the focal dis- 
 tance of the glass nearest the eye. By this means, tlie field 
 of view is lessened, but the advantage of greater distinctness 
 is obtained. 
 
 To ascertain the goodness of object-glasses, the best expe- 
 dient is, to place them successively in a tube, and examine 
 some distant object with different eye-glasses ; that glass which 
 represents objects the brightest and most distinct, and which 
 bears the greatest aperture with the sho.rtest eye-glass, without 
 colouring, is the best. 
 
 In double convex lenses, the most perfect is that whose radius 
 of curvature of the first surface, is to that of the second as one to 
 six ; its aberration being the least possible, viz. -jf of its thick- 
 ness : but if this glass is turned with its other side to the rays, the 
 aberration will be \*f, and therefore would be much worse. 
 The same ratio holds with respect to concave lenses. 
 
 The lenses of a good telescope should all be perfectly well 
 polished, and should be as thin as possible for their size and 
 curvature. When a lens is exactly circular, and yet is found 
 to be thicker on one side than another, it is a proof that the 
 axes of the two surfaces are not coincident, and it ought to be 
 rejected. 
 
 Of reflecting telescopes, the Gregorian is almost the only 
 one made for common use. This preference seems to be owing 
 to the neatness of its appearance, and to its being pointed 
 towards objects in the same manner as refracting telescopes, 
 rather than to any really intrinsic advantages which it has over 
 the Newtonian construction. In all the reflecting telescopes 
 •we have noticed, the diameter of the large mirror, on account 
 of the interposition of the small one, at the aperture, must 
 be greater than would otherwise be required, in order to en- 
 lighten, the field of view sufficiently ; and it may also be ob- 
 served, that as specula have probably never been ground to a 
 truly parabolic figure, the central part, the benefit of which is 
 lost, is the most valuable portion of the whole, because it 
 would reflect the light with the greatest regularity. To this 
 it may be added, that the difficulty of grinding a speculum 
 increases perhaps in so great a proportion as the square of the 
 diameter, and it is therefore of some importance to those who 
 
6PTICS. 499 
 
 - ^ii • I 
 
 Miscellaneous remarks relative to telescopes. 
 
 may undertake the construction of these instruments for them- 
 selves, to make the large speculum as small as possible, con- 
 sistently with a given power and quantity of light. We shall 
 therefore mention the form of a reflecting telescope, which we 
 recollect to have been shewn many years ago, by a self-taught 
 optician, who, so far as we can recollect, on the question being 
 put to him, declared himself to be the first who had adopted 
 it. It is represented by fig. 5, pi. VII, where the great mirror 
 is as usual at the bottom of the principal tube, but instead of 
 having its axis coincident with that of the tube, it is inclined, 
 so as to throw the image upwards, through an aperture for 
 that purpose, to n o, whence the rays diverge, and falling upon 
 the small speculum, are reflected by it Lo the eye, as in the 
 Gregorian telescope. Here we have supposed the small mirror 
 to be concave, but we are not certain whether it was so or not; 
 it is clear, the principle of it is preserved, if the whole diame- 
 ter of the mirror is employed to reflect light; and therefore it 
 may have the small mirror convex like Cassegrain's reflector, 
 or plane, like Newton's. We can speak with confidence of 
 the excellence of the instrument we allude to, and we gladly 
 bear this testimony to the merit of an obscure individual, who, 
 we have reason to believe, is now deceased. He was tenacious 
 of his optical knowledge, and reserved in speaking of it ; yet 
 he was far from an illiberal character. His mirrors were cast 
 by himself in his own house, and every other part of the work- 
 manship of his instruments, and the tools with which they 
 were made, were constructed by himself: in solitude and 
 silence, by day and by night, though surrounded by the chil- 
 ling difficulties of poverty, he seems to have pursued his 
 studies with an assiduity seldom paralleled ; and the know- 
 ledge which it cost him so much to acquire, he might feel 
 it difficult to part with to a casual acquaintance. Perhaps 
 the notice we have taken of him in this place, may be more 
 than is called for by any proof we have given of his abilities, 
 in the new arrangement of a telescope ; but the example 
 may be an encouragement to others, and ought not to be 
 lost. In another place, we may have an opportunity of dis- 
 closing his name, and of investigating his particular merits ; 
 here we will only add, that he had taken no scanty draught 
 of the " Pierian spring," and that his poetical eff'usions were 
 as elegant and impassioned, as the work of his hands was cor- 
 rect and effectual. 
 
 The two mirrors of a telescope should not have the same 
 curve. Maskelyne has observed, that when the two specula 
 are parabolas, they cause a very considerable aberration, which 
 is negative, that is to say, the focus of the extreme ray is 
 
500 OPTICS. 
 
 - ~ ■ -: • — ■ 
 
 Mucelluoi-oiis remarks reliUTe to telescopes. — Micrometers. 
 
 longer than those of the middle ones. If the large speculum 
 is a parabola, the small one ought to be an ellipse ; but when 
 the small speculum is spherical, which is generally the case in 
 practice, if concave, the figure of the large speculum ought to 
 be an hyperbola; if convex, the large speculum ought to be an 
 ellipse, to free the telescope from aberration. This will be easier 
 understood, by attending to the positions of the first and second 
 images ; when a curve is of such a form that lines drawn from 
 each image, and meeting in any point of the curve, make equal 
 angles with the tangent to the curve at that point, it is evident 
 that such curve will be free from aberration. 
 
 From a property of reflecting telescopes, that the apertures 
 of the two specula are to each other very nearly in the propor- 
 tion of their focal lengths, it follows that their aberrations will 
 be to each other in the same proportion ; and these aberra- 
 tions are in the same direction, if the two specula are both 
 concave; or in contrary directions, if one speculum is con- 
 cave, and the other convex. 
 
 In the Gregorian telescope, both specula being concave, the 
 aberration at the second image will be the sum of the aberra- 
 tions of the two mirrors ; but in Cassegrain's construction, one 
 mirror being concave, and the other convex, the aberration at 
 the second image will be the difference between their aberra- 
 tions. By assuming such proportions for the foci of the spe- 
 cula as are generally used in the reflecting telescope, which is 
 about as 1 to 4, the aberration in the Cassegrainian construc- 
 tion will be to that in the Gregorian as 3 to 5. Hence ap- 
 pears the service which might be rendered to artists by a 
 complete mathematical investigation of the curves adapted to 
 optical purposes. 
 
 Of Micrometers. 
 
 A micrometer is an instrument by which the apparent mag- 
 nitudes of objects viewed through microscopes or telescopes 
 are measured. 
 
 A great variety of micrometers have been contrived, some 
 of which are very complicated and expensive, others possess 
 great simplicity. The accuracy of their performance has not 
 always been proportionate to their cost. We shall notice only 
 one or two of the most simple micrometers, as our limits will 
 not permit us to enlarge much beyond what maj' prove in- 
 teresting to the generality of our readers. 
 
 Lewenhoeck's method of estimating the size of small objects 
 was by comparing them with grains of sauid, of which one 
 hundred placed in a line took up an inch. These grains he 
 laid on the same plate with his objects, and viewed them atthe 
 
OPTICS. 
 
 TLTIL 
 
 T^iblisliea. "by Henry Hster. Ca3to-n. London . 1322. 
 
OPTICS. 501 
 
 Dr. Hook's micrometer. — Cavallo's. 
 
 same time. Dr. Jurin's method was similar to this, for he 
 wrapped a piece of fine silver wire very closely round a pin, 
 and observed how many rings made an inch; and he used this 
 wire as Lewenhoeck used his sand. Dr. Hook used to look 
 upon the magnified object with one eye, while, at the same 
 time, he viewed other objects placed at the same distance, with 
 the other eye. In this manner he was able, by the help of a 
 ruler, divided into inches and small parts, and laid on the pe- 
 destal of the microscope, to cast the magnified appearance of 
 the object upon the ruler, and thus exactly to measure the dia- 
 meter which it appeared to have through the glass ; which 
 being compared with the diameter as it appeared to the naked 
 eye, easily shewed the degree in which it was magnified. A 
 little practice renders this method very easy. 
 
 Cavallo's micrometer, which he has described in the third 
 volume of his '* Elements of Natural and Experimental Phi- 
 losophy," is simple and valuable. It consists of a small semi- 
 transparent scale or slip of mother-of-pearl, about the twentieth 
 part of an inch broad, and of the thickness of common writing 
 paper. It is divided into a number of equal parts by means 
 of parallel lines, every fifth and tenth of which divisions is a 
 little longer than the rest. 
 
 This micrometer, or divided scale, is situated within the 
 tube, at the focus of the eye-lens of the telescope, where the 
 image of the object is formed, and with its divided edge pass- 
 ing through the centre of the field of view ; though this is not 
 absolutely necessary. It is immaterial whether the telescope 
 be a refractor or a reflector, provided the eye-lens be convex, 
 and not concave, as in the Galilean telescope. 
 
 The simplest way of fixing it, is to stick it upon the dia- 
 phragm, which generally stands within the tube, at the focal 
 distance of the eye-lens. 
 
 By looking through the telescope, the image of the object 
 and the micrometer will appear to coincide : hence the observer 
 may easily see how many divisions of the latter measure the 
 length or breadth of the former ; and knowing the value of the 
 divisions of the micrometer, he may easily d.etermine the angle 
 which is subtended by the object. 
 
 There are several methods of ascertaining the value of the 
 divisions of a micrometer in a given telescope. The following 
 is one of the easiest : Direct the telescope to the sun, and 
 observe how many divisions of the micrometer measure its 
 diameter exactly ; then take out of the Nautical Almanack the 
 diameter of the sun for the day in which the observation is 
 made ; divide it by the above-mentioned number of divisions, 
 and the c^uotient is the value of one division of the micrometer. 
 
502 ABSTRACT OF OPTICS. 
 
 Cavallu's micrometer. 
 
 Thus, suppose that 265 divisions of the micrometer measure 
 the diameter of the sun, and the Nautical Almanack gives for 
 the measure of the angle which is subtended by the same dia- 
 meter, 31 minutes, 22 seconds; or (by reducing the whole into 
 seconds) 1882 seconds. Divide 1882 by 26.5, and the quo- 
 tient, neglecting a small remainder, is 71 seconds, or 1 minute 
 11 seconds; which is the value of one division of the micro- 
 meter; and therefore the value of any greater number of divi- 
 sions is easily ascertained. 
 
 The mother-of-pearl micrometer may be applied to a micro- 
 scope, and it will thus serve to measure the lineal dimensions 
 of the object. The value of its divisions may be ascertained 
 by placing an object of a known dimension before the micro- 
 scope, and by observing how many divisions of the micrometer 
 measure its magnified image. For instance, place a piece of 
 paper which is exactly one-tenth of an inch long, before the 
 microscope, and if you find that 60 divisions of the microme- 
 ter measure its magnified image, you may conclude that each 
 division denotes an extension of the oOOdth part of an inch in 
 the object; for if 50 divisions measure one-tenth, 500 divisions 
 must measure the whole inch. 
 
 Mother-of-pearl was found, by Ca\ alio, after many trials, to 
 be a much more convenient substance than either glass, 
 ivory, horn, or wood, as it is a very steady substance, the 
 divisions are easily marked upon it, and when made as thin 
 as common writing paper, it has a very useful degree of trans- 
 parency. 
 
 Abstract of Optics. 
 
 1. The particles of light, which are inconceivably small, 
 proceed from luminous bodies in right lines. 
 
 2. Consequently the density of light is inversely as the 
 square of the distance from the luminous centre. 
 
 3. Light moves at the rate of nearly 200,000 miles in one 
 second of time. 
 
 4. Its impression on the retina is not instantaneous ; hence 
 though its particles may be separately projected, so as to be, 
 in their progress, at the rate of 1000 miles apart, its velocity 
 is sufficient to produce distinct vision. 
 
 5. Every ray of light carries with it the image of the point 
 from which it was emitted ; when, therefore, pencils of rays 
 
ABSTRACT OF OPTICS. 503 
 
 Light. — Lenses. — Mirrors. 
 
 from every point of an object are united in the same order in 
 which they were emitted, they form an image or representation 
 of that object, at the place where they are thus united. 
 
 6. All the rays of light, which enter another medium ob- 
 liquely, suffer refraction ; that is, they either move farther 
 from, or nearer to, the perpendicular, as the medium into 
 which they enter is rarer or denser than the other medium. 
 
 7. On the refrangibility of light depends the properties of 
 lenses. 
 
 8. Convex lenses collect the rays of light, and make them 
 converge to a centre or focus. 
 
 9. Concave lenses disperse the rays of light, their power of 
 refraction not being towards their centre, but towards their 
 circumference. 
 
 10. When light strikes upon a surface, it is reflected so 
 that the angle of reflection is equal to the angle of incidence; 
 on this the properties of mirrors depend. 
 
 11. Plane mirrors have no other effect than that of chang- 
 ing the direction of the incident rays. 
 
 12. Convex mirrors cause parallel rays to diverge. 
 
 13. Concave mirrors collect parallel rays, or cause them to 
 converge to a focus. 
 
 14. Mixed mirrors exhibit distorted images, because they 
 increase or lessen the divergence or convergence of the rays 
 in one or two directions only. 
 
 15. The solar beam is composed of rays possessed of dif- 
 ferent degrees of refrangibility, and these diff'erences of re- 
 frangibility, which are dependent on the size of their particles, 
 produce all the phenomena of colours. 
 
 16. The solar beam, or white light, contains rays of seven 
 diff"erent colours, viz. red, orange, yellow, green, blue, indigo, 
 and violet. These are called the primitive colours, because 
 they are immutable, except by intermixture. 
 
 17. It is inferred that red light is composed of particles 
 of the largest size, because it is found to be capable of 
 struggling through thick and resisting mediums, which stop 
 every other colour. 
 
 18. The size of the particles of other colours are in the 
 order of their enumeration, the violet being the smallest. 
 
 19. The rainbow is owing to the separation of the light 
 into its primitive colours, by the drops of falling rain, which 
 act like a prism. 
 
 20. The rays of light are inflected when they pass very near 
 a body, and deflected when they pass at a greater distance. 
 
 21. Those rays which deviate the least by refraction, deviate 
 the most by flection. 
 
504 ABSTRACT OF OPTICS. 
 
 \'isioD. — Mli:ro(copes. — Telescopes. 
 
 22. The images of all visible objects are depicted on the 
 retina, in an inverted position. 
 
 23. With two eyes, vision is not only more distinct, but 
 more accurate than with one. 
 
 24. A good eye can see most distinctly when the rays fall 
 parallel upon it, because the foci of such rays fall exactly on 
 the retina. 
 
 25. The best eye can hardly distinguish any object that 
 subtends an angle of less than half a minute. 
 
 26. The apparent magnitude of objects is dependent on the 
 angle under which they are seen, or on the size of their images 
 depicted on the retina. 
 
 27. The long-sighted require convex spectacles, the short- 
 sighted, concave ones. 
 
 28. Burning lenses must be convex, and burning mirrors 
 concave, as the effects of both these instruments are depen- 
 dent on the condensation of the incident light. 
 
 29. Microscopes are instruments for viewing small objects. 
 They appear to magnify objects, because they enable us to 
 see them with distinctness, nearer than the natural limits of 
 vision. 
 
 30. Refracting telescopes are formed by lenses only ; when 
 manufactured in the best manner, they are either furnished 
 with an achromatic object-glass, which corrects the defect 
 arising from the unequal refraction of the different rays, by a 
 combination of one or two convex lenses with a concave one 
 of a different sort of glass ; or though more rarely, they have 
 an aplanatic object-glass, which corrects the same defect by 
 the combination of a plano-convex and meniscus glass, with 
 a fluid between them that acts like a third lens. 
 
 31. Reflecting telescopes consist of lenses and at least of 
 one speculum. When there is more than one speculum, the 
 second is only about one-fourth of the size of the other, and 
 may be either convex, concave, or plane. 
 
 32. Reflecting telescopes admit of a much greater magnify 
 jog power in a given length, than refracting telescopes. 
 
 33. The binocular telescope consists of two telescopes so 
 combined, that both eyes may be employed in looking at the 
 same abject. 
 
ASTRONOMY. 505 
 
 Historical remarks. 
 
 ASTRONOMY. 
 
 Astronomy is the science which treats of the motions, eclipses, 
 magnitudes, periods, and o-dier phenomena of the heavenly 
 bodies. 
 
 The early history of astronomy, as of all other ancient 
 sciences, is enveloped in much obscmity. It may be presumed, 
 that some knowledge of it would be nearly coeval with the 
 human race, because it would excite attention, as well from 
 motives of curiosity, as from its connection with the common 
 concerns of life; but in what age or country the united obser- 
 vations of many, were first so far methodized, as to raise it to 
 the dignity of a science, is now beyond the reach of unques- 
 tionable discovery, and it would be useless to multiply con- 
 jectures on the subject. 
 
 Those delightful regions of Asia, which were the first abodes 
 of mankind, were peculiarly calculated to favour the growth 
 of astronomical knowledge. Accordingly, it appears that as- 
 tronomy was much cultivated among the Chaldeans. The level 
 and extensive plains of that country, the nights which they 
 passed in the open air, an unbroken horizon, a pure and serene 
 sky, all conspired to engage that people to contemplate the- 
 motions of the stars, and to lead them to conjecture on the laws 
 by which they were governed. 
 
 From Chaldea, astronomy passed into Egypt, and was soon 
 afterwards carried into Phoenicia, where the people began to 
 apply the observations which had been made to the uses of 
 navigation, and thus rendered themselves the masters of the 
 sea and of commerce. Their guide in steering their ships, when 
 far from land, was one of the stars in the constellation called 
 the Little Bear, which, unlike other stars, appeared always to 
 retain the same situation. Other nations, less skilful in astro- 
 nomy, observed only the Great Bear in their voyages, a guide 
 too imperfect to enable them to lose sight of land with safety. 
 
 Thales, the IMilesian, who flourished about 700 years before 
 the Christian era, brought the science of the stars from Phoeni- 
 cia into Greece, where he taught the utility of the constellation 
 of the Little Bear in navigation. He also taught the theory 
 of the motion of the sun and moon, by which he accounted for 
 the length and shortness of the days, determined the number 
 of the days of the solar year, and not only explained the cause 
 of eclipses, but the art of predicting them, which he even 
 reduced to practice, foretelling an eclipse which took place 
 soon after, and was rendered memorable by its happening on 
 a day of battle between the Medes and Lydians. 
 22.— Vol. I. 3 T 
 
506 ASTROXOMY. 
 
 Historical remarks. 
 
 To Anaximander, one of the disciples ot'Thales, is ascribed 
 the invention of the terrestrial globe, and of a gnomon which 
 he erected at Sparta, by means of which he observed the equi- 
 noxes and solstices, and determined the obliquity of the 
 ecliptic more exactly than had ever been done before. The 
 Greeks, assisted by the instructions they had received from 
 Thales and Anaximander, ventured to make considerable voy- 
 ages, and planted several colonies in remote countries; yet 
 the latter philosopher and his children were proscribed by the 
 Athenians, and their lives would have been sacrificed but for 
 Pericles, through whose influence the sentence was commuted 
 for banishment. The charge against him was the discovery 
 of truth; for it was thought impious to suppose the works of 
 the gods subject to immutable laws! 
 
 Pythagoras, another of the disciples of Thales, taught 
 many sublime astronomical truths. To him is attributed the 
 discovery of the true system of the world, which, after the 
 lapse of many centuries, was revived bv Copernicus, and which 
 is now settled on the basis of so many proofs, that it can never 
 be overthrown. It was even thought, in his school, that the 
 planets were inhabited like the earth, and that the stars which 
 are disseminated through infinite space, are suns, and the 
 centres of other planetary systems. He is also said to have 
 considered the comets as permanent bodies, moving round 
 the sun; and not as perishing meteors, tormed in the atmo- 
 sphere, as they were thought to be in after times. Pythagoras 
 died about the year 497 before Christ. 
 
 Pythias was the first who taught the method of distinguish- 
 ing climates by the length of the days and nights, and about 
 his time, a remarkable emulation to excel in astronomy 
 prevailed among the Greeks. Eudoxus, a disciple of Plato, 
 not satisfied with what he could learn at Athens, repaired 
 to Egypt, to cultivate astronomy at its source. At his 
 return he compiled several books on astronomy, and among 
 others, a description of the constellations. Eudoxus attempted 
 to explain the celebrated cycle of nineteen years, which 
 had been imagined by ^Meton, in order to conciliate the 
 solar and lunar motions. This is the most accurate period, 
 for a short interval of time, that could have been devised, 
 for embracing an exact number of revolutions of the sun 
 and moon; and is so simple and useful, that when Meton 
 proposed it to the Greeks, assembled at the Olynipic games, 
 as the basis of their calendar, it was received with great 
 approbation, and unanimously adopted by all their colonies. 
 The year of the cycle is called the golden number, and is still 
 placed in tlie calendar. 
 
ASTRONOMY. 507 
 
 Historical remarks. 
 
 Aristotle, the disciple of Plato, and the contemporary of 
 Eudoxus, made use of astronomy for improving physics and 
 geography. He attempted to determine, by astronomical 
 observations, both the figure and the magnitude of the earth. 
 He demonstrated that it was of a spherical form, by the cir- 
 cular appearance of its shadow on the disc of the moon in 
 eclipses ; and from the inequality of the meridian altitudes of 
 the sun, which are difl'erent in different latitudes. Callisthenes, 
 who attended Alexander the Great, having been sent to Baby- 
 lon, found there astronomical observations made by the Baby- 
 lonians during the space of 1903 years, and sent them to 
 Aristotle. 
 
 But oi" all the schools of antiquity in which astronomy was 
 taught, that of Alexandria has justly attained the highest 
 celebrity. Here we have the first record of a combined series 
 of observations, made with instruments proper for measuring 
 angles, and calculated trigonometrically. Here the position 
 of the stars began to be minutely determined; the course of 
 the planets to be traced with care ; and the inequalities of the 
 solar and lunar motions to be better known. The theory 
 adopted, aimed at the explanation of all the celestial motions ; 
 and though inferior to that of Pythagoras, yet the numerous 
 observations which were made, furnished the means of detect- 
 ing its fallacy, and of enabling succeeding astronomers to 
 discover the true system of nature. — Hipparchus of Bithynia, 
 who flourished at Alexandria about the year 162 before Christ, 
 is particularly famous for the excellence of his observations; 
 and he determined the length of the tropical year with a pre- 
 cision never attained before, his result not varying more than 
 41 minutes from the truth. 
 
 Ptolemy, an Egyptian, who has always been considered the 
 prince of astronomers among the ancients, flourished in the 
 second century of the Christian era. He has preserved and 
 transmitted to us the observations and principal discoveries 
 of the ancients, much enriched and enlarged by his own 
 labours, in a treatise called " The Great Syntaxis," in which 
 he gave the theory and tables of the motion of the sun and 
 moon, the planets and the fixed stars. He adopted the 
 most ancient system, which supposed the earth to be in 
 the centre of the universe, and this system, to distinguish 
 it from others, has been called the Ptolemaic System. 
 The defects of his system did not, however, prevent him 
 from calculating all the eclipses that were to happen for 600 
 years to come. 
 
 About the year 826, Ptolemy's great work was translated by 
 the Arabians into their language, in which it was called the 
 
50S ASTRONOMY. 
 
 Historical remarks. 
 
 Alma"-est. About the year 1230, it was translated from the 
 Arabic into Latin, under the auspices of the emperor Frederic 
 II. who was willing that the Christians should understand 
 astronomy as well as those whom they styled Barbarians. 
 Alphonso', king of Castile, went further, for he assembled the 
 most able astronomers from all parts, who composed new 
 tables, called after him the Alphonsine tables. 
 
 By these means the attention of the learned men in Europe 
 became directed to a science, the knowledge of which pro- 
 mised so much gratification, utility, and fame. Calculations 
 and treatises became very frequent, as well as the invention 
 of instruments to facilitate observations. The most memorable 
 event of this period was the revival of the ancient or Pytha- 
 gorean system of the world, which had been set aside ever 
 since the time of Ptolemy. This was done by Nicholas 
 Copernicus, a native of Thorn, in Prussia, born in 1472. The 
 Ptolemaic system, which supjjoses the earth to be fixed in the 
 centre of the universe, and the Sun and INIoon, with ^lercury, 
 Venus, and the other planets, to revolve about it in concentric 
 circles, he perceived to be inconsistent with the phenomena, 
 and encumbered with many absurdities which did not afi'ect 
 the hypothesis which considered the Sun to be in the centre, 
 and the Earth a planet revolving annually with the rest about 
 the Sun, and daily on its own axis. He established his 
 theory by such incontrovertible reasoning, that it gradually 
 prevailed from that time, and continuing to spread with the 
 lapse of ages, it is now the only one received in Europe, and 
 wherever true science is known. But Copernicus had not 
 himself the satisfaction of seeing the triumph of the theory 
 he defended: threatened with persecution by the religious 
 bigots on one side, and with a perverse opposition from those 
 who called themselves philosophers on the other, he could not 
 be prevailed upon to publish the work which contained the 
 result of his observations, till long after it had been finished. 
 His consent to the publication of it was wrung from him by 
 the importunities of his friends, and a copy was brought to 
 him only a few hours before his death, which happened in his 
 seventy-first year. In this age, or rather in this country, we 
 reo-ard with astonishment the deo-rading thraldom to which the 
 human mind was in these times subjected. 
 
 The only opposition of any consequence which the theory 
 of Copernicus ever met with from science and argument, pro- 
 ceeded from Tycho Brahe, a celebrated Danish astronomer, 
 who attempted to set up against it a theory of his own. — 
 His system is not very different from the Ptolemaic, but is 
 generally called by his name. He supposed the earth to be 
 
ASTROXOMY. 509 
 
 Historical remarks. 
 
 immoveable in the centre of the universe, and the Sun to 
 revolve about it every twenty-four hours: the planets he 
 thought went round the Sun in their periodical times. Mercury 
 being nearest to the Sun, then Venus, Mars, Jupiter, and 
 Saturn, and of course to revolve also about the earth. But some 
 of Brahe's disciples supposed the earth to have a diurnal mo- 
 tion round its axis, and the sun, with all the planets, to move 
 round the earth in one year. The inconsistencies with which 
 this hypothesis abounded, will be most obvious when we have 
 explained the Copernican or true system. In defence of 
 Brahe's just fame, it is proper to add, that though he adopted 
 an erroneous theory, he was actuated by pious motives, and 
 that he rendered very important services to astronomy, by the 
 correctness and number of his observations. 
 
 Kepler was one of the pupils of Tycho Brahe, and a man of 
 a truly original and admirable genius. Hipparchus, Ptolemy, 
 Tycho Brahe, and even Copernicus himself, were indebted for 
 a great part of their knowledge to the Egyptians, Chaldeans, 
 and Indians; pursuing paths already pointed out, they did 
 little more than separate fancy from fact, with more or less 
 success; but Kepler, by his own talents and industry, has 
 made discoveries of which no traces are to be found in the 
 annals of antiquity. Galileo was contemporary with Kepler, 
 and while the latter was tracing the orbits of the planets, and 
 settling the laws of their motions, he was investio-atinp- the 
 doctrine of motion in general, which had been neglected for 
 2000 years; and from the results of their united labours, New- 
 ton and Huygens were afterwards enabled to establish the most 
 complete theories of all the planetary motions. Though Galileo 
 proved, in the most incontestable manner, the annual and 
 diurnal motion of the earth, his doctrine was declared heretical 
 by a congregation of cardinals assembled for the purpose, and 
 though not only venerable for his years, but also one of the most 
 virtuous and enlightened men of his age, he was condemned to 
 perpetual imprisonment, for believing and promulgating truths 
 which accorded with the order of nature, and which he therefore 
 believed to be written with the finger of God. Through the soli- 
 citations of his great patron, the Grand Duke of Tuscany, he 
 partially regained his liberty at the end of a year, and in 1642 
 he died, regretted by the learned and liberal of all Europe. 
 
 From the time of Newton, who carried the theoretical part 
 of the science to perfection, astronomy has never been without 
 an illustrious phalanx of supporters, whose particular merits 
 and discoveries it would be impossible to mention in this con- 
 cise historical abstract. We shall now therefore proceed with 
 the elementary details of the science. 
 
510 ASTRONOMY. 
 
 Detluitioiis 
 
 Definitions and preliminary Explanations. 
 
 All the visible bodies which are dispersed throughout the 
 u'hole heavens, receive in ordinary discourse the appellation of 
 stars; but they are divided by astronomers into several classes, 
 according to certain differences which distinguish them. 
 
 Thejixed stars, or simple stars, are so called because they 
 always appear to be at the same distance from each other. 
 The sun is a fixed star, its apparent motion being occasioned 
 only by that of the earth, and all the other fixed stars are 
 supposed to be suns, which to us appear small, on account of 
 the immensity of their distances from us. Another property 
 supposed to be peculiar to the fixed stars, is, that they shine 
 by their own light. 
 
 The term planet is derived from a Greek word signifying 
 wanderer, and is given to those stars which are continually 
 changing their position with respect to each other, as well as 
 with respect to the fixed stars. 
 
 Dr. Herschel more particularly defines planets to be celestial 
 bodies of a considerable size and small eccentricity of orbit, 
 moving in planes that do not deviate many degrees from that 
 of the earth, in a direct course, and in orbits at considerable 
 distances from each other, with atmospheres of considerable 
 extent, but bearing hardly any sensible proportion to their 
 diameters, and having satellites or rings. 
 
 The planets are further distinguished into primary and 
 secondary. The primary ones, called, by way of eminence, 
 planets, are those which revolve round the sun as a centre : antl 
 the secondary planets, more usually called satellites or moons, 
 are those which revolve round a primary planet as a centre, 
 and constantly attend it in its revolution round the sun. 
 
 The primary planets are again distinguished into superior 
 and inferior. The superior planets are those farther from the 
 sun than our Earth, as INIars, Jupiter, Saturn, and the Herschel : 
 and the inferior planets are those nearer the sun than our 
 Earth, as Venus and jNIercury. 
 
 The newly discovered planets, Ceres Ferdinandea, Juno, 
 Pallas, and Vesta, and all such as may hereafter be discovered, 
 and found to resemble them, Dr. Herschel proposes to call by 
 the name of asteroids, which he defines as celestial bodies, 
 moving in orbits either of little or of considerable eccentricity 
 round the sun, the plane of which may be inclined to the 
 ecliptic in any angle whatsoever. This motion may be direct 
 or retrograde; and they may or may not have considerable 
 atmospheres, very small comas, discs, or nuclei. 
 
 Comets are celestial bodies moving in directions wholly 
 
ASTROXOMV. oil 
 
 Dednitions. 
 
 undetermined, in very eccentric orbits, situated in every variety 
 of position, and having very extensive atmospheres. 
 
 Planets, satellites, asteroids, and comets, all shine by 
 reflected light. 
 
 The planets, for the convenience of expressing them on 
 globes, or in tables, &:c. are often denoted by peculiar 
 characters, as follows: Mercury § ; Venus $; the Earth 0; 
 Mars $ ; Jupiter %; Saturn T? ; and the Georgium Sidus, or 
 Georgium Planet, which the foreign astronomers usually call 
 Uranus, or Herschel ^. The four lately discovered planets, 
 or asteroids, have not yet any peculiar characters assigned to 
 them. 
 
 The orbit of a planet or comet, is its path, or the curve it 
 describes in its revolution round its central body. The orbits 
 of all the planets are elliptical, yet they deviate but very little 
 from circles; while the orbits of comets deviate widely from 
 circles, and are therefore said to have great eccentricity. 
 
 The direct motion of a planet, is when it appears to move 
 from west to east. Its retrograde motion, is when it appears to 
 move the contrary way, that is, from east to west. When a 
 planet seems to remain a certain time in the same place, it is 
 said to be stationary. 
 
 The ecliptic is that path or way which the earth appears to 
 describe among the fixed stars, to an eye placed in the sun ; 
 or, which amounts to the same thing, it is the path which the 
 sun appears to describe among the stars, to an eye on the earth. 
 
 The zodiac is a zone extending eight de2:rees on each side of 
 the ecliptic all round the heavens. It is divided into 12 equal 
 parts, called signs ; and as every circle, whether great or small, 
 is supposed to be divided into 360 degrees, each sign of the 
 zodiac contains 30 degrees. 
 
 Each sign of the zodiac is distinguished by a particular name, 
 and each name is denoted by a particular symbol, which is some- 
 times used alone. These names and symbols are as follows : 
 Aries t; Taurus t^ ; Gemini n; Cancers; Leo il; Virgo nc; 
 Libra :^; Scorpio nv; Sagittarius^; Capricornusc'; Aquarius 
 m, and PiscesK. The signs of the zodiac are situated in the 
 order in which they are here enumerated, reckoning from west 
 to east, which is called the order of the signs. 
 
 The fixed stars are divided into groups, called constellations, 
 which comprise a number of stars that appear to be near each 
 other. The ancients distinguished the constellations by the 
 names of men, birds, fishes, &.c. such a name being selected, 
 that the stars of the constellation it is given to, could be con- 
 tained within a figure of the original. Then, in order to specify 
 any particular star, they spoke, for instance, of the star on the 
 
512 ASmONOMY. 
 
 Dflinitious. 
 
 shoulder of Orion, or on the tail of the Fish, &.c. The assist- 
 ance these imaginary figures afford the memory, has induced 
 the moderns to continue the use of them, with some improve- 
 ments. They use these constellations to indicate the general 
 assemblage of stars in a certain portion of the heavens; but 
 they distinguish each particular star by a Greek letter or by 
 figures, as, 1, 2, 3, &.c. and mark its true place by mentioning 
 its distances from particular points. 
 
 The horizon is either sensible or rational: the sensible horizon 
 is that circle which limits our view; the rational or true 
 horizon is parallel to the former, and is a circle in the heavens 
 supposed to be formed by the continuation of a plane which 
 parses through the centre of the earth. The planes of these 
 two horizons, are separated by the earth's semi-diameter, but 
 with respect to the heavens, they may be considered as coin- 
 cident, the distance of the fixed stars being so prodigiously 
 great, that the error of considering the earth's semi-diameter 
 as a point, is insignificant. 
 
 The whole concave orb or expanse which invests our globe, 
 and in which the heavenly bodies are seen, is called the sphere. 
 
 The sphere appears to turn round two points which are oppo- 
 site each other, and are called the poles cfthe icorld, one of them 
 being called the arctic, or north pole; and the other the antarctic, 
 or south pole. The axis itself, or that imaginary line which 
 joins the two poles, is called the axis of the world. 
 
 The highest point of the heavens, or that directly over our 
 heads, is called the zenith. 
 
 The opposite, or lowest point of the heavens, which is 
 directly under our feet, is called the nadir. 
 
 The Jiodes are the two points in which the orbit of a planet in- 
 tersects the ecliptic. That node from which the planet ascends 
 northward, above the plane of the ecliptic, is called the ascend- 
 ing node: the other node, from which the planet descends 
 southward, is called the descending node. A line passing from 
 one node to the other, is called the line of nodes. 
 
 The orbit of each planet being an ellipse, in one focus* of 
 
 * The nalnre of the foci of an ellipse is well shewn by the common mechanical method 
 of describing that figure : two pins or nails are driven far enough into a board to bear 
 a slight pressure ; a cord with its two ends fastened together, so as to form what is, in 
 the arts, called an endless cord, is then put over them; and a pencil or any marking in- 
 strument is employed to stretch one point of the cord parallel to the board, and at the 
 same time to mark out its own course, while the hand carries it round; the Hue pro- 
 duced falls into itself, and forms an ellipse, of which the nails occupy the places called 
 the foci. Those who have once tried the experiment, immediately perceive, that the 
 longer the endless cord, while the nails retain the same distance, the nearer the ellipse 
 approaches to a circle; and on the contrary, the shorter it is, the nearer the foci are to 
 the sides. The distance between either of the foci and the centre of an ellipse, is 
 called its eccentricity. 
 
ASTROXQMY. 513 
 
 Apparent motions of the heavenly bodies. 
 
 which the sun is situated, it is obvious that the distances of the 
 planets from the sun is different at different times. The points 
 of the greatest and least distance, when spoken of indifferently, 
 are called the apsides of the planet; but when contradistinction 
 is necessary, the point of greatest distance is called the higher 
 apsis, or aphelion ; the point of least distance is called the lower 
 apsis, or perihelion. The line connecting these two points, is 
 called the line of the apsides, and is supposed to pass through 
 the centre of the sun. 
 
 When the sun or moon is at its least distance from the earth, 
 it is said to be in perigee. — When either is at its greatest dis- 
 tance from the earth, it is said to be in apogee. 
 
 To those who live exactly under the equator, the poles of the 
 world appear in the horizon ; hence that situation is called the 
 right position of the sphere. 
 
 To those who have either of the poles of the world in their 
 zenith, the horizon coincides with, or is parallel to, the equator ; 
 hence that situation is called a parallel sphere. 
 
 To all the other inhabitants of the earth, the sphere is said to 
 be in vox oblique position, because the equator is neither perpen- 
 dicular nor parallel, but oblique to the horizon. 
 
 These definitions and explanations are brought together, for 
 the more easy reference of the reader ; but a variety of others, 
 with more enlarged explanations of some already enumerated, 
 will be necessary as we proceed. 
 
 Of the apparent Motions of the heavenly Bodies. 
 
 As the true motions of the heavenly bodies can only be ob- 
 tained by a careful observation of their apparent ones, it is 
 absolutely necessary, for those who wish to become acquainted 
 with them, to know perfectly the different changes which take 
 place in the heavens as seen from the earth, the only place from 
 which man can make any observation. By carefully attending 
 to these, a little knowledge of optics will enable us to under- 
 stand with great certainty, not only the true system of nature, 
 })ut also what appearance the heavens would make to a spec- 
 tator placed in any part of the visible creation. 
 
 When we cast our eyes towards the heavens, we perceive a 
 vast concave hemisphere, at an unknown distance, and of which 
 the eye seems to constitute the centre. The earth, or our sen- 
 sible horizon, stretching on every side like an immense plain, 
 appeal's to meet and to bound the heavenly expanse. The sun 
 we observe to rise in the east, and to set in the west; after 
 which the moon and stars appear, still keeping the same wes- 
 terly course, till we lose sight of them altogether. We further 
 discover, upon attentive and repeated observation, that neither 
 
 22.— Vol I, 3 U 
 
514 ASTRONOMY. 
 
 Apparent motioDS of the hcavenlj bodies. 
 
 the sun nor the moon always rise exactly in the same point of 
 the heavens. If we begin to observe the sun, for instance, in 
 the beginning of jNIarch, we find that he seems to rise almost 
 every dav more to the northward than he did the day before, to 
 continue longer above the horizon, and to be nearer the zenith 
 at mid-day. This continues nearly to the end of June, when he 
 is observed to move backward in the same manner ; and his 
 retrograde motion continues till nearly the end of December, 
 after which he gradually attains the point from which he set 
 CHit, and acain renews the phenomenon. 
 
 The motion of the moon through the heavens, as well as her 
 appearance at different times, are still more remarkable than 
 those of the sun. When she first becomes visible, at the time 
 she is called the new moon, she is in the western parts of the 
 heaveni!, and appears to be at no great distance from the sun 
 himself. Every night she not only appears larger but at a 
 greater distance from the sun, till at last she rises from the 
 eastern part of the horizon, just at the time the sun disappears 
 in the western, and presents, at the same time, a completely 
 circular face. She then gradually moves farther eastward, 
 diminishing in size, and rising later and later every night, till 
 at last she seems to approach the sun as nearly in the east as 
 she did in the west, and rises only a little before him in the 
 morning, as in the first part of her course she set in the west not 
 long after him. All these different appearances are completed 
 in the space of a month, after which they begin in the same 
 order as before. They are not, however, at all times regular; 
 for at some seasons of the year, particularly in harvest, the 
 moon appears for several days to recede no farther from the sun, 
 and to rise for several nights nearly at the same hour. 
 
 In contemplating the stars, on a clear evening, we observe 
 continual chancres. New stars keep rising in the east, while 
 others are setting in the west. Looking towards the south, some 
 stars just appear on the horizon, skim its boundary for a little 
 while without rising above it, and then vanish; others, a little 
 farther from the south, rise above the horizon, make a small arc, 
 and then go down; while some again describe a large arc, and 
 take a long time before thev set. If we look toward the north, 
 we find that some just skim the horizon, then mount to the 
 middle of the heavens; then again descend and remount, with- 
 out ever disappearingr. Others describe complete circles, 
 without descending to the horizon, and these circles diminish, 
 till at last we arrive at a star that the eye cannot perceive to 
 have any motion, but which seems to be in that particular point 
 round which the whole hemisphere appears to turn. This star, 
 is called the pole-star. 
 
ASTRONOMY. 615 
 
 Apparent motions of the heavenly bodies. 
 
 Upon considering these phenomena, it is easy to conceive, 
 that as there is a hemisphere above us, there is also its coun- 
 terpart below us, though invisible ; and that, of course, the 
 earth, with all its inhabitants, is suspended in the middle of 
 this heavenly sphere, which the horizon divides into two parts. 
 Consonantly with this idea, we find, that the remarkable star 
 called the pole-star is more or less elevated, according to the 
 different parts of the earth from which we take our view. The 
 inhabitants of Lapland, for instance, see it much nearer the 
 zenith that we do; we see it nearer the zenith than the inhabit- 
 ants of France and Spain ; and they again see it nearer the 
 zenith than the inhabitants of Barbary. By continually travel- 
 ling south, this star would be seen nearer and nearer to the 
 horizon, and would at length become invisible. Another point 
 would then appear in the south part of the horizon, round which 
 the stars in that quarter would seem to turn. In the southern 
 hemisphere, however, where we should now be arrived, there 
 is no star so near the pole as in the northern, nor is the num- 
 ber of stars so great. 
 
 Hence we know that if the opacity of the earth did not limit 
 our view, the general appearance of the heavens would be that 
 of a vast concave sphere, turning round two points fixed in the 
 north and south parts of it once in twenty-four hours. 
 
 On further observation of the stairs, we find the greatest 
 number of them to keep their places with respect to one another, 
 that is, if we observe two stars having a certain apparent dis- 
 tance from each other this night, we find that they have the 
 same to-morrow, and every succeeding night; but we by no 
 means observe them to have the same places with respect to the 
 sun and moon. The stars that do not appear to be of this fixed 
 kind, are very few in number ; the naked eye can see but seven, 
 and astronomers, with the use of the best telescopes, have not 
 added more than four to that number. These eleven stars are 
 the planets and asteroids defined in the preceding section, and 
 they change their places very remarkably with regard to the 
 stars, and with regard to each other. Sometimes th^y seem to 
 be moving to the westward, sometimes to the eastward, and 
 sometimes they appear stationary for a considerable time. 
 There are other bodies, which appear only occasionally, move 
 for some time with prodigious velocity, and then recede from 
 us so far, that they cease to be visible. These are comets, of 
 which the number is unknown. 
 
 To obtain correct views of the motions of the heavenly 
 bodies, it is necessary to possess the means of indicating with 
 precision, the places they occupy. This is done by several 
 imaginary lines or circles supposed to be described upon th« 
 
516 ASTRONOMY. 
 
 Artificial divisions of the sphere. 
 
 surface of the sphere. These circles are divided into degrees, 
 minutes, and seconds : a degree, it has been already mentioned, 
 is the 36Uth part of the circumference of a circle ; a minute 
 is the 60th part of a degree; and a second the 60th part of a 
 minute. When these divisions are not expressed by words, de- 
 grees are marked by a small circle o placed near the upper part 
 of the figures; minutes by a small line'in the same situation, 
 and seconds bvtwo lines" of the like sort. This being premised, 
 ■\ve shall first illustrate by a diagram, the position of the axis of 
 the world, to which the imaginary circles of the sphere have a 
 reference. Let HO, fig. 1, plate II, represent the circle of the 
 horizon, seen edgeways, in which case it will appear as a straight 
 line: letHMON represent the complete sphere of the heavens, of 
 ■which, as only half of it can be seen at once, HMO may be 
 considered the visible hemisphere, and HNO the invisible 
 hemisphere : then P will be the pole or fixed point among the 
 stars, visible to us, round which the sphere will appear to 
 turn, and as the opposite point will be at R, the line PR will 
 be the axis of the sphere. 
 
 If through the centre of the earth c, there be drawn a line 
 QE, it will represent the edge of a great circle, which will be 
 a quarter of a circle, or 90 deorees, distant from each pole. It 
 is called the equator, because it divides the heavens into two 
 equal parts, the north and south. 
 
 The equator having reference to the pole, is always the same, 
 but this is not the case with the horizon, the position of which 
 a spectator chanses at every remove, consequently also the 
 hemisphere he beholds, and the position of his zenith and 
 nadir; these two points beino- always 90 degrees distant from 
 his horizon. If HO be the horizon, >I will be the zenith, and 
 the opposite point N the nadir. 
 
 The horizon of a spectator can never be coincident with the 
 equator, nor his zenith and nadir with the axis of the sphere, 
 unless he be situated exactly at the pole. The zenith and 
 nadir are often called ih^ poles of the horizon ; the young stu- 
 dent must therefore be careful to distinguish these poles, w hich 
 vary with the place of the spectator, from those of the world, 
 which are constantly the same. 
 
 All circles drawn through the zenith and nadir, are perpen- 
 dicular to the horizon, and are therefore called vertical cinlet, 
 or azimuths. 
 
 Two of these vertical circles or azimuths, are particularly 
 important, oneof themis called the meridian, and passes through 
 the poles, through the zenith and nadir, and cuts the equator 
 perpendicularly. As this circle passes through the poles, it is 
 obvious that if a spectator travels exactly north or south, he 
 
ASTRONOMY. 517 
 
 Artificial divisions of the sphere. 
 
 may go round the globe without changing his meridian ; but if 
 he travel east or west, he changes it every instant. The number 
 of meridians may therefore be supposed to be infinite, but a very 
 small number, seldom more than thirty-six, are drawn upon maps 
 and globes. At mid-day, when the sun has attained his greatest 
 elevation, his centre is precisely on the meridian of the place 
 where he is observed, and the moment he has passed the meri- 
 dian he declines towards the west. The meridian divides the cir- 
 cles described by the stars into two equal parts, and those stars 
 which never sink below the horizon, maybe observed to cross 
 it twice in twenty-four hours. All the rest of the stars, as well 
 as the sun, do the same, but when they pass the meridian 
 below our horizon, they are of course invisible to us. 
 
 The second remarkable azimuth is called the prime vertical. 
 It divides the eastern and western sides of the horizon into two 
 equal parts, and the points of intersection are called the true 
 east and west points; so that the meridian and prime vertical 
 divide the horizon into four equal parts, and the points of divi- 
 sion, viz. the north, the east, the south, a.nd the west, are called 
 the principal points of the horizon, ov cardinal points. 
 
 These three great circles, viz. the equator, the meridian, and 
 prime vertical, form the basis of reference in observations on 
 the heavenly bodies, and it is therefore necessary to determine 
 their relative situations. If the pole-star had been accurately 
 at the pole of the heavens, nothing more would be necessary, 
 in order to obtain the altitude of the pole, but to take the 
 altitude of this star ; but as it is at the distance of 2 degrees 
 from the pole, 2 degrees must be added to this altitude, to find 
 that of the pole. The elevation of the pole being known, it is 
 easy to find that of the equator. Thus, in fig. 1, pi II, HMO, 
 or the visible part of the heavens, contains 180 degrees ; and 
 it is ninety degrees from the pole P, to E the equator. If, 
 therefore, we take away PE from the semi-circle HMO, there 
 remains 90 degrees for the other two arcs, PH, and EO, that 
 is, the elevation of the pole and the equator are together equal 
 to 90 degrees ; so that the one being known, and subtracted 
 from 90, it will give the other. Hence the elevation of the 
 pole, at any place, is equal to what the elevation of the equa- 
 tor wants of 90 degrees : and the elevation of the equator is 
 equal to the distance from the pole P to the zenith M. 
 
 We cannot, under ordinary circumstances, observe the sun's 
 motion among the fixed stars, because his splendour prevents 
 their being seen ; but we can observe the instant of his coming 
 to the meridian, and his meridional altitude. We can also com- 
 pute what point of the starry heavens comes to the same meri- 
 dian at the same time, and with the same altitude. Or we can 
 
518 ASTRONOMY. 
 
 Artificial diriiions of the sphere. 
 
 observe that point in the heavens, which comes to the meridian 
 at midnight, with a declination as far from the equator at one 
 side, as the sun's is on the other side; and it is evident that the 
 sun must be in that part of the heavens which is diametrically 
 opposite to this point. By either of these methods, we obtain a 
 series of points in the heavens, through which the sun passes in a 
 vear, and a line supposed to be drawn through them all, forms 
 that circle called the ecliptic. This appellation is derived from 
 the circumstance, that all the eclipses of the sun and moon 
 are performed either actually in or very near it. 
 
 The ecliptic or annual path of the sun coincides with the 
 equator only in two points ; for the sun rises above the equa- 
 tor in summer, and does not rise so high in winter. Those two 
 poinis of the ecliptic, where the sun is situated, when he is 
 most distant from the equator, are called the sodtitial points, 
 and the points of intersection of the ecliptic and equator, are 
 called the erjuinoctial points. 
 
 The equinoctial colure is the great circle which passes at right 
 angles to the equator, through the equinoctial points. The 
 solstitial coliire, is another great circle cutting the equator at 
 riorht angles, but passing through the solstitial points, and 
 through the poles of the ecliptic. 
 
 The angle which the plane of the ecliptic makes with that of 
 the equator, is called the obliquitif of the ecliptic. The obliquity 
 of the ecliptic is equal to the elevation of the sun above the 
 equator when in either solstice. It amounts nearly to 23 A". 
 
 The smaller circles of the sphere are all those which have their 
 centre in the axis, but not in the centre of the sphere; two of 
 them cut the solstitial points, and their planes are at right 
 angles to the axis of the world, as AC, BD, fig. 1, pi. II ; these 
 are called /ro/)/r5, of which that on the south side of the equator 
 is called the tropic of Capricorn, and that on the north side of 
 the equator, the tropic of Cancer. The two polar circles, FG, 
 IK, are at the same distance from the two poles that the tro- 
 pics are from the equator, namely 285°. 
 
 The distanceof theheavenlybodiesfrom the equator is called 
 their declination, which is used with the addition of Xorth or 
 iSouth, to denote on which side of the equator they are. The 
 sun's declination can never exceed the obliquity of the eclip- 
 tic ; but starshave all degrees of declination, because they have 
 all degrees of altitude. Great circles, drawn through the poles of 
 the equator, are called circles of declination, or meridians, because 
 uponthemdeclinationismeasured. Twenty-fourof these circles 
 of declination, are called hour-circles; because each contains 
 15 degrees, which space the sun passes over every hour. 
 
 The right ascension of the heavenly bodies is their distance 
 
ASTROXOMV. 519 
 
 Aitificial divisions of the spliere. 
 
 from the first point of Aries nn, estimated in time on the equa- 
 tor, where cut by a declination circle passing through the body, 
 reckoning, as above stated, 15 degrees for every hour. 
 
 The oblique ascension of a celestial body is an arc of the equa- 
 tor, extending, according to the order of the signs, from Aries 
 to that point of tiie equator that rises with the star in an oblique 
 sphere ; and the difference between the right and the oblique 
 ascension of the body, is called the ascensional clifj'eience. 
 
 The latitude of a star is its distance from the ecliptic, mea- 
 sured on A circle of latitude, which is a circle perpendicular to 
 the ecliptic, and is either north or south, as the star is situated 
 either on the north or south side of the ecliptic. 
 
 The longitude of a star is estimated from the first point of 
 Aries tc the place where a line from the star would cut the 
 ecliptic perpendicularly, or would cut the star's circle of longi- 
 tude. Circles of longitude are lesser circles parallel to the eclip- 
 tic, and which diminish as they recede from it. The longitude 
 of a star, is of course, like its latitude, either north or south. 
 
 When the latitude or longitude of a celestial body is spoken 
 of as if that body were seen from the centre of the sun, it is 
 said to be heliocentric ; but when the body is supposed to be 
 seen from the earth, its latitude or longitude is said to be 
 geocetitric. 
 
 Of the Figure, Motion, and Magnitude of the Earth — Magni- 
 tude and Distance of the Sun and Moon — and of planetary 
 Motion. 
 
 Having now taken a superficial view of the general pheno- 
 mena of the heavens, and of the principal lines or circles which 
 constitute the artificial means that have been adopted to give 
 precision to our ideas in speaking of these phenomena ; — we 
 may now proceed to inquire whether the evidence of vision 
 does not require correction from our judgment. This examina- 
 tion of our first impressions will lead us to ascertain, if we 
 can, the real figure and size of the earth; and when we have 
 satisfied ourselves on these points, we may then proceed to 
 consider whether the earth is in motion or at rest, for till we 
 have done this, it is clear we can form no rational judgment of 
 the real motions of the stars. 
 
 Mankind at first considered the earth to be an illimitable 
 plain, and we have every reason to suppose, that they found it 
 difficult to divest themselves of a prejudice so natural. But 
 when observations of certain appearances on the earth, and of 
 the motions of the heavenly bodies, became considerably multi- 
 plied, and compared with each other, the more penetrating of our 
 
520 ASTRONOMY, 
 
 Figure of tlie earlh. 
 
 forefathers would perceive that the eartli could not possibly be 
 a plain, and that if it was not a yjlain, it was in all probability 
 not boundless. In the flat countries of the East, on approach- 
 ino^ any elevated and very distant object, it would be perceived 
 that the top of that object firet became visible, that the lower 
 parts appeared in regular succession, and that the base was 
 seen the last of all. This would be found a constant phenome- 
 non, depending; upon no accidental circumstance, but observ- 
 able in every direction ; and the clearer the atmosphere, and 
 the more distinct the difference between the base and the sum- 
 mit, tiie more obvious it would appear. If the earth were a 
 plain, the surface of water might certainly be expected to 
 exhibit perfect flatness; the local inequalities of the land were 
 easily accounted for, but water could rest only at a perfect 
 level. When, therefore, in sailing from land, it was found 
 that the base of all elevated objects disappeared first, and all 
 the other parts in succession, in a still more regiilar and re- 
 markable manner than upon land, a doubt could hardly remain 
 of the curvature of the earth's general surface ; the progress 
 of knowledge gradually rendered the fact more evident, and at 
 last it received the most ample confirmation, from a circum- 
 stance within the comprehension of every one. Magellan, Sir 
 Francis Drake, Lord Anson, Captain Cooke, and others, have 
 all, at different times, sailed round the earth. They set out 
 from European ports, and by steering their course westward, 
 arrived at length at the very place they departed from ; which 
 could not have happened, had the earth been of any other than 
 a globular figure. These, as well as all other navigators and 
 travellers, whatever the direction or length of their course, 
 still find themselves surrounded, as much as those who never 
 change their residence, by the same appearance of an immense 
 vault, in which the stars are beheld. It is clear, therefore, 
 that the earth has nothing analogous to what men would 
 call a support, but that it exists by itself, perfectly detached 
 from all other bodies, " in the unfathomable expanse of the 
 Universe." 
 
 From the spherical form of the earth, it follows, that all its in- 
 habitants, being directed, with their feettowards the centre of it, 
 must be variously inclined to one another, in a manner easily re- 
 presented by drawing lines from the circumference of a circle so 
 as to point to its centre, or still better by sticking pins into a 
 ball in the same direction. The inhabitants of countries diame- 
 trically opposite, are called the antipodes of each other. 
 
 To the mind just enteringthe confines of philosophy, itseems 
 a strange and inconceivable thing, that people can stand onever^ 
 feide of a globe, and that there can be those who have the^ 
 
ASTRONOMY. 621 
 
 Figure and size of the earth. 
 
 feet opposite to ours ; but when they have considered a little the 
 nature of the attraction of gravitation, for the general doctrine 
 of which we must refer to the pages we have devoted to the sub- 
 ject, they will soon perceive that the greatest absurdity would 
 be to suppose the contrary. They \v\\\ perceive also, that the 
 terms up and down, with respect to the universe, have no ab- 
 solute or constant meaning ; for the region which we call up- 
 permost, is opposite the feet of our antipodes ; and the region 
 opposite our feet, is consequently above their heads. But 
 with respect to the earth, the words up or down, above or be- 
 low, mean the situations nearer to, or farther from its centre, 
 to which all bodies on its surface are unceasingly drawn, in a 
 right line, by the influence of gravitation. 
 
 When the globular form of the earth became clearly under- 
 stood, a tolerable idea of its size could not be long a secret. 
 We have the record of the attempt to measure it being made 
 550 years before the Christian era; but the first time that great 
 accuracy was shewn, was in 1635, by Richard Norwood, who 
 measured the distance between London and York with a chain ; 
 then having ascertained the sun's meridian altitude, for the 
 same moment, at each extremity of this known base, he de- 
 duced that a degree, or the 360th part of the earth's circum- 
 ference, amounted to 69| miles, and 14 poles, from which he 
 found that the circuit of the earth was about 25,036 miles. 
 
 When the size of the earth was supposed to be known, the 
 next object of astronomical investigation might very properly 
 be, to discover the size and distance of the sun, moon, and stars. 
 To offer conjectures respecting these bodies, before something 
 like adequate ideas of their distances were obtained, would 
 be folly; and to do it before the figure and size of the earth 
 were demonstrated, could not lead to any satisfactory conclu- 
 sions. 
 
 When the effects of the sun, in giving light and beat, were 
 compared with the effects of any artificial fire, it would be 
 admitted by the most ignorant, that the size of that luminary 
 must be very great ; but the ideas of the earliest ages, on such 
 a subject, could never be commensurate to the truth. We 
 must, however, leave the speculations of the ancients, with ob- 
 serving, that every method which ingenuity could devise, has 
 been tried, to resolve the interesting questions of the sun and 
 and moon's distances in particular ; but the only one which 
 can satisfy the mind, is that which depends upon mathematical 
 principles, and which is similar to that employed in the mea- 
 surement of distant terrestrial objects. From the two extre- 
 mities of a base, whose length is known, the angles which the 
 visual rays from the object, whose distance is to be measured, 
 22.— Vol. I. 3 X 
 
522 ASTRONOMY. 
 
 Mcthiiit of discovering the distance of tbe tiioou. 
 
 make with the base, are measured by means of a quadrant; 
 their sum, subtracted from IKO"^, gives the angle which these 
 ravs form at tlie object where they intersect. This angle is 
 called the parallax, and when it is once known, mathematicians 
 find it easy, by means of trigonometry, to ascertain the distance 
 of the object. Let AB, in fig. 2, pi. II, be the given base, and 
 C the object whose distance we wish to ascertain. The angles 
 CAB and CBA,* formed by the rays CA and CB with the base, 
 may be ascertained by observation; and their sum, subtracted 
 from 180"^, leaves the angle ACB, which is the parallax of the 
 object C. It gives us the apparent size of the base AB, as 
 seen from C. When this method is applied to the heavenly 
 bodies, it is necessary to have the largest possible base, and 
 in the usual astronomical sense, the parallax of a star is the 
 difference between its true and apparent distance from the 
 zenith. In explanation of this subject we may observe, that 
 when a star is in the zenith, it is seen bv the spectator on the 
 earth's surface, where it would appear if it could be seen from 
 the earth's centre; it is therefore seen in what is called its true 
 place ; but when the star is not in the zenith, a spectator does 
 not see it in the same place that he would, could he view it 
 from the centre of the earth ; but, in all cases, the difference 
 between the true and apparent place of an object is less in 
 proportion as its distance is greater. Of the popular illustra- 
 tions of the method of taking a parallax, the following by 
 Walker is perhaps one of the plainest : Suppose the distance 
 of the moon were our first object, and that we had a sea hori- 
 zon, and were situated so that the moon passed over our 
 zenith ; then let A, fig. 3, pi. II, be the earth, and a the place 
 of the observer, who must be supposed to have a quadrant, 
 b c, by which he can note the moment the moon comes to the 
 zenith. Now as the moon apparently passes from a meridian 
 to that meridian again in twenty-four hours and forty-eight 
 minutes, she will perform a quarter of that circuit, viz. from 
 d to e, in a quarter of that time, or in six hours and twelve 
 minutes. But the observer will find that she will set before 
 the six hours and twelve minutes are expired, which he must 
 note; for when she comes to his sensible horizon, a c g, she 
 sets to him. Now as the sensible horizon is parallel to the 
 rational horizon k e, a diagonal a e, will make the angle c z 
 equal to n, its opposite angle. To find what the angle c z is, 
 say, by the rule of three, if six hours and 12 minutes be re- 
 quired for a quarter of the moon's circuit, or 90°, viz. from 
 
 " When an angle is referred to, the middle letter indicates its vertex, or the centre 
 ©f the circle upon wtich it is meastued. 
 
ASTRONOMY. 523 
 
 Method of discovering the distance of the sun. 
 
 c? to e; how many degrees of the ninety would she pass through 
 in the time of her going from the zenith to the sensible hori- 
 zon, or from d to g, which being taken from 90^*, or the qua- 
 drant de, will leave the quantity of the arch ge, or the angle cz. 
 Now as the angle « is equal to c z, we are in possession of a 
 right-angled triangle a k e, with a side and an angle known j 
 for a k, the semi-diameter of our globe, is nearly 3960 miles : 
 it is the property of a right-angled triangle to have its sides 
 proportional to the sides of its opposite angles ; therefore as 
 the angle 7i is to its opposite side a k, so is the angle o to its 
 opposite side k e, or the mean distance of the earth's centre 
 from that of the moon, viz. 240,000 miles. The parallax or 
 angle u, is on a medium about 57' . 
 
 The sun is so distant, that his parallax or angle g a S is too 
 small to be measured with any certainty by a horizontal paral- 
 lax. The radius of the earth is indeed found to be an almost 
 insignificant space compared with the sun's distance. Dr. 
 Halley therefore recommended the transits of Venus, which 
 were to happen in the years 1761 and 1769, as affording the 
 best means of ascertaining the distance of the sun. The tran- 
 sit of a planet takes place when the planet comes exactly be- 
 tween the earth and the sun, whose disc it traverses in the 
 form of a small round, dark spot. Transits can only be ex- 
 hibited by the planets Mercury and Venus, because no other 
 have their orbits within that of the earth. Only two transits 
 of Venus occurred during the last century, and there will not 
 be another till 1874. By these transits, which were very care- 
 fully observed, almost the whole diameter of the earth formed 
 a parallax, instead of the above semi-diameter, by which a 
 commensurate angle was thus procured : Venus moves in her 
 orbit in the direction z ?i, fig. 4. pi. II, and from the centre of 
 the earth c, would be seen to move over the sun's disc from s 
 to v; an observer therefore at a, would see the contact ats, at 
 the same instant that one at b would see the planet at u; and 
 one at d would see it at its egress at v, along the line d V v. 
 This would be the case were the earth at rest ; but it is turning 
 on its axis in the direction ah d. Now if the planet stood still 
 at V, while the earth turned from a to d, that time would be 
 easily turned into the parallactic angle a V d, and might be 
 treated like that of the moon; but the motion of Venus, as 
 well as that of the earth, was to be taken into this calculation, 
 as well as observations made in different latitudes; after taking 
 every possible precaution, and making every requisite allow- 
 ance, the sun's parallax was found to be about 8"; the angle, 
 though so very small, admits of calculation, and the sun's dis- 
 tance was from it deduced to be about 96 millions of miles. 
 
i24 ASTRO \OMY. 
 
 Mugoitades of the ^nio and uooii. 
 
 The distances of the sun and moon being thus determined, 
 the facility of ascertaining their size is comparatively great ; 
 because it may be approximated by very easy calculations, ac- 
 cording to the principles of optics. The angle subtended at 
 the eye by an object is easily measured, and is inversely as the 
 distance of that object. Now the distance gives the radius of 
 a circle, the centre of which is at the |)upil of the spectator's 
 eye, and on the circumference of which is the object, which 
 we will suppose to be the sun. Having the radius, the circum- 
 ference of this immense circle is obtained by the common arith- 
 metical method ; and when we have the circumference, the value 
 or extent of its degrees and minutes may also be found. There- 
 fore, if the number of minutes subtended by the sun, be mul- 
 tiplied by the value of a minute, we have the diameter of that 
 luminary. In ascertainino- the distances as well as the majr- 
 nitudes of the sun and moon, a variety of mathematical and 
 mechanical means are resorted to, in order to secure the most 
 accurate results ; but it would be useless on the present occa- 
 sion to enter upon the consideration of the details. It is 
 enough to give, with their results, a general idea of the pro- 
 cesses employed, and to observe, that by the latest and most 
 careful calculations, the moon has been found to be 2180 miles 
 in diameter; and the diameter of tlie sun 883,246 miles. The 
 evidence in favour of these dimensions being very nearlv cor- 
 rect, is so conclusive, as to enforce conviction in every mind 
 open to the investigation of truth. 
 
 When the sphericity of the earth, its insulated existence, 
 and its magnitude, have become familiar to the voung student; 
 and, keeping in mind the oeneral doctrine of attraction above 
 referred to, when he has further compared these three circum- 
 stances with the correspondent circumstances of the sun and 
 moon, he w ill then perhaps begin to inquire, whether the real 
 motions of the sun and moon are such as they appear to be, 
 and whether the earth revolves round them, or they round the 
 earth. With respect to the moon, a very momentary atten- 
 tion will give satisfaction. This satellite always presents to 
 us the same face or side; therefore she revolves round the 
 earth, otherwise this would be impossible. But with respect 
 to the sun, in thus consulting his jud2;ment, he will have 
 much reason for doubt, and to believe that the evidence of 
 sense is fallacious. The sun is a million of times larger than 
 the earth, and if he revolves round the earth in twenty-four 
 hours, he must each hour advance over a space of more than 
 24,000,000 of miles. That a body of such prodigious size 
 should revolve at such a rate round another body, which 
 (like the earth to the sun) is a mere point when compared 
 
ASTRONOMY. 525 
 
 Absurd to suppose the earth at rest. 
 
 with it, is as absurd as to suppose that a grain of sand 
 should command the motion of a mill-stone, or a pepper- 
 corn that of a mountain. It would be totally irreconcileable 
 with all that we know of the simplicity of the laws of nature, 
 which always accomplish the grandest objects by the simplest 
 means; and when we advanced to the consideration of other 
 celestial phenomena, we should find our difficulties increase 
 with our progress, till at last inextricable confusion beset us. 
 It is absolutely necessaryto relinquish the idea that the earth 
 is immoveable, and philosophers, after the most minute inves- 
 tigation of the phenomena, have completely shewn, that the 
 earth has at least two motions, one on its own axis, called its 
 diurnal motion, which causes it to revolve from east to west 
 in twenty-four hours, and the other a progressive motion, 
 called its annual motion, which carries it round the sun once 
 in a year. The diurnal motion produces the regular return of 
 day and night, and the apparent revolution of the sphere ; the 
 annual motion produces the vicissitudes of the seasons, sum- 
 mer, winter, spring, and autumn. 
 
 It is no reason against the rotation of the earth, tnat we 
 are unable to perceive it. For as the motion of a ship at 
 sea, when swiftly sailing over the smooth surface of the 
 water, is not perceptible to the company on board ; cer- 
 tainly we may expect that so large a body as the earth, 
 which has no impediment in its way, or resistance to over- 
 come, will in this respect deceive us effectually. It has 
 been asserted again, that if the earth moved, a stone drop- 
 ped from the top of a tower, or any other high building, 
 would not fall just at the bottom of it, as the building must 
 have advanced considerably forward, during the time of 
 the full. But this is evidently a mistake ; for it is well 
 known, by repeated experiments, that if a body be projected 
 from another body in motion, it will always partake of the 
 motion of that other body. Thus, a stone dropped from the 
 top of a mast, while the ship is under sail, is not left by the 
 vessel, but falls exactly at the foot of the mast. And if a 
 bottle of water be hung up in the cabin, with its neck down- 
 wards, it will empty itself, drop by drop, into another vessel 
 phiced exactly underneath it, though the ship shall have run 
 many feet whilst each drop was in the air. 
 
 The motion of the earth, in the manner stated by astrono- 
 mers, being admitted, as also the magnitudes and distances 
 of the sun and moon, the remaining wonders of astronomy, 
 though they may overwhelm with the idea of immensity, will 
 scarcely excite disbelief. It is found by data as indisputable 
 as those which establish the facts hitherto mentioned, that 
 
526 ASTRONOMY. 
 
 Origin of Sir I^aac Newton's lheor\ of natun 
 
 the planets or wandering stars, and the comets, all revolve 
 round the sun in different orbits, and that these bodies are in 
 reality all ot them worlds. We shall therefore make a few re- 
 marks on that power which preserves them in the order we 
 observe, and afterwards proceed to the separate consideration 
 of them as parts of the solar system. 
 
 When Sir Isaac Newton undertook the reformation of phi- 
 losophy, he proceeded upon a very different plan to those 
 who had gone before him; he proposed to assun:»e nothing as 
 an hypothesis which was not deduced from data acknow- 
 ledged and obvious to our eyes, and thus, by arguing from 
 those things which are within our reach, he thought we 
 might come to know with certainty what must happen in the 
 celestial regions. The manner in which he was first led to 
 form his system of gravitation, which is now universally re- 
 ceived, is said to have been as follows : He was sitting alone 
 in a garden, when some apples falling from a tree, directed 
 his thoughts to the subject of gravity, or the cause of their 
 fall ; and reflecting on that principle, he began to consider, 
 that, as its power is not found to be sensibly diminished at 
 the most remote distance from the centre of the earth to 
 which we can rise, neither at the tops of the loftiest build- 
 ings, nor on the summits of the highest mountains, it was 
 reasonable to conclude that its influence must extend much 
 further than was usually thought. Then a train of thought 
 arose in his mind, " Might it not extend as far as the moon ; 
 and if so, must not her motion be influenced by it? perhaps 
 it retains her in her orbit: however, though the power of gra- 
 vity is not sensibly weakened in the little change of distance, 
 at which we can place ourselves from the centre of the earth, 
 yet it is very possible, that, as high as the moon, this power 
 may differ much in strength from what it is here." To make 
 an estimate of the degree of this diminution, he considered, 
 that if the moon be retained in her orbit by the force of gra- 
 vity, no doubt the primary planets were retained in their 
 orbits by a similar gravitation towards the sun ; and by com- 
 paring the periods of the several planets with their distances 
 from the sun, he found, that if any power like gravity held 
 them in their courses, its strength must decrease in the dupli- 
 cate proportion of the increase of distance. This was con 
 eluded from a supposition that these bodies moved in perfect 
 circles round the sun 5 which, though they are not found to do 
 exactly, yet the error was but of little consequence. 
 
 To account then for the perpetual motions of the planets and 
 comets in their orbits, Newton had recourse to the force of 
 gravity, and a projectile force compounded with it. These two 
 
ASTRONOMY. 527 
 
 Esplanatiou of planetary motiou. 
 
 forces had indeed been proposed by Horrox, before his time, 
 with the same view ; and the power of gravity, as existing in 
 the celestial regions, had been intimated by several philoso- 
 phers ; but their ideas appeared as little better than vague 
 speculations, and were almost unnoticed by the world. As 
 Newton was ignorant of any natural power by which the pla- 
 nets could be impelled in the direction of a tangent line to 
 any part of their orbits, he had recourse, for one of the forces, 
 to the immediate action of the Deity. According to him, 
 God having created this world, impressed the universal law of 
 attraction or gravitation upon matter, and impelled each of the 
 planets in the direction of a right line touching their orbits. 
 Being immediately acted upon by the attraction of the sun, 
 their courses were bent from a straight line into a curve; and 
 the same causes still continuing to act, the original rectilinear 
 direction was changed into one nearly circular, which has con- 
 tinued ever since. The same mode of reasoning is applicable 
 to the continued motion of the secondary planets round their 
 primaries. The manner in which Sir Isaac Newton demon- 
 strates the operation of the projectile and gravitating forces 
 upon the planets, so as to direct them in circles round the sun, 
 is by supposing the orbits they describe divided into infinitely 
 small parts, each of which will not differ from a right line, and 
 consequently the whole curve may be considered as consisting 
 of the diagonals of parallelograms infinitely small, one of whose 
 sides is represented by the space which the planet would have 
 moved through by the projectile force alone, and the other by 
 that which it would have moved through by the force of gravity 
 alone, in the same time. To those who would enter deeply 
 into the subject, Newton's Principia will afford ample gratifi- 
 cation, but it will better suit our purpose of popular illustra- 
 tion to adopt the following explanation of Ferguson : 
 
 " From tlie uniform projectile motion of bodies in straight 
 lines, and the universal power of attraction which draws them 
 off from these lines, the curvilineal motions of all the planets 
 arise. If the body A be projected along the right line ABX, 
 fig- 5, pi. II, in open space, where it meets with no resist- 
 ance, and is not drawn aside by any other power, it will for 
 ever go on with the same velocity and in the same direction. 
 For the force which moves it from A to B in any given time, 
 will carry it from B to X in as much more time, and so on; 
 there being nothing to obstruct or alter its motion. But 
 if, when this projectile force has carried it, suppose to B, the 
 body S begins to attract it, with a power duly adjusted, and 
 perpendicular to its motion at B, it will then be drawn from 
 the straight line ABX, and forced to revolve about S in 
 
528 ASTRONOMY. 
 
 Explaualinii of plantftiirv mutinii. 
 
 the circle BYTU. When the body A comes to U, or any 
 other part of its orbit, if the small body //, within the sjihere 
 of L 's attraction, be projected as in the right line Z, with a 
 furce perpendicular to the attraction of U, then u will go 
 round L in the orbit W, and accompany it in its whole course 
 round the body S. Here, S may represent the sun, U the 
 earth, and u the moon. 
 
 " if a planet at B gravitates, or is attracted towards the sun, 
 so as to fall from B to t/, in the time that the projectile force 
 would have carried it from B to X, it will describe the curve 
 BY, by the combined action of these two forces, in tlie same 
 time that the projectile force singly would have carried it from 
 B to X, or the gravitating power singly have caused it to de- 
 scend from B to y; and these two forces being duly propor- 
 tioned and perpendicular to each other, the planet obeying 
 them both, will move in the circle BY'TU. To make the pro- 
 jectile force balance the gravitating power so exactly, as that 
 the body may move in a circle, the projectile velocity of the 
 body must be such as it would have acquired by gravity alone 
 in falling through half the radius of the circle. 
 
 " But if, whilst the projectile force would carry the planet 
 from B to b, the sun's attraction (which constitutes the pla- 
 net's gravitation) should bring it down from B to 1, the gra- 
 vitating power would then be too strong for the projectile 
 force, and would cause the planet to describe the curve 
 BC. When the planet comes to C, the gravitating power 
 (which always increases as the square of the distance from 
 the sun, S, diminishes) will be yet stronger for the projec- 
 tile force, and by conspiring in some degree therewith, will 
 accelerate the planet's motion all the way from C to K ; 
 causing it to describe the arcs BC, CD, DE, EF, Sec. all in 
 equal times. Having its motion thus accelerated, it thereby 
 gains so much centrifugal force, or tendency to fly oti" at K, 
 in the line K A-, as overcomes the sun's attraction ; and the 
 centrifugal force being too great to allow the planet to be 
 brought nearer the sun, or even to move round him in the 
 circle K / m n, it goes off, and ascends in the curve KL MX, 
 &c. its motion decreasing gradually from K to B, as it in- 
 creased from B to K, because the sun's attraction now acts 
 against the planet's projectile motion just as much as it acted 
 with it before. When the planet has got round to B, its pro- 
 jectile force is as much diminished from its mean state about 
 F or M, as it was augmented at K; and so the sun's attraction 
 being more than sufficient to keep the planet from going off 
 at B, it describes the same orbit over again, by virtue of the 
 same forces or powers. 
 
ASTRONOMY. 529 
 
 Explanation of planetary motion. 
 
 " A double projectile force will always balance a quadruple 
 power of gravity. Let the planet at B have twice as great an 
 impulse from thence towards X, as it had before ; that is, in 
 the same length of time that it was projected from B to b, as in 
 the last example, let it now be projected from B to c; and it 
 will require four times as much gravity to retain it in its orbit: 
 that is, it must fall as far as from B to 4, in the time that the 
 projectile force would carry it from B to c; otherwise it would 
 not describe the curve BD, as is evident by the figure. But 
 in as much time as the planet moves from B to C, in the higher 
 part of its orbit, it moves from I to K, or from K to L, in the 
 lower part thereof; because, from the joint action of these 
 two forces, it must always describe equal areas in equal times 
 throughout its annual course. These areas are represented 
 by the triangles BSC, CSD, DSE, ESF, 8cc. whose contents 
 are equal to one another quite round the figure. 
 
 "As the planets approach nearer the sun, and recede far- 
 ther from him, in every revolution; there may be some diffi- 
 culty in conceiving the reason why the power of gravity, when 
 it once gets the better of the projectile force, does not bring 
 the planets nearer and nearer tlie sun in every revolution, till 
 they fall upon and unite with him; or why the projectile force, 
 when it once gets the better of gravity, does not carry the 
 planets farther and farther from the sun, till it removes them 
 quite out of the sphere of his attraction, and causes them to 
 go on in straight lines for ever afterward. But by considering 
 the effects of these powers, as already described, this difficulty 
 win be removed. Suppose a planet at B to be carried by the 
 projectile force as far as from B to b, in the time that gravity 
 would have brought it down from B to 1 : by these two forces 
 it will describe tlie curve BC. When the planet comes down 
 to K, it will be but half as far from the sun, S, as it was at B; 
 and therefore, by gravitating four times as strongly towards 
 him, it would fall from K to V in the same length of time that 
 it would have fallen from B to 1 in the higher part of its orbit, 
 that is, through four times as much space; but its projectile 
 force is then so much increased at K, as would carry it from 
 K to A: in the same time, being double of what it was at B; and 
 is therefore too strong for the gravitating power, either to 
 draw the planet to the sun, or cause it to go round in the 
 circle K Z m n, which would require its falling from K to lo, 
 through a greater space than gravity can draw it, while the 
 projectile force is such as would carry it from K to k; and 
 therefore the planet ascends in its orbit KLMN, decreasing in 
 its velocity, for the causes already assigned. 
 
 " By the above-mentioned law, bodies will move in all kinds 
 23.— Vol. I. 3Y 
 
536 ASTRONOMY. 
 
 Planetary motion. — Kepler's laws. 
 
 of ellipses, wliether long or short, if the spaces they move in 
 be void of resistance. Only, those which move in the longer 
 ellipses, have so much the less projectile force impressed upon 
 them in the higher parts of their orbits; and their velocities, 
 in coming down towards the sun, are so prodigiously in- 
 creased by his attraction, that their centrifugal forces, in the 
 lower parts of their orbits, are so great as to overcome the 
 sun's attraction there, and cause them to ascend again 
 towards the higher parts of their orbits; during which time 
 the sun's attraction acting so contrary to the motion of those 
 bodies, causes them to move slower and slower, until the 
 projectile forces are diminished almost to nothing; and then 
 they are brought back again by the sun's attraction, as 
 before." 
 
 The celebrated Kepler discovered, by assiduous observation, 
 the two remarkable laws of planetary motion; and in honour 
 of him they are called Kepler's laws. Sir Isaac Newton 
 demonstrated them according to the above theory of attrac- 
 tion, and they shew, in a remarkable manner, the hamiony of 
 the celestial motions: 
 
 I. If a straight line be drawn from a planet to the sun, and 
 this line be supposed to be carried along by the periodical 
 motion of the planet, then the areas which are described by 
 this right line and the path of the planet, are proportional to 
 the times of the planet's motion; for instance, the area thus 
 described in two hours, is the double of that which is de- 
 scribed in one hour, and a third part of that which is described 
 in six hours; though the arc which is described by the planet 
 itself in two hours, is not the double of the arc which is 
 described by the same in one hour; nor the third part of that 
 which is described in six hours. 
 
 II. The planets are at different distances from the sun, and 
 perform their periodical revolutions in different times ; but the 
 cubes of their distances, or of the principal axes of their 
 elliptical orbits, are constantly as the squares of their periodi- 
 cal simes, viz. of the times of their performing their periodical 
 revolutions. 
 
 The sun forms the centre of attraction, round which all the 
 plc.nets move; but all the planets and comets, according to 
 the quantities of matter they contain, and their distance, exert 
 their power of attraction upon the sun. The sun, therefore, is 
 not at rest ; but as his magnitude so greatly exceeds that of all 
 the planets and comets put together, and as these exert their 
 attractive powers in opposite directions, from being on oppo- 
 site sides, the centre of gravity of the whole system, which is 
 the point round which the sun performs a circuit, is always 
 
ASTRONOMY. 531 
 
 Effects of the mutual attractions of the heavenly bodies. — Solar system. 
 
 near him, and generally within his body. From th€ grand 
 principle of universal attraction, also, it follows that the 
 planets must attract each other; and in fact it has not escaped 
 the observation of philosophers, that they sensibly disturb 
 each other's motions. When any two planets are in con- 
 junction, their mutual attractions, which tend to bring them 
 nearer to one another, draws the inferior one a little farther 
 from the sun, and the superior one a little nearer to him; by 
 which means the figure of their orbits is somewhat altered, 
 though the alteration is too small to be discovered in several 
 ages. 
 
 The orbits of such planets as have one or more moons, are 
 still more remarkably disfigured by the attraction of these 
 attendants ; for example, the point which describes the orbit 
 of the earth round the sun, is not the centre of the earth, but 
 the common centre of gravity of the earth and moon, and which 
 is found by calculation, on comparing their quantities of mat- 
 ter, to be about 2000 miles from the earth's surface. But these 
 irregularities, while they shew the truth of the theory that 
 accounts for them all, are too inconsequential to require much 
 notice in a popular view of the solar system. 
 
 Of the Solar System. 
 
 The plate, marked Solar System, is intended to give an idea 
 of the appearance of that system, could a spectator view it from 
 a considerable distance above the sun, in a line perpendicular 
 to the earth's orbit. In every plan of thi.s kind, however, some 
 sacrifices of strict propriety are made for the sake of conve- 
 nience. The sun is only supposed to be seen by the radiance 
 he throws around ; the planets and their satellites, with their 
 distances from the centre of the sun, are not exhibited in due 
 proportion; and the orbits of all the planets are represented 
 by circles, from which the reader will recollect they differ so 
 little, that though in reality ellipses, they could scarcely on so 
 small a scale be distinguished from circles. Of the comets, 
 but four are noticed, each moving in an orbit of different 
 eccentricity, but the actual number of these bodies is known 
 to be very considerable, though by no means determined. 
 
 To a spectator, situated where we have supposed, above the 
 sun, the planetary motions would appear extremely regular; 
 but it is easy to conceive, that to one situated sideways, or 
 viewing them, for example, as from the earth, they will exhibit 
 great anomalies. Accordingly, the planets sometimes appear 
 to be going forwards, sometimes backwards, and sometimes 
 appear to be stationary. These irregularities are such as 
 
532 ASTROXOMV. 
 
 Nuuibt-r ol' the planets and llicir satellites. — Tbe Sun. 
 
 must take place, if the Copernican be the true system of 
 nature, and therefore the observation of them is a proof of the 
 correctness of that system. 
 
 The planetary bodies of the Solar System, so far as at pre- 
 sent known, and commencing with the one nearest the sun, 
 are the following : 
 
 1. ^Mercury, 
 
 2. Venus, 
 
 3. The Earth, with one Satellite or Mocn, 
 4 Mars, 
 
 6. Vesta, 
 
 6. Juno, 
 
 7. Ceres, 
 
 8. Pallas, 
 
 9. Jupiter, with four Satellites, 
 
 10. Saturn, with seven Satellites, 
 
 11. Herschel, with six Satellites. 
 
 fth, the sixth, the seventh, and eighth, are the four 
 ■uhich Dr. Herschel calls asteroids. After treating of the sun, 
 "we shall notice all the planetary bodies in their order. 
 
 The Sun, © 
 
 The sun is an immense globe, the mean diameter of which 
 amounts to 88-3,246 miles. When viewed through a telescope, 
 dark spots, of various sizes and duration, are perceived on its 
 surface; and from observations on the motions of these 
 spots, it has been found that the sun revolves on its axis, 
 from east to west, once in 25 days, 14 hours, 8 minutes, and 
 his axis deviates about 8'^ from a perpendicular to the plane 
 of the ecliptic. 
 
 The dark spots of the sun's surface, are often called macula; 
 and those parts which are brighter than the rest, are called 
 facula. 
 
 A variety of very different and heterogeneous conjectures 
 have been offered to account for the macules. As the constant 
 emanation of light and heat, induced the opinion that the sun 
 was nothing else than one prodigious mass of fire, so it was 
 interwoven with the idea that so vast a combustion required 
 adequate accessions of matter to keep it up; on this account, 
 it was supposed that the maculae might be only the scoria or 
 unconsumed portions of the additional fuel, swimming on the 
 liquid surface of the sun. Faculte, on the contrary, have been 
 called clouds of li^ht, and luminous vapours; and because 
 maculie have sometimes been observed to change into faculae, 
 it has been conjectured, that the latter were the dark fumes of 
 
INSERT FOLDOUT HERE 
 
ASTRONOMY. 533 
 
 Solar system reviewed. — The Sun. 
 
 volcanoes rapidly blazing out, after the dark smoky matter, 
 which produced the maculae, became dissipated by combus- 
 tion. But these ideas partook very much of the vagaries of 
 imagination; they possessed nothing admirable in themselves, 
 and no attempt was made to render them, in a satisfactory man- 
 ner, consistent with all the phenomena. In the year 1788, how- 
 ever, appeared a dissertation by Edward King, F.R.S. which 
 contained many original remarks: this philosopher asserted, 
 that the real body of the sun is less than its apparent diameter ; 
 that we never discover the real body of the sun itself, except 
 when we behold its spots, (maculse,) that the sun is inhabited as 
 well as our earth, and is not necessarily subject to burning heat; 
 and that there is in reality no violent elementary heat existing 
 in the rays of the sun themselves essentially. 
 
 Several years after the publication of these opinions by 
 King, Dr. Herschel began to publish, in the Philosophical 
 Transactions, his theory concerning the nature of the sun, 
 and to which he was led by his numerous observations, made 
 with his admirable instruments, and the most persevering 
 industry. This theory has received a degree of acceptance 
 proportionate to the skill with which it was established, and 
 we subjoin the following outline of it: 
 
 The Doctor considers the sun to be a magnificent habitable 
 globe, surrounded by a double set of clouds. Those which 
 are nearest its opaque body, are less bright and more closely 
 connected together than those of the upper stratum, which 
 form the luminous apparent globe we behold. This luminous 
 external matter, he observes, is neither a liquid nor an elastic 
 fluid of an atmospheric nature; for in either of these two 
 cases, it would not admit of any chasms or openings. There- 
 fore, it must be concluded that this shining matter exists 
 it the manner of empyreal, luminous, or phosphoric clouds, 
 residing in the higher regions of the solar atmosphere. 
 The Doctor then is of opinion that the spots are only 
 accidental openings between the luminous clouds, through 
 which we behold the opaque body of the sun, or the inferior 
 less luminous clouds; hence the spots appear of different 
 shades. The Doctor rejects the terms maculae, faculee, luculi, 
 and others previously in use, and substitutes the following, as 
 better adapted to express what he considers the true pheno- 
 mena of the sun. 
 
 Openings are those places where, by the accidental removal 
 of the luminous clouds of the sun, its own solid body may be 
 seen; and this not being lucid, the openings through which 
 we see it may, by a common telescope, be mistaken for mere 
 black spots or their nuclei. 
 
534 ASTRONOMY. 
 
 Solar svs4eni reviewed. — The Sun. 
 
 S/i(i//ows are extensive and level depressions of the luminous 
 solar clouds, generally surrounding the openings to a con- 
 siderable distance. As they are less luminous than the rest 
 of the sun, they seem to have some distant, though very 
 imperfect, resemblance to penumbrae; which might occasion 
 their having been called so formerly. 
 
 Ridges are bright elevations of luminous matter extended in 
 rows of an irregular arranirment. 
 
 y>odu/es are also bright elevations of luminous matter, but 
 confined to a small space. These nodules and ridges cor- 
 respond to what have hitherto been called faculae and 
 iuculi. 
 
 Corrugations, he applies to that very particular and remark- 
 able unevenness, ruggedness, or asperity, which is peculiar to 
 the luminous solar clouds, and extends all over the surface of 
 the globe of the sun. As the depressed parts of the corruga- 
 tions are less luminous than the elevated ones, the disc of the 
 sun has an appearance which may be called mottled. 
 
 Indentations are the depressed or low parts of the corruga- 
 tions; they also extend over the whole surface of the luminous 
 solar clouds. 
 
 Pores are very small holes or openings, about the middle of 
 the indentations. 
 
 As the Doctor considers the solar clouds to consist rather of 
 a flame than a liquid substance, he attributes the spots to the 
 emission of an aeriform fluid, not yet in combustion, which 
 displaces the general luminous atmosphere, and which is after- 
 wards to serve as fuel for supporting the process; hence he 
 supposes the appearance of copious spots to be indicative of 
 the approach of warm seasons on the surface of the earth, a 
 theory which he has attempted to maintain by historical 
 evidence. The shallows he considers as parts of an inferior 
 stratum of opaque clouds, capable of protecting the immediate 
 surface of the sun from the excessive heat produced by com- 
 bastion in the superior stratum, and probably of rendering 
 this stupendous world fit for animated existence. In general, 
 more or less of the real body of the sun is supposed to be 
 visible through its lucid atmosphere, where the substance is 
 not very intense, or where it is removed by the varying cir- 
 cumstances of the combustion. As some of the spots appear 
 below the surface of the shining fluid, it may be presumed 
 that they shew us the lower parts of the sun's surface; and as 
 other spots appear above the shining fluid, it is equally rea- 
 sonable to conclude, that they are the solar mountains. The 
 former kind of spots are very variable in situation, and it is 
 clear, that if we have the true theory of the sun before us, 
 
ASTRONOMY. 535 
 
 Solar system reviewed. — The Sun. 
 
 they may never occur twice precisely at the same place* 
 The spots attributed to the projection of the solar mountains, 
 are fixed with respect to the sun's surface, and are those by 
 which the sun's rotation on his axis has been determined. 
 Dr. Herschel, taking into consideration the great attraction 
 exerted by the sun upon bodies placed at its surface, and the 
 slow revolution it has about its axis, thinks that the solar 
 mountains may be more than 300 miles high, and yet stand 
 very firmly. 
 
 Dr. Young, in opposition to the theory of Dr. Herschel, 
 argues, that if we inquire into the intensity of the heat which 
 must necessarily exist wherever this combustion is performed, 
 we shall soon be convinced that no clouds, however dense, 
 could impede its rapid transmission to the parts below. Be- 
 sides, the diameter of the sun is 111 times as great as that of 
 the earth; and at its surface, a heavy body would fall through 
 no less than 450 feet in a single second; so that if every other 
 circumstance permitted human beings to reside on it, their 
 own weight would present an insuperable difficulty, since it 
 would become thirty times as great as upon the surface of the 
 earth, and a man of moderate size would weigh above two tons. 
 This hypothesis is not so satisfactory as Dr. Herschel's. If 
 the sun be not inhabited, the only adequate purpose of its 
 existence which we can conceive, is that of maintaining the 
 existence of the other parts of the solar system ; and that this 
 is the only or chief use of it, is inconsistent with that frugality 
 of means which appears the more eminent, the more closely 
 we examine the works of nature. As the sun is so much larger 
 than the aggregate mass of the planets, if it had been endued 
 with no more inherent heat than we can easily reconcile to 
 our gross ideas of organization, it would have supported a 
 greater number of inhabitants than all the planets, supposing 
 each of them to be the residence of living beings. It seems 
 absurd, then, to suppose that the sun is created solely for the 
 use of the comparatively mean and diminutive worlds that sur- 
 round him; and a strange, unphilosophical limitation of the 
 Deity's power and beneficence, to conclude that he cannot call 
 into existence beings whose organs are as admirably suited to 
 their peculiar circumstances on the sun, as we are to our own 
 situation. With the knowledge we possess of modes of exist- 
 ence infinitely diversified, we cannot deny the existence of the 
 power to produce such beings ; and when we consider the ful- 
 ness of creation within the sphere of our observation, it seems 
 as difficult to deny the presence of the will to create them, 
 rather than leave a mighty blank of that possible happiness, 
 which myriads of beings might enjoy. The arguments against 
 
536 ASTRONOMY. 
 
 Solar system reviewed. — The Sun. 
 
 the inhabitable condition of the sun, drawn from the circum- 
 stance of the excessive weight of bodies on its surface, are 
 inconclusive, unless inhabitants could be formed only of the 
 same kind of matter as ourselves, which is not certainlv the 
 most probable conjecture. It may also be observed, that the 
 strenfjth or muscular energy of ordinary men is not more 
 inferior to the muscular energy of some individuals that have 
 occasionally appeared in the world, than the muscular energy 
 of these remarkable men is inferior to what an inhabitant of 
 the sun would require, provided greater muscular energy were 
 the only means he possessed to overcome the disadvantage of 
 liis weight. 
 
 With respect to the excessive heat at the surface of the sun, 
 there are many facts which shew that heat is produced by the 
 sun's rays only when they act on a calorific medium; they 
 produce heat bv uniting with the matter of fire which is con- 
 tained in the substances that are heated. The collision of a 
 flint and steel will inflame a magazine of gunpowder, by 
 putting all the latent fire which it contains into action; while 
 the same spark upon a heap of sand would not have the 
 slightest effect. Now if we suppose the matter on the surface 
 of the sun, not to admit of any extensive chemical combination 
 with its rays, it is easy to perceive, that the heat may not be 
 such as to be inconsistent with organized existence. 
 
 Walker's speculations on the sun are very ingenious. He 
 supposes the particles of light, which constitute the solar atmo- 
 sphere, strongly to repel each other; that this property of re- 
 pulsion is one cause of the light being dispersed or projected 
 in every direction through space; but that, at the equatorial 
 regions of the sun, the centrifugal force created by his rotation, 
 co-opeiating with repulsion, produces a more copious emission 
 of the vital ocean, than at the polar refjions. The sun turns on 
 an axis inclined but 8^ from the plane of the ecliptic ; his equa- 
 torial, or greatest centrifugal discharge of particles, will there- 
 fore be nearly confined to the zodiac or track of the planets, 
 where the greatest supply is required. Taking for granted, 
 (and we shall afterwards see the reasonableness of the belief,) 
 that the fixed stars are suns, he supposes that these suns recruit 
 each other, by imbibing, at and near their poles, most of the 
 light that reaches them; for towards their poles, the strong 
 attraction of the sun's body overcomes the repulsion of light, 
 when that repulsion is but little assisted by the centrifugal force, 
 and the light thus imbibed, sinks down for the supply of the 
 equatorial regions. But a still more beautiful part of this hypo- 
 thesis, is, that it explains the cause of the diurnal motion of 
 the earth and other planets, which Newton left unaccounted for 
 
ASTRONOMY. 537 
 
 Solar svstem reviewed. — The Sun. 
 
 Light has been shewn (see Optics) to possess momentum ; 
 suppose then, S, fig. 6, plate II, to be the sun, and E the earth, 
 and that the lines a, b, c, d, Sec. represent rays of light issuing 
 centrifugallv from the sun. A line from the sun's centre of 
 gravity, 5, to that of the earth, 0, will shew the direction in 
 which the two bodies mutually struck each other, or the line 
 in w hich they would fall together, if no counteracting^ principle 
 prevented it. The equal distances of the lines which repre- 
 sent the rays, being agreeable to the equal distribution of light 
 through the zodiac, it may be seen that twice the quantity falls 
 on one side of the line of direction, 50, as on the other ; thus 
 ah c d are on one side, while e and /^" only fall on the opposite 
 side of the line of direction. Though this is an exaggerated 
 proportion, yet in all the positions in which the earth stands to 
 the sun, during its annual revolution round him, it will be 
 found that more rays fail on one side of its axis than on the 
 other; and the consequence of such an inequality of impulse 
 must be its rotation on its axis, if not also of its annual revo- 
 lution. The author further inquires, whether the repulsion of 
 light may not be a balance to the gravitation of the planets 
 towards the sun, and whether their several distances may not 
 be produced by dilFerences in their densities, because the 
 larger the surface of a given quantity of matter, the more it 
 will be repelled by the action of light? To put these ideas to 
 the test of experiment, he had recourse to the following 
 means : He took a circular wooden box, BB, fig. 1, plate III, 
 and made oblique holes, represented by the tubes, in its side. 
 This box was close, both at top and bottom, with the excep- 
 tion of an aperture, o, to admit the mouth of a double pair of 
 bellows, to make the current of air equal. A very thin glass 
 globe was hung by a very long thread, exactly over the centre a. 
 This gave it a constant tendency towards that centre, and re- 
 presented the gravitation of the planets towards the sun. The 
 ball being now drawn to the side of the box, and the bellows 
 blowai, the box instantly began to turn on its axis, and to 
 revolve in an ellipse. This experiment is ingeniously con- 
 trived, and w ill gratify those who may not be disposed to give 
 full credence to the theory it was instituted to support ; yet 
 the theory itself is certainly more entitled to respectful atten- 
 tion than many that have been proposed ; the present state of 
 our knowledge does not enable us to decide against it; and it 
 therefore remains for future investigations to establish or 
 subvert. 
 
 Vol I.— 23. 3 Z 
 
53vS ASTRO XO MY 
 
 Solar STstcni ret iewed. — Mercury- 
 
 yicrcury. 5 
 
 Mercury is the nearest planet to the sun, yet his distance from 
 that luminary is not less than 37,000,000 of miles. He emits a 
 very bright white light, always to us appears near the sun, and 
 as he is generally lost in the splendour of the solar beams, 
 astronomers have but scanty opportunities of making observa- 
 tions upon him. His diameter is 3224 miles ; and his revolu- 
 tion roinid the sun is performed in 87 days 23 hours, 14 mi- 
 nutes, 33 seconds ; his summers and winters cannot therefore 
 be more than 44 days each. As no spots have yet been dis- 
 covered on his disc, the time of his diurnal rotation is not 
 known with certainty, though Schroeter is induced, by some of 
 his observations, to believe that it is performed in 24 hours and 
 5 minutes. The position of his axis is also undetermined. 
 
 Mercury never goes to a greater distance from the sun than 
 about 27"^ o ; so that he is never lonoer in setting after the sun 
 than an hour and fifty minutes ; nor does he ever rise sooner 
 than the same space of time before the sun. When he 
 begins to make his appearance in the evening after sun- 
 set, he can scarcely at first be distinguished on account 
 of the twilight ; but he is seen at a greater distance from the sun 
 every successive evening, till he has attained the distance of 
 about 22° 5', when he begins to return again. During this 
 interval, his motion, referred to the stars, is direct ; but when 
 he approaches within 18*^ of the sun, he appears for some 
 time stationary, and then his motion begins to be retrograde, 
 till at last his proximity to the sun prevents his being seen. 
 Soon after, he may be perceived in the morning before sun- 
 rise, receding further and further from that luminary, as before 
 he disappeared. Having attained the distance of 18°, he again 
 becomes stationary, then assumes a direct motion, but con- 
 tinues to separate till his distance is 22° 5', after which he 
 again returns to the sun, y^lunges into his rays, and appears 
 soon after in the evening, after sun-set, to repeat the same 
 career. 
 
 The inclination of Mercury's orbit, to the plane of the 
 ecliptic, is about 7°. He may be much better observed near 
 our equator than in high latitudes. When examined with a 
 telescope magnifying 200 or 300 times, he exhibits the same 
 differences of phases as the moon, being sometimes horned, 
 sometimes gibbous, and sometimes shining almost with a round 
 face — it is not entirely full, because his enlightened side is never 
 turned directly towards us ; but he is at all times well defined, 
 without any ragged edge, and perfectly bright; like the moon, 
 the crescent is always turned towards the sun. The apparent 
 
ASTIR ©:h©m^ 
 
 FLU. 
 
 JnKhakei "ay Benr-r "HVter ^rrr^^- LcciorL. 
 
ASTRONOMY. 539 
 
 Solar svstein reviewed. — Venus. 
 
 diameter of Mercury varies with his position; it is least when 
 the planet plunges into the solar rays in the morning, or when 
 it disengages itself from them; it is greatest when the planet 
 plunges into the solar rays in the evening, or when it disen- 
 gages itself from them in the evening; that is, when the planet 
 passes the sun in its retrograde motion, its diameter appears 
 the greatest possible ; and when it passes the sun into its 
 direct motion, it is the smallest possible. Sometimes, when 
 the planet disappears during its retrograde motion, that is, 
 when it plunges into the sun's rays in the evening, it may be 
 seen crossing the sun, in a right line, under the form of a black 
 spot. This black spot is recognized to be the planet, by its 
 position, its apparent diameter, and its retrograde motion. 
 These transits, as they are termed, are miniature eclipses ; 
 they demonstrate that the planet is itself an opaque body, and 
 shines only by the light it borrows from the sun. A transit 
 of Mercury does not occur at every one of his revolutions, 
 because his orbit is inclined to the ecliptic, and coincides with 
 it only at the two nodes; hence the planet cannot be seen to 
 pass over the disc of the sun, unless its nodes are in or very 
 near the line which joins the sun and the earth. Transits of 
 Mercury are expected in the years 1815, 1822, 1832, 1835, 1845, 
 and 1848. 
 
 Mercury moves in his orbit about 110,000 miles per hour; 
 the sun to him appears three times as large as to us, and affords 
 him, at a medium, seven times the heat felt at our torrid zone. 
 Hence it appears that if the materials of which the planet is 
 composed were as susceptible of being heated as those of our 
 earth, they would be either melted or vitrified, and the planet 
 would have acquired such a brightness, as to prevent its ap- 
 pearing like a black spot at its transit. Why then may we not 
 suppose that he has inhabitants as different from us as his 
 matter ? 
 
 Venus. ? 
 
 Venus is the most beautiful star of the heavens. She is called 
 the Morning or Evening Star, according as she precedes or 
 follows the apparent course of the sun, and retains each title 
 about 290 days. Her light is remarkable for its brightness and 
 whiteness, casting a sensible shadow, and so vivid, at her 
 greatest elongations, that she is visible to the naked eye in full 
 day-light ; and the light of the moon is frequently observed to 
 be comparatively dull. She is the second planet from the 
 sun, round which she performs her annual revolution, at the 
 medium distance of 68,000,000 of miles, in 224 days, 16 hours, 
 41 minutes, 27 seconds. The time of her diurnal revolution. 
 
540 ASTRONOMY. 
 
 Solar system reviewed. — \'enus. 
 
 or the length of her day, is supposed to be 23 hours, 21 
 minutes. It is necessary to remark, that with respect to this 
 
 Eoint, and conclusions drawn from it, there is some uncertainty, 
 ir. Herschel is among those observers who have not satisfied 
 themselves as to the existence of spots on this planet, and 
 therefore do not consider the position of her axis, and the period 
 of her diurnal rotation, as fully determined. Her diameter is 
 7687 miles, and she is therefore very nearly the size of the 
 Earth. The inclination of her orbit to the ecliptic, is about 
 3" 23' 35". Her greatest distance from the sun varies from 45" 
 to nearly 48°. 
 
 The transits of Venus are very rare phenomena ; only two, 
 we have before observed, occurred during the last century, and 
 another will not happen until the year 1874. During a transit, 
 the black spot, under the form of which she appears, has an 
 apparent diameter of 59 ". A few days after this has been ob- 
 served, she is seen in the morning, west of the sun, and ap- 
 pears through a good telescope in the form of a fine crescent, 
 with the convexity turned towards the sun. She moves gra- 
 dually westward with a retarded motion, and the crescent 
 become more full. In about ten weeks, she has moved about 
 46*^ west of the sun, and is now a semi-circle, and her diameter 
 is 26'. She is now stationary. She then moves eastward with 
 a motion gradually accelerated, and overtakes the sun in 9i 
 months after having been seen on his disc. Some time after 
 she is seen in the evening, east of the sun, round, but very 
 small. She moves eastward, and increases in diameter, but 
 loses of her roundness, till she gets about 46*^ east of the sun, 
 when she is again a semi-circle. She now moves westward, 
 increasing in diameter, but becoming a crescent, like the 
 waning moon ; and, at last, after a period of nearly 584 days, 
 comes again into conjunction with the sun, with an apparent 
 diameter of 59". It may perhaps excite the surprise of some, 
 that Venus should keep longer on the east or west of the sun, 
 than the whole time of her period round him. But the diflH- 
 culty vanishes when we consider that the earth is all the while 
 going round the sun the same way, though not so quickly, as 
 Venus ; and therefore her relative motion to the earth must in 
 every period be as much slower than her absolute motion in 
 her orbit, as the earth durins: that time advances forward in 
 the ecliptic, which is 220 degrees. 
 
 To the inhabitants of Venus, the sun will appear nearly twice 
 as large as to us, and iSIercury will be a morning and evening 
 star to them, as she is to us. Her atmosphere has been calcu- 
 lated at fifty miles in height, from a shade appearing on the 
 sun's face, about five seconds before the dark body of Venus 
 
ASTRONOMY. 541 
 
 Solar system reviewed. — The Earth. 
 
 appeared to touch his edge, at the time of her transit. During 
 the transit, also, the greatest attention was exerted to discover 
 whether she was attended by any satellite, but none was ob- 
 served, and she is therefore not known to have one. 
 
 The axis of Venus is generally supposed to be inclined 75 
 degrees from the axis of her orbit, which is Slg degrees more 
 than our earth's axis is inclined from the axis of the ecliptic; 
 and therefore the variation of her seasons is proportionately 
 greater. The north pole of her axis inclines towards the 20th 
 degree of Aquarius, our earth's to the beginning of Cancer : 
 consequently the northern regions of Venus have summer in 
 the signs where those of our earth have winter, and vice versa. 
 As the sun's greatest declination on each side of her equator 
 amounts to 75 degrees, her tropics are only 15 degrees from 
 her poles, and her polar circles as far from the equator. She 
 has at her equator two summers and two winters, in each of 
 her annual revolutions. 
 
 The Earth. ® 
 
 The Earth is the next planet above Venus in the Solar 
 System. Her distance from the sun, is at a mean 95,000,000 
 of miles, and her annual revolution is performed in 365 
 days, 5 hours, 48 minutes, 49 seconds ; this is called her 
 tropical year; but the time she takes to perform an annual 
 revolution from any fixed star to the same again, as seen 
 from the sun, is 365 days, 6 hours, 9 minutes, 12 seconds, 
 which is called a sidereal year. Her rotation on her axis is 
 performed in 24 hours, which is called the length of a natu- 
 ral day. Her diameter is 7911 miles. Though she travels 
 along her orbit at the rate of 68,000 miles per hour, a motion 
 about 140 times swifter than that of a cannon ball, yet her 
 motion is little more than half as swift as that of Mercury 
 in his orbit. Her diurnal rotation being from west to east, 
 the diurnal motion of all the heavenly bodies appears to be 
 from east to west. Besides the 68,000 miles of progressive 
 advance, to which the inhabitants of every part are subjected 
 alike, the inhabitants of the equator are carried 1030 miles 
 every hour by the earth's motion on its axis ; but from the 
 equator to the poles, this motion diminishes, and at the 
 latitude of London, it amounts only to about 580 miles per 
 hour. 
 
 If the axis of the earth were perpendicular to the ecliptic, 
 or plane of its orbit, the days and nights would be of equal 
 length in every part of it; but its axis is inclined 23^ degrees 
 to the ecliptic, and this produces the seasons ; the excessive heat 
 
542 ASTRONOMY. 
 
 Solar system reviewed. — The Earth. 
 
 which would be felt at the equator, and the excessive cold 
 which would be felt at the poles, being thereby both 
 moderated. The earth's axis keeps parallel with itself, that 
 is, constantly preserves the same oblique direction, during the 
 whole of its annual journey, with a very small exception, to 
 be afterwards noticed. 
 
 The following experiment will give a plain idea of the diur- 
 nal and annual motions of the earth, and the phenomena of 
 the seasons: Take about seven feet of strong wire, and bend 
 it circularly, ns a b c d, fig, 2, pi. Ill, which being viewed 
 obliquely, appears elliptical as in the figure. Place a lighted 
 candle on a table; and having fixed one end of a silk thread, 
 K, to the north pole of a small terrestrial globe H, about 
 three inches diameter, cause another person to hold the wire 
 circle, so that it may be parallel to the table, and as high as 
 the flame of the candle, 1, which should be in or near the 
 centre. Then having twisted the tliread as towards the left 
 hand, that by untwisting, it may turn the globe round east- 
 ward, or contrary to the way in which the hands of a watch 
 move, hang the globe by the thread within this circle, almost 
 contiguous to it; and, as the thread untwists, the globe 
 (which is enlightened half round by the candle, as the earth 
 is by the sun) will turn round its axis, and the different places 
 upon it will be carried through the light and dark hemi- 
 spheres, and have the appearance of a regular succession of 
 days and nights, as our earth has in reality by such motion. 
 As the globe turns, move your hand slowly, so as to carry the 
 globe round the candle, according to the order of the letters 
 abed, keeping its centre even with the wire circle, and you 
 will perceive, that the candle, being still perpendicular to the 
 equator, will enlighten the globe from pole to pole in its whole 
 motion round the circle; and that every place on the globe 
 goes equally through the light and dark, as it turns round by the 
 untwisting of the thread, and therefore has a perpetual equinox. 
 The globe thus turning round, represents the earth turning round 
 its axis ; and the motion of the globe round the candle represents 
 the earth's annual motion round the sun; and shews, that if the 
 earth's axis had no inclination to the plane of its orbit, all the days 
 and nights of the year would be equallylong, consequently there 
 would be no difference of seasons. This peculiarity distinguishes 
 the planet Jupiter, whose inhabitants have a perpetual equinox, 
 namely, his axis is perpendicular to the plane of his orbit, as the 
 thread, round which the globe turns in this experiment, is per- 
 pendicular to the plane of the area inclosed by the wire. But 
 now let the person who holds the wire, hold it obliquely in the 
 position A BCD, raising the side s just as much as he depresses 
 
ASTRONOMY. 543 
 
 Solar system reviewed. — Tke Earth. 
 
 the side V5', that the flame may be still in the plane of the 
 circle; and twisting the thread as before, that the globe may 
 turn round its axis in the same way it is carried round the 
 candle, that is, from west to east; let the globe down into the 
 lowermost part of the wire circle at '/f ; and if the circle be 
 properly inclined, the candle will shine perpendicularly on the 
 tropic of Cancer; and the frigid zone, lying within the arctic 
 or north polar circle, will be enlightened as in the figure; and 
 will keep in the light, however often the globe turns round its 
 axis. From the equator to the north polar circle, all the 
 places have longer days and shorter nights; but from the 
 equator to the south polar circle, just the reverse. The sun 
 does not set to any part of the north frigid zone, as shewn by 
 the candle's shining on it, so that the motion of the globe 
 can carry no place of that zone into the dark; and at the 
 same time the south frigid zone is involved in darkness, and 
 the turning of the globe brings none of its places into the light. 
 If the earth were to continue in the like part of its orbit, the 
 sun would never set to the inhabitants of the north frigid 
 zone, nor rise to those of the south. At the equator, it would 
 be always equal day and night; and as places were gradually 
 more and more distant towards the arctic circle, they would 
 have longer days and shorter nights; whilst those on the 
 south side of the equator would have their nights longer than 
 their days. In this case there would be a continual summer 
 on the north side of the equator, and continual winter on tli ^ 
 soutli side of it. But as the globe turns round its axis, move 
 your hand slowly forward, so as to carry the globe from H 
 towards E, and the boundary of light and darkness will 
 approach towards the north pole, and recede towards the 
 south pole: the nortliprn places will go through less and less 
 of the light, and the southern places through more and more 
 of it; shewing how the northern days decrease in length, and 
 the southern days increase, whilst the globe proceeds from 
 H to E. When the globe is at E, it is at a mean state be- 
 tween the lowest and highest parts of its orbit; the candle is 
 directly over the equator; the boundary of light and darkness 
 just reaches to both the poles, and all places on the globe go 
 equally through the light and dark hemispheres, shewing that 
 the days and nights are then equal at all places of the earth, the 
 poles only excepted ; for the sun is then setting to the north 
 pole, and rising to the south pole. 
 
 Continue moving the globe forward, and as it goes through 
 the quarter A, the north pole recedes still farther into the dark 
 hemisphere, and the south advances more into the light, as the 
 
544 ASTRONOMY. 
 
 Solar svstem reviewed. — Tbe Earth. 
 
 globe comes nearer to g; and when it comes there at F, the 
 candle is directly over the tropic of Capricorn; the days are at 
 the shortest, and the nights at the longest, in the northern 
 hemisphere, all the way from the equator to the arctic circle; 
 and the rever&e in the southern hemisphere, from the equator 
 to the antarctic circle; within which circle it is for many 
 weeks continual daylight. 
 
 Continue both motions, and as the globe moves through the 
 quarter B, the north pole advances towards the light, and the 
 south pole recedes towards the dark; the days lengthen in the 
 northern hemisphere, and shorten in the southern; and when 
 the globe comes to G, the candle will be again over the equator, 
 (as when the globe was at E,) the days and nights will there- 
 fore again be equal, and the north pole will be just commencing 
 her long day, while the south is left for an equal space of 
 time in darkness. 
 
 Thus we see the reason why the days lengthen and shorten 
 from the equator to the polar circles every year; why there is 
 periodically no day or night for many turnings of the earth 
 within the polar circles; why there is but one day and one 
 night in the whole year at the poles; and why the days 
 and nights are equally long all the year round at the equator, 
 which is always equally cut by the circle bounding light and 
 darkness. The inclination of an axis, or of the plane of an 
 orbit, is merely relative, arising out of the comparison we 
 make with some other axis or orbit, which we do not con- 
 sider as inclined at all. If the axis of the earth be inclined 
 285 degrees to the axis of this orbit, the axis of this orbit 
 must be inclined 23h degress to the axis of the earth; and 
 it amounts to the same thing, whether we use the former ex- 
 pression or the latter. 
 
 Though we say that the inhabitants within the arctic and 
 antarctic circles are at opposite times of the year deprived 
 of the sight of the sun, yet they are not altogether deprived of 
 his light; for the atmosphere reflecting and refracting the sun's 
 light, forms a twilight at the distance of even 18 degrees. 
 Hence the day breaks to us when the sun is still 18 degrees 
 below the eastern horizon ; and we have his light in the evening 
 till he sinks 18 degrees below the western. But as the sun in 
 summer rises considerably to the north of the east, and sets 
 also to the noith of the west with us, he rises and sets very 
 obliquely to the horizon, and the twilight is of long duration. 
 At midsummer. Great Britain has no night, for the whole 
 island is within 18 degrees of the horizon, or boundary of the 
 sun's light. In high southern latitudes, they enjoy in their 
 
ASTRONOMY. 545 
 
 Solar sjstem reviewed. — The Earth. 
 
 turn, the same advantage ; but at the equator, the sun sinks 
 abruptly from the horizon, because his path is at right angles 
 to it, or very nearly so ; and the quick transition from the glare 
 of day, to utter darkness, cuts off the enjoyment of the twi- 
 light hour. 
 
 From the revolution of the earth round its axis. Sir Isaac 
 Newton was led to suppose, that its shape was not a perfect 
 sphere, but that of an oblate spheroid, flattened at the poles ; 
 because in revolving, its parts have a tendency to recede from 
 the axis, from the equator particularly, consequently the poles 
 press internally, and raise the equatorial parts, till an equili- 
 brium occurs. Sir Isaac Newton calculated that the equato- 
 rial exceeds the polar diameter 34 miles and one-fifth ; and this 
 instance of his penetration was afterwards fully confirmed by 
 the admeasurements and observations of two deputations of 
 mathematicians, who visited the vicinity of the northern and 
 southern poles in 1735, and agreed in pronouncing them flat- 
 tened, making the difference between the diameters as 266 to 
 265, or as 179 to 178. Another proof of the flatness of the 
 poles has been mentioned at page 277, where we have observ- 
 ed, that a pendulum, calculated to swing seconds at the poles, 
 or in a high latitude, will not swing seconds at the equator 
 unless it be shortened; therefore, as a pendulum swings by the 
 power of gravity, that power is rather less at the equator than 
 at the poles ; partly on account of the greater distance of those 
 parts from tlie equator, and partly from their greater centrifu- 
 gal force, which tends to throw every thing in a tangent from 
 its surface. If tlie diurnal motion of the earth were to cease, 
 its centrifugal force would of course be destroyed at the same 
 time, in which case, the waters of the ocean would flow to- 
 wards the poles, in order to restore the perfectly globular form 
 of the earth, and would overwhelm those regions at the same 
 time that they retired from the shores of the equator. An ex- 
 periment may easily be contrived to shew the tendency of a 
 swift whirling motion to produce an obl-ate spheroid like the 
 earth. Let two slips of pasteboard, or any tolerably flexible 
 material, be bent into a circular form, and fitted upon an axis, 
 as shewn by fig. 3, plate III ; so as to turn with that axis. 
 When they are made to revolve very slowly by turning the 
 winch G, the change in their form will not be perceptible ; but 
 if the winch be turned with rapidity, the poles of the hoops 
 are depressed, and their sides bulge out. 
 
 That the apparent diameter of the sun is greater in winter 
 than in summer, is obvious to common observation. The rea- 
 son is, the earth's elliptical orbit, in the lower focus of which 
 the sun is situated, and as this point is 1,617,941 miles from 
 Vol. T.— 23. 4 A 
 
54G ASTROXOMY. 
 
 Solar system reviewed. — The Earth. 
 
 the centre of the orbit, the earth comes twice that distance, or 
 3,235,882 miles nearer the sun at one time of the year than 
 another, and its nearest approach is in our winter, for it is then 
 the sun appears under the largest angle. Hence it may be 
 inquired, why have we not the hottest weather when the earth 
 is nearest the sun? In the tirst place, it must be observed in 
 answer, that the eccentricity of the earth's orbit, or 1,617,9-11 
 miles, bears no greater proportion to the earth's mean distance 
 from the sun, than 17 does to 1,000; and therefore this small 
 diti'erence of distance is in itself of little consequence. But 
 the principal cause of the difference between winter and sum- 
 mer is, that in the former season, the sun's rays fall so ob- 
 liquely upon us, that any siiven number of them is spread over 
 a much greater surface in the latitude in which we live. An- 
 other cause contributing greatly to tiie winter's cold, is the 
 length of the nights, which carry oft' more heat than the sun 
 imparts during the day. In summer, on the contrary, the sun 
 attaining a considerable altitude, his rays dart down upon us 
 more nearly perpendicular, a sreat number of them fall upon 
 the same space, and, by their long continuance, more heat is 
 imparted by day than can fly ofl' by night. The atmosphere 
 is heated, not by the passage of the sun's rays through it, but 
 from the earth, which absorbs tlie rays, and when the earth is 
 once well heated, it does not lose its temperature very readily, 
 because the air carries off heat slowly. We have therefore a 
 greater number of warm days, and all our hottest weather, 
 after the longest day, though the sun's meridian altitude from 
 that day begins to decline; and this is only stating with regard 
 to a season, a fact of which we have diurnally a proof, viz. that 
 it is hotter at tu o or three in the afternoon, when the sun has 
 gone towards the west, than at noon when he is on the meri- 
 dian. By parity of reasoning, it is equally evi'dent, that we 
 shall have the greatest number of cold days, and the coldest 
 weather, after the shortest day, though after that time, the sun 
 is actually higher every day ; but the heat received by the 
 Earth is so trifling, that the long night is not only adequate 
 to counteract its influence, but has power enough to reduce 
 it for some time lower and lower. 
 
 The Moon, d 
 
 Ne.xt to the sun, the Moon appears to us the most splendid 
 of all the heavenly bodies. Her orbit is an ellipse, with the 
 Earth in one of its foci; her revolution round the earth is per- 
 formed in 29 days, 17 hours, 44 minutes, 3 seconds, and with 
 the earth she revolves round the sun in a year. Her diameter 
 is 2180 miles, and her distance 240,000 miles. It is therefore 
 
A fSIl?. ©Zf (D)M!K 
 
 fLja. 
 
 JPriHsLei tiy JT enry tiker . Caslan i-aaion - 
 
ASTRONOMY. 547 
 
 Solar system reviewed. — The iuood. 
 
 her comparative nearness to us, that makes her appear so large, 
 and afford so much light. Her rotation on her axis is per- 
 formed in the same time as her revolution round the earth, 
 hence she always keeps the same side towards us, except that 
 we sometimes see a little more of one side, and at other times 
 a little more of another side. This is called her libratioii. 
 This libration is produced by her unequal motion in her orbit, 
 and from the difference in its direction, it is distinguished into 
 two kinds; 1, Libration in latitude; 2, Libration in longitude. 
 Her libration in longitude, is her seeming motion to and fro, so 
 as to shew sometimes more of her eastern edge, and sometimes 
 more of her western edge. Her libration in latitude, is 
 when either of her poles appears to dip a little towards the 
 earth. 
 
 The moon is an opaque globe like the earth, and shines 
 only by the light she receives from the sun, and reflects to us. 
 She therefore disappears when she comes between us and the 
 sun, because her dark side is then towards us. Thus when 
 she is at A, fig. 4, plate III, in conjunction with the sun S, 
 her dark half is towards the earth, and she disappears as at a, 
 there being no light on that half to render it visible. On the 
 innermost circle, the moon is delineated as she would appear 
 to a spectator in the sun ; on the outermost circle, is shewn 
 her appearance to a spectator on the earth at T. When she 
 comes to her first octant at B, or has gone an eighth part of 
 her orbit from her conjunction, a quarter of her enlightened 
 side is towards the earth, and she appears horned as at b. 
 When she has gone a quarter of her orbit from between the 
 earth and sun to C, she shews us one half of her enlightened 
 side, as at c, and we say she is a quarter old. At D she is in 
 her second octant; and by shewing us more of her enlightened 
 side, she appears gibbous as at d. At E her whole enlightened 
 side is towards the earth ; and therefore she appears round, 
 as at e, and we say it is full moon. In her third octant at F, 
 part of her dark side being towards the earth, she again ap- 
 pears gibbous, and is on the decrease as at f. At G we see 
 just one half of her enlightened side; and she appears half 
 decreased, or in her third quarter, as at g. At H we only see 
 a quarter of her enlightened side ; being in her fourth octant, 
 where she appears horned as at li. And at A, having com- 
 pleted her course from the sun to the sun again, she disap- 
 pears ; and we say it is new moon. Thus in going from A to 
 E, the moon seems continually to increase ; and in going from 
 E to A, to decrease in the same proportion ; having like 
 phases at equal distances from A to E, but as seen from the 
 sun S, she is always full. 
 
543 ASTRONOMY. 
 
 Solar system re\iewed. — The Moon. 
 
 The moon appears not perfectly round when she is full in 
 the hii^hest or lowest part of her orbit, because we have not a 
 full view of her enliohtened side at that time. When full in 
 the highest part of her orbit, a small deficiency appears on 
 her lowest edge ; and the contrary when full in the lowest part 
 of her orbit. 
 
 It is evident from the figure, that when the moon changes 
 to the earth, the earth appears full to the moon, and vice 
 versa. For when the moon is at A, new to the earth, the 
 whole enlightened side of the earth is towards the moon ; and 
 when the moon is at E, full to the earth, its dark side is to- 
 wards her. Hence a new moon answers to a full earth, and a 
 full moon to a new earth. The quarters are also reversed to 
 each other. 
 
 Between the third quarter and change, the moon is fre- 
 quently visible in the forenoon, even when the sun shines ; 
 and then she affords us an opportunity of trying an amusincr 
 experiment, if we can find a globular stone above the level of 
 the eye, as suppose on the top of a gate. For if the sun 
 shine on the stone, and we place ourselves so that the upper 
 part of the stone may just seem to touch the point of the 
 moon's lowermost horn, we shall then see the enlightened 
 part of the stone exactly of the same shape with the moon, 
 horned as she is, and inclined the same way to the horizon. 
 The reason is plain ; for the sun enlightens the stone the 
 same way that he does the moon ; and both being globes, 
 •when we put ourselves into the above situation, the moon 
 and the stone have the same position to our eyes, and there- 
 fore we can see no more of the illuminated side of the one 
 than of the other. 
 
 The position of the moon's cusps, or a right line touching 
 the points of her horns, is very differently inclined to the 
 horizon, at difterent hours of the same days of her age. Some- 
 times this line is perpendicular to the horizon ; when this hap- 
 pens, she is in what astronomers call the /tonagesimal degree, 
 ■which is the highest point of the ecliptic above the horizon at 
 that time, and is 90 degrees from both sides of the horizon 
 where it is then cut by the ecliptic. But this never happens 
 •when the moon is on the meridian, except when she is at the 
 very beginning of Cancer or Capricorn. 
 
 It is apparent to the naked eye, that the surface of the moon 
 is irregular, from the different shades of colour which it exhi- 
 bits; and when viewed through a good telescope, it appears 
 diversified with every variety of hill and dale. In every 
 situation of the moon, the elevated parts of her surface cast a 
 shadow on the adjoining lower parts in the direction from the 
 
ASTROXOMY. 549 
 
 Solar svstera reviewed — The Moon. 
 
 sun, and the cavities are always dark on the side next the sun. 
 The darker parts were at one time very generally believed to 
 be water, but it is now generally admitted that they are deep 
 cavities or valleys, and that the appearance of the moon's disc 
 is such as our earth would present to her inhabitants, if the 
 ocean was dried up. The heights of the lunar mountains were 
 formerly supposed to be much greater than those of the earth; 
 But Dr. Herschel has demonstrated that very few are more 
 than half a mile high, and the highest little more than a mile. 
 Before or after the moon is at full, the boundary of the light 
 and dark part of the side presented to us, in consequence of 
 the multiplicity of her caverns and mountains, appears ex- 
 ceedingly jagged or uneven; while, contrary to what we 
 should expect, the real boundary of her disc, or the boun- 
 dary between the hemisphere which we see, and that which 
 is turned away from us, appears smooth and well defined, 
 though examined with a considerable magnifying power. In 
 explanation of this difficulty, it has been observed, that as 
 all the parts adjoining to the real boundary or edge of the 
 moon are full of irregularities, the elevations of some parts 
 may stand before the hollows of others, so as to form, upon 
 the whole, the appearance of a smooth surface, and perhaps 
 the atmosphere of the moon may likewise contribute to this 
 appearance. 
 
 The ruggedness of the moon's surface adds greatly to her 
 utility as an attendant of the earth ; for if her disc were free 
 from all asperities, or covered with water, her light would 
 only in certain situations shew us the image of the sun, appa- 
 rently nothing more than a brilliant point. But her inequali- 
 ties scattering the light on every side, her disc becomes en- 
 tirely resplendent. 
 
 Bright specks have been observed on the dark parts of the 
 moon's disc, and so far from the illuminated portion as not 
 to depend on the sun's rays. These spots, becoming extinct 
 after a certain time, have been supposed to be volcanoes. 
 Dr. Herschel, in 1787, saw three of these volcanoes at once 
 in the dark part of the moon : two of them were barely 
 visible ; but the third was more vivid, and exhibited an elon- 
 gation like an eruption or lava of luminous, matter, resem- 
 bling a small piece of burning charcoal, covered by a very 
 thin coat of ashes. If volcanoes or fire exist in the moon, 
 it seems reasonable to conclude, that the moon has an atmo- 
 sphere, and that this is the case, there is good reason to 
 believe, though it has been much controverted, and even 
 denied till lately. Some astronomers think that they can 
 attribute the variable brightness of the moon, at certain 
 
550 ASTKONOMY. 
 
 Sular system reviewed. — The Moon. 
 
 times, to no other cause than the variable state of her atmo- 
 sphere, more or less loaded with vapours. Cassini observed, 
 that Saturn, Jupiter, and the fixed stars, had their circular 
 figures changed into elliptical ones, when they approached 
 either the dark or the illuminated edge of" the moon; and to 
 these indications of a lunar atmosphere, Schroeter adds the 
 following; that the two cusps or apices of the luminous horns, 
 in a new moon, appear tapering in a very sharp and faint pro- 
 longation. He also observed that, when once Jupiter came 
 very near the moon, two of his satellites appeared indistinct 
 for a short time before they went quite behind the body of 
 the moon. If the moon have really an atmosphere, still we 
 need not be surprised at the difficulty with which it is distin- 
 guished, for it probably is extremely attenuated ; Laplace in- 
 deed calculates that it must be more rare than what we call 
 the vacuum of our best air-pumps. 
 
 Excepting the occasional changes attributed to the vol- 
 canoes and to the atmosphere of the moon, the general ap- 
 pearance of this luminary is constantly the same ; and seleno- 
 gnip/iia, or maps of the face of the moon, have frequently 
 been published ; the most remarkable spots in these maps, 
 have been sometimes distino;uished by the names of places 
 on the earth, but mostly by the names of eminent persons, 
 as Plato, Archimedes, &,c. The most accurate transcript 
 of the moon's disc is perhaps that executed by John Rus- 
 sel, R. A. 
 
 The moon has scarcely any difference of seasons, her axis 
 being almost perpendicular to the ecliptic. What is very 
 singular, one half of her has no darkness at all ; the earth 
 constantly affording it a strong light in the sun's absence ; 
 while the other half has a fortnight's darkness and a fort- 
 night's light by turns. The earth, as before observed, acting 
 as a moon to the moon, will from her be seen to wax and 
 wane regularly, but will appear about 13 times as large, and 
 afford about 13 times as much light as she does to us. 
 
 From the moon, one half of the earth is never seen ; from 
 the middle of the other half of the moon, tlie earth is always 
 seen over head, turning round almost 30 times as quickly as 
 the moon does. From the circle which limits our view of the 
 moon, only one half of the earth's side next her is seen ; the 
 other half being hid below the horizon of all the places on 
 that circle. 
 
 As the earth turns round its axis, the several continents, 
 seas, and islands, appear to the moon's inhabitants like so 
 many spots of different forms and brightness, moving over 
 its surface ; but much fainter at some times than others^ 
 
ASTRONOMY. 551 
 
 S'olar system reviewed. — The Moon. 
 
 as our clouds cover them or leave them. By these spots 
 the lunarians can determine the time of the earth's diurnal 
 motion, just as we do the motion of the sun; and their obser- 
 vations will at the same time furnish them with an accurate 
 measure of time. 
 
 The moon's axis being nearly perpendicular to the ecliptic, 
 the sun never removes sensibly from her equator ; and the 
 obliquity of her orbit, which is next to nothing as seen from 
 the sun, cannot cause the sun to decline sensibly from her 
 equator. Yet her inhabitants are not destitute of means for 
 ascertaining the length of their year, though their method 
 and ours must differ. We can know the length of our year 
 by the return of our equinoxes ; but the lunarians having 
 always equal day and night, must have recourse to another 
 method ; and we may suppose, they measure their year by 
 observing when either of the poles of our earth begins to be 
 enlightened, and the other to disappear, which is always 
 at our equinoxes; they being conveniently situated for ob- 
 serving great tracts of land about tlie poles of our earth, 
 which are entirely unknown to us. Hence we conclude, that 
 the year is of the same absolute length both to the earth and 
 moon, though very different as to the number of days ; we 
 having 365^ natural days, and the lunarians only 12^\%, the 
 day and night in the moon being together as long as 2y^ on 
 the earth. 
 
 The lunar rays excite no perceptible warmth, not even 
 when concentrated upon a thermometer, by the largest burn- 
 ing mirror or lens. This is accounted for on the supposition 
 of their extreme diffusion, so that we are unable to collect a 
 sufficient quantity of them to evidence their heating effects. 
 The calculations which have been made of the tenuity of the 
 moon's light, differ so widely from each other, that we know 
 not which to credit; some having asserted that it is ninety 
 thousand, and others that it is three hundred thousand times 
 less than that of common day-light. But all calculations 
 founded on the supposed fact that the reflected light and 
 heat are diminished in equal proportions, are probably fal- 
 lacious: she may reflect little or no heat, yet her light may 
 reach us in a far greater proportion than has yet been al- 
 lowed. 
 
 The moon has not received more attention from the astro- 
 nomer than the poet; but while the recondite speculations of 
 the one please but few, and often almost bewilder the keenest 
 understanding, the other depicts only those benignant effects 
 which every eye can behold, and every heart enjoy. The 
 following night-piece from Homer, is esteemed by Pope, its 
 
552 ASTRONOMY 
 
 Solar svsteni reviewed. — ^Iar?<. 
 
 translator, to be supereminent in the orisjinal, and it will not be 
 denied the tribute of admiration in its Enfjlish dress: 
 
 " So when the Moon, refulgent lamp of night, 
 O'er heaven's clear azure spreads her sacred light. 
 When not a breath disturbs the d'tep serene. 
 And not a cloud o'ercasts the solemn scene; 
 Around her throne the vivid planets roll, 
 And stars unnuraber'd gild the glowing pole. 
 O'er the dark trees a vellower verdure shed, 
 And tip with silver every mountain's head \ 
 Then shine the vales ; the rocks in prospect rise ; 
 A flood of glory hursts from all the skies ; 
 The conscious swains, rejoicing in the sight. 
 Eye the blue vault, and bless the useful light." 
 
 Mars. $ 
 
 The next planet beyond the earth in the solar system, is 
 Mars, whose distance from the sun is 144,000,000 of miles, and 
 •who requires nearly 687 days to complete his annual revolu 
 tion. The colour of this planet is a duskv red, an appearance 
 attributed to the 2:;reat density of his atmosphere, which per- 
 mits only the red rays to be reflected to us. From the spots 
 which have been observed on his disc, his rotation on his axis 
 has been ascertained to be performed in 24 hours, 39 minutes, 
 22 seconds, and the inclination of his axis to his orbit is 59*^ 
 22'. His equatorial is to his polar diameter as 16 to 15, and 
 his mean diameter is 4189 miles. 
 
 INIars is not subject to the same limitation in his motion as 
 Mercury or Venus, but appears sometimes very near the sun, 
 and at other times at a great distance from him ; sometimes 
 rising when the sun sets, or setting when the sun rises. He 
 sometimes appears gibbous, but never horned, like the moon, 
 a proof that his orbit includes that of the earth, and that he 
 shines by a borrowed light. When he is opposite to the sun, 
 or when we see him on the meridian at midnight, he is much 
 more brilliani than in another situation, being five times nearer 
 to us than at a conjunction. 
 
 The sun affords to Mars only about a third of the light he 
 affords to the earth, and it has therefore been thought singu- 
 lar, that he has not been discovered to have a moon ; but per- 
 haps he is compensated for this apparent want, and the light 
 he receives may be prolonged, by the height and density of 
 his atmosphere, which is so remarkable, \hat when he ap- 
 proaches any of the fixed stars, they change their colour, 
 grow dim, and often become nearly invisible, though at some 
 little distance from the body of the planet. 
 
ASTRONOMY. 553 
 
 Solar system reviewed. — Mars. 
 
 Mars appears to move from west to east round the earth, 
 but his apparent motion is very unequal. When we first per- 
 ceive him in the morning, separating from the sun, his motiori 
 is the most rapid; but this rapidity diminishes gradually, and 
 ceases altogether when the planet is about 137'^ distant from 
 the sun; then his retrograde motion commences, and increases 
 in rapidity till he comes into opposition with the sun. It then 
 gradually diminishes again, till the planet is within 137*^ of 
 the sun; it then becomes direct, till at last the planet is lost 
 for a time in the evening rays of that luminary. As his dis- 
 tance from us is so different at different times, so is his appa- 
 rent diameter, which at a mean is 27", but when in opposition, 
 his apparent diameter is 81". 
 
 Besides the dark spots, which serve to determine the diurnal 
 revolution of Mars, several early astronomers took notice that 
 a segment of his globe about the south pole exceeded the rest 
 of his disc so much in brightness, as to appear as if it were 
 the segment of a larger globe. Maraldi informs us, that this 
 bright spot had been taken notice of for sixty years, and was 
 more permanent than the other spots on the planet. One part 
 of it is brighter than the rest, and the least bright part is sul)- 
 ject to great changes, and has sometimes disappeared. A 
 similar brightness about the north pole was also sometimes 
 observed, and these observations are now confirmed by Dr. 
 Herschel, who has viewed the planet with much better instru- 
 ments, and much higher magnifying powers, than any other 
 astronomer. "The analogy," says the Doctor, " between Mars 
 and the earth, is by far the greatest in the whole solar system. 
 Their diurnal motion is nearly the same ; the obliquity of their 
 respective ecliptics not very different. Of all the superior 
 planets, the distance of Mars from the sun is by far the nearest 
 alike to that of the earth ; nor will the length of the Martial 
 year appear very different from what we enjoy, when com- 
 pared to the surprising duration of the years of Jupiter, Sa- 
 turn, and the Georgium Sidus. If then we find that the globe 
 we inhabit has its polar region frozen and covered with moun- 
 tains of ice and snow, that only partly melt when alternately 
 exposed to the sun, I may well be permitted to surmise, that 
 the same causes may probably have the same effect on the 
 globe of Mars ; that the bright polar spots are owing to the 
 vivid reflection of light from frozen regions, and that the re- 
 duction of those spots is to be ascribed to their being exposed 
 to the sun. In the year 1781, the south polar spot was ex- 
 tremely large, which we might well expect, as that pole had 
 but lately been involved in a whole twelvemonth's darkness and 
 absence from the sun ; but in 1783, I found it considerably 
 Vol. I.— 24. 4 B 
 
554 ASTROVOMY. 
 
 Solar syilem reviewed. — Vesta. — Jano. — Cerc». 
 
 smaller than before, and it decreased continually from the 
 20th of May, till about the middle of September, when it 
 seemed to be at a-stand. During this last period, the south 
 pole had already been above eight months enjoying the benefit 
 of summer, and still continued to receive the sun-beams, 
 tliough, towards the latter end, in such an oblique direction 
 as to be but little benefited by them. On the otlier hand, in 
 the year 1781, the north polar spot, which had been its twelve- 
 month in the sunshine, and was but lately returning into ilark- 
 ness, appeared small, though undoubtedly increasing in size. 
 Its not being visible in the year 1783, is no objection to these 
 phenomena, being owing to the position of the axis, by which 
 it was removed out of sight." 
 
 Vesta. 
 
 This small planet was discovered by Dr. Olbers, of Bremen, 
 on the 29th of March, 1807. Its diameter is about 238 miles. 
 Its distance from the sun is almost 215,000,000 of miles, and 
 its annual revolution round the sun is performed in 1335 days. 
 The inclination of its orbit to the ecliptic is 7°. Examined 
 by Dr. Herschel, with an excellent hfteen-feet reflector, and 
 a power of 300, it exhibited no appearance of a disc, but ap- 
 peared like a fixed star of the sixth magnitude ; that is, it was 
 nearly a point, with an intense radiating light. When the sky 
 was clear, it might be discerned with the naked eye. 
 
 Juno. 
 
 This is another small planet, which was discovered by Hard- 
 ing, of Lilienthal, on the 1st of September, 1803. Its diame- 
 ter is 1425 miles, its distance from the sun 243,000,000 of 
 miles; and its annual period 1590 days. The inclination of 
 its orbit to the ecliptic is 14"^. 
 
 Ceres. 
 
 This planet was discovered on the 1st of January, 1801, by 
 Piazzi, an Italian astronomer, whose name it not unfrequently 
 receives. Its distance from the sun is nealy 263,000,000 of 
 miles. Its diameter is only about 163 miles. The time of its 
 diurnal motion is unknown; but its annual sidereal revolution 
 is calculated, by Laplace, to be performed in 1681 days, 17 
 hours, 57 seconds. The inclination of its orbit to the ecliptic 
 is about 11*^ 48'. It appeared to Herschel of a ruddy colour, 
 but not very deep; and he is of opinion that it has an atmo- 
 
ASTRONOMY. 555 
 
 Solar system reviewed. — Pallas. 
 
 sphere. Its disc seldom appears well defined, apparently 
 because a very slight degree of an unfavourable state in our 
 atmosphere, for viewing it, affects its feeble light. 
 
 Pallas. 
 
 This planet was also discovered by Dr. Olbers, of Bremen, 
 on the 28th of March, 1802. Its distance from the sun is 
 nearly 264,000,000 miles; its sidereal period is 1681 days, 17 
 hours, 57 seconds ; and its diameter is only 80 miles, so that 
 it is the smallest known planet of the Solar System. The in- 
 clination of its orbit to the ecliptic amounts to about 38*^. 
 It is of a dusky whitish colour, and appears very indistinct 
 with a power of 500, unless the atmosphere be remarkably 
 clear. 
 
 Dr. Olbers received from the French National Institute, La- 
 lande's prize of 10,000 francs (£417) a sum appropriated to the 
 person whose labours most contributed to the promotion of 
 astronomical knowledge, in the year of its discovery. 
 
 The orbits of the Ceres and Pallas would have been most 
 properly represented in the plate of the Solar System, by 
 circles of nearly equal size, but with their centres a little on 
 opposite sides of that of the sun. 
 
 Soon after the Ceres and Pallas had been discovered. Dr. 
 Herschel published some observations upon them; and taking 
 into consideration their great deviation from the zodiac, or 
 track of the planets previously known, he observed, that if 
 they were admitted into the order of planets, we must give up 
 the zodiac ; for by extending it to them, should a few more be 
 discovered, still farther deviating from the path of the earth, 
 we might soon be obliged to convert the whole firmanent into 
 a zodiac, that is, we should have none at all. Hence he pro- 
 posed the name of asteroids, for planetary bodies of the de- 
 scription in question; and made the definition of that term so 
 comprehensive, as to include all that were likely to be disco- 
 vered : and he distinctly stated his belief that more planetary 
 bodies deviating from the zodiac would be discovered : a con- 
 jecture which was realized in a short time by the discovery of 
 the Juno and Vesta. Dr. Thompson, in his History of the 
 Royal Society, of which society Dr. Herschel is an illustrious 
 member, intimates his disapprobation of the term asteroids, 
 observing that he can see no reason why the stars it is intended 
 to designate ought not to be called planets, unless it were to 
 prevent their discoverers from ranking so high as the disco- 
 verer of the Georgium Sidus. This insinuation appears too 
 gratuitous to be considered either generous or just. 
 
556 ASTRONOMY. 
 
 Sular svstein re»iewed. — Jnpiter and bis foar Satellite- 
 
 Jupiter. % 
 
 Jupiter is the largest of all the planetary bodies, and next 
 to Venus, appears to us the brightest. His prodigious dis- 
 tance, which is 490,000,000 of miles from the sun, is the rea- 
 son of his apparent size being less than that of Venus, though 
 his real magnitude is above 1500 times greater, his diameter 
 being 89,170 miles. The length of his year is 4330 days, 14 
 hours, 39 minutes, 2 seconds. His diurnal rotation is per- 
 formed in the short space of 9 hours, 55 minutes, 37 seconds. 
 This rotation is so amazingly rapid, that it carries his equa- 
 torial inhabitants 28,800 miles every hour, which is nearly 
 4000 miles more than the inhabitants of our earth's equator 
 are carried by the same motion in 24 hours, and almost equals 
 the hourly advance in his orbit. The remarkable spot, by the 
 motion of which Jupiter's rotation on his axis was determined, 
 appeared in 1694, and was lost till the year 1708, when it re- 
 Jtppeared, on the same part of his face, and has been occasion- 
 ally seen ever since. 
 
 iFrom the greater rapidity of his diurnal rotation, his figure 
 is that of a much flatter oblate spheroid than the earth, his 
 equatorial being to his polar diameter as 13 to 12, a proportion 
 which makes the former measure 6000 miles more than the 
 latter. His axis is very nearly perpendicular to the plane of 
 his orbit, so that his inhabitants have no perceptible change 
 of seasons. If his axis had been inclined any considerable 
 number of detrrees, just so many degrees round each pole 
 rtould alternately be involved in darkness for almost six of our 
 years together ; and as each degree of a great circle of Jupiter 
 contains at the least 800 of our miles, the tracts of land thus 
 deprived of the sun, would be immense. The sun appears to 
 him only a fifth of the size he does to us, and his light and 
 heat are onl^ a twentv-fifth of ours. But his night is on no 
 part of his surface five hours Ions: ; at his polar regions, the sun 
 never sets, he enjovs a perpetual sprins:, and he has the splen- 
 did attendance of four moons, one or more of which are always 
 visible to enliven his short night. It is easy, from all these 
 circumstances, to conceive, that this spacious orb may be 
 tlie residence of a race af beings very little dissimilar to our- 
 selves. 
 
 AVhen Jupiter is viewed through a good telescope, we per- 
 ceive a number of zones or belts, of a darker colour than the 
 rest of his disc. They are generally parallel to his equator, 
 which is very nearly parallel to the ecliptic, but in other 
 respects are subject to great variations. Sometimes only one 
 
ASTRONOMY. 557 
 
 Solar system reviewed. — Jupiter and bis four Satellites. 
 
 can be perceived, at other times eight. They are sometimes 
 not parallel to one another, and their breadth is very variable, 
 one belt having been observed to grow narrow, while another 
 in its neighbourhood has increased in breadth, as if the one 
 had flowed into the other; and in this case Dr. Long ob- 
 serves that a part of an oblique belt lay between them, as if 
 to form a communication for this purpose. The time of their 
 continuance is very uncertain ; sometimes remaining un- 
 changed for three months ; at others, new belts have been 
 formed in an hour or two. The continuity of the belts is 
 sometimes interrupted, so as to give them a broken appear- 
 ance. The spots and belts seen on the 7th of April, 1792, are 
 represented by fig. 5, pi. III. The belts and spots are con- 
 sidered to be the body of the planet, and the light parts the 
 clouds, transported by the winds with diflerent velocities, and 
 in different directions. 
 
 The four satellites of Jupiter are distinguished by their situa- 
 tion, that being called the first which is nearest to its primary. 
 Their periods and distances are as follows : 
 
 Day. h. lu. s. Miles. 
 
 1st satellite revolves in 1 18 27 33 at the distance of 259,170 
 
 2nd .. .. 3 13 13 42 •• 412,352 
 
 3rd .. .. 7 3 42 33 •• 657,735 
 
 4th .. .. 16 16 32 8 •• 1,156,640 
 
 These satellites move round Jupiter from west to east, and 
 their orbits are supposed to be ellipses, though the eccentri- 
 city of all of them is too small to be measured, except that of 
 the fourth ; and even this amounts to more than 0,007 of its 
 mean distance from the primary. The motions of the first 
 three satellites, Laplace has observed, are related to each other 
 by a most singular analogy. The mean sidereal or synodical 
 motion of the first, added to twice that of the third, is con- 
 stantly equal to three times the mean motion of the second. 
 And the mean sydereal or synodical longitude of the first, 
 minus three times that of the second, plus twice that of the 
 third, is always equal to two right angles. 
 
 When the satellites fall into the shadow of the primary, we 
 loose sight of them for a time, and they are said to b€ eclipsed. 
 The three nearest are eclipsed every revolution; but the orbit 
 of the fourth is so much inclined, that it passes by its opposi- 
 tion to Jupiter, without falling into his shadow, two years in 
 six. From the singular analogy, above alluded to, it follows 
 that (for a great number of years at least) the first three satel- 
 
 tes cannot be eclipsed at the same time : for in the simul- 
 
558 ASTRONOMY. 
 
 Solar sjstem reviewed. — Jupiter's Satellites. 
 
 taneous eclipses of the second and third, the first will always 
 be in conjunction with Jupiter, and vice versa. By these 
 eclipses, has been discovered the velocity of light, (see Op- 
 tics,) and they serve a still more important purpose, — that of 
 determining the longitude of places on the earth, with great 
 facility and certainty. The longitude of a place is its distance 
 east or west from another, reckoned in degrees of the equator, 
 and it can always be ascertained with certainty, provided we 
 know the time of the day at the place from which we reckon, 
 as well as that of the place where we make the observation. 
 Since by the motion of the earth round its axis, every point 
 on its surface describes the circumference of a circle, or 360", 
 in twenty-four hours, it must consequently describe 16^ in one 
 hour, because 15 is a 24th part of 360. Hence the difference 
 of longitude may be converted into time, by allowing one hour 
 for every 15'^, and proportionally for minutes and seconds; or 
 conversely, with equal ease, difference of time may be converted 
 into longitude. Voyagers, therefore, if provided with a fault- 
 less timekeeper, set at their departure exactly to the time of 
 the place from which they sailed, may at any time ascertain 
 their longitude, or distance east or west from that place, with 
 great facilit}-. Timekeepers are, however, subject to so many 
 causes of imperfection, that they cannot be depended on, for 
 any long period : but as the eclipses of Jupiter's satellites are 
 more numerous than the days of the year, and Jupiter can be 
 seen for eleven months in the year, and as there is generally 
 one, and often two or three in one night, the observation of 
 them may be made to answer the same purpose as a time- 
 keeper. For example, the exact times at which the eclipses 
 of Jupiter's satellites will occur, at the meridian of Greenwich, 
 are given for several years to come, in the nautical ephemeris; 
 let a person then, at any distance east or west from Greenwich, 
 notice the eclipse of a satellite, and compare it with the time 
 given for the same eclipse in his tables; he then perceives at 
 once the difference of time, and consequently his longitude. 
 To observe these eclipses with precision, does not require a 
 telescope of great power; but the principal disadvantage of 
 the plan at sea arises from the motion of the ship. 
 
 Satiini. b 
 
 Saturn is next to Jupiter in order from the sun, and the 
 next to that ])lanet in magnitude. His distance from the sun 
 is 900 millions of miles, and therefore we may readily account 
 for the paleness and feebleness of the light he reflects to us, 
 and that he can hardly be distinguished from a fixed star by 
 
ASTRONOMY. 559 
 
 Solar system reviewed. — Saturn. 
 
 the naked eye. His diameter is 79,042 miles, and he em- 
 ploys 10,746 days, 19 hours, 16 minutes, 15 seconds, in re- 
 volving once round the sun, so that the length of his year is 
 almost thirty of ours. His diurnal rotation is believed to be 
 performed in 10 hours, 16 minutes, 2 seconds; but the spots, 
 the motion of which determine this point, are not well de- 
 fined. The inclination of his orbit to the ecliptic is about 2g 
 degrees ; but the inclination of his axis to his orbit is sup- 
 posed to be 60°. 
 
 Saturn, when viewed through a good telescope, makes a 
 more remarkable appearance than any of the other planets. 
 He has belts, somewhat like those of Jupiter, though fainter : 
 but his distinguishing characteristic is a broad, double, lumi- 
 nous ring, entirely surrounding him. The appearance of the 
 planet through a good telescope is shewn by fig. 6, plate III. 
 The ring is detached from his body in such a manner, that 
 the distance between the innermost part of the ring and the 
 body of the planet is equal to its breadth. Both the outward 
 and inward rim of the ring is projected into an ellipsis, more 
 or less oblong, according to the different degrees of obliquity 
 with which it is vi3wed. Sometimes our eye is in the plane 
 of the ring, and then it becomes invisible even when sought 
 for with very good telescopes, probably because it is in that 
 position too thin to subtend a sufficient angle, at such a 
 distance. But sometimes it may be detected with a tele- 
 scope of peculiar excellence, and a favourable night, and 
 then it appears like a very slender ray of light across the 
 disc of the planet. As the plane of this ring is always pa- 
 rallel to itself, that is, its situation in one part of the orbit 
 is always parallel to that in any other part, it disappears 
 about every 15 years, or twice in every revolution of the 
 planet, which sometimes app-^iars quite round for nine months 
 together. At other times, the distance between the body of 
 the planet and the ring is very perceptible, so that stars may 
 be seen through it. 
 
 When Saturn appears round, if our eye be elevated above 
 the plane of the ring, a shadowy belt will be visible, caused 
 by the shadow of the ring, as well as by the interposition of 
 part of it between the eye and the planet. The shadow of the 
 ring is broadest when the sun is most elevated, but its obscure 
 parts appear broadest when our eye is most elevated above the 
 plane of it. When it is seen double, the ring next the body 
 of the planet appears brightest. When the ring appears of an 
 elliptical form, the parts about the ends of the largest axis are 
 called the ansa. These, a little before and after the disappear- 
 ing of the ring, are of an unequal magnitude; the largest ansa 
 
560 ASTRO XOMY 
 
 Sular »T^tem reviewed. — Saturn's Riog. 
 
 is lonojer visible before the planet's round phase, and appears 
 again sooner than the other. Herschel demonstrates, that 
 the ring revolves in its own plane, in lOh. 32 15.4'. This 
 philosopher has paid great attention to Saturn's ring. Ac- 
 cording to him, there is one single, dark, considerably broad 
 line, belt, or zone, which he has constantly found on the 
 north side of the ring. As this dark belt is subject to no 
 change whatever, it is probably owing to some permanent 
 construction of the surface of the ring: this construction 
 cannot be owing to the shadow of a chain of mountains, 
 since it is visible all around on the ring ; for there could 
 be no shade at the ends of the ring; and a similar argument 
 will apply against the opinion of very extended caverns. It 
 is pretty evident that this dark zone is contained between 
 two concentric circles, for all the phenomena correspond 
 ■with the projection of such a zone. The nature of the ring 
 the Doctor thinks no less solid than that of the planet itself, 
 and it is observed, as above mentioned, to cast a strong sha- 
 dow on the planet. The light of tiie ring is also generally 
 brighter than that of the planet, for the rins; appears suffici- 
 ently bright, with a telescope that affords scarcely light 
 enough to see Saturn. He concludes that the edge of the 
 ring is not flat, but spherical, or spheroidal. The dimensions 
 of the ring, or of the two rings with the space between them, 
 he estimates as follows : 
 
 Miles. 
 
 Inner diameter of the small ring 145',345 
 
 Outside diameter of ditto 184,393 
 
 Inner diameter of the larger ring 190,248 
 
 Outside diameter of ditto 204,883 
 
 Breadth of the inner ring 20,000 
 
 Breadth of the outer ring 7,200 
 
 Breadth of the vacant space or dark zone 2,839 
 
 If this ring be opaque, as the sun shines fifteen years on its 
 north side, and the same time on its south side, it will have 
 equal day and night, each fifteen years long. 
 
 The swiftness of Saturn's motion on his axis produces an 
 oblate figure; and his equatorial diameter is calculated to be 
 to his polar diameter, as 11 to 10. 
 
 The sun scarcely affords to Saturn an eightieth part of the 
 direct light that we receive from him; but this planet, besides 
 his magnificent ring, is accompanied by not less than seven 
 satellites, which revolve round him beyond his ring. The 
 
ASTRO XO MY. 
 
 5GI 
 
 Solar system reviewed. — Saturn's satellites. 
 
 periods of these satellites, and their distances from their 
 primary, are as follows : 
 
 D. h. m. 
 
 Mile 
 
 1st satellite revolves in 1 21 18 27 at the distance of 170,000 
 
 2nd 
 
 3d 
 
 4th 
 
 5th 
 
 6th 
 
 7 th 
 
 
 2 17 41 22 
 
 
 4 12 25 12 
 
 
 15 22 41 13 
 
 
 79 7 48 
 
 
 1 8 53 9 
 
 
 22 40 46 
 
 217,000 
 303,000 
 704,000 
 205,000 
 135,000 
 107,000 
 
 The satellites of Jupiter, we have observed, are enumerated 
 and distinguished in a regular manner, beginning at the one 
 nearest the primary; so that the second satellite is the second 
 in size and distance, and so of the rest. This, it appaars 
 above, is not exactly the case with the satellites of Saturn, 
 and the reason is, that the first five satellites were discovered 
 in 1685 by Cassini and Huygens; no more were supposed to 
 exist, and they were called the 1st, 2nd, 3d, Sec. reckoning- 
 from the planet; but, about a century afterwards. Dr. Herschel 
 discovered two others, which were smaller and nearer to the 
 planet; these ought therefore to have been called the first 
 and second, and the numerical order of the others changed, 
 so that the fifth would have become the seventh. But to 
 avoid the confusion that might arise, in reading astronomical 
 works, written previous to the Doctor's discovery, the anomaly 
 of their enumeration was allowed, and they are constantly 
 reckoned the sixth and seventh. 
 
 The inclinations of the orbits of the first, second, third, 
 and fourth satellites, to the ecliptic, are from 30*^ to 31*^. That 
 of the fifth, is from 17'^ to 18°. This satellite is further remark- 
 able for being the only one of the solar system, except our 
 own, that has been observed to have any spots. From these 
 spots, its rotation on its axis has been determined; and it is 
 singular, that, like the moon, it revolves round its axis in the 
 same time that it revolves round its primary. 
 
 The satellites of Saturn are frequently eclipsed, and these 
 eclipses would be of the same use as those of Jupiter, for dis- 
 covering the longitude of places on the earth, but their vast 
 distances render them so much less susceptible of accurate 
 observation. 
 
 Vol. I.— 24. 
 
 4C 
 
5&2 ASTRaN'OMV. 
 
 Solar system reviewed. — llcrscliel. 
 
 27te Herschel. y 
 
 At the distance of 1800 millions of miles from the sun, the 
 planet Herschel, the last of the system, advances in his course. 
 This planet was discovered by Dr. Herschel, on the 13th of 
 IMarch, 1781, and though commonly distinguished by his 
 name, it is also known by other appellations; the discoverer 
 himself called it the Georgium Sidus, or Georgian Planet, in 
 honour of his sovereign ; Lalande, and the rest of the French 
 philosophers after him, called it the Herschel; while the 
 Germans, preferring the ancient mode of following the heathen 
 mythology, called it Uranus, who, in fabulous history, was the 
 father of Saturn. The Herschel seems the name most likely 
 to content all parties; the public voice, where the evidence 
 is clear, is always in favour of those who have the strongest 
 claim to distinction: the English will not relinquish both the 
 names which may indicate the country where the discovery 
 was made; and foreign philosophers, while they willingly give 
 the honour to one of their own republic, are not disposed to 
 give it to any particular king. Galileo, who discovered the 
 four satellites of Juj^iter, called them Medicetni stars, in honour 
 of the family of the Medici, his patrons; but the title is now 
 almost forgotten, and never used. 
 
 The Herschel is 35,112 miles in diameter, and performs its 
 annual revolution in 30,637 days, 4 hours, that is, very nearly 
 eighty-four of our years. The period of its diurnal rotation 
 has not been determined. On a clear evening, it may be seen 
 by the naked eye, in the absence of the moon; when viewed 
 through a good telescope, it is of a bluish white colour, and 
 its disc is well defined. The light and heat which it receives 
 directly from the sun is about the 362nd part of what we enjoy. 
 
 When the discoverer of the Herschel first observed it, he 
 supposed it to be a comet; but it was soon proved to be a 
 planet, the suspicion of which was naturally suggested by its 
 nearness to the ecliptic. It had always been considered a fixed 
 star, on account of the slowness of its motion ; and it was no 
 sooner known to be a ])lanet, than it was perceived to be the 
 star marked 34 in Flamsteed's catalogue, and 964 in Meyer's. 
 By these means, astronomers were in possession of a whole 
 century of observations respecting it. 
 
 Dr. Herschel had the singular merit of not only discovering 
 this star to be a planet, but of discovering all the satellites it is 
 supposed to have, and which are six in number. These satel- 
 lites cannot be seen witiiout a powerful telescope. Their 
 periods of revolution round their primary, and their distances 
 from it, are as follows : 
 
ASTROXOMY. 563 
 
 Solar system reviewed. — Satellites of Herschel. — Synopsis. 
 
 1st satel 
 
 2nd 
 
 3d 
 
 4th 
 
 6th 
 
 6th 
 
 
 D. Ii. 
 
 ite revolves 
 
 in 5 21 
 
 
 8 18 
 
 
 10 23 
 
 
 13 12 
 
 
 38 1 
 
 
 107 16 
 
 m. Miles. 
 
 25, at the distance of 230,334 
 
 . . 298,838 
 
 4 . . 348,398 
 
 . . 399,434 
 
 49 • . 798,920 
 
 40 .. 1,597,736 
 
 These satellites move in a plane nearly perpendicular to the 
 plane of the planet's orbit, and contrary to the order of the 
 signs. 
 
 The following Synopsis of the planetary system, will 
 enable the reader to compare the corresponding circum- 
 stances of the planets with each other, and to refer to them 
 with facility: 
 
 Distances of the Planets from the Sun in English miles. 
 
 Mercury 37,000,000 
 
 Venus 68,000,000 
 
 Earth 95,000,000 
 
 Mars 144,000,000 
 
 Vesta 215,000,000 
 
 Juno 243,000,000 
 
 Ceres 263,000,100 
 
 Pallas 264,000,000 
 
 Jupiter 490,000,000 
 
 Saturn 900,000,000 
 
 Herschel 1,800,000,000 
 
 Diameter of the Sun and Planets in English miles. 
 
 Sun 883,246 
 
 Mercury 3,224 
 
 Venus 7,687 
 
 Earth 7,911 
 
 Moon 2,180 
 
 Mars 4,189 
 
 Vesta 238 
 
 Juno 1,425 
 
 Ceres 163 
 
 Pallas 80 
 
 Jupiter 89,170 
 
 Saturn 79,042 
 
 Herschel 35,112 
 
5o4 ASTRONOMY. 
 
 Solar svstem reTiewed. — Svnopsis 
 
 Densitj/ oj the Suit and Planets, that of Water being 1. 
 
 Sun lis 
 
 Mercury 9^ 
 
 Venus 5 ji 
 
 Earth 4^ 
 
 Mars 3^ 
 
 Jupiter 1^ J 
 
 Saturn J j 
 
 Herschel 1 + 
 
 Quantity of Matter in the Planetary bodies, the Earth being 1. 
 
 Sun 3-29.630 
 
 IVIercury 0.135 
 
 A'enus • 0.135 
 
 Earth 1.000 
 
 Moon 0.025 
 
 Jupiter 330.600 
 
 Saturn 103.950 
 
 Herschel 16.840 
 
 Number of feet per second through trhich a heavy body uou/d 
 fall at the surface of the Sun, and of each of these Planets. 
 
 Sun 450 
 
 Mercurv 12 
 
 Venus .'. 28 
 
 Earth 16 + 
 
 Moon 3 
 
 Jupiter 42 
 
 Saturn 15 
 
 Herschel 4.2 
 
 Rotation of the Sun and Planets round their axes. 
 
 D. h. m. s. 
 
 Sun 25 10 
 
 Venus 23 21 
 
 Earth 24 
 
 Mars 24 39 22 
 
 Jupiter 9 55 37 
 
 Saturn 10 16 2 
 
 Saturn's Ring 10 32 15 
 
 Moon, a lunation, at a mean. 29 12 44 3 
 
ASTRONOMY. 565 
 
 Solar svstem reviewed. — Comets. 
 
 Tropical Revolutions of the Planets r&imd the Snn, 
 
 D. h. m. s. 
 
 Mercury 87 23 14 33 
 
 Venus 224 16 41 27 
 
 Earth 365 5 48 49 
 
 Mars 686 22 18 27 
 
 Jupiter 4,330 14 39 2 
 
 Saturn 10,746 19 16 15 
 
 Herschel 30,637 4 
 
 Moon (round the earth) .. 27 7 43 5 
 
 Of Comets. 
 
 We have yet to mention a numerous class of bodies, uliich 
 frequently appear within the limits of the planetary orbits, 
 and some or all of which certainly belong to the solar 
 system. We allude to comets, respecting the nature, num- 
 ber, and use of which the opinions of philosophers have 
 been exceedingly divided, and very little has yet been 
 determined in a satisfactory manner, unless it be the mani- 
 fest truth, that they are not to be considered, as formerly, 
 the portentous heralds of calamities to mankind. They 
 are, doubtless, noble parts of the creation, traversing their 
 appointed course, without any connection with the earth, 
 except as parts of the same whole; and the appearances 
 they present to us arise only from peculiarities of their 
 constitution, indispensable probably to the comforts of their 
 inhabitants. 
 
 The curves in which comets move are generally considered 
 to be very eccentric ellipses, so that comets can only be seen 
 during their vicinity to the sun, and are invisible to us during 
 the greater part of their course. Their periodical times are 
 so very long, and so difficult to be ascertained, that very few 
 of them have been observed twice, and when their appear- 
 ance agrees with the time foretold, it is almost impossible to 
 identify them. Their appearances are very different. Some 
 of them resemble a faint vapour; others have a perceptible 
 nucleus or solid part in the middle. WTien they approach 
 near the sun, they put forth the appearance of a beard or 
 tail of luminous matter, which is differently denominated 
 according to their position. When the comet is eastward 
 of the sun, and mores from the sun, it is said to be bearded, 
 because the luminous elongation goes before it. When the 
 comet is westward of the sun, and sets after him, it is then 
 said to have a tail, because the luminous elongation follows it ; 
 
666 ASTRONOMY. 
 
 Solar svstem reviewed. Comets. 
 
 and when the earth happens to be directly between the sun 
 and the comet, then the train of light is so much hidden by 
 the body of the comet, that only a little of it can be seen on 
 each side of the comet, which is therefore said to have a coma 
 or hairy appearance. 
 
 The more conspicuous comets have been long known to the 
 vulgar under the appellation of blazing stars ; but many of them 
 are only to be seen by the help of telescopes, and are discovered 
 by accident, because they appear and disappear in a few nights, 
 and traverse the heavens in every direction. The actual motion 
 of some of them is from east to west, while others, like the 
 planets, move from west to east. Some of them move in the 
 plane of the ecliptic, or within the zodiac ; whilst others go in 
 different directions, even perpendicular to the ecliptic. 
 
 Kepler and some other philosophers were of opinion that 
 comets were in reality nothing more than a congeries of 
 vapours, or exhalations from the sun and planets ; but Sir 
 Isaac Newton proved the absurdity of this hypothesis, by 
 shewing that no vapours or exhalations could subsist in the 
 regions where they were found to be the brightest; that is, 
 very near the sun. The heat of the sun is as the density of 
 his rays, or reciprocally as the squares of the distances of places 
 from the sun. Therefore, as the distance of the comet of 1680, 
 in its perihelion, December the 8th, was observed to be to the 
 distance of the earth from the sun nearly as 6 to 1000 ; the sun's 
 heat at the comet, was to his heat with us at midsummer as 
 1,000,000 to 36; or 28,000 to 1. Now the heat of boiling water 
 is little more than three times the heat of our dry earth, when 
 exposed to the midsummer's sun; assuming then the heat of 
 red-hot iron to be about three or four times as great as boiling 
 water, Newton concludes, that the heat of the dried earth, or 
 body of the comet in its perihelion, must be nearly 2000 times 
 as great as that of red-hot iron. He computes, that a globe of 
 red-hot iron, of the dimensions of our earth, would scarcely be 
 cool in 50,000 years. If then the comet be supposed to cool 
 100 times as fast as red-hot iron, yet, since its heat was 2000 
 times greater, supposing its magnitude to be equal to that of 
 the earth, it would not be cool in a million of years. Upon the 
 whole, therefore, he concluded that the comets were compact, 
 solid, fixed, and durable bodies, in one word, a kind of planets, 
 which move in very oblique orbits, every way with the greatest 
 freedom; persevering in their motions, even against the course 
 and direction of the planets; that their tail is a very thin 
 vapour, emitted by the head or nucleus of the comet, ignited 
 or heated by the sun; and he entertained and announced the 
 grand idea that they might re-appear at every revolution. 
 
ASTRONOMY. 567 
 
 Solar system reviewed. — Comets. 
 
 Frequently a nucleus cannot be discerned : of sixteen comets 
 observed by Dr. Herschel, fourteen were of this description; 
 and the other two had only a very ill-defined central light, 
 ■which perhaps might be called a nucleus, but not a disc. It is 
 this want of a well-defined disc or nucleus, that renders many 
 of the observations on comets uncertain. 
 
 From observations of the comet of 1680, Sir Isaac Newton 
 found, that the vapour in the extremity of the tail, January 
 25th, began to ascend from the head before December 11th, 
 and had therefore spent more than forty -five days in its ascent; 
 but that all the tail which appeared December 10th, ascended 
 in the space of those two days, then just past since its peri- 
 helion. The vapour, therefore, at the beginning, when the 
 comet was near the sun, ascended with a prodigious swiftness; 
 and afterwards continued to ascend with a motion retarded 
 by the gravity of its particles, and by that ascent increased 
 the length of the tail; but the tail, notwithstanding its length, 
 consisted almost wholly of vapours, which had ascended from 
 the time of its perihelion; and the vapour which ascended 
 first, and composed the extreme of the tail, did not vanish till 
 it was too far from the sun to be illuminated by him, and from 
 us to be visible. Hence, also, the tails of comets that are 
 shorter, do not ascend with a quick and continual motion from 
 the head, and then presently disappear; but are permanent 
 columns of vapours and exhalations, gathered from the head, 
 by a very gentle motion, and a great space of time; which 
 yet, by participating of that motion of their heads they had 
 at the beginning, continue easily to move along with their 
 heads through the celestial regions. 
 
 The ascent of the vapours from a comet, will be promoted 
 by their circular motion round the sun, and will endeavour to 
 recede from the sun, as smoke recedes from a fire ; and there- 
 fore, the more the comets advance into the dense atmosphere 
 of the sun, their tails will be the greater. The vapours 
 being thus dilated, rarefied, and diffused throughout all the 
 celestial regions, Newton thought they might probably, by 
 little and little, through their own gravity, be attracted down 
 to the planets, and become intermingled with their atmo- 
 spheres, to which they would furnish fresh accessions of 
 pure particles, and supply the loss of air and moisture 
 sustained by the processes of life and vegetation. Another 
 use he supposed comets may be designed to serve, is that 
 of supplying the sun with fresh fuel. This may happen from 
 their suffering a diminution of their projectile force, when 
 in perihelion, from the resistance of the solar atmosphere ; 
 so that by degrees, their gravitation towards the sun may 
 
568 ASTRONOMY 
 
 Solar sTStein reviewed. — Comets. 
 
 be SO far increased as to precipitate their fall into his body. 
 Thus, also, fixed stars which have been gradually wasted, may 
 be supplied with fresh fuel, acquire new splendour, and be 
 taken for new stars: of this kind may be supposed those 
 fixed stars which appear on a sudden, or shine at first 
 with a surprising brightness that gradually decays, till they 
 disappear. If this theory were true, the comets might be 
 regarded as the conservators of the solar system, while it 
 was shewn, at the same time, how little cause there was 
 for that superstitious dread of their appearance, which had 
 prevailed so generally among the ignorant of all creatures. 
 'J'he ideas which Sir Isaac ISev\ton entertained of comets, 
 were certainly far removed from the current opinions even 
 of philosophers in his day, and we cannot say that the 
 investigations of the century since they were published, 
 have carried us much beyond them, though substantial ob- 
 jections may be otl'ered to some parts of his hypothesis. The 
 idea of the periodical revolutions of the comets is extremely 
 likely to be confirmed, but the supposition that the planets 
 are supplied with pure air and moisture diffused by them, 
 the late discoveries in chemistry render exceedingly doubtful, 
 by shewing abundant other means of nature's repletion in 
 these respects. Modern chemistry indisputably proves to 
 us, that water is composed of two invisible gases ; and 
 that by the operations constantly going on in nature, it 
 is produced, resolved into its elements, and reproduced: 
 on the one hand, water produces air; on the other, air 
 produces water: under certain circumstances, water unites 
 in a solid form with various substances; under other cir- 
 cumstances, it is liberated from its solid form, and assumes 
 that of vapour or air. These operations, there is reason to 
 believe, balance each other; the atmosphere and the water, 
 though constantly changing, remain constantly the same in 
 quantity, or as nearly so as the existence of mankind requires; 
 and we find it impossible to point out the source of a decay or 
 the need of a foreign supply. With respect to the supply of 
 combustible matter which comets may afford to the sun, and the 
 cause which Sir Isaac Newton, as above stated, has assigned for 
 their falling into it, we may consider, that if on the one hand 
 the density of the solar atmosphere has a tendency to lessen the 
 projectile force of a comet passing through it, that tendency, it 
 is reasonable to suppose, will be simultaneously checked and 
 overcorae,by the greater momentum with which the rays fall upon 
 the comet. It is further to be considered, that if the comets 
 are to be regarded merely as fuel for the sun, it were inconsist- 
 ent to suppose them at the same time to be habitable worlds. 
 
ASTROXOMY. 5G9 
 
 Solar system reviewed. — Comets. 
 
 Is it not then at least reasonable to adopt the opinion, that the 
 comets are co-existent with the planets; andthat the sun's waste 
 of matter is supplied by other means than their destruction? 
 
 Rowning, who is not satisfied with Sir Isaac Newton's 
 theory of comets, accounts for their tails in the following 
 manner: It is well known, that when the light of the sun 
 passes through the atmosphere of any body, as the earth, that 
 which passes on one side is by the refraction thereof made to 
 converge towards that which passes on the opposite one ; and 
 tile co.nvergency is not wholly effected either at the entrance of 
 the light into the atmosphere, or at its going out ; but begin- 
 ning at its entrance, it increases in every point of its progress. 
 It is also agreed, that the atmospheres of comets are very large 
 and dense. He therefore supposes that by such time as the 
 light of the sun has passed through a considerable part of the 
 atmosphere of a comet, the rays are so far refracted toward 
 each other, that they then begin sensibly to illuminate it, or 
 rather the vapours floating in it; and so render that part they 
 have yet to pass through visible to us : and that this portion 
 of the atmosphere of a comet, thus illuminated, appears to us 
 in the form of a beam of the sun's light, and passes under the 
 denomination of a comet's tail. Some comets have had tails 
 extending 80 millions of miles. 
 
 This opinion, that the comet's tail is merely an optical ap- 
 pearance, produced by the light streaming through a dense 
 and very extensive atmosphere, is at the least exceedingly spe- 
 cious. If the tail be nothing but light, we can readily account 
 for its not distorting the rays from the smallest star seen 
 through it; and it is not difficult to admit that such an atmo- 
 sphere must render the solid opaque body of the comet very 
 indistinct, if not altogether invisible. This theory of comets 
 it has been proposed to elucidate by experiment: Take a glass 
 globe, suspend a small opaque globe in the middle of it, and 
 then expose it to a strong stream of light, as from a fire or a 
 number of candles, while it is so placed in an aperture of a 
 wall or '^door, that the light which passes through it may be 
 seen in a darkened room : but the imitation of the cometary 
 phenomena will not be good, from the envelope of the glass, 
 nor equal to what it would be if the water were in a state of 
 vapour. 
 
 Kepler's law of the analogy between the periodical times of 
 the planets, and their distances from the sun, is applicable also 
 to the comets. Hence the mean distance of a comet from the 
 sun may be found by comparing its period witkthe time of the 
 earth's revolution round the sun: thus the period of the comet 
 that appeared in 1531, 1607, 1682, and 1759, being about 76 
 Vol. I.— 24. 4 D 
 
570 ASTRONOMY. 
 
 Solar system reviewed. — Comets. 
 
 years, its mean distance is found by this proportion : as 1, the 
 square of one year, the earth's periodical time, is to 5776, the 
 square of 76, the comet's periodical time, so is 1,000,000 the 
 cube of 100, the earth's mean distance from tiie sun, to 
 5,776,000,000 the cube of the comet's mean distance ; the cube 
 root of which is 1794, the mean distance itself, in such parts 
 as the mean distance of the earth contains 100. If the peri- 
 helion distance of this comet, 58, be taken from 3588, double 
 the mean distance, we shall have the aphelion distance 3530 
 of such parts as the distance of the eartli contains 100; and 
 this is a little more than 35 times the distance of the earth 
 from the sun. By a like method, the aphelion distance of the 
 comet of 1680, comes out 138 times the mean distance of the 
 earth from the sun, supposing- its period to be 575 years ; so 
 that this comet, in its aphelion, goes to more than fourteen 
 times the distance of Saturn from the sun. 
 
 Dr. Halley compiled a table of the elements of comets, ac- 
 cording to the best information he could obtain ; the end for 
 ^vhich he compiled this table, he observes, was this, tiiat wiien- 
 ever a new comet shall appear, we may be able to know, by 
 comparing together the elements, whether it be any of those 
 which have appeared before, and consequently to determine 
 its period, and the axis of its orbit, and to foretell its return. 
 He asserted his belief that the comet which Appian observed 
 in the year 1531, was the same with that which Kepler and 
 Longomontanus more accurately described in the year 1607 ; 
 and which he himself observed in the year 1682. AH the ele- 
 ments agreed, and nothing appeared to contradict his conclu- 
 sion but a trifling inequality of the periodic revolution. This, 
 he remarked, might arise from physical causes ; for the motion 
 of Saturn is so disturbed by the rest of the planets, especially 
 Jupiter, that the periodic time of the planet is uncertain for 
 some whole days together. How much more therefore will a 
 comet be subject to such like errors, which rises almost four 
 times higher than Saturn, and whose velocity, though in- 
 creased but very little, would be sufficient to change its orbit 
 from an ellipsis to a parabola. On looking over the histories 
 of comets, he found that one had been seen in the year 1456, 
 and also in the year 1305, which latter year he considered to 
 be at the distance of a double period from the former. From 
 all these considerations, he ventured to foretell that the return 
 of the comet would take place about the year 1758- The effect 
 of Jupiter he computed would increase its periodic time above 
 a year, in consequence of which he predicted its return at the 
 end of the year 1758 or the beginning of 1759. But Clairaut 
 computed the effects both of Saturn and Jupiter, and found 
 
ASTRONOMY. 571 
 
 Comets. — Fixed Stars. 
 
 that the former would retard its return in the last period 100 
 days, and the latter 511 days; and he determined the time 
 when the comet would come to its perihelion to be April 15, 
 1759, observing, that he might err a month, from neglecting 
 small quantities in the computation. It passed the perihelion 
 on iMarch 13, within thirty-three days of the time computed. 
 This is the first comet the period of which was foretold, and 
 the prediction was verified. It did not, however, make any 
 considerable appearance, by reason of the unfavourable situa- 
 tion of the earth, all the time its tail might otherwise have 
 been conspicuous, the comet being then too near the sun to 
 be seen by us. "Let us remark," says Laplace, "for the 
 honour of the human understanding, that this comet, which in 
 this century only excited the curiosity of astronomers and 
 mathematicians, had been regarded in a very different manner, 
 four revolutions before, when it appeared in 1456. Its long 
 tail spread consternation over all Europe, already terrified by 
 the rapid success of the Turkish arms, which had just destroyed 
 the great empire. Pope Callixtus, on this occasion, ordered 
 a prayer, in which both the comet and the Turks were included 
 in one anathema !" 
 
 Five hundred comets are supposed to have appeared since 
 the commencement of the Christian era. Of these probably 
 a considerable number are the different appearances of the 
 same comet; but our knowledge of these bodies is so much in 
 its infancy, the period of accurate observations has so lately 
 commenced, that we have as yet but slender means of deter- 
 mining any question respecting them. 
 
 Of the Fixed Stars. 
 
 When we have separated the planets and comets from our 
 consideration, we find that all the other bodies of the sphere 
 constantly preserve the same situation with respect to each 
 other; for this reason they are cMed Jixed stars. They ob- 
 viously diflfer again from the planets, by the twinkling of their 
 light, however clear the atmosphere; to the unphilosophic eye 
 they appear to be innumerable, but this is only in consequence 
 of their being scattered over the expanse of heaven apparently 
 without order, and our not being able to see them all at once. 
 Of this deception of sight, we may soon convince ourselves, 
 by selecting any particular portion of the heavens, and count- 
 ing the stars it contains ; the number of which will be found 
 very small. The whole number of stars which can be seen at 
 once by the naked eye, is not above 500 ; but soon after the 
 invention of the telescope, the number actually noted down in 
 catalogues amounted to 3000. The further telescopes have 
 
572 ASTRO NO MV 
 
 Fi^ed stars destitute ol' parallax. 
 
 been improved, the more have been discovered, and Dr. Her- 
 schel has, in our own time, added at least 30,000 to those for- 
 merly known. 
 
 Before we venture upon any speculations with respect to the 
 nature of the fixed stars, it will be proper to obtain, if possible, 
 some idea of their distances. For this purpose, we must re- 
 mind the reader of the nature of a parallax, which signifies the 
 angle subtended by the visual rays coming from an object 
 viewed at two different situations. The moon, it has been 
 shewn at page 522, is so near us, that its distance may be 
 ascertained tolerably well by a horizontal parallax, which is 
 equivalent to viewing it from two situations separated by the 
 semi-diameter of the earth, or nearly iOOO miles. But the dis- 
 tance of the sun is found to be so great, that a horizontal pa- 
 rallax will not give a satisfactory result; philosophers there- 
 fore, it has been shewn, turned their attention, for ascertaining 
 the distance of this luminary, to the transits of Venus, by which 
 nearly the whole diameter of the earth formed a parallax, and 
 enabled them to solve the problem. Two stations, however, 
 separated bv the whole diameter of the earth, are utterly in- 
 capable of shewing us the distance of a fixed star. What re- 
 source then has human ing;enuity left? One which might surely 
 be thouffht sufficient, thai of viewingr and taking; the parallax 
 of these bodies from two opposite parts of the earth's orbit. 
 This is called the great parallax, or annual parallax. By means 
 of the zenith sector. Hook, Flamsteed, and Bradley observed, 
 for some time, at the spring and autumnal equinox, the transit 
 of 7 Draconis over this perpendicular telescope, hoping that 
 the diameter of the earth's orbit might make an angle or pa- 
 rallax with it. They were disappointed. The star was seen 
 so near the same place, at each of the earth's two stations, 
 which are nearly 200 millions of miles distant from each other, 
 that no estimate could be made! Bradley guessed there might 
 be an angle of about two seconds, which would make the star 
 400,000 times further from us than the sun. Cassini supposed 
 the annual parallax of Sirius, which of all the fixed stars is 
 considered the nearest, to be six seconds, from which it was 
 calculated to be 18,000 times further from us than the sun. 
 Nothing, however, on this subject, has been determined with 
 certainry, except the simple but truly astonishing truth, that 
 the space of 200 millions of miles is an insensible point, when 
 compared with the distance of the nearest fixed star. 
 
 The next telescopic peculiarity which we must notice, is, 
 that the fixed stars have no commensurable magnitude. This 
 is the more remarkable, as thev differ from each other so much 
 in brilliancy ; but when examined with a telescope, however 
 
ASTRONOMY. 573 
 
 Fixed stRrs sliine by native ligbt. 
 
 great the magnifying power, the most sparkling appear only 
 to be lucid points, intensely bright, indeed, but indivisible. 
 Their twinkling is occasioned by the motion of particles in 
 our atmosphere, intercepting their light; and their sensible 
 magnitude to the naked eye arises from adventitious rays, cut 
 off by the telescope. 
 
 From the two circumstances which have been just mention- 
 ed, to wit, the indefinite distances and magnitudes of the fixed 
 stars, we may justly infer, that they shine by native light, 
 otherwise they would be invisible to the naked eye : for the 
 satellites of Jupiter and Saturn, though we can measure their 
 distances, and they also appear of a very distinguishable mag- 
 nitude through a telescope, reflect too little light to be visible 
 without that instrument. We may calculate the distance at 
 which 200 millions of miles would appear to our sight as a 
 point; and supposing we assume that distance to be the dis- 
 tance of the nearest fixed star, we shall be satisfied, that our 
 sun, if equally far off, would only appear like a star. Are we 
 not then compelled to draw the conclusions, that every fixed 
 star is itself a sun ; that each of them is the centre of a system 
 of planetary worlds; that it directs their motions, and supplies 
 them with light and heat, in the same manner that the sun 
 governs the several bodies of which our solar system is com- 
 posed ; that these planets of other systems, are furnished like 
 ours with satellites, though these as well as their primaries are 
 altogether invisible to us; and finally, that the fixed stars are, 
 generally speaking, as distant from each other as the nearest 
 of them is from the earth. 
 
 The difference of the apparent magnitudes of the fixed stars 
 is verj' great ; yet it is found to answer sufficiently well the 
 purposes of description and discrimination, to distinguish them 
 into six orders, calling the largest of them stars of the Jirst 
 magnitude ; the next, stars of the second magnitude, and so on. 
 The stars of the sixth magnitude are those which can barely 
 be distinguished by the naked eye. Those which can only 
 be seen by the help of the telescope, are usually called tele 
 scopic stars. 
 
 A better method than the above of distinguishing the bright- 
 ness and magnitude of the stars, is that adopted by Dr. Her- 
 schel, in his catalogues of the comparative brightness of the 
 fixed stars. It consists in referring a given star to two other 
 stars, one of which is somewhat brighter, and the other rather 
 less bright, than the one to be designated. "I place," says 
 he, "each star, instead of giving its magnitude, into a short 
 series, constructed upon the order of brightness of the nearest 
 proper stars. For instance, to express the lustre of D, I say 
 
574 ASTRONOMY, 
 
 Classification oi' the fixed .stars. 
 
 CDE. By this short notation, instead of referring the star D 
 to an imaginary, uncertain standard, I refer it to a precise and 
 determinate existing one. C is a star that has a greater lustre 
 than D; and E is another of less brightness than D. Both C 
 and E are neighbouring stars, chosen in such a manner that I 
 may see them at the same time with D, and therefore may be 
 able to compare them properly. The lustre of C is in the same 
 manner ascertained by BCD; that of B by ABC; and also the 
 brightness of E by DEF ; and that of F by EFG." 
 
 Astronomers divide the heavens into three regions ; a 
 northern and a southern hemisphere, and the zodiac. Stars 
 of various magnitudes are seen in all these regions, and are 
 classed into what are called constellations, or systems of stars, 
 according as they lie near one another, so as to occupy those 
 spaces which the figures of different sorts of animals, &c. 
 would take up, if they were delineated on what appears to be 
 the concave surface of the heavens. Those stars which the 
 ancients could not bring into any particular constellation, 
 they called unj'ormed siars, but most of these are comprehended 
 in the new constellations of the moderns. 
 
 This division of the stars into different constellations, or 
 asterims, serves to distinguish them in such a manner, that 
 any particular star may be readily found in the heavens, by 
 means of a celestial globe, on which the constellations are so 
 delineated as to put the most remarkable stars into those parts 
 of the figures which may be most easily pointed out. A great 
 improvement of this ancient mode of pointing out the stars, 
 consists in annexing a Greek or a Roman letter, or a number, 
 to each star, the brightest star having the first letter in the 
 alphabet, or the lowest number, and the rest following in regu- 
 lar order. By these means, every star may be pointed out 
 with as much ease, as if it had a distinct name. Thus if we 
 see a in Orion, and a in the Twins, the former is called Alpha 
 Orionis, and the latter Alpha Geminorum, 8cc. But before 
 this arrangement prevailed, remarkable stars in some of the 
 constellations obtained a particular name, which they still 
 retain, as Aldebaran, Castor and Pollux, &c. 
 
 To the following list of the constellations, and of the num- 
 ber of stars observed in each of them, as far as those of the 
 sixth magnitude, is subjoined the names and magnitudes of 
 those stars to which independent names have been given: 
 
ASTRONOMY. 
 
 575 
 
 List of the constellations. 
 
 Constellations of the Zodiac 
 
 No. 
 
 CoDstellatious. of Stars. 
 
 Aries The Ram 66 
 
 Taurus The Bull 140 
 
 Gemini The Twins 8.5 
 
 Cancer The Crab 83 
 
 Leo The Lion 95 
 
 Virgo The Virgin 110 
 
 Libra The Scales 51 
 
 Scorpio The Scorpion . . . , 44 
 
 Sagittarius The Archer 69 
 
 Capricornus The Goat 51 
 
 Aquarius The Water-carrier 108 
 
 Pisces The Fishes 112 
 
 Principal Stars and 
 their Magnitudes. 
 
 Aldebaran 
 
 Castor and Pollux 
 
 Regulus 
 
 Spica Virginis . 
 Zubenisch Mali 
 Antares 1 
 
 1 
 
 1.2 
 
 1 
 
 1 
 
 2 
 
 Scheat 3 
 
 Constellations on the North Side of the Zodiac. 
 
 Constellations. of 
 
 Ursa Minor The Little Bear . . . 
 
 Ursa Major The Great Bear . . . 
 
 Cassiopeia Lady in her Chair. . 
 
 Perseus Perseus 
 
 Auriga The Waggoner .... 
 
 Bootes The Bear Driver . . 
 
 Draco , The Dragon 
 
 Cepheus 
 
 CanesVenatici.viz. As- rr-i -^ , j 
 terian and Chara. . ^he Greyhounds . . 
 
 Corona Caroli Charles' Crown .... 
 
 Triangulum The Triangle 
 
 Triangulum Minus .. . The Lesser Triangle 
 
 Musca The Bee 
 
 Lynx 
 
 Leo Minor The Little Lion .... 
 
 Coma Berenicis Berenice's Hair .... 
 
 Camelopardalus The Camelopard . . 
 
 Mons Menelaus 
 
 Corona Borealis Northern Cro%vn . . 
 
 Serpens The Serpent 
 
 Scutiun Sobieski .... Sobieski's Shield . . 
 
 No. Principal Stars and 
 
 Stars. their Magnitudes. 
 
 24 Stella Poloris 2 
 
 87 Dubhe 1 
 
 55 
 
 59 Algenib 2 
 
 56 Capella 1 
 
 54 Arcturus 1 
 
 60 Rastaber 3 
 
 35 Alderamin 3 
 
 25 
 
 3 
 16 
 
 5 
 
 6 
 44 
 24 
 40 
 58 
 11 
 21 
 50 
 
 8 
 
570 
 
 ASTRONOM V 
 
 IJst of the Constellation*. 
 
 Constellations on the North Side of the Zodiac (continued.) 
 
 No. 
 of Stars. 
 
 Constellations 
 
 Herculus.cumRamo ) „ i i v iio u ai • .u 
 P / > Hercules kneeling 113 Ras Algiatha . 
 
 Principal Stars and 
 their Magnitudes. 
 
 Serpentarius 67 Ras Alhagus 
 
 Veo-a 
 
 Serpentarius sive 
 Ophiuchus .... 5 
 
 Taurus Poniatowski 7 
 
 Lyra The Harp 22 
 
 Vulpecula et Anser . . The Fox and Goose 37 
 
 Sagitta The Arrow 18 
 
 Aquila The Eagle 40 Altair 
 
 Delphinus The Dolphin 18 
 
 Cygnus ^ The Swan 73 Deneb Adige 
 
 Equule^is The Horse's Head.. 10 
 
 Lacerta The Lizard 16 
 
 Pegasus The Flying Horse 85 
 
 Andromeda 66 
 
 Markab 2 
 
 Almaac 2 
 
 Constellations on the South Side of the Zodiac. 
 
 "So. Principal Stars and 
 
 Constellations. of Stars. their Magnitndes. 
 
 Phoenix Phenix 13 
 
 Officina sculptoria 12 
 
 Eridanus The River 76 Achernar 1 
 
 Hydrus The Hvdra 10 
 
 Cetus The Whale 80 Menekar 2 
 
 Fornax Chemica 14 
 
 Horologium 12 
 
 ReticulusRhomboidalis 10 
 
 Xiphias The Sword-fish .... 7 
 
 Celapraxitellis 16 
 
 Lepus The Hare 19 
 
 Columba Noachi .... Noah's Dove 10 
 
 Orion 78 Betelguese 1 
 
 Argo Navis The Ship 50 Canopus 1 
 
 Canis Major The Great Dog. ... 30 Sirius 1 
 
 Equuleus Pictorius 8 
 
 Monoceros The Unicorn 31 
 
 Canis Minor The Little Dog. .. . 14 Procyon 1 
 
 Chamaeleon Chameleon 10 
 
 Pixis Nautica 4 
 
 Piscis Volans ....... The Flying Fish , . 8 
 
ASTRONOMY. 577 
 
 List of the constellations. 
 
 Constellations on the South Side of the Zodiac (continued.) 
 
 No. Principal stars and 
 
 Constellations. of Stars. their Magnitades. 
 
 Hydra ^ ^ 60 CorHydrae 1 
 
 Sextans The Sextant 4 
 
 Robur Carolinum .... The Royal Oak .... 12 
 
 IMachina Pneumatica 3 
 
 Crater The Cup 11 Alkes 3 
 
 Corvus The Crow 9 Algorab 3 
 
 Crux.......... The Cross 6 Crucis 1 
 
 Muca The Bee 4 
 
 Apus The Bird of Paradise 11 
 
 Circinus 4 
 
 Centaurus The Centaur 36 
 
 Lupus The Wolf 24 
 
 Quadra Euclidis 12 
 
 Triangulum Australe . Southern Triangle . 5 
 
 Ara The Altar 9 
 
 Telescopium The Telescope .... 9 
 
 Corona Australis .... Southern Crown . . 12 
 
 Pavo The Peacock 14 
 
 Indus The Indian 12 
 
 Microscopium The Microscope. . . 10 
 
 Octans Hadleianus ... Hadley's Quadrant 43 
 
 Grus The Crane 14 
 
 Toucan American Goose . . 9 
 
 Piscis Australis The Southern Fish . 20 Tomalhaut « . 1 
 
 The whole number of stars reckoned in the preceding lists, 
 amounts to 8192, but the number which may be discerned 
 with the assistance of telescopes is incalculable. Dr. Hook, 
 with a telescope of twelve feet, saw 78 stars among the Plei- 
 ades ; and with a longer telescope still more ; and in the con- 
 stellation Orion, which is usually reckoned to contain 78 stars, 
 there have been seen 2000. Dr. Herschel, with a telescope 
 that magnifies from five to six thousand times, has counted 
 forty-four thousand in a space of sky eight degrees in length, 
 and three in width ; and supposing this quantity to be the same 
 in all portions of the sky, of equal dimensions, +he whole num- 
 ber of stars, that may be rendered visible by such a telescope, 
 cannot be fewer than seventy-four millions. 
 
 Kepler has made a very ingenious observation upon the 
 
 magnitudes and distances of the fixed stars. He observes, that 
 
 there can be only thirteen points upon the surface of a sphere, 
 
 as far distant from each other as from the centre, and supposing 
 
 25.— Vol. 1. 4 £ 
 
578 ASTRONOMY. 
 
 Cliiinges amoot; tLe tixeil stars. 
 
 the nearest fixed stars to be as far distant from each other as 
 from the sun, he concludes that there can, strictly speaking, be 
 only 13 stars of the first magnitude. Hence at twice that dis- 
 tance from the sun there may be placed four times as many, and 
 so on ; and this mode of calculation gives us pretty nearly the 
 number of stars of the first, second, third, &,c. magnitudes. 
 
 It is a fact no less singular than well ascertained, that stars 
 which were observed by the ancients are now no longer to be 
 seen, and new ones have appeared in different places, which 
 were unknown to the ancients, and some of these have also dis- 
 appeared and again become visible. Hipparchus, the ancient 
 astronomer, having observed the disappearance of a star, was 
 induced to make a catalogue of the fixed stars, that posterity 
 might judge whether any other change took place among 
 them. Many ages afterwards, a new star having been observed 
 by Tycho Brahe and his contemporaries, this astronomer also 
 determined to make a catalogue with the same view as Hip- 
 parchus. Of the new star seen by Brahe, and on the subject 
 of changes among the stars in general, we have the following 
 interesting account by Dr Halley : " The first new star in the 
 Chair of Cassiopeia was not seen by Cornelius Gemma on the 
 8th of November 1572, who says, he that night considered 
 that part of the heaven in a very serene sky, and saw it not; 
 but that the next night, November 9, it api)eared, with a splen- 
 dour surpassing all the fixed stars, and scarcely less bright 
 than Venus. This was not seen by Tycho Brahe before the 
 11th of the same month ; but from thence he assures us that it 
 gradually decreased and died away; so as in March, 1574, after 
 16 months, to be no longer visible ; and at this day no signs 
 of it remain. Its place in the sphere of fixed stars, by the ac- 
 curate observations of the same Tycho, was 0* 9'^ 17' a l»ua*<Y), 
 with 5'd'^ 45' north latitude. Such another star was seen and 
 observed by the scholars of Kepler, to begin to appear on Sep- 
 tember 30, St. Vet. anno 1604, which was not to be seen the 
 day before ; but it broke out at once with a lustre surpassing 
 that of Jupiter; and like the former, it died away gradually, 
 and in much about the same time disappeared totally, there 
 remaining no footsteps thereof in January, 1605-6. This was 
 near the ecliptic, following the right leg of Serpentarius ; and 
 by the observations of Kepler and others, was in 7* 28" 0' a 
 luja*fy>^ with north latitude 1*^ 56'. These two seem to be of 
 a distinct species from the rest, and nothing like them has 
 appeared since. But between them, viz. in the year 1596, we 
 have the first account of the wonderful star in Collo Ceti, seen 
 by David Tabricius on the 14th of August, as bright as a star 
 of the third magnitude, which has been since found to appear 
 
ASTRONOMY. 579 
 
 Changes among the fixed stars. 
 
 and disappear periodically; its period being precisely enough 
 seven revolutions in six years, though it returns not always 
 with the same lustre. Nor is it ever totally extinguished, but 
 may at all times be seen with a six feet tube, [telescope.] This 
 was singular in its kind, till that in Collo Cigni was discover- 
 ed. It precedes the tirst star of Aries 1*^ 40', with 15*^ 57' 
 south latitude. Another new star was first discovered by 
 William Jansonius, in the year 1600, in Pectore, or rather in 
 Eductione Colli Cygni, which exceeded not the third magni- 
 tude. This having continued some years, became at length so 
 small, as to be thought by some to have disappeared entirely; 
 but in the years 1657, 1658, and 1659, it again arose to the 
 third magnitude ; though soon after it decayed by degrees to 
 the fifth or sixth magnitude, and at this day is to be been as 
 such in Qs 18^ 38' a lnia#^^ with 55^ 29' north latitude. A 
 fifth new star was first seen by Hevelius in the year 1670, on 
 July 15, St. Vet. as a star of the third magnitude ; but by the 
 beginning of October was scarcely to be perceived by the 
 naked eye. In April following, it was again as bright as 
 before, or rather greater than of the third magnitude, yet 
 wholly disappeared about the middle of August. The next 
 year, in March 1672, it was seen again, but not exceeding 
 the sixth magnitude : since when, it has been no further visi- 
 ble, though we have frequently sought for its return ; its place 
 is 9s 3"^ 17' a ln>a*r^, and has lat. north 47" 28'. The sixth 
 and last is that discovered by G. Kirch in the year 1686, and 
 its period determined to be of 404| days, and though it rarely 
 exceeds the 5th magnitude, yet it is very regular in its re- 
 turns, as we found in the year 1714. Since then we have 
 watched, as the absence of the moon and the clearness of the 
 weather would permit, the first beginning of its appearance in 
 a six-feet tube, that, bearing a very great aperture, discovers 
 most minute stars. And on June 15th last, it was first per- 
 ceived like one of the first telescopical stars ; but in the rest 
 of that month and July, it gradually increased, so as to be- 
 come in August visible to the naked eye, and so continued 
 till the month of September. After that it again died away by 
 degrees ; and on the 8th of December, at night, was scarcely 
 discernible by the tube ; and as near as could be guessed, 
 equal to what it was at its first appearance on June 15, so 
 that this year it has been seen in all nearly six months, which 
 is but little less than half its period ; and the middle, and 
 consequently the greatest brightness, falls about the 10th of 
 September." 
 
 In the 76th volume of the Philosophical Transactions, a 
 paper, by Edward Pigot, gives a dissertation on the stars sus- 
 
580 ASTRONOMY. 
 
 Changes among the tixed stars. 
 
 pectetl by the astronomers of the last century to be changeable. 
 The author divides thera into two classes; one containing those 
 which are undoubtedly changeable, and the other those which 
 are only suspected to be so. The first class contains a list of 
 12 stars from the first to the fourth magnitude, including the 
 new one which appeared in Cassiopeia, in 1672, and tliat in 
 Serpentarius in 1604 : the second contains the names of 30 
 stars of different magnitudes, from the first to the seventh. 
 He is of opinion, that the celebrated new star in Cassiopeia is 
 a periodical one, and that it returns once in 150 years. Keill 
 is of the same opinion; and Pigot thinks that its not being 
 observed at tlie expiration of each period, is no argument 
 against the truth of that opinion, remarking, that, "like most 
 of the variables, it may at different periods have different 
 degrees of lustre, so as sometimes only to increase to the 
 ninth magnitude ; and if this should be the case, its period is 
 probably much shorter." For this reason, in September, 1782, 
 lie took a plan of the small stars near the place where it for- 
 merly appeared, but in four years had observed no alteration. 
 He also examined the star in the neck of the Whale, from the 
 end of 1782, to 1786, but he never found it exceed the sixth 
 magnitude, though it had been observed by others at diff'erent 
 times to be of the third magnitude. He deduced its perio<l, 
 from its apparent equality with a smaller star in the neigh- 
 bourhood, and thence found it to be 320, 328, and 337 days. 
 Another changeable star, and one of the most remarkable of 
 the class, is that called Algol, in the head of Medusa ; it had 
 long been known to be variable, but its period was first as- 
 certained by Goodricke, of York, who began to observe it in 
 the beginning of 1783. It seems to have a period of about 
 2 days, 21 hours, during which it varies gradually from the 
 size of the second to that of the fourth magnitude in about 3| 
 hours; after which time it gradually recovers its greatest 
 lustre, which it preserves to the end of its period, and then 
 again begins to diminish. 
 
 These changes among the fixed stars have given rise to a 
 great variety of hypotheses to account for them. The sug- 
 gestion of Sir Issac Newton, that the transitory blaze of some 
 of them, might be owing to the addition of fuel they have re- 
 ceived from the influx of a comet, has already been noticed. 
 Maupertuis thought some stars, by their prodigiously quick 
 rotations on their axis, may not only assume the figures of 
 oblate spheroids, but that, from the great centrifugal force 
 arising from such rotations, they may become of the figures 
 of mill-stones ; or be reduced to flat circular planes, so thin 
 as to be quite invisible when their edges are turned towards 
 
ASTRONOMY. 581 
 
 Possible causes of the changes among the fixed stars. — Galaxy and nebulae. 
 
 US, as Saturn's ring is in such positions. But when very ec- 
 centric planets or comets go round any flat star, in orbits 
 much inclined to its equator, the attraction of the planets or 
 comets in their perihelions, must alter the inclination of the 
 axis of that star; on which account, it will appear more or 
 less large and luminous, as its broad side is more or less 
 turned towards us. A third opinion is, that the changes in 
 question may be owing to spots on the surfaces of the stars, 
 so that when, by their periodical rotations on their axis, these 
 spots are turned towards us, the star is either not seen at all, 
 or appears less bright than at other times. Lastly, it is con- 
 jectured that they revolve in very considerable orbits, and 
 that we only see them when they are nearest to us. It pro- 
 bably is not one uniform cause that occasions the phenomena, 
 which may be owing sometimes to one and sometimes to 
 another of the causes assigned, or to various others of which 
 we have not the least apprehension ; indeed, ages of assiduous 
 observation may elapse before any rational conjecture can be 
 offered on the subject. 
 
 On almost any evening which is clear enough for the stars 
 to be seen with tolerable distinctness, we may observe in the 
 heavens, a broad tract, of a whitish colour. This remarkable 
 tract is called, from its whiteness, the galaxy, via lactea, or 
 milky way. On more minute inspection, we may observe a 
 number of separate whitish places, affording the same kind 
 of light as the milky way, but not so bright ; these are called 
 nebul(£. 
 
 The milky way encircles the celestial concave ; it is irregu- 
 larly extended, sometimes double, but for the most part single, 
 and varies in breadth from 4*^ to 20^. It passes through Cas- 
 siopeia, Perseus, Auriga, the foot of Gemini, Orion's club, part 
 of Monoceros, the tail of Canis Major, through Argo Navis, 
 Robur Carolinum, Crux, and the feet of the Centaur ; here it 
 divides into two parts ; its eastern branch passes through Ara, 
 the tail of Scorpio, the eastern foot of Serpentarius, the bow 
 of Sagittarius, Scutum Sobiescianum, the feet of Antinous, and 
 Cygnus. Its western branch passes through the upper part of 
 the tail of Scorpio, the right of Serpentarius and Cygnus, and 
 ends in Cassiopeia. With a powerful telescope, Dr. Herschel 
 first began to survey the via lactea, and found that it com- 
 pletely resolved the whitish appearance into stars which the 
 telescope he formerly used had not power enough to do. The 
 portion he first observed, was that about the hand and club of 
 Orion ; he perceived in it an astonishing multitude of stars, 
 the number of which he endeavoured to estimate, by counting 
 many different telescopic fields of view, and computing from a 
 
582 ASTRONOMY. 
 
 Galaxy and nebula?. 
 
 mean of these how many might be contained in a given por- 
 tion of the milky way. In the most vacant place to be met 
 with in that neighbourhood, he found 63 stars; other six fields 
 contained 1 10, 60, 70, 90, 70, and 74 stars ; a mean of all which 
 gave 76 for the number of stars to each field; and thus he found, 
 that by allowing fifteen minutes for the diameter of his held 
 of view, a belt of 16 degrees long, and two broad, wliich he 
 had often seen pass before his telescope in an hour's time, 
 could not contain less than 50,000 stars, large enough to be 
 distinctly numbered; besides which, he suspected twice as 
 many more, which could be seen only now and then by faint 
 glimpses, for want of sufficient light. 
 
 Dr. Herschel's success in examining the milky way, induced 
 him to turn his telescope to the nebulous parts of the heavens. 
 Most of these yielded to a Newtonian refiector, of 20 feet fo- 
 cal distance, and 12 inches aperture; which plainly discovered 
 them to be composed of stars, or at least to contain stars, and 
 to afford very strong indications of their consisting of them 
 entirely. "The nebulae," says he, "are arranged into strata, 
 and run on to a great length, and some of them I have been 
 able to pursue, and to guess pretty well at their form and di- 
 rection. It is probable enough that they may surround the 
 whole starry sphere of the heavens, not unlike the milky way, 
 which undoubtedly is nothing but a stratum of fixed stars ; and 
 as this immense starry bed is not of equal lustre in every part, 
 nor runs on in one straight direction, but is curved, and even 
 divided into two streams along a very considerable portion of 
 it ; we may likewise expect the greatest variety in the strata 
 of the cluster of the stars and nebulaj. One of these nebulous 
 beds is so rich, that, in passing through a section of it in the 
 time of only 36 minutes, I have detected no less than 31 ne- 
 bulae., all distinctly visible upon a fine blue sky. Their situa- 
 tion and shape, as well as condition, seem to denote the 
 greatest variety imaginable. In another stratum, or perhaps 
 a different branch of the former, I have often seen double and 
 treble nebulae variously arranged; large ones, with small seem- 
 ing attendants; narrow, but much extended lucid nebulae or 
 bright dashes ; some of the shape of a fan, resembling an elec- 
 tric brush issuing from a lucid point ; others of the cometic 
 shape, with a seeming nucleus in the centre, or like cloudy 
 stars, surrounded with a nebulous atmosphere: a difierent sort 
 of orb again, contain a nebulosity of the milky kind, like 
 that wonderful, inexplicable phenomena about Orionis ; while 
 others shine with a fainter mottled kind of light, which de- 
 notes their being resolvable into stars. 
 
 " It is very probable that the great stratum called the milky 
 
ASTRONOMY. 583 
 
 Galas.j and nebula?> 
 
 way, is that in which the sua is placed, though perhaps not 
 in the very centre of its thickness. We gather this from the 
 appearance of the galaxy, which seems to encompass the whole 
 heavens, as it certainly must do if the sun is within the same: 
 for, suppose a number of stars arranged between two parallel 
 planes, indefinitely extended every way, but at a given consi- 
 derable distance from one another, and calling this a sidereal 
 stratum, an eye placed somewhere within it, will see all the 
 stars in the direction of the planes of the stratum projected 
 into a great circle, which will appear lucid, on account of the 
 accumulation of the stars, while the rest of the heavens, at 
 the sides, will only seem to be scattered over with constella- 
 tions, more or less crowded, according to the distance of the 
 planes, or number of stars contained in the thickness or sides 
 of the stratum." 
 
 The nebulae have been divided into three kind . The first 
 kind comprises those which consist of a great number of 
 stars crowded together, and which are seen to be distinct 
 through a telescope. Among these, is the famous nebulae of 
 Cancer, or the prcesepe cancri, forming a collection of 25 or 
 30 stars, and many similar groups in various parts of the 
 heavens. — The second kind consist of one or more stars, 
 surrounded by a whitish spot, through which they seem 
 to shine. There are several of this sort, but one of the 
 most remarkable is that in Orion, which, through a tele- 
 scope, appears as a whitish spot nearly triangular : it con- 
 tains seven stars, one of which is itself surrounded by a 
 small cloud brighter than the rest of the spot. — Nebulae 
 of the third kind are white spots in which no stars are 
 seen when viewed with a telescope. — Fourteen of these have 
 been observed in the austral hemisphere, among which the 
 celebrated spots, near the south pole, called by sailors the 
 Magellanic clouds, hold the first rank. — Dr. Herschel has 
 given catalogues of 2000 nebulae and clusters of stars disco- 
 vered by himself. 
 
 We will now, says the Doctor, in one of his papers on this 
 subject, retreat to our own retired situation in one of the 
 planets, attending a star in the great combination, with num- 
 berless others ; and in order to investigate what will be the 
 appearances from this contracted situation, let us begin with 
 the naked eye. The stars of the first magnitude being in all 
 probability the nearest, will furnish us with a step to begin 
 our scale ; setting off therefore with the distance of Sirius, or 
 Arcturus, for instance, as unity, we will at present suppose, 
 that those of the second magnitude are at double, and those 
 of the third at treble the distance, and so forth. Taking it 
 
584 ASTRONOMY. 
 
 GalajLT. — Power of Herschel's telescope. 
 
 then for granted, that a star of the seventh magnitude is about 
 seven times as far from us as one of the first, it follows that 
 an observer, who is enclosed in a globular cluster of stars, and 
 not far from the centre, will never be able, with the naked eye, 
 to see the end of it : for since, according to the above estima- 
 tions, he can only extend his view about seven times the dis- 
 tance of Sirius, it cannot be expected that his eyes should 
 reach the borders of a cluster, which has perhaps fifty stars 
 in depth every where around him. The whole universe, there- 
 fore, to him, will be comprised in a set of constellations, 
 richly ornamented with scattered stars of all sizes. Or if the 
 united brightness of a neighbouring cluster of stars should, 
 in a remarkably clear night, reach his sight, it will put on the 
 appearance of a ' nail, faint, nebulous cloud, not to be per- 
 ceived without the grreatest attention. Allowing him the use 
 of a common telescope, he begins to suspect that all the 
 milkiness of the bright path which surrounds the sphere may 
 be owing to stars. By increasing his power of vision, he 
 becomes certain that the milky way is, indeed, no other than 
 a collection of very small stars, and the nebulae nothing but 
 clusters of stars. 
 
 Dr. Herschel then solves a general problem for computing 
 the length of the visual ray : that of the telescope which he 
 uses, will reach to stars 497 times the distance of Sirius. 
 Now according to his reasoning, Sirius cannot be nearer than 
 100,000x194,000,000 of miles'^ therefore his telescope will, 
 at least, reach to 100,000x194,000,000x497 miles. And he 
 observes, that in the most crowded part of the milky way, 
 he has had fields of view that contained no less than 588 
 stars, and these were continued for many minutes ; so that, 
 in a quarter of an hour, he has seen 116,000 stars pass 
 through the field of view of a telescope of only 15' aperture ; 
 and at another time in 41 minutes he saw 258,000 stars 
 pass through the field of his telescope. Every improve- 
 ment in his telescopes has discovered stars not seen before, 
 so that there appears no bounds to their number, or to the 
 extent of the universe. 
 
 The sun, like many other stars, has probably a progressive 
 motion directed towards the constellation Hercules, carrying all 
 its attendant planets along with it; and with respect to this 
 motion, Dr. Herschel observes, that the apparent proper mo- 
 tions of 44 stars out of 56, are nearly in the direction which 
 would be the result of such a real motion of the solar system ; 
 and that the bright stars i^rcturus and Sirius, which are pro- 
 bably the nearest to us, have, as they ought, according to this 
 theory, the greatest apparent motions. Again, the star Castor, 
 
ASTRONOMY. 585 
 
 Fixed stars. — Concludinsr observations. 
 
 appears when viewed with a telescope, to consist of two stars, 
 of nearly equal magnitude ; and though they have an apparent 
 motion, they have never been found to change their distance 
 with respect to one another a single second, a circumstance 
 easily understood, if both their apparent motions are supposed 
 to arise from the real motion of the sun. 
 
 In the infancy of science, when the distances of the sun 
 and planets were unknown, man, unable to draw any just con- 
 clusion respecting objects so much beyond his reach, never 
 doubted that he was the on'y being of his kind in the uni- 
 verse, or that the earth was the most consequential aggregate 
 of matter in existence. It was not without many a struggle, 
 and by slow degrees, that he withdrew from the dominion of 
 so contemptible a belief. At length, when the evidence, con- 
 tinually accumulating, triumphed over all contradiction; when 
 the magnificence of the planetary orbs became perfectly 
 evident; when it was proved that they certainly experienced 
 the regular return of day and night, spring, summer, autumn, 
 and winter; it became difficult to deny the plurality of in- 
 habited worlds, with any hope of rationally explaining the 
 design of that exertion of Creative Power which called those 
 orbs into existence, and subjected them to the same laws as 
 the earth; and man was constrained to admit, that a grain of 
 sand was not more completely an atom to the earth, than the 
 earth itself was an atom to the solar system. These ideas had 
 scarcely obtained general acceptance, when it was found that 
 the distances of all the fixed stars is immeasurably great; 
 then it became obvious that they shine by their own light ; 
 that they are each of them comparable in real splendour and 
 magnitude to the sun ; that they are as distant from each 
 other as any one of them must be from us; that therefore, 
 instead of being merely as ornaments of the sky, the most 
 reasonable conclusions of analogy inculcated, that they were 
 the centres of so many systems, dispersed throughout the 
 universe, and dispensing light and heat to planetary worlds 
 around them; and the use of the telescope, at the same time 
 that it imparted the power of counting millions of stars which 
 the naked eye never beheld, yielded a convincing evidence, 
 that millions of millions exist, at distances too great to be 
 separately distinguished. From the insignificance of the 
 earth compared with the whole solar system, man now 
 advanced to the perception of the insignificance of the whole 
 solar system, compared with the remaining immensity of 
 25.— Vol. I. 4 F 
 
586 ASTROXOJIY. 
 
 Gravitation the cause of the tides. 
 
 creation; so that the total annihilation of tlie system that 
 appears so vast to him, would be like the abstraction of 
 nothing from the universe. On the wings of conjecture, 
 indeed, but still with analogy by our side, we may take 
 a still more eminent if not expansive view. If there are 
 reasons to believe that every orb of the solar system is in- 
 habited, there are reasons scarcely less strong for believing 
 that every star, and every orb attending every star, is created 
 and exists only for the same great purpose. What incom- 
 municable feelings do these views excite! Perhaps all the 
 stars that we can discover, and all, of the existence of which 
 we can obtain any evidence, form a very finite part of the 
 total number; yet, is it possible by any otlier means, to ob- 
 tain so admirable a view of Infinite Power and Infinite Wis- 
 dom, as may be derived from the contemplation of the at- 
 tributes of that BeincT who has created, and maintains, even 
 that portion which we can actually discern of the stupendous 
 fabric of Universal Nature? 
 
 Of the Tides. 
 
 Having now ranged over the mighty field of astronomical 
 research, we must next proceed to consider a variety of 
 phenomena, resulting from the general laws of nature, which 
 are particularly and immediately interesting to us as in- 
 habitants of the earth, but which would have too much 
 interrupted our general view, if we had entered sufficiently 
 into the details of them in the course of our progress. Here 
 then we shall in the first place advert to those remarkable 
 fluctuations of the ocean called Tides. 
 
 The apparent connection subsisting between the movements 
 of the ocean and those of the moon, has been observed from 
 the earliest periods of antiquity ; but it was Kepler who first 
 asserted that the moon's attraction was the real cause; " If," 
 says he, " the earth ceased to attract its waters towards itself, 
 all the water in the ocean would rise and flow to the moon. 
 The sphere of the moon's attraction extends to our earth, and 
 draws up the water." Sir Isaac Newton afterwards demon- 
 strated the consonance of the cause assigned by Kepler, with 
 his theory of universal gravitation, and explained at the same 
 time the cause of the tides on the side of the earth opposite to 
 the moon, and from his time to the present, not a doubt has 
 been entertained on the subject. 
 
 The principal phenomena of the tides are as follows : 
 
 1. The sea is observed to flow for about six hours from south 
 to north, gradually swelling; after tliis it seems to rest for 
 a quarter of an hour; and then to ebb or retire back again 
 
ASTRONOMY. 587 
 
 General phenomena of tlie tides. 
 
 from north to south, for six hours more. Then after another 
 pause of about a quarter of an hour, the sea again begins to 
 flow; and so on alternately. 
 
 2. The time of a flux and reflux, is on an average about 
 12 hours 25 minutes, and twice this time, or 24 hours 50 
 minutes, is the period of a lunar day, or the time between the 
 moon's passing a meridian, and coming to the same point of 
 it again. So that the sea flows as often as the moon passes 
 the meridian, that is, as well when she comes to the arch 
 above the horizon, as when she comes to that below the 
 horizon ; and ebbs as often as the moon passes the horizon, 
 both on the eastern and western side. 
 
 3. The elevation of the waters on that side of the earth im- 
 mediately under the moon, somewhat exceeds the elevation 
 of the opposite side, and in all cases the elevation diminishes 
 from the equator to the poles. 
 
 4. The sun raises and depresses the sea twice every day, 
 in the same manner as the moon; but the solar influence in 
 this respect is less than that of the moon, in the proportion 
 of 1 to 3. 
 
 5. The tides which depend on the actions of the sun and 
 moon, are not distinguished but compounded; and thus they 
 form, to appearance, one united tide, which increasing and 
 decreasing, produces Neap and Spring tides. 
 
 6. In the syzygies, that is, when the moon is either new or 
 full, the action of both luminaries concur, and the tides are 
 highest; but they are least in the quadratures, or when the 
 lines of their action are 90 degrees apart, for where the water 
 is elevated by the moon, it is depressed by the sun, and vice 
 versa. Therefore while the moon passes from the syzygy to 
 the quadrature, the daily elevations are continually diminished : 
 on the contrary, they are increased, while the moon passes 
 from the quadrature to the syzygy. At the new moon, also, 
 the tides are the greatest, because the sun and moon are not 
 only in the same line, but on the same side of the earth ; and 
 at this time the tides of the same day are more different than 
 those at full moon. 
 
 7. The greatest elevations and depressions take place on 
 the second or third day after the new or full moon; and they 
 are the greater the nearer the luminaries are to the plane of 
 the equator; they are therefore greatest in the syzygies at the 
 time of the equinoxes. 
 
 8. The actions of the sun and moon are greater, the nearer 
 these bodies are to the earth ; and the greatest tides happen 
 -when the sun is a little to the south of the equator; but this 
 does not happen regularly every year, because some variation 
 
588 ASTROXOMY 
 
 Tlieory of ihe tides. 
 
 may 'arise from the situation of the moon's orbit, and th-j 
 distance of the syzygy from the equinox. 
 
 V. As the mean force of the moon to move the sea, is to 
 that of the san nearly as 3 to 1 ; if the action of the sun alone 
 would produce a tide of two feet, which it is said to do, then 
 that of the moon will be six feet; hence the spring-tides will 
 be eight feet, and the neap tides four feet. 
 
 These phenomena take place where the ocean is sufficiently 
 extended to admit of those motions which the actions of the 
 sun and moon have a tendency to impart; but they are vari- 
 ously modified by the obstacles which the water meets with 
 in its course; from the direction of the wind; from straits 
 and gulfs; from capes, bays, and other peculiarities of the 
 shores of different countries; so that on some shores, little or 
 no tide is observed, and on others they rise far beyond the 
 amount determined, without taking into consideration the 
 modifying circumstances. 
 
 The tides are propagated to great distances in large rivers; 
 and at the strait of Pauxis, in the river of the Amazons, they 
 are sensible at 600 miles from the sea. 
 
 In explaining the general principles on which the tides 
 depend, it must be observed and remembered, that if the ac- 
 tion of the moon were equal upon every part of the ocean, we 
 should have no tides; it is the inequality/ of its action that pro- 
 duces them. The action of the moon is greatest in the direc- 
 tion of a line that would join its centre of gravity with that of 
 the earth; to this line as an axis, the ocean rushes up from 
 every side, and as this line moves, the water which flowed up 
 to it at the first moment of the moon's action, continues to 
 move along with it. The inequality of the moon's action is 
 not so great as to counteract the cohesion which the solid 
 parts of the earth have to each other, and therefore they are 
 unatlected by it; but the particles of fluids move among each 
 other with such facility, that they receive it readily. To under- 
 stand more fully why the moon exerts an unequal action on the 
 earth, it is necessary to recollect that the power of gravity di- 
 minishes as the square of the distance increases; and therefore 
 the waters of the earth ABCDEFGH, fig. 1, pi. IV, must be 
 more attracted at Z, on the side next the moon, M, than at the 
 central parts of the earth, O ; and the central parts are more at- 
 tracted by the same power than the waters on the opposite side 
 of the earth at X ; and therefore the distance from the earth's 
 centre, and the waters on its surface under and opposite to the 
 moon, will be increased. For, let there be three bodies at H, 
 O, and D ; if they are all equally attracted by the body at M, 
 they will all move equally fast towards it, their mutual distances 
 
ASTRONOMY. 589 
 
 Theory of tLe tides. 
 
 from each other continuing the same. If the attraction of M is 
 unequal, then that body which is most strongly attracted will 
 move fastest, and this will increase its distance from the other 
 body. Therefore, by the law of gravitation, M will attract H 
 more strongly than it will attract O, by which the distance be- 
 tween H and O will be increased; and a spectator at O will 
 perceive H rising higher towards Z. In like manner, O being 
 more strongly attracted than D, it will move faster towards 
 M than D; consequently the distance between O and D will 
 be increased; and a spectator at O, not perceiving his own 
 motion, will see D receding farther from him towards N; all 
 effects and appearances being the same, whether D recedes 
 from O, or O from D. 
 
 Suppose now there are a number of bodies, as A, B, C, D, 
 E, F, G, H, or in one word, a fluid ring, placed about O ; then 
 as the whole is attracted towards M, the parts at H and D 
 will have their distance from O increased ; whilst the parts at 
 B and F, being nearly at the same distance as O from M, will 
 not recede from one another; but rather, by the oblique at- 
 traction of M, they will approach nearer to O. The fluid ring 
 therefore will form itself into an ellipse ZIBLNKFRZ, whose 
 longer axis, NOZ, it produced, will pass through M, and its 
 shorter axis EOF, will terminate in B and F. Let the ring 
 be filled with fluid particles, so as to form a sphere round O; 
 then as the whole moves towards M, the fluid sphere being 
 lengthened at Z and N, will assume an oblong or spheroidal 
 form. If M is the moon, O the earth's centre, ABCDEFGH 
 the sea covering the earth's surface, it is evident, by the above 
 reasoning, that whilst the earth, by its gravity, falls towards 
 the moon, the water directly below the moon at B, will swell 
 and rise gradually towards her; also the water at D will re- 
 cede from the centre, and rise on the opposite side of the 
 earth, whilst the water at B and F is depressed, and falls below 
 the former level. Hence as the earth turns round its axis 
 from the moon to the moon again in 24 hours 50', there will 
 be two tides of flood, and two of ebb, in that time. By the 
 motion of the earth on its axis, the most elevated part of the 
 water is carried beyond the moon, in the direction of the 
 rotation; but the water, from the accumulation of impulse, 
 still continues to rise after it has passed directly under the 
 the moon, though the greatest immediate action of the moon 
 has then begun to decrease. The greatest elevation is not 
 attained, till the moon has for the space of an hour or more 
 ceased to be vertical to the place where it occurs. Thus, in 
 open seas, where the water flows freely, the moon M is past 
 the north and south meridian, as at^, when it is high water 
 
590 ASTRONOMY. 
 
 Theory of Uie tides. 
 
 at Z and N. For it will be understood, tliat thouoh the moon's 
 attraction were to cease altogether when she was past the 
 meridian, yet the motion of ascent previously communicated to 
 the water, would make it continue to rise for some time after; 
 much more must it do so when the attraction is only dimi- 
 nished; this is agreeable to the ordinary phenomenon that the 
 heat is greater at two in the afternoon, than when the sun is on 
 the meridian. The tides do not always answer to the same dis- 
 tance of the moon from the meridian at the same places; but 
 are variously aftected by the action of the sun, which brings 
 them on sooner when the moon is in her first and third quar- 
 ters, and keeps them back when she is in her second and fourth ; 
 because, in the former case, the tides raised by the sun alone 
 would be earlier than the tides raised by the moon; and in the 
 latter case they would be later. 
 
 It is not difficult for the novice to admit, that the moon 
 should have the stated effect on the waters of the hemisphere 
 immediately beneath it; but it excites surprise, and is not so 
 easily understood, that it should produce the like efi'ect on 
 the opposite hemisphere, and that it should be flood or ebb 
 tide with our antipodes at the same time that it is flood-tide 
 or ebb-tide with us. Here then we must remark again, that 
 it is not the absolute or total action of the moon, but the 
 difference between its action on one part and its action on ano- 
 ther, that constitutes the force which occasions any tide. The 
 point Z is nearer to the moon than any other part of the 
 hemisphere BZF; the waters therefore about that point are 
 more strongly attracted by the moon jNI, than at others more 
 remote; and since this attraction acts in a contrary direction 
 to that of the earth, the waters of all parts from BF to Z must 
 have their gravity or tendency towards the centre, O, dimi- 
 nished, and as this tendency to the centre is the least at the 
 point Z, they will consequently stand higher there than in any 
 other part of the hemisphere. Now on the opposite hemi- 
 sphere, BFN, the moon's attraction is also unequally exerted, 
 and the necessary consequence of ihat unequal action pro- 
 duces tides. From B and F to jN, the force of the moon's 
 attraction is diminished, agreeably to the general rule, as the 
 square of the distance increases. At N it is the least of 
 all, and as the attraction of the waters to the centre of the 
 earth is also the least there, because in the hemisphere BFN, 
 the moon's attraction in proportion to the nearness of the 
 parts to B and F, favours the earth's, the water will be higher 
 there than at any other part of the hemisphere. Hence the 
 moon raises the waters, both at that point of the earth which 
 is nearest to her, and at that which is furthest from her, at the 
 
ASTRONOMY. 591 
 
 Theory of the tides. 
 
 same time. At any port or harbour, therefore, which lies open 
 to the ocean, the action of the moon will tend to elevate the 
 waters there, when she is on the meridian of that place, whe- 
 ther she be on the arch of the meridian above, or on that 
 below the horizon. But the water cannot be raised at one 
 place, without flowing from and being depressed at another, 
 and these elevations and depressions will obviously be the 
 greatest at opposite points of the earth's surface. When the 
 moon raises the waters at Z and N, they will be depressed at 
 B and F; and when by the earth's rotation they are raised by 
 her at B and F, they will be depressed at Z and N. And as 
 the moon passes over the meridian, and is in the horizon twice 
 every day, there must therefore, as we find by experience, be 
 two tides of flood and two of ebb in that time, at the interval 
 of about six hours and twelve minutes. 
 
 The whole attractive force exerted by the sun on the earth, 
 is far superior to that of the moon, but as his distance from 
 the earth is nearly 400 times greater, the intensity of the 
 forces with which he acts upon difl'erent parts of it, are much 
 nearer to equality than those of the moon ; and as it is the 
 diff"erence of intensity that is alone exerted in producing tides, 
 his action in this respect is inferior to that of the moon. New- 
 ton calculated the effect of the sun's influence to be three 
 times less than that of the moon. The action of the sun is 
 therefore sufficient to produce a flux and reflux of the sea, 
 and in fact there are two tides, a solar one and a lunar one; 
 which have a joint or opposite effect, according to the 
 situation of these bodies. At fig. 2, plate IV, they conspire 
 together, on the same side as the earth, the new moon at A, 
 and the sun at S, their centres being nearly in the same line; 
 and they raise the water at Z and N higher than either of them 
 could do alone. When the moon is full, as at B, directly 
 opposite the sun, and on the opposite side of the earth, it 
 again acts in the same line as the sun, and the eff'ect produced 
 on the ocean is nearly the same as before ; and in both cases 
 they occasion w-hat are called spring tides. Now, as shewn 
 by fig. 3, let us suppose the sun to be at S, while the moon is 
 at Q, in her first quarter. Here the sun and moon tend to 
 counteract each other's eff'ect; for the one would raise the 
 ocean highest where the other would make it least. If the 
 forces then were equal, there would be no tide, but as that of 
 the moon is strongest in the proportion of 3 to 1, the differ- 
 ence between the forces is the moon's effect, and the water is 
 therefore raised twice as high under the moon as under the sun. 
 When the moon has arrived at her third quarter at R, her 
 situation relatively to the sun is the same as v/hen she was in 
 
592 ASTRONOMY. 
 
 Tbeorv of the tides. 
 
 her tirst, her force and that of the sun act in lines perpendicular 
 to each other as before, and therefore the eftect corresponds. 
 
 The effects of the sun and the moon, on the waters of the 
 ocean, are sensibly varied by the different distances of those 
 bodies, produced by the elliptical orbit of the earth round the 
 sun, and of that of the moon round the earth. Sir Isaac New- 
 ton skews that their actions increase as the cubes of the dis- 
 tances decrease; so that the moon at half its mean distance 
 would produce a tide eight times greater than at present. 
 Other variations likewise take place in consequence of the 
 different declinations of the sun and moon at different times ; 
 for if either of these luminaries were at the pole, it would 
 occasion a cuubtaut elevation uL both poles, and a constant de- 
 pression at the equator Hence by the declination or departure 
 of these bodies either northward or southward from the ecjuator, 
 their effect is lessened, and the tides are less. The highest 
 tides therefore occur when both the sun and moon are in the 
 equator, where the cpntrifugal force is the greatest; and the 
 moon is not only either new or full at the same time, but also 
 in her perigee, or at her least distance from the earth. The 
 effect would be still greater if the sun could be nearest the 
 earth at the same time that he is in the equator; but as this 
 is impossible, because he is nearest to us in winter, when he 
 is southward of the equator, therefore the spring tides are 
 highest and the neap tides lowest about the time of the equi- 
 noxes in March and September, for at these two seasons the 
 circumstances upon which the elevation of the waters depends, 
 conspire to produce the greatest effect. The highest tides 
 happen a little before the vernal equinox, when the sun is re- 
 tiring from the earth ; and a little after the autumnal equinox, 
 when the sun is approaching nearer to the earth. 
 
 From the foregoing theory it follows, that when the earth's 
 axis inclines to the moon, the northern tides, if not retarded in 
 their passage through shoals and channels, nor affected by the 
 winds, ought to be greatest when the moon is above the horizon, 
 and least when she is below it; and quite the reverse when the 
 earth's axis declines from her; but in those cases they return at 
 equal intervals of time. When the earth's axis inclines side- 
 ways to the moon, both tides are equally high ; but they happen 
 at unequal intervals of time. In summer, the earth's axis in- 
 clines towards the moon when new; and therefore the day- 
 tides ought to be highest, and night-tides lowest about the 
 change : at the full, the reverse. At the quarter, they ought to 
 be equally high, but unequal in their returns; because the 
 earth's axis then inclines sideways to the moon. In winter, the 
 phenomena are the same at full moon, as in summer at new. In 
 
ASTRONOMY. 593 
 
 Theory of the tides. 
 
 autumn, the earth's axis inclines sideways to the moon, when 
 new and full; therefore the tides ought to be equally high, and 
 unequal in their returns at these times. At the first quarter, 
 the tides of flood should be least when the moon is above the 
 horizon, greatest when she is below it ; and the reverse at her 
 third quarter. In spring, the phenomena of the first quarter 
 answer to those of the third quarter in autumn; and vice versa. 
 The nearer any time is to either of these seasons, the more 
 the tides partake of the phenomena of the nearest season ; 
 and in the middle between them, the tides are at a mean state 
 between those of both. 
 
 The tides cannot have their full motion, unless the ocean to 
 which the moon is vertical, be of a uniform depth, and ex- 
 tended from east to west, over the whole globe. They are 
 so retarded in their passage through different shoals and 
 channels, and otherwise so variously affected by striking 
 against capes and headlands, that to different places they 
 happen at all distances of the moon from the meridian; con- 
 sequently at all hours of the lunar day. The tide propagated 
 by the moon in the German ocean, when she is three hours 
 past the meridian, takes twelve hours to come from thence to 
 London bridge, where it arrives by the time that a new tide 
 is raised in the ocean ; and therefore when the moon has 
 north declination, and we should expect the tide at London 
 to be greatest when the moon is above the horizon, we find 
 it is least; and the contrary when she has south declination. 
 At several places it is high water three hours before the 
 moon comes to the meridian ; but the tide that appears to 
 precede the moon, is only the tide opposite to that which 
 was raised by her when she was nine hours past the opposite 
 meridian. 
 
 There are no tides in lakes, because they are generally so 
 small, that when the moon is vertical she attracts every part of 
 them almost alike, and therefore no part of the water is raised 
 perceptibly higher than the rest. She passes over them, like- 
 wise, so quickly, that the equilibrium of the water can at most 
 be disturbed for a very transient space of time. The Medi- 
 terranean and Baltic seas have very small elevations, because 
 the inlets by which they communicate with the ocean are so 
 narrow, that they cannot, in so short a time, receive or dis- 
 charge enough to raise or sink their surfaces sensibly. 
 
 Among the islands in the West Indies, the tides seldom 
 rise higher than twelve or fourteen inches, which appears 
 remarkable, because, from their proximity to the equator, these 
 islands are more under the influence of the moon than some 
 other places where the fluctuation is twenty or thirty times as 
 
 25.— Vol. L 4G 
 
694 ASTRONOMY. 
 
 Theory of Uie tides. 
 
 great. But it must be observed, that as the Atlantic ocean 
 turns from the moon, from west to east, her inHuence on its 
 ■water must be from east to west; this tide, like a prodigious 
 wave, is stopped by America, and reflected back as the moon 
 passes over the Pacific ocean. But in the same direction that 
 the moon drags the waters of the Atlantic, the trade winds 
 constantly blow ; and so great is the power of wind, that 
 tides every-where are made greater or less according as the 
 wind is with them or acrainst them. Now the sfulf of I\Iex- 
 ico is a cavity between North and South America, into which 
 the winds and tides are perpetually pouring water ; so that 
 the first tide that ever flowed into it, may be said to be kept 
 up in it by this unremitting influx, and, of course, the tides 
 cannot rise and fall as in places less in the way of these 
 causes. But water raised above the general level, always 
 endeavours to fall back to that level ; the trade winds in 
 some measure prevent this, by blowing perpetually from the 
 east. As water so accumulated, cannot return in opposition 
 to the trade winds, it (turns round the west end of Cuba, 
 and meeting with the Bahama Islands, is turned northward 
 along the coast of America ; forming that remarkable 
 stream, the strong current of the Gulf of Florida. To 
 shew that this accumulation actually takes place in the 
 Gulf of Mexico, a survey was made across the isthmus 
 of Darien, by which it was found that the water on the 
 Atlantic side was fourteen feet higher than the water on the 
 Pacific side. 
 
 From the lightness of the air, its freedom of motion, and 
 its greater nearness to the moon than the ocean, it may be sup- 
 posed that there are aerial tides of great elevation ; but it seems 
 to contradict this supposition, when we find that there are no 
 changes of the barometer indicating this to be extensively tlie 
 case. A little consideration, however, will shew, that such a 
 change of the barometer is not to be expected. By the preced- 
 ing theory of the tides, it appears that water is higher immedi- 
 ately under the moon, and at the same time to the antipodes of 
 the point immediately under her, that is, at those two |)laces on 
 the earth where her attraction is greatest and least. Now though 
 the water is higher at these two points than elsewhere, yet it 
 does not follow that the water, from its increased quantity, 
 presses with a proportionately greater weight on the bottom of 
 the ocean than before it was thus raised up, because the moon's 
 attraction has diminished that tendency to the centre of the 
 earth which constitutes the weight of bodies. Hence we may 
 suppose, that though the moon draws up the atmosphere to a 
 great height, yet a higher column of mercury is not supported 
 
ASTRONOMY* 595 
 
 Harvest mooo and hunter's moon. 
 
 through this circumstance, because the actual weight of the 
 atmosphere remains but little affected. 
 
 Of' the Harvest Moon, and Hunter's Moon. 
 
 When the earth, in turning on its axis from west to east, 
 has revolved from one meridian to the same meridian again, 
 the moon, which also revolves from west to east, has advanced 
 over little more than a thirtieth part of her orbit, or 12° and 
 some minutes ; hence the tides do not occur till after the 
 earth has overtaken her, and therefore are generally observed 
 and supposed to be about 50 minutes later every day. This 
 is not, however, constantly the case, except at or near the 
 equator ; for on account of the different angles made by the 
 horizon and different parts of the moon's orbit, this retarda- 
 tion differs considerably in places of high latitude. Twice in 
 the year she rises for a week together nearly at the same 
 time, and these phenomena happening successively in au- 
 tumn, the earlier is called the harvest moon, and the later the 
 hunter's moon. We shall chiefly be indebted to Ferguson for 
 the explanation of these problems. 
 
 The plane of the equinoctial is perpendicular to the earth's 
 axis; and therefore as the earth turns round its axis, all parts 
 of the equinoctial make equal angles with the horizon, both 
 at rising and at setting ; so that equal portions of it always 
 rise or set in equal times. Consequently, if the moon's mo- 
 tion were equable, and in the equinoctial, at the rate of 
 12° ir from the sun every day, as it is in her orbit, she 
 would rise and set 50 minutes later every day than on the 
 preceding day: for 12° 11' of the equinoctial rise or set in 
 50' of time in all latitudes. But the moon's orbit is so nearly 
 in the same plane as the ecliptic, that we may consider her 
 at present as moving in the ecliptic. Now the different parts 
 of the ecliptic, on account of its obliquity to the earth's axis, 
 make very different angles with the horizon as they rise or 
 set. Those parts or signs which rise with the smallest 
 angles, set with the greatest, and vice versa. In equal times, 
 whenever this angle is least, a greater portion of the eclip- 
 tic rises than when the angle is larger, as may be easily per- 
 ceived by referring to a globe. Thus in fig. 4 and 5, pi. IV, 
 L represents the latitude of London, AB the horizon to that 
 place, FP the axis of the world, E e the equator, K k the 
 ecliptic. On account of the oblique position of the sphere in 
 the latitude of London, the ecliptic has a high elevation above 
 the horizon, making the angle AVK, in fig. 4, about 62^ 
 degrees, when the sign of Cancer is upon the meridian, at 
 
59G ASTRONOMY. 
 
 Harvest moon and lianter's niooa. 
 
 which time Libra rises in the east. But when the other part 
 of the ecliptic is above the horizon, that is, when the sign 
 of Capricorn is upon the meridian, and Aries rises in the 
 east, then the ecliptic will make with the horizon the much 
 smaller angle k VA, as represented by fig. 5, which angle is 
 only about 15 degrees, that is, 47^ degrees smaller than the 
 former angle. Thus it may be conceived, that as the celes- 
 tial sphere appears to turn round the axis FP, a greater por- 
 tion of the ecliptic will rise in a given portion of time, as for 
 instance, threo or four hours, when the ecliptic is in the situ- 
 ation of fig. 5, than when it is in the situation of fig. 4. 
 
 In northern latitudes, the smallest angle made by the eclip- 
 tic and horizon, is when Aries rises, at which time Libra sets; 
 the greatest when Libra rises, at which time Aries sets. From 
 the rising of Aries, to the rising of Libra (which is twelve 
 sidereal hours) the angle increases ; and from the rising of 
 Libra to the rising of Aries, it decreases in the same propor- 
 tion ; hence it appears that the ecliptic rises fastest about 
 Aries, and slowest about Libra. 
 
 On the parallel of London, as much of the ecliptic rises 
 about Pisces and Aries in two hours, as the moon goes through 
 in six days ; therefore, whilst the moon is in these signs, she 
 differs but two hours in rising for six days together ; that is, 
 about twenty minutes later every day or night than on the 
 preceding, at a mean rate. But in 14 days afterwards, the 
 moon comes to Virgo and Libra, which are the opposite signs 
 to Pisces and Aries; and then she differs almost four times 
 as much in rising; namely, one hour and about 15' later 
 every day or night than the preceding, whilst she is in these 
 signs. As the signs Taurus, Gemini, Cancer, Leo, Virgo, 
 and Libra, rise successively, the angle of the ecliptic with the 
 horizon increases gradually ; and decreases in the saine pro- 
 portion as they set ; and for that reason, the moon differs 
 gradually more in the time of her rising every day whilst 
 she is in these signs, and less in her setting : after which, 
 through the other six signs, viz. Scorpio, Sagittary, Capri- 
 corn, Aquarius, Pisces, and Aries, the rising difterence 
 becomes less every day, until it be at the least of all, namely, 
 in Pisces and Aries. 
 
 The moon goes round the ecliptic in about 27 days and 8 
 hours; but not from change to change in less than about 29^ 
 days : so that she is in Pisces and Aries at least once in every 
 lunation, and in some lunations twice. 
 
 If the earth had no annual motion, the sun would never 
 appear to shift his place in the ecliptic; and then every new 
 moon would fall in the same sign and degree of the ecliptic. 
 
ASTRONOMY. 597 
 
 Harvest moon and hunter's moon. 
 
 and every full moon in the opposite ; for the moon would go 
 precisely round the ecliptic from change to change. So that 
 if the moon was once full in Pisces or Aries, she would al- 
 ways be full when she came round to the same sign and degree 
 again. And as the full moon rises at sun-set, (because when 
 any point of the ecliptic sets, the opposite point rises,) she 
 would constantly rise within two hours of sun-set, on the 
 parallel of London, during the week in which she was full. 
 But in the time that the moon goes round the ecliptic from 
 any conjunction or opposition, the earth goes almost a sign for- 
 ward ; and therefore the sun will seem to go as far forward in 
 that time, namely 27^^; so that the moon must go 271^ more 
 than round, and as much farther as the sun advances in that 
 interval, which is 2-^^'^^ before she can be in conjunction with, 
 or opposite to, the sun again. Hence it is evident, that tliere 
 can be but one conjunction or opposition of the sun and 
 moon in a year in any particular part of the ecliptic. This 
 may be familiarly exemplified by the hour and minute hands 
 of a watch, which in twelve hours are never in conjunction or 
 opposition in that part of the dial-plate where they were so 
 last before. 
 
 As the moon can never be full but when she is opposite to 
 the sun, and the sun is never in Virgo and Libra but in our 
 autumnal months, it is plain that the moon is never full in the 
 opposite signs, Pisces and Aries, but in these two months; 
 and therefore we can have only two full moons in the year 
 which rise nearly at the time of sun-set for a week together, 
 as has been mentioned above. 
 
 When the moon is in Pisces and Aries, she must rise with 
 nearly the same difference of time in every revolution through 
 her orbit, which is exactly the phenomenon of the harvest 
 moon ; but it passes unobserved, because in winter those signs 
 rise at noon, and being then only a quarter of a circle distant 
 from the sun, the moon in them is in her first quarter, and 
 rises about noon, at which time her rising is not noticed. In 
 spring those signs rise with the sun, for the sun is in them, 
 consequently the moon being in them too, is in conjunction 
 with the sun, and therefore her rising is invisible. In summer, 
 those signs rise about midnight; and the sun is three signs, or 
 about 90* before them ; therefore the moon in them must be in 
 her third quarter, when she gives little light, and rises late, on 
 which accounts, the phenomenon of her rising for some nights 
 with little difference of time, passes unnoticed. In autumn, 
 however, the case is different ; for the signs of Pisces and Aries 
 then rise about sun-set, and therefore the moon being in 
 them, is in opposition to the sun, consequently full, and rises 
 
598 ASTROXOMV. 
 
 Harvest uiuoo and hunter's iiioon. 
 
 in great splendour when the sun sets, and seems to prolong 
 the day, for the advantage of the husbandman at the time of 
 harvest. 
 
 in northern latitudes, the autumnal full moons are in Pisces 
 and Aries; and the vernal full moons in Virgo and Libra. In 
 southern latitudes, just the reverse, because the seasons are 
 contrary. But Virgo and Libra rise at as small angles witit 
 the horizon in southern latitudes, as Pisces and Aries do in 
 the northern; and therefore the harvest moons are just as re- 
 gular on one side of the equator as on the other. 
 
 As those signs, which rise with the least angle, set with 
 the greatest, the vernal full moons differ as much in their 
 times of rising every night, as the autumnal full moons differ 
 in their times of setting; and set with as little difference as 
 the autumnal full moons rise ; the one being in all cases the 
 reverse of the other. 
 
 Hitherto, to avoid the complication of the subject, the 
 moon's orbithas been supposed to coincide with the ecliptic; but 
 since her orbit makes an angle with the ecliptic, varying from 5*^ 
 to 5*^ 18', one half of it is on one side of the ecliptic, and the 
 other on the other side, and she only coincides with it when at 
 the points of intersection, called her nodes, which coincidence 
 cannot happen less than twice, but sometimes happens thrice, 
 between change and change. For as the moon goes almost a 
 whole sign more than round her orbit from change to change, if 
 she passes through either node at or a little before the time of 
 change, she will pass by the other about fourteen days after- 
 wards, and come round to the former node before the next 
 change. When the moon is northward of the ecliptic, she 
 rises sooner and sets later than if she moved in the ecliptic: and 
 when she is southward of the ecliptic, she rises later and sets 
 sooner. This difference is variable, even in the same signs, 
 because the nodes shift backwards about 19i*^ in the ecliptic 
 every year ; and so go round it contrary to the order of the 
 signs in 18 years 228 days. 
 
 When the ascending node is in Aries, the southern half of 
 the moon's orbit makes an angle of 5;^*^^ less with the horizon 
 than the ecliptic does, when Aries rises in northern latitudes: 
 for which reason the moon rises with less difference of time when 
 she is in Pisces and Aries, than if she moved in the ecliptic, 
 and will differ only 1 hour and 40' for the whole of seven days. 
 But in 9 years 114 days afterwards, the descending node conies 
 to Aries, and then the moon's orbit makes an angle of 5^^ 
 greater with the horizon when Aries rises, than the ecliptic 
 does at that time ; which causes the moon to rise w'\ih greater 
 diflerence of time in Pisces and Aries than if she moved in the 
 
ASTROXOMV. 599 
 
 Harvest mooD and banter's moon. — Less moonlight in summer than winter. 
 
 ecliptic ; this difference, in the course of a week, will amount 
 to full Sg hours. Hence, though we observe the phenomenon 
 of the harvest moon every year, yet it is not every year equally 
 remarkable, but alternately for a period of nearly nine years 
 and a half, it is greatest and least; from 1813 to 1815 is the 
 remainder of a period during which the harvest moon varies 
 most in the time of rising; from 1816 to 1825 includes a period 
 during which the difference is the least. 
 
 At the polar circles, when the sun touches the summer 
 tropic, he continues 24 hours above the horizon ; and 24 
 hours below it when he touches the winter tropic. For the 
 same reason, the full moon neither rises in summer, nor 
 sets in winter, considering her as moving in the ecliptic. 
 For the winter full moon being as high in the ecliptic as the 
 summer sun, must therefore continue as long above tlie lio- 
 rizon ; and the summer full moon being as low in the ecliptic 
 as the winter sun, can no more rise than he does. But 
 these are the only two full moons which happen about the 
 tropics ; for all the others rise and set. In summer, the full 
 moons are low, and their stay is short above the horizon, 
 when the nights are short, and we have the least occasion for 
 moonlight: in winter the full moons rise high, and stay long 
 above the horizon, when the nights are long, and we want the 
 greatest quantity of moonlight. 
 
 At the poles, one half of the ecliptic never sets, and the other 
 half never rises: and therefore, as the sun is always half a year 
 in describingonehalf of the ecliptic, it is natural to imagine that 
 the sun continueshalf a year together above the horizon of each 
 pole in its turn, and as long below it ; rising to one pole when 
 he sets to the other. This would be exactly the case if there were 
 no refraction ; butbythe atmosphere's refracting the sun's rays, 
 he becomes visible some days sooner, and continues some days 
 longer in sight, than he would otherwise do : so that he appears 
 above the horizon of either pole before he has got below the 
 horizon of the other. And, as he never goes more than 23h° 
 below the horizon of the poles, they have very little dark night ; 
 it being twilight there, as well as at other places, till the sun is 
 18* below the horizon. The full moon being always opposite to 
 the sun, can never be seen while the sun is above the horizon, 
 except when she is in the northernhalf of her orbit; for when- 
 ever any point of the ecliptic rises, the opposite point sets. 
 Therefore, as the sun is above the horizon of the north pole, 
 from the 20th of March till the 23d of September, it is plain 
 that the moon, when full, being opposite to the sun, must be 
 below the horizon that half of the year. But when the sun is 
 in the southern half of the ecliptic, he never rises to the north 
 
600 ASTRONOMY. 
 
 HorizuDtal niuoD. 
 
 pole ; during which half of the year, every full moon happens 
 in some part of the northern half of the ecliptic which never 
 sets. Consequently, as the polar inhabitants never see the 
 full moon in summer, they have her always in the winter, be- 
 fore, at, and after the full, shining for fourteen of our days 
 and nights. Thus the poles are supplied, during one half the 
 winter, with constant moonlight in the sun's absence; and 
 only lose sight of the moon from her third to her first quar- 
 ter, while she gives but little light, and would be of the least 
 benefit. 
 
 The Horizontal Moon. 
 
 When the moon is near the horizon, she appears of a shape 
 somewhat elliptical, and larger than when she is in the zenith, 
 or on the meridian. This phenomenon is called the horizontal 
 moon. 
 
 The longest axis of the elliptical disc of the horizontal 
 moon is parallel with the horizon, but the circular shape is 
 gradually attained, as the moon rises higher. To account for 
 these appearances has greatly exercised the ingenuity of several 
 philosophers, though no solution of the problem has yet been 
 offered, which has received general assent. As the moon when 
 in the horizon is further from us by the semi-diameter of the earth 
 than when in the zenith, the real angle it subtends must at that 
 time be least ; but the difference is so small, amounting only to 
 half a minute of a degree, that it would scarcely be noticed by 
 the sharpest eye. Gassendus thought, as the moon was less 
 bright in the horizon than in the meridian, we looked at it, in 
 the former situation, with a greater pupil of the eye, and there- 
 fore it appeared larger. An explication of this kind has been 
 lately offered, and experiments are given which appear to 
 shew that a difference of aperture will inake a difference in the 
 magnitude of the image of a lens ; but more careful experiments 
 shew this conclusion, so opposite to the principles of optics, to 
 be erroneous, and therefore that the variations in the aperture 
 of the pupil of the eye do not occasion variations in the 
 magnitude of the image on the retina. Some have supposed 
 that the image of the horizontal moon on the retina, is not 
 actually larger than at other times, but that we observe such an 
 extent of intermediate objects in that situation, as to give it a 
 greater apparent distance than at other times, and consequently 
 we consider it to be larger, although it subtends only the same 
 angle ; the dimness with which it appears, from the fewness of 
 the rays which reach us, contributing at the same time to the 
 deception. The circumstance of faiutness does not, however, 
 
ASTRONOMY. 601 
 
 Horizontal moon. 
 
 seem essential to the phenomenon, otherwise, the moon on the 
 meridian would appear as large as when at the horizon, or 
 nearly so, while it happens to be obscured by clouds; and it 
 may also be remarked, that the lower part of a rainbow appears 
 broader than the upper part, at the same time that it often ap- 
 pears brighter, and the breadth of the moon and of the rain- 
 bow in this case are doubtless collateral phenomena. It is evi- 
 dent to the slightest consideration, that the rays of light from 
 the moon, pass through the greatest tract of air, when the moon 
 is in the horizon, and through the least when that luminary is 
 in the zenith ; and at all intermediate distances, the quantity 
 of air passed through is of an intermediate extent, and propor- 
 tionate to the elevation. This is shewn by fig. 6, pi. IV, where 
 AB represents an arc of the top of the atmosphere, and CD an 
 arc of the surface of the earth. When the moon is at H, she 
 is seen by a spectator at E, through the tract f g E, which is 
 about twice as long as the tract traversed when she has arrived 
 at the altitude I, and three times the extent of that passed 
 through when she is in the zenith K. May we not then sup- 
 pose, that the difference in the refraction, produced by the 
 difference in the length of the tract of air passed through by 
 the rays from the moon, is the chief cause of the apparent en- 
 largement of the lunar disc, particularly as we find by experi- 
 ment, that an object appears larger in proportion as it is viewed 
 at a greater depth in water, and that though a thin piece of flat 
 glass has no perceptible magnifying power, yet a thick piece 
 certainly has, and ought to have, according to the principles 
 of optics. The variations of the density of the vapours through 
 which the rays pass, account for the variations of the horizontal 
 moon, which at equal altitudes sometimes appenr larger than 
 at other times. With respect to the elliptical figure of the 
 moon, in the situation mentioned, it must be observed, that the 
 rays from the extremities of its horizontal diameter, strike the 
 convex surface of the atmosphere, and come to the eye with an 
 equal angle, but this is not the case with the rays in the direc- 
 tion of the vertical diameter, and therefore a distortion takes 
 place. A watch-glass fastened upon the surface of a piece of 
 plain thin glass, and the enclosed space filled with water, will 
 illustrate this account of the horizontal moon; for if a small 
 circle be made on a slip of paper, and the circle be held upright 
 nearly on a level with the upper surface, while it is viewed 
 through the under surface of the plain glass, and at the same 
 time through at least half the diameter of the water, the circle 
 will appear elliptical, enlarged in its horizontal and lessened 
 in its vertical diameter; but the distortion will diminish in pro- 
 portion as it is placed and viewed in a zenith direction. 
 26.— Vol. I. 4 H 
 
602 ASTRONOMY, 
 
 Aberration of liglit. 
 
 The sun and stars also appear larger in the horizon, and any 
 two stars appear further apart, than at a greater elevation. 
 The causes of these appearances must be considered the same 
 as that producing the horizontal moon, whether the above ex- 
 plication of this appearance be accurate or not. 
 
 Aberration of light. 
 
 The fixed stars, and other heavenly bodies, are not seen in 
 their real directions. This circumstance is independent of the 
 refractive power of our atmosphere, and is always present, whe- 
 ther we observe them in the zenith, or any other situation ; it 
 is a deviation which arises from fhe progressive motion of light, 
 and the annual motion of the earth; for the light by which a star 
 is seen, at the time it sets out from the star, is not directed to 
 the spectator, but to a point beyond him, at which, or into the 
 same line with which, his eye will arrive, by the progression 
 of the earth, in exactly the same time that the light requires 
 to reach the earth. This phenomenon is called the aberration 
 of light, or aberration of the fxed stars. 
 
 The aberration of the fixed stars was discovered by Dr. 
 Bradley, in his attempts to ascertain their annual parallax, and 
 he explained it in the following manner: He imagined CA, fig. 
 7, pi. IV. to be a ray of light falling perpendicularly upon the 
 line BD ; that, if the eye is at rest at A, the object must ap- 
 pear in the direction AC, whether light be propagated in time 
 or in an instant. But if the eye is moving from B towards A, 
 and light is propagated in time, with a velocity that is to the 
 velocity of the eye, as CA to BA ; then light moving from 
 C to A, whilst the eye moves from B to A, that particle of it 
 by which the object will be discerned when the eye comes to 
 it, is at C when the eye is at B. Joining the points BC, he 
 supposed the line CB to be a tube, inclined to the line BD in 
 the angle DBC, of such diameter as to admit but one particle 
 of light. Then it was easy to conceive, that the particle of 
 light at C, by which the object must be seen, when the eye, as 
 it moves along, arrives at A, would pass through the tube BC, 
 if it is inclined to BD in the angle DBC, and accompanies the 
 eye in its motion from B to A ; and that it could not come to 
 the eye placed behind such a tube, if it had any other inclina- 
 tion to the line BD. If, instead of supposing CB so small a 
 tube, we imagine it to be the axis of a larger, then, for the 
 same reason, the particle of light at C would not pass through 
 the axis, unless it is inclined to BD in the angle CBD. In 
 like manner, if the eye moved the contrary way, from D 
 towards A, with the same velocity, then the tube must be in- 
 clined in the angle BDC. 
 
ASTRONOMY. 603 
 
 Aberration of light. 
 
 Although, therefore, the true or real place of an object is 
 perpendicular to the line in which the eye is moving, yet the 
 visible place will not be so ; since that no doubt must be in the 
 direction of the tube ; but the difference between the true and 
 apparent place will be greater or less, according to the different 
 proportion between the velocity of light and that of the eye ; 
 so that if we could suppose that light was propagated in an 
 instant, then there would be no difference between the real and 
 visible place of an object, although the eye was in motion ; for 
 in that case, AC being infinite with respect to AB, the angle 
 ACB, the difference between the true and visible place, va- 
 nishes. But if light be propagated in time, it is evident from 
 the foregoing considerations, that there will be always a differ- 
 ence between the real and visible place of an object, unless the 
 eye is moving either directly from or to the object. And in 
 all cases, the sine of the difference between the real and visible 
 place of the object, will be to the sine of the visible inclina- 
 tion of the object to the line in which the eye is moving, as 
 the velocity of the eye is to the velocity of light. 
 
 Dr. Bradley then shews, that if the earth revolve round the 
 sun annually, and the velocity of the light be to the velocity 
 of the earth's motion in its orbit, as 1000 to 1, that a star 
 really placed in the very pole of the ecliptic, would, to an eye 
 carried along with the earth, seem to change its place continu- 
 ally ; and neglecting the small difference arising from the 
 earth's diurnal revolution on its axis, would seem to describe 
 a circle round that pole, every way distant from it 3g'; so that 
 its longitude would be varied through all the points of the 
 ecliptic every year, but its latitude would always remain the 
 same. Its right ascension would also change, and its declina- 
 tion, according to the different situation of the sun with re- 
 spect to the equinoctial points, and its apparent distance from 
 the north pole of the equator, would be 7' less at the autumnal 
 than the vernal equinox. 
 
 The greatest alteration of the place of a star in the pole 
 of the ecliptic, or which in effect amounts to the same thing, 
 the proportion between the velocity of light and the earth's 
 motion in its orbit being known, it will not be difficult, he ob- 
 serves, to find what would be the difference, upon this account, 
 between the true and apparent place of any other star at any 
 time, and on the contrary, the difference between the true and 
 apparent place being given, the proportion between the velo- 
 city of light, and the earth's motion in its orbit, may be found. 
 — Therefore, since the apparent declination of the star y Dra- 
 conis, on account of the successive propagation of light, 
 would be to the diameter of the little circle which a star would 
 
604 ASTRONOMY. 
 
 Aberration of light. 
 
 seem to describe about the pole of the ecliptic, as 39" to 40.4' ; 
 the half of this is the angle ACB. This, therefore, being 20.2", 
 AC will be to AB, that is, the velocity of light will be to the 
 velocity of the eye, (or the velocity of the earth in its annual 
 motion,) as 10,210 to 1 ; from which it follows, that light moves 
 as far as from the sun to the earth in 8 12 . By the near 
 agreement of multiplied observations. Dr. Bradley proved this 
 statement to be very near the truth. 
 
 The aberration of tiie sun and planets is inconsiderable, 
 because while the light is coming from them, the earth has 
 moved but a little way ; yet for accurate calculations, it is 
 always taken into account; the sun's aberration is about 20", 
 that being the space passed over by the earth while his light 
 is coming to us. 
 
 Dr. Bradley's explanation of the aberration of the fixed stars, 
 was no sooner proposed than acceded to by all philosophers, 
 and has never since been questioned. The phenomenon is a 
 strong proof of the annual motion of the earth round the sun, 
 and as the discovery of it so justly gives celebrity to this as- 
 tronomer's name, the incident which enabled him to explain 
 it so well, and which is given by Dr. Thompson, in his History 
 of the Royal Society, deserves our notice. It shews how much 
 we may improve by habits of observation, and that the most 
 common occurrences may frequently be made subservient to 
 the illustration of the most abstruse points of philosophy. 
 When Dr. Bradley had discovered what is now called the aber- 
 ration of the fixed stars, and had even completed a whole year's 
 obseivations,he perplexed himself continually withi)ut success 
 to find out the cause of the phenomenon. At last, a satisfac- 
 tory explanation of it at once occurred to him, when he was 
 not in search of it. He accompanied a pleasure-party in a sail 
 upon the river Thames. The boat in which they weri;, was pro- 
 vided with a mast, that had a vane at the top of it. It blew a 
 moderate wind, and the party sailed up and down the river for 
 a considerable time. Dr. Bradley remarked, that every time 
 the boat put about, the vane at the top of the boat's mast shifted 
 a little, as if there had been a slight change in the direction of 
 the wind. He observed this three or four times without speak- 
 ing ; at last he mentioned it to the sailors, and expressed his sur- 
 prise that the wind should shift so regularly every time they 
 put about. The sailors told him the wind had not shifted, but 
 that the apparent chanoe was owing to the change in the di- 
 rection of the boat, and assured him that the same thing in- 
 variably happened in all cases. This accidental observation 
 led him justly to conclude, that the aberration of the stars was 
 owing to the combined motion of light and the earth. 
 
A.ST1{<>JS"031V. 
 
 77/-/.. /i./. 
 
 ibi'sied "by Eenrjr Hsier. C ax: j^l , ZLomJinn . 1825 ■ 
 
ASTRONOMV. 605 
 
 ElVects of the spheroidal figure of the earth. 
 
 Nutation of the Earth's Axis. 
 
 The discovery of the aberration of the fixed stars, was fol- 
 lowed up by Dr. Bradley with an almost incessant assiduity 
 of observation. All his subsequent investigations afforded con- 
 firmations of what he had advanced on the subject; and his 
 perseverance was further rewarded, in his own opinion, no 
 doubt, and certainly in that of all other philosophers, by 
 another brilliant discovery, which is called the nutation of the 
 earth's axis. This is a result and a proof of the spheroidal 
 figure of the earth. The moon, exerting a greater power of 
 attraction on the equatorial regions of the earth, than on the 
 polar regions, because the mass of matter at the equator is 
 greater than at the poles, causes a libratory motion of the 
 earth's axis, the inclination of which to the ecliptic is, though 
 in a very small degree, continually varying backwards and 
 forwards, so that its extremity describes a small ellipse, the 
 greater diameter of which is about 19.1" and the less 14.2". 
 This ellipse is described in the same time as a cycle of the 
 moon, or about 18 years and 7 months. 
 
 Precession of the Equinoxes. 
 
 When we examine a celestial globe, we find that all the 
 symbols of the constellations are 30 degrees out of what we 
 should consider their true place, or from the constellations 
 themselves. This apparent irregularity has attained its pre- 
 sent amount, by the very slow accumulation of 50" annually. 
 The reason of it is, that the sun does not cut the equator at 
 the same point every year. If, on a certain day, he cuts the 
 equator in a particular point, he will on the same day of the 
 following year cut it in a point 50.25' westward of the former, 
 and thereby comes to the equinox 20'. 23" of time before he 
 has completed his circuit of the heavens, or gone from fixed 
 star to fixed star. This phenomenon is called the precession of 
 the equinoxes. It makes the tropical year, or true year of the 
 seasons, shorter than the revolution of the sun, or sidereal year, 
 because it is a deviation contrary to the order of the signs. 
 But the intersections of the equator and the ecliptic are still 
 called the beginning of m, and the beginning of ^, though 
 the constellations (being always at the same place) are actu- 
 ally at the distance above stated, from those intersections. 
 The precession of the equinoxes, like the nutation of the earth's 
 axis, is produced by the protuberance of the equator; for when 
 the sun is on either side of the equator, his return to it is has- 
 
606 ASTRONOMY, 
 
 Precession of the equinoxes. 
 
 tened by his attraction for the greater quantity of matter in 
 that direction. Receding at the rate of 50.25" westward annu- 
 ally, the equinoxes make an entire revolution in 25,791 years, 
 when the variationcommences another period. In consequence 
 of it, those stars which in the infancy of astronomy were in 
 Aries, are now got into Taurus; those of Taurus into Gemini, 
 &c. Hence, likewise, it is, fhat the stars which rose at any 
 particular season, in the times of the ancients, by no means 
 answer at this time to the description then given of them. 
 
 In consequence of the precession, or retrograde motion of 
 the equinoctial points in the heavens, the earth's axis does not 
 preserve an undeviating parallelism, but, with a conical motion 
 describes a small circle, the diameter of which is equal to 
 twice its inclination to the ecliptic, or 47^. This motion may 
 be explained by a diagram : let NZSVL, (tig. 1, pi. V,) be the 
 earth ; SONA its axis, produced to the starry heavens, and 
 terminating in A, the present north pole of the heavens, which 
 is vertical to N, the north pole of the earth. Let EOQ be the 
 equator, TsZ the tropic of Cancer, and VTyf the tropic of 
 Capricorn ; VOZ the ecliptic, and BO its axis, which must be 
 considered immoveable, because the ecliptic always passes 
 over the same stars. But as the equinoctial points recede in 
 the ecliptic, the earth's axis, SON, is in motion upon the earth's 
 centre O, in such a manner as to describe the double cone 
 IS'O n and SO s, round the axis of the ecliptic BO, in the time 
 that the equinoctial points move round the ecliptic, that is, in 
 25,791 years ; and in that length of time, the north pole of the 
 earth's axis produced, describes the circle ABCDA in the 
 starry heavens, round the pole of the ecliptic, which keeps 
 immoveable in the centre of that circle. The earth's axis being 
 nearly 23^° inclined to the axis of the ecliptic, the circle 
 ABCDA, described by the north pole of the earth's axis pro- 
 duced to A, will therefore be almost 47*^ in diameter, or double 
 the inclination of the earth's axis. In consequence of this, 
 the point A, wliich is at present the north pole of the heavens, 
 and near to a star of the second magnitude, in the end of the 
 Little Bear's tail, must be deserted by the earth's axis; which, 
 moving back one degree in every 71 > years nearly, will be di- 
 rected towards the star or point B in 6447| years hence ; and 
 in double that time, or 12,895.1 years, it will be directed to- 
 wards the star or point C ; which will then be the north pole 
 of the heavens, although it is at present 8.3*^ south of the zenith 
 of London L. The present position of the equator, EOQ will 
 then be changed into eO q; the tropic of Cancer, Ts, into 
 V /«3 ; and the tropic of Capricorn, VTc\ into t /^K\ and the 
 sun, in the same part of the heavens where he is now over the 
 
ASTROXOMV. 607 
 
 Eclipses of the moon. 
 
 earthly tropic of Capricorn, and makes the shortest days and 
 longest nights in the northern hemisphere, will then be over 
 the earthly tropic of Cancer, and make the days longest and 
 nights shortest. So that it will require 12,895| years yet more, 
 or 25,791 years from that time, to bring the north pole N quite 
 round, so as to be directed towards that point of the heavens 
 which is vertical to it at present. And then, but not before, 
 the same stars which at present describe the equator, tropics, 
 polar circles, &c. by the earth's diurnal motion, will describe 
 them over again. 
 
 Of Eclipses. 
 
 The words transit, occultation, and eclipse, are all employed 
 to designate the same general phenomenon, of one heavenly 
 body being lost sight of, either wholly or in part, by the inter- 
 position of another ; but they are not used indiscriminately. 
 The word transit denotes the passage of the inferior planets, 
 Venus and Mercury, over the sun's disc; occultation, the disap- 
 pearance of the stars or planets by the interposition of the 
 moon ; and eclipse, 1st, to the obscuration of the moon, when 
 this luminary falls into the earth's shadow, called an eclipse of 
 the moon; 2nd, to the obscuration of the sun, when the earth 
 falls into the moon's shadow, called an eclipse of the sun ; and 
 3rd, to the obscuration of the satellites of any of the planets, 
 by their coming within the shadows of their respective prima- 
 ries. One of the two first of these three phenomena is always 
 meant, when the word eclipse is used alone. 
 
 Eclipses, like comets, were formerly the objects of popular 
 dread, but the age of these vain terrors is now gone by, because 
 it has become a part of popular belief, that they happen ac- 
 cording to the common course of nature, which belief is en- 
 forced by the notoriety of the fact, that they may be predicted 
 with precision, like the alternations of day and night. 
 
 Eclipses of the Moon. 
 
 As the earth, like all the rest of the celestial bodies, is glo- 
 bular, the sun can only enlighten one half of it at once ; and 
 the hemisphere turned from the sun, will cause a shadow in 
 the regions of space, on account of the light intercepted by its 
 opacity. If the sun and earth were of the same size, this sha- 
 dow would be almost cylindrical, and of infinite extent; it 
 would not be perfectly cylindrical, because the rays proceed 
 from the sun divergently; but as the earth is very considerably 
 
COS ASTRONOMY. 
 
 Eclipses of tbe rooon. 
 
 less than the sun, the shadow is proportionately conical, and 
 though sufficiently long to reach and envelop the moon, it 
 does not reach to tiie planet Mars. In plate V, fiij. 2, S is the 
 sun, E the earth, and JNI the moon. The dark conical shadow 
 terminates at^", just where rays from the upper and lower Hmbs 
 of the sun, after passing the contour of the earth, meet in a 
 point : on the sides of this conical shadow, there is a diverging 
 shadow abed, the density of which decreases in proportion 
 as it recedes from the sides of the former conical shadow; this 
 is called iha penumbra. 
 
 When the moon passes entirely through the earth's shadow, 
 the eclipse is total ; but when only a part of it passes through 
 the shadow, the eclipse is partial. 
 
 The quantity of the moon's disc which is eclipsed, is ex- 
 pressed by twelve parts, called digits; that is, the disc is sup- 
 posed to be divided by twelve parallel lines; then if half the 
 disc is eclipsed, the quantity of the eclipse is said to be six 
 diu;its, and so of other proportions. 
 
 When the diameter of the shadow through which the moon 
 must pass, is greater than the diameter of the moon, the quan- 
 tity of the eclipse is said to be more than twelve digits; thus, 
 if the diameter of the moon is to that of the shadow as four to 
 five, then the e-clipse is said to be 15 digits. 
 
 The general phenomena of lunar eclipses may be classed as 
 follows : — 1. All lunar eclipses are universal, or visible in all 
 parts of the earth which have the moon above the horizon; and 
 are every where of the same magnitude, with the same begin- 
 ning and end. — 2. In all lunar eclipses, the eastern side is that 
 which tirst immerges and emerges again ; that is, the left side of 
 the moon as we look towards her from the north; for the pro- 
 per motion of the moon being swifter than that of the earth's 
 shadow, the moon approaches it from the west, overtakes it, 
 and passes through it, with her east side foremost, leaving the 
 shadow behind, or to the westward. — 3. Total eclipses, and 
 those of the longest duration, happen in the very nodes of the 
 ecliptic, because the section of the earth's shadow then falling 
 on the moon, is considerably larger than her disc, and the 
 eclipse is more than 12 digits. There may, however, be total 
 eclipses within a small distance of the nodes; but their dura- 
 tion is the less as they are farther from them, till they become 
 only partial ones, and at last the moon escapes the shadow. — 
 4. The moon, even in the middle of a total eclipse, is not 
 invisible, when the sky is clear, but has an appearance resem- 
 bling tarnished copper; this appearance is attributed to the 
 rays refracted by the earth's atmosphere into the shadow, of 
 which it consequently diminishes the intensity. — 5. the moon 
 
ASTRONOMY. G09 
 
 Eclipses of tbe moon. — Of the sun. 
 
 becomes sensibly paler and dimmer, before entering into the 
 real shadow ; this is owing to the penumbra. 
 
 If the moon's orbit were in the same plane as the ecliptic, 
 or plane of the earth's orbit, she would go through the middle 
 of the earth's shadow, and suffer a total eclipse at every full, 
 and would be totally darkened for above an hour and a half. 
 But one-half of the moon's orbit is elevated five degrees 
 above the ecliptic, and the other half as much depressed 
 below it; consequently the moon's orbit intersects the 
 ecliptic only in the two points before mentioned, called the 
 nodes. When either of these points is in a right line with 
 the centre of the sun, at new or full moon, the sun, moon, 
 and earth, are all in a right line; and if the moon be then 
 full, she falls into the earth's shadow. When the moon is 
 more than 12^^ from either of the nodes at the time of full 
 moon, she is then generally too high or too low in her orbit 
 to go through any part of the earth's shadow, and no eclipse 
 occurs. But when she is less than 12*^ from either node at 
 the time of opposition or full, she goes through a greater or 
 less portion of this shadow as she is more or less within this 
 limit. Her orbit contains 360'^, of which 12*^, the limit of 
 lunar eclipses on either side of the nodes, are but small por- 
 tions, and therefore, as the sun commonly passes the nodes but 
 twice in a year, it is not extraordinary that the eclipses are so 
 few in number. 
 
 The eclipses of the moon, as observed above, are visible 
 from all parts of the earth that have the moon above the hori- 
 zon at the time, and wherever observed, they are of equal ex- 
 tent; but the time at which they are seen, differs with the 
 difference of longitude, in the same manner as the observations 
 of the satellites of Jupiter, mentioned in page 558, and there- 
 fore they are of similar use in determining the longitude of 
 places on the earth. The lunar eclipses are very variable in 
 duration, but never exceed two hours. 
 
 Eclipses of the Sun. 
 
 As the light of the sun, in its passage to the earth, can only 
 be intercepted by the moon, when the moon is in conjunction 
 with the sun, therefore an eclipse of the sun, or the falling of 
 the earth into the moon's shadow, can only occur at new moon. 
 The moon being a body similar to the earth, its shadow and 
 penumbra are like those occasioned by the earth. This is 
 shewn by fig. 3, plate V; but as the moon is less than the 
 earth, it could not, however near, intercept the whole of the 
 sun's light from the earth, therefore, at the distance of 240,000 
 miles, and casting a conical shadow, a total eclipse affects 
 26.— Vol. I. 4 1 
 
610 ASTRONOMY. 
 
 EcliiiscD of ibe son. 
 
 but a small part of the «artli at once, and even the penumbra 
 is far from beino; broad enougrh to cover the whole terrestrial 
 disc. 
 
 The quantity of a solar eclipse is estimated by digits, like 
 an eclipse of the moon. 
 
 The general circumstances of solar eclipses are the follow- 
 ing : 1. None of them are universal; that is, none of them are 
 seen throughout the whole hemisphere which the sun is tlien 
 above. Commonly the moon's dark shadow covers only a 
 spot on the earth's surface, about 18U miles broad, as indicated 
 by fig. 3, when the sun's distance is greatest, and the moon's 
 least; but her partial shadow, or penumbra, may then cover a 
 circular space of 4900 miles in diameter, within which the sun 
 is more or less eclipsed, as the places are nearer to or further 
 from the centre of the penumbra. In this case, the axis of 
 the shade passes through the centre of the earth, or the new 
 moon happens exactly in the node, and then it is evident that 
 the section of the shadow is circular; but in every other case, 
 the conical shadow is cut obliquely by the surface of the earth, 
 and the section will be elliptical. — 2. A solar eclipse does 
 not appear the same in all parts of the earth where it is seen, 
 but when in one place it is total, in another it is only partial. 
 Also, when the moon appears much less than the sun, as is 
 chiefly the case when she is in apogee and he in perigee, the ver 
 tex of the lunar shadow is then too short to reach the earth, and 
 tliough she be in a central conjunction with the sun, is yet not 
 large enough to cover his whole disc ; the portion we see of him 
 resembles a lucid ring or bracelet, an appearance constituting 
 what is called an amiitlar eclipse. — 3. A solar eclipse does n t 
 happen at the same time in all places where it is seen; but ap 
 pears more early to the western parts than the eastern, as the 
 motion of the moon, and consequently of her shadow, is from 
 west to east. — 4. In most solar eclipses, the moon's disc is 
 covered with a faint light, which is attributed to the reflection 
 of the light from the illuminated part of the earth. — 5. In total 
 eclipses of the sun, the moon's limb is seen surrounded by a pale 
 circle of light ; which some astronomers consider as an indica- 
 tion of a solar atmosphere, because it has been observed to 
 move equally with the sun, and not with the moon. 
 
 Solar eclipses do not happen at every new moon, for the 
 same reason that lunar eclipses do not happen at every full 
 moon, that is, because the orbit of the moon is not coincident 
 with the ecliptic, except in the two nodes; but solar eclipses 
 may hapjjen at the distance of 17*^ from the nodes, which is a 
 range of j''" on either side more than the arc that comprehends 
 the lunar eclipses. 
 
ASTRONOMY. 611 
 
 Eclipses of the sao. 
 
 The moon's apparent diameter when largest, exceeds the 
 sun's when least, only 1' 38"; and in the greatest eclipse of the 
 sun that can happen at any time and place, the total darkness 
 continues no longer than whilst the moon is going 1' 38" of a 
 a degree in her orbit, which is about 3' 13 " of an hour. 
 
 When the penumbra (a b, fig. 3) first touches the earth, the 
 general eclipse begins: when the penumbra leaves the earth, 
 the general eclipse ends; from the beginning to the end, the 
 sun appears eclipsed to some part of the earth or other. When 
 the penumbra touches any place, the eclipse begins at that 
 place, and ends when the penumbra leaves it. W'hen the moon 
 changes in the node, the penumbra goes over the centre of the 
 earth's disc as seen iVom the moon ; and consequently, by de- 
 scribing the longest line possible upon the earth, continues 
 the longest upon it, namely, at a mean rate, 5 hours 50'; 
 longer, if the moon be at her greatest distance from the earth, 
 because she then moves slowest; not so long, if she be at her 
 least distance, because of her quicker motion. 
 
 The general phenomena of eclipses will perhaps be made 
 still plainer by reference to the diagram, fig. 4, plate V. Let 
 S be the sun, E the earth, M the moon, and AMP the moon's 
 orbit. Draw the right line W e from the western side of the 
 sun at W, touching the western side of the moon at c, and t'he 
 earth at e: draw also the right line V d, from the eastern side 
 of the sun at V, touching the eastern side of the moon at d^ 
 and the earth at e; the dark space c e d, included between 
 those lines, is the moon's shadow, ending in a point at e, 
 where it touches the earth; because in this case the moon is 
 supposed to change at M, in the middle, between A, the 
 apogee, or farthest point of her orbit from the earth, and P, 
 the perigee, or nearest point of it. But as any point of the 
 moon's orbit may be directed to the sun, if the point P had 
 been at M, the moon would of course have been nearer the 
 earth, and her dark shadow at e would have covered a space 
 upon it about 180 miles broad, and the sun would for a time 
 be totally darkened ; but had the point A been at M, the 
 moon would have been farther from the earth, and her shadow 
 would have ended in a point a little above e, and therefore the 
 sun would have appeared like a luminous ring all round the 
 moon. Draw the right lines WX d h, and VX eg, touching 
 the contrary sides of the sun and moon, and ending on the 
 earth at a and h; draw also the right line SXM, from the 
 centre of the sun's disc, through the moon's centre, to the 
 earth; and suppose the two former lines WX d h and VX eg 
 to revolve on the line SXM as an axis, and their points a and 
 h will describe the limits of the penumbra TT on the earth's 
 
012 ASTRONOMY. 
 
 Eclipses of the sun. 
 
 surface, including the large space ah; within which penumbra 
 the sun appears more or less eclipsed, as the places are more 
 or le-ss distant from its verge. 
 
 Draw the right line y 12 across the sun's disc, perpendi- 
 cular to SXM, the axis of the penumbra; then divide the line 
 y 12 into twelve equal parts, as in the figure, for the twelve 
 digits or equal parts of the sun's diameter; and, at equal dis- 
 tances from the centre of the penumbra at e, on the earth's 
 surface YY, to its edge a b, draw 12 concentric circles, marked 
 witli the numeral figures 1 to 12, and remember that the moon's 
 motion in her orbit AMP, is from west to east, as from I\I to P ; 
 then, to an observer on the earth at b, the eastern limb of 
 the moon at d, seems to touch the western limb of the sun at 
 W, when the moon is at M ; and the sun's eclipse begins at 
 b; but at the same moment of absolute time, to an observer 
 at a, the western edge of the moon at c leaves the eastern 
 edge of the sun at V. At the very same instant, to all those 
 who live on the circle marked 1 on the earth, the moon ]\I 
 cuts off' or darkens a twelfth part of the sun S, and eclipses 
 him one digit; to those who live on the second circle, the 
 moon cuts off" two digits ; to those on the circle 3, three digits ; 
 and so on to the centre at 12, where the sun is centrally- 
 eclipsed. It is evident by the figure, that the sun is totally 
 or centrally eclipsed but to a small part of the earth at any 
 time, because the dark conical shadow at e of the moon M, 
 falls but on a small part of the earth, even when it covers the 
 largest space possible ; and that the partial eclipse is con- 
 fined at that time to the space included by the circle a b, of 
 which only one half can be projected in the figure, the other 
 half being supposed to be hidden by the convexity of the 
 earth E: and likewise, that no part of the sun is eclipsed to 
 the large space YY of the earth, because the moon is not be- 
 tween the sun and any of that part of the earth ; and therefore 
 to all that part the eclipse is invisible. The earth turns east- 
 ward on its axis, as from g to h, which is the same way that 
 the moon's shadow moves ; but the moon's motion in her orbit 
 is much swifter than the earth's on its axis; and therefore, 
 although eclipses of the sun are of longer duration, on account 
 of the earth's motion on its axis, than they would be if that 
 motion were stopped, yet, in four minutes of time at the most, 
 the moon's swifter motion carries her dark shadow quite over 
 any place that its centre touches at the time of greatest ob- 
 scuration. The motion of the shadow on the earth's disc, is 
 equal to the moon's motion from the sun, which is about SOV' 
 of a degree every hour at a mean rate; but so much of the 
 moon's orbit is equal to SOa degrees of a great circle on the 
 
AST1!02C0.\1T. 
 
 
 ■^ 
 
 
 
 
 
 U 
 
 
 '■■ T. 
 //> — ' 
 
 2 
 
 
 ■9-X 
 
 73)>*^fi^i 
 
 "by "Z-szTy Jiiicr Cszcxan. Loin don, 1323 
 
ASTRONOMY. 613 
 
 Scenery of nature, during a solar eclipse, delineated. 
 
 earth ; and therefore the moon's shadow goes SO^ degrees, or 
 1830 geographical miles, on the earth in an hour, or SOg miles 
 in a minute, which is about four times as swift as the motioa 
 of a cannon-ball. 
 
 Description of a total Eclipse of the Sun. 
 
 As total eclipses of the sun are very rare phenomena, we 
 shall here give the excellent description of one sent by 
 Dr. Stukely, to his friend Dr. Edmund Halley. Its general 
 interest is much increased by the circumstance that the 
 author, from being unprovided with instruments, describes 
 the spectacle rather as a general, though intelligent, observer 
 of nature, than an astronomer. 
 
 " According to my promise, I send you what I observed of 
 the solar eclipse, though I fear it will not be of any great use 
 to you. I was not prepared with any instruments for mea- 
 suring time or the like, and proposed to myself only to watch 
 all .the appearances that nature would present to the naked 
 eye upon so remarkable an occasion, and which generally are 
 overlooked, or but grossly regarded. I chose for my station 
 a place called Haradon Hill, two miles eastward from Amsbury, 
 and full east from the opening of Stonehenge avenue, to which 
 it is as the point of view. Before me lay the vast plain where 
 that celebrated worjc stands, and 1 knew that the eclipse 
 would appear directly over it; besides, I had the advantage 
 of a very extensive prospect every way, this being the highest 
 hill hereabouts, and nearest the middle of the shadow; full 
 west of me, and beyond Stonehenge, is a pretty copped hill, 
 like the top of a cone, lifting itself above the horizon; this is 
 Clay-hill, near Warminster, 20 miles distant, and near [the 
 central line of darkness, which must come from thence, so that 
 I could have notice enough before-hand of its approach. 
 Abraham Sturgis and Stephen Ewens, both of this place, and 
 sensible men, were with me. Though it was very cloudy, yet 
 now and then we had gleams of sunshine, rather more than I 
 could perceive at any other place around us. These two per- 
 sons looking through smoked glasses, while I was taking some 
 bearings of the country with a circumferentor, botii confi- 
 dently affirmed the eclipse was begun, when by my watch I 
 found it just half an hour after 5; and accordingly, from thence 
 the progress of it was visible, and very often to the naked eye; 
 the thin clouds doing the office of glasses. From the time of 
 the sun's body being half covered, there was a very conspicuous 
 circular iris round t/ie sun with perfect colours. On all sides, 
 we beheld the shepherds hurrying their flocks into fold, the 
 
614 ASTRON'OMV. 
 
 Scenery of uuture, durlni; a solar eclipse, delineated. 
 
 darkness coming on; for they expected nothing less than a 
 total eclipse for an hour and a quarter. 
 
 " When the sun looked very sharp like a new moon, the 
 sky was pretty clear in that spot; but soon after a thicker 
 cloud covered it, at which time the iris vanished, the copped 
 hill before mentioned grew very dark, together with the hori- 
 zon on both sides, that is, to the north and south, and looked 
 blue; just as it appears in the east at the declension of day. 
 We had scarce time to tell ten, when S-alisbury steeple, six 
 miles off southward, became very black ; the copped hill quite 
 lost, and a most gloomy night with full career came upon us. 
 At this instant we lost sight of the sun, whose place among 
 the clouds was hitherto sufficiently distinguishable, but now 
 not the least trace of it to be found, no more than if really 
 absent: then I saw by my watch, though with difficulty, and 
 only by help of some light from the northern quarter, that it 
 was G hours 35 minutes. Just before this, the whole compass 
 of the heavens and earth looked of a lurid complexion, pro- 
 perly speaking, for it was black and blue, only on the earth, 
 upon the horizon, the blue prevailed; there was likewise in 
 the heavens, among the clouds, much green interspersed, so 
 that the whole appearance was really very dreadful, and as 
 symptoms of sickening nature. 
 
 " Now I perceived us involved in total darkness and palpa- 
 ble, as I may aptly call it; though it came quick, yet 1 was so 
 intent, that 1 could perceive its steps, and feel it as it were drop 
 upon us, and fall on the right shoulder (we looking westward) 
 like a great dark mantle or coverlet of a bed thrown over us, or 
 like the drawing of a curtain on that side; and the horses we 
 held in our hantls were very sensible of it, and crowded close 
 to us, startling with great surprise ; as much as I could see of 
 the men's faces that stood by me, had a horrible aspect. At 
 this instant I looked around me, not without exclamations of 
 admiration, and could discern colours in the heavens, but the 
 earth had lost its blue, and was wholly black; for some time 
 among the clouds, there were visible streaks of rays tending 
 to the place of the sun as their centre; but immediately after 
 the whole appearance of earth and sky was entirely black : of 
 all things I ever saw in my life, or can by imagination fancy, 
 it was a sight the most tremendous. 
 
 " Towarci the north-west, whence the eclipse came, I could 
 not in the least find any distinction in the horizon between 
 heaven and earth, for a good breadth of about sixty degrees or 
 more; nor the town of Amsbury underneath us, nor scarce the 
 ground we trod on. I turned myself round several times during 
 thi.<s total darkness^ and remarked at a good distance from the 
 
ASTRONOMY. 615 
 
 Scenery of nature, daring a solar eclipse, delineated. 
 
 west on both sides, that is to the north and south, the horizon 
 very perfect; the earth being black, the lower part of the hea- 
 vens light; for the darkness above hung over us like a canopy, 
 almost reaching the horizon in those parts, or as if made with 
 skirts of a lighter colour; so that the upper edges of all the 
 hills were as a black line, and I knew them very distinctly by 
 their shape or profile; and northward I saw perfectly, that the 
 interval of light and darkness in the horizon was between 
 Martinsal-hill and St. Ann's hill ; but southward it was more 
 indefinite. I do not mean that the verge of the shadow passed 
 between those hills, which were but 12 miles distant from us; 
 but, so far I could distinguish the horizon, beyond it not at 
 all: the reason of it is this: the elevation of ground I was 
 upon, gave me an opportunity of seeing the light of the hea- 
 vens beyond the shadow ; nevertheless, this verge of light 
 looked of a dead, yellowish, and greenish colour; it was 
 broader to the north than south, but the southern was of a 
 tawny colour. At this time behind us, or eastward toward 
 London, it was dark too, where otherwise I could see the hills 
 beyond Andover; for the foremost end of the shadow was past 
 thither; so that the whole horizon was now divided into four 
 parts of unequal bulk, and degrees of light and dark : the 
 part to the north-west broadest and blackest; to the south- 
 west lightest and longest. All the change I could perceive 
 during the totality, was that the horizon by degrees drew into 
 two parts, light and dark; the northern hemisphere growing 
 still longer, lighter, and broader; and the two opposite dark 
 parts uniting into one, and swallowing up the southern en- 
 lightened part. 
 
 " As at the beginning, the shade came feelingly upon our 
 right shoulders, so now the light from the north, where it 
 opened as it were; though I could discern no defined light or 
 shade upon the earth that way, which I earnestly watched for, 
 yet it was manifestly by degrees, and with oscillations, going 
 back a little, and quickly advancing further, till at length, 
 upon the first lucid point appearing in the heavens, where the 
 sun was, I could distinguish pretty plainly a rim of light run- 
 ning alongside of us a good while together, or sweeping by 
 at our elbows from west to east ; just then having good rea- 
 son to suppose the totality ended with us, I looked on my 
 watch, and found it to be full three minutes and a half more. 
 Now the hill-tops changed their black into blue again, and I 
 could distinguish an horizon where the centre of darkness was 
 before; the men cried out, they saw the copped hill again, 
 which they had eagerly looked for; but still it continued dark 
 to the south-east, yet I cannot say that ever the horizon that 
 
G16 ASTRONOMY. 
 
 Scenerv of iiiturc, during a solar eclipse, dclineati.'d. 
 
 way was undistinguishable; immediately we lieard the larks 
 chirping, and singing very briskly, for joy of the restored 
 luminary, after all things had been hushed into a most pro- 
 found and universal silence. The heavens and earth now ap- 
 peared exactly like morning before sun-rise, of a grayish cast, 
 but rather more blue interspersed ; and the earth, so far as 
 the verge of the hill reached, was of a dark green or russet 
 colour. 
 
 " As soon as the sun emerged, the clouds grew thicker, and 
 the light was very little amended for a minute or more, like a 
 cloudy morning slowly advancing. After about the middle of 
 the totality, and so after the emersion of the sun, we saw- 
 Venus very plainly, but no other star. Salisbury steeple now 
 appeared. The clouds never removed, so that we could take 
 no account of it afterwards, but in the evening it lightened 
 very much. I hasted home to write this letter; and the im- 
 pression was so vivid upon my mind, that I am sure, I could 
 for some days after have wrote the same account of it, and 
 very precisely. After supper I made a drawing of it from my 
 imagination, upon the same paper I had taken a prospect of 
 the country before. 
 
 " I must confess to you, that I was (I believe) the only 
 person in England that regretted not the cloudiness of the day, 
 which added so much to the solemnity of the sight, and which 
 incomparably exceeded, in my apprehension, that of 1715, 
 which I saw very perfectly from the top of Boston steeple, in 
 Lincolnshire, where the air was very clear; but the night of 
 this was more complete and dreadful: there, indeed, I saw 
 both sides of the shadow come from a great distance, and pass 
 beyond us to a great distance ; but this eclipse had much niore 
 of variety and majestic terror; so that I cannot but ft.licitate 
 myself upon the opportunity of seeing these two rare accidents 
 of nature, in so diderent a manner: yet 1 should willingly 
 have lost this pleasure for your more valuable advantage of 
 perfecting the noble theory of the celestial bodies, which last 
 time you gave the world so nice a calculation of; and wish 
 the sky had now as much favoured us, for an addition to your 
 honour and great skill, which I doubt not to be as exact in 
 tliis as before." 
 
ASTRONOMY. 617 
 
 The astronomical aud civil day. 
 
 Division of Time. 
 
 Time is distinguished into absolute and relative. 
 
 Absolute time, is time considered in itself, and without any 
 relation to bodies or their motions. 
 
 Relative time, is the time measured by means of motion, as 
 by the motion of the heavenly bodies, timekeepers, &c. Mo- 
 tion affords us the only means we possess, of rendering the 
 flow of absolute time perceptible. 
 
 Astronomical time, is that measured by the motion of the 
 heavenly bodies. 
 
 Civil time, is astronomical time accommodated to civil uses, 
 and distinguished into days, months, years, 8cc. 
 
 Of the Day, and smaller portions of Time. 
 
 In common language, the day is the interval of time which 
 elapses from the rising to the setting of the sun; this is called 
 the artifcial day, the night is the interval that the sun con- 
 tinues below the horizon. 
 
 The time which elapses during an entire revolution of the 
 sun, from a meridian to the same meridian again, is called an 
 astronomical or natural day. It commences at 12 o'clock at 
 noon, when the sun's centre is on the meridian, and termi- 
 nates at the next noon, when the sun's centre is again on the 
 meridiun. It is divided into 24 hours, reckoning in a nume- 
 rical succession from 1 to 24 : the first 12 are sometimes dis- 
 tinguished by the mark P.M. s'lgm^y'iug post meridian, or after- 
 noon; and the latter twelve are marked A. M. signifying a«/e 
 meridiem, or before noon. But astronomers generally reckon 
 through the 24 hours, from noon to noon; and those hours 
 which are by the civil or common way of reckoning called 
 morning hours, they reckon in succession from 12 or midnight, 
 to 24 hours. Thus 10 o'clock in the morning of June the 
 11th, they would call the 22d hour of June the 11th. 
 
 The length both of the astronomical and the civil day is 
 constantly changing. This variation arises from two causes : 
 1. The unequal motion of the earth in its orbit, 2. The obli- 
 quity of that orbit to the plane of the equator. The mean and 
 apparent solar days are never equal, except when the sun's 
 daily motion in right ascension is 59' 8". This is the case 
 about April 16, June 16, September 1, and December 25 : on 
 these days the difference vanishes, or nearly so. It is at its 
 greatest about November 1, when it is 16' 16". 
 
 26.— Vol. I. 4 K 
 
618 ASTRONOMY, 
 
 Division of time. — The daj. 
 
 An aslrononiical day is somewliat greater tlian a sidereal 
 year, whicli is the time employed by tlie earth in revolving on 
 its axis, and is at all times of the year immutably tiie same. 
 It will easily be understood, that at every complete rotation 
 of the earth, the same fixed stars will be on the meridian; 
 but that this cannot be the case vvitii the sun, because, during 
 the earth's rotation, the sun* has advanced eastward and left 
 the star, and therefore the earth has to make somewhat more 
 than one turn before the sun comes a second time to the 
 same meridian. If the mean astronomical or civil day be 
 taken equal to 24 hours, the duration of the sidereal day will 
 be 23h. 56' 4.1". Hence a star which was on the meridian 
 with the sun, will be on the meridian 3' 56" before the next 
 noon ; 3' 56" being the difference between a solar and sidereal 
 day. 
 
 The civil day, or astronomical day accommodated to civil 
 purposes, begins with different nations at different times : The 
 Egyptians began their day at midnight; the same method 
 prevails in Great Britain, France, Spain, and most parts of 
 Europe. The Babylonians began their day at sun-rising, 
 reckoning the hour immediately before its rising again, the 
 24th hour of the day, whence the hours reckoned in this way 
 are called the Babylonic. In several parts of Germany and 
 Italy, they begin their day at sun-setting, and reckon on till 
 it sets next day, calling that the 24th hour : these are gene- 
 rally termed Italian hours. The Jews also began their day at 
 sun-setting, but then, like us, they divided it into parts of 
 12 hours each. The Romans also reckoned their hours in this 
 manner, and the Turks do the same at this day. 
 
 An hour may be considered either as an equal or unequal 
 portion of time. Equal hours, or those in common use, are the 
 twenty-fourth part of a day and night precisely. They are 
 likewise called equinoctial hours, because they are measured on 
 the equinoctial; and astronomical, because used by astrono- 
 mers. Unequal hours are those by which the artificial day is 
 
 * Nothing is more common, in astronomical treatises, than to speak of the sun's 
 motion, us if that luminary had a real motion in the ecliptic or throngh the zodiac. This 
 mode of speech is used, because it is easily' explained, that the sun's motion, like that of 
 the shore to a person in a ship, is only apparent, the real motion belonging to the earth, 
 wliich, in diflerent situations of its orbit, necessarily induces the terrestrial spectator to 
 transfer the situation of the sun to a dilTerent position in the sphere. It is an accommo- 
 dalion to the ordinary testimony of our senses, that introduces no confusion of ideas ; 
 but it shews at the same time, that even in this day, a mode of speech is thought expe- 
 dient, that in early ajjes was certainly indispensable, when the sun was commanded to 
 stand still, and virtually obeyed. 
 
ASTRONOMY. 619 
 
 Division of time. — The week. — The month. 
 
 divided into 12 parts, and the night into the same number. 
 As it is only at the time of the equinoxes that the artificial 
 day is equal to the night, they wiH, except at these times, be 
 always either increasing or decreasing. 
 
 The hour is divided into 60 equal parts called minutes; each 
 minute into 60 equal parts called seconds; and each second 
 into 60 equal parts called thirds. The Jews, Chaldeans, and 
 Arabians, divided the hour into 1080 equal parts, called scru- 
 ples ; which number containing 18 times 60, each minute con- 
 tains 18 scruples. 
 
 Of the Week. 
 
 Various divisions of this sort have obtained in different 
 countries, and at different times. The early Greeks divided 
 their month into three portions of ten days each ; the North- 
 ern Chinese had a week of fifteen days, and the Mexicans 
 one of thirteen ; but the most general division of the month 
 was that of the Jews, into periods of 7 days. This period 
 was adopted by the Chaldeans, and most other Oriental na- 
 tions, it is at this day in use wherever Christianity has been 
 introduced. The French abandoned it for the decade during 
 the season of revolutionary frenzy, but their late Dictator 
 brought it back to its former standard. The Mahometans 
 also use the week of 7 days. 
 
 Of the Moon. 
 
 The periodical changes of the phases of the moon, it seems 
 very apparent, gave rise to the division of time called a month ; 
 but the difficulty of adjusting the lunar month to the annual 
 revolution of the earth, led to the introduction of other divi- 
 sions of time under the same name. 
 
 Months are now divided into astronomical and civil. 
 
 The astronomical months, with which chronology is concerned, 
 are measured by the revolutions of the moon, and are either 
 periodical or si/nodical. 
 
 The periodical lunar month, or the exact time in which the 
 moon passes through the zodiac, from any point of her orbit 
 to the same point again, consists of 27 days, 7 hours, 43' 5". 
 
 The synodical lunar month, called a lunation, is reckoned 
 from one conjunction of the sun with the moon to another. 
 It is a variable period, being subject to the variation of the 
 sun eastward on the ecliptic; at a mean rate, a lunation con- 
 sists of 29 days, 12 hours, 44' 3". In ancient times, this was 
 the month in oeueral use. 
 
C20 ASTRONOMY. 
 
 I)i' ision of lime. — The month. — Tlie >ear. 
 
 The iula)- muiith is the time in which the sun passes throuL;h 
 one sign of the zodiac ; its length is variable, because the 
 motion of the sun is so : at a mean rate, it consists of 3U days, 
 10 hours, 29' 5'. 
 
 A civil ov political month is a portion of time set out by the 
 custom of particular nations, without any precise regard to 
 the celestial motions. The British and most European na- 
 tions make twelve 'months in the year. 
 
 Civil lunar months consist alternately of 29 and 30 days 
 Thus two of these months are nearly equal to two astrono- 
 mical months, and the new moon will be kept to the first 
 day of such civil months for a lonoer time t02;ether. This 
 was the month in common use till the time of Julius Cesar. 
 
 The civil solar month is the one at present in use. It was 
 introduced by Julius Cesar. It consisted at first alternately of 
 30 and 31 days, excepting one month of the twelve, which in 
 every fourth year consisted of thirty, and for the other three 
 years of 29 days. This arrangement was altered a little by 
 Augustus, whose name was given to the month ])reviously 
 called Sextilis, and to make it equal to any of the rest, the 
 number of the days it contained was increased from 30 to 31. 
 The day thus added to it, was taken from February, which 
 month, from that time, had only 28 days, except in every 
 fourth year, when it received the intercalary day. The civil 
 months thus determined in the reign of Augustus, have 
 remained in use to the present time, and are common to all 
 Europe ; they are the same with what we usually call calen- 
 dar months. 
 
 Of the Year. 
 
 The smaller divisions of time, under the same name, differ- 
 ing from each other according to the motion by which they 
 are measured, it will necessarily follow that the large divisions 
 will admit variations, according to the value of their integral 
 parts. 
 
 The solar ijear, or tropical year, is the time that the sun takes 
 to pass through the twelve sis;ns of the zodiac, or from one 
 equinox to the same again, and constitutes a natural division 
 of time, because the seasons always fall in the same months. 
 Its length is equal to 365 days, 5 hours, 48' 49". 
 
 The tropical year is divided by astronomers into four parts, 
 determined by the two equinoxes and solstices. The interval 
 between the vernal and autumnal equinoxes is (on account of 
 the eccentricity of the earth's orbit, and its unequal velocity 
 therein) nearly 8 days longer than the interval between the 
 
ASTRONOMY. 621 
 
 Division of time. — The year. 
 
 autumnal and vernal equinoxes. These intervals are at pre- 
 sent nearly as follows : — 
 
 D. h. m. D. L. m. 
 
 From lie Spring eqainox to the summer solstice 92 21 45) 
 
 Summer solstice to the autumnal equinox.. . .93 13 35^ ^^ 
 
 Autumnal equinox to tbe winter solstice 89 16 47 jf 
 
 Winter solstice to the spring eqainox 89 1 42 ^ 
 
 7 16 51 
 
 The Julian year, so named by Julius Cesar, who established 
 it, consists of 365 days, 6 hours ; but the 6 hours were not 
 reckoned till the expiration of every fourth year, when, amount- 
 ing just to one day, they were added to the end of February, 
 and the year in which they were thus reckoned was called bis- 
 sextile or leap year. The Julian year exceeding the true solar 
 year more than 11 minutes, the excess amounts to a whole 
 day in 131 years 5 in consequence of which, the vernal equi- 
 nox, which in the first Julian year fell on the 25th of March, 
 had in the year of our Lord 325, at the time of the council of 
 Nice, gone back to fhe2lst, and in 1582 to the 11th of March. 
 To correct this growing error, the calendar was again amend- 
 ed, under the auspices of Pope Gregory XIII. by taking 10 
 entire days out of it. Accordingly, in the year 1582, the day- 
 following the 4th of October, instead of being called the 5th 
 was called the 15th ; by means of which the real equinox was 
 restored to the 21st of March. A provision was at the same 
 time made, to prevent the recurrence of so important an error 
 again. The intercalary or bissextile day, which had been 
 regularly added to February every fourth year, was ordered 
 to be suppressed at the end of every century not divisible by 
 4. The last year of the 17th, 18th, and 19th centuries are 
 not leap-years, because 4 will not divide these numbers with- 
 out a remainder ; but the last year of the 20th century, or 
 year 2000, will be a leap-year. This pope's correction of the 
 calendar is so near the truth, that it will not err a day in 3000 
 years ; it is called the Gregorian, or the New Style, and ob- 
 tains in almost all Christian nations. It was not used in 
 England before 1752, when it was adopted pursuant to act of 
 parliament, and the third of September was reckoned the 
 fourteenth, the error of the Julian calendar having at that 
 time increased to 11 days. 
 
 The lunar year is the space of 12 lunar months, and is 
 either astronomical or civil. 
 
 The lunar astronomical year consists of twelve lunar synodi- 
 
622 ASTRONOMY. 
 
 Division of time — The jear. — Cycles and indiclion. 
 
 cal months; and is, therefore, 354 days, 8 hours, 48', 38', 
 bein^ 10 days, 21 hours, 11' shorter than the solar year. 
 
 The lunar civil i/ear is either common or embolimic. The 
 common lunar year consists of 12 lunar civil months, and 
 contains 354 days. The embolimic lunar year consists of 13 
 lunar civil months, and contains 384 days. 
 
 The ancient "Roman year, as first settled by the Romans, 
 contained only 10 months, and in all 304 days. 
 
 The Egyptian year, called also the year of Nabonassar, con- 
 tains only 365 days, divided into 12 months of 30 days each, 
 %vith five intercalary days added at the end. Thus the Egyp- 
 tian year loses a whole day of the Julian year every four years, 
 and after the space of 1460 years, it begins with the Julian 
 year, which lencrth of time is called the Lofhic period. 
 
 The ancient Greek year consisted of 12 months, which at 
 first were divided into 30 days each; but afterwards each 
 month contained 29 and 30 days alternately ; and this year 
 was computed from the first appearance of the new moon, 
 with the addition of an embolimic month of 30 days, every 3d, 
 5th, 8th, Uth, 14th, 16th, and 19th year, in order to keep the 
 new and full moons to the same seasons of the year. 
 
 Xot only the length of the year, but the time of its com- 
 mencement, has been differently reckoned among different 
 nations. The Chaldeans and Egyptians commenced their 
 year at the autumnal equinox. The Jews reckoned their civil 
 year from the same period, but began their ecclesiastical year 
 in the spring. Some of Grecian states commenced their year 
 at the vernal equinox, others at the autumnal equi-nox, and 
 some at the summer solstice. Tiie Roman year at one time 
 began in March, but afterwards was made to commence in 
 January. The new year's day of the church of Rome is fixed 
 en the Sunday nearest the full moon of the vernal equinox. 
 In England, the year began in March, till 1752, when the 
 style was altered, and the year was at the same time settled 
 to commence on the first of January. The period between 
 the 1st of January and the 25th of March, was for some time 
 subsequently to the alteration of the style, expressed thus : 
 1735-6, or 173|, forms of writing which have now become 
 obscure. 
 
 Of the Lunar and Solar Cycles and Indiction. 
 
 A cycle is a perpetual circulation of a certain fixed and 
 determinate space of time. 
 
 The cycle of the sun consists of 28 years ; at the end of 
 which time, the days of the months return again to the same 
 
ASTRONOMY. 623 
 
 Division of tisie. — Cycles. — Epochs and eras. 
 
 days of the week, and the sun is in the same sign and degree 
 of the ecliptic, which he was in at its commencement; at 
 least, this is the case within a degree for 100 years. The leap- 
 years, also, at the expiration of the solar cycle, begin the 
 same course over again with respect to the days of the week 
 on which the days of the months fall. 
 
 The cycle of the moon, the year of which is called the golden 
 number, is a period of 19 years, at the end of which time, the 
 sun and moon return to the same situation in the heavens, or 
 very nearly so : the conjunctions, oppositions, &c. of these 
 two luminaries being, within about an hour and a half, the 
 same that they were on the same days of the months at the 
 commencement of the period. 
 
 The cycle of indiction, or Roman indiction, has no reference 
 to any natural period. It consisted of 15 years, and was in 
 use only among the Romans, for some civil purposes which 
 cannot now be completely ascertained. 
 
 The year of our Saviour's birth, according to the usual 
 computation, was the ninth year of the solar cycle; the first 
 year of the lunar cycle ; and the 312th year after his birth 
 was the first year of the Roman indiction. With this data, 
 the year of any subsequent cycle or indiction may be found. 
 
 Of Epochs and Eras. 
 
 Any remarkable event or period from which time is reckon- 
 ed, is called an epoch; the years computed from an epoch, or 
 from such event or period, are called an era, whatever be their 
 number. 
 
 The earliest and most memorable epoch connected with 
 mundane affairs, is that of the creation of the world ; but 
 the era referring to it is only continued to the birth of Christ. 
 According to Archbishop Usher, whose chronology is adopt- 
 ed in the current translation of the Bible, it began in the 
 year 4004 before Christ. Playfair places it in 4007. The 
 next epoch was the Deluge, and like the former it was used 
 by the Jews. 
 
 The epoch adopted by all Christians is that of the birth of 
 Christ; the common estimation of which, making the present 
 year of the Christian era 1814, is supposed to be 4 years too 
 late. The reason of this uncertainty is, that the Christian era 
 was not used till the sixth century after the birth of Christ, 
 when it was too late to fix its commencement with indisput- 
 able accuracy. But as all the nations who use it, begin at the 
 same time, and the year 1814, for example, is with all of them 
 the year 1814, the error introduces no confusion. 
 
G24 ASTRONOMY. 
 
 Division of time. — Olvmpiads. — Julian period. — Abstract of astroDomy. 
 
 The mode of reckoning by olyvipiads, or periods of four 
 years, was used by the Greeks. The first olympiad began 775 
 years before the birth of Christ. 
 
 The Romans reckoned from the building of Rome ; the date 
 of that event is, however, not well ascertained, from the want 
 of authentic records in those early times ; but the computa- 
 tion adopted by the Romans themselves, places it in the year 
 753 before Christ. 
 
 The Hegira, or Mahometan era, takes date from the flight 
 of ^Mahomet to Medina, 622 years after the commencement 
 of the Christian era. 
 
 To obtain a common standard in the comparison of dates 
 and eras, Joseph Scaliger adopted a very ingenious method : 
 By multiplying into each other, the cycle of the sun, which 
 is 28 years ; the cycle of the moon, which is 19 years ; and 
 cycle of indiction, which is 15 years, he obtained the number 
 7980, which is called a Julian period. This period is supposed 
 to commence 706 years antecedently to the creation, and till 
 the expiration of it, or till the world is 727 1 ypars old. the 
 first years of each of these cycles will not come together. — 
 The year of the Julian period corresponding with any given 
 vear before or since the commencement of the Christian era 
 may easily be found. If the year required be since the Chris- 
 tian era, add to it 4713, the number of years elapsed before 
 the Christian era, and the sum will be the year required. 
 If the year required precede the Christian era, subtract the 
 years before Christ from 4713, and the difference will be the 
 answer. 
 
 Abstract of Astronomy 
 
 1. The solar system, comprises the sun and all the bodies 
 that revolve round him, viz. the comets, the planets with their 
 respective satellites, and the asteroids. 
 
 2. The number of the comets is unknown ; that of the pla- 
 nets, so far as yet discovered, is seven, the satellites eighteen ; 
 and the asteroids four. 
 
 3. The figure of the earth is not that of a perfect globe, but 
 an oblate spheroid, flattened a little at the poles, by its re- 
 volution on its axis. 
 
 4. The planets Jupiter and Saturn are also observed to be 
 flattened at the poles like the earth, but in a greater degree, 
 evidently because their diurnal revolution is swifter. 
 
ASTRONOMY. 625 
 
 Abstract of astrouomy. 
 
 5. The orbits of all the planets, asteroids, and comets, are 
 ellipses, having the sun in one of their foci ; but the orbits of 
 the two former classes of bodies are nearly circular, while the 
 orbits of the comets are all very eccentric. 
 
 6. The orbits of the satellites are also ellipses in one of the 
 foci of which is situated the primary planet round which they 
 move. 
 
 7. The periods, distances, and magnitudes of the planets, 
 have all been determined with very considerable exactness ; 
 the same circumstances respecting the asteroids, are also evi- 
 dently determinable, though the results yet laid down, have 
 not, from the recent date of their discovery, been so amply 
 confirmed, as to be fully relied on ; but the comets recede to 
 such immense distances, and there is so much uncertainty in 
 identifying them, that their elements are hypothetical. 
 
 8. The planets, comets, and asteroids, are preserved in their 
 orbits, by the joint effects of the power of attraction, which 
 acts in a right line from them to the sun, and a projectile or 
 centrifugal force, which would carry them off ia a tangeat to 
 the curve of revolution. 
 
 9. The powers which preserve the satellites in their orbits, 
 are the same as those that act upon the planets and comets, 
 but the centripetal force is exercised by the primary. 
 
 10. The body of the sun is supposed to be opaque, and to be 
 surrounded with a double set of clouds, the upper stratum of 
 which forms the luminous globe we behold. 
 
 11. The panets revolve round an imaginary line or axis 
 within themselves, and the time in which they perform this 
 rotation, constitutes their day and night. 
 
 12. The time in which a planet revolves round the sun, 
 forms its year. 
 
 13. The diversity of seasons is occasioned by the inclina- 
 tion of the axis of a planet to the plane of its orbit. 
 
 14. The annual and diurnal revolutions of the planets are 
 all performed from west to east. 
 
 15. The satellites also revolve from west to east, with the 
 exception of the satellites of the Herschel, which appear to 
 move in a contrary direction. 
 
 16. The fixed stars are distinguished from the bodies of the 
 solar system, by the twinkling light they afford, by their having 
 no parallax, and by their having, even through the best tele-^ 
 scopes, no sensible magnitude. 
 
 17. The naked eye cannot behold above five hundred stars 
 in the whole hemisphere ; but the number discovered with the 
 assistance of a telescope exceeds all calculation. 
 
 18. Every fixed star is supposed to be a sun, shining by its 
 27.— Vol. I. 4 L 
 
626 ASmONOMY. 
 
 Abstract of astronomj. 
 
 own light, and surrounded by planetary worlds like those of 
 the solar system. 
 
 19. The tides are an effect of the attraction of the sun and 
 moon upon the ocean. When these luminaries act together, 
 or in the same line, they occasion spring tides ; when they 
 counteract each other's attraction, neap tides take place. 
 
 20. Eclipses of the moon are owing to the shadow of the 
 earth falling upon the moon. 
 
 21. Eclipses of the sun occur, when the moon, coming be- 
 tween the earth and sun, throws a shadow on the earth. 
 
 22. Motion is the measure of time, and the motions of the 
 heavenly bodies are the basis by which all other motions are 
 measured. 
 
 23. The day is a natural division of time, that is, it com- 
 prises a portion of time measured out by the completion of 
 certain phenomena, successive according to regular laws. 
 
 24. The periodical and synodical lunar months aro also na- 
 tural divisions of time, but no other ; the year, and lunar and 
 solar cycles, are of the same character as the lunar months ; 
 the cycle of indiction, and the olympiad, are examples of the 
 artificial division of time. 
 
 END OF VOLUME FIRST. 
 
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