REESE LIBRARY' 
 
 <JF THK 
 
 UNIVERSITY OF CALIFORNIA. 
 
 Received-- 
 
 -.~ 
 
 Accessions No..... Shelf No. 
 

A SYSTEM 
 
 OF 
 
 NATURAL PHILOSOPHY: 
 
 IN WHICH ARE EXPLAINED THE 
 
 PRINCIPLES OF MECHANICS, 
 
 HYDROSTATICS, HYDRAULICS, PNEUMATICS, ACOUSTICS, OPTICS, 
 
 ASTRONOMY, ELECTRICITY, MAGNETISM, STEAM-ENGINE, 
 
 ELECTRO-MAGNETISM, ELECTROTYPE, PHOTOGRAPHY, 
 
 AND DAGUERREOTYPE; 
 
 \ 
 
 TO WHICH ARE ADDED 
 
 QUESTIONS FORTHE EXAMINATION OF PUPILS, 
 
 DESIGNED FOR 
 
 THE USE OF SCHOOLS AND ACADEMIE& 
 
 ILLUSTRATKD BY THREE HUNDRED EN GRAVING!. 
 
 ~BY^L L. COMSTOCK;*M. D. 
 
 OT INTRODUCTION TO MINERALOGY, ELEMENTS OP CHEMISTRY, IXTRODCCTTOIf TO BO 
 OF GEOLOGY, OUTLINES Of PHYSIOLOGY, R'ATTTRAL HISTORY OF BIRDS, CTC. 
 
 ONE BUVr-^-n AND SIXTIETH EDITION. 
 REVISED AND ENLARGED. 
 
 PRATT, WOODFORD, AND CO. 
 1853. 
 
ENTERED, according to Act of Congress, in the year of our Lord 
 thousand eight hundred and fifty -two, 
 
 By J. L. COMSTOCK, 
 In the Clerk's Office of the District Court of Connecticut. 
 
 STEREOTYPED BY PRINTED BY 
 
 RICHARD H. HOBBS, CASE, TIFFANY, AND CO., 
 
 HARTFORD, CONN. HARTFORD, CONN. 
 
NOTICE BY THE PUBLISHERS 
 
 THE publishers of Dr. Comstock's Natural Philosophy will not 
 withhold from the public an expression of the gratification they 
 feel as Americans, at the manner in which the work has been re- 
 ceived and appreciated in Europe. 
 
 It has been twice edited and republished in the Queen's do- 
 minions. First in Scotland, the editor being Prof. Lees "of the 
 Naval and Military Academy, and Lecturer on Natural Philoso- 
 phy, Edinburgh." 
 
 In his Preface the editor says: "Among the many works on 
 Natural Philosophy which have made their appearance of late 
 years, we certainly have not met with one uniting in a greater 
 degree the two grand requisites of precision and simplicity than 
 Jie work of Dr. Comstock. * * * * The principle* of the 
 science are stated with singular clearness, and illustrated by the 
 most apt, and interesting examples. * * * * The develop- 
 ment of the various branches is effected by the help of well-de- 
 signed diagrams, and these by no means sparingly introduced. 
 Published by Scott, Webster, $ Geary, London. 
 
 During the last year the Philosophy was again edited by Prof 
 Hoblyn of Oxford, now " Lecturer in the Institute of Medicine 
 and Arts," London ; and author of a Medical Dictionary repub~ 
 lished in this country. This edition is dedicated to Marshall Hall, 
 M. D., F. R. S., one of the chiefs of the Medical profession in 
 the metropolis, and who, it appears, has introduced it to his pupils 
 in the lecture room. 
 
 The following is an extract from Prof. Hoblyn's Preface to his 
 edition : 
 
 " This Manual of Natural Philosophy claims no higher merit 
 than that of being a republication of the popular treatise of Dr. 
 Comstock of Hartford, in the United States, enlarged and to a 
 certain extent remodeled. His colleague feels a peculiar pleasure 
 
IV NOTICE BY THE PUBLISHERS. 
 
 in the association of his own name with that of an author, who 
 has earned a well-merited reputation in the pursuit of physical 
 science. As an elementary work, requiring for its perusal no 
 mathematical attainment, nor indeed any previous knowledge of 
 Natural Philosophy, it is at once simple, intelligible, and in most 
 Darts familiar." Published by Adam Scott, Charter House 
 Square, London. 
 
 Besides these two editions of the entire work, Dr. Comstock's 
 Philosophy has been published in parts, in the form of scientific 
 tracts, at a shilling each, for general circulation in England. We 
 understand also, that the work has been translated into German, 
 for the use of the public schools in Prussia. 
 
 Having thus undergone the critical examination of two Profes- 
 sors of high attainments abroad, who have each corrected its 
 errors, and added to its pages, and of whose labors, we have no 
 doubt the author has availed himself, we now offer this revised 
 edition to the public, with renewed confidence in its correctness, 
 as well as its adaptation to the purpose for which the work is 
 intended. 
 
 NEW YORK. 
 
PREFACE 
 
 TO THE FIFTH STEREOTYPE REVISION. 
 
 THE author has me satisfaction of being again called upon by 
 his well-known, and enterprising publishers to superintend the 
 proofs of a new set of stereotype plates for this work, being the 
 fifth which its circulation has required in this country. 
 
 In this improved edition, although large portions of the contents 
 have been re- written, and much new matter added, the author 
 has been careful not to make such changes as to render it unfit to 
 be used in classes with the last editiorb- 
 
 In attempting to make this copy more complete and useful 
 than the former ones, the author has availed himself of the addi- 
 tions and corrections, made by Professor Lees of " the Naval and 
 Military Academy of Edinburgh," and of those of Professor Hob- 
 lyn, Lecturer in " The Institute of Medicine and Arts," London, 
 these two gentleman having done him the honor to associate 
 their names with that of the author, in two several editions of 
 this work. 
 
 In the Chapters on the Steam Engine, and Astronomy, it was 
 found that many paragraphs might be omitted with advantage 
 to the essential parts of the subjects. These erasures have 
 been replaced by a description of the Railroad Locomotive, and 
 by the insertion of Tables containing the names of the new 
 Asteroids, and of those of their discoverers, with the date of each 
 discovery. 
 
 Among the additions will be found descriptions and illustrations 
 cf McCormick's Reaper ; Sharps' Rifle ; Printing Presses , 
 House's Printing Telegraph; Manufacture of Percussion Caps; 
 The Organ ; Monochord ; Hygrometer ; Harmonicon ; Air-Gun ; 
 Dipping Needle, and much new matter on Electro-Magnetism. 
 
 The questions, in this edition, are numbered, to correspond 
 
Vi PREFACE. 
 
 with those of the paragraphs ; and nearly all the old cuts have 
 been re-engraved, and about fifty new ones added. 
 
 And, finally, perhaps the author will be excused for adding, 
 that besides the circulation of this work in England, Scotland, 
 and Prussia, there have been printed, and sold more than half 
 a million copies in this country. 
 
 J. L. 0. 
 
 HARTFORD, April, 1852, 
 
or THE 
 
 'UNIVERSITY 
 
 %JF( 
 
 NATURAL PHILOSOPHY, &c. 
 
 CHAPTER I. 
 
 THE PROPERTIES OF BODIES 
 
 NATURAL PHILOSOPHY, or the Science of Nature, has for its 
 objects the investigation of the properties of all natural bodies 
 and their mutual action on each other. The term Physics has 
 a similar meaning. * 
 
 1. A BODY is any substance of which we can gain a knowl- 
 edge by our senses. Hence, air, water, and earth, in all their 
 modifications, are called bodies. 
 
 2. There are certain properties which are common to all 
 bodies. These are called the essential properties of bodies. 
 They are Impenetrability, Extension, Figure, Divisibility, In- 
 ertia, and Attraction. 
 
 3. IMPENETRABILITY. By impenetrability, is meant that two 
 bodies can not occupy the same space at the same time, or, that 
 the ultimate particles of matter can not be penetrated. Thus, 
 if a vessel be exactly filled with water, and a stone or any other 
 substance heavier than water, be dropped into it, a quantity of 
 water will overflow, just equal to the size of the heavy body. 
 This shows that the stone only separates or displaces the parti- 
 cles of water, and therefore that the two substances can not ex- 
 ist in the same place at the same time. If a glass tube open 
 at the bottom, and closed with the thumb at the top, be pressed 
 down into a vessel of water, the liquid will not rise up and fill 
 the tube, because the air already in the tube resists it ; but if 
 the thumb be removed, so that the air can pass out, the water 
 will instantly rise as high on the inside of the tube as it is on 
 
 What are the objects of natural philosophy 7 1. What is a body? 2. Mention 
 several bodies. What are the essential properties of bodies 1 3. What is meant by 
 impenetrability ? How is it proved that air and water are impenetrable 7 
 
8 PROPERTIES OF BODIES. 
 
 the outside. This shows that the air is impenetrable to the 
 water. 
 
 4. If a nail be driven into a board, in common language, it 
 is said to penetrate the wood, but in the language of philoso- 
 phy, it only separates, or displaces the particles of the wood. 
 The same is the case, if the nail be driven into a piece of lead ; 
 the particles of the lead are separated from each other, and 
 crowded together, to make room for the harder body, but the 
 particles themselves are by no means penetrated by the nail. 
 
 5. When a piece of gold is dissolved in an, acid, the parti- 
 cles of the metal are divided, or separated from each other, and 
 diffused in the fluid, but the particles of gold are supposed still 
 to be entire, for if the acid be removed, we obtain the gold 
 again in its solid form, just as though its particles had never 
 been separated. 
 
 6. EXTENSION. Every body, however small, must have length, 
 breadth, and thickness, since no substance can exist without them. 
 By extension, therefore, is only meant these qualities. Exten- 
 sion has no respect to the size, or shape ofia body. 
 
 7. The size and shape of a block of wood a foot square is 
 quite different from that of a walking-stick. But they both 
 equally possess length, breadth, and thickness, since the stick 
 might be cut into little blocks, exactly resembling in shape the 
 large one. And these little cubes might again be divided until 
 they were only the hundredth part of an inch in diameter, and 
 still it is obvious that they would possess length, breadth, and 
 thickness, for they could yet be seen, felt, and measured. " But 
 suppose each of these little blocks to be again divided a thou- 
 sand times, it is true we could not measure them, but still they 
 would possess the quality of extension, as really as they did be- 
 fore division, the only difference being in respect to dimensions. 
 
 8. FIGURE OR FORM is the result of extension, for we can not 
 conceive that a body has length and breadth, without its also 
 having some kind of figure, however irregular. 
 
 9. Some solid bodies have certain or determinate forms 
 which are produced by nature, and are always the same, 
 wherever they are found. Thus, a crystal of quartz has six 
 sides, while a garnet has twelve sides, these numbers being in- 
 variable. Some solids are so irregular, that they can not be 
 
 4. When a nail is driven into a board or piece of lead, are the particles of these 
 bodies penetrated or separated 1 5. Are the particles of gold dissolved, or only sep- 
 arated by the acid ? 6. What is meant by extension ? 7. In how many directions 
 do bodies possess extension 1 8. Of what is figure or form the result 1 Do all 
 bodies possess figure 7 9. What solids are regular in their forms 7 
 
PROPERTIES OF BODIES. 9 
 
 compared with any mathematical figure. This is the case 
 with the fragments of a broken rock, chips of wood, fractured 
 glass, &c. ; these are called amorphous. 
 
 10. Fluid bodies have no determinate forms, but take their 
 shapes from the vessels in which they happen to be placed. 
 
 11. DIVISIBILITY. By the divisibility of matter, we mean 
 that a body may be divided into parts, and that these parts may 
 again be divided into other parts. 
 
 12. It is quite obvious, that if -we break a piece of marble 
 into two parts, these two parts may again be divided, and that 
 the process of division may be continued until these parts are 
 so small as not individually to be seen or felt. But as every 
 body, however small, must possess extension and form, so we 
 can conceive of none so minute but that it may again be di- 
 vided. There is, however, possibly a limit, beyond which 
 bodies can not actually be divided, for there may be reason to 
 believe that the atoms of matter are indivisible by any means 
 in our power. But under what circumstances this takes place, 
 or whether it is in the power of man during his whole life, to 
 pulverize any substance so finely, that it may not again be 
 broken, is unknown. 
 
 13. We can conceive, in some degree, how minute must be 
 the particles of matter, from circumstances that every day come 
 within our knowledge. 
 
 14. A single grain of musk will scent a room for years, and 
 still lose no appreciable part of its weight. Here, the particles 
 of musk must be floating in the air of every part of the room, 
 otherwise they could not be every where perceived. 
 
 15. Gold is hammered so thin, as to take 282,000 leaves to 
 make an inch in thickness. Here, the particles still adhere 
 to each other, notwithstanding the great surface which they 
 cover, a single grain being sufficient to extend over a surface 
 of fifty square inches. 
 
 16. INDESTRUCTIBILITY. This term means that nothing is 
 destroyed. The ultimate particles of matter, however widely 
 they may be diffused, are not individually destroyed, or lost, 
 but under certain circumstances, may again be collected into a 
 body without change of form. Mercury, water, and many 
 other substances, may be converted into vapor, or distilled in 
 
 9. What bodies are irregular? 11. What is meant by divisibility of matter? Is 
 there any limit to the divisibility of matter ? 12. Are the atoms of matter divisible ? 
 14. What examples are given of the divisibility of matter ? 15. How many leaves of 
 gold does it take to make an inch in thickness 1 How many square inches may a 
 grain of gold be made to cover ? 16. Under what circumstances may the particles 
 of matter again be collected in their original form ? 
 
 1* 
 
10 PROPERTIES OF BODIES. 
 
 close vessels, without any of their particles being lost. In such 
 cases, there is no decomposition of the substances, but only a 
 change of form by the heat, and hence the mercury and water 
 assume their original state again on cooling. 
 
 17. Where bodies suffer decomposition or decay, their ele- 
 mentary particles, in like manner, are neither destroyed nor lost, 
 but only enter into new arrangements or combinations with 
 other bodies. 
 
 18. When a piece of wood is heated in a close vessel, such 
 as a retort, we obtain water, an acid, several kinds of gas, and 
 there remains a black, porous substance, called charcoal. The 
 wood is thus decomposed, or destroyed, and its particles take a 
 new arrangement, and assume new forms, but that nothing is 
 lost, is proved by the fact tha^ if the water, acid, gases, and 
 charcoal, be collected and weighed, they will be found exactly 
 as heavy as the wood was before distillation. 
 
 19. Bones, flesh, or any other animal substance, may in the 
 sawie manner be made to assume new forms, without losing a 
 particle of the matter which they originally contained. 
 
 20. The decay of animal or vegetable bodies in the open air, 
 or in the ground, is only a process by which the particles of 
 which they were composed, change their places and assume 
 new forms. 
 
 21. The decay and decomposition of animals and vegetables 
 on the surface of the earth form the soil, which nourishes the 
 growth of vegetables ; and these, in their turn, form the nu- 
 triment of animals. Thus is there a perpetual change from 
 life to death, and from death to life, and as constant a succes- 
 sion in the forms and places, which the particles of matter as- 
 sume. Nothing is lost, and not a particle of matter is struck 
 out of existence. The same matter of which every living ani- 
 mal, and every vegetable was formed since the creation, is still 
 in existence. As nothing is lost or annihilated, so it is proba- 
 ble that nothing has been added, and that we, ourselves, are 
 composed of particles of matter as old as the creation. In time, 
 we must, in our turn, suffer decomposition, as all forms have 
 done before us, and thus resign the matter of which we are 
 composed, to form new existences. 
 
 22. INERTIA. Inertia means pa-ssivcness or want of power 
 
 17. What is meant by indestructibility ? 18. When bodies suffer decay, are their 
 particles lost? 19. What becomes of the particles of bodies which decay 1 21. Is it 
 probable that any matter has been annihilated or added, since the first creation? 
 What is said of the particles of matter of which we are made? 
 
PROPERTIES OF BODIES. 11 
 
 Thus, matter is, of itself, equally incapable of putting itself in 
 motion, or of bringing itself to rest when in motion. 
 
 23. It is plain that a rock on the surface of the earth never 
 changes its position in respect to other things on the earth. It 
 has of itself no power to move, and would, therefore, forever lie 
 still, unless moved by some external force. This fact is proved 
 by the experience of every person, for we see the same objects 
 lying in the same positions all our lives. Now, it is just as true, 
 that inert matter has no power to bring itself to rest, when once 
 put in motion, as it is that it can not put itself in motion when 
 at rest, for having no life, it is perfectly passive, both to motion 
 and rest ; and therefore either state depends entirely upon cir- 
 cumstances. 
 
 24. Common experience proving that matter does not put 
 itself in motion, we might be led to believe, that rest is the nat- 
 ural state of all inert bodies ; but a few considerations will show 
 that motion is as much the natural state of matter as rest, and 
 that either state depends on the resistance, or impulse, of ex- 
 ternal causes. 
 
 25. If a cannon-ball be rolled upon the ground, it will soon 
 cease to move, because the ground is rough, and presents imped- 
 iments to its motion ; but if it be rolled on the ice, its motion 
 will continue much longer, because there are fewer impediments, 
 and consequently, the same force of impulse will carry it much 
 further. We see from this, that with the same impulse, the 
 distance to which the ball will move must depend on the im- 
 pediments it meets with, or the resistance it has to overcome. 
 But suppose that the ball and ice were both so smooth as to re- 
 move as much as possible the resistance caused by friction, then 
 it is obvious that the ball would continue to move longer, and 
 go to a greater distance. Next, suppose we avoid the friction 
 of the ice, and throw the ball through the air, it would then 
 continue in motion still longer with the same force of projection, 
 because the air alone presents less impediment than the air and 
 ice, and there is now nothing to oppose its constant motion, ex- 
 cept the resistance of the air, and its own weight, or gravity. 
 
 26. If the air be exhausted, or pumped out of a vessel by 
 means of an air-pump, and a common top, with a small, hard 
 point, be set in motion in it, the top will continue to spin for 
 
 22. What does inertia mean 1 24. Is rest or motion the natural state of matter ? 
 25 Why floes the ball roll farther on the ice than on the ground ? What does this 
 prove? Why, with the same force of projection, will a ball move further through 
 the air than on the ice ? 26. Why will atop spin, or a pendulum swing, longer inaa 
 exhausted vessel than in the u.ir ? 
 
12 PROPERTIES OF BODIES. 
 
 hours, because the air does not resist its motion. A pendulum, 
 set in motion, in an exhausted vessel, will continue to swing, 
 without the help of clock-work, for a whole day, because there 
 is nothing to resist its perpetual motion but the small friction at 
 the point where it is suspended, and gravity. 
 
 27. We see, then, that it is the resistance of the air, of fric- 
 tion, and of gravity, which cause bodies once in motion to 
 'cease moving, or come to rest, and that dead matter, of itself, 
 is equally incapable of causing its own motion, or its own rest. 
 
 28. We have perpetual examples of the truth of this doc- 
 trine, in the moon, and other planets. These vast bodies move 
 through spaces which are void of the obstacles of air and fric- 
 tion, and their motions are the same that they were thousands 
 of years ago, or at the beginning of creation. 
 
 29. ATTRACTION. By attraction is meant that property or 
 quality in the particles of bodies, which makes them tend toward 
 each other. 
 
 30. We know that substances are composed of small atoms 
 or particles of matter, and that it is a collection of these, united 
 together, that form all the objects with which we are ac- 
 quainted. Now, when we come to divide, or separate any sub- 
 stance into parts, we do not find that its particles have been 
 united or kept together by glue, little nails, or any such me- 
 chanical means, but that they cling together by some power, 
 not obvious to our senses. This power we call Attraction, but 
 of its nature or cause, we are entirely ignorant. Experiment 
 and observation, however, demonstrate that this power pervades 
 all material things, and that under different modifications, it 
 not only makes the particles of bodies adhere to each other, but 
 is the cause which keeps the planets in their orbits as they pass 
 through the heavens. 
 
 3 1 . Attraction has received different names, according to the 
 circumstances under which it acts. 
 
 32. The force which keeps the particles of matter together, 
 to form bodies, or masses, is called Attraction of cohesion. 
 That which inclines different masses toward each other, is 
 called Attraction of gravitation. That which causes liquids to 
 rise in tubes, is called Capillary attraction. That which forces 
 
 27. What are the causes which resist the perpetual motion of bodies ? v 28. Where 
 have we an example of continued motion without the existence of air and friction 1 
 29. What is meant by attraction 1 30. What is known about the cause of attraction ? 
 Is attraction common to all kinds of matter, or not? What effect does this power 
 have upon the planets'? 31. Why has attraction received different names? 32. How 
 many kinds of attraction are there 1 How does the attraction of cohesion operate? 
 What is meant by attraction of gravitation ? What by capillary attraction ? 
 
PROPERTIES OF BODIES. 13 
 
 the particles of substances of different kinds to unite, is known 
 under the name of Chemical attraction. That which causes 
 the needle to point constantly toward the poles of the earth, is 
 Magnetic attraction ; and that which is excited by friction in 
 certain substances, is known by the name of Electrical at- 
 traction. 
 
 33. The following illustrations, it is hoped, will make each 
 kind of attraction distinct and obvious to the mind of the student. 
 
 34. ATTRACTION OF COHESION acts only at insensible distances, 
 as when the particles of bodies apparently touch each other. 
 
 35. Take two 
 
 pieces of lead, Fig. FIG - L 
 
 1, of a round form, 
 an inch in diame- 
 ter and two inches 
 long ; flatten one 
 end of each and Cohesion. 
 
 make 1 through it 
 
 an eye-hole for a string. Make the other ends of each as 
 smooth as possible, by 'cutting them with a sharp knife. If now 
 the smooth surfaces be brought together, with a slight turning 
 pressure, they will adhere with such force that two men can 
 hardly pull them apart by the two strings. 
 
 36. In like manner, two pieces of plate glass, when their 
 surfaces are cleaned from dust, and they are pressed together, 
 will adhere with considerable force. Other smooth substances 
 present the same phenomena. 
 
 37. This kind of attraction is much stronger in some bodies 
 than in others. Thus, it is stronger in the metals than in most 
 other substances, and in some of the metals it is stronger than 
 in others. In general it is most powerful among the particles 
 of solid bodies, weaker among those of liquids, and probably 
 entirely wanting among elastic fluids, such as air and the gases. 
 
 38. Thus, a small iron wire will hold a suspended weight 
 of many pounds, without having its particles separated ; the 
 particles of water are divided by a very small force, while those 
 of air are still more easily moved among each other. These 
 different properties depend on the force of cohesion with which 
 the several particles of these bodies are united. 
 
 32. What by chemical attraction ? What is that which makes the needle point 
 toward the pole ? How is electrical attraction excited ? 53. Give an example of cohe- 
 sive attraction 1 37. In what substances is cohesive attraction the strongest? la 
 what substances is it the weakest } 38. Why are the particles <> f "aids more easily 
 separated than those of solids 1 
 
14 PROPERTIES OF BODIES. 
 
 39. When the particles of fluids are left to arrange them- 
 selves according to the laws of attraction, the bodies which they 
 compose assume the form of a globe or ball. 
 
 40. Drops of water thrown on an oiled surface, or on wax, 
 globules of mercury, hailstones, a drop of water adhering to 
 the end of the finger, tears running down the cheeks, and dew- 
 drops on the leaves of plants, are all examples of this law of 
 attraction. The manufacture of shot is also a striking illustra- 
 tion. The lead is melted and poured into a sieve, at the height 
 of about two hundred feet from the ground. The stream of 
 lead, immediately after leaving the sieve, separates into round 
 globules, which, before they reach the ground, are cooled and 
 Income solid, and thus are formed the shot used by sports- 
 men. 
 
 41. To account for the globular form in all these cases, we 
 have only to consider that the particles of matter are mutually 
 attracted toward a common center, and in liquids being free to 
 move, they arrange themselves accordingly. 
 
 42. In all figures except the globe or ball, some of the parti- 
 cles must be nearer the center than others. But in a body that 
 is perfectly round, every part of the outside is exactly at the 
 same distance from the center. 
 
 43. Thus, the corners of a cube, or 
 
 square, are at much greater distances FIG. 2. 
 
 from the center than the sides, while 
 the circumference of a circle or ball is 
 every where at the same distance from 
 it. This difference is shown by Fig. 2, 
 and it is quite obvious, that if the parti- 
 cles of matter are equally attracted to- 
 ward the common center, and are free 
 to arrange themselves, no other figure 
 could possibly be formed, since then Globular form. 
 
 every part of the outside is equally at- 
 tracted. 
 
 44. The sun, earth, moon, and indeed all the heaven y bodies, 
 are illustrations of this law, and therefore were probably in so 
 soft a state when first formed, as to allow their particles freely 
 to arrange themselves accordingly. 
 
 39. What form do fluids take, when their particles are left to their own arrange- 
 ment ? 40. Give examples of this law. 41. How is the globular form which liquids 
 assume accounted for ? If the particles of a body are free to move, and are equal- 
 ly attracted toward the center, what must be its figure ? 43. Why must the figure 
 be a globe 7 44. What great natural bodies are examples of this law 1 
 
PROPERTIES OF BODIES. 
 
 15 
 
 FIG. 3. 
 
 45. ADHESION. The attraction between solids and liquids 
 is termed adhesion. This is well illustrated by means of Fi(j. 3. 
 
 First,very nice- 
 ly balance the 
 plate of copper, 
 C,~by means of 
 weights in the 
 cup,A, and then 
 slide the vessel 
 of water, B, un- 
 der the copper, 
 pouring in more 
 of the fluid un- 
 
 Adhesion between solids and liquids. 
 
 FIG. 4. 
 
 til the metal just 
 touches it. Now 
 
 on placing weights in A, it will be found that the metal ad- 
 heres to the water with so much force, that -if the plate has an 
 area of about seven inches, it will require a weight of more than 
 1000 grains to raise it from the surface of the water. 
 
 46. ATTRACTION OF GRAVITATION. As the attraction of 
 cohesion unites the particles of matter into masses or bodies, so 
 the attraction of gravitation tends to force these masses toward 
 each other , to form those of still greater dimen- 
 sions. The term gravitation, does not here 
 
 strictly refer to the weight of bodies, but to 
 the attraction of the masses of matter toward 
 each other, whether downward, upward, or 
 horizontally. 
 
 47. The attraction of gravitation is mutual, 
 since all bodies not only attract other bodies, 
 but are themselves attracted. 
 
 48. Two cannon-balls, when suspended by 
 long cords, so as to hang "quite near each other, 
 are found to exert a mutual attraction, so that 
 neither of the cords are exactly perpendicular, 
 but they approach each other as in Fig. 4. 
 
 49. In the same manner, the heavenly bodies, 
 when they approach each other, are drawn out 
 of the line of their paths, or orbits, by mutual 
 
 attraction. Attraction. 
 
 45. What explanation can yon give of Fig. 3? 46. What is meant by attraction of 
 gravitation? 47. Can one body attract another without being itself attracted? 
 43. How is it proved that bodies attract each other 1 
 
16 PROPERTIES OF BODIES. 
 
 50. The force of attraction increases in proportion as bodies 
 approach each other, and by the same law it must diminish in 
 proportion as they recede from each other.. 
 
 51. Attraction, in technical language, is inversely as the 
 squares of the distances between the two bodies. That is, in 
 proportion as the square of the distance increases, in the same 
 proportion attraction decreases, and so the contrary. Thus, 
 if at the distance of 2 feet, the attraction be equal to 4 pounds, 
 at the distance of 4 feet, it will be only 1 pound; for the 
 square of 2 is 4, and the square of 4 is 16, which is 4 times 
 the square of 2. On the contrary, if the attraction at the dis- 
 tance of 6 feet be 3 pounds, at the distance of 2 feet it will be 
 9 times as much, or 27 pounds, because 36, the square of 6, 
 is equal to 9 times 4, the square of 2. 
 
 52. The law 
 
 of attraction in FIG. 5. 
 
 masses is very sat- 
 isfactorily shown 
 by the two little 
 cork balls in Fig. 
 5. They are cover- 
 ed with Varnish, Attraction of cork balls. 
 
 or beeswax, to re- 
 pel the water. 
 
 Two such balls placed on the surface of a dish of water, two 
 or three inches apart, and not near the side of the dish, will 
 soon begin to approach each other; their velocities being 
 in proportion to their sizes, and increasing as their distances di- 
 minish, until quite near, when they rush together as though 
 they had life. 
 
 53. The intensity of light is found to increase and diminish 
 in the same proportion. Thus, if a board a foot square, be 
 placed at the distance of one foot* from a candle, it will be 
 found to hide the light from another board of two feet square, 
 at the distance of two feet from the candle. Now a board of 
 two feet square is just four times as large as one of one foot 
 square, and therefore the light at double the distance being 
 spread over four times the surface, has only one fourth the in- 
 tensity. 
 
 50. By what law or rule, does the force of attraction increase ? 5t. Give an exam, 
 pie of this rule. 52. How is attraction illustrated by Fig 5 ? 53. How is it shown 
 that the intensity of light increases and diminishes in the same proportion as the at 
 traction of matter 7 
 
PROPERTIES OF BODIES! 17 
 
 54. The force of the attraction of gravitation, is in proportion 
 to the quantity of matter the attracting body contains. 
 
 55. Some bodies of the same bulk contain a much greater 
 quantity of matter than others : thus, a piece of lead contains 
 about twelve times as much matter as a piece of cork of the 
 same dimensions, and therefore a piece of lead of any given 
 size, and a piece of cork twelve times as large, will attract each 
 other equally. 
 
 56. CAPILLARY ATTRACTION. The force by which small 
 tubes, or porous substances, raise liquids above their levels, is 
 called capillary attraction. 
 
 5*7. If a small glass tube be placed in water, the water on 
 the inside will be raised above the level of that on the outside 
 of the tube. The cause of this seems to be nothing more than 
 the ordinary attraction of the particles of matter for each other. 
 The sides of a small orifice are so near each other as to attract 
 the particles of the fluid on their opposite sides, and as all at- 
 traction is strongest in the direction of the greatest quantity of 
 matter, the water is raised upward, or in the direction of the 
 length of the tube. On the outside of the tube, the opposite 
 surfaces, it is obvious, can not act on the same column of water, 
 and therefore~the influence of attraction is here hardly percep- 
 tible in raising the fluid. This seems to be the reason why the 
 fluid rises higher on the inside than on the outside of the tube. 
 
 58. Height and size of the bore. The 
 
 height to which the fluid will rise, PJQ 6 
 
 seems to depend, not on the specific 
 gravity of the fluid, but on the size of 
 the bore. 
 
 59. Thus, if the four glass tubes, 
 shown by Fig. 6, are respectively the 
 
 10th, 20th, 40th, and 80th of an" inch 
 
 in diameter, then the height of the 
 
 a ., . T_ -11 i ,T_ CapiSary attraction. 
 
 fluid in each will be inversely as their 
 several diameters. 
 
 60. On comparing the elevation of several fluids in tubes of 
 the same diameter, it has been found that water rose more than 
 three times as high as sulphuric acid, though the latter is nearly 
 
 54. Do bodies attract in proportion to bulk, or quantity of matter ? 55. What 
 Would be the difference of attraction between a cubic inch of'lead. aud a cubic inch of 
 cork ? Why would there be so much difference 1 56. What is meant by capillary 
 attraction 1 57. How is this kind of attraction illustrated with glass tubes ? Why 
 does the water rise higher in the tube than it does on the outside 1 58. On what / 
 does the height of the fluid in capillary tubes depend ? 59. Explain Fig. 6. 60. What 
 is the difference iu height between sulphuric acid and water 7 
 
18 PROPERTIES OF BODIES. 
 
 twice as heavy as the former, and therefore contains a propor- 
 tionate quantity of attractive matter. The cause of this differ- 
 ence is unknown. 
 
 61. Prevents evaporation. It is very remarkable that capil- 
 lary attraction prevents ' evaporation. Thus, fine glass tubes, 
 open at both ends, and containing water, were exposed to the 
 influence of the sun, in the open air, for months, without losing 
 the least portion of their contents. 
 
 62. It is well known that mercury in a small vertical tube is 
 depressed around the sides next to the glass ; but rises in the 
 center, forming the section of a ball. This is owing to the 
 strong attraction the particles of this metal have for each other, 
 while they appear to have none for the glass. This attraction 
 is beautifully shown by the little bright globules which mercu- 
 ry forms on being, thrown on a smooth surface. 
 
 63. A great variety of porous substances are capable of ca- 
 pillary attraction. If a piece of sponge or a lump of sugar be 
 placed so that its lowest corner touches the water, the fluid;; wll 
 rise up and \?^lhj whole mass. In the same manner, the 
 wick of a lamp \v:fl carry up the oil to supply the flame,-fchough 
 the flame is sevejal inches above the level of the oil. K the 
 end of a towel haaMns to be left in a basin of water, it will 
 empty the basin dp^s contents. And on the same principle, 
 when a dry w#dge of wood is driven into the crevice of a rock, 
 and afterward moistened with water, as when the rain falls 
 upon it, it will absorb the water, swell, and sometimes split the 
 rock. In Germany mill-stone quarries are worked in this 
 manner. 
 
 64. CHEMICAL ATTRACTION takes place between the particles 
 of substances of different kinds, and unites them into one com- 
 pound. 
 
 65. This species of attraction takes place only between the 
 particles of certain substances, and is not, therefore, a universal 
 property. It is also known by the name of chemical affinity, 
 because the particles of substances having an affinity between 
 them, will unite, while those having no affinity for each other 
 do not readily enter into union. 
 
 66. There seems, indeed, in^s respect, to be very singular 
 preferences, and dislikes, existing among the particles of matter. 
 
 61. What is said of its preventing evaporation? 62. Why does mercury form a 
 section of a ball in a glass tube ? 64. What is the effect of chemical attraction ? 
 65. By what other name is this kind of attraction known ? 66. What effect is pro- 
 duced when marble and sulphuric acid are brought together? What is the effect 
 when glass and this acid are brought together 7 What is the reason of this difference ? 
 
PROPERTIES OF BODIES. 19 
 
 Thus, if a piece of marble be thrown into sulphuric acid, their 
 particles will unite with great rapidity and commotion, and there 
 will result a compound differing in all respects from the acid 
 or the marble. But if a piece of glass, quartz, gold, or silver, 
 be thrown into this acid, no change is produced on either, be- 
 cause their particles have no affinity. 
 
 67. Sulphur and quicksilver, when heated together, will form 
 a beautiful red compound, known under the name of vermilion, 
 and which has none of the qualities of sulphur or quicksilver. 
 
 68. Oil and water have no affinity for each other, but pot- 
 ash has an attraction for both, and therefore oil and water will 
 unite when potash is mixed with them. In this manner, the 
 well-known article called soap is formed. But the potash has 
 a stronger attraction for an acid than it has for either the oil or 
 the water ; and therefore, when soap is mixed with an acid, the 
 potash leaves the oil, and unites with the acid, thus destroying 
 the old compound, and at the same instant forming a new one. 
 liic same happens when soap is dissolved in any water con- 
 
 , tainmg an acid, as the waters of the seamaid of certain wells. 
 
 .,-The potash forsakes the oil, and unites with fRe acid, thus leav- 
 ing the oil to rise to the surface of the water. % Such waters are 
 called hard, and will not wash, becauseflfc| acid renders the 
 potash a neutral substance. ^H 
 
 69. MAGNETIC ATTRACTION. There is a certain ore of iron, 
 a piece of which, being suspended by a thread, will always turn 
 one of its sides to the north. This is called the loadstone, or 
 natural magnet, and wj^en it is brought near a piece of iron, or 
 steel, a mutual attraction takes place, and under certain circum- 
 stances the two bodies will come together, and adhere to each 
 other. This is called Magnetic Attraction. When a piece of 
 steel or iron is rubbed with a magnet, the same virtue is com- 
 municated to the steel, and it will attract other pieces of steel, 
 and if suspended by a string, one of its ends will constantly 
 point toward the north, while the other, of course, points 
 toward the south. This is called an artificial magnet. The mag- 
 netic needle is a piece of steel, first touched with the loadstone, 
 and then suspended, so as to turn easily on a point. By means 
 of this instrument, the mariner guides his ship through the 
 pathless ocean. See Magnetism. 
 
 67. When sulphur and quicksilver are combined, what is formed ? 68. How may 
 oil and water be made to unite 7 What is the composition thus formed called J 
 How does au acid destroy this compound 1 What is the reason that hard water will 
 not wash ? 69. What is a natural magnet 1 What is meant by magnetic attraction 7 
 What is an artificial magnet 1 What is a magnetic needle 1 What is its use 1 
 
20 PROPERTIES OF BODIES. 
 
 70. ELECTRICAL ATTRACTION. When a piece of glass, 01 
 sealing-wax, is rubbed with the dry hand, or a piece of cloth, 
 and then held toward any light substance, such as hair or 
 thread, the light body will be attracted by it, and will adhere 
 for a moment to the glass or wax. The influence which thus 
 moves the light body is called Electrical Attraction. When 
 
 . the light body has adhered to the surface of the glass for a mo- 
 ment, it is again thrown off, or repelled, and this is called Elec- 
 trical Repulsion. See Electricity. 
 
 71. We have thus described and illustrated all the universal 
 or inherent properties of bodies, and have also noticed the seve- 
 ral kinds of attraction which are peculiar, namely, Chemical, 
 Magnetic, and Electrical. There are still several properties to 
 be mentioned. Some of them belong to certain kinds of mat- 
 ter in a peculiar degree, while other kinds possess them but 
 slightly, or not at all. These properties are as follows : 
 
 72. DENSITY. This property relates to the compactness of . 
 bodies, or the number of particles which a body contains within 
 a given bulk. It is closeness of texture. 
 
 73. Bodies which are most dense, are those which contain 
 the least number of pores. Hence, the density of the metals is 
 much greater than that of wood. Two bodies being of equal 
 bulk, that which weighs most is most dense. Some of the 
 metals may have this quality increased by hammering, by 
 which their pores are filled up, and their particles are brought 
 nearer to each other. The density of air is increased by forcing 
 more into a close vessel than it naturallv contained. 
 
 74. RARITY. This is the quality opposite to density, and 
 means that the substance to which it is applied is porous and 
 light. Thus, air, water, and ether are rare substances, while 
 gold, lead, and platina are dense bodies. 
 
 75. HARDNESS. This property is not in proportion, as 
 might be expected, to the density of the substance, but to the 
 force with which the particles of a body cohere, or keep their 
 places. 
 
 76. Glass, for instance, will scratch gold or platina, though 
 these metals are much more dense than glass. It is probable, 
 therefore, that these metals contain the greatest number of par- 
 ticles, )>ut that those of the glass are more firmly fixed in their 
 places. 
 
 70. What is meant by electrical attraction 7 What is electrical repulsion 1 71. What 
 properties of bodies have been described 7 72. What is density 7 73. What bodies 
 are most dense 7 How may this quality be increased in metals 7 74. What, is rarity 7 
 What are rare bodies 7 What are dense bodies 7 75. How does hardness differ from 
 density 7 76. Why will glass scratch gold or platiua 7 
 
PROPERTIES OF BOD] 
 
 P.3IT1 
 
 77. Some of the metals can be made hard or soft at pleasure. 
 Thus steel, when heated, and then suddenly cooled, becomes 
 harder than glass ; while, if allowed to cool slowly^t'lsioft, and 
 flexible. 
 
 78. ELASTICITY is that property in bodies by which, after 
 being forcibly compressed, or bent, they regain tfieir original 
 state when the force is removed. 
 
 79. Some substances are highly elastic, while others want 
 this property entirely. The separation of two bodies after im- 
 pact, is a proof that one or both are elastic. In general, most 
 hard and dense bodies possess this quality in greater or less de- 
 gree. Ivory, glass, marble, flint, and ice, are elastic -solids. 
 An ivory ball, dropped upon a marble slab, will bound nearly 
 to the height from which it fell, and no mark will be left on 
 either. India rubber is exceedingly elastic, and, on being thrown 
 forcibly against a hard body, will bound to an amazing distance. 
 Steel, when hardened in a particular manner, and wrought into 
 certain forms, possesses this property in the highest degree. 
 Watch-springs, and those of carriages, as well as sword-blades, 
 are examples. Gold, silver, copper, and platina, also have this 
 property in a degree. 
 
 80. Putty, dough, and wet clay are examples of the entire 
 want of elasticity ; and if either of these be thrown against an 
 impediment, they will be flattened, stick to the place they touch, 
 and never, like elastic bodies, regain their former shapes. 
 
 81. Among fluids, water, oil, and in general all such substances 
 as are denominated liquids, are nearly inelastic, while air, and 
 the gaseous fluids, are the most elastic of all bodies. 
 
 82. Change of Form. The change of 
 
 form in an elastic body, as an India rubber FIQ 7 - 
 
 ball, is shown by Fig. 7, where its side, 
 
 on striking an impediment, is compressed 
 
 to a, but instantly springs to b ; the dark 
 
 line between them being the surface in 
 
 the natural state. 
 
 83. BRITTLENESS is the property which 
 renders substances easily broken, or sepa- 
 rated into irregular fragments. This change h of form, 
 property belongs chiefly to hard bodies. 
 
 84. It does not appear that brittleness is entirely opposed to 
 
 77. What metal can be made hard or soft at pleasure ? 78. What is meant by 
 elasticity? 79. How is it known that bodies possess this property ? Mention seve- 
 ral elastic solids. 80. Give examples of inelastic solids. 81. Do liquids possess this 
 property ? What are the most elastic of all substances? 82. Explain Fig. 7. 83. What 
 is brittleness? 
 
22 PROPERTIES OF BODIES. 
 
 elasticity, since, in many substances, both these properties are 
 united. Glass is the standard, or type of brittleness ; and yet 
 a ball, or fine threads of this substance, are highly elastic, as 
 may be seen by the bounding of the one, and the springing of 
 the other. Brittleness often results from the treatment to 
 which substances are submitted. Iron, steel, brass, and copper, 
 become brittle when heated and suddenly cooled ; but if cooled 
 slowly, they are not easily broken, 
 
 85. MALLEABILITY. Capability of being drawn under the 
 hammer or rolling-press. 
 
 This property belongs to some of the metals, but not to all, 
 and is of vast importance to the arts and conveniences of life. 
 
 86. The malleable metals are platina, gold, silver, iron, cop- 
 per, lead, tin, and some others. Antimony, bismuth, and co- 
 balt, are brittle metals. Brittleness is, therefore, the opposite 
 of malleability. 
 
 87. Gold is the most malleable of all substances. It may 
 be drawn under the hammer so thin that light may be seen 
 through it. Copper and silver are also exceedingly malleable. 
 
 88. DUCTILITY is that property in substances which renders 
 them susceptible of being drawn into wire. 
 
 89. We should expect that the most malleable metals would 
 also be the most ductile ; but experiment proves that this is 
 not the case. Thus, tin and lead may be drawn into thin 
 leaves, but can not be drawn into small wire. Gold is the most 
 malleable of all the metals, but platina is the most ductile. 
 Dr. Wollaston drew platina into threads not much larger than 
 a spider's web. 
 
 90. TENACITY, in common language called toughness, refers 
 to the force of cohesion among the particles of bodies. 
 
 Tenacious bodies are not easily pulled apart. There is a re- 
 markable difference in the tenacity of different substances. 
 Some possess this property in a surprising degree, while others 
 are torn asunder by the smallest force. 
 
 91. Tenacity of Wood. The following is a tabular view of 
 the absolute cohesion of the^ principal kinds of timber employed 
 in the arts and in building, showing the weight which would 
 rend a rod an inch square, and also the length of the rod, 
 which, if suspended, would be torn asunder by its own weight. 
 
 84. Are brittleness and elasticity ever found in the same substance ? Give exam- 
 ples. How are iron, steel, and brass made brittle? 85. What does malleability 
 mean! 86. What metals are malleable, and what are brittle? 87. Which is the 
 most malleable metal ? 88. What is meant by ductility ? 89. Are the most mallea- 
 ble metals the most ductile ? 90. What is meant by tenacity 1 From what does this 
 property arise 7 
 
PROPERTIES OF BODIES. 23 
 
 92. It appears, by experiment, that the following is the ave- 
 rage tenacity of the kinds of woods named ; but it is found that 
 there is much difference in the strength of the same species, 
 and even of the different parts of the same tree. 
 
 93. The first line refers to the weight, and the other to the 
 length, the wood being an inch square. 
 
 Pounds. Feet. 
 
 Teak, .... 12,915 36,049 
 
 Oak, .... 11,880 32,900 
 
 Sycamore, . . . 9,630 35,800 
 
 Beech, .... 12,225 38,940 
 
 Ash, .... 14,130 39,050 
 
 Elm, .... 9,540 40,500 
 
 Larch, .... 12,240 42,160 
 
 94. Tenacity of the Metals. The metals differ much more 
 widely in their tenacity than the woods. According to the ex- 
 periments of Mr. Rennie, the cohesive power of the several 
 metals named below, each an inch square, is equal to the num- 
 ber of pounds marked in the table, while the feet indicate the 
 length required to separate each metal by its own weight. 
 
 Pounds. Feet. 
 
 Cast steel, . . 134,256 39,455 
 
 Malleable iron, . . 72,064 19,740 
 
 Cast iron, . . .19,096 6,110 
 
 Yellow brass, . .17,958 5,180 
 
 Cast copper, . . . 19,072 5,093 
 
 Cast tin, . . . 4,736 1,496 
 
 Cast lead, . . . 1,824 348 
 
 The cohesion of fluids it is difficult to measure, though some 
 indication of this property is derived by the different sizes of 
 the drops of each on a plane surface. ' 
 
 95. RECAPITULATION. The common or essential properties 
 of bodies are, Impenetrability, Extension, Figure, Divisibility, In- 
 ertia, and Attraction. Attraction is of several kinds, viz. attraction 
 of Cohesion, attraction of Gravitation, Capillary attraction, Chem- 
 ical attraction, Magnetic attraction, and Electrical attraction. 
 
 96. The peculiar properties of bodies are, Density, Rarity, Hard- 
 ness. Elasticity, Brittleness, Malleability, Ductility, and Tenacity. 
 
 93. Give the names of the most tenacious sorts of wood. 94. What metals are most 
 tenacious 7 What metals are least tenacious ? 95. What are the essential properties 
 of bodies ? How many kinds of attraction are there ? 96. What are the peculiaj 
 properties of bodies 1 
 
CHAPTER II. 
 
 GRAVITY. 
 
 97. THE force by which bodies are drawn toward each other 
 in the mass, and by which they descend toward the earth when 
 let fall from a height, is called the force of gravity. 
 
 98. The attraction which the earth exerts on all bodies 
 near its surface, is called terrestrial gravity / and the force 
 with which any substance is drawn downward, is called its 
 weight. 
 
 99. All falling bodies tend downward, or toward the center 
 of the earth, in a straight line from the point where they are 
 let fall. If, then, a body descends, in any part of the world, 
 the line of its direction will be perpendicular to the earth's sur- 
 face. It follows, therefore, that two falling bodies, on opposite 
 parts of the earth, mutually fall toward each other. 
 
 100. Suppose a cannon-ball to be disengaged from a height 
 opposite to us, on the other side of the earth, its motion in re- 
 spect to us would be upward, while the downward motion from 
 where we stand would be upward in respect to those who stand 
 opposite to us on the other side of the earth. * 
 
 101. In like manner, if the falling body be a quarter, in- 
 stead of half the distance round the earth from us, its line of 
 direction will be directly across, or at right-angles with the line 
 already supposed. 
 
 102. This will be readily understood by Fig. 8, where the 
 circle is supposed to be the circumference of the earth, A, the 
 ball falling toward its upper surface, where we stand ; B, a ball 
 falling toward the opposite side of the earth, but ascending in 
 respect to us ; and D, a ball descending at the distance of a 
 quarter of the circle from the other two, and crossing the liue 
 of their direction at right-angles. 
 
 103. It will be obvious, therefore, that what we call up and 
 down, are merely relative terms ; and that what is down in re- 
 
 97. What is gravity ? 98. What is terrestrial gravity 1 99. To what point in the 
 earth do falling bodies tend? 100. In what direction will two falling bodies, from 
 opposite parts of the earth, tend in respect to each other 1 101. In what direction 
 will one from half-way between them meet their line ? 102. How is this shown by 
 Fig. 8 ? 103. Are the terms up and down relative or positive in their meaning ? 
 
GRAVITY. 
 
 25 
 
 Direction of Falling Bodies. 
 
 spect to us, is up in respect 
 to those who live on the 
 opposite side of the earth, 
 and so the contrary. Conse- 
 quently, down every where 
 means toward the center of 
 the earth ; and up, from the 
 center of the earth, "because 
 all bodies descend toward the 
 earth's center from whatever 
 part they are let fall. This 
 will be apparent when we 
 consider that, as the earth 
 turns over every 24 hours, 
 we are carried with it through 
 the points A, D, and B, Fig. 
 8 ; and, therefore, if a body 
 is supposed to fall from the 
 point A, say at 12 o'clock, and the same to foil again from the 
 same point above the earth at 6 o'clock, the two lines of directi >n 
 will be at right-angles, as represented in the figure, for that part 
 of the earth which was under A at 12 o'clock, will be under D at 
 6 o'clock, the earth having in that time performed one quarter 
 of its daily revolution. At 12 o'clock at night, if the body be 
 supposed to fiill again, its line of direction will be at right-a i- 
 gles with that of its last descent, and consequently, it will as- 
 cend in respect to the point from which it fell 12 hours before, 
 because the earth would have then gone through one half her 
 daily rotation, and the point A would be at B. 
 
 VELOCITY OF FALLING BODIES. 
 
 104. The velocity of every falling body is uniformly accele- 
 rated in its approach toward the earth, from whatever height it 
 falls. 
 
 105. If a rock is rolled from a steep mountain, its motion is 
 at first slow and gentle ; but, as it proceeds downward, it moves 
 with perpetually increased velocity, seeming to gather frjsh 
 speed every moment, until its force is such that every obstacle 
 is overcome. 
 
 106. The principle of increased velocity as bodies descend 
 
 What is understood by dmcn in any part of the earth ? Suppose a ball be let all 
 at 12 and then at 6 o'clock, in what direction would the lines of their descent meet 
 each other 1 104. What is said concerning the motions of falling bodies 7 105. How 
 is this increased velocity illustrated ? 106. Explain Fig. 9 
 
 o 
 
26 
 
 GRAVITY. 
 
 from a height, is curiously illustrated by 
 pouring molasses or thick syrup from an F1G 9- 
 
 elevation to the ground. The bulky stream, 
 Fig. 9, of perhaps two inches in diameter 
 where it leaves the vessel, as it descends, is 
 reduced to the size of a straw, or knitting- 
 needle ; but what it wants in bulk is made 
 up in velocity, for the small stream at the 
 ground will fill a vessel just as soon as the 
 large one at the outlet. 
 
 107. For the same reason, a man may 
 leap from a chair without danger, but if he 
 jumps from the house-top, his velocity be- 
 comes so much increased before he reaches 
 the ground, as to endanger his life by the 
 blow. 
 
 It is found, by experiment, that the mo- 
 tion of a falling body is accelerated in regu- 
 lar mathematical proportions. 
 
 These increased proportions do not de- 
 pend on the increased weight of the body, increased Velocity. 
 because it approaches nearer the center of 
 the earth, but on the constant operation of the force of gravity, 
 which perpetually gives new impulses to the falling body, and 
 increases its velocity. 
 
 108. It has been ascertained, by experiment, that a body 
 falling freely, and without resistance, passes through a space 
 of 16 feet and 1 inch during the first second of time. Leaving 
 out the inch, which is not necessary for our present purpose, 
 the ratio of descent is as follows : 
 
 109. If the height through which a body falls in one second 
 of time be known, the height which it falls in any proposed 
 time may be computed. For since the height is proportional 
 to the square of the time, the height through which it will fall 
 in two seconds will be four times that which it falls through in 
 one second. In three seconds it will fall through nine times that 
 space ; in four seconds sixteen times that of the first second ; in 
 five seconds twenty-five times, and so on in this proportion. 
 
 The following, therefore, is a general rule to find the height 
 through which a body will fall in any given time. 
 
 107. Why is there more danger in jumping from the house-top than from a chair ? 
 
 108. What number of feet does a falling body pass through in the first second? 
 
 109. If a body fall from a certain height in two seconds, what proportion to this will 
 it fall in four seconds 1 
 
GRAVITY. 
 
 27 
 
 110. Rule. Reduce the given time to seconds ; take the 
 square of ihe number of seconds in the time, and multiply the 
 height through which the body falls inone second by that num- 
 ber, and the result will be the height sought. 
 
 111. The following table exhibits the height in feet, and the 
 corresponding times in seconds. 
 
 Time 
 Height 
 
 1 
 1 
 
 2 
 4 
 
 3 
 9 
 
 4 
 
 16 
 
 5 
 
 25 
 
 6 
 36 
 
 7 
 49 
 
 8 
 64 
 
 9 
 81 
 
 10 
 100 
 
 Now, as the body falls at the rate of 16 feet during the first 
 second, this number, according to the rule, multiplied by the 
 square of the time, that is, by the numbers expressed in the sec- 
 ond line, will show the actual distance through w r hich the body 
 falls. 
 
 112. Thus we have for the first second 16 feet ; for the end 
 of the second ; 4 X 16 = 64 feet ; third, 9 X 16 = 144 ; fourth, 
 16x16 = 256; fifth, . 25 X 16 = 400 ; sixth, 36x16 = 576; 
 seventh, 49 x 16 = 784 ; and for the 10 seconds 1600 feet 
 
 113. If, on dropping a stone from a precipice, or into a well, 
 we count the seconds from the instant of letting it fall until we 
 hear it strike, we may readily estimate the height of the preci- 
 pice, or the depth of the well. Thus, suppose it is 5 seconds in 
 falling, then we only have to square the seconds, and multiply 
 this by the distance the body falls in one second. We have 
 then 5 X 5 = 25, the square, which 25 X 16 = 400 feet, the depth 
 of the well. 
 
 114. Thus it appears, that to ascertain the velocity with 
 which a body falls in any given time, we must know how many 
 feet it fell during the first second : the velocity acquired in one 
 second, and the space fallen through during that time, being the 
 fundamental elements of the whole calculation, and all that are 
 necessary for the computation of the various circumstances of 
 falling bodies. 
 
 115. The difficulty of calculating exactly the velocity of a 
 falling body from actual measurement of its height, and the 
 time which it takes to reach the ground, is so great, that no ac- 
 curate computation could be made from such an experiment. 
 
 116. ATWOOD'S MACHINE. This difficulty has, however, been 
 overcome by a curious piece of machinery invented by Mr. At- 
 wood. This consists of an upright pillar, with a wheel on the 
 
 110. What is the rule by which the height from which a body falls may be found ? 
 112. How many feet will a body fall in 10 stco ids I 113. If the stone is 5 seconds in 
 falling, how de'ep is the well 1 116. Is the velocity of a falling body calculated from 
 actual measurement, or by a machine ? 
 
28 
 
 GRAVITY. 
 
 no. 
 
 top, as shown by Fig. 10. The 
 weights A and B are of the same 
 size, and are made to balance each 
 other exactly, being connected by 
 a thread passing over the wheel. 
 The ring, R, admits the weight, A, 
 to fall through it in its passage to 
 the stage, S, on which it rests. 
 The ring and stage slide lip and 
 down, and are fastened by a thumb- 
 screw. The pillar is a graduated 
 scale, and M is a small bent wire, 
 weighing a quarter of an ounce, and 
 longer than the diameter of the 
 ring. 
 
 117. When the machine is to 
 be used, the weight, A, is drawn up 
 to the top of the scale, and the ring 
 and stage are placed a certain num- 
 ber of inches from each other. The 
 small bar, M, is then placed across 
 the weight, A, by means of which 
 it is made slowly to descend. When 
 it has descended to the ring, the 
 small weight, M, is taken off by the 
 ring, and thus the two weights are 
 left equal to each other. Now it 
 must be observed, that the motion 
 and descent of the weight, A, are en- 
 tirely owing to the gravitating force 
 
 of the weight, M, until it arrives at the ring, R, when the ac- 
 tion of gravity is suspended, and the large weight continues to 
 move downward to the stage, in consequence of the velocity it 
 had acquired previously to that time. 
 
 118. To comprehend the accuracy of this machine, it must 
 be understood that the velocities of gravitating bodies are sup- 
 posed to be equal, whether they are large or small, this being 
 the case when no calculation is made for the resistance of the 
 air. Consequently, the weight of a quarter of an ounce placed 
 on the large weight, A, is a representative of all other solid 
 
 116 Describe the operation of Mr. Atwood's machine for estimating the velocities 
 of falling bodies. 117. After the sm;Jl weight is taken off hy the ring, why does the 
 large weight continue to descend 1 118. Does his machine show the actual velocity 
 of a falling body, or only its inerease 1 
 
 Atwoo&s Machine. 
 
GRAVITY. 29 
 
 descending bodies. The slowness of its descent, \vhen com- 
 pared with freely gravitating bodies, is only a convenience by 
 which its motion can be accurately measured, for it is the in- 
 crease of velocity which the machine is designed to ascertain, 
 and not the actual velocity of falling bodies. 
 
 119. Xow it will be readily comprehended, that in this re- 
 spect it makes no difference how slowly a body falls, provided 
 it follows the same laws as other descending bodies, and it has 
 already been stated, that all estimates on this subject are made 
 from the known distance a body descends during the first sec- 
 ond of time. 
 
 120. It follows, therefore, that if it can be ascertained ex- 
 actly, how much faster a body falls during the third, fourth, or 
 fifth second, than it did during the first second, we should be 
 able to estimate the distance it would fall during all succeeding 
 seconds. 
 
 121. If, then, by means of a pendulum beating seconds, the 
 weight, A, should be found to descend a certain number of inches 
 during the first second, and another certain number during the 
 next second, and so on, the ratio of acceleration would be pre- 
 cisely ascertained, and could be easily applied to the falling of 
 other bodies; and this is the use to" which this instrument is 
 applied. 
 
 122. It will be readily conceived, that solid bodies falling from 
 great heights, must ultimately acquire an amazing velocity by 
 this proportion of increase. An ounce ball of lead, let fall from 
 a certain height toward the earth, would thus acquire a force 
 ten or twenty times as great as when shot out of a rifle. 
 
 123. By actual calculation, it has been found that were the 
 moon to lose her projectile force, which counterbalances the 
 earth's attraction, she would fall to the earth in four days and 
 twenty hours, a distance of 240,000 miles. And were the 
 earth's projectile force destroyed, it would fall to the sun, with- 
 out resistance, in sixty-four days and ten hours, a distance of 
 95,000,000 of miles. 
 
 124. Every one knows, by his own experience, the different 
 effects of the same body felling from a great, or small height. 
 A boy will toss up his leaden bullet and catch it with his 
 hand, but he soon learns, by its painful effects, not to throw it 
 
 121. By what means is the ratio of descent found ? 122. Would it be possible for a 
 rifl- -ball to acquire a greater force by falling, than if shot from a rifle ? 123. How 
 long would it fate the moon to come to the earth, according to the Jaw of increased 
 velocity ? How long would it take the earth to fall to the sun ? 124. What familiar 
 illustrations are given of the force acquired by the velocity of falling bodies 1 
 
30 GRAVITY. 
 
 too high. The effects of hailstones on window-glass., animals, 
 and vegetation, are often surprising, and some times calamitous 
 illustrations of the velocity of falling bodies. 
 
 125. It has been already stated that the velocities of solid 
 bodies, falling from a given height toward the earth, are equal, 
 or in other words, that an ounce ball of lead will descend in the 
 same time as a pound ball of lead. 
 
 This is true in theory, and in a vacuum, but there is a slight 
 difference in this respect in favor of the velocity of the larger 
 body, owing to the resistance of the atmosphere. We, how- 
 ever, shall at present consider all solids, of whatever size, as de- 
 scending through the same spaces in the same times, this being 
 exactly true when they pass without resistance. 
 
 126. To comprehend the reason of this, we have only to con- 
 sider, that the attraction of gravitation in acting on a mass of 
 matter, acts on every particle it contains ; and thus every parti- 
 cle is drawn down equally, and with the same force. The ef- 
 fect of gravity, therefore, is in exact proportion to the quantity 
 of matter the mass contains, and not in proportion to its bulk. 
 
 127. A ball of lead of a foot in diameter, and one of wood 
 of the same diameter, are obviously of the same bulk ; but the 
 lead contains twelve particles of matter where the wood con- 
 tains only one, and consequently will be attracted with twelve 
 times the force, and therefore will weigh twelve times as much. 
 
 128. Attraction proportionable to the quantity of matter. 
 If, then, bodies attract each other in proportion to the quantities 
 of matter they contain, it follows that if the mass of the earth 
 were doubled, the weights of all bodies on its surface would also 
 be doubled ; and if its quantity of matter were tripled, all bodies 
 would weigh three times as much as they do at present. 
 
 129. It follows, also, that two attracting bodies, when free to 
 move, must approach each other mutually. If the two bodies 
 contain .like quantities of matter, their approach will be equally 
 rapid, and they will move equal distances toward each other. 
 But if the one be small and the other large, the small one will 
 approach the other with a rapidity proportioned to the less 
 quantity of matter it contains. 
 
 130. It is easy to conceive, that if a man in one boat pulls at 
 
 125 Will a small and a large body fill through the same space in the same time ? 
 126 On what parts of a mass of matter does the force of cnivity act ? Is the effect 
 of gravity in proportion to bulk, or qu-uitity of matter ? 127. What is the difference 
 between a ball of lead and one of wood, of the same size"? 128. Were the mass of 
 the earth doubled, how much more should we weigh 1 129. Suppose one body 
 moving toward another, three times as larsre, by the force of gravity what would be 
 their proportional velocities 1 130. How is this illustrated J 
 
GRAVITY. 31 
 
 a rope attacned to another boat, the two boats, if of the same 
 size, will move toward each other at the same rate ; but if the 
 one be large, and the other small, the rapidity with which each 
 moves will be in proportion to its size, the large one moving 
 with as much less velocity as its size is greater. 
 
 131. A man in a boat, pulling a rope attached to a ship, 
 seems only to move the boat ; but that he really mo\ es the 
 ship is certain, when it is considered that a thousand boa s pull- 
 ing in the same manner would make the ship meet them half 
 way. 
 
 It appears, therefore, that an equal force acting on bodies 
 containing different quantities of matter, moves them with dif- 
 ferent velocities, and that these velocities are in an inverse pro 
 portion to their quantities of matter. 
 
 In respect to equal forces, it is obvious that in the case of 
 the ship and single boat, they were moved toward each other 
 by the same force, that is, the force of a man pulling by a 
 rope. The same principle holds in respect to attraction, for all 
 bodies attract each other equally, according to the quantities of 
 matter they contain ; and since all attraction is mutual, no body 
 attracts another with a greater force than that by which it is at- 
 tracted. 
 
 132. Suppose a body to be placed at a distance from the 
 earth, weighing two hundred pounds ; the earth would then 
 attract the body with a force equal to two hundred pounds, and 
 the body would attract the earth with an equal force, other- 
 wise their attraction would not be equal and mutual. Another 
 body, weighing ten pounds, would be attracted with a force 
 equal to ten pounds, and so of all bodies according to the quan- 
 tity of matter they contain ; each body rJeing attracted by the 
 earth with a force equal to its own weight, and attracting the 
 earth with an equal force. 
 
 133. If, for example, two boats be connected by a rope, and 
 a man in one of them pulls with a force equal to 100 pounds, 
 it is plain that the force on each vessel would be 100 pounds. 
 For if the rope were thrown over a pulley, and a man were 
 to pull at one end with a force of 100 pounds, it is plain it 
 would take 1 00 pounds at the other end to balance. See Fig. 11. 
 
 131. Does a large body attract a small one with any more force than it is attracted ? 
 13:2. Suppose a body weighing 200 pounds to be placed at a distance from the earth, 
 with how much force does the earth attract the body ? With what force does rhe 
 body aitract the earth ? 133. Suppose a man in one boat pulls with 'a force of 100 
 pounds at a rope fastened to another boat, what would be the force on each 
 How is this illustrated 1 
 
32 ASCENT OF BODIES. 
 
 FIG. 11. 
 
 Attraction illustrated. 
 
 134. Attracting bodies approach each other. It is inferred 
 from the above principles, that all attracting bodies which are 
 free to move, mutually approach each other, and therefore that 
 the earth moves toward every body which is raised from its sur- 
 face, with a velocity and to a distance proportional to the quan- 
 tity of matter thus elevated from its surface. But the velocity 
 of the earth being as many times less than that of the falling 
 body as its mass is greater, it follows that its motion is not per- 
 ceptible to us. 
 
 The following calculation will show what an immense mass 
 of matter it would take, to disturb the earth's gravity in a per- 
 ceptible manner. 
 
 135. If a ball of earth, equal in diameter to the tenth part 
 of a mile, were placed at the distance of the tenth part of a 
 mile from the earth's surface, the attracting -powers of the two 
 bodies would be in the ratio of about 512 millions of millions 
 to one. For the earth's diameter being about 8000 miles, the 
 two bodies would bear to each other about this proportion. 
 Consequently, if the tenth part of a mile were divided into 512 
 millions of millions of equal parts, one of these parts would be 
 nearly the space through which the earth would move toward 
 the falling body. Now, in the tenth part of a mile there are 
 about 6400 inches, consequently this number must be divided 
 into 512 millions of millions of parts, which would give the 
 eighty thousand millionth part of an inch through which the 
 earth would move to meet a body the tenth part of a mile in 
 diameter. 
 
 ASCENT OF BODIES. 
 
 136. Having now explained and illustrated the influence of 
 gravity on bodies moving downward and horizontally, it remains 
 to show how matter is influenced by the same power when 
 bodies are thrown upward, or contrary to the force of gravity. 
 
 134. Do all attracting bodies approach each other 1 Suppose the body falls toward 
 the earth, is the earth set in motion by its attraction ? Why is not the earth's motion 
 toward it perceptible ? 135. What d.stance would a body, the tenth part of a mile in 
 diameter, placed at the distance of a tenth part of a mile, attract the earth toward it? 
 
FALLING BODIES. 33 
 
 137. What has been stated in respect to the ve- FIG 12 - ^ 
 locity of falling bodies is reversed in respect to d [" 
 those which are thrown upward, for as the motion 
 
 of a falling body is increased by the action of gravi- 
 ty, so it is retarded by the same force when pro- 
 jected from the center of gravity. 
 
 A bullet shot upward, every instant loses a part 
 of its velocity, until having arrived at the highest 
 point from whence it was thrown, it then returns 
 again to the earth. 
 
 138. The same law that governs a descending 
 body, governs an ascending one, only that their mo- 
 tions are reversed. 
 
 139. The same ratio is observed to whatever dis- 
 tance the ball is propelle'd, for as the height to 
 which it is thrown may be estimated from the space, 
 it passes through during the first second, so its re- 
 turning velocity is in a like ratio to the height to 
 which it was sent. 
 
 140. This will be understood by Fig. 12. Sup- 
 pose a ball to be propelled from the point a, with 
 a force which would carry it to the point b in the 
 first second, to c in the next, and to d in the third 
 second. It would then remain nearly stationary for 
 an instant, and in returning would pass through the 
 same spaces in the same time, only that its direc- 
 tion would be reversed. Thus, it will fall from d 
 to c in the first second, to b in the next, and to a in 
 the third. 
 
 141. Now the momentum of a moving body is as 
 its velocity and its quantity of matter, and hence 
 
 the same ball will fall with the same force that it rises. For in- 
 stance, a ball shot out of a rifle, with force sufficient to overcome 
 a certain impediment, on returning would again overcome the 
 same impediment. 
 
 142. It has been doubted, even by good authority, whether 
 the principle above enunciated is true that is, whether a rising 
 and a falling body observe the same law of motion, only, that 
 they are reversed. On this point we quote Dr. Lardner, who, 
 perhaps, is not inferior to any other authority. 
 
 137. What effVct does the force of gravity have on bodies moving upward ? 138. Are 
 upward and downward motion governed by the same laws 1 140. Explain Fig. 12. 
 What is the difference between the upward and returning velocity of the same body? 
 HI. What is said of (he returning force ol a rifle-ball 1 142. What doubts have been 
 expressed on this subject ? ^ 
 
34 MOTION ON INCLINED PLANES. 
 
 ' 143. All the circumstances attending the accelerated de- 
 scent of falling bodies, are exhibited in a reversed order when a 
 body is projected upward. 
 
 " Thus, if a body be projected vertically upward, with the ve- 
 locity which it would acquire in falling freely during one second, 
 the body so projected will rise exactly to the height from which 
 it would have fallen in one second, and at that point of its as- 
 cent, it will have the velocity which it would have at the same 
 point, if it had descended." Hand Book of Natural Philoso- 
 phy, (London, 1851,) p. 116. 
 
 144. It has been estimated that a leaden ball (122) falling 
 from a sufficient height, would acquire a much greater force 
 than if shot from a rifle. 
 
 It is understood that these estimates refer only to dense bullets, 
 as those of lead, or other metals, on which the atmosphere has 
 the least resistance. 
 
 145. It is stated that attempts have been made to test this 
 principle by shooting rifle-balls vertically, and observing with 
 what force they descended, by the depth they penetrated wood- 
 
 ,en impediments. 
 
 But this would hardly be within the art of gunnery, unless 
 the mark erected for the returning ball should be more exten- 
 sive than experimenters would be willing to construct. 
 
 MOTION ON INCLINED PLANES. 
 
 146. Bodies falling down inclined planes follow the same 
 laws of motion as those falling freely, only that their velocities 
 are diminished in proportion as the planes are more or less in- 
 clined. 
 
 14Y. This is illustrated by Fig. 13, where let b be an inclined 
 plane, and A, G, the vertical line of the same length, the letters 
 on each marking the points to which the falling body is sup- 
 posed to reach in 1, 2, 3, 4, and 5 seconds. Now suppose two 
 balls to be dismissed at the same instant from A, the one fall- 
 ing freely, and the other along the plane. Then, to find the 
 difference in their velocities, draw perpendicular lines from the 
 points, 1, 2, 3, 4, and 5, along the inclined plane, and extend these 
 lines to B, C, D, E, F, G, of the vertical line, and these points 
 will respectively mark the difference in their velocities. Thus, at 
 the end of the first second, one of the balls will arrive at B, and 
 
 143. What is the quotation from Dr. Lardner? 144. What estimates have been 
 made with respect to the fall of a rifle-ball ? 145. What is said of the experiment of 
 shooting rifle-balls vertically 1 146. What are the laws of motion down inclined 
 planes ? 147. Explain Fig. 13. 
 
FALL OF LIGHT BODIES. 
 
 35 
 
 Fall on Inclined Plane. 
 
 the other at b, and so in these propor- FIG< 13 * 
 
 tions until they fall to the earth. 
 
 148. It will therefore be observed, 
 that although the ball which falls down 
 the plane is retarded in its motion by 
 friction, still it follows the same law as the 
 other, both being uniformly accelerated 
 in their descent by the force of gravity. 
 
 FALL OF LIGHT BODIES. 
 
 149. Jt has been stated that the 
 earth's attraction acts equally on all 
 bodies containing equal quantities of 
 matter, and that in v'acuo, all bodies, 
 whether large or small, descend from the 
 same heights in the same time. (125.) 
 
 There is, however, a great differ- 
 ence in the quantities of matter which 
 bodies of the same birlk contain, and 
 consequently a difference in the resist- 
 ance which they meet with in passing 
 through the air. 
 
 150. Now, the fall of a body containing a large quantity of 
 matter in a small bulk, meets with little comparative resistance, 
 while the fall of another, containing the same quantity of mat- 
 ter, but of larger size, meets with more in comparison, for two 
 bodies of the same size, meet with exactly the same resistance. 
 Thus, if we let fall a ball of lead, and another of cork, of two 
 inches in diameter each, the lead will reach the ground before 
 the cork, because, though meeting with the same resistance, the 
 lead has the greatest power of overcoming it. 
 
 151. This, however, does not affect the truth of the general 
 law, already established, that the weights of bodies are as the 
 quantities of matter they contain. It only shows that the pres- 
 sure of the atmosphere prevents bulky and porous substances 
 from falling with the same velocity as those which are compact 
 or dense. 
 
 152. Were the atmosphere removed, all bodies, whether light 
 or heavy, large or small, would descend with the same velocity. 
 This has besn ascertained by experiment in the following manner : 
 
 143. What does the explanation of the fiirure prove ? 149. What is said of the fall 
 of bodies ? 150 Why will not a sack of feathers and a stone of the same size fall 
 through the air in the same time ? 151. Does this afftct the truth of the general law, 
 that the weights of bodies are as their quantities of matter! 152. What would be the 
 effect on the fall of light and heavy bodies, were the atmosphere removed 7 
 
36 
 
 MOTION. 
 
 The air-pump is an instrument ' by 
 means of which the air can be pumped 
 out of a close vessel, as will be seen under 
 the article Pneumatics. Taking this for 
 granted at present, the experiment is made 
 in the following manner : 
 
 153. On the plate of the air-pump, A, 
 place the tall jar, B, which is open at the 
 bottom, and has a brass cover fitted close- 
 ly to the top. Through the cover let a 
 wire pass, air-tight, having a small cross 
 at the lower end. On each side of this 
 cross place a little stage, and so contrive 
 them that by turning the wire by the 
 handle, C, these stages shall be upset. On 
 one of the stages place a guinea or piece 
 of lead, and on the other place a feather. 
 When this is arranged, let the air be ex- 
 hausted from the jar by the pump, and 
 then turn the handle, C, so that the guinea 
 and feather may fall from their places, and 
 it will be found that they will both strike 
 the plate at the same instant. Thus is it 
 demonstrated, that were it not for the re- 
 sistance of the atmosphere, a bag of feathers and one of 
 guineas would fall from a given height with the same velocity 
 and in the same time. 
 
 CHAPTER III. 
 
 MOTION. 
 
 154. MOTION may be defined, a continued change of place, 
 with regard to a fixed point. 
 
 155. Without motion there would be no rising or setting 
 of the sun no change of seasons no fall of rain no building 
 of houses, and finally no animal life. Nothing can be done 
 without motion, and therefore without it, the whole universe 
 would be at rest and dead. 
 
 153. How is it proved that a feather and a guinea will fall through equal spaces in 
 the same time, where there is no resistance? 154. How will you define 
 165. What would be the consequence were all motion to cease ? 
 
VELOCITY OF MOTION. 37 
 
 156. In the language of philosophy, the power which puts 
 % body in- motion is called force. Thus, it is the force of gravi- 
 ty that overcomes the inertia of bodies, and draws them toward 
 the earth. The force of water and steam gives motion to ma- 
 chinery, &c. 
 
 157* For the sake of convenience, and accuracy in the use of 
 terms, motion is divided into two kinds, viz. absolute and relative. 
 
 158. Absolute motion is a change of place with regard to a 
 fixed point, and is estimated without reference to the motion of 
 any other body. When a man rides along the street, or when 
 a vessel sails through the water, they are both in absolute motion. 
 
 159. Relative motion is a change of place in a body, with 
 respect to another body, also in motion, and is estimated from 
 that other body exactly as absolute motion is from a fixed point. 
 
 160. The absolute velocity of the earth in its orbit from west 
 to east, is 68,000 miles in an hour ; that of Mars, in the same 
 direction, is 55,000 miles per hour. The earth's relative ve- 
 locity, in this case, is 13,000 miles per hour from west to east. 
 That of Mars, comparatively, is 13,000 miles from east to west, 
 because the earth leaves Mars that distance behind her, as she 
 would leave a fixed point. 
 
 161. Best, in the common meaning of the term, is the op- 
 posite of motion, but it is obvious that rest is often a relative 
 term, since an object may be perfectly at rest with respect to 
 some things, and in rapid motion in respect to others. 
 
 162. Thus, a man sitting on the deck of a steamboat, may 
 move at the rate of fifteen miles per hour, with respect to the 
 land, and still be at rest with respect to the boat. And so, if 
 another man was running on the deck of the same boat at the 
 rate of fifteen miles the hour in a contrary direction, he would 
 be stationary in respect to a fixed point, and still be running 
 with all his might, with respect to the boat. 
 
 VELOCITY OF MOTION. 
 
 163. Velocity is the rate of motion at which a body moves 
 from one place to another. 
 
 Velocity is independent of the weight or magnitude of the 
 moving body. Thus, a cannon-ball and a musket-ball, both 
 living at the rate of a thousand feet in a second, have the same 
 velocities. 
 
 156 What is that power called which puts a body in motion 7 157. How is motion 
 divided 1 158. What is absolute motion 1 159. What is relative motion ? 160. What 
 is the earth's relative velocity in respect to Mars 7 161. What is rest 1 162. In what 
 respect is a man in a stea'mboat at rest, and in what respect does he move 11 
 163. What is velocity 7 
 
38 
 
 VELOCITY OF MOTION. 
 
 164. Velocity is said to be uniform, when the moving body 
 passes over equal spaces in equal times. If a steamboat moves 
 at the rate of ten miles every hour, her velocity is uniform. The 
 revolution of the earth from west to east is a perpetual exam- 
 ple of uniform motion. 
 
 165. Velocity is accelerated, when the rate of motion is in- 
 creased, and the moving body passes through unequal spaces in 
 equal times. Thus, when a falling body moves sixteen feet 
 during the first second, and forty-eight feet during the next 
 second, and so on, its velocity is accelerated. A body falling 
 from a height freely through the air, is the most perfect exam- 
 ple of this kind of velocity. 
 
 166. Retarded velocity, is when the rate of motion of the 
 body is constantly decreased, and it is made to move slower 
 and slower. . A ball thrown upward into the air, has its veloci- 
 ty constantly retarded by the attraction of gravitation, and con- 
 sequently, it moves slower every moment. (137.) 
 
 VELOCITIES OF CERTAIN MOVING BODIES. 
 
 167. Objects moving : 
 Man walking, 
 
 Miles pe 
 hour. 
 . . 3 
 
 r 
 
 Feet per 
 second. 
 44 
 
 Horse trotting, 
 Swiftest race-horse, ..... 
 Railway train, (Eno-lish) 
 
 . . 7 
 . . 60 
 . . 32 
 
 
 Itt 
 
 88 
 47 
 
 
 . . 18 
 
 
 26 
 
 " (Belgian) . . . 
 
 25 
 
 
 36 
 
 " (French) . . . 
 
 . . 27 
 
 
 40 
 
 " (German) . . . 
 
 24 
 
 
 35 
 
 Swift "EnoTish steamers 
 
 14 
 
 
 20 
 
 American steamers on the Hudson, 
 Fast sailing vessels, 
 
 . . 18 
 10 
 
 
 26 
 14 
 
 Current of slow rivers 
 
 3 
 
 
 41 
 
 " of rapid rivers, 
 
 . . 7 
 
 
 10 
 
 Moderate wind, ''- -"." : " 
 
 7 
 
 
 10 
 
 A storm, with wind, 
 
 36 
 
 
 52 
 
 A hurricane, in hot climates, . . 
 Air rushing into a vacuum, . . 
 Common musket-ball, . . 
 A rifle -ball, . . . 
 
 . . 80 
 . . 884 
 . . 850 
 . . 1000 
 
 
 117 
 
 1296 
 1246 
 1466 
 
 A 24-lb. cannon-ball, . . . 
 
 . . 1600 
 
 
 2346 
 
 
 . . 466 
 
 
 683 
 
 Sound, heat at 32, . . . 
 
 748 
 
 
 1090 
 
 " do at 60 
 
 762 
 
 
 1118 
 
 Earth's velocity round the sun, 
 " diurnal motion at equator, 
 
 . . 67,374 
 . . 1037 
 
 
 98,815 
 1520 
 
 164. When is velocity uniform ? 165. When is velocity accelerated ? Give illus- 
 trations of these two kinds of velocity. 166. What is meant by retarded velocity 1 
 Give an example of retarded velocity 
 
MOMENTUM. 39 
 
 168. The above, from Lardner's Mechanics, may be useful for 
 occasional reference. We have omitted the fractional parts with 
 respect to the seconds, as being difficult to remember, and 
 useless for the present purpose. In regard to American loco- 
 motive speed, it is at the present time probably nearly one-third 
 too small. The comparative velocities of balls from fire-arms 
 differ from those given by some other authorities, but on this 
 subject we have made no experiments. 
 
 FORCE, OR MOMENTUM OF MOVING BODIES. 
 
 169. The velocities of bodies are equal, when they pass over 
 equal spaces in the same times ; but the force with which bodies, 
 moving at the same rate, overcome impediments, is in propor- 
 tion to the quantity of matter they contain. This power, or 
 force, is called the momentum of the moving body. 
 
 170. Thus, if two bodies of the same weight move with the 
 same velocity, their momenta will be equal. 
 
 171. Two vessels, each of a hundred tons, sailing at the rate 
 of six miles an hour, would overcome the same impediments or 
 be stopped by the same obstructions. Their momenta would 
 therefore be the same. 
 
 The force or momentum of a moving body, is in proportion 
 to its quantity of matter, and its -velocity. 
 
 172. A large body moving slowly, may have less momenta 
 than a small one moving rapidly. Thus, a bullet shot out of a 
 gun, moves with much greater force than a stone thrown by the 
 hand. 
 
 173. The momentum of a body is found by multiplying its 
 quantity of matter by its velocity per second. Thus, if the 
 velocity be 2, and the weight 2, the momentum will be 4. If 
 the velocity be 6, and the weight of the body 4, the momentum 
 will be 24. 
 
 174. If a moving body strikes an impediment, the force with 
 which it strikes, and the resistance of the impediment, are equal. 
 Thus, if a boy throw his ball against the side of the house, with 
 the force of 3, the house resists it with an equal force, and the 
 ball rebounds. If he throws it against a pane of glass with the 
 same force, the glass having only the power of 2 to resist, the ball 
 will go through the glass, still retaining one-third of its force. 
 
 168. What is said of the speed of our locomotives 1 169. What is meant by ths 
 momentum of a body ? 170. When will the momentum of two bodies be equal ? 
 171 Gire an example. 172. When has a small body a greater momentum than a 
 lanreone? 173 By what rule is the momentum of a body found ? 174. When a 
 moving body strikes au impediment, which receives the greatest shock ? 
 
40 
 
 MOMENTUM. 
 
 FIG. 15. 
 
 175. PILE DRIVER. This machine consists of a frame and 
 pulley, by which a large piece of cast iron, called the hammer, 
 is raised to the height of 30 or 40 feet, and then let fall on the 
 end of a beam of wood called a pile, and by which it is driven 
 into the ground. When the hammer is large, and the height 
 considerable, the force, or momentum, is tremendous, and unless 
 ths pile is hooped with iron, will split it into fragments. 
 
 176. Now the momentum of a body being in proportion to 
 its weight and velocity conjointly, to find it, we must multiply 
 their two sums together. 
 
 Suppose then the hammer, weighing 2000 pounds, is ele- 
 vated two seconds of time abovje the head of the pile, then, 
 according to the law of falling bodies, (110,) it would fall 64 feet, 
 this being the rate of its velocity. Then 64 X 2000, being the 
 velocity and quantity of matter, gives 64 tons as the momen- 
 tum. But according to the same law, this force is immensely 
 increased by a small increase of time, for if we add two seconds 
 of time, the rate of velocity at the instant of striking would be 
 256 feet per second, and thus 256 X 2000 = 512,000 pounds, or 
 256 Ions. 
 
 177. ACTION AND REACTION 
 EQUAL. From observations 
 made on the effects of bodies 
 striking each other, it is found 
 that action and reaction are 
 equal or, in other words, that 
 force and resistance are equal. 
 Thus, when a moving body 
 strikes one that is at rest, the 
 body at rest returns the blow 
 with equal force. 
 
 This is illustrated by the 
 well-known fact, that if two 
 persons strike their heads to- 
 gether, one being in motion, 
 and the other at rest, they are 
 both equally hurt. 
 
 178. The philosophy of ac- 
 tion and reaction is finely illustrated by a number of ivory 
 balls, suspended by threads, as in Fig. 15, so as to touch each 
 
 175. What is a pile driver ? 176. If the hammer of this machine weighs 2000 
 pounds, and falls 2 seconds, what will be the momentum ? If the fall be 3 seconds, 
 what is the momentum 1 177. What is the law of action and reaction ? How is this 
 illustrated 7 
 
 e d C b 
 
 Action and Reaction. 
 
REFLECTED MOTION. 41 
 
 other. If the ball a be drawn from the^erpendicular, and then 
 let fall, so a? to strike the one next to it, the motion of the falling 
 ball will be communicated through the whole series, from one to 
 the other. None of the balls except /, will, however, appear to 
 move. This will- be understood, when we consider that the reac- 
 tion of b is just equal to the action of a, and that each of -the 
 other balls, in like manner, act, and react, on. the other, until 
 the motion of a arrives at /, which, having no impediment, or 
 nothing to act upon, is itself put in motion. It is therefore, 
 reaction, which causes all the balls, except/, to remain at rest. 
 
 180. It is by a modification of the same principle, that rock- 
 ets are impelled through the air. The stream of expanded air, 
 or the fire, which is emitted from the lower end of the rocket, 
 not only pushes against the rocket itself, but against the atmos- 
 pheric air, which, reacting against the air so expanded, sends 
 the rocket along. 
 
 181. It was on account of not understanding the principles 
 of action and reaction, that the man undertook to make a fair 
 wind for his pleasure-boat, to be used whenever he wished to 
 sail. He fixed an immense bellows in the stern of his boat, not 
 doubting that the wind from it would carry him along. But 
 on making, the experiment, he found that his boat went back- 
 ward instead of forward. The reason is plain. The reaction 
 of the atmosphere on the stream of wind from the bellows, 
 before it reached the sail, moved the boat in a contrary direction. 
 
 182. Had the sail received the whole force of the wind from 
 the bellows, the boat would not have moved at all, for then, 
 action and reaction would have been exactly equal, and it would 
 have been like a man's attempting to raise himself over a fence 
 by the straps of his boots. 
 
 REFLECTED 3IOTIOX. 
 
 183. It has been stated (27) that all bodies when once set in 
 motion, would continue to move straight forward, until some 
 impediment, acting in a contrary direction, should bring them 
 to rest ; continued motion without impediment being a conse- 
 quence of the inertia of matter. 
 
 184. 'Such bodies are supposed to be acted upon by a single 
 force, and that in the direction of the line in which they move. 
 
 179 When one of the ivory balls strikes the other, why does the most distant one only 
 move? 1^ On what principle are rockets impelled through the air? 181. In the 
 experiment With the boat and bellows, why did the boat move backward? 182. 
 Why would it not have moved at all had the sail received all the wind from the bel- 
 lows? 183. What is said of the continuity of motion? 
 
42 REFLECTED MOTION. 
 
 TLi;s, a ball sent out of a gun, or struck by a bat, turns neither 
 to the right nor left, but makes a curve toward the earth, in 
 consequence of another force, which is the attraction of gravita- 
 tion, -and by which, together with the resistance of the atmos- 
 phere, it is finally brought to the ground. 
 
 The kind of motion now to be considered, is that which is 
 produced when bodies are turned out of a straight line by some 
 force, independent of gravity. 
 
 A single force, or impulse, sends the body directly forward, 
 but another force, not exactly coinciding with this, will give it 
 a new direction, and bend it out of its former course. 
 
 1-85. If, for instance, two moving bodies strike each other 
 obliquely, they will both be thrown out of the line of their for- 
 mer direction. This is called reflected motion, because it 
 observes the same laws as reflected light. 
 
 186. The bounding of a ball; the skipping of a stone over 
 the smooth surface of a pond ; and the oblique direction of an 
 apple, when it touches a limb in its fall, are examples of reflected 
 motion. 
 
 By experiments on this kind of motion, it is found that mov- 
 ing bodies observe certain laws, in respect to the direction they 
 take in rebounding from any impediment they happen to strike. 
 
 187. Thus, a ball, striking on the floor, or wall of a room, 
 makes the same angle in leaving the point where it strikes, that 
 it does in approaching it. 
 
 188. Suppose a, 6, 
 Fig. 16, to be a mar- 
 ble floor, and c, to be 
 an ivory ball, which 
 has been thrown to- 
 ward the floor in the 
 
 direction of the line Reflected Motion. 
 
 c e ; it will rebound 
 
 in the direction of the line e d, thus making the tw T o angles f 
 
 and g exactly equal. 
 
 189. If the ball approaches the floor under a larger or 
 smaller angle, its rebound will observe the same rule. Thus, 
 if it fell in the line h k, Fig. 17, its rebound would be in the line 
 
 184. Suppose a body is a-'ted on. and set in motion by a single force, in what direc- 
 tion will it move? 185. What is the motion called, when a body is turned out of a 
 straight line by another force? 186. What illustrations can you pive of reflected 
 motion? What laws are observed in reflected motion? 187. Suppose a ball to be 
 thrown on the floor in a certain direction, what rule will it observe in rebounding? 
 188. Explain Fig 16. 
 
COMPOUND* MOTION. 
 
 43 
 
 k i, and if it was drop- FIG. 17. 
 
 ped perpendicularly 
 
 from I to k, it would 
 
 return in the same line 
 
 to I. The angle which 
 
 the ball makes with 
 
 the perpendicular line, 
 
 I k, in its approach to 
 
 the floor, is called the 
 
 angle of incidence, and 
 
 that which it makes 
 
 in departing from the 
 
 floor with the same line, is called the angle of reflection, and 
 
 these angles are always equal. 
 
 /c 
 
 Equal Angles. 
 
 FIG. 18. 
 
 COMPOUND MOTION. 
 
 190. Compound motion is ( that which is produced by two or 
 more forces, acting in different directions, on the same body, at 
 the same time. This will be readily understood by a diagram. 
 
 191. Suppose the ball 
 a, Fig. 18, to be moving 
 with a certain velocity in 
 the line 6 c, and suppose 
 that at the instant when it 
 came to the point a, it 
 should be struck with an 
 equal force in the direction 
 of d e, then, as it can not 
 obey the direction of both 
 these forces, it w T ill take a 
 course between them, and 
 fly off in the direction of/. 
 
 The reason of this 
 
 is plain. The first force Compound Motion. 
 
 would carry the ball from 
 
 b to c ; the second would carry it from d to e ; and these two 
 forces being equal, gives it a direction just half way between the 
 two, and therefore it is sent toward /. 
 
 The line a f, is called the diagonal of the square, and 
 
 189. What is the angle called, which the ball makes in approaching the floor 1 
 What is !he angle called, which it makes in leaving the floor ? What is the difference 
 between these angles 7 190. What is compound motion ? 191. Suppose a ball, mov 
 ing with a certain" force, to be struck crosswise with the same force, in what direc- 
 tion will it move? 
 
44 
 
 COMPOUND MOTION. 
 
 Diagonal Motion. 
 
 results from the cross forces, b and d, being equal to each other. 
 If one of the moving forces is greater than the other, then the 
 diagonal lijie will be lengthened in the direction of the greater 
 force, and instead of being the diagonal of a square, it will 
 become that of a parallelogram. 
 
 192. Suppose the force 
 
 in the direction of a 6, ^ G - 19 
 
 should drive the ball with 
 twice the velocity of the 
 cross force c d, Fig. 19, 
 then the ball would go 
 twice as far from the line 
 c d, as from the line b a, 
 and ef would be the diag- 
 onal of a parallelogram 
 whose length is double its 
 breadth. 
 
 193. Suppose a boat, in 
 
 crossing a river, is rowed forward at the rate of four miles an 
 hour, and the current of the river is at the same rate, then the 
 two cross forces will be equal, and the line of the boat will be 
 the diagonal of a square, as in Fig. 18. But if the current be 
 four miles an hour, and the progress of the boat forward only 
 two miles an hour, then the boat will go down stream twice as 
 fast as she goes across the river, and her path will be the diago- 
 nal Of a parallelogram, as in Fig. 19, and therefore, to make the 
 boat pass directly across the stream, it must be rowed toward 
 some point higher up the river than the landing-place ; a fact 
 well known to boatmen. 
 
 194. CIRCUS RIDER. Those who have seen feats of horse- 
 manship at the circus, are often surprised that when the man 
 leaps directly upward, the horse does not pass from under him, 
 and that in descending he does not fall behind the animal. 
 But it should be considered that, on leaving the saddle, the 
 body of the rider has the same velocity as that of the horse ; 
 nor does his leaving the horse by jumping upward, in nny 
 degree diminish his velocity in the same direction ; his motion 
 being continued by the impulse he had gained from the animal. 
 In this case, the body of the man describes the diagonal of a 
 parallelogram, one side of which is in the direction of the horse's 
 
 192. Suppose it to be struck with twice its former force, in what direction will it 
 move? What is the line af Fig. 18, called ? What is the line e f. Fig. 19, called. 
 How are these figures illustrated? Explain Figs. 18 and 19. 193. Explain the motion of 
 the boat ? Why does the leaping circus rider form the diagonal of a parallelogram ? 
 
CIRCULAR MOTION. 
 
 FIG. 20. 
 
 motion, and the other perpendicularly upward, in the direction 
 in which he makes the leap. 
 
 195. This will be 
 better understood 
 by Fig. 20, where 
 the two forces are 
 illustrated. Had 
 the rider remained 
 on the horse, he 
 would have reached 
 that point, where he 
 meets him after the 
 leap over the iron 
 
 bar, under which the animal passes. This force the rider gains 
 from the horse. The diagonal force is the result of his own 
 muscular exertion, and by which he raises himself above the 
 bar, still retaining in his leap the velocity of the horse, and thus 
 regains the saddle, as though he had not left it. 
 
 The motion of the rider is through the section of a sphere, as 
 shown by the figure, where the horse and rider are shown before 
 and after the leap. 
 
 Circus Rider. 
 
 CIRCULAR MOTION. 
 
 196. Circular motion is that of a body in a ring, or circle, 
 and is produced by the action of two forces. By one of these 
 
 forces, the moving body tends to fly off in a straight line, while 
 by the other it is drawn toward the center, and thus it is made 
 to revolve, or move round in a circle. 
 
 197. The force by which a body tends to go off in a straight 
 line, is called the centrifugal force ; that which keeps it from 
 flying away, and draws it toward the center, is called the cen- 
 tripetal force. 
 
 198. Bodies moving in circles are constantly acted upon by 
 these two forces. If the centrifugal force should cease, the 
 moving body would no longer perform a circle, but would 
 approach the center of its own motion. If the centripetal force 
 should cease, the body would instantly begin to move off in a 
 straight line, this being, as we have explained, the direction 
 which all bodies take when acted on by a single force. 
 
 195. Expla'n Fi2. 20. and show on what principle the rider meets his horse after 
 the leap? 196. What is circular motion? How is this motion produced? 197. 
 What is the centrifugal force? What is the centripetal force? 19S. Suppose the 
 centrifugal force should cease, in what di' ection would the body move 1 Suppose 
 the centripetal force should cease, where would the body go ? 
 
46 
 
 CIRCULAR MOTION. 
 
 FIG. 21 
 
 Centrifugal Force. 
 
 199. Suppose a cannon-ball, 
 Fig'. 21, tied with a string to 
 the center of a slab of smooth 
 marble, and suppose an at- 
 tempt be made to push this 
 ball with the hand in the di- 
 rection of b ; it is obvious that 
 the string would prevent its 
 going to that point ; but would 
 keep it in the circle. In this 
 case the string is the centripe- 
 tal force. 
 
 200. Now suppose the ball 
 to be kept revolving with ra- 
 pidity, its velocity and weight 
 would cause its centrifugal 
 
 force ; and if the string were cut, when the ball was at the 
 point c, for instance, this force would carry it off in the line 
 toward b. 
 
 The greater the velocity with which a body moves round in 
 a circle, the greater will be the force with which it would tend 
 to fly off in a right line. 
 
 Thus, when one wishes to sling a stone to the greatest dis 
 tance, he makes it whirl round with the greatest possible 
 rapidity, before he lets it go. Before the invention of other 
 warlike instruments, soldiers threw stones in this manner, with 
 great force and dreadful effects. 
 
 201. The line about which a body revolves, is called its axis 
 of motion. The point round which it turns, or on which it 
 rests, is called the center of motion. In Fig. 21, the point c/, to 
 which the string is fixed, is the center of motion. In the spin 
 ning-top, a line through the center of the handle to the poirt 
 on which it turns, is the axis of motion. 
 
 In the revolution of a wheel, that part which is at the greatest 
 distance from the axis of motion, has the greatest velocity, and, 
 consequently, the greatest centrifugal force. 
 
 202. Suppose the wheel, Fig. 22, to revolve a certain number 
 of times in a minute, the velocity of the end of the arm at the 
 point a, would be as much greater than its middle at the 
 point 6, as its distance is greater from the axis of motion, because 
 
 199. What constitutes the centrifugal force of the body moving round in a circle 1 
 200. How is this illustrated ? 201. What is the axis of motion ] What is the centei 
 of motion? Give illustrations? 202. What part of a revolving wheel has the great 
 est centrifugal force ? 
 
CENTER OF GRAVITY. 
 
 4T 
 
 Revolving Wheel. 
 
 it moves in a larger circle, and 
 consequently the centrifugal 
 force of the rim c, would, in like 
 manner, be as its distance from 
 the center of motion. 
 
 203. Large wheels, which are 
 designed to turn with great ve- 
 locity, must, therefore, be made 
 with corresponding strength, 
 otherwise the centrifugal force 
 will overcome the cohesive attrac- 
 tion, or the strength of the fast- 
 enings, in which case the wheel 
 will fly in pieces. This some- 
 time^ happens to the large grindstones used in gun factories, 
 and the stone either flies away piecemeal, or breaks in the mid- 
 dle, to the great danger of the workmen. 
 
 204. Were the diurnal velocity of the earth about seventeen 
 times greater than it is, those parts at the greatest distance from 
 its axis would begin to fly off in straight lines, as the water does 
 from a grindstone when it is turned rapidly. 
 
 CENTER. OF GRAVITY. 
 
 205. The center of gravity, in any body or system of bodies, 
 is that point upon which the body, or system of bodies, acted 
 upon only by gravity, will balance itself in all positions. 
 
 206. The center of gravity, in a 
 wheel made entirely of wood, and of 
 equal thickness, would be exactly in 
 its center of motion. But if one side 
 of the wheel were made of iron, and 
 the other part of wood, its center of 
 gravity would be changed to some 
 point, aside from the center of the 
 wheel. 
 
 207. Thus, the center of gravity 
 in the wooden wheel, Fig. 23, is at 
 the axis on which it turns ; but were 
 the arm a, of iron, its center of mo- 
 
 no . 23. 
 
 Center of Gravity. 
 
 203. Why must large wheels, turning with great velocity, be strongly marie ? 204. 
 vVhat would be the "consequence, were the velocity of the earth seventeen times 
 greater than it is ? 205. Where is the center of gravity in a body! 206 Where is 
 the center of gravity in awheel made of wood? If one side is made of wood, and the 
 other of iron, where is the center ? 207. Is the center of motion and of gravity 
 alwayg the same"? 
 
48 
 
 CENTER OF GRAVITY. 
 
 tion and of gravity would no longer be the same, but while the 
 center of motion remained as before, the center of gravity would 
 falf to the point a. Thus the center of motion and of gravity, 
 though often at the same point, are not always so. 
 
 When a body is shaped irregularly, or there are two or more 
 bodies connected, the center of gravity is the point on which 
 they will balance without falling. 
 
 FIG. 24. 
 
 FIG. 25. 
 
 208. If the two balls A and B, Fig. 24, weigh each four 
 pounds, the center of gravity will be a point on the bar equally 
 distant from each. 
 
 But if one of the balls be heavier than the other, then the 
 center of gravity will, in proportion, approach the larger ball. 
 Thus, in Fig. 25, if C weighs two pounds, and D eight pounds, 
 this center will be four times the distance from C that it is 
 from D. 
 
 209. In a body of equal thickness, as a board, or a slab of 
 marble, but otherwise of an irregular shape, the center of gravity 
 may be found by suspending it, first from one point, and then 
 from another, and marking, by means of a plumb-line, the per- 
 pendicular ranges from the point of suspension. The center of 
 gravity will be the point where these two lines cross each other. 
 
 FIG. 26. 
 
 FIG. 27. 
 
 FIG. 28. 
 
 Finding the Center of Gravity. 
 
 Thus, if the irregular shaped piece of board, Fig. 26, be sus- 
 pended by making a hole through it at the point A, and at the 
 
 208. When two bodies are connected, as by a bar between them, where is the cen- 
 ter of gravity ? 209. In a board of irregular shape, by what method is the center of 
 gravity found 1 
 
CENTER OF GRAVITY. 
 
 same point suspending the plumb-line C, 
 
 v ill hang in the position represented in 
 
 marked this line across the board, let it be suspfc^ggMg/iiu in 
 
 the position of Fig. 27, and the perpendicular line again marked. 
 
 The point where these lines cross, is the center of gra\ity, as 
 
 seen by Fig. 28. 
 
 210. Importance of the subject. It is often of great conse- 
 quence, in the concerns of life, that the subject of gravity should 
 be well considered, since the strength of buildings, and of ma- 
 chinery, often depends chiefly on the gravitating point. 
 
 Common experience teaches, that a tall object, with a nar- 
 row base, or foundation, is easily overturned ; but common 
 experience does not teach the reason, for it is only by under- 
 standing principles, that practice improves experiment. 
 
 211. An upright object will fall to the ground, when it leans 
 so much that a perpendicular line from its center of gravity falls 
 beyond its base. A tall chimney, therefore, with a narrow 
 foundation, such as are commonly built at the present day, will 
 fall with a very slight inclination. 
 
 212. Now, in falling, the center of gravity passes through 
 the part of a circle, the center of which is at the extremity of 
 the base on which the body stands. This will be comprehended 
 by Fig. 29. 
 
 Suppose the figure FIG 29. FIG. so. 
 
 to be a block of mar- 
 ble, which is to be 
 turned over, by lift- 
 ing at the corner A, 
 the corner B would 
 be the center of its 
 motion, or the point 
 
 on which it would turn. The center of gravity, C, would, there- 
 fore, describe the part of a circle, of which the corner, B, is the 
 center. 
 
 213. It will be found that the greatest difficulty in turning 
 over a square block of marble, is in first raising up the center 
 of gravity, for the resistance will constantly become less, in pro- 
 portion as the point approaches a perpendicular line over the 
 CDrner B, which, having passed, it will fall by its own gravity. 
 
 210 Why is findjns the center of gravity of importance ? 211. In what direction 
 must the center of gravity be from the outs de of the base, before the object will fall^ 
 212. In falling, the center of gravity passes through part of a circle : where is the cen- 
 ter of this circle 1 213. In turning over a body why does the force required con 
 tantly become less and less ? 
 
 3 
 
60 CENTER OF GRAVITT. 
 
 The difficulty in turning over a body of particular form, will 
 be more strikingly illustrated by the figure of a triangle, or low 
 pyramid. 
 
 214. In Fig. 30, the center of gravity is so low, and the base 
 so broad, that in turning it over, a great proportion of its whole 
 weight must be raised. Hence we see the firmness of the pyra- 
 mid in theory, and experience proves its truth ; for buildings are 
 found to withstand the effects of time, and the commotions of 
 earthquakes, in proportion as they approach this figure. 
 
 The most ancient monuments of the art of building, now 
 standing, the pyramids of Egypt, are of this form. 
 
 215. Movement of a Ball. When a ball is rolled on a hori- 
 zontal plane, the center of gravity is not raised, but moves in a 
 straight line, parallel to the surface of the plane on which it 
 rolls, and is consequently always directly over its center of 
 motion. 
 
 216. Suppose, Fig. 31, A is the FIG. 31. 
 plane on which the ball moves, B 
 
 the line on which the center of grav- 
 ity moves, and C a plumb-line, show- 
 ing that the center of gravity must 
 always be over the center of motion, 
 when the ball moves on a horizontal 
 plane then we shall see the reason 
 why a ball moving on such a plane, 
 will rest with equal firmness in any 
 position, and why so little force is required to set it in motion. 
 For in no other figure does the center of gravity describe a 
 horizontal line over that of motion, in whatever direction the 
 body is moved. 
 
 217. If the plane is inclined downward, the ball is instantly 
 thrown into motion, because the center of gravity then falls for- 
 ward of that of motion, or the point on which the ball rests. 
 
 218. This is explained by Fig. 32, where A is the point on 
 which the ball rests, or the center of motion, 13 the perpendicular 
 line from the center of gravity, as shown by the plumb- weight C. 
 
 219. If the plane is inclined upward, force is required to 
 move the ball in that direction, because the center of gravity 
 
 214. Why is there less force required to overturn a cube, or square, than a pyra- 
 m : d of the same weight? 215. When a ball is rolled on a horizontal plane, in what 
 direction does the center of gravity move 1 Explain Fig. 31. 21t>. Why does a ball 
 on a horizontal plane ret;t equally well in all positions 1 Why does it move with little 
 force? 217. If the plane is inclined downward, why does the ball roll in that direc- 
 tion 1 218. Explain Fijj. 32. 2|.9. Why is force required to move a hall UD an in- 
 e.inedpla1 
 
CENTER OP GRAVITY. 
 
 51 
 
 Cii 
 
 FIG. 33. 
 
 then falls behind that of motion, and 
 therefore this point has to be constantly 
 lifted. This is also shown by Fig. 32, 
 only considering the ball to ba moving 
 up the inclined plane, instead of down it. 
 From these principles, it will be read- 
 ily understood why so much force is 
 required to roll a heavy body, as a 
 hogshead of sugar, for instance, up an 
 inclined plane. The center of gravity 
 falling behind that of motion, the weight 
 
 is constantly acting against the force employed to raise the 
 body. 
 
 220. Illustration by Blocks. One of the best illustrations of 
 this subject may be made by a number of square blocks of wood, 
 placed on each other, as in Fig. 33, forming a 
 
 leaning tower. Where five blocks are placed 
 in this position, the point of gravity is near the 
 center of the third block, and is within^ the 
 base, as shown by the plumb-line. But on 
 adding another block, the gravitating point 
 falls beyond the base, and the whole will now 
 fall by its own weight. 
 
 221. The student having such blocks, (and 
 they may be picked up about any joiner's 
 shop,) will convince himself, that however care- 
 fully his leaning tower is laid up, it will not 
 stand when the center of gravity falls an inch 
 or two beyond the support. 
 
 222. We may learn, from these compari- 
 sons, that it is more dangerous to ride in a 
 high carriage than a low one, in proportion to 
 the elevation of the vehicle, and the nearness 
 
 of the wheels to each other, or in proportion to the narrowness 
 of the base, and the height of the center of gravity. A load of 
 hay, Fig. 34, upsets where one wheel rises but little above the 
 other, because it is broader on the top than the distance of the 
 wheels from each other ; while a load of stone is very rarely 
 turned over, because the center of gravity is near the earth, and 
 its weight between the wheels, instead of being far above them. 
 
 220. Why is a body, shaped like Fig. 33, easily thrown down ? Hence, in riding 
 in a carriage, how ; s the danger of upsetting proportioned? Explain Fig. 33. 221. 
 How may the point of gravity be found by means of a number of square blocks 1 
 222. Why will a load of hay upset more readily than one of stone! 
 
 Leaning Tower. 
 
52 
 
 CENTER OF GRAVITY. 
 
 Load of Hay. 
 
 223. Center of gravity in man. : FIQ - 
 In man the center of gravity is be- 
 tween the hips, and hence, were his 
 
 feot tied together, and his arms tied 
 to his sides, a very slight inclination 
 of his body would carry the perpen- 
 dicular of his center of gravity be- 
 yond the base, and he would fall. 
 But when his limbs are free to move, 
 he widens his base, and changes this 
 center at pleasure, by throwing out 
 his arms, as circumstances require. 
 
 When a man runs, he inclines for- 
 ward, so that the center of gravity 
 may hang before his base, and in 
 this position he is obliged to keep his feet constantly advancing, 
 otherwise he would fall forward. 
 
 A man standing on one foot, can not throw his body forward 
 without, at the same time, throwing his other foot backward, in 
 order to keep the center of gravity within the base. 
 
 224. A man, therefore, standing with his heels against a per- 
 pend'cular wall, can not stoop forward without falling, because 
 the wall prevents his throwing any part of his body backward. 
 A person, little versed in such things, agreed to pay a certain 
 sum of money for an opportunity of possessing himself of double 
 the sum, by taking it from the floor with his heels against the 
 wall. The man, of course, lost his money, for in such a posture, 
 one can hardly reach lower than his own knee. 
 
 225. The base on which a man is supported, in walking or 
 standing, is his feet, and the space between them. By turning 
 the toes out, this base is made broader, without taking much 
 from its length, and hence persons who turn their toes outward, 
 not only walk more firmly, but more gracefully, than those who 
 turn them inward. 
 
 226. In consequence of the upright position of man, he is 
 constantly obliged to employ some exertion to keep his balance. 
 This seems to be the reason why children learn to walk with so 
 much difficulty ; for after they have strength to stand, it re- 
 quires considerable experience so to balance the body as to set 
 one foot before the other without falling. 
 
 , 223 Where is the center of man's gravity ? Why will a man fall with a slight in- 
 clination, when his feet and arms are tied ? 224. Why can not one who stands with 
 his heels against a wall stoop forward ? 225. Why does a person walk most firmly, 
 who turns his toes outward! 226. Why does not a child walk as soon as he can 
 stand 7 
 
CENTER OF GRAVITY. 
 
 53 
 
 FIG. 35. 
 
 227. By experience in the art of balancing, or of keeping the 
 center of gravity in a line over the base, men sometimes perform 
 things that, at first sight, appear altogether beyond human 
 pow^r, such as dining with the table and chair -standing on a 
 single rope, dancing on a wire, &c. 
 
 228. Illustration by Tr$es. No form, under which matter 
 exists, escapes the general law of gravity, and hence vegetables, 
 as well as animals, are formed with reference to the position of 
 this center, in respect to the base. 
 
 It is interesting, in reference to this circumstance, to observe 
 how exactly the tall trees of the forest conform to this law. 
 
 The pine, which grows a hundred feet high, shoots up with 
 as much exactness, with respect to keeping its center of gravity 
 within the base, as though it had been directed by the plumb- 
 line of a master builder. Its limbs toward the top are sent off 
 in conformity to the same law ; each one growing in respect to 
 the other, so as to preserve a due balance between the whole. 
 
 229. SHEPHERDS OF LANDES. Men, as already noticed, by 
 practice in the art of balancing, 
 
 pei-form feats which are won- 
 derful to all beholders. The 
 shepherds of Landes, in the 
 south of France, are perhaps 
 the only people who apply this 
 art to the common business of 
 life. These men walk on stilts 
 from four to five feet high ; and 
 their children, when quite 
 young, who are intended to 
 take the places of their fathers 
 as shepherds, are taught this 
 art in order to qualify them for 
 business. 
 
 To strangers, passing then- 
 district, these men cut a figure 
 at once ludicrous and surpris- 
 ing. Fig. 35. But it is for 
 their own convenience that this 
 singular custom has been adopt- 
 ed, for by this means the feet 
 are kept out of the water which Shepherd of Land 
 
 227. In what does the art of balancing, or walking on a rope, consist ? 228. What 
 is observed in the growth of the trees of the forest, in respect to the laws of gravity 1 
 229. What principle is involved in Fig. 35. 
 
54 CENTER OF INERTIA. 
 
 covers their land in the winter, and from the heated sand in the 
 summer. Besides these comforts, the sphere of vision over a 
 flat country is materially increased by the elevation, so that the 
 shepherd can see his flock at a much greater distance than from 
 the ground. 
 
 By -habit, it is said these men acquire the art of balancing 
 themselves so perfectly as to run, jump, and dance on these 
 stilts with perfect ease. They walk with surprising quickness, 
 so that footmen have to do their best to keep up with them. 
 
 CENTER OF INERTIA, OR INACTIVITY. 
 
 230. It will be remembered that inertia (22) is one of the 
 inherent, or essential properties of matter, and that it is in con- 
 sequence of this property, when bodies are at rest, that they never 
 move without the application of force, and when once in motion, 
 that they never cease moving without some external cause. (27.) 
 
 231. Now, inertia, though like gravity, it resides equally in 
 every particle of matter, must have, like it, a center in each par- 
 ticular body, and this center is the same with that of gravity. 
 
 232. In a bar of iron, six feet long and two inches square, 
 this center is just three feet from each end, or exactly in the 
 middle. If, therefore, the bar is supported at this point, it will 
 balance equally, and because there are equal weights on both 
 ends, it will not fall. 
 
 Now suppose a bar should be raised by raising up the center 
 of gravity, then the inertia of all its parts would be overcome 
 equally with that of the middle. The center of gravity is, 
 therefore, the center of inertia. 
 
 233. But, suppose FIQ 36 
 the same bar of iron, 
 
 whose inertia was over- 
 come by raising the 
 center, to have balls of 
 
 different weights at- Center of Inertia. 
 
 tacned to its ends; 
 then the center of inertia would no longer remain in the middle 
 of the bar, but would be changed to the point A, Fig. 36, so 
 that, to lift the whole, this point must be raised, instead of the 
 middle, as before. 
 
 230. \V1iat effect does inertia exert on bodies at rest ? What effect does it have on 
 bodies in motion ? 231. Is the center of inertia, and that of gravity, the same 1 232. 
 Where is the center of inertia in a body, or a system of bodies 1 233. Why is the 
 point of inertia changed, by fixing different weights to the ends of the iron bar 7 
 
EQUILIBRIUM. 
 
 EQUILIBRIUM. 
 
 234. When two forces counteract, or balance each other, tftey 
 are said to be in equilibrium. 
 
 235. It is not necessary for this purpose that the weights 
 opposed to each other should be equally heavy, for we have 
 just seen that a small weight, placed at a distance from the 
 center of inertia, will balance a large one placed near it. To 
 produce equilibrium, it is only necessary that the weights on 
 each side of the support should mutually counteract each other, 
 or if set in motion, that their momenta should be equal. 
 
 236. A pair of scales are in equilibrium when the beam is 
 in a horizontal position. 
 
 To produce equilibrium in solid bodies, therefore, it is only 
 necessary to support the center of inertia, or gravity. 
 
 237. If a body, or sev- 
 eral bodies, connected, be FIG. 37. 
 suspended by a string, as 
 
 in Fig. 37, the point of 
 support is always in a per- 
 pendicular line above the 
 center of inertia. The 
 plumb-line, D, cuts the bar 
 connecting the two balls at 
 this point. Were the two 
 weights in this figure equal, 
 
 it is evident that the hook, Equilibrium. 
 
 or point of support, must 
 
 be in the middle of the string, to preserve the horizontal 
 position. 
 
 238. When a man stands on his right foot, he keeps himself 
 in equilibrium, by leaning to the right, so as to bring his center 
 of gravity in a perpendicular line over the foot on which he 
 stands. 
 
 CURVILINEAR, OR BENT MOTION. 
 
 239. We have seen that a single force acting on a body, (183,) 
 drives it straight forward, and that two forces acting crosswise, 
 drive it midway between the two, or give it a diagonal direc- 
 tion, (190.) 
 
 234. What is meant by equilibrium? 235. To produce equilibrium, must the 
 weights be equal 1 236. When is a pair of scales in equilibrium ? 237. When a body 
 is suspended by a string, where must the support be with respect to the point ol in- 
 ertia J 239. What is meant by curvilinear motion 1 
 
50 CURVILINEAR MOTION. 
 
 Curvilinear motion differs from both these ; the direction of 
 the body being neither straight forward nor diagonal, but through 
 a line which is curved. 
 
 This kind of motion may be in any direction ; but when it is 
 produced in part by gravity, its direction is always toward the 
 earth. 
 
 239. A stream of water from an aperture in the side of a 
 vessel, as it falls toward the ground, is an example of a curved 
 line ; and a body passing through such a line, is said to have 
 curvilinear motion. Any body projected forward, as a cannon- 
 ball, or rocket, falls to the earth in a curved line. 
 
 240. It is the action of gravity acioss the course of the stream, 
 or the path of the ball, that bends it downward and makes it 
 form a curve. The motion is, therefore, the result of two forces, 
 that of ] -rojection, and that of gravity. 
 
 241. In jets of water, the shape of the curve will depend on 
 the velocity of the stream. When the pressure of the water is 
 great, the stream, near the vessel, is nearly horizontal, because 
 its velocity is in proportion to the pressure. When a ball first 
 leaves the cannon, it describes but a slight curve, because its 
 projectile velocity is then greatest. 
 
 242. The curves described, by jets of water under different 
 degrees of pressure, are readily illustrated by tapping a tall 
 vessel in several places, one above the other. 
 
 243. The action of gravity being always the same, the shape 
 of the curve described must depend on the velocity of the mov- 
 ing body ; but whether the projectile force be great or small, 
 the moving body, if thrown horizontally, will reach the ground 
 from the same height in the same time. 
 
 This, at first thought, would seem improbable ; for, without 
 consideration, most persons would assert, that, if two cannons 
 were fired from the same spot at the same instant, and in the same 
 direction, one of the balls falling half a mile, and the other a 
 mile distant, that the ball which went to the greatest distance 
 would take the most time in performing its journey. 
 
 244. But it must be remembered, that the projectile force 
 does not in the least interfere with the force of gravity. A ball 
 
 239. What are examplps of this kind of motion? 240. What two forces produce 
 this motion? 241 On what does the shape of the curve depend ? 242. How are the 
 curves described by jets of water illustrated? 243 What difference is there in re- 
 spect to the time taken by a body to reach the ground, whether the curve be grf at or 
 small ? 244. Why do bod es, forming different curves from the same height, reach 
 the srrouml at the same time ? Suppose two balls, one flying at the rate of a thousand, 
 and the other at the rate of a hundred feet per second, which would descend most 
 during the second 7 
 
CURVILINEAR MOTION". 57 
 
 flying horizontally, at the rate of a thousand feet per second, is 
 attracted downward with precisely the same force as one flying 
 only a hundred feet per second, and must, therefore, descend 
 the same distance in the same time. 
 
 245. The distance to which a ball will go, depends on the 
 force of impulse given it the first instant, and, consequently, on 
 its projectile velocity. If it moves slowly, the distance will be 
 short; if more rapidly, the space passed over will be greater. 
 It makes no difference, then, in respect to the descent of the 
 ball, whether its projectile motion be fast or slow, or whether it 
 moves forward at all. 
 
 246. Falling of Cannon Balls. This may be shown by ex- 
 periment. Suppose a cannon to be loaded with a ball, and 
 placed on the top of a tower, at such a height from the ground, 
 that it would take just four seconds for the ball to descend from 
 it to the ground, if let fall perpendicularly. Now, suppose the 
 cannon to be fired in an exact horizontal direction, and, at the 
 same instant, the ball to be dropped toward the ground. They 
 will both reach the ground at the same instant, provided its 
 surface be a horizontal plane from the foot of the tower to the 
 place where the projected ball strikes. 
 
 247. Demonstration. This is demonstrated by Fig. 38, where 
 A is the cannon from which the ball is to be fired, a the ver- 
 tical line of the descending ball, A, B, 1, a, the parallelogram 
 through which the ball passes during the first second. 
 
 Now the ball dropped in the vertical direction, will descend 
 16 feet the first second, increasing its velocity according to the 
 law of falling bodies already explained. Meantime the pro- 
 jected ball passing through the diagonal of the upper parallelo- 
 gram, will arrive at 1, while the other falls to a. During the 
 next second the vertical ball will fall to 6, while the other, in 
 consequence of its projectile force, will pass through the diago- 
 nal of the parallelogram 6, 2, C, A. 
 
 The same laws of descent being continued, it is obvious, that 
 the two balls will reach d, 4 at the same instant. 
 
 248. From these principles it may be inferred, that the hoi- 
 izontal motion of a body through the air, does not interfere 
 with its gravitating motion toward the earth, and, therefore, 
 
 245. Does it make any difference in respect to the descent of the ball, whether it 
 has a projectile motion or not 1 246. Suppose, then, one hall be fired from a cannon, 
 and another let fall from the same height at the same instant, would they both reach 
 the ground at the same time 1 247. Explain Fig. 33. showing the reason why the 
 two bails will reach the sround at the same time. Why does the ball approach the 
 earih more rapidly in the last part of the curve than in the first parti 248. What ia 
 t:ie inference from these princ pies ? 
 
 3* 
 
58 
 
 CURVILINEAR MOTION. 
 
 Path of a Cannon Ball. 
 
 that a rifle-ball, or any other body projected horizontally, will 
 reach the ground in the same period of time as one that is let 
 fall perpendicularly from the same height. 
 
 249. The two forces acting on bodies which fall through 
 curved lines, are the same as the centrifugal and centripetal 
 forces, already explained ; the centrifugal, in case of the ball, 
 being caused by the powder the centripetal, being the action 
 of gravity, (199.) 
 
 250. Now the space through which a cannon-ball, or any 
 other body, can be thrown, depends on the velocity with which 
 it is projected ; for the attraction of gravitation, and the resist- 
 ance of the air, acting perpetually, the time which a projectile 
 can be kept in motion through the air is only a few moments. 
 
 Perpetual Revolution. If the projectile be thrown from an 
 elevated situation, it is plain that it would strike at a greater 
 distance than if thrown on a level, because it would remain 
 longer in the air. Every one knows that he can throw a stone 
 to a greater distance when standing on a steep hill, than when 
 standing on the plain below. 
 
 251. Suppose the circle, Fig. 39, to be the earth, and A, a 
 high mountain on its surface. Suppose that this mountain 
 
 249. What is the force called which throws a ball forward ? What is that called 
 which brings it to the ground 1 250. On what does the distance to which a projected 
 body may be thrown, depend ? Why does the distance depend on the velocity ? 
 251. Suppose the velocity of a cannon-ball shot from a mountain 50 miles high to be 
 ten times its usual rate, where would it stop ? 
 
GUNNERY. 
 
 reaches above the atmos- FIG. 39. 
 
 phere, or is fifty miles 
 high, then a cannon-ball 
 might perhaps reach 
 from A to B, a distance 
 of eighty or a hundred 
 miles, because the resist- 
 ance of the atmosphere 
 being out of the calcula- 
 tion, it would have noth- 
 ing to contend with, ex- 
 cept the attraction of 
 gravitation. Ifj then, one 
 degree of force, or veloc- 
 ity, would send it toB, 
 another would send it to 
 C ; and if the force was 
 increased three times, it would fall to D, and if four times, it 
 would pass to E. If, now, we suppose the force to be about ten 
 times greater than that with which a cannon-ball is projected, 
 it would not fall to the earth at any of these points, but would 
 continue its motion until it again came to the point A, the place 
 from which it was first projected. 
 
 252. It would now be in equilibrium, the centrifugal force 
 being just equal to that of gravity, and, therefore, it would per- 
 form another and another revolution, and so continue to revolve 
 around the earth perpetually. 
 
 253. It is these two forces which retain the heavenly bodies 
 in their orbits ; and in the case we have supposed, our cannon- 
 ball would become a little satellite, moving perpetually round 
 the earth. 
 
 Perpetual Revolution of a 
 
 254. LAW AND FORCE OF PROJECTILES. Ever since the dis- 
 covery of gunpowder, the laws of projectiles have been studied 
 with attention, as being of importance in the art of war. Many 
 learned and elaborate works have been published on the sub- 
 ject, but our limits will only admit the insertion of a few of the 
 most important principle's of Gunnery. 
 
 255. A projectile, as a bullet from a gun, unless it has a 
 
 Explain Fig. 39. 252. When would this ball be in equilibrium ? Why would not 
 the force of gravity ultimately brin? this hall to the earth ? 253. After the fiist revo- 
 lution, if the two forces continued the same, would not the motion of the ball be per 
 petuall 254. Why are the laws of projectiles viewed important? 255. What two 
 forces act on projectiles 7 What is the path of a projectile called 7 
 
60 
 
 GUNNERY. 
 
 vertical direction, is acted on by two forces, that of projection, 
 which carries it forward, and that of gravity, which draws it 
 downward. Its path, therefore, is a curve, called a parabola. 
 
 256. The distance to which the ball will fly, depends on the 
 force of projection, since, if its direction is horizontal, its fall 
 toward the earth by the force of gravity (250) will be the same, 
 whether its velocity be great or small. 
 
 257. The resistance of the atmosphere, is the great impedi- 
 ment to the effects of projectile forces. Thus it has been de- 
 monstrated that a 24-lb. cannon-ball, discharged at an elevation 
 of 45, and at the velocity of 2000 feet per second, would, in 
 vacuo, reach the horizon -distance of 125,000 feet, but the re- 
 sistance of the air limit's its range to 7,300 feet. 
 
 258. VELOCITY OF THE BALL. There are several methods 
 of computing the velocity of the ball, one of which is by means 
 of the Ballistic pendulum. This is a thick, heavy block of 
 wood, so suspended as to swing freely about on axis, and into 
 this the ball is fired. The weight of the ball, and that of the 
 block being known, the velocity is found, by the degrees of mo- 
 tion given to the pendulum, which is accurately measured by 
 machinery. 
 
 259. Recoil of the Gun. Another method of finding the 
 velocity of the ball, is by means of the recoil of the gun. This 
 method is founded on the supposition that the explosive force 
 of the powder, communicates equal quantities of motion to the 
 gun and ball, in opposite directions. Hence, by suspending 
 the gun, loaded with weights, like a pendulum, the extent of its 
 arc of vibration, will indicate the force of the charge, and by 
 knowing the weights of the gun and ball, its velocity is indica- 
 ted. By such means Dr. Hutton constructed the following 
 table : 
 
 POWDER. 
 
 VELOCITY PER SECOND. 
 
 DISTANCE. 
 
 TIME OF FLIGHT. 
 
 Ounces. 
 
 Feet. 
 
 Feet. 
 
 Seconds. 
 
 2 
 
 800 
 
 4100 
 
 9 
 
 4 
 
 1230 
 
 5100 
 
 12 
 
 8 
 
 1640 
 
 6000 
 
 14|- 
 
 12 
 
 1680 
 
 6700 
 
 151 
 
 260. Experiment shows that the velocity of the ball increases 
 
 256. On what does the distance of a projectile depend ? 257. What is said of at- 
 mospheric resistance 1 258. What is the construction of the ballistic pendulum 1 
 859. What is the other method of estimating the velocity of the ball 1 ? 
 
PERCUSSION CAPS. 61 
 
 with the charge, to a certain extent, which is peculiar to each 
 gun, after which the increase diminishes the force, until the 
 bore is quite full. 
 
 261. The greatest velocity of a ball known, is about 2000 
 feet per second, and this from a cannon. This velocity dimin- 
 ishes, soon after it leaves the gun. 
 
 262. Power and Destruction. The penetration of the ball 
 is as the square of its velocity. Hence, when the object is 
 merely to penetrate, as in the breaching of a fortification, the 
 greatest velocity is given. But in naval combats, the utmost 
 velocity is not the most injurious, the most destructive balls 
 being such as merely pierce the ship's sides. 
 
 MANUFACTURE OF PERCUSSION CAPS. 
 
 263. The processes by which percussion caps are made at the 
 establishment of Mr. Mclntyre, in the city of Hartford, Ct., are 
 as follow : 
 
 264. First. The copper is rolled to about the thickness of 
 stout brown paper, and is then cut into strips three-fourths of an 
 inch wide, and several yards long. The end of such a strip 
 being placed between the rollers of a cutting and punching 
 machine, invented for this purpose, the whole, without further 
 attention, is cut into star-like pieces of the form and size repre- 
 sented by Fig. 40, A being the piece cut out, and B, the ap- 
 pearance of the strip of copper after the operation. 
 
 FIG. 40. 
 
 First shape of the Copper. 
 
 These pieces are instantly moved, by the same engine, under 
 the punch, by which they are driven through a finely creased 
 die, and are thus formed into caps which fall into a vessel 
 below. 
 
 These stellate pieces, being struck by the punch in the cen- 
 ter, the extremities are thus brought into contact, but not 
 joined, so that the caps consist of four portions connected at the 
 bottom, like the four quarters of an orange peel. 
 
 260. How far does the velocity of the ball increase with that of the charge! 261. 
 What is the greatest velocity of a ball 7 262. What velocity of the ball is most de- 
 structive ? 
 
62 
 
 PERCUSSION CAPS. 
 
 When the caps are exploded by. the hammer, these quarters 
 open and thus prevent the tearing of the metal which, if solid, 
 would be apt to fly into fragments and thus endanger the 
 eyes. 
 
 265. Second. The caps are next placed in a revolving cylin- 
 der containing saw-dust, by which they are made clean and 
 bright. 
 
 They are now ready to receive the fulminating powder, the 
 explosion of which sets fire to the powder in the gun-barrel. 
 
 The caps are now placed, a handful at a time, on a sheet of 
 iron, three feet long, eight inches wide, and the fourth of an 
 inch thick, pierced with holes a quarter of an inch apart, as 
 shown by Fig. 41. This being placed in a horizontal position, 
 and shaken, the caps find their way into the apertures, with 
 their open ends up, in a manner that is quite surprising. 
 
 FIG. 41. 
 
 Mode of Placing the Caps for Fitting. 
 
 266. Third. A piece of brass plate, of the exact size of that 
 containing the caps, is pierced with apertures to correspond with 
 each and every cap, but smaller in size, as shown by Fig. 42. 
 
 FIG. 42. 
 
 
 
 Mode of Filling. 
 
 This plate, being about the sixth of an inch thick, is laid on 
 a smooth surface, and the fulminating compound, a little moist- 
 ened by gum-water, is rubbed into the apertures with the hand, 
 where it adheres, that remaining on the surface being rubbed off. 
 
RESULTANT MOTION. 63 
 
 267. Fourth. This brass plate, being laid on that Contain- 
 ing the caps, each aperture corresponding to a cap, the powder, 
 by means of a brush, is made to fall into the caps. 
 
 ^268. Fifth. The caps are now charged with the powder, 
 in a loose state, and requires a gentle pressure to fix it in its 
 place. 
 
 This is done by placing the plate, containing them, as above 
 described, under rows of punches, which are worked with a little 
 cam engine, and by which the punches are lifted, while the 
 plate is moved forward, by means of a click and notches, so as 
 to correspond exactly with the fall of the punches, by the press- 
 ure of which, the powder is fixed in its place. 
 
 269. Sixth. The best caps are varnished, in order to pro- 
 tect them from moisture. It being the powder only which 
 requires this protection, in France it is done with a little brush 
 on each cap held in the fingers. But Mr. Mclntyre has invented 
 a much more expeditious way, and which insures the same 
 quantity in each cap. 
 
 This is done by a small machine, consisting of two cams ; a 
 click working in horizontal notches, and a crank, by which 
 the whole is moved. On the platform or bed of this, is laid the 
 plate, Fig. 41, containing the caps, (268,) and on working the 
 machine, two dozen blunt metallic points are alternately dipped 
 into a little trough containing copal varnish, and then into the 
 caps, these being moved by the click, to correspond with the 
 revolution of the cams by which the motions of these points 
 are produced. In this way hundreds of caps are varnished in 
 a few minutes. 
 
 270. Seventh. The edges of the best caps are polished, one 
 at a time, by holding them with pliers for a second on a spindle 
 of steel, revolving a thousand times a minute, the point of 
 which enters the cap, the edge rubbing against a shoulder, by 
 which the work is done. 
 
 With two engines, as above described, the proprietor esti- 
 mates the number of caps made per day, to be about 100,000, 
 a market being always ready for all he can make. 
 
 RESULTANT MOTION. 
 
 271. Resultant motion consists in the operation of two, or 
 inore, forces, the joint action of which, results in unity of effect. 
 
 271. What is meant by resultant motion ? Suppose two boats sailing at the same 
 rate and in the same direction, if an apple be tossed from one to the other, what wUJ 
 be its direction in respect to the boats ? 
 
64 
 
 RESULTANT MOTION. 
 
 FIG. 43. 
 
 Suppose two men to be sailing in two boats, each at the 
 rate of four miles an hour, at a short distance opposite to each 
 other, and suppose as they are sailing along in this manner, one 
 of the men throws the other an apple. In respect to the boats, 
 the apple would pass directly across from one to the other, that 
 is, its line of direction would be at right-angles with the sides 
 of the boats. But its actual line through the air would be 
 oblique, or diagonal, in respect to the sides of the boats, because, 
 in passing from boat to boat, it is impelled by two forces, viz., 
 the force of the motion of the boat forward, and the force by 
 which it is thrown by the hand across this motion. 
 
 272. This diagonal motion 
 of the apple is called the re-, 
 sultant, or the resulting mo- ^ 
 tion, because it is the effect or 
 result of two motions resolved 
 into one. Perhaps this will 
 
 be more clear by Fig. 43, 
 
 where A B, and C D, are sup- 
 
 posed to be the sides of the Diagonal Motion. 
 
 two boats, and the line E F, 
 
 that of the apple. Now the apple, when thrown, has a motion 
 
 with the boat at the rate of four miles an hour, from C toward 
 
 D, and this motion is supposed to continue just as though it had 
 
 remained in the boat. 
 
 273. Had it remained in the boat during the time it was 
 passing from E to F, it would have passed from E to H. But we 
 suppose it to have been thrown at the rate of eight miles an 
 hour, in the direction toward G ; and if the boats are moving 
 south, and the apple thrown toward the east, it would pass in 
 the same time twice as far toward the east as it did toward the 
 south. Therefore, in respect to the boats the apple would pass 
 at right-angles from the side of one to that of the other, because 
 they are both in motion. But in respect to a right line, drawn 
 from the point where the apple -was thrown, and a parallel line 
 with this, drawn from the point where it strikes the other boat, 
 the line of the apple would be oblique. This will be clear, when 
 we consider that, when the apple is thrown, the boats are at the 
 points E and G, and that when it strikes, they are at II and F, 
 these two points being opposite to each other. 
 
 What would be its line through the air in respect to the boats? 272. What is this 
 kind of motion called f Why is it called resultant motion I Explain Fig 43 273. 
 Why woul'J the line of the apple he actually at right-angles in respect to the boats, 
 but oblique in respect to parallel lines drawn from where it was thrown and where 
 it struck 1 How is this further illustrated 1 
 
HOROLOQT. 65 
 
 The line E F, through which the apple is thrown, is called 
 the diagonal of a parallelogram, as already explained under 
 compound motion. 
 
 274. On the above principle, if two ships, during a. battle, are 
 sailing before the wind at equal rates, the aim of the gunners 
 will be exactly the same as though they stood still ; whereas, if 
 the gunner fires from a ship standing still, at another under sail, 
 he takes his aim forward- of the mark he intends to hit, because 
 the ship would pass a little forward while the ball is going to her. 
 
 275. And so, on the contrary, if a ship in motion fires at an- 
 other standing still, the aim must be behind the mark, because, 
 as the motion of the ball partakes of that of the ship, it will 
 strike forward at the point aimed at. 
 
 276. For the same reason, if a ball be dropped from the top- 
 mast of a ship under sail, it partakes of the motion of the ship 
 forward, and will fall in a line with the mast, and strike the same 
 point on the deck as though the ship stood still. 
 
 If a man upon the full run drops a bullet before him from 
 the height of his head, he can not run so fast as to overtake it 
 before it reaches the ground. 
 
 It is on this principle, that if a cannon-ball be shot up verti- 
 cally from the earth, it will fall back to the same point ; for. 
 although the earth moves forward while the ball is in the air, 
 yet, as it carries this motion with it, so the ball moves forward, 
 also, in an equal degree, and, therefore, comes down at the 
 same place. 
 
 HOROLOGY. 
 
 277. This term, derived from the Greek, means, to indicate 
 the hour. It is the science of time-keeping. 
 
 278. For this purpose, a great variety of instruments have 
 been invented, by some of which, time was measured by the 
 dropping of water, -as in the clepsydra, or water-clock, in others, 
 by the running of sand, as in the hour-glass, or by the revolu- 
 tion of the sun, by means of the gnomon, or sun-dial. But 
 these ancient methods have given place to the modern inven- 
 tion of clocks, regulated by the pendulum, and watches, regu- 
 lated bv a balance -wheel. 
 
 274. When the ships are in equal motion, where does the gunner take his aim? 
 Why does he aim forward of the mark when the other ship is in motion ? 275 If a 
 eh p-'m motion fires af one Handing still, where must be the aim ? Why. in this case, 
 imiM the aim be brhinri the mark ? 276. What other illustrations are given of result- 
 ant motion 1 277. What is the meaning of horology 1 278. What were the ancient 
 methods of keeping time ? 
 
PENDULUM. 
 
 PENDULUM. 
 
 FIG, 44. 
 
 Pendulum. 
 
 2*79. A pendulum is a heavy body, suck as a piece of brass 
 or lead, suspended by a wire or cord, so as to swing backward 
 and forward. 
 
 When a pendulum swings, it is said to vibrate ; and that 
 part of a circle through which it vibrates, is called its arc. 
 
 280. The times of the vibration of a pendulum are very 
 nearly equal, whether it pass through a greater or less part of 
 its arc. 
 
 Suppose A and 
 B, Fig. 44, to be 
 two pendulums of 
 equal length, and 
 suppose the weights 
 of each are carried, 
 the one to C, and 
 the other to D, and 
 both let fall at the 
 same instant ; their 
 vibrations would be 
 
 equal in respect to time, the one passing through its arc from 
 C to E, and so back again in the same time that the other 
 passes from D to F, and back again. 
 
 281. The reason of this appears to be, that when the pendu- 
 lum is raised high, the action of gravity draws it more directly 
 downward, and it therefore acquires in falling a greater com- 
 parative velocity than is proportioned to the trifling difference 
 of height. 
 
 282. Common Clock. In the common clock, the pendulum 
 is connected with wheel-work, to regulate the motion of the 
 hands, and with weights, by which the whole is moved. The 
 vibrations of the pendulum are numbered by a wheel or 
 escapement, having thirty teeth, which revolves once in a 
 minute. Each tooth, therefore, answers to one vibration of the 
 pendulum, and the wheel moves forward one tooth in a second, 
 Thus the second-hand revolves once in every sixty beats of the 
 pendulum ; and, as these beats are seconds, it goes round once 
 in a minute. By the pendulum the whole machine is regu- 
 lated, for the clock goes faster or slower, according to its num- 
 
 279. What is a pendulum 1 280. What is meant by the vibration of a pendulum 1 
 What is that part of a circle called through which it swings? 281. Why doe? the 
 pendulum vibrate in equal time whether it goes through a small or large part of its 
 arc { 282. Describe the common clock. How many vibrations has the pendulum in 
 a minute 1 
 
PENDULUM. 
 
 ber of vibrations in a given time. The number of vibrations 
 which a pendulum makes in a given time depends upon its 
 length, because a long pendulum does not perform its jour- 
 ney to and from the corresponding points of its arc so soon as a 
 short one. 
 
 283. As the motion of the clock is regulated entirely by the 
 pendulum, and as the number of vibrations are as its length, 
 the least variation in this respect will alter its rate of going. 
 To beat seconds, its length must be about thirty-nine inches. 
 In the common clock, the length is regulated by a screw, which 
 raises and lowers the weight. But as the rod to which the 
 weight is attached is subject to variations of length, in conse- 
 quence of the change of the seasons, being contracted by cold 
 and lengthened by heat, the common clock goes faster in win- 
 ter than in summer. 
 
 In the small clocks of the present day, 
 the pendulum oscillates twice and some- 
 times more in a second, and consequently 
 the escapement must have 60 or more 
 teeth, the second-hand performing two 
 revolutions in a minute. 
 
 The length of a pendulum beating two 
 seconds is the square of that beating 
 seconds. If the length of the seconds 
 pendulum be 39-}- inches, then that beat- 
 ing two seconds will be about 13 feet. 
 
 A pendulum beating half seconds is in 
 length, as the square root of that beating 
 seconds, or about 10 inches long. 
 
 284. Gridiron Pendulum. Various 
 means have been contrived to counteract 
 the effects of these changes, so that the 
 pendulum may continue the same length 
 the whole year. Among inventions for 
 this purpose, the gridiron pendulum is 
 considered among the best. It is so called, 
 because it consists of several rods of dif- 
 ferent metals connected together at each 
 end. 
 
 Gridiron Pendulum. 
 
 233. On what depends the number of vibrations which a pendulum makes in a 
 given time ? What is the medium length of the pendulum beating seconds 1 Whj 
 does a common clock go faster in winter than in summer? What is necessary in 
 respect to the pendulum, to make the clock go true the year round 1 281. What if 
 the principle on which the gridiron pendulum is constructed 1 
 
68 PENDULUM. 
 
 285. The principle on which this pendulum is constructed is 
 derived from the fact that some metals dilate more by the same 
 degrees of heat than others. Thus, brass will dilate about twice 
 as much by heat, and, consequently, contract twice as much by 
 cold, as steel. If, then, these differences could be made to 
 counteract each other mutually, given points at each end of a 
 system of such rods would remain stationary the year round, 
 and thus the clock would go at the same rate in all climates, and 
 during all seasons. 
 
 286. Suppose, then, steel bars A B, are firmly fixed to cross 
 bars at each end, as seen by Fig. 45, and that on the lower 
 cross bar, the brass rods 1 2, are also fixed, then the steel bars 
 can expand only downward, and the brass ones, only upward. 
 Now as the pendulum rod passes through the lower cross bar, 
 and is fixed to the upper cross piece of the brass rods, it will be 
 seen that the elongation of the two metals by heat mutually 
 counteract each other, and therefore that the point of suspen- 
 sion, a, and the pendulum weight, 6, will always remain at the 
 same 'distance from each other. It is found by experiment that 
 the expansion of brass to that of steel is in the proportion of 
 100 to 61. 
 
 287. Gravity varies the Vibrations. As it is the force of 
 gravity which draws the weight of the pendulum from the 
 ighest x point of its arc downward, and as this force increases or 
 diminishes as bodies approach toward the center of the earth, 
 or recede from it, so the pendulum will vibrate faster or slower 
 in proportion as this attraction is stronger or weaker. 
 
 288. Now it is known that the earth at the equator rises 
 higher from its center than it does at the poles, for toward the 
 poles it is flattened. The pendulum, therefore, being more 
 strongly attracted at the poles than at the equator, vibrates 
 more rapidly. For this reason, a clock that would keep exact 
 time at the equator would gain time at the poles, for the rate at 
 which a clock goes depends on the number of vibrations its pen- 
 dulum makes. Therefore, pendulums, in order to beat second;*, 
 must be shorter at the equator, and longer at the poles. 
 
 For the same reason, a clock which keeps exact time at the 
 foot of a high mountain, would move slower on its top. 
 
 2$5. What are the metals of which this instrument is made ? 286. Explain Fig. 45, and 
 give the reason why the length of the pendulum will not change by the variations ot 
 temperature. 287. What is the downward force which makes the pendulum vibrate? 
 Explain the reason why the same clock would go faster at the poles and slower at 
 the equator. 288. How can a clock which goes true at the equator be made to go 
 true at the poles 1 Will a clock keep equal time at the foot and on the top of a high 
 mountain 1 Why will it not ? 
 
PENDULUM. 
 
 69 
 
 FIG. 46. 
 
 289. METRONOME. There is a short pendulum, used by mu- 
 sicians for marking time, which may be made to vibrate fast or 
 slow, as occasion requires. This little instrument is called a 
 metronome, and besides the pendulum, consists of several wheels, 
 and a spiral spring, by which the whole is moved. This pen- 
 dulum is only ten or twelve inches long, and instead of being 
 suspended by the end, like other pendulums, the rod is pro- 
 longed above* the point of suspension, and there is a ball placed 
 near the upper, as well as at the lower extremity. 
 
 290. This arrangement will be under- 
 stood by Fig. 46, where A is the axis 
 of suspension, B the upper ball, and C 
 the lower one. Now, when this pendu- 
 lum vibrates from the point A, the up- 
 per ball constantly retards the motion of 
 the lower one, by in part counterbalanc- 
 ing its weight, and thus preventing its 
 full velocity downward. 
 
 291. Perhaps this will be more ap- 
 parent, by placing the pendulum, Fig. 
 47, for a moment on its side, and across 
 a bar, at the point of suspension. In 
 this position, it will be seen that the 
 little ball would prevent the large one 
 from falling with its full weight, since, 
 were it moved to a cer- 
 tain distance from the 
 
 point of suspension, it 
 would balance the large 
 one so that it would not 
 descend at alt. It is plain, 
 
 therefore, that the comparative velocity of the large ball will be 
 in proportion as the small one is moved to a greater or less dis- 
 tance from the point of suspension. The metronome is so con- 
 structed, the little ball being made to move up and down on 
 the rod at pleasure, that its vibrations are made to beat the 
 time of a quick or slow tune, as occasion requires. 
 
 By this arrangement, the instrument is made to vibrate every 
 two seconds, or every half, or quarter of a second, at pleasure. 
 Metronome means time measurer. 
 
 FIG. 47. 
 
 Metronome. 
 
 2S9 What is the metronome 1 How does this pendulum differ from the common 
 pendulums? 290. Explain Fig. 46. 291. How does the upper ball retard the motion 
 of the lower one ? How is the metronome made to go faster or slower, at pleasure ? 
 
CHAPTER IV. 
 
 MECHANICS. 
 
 292. Mechanics is a science which investigates the laws and 
 effects of force and motion. 
 
 293. The practical object of this science is, to teach the best 
 modes of overcoming resistances by means of mechanical powers, 
 and to apply motion to useful purposes, by means of machinery. 
 
 294. A machine is any instrument by which power, motion, 
 or velocity, is applied or regulated. 
 
 295. A^ machine maybe very simple, or exceedingly com- 
 plex. Thus, a pin is a machine for fastening clothes, and a 
 steam-engine is a machine for propelling mills and boats. 
 
 As machines are constructed for a vast variety of purposes, 
 their forms, powers, and kinds of movement, must depend on 
 their intended uses. 
 
 Several considerations ought to precede the actual construc- 
 tion of a new or untried machine ; for if it does not answer the 
 purpose intended, it is commonly a total loss to the builder. 
 
 Many a man, on attempting to apply an old principle to a 
 new purpose, or to invent a new machine for an old purpose, 
 has been sorely disappointed, having found, when too late, that 
 his time and money had been thrown away, for want of proper 
 reflection, or requisite knowledge. 
 
 If a man, for instance, thinks of constructing a machine for 
 raising a ship, he ought to take into consideration the inertia or 
 weight, to be moved the force to be applied the strength of 
 the materials, and the space or situation he has to work in. 
 For, if the force applied, or the strength of the materials be in- 
 sufficient, his machine is obviously useless ; and if the force and 
 strength be ample, but the space be wanting, the same result 
 must follow. 
 
 If he intends his machine for twisting the fibers of flexible 
 substances into threads, he may find no difficulty in respect to 
 power, strength of materials, or space to work in, but if the 
 
 292. What is mechanics 7 293. What is the object of this science ? 294 What is 
 a machine? 295. Mention one of the most simple, and one of the most complex of 
 machines. 
 
DEFINITIONS. 71 
 
 velocity, direction, and kind of motion he obtains, be not appli- 
 cable to the work intended, he still loses his labor. 
 
 Thousands of machines have been constructed, which, so far 
 as regarded the skill of the workmen, the ingenuity of the con- 
 triver, and the construction of the individual parts, were models 
 of art and beauty ; and, so far as could be seen without trial, 
 admirably adapted to the intended purpose. But on putting 
 them to actual use, it has too often been found, that their only 
 imperfection consisted in a stubborn refusal to do any part of 
 the work intended. 
 
 Now, a thorough knowledge of the laws of motion, and the 
 principles of mechanics, would, in many instances,, at least, have 
 prevented all this loss of labor and money, and spared him so 
 much vexation and chagrin, by showing the projector that his 
 machine would not answer the intended purpose. 
 
 The importance of this kind of knowledge is therefore ob- 
 vious, and it is hoped will become more so as we proceed. 
 
 DEFINITIONS. 
 
 296. In mechanics, as well as in other sciences, there are 
 words which must be explained, either because they are com- 
 mon words used in a peculiar sense, or because they are terms 
 of art, not in common use. All technical terms will be as much 
 as possible avoided, but still there are a few, which it is neces- 
 sary here to explain. 
 
 297. Force is the means by which bodies are set in motion, 
 kept in motion, and when moving, are brought to rest. 
 The force of gunpowder sets the ball in motion, and keeps 
 it moving, until the force of the resisting air, and the force 
 of gravity, bring it to rest. 
 
 298. Power is the means by which the machine is moved, 
 and the force gained. Thus we have horse-power, water- 
 power, and the power of weights. 
 
 299. Weight is the resistance, or the thing to be moved by 
 the force of the power. Thus the stone is the weight to 
 be moved by the force of the lever or bar. 
 
 300. Fulcrum, or prop, is the point on which a thing is sup- 
 ported, and about which it has more or less motion. In 
 raising a stone, the thing on which the lever rests, is the 
 fulcrum. 
 
 297. What is meant by force in mechanics ? 295. What is meant by power ? 299. 
 What is understood by weight 1 300. What is the fulcrum 1 301. Are the mechan- 
 ical powers numerous, or only few in number 7 
 
72 LEVER. 
 
 301. In mechanics, there are a few simple machines called 
 the mechanical powers, and however mixed, or complex, a com- 
 bination of machinery may be, it consists only of these few in- 
 dividual powers. 
 
 We shall not here burden, the memory of the pupil with the 
 names of these powers, of the nature of which he is at present 
 supposed to know nothing, but shall explain the action and use 
 of each in its turn, and then sum up the whole for his accom- 
 modation. 
 
 THE LEVER. 
 
 302. Any rod, or bar, which is used in raising a weight, or 
 surmounting a resistance, by being placed on a fulcrum, or prop, 
 becomes a lever. Levers are simple and compound. 
 
 303. Simple levers are of three kinds, namely : first, where 
 the fulcrum is between the power and the weight; second, 
 where the weight is between the fulcrum and the power ; third, 
 where the power is between the fulcrum and the weight* 
 
 304. First Kind. The first kind is represented by Fig. 48, 
 being a straight 
 
 rod of iron, called FIG 48 - 
 
 a crowbar, in com- 
 mon use for rais- 
 ing rocks and oth- 
 er heavy bodies. 
 The stone, B, is 
 the weight. A the 
 
 7 -j-xs.il Simple Lever. 
 
 lever, and C the 
 
 fulcrum j the power being the hand of a man applied at A. 
 
 It will be observed, that by this arrangement the application 
 of a small power may be used to overcome a great resistance. 
 
 305. The force to be obtained by the lever, depends on its 
 length, together with the power applied, and the distance of the 
 weight and power from the fulcrum. 
 
 306. Suppose, Fig. 49, that A is the lever, B the fulcrum, D 
 the weight to be raised, and C the power. Let D be considered 
 three times as heavy as C, and the fulcrum three times as far 
 from C as it is from D ; then the weight and power will ex- 
 actly balance each other. Thus, if the bar be four feet long v 
 
 302. What is a lever ? 303. What are the three kinds of simple levers ? 304. What 
 is the simplest of all mechanical powers \ Explain Fig. 48. Which is the weight 1 
 Where is the fulcrum ? Where is the power applied ! What is the power in this 
 ease? 305. On what does the force to be obtained by the lever depend } 306 Sup- 
 pose a lever four feet long, and the fulcrum one foot from the end, what number of 
 pound* will balance each other at the ends 1 
 
LEVER. 73 
 
 FIG. 49. 
 
 Lever Unequal Arms. 
 
 and the fulcrum three feet from the end, then three pounds on 
 the long arm will weigh just as much as nine pounds on the 
 short arm, and these proportions will be found the same in all 
 cases. 
 
 307. When two weights balance each other, the fulcrum is 
 always at the center of gravity between them, and therefore, 
 to make a small weight raise a large one, the fulcrum must be 
 placed as near as possible to the large one, since the greater the 
 distance from the fulcrum the small weight or power is placed, 
 the greater will be its force. 
 
 FIG. 50. 
 
 I 
 
 Q~ 
 
 Lever Double Weights. 
 
 308. Suppose the weight B, Fig. 50, to be sixteen pounds, 
 and suppose the fulcrum to be placed so near it, as to be raised 
 by the power A, of four pounds hanging equally distant from 
 the fulcrum and the end of the lever. If now tlie power A be 
 removed, and another of two pounds, C, be placed at the end 
 of the lever, its force will be just equal to A, placed at the 
 middle of the lever. 
 
 309. But let the fulcrum be moved along to the middle of 
 the lever, with the weight of sixteen pounds still suspended to 
 it, it would then take another weight of sixteen pounds, instead 
 cf two pounds, to balance it, Fig. 51. 
 
 307. When weights ba'ance each other, at what point between them must the ful- 
 crum be ? 38 Suppose a weight of 16 pounds on the short arm of a lever is coun- 
 terbalanced by 4 pounds in the middle of the long arm. what power would balance 
 this weight at the end of the lever ? 309. Suppose the fulcrum to be moved to the 
 middle of the lever, what power would then be equal to 16 pounds? 
 
 4 
 
74 LEVER. 
 
 FIG. 61. 
 
 Lever Equal Arms. 
 
 Thus, the power which would balance sixteen pounds, when 
 the fulcrum is in one place, must be exchanged for another power 
 weighing eight times as much, when the fulcrum is in another 
 place. 
 
 310. From these investigations, we may draw the following 
 general truth, or proposition, concerning the lever : " That the 
 force of the lever increases in proportion to the distance of the 
 power from the fulcrum, and diminishes in proportion as the 
 distance of the weight from the fulcrum increases." 
 
 311. From this proposition, may be drawn the following rule, 
 by which the exact proportions between the weight or resist- 
 ance, and the power, may be found. Multiply the weight by 
 its distance from the fulcrum ; then multiply the power by its 
 distance from the same point, and if the products are equal, the 
 weight and the power will balance each other. 
 
 312. Suppose a weight of 100 pounds on the short arm of 
 a lever, 8 inches from the fulcrum, then another weight, or 
 power, of 8 pounds, would be equal to this, at the distance 
 of 100 inches fro n the fulcrum ; because 8 multiplied by 100 
 is equal to 800 ; and 100 multiplied by 8 is equal to 800, and 
 thus they would mutually counteract each other. 
 
 313. Many instruments 
 
 in common use are on the FIG. 52. 
 
 principle of this kind of 
 
 lever. Scissors, Fig. 52, 
 
 consist of two levers, the 
 
 rivet being the fulcrum 
 
 for both. The fingers are 
 
 the power, and the cloth 
 
 to be cut, the resistance to 
 
 be overcome. TWO Levers. 
 
 Pincers, forceps, and 
 sugar-cutters, are examples of this kind of lever. 
 
 310. What is the general proposition drawn from these results? 311. What is the 
 rule for finding the proportions between the weight and power 1 312. Give an illus- 
 tration of this rulp. 313. What instruments operate on the principle of this lever 7 
 
LEVIR. 75 
 
 314. A common scale-beam, used for weighing, is a lever, 
 suspended at the center of gravity, so that the two arms bal- 
 ance each other. Hence the machine is called a balance. The 
 fulcrum, or what is called the pivot, is sharpened, like a wedge, 
 and made of hardened steel, so as much as possible to. avoid 
 friction. 
 
 315. A dish is suspended by FIG. 53. 
 cords to each end or arm of the 
 
 lever, for the purpose of hold- 
 ing the articles to be weighed. 
 "When the whole is suspended 
 at the point a, Fig. 53, the 
 beam or lever ought to remain 
 in a horizontal position, one of common Scale*. 
 
 its ends being exactly as high 
 
 as the other. If the weights in the two dishes are equal, and 
 the support exactly in the center, they will always hang as 
 represented in the figure. 
 
 316. A very slight variation of the point of support toward 
 one end of the lever, will make a difference in the weights em- 
 ployed to balance each other. In weighing a pound of sugar, 
 with a scale-beam of eight inches long, if the point of support 
 is half an inch too near the weight, the buyer would be cheated 
 nearly one ounce, and consequently nearly one pound in every 
 sixteen pounds. This fraud might instantly be detected by 
 changing the places of the sugar and weight, for then the dif- 
 ference would be quite material, since the sugar would then 
 seem to want twice as much additional weight as it did really 
 want. 
 
 317. The steelyard dif- ^ 
 fers from the balance, in 
 
 having its support near 
 one end, instead of in the 
 middle, and also in hav- 
 ing the weights suspend- 
 ed by hooks, instead of steelyard. 
 being placed in a dish. 
 
 If we suppose the beam to be 7 inches long, and the hook, 
 C, Fig. 54, to be one inch from the end, then the pound weight, 
 A, will require an additional pound at B, for every inch it is 
 
 314 In the common scale-beam, where is the fulcrum ? 315. In what position 
 ought the scale-beam to hang ] 316. How may a fraudulent scale-beam be made 7 
 How may the cheat be detected '} 317. How does the steelyard differ Irom the 
 balance ? 
 
76 LEVER. 
 
 moved from it. This, however, supposes that the bar will bal- 
 ance itself, before any weights are attached to it. 
 
 FIG. 55. 
 
 Lever of the Second Kind. 
 
 318. Second Kind. The second kind of lever is represented 
 by Fig. 55, where W is the weight, L the lever, F the fulcrum, 
 and P a pulley, over which a string is thrown, and a weight 
 suspended, as the power. In the common use of a lever of the 
 first kind, the force is gained by bearing down the long arm, 
 which is called prying. In the second kind, the force is gained 
 by carrying the long arm in a contrary direction, or upward, 
 and this is called lifting. 
 
 319. Levers of the second kind are not so common as the 
 first, but are frequently used for certain purposes. The oars of 
 a boat are examples of the second kind. The water against 
 which the blade of the oar pushes, is the fulcrum, the boat is 
 the weight to be moved, and the hands of the man, the power. 
 
 320. Two men carrying a load between them on a pole, is 
 also an example of this kind of lever. Each man acts as the 
 power in moving the weight, and at the same time each be- 
 comes the fulcrum in respect to the other. 
 
 If the weight happens to slide on the pole, the man toward 
 whom it goes has to bear more of it in proportion as its dis- 
 tance from him is less than before. 
 
 321. A load at A, Fig. 56, is borne equally by the two men, 
 being equally distant from each other ; but at B, three quarters 
 of its weight would be on the man at that end, because three 
 quarters of the length of the lever would be on the side of the 
 other man. 
 
 318. la the first kind of lever, where is the fulcrum, in respect to the weight and 
 power 1 In the second kind, where is the fulcrum, in respect to the weight and 
 power 1 What is the action of the first kind called ? What is the action of the see* 
 ond kind called 1 319. Give examples of the second kind of lever. 320. In rowing a 
 boat, what is the fulcrum, what the weight, and what the power ? 321. What other 
 illustrations ef this principle ar given 1 
 
LEVER. 
 
 FIG. 56. 
 .A 
 
 Lever Unequal Arm*. 
 
 Lever of the Third Kind. 
 
 I 
 
 322. Third Kind. In the third and last kind of lever, the 
 weight is placed at one end, the fulcrum at the other end, and 
 the power between them, or the hand, is between the fulcrum 
 and the weight. 
 
 This is represented by Fig. 57, where C is the fulcrum, A 
 the power, suspended over the pulley B, and D is the weight 
 to be raised. 
 
 323. This kind of lever works to great disadvantage, since 
 the power must be greater than the weight. It is therefore 
 seldom used, except in cases where velocity and not force is 
 required. In raising a ladder from the ground to the roof of 
 a house, men are obliged sometimes to make use of this princi- 
 ple, and the great difficulty of doing so, illustrates the mechan- 
 ical disadvantage of this kind of lever. 
 
 We have now described the three kinds of levers, and, we 
 hope, have made the manner in which each kind acts plain, by 
 illustrations. But to make the difference between them still 
 more obvious, and to avoid all confusion, we will here compare 
 them together. 
 
 322 In the third kind of lever where are the respective places of the weight, 
 power, and fulcrum ? 323. What is the disadvantage of this kind of lever ? Give an 
 example of the use of the third kind of lever 7 
 
LEVflR. 
 FIG. 58. 
 
 i 
 
 FIG. 69. 
 
 no. GO. 
 
 The Levers Compared. 
 
 324. In Fig. 58, the weight and hand both act downward. 
 In 59, the weight and hand act in contrary directions, the hand 
 upward and the weight downward, the weight being between 
 them. In 60, the hand and weight also act in contrary direc- 
 tions, but the hand is between the fulcrum and the weight. 
 
 325. COMPOUND LEVER. When several simple levers ar<? 
 connected together, and act one upon the other, the machine is 
 
 324. In what direction dp the hand and weight act, in the first kind of lever? In 
 what direction do they act in the second kind ] In what direction do they act in the 
 third kind 1 325. What Is a compound lever 7 
 
LEVER. 79 
 
 called a compound lever. In this machine, as each lever acts as 
 an individual, and with a force equal to the action of the next 
 lever upon it, the force is increased or diminished, and becomes 
 greater or less, in proportion to the number or kind of levers 
 employed. 
 
 We will illustrate this kind of lever by a single example, but 
 must refer the inquisitive student to more extended works for a 
 full investigation of the subject. ' 
 
 FIG. 61. 
 E T P 
 
 Compound Levtr. 
 
 Fig. 61 represents a compound lever, consisting of three sim- 
 ple levers of the first kind. 
 
 326. In calculating the force of this lever, the rule applies 
 which has already been given for the simple lever, namely : 
 The length of the long arm is to be multiplied by the moving 
 power, and that of the short one, by the weight, or resistance. 
 
 327. Let us suppose, then, that the three levers in the figure 
 Sire of the same length, the long arms being six inches, and the 
 short ones two inches long ; required, the weight which a mov- 
 ing power of 1 pound at A will balance at B. In the first place, 
 1 pound at A, would balance 3 pounds at E, for the lever being 
 6 inches, and the power 1 pound, 6x1 = 6, and the short one 
 being 2 inches, 2x3 = 6. The long arm of the second lever 
 being also 6 inches, and moved with a power of 3 pounds, mul- 
 tiply the 3 by 6 = 18; and multiply the length of the short 
 arm, being 2 inches, by 9 = 18. These two products being 
 equal, the power upon the long arm of the third lever, at D, 
 would be 9 pounds. 9 pounds x6=54, and 27 X 2, is 54 ; so 
 that 1 pound at A would balance 27 at B. 
 
 The increase of force is thus slow, because the proportion be- 
 tween the long and short arms is only as 2 to 6, or in the pro- 
 portions of 1, 3, 9. 
 
 326. By what rule is the force of the compound lever calculated ? 327. How many 
 pounds weight will be raised by three levers connected, of six inches each, with the 
 fulcrum two inches from the end, by a power of one pound 1 
 
80 
 
 WHEEL AND AXLE. 
 
 FIG. 62. 
 
 328. Now suppose the long arms of these levers to be 18 
 inches, and the short ones 1 inch, and the result will be sur- 
 prisingly different, for then 1 pound at A would balance 18 
 pounds at E, and the second lever would have a power of 
 18 pounds. This being multiplied by the length of the lever, 
 18x18 324 pounds at D. The third lever would thus be 
 moved by a power of 324 pounds, which, multiplied by 18 
 inches for the weight it would raise, would give 5832 pounds. 
 
 329. The compound lever is employed in the construction of 
 weighing machines, and particularly in cases where great weights 
 are to be determined, in situations where other machines would 
 be inconvenient, on account of their occupying too much space. 
 
 330. KNEE LEVER. A compound instrument, called the 
 Knee Lever, is used in various kinds of machinery, the principle 
 of which. is explained by Fig. 62. 
 
 This combination consists of 
 a metal rod, A B, having a 
 joint at A, above which there 
 is a firm support. At C is an- 
 other rod, or bar, jointed to the 
 long lever, and terminating at 
 G, where there is another joint, 
 attached to a movable plat- 
 form, on which the- force of the 
 two levers are exerted. 
 
 Now when B is pushed to- 
 ward the vertical position, the 
 force on the joints A and G, is 
 constantly increased, until the 
 two bars become perpendicular, 
 when the pressure exerted, is 
 augmented to nearly an indefi- Knee Lever. 
 
 nite degree. 
 
 331. Various engines for pressing paper, and for printing, are 
 constructed on this principle, and it is said they are unequaled 
 m power, except by the Hydrostatic press. 
 
 WHEEL AND AXLE. 
 
 332. The mechanical power, next to the lever in arrange- 
 
 328. If the long arms of the levers be eighteen inches, and the short ones one inch 
 how much will a power of one pound balance ? 329. In what machines is the com 
 pound lever employed 1 330. Explain the principle of the Knee Lever, Fig 62. 331 
 What machines are on this principle 1 
 
WHEEL AND AXLE. 
 
 81 
 
 Wheel and Axle. 
 
 ment, is the wheel and axle. It is, however, much more com- 
 plex than the lever. 
 
 333. It consists ot two wheels, FIG. 63. 
 one of which is larger than the 
 
 other, but the small one passes 
 through the larger, and hence both 
 have a common center, on which 
 they turn. 
 
 334. The manner in which this 
 machine acts will be understood by 
 Fig. 63. The large wheel, A, on 
 turning the machine, will take up, 
 or throw off, as much more rope 
 than the small wheel or axle, B, as 
 
 its circumference is greater. If we suppose the circumference 
 of the large wheel to be four times that of the small one, then 
 it will take up the rope four times as fast. And because A is 
 four times as large as B, 1 pound at D will balance 4 pounds 
 at C, on the opposite side. 
 
 335. The principle of this machine is 
 that of the lever, as will be apparent 
 by an examination of Fig, 64. 
 
 336. This figure represents the ma- 
 chine endwise, so as to show in what 
 manner the lever operates. The two 
 weights hanging in opposition to each 
 other, the one on the wheel at A, and 
 the other on the axle at B, act in the 
 same manner as if they were connected 
 by the horizontal lever A B, passing from 
 
 one to the other, having the common wheel and Axle. 
 
 center, C, as a fulcrum between them. 
 
 337. The wheel and axle, therefore, acts like a constant suc- 
 cession of levers, the long arm being half the diameter of the 
 wheel, and the short one half the diameter of the axle ; the 
 common center of both being the fulcrum. The wheel and axle 
 has, therefore, been called the perpetual lever. 
 
 338. The great advantage of this mechanical arrangement is, 
 that while a single lever of the same power can raise a weight 
 
 FIG. 64. 
 
 332. What is the next mechanical power to the lever ? 333. Describe this ma- 
 chine. 334. Explain Fig. 63. 335. On what principle does this machine act 7 336. 
 In Fig 64, which is the fulcrum, and which the two arms of the lever? 337. What 
 is this machine called, in reference to the principle on which it acts? 338 What is 
 the great advantage of this machine over the lever aiid other mechanieal powers 7 
 
 4* 
 
TTHEEL AND AXLK. 
 
 but a few inches at a time, and then only in a certain direction, 
 this machine exerts a continual force, and in any direction 
 wanted. To change the direction, it is only necessary that the 
 rope by which the weight is to be raised, should be carried in a 
 line perpendicular to the axis of the machine, to the place belovr 
 where the weight lies, and there be let fall over a pulley. 
 
 339. Suppose the wheel 
 
 and axle, Fig. 65, is erect- FIG. 65. 
 
 ed in the third story of a 
 store-house, with the axle 
 over the scuttles, or doors 
 through the floors, so that 
 goods can be raised by it 
 from the ground-floor, in 
 the direction of the weight 
 A. Suppose, also, that the 
 same store stands on a 
 wharf, where ships come 
 up to its side, and goods 
 are to be removed from the 
 vessels into the upper sto- 
 ries. Instead of removing 
 the goods into the store, and hoisting them in the direction of 
 A, it is only necessary to carry the rope B, over the pulley C, 
 which is at the end of a strong beam projecting out from the 
 side of the store, and then the goods will be raised in the direc- 
 tion of B, thus saving the labor of moving them twice. 
 
 The wheel and axle, under different forms, is applied to a 
 variety of common purposes. 
 
 340. The capstan, in universal FIG. 66. 
 use, on board of ships, is an axle 
 
 placed upright, with a head, or 
 drum, A, Fig. 66, pierced with 
 holes for the levers B, C, D. The 
 weight is drawn by the rope, E, 
 passing two or three times round 
 the axle to prevent its slipping. 
 
 341. This is a very powerful 
 and convenient machine. When 
 not in use, the levers are taken out 
 
 Modified Wheel and Axle. 
 
 Capstan. 
 
 339. Describe Fig. 65, and point out the manner in which weights can be raised by 
 letting fall a rope over the pulley. 340. What is the capstan 1 Where is it chiefly 
 usedl 341. What are the peculiar advantages of this form of the wheel and axle? 
 
WHEEL AND AXLE. 
 
 83 
 
 Windlass. 
 
 of their places and laid aside, and when great force is required, 
 two or three men can push at each lever. 
 
 342. WINDLASS. The 
 common windlass for 
 drawing water is another 
 modification of the wheel 
 and axle. The winch, 
 or crank, by which it is 
 turned, is moved around 
 by the hand, and there 
 is no difference in the 
 principle, whether a 
 whole wheel is turned, 
 or a single spoke. The 
 winch, therefore, an- 
 swers to the wheel, while the rope is taken up, and the weight 
 raised by the axle, as already described. 
 
 In cases where great weights are to be raised, and it is 
 required that the machine should be as small as possible, on 
 account of room, the simple wheel and axle, modified as repre- 
 sented by Fig. 67, is sometimes used. 
 
 343. The axle may be considered in two parts, one of which 
 is larger than the other. The rope is attached by its two ends, 
 to the ends of the axle, as seen in the figure. The weight to 
 be raised is attached to a small pulley, around which the rope 
 passes. The elevation of the weight may be thus described. 
 Upon turning the axle, the rope is coiled around the larger part, 
 and, at the same time, it is thrown off the smaller part. At 
 every revolution, therefore, a portion of the rope will be drawn 
 up, equal to the circumference of the thicker part, and at the 
 same time a portion, equal to that of the thinner part, will be 
 let down. On the whole, then, one revolution of the machine 
 will shorten the rope where the weight is suspended, just as 
 much as the difference is between the circumference of the two 
 parts. 
 
 344. Illustration. Now to understand the principle on which 
 this machine acts, we must refer to Fig. 68, where it is obvious 
 that the two parts of the rope, A and B, equally support the 
 weight D, and that the rope, as the machine turns, passes from 
 the small part of the axle E to the large part H, consequently, 
 
 342. In the common windlass, what part answers to the wheel ? Explain Fig. 67 
 343. Wliy is the rope shortened, and the weight raised J 344 What is the design o. 
 Fig. 68 ? Does the weight rise perpendicular to the axis of motion ? 
 
84 
 
 WHEEL AND AXLE. 
 
 Windlass. 
 
 the weight does not rise in a perpendicular FIG. 68. 
 
 line toward C, the center of both, but in a 
 line between the outsides of the large and 
 small parts. 
 
 345. Let us consider what would be the 
 consequence of changing the rope A to the 
 larger part of the axle, so as to place the 
 weight in a line perpendicular to the axis of 
 motion. In this case, it is obvious that the 
 machine would be in equilibrium, since the 
 weight D would be divided between the two 
 sides equally, and the two arms of the lever 
 passing through the center C, would be of 
 equal length, and therefore no advantage 
 would be gained. 
 
 346. But in the actual arrangement, the weight being sus- 
 tained equally by the large and small parts, there is involved a 
 lever power, the long arm of which is equal to half the diameter 
 of the large part, while the short arm is equal to half the diam- 
 eter of the small part, the fulcrum being between them. 
 
 A Varying Power, producing a Constant Force. If a 
 power, varying under any given conditions, be required to over- 
 come a resistance which varies according to some other given 
 conditions, the one may be accommodated to the other by pro- 
 ducing a variation in the leverage, by which one or both acts. 
 
 347. This is done in the 
 
 mechanism of the watch, of, FIG. 69. 
 
 which A, Fig. 69, is the bar- 
 rel containing the power in 
 the form of a convoluted 
 spring, and B the fusee which 
 acts as a varying lever, and 
 through which motion is 
 conveyed to the hands of the 
 watch. 
 
 348. Now when the watch is first wound up, the main-spring 
 within the barrel is closely coiled, and of course acts with much 
 more power than afterward, when it is partly unrolled ; hence, 
 were no means used to equalize this power, every watch would 
 
 Barrel and Fusee. 
 
 345. Suppose the cylinder was. throughout, of the same size, what would be the 
 consequence? 3-)6. On what principle does this machine act 1 Which are the long 
 and short arms of the lever, and where is the fulcrum? 347. What is the main 
 spring of a watch ? 348. Where is it contained ? What is the fusee of a watch 1 
 What is its form 1 When does the main spring act with most force 1 
 
WHEEL AND AXLE. 
 
 85 
 
 run two or three times as fast, when first wound up, as 
 afterward. 
 
 349. We shall see that the fusee is a complete remedy for 
 the varying action of the main-spring. Its form is a low cone, 
 with its surface cut into a spiral groove, to receive the chain, 
 which runs round the barrel. Now when the watch is wound 
 up, by applying the key to the axis of the fusee at C, the main- 
 spring, one end of which is attached to the diameter of the 
 barrel, and the other to its axis, is closely coiled ; but as the ac- 
 tion begins on the smallest part of the fusee, the leverage is 
 short, and the power weak ; but as the fusee turns, and the 
 spring uncoils, the leverage increases in proportion as the strength 
 of the spring becomes weaker, and thus the two forces mutually 
 equalize each other, and the watch runs at the same rate until 
 the chain which connects them has run from the barrel to the 
 fusee, when it again requires winding, and the same process 
 begins again. 
 
 350. SYSTEM OF WHEELS. As the wheel and axle is only a 
 modification of the simple lever, so a system of wheels acting on 
 each other, and transmitting the power to the resistance, is only 
 another form of the compound lever. 
 
 351. Such a combina- 
 tion is shown in Fig. 70. 
 The first wheel, A, by 
 means of the teeth, or cogs, 
 around its axle, moves the 
 second wheel, B, with a 
 force equal to that of a 
 lever, the long arm of 
 which extends from the 
 center to the circumference 
 of the wheel, where the 
 power P is suspended, and 
 the short arm from the 
 same center to the ends of 
 the cogs. The dotted line 
 C, passing through the cen- 
 ter of the wheel A, shows the position of the lever, as the wheel 
 now stands. The center on which the wheel and axle turns, is 
 the fulcrum of this lever. As the wheel turns, the short arm 
 
 349. ITow does the fusee equali/.e this ferce 1 Explain how the forces of the spring 
 and fusee mutually equalize each other. 350. On what principle does a system of 
 wheels act, as represented in Fig. 701 351. Explain Fig. 70, and show how the 
 power P is transferred by the action of levers? 
 
 System of Wheels. 
 
86 WHEEL AND AXLE. 
 
 of this lever will act upon the long arm of the next lever by 
 means of the teeth on the circumference of the wheel B, and this 
 again through the teeth on the axle of B, will transmit its force 
 to the circumference of the wheel D, and so by the short arm 
 of the third lever to the weight W. As the power or small 
 weight falls, therefore, the resistance W, is raised, with the mul- 
 tiplied force of three levers acting on each other. 
 
 352. In respect to the force to be gained by such a machine, 
 suppose the number of teeth on the axle of the wheel A to be 
 six times less than the number of those on the circumference of 
 the wheel B, then B would only turn round once, while A turns 
 six times. And, in like manner, if the number of teeth on the 
 circumference of D, be six times greater than those on the axle 
 of B, then D would turn once, while B is turned six times. Thus 
 six revolutions of A would make B revolve once, and six revolu- 
 tions of B would make D revolve once. Therefore, A makes 
 thirty-six revolutions while D makes only one. 
 
 353. The diameter of the wheel A, being three times the 
 diameter of the axle of the wheel D, and its velocity of motion 
 being 36 to 1, 3 times 36 will give the weight which a power 
 of 1 pound at P would raise at W. Thus 36 X 3 = 108. One 
 pound at P would therefore balance 108 pounds at W. 
 
 354. No MACHINE CREATES FORCE. If the student has 
 attended closely to what has been said on mechanics, he will 
 now be prepared to understand, that no machine, however 
 simple or complex, can create the least degree of force. It is 
 true, that one man with a machine may apply a force which a 
 hundred could not exert with their hands, but then it would 
 take him a hundred times as long. 
 
 355. Suppose there are 20 blocks of stone to be moved a 
 hundred feet ; perhaps twenty men, by taking each a block, 
 would move them all in a minute. One man, with a capstan, 
 we will suppose, may move them all at once, but this man, with 
 his lever, would have to make one revolution for every foot he 
 drew the whole load toward him, and therefore to make one 
 hundred revolutions to perform the whole work. It will also 
 take him twenty times as long to do it, as it took the twenty 
 men. His task, indeed, would be more than twenty times 
 harder than that performed by the twenty men, for, in addition 
 to moving the stone, he would have the friction of the machinery 
 
 353. What weight will one pound at P balance at W? 354. Ts there any actual 
 power framed by the use of machinery? 355. Suppose twenty men to move twenty 
 Btonestoa certain distance with their hands, and one man moves them back to the 
 same place with a capstan, which performs the most actual labor ? Why ? 
 
WHEEL AND AXLE. 87 
 
 to .overcome, which commonly amounts to nearly one third of 
 the force employed. 
 
 356. Hence there would be an actual loss of power by the 
 use of the capstan, though it might be a convenience for the 
 one man to do his work by its means, rather than to call in 
 nineteen of his neighbors to assist him. 
 
 357. Any power by which a machine is moved, must be 
 equal to the resistance to be overcome, and, in all cases where 
 the power descends, there will be a proportion between the 
 velocity with which it moves downward, and the velocity with 
 which the weight moves upward. 
 
 358. There will be no difference in this respect, whether the 
 machine be simple or compound, for if its force be increased by 
 increasing the number of levers, or wheels, the velocity of the 
 moving power must also be increased, as that of the resistance 
 is diminished. 
 
 359. There being, then, always a proportion between the 
 velocity with which the moving force descends, and that with 
 which the weight ascends, whatever this proportion may be, it 
 is necessary that the power should have to the resistance the 
 same ratio that the velocity of the resistance has to the velocity 
 of the power. In other words, " The power multiplied by the 
 space through which it moves, in a vertical direction, must be 
 equal to the weight multiplied by the space through which it 
 moves in a vertical direction" 
 
 This law is known under FIG. 71. 
 
 the name of "the law of 
 virtual velocities," and is con- 
 sidered the golden rule of 
 mechanics. 
 
 360. This principle has al- 
 ready been explained, while / 
 treating of the lever, (312 ;) 
 
 but that the student should 
 want nothing to assist him in 
 clearly comprehending so im- 
 portant a law, we will again 
 
 illustrate it in a different Weight and Space. 
 
 manner. 
 
 356. Why, then, is machinery a convenience? 357 In the use of the lever, what 
 proportion is there between the force of the short arm, and the velocity of the long 
 arm ? Is it said, tnat the velocity of the power downward, must be in proportion to 
 that of the weurht upward ? 35S Doe* it make any difference, in this respect, 
 whether the machine be simple or compound 1 359. What is the golden rule of me 
 chanics ? Explain Fig. 71, and show how the rule is illustrated by it. 
 
88 PULLEY. 
 
 Suppose the lever, Fig. 71, to be thirty inches long from the 
 fulcrum to the point where the power, P, is suspended, and 
 that the weight. W, is two inches from the fulcrum. If the 
 power be 1 pound, the weight must be 15 pounds, to produce 
 equilibrium, and the power, P, must fall thirty inches to raise 
 the weight, W, two inches. Therefore, the power being 1 
 pound, and the space 30 inches, 30 x 1 =30. The weight being 
 15 pounds, and the space 2 inches,. 15 X 2 =30. 
 
 Thus, the power multiplied by the space through which it 
 falls, and the weight multiplied by the space through which 
 it rises, are equal. 
 
 361. However complex the machine may be, by which the 
 force of a descending power is transmitted to the weight to be 
 raised, the same rule will apply as it does to the action of the 
 simple lever. 
 
 THE PULLEY. 
 
 362. A pulley consists of a wheel which is grooved on the 
 edge, and which is made to turn on its axis, by a cord passing 
 over it. 
 
 363. Simple Pulley. Fig. 72,repre- 
 sents a simple pulley, with a single fixed 
 wheel. In other forms of the machine, 
 the wheel moves up and down with the 
 weight. 
 
 364. The pulley is arranged among 
 the simple mechanical powers ; but when 
 several are connected, the machine is 
 called a system of pulleys, or a com- 
 pound pulley. 
 
 365. One of the most obvious ad van- simple Pulley 
 tages, of the pulley is, its enabling men 
 
 to exert their own power in places where they can not go them- 
 selves. Thus, by means of a rope and wheel, a man can stand 
 on the deck of a ship, and hoist a weight to the topmast. 
 
 366. By means of two fixed pulleys, a weight may be raised 
 upward, while the power moves in a horizontal direction. The 
 weight will also rise vertically through the same space that the 
 rope is drawn horizontally. 
 
 361. What is said of the application of this rule to complex machines'? 362. What 
 is a pulley 1 363. What is a simple pulley 1 364. What is a system of pulleys, or a 
 compound pulley 1 365. What is the most obvious advantage of the pulley? 366 
 How must two fixed pulleys be placed to raise a weight vertically as far as the 
 power goes horizontally ? 
 
PULLEY. 
 
 89 
 
 Fig. 73, represents two fixed 
 pulleys, as they are arranged for 
 such a purpose. In the erection 
 of a lofty edifice, suppose the up- 
 per pulley to be suspended to 
 some part of the building ; then 
 a horse pulling at the rope, A, 
 would raise the weight, W, ver- 
 tically, as far as he went hori- 
 zontally. 
 
 367* In the use of the wheel 
 of the pulley, there is no mechan- 
 ical advantage, except that which 
 arises from removing the friction, 
 
 FIG. 73. 
 
 Simple PuOey. 
 
 FIG 
 
 and diminishing the imperfect flexibility of the rope. 
 
 In the mechanical effects of this machine, the result would 
 be the same did it slide on a smooth surface with the same 
 ease that its motion makes the wheel revolve. 
 
 368. The action of the pulley is on a dif- 
 ferent principle from that of the wheel and 
 axle. A system of wheels, as already ex- 
 plained, acts on the same principle as the 
 compound lever. But the mechanical effica- 
 cy of a system of pulleys is derived entirely 
 from the division of the weight among the 
 strings employed in suspending it - 
 
 369. In the use of the single fixed pul- 
 ley, there can be no mechanical advan- 
 tage, since the weight rises as fast as the' 
 power descends. This is obvious by Fig. 
 74, where it is also apparent that the power 
 and weight must be equal, to balance each 
 other, as already shown. 
 
 In the single movable pulley, Fig. 74, the 
 same rope passes from the fixed point, A, to 
 the power, P. It is evident here, that the weight is supported 
 equally by the two parts of the string between which it hangs. 
 Therefore, if we call the weight, W, ten pounds, five pounds 
 will be supported by one string, and five by the other. The 
 power, then, will support twice its own weight ; so that a per- 
 
 367. What is the advantage of the wheel of the pulley ? 368. How does the action 
 of the pulley differ from that of.the wheel and axle"? 369 Is there any mechanical 
 advantage in the fixed pulley 1 What weight at P, Fig. 74, will balance ten pounds 
 at W? 
 
 Movable Pulley. 
 
PULLET. 
 
 son pulling with a force of five pounds at P, will raise ten 
 pounds at W. The mechanical force, therefore, in respect to the 
 power, is as two to one. 
 
 In this example, it is supposed there are only two ropes, each 
 of which bears an equal part of the weight. 
 
 FIG. 75. 
 
 FIG. 76. 
 
 Compound Pulley. 
 
 System of Putteys. 
 
 370. Compound Pulley. If the number of ropes be in- 
 creased, the weight may be increased with the same power ; or 
 the power may be diminished in proportion- as the number of 
 ropes is increased. In Fig. 75, the number of ropes sustaining 
 the weight is four, and therefore, the weight may be four times 
 as great as the power. This principle must be evident, since it 
 is plain that each rope sustains an equal part of the weight. 
 The weight may, therefore, be considered as divided into four 
 parts, and each part sustained by one rope. 
 
 370. Suppose the number of ropes be increased, and the weight increased, must the 
 power be increased also ? Suppose the weight, Fig. 75, to be thirty-two pounds, 
 what will each rope bear 7 
 
PULLEY. 91 
 
 371. In Fig. 76, there is a system of pulleys represented, in 
 which the weight is sixteen times the power. 
 
 The tension of the rope, D E, is evidently equal to the power, 
 P, because it sustains it. D, being a movable pulley, must sus- 
 tain a weight equal to twice the power ; but the weight which 
 it sustains, is the tension of the second rope, D C. Hence, the 
 tension of the second rope is twice that of the first ; and, in 
 like manner, the tension of the third rope is twice that of the 
 second, and so on, the weight being equal to twice the tension 
 of the last rope. 
 
 372. Suppose the weight, W, to be sixteen pounds ; then the 
 two ropes, 8 and 8, would sustain 8 pounds each, this being 
 the whole weight divided equally between them. The next 
 two ropes, 4 and 4, would evidently sustain but half this whole 
 weight, because the other half is already sustained by a rope 
 fixed at its upper end. The next two ropes sustain but half of 
 4, for the same reason ; and the next pair, 1 and 1, for the 
 same reason, will sustain only half of 2. Lastly, the power, P, 
 will balance two pounds, because it sustains but half this weight, 
 the other half being sustained by the same rope, fixed at its 
 upper end. 
 
 It is evident that, in this system, each rope and pulley which 
 is added will double the effect of the whole. Thus, by adding 
 another rope and pulley beyond 8, the weight, W, might be 32 
 pounds, instead of 16, and still be balanced by the same power. 
 
 373. In our calculations of the effects of pulleys, we have 
 allowed nothing for the weight of the puHeys themselves, or for 
 the friction of the ropes. In practice, however, it will be found 
 that nearly one-third must be allowed for friction, and that the 
 power, therefore, to actually raise the weight must be about 
 one- third greater than has been allowed. 
 
 374. The pulley, like other machines, obeys the laws of 
 virtual velocities, already applied to the lever and wheel. Thus, 
 " in a system of pulleys, the ascent of the weight, or resistance, 
 is as muck less than the descent of the power as the weight is 
 greater than the poioer" If, as in the last example, the weight 
 is 1 6 pounds, and the power 1 pound, the weight will rise only 
 1 foot, while the power descends 16 feet. 
 
 371. Explain Fi?. 76, and show what part of the weight each rope pustains. and 
 why one pound at P, will balance sixteen pounds at W. 372. Explain the reason 
 why each additional rope and pulley will double the effect of the whole, or why its 
 weight may be double that of all the others with the same power. 373. In compound 
 machines, how much of the power must be allowed for the friction 1 374. What 
 general law applies to the pulley 7 
 
92 
 
 PULLED. 
 
 375. In the single fixed pulley, the weight and power are 
 equal, and, consequently, the weight rises as fast as the power 
 descends. 
 
 With such a pulley, a man may raise himself up to the mast- 
 head by his own weight. Suppose a rope is thrown over a 
 pulley, and a man ties one end of it round his body, and takes 
 the other end in his ha'nds ; he may raise himself up, because, 
 by pulling with his hands, he has the power of throwing more 
 of his weight on that side than on the other, and when he does 
 this, his body will rise. Thus, although the power and the 
 weight are the same individual, still the man can change his 
 center of gravity so as to make the power greater than the 
 weight, or the weight greater than the power, and thus can 
 elevate one half of his weight in succession. 
 
 WHITE 8 PULLEY. 
 
 376. In all the pulleys we have described, 
 there is a great detect, in consequence of the 
 different velocities at which the several wheels 
 turn, and the consequent friction to which 
 some of them are subjected. 
 
 377. It has been an object among mechan- 
 ical philosophers, to remedy this defect by 
 inventing a system of pulleys, the wheels of 
 which should all revolve on their axles in the 
 same time, each making the same number of 
 revolutions, notwithstanding the different 
 lengths of rope passing over them, and thus 
 avoid a defect common to those in use. 
 
 378. This object seems to have been fully 
 attained by Mr. James White, whose inven- 
 tion is represented by Fig. 77, and which will 
 be understood by the following description. 
 In order that the successive wheels should re- 
 volve in the same time, and their circumfer- 
 ences should be just equal to the length of 
 rope passing over them, Mr. White made 
 them all of different diameters. By this con- 
 struction, although the length of rope passing 
 over each was different, yet their revolutions 
 are equal, both with respect to time and num- 
 ber. 
 
 By this arrangement all the friction is 
 
 FIG. 77. 
 
 Whiles Pulley 
 
INCLINED PLANK. 
 
 93 
 
 Inclined Plane. 
 
 avoided, except that of a pivot at each end, and the lateral fric- 
 tion of a single wheel. A single rope sustains the whole, and as 
 in other systems, the weight is as many times the power as there 
 are ropes sustaining the lower block. This is considered the 
 most perfect system of pulleys yet inrented. 
 
 THE INCLINED PLANE. 
 
 379. This power, the most simple of all machines, consists of 
 a hard, smooth plane, inclined to the horizon in various degrees. 
 
 It is the fourth me- 
 chanical power, and is FIG - 78 - 
 represented by Fig. 78, 
 where from A to B is 
 the inclined plane ; the 
 line from D to A, is its 
 height, and that from 
 B to D, its base. 
 
 A board with one end 
 
 on the ground, and the other resting on a block, becomes an 
 inclined plane. 
 
 380. This machine being both useful and easily constructed, 
 is in very general use, especially where heavy bodies are to be 
 raised only to a small height. Thus a man, by means of an 
 inclined plane, which he can readily construct with a board, or 
 couple of bars, can raise a load into his wagon, which ten men 
 could not lift with their hands. 
 
 381. The power required to force a given weight up an in- 
 clined plane, is in proportion to its height, and the length of its 
 base, or, in other words, tJte force must be in proportion to the 
 rapidity of its inclination. 
 
 382. The power, P, nc - 79 - 
 Fig. 79, pulling a weight 
 
 up the inclined plane, 
 from C to D, only raises 
 it in an oblique direction 
 from E to D, by acting 
 along the whole length 
 
 Of the plane. If the Inc'ined Plane. 
 
 375. How may a man raise himself up by means of a rope and single fixed pulley ? 
 376. What is a great defect in the common pulley? 377. In what manner is it said 
 that the defect with respect to friction might be remedied ? 378. Describe White's 
 
 alley, and show how the defects in other "pulleys are remedied by this. 379. What 
 an inclined plane? 330. On what occasions is this power chiefly used? Suppose 
 a man wants to put a barrel ef cider into his wagon, how does he make an inclined 
 plane for this purpoee? 
 
94 
 
 INCLINED PLANE. 
 
 Inclined Plane. 
 
 plane be twice as long as it is high, that is, if the line from C 
 to D be double the length of that from E to D, then one pound 
 at P will balance two pounds any where between D and C. It 
 is evident, by a glance at this figure, that were the base length- 
 ened, the height from E to D being the same, a less power 
 at P would balance an equal weight any where on the inclined 
 plane ; and so, on the contrary, were the base made shorter, 
 that is, the plane more steep, the power must be increased in 
 proportion. 
 
 383. Suppose two in- FIG. so. 
 clined planes, Fig. 80, 
 
 of the same height, with 
 
 bases of different lengths; 
 
 then the weight and 
 
 power will be to each 
 
 other as the length of 
 
 the planes. If the length 
 
 from A to B is two feet, and that from B to C one foot, then 
 
 two pounds at D will balance four pounds at W, and so in this 
 
 proportion, whether the planes be longer or shorter. 
 
 384. The same principle, with respect to the virtual veloci- 
 ties of the weight and power, applies to the inclined plane, in 
 common with the other mechanical powers. 
 
 Suppose the inclin- 
 ed plane, Fig. 81, to FIG. 81. 
 be two feet from A to 
 B, and one foot from 
 C to B, then, as we 
 have already seen by 
 Fig. 79, a power of 
 one pound at P, would 
 balance a weight of 
 two pounds at W. 
 Now, in the fall of the 
 power to draw up the 
 weight, it is obvious 
 that its vertical de- 
 scent must be just twice the vertical ascent of the weight ; for 
 
 Indined Plane. 
 
 381. To roll a given weight up an inclined plane, to what must the force be pro- 
 portioned? 382. Explain Fig. 79. 383. If (he length of the long plane, Fig. 80, be 
 double that of the short one, what must be the proportion between the power and 
 the weight? 384. What is said of the application of the law of virtual velocities to 
 the inclined plane ? Explain Fig. 81, and show why the power must fall twice as far 
 as the weight rises. 
 
WEDGE. 95 
 
 the power must fall down the distance from A to B, to draw, 
 the weight that distance ; but the vertical height to which the 
 weight W is raised, is only from C to B. Thus the power, be- 
 ing two pounds, must fall two feet, to raise the weight, four 
 pounds, one foot ; and thus the power and weight, multiplied 
 by their several velocities, are equal. 
 
 When the power of an inclined plane is considered as a ma- 
 chine, it must therefore be estimated by the proportion which 
 the length bears to the height ; the power being increased in 
 proportion as the elevation of the plane is diminished. 
 
 385. APPLICATION TO ROADS. Hilly roads may be regarded 
 as inclined planes, and kteds drawn upon them in carriages, 
 considered in reference to the powers which draw them, are 
 subject to all the conditions which we have stated, with respect 
 to inclined planes. 
 
 The power required to draw a load up a hill, is in proportion 
 to the elevation of the inclined plane. On a road perfectly 
 horizontal, if the power is sufficient to overcome the friction, 
 and the resistance of the atmosphere, the carriage will move. 
 But if the road rise one foot in lifteen, besides these impedi- 
 ments, the moving power will have to lift one fifteenth part of 
 the load. 
 
 386. Now, where is there a section of country in which the 
 traveler is not vexed with roads, passing straight over hills, 
 when precisely the same distance would carry him around them 
 on a level plane ? To use a homely, but very pertinent illustra- 
 tion, " the bale of a pot is no longer, when it lies down, than 
 when it stands up." Had this simple fact been noticed, and its 
 practical bearing carried into effect by road makers, many a 
 high hill would have been shunned for a circuit around its base, 
 and many a poor horse, could he speak, would thank the wis- 
 dom of such a decision. 
 
 THE WEDGE. 
 
 387. The next simple mechanical power is the wedge. This 
 instrument may be considered as two inclined planes, placed 
 base to base. 
 
 It is much employed for the purpose of splitting or dividing 
 solid bodies, such as wood and stone. 
 
 385. How do the principles of the inclined plane apply to roads 1 386. What i& 
 said about the bale of a pot, as applied to road making? 387. On what principle 
 does the wedge act ? In what case is this power useful 1 
 
96 
 
 SCREW. 
 
 . 388. Fig. 82 represents such a wedge as is FIG - 82 - 
 usually employed in cleaving timber. This in- 
 strument is also used in raising ships, and pre- 
 paring them to launch, and for a variety of other 
 purposes. Nails, awls, needles, and many cut- 
 ting instruments, act, more or less, on the prin- 
 ciple of this machine. 
 
 389. There is much difficulty in estimating 
 the power of the wedge, since this depends on 
 the force, or the number of blows given it, to- 
 gether with the obliquity of its sides. A wedge 
 of great obliquity would require hard blows to 
 drive it forward, for the same reason that a 
 plane, much inclined, requires much force to roll 
 a heavy body up it. But were the obliquity of 
 the wedge, and the force of each blow given, still it would be 
 difficult to ascertain the exact power of the wedge in ordinary 
 cases, for, in the splitting of timber and stone, for instance, the 
 divided parts act as levers, and thus greatly increase the power 
 of the wedge. Thus, in a log of wood, six feet long, when split 
 one half of its length, the other half is divided with ease, be- 
 cause the two parts act as levers, the lengths of which con- 
 stantly increase, as the cleft extends from the wedge. 
 
 Wedge. 
 
 THE SCREW. 
 
 FIG. 
 
 390. The screw is the sixth and last simple mechanical power. 
 It may be considered as a modification of the inclined plane, or 
 as a winding wedge. 
 
 391. It is an inclined plane run- 
 ning spirally round a spindle, as will 
 be seen by Fig. 83. Suppose a to 
 be a piece of paper, cut into the form 
 of an inclined plane and rolled round 
 the piece of wood d ; its edge would 
 form the spiral line, called the thread 
 of the screw. If the finger be placed 
 between the two threads of a screw, 
 and the screw be turned round once, 
 
 the finger will be raised upward equal to the distance of the two 
 
 Winding Wedge. 
 
 388. What common instruments act on the principle of the wedge? 389. What 
 difficulty is there in estimating the | ower of the wedge? 390. On what principle 
 does the screw act 1 391. How is it shown that the screw is a modification of the in- 
 clined plane? 
 
SCREW. 97 
 
 threads apart. In this manner, the finger is raised up the 
 inclined plane, as it runs round the cylinder. 
 
 The power of the screw is transmitted and emp'oyed by 
 means of another screw called the nut, through which t passes. 
 This has a spiral groove running through it, which exactly fits 
 the thread of the screw. 
 
 If the nut is fixed, the screw itself, on turning it roun.. 1 , ad- 
 vances forward ; but if the screw is fixed, the nut, when tui'ied, 
 advances along the screw. 
 
 392. Fig. 84 represents the first kind of screw, being such 
 as is commonly used in pressing paper, and other substances. 
 The nut, N, through which the screw passes, answers also for 
 one of the beams of the press. If the screw be turned to u e 
 right, it will advance downward, while the nut stands still. 
 
 FIG. 84. 
 
 Nut Fixed. 
 
 Screw Fixed. 
 
 393. A screw of the second kind is represented by Fig. 85. 
 In this, the screw is fixed, while the nut, N, by being turned by 
 the lever, L, from right to left, will advance down the screw. 
 
 394. In practice, the screw is never used as a simple me- 
 chanical machine ; the power being always applied by means 
 of a lever, passing through the head of the screw, as in Fig. 
 84, or into the nut, as in Fig. 85. 
 
 395. POWER OF THE SCREW. The screw acts with the corn- 
 lined power of the inclined plane and the lever, and its force is 
 
 392. Explain Fi. 84. Which is the screw, and which the nut ? Which way must 
 the screw be turned to make it advance through the nut. 1 393. How does the screw, 
 Fig. 84, differ from Fig. 85 ? 394. Is the screw ever used as a simple machine ] By 
 what simple power is it moved ? 395. What two simple mechanical powers ar 
 concerned in the force of the screw ? 
 
08 SCREW. 
 
 such as to be limited only by the strength of the materials of 
 which it is made. 
 
 In investigating the effects of this machine, we must, there- 
 fore, take into account both these simple mechanical powers, so 
 that the screw now becomes really a compound engine. 
 
 396. In the inclined plane, we have already seen, that the 
 less it is inclined, the more easy is the ascent up it. In apply- 
 ing the same principle to the screw, it is obvious, that the 
 greater the distance of the threads from each other, the more 
 rapid the inclination, and consequently, the greater must be 
 the power to turn it, under a given weight. On the contrary, 
 if t> e thread inclines but slightly, it will turn with less power, 
 fr the same reason that a man can roll a heavy weight up a 
 plane but little inclined. Therefore, the finer the screw, or the 
 nearer the threads to each other, the greater will be the pres- 
 sure under a given power. 
 
 397. Let us- suppose two screws, the one having the threads 
 one inch apart, and the other half an inch apart; then the 
 force which the first screw will give with the same power at the 
 lever, will be only half that given by the second. The second 
 screw must be turne<i twice as many times round as the first, to 
 go through the same space ; but what is lost in velocity is gained 
 in power. At the lever of the first, two men would raise a 
 given weight to a given height, by making one revolution ; 
 while at the lever of the second, one man would raise the same 
 weight to the same height, by making two revolutions. 
 
 398. It is apparent that the length of the inclined plane, up 
 which a body moves in one revolution, is the circumference of 
 the screw, and its height the interval between the threads. 
 The proportion of its power would therefore be "as the circum- 
 ference of the screw, to the distance between the threads, so is 
 the weight to the power.''' 
 
 399. By this rule the power of the screw alone can be found ; 
 but as this machine is moved by means of the lever, we must 
 estimate its force by the combined power of both. In this case, 
 the circumference described by the end of the lever employed, 
 is taken, instead of the circumference of the screw itself. The 
 means by which the force of the screw may be found, is there- 
 fore, by multiplying the circumference which the lever describes 
 by the power. 
 
 396. Why does the nearness of the threads make a difference in the force of the. 
 screw? 307. Suppose one screw, with its threads one inch apart, and another half 
 an inch apart, what will be their difference in force? 398. What is the length of the 
 inclined plane, up whi*h a body moves by one revolution of the screw ? 
 
SCREW. 
 
 400. Thus, " the power multiplied by the cirwnfe rente which it 
 describes, is equal to the weight or resistance,^xuj$c$aj[ by the 
 distance between the two contiguous threads^jjOiiite the 
 efficacy of the screw may be increased, by increasingrfte length 
 of the'lever, or by diminishing the distance between the threads. 
 If, then, we know the length of the lever, the distance between 
 the threads, and the weight to be raised, we can readily calcu- 
 late the power ; or, the power being given, and the distance of 
 the threads and the length of the lever known, we can estimate 
 the weight the screw will raise. 
 
 401. "Thus, suppose the length of the lever to be forty inches, 
 the distance of the threads one inch, and the weight 8000 
 pounds ; required, the power, at the end of the lever, to raise 
 the weight. . 
 
 The lever being 40 inches, the diameter of the circle, whicli 
 the end describes, is 80 inches. The circumference is a little 
 more than three times the diameter, but we will call it just 
 three times. Then, 80x3 = 240 inches, the circumference of 
 the circle. The distance of the threads is 1 inch, and the weight 
 8000 pounds. To find the power, multiply the weight by the 
 distance of the threads, and divide by the circumference of the 
 circle. Thus, 
 
 Circum. In. Weight. Power. 
 
 240 1 : : 8000 : 33* 
 
 The power at the end of the lever must therefore be 33* 
 pounds. In practice, this power would require to be increased 
 about one-third, on account of friction. 
 
 402. PERPETUAL SCREW. The force of the screw is some- 
 times employed to turn a wheel, by acting on its teeth. In this 
 case it is called the perpetual screw. 
 
 403. Fig. 86 represents such a machine. It is apparent, 
 that by turning the crank C, the wheel will revolve, for the 
 thread of the screw passes between the cogs of the wheel. By 
 means of an axle, through the center of this wheel, like the 
 common wheel and axle, this becomes an exceedingly powerful 
 machine, but like all other contrivances for obtaining great 
 power, its effective motion is exceedingly slow. It has, how- 
 ever, some disadvantages, and particularly the great friction be- 
 
 400. How is the force of the screw estimated ? How may the efficacy of the screw 
 be increased ? 401. The length of the lever, the distance between the threads, and 
 the weiht being known, how can the power be found? Give an example. 402. 
 What is the screw called when it is employed to turn a wheel? 403. Explain Fig 
 86. What is the objection to this machine for raising weights? 
 
100 
 
 SCREW. 
 
 tween the thread of the screw and FIG. 86. 
 
 the teeth of the wheel, which pre- 
 vents it from being generally em- 
 ployed to raise weights. 
 
 404. ALL THESE MECHANICAL 
 
 POWERS RESOLVED INTO THREE. 
 
 We have now enumerated and de- 
 scribed all the mechanical powers 
 usually denominated simple. They 
 are six in number, namely, the 
 Lever, Wheel and Axle, Pulley, 
 Wedge, Inclined Plane, and Screw. screw and Wheel. 
 
 405. In respect to the principles 
 
 o-n which they act, they may be resolved into three simple 
 powers, namely, the lever, the inclined plane, and the pulley ; 
 for it has been shown that the wheel and axle is only another 
 form of the lever, and that the screw is but a modification of 
 the inclined plane. 
 
 It is surprising, indeed, that these simple powers can be so 
 arranged and modified, as to produce the different actions in all 
 that vast variety of intricate machinery which men have in- 
 vented and constructed. 
 
 406. CARD MACHINE. The variety of motions we witness in 
 the little engine which makes cards, by being supplied with 
 wire for the teeth, and strips of leather to stick them through, 
 would itself seem to involve more mechanical powers than those 
 enumerated. This engine takes the wire from a reel ; bends it 
 into the form of teeth ; cuts it off; makes two holes in the 
 leather for the tooth to pass through ; sticks it through ; then gives 
 it another bend on the opposite side of the leather ; graduates 
 the spaces between the rows of teeth, and between one tooth 
 and another ; and, at the same time, carries the leather back- 
 ward and forward, before the point where the teeth are intro- 
 duced, with a motion so exactly corresponding with the motions 
 of the parts which make and stick the teeth, as not to produce 
 the difference of a hair's breadth in the distance between them. 
 
 All this is done without the aid of human hands, any further 
 than to put the leather in its place, and turn a crank ; or, in 
 some instances, many of these machines are turned at once, by 
 means of three or four dogs, walking on an inclined plane which 
 revolves. 
 
 404. How many simple mechanical powers are there, and what are they called 7 
 405. How can they be resolved into three simple powers 1 406. What is said of the 
 card-making machine ? 
 
SCREW. 101 
 
 407. Such a machine displays the wonderful ingenuity and 
 perseverance of man, and at first sight would seem to set at 
 naught the idea that the lever and wheel are the chief simple 
 powers concerned in its motions. But when these motions are 
 examined singly and deliberately, we are soon convinced that 
 the wheel variously modified, is the principal mechanical power 
 in the whole engine." 
 
 408. USE OF MACHINERY. It has already been stated, (354,) 
 that notwithstanding the vast deal of time and ingenuity which 
 men have spent on the construction of machinery, and in 
 attempting to multiply their powers, there has, as yet, been 
 none produced, in which the power was not obtained at the 
 expense of velocity, or velocity at the expense of power ; and, 
 therefore, no actual force is ever generated by machinery. 
 
 When men employ the natural elements as a power to over- 
 come resistance by means of machinery, there is a vast saving 
 of animal labor. Thus mills, and all kinds of engines, which 
 are kept in motion by the power of water, or wind, or steam, 
 save animal .labor equal to the power it takes to keep them in 
 motion. 
 
 409. Fine Mechanical Powers in one Machine. An engineer, 
 it is said, for the purpose of drawing a ship out of the water to 
 be repaired, combined the mechanical powers represented by 
 Fig. 87, and perhaps no machine ever constructed gives greater 
 force with so small a power. 
 
 FIG. 87. 
 
 The Fire Mechanical Powers. 
 
 It involves the lever A, wheel and axle B, the pulley C, the 
 inclined plane D, and the screw E. 
 
 407. What are the chief mechanical powers concerned in its motions 1 408. Is 
 there any actual force generated by machinery I What is said of employing the 
 natural elements as a power 1 409. What are the five mechauieal powers employed 
 hi Fig. 87 1 Point out on the cut the place of each nower. 
 
102 HYDROSTATICS. 
 
 To estimate the force of this engine, it is necessary to know 
 the length of the lever, diameter of the wheel, &c. 
 
 Suppose then, the sizes of the different powers are as fol- 
 lows, viz. : 
 
 Length of the lever A, 18 inches. 
 
 Distance of the thread E, 1 inch. 
 
 Diameter of the wheel B, 4 feet. 
 
 Diameter of the axle, 1 foot. 
 
 Pulleys C and D, D fixed, 4 strings. 
 
 Height of the plane D, one-half its length, . . 2 feet. 
 
 Suppose the man turns the lever A, with the power equal to 
 100 pounds, the force on the ship would thus be found, for the 
 different laws and rules referring to each mechanical power. 
 
 1. One hundred pounds on the lever A, would be- 
 come a force by means of the screw on the wheel Pounds. 
 
 B of 11,309.76 
 
 2. Diameter of wheel four times that of the axle, . 4 
 
 45,239.04 
 
 3. The number of pulley strings, 4 
 
 180,956.16 
 
 4. Height of the inclined plane half its lenth, . . 2 
 
 361,912.32 
 
 The force on the ship therefore would be equal to 361,912 
 pounds, or about 161 tons. 
 
 CHAPTER V. 
 
 HYDROSTATICS. 
 
 410. Hydrostatics is the science which treats of the weight, 
 pressure, and equilibrium of water, or other fluids, when in a 
 state of rest. 
 
 411. Hydraulics is that part of the science of fluids which 
 treats of water in motion, and the means of raising and con- 
 ducting it in pipes, or otherwise, for all sorts of purposes. 
 
 409. What must be known to estimate the power of this machine? What is the 
 amount of force on the ship? 410. What is hydrostatics 1 411. How does hydraulics 
 differ from hydrostatics 1 
 
HYDROSTATICS. 103 
 
 The subject of water at rest, will first claim investigation, 
 since the laws which regulate its motion will be best understood 
 by first comprehending those which regulate its pressure. 
 
 412. A fluid is a substance whose particles are easily moved 
 among each other, as air and water. 
 
 413. The air is called an elastic fluid, because it is easily 
 compressed into a smaller bulk, and returns again to its original 
 state when the pressure is removed. Water is called a non- 
 elastic fluid, because it admits of little diminution of bulk under 
 pressure. 
 
 414. The non-elastic fluids are perhaps more properly called 
 liquids, but both terms are employed to signify water and othei 
 bodies possessing its mechanical properties. The term fluid, 
 when applied to the air, has the word elastic before it. 
 
 415. One of the most obvious properties of fluids, is the 
 facility with which they yield to the impressions of other bodies, 
 and the rapidity with which they recover their former state, 
 when the pressure is removed. The cause of this, is the free- 
 dom with which their particles slide over, or among each other; 
 their cohesive attraction being so slight as to be overcome by 
 the least impression. On this want of cohesion among their 
 particles seems to depend the peculiar mechanical properties of 
 these bodies. 
 
 416. In solids, there is such a connection between the parti- 
 cles, that if one part moves, the other part must move also. 
 But in fluids, one portion of the mass may be in motion, while 
 the other is at rest. In solids, the pressure is always downward, 
 or toward the center of the earth's gravity ; but in fluids, the 
 particles seem to act on each other as wedges, and hence, when 
 confined, the pressure is sideways, and even upward, as well as 
 downward. 
 
 417. Elasticity of Water. Water has commonly been called 
 a non-elastic substance, but it is found that under great pressure 
 its volume is slightly diminished, and hence it is proved to be 
 elastic. The most decisive experiments on this subject were 
 made many years ago by Mr. Perkins. 
 
 418. These experiments were made by means of a hollow 
 cylinder, Fig. 88, which was closed at the bottom, and made 
 water-tight at the top, by a cap, screwed on. Through this 
 
 412. What is a fluid 1 413 What is an elastic fluid ? Why is air called an elastic 
 flu (1 ? 414 What substances are called liquids? 415. What is one of the most ob- 
 vious properties of liquids? 416. On what do the peculiar mechanical properties of 
 fluids depend ? In what respect does the pressure of a fluid diflVr from that of a 
 solid ? 417. Is water an elastic, or a non-elastic fluid J 413. Describe Fig. 88, and 
 show how water was found to be elastic. 
 
Water 
 
 104 PRESSURE OF WATER. 
 
 cap, at A, passed the rod B, which was five-sixteenths FIG. s& 
 
 of an inch in diameter. The rod was so nicely fitted 
 
 to the cap, as also to be water-tight. Around the 
 
 rod at C, there was placed a flexible ring, which could 
 
 be easily pushed up or down, but fitted so closely as 
 
 to remain on any part where it was placed. 
 
 A cannon of sufficient size to receive this cylinder, 
 which was three inches in diameter, was furnished 
 with a strong cap and forcing pump, and set verti- 
 cally into the ground. The cannon and cylinder 
 were next filled with water, and the cylinder, with 
 its rod drawn out, and the ring placed down to the 
 cap, as in the figure, was plunged into the cannon. 
 The water in the cannon was then subjected to an 
 immense pressure by means of the forcing pump, Elastic. 
 after which, on examination of the apparatus, it was 
 found that the ring C, instead of being where it was placed, w^i 
 eight inches up the rod. The water in the cylinder being com- 
 pressed into a smaller space, by the pressure of that in the can- 
 non, the rod was driven in, while under pressure, but was forced 
 out again by the expansion of the water, when the pressure 
 was removed. Thus, the ring on the rod would indicate the 
 distance to which it had been forced in, during the greatest 
 pressure. 
 
 419. This experiment proved that water, under the pressure 
 of one thousand atmospheres, that is, the weight of 15,000 
 pounds to the square inch, was reduced in bulk about one part 
 iir 24. 
 
 So slight a degree of elasticity under such immense pressure, 
 is not appreciable under ordinary circumstance, and therefore 
 in practice, or in cpmrnon experiments on this fluid, water is 
 considered as non-elastic. 
 
 EQUAL PRESSURE OF WATER. 
 
 420. The particles of water, and other fluids, when confined, 
 press on the vessel which confines them, in all directions, both 
 upward, downward, and sideways. 
 
 From this property of fluids, together with their weight, very 
 unexpected and surprising effects are produced. 
 
 The effect of this property, which we shall first examine, is, 
 
 419. In what proportion does the bulk of water diminish under a pressure of 15,000 
 pounds to the square inch 1 In common experiments, is water considered elastic, 
 or non-elastic? 420. When water is confined, in what direction does it press 1 
 
PRESSURE OF WATER. 
 
 105 
 
 FIG. 89. 
 
 that a quantity of water, however small, will balance another 
 quantity, however large. Such a proposition at first thought 
 might seem very improbable. But on examination, we shall 
 find that an experiment with a very simple apparatus will con- 
 vince any one of its truth. Indeed, we every day see this prin- 
 ciple established by actual experiment, as will be seen directly. 
 
 421. Fig. 89, represents a common 
 coffee-pot, supposed to be filled up to the 
 dotted line A, with a decoction of coffee, 
 or any other liquid. The coffee, we know, 
 stands exactly at the same height, both in 
 the body of the pot, and in its spout. 
 Therefore, the small quantity in the spout, 
 balances the large quantity in the pot, or 
 presses with the same force downward, as 
 that in the body of the pot presses up- 
 ward. This is obviously true, otherwise, the large quantity 
 would sink below the dotted line, while that in the spout would 
 rise above it, and run over. 
 
 422. The same principle is more strik- 
 ingly illustrated by Fig. 90. 
 
 Suppose the cistern A to be capable of 
 holding one hundred gallons, and into its 
 bottom there be fitted the tube B, bent, 
 as seen in the figure, and capable of con- 
 taining one gallon. The top of the cis- 
 tern, and that of the tube, being open, 
 pour water into the tube at C, and it will 
 rise up through the perpendicular bend 
 into the cistern, and if the process be con- 
 tinued, the cistern will be filled by pour- 
 ing water into the tube. Now it is plain, 
 
 that the gallon of water in the tube presses against the hun- 
 dred gallons in the cistern, with a force equal to the pressure of 
 the hundred gallons, otherwise, that in the tube would be forced 
 upward higher than that in the cistern, whereas, we find that 
 the surfaces of both stand exactly at the same height. 
 
 423. From these experiments we learn, " that the pressure 
 of a Jluid is not in proportion to its quantity, but to its height, 
 
 Coffee-Pot. 
 
 FIG. 90. 
 
 Pressure of Water. 
 
 421. now does the experiment with the coffee-pot show that a small quantity of 
 liquid will balance a large one! 422. Explain Fig. 90, and show how the pressure 
 in the tube is equal to the pressure in the cistern. 423. What conclusion, or gen- 
 eral truth, is to be drawn from these experiments ? 
 
 5* 
 
106 
 
 PRESSURE OF WATER. 
 
 and that a large quantity of water in an open vessel, presses 
 with no more force than a small quantity of the same height?' 
 424. Pressure equal in Vessels of all Sizes and Shapes. 
 The size or shape of a vessel is of no consequence, for if a num- 
 ber of vessels, differing entirely from each other in figure, posi- 
 tion, and capacity, have a communication made between them, 
 and one be filled with water, the surface of the fluid, in all, will 
 be at the same elevation. If, therefore, the water stands at an 
 e^ual height in all, the pressure in one must be just equal to 
 that in another, and so equal to that in all the others. 
 
 Equal Pressure of Water. 
 
 425. To make this obvious, suppose a number of vessels, of 
 different shapes and sizes, as represented by Fig. 91, to have a 
 communication between them, by means of a small tube, pass- 
 ing from the one to the other. If, now, one of these vessels be 
 filled with water, or if water be poured into the tube A, all the 
 other vessels will be filled at the same instant, up to the line 
 B C. Therefore, the pressure of the water in A, balances that 
 in 1, 2, 3, &c., while the pressure in each of these vessels is 
 equal to that in the other, and so an equilibrium is produced 
 throughout the whole series. 
 
 426. If an ounce of water be poured into the tube A, it will 
 produce a pressure on the contents of all the other vessels, equal 
 to the pressure of all the others on the tube : for, it will force 
 the water in all the other vessels to rise upward to an equal 
 height to that in the tube itself. Hence, we must conclude, 
 that the pressure in each vessel is not only equal to that in any 
 
 424. What difference does the shape or size of a vessel make in respect to the pres- 
 sure of a fluid on ite bottom? 425. Explain Fisr. 91, and show how the equilibrium 
 is produced. 426. Suppose an ounce of water be poured into the tube A, what will 
 be its effect on th contents of the other vessels 1 
 
PRESSURE OF WATER. 
 
 107 
 
 FIG. 92. 
 
 of the others, but also th^t the pressure in any one is equal to 
 that in all the others. 
 
 427. From this, we learn that the shape or size of a vessel 
 has no influence on the pressure of its liquid contents, but that 
 the pressure of water is as its height, whether the quantity be 
 great or small. We learn, also, that in no case will the weight 
 of a quantity of liquid, however large, force another quantity, 
 however small, above the level of its own surface. 
 
 428. Now, by other experiments, it is ascertained, that the 
 pressure of a liquid is in proportion to its height, and the area 
 of its base. 
 
 Suppose a vessel, ten feet high, and two 
 feet in diameter, such as is represented at 
 A, Fig. 92, to be filled with water ; there 
 would be a certain amount of pressure, at 
 C, near the bottom. Let D represent an- 
 other vessel, of the same diameter at the 
 bottom, but only a foot high, and closed 
 at the top. Now if a small tube, the fourth 
 of an inch in diameter, be inserted into th,e 
 cover of this vessel, and the tube be car- 
 ried to the height of the vessel A, and then 
 the vessel and tube be filled with water, 
 the pressure on the bottoms and sides of 
 both vessels at the same height will be 
 equal, and jets of water starting from D 
 and C will have exactly the same force, 
 and spout to the same distance. 
 
 This might at first seem improbable, but to convince our- 
 selves of its truth, we have only to consider, that any impres- 
 sion made on one portion of the confined fluid in the vessel D, 
 is instantly communicated to the whole mass. Therefore, the 
 water in the tube B, presses with the same force on every other 
 portion of the water in D, as it does on that small portion over 
 which it stands. 
 
 429. Bursting a Cask. This principle is illustrated in a 
 very striking manner, by the experiment, which has often been 
 made, of bursting a common wine cask with a few ounces of 
 water. 
 
 427. What conclusion is to be drawn from pouring the ounce of water into the tube 
 A 1 What is the reason that a large quantity of water will not ibrce a small quantity 
 above its own level ? Is the force of water in proportion to its height, or its quan- 
 tity ? 428 How is a small quantity of water shown to press equal to a large quantity 
 by Fig. 92 7 429. Explain the reason why the pressure is as great at D, as at O. 
 
108 
 
 PRESSURE OF WATER. 
 
 FIG. 93. 
 
 Suppose A, Fig. 93, to be such a cask, already 
 filled with water, and suppose the tube B, fifty 
 feet high, to be screwed, water-tight, into 'its 
 head. When water is poured into the tube, 
 so as to fill it gradually, the cask will show in- 
 creasing signs of pressure, by emitting the 
 water through the pores of the wood, and be- 
 tween the joints ; and, finally, as the tube is 
 filled, the cask will burst asunder. 
 
 430. The same apparatus will serve to illus- 
 trate the upward pressure of water ; for, if a 
 small stop-cock be fitted to the upper head, on 
 turning this, when the tube is filled, a jet of 
 water will spirt up with a force, and to a height, 
 that will astonish all who never before saw such 
 an experiment. 
 
 In theory, the water will spout to the same 
 height with that which gives the pressure, but, Bursting a Cask 
 in practice, it is found to fall short in the fol- 
 lowing proportions : 
 
 431. If the tube be twenty feet high, and the orifice for the 
 jet half an inch in diameter, the water will spout nearly nine- 
 teen feet. If the tube be fifty feet high, the jet will rise up- 
 ward of forty feet, and if a hundred feet, it will rise above eighty 
 feet. It is understood, in every case, that the tubes are to be 
 kept fall of water. 
 
 The height of these jets shows the astonishing effects that a 
 small quantity of fluid produces when pressing from a perpen- 
 dicular elevation. 
 
 432. HYDROSTATIC PARADOX. This paradox, illustrated by 
 Fig. 94, consists in experimental proof of the principle already 
 insisted on, that water presses according to its height, and not 
 to its quantity. Fill a glass jar with water, and balance it on 
 the scale-beam F, E, with small weights. Then pour out the 
 water, leaving only an inch or two deep, letting the balance 
 weights remain. Replacing the jar, which will now stand 
 higher than before, owing to the loss of water, introduce into 
 it, by means of the crane, II, a piece of wood a few lines smaller 
 in all directions than the inside of the jar. The wood being 
 
 How is the same principle illustrated by Fig. 93 1 430. How may Fijr. 93 be made 
 to illustrate the upward pressure of water! 431. Under the pressure of a column 
 of water twenty feet high, what will be the height of the jet, I Under a pressure of a 
 hundred feet, how high will it rise ? 432. What does the hydrostatic paradox show 
 Explain by the figure how the experiment is made. 
 
PRESSURE OF WATBB. 109 
 
 FIG. 91 
 
 Hydrostatic Paradox. 
 
 adjusted by means of the thumb-screw, so that the water is 
 made to rise around it exactly to the brim, or as high as it 
 stood before any was poured out, (the wood not touching the 
 glass,) and it will be found that it will exactly balance the 
 weights, as it did when full of water, though it now contains 
 only a tenth as much as before. 
 
 The result will be the same if. instead of the wood, the same 
 bulk of cork or lead be placed in the jar, the only point being, 
 that, in each case, the water should rise to the same height. 
 
 The above experiment proves, in a very striking manner, that 
 the pressure of water is as its height ; and the reason why it 
 makes no difference in the result whether the body placed in 
 the jar be of wood, cork, or lead is, that the solid merely takes 
 the place of the fluid, displacing its own bulk, and thus the 
 weight remains just as though the water itself had remained in 
 the jar. Thus, the pressure of a tenth part of the water, of 
 equal height, equals the whole. 
 
 433. Proof by Mercury. In addition to the above proofs, 
 that a small, will balance a large quantity of water, we add the 
 following, perhaps the most satisfactory of all. 
 
 Let A, B, C, Fig. 95, represent a glass tube, having at A, a 
 collar cemented to the glass, into which vessels of different ca- 
 pacities and shapes, may be screwed. The tube is first filled 
 with mercury up to the level of the dotted line A C, and the 
 tube G jt?, fitted in its place. The vessel D, is then screwed 
 into A, and water is poured in as far as A, the base of the column 
 
 433. Explain Fig. 95, and show in what manner different quantities of water will 
 balance the same weight of mercury. 
 
110 
 
 PRESSURE OF WATER. 
 
 Proof by Mercury. 
 
 FIG. 96. 
 
 of water being, as seen, '' IQ - 
 
 equal to that of the mer- 
 cury. The fluid metal 
 will rise, by the pressure 
 of the water on A, up to 
 p in the small tube. Then 
 unscrew D, and in its 
 place fix the conical ves- 
 sel E, and pour in water 
 as before, and the same 
 result will follow, and so 
 with the small tube F; 
 in each case, the height 
 of the water, notwithstanding the difference in quantity, will 
 force the mercury to exactly the same elevation. 
 
 434. HYDROSTATIC BELLOWS. An instrument called the 
 hydrostatic bellows, also shows, in a striking manner, the great 
 force of a small quantity of water, pressing in a perpendicular 
 direction. 
 
 This instrument consists of two 
 boards, connected together with strong 
 leather, in the manner of the common 
 bellows. It is then furnished with a 
 tube A, Fig. 96, which communicates 
 between the two boards. A person 
 standing on the upper board may raise 
 himself up by pouring water into the 
 tube. If the tube holds an ounce of 
 water, and has an area equal to a 
 thousandth part of the area of the top 
 of the bellows, one ounce of water in 
 the tube will balance a thousand ounces 
 placed on the bellows. 
 
 435. HYDROSTATIC PRESS. This 
 property of water was applied by Mr. 
 
 Bramah, to the construction of his hydrostatic . press. But 
 instead of a high tube of water, which in most cases could not 
 be so readily obtained, he substituted a strong forcing-pump, 
 and instead of the leather bellows, a metallic pump, barrel, and 
 piston. 
 
 434. What is the hydrostatic bellows! What property of water is this instrument 
 designed to show? 435. Explain Fig. 97. Where is the piston? Which is the 
 pump-barrel in which it wftrks 1 
 
 Hydrostatic Bellows. 
 
PRESSURE OF WATER. 
 
 Ill 
 
 Hydrostatic Press. 
 
 This arrangement will be . FIG. 97. 
 
 understood by Fig. 97, where 
 the pump-barrel, A, B, is repre- 
 sented as divided lengthwise, 
 in order to show the inside. 
 The piston, C, is fitted so ac- 
 curately to the barrel, as to 
 work up and down water-tight ; 
 both ban-el and piston being 
 made of iron. The thing to 
 be broken or pressed, is laid 
 on the flat surface, I, there be- 
 ing above this, a strong frame 
 
 to meet the pressure, not shown in the figure. The small 
 forcing-pump, of which D is the piston, and H, the lever by 
 which it is worked, is also made of iron. 
 
 Now, suppose the space between the small piston and the 
 large one, at W, to be filled with water, then, on forcing down 
 the small piston, D, there will be a pressure against the large 
 piston, C, the whole force of which will be in proportion as the 
 aperture in which C works, is greater than that in which D 
 works. 
 
 436. If the piston, D, is half an inch in diameter, and the 
 piston, C, one foot in diameter, then the pressure on C will be 576 
 times greater than that on D. Therefore, if we suppose the 
 pressure of the small piston to be one ton, the large piston 
 will be forced up against any resistance, with a pressure equal 
 to the weight of 576 tons. 
 
 437. It would be easy for a single man to give the pressure 
 of a ton at D, by means of the lever, and, therefore, a man, with 
 this engine, would be able to exert a force equal to the weight 
 of near 600 tons. 
 
 438. It is evident that the force to be obtained by this prin- 
 ciple, can only be limited by the strength of the materials of 
 which the engine is made. Thus, if a pressure of two tons be 
 given to a piston, the diameter of which is only a quarter of an 
 inch, the force transmitted to the other piston, if three feet in 
 diameter, would be upward of 40,000 tons ; but such a force 
 
 436 In the hydrostatic press, what is the proportion between the pressure given 
 by the small piston, and the force exerted on the Jarge one 1 437. What is the esti- 
 mated force which a man could s\\e by one of these ensines 1 ? 433. If the pressure 
 of two tons be made on a piston of a quarter of an inch in diameter, what will be the 
 force transmitted to the other piston of three feet in diameter? 
 
112 PRESSURE OF WATER. 
 
 is much too great for the strength of any material with which 
 we are acquainted. 
 
 A small quantity of water, extending to a great elevation, 
 would give the pressure above described, it being only for the 
 sake of convenience, that the forcing-pump is employed instead 
 of a column of water. 
 
 439. Rupture of a Mountain. There is no doubt, but in the 
 operations of nature, great effects are sometimes produced amor g 
 mountains, by a small quantity of water finding its way to a 
 reservoir in the crevices of the rocks far beneath. 
 
 FIG. 98. 
 
 Rupture of a Mountain. 
 
 Suppose, in the interior of a mountain, at A, Fig. 98, there 
 should be a space of ten yards square, and an inch deep, filled 
 with water, and closed up on all sides ; and suppose that, in the 
 course of time, a small fissure, no more than an inch in diam- 
 eter, should be opened by the water, from the height of two 
 hundred feet above, down to this little reservoir. The conse- 
 quence might be, that the side of the mountain would burst 
 asunder, for the pressure, under the circumstances supposed, 
 would be equal to the weight of five thousand tons. 
 
 440. Pressure on Vessels with Oblique Sides. It is obvious, 
 that, in a vessel, the sides of which are every where perpendic- 
 ular to each other, the pressure on the bottom will be as 
 the height, and that the pressure on the sides will every where 
 be equal,, at an equal depth of the liquid. 
 
 But it is not so obvious, that in a vessel having oblique sides, 
 
 439. What is paid of the pressure of water in the crevices of mountains and its 
 effects! 440. What is the pressure on the bottom of a vessel containing a fluid equal 
 to 7 Suppose the sides of the vessel slope outward, what effect does this produce on 
 the pressure 1 
 
PRESSURE OP WATER. 113 
 
 that is, diverging outward from the bottom, or converging from 
 the bottom toward the top, in what manner the pressure will 
 be sustained. 
 
 441. Now, the pressure on the bottom of any vessel, no mat- 
 ter what the shape may be, is equal to the height of the fluid, 
 and the area of the bottom, (428.) 
 
 Hence the pressure on 
 
 the bottom of the vessel FIG. 99. 
 
 sloping outward, Fig. 99, \^^^___-__-f^---- - -- -- ^/ 
 
 will be just equal to what ^ f 
 
 it would be, were the sides gjf 
 
 perpendicular, and the same 
 
 Would be the Case did the Pressure on Diverging Sides. 
 
 sides slope inward instead 
 cf outward. 
 
 In a vessel of this shape, the sides sustain a pressure equal to 
 the perpendicular height of the fluid, above any given point. 
 Thus, if the point 1 sustain a pressure of one pound, 2, being 
 twice as far below the surface, will have a pressure equal to two 
 pounds, and so in this proportion with respect to the other eight 
 parts marked on the side of the vessel. On the contrary, did 
 the sides of the vessel slope inward instead of outward, still the 
 same consequences ensue, the vertical height in both cases mak- 
 ing the pressure equal. For although in the latter, the eleva- 
 tion is not above the point of pressure, the effect is the same in 
 each case. 
 
 PRESSURE OF WATER IN POUNDS, AT VARIOUS DEPTHS. 
 
 442. The weight of a cubic inch of water at the temperature 
 of 62, is the 0.036065 fraction of a pound. A column of wa- 
 ter one foot high, being twelve times the above, would there- 
 fore be 0.4328 pounds. 
 
 443. Xow a square foot is 144 square inches, and therefore 
 the pressure, or weight, of a square foot of water will be found 
 by multiplying the above fraction by 144, which gives 62.3232. 
 ?>r nearly 62 and a third pounds. Omitting the decimals, a 
 cubic foot of water is commonly estimated at 62 pounds. 
 
 441. On the contrary, did the sides of the vessel slope inward exactly the same 
 amount of pressure according to Ihe height, what would be the result 1 442. What 
 is the weight of a cubic inch of water ? 443. What is the weight of a cubic foot of 
 water 7 
 
 10* 
 
114 
 
 WATER LEVEL. 
 
 The following table, founded on the above estimates, may be 
 useful in determining the pressure of water in pipes or other 
 vessels, of known depth. 
 
 DEPTH IN FEET. 
 
 PRESSURE PER SQUARE 
 INCH. 
 
 PRESSURE PER SQUARE 
 FOOT. 
 
 
 
 Pounds. 
 
 Pounds. 
 
 1 
 
 0.4328 
 
 62.3232 
 
 2 
 
 0.8656 
 
 124.6464 
 
 3 
 
 1.2984 
 
 186,9696 
 
 4 
 
 1.7312 
 
 249.2928 
 
 5 
 
 2.1640 
 
 311.6160 
 
 6 
 
 2.5968 
 
 373.9392 
 
 7 
 
 3.0296 
 
 436.2624 
 
 8 
 
 3.4624 
 
 498.5856 
 
 9 
 
 3,8952 
 
 560.9088 
 
 10 
 
 4.3280 
 
 623.2320 
 
 Suppose it is required to know the pressure on the bottom 
 of a vessel of water, 1 foot square and 20 feet deep, then it is 
 found by doubling that of 10 feet deep, thus 623.2320x2 = 
 1246.464 pounds. The pressure on a tube equal to an inch 
 square, and of an equal depth, is found by substituting inches for 
 feet, as above seen. 
 
 WATER LEVEL. 
 
 444. We have seen, that in whatever situation water is 
 placed, it always tends to seek a level. Thus, if several vessels 
 communicating with each other be filled with water, the fluid 
 will be at the same height in all, and the level will be indica- 
 ted by a straight line drawn through all the vessels as in Fig. 91. 
 
 It is on the principle of this tendency that the little instru- 
 ment called the water level is constructed. 
 
 445. Let A, Fig. 100, be a straight glass tube having two 
 legs, or two other glass tubes rising from each end at right- 
 angles. Let the tube A, and a part of the legs, be filled with 
 mercury or some other liquid, and on the surfaces, a 6, of the 
 liquid, let floats be placed, carrying upright wires, to the ends 
 of which are attached sights at 1, 2. These sights are repre- 
 sented by 3, 4, and consist of two fine threads, or hairs, stretched 
 
 445. Explain by Fig. 100, how an exact line may be obtained by adjusting th 
 lights. 
 
WATER LEVEL. 
 FIG. 100. 
 
 Improved Water Level. 
 
 at right-angles across a square, and are placed at right-angles to 
 the length of the instrument. 
 
 They are so adjusted that the point where the hairs intersect 
 each other, shall be at equal heights abore the floats. This ad- 
 justment may be made in the following manner : 
 
 Let the eye be placed behind one of the sights, looking 
 through it at the other, so as to make the points, where the 
 hairs intersect, cover each other, and let some distant object, 
 covered by this point, be observed. Then let the instrument 
 be reversed, and let the points of intersection of the hairs be 
 viewed in the same way, so as to cover each other. If they are 
 observed to cover the same distant object as before, they will 
 be of equal heights above the surfaces of the liquid. But, if the 
 same distant points be not observed in the direction of these 
 points, then one or the other of the sights must be raised or 
 lowered, by an adjustment provided for that purpose, until the 
 points of intersection be brought to correspond. The points 
 will then be properly adjusted, and the line passing through 
 them will be exactly "horizontal. All points seen in the direc- 
 tion of the sights will be on the level of the instrument. 
 
 44fi. The principles on which this adjustment depends are 
 easily explained : if the intersections of the hairs be at the same 
 distance from the floats, the line joining these intersections will 
 evidently be parallel to the lines joining the surfaces a, 6, of the 
 
 446. Explain the principle on which the water level with sights is constructed. 
 
116 SPECIFIC GRAVITY. 
 
 liquid, and will therefore be level. But if one of these points 
 be more distant from the floats than the other, the line joining 
 the intersections will point upward if viewed from the lower 
 sight, and downward if viewed from the higher one. 
 
 The accuracy of the results of this instrument, will be greatly 
 increased by lengthening the tube A. 
 
 447. Spirit Level The common G m - 
 spirit level consists of a glass tube, a 
 
 Fig. 101, filled with spirit of wine, ex- <*}~" }* 
 
 cepting a small space in which there 
 
 is left a bubble of air. This bubble, spirit Level. 
 
 when the instrument is laid on a level 
 
 surface, will be exactly in the middle of the tube, and therefore, 
 
 to adjust a level, it is only necessary to bring the bubble to this 
 
 position. 
 
 The glass tube is inclosed in a brass case, which is cut out 
 on the upper side, so that the bubble may be seen, as repre- 
 sented in the figure. 
 
 448. This instrument is employed by builders to level their 
 work, and is highly convenient for that purpose, since it is only 
 necessary to -lay it on a beam to try its level. 
 
 SPECIFIC GRAVITY. 
 
 449. If a tumbler be filled with water to the brim, and an 
 egg, or any other heavy solid, be dropped into it, a quantity of 
 the jluid, exactly equal to the size of the egg, or other solid, will 
 be displaced, and will flow over the side of the vessel. Bodies 
 which sink in water, therefore, displace a quantity of the fluid 
 equal to their own bulks. 
 
 450. Now, it is found by experiment, that when any solid 
 substance sinks in water, it loses, while in the fluid, a portion of 
 its weight, just equal to that of the bulk of water which it dis- 
 places. This is readily made evident by experiment. 
 
 451. Take a piece of ivory, or any other substance that will 
 sink in water, and weigh it accurately in the usual manner ; 
 then suspend it by a thread, or hair, in the empty cup A, Fig. 
 102, and balance it, as shown in the figure. Now pour water 
 into the cup, and it will be found that the suspended body will 
 lose a part of its weight, so that a certain number of grains 
 
 447. Describe the common spirit level, and the method of using it. 448. What is 
 the use of the level 7 449. How much water will an eg* displace? 450. How much 
 less will a cubic inch of any substance weigh in water than in air ] 451. How is it 
 proved by Fig. 102, that a body weighs less in water than in air 1 
 
SPECIFIC GRAVITY. 
 
 117 
 
 Weighing in Water. 
 
 must be taken from the 
 opposite scale, in order to 
 make the scales balance as 
 before the water was pour- 
 ed in. The number of 
 grains taken from the op- 
 posite scale, show the 
 weight of a quantity of 
 water equal to the bulk of 
 the body so suspended. 
 
 452. It is on the prin- 
 ciple, that bodies weigh less 
 in the water than they do 
 when weighed out of it, or 
 in the air, that water be- 
 comes the means of ascertaining their specific gravities, for it is 
 by comparing the weight of a body in the water, with what it 
 weighs out of it, that its specific gravity is determined. 
 
 Thus, suppose a cutfic inch of gold weighs 19 ounces, and on 
 being weighed in water, weighs only 18 ounces, or loses a nine- 
 teenth part of its weight, it will prove that gold, bulk for bulk, 
 is nineteen times heavier than water, and thus 19 would be the 
 specific gravity of gold. And so if a cube of copper weigh 9 
 ounces in the air, and only 8 ounces in the water, then copper, 
 bulk for bulk, is 9 times as heavy as water, and therefore has a 
 specific gravity of 9. 
 
 453. If the body weighs less, bulk for bulk, than water, it is 
 obvious that it will not sink in it, and therefore weights must 
 be added to the lighter body, to ascertain how much less it 
 weighs than water. 
 
 The specific gravity of a body, then, is merely its weight 
 compared with the same bulk of water ; and water is thus made 
 the standard by which the weights of all other bodies are 
 compared. 
 
 454. How to take the Specific Gravity. To take the specific 
 gravity of a solid which sinks in water, first weigh the body in 
 the usual manner, and note down the number of grains it 
 weighs; then, with a hair, or fine thread, suspend it from the 
 bottom of the scale-dish, in a vessel of water, as represented by 
 Fig. 102. As it weighs less in water, weights must be added 
 to the side of the scale where the body is suspended, until they 
 
 452. On what principle are specific gravities found? 453. What is the specific 
 gravity of a body 7 454. How are the specific gravities of solid bodies taken ? 
 
118 
 
 SPECIFIC GRAVITY. 
 
 exactly balance each other. Next, note down the number of 
 grains so added, and they will show the difference between the 
 weight of the body in air and in water. 
 
 455. It is obvious that the greater the specific gravity of the 
 body, the less, comparatively, will be this difference, because 
 each body displaces only its own bulk of water, and some bodies 
 of the same bulk will weigh many times more than others. 
 
 456. For example, suppose that a piece of platina, weighing 
 22 ounces, will displace an ounce of water, while a piece of 
 silver, weighing 22 ounces, will displace two ounces of water. 
 The platina, therefore, when suspended as above described, will 
 require one ounce to make the scales balance, while the same 
 weight of silver will require two ounces for the same purpose. 
 The platina, therefore, bulk for bulk, will weigh twice as much 
 as the silver, and will have twice as much specific gravity. 
 
 Having noted down the difference between the weight of the 
 body in air and in water, as above explained, the specific gravity 
 is found by dividing the weight in air by the loss in water. 
 The greater the loss, therefore, the less will be the specific 
 gravity, the bulk being the same. 
 
 457. Thus, in the above example, 22 ounces of platina was 
 supposed to lose one ounce in water, while 22 ounces of silver 
 lost two ounces in water. Now, 22 divided by 1, the loss of 
 the platina, is 22 ; and 22 divided by 2, the loss in the silver, 
 is 11. So that the specific gravity of platina is 22, while that 
 of silver is 11. The specific gravities of these metals are, how- 
 ever, a little less than here estimated. 
 
 458. TABLE OF SPECIFIC GRAVITIES. 
 
 Antimony, 7 
 
 Zinc, 7 
 
 Cast Iron, 7 
 
 Tin, 8 
 
 Cobalt, 8 
 
 Steel, 8 
 
 Copper, 9 
 
 Bismuth, 10 
 
 Silver, 10 
 
 Lead, 11 
 
 Gold, 19 
 
 Platinum, 20 
 
 " hammered, ... 22 
 
 Mercury, 14 
 
 Agate, 2* 
 
 Sulphur, 2 
 
 Glass, crown, 2$ 
 
 " flint, 3i 
 
 Rock, crystal, 24 
 
 Marble, 24 
 
 Diamonds, 3i 
 
 Ruby, (oriental,) 4i 
 
 This table being intended for common use, the fractions are 
 omitted, and the nearest round numbers only given. 
 
 455 Why does a heavy body weigh comparatively less in the water than a lijrht 
 one? 456 Having taken the difference between the weight of a body in air and in 
 water, by what rule is its specific gravity found? 457. Give the example stated, and 
 show how the difference between the specific gravities of platina and silver is found- 
 
HYDROMETER. 119 
 
 HYDROMETER. 
 
 459. The hydrometer is an instrument by which the specific 
 gravities of fluids are ascertained by the depth to which the in- 
 strument sinks below their surfaces. 
 
 460. Suppose a cubic inch of lead loses, when weighed in 
 water, 253 grains, and, when weighed in alcohol, only 209 
 grains, then, according to the principle already recited, a cubic 
 inch of water actually weighs 253, and a cubic inch of alcohol 
 209 grains, for when a body is weighed in a fluid, it loses just 
 the weight of the fluid it displaces. 
 
 461. Water, as we have already seen, (453,) is the standard 
 by which the weights of other bodies are compared, and by 
 ascertaining what a given bulk of any substance weighs in wa- 
 ter, and then what it weighs in any other fluid, the compara- 
 tive weight of water and this- fluid will be known. For if, as in 
 the above example, a certain bulk of water weighs 253 grains, 
 and the same bulk of alcohol only 209 grains, then alcohol has 
 a specific gravity nearly one-fourth less than water. 
 
 462. It is on this principle that the hydrom- 
 eter is constructed. It is composed of a hoi- 
 low ball of glass, or metal, with a graduated 
 scale rising from its upper part, and a weight 
 on its under part, which serves to balance it in 
 the fluid. 
 
 Such an instrument is represented by Fig. 
 103, of which B is the graduated scale, and A, 
 the weight, the hollow ball being between 
 them. 
 
 463. To prepare this instrument for use, 
 weights in grains, or half-grains, are put into 
 the little cup, A, until the scale is carried down 
 so that a certain mark on it coincides exactly 
 with the surface of the water. This mark, 
 
 then, becomes the standard of comparison be- Hydrometer. 
 tween water and any other liquid in which the 
 hydrometer is placed. 
 
 464. If plunged into a fluid lighter than water, it will sink 
 below the mark, and, consequently, the fluid will rise higher on 
 
 459. What is the hydrometer ? 460. Suppose a cubic inch of any substance weighs 
 253 grains less in water than in air. what is the actual weight of a" cubic inch of wa- 
 ter! 461. On what principle is the hydrometer founded ? 462. How is this instru- 
 ment formed ? 463. How is the hydrometer prepared for use ? 464. How is it 
 known by this instrument whether the lluid is lighter or heavier than wafer 7 
 
120 SIPHON. 
 
 the scale. If the fluid is heavier than water, the scale will rise 
 above the surface in proportion, and thus it is ascertained in a 
 moment whether any fluid has a greater or less specific gravity 
 than water. 
 
 To know precisely how much the fluid varies from the 
 standard, the scale is marked off into degrees, which indicate 
 grains by weightj so that it is ascertained very exactly how much 
 the specific gravity of one fluid differs from that of another. 
 
 465. Water being the standard by which the weights of 
 other substances are compared, it is placed as the unit, or point 
 of comparison, and is, therefore, 1, '10, 100, or 1000, the 
 ciphers being added whenever there are fractional parts ex- 
 pressing the specific gravity of the body. It is always under- 
 stood, therefore, that the specific gravity of water is 1 ; and 
 when it is said a body has a specific gravity of 2, it is only 
 meant that such a body is, bulk for bulk, twice as heavy as 
 water. 
 
 466. If the substance is lighter than water, it has a specific 
 gravity of 0, with a fractional part. Thus, alcohol has a specific 
 gravity of 0.809, that is, 809, water being 1000. 
 
 467. By means of this instrument, it can be told with great 
 accuracy how much water has been added to spirits, for the 
 greater the quantity of water, the higher will the scale rise 
 above the surface. 
 
 The adulteration of milk with water, can also be readily de- 
 tected with it, for as new milk has a specific gravity of 1032, 
 water being 1000, a very small quantity of water mixed with 
 it would be indicated by the instrument. 
 
 THE SIPHON. 
 
 468. Take a tube bent like the letter U, and, having filled it 
 with water, place a finger on each end, and in this state plunge 
 one of the ends into a vessel of 'water, so that the end in the 
 water shall be a little the highest ; then remove the fingers and 
 the liquid will flow out, and continue to do so until the vessel 
 is exhausted. 
 
 469. A tube acting in this manner is called a siphon, and is 
 
 465. What is the standard by which the weights of other bodies are compared ? 
 What is the specific gravity of water? When it is said that the specific gravity of a 
 borly is 2, or 4, what meaning is intended to be conveyed? 466. If alcohol has a 
 specific gravity of 809; what, in reference to this, is the specific gravity of water? 
 467. In what cases will the hydrometer detect fraud ? 468. In what manner is a 
 Biphon made 1 469. Explain the reason why the water ascends through one leg of 
 .he siphon, and descends through the other 1 
 
BIPHON. 
 
 121 
 
 represented by Fig. 104. The reason FIG. lot 
 
 why the water flows from the end of 
 
 the tube, A, and, consequently, ascends 
 
 through the other part, is, that there 
 
 is a greater weight of the fluid from B 
 
 to A, than from C to B, because the 
 
 perpendicular height from B to A, is 
 
 the greatest. The weight of the water 
 
 from B to A, falling downward by its 
 
 gravity, tends to form a vacuum, or 
 
 void space, in that leg of the tube ; but 
 
 the pressure of the atmosphere on the 
 
 water in the vessel, constantly forces Siphon. 
 
 the fluid up the other leg of the tube, 
 
 to fill the void space, and thus the stream is continued as long 
 
 as any water remains in the vessel. 
 
 470. Ajjplicatian of the Siphon.- The siphon is employed 
 in draining mines, when there is a sufficient fall in the vicinity : 
 it may also be used to convey water over a hill, provided the 
 place where it is wanted is a foot or two lower than the fountain. 
 
 Application of the Siphon. 
 
 For this purpose, let A be a spring, Fig. 105, situated be 
 hind a hill, and it is desired to bring the water to B for family 
 use. To do this, a lead tube, with a stop-cock at C, is carried 
 over the hill, having also a stop-cock at each end. This done, 
 and the two ends being closed, fill the two legs of the tube by 
 pouring in water at C ; then C being closed, let one person open 
 the stop-cock at B, and a moment after, open that at A, and 
 the water will instantly begin to flow from the spring to the 
 reservoir, and if C is kept closed, will continue to run so long 
 as the fountain furnishes water. 
 
 470. Explain by Fig. 105, how the siphon o/oveys water over a hilL 
 
 6 
 
122 
 
 INTERMITTING SPRINGS. 
 
 The principle of the siphon has been explained under 
 Fig. 104. 
 
 INTERMITTING SPRINGS. 
 
 471. The action of the siphon depends upon the same prin- 
 ciple as the action of the pump, namely, the pressure of the 
 atmosphere, and, therefore, its explanation properly belongs to 
 Pneumatics. It is introduced here merely for the purpose of 
 illustrating the phenomena of intermitting springs, a subject 
 which belongs to Hydrostatics. 
 
 Some springs, situated on the sides of mountains, flow, for a 
 while, with great violence, and then cease entirely. After a 
 time they begin to flow again, and then suddenly stop, as be- 
 fore. These are called intermitting springs. Among ignorant 
 and superstitious people, these strange appearances have been 
 attributed to witchcraft, or the influence of some supernatural 
 power. But an acquaintance with the laws of nature will dis- 
 sipate such ill-founded opinions, by showing that they owe their 
 peculiarities to nothing more than natural siphons, existing in 
 the mountains from whence the water flows. 
 
 FIG. 106. 
 
 Intermitting Spring. 
 
 472. Fig. 106 is the section of a mountain and spring, show- 
 ing how the principle of the siphon operates to produce the 
 effect described. Suppose there is a crevice, or hollow, in the 
 rock from A to B, and a narrow fissure leading from it, in 
 the form of the siphon, B C. The water from the rills F E, 
 
 471. What is an intermitting spring 1 472, How is the phenomenon of the inter 
 mitting spring explained 1 
 
HYDRAULICS. 123 
 
 filling the hollow, up to the line A D, it will then discharge 
 itself through the siphon, and continue to run until the water 
 is exhausted down to the leg of the siphon B, when it will 
 cease. Then the water from the rills continuing to run until 
 the hollow is again filled up to the same line, the siphon again 
 begins to act, and again discharges the contents of the reservoir 
 as before, and thus the spring P, at oi*e moment flows with 
 great violence and the next moment ceases entirely. 
 
 473. The hollow, above the line A D, is supposed not to be 
 filled with the water at all, since the siphon begins to act when- 
 ever the fluid rises up to the bend D. 
 
 During the dry seasons of the year, it is obvious, that such a 
 spring would cease to flow entirely, and would begin again only 
 when the water from the mountain filled the cavity through 
 the rills. 
 
 CHAPTER VI. 
 
 HYDRAULICS. 
 
 474. It has been stated, (410,) thai Hydrostatics is that 
 branch of Natural Philosophy, which treats of the weight, pres- 
 sure, and 'equilibrium of fluids, and that Hydraulics has for 
 its object, the investigation of the laws which regulate fluids in 
 motion. 
 
 If the pupil has learned the principles on which the pres- 
 sure and equilibrium of fluids depend, as explained under the 
 former article, he will now be prepared to understand the laws 
 which govern fluids when in motion. 
 
 475. The pressure of water downward, is in the same pro- 
 portion to its height, as is the pressure of solids in the same 
 direction. 
 
 476. Suppose a vessel of three inches in diameter has a billet 
 of wood set up in it, so as to touch only the bottom, and sup- 
 pose the piece of wood to be three feet long, and to weigh nine 
 pounds ; then the pressure on the bottom of the vessel will be 
 
 472. Explain Fi2. 106, and show the reason why such a spring will flow and cease 
 lo flow, alternately. 474. How does the science of Hydrostatics differ from that of 
 Hydraulics ? 475. Does the downward pressure of water differ from the downward 
 pressure of solids, in proportion 1 476. How is the downward pressure of water 
 illustrated 1 
 
124 
 
 HYDRAULICS. 
 
 nine pounds. If another billet of wood be set on this, of the 
 same dimensions, it will press on its top with the weight of 
 nine pounds, and the pressure at the bottom will be eighteen 
 pounds, and if a like billet be set on this, the pressure at the 
 bottom will be twenty-seven pounds, and so on, in this ratio, to 
 any height the column is carried. 
 
 Now the pressure cf fluids is in the same proportion; and 
 when confined in pipes, may be considered as one short column 
 set on another, each of which increases the pressure of the 
 lowest, in proportion to their number and height. 
 
 477. If a vessel, 
 
 Fig. 107, be filled FIG - 107 
 
 with water,and three 
 apertures be made 
 in its side at E F G, 
 the fluid will be 
 thrown out in jets, 
 falling to the earth 
 in the curved lines 
 shown. The reason 
 why these curves 
 differ in shape, is, 
 that the fluid is act- 
 ed on by two forces, 
 namely,- the pres- 
 sure of the water 
 above the jet, which produces its velocity forward, and the ac- 
 tion of gravity, which impels it downward. It therefore obeys 
 the same laws that solids do when projected forward, and falls 
 down in curved lines, the shapes of which depend on their rela- 
 tive velocities, (246.) 
 
 478. The quantity of water discharged, being in proportion 
 to the pressure, when the orifices are the same, that discharged 
 from each orifice will differ in quantity, according to the height 
 of the water above it. 
 
 479. It is found, however, that the velocity with which a 
 vessel discharges its contents, does not depend entirely on the 
 pressure, but in part on the kind of orifice through which the 
 liquid flows. It might be expected, for instance, that a tin v^es- 
 
 Velocity and Gravity. 
 
 477. Why do the lines described by the jets from the vessel, Fig. 107, -differ in 
 shape? What two forces act upon the fluid as it is discharged, and how do these 
 forces produce a curved line? 478. In what proportion do the orifices discharge the 
 fluid 7 479. Does th velocity with which a lluid is discharged, depend entirely on 
 thepressuro? 
 
HYDRAULICS. 125 
 
 sel of a gwen capacity, with an orifice of, say an inch in diam- 
 eter, would part with its contents^ sooner than another of the 
 same capacity and orifice, whose side was an inch or two thick, 
 since the friction through the tin might be considered much 
 less than that presented by the other orifice. 
 
 480. But it has been found, by experiment, that the tin ves- 
 sel does not part with its contents so soon as another vessel, of 
 the sarne height and size of orifice, from which the water flowed 
 through a short pipe. And, on varying the length of these 
 pipes, it is found that the most rapid discharge, other circum- 
 stances being equal, is through a pipe, whose length is twice the 
 diameter of its orifice. Such an aperture discharged 82 quarts, 
 in the same time that another vessel of tin, without the pipe, 
 discharged 62 quarts. 
 
 481. This surprising difference is accounted for, by supposing 
 that the cross currents, made by the rushing of the water from 
 different directions toward the orifice, mutually interfere with 
 each other, by which the whole is broken, and thrown into con- 
 fusion by the sharp edge of the tin, and hence the water issues 
 in the form of spray, or of a screw, from such an orifice. A 
 short pipe seems to correct this contention among opposing 
 currents, and to smooth the passage of the whole, and hence 
 we may observe, that from such a pipe, the stream is round and 
 well defined. 
 
 482. Proportion between the Pressure and the Velocity of 
 Discharge. If a small orifice be made in the side of a vessel 
 filled with any liquid, the liquid will flow out with a force and 
 velocity equal to the pressure which the liquid before exerted 
 on that portion of the side of the vessel before the orifice was 
 made. 
 
 Now, as the pressure of fluids is as their heights, it follows, 
 that if several such orifices are made, the lowest will discharge 
 the greatest, while the highest will discharge the least quantity 
 of the fluid. 
 
 The velocity of discharge, in the several orifices of such a 
 vessel, will show a remarkable coincidence between the ratio 
 of increase in the quantity of liquid, and the increased ve- 
 locity of a falling body. 
 
 480. What circumstance, besides pressure, facilitates the discharge of water from 
 an orifice ? In a tube discharging water with the greatest velocity, what is the pro- 
 portion between its diameter and its length ] What is the proportion between the 
 quantity of fluid discharged through an orifice of tin and through a short pipe ? 481. 
 How is this difference explained } 482. What are the proportions between the ve- 
 locities of discharge and the heights of the orifices, as above explained 7 
 
126 
 
 HYDRAULICS. 
 
 483. Thus, if the tall vessel, Fig. 108, 
 of equal dimensions throughout, be filled 
 with the water, and a small orifice be 
 made at one inch from the top, or below 
 the surface, as at 1 ; and another at 2, 4 
 inches below this ; another at 9 inches ; 
 a fourth at 16 inches ; and a fifth at 25 
 inches ; then the velocities of discharge, 
 from these several orifices, will be in pro- 
 portion of 1, 2, 3, 4, 5. 
 
 484. To make this more obvious, we 
 will place the expressions of the several 
 velocities in the upper line of the following 
 table, the lower numbers expressing the 
 depths of the several orifices. 
 
 FIG. 108. 
 
 -i 
 
 Velocity of Discharge. 
 
 Velocity,. ... 
 Depth, ..... 
 
 41 51 61 71 81 91 10 
 16 25 36 49 64 81 100 
 
 Thus it appears, as in falling bodies, that to produce a two- 
 fold velocity a fourfold height is necessary. To obtain a three- 
 fold velocity of discharge, a ninefold height is required, and for 
 a fourfold velocity, sixteen times the height, and so in this pro- 
 portion, as shown by the table, (111.) 
 
 In order to establish the fact, that the velocity with which a 
 liquid spouts from an orifice, is equal to the velocity which 
 a body would acquire in falling unobstructed from the^surface 
 of the liquid to the depth of the orifice, it is only necessary to 
 prove the truth of the principle in any one particular case. 
 
 Now it is manifestly true, if the orifice be presented down- 
 ward, and the column of fluid over it be of small height, then 
 this indefinitely small column will drop out of the orifice by the 
 mere effect of its own weight, and, therefore, with the same 
 velocity as any other falling body ; but as fluids transmit pres- 
 sure in all directions, the same effect will be produced, whatever 
 may be the direction of the orifice. 
 
 FRICTION BETWEEN SOLIDS AND FLUIDS. 
 
 485. The rapidity with which water flows through pipes of 
 the same diameter, is found to depend much on the nature 
 
 483. If in Fig. 108. orifices are made at the distance of 1.4, 9, 16, and 25 inches from 
 the top, then in what ratio of velocity will the water be discharged 1 484. How is it 
 proved that the velocity of the spouting liquid is equal to that of a falling body ? 485. 
 Suppose a lead and a glass tube, of the same diameter ; which will deliver the greatest 
 quantity of liquid in the same time 7 Why will a glass tube deliver most 7 
 
SOLIDS AND FLUIDS. 127 
 
 of their internal surfaces. Thus a lead pipe, with a smooth 
 aperture, under the same circumstances, will convey much more 
 water than one of wood, where the surface is rough, or beset 
 with points. In pipes, even where the surface is as smooth as 
 glass, there is still considerable friction, for in all cases, the wa- 
 ter is found to pass more rapidly in the middle^f the stream 
 than it does on the outside, where it rubs against the sides of 
 the tube. 
 
 486. The sudden turns, or angles of a pipe, are also found to 
 be a considerable obstacle to the rapid conveyance of the water, 
 for such angles throw the fluid into eddies or currents by which 
 its velocity is arrested. 
 
 In practice, therefore, sudden turns are generally avoided, and 
 where it is necessary that the pipe should change its direction, 
 it is done by means of as large a circle as convenient. 
 
 48*7. Water in Pipes. Where it is proposed to convey a 
 certain quantity of water to a considerable distance in pipes, 
 there will be a great disappointment in respect to the quantity 
 actually delivered, unless the engineer takes into account the 
 friction, and the turnings of the pipes, and makes large allow- 
 ances for these circumstances. K the quantity to be actually 
 delivered ought to fill a two-inch pipe, one of three inches will 
 not be too great an allowance, if the water is to be conveyed to 
 any considerable distance. 
 
 In practice, it will be found that a pipe of two inches in diam- 
 eter, one hundred feet long, will discharge about five times as 
 much water as one of one inch in diameter of the same length, 
 and under the same pressure. 
 
 488. This difference is accounted for, by supposing that both 
 tubes retard the motion of the fluid, by friction, at equal dis- 
 tances from their inner surfaces, and consequently that the effect 
 of this cause is much greater in proportion, in a small tube, 
 than in a large one. 
 
 489. flowing of Rivers. The effect of friction in retarding 
 the motion "of fluids is perpetually illustrated in the flowing of 
 rivers and brooks. On the side of a river, the water, especially 
 where it is shallow, is nearly still, while in the middle of a 
 stream it may run at the rate of five or six miles an hour. For 
 the same reason, the water at the bottoms of rivers is much less 
 
 486. What is said of the sudden turnings of a tube, in retarding the motion of the 
 fluid? 487. How much more water will a two-inch tube of a hundred feet long dis- 
 charge, than a one-inch tube of the same length ? 488. How is this difference ac- 
 counted for ? 489. How do rivers show the effect of friction in retarding the motion 
 of their waters? 
 
128 RAISING WATER. 
 
 rapid than at the surface. This is often proved by the oblique 
 position of floating substances, which in still water would assume 
 a vertical direction. 
 
 Thus, suppose the stick of w r ood E, FIG - 109- 
 
 Fiff. 109, to be loaded at one end with 
 lead, of the sSne diameter as the wood, 
 so as to make it stand upright in still 
 water. In the current of a river, where 
 the lower end nearly reaches the bottom, 
 it will incline as in the figure, because 
 the water is more rapid toward the 
 surface than at the bottom, and hence 
 the tendency of the upper end to move 
 faster than the lower one, gives it an 
 inclination forward. River Current. 
 
 MACHINES FOR RAISING WATER. 
 
 490. The common pump, though a hydraulic machine, de- 
 pends on the pressure of the atmosphere for its effect, and there- 
 fore its explanation comes properly under the article Pneumatics, 
 where the consequences of atmospheric pressure will be illus- 
 trated. 
 
 Such machines only as raise water without the assistance of 
 the atmosphere, come properly under the present article. 
 
 491. ARCHIMEDES' SCREW. Among these, one of the most 
 curious, as well as ancient machines, is the screw of Archimedes, 
 and which was invented by that celebrated philosopher, two 
 hundred years before the Christian era, and then employed for 
 raising water, and draining land in Egypt. 
 
 492. It consists of a tube, made of lead, or strong leather, 
 coiled round a cylinder of wood or iron, as represented by Fig. 
 110. It has a support at each end, turning on gudgeons, the 
 upper end being sometimes furnished with cog-wheels to give a 
 more easy and rapid motion. Both ends are open, the lower 
 one being placed so far under the water as not to allow the 
 orifice to come above the surface in turning ^ the other dis- 
 charges the water in an uninterrupted stream. 
 
 493. The angle at which these machines work depends on 
 the manner of winding the tube on the cylinder ; that is, 
 
 Explain Fig. 109. 490. What is said of the common pump ? 491. Who is said to 
 have been the inventor of Archimedes' screw? When was this screw invented? 
 492. Explain this machine, as represented in Fig. 110. and show how the water is 
 elevated by turning it. 493. What must be the inclination of this machine 1 
 
RAISING TTATER. 129 
 
 FIG. 110. 
 
 whether the folds touch each other, or are at a distance apart, 
 for it is obvious that if the tube passes only a few times around 
 the support, this must be in nearly a horizontal position to act ; 
 but if the folds nearly touch, as in the figure, it may be placed 
 at an angle of about 50 with the horizon. It will be apparent 
 that the direction of each fold must be toward the horizon, as 
 the screw turns, otherwise the water would not run. This is 
 shown by the figure. This machine, as above stated, is a very 
 ancient invention, but has been re-invented in modern times, and 
 employed in most parts of Europe. 
 
 It has been constructed in various ways besides that here 
 represented. One was, to cut a spiral groove in a large log of 
 wood, and cover this with metal, leather, or boards, so as to 
 make it hold the water. The screw being thus sunk into the 
 wood, instead of being on the outside, as commonly represented. 
 
 494. When it was necessary to raise the water to a great 
 height, a series, one obliquely above the other, were employed, 
 platforms being constructed, with vessels to contain the water, 
 the lower end of the second screw taking that which was eleva- 
 ted by the first ; the third receiving that carried up by the 
 second, and so on. At present we believe this engine is no 
 where used except as a curiosity, there being better means of 
 raising water. 
 
 495. This principle is readily illustrated by winding a piece 
 of lead tube round a walking-stick, and then turning the whole 
 with one end in a dish of water, as shown in the figure. 
 
 494. How was water raised to great heights by this machine! 495. How may the 
 principle of Archimedes' screw be readily illustrated ? 
 
 6* 
 
180 RAISING WATER. 
 
 Theory of Archimedes 1 Screw. By the following cut and 
 explanation,. the manner in which this machine acts will be un- 
 derstood. 
 
 496. Suppose the FIG. in. 
 extremity 1, Fig. Ill, 
 
 to be presented up- 
 ward, as. in the figure, 
 the screw itself being 
 inclined as represent- 
 ed. Then, from its 
 peculiar form and po- 
 sition, it is evident, 
 that commencing at 
 1, the screw will de- 
 scend until we arrive 
 
 at a certain point, 2 ; in proceeding from 2 to 3, it will ascend. 
 Thus, 2 is a point so situated that the parts of the screw on 
 both sides of it ascend, and therefore if any body, as a ball, 
 were placed in the tube at 2, it could not move in either direc- 
 tion without ascending. Again, the point 3 is so situated, that 
 the tube on each side of it descends ; and as we proceed we 
 find another point, 4, which, like 2, is so placed, that the tube 
 on both sides of it ascends, and, therefore, a body placed at 4, 
 could not move without ascending. In like manner, there is a 
 series of other points along the tube, from which it either de- 
 scends or ascends, as is obvious by inspection. 
 
 Now let us suppose a ball, less in size than the bore of the 
 tube, so as to move freely in it, to be dropped in at 1. As the 
 tube descends from 1 to 2, the ball of course will descend down 
 to 2, where it will remain at rest. 
 
 Next, suppose the ball to be fastened to the tube at 2, and 
 suppose the screw to be turned nearly half round, so that the 
 end 1 shall be turned downward, and the point 2 brought to 
 the highest point of the curve 1, 2, 3. 
 
 497. The last movement of the spiral, it is evident, would so 
 change the positions of the ascending and descending parts, as 
 to -continue the motion upward, but it must be remembered 
 that the water differs from the ball used for illustration, in hav- 
 ing a constant pressure downward, and consequently upward, 
 and that the ascent of the water depends on this property of 
 the action of fluids. 
 
 4%. Explain the manner in which a ball would ascend, Fig. Ill, by turning the 
 screw. 497. On what property of fluids does the ascent of the water depend ? 
 
RAISING WATER. 
 
 131 
 
 498. BARKER'S MILL. For the different modes of applying 
 water as a power for driving mills, and other useful purposes, 
 we must refer the reader to works on practical mechanics. 
 There is, however, one method of turning machinery by water, 
 invented by Dr. Barker, which is strictly a philosophical, and, 
 at the same time, a most curious invention, and therefore is 
 properly introduced here. 
 
 This machine is called Barker's FIG. 112. 
 
 centrifugal mill, and such parts D 
 
 of it as are necessary to understand 
 the principle on which it acts are 
 represented by Fig. 112. 
 
 The upright cylinder A, is a 
 tube which has a funnel-shaped 
 mouth for the admission of the 
 stream of water from the pipe B. 
 This tube is six or eight inches in 
 diameter, and may be from ten to 
 twenty feet long. The arms, N 
 and O, are also tubes communica- 
 ting freely with the upright one, 
 from the opposite sides of which 
 they proceed. The shaft D is 
 firmly fastened to the inside of 
 the tube, openings at the same 
 time being left for the water to 
 
 pass to the arms O and N. The Centrifugal Mm. 
 
 lower part of the tube is solid, 
 
 and turns on a point resting on a block of stone or iron, C. 
 The arms are closed at their ends, near which are the orifices 
 on the sides opposite to each other, so that the water spouting 
 from them will fly in opposite directions. The stream from the 
 pipe B, is regulated by a stop-cock, so as to keep the tube A. 
 constantly full without overflowing. 
 
 499. To set this engine in motion, nothing is required but 
 the force of the water, which being let in by the pipe, descends, 
 and spouting from the opposite orifices, the motion immediately 
 begins, and if the main tube is of sufficient length, and kept full 
 of water, it will in a few minutes acquire a whirling velocity 
 which will astonish any one who has not before seen this curious 
 machine. 
 
 498. Describe Barker's centrifugal mill, Fig. 112. 499 How is this mill set In 
 motion 7 
 
132 CHAIN PUMP. 
 
 500. With respect to the theory of its motion, Euler, Greg- 
 ory, Brande and others, have written ; and it was formerly sup- 
 posed to depend in part on the resistance of the atmosphere, 
 but on trial it is said to revolve most rapidly in a vacuum. It 
 is therefore difficult to explain very clearly on what its motion 
 does depend. Dr. Gregory says, " In this machine the water 
 does not act by its weight, or momentum, but by its centrifugal 
 force, and the reaction that is produced by the flowing of the 
 water on the point immediately behind the orifice of discharge." 
 Dr. Brande says, " The resistance, or reaction generated by the 
 water issuing from the holes, is such as to throw the vertical 
 pipe with its arms and axis into rapid rotatory motion." 
 
 A model of the running part of this mill may be made by 
 any tinner, for a few shillings, and may be kept in constant mo- 
 tion, as a curiosity, by the waste water from the water ram de- 
 scribed a few pages hence. The shaft may be from two to four 
 feet. in length, and an inch or two in diameter, the arms being 
 one-half or one-third this size. The orifices in the arms must 
 be small, otherwise too much water will be required, the quan- 
 tity discharged being much greater than might be supposed. 
 
 After a few revolutions, the machine will receive an addi- 
 tional impulse by the centrifugal force generated in the arms, 
 and in consequence of this, a much more violent and rapid dis- 
 charge of the water takes place, than would occur by the pressure 
 of that in the upright tube alone. The centrifugal force, and the 
 force of the discharge thus acting at the same time, and each in- 
 creasing the force of the other, this machine revolves with great ve- 
 locity and proportionate power. The friction which it has to 
 overcome, when compared with that of other machines, is very 
 slight, being chiefly at the point C, where the weight of the 
 upright tube and its contents is sustained. 
 
 By fixing a cog-wheel to the shaft at D, motion may be given 
 to any kind of machinery required. 
 
 Where the quantity of water is small, but its height consid- 
 erable, this machine may be employed to great advantage, it 
 being under such circumstances one of the most powerful engines 
 ever invented. 
 
 CHAIN PUMP. 
 
 501. The principle of this machine is ancient, but instead of 
 flat boards, as in Fig. 113, pots, or deep buckets, were em- 
 
 500. What is the theory of Barker's mill ? 501. What is said of the antiquity and 
 use of the chain-pump 1 Describe the construction and action of this machine. 
 
WATER WHEELS. 
 
 133 
 
 ployed. Such engines are numerous along the banks of tho 
 Xile, and in Nubia and Hindostan, at the present day. 
 
 The construction, as well as the action FIG. 113. 
 
 of the chain-pump, will be understood 
 by the figure. It consists of a number 
 of square pieces of board, or of thin 
 iron, connected together through their 
 centers by iron rods, so that they can 
 have no lateral motion. These rods are 
 fastened to each other by hooks and 
 eyes, thus forming a chain with long 
 links. The ascending side of this chain 
 passes through a square box, to which 
 these pieces or buckets are closely fitted, 
 but not so as to create much friction. 
 The lower wheel, as well as the lower 
 end of the box, must be placed below 
 the surface of the water to be raised. 
 
 The action of this machine is described 
 in few words. To the upper wheel is 
 attached a crank ; or if large quantities 
 of water are to be raised, as on board of 
 ships, mill work is added, to multiply 
 the motion of the wheel, in order to 
 give the buckets a more rapid ascent 
 
 through the box. As the end of the box is constantly under 
 the water, every board necessarily carries up a portion in its 
 ascent, and although a single bucket would elevate but a small 
 quantity up to the end of the box, yet as they follow each other 
 in rapid succession, a constant stream is produced, and thus, 
 when the trunk is a foot in diameter, and the power is sufficient, 
 it will be obvious that a large quantity of water may, in a short 
 time, be elevated by this means. 
 
 602. Although this machine is called a pump, it will be ob- 
 served that the atmosphere is not concerned in its action. 
 
 WATER WHEELS. 
 
 503. Water wheels generally consist of a drum, or hollow 
 cylinder, revolving on an axis, while the diameter or exterior 
 sui-tace is covered with flat-boards, vanes, or cavities called 
 
 r/T2. Does the chain-pump act by the pressure of the atmosphere or not 1 503. Of 
 what do all water wheels consist ? How many kinds of water wheels are there, and 
 what are their names 7 
 
134 
 
 WATER WHEELS. 
 
 buckets, upon which the water acts ; first, to give motion to the 
 wheel, and then to machinery. Such wheels are of three kinds, 
 namely : the overshot, undershot, and breast wheels. 
 
 504. Overshot Wheel. This wheel of all others, gives the 
 greatest power with the least quantity of water, and is, there- 
 fore, generally used when circumstances will permit, or where 
 there is a considerable fall, with a limited quantity of water. 
 
 505. The overshot wheel, 
 Fig. 114, requires a fall 
 equal at least to its own 
 diameter, arid it is custom- 
 ary to give it a greater 
 length than other wheels, 
 that the cells or buckets 
 may contain a large quan- 
 tity of water, for it is chiefly 
 by the weight, and not the 
 momentum of the fluid that 
 this wheel is turned. 
 
 506. In its construction, 
 the drum, or circumference 
 is made water-tight, and to 
 
 this are fixed narrow Overshot Wheel. 
 
 troughs or buckets, formed 
 
 of iron, or boards, running the whole length of the drum. The 
 water is conducted by a trough nearly level, and sometimes in 
 width equal to the length of the wheel. It falls into the buckets 
 on the top of the wheel, and hence the name overshot. 
 
 507. The buckets are so constructed as to retain the water 
 until the wheel has made about one-third of a revolution from 
 the place of admission, when it escapes as from an inverted ves- 
 sel, and the wheel ascends with empty buckets, while on the 
 opposite side they are filled with water, and thus the revolution 
 is perpetuated. This whole machine and its action are so plain 
 and obvious as to require no particular reference. 
 
 508. From the experiments of Mr. Smeaton, it appears, that 
 the fall and quantity of water, and the diameter of the wheel 
 being the same, the overshot, will produce about double the 
 effect of the undershot wheel. 
 
 509. Undershot Wheel. This is so called because the water 
 
 504. What is the chief advantage of the overshot wheel? 505. Is this wheel turned 
 by the weight or momentum of the water? 506. Describe its construction. 507. 
 What is said of the construction of the buckets? 508. Circumstances being equal, 
 how much gre&ter powe r has the overshot than the undershot wheel? 
 
WATER WHEELS. 
 
 135 
 
 Undershot Wheel. 
 
 passes under instead of over 
 the circumference, as in that 
 above described. Hence it 
 is moved by the momentum, 
 not the weight of the water. 
 
 510. Its construction, as 
 shown by Fig. 115, is dif- 
 ferent from the overshot, 
 since instead of tight buckets 
 to retain the water, it has 
 flat-boards, standing like 
 rays around the circumfer- 
 ence. 
 
 511. Thus constructed, 
 
 this wheel moves equally well whether the water acts on one or 
 the other side cf the boards, and hence is employed for tide- 
 wheels, which turn in one direction when the tide is going out, 
 and in the other when it is coming in. 
 
 This wheel requires a rapid flow, and a large quantity of wa- 
 ter, to give it an efficient motion. 
 
 512. Breast Wheel 
 This wheel, in its construc- 
 tion, or rather in the ap- 
 plication of the moving 
 power, is between the two 
 wheels already described. 
 In this the water, instead 
 of passing over, or entirely 
 under the wheel is deliv- 
 ered in the direction of its 
 center, Fig. 116. This is 
 one of the most common 
 wheels, and is employed 
 where there is not a suffi- 
 cient fall for the construc- 
 tion of the overshot kind. 
 
 513. The breast wheel is moved partly by the weight, and 
 partly by the momentum of the water. But notwithstanding 
 this double force, this wheel is greatly inferior to the overshot, 
 
 509. Where does the water pass in the undershot wheel? What kind of force 
 moves this wheel ? 510. How does its construction differ from the overshot wheel ? 
 511. What is a tide-wheel? 512. How does the breast wheel differ from the overshot 
 and undershot wheels \ Where does the water strike this wheel? 513. By what 
 power is the breast wheel moved ? Why is this wheel inferior to the overshot 1 
 
 FIG. 116. 
 
 Breast Wheel. 
 
136 
 
 WATER WHEELS. 
 
 in effect, not only because the lever power is diminished by the 
 smaller diameter, but also on account of the great waste of wa- 
 ter which always attends the best constructed wheels of this 
 kind. 
 
 514. General Remarks. In order to allow any of the above 
 wheels to act with freedom, and to their fullest power, it is ab- 
 solutely necessary that the water which is discharged, at the 
 bottom of the wheel should have a wide and uninterrupted 
 passage to run away, for whenever this is not the case it ac- 
 cumulates and forms a resistance to the action of the buckets or 
 flat- boards, and thus subtracts just so much from the velocity 
 and power of the machine. 
 
 515. HYDRAULIC, OR WATER RAM. This beautiful engine 
 was invented by Montgolfier, a Frenchman, (and the same who 
 first ascended in a balloon,) in about 1796. 
 
 FIG. 117. 
 
 Hydraulic or Water Ram, 
 
 The .form and construction of this useful machine, which is 
 very simple in all its- parts, will be understood by Fig. 117. 
 Suppose the pipe A, comes from a spring, elevated a few feet 
 above the horizontal line B, and that it conveys a constant 
 stream of water. At the termination of this pipe, there is a 
 valve, called a spindle valve, capable of closing its orifice when 
 drawn upward ; on the spindle t, are several small weights, by 
 which the valve is made to drop down and remain open when 
 the water is still ; the weight of the whole being so nicely ad- 
 justed, that the movement of the running water will elevate it 
 
 514. What cautions are necessary in order to permit any of the wheels described to 
 produce their full effects? 515. Who invented the hydraulic, or water ram 7 Ex- 
 plain its construction by Fig. 117. 
 
WATER WHEELS. 18Y 
 
 to its place, ana thus stop the discharge. The weight of this 
 valve, a nice point in the construction of the machine, must be 
 just sufficient to make it rise by the force of the stream, and 
 sink again when the water ceases to flow, thus rising and falling, 
 and in effect causing the Laid to stop for an instant, and then 
 renew its motion. 
 
 516. Now water in motion acquires a momentum in propor- 
 tion to the length of the column, and the height of the source, 
 and when in action exerts a force equal to that of a solid body 
 of the same length and gravity, pressing downward from the 
 same elevation. The inelasticity of the fluid gives it the prop- 
 erty of acquiring motion through the whole length of a tube 
 elevated at one extremity, whenever only a small portion is 
 allowed to escape by its own pressure. Hence, when the valve 
 opens by dropping down, all the water in the pipe, however 
 long it may be, instantly moves forward to supply the place of 
 that which has thus escaped ; and if the pipe is long and the 
 fountain high, ordinary metallic conductors are burst asunder 
 by the shock whenever the stream is interrupted. It is on these 
 principles of the force of water, that the Hydraulic Ram is 
 founded ; for when the stream is stopped by the rising of the 
 valve, as already explained, an outlet is provided by another 
 valve, u, opening upward into an air vessel, having a discharg- 
 ing .pipe, x, and consequently when the spindle valve, , is closed, 
 this valve instantly opens, and the water is thrown with great 
 force into the air vessel, and through the discharging pipe to 
 the place where it is wanted. The stream being thus inter- 
 rupted, and the water becoming still under the lower valve, this 
 instantly opens by falling down, thus allowing the fluid to dis- 
 charge itself at.r, when the motion again raises the valve, and 
 it is stopped, the valve u being raised for its escape as before ; 
 and thus this curious machine, if well constructed, will act with 
 no other power or help, but a little stream of water, for weeks 
 or months. 
 
 517. This engine affords the most efficient, cheap, and con- 
 venient means of raising water, for ornamental or farming pur- 
 poses, ever invented. A spring on a hill near the house, or a 
 running brook with an elevation of a few feet, is all the power 
 required to supply an abundance of water for any private, or 
 even public establishment. Mr. Millington, who erected many 
 
 516. On what does the momentum of water in a tube depend 1 What is said of the 
 motion of the water in the tube 7 517. What is said of the economy and convenience 
 of the water ram 7 To what heights will it throw water in proportion to the falll 
 
140 PNEUMATICS. 
 
 52 Y. Expansion of the Air. On the contrary, when the 
 usual pressure of the atmosphere is removed from a portion of 
 air, it expands and occupies a space larger than before ; and it 
 is found by experiment, that this expansion is in a ratio, as the 
 removal of the pressure is more or less cohiplete. Air also ex- 
 pands or increases in bulk, when heated. 
 
 528. If the stop-cock, C, Fig. 118, Be opened, the piston, A, 
 may be pushed down with ease, because the air contained in 
 the barrel will be forced out at the aperture. Suppose the pis- 
 ton to be pushed down to within an inch of the bottom, and 
 then the stop-cock closed, so that no air can enter below it. 
 Now, on drawing the piston up to the top of the barrel, the 
 inch of air will expand and fill the whole space, and were this 
 space a thousand times as large, it would still be filled with the 
 expanded air, because the piston removes the pressure of the 
 external atmosphere from that within the barrel. 
 
 529. It follows, therefore, that the space which a given por- 
 tion of air occupies, depends entirely on circumstances. If it is 
 under pressure, its bulk will be diminished in exact proportion ; 
 and as the pressure is removed, it will expand in proportion, so 
 as to occupy a thousand, or even a million times as much space 
 as before. 
 
 530. Weight of Air. Another property which air possesses 
 is weight, or gravity. This property, it is obvious, must be 
 slight, when compared with the weight of other bodies. But 
 that air has a certain degree of gravity in common with other 
 ponderous substances, is proved by direct experiment. Thus if 
 the air be pumped out of a close vessel, and then the vessel be 
 exactly weighed, it will be found to weigh more when the air is 
 again admitted. 
 
 531. Pressure of the Atmosphere. It is, however, the weight 
 of the atmosphere which presses on every part of the earth's 
 surface, and in which we live and move, as in an ocean, that 
 here particularly claims our attention. 
 
 The pressure of the atmosphere may be easily shown by the 
 tuba and piston, Fig. 119. 
 
 Suppose there is an orifice to be opened or closed by the 
 valve B, as the piston A is moved up or down in its barrel. 
 The valve being fastened by a hinge on the upper side, on 
 
 526. In what proportion ti the force employed is the bulk of air lessened 1 527. In 
 what proportion will a quantity of air increase in bulk as the pressure is removed 
 from it? 528. How is this illustrated by Fig. 118! 529. On what circumstances, 
 therefore, will the bulk of a given portion of air depend ? 530. How is it proved that 
 air has weight ? 531. Explain in what manner the pressure of the atmosphere is 
 shown by Fig. 119. 
 
PNEUMATICS. 141 
 
 pushing the piston aown, it will open by the pres- FIG - 119 - 
 sure of the air against it, and the air will make 
 its escape. But when the piston is at the bottom 
 of the barrel, on attempting to raise it again, to- 
 ward the top, the valve is closed by the force of 
 the external air acting upon it. 
 
 532. If, therefore, the piston be drawn up in 
 this state, it must be against the pressure of the 
 atmosphere, the whole weight of which, to an ex- 
 tent equal to the diameter of the piston, must be 
 lifted, while there will remain a vacuum or void 
 space below it in the tube. 
 
 533. If the piston be only three inches in diam- 
 eter, it will require the full strength of a man to draw it to the 
 top of the barrel, and when raised, if suddenly let go, it will be* 
 forced back again by the weight of the air, and will strike the 
 bottom with great violence. 
 
 534. Supposing the surface of a man to be equal to 14-J- 
 square feet, and allowing the 'pressure on each square inch to 
 be 15 Ibs., such a man would sustain a pressure on his whole 
 surface equal to nearly 14 tons. 
 
 Now, that it is the weight of the atmosphere which 
 presses the piston down, is proved by the fact, that if its diam- 
 eter be enlarged, a greater force, in exact proportion, will be 
 required to raise it. And further, if when the piston is drawn 
 to the top of the tube, a stop-cock, as at Fig. 118, be opened, 
 and the air admitted under it, the piston will not be forced 
 down in the least, because then the air will press as much on 
 the under, as on the upper side of the piston. 
 
 535. By accurate experiments, an account of which it is not 
 necessary here to detail, it is found that the weight of the at- 
 mosphere on every square inch of the surface of the earth is 
 equal to fifteen pounds. If, then, a piston working air-tight in 
 a barrel, be drawn up from its bottom, the force employed, be- 
 sides the friction, will be just equal to that required to lift the 
 same piston, under ordinary circumstances, with a weight laid 
 on it equal to fifteen pounds for every square inch of surface. 
 
 536. The number of square inches in the surface of a piston 
 
 532. The force pressing on the piston, when drawn up.ward. is sometimes called 
 suction. 533. Huw is it proved that it is the weight of the atmosphere, instead of 
 suction, which makes the piston rise with difficulty ? 534. What is the pressure of 
 the atmosphere on the surface of a man ? 535. What is the pressure of the atmos- 
 phere on every square inch of surface on the earth ? 536. What is the number of 
 square inches in a circle of one foot in diameter? What is the weight of the atmos- 
 phere on the surface of afoot in diameter? 
 
142 AIR PUMP. 
 
 of a foot in diameter, is 113. This being multiplied by the 
 weight of the air on each inch, which, being 1 5 pounds, is equal 
 to 1695 pounds. Thus the air constantly presses on every sur- 
 face, which is equal to the dimensions of a circle one foot in 
 diameter, with a weight of 1695 pounds. 
 
 537. The air pump is an engine by ivhich the air can be 
 pumped out of a vessel, or withdrawn from it. The vessel so 
 exhausted, is called a receiver, and the space thus left in the 
 vessel, after withdrawing the air, is called a vacuum. 
 
 Tjie principles on which the air pump is constructed are 
 readily understood, and are the same in all instruments of this 
 ^ind, though the form of the instrument itself is often consider- 
 ably modified. 
 
 539. The general principles of its FIG. 120. 
 
 construction will be comprehended 
 by an explanation of Fig. 120. Jn 
 this figure let R be a glass vessel, or 
 receiver, closed at the top, and open 
 at the bottom, standing on a per- 
 fectly smooth surface, which is called 
 the plate of the air pump. Through 
 the plate is an aperture, which com- 
 municates with the inside of the re- 
 ceiver, and the barrel of the pump. 
 The piston-rod works air-tight Air Pump. 
 
 through the barrel. At the extrem- 
 ity of the barrel, there is a valve which opens upward, and is 
 closed as the piston rises. 
 
 539. Now suppose the piston to be drawn up, it will then 
 leave a free communication between the receiver R, through the 
 orifice to the pump-barrel in which the piston works. Then if 
 the piston be forced down, it will compress the air in the barrel 
 between V and V, and, in consequence, the valve E will be 
 opened, and the air so condensed will be forced out. On draw- 
 ing the piston up again, the valve will be closed, and the ex- 
 ternal air not being permitted to enter, a partial vacuum will 
 be formed in the barrel, from V to V. When the piston rises 
 again, the air contained in the glass vessel, together with that 
 
 537. What is an air pump 1 538. Give the names of the different parts of the air 
 pump by Fig. 120. 539. Show the manner in which the air pump works to produce 
 a vacuum. 
 
AIR PUMP. 148 
 
 in the passage between the vessel and the pump-barrel, will 
 rush in to fill the vacuum. Thus, there will be less air in the 
 whole space, and consequently in the receiver, than at first, be- 
 cause all that contained in he barrel is forced out at every 
 stroke of the piston. On repeating the same process, that is 
 drawing up and forcing down the piston, the air at each time in 
 the receiver will become less and less in quantity, and, in conse- 
 quence, more and more rarefied. For it must be understood, 
 that although the air is exhausted at every stroke of the pump, 
 that which remains, by its elasticity, expands, and still occupies 
 the whole space. The quantity forced out at each successive 
 stroke is therefore diminished, until, at last, it no longer has 
 sufficient force before the piston to open the valve, when the 
 exhausting power of the instrument must cease entirely. 
 
 540. Now it will be obvious, that as the exhausting power of 
 the air pump depends on the expansion of the air within it, a 
 perfect vacuum can never be formed by its means, for so long 
 as exhaustion takes place, there must be air to be forced out, 
 and when this becomes so rare as not to force open the valves, 
 then the process must end. 
 
 DOUBLE-ACTING AIR PUMP. 
 
 541. The double air pump has two similar barrels to that 
 above described, and therefore the process of exhaustion is per- 
 formed in half the time. 
 
 This is represented by Fig. 121, where P P are the cylinders 
 of brass, in which the pistons work, and of which V V are the 
 valves. The piston rods, E E, are toothed to correspond with 
 the teeth of the wheel W, which is worked by the crank D. 
 The exhausting tube T, also of brass, opens by the valves V V, 
 into the cylinders. This has a stop-cock, C, to prevent the ingress 
 of air after the vacuum is made, in case the pistons leak. The 
 receiver, R, is of glass, ground to fit, air-tight, to the plate of 
 brass on which it stands. The exhausting tube opens at O, 
 into the interior of the receiver. The barometer tube H, at its 
 upper end opens into this tube, while at the lower end, M, it is 
 inserted into a cup of mercury. 
 
 542. The barometer tube is designed to show the degree of 
 exhaustion, in the receiver, with which it communicates, as 
 shown in the figure. As the exhaustion proceeds, the external 
 
 540. Will the air pump form a perfect vacuum"? Why not ? 541. Name the sev- 
 eral parts of the double-acting air pump by Fig. 121, and show how it works. 642. 
 What is the use of the barometer tube, as applied to the air pump! 
 
144 
 
 AIR PUMP. 
 FIG. 121, 
 
 Double-Acting Air Pump. 
 
 air, pressing on the mercury in the cup, elevates that in the 
 tube, in proportion to the rarety of the air in the receiver. 
 
 Action. The manner in which the double pump acts, is ex- 
 actly similar to the single one, only that it has two barrels, or 
 cylinders, instead of one. It is, therefore, unnecessary to repeat 
 the explanation given under the last figure. 
 
 543. External View of the Air Pump. Having explained 
 the principles and action of the air pump, by figures showing 
 its interior construction, we here present the student with an 
 external view, Fig. 122, of the whole machine. 
 
 544. It is a small single-barrel pump, those with more bar- 
 rels being of course more complex in structure, and less easily 
 understood. The barrel, A, is seven inches high and two in 
 diameter ; the plate, K, is eight inches in diameter ; the piston 
 rod, B, works air-tight by means of the packing screw J, which 
 is fitted to the barrel case, I. The piston is kept perpendicular 
 by the guide E, through which it works; the fulcrum prop, H, 
 is eighteen inches high, and the parallel roas, D, connect the 
 piston rod and cross-head, C, with the lever. 
 
 The dome cap, I, contains a valve opening upward, for the 
 escape of the air when the piston rises, This is the only valve 
 in this pump, except that in the piston, which, as already shown, 
 opens to admit the expanded air from the receiver, and force it 
 
 544. Explain all parts of the air pump by Fig. 122. 
 
145 
 
 Single-Barrel Air Pump. 
 
 out at the upper valve. To the dome cap, above the valve, is 
 fitted a curved tube, leading to the cistern, F ; its use is to re- 
 ceive the waste oil which may escape from that used to lubricate 
 the piston. Th globular bell -glass, or receiver, L, is fitted by 
 grinding to the brass plate on which it stands ; the barometer 
 gauge, G. contains mercury, and communicates with the tube 
 leading from the barrel to the receiver ; this shows by its scale 
 what proportion of air is exhausted from the receiver ; within 
 the receiver there is seen a protuberance, showing the end of 
 the exhausting tube, and into which may be screwed receivers 
 or tubes for various experiments. 
 
 545. UPWARD ATMOSPHERIC PRESSURE. The atmosphere, 
 as we have seen, presses in every direction. Its upward pressure 
 is shown by the apparatus, Fig. 123. 
 
 It consists of a hand air pump, a, with a valve opening up- 
 ward, not shown. This pump is attached to a cylinder of larger 
 size, 6, in which is the piston, c, to which a 56-lb. weight is 
 attached by a cord. This piston must be air-tight, and at the 
 
 645. Describe Fig. 123, and show how the weight is elevated and sustained. 
 7 
 
146 
 
 AIR PUMP. 
 
 Upward Atmospheric Pressure. 
 
 lower part of the cylinder when FIG. 123. 
 
 the experiment begins. Now, 
 on working the pump, a vacu- 
 um is formed between the pis- 
 tons in the cylinder 6, and con- 
 sequently the pressure of the 
 air on the under part of c, the 
 cylinder being open, forces it 
 upward, drawing the weight 
 with it. On admitting the air 
 into the large cylinder, from 
 above, the weight instantly de- 
 scends, showing that it is the 
 pressure of the atmosphere 
 from below which sustained 
 the weight. 
 
 546. I. If a withered apple 
 be placed under the receiver, 
 and the air is exhausted, the 
 apple will swell and become 
 plump, in consequence of the 
 
 expansion of the air which it contains within the skin. 
 
 II. Ether, placed in the same situation, soon begins to boil 
 without the influence of heat, because its particles, not having 
 the pressure of the atmosphere to force them together, fly off 
 with so much rapidity as to produce ebullition. 
 
 III. If a bladder partly filled with air, and the neck well se- 
 cured, has the external air exhausted, that within will so expand 
 as to burst the membrane. 
 
 IV. If a flask partly filled with water, be placed, with its 
 neck in a jar of the same fluid, under the receiver, the rarefied 
 air within the flask will drive the water out, but it will rush in 
 again when the air is again let into the receiver. 
 
 V. If a burning taper be placed under it, the flame soon 
 ceases for want of oxygen to support it. For the same reason 
 no light is seen from the collision of flint and steel in a vacuum. 
 
 VI. If a bell be struck under the receiver, the sound will 
 grow faint as the air is exhausted, until it is no longer audible. 
 See Acoustics. 
 
 547. Magdeburgh Hemispheres. One of the most striking 
 
 546. Why does an apple placed in the exhausted receiver grow plump ? Why does 
 ether boil in the same situation? Why does flame cease in a vacuum? Why is a 
 bell inaudible in a vacuum 1 547. Describe the Magdeburgh hemispheres. 
 
AIR PUMP. 
 
 147 
 
 illustrations of atmospheric pressure is FIG. 124. 
 
 made by means of the before named in- 
 strument, Fig. 124. It consists of two 
 hemispheres of brass, A and B, fitted to 
 each other by grinding, so that when put 
 together they perfectly exclude the air. 
 AYlien put together without preparation, 
 or in the usual manner, they hold no 
 stronger than the parts of a snuff-box; 
 but when the air is exhausted from within, 
 it will take two strong men, if the diam- 
 eter of the hemispheres are six inches, to 
 pull them apart. The air is exhausted by 
 unscrewing the lower handle and connect- 
 ing that part with the exhausting tube of 
 the air pump, and then by turning the 
 key its return is prevented. 
 
 548. The amount of force required to 
 separate them, will of course depend on 
 
 their diameter and may be calculated by Ma s^ur g h Hemisphere* 
 estimating the pressure to be equal to fif- 
 teen pounds for every square inch of surface, this, as we have 
 seen, (536,) being the pressure of the atmosphere. 
 
 549. The same principle is involved FIG. 125. 
 when a piece of wet leather, with a 
 
 string in the center, is pressed on a 
 smooth stone, and then pulled by the 
 string. 
 
 550. EXPANSION FOUNTAIN. A very 
 pretty experiment is made, with the 
 air pump, by means of the apparatus, 
 Fig. 125. 
 
 It consists of two glass globes, the 
 upper one, a, being open at the top, 
 and furnished with a stop-cock and jet 
 tube, reaching nearly to the bottom of 
 the lower globe. 
 
 The lower one, being nearly filled 
 with some colored liquid, the upper 
 one, with the jet, is screwed to it, as 
 
 Seen in the fio-urei. Expansion fountain. 
 
 548. What is the force required to pull them apart 1 549. Why does a piece of wet 
 leather adhere to a smooth surface! 550. Explain in what manner the fluid in the 
 globes is made to rise, or fall, at pleasure. 
 
146 
 
 AIR PU1AP. 
 
 Upward Atmospheric Pressure. 
 
 lower part of the cylinder when FIG. 123. 
 
 the experiment begins. Now, 
 on working the pump, a vacu- 
 um is formed between the pis- 
 tons in the cylinder 6, and con- 
 sequently the pressure of the 
 air on the under part of c, the 
 cylinder being open, forces it 
 upward, drawing the weight 
 with it. On admitting the air 
 into the large cylinder, from 
 above, the weight instantly de- 
 scends, showing that it is the 
 pressure of the atmosphere 
 from below which sustained 
 the weight. 
 
 546. I. If a withered apple 
 be placed under the receiver, 
 and the air is exhausted, the 
 apple will swell and become 
 plurnp, in consequence of the 
 
 expansion of the air which it contains within the skin. 
 
 II. Ether, placed in the same situation, soon begins to boil 
 without the influence of heat, because its particles, not having 
 the pressure of the atmosphere to force them together, fly off 
 with so much rapidity as to produce ebullition. 
 
 III. If a bladder partly filled with air, and the neck well se- 
 cured, has the external air exhausted, that within will so expand 
 as to burst the membrane. 
 
 IV. If a flask partly filled with water, be placed, with its 
 neck in a jar of the same fluid, under the receiver, the rarefied 
 air within the flask will drive the water out, but it will rush in 
 again when the air is again let into the receiver. 
 
 V. If a burning taper be placed under it, the flame soon 
 ceases for want of oxygen to support it. For the same reason 
 no light is seen from the collision of flint and steel in a vacuum. 
 
 VI. If a bell be struck under the receiver, the sound will 
 grow faint as the air is exhausted, until it is no longer audible. 
 See Acoustics. 
 
 547. MagdelurgJi Hemispheres. One of the most striking 
 
 546. Why does an apple placed in the exhausted receiver grow plump ? Why does 
 (ether boil in the same situation 1 Why does flame cease in a vacuum 1 Why is a 
 bell inaudible in a vacuum 1 547. Describe the Magdcburgb. hemispheres. 
 
AIR PUMP. 
 
 147 
 
 illustrations of atmospheric pressure is FIG. 134. 
 
 made by means of the before named in- 
 strument, Fig. 124. It consists of two 
 hemispheres of brass, A and B, fitted to 
 each other by grinding, so that when put 
 together they perfectly exclude the air. 
 AVlien put together without preparation, 
 or in the usual manner, they hold no 
 stronger than the parts of a snuff-box; 
 but when the air is exhausted from within, 
 it will take two strong men, if the diam- 
 eter of the hemispheres are six inches, to 
 pull them apart. The air is exhausted by 
 unscrewing the lower handle and connect- 
 ing that part with the exhausting tube of 
 the air pump, and then by turning the 
 key its return is prevented. 
 
 548. The amount of force required to 
 separate them, will of course depend on 
 
 their diameter and may be calculated by Ma debur sk Hemisphere* 
 estimating the pressure to be equal to fif- 
 teen pounds for every square inch of surface, this, as we have 
 seen, (536,) being the pressure of the atmosphere. 
 
 549. The same principle is involved FIG. 125. 
 when a piece of wet leather, with a 
 
 string in the center, is pressed on a 
 smooth stone, and then pulled by the 
 string. 
 
 550. EXPANSION FOUNTAIN. A very 
 pretty experiment is made, with the 
 air pump, by means of the apparatus, 
 Fig. 125. 
 
 It consists of two glass globes, the 
 upper one, a, being open at the top, 
 and furnished with a stop-cock and jet 
 tube, reaching nearly to the bottom of 
 the lower globe. 
 
 The lower one, being nearly filled 
 with some colored liquid, the upper 
 one, with the jet, is screwed to it, as 
 
 Seen in the fio*UrU Expansion Fountain. 
 
 548. What is the force required to pull them apart 1 549. Why does a piece of wet 
 leather adhere to a smooth surface! 550. Explain in what manner the fluid in the 
 globes is made to rise, or fall, at pleasure. 
 
148 
 
 CONDENSER. 
 
 Thus prepared, they are placed under the receiver of the air 
 pump, and as the air is exhausted, that contained in the lower- 
 globe expands, and forces the liquid through the tube into the 
 upper globe. On admitting the air into the receiver, the fluid 
 again returns into the lower one, and this may be repeated any 
 number of times, affording a very interesting experiment. 
 
 THE CONDENSER. 
 
 FIG. 126. 
 
 551. The opera tion of the condenser is the reverse of that of the 
 air pump, and is a much more simple machine. The air pump, 
 as we have just seen, will deprive a vessel of its ordinary quan- 
 tity of air. The condenser, on the contrary, will double or 
 treble the ordinary quantity of air in a close vessel, according to 
 the force employed. 
 
 This instrument, Fig. 126, consists of a pump- 
 barrel and piston, A, a stop-cock B, and the vessel 
 C, furnished with a valve opening downward. 
 The orifice, D, is to admit the air, when the pis- 
 ton is drawn up to the top of the barrel. 
 
 552. To describe its action, let the piston be 
 above D, the orifice being open, and therefore the 
 instrument filled with air, of the same density as 
 the external atmosphere. Then, on forcing the 
 piston down, the air in the pump-barrel, below 
 the orifice D, will be compressed, and will rush 
 through the stop-cock, B, into the vessel C, where 
 it will be retained, because, on again moving the 
 piston upward, the elasticity of the air will close 
 the valve through which it was forced. On draw- 
 ing the piston up again, another portion of air 
 will rush in at the orifice D, and on forcing it 
 clown, this will also be driven into the vessel C ; 
 
 and this process may be continued as long as sufficient force is 
 applied to move the piston, or there is sufficient strength in the 
 vessel to retain the air. "When the condensation is finished, the 
 stop-cock B may be turned, to render the confinement of the air 
 more secure. 
 
 553. AIR GUN. The magazines of air guns formerly con- 
 sisted of a copper ball, which after being charged with condensed 
 air, was screwed to the barrel, presenting an unseemly and in- 
 
 Condenser. 
 
 551. How does the condenser differ from the air pump ? 552. Explain Fig. 126, and 
 show in what manner the air is condensed. 553. Explain the principle of the air 
 gun. 
 
CONDENSER. 
 
 149 
 
 convenient appendage. That here described, TO - 127 - 
 is a more recent and greatly improved inven- 
 tion. 
 
 In this, the breach of the gun is made of 
 copper, and without much increasing the size, 
 answers for the magazine, while the barrel 
 serves as the tube of the condenser. 
 
 554. At A, Fig. 127, the barrel is screwed 
 on to the breach, in which the air is condensed, 
 by means of the piston, rod, and handle, as 
 shown by the figure. 
 
 The piston is then withdrawn, the condensed 
 air being prevented from escape by the valve, 
 opening outward, as shown by the figure. 
 
 The ball being introduced, is fired by pull- 
 ing back the trigger, which opens the valve, 
 and allows a portion of the air to escape. 
 
 The velocity and force of the ball will de- 
 pend on the amount of condensation in the 
 magazine, and the smaller the tube and piston 
 by which this is made, the greater of course 
 will be the density of the confined air, and the 
 more powerful the force by which the ball is 
 impelled. Where the piston is no more than 
 half or three quarters of an inch in diameter, 
 it is said the ball will have a force not much 
 short of that of a musket-shot. 
 
 555. Bottle Imp. A curious philosophical 
 toy, called the Bottle Imp, shows in a very 
 
 striking manner the effects of condensing a small portion of air. 
 
 Procure a glass jar, with a neck, as represented by Fig. 128, 
 also a piece of India rubber, and a string to secure it over the 
 mouth of the jar, so that it shall be perfectly air-tight. Next, 
 take a piece of glass tube, about three-eighths of an inch in 
 diameter, and with a file cut off pieces an inch long, and into 
 one end of each put a cork stopper of such size as to make most 
 of the cork swim on the surface when the tube is placed in the 
 water. The tubes must now be partly filled with water, so that 
 they will just balance themselves in the fluid without sinking, 
 the air remaining in their upper halves. 
 
 Having prepared the tubes with their corks in this manner, 
 
 Air Gun. 
 
 554. With what force will the air gun fhrow a ball 1 555. Explain the manner of 
 constructing the bottle imp. 
 
150 
 
 BAROMETER. 
 
 FIG. 128. 
 
 and placed them in the jar nearly filled with water, 
 tie on the rubber cap with a good long string, so 
 that no air can escape, and this little apparatus is 
 finished. 
 
 Now press upon the rubber with the hand, and 
 the floating tubes will immediately begin to de- 
 scend, and will strike the bottom of the jar, one 
 . after the other, with an audible stroke, and will 
 rise again when the pressure ceases. 
 
 Many a philosophical head, on seeing this ex- 
 periment for the first time, has been puzzled to 
 assign any cause why these little objects should fall 
 and rise in this manner, the hand not going near 
 them, there being several inches of air between the 
 cap and the water. 
 
 556. The explanation will be obvious on setting 
 the jar between the light and the eye, and watch- Bottle imp. 
 ing a tube when the pressure is made, for the wa- 
 ter will be seen to rise in it at the moment it begins to fall, and 
 sink again as it rises. The pressure of the hand is transmitted 
 through the elastic rubber and air, to the water, and so to the 
 air in the tube, which being thus condensed, takes in more wa- 
 ter than its buoyancy can sustain, and it sinks rising again 
 when the air is allowed to expand, and drive" out the water. 
 
 BAROMETER. 
 
 55*7. The Barometer is an instrument which, by means of a 
 column of mercury in a glass tube, shows, by its elevation and 
 depression, the different degrees of atmospheric pressure. 
 
 558. Suppose A, Fig. 129, to be a long tube, with the piston 
 B so nicely fitted to its inside, as to work air-tight. If the 
 lower end of the tube be dipped into water, and the piston 
 drawn up by pulling at the handle C, the water will follow the 
 piston so closely, as to be in contact with its surface, and ap- 
 parently to be drawn up by the piston, as though the whole 
 was one solid body. If the tube be thirty-five fejet long, the 
 water will continue to follow the piston, until it comes to the 
 height of about thirty-three feet, where it will stop. 
 
 559. If the piston be drawn up still further, the water will 
 
 556. Explain the reason why the floats in the water imp are influenced by the pres- 
 sure 557. What is the barometer ? 558. Supi ose the tube, Fitf. 129, to stand with 
 its lower end in the water, and the piston A to be drawn upward thirty five feet, how 
 far will the water follow the piston 1 559. What will remain in the tibe between the 
 piston and the water, after the piston rises higher than thirty-three teet 1 
 
BAROMETER. 
 
 151 
 
 FIG. 129. 
 
 m 
 
 not follow it, but will remain stationary, the 
 space from this height, between the piston and 
 the water, being left a void space or vacuum. 
 
 560. The rising of the water in the above 
 case, which only involves the principle of the 
 common pump, is thought by some to be caused 
 by suction, the piston sucking up the water as it is 
 drawn upward. But according to the common 
 notion attached to this term, there .is no reason 
 why the water should not continue to rise above 
 the" thirty-three feet, or why the power of suc- 
 tion should cease at that point, rather than at 
 any other. 
 
 561. Without entering into any discussion 
 on the absurd notions concerning this power, it 
 is sufficient here to state, that it has long since 
 been proved, that the elevation of the water, in 
 the case 'above described, depends entirely on 
 the weight and pressure of the atmosphere on 
 that portion of the fluid which is on the outside 
 of the tube. Hence, when the piston is drawn 
 up under circumstances where the air can not 
 act on the water around the tube, or pump- 
 barrel, no elevation of the fluid will follow. 
 
 562. If an atmospheric pump, or even the suction-hose of a 
 fire engine, be inserted into the side of a tight cask filled with 
 fluid, all the force of what is called suction may be exerted by 
 the pump or engine in vain ; for the liquid will not rise until an 
 aperture, admitting the atmosphere, is made in some part of the 
 cask. It may be objected that wells, though covered several 
 feet deep with earth, still admit water to be drawn from them 
 by pumps, with all the facility of those which are open. But it 
 must be remembered that the ground is porous, admitting the 
 atmosphere to an unknown depth from the surface, and hence 
 wells can not be covered by any common means so as to ex- 
 clude sufficient atmospheric pressure for the purpose in question. 
 That the pump will not raise water without the influence of the 
 atmosphere, will be seen by the following experiment. 
 
 503. Proof that the Pump acts by External Pressure. 
 Suppose Fig. 130 to be the sections or halves, of two tubes, one 
 
 560. What is commonly supposed to make the water rise in such cases'? Is there 
 any reason why the suction should cease at thirty-three feet ? 561. What is the true 
 cause of the elevation of the water, when the piston, Fig. 129, is drawn up > 562. 
 Will the suction-hose of a fire-engine raise water from a tight cask 7 
 
152 
 
 BAROMETER. 
 
 within the other, the outer one being made en- 
 tirely close, so as to admit no air, and the space 
 between the two being also made air-tight at the 
 top. Suppose, also, that the inner tube being left 
 open at the lower end, does not reach the bottom 
 of the outer tube, and thus that an open space be 
 left between the two tubes every where, except at 
 their upper ends, where they are fastened to- 
 gether ; and suppose that there is a valve in the 
 piston, opening upward, so as to let the air which 
 it contains escape, but which will close on draw- 
 ing the piston upward. Now, let the piston be 
 at A, and in this state pour water through the 
 stop-cock, C, until the inner tube is filled up to 
 the piston, and the space between the two tubes 
 filled up to the same point, and then let the stop- 
 cock be closed. If now the piston be drawn up 
 to the top of the tube, the water will not follow 
 it, as in the case of Fig. 129 ; it will only rise a 
 few inches, in consequence of the elasticity of the 
 air above the water, between the tubes, and in 
 the space above the water, there will be formed a 
 vacuum between the water and the piston, in the 
 inner tube. 
 
 The reason why the result of this experiment differs from that 
 before described, is, that the outer tube prevents the pressure of 
 the atmosphere from forcing the water up the tube as th piston 
 rises. This may be instantly proved, by opening the stop-cock 
 C, and permitting the air to press upon the water, when it will 
 be found, that as the air rushes in, the water will rise and fill 
 the vacuum,' up to the piston. 
 
 564. For the same reason, if a common pump be placed in a 
 cistern of water, and the water is frozen over on its surface, so 
 that no air can press upon the fluid, the piston of the pump 
 might be worked in vain, for the water would not, as. usual, 
 obey its motion. 
 
 , 565. It follows, as a certain conclusion from such experi- 
 ments, that when the lower end of a tube is placed in water, 
 and the air from within removed by drawing up the piston, 
 that it is the pressure of the atmosphere on the water around 
 
 563. How is it shown by Fig. 130. that it is the pressure of the atmosphere wh'ch 
 causes the water to rise in the pump-barrel ? 564. Suppose the ice prevents the at 
 mosplvre from pressing on the water in a vessel, can the water be pumped out ? 
 665. What conclusion follows from the experiments above described? 
 
BAROMETER. 
 
 153 
 
 the tube, which forces the fluid up to fill the space thus left by 
 the air. 
 
 566. It is also proved, that the weight, or pressure of the at- 
 mosphere, is equal to the weight of a perpendicular column of 
 water 33 feet high, for it is found (Fig. 129) that the pressure 
 of the atmosphere will not raise the water more than 33 feet, 
 though a perfect vacuum be formed to any height above this 
 point 
 
 567. Experiments on other fluids, prove -that this is the 
 weight of the atmosphere, for if the end of the tube be dipped 
 in any fluid, and the air be removed from the tube, above the 
 fluid, it will rise to a greater or less height than water, in pro- 
 portion as its specific gravity is less, or greater than that fluid. 
 
 568. Mercury, or quicksilver, has a specific gravity of about 
 13 times greater than that of water, and mercury is found to 
 rise about 29 inches in a tube under the same circumstances that 
 water rises 33 feet. Xow, 33 feet is 396 inches, which being 
 divided by 29, gives nearly 13-^, so that mercury being 13-J- 
 times heavier than water, the water will rise under the same 
 pressure 13- times higher than the mercury. 
 
 569. Construction of the Barometer. 
 The barometer is constructed on the princi- 
 ple of atmospheric pressure. This term is 
 compounded of two Greek words, baros, 
 weight, and metron, measure, the instrument 
 being designed to measure the weight of the 
 atmosphere. 
 
 Its construction is simple and easily un- 
 derstood, being m4My a tube of glass, nearly 
 filled with mercury, with its lower end placed 
 in a dish of the same fluid, and the upper 
 end furnished with a scale, to measure the 
 height of the mercury. 
 
 570. Let A, Fig. 131, be such a tube, 
 thirty-four or thirty-five inches long, closed 
 at one end, and open at the other. To fill 
 the tube, set it upright, and pour the mer- 
 cury in at the open end, and when it is en- 
 tirely full, place the fore-finger forcibly on 
 
 FIG. 131. 
 
 Barometer. 
 
 566. How is it proved that the pressure of the atmosphere is equal to the weight of 
 a column of water 33 feet hisrh ? 567. How do experiments on other fluids show that 
 the pressure of the atmosphere is equal to the weight of a column of water 33 feet 
 hijih } 563. How hish does mercury rise in an exhausted tube? How does the 
 height of mercury in the barometer indicate that of water I 
 
 7* 
 
154 BAROMETER. 
 
 this end, and then plunge the tube and finger under the surface 
 of the mercury, before prepared in the cup, B. Then withdraw 
 the finger, taking care that in doing this, the end of the tube is 
 not raised above the mercury in the cup. When the finger 
 i* removed, the mercury will descend four or five inches, and 
 after several vibrations, up and down, will rest at an elevation 
 of 29 or 30 inches above the surface of that in the cup, as at C. 
 Having fixed a scale to the upper part of the tube, to indicate 
 the rise and fall of the mercury, the barometer would be fin- 
 ished, if intended to remain stationary. It is usual, however, to 
 have the tube inclosed in a mahogany or brass case, to prevent 
 its breaking, and to have the cup closed at the top, and fastened 
 to the tube, so that it can be transported without danger of 
 spilling the mercury. 
 
 571. Cup of the Portable Barometer. The cup of the port- 
 able barometer also differs from that described, for were the 
 mercury inclosed on all sides, in a cup of wood, or brass, the air 
 would be prevented from acting upon it, and therefore the in- 
 strument would be useless. To remedy this defect, and still 
 have the mercury perfectly inclosed, the bottom of the cup is 
 made of leather, which, being elastic, the pressure of the atmos- 
 phere acts upon the mercury in the same manner as though it 
 was not inclosed at all. 
 
 572. Below the leather bottom, there is a round plate of 
 metal, an ift^.in diameter, which is fixed on the top of a screw, 
 so that when the instrument is to be transported, by elevating 
 this piece of metal, the mercury is thrown up to the top of the 
 tube, and thus kept from playing backward ^jid forward, when 
 the barometer is in motion. 
 
 573. A person not acquainted with the principles of this in- 
 strument, on seeing the tube turned bottom upward, will be 
 perplexed to understand why the mercury does not follow the 
 common law of gravity, and descend into the cup ; were the 
 tube of glass 33 feet high, and filled with water, the lower end 
 being dipped into a tumbler of the same fluid, the wonder would 
 be still greater. But as philosophical facts, one is no more 
 wonderful than the other, and both are readily explained by th<> 
 principles above illustrated. 
 
 569. What is the principle on which the barometer is constructed ? 570. Describe 
 the construction of the barometer, as represented by Fig. 131. 571. How is the cup 
 of the portable barometer made so as to retain the mercury, and still allow the air to 
 press upon it ? 572. What is the use of the metallic plate and screw, under the bot- 
 tom of the cup ? 573. Explain the reason why the mercury does not fall out of the 
 barometer tube when its open end is downward. 
 
BAROMETER. 155 
 
 574. WATER BAROMETER. It has already been shown, (563,) 
 that it is the pressure of the atmosphere on the fluid around 
 the tube, bv which the fluid within it is forced upward, when 
 the pump is exhausted of* its air. The pressure of the air, we 
 have also seen, is equal to a column of water 33 feet high, or 
 of a column of mercury 29 inches high. Suppose, then, a tube 
 33 feet high is filled with water, the air would then be entirely 
 excluded, and were one of its ends closed, and the other end 
 dipped in water, the effect would be the same as though both 
 ends were closed, for the water would not escape, unless the air 
 was permitted to rush in and fill up its place. The upper end 
 being closed, the air could gain no access in that direction, and 
 
 'the open end being under water, is equally secure. The quan- 
 tity of water in which the end of the tube is placed, is not essen- 
 tial, since the pressure of a column of water, an inch in diameter, 
 provided it be 33 feet high, is just equal to a column of air of 
 an inch in diameter, of the whole height of the atmosphere. 
 Hence the wnter on the outside of the tube serves merely to 
 guard against the entrance of the external air. 
 
 575. The same happens to the barometer tube, when filled 
 with mercurv. The mercury, in the first place, fills the tube 
 perfectly, and therefore entirely excludes the air, so that when 
 it is inverted in the cup or cistern, all the space above 29 inches 
 is left a vacuum. The same effect precisely would be produced, 
 were the tube exhausted of its air, and the open end placed in 
 the cup ; the mercury would run up the tube 29 inches, and 
 then stop, all above that point being left a vacuum. 
 
 576. The mercury, therefore, is prevented from falling out 
 of the tohpi, by the pressure of the atmosphere on that which 
 remains in the cistern ; for if this be removed, the air will enter, 
 while the mercury will instantly begin to descend. This is 
 called the Cistern barometer. 
 
 577. WHEEL BAROMETER. In the common barometer, the' 
 rise and fell of the mercury is indicated by a scale of inches, and 
 tenths of inches, fixed behind the tube ; but it has been found 
 that vpry slight variations in the density of the atmosphere are 
 not readily perceived by this method. It being, however, de- 
 sirable that these minute changes sh'ould be rendered more 
 obvious, a contrivance for increasing the scale, called the wheel 
 barometer, was invented. 
 
 574. How high does the fluid stand in the water barometer 7 575. What fills the 
 space above 29 inches, in the barometer tube? 576. What prevents the mercury 
 from falling out of the barometer tube! 577. In the common barometer, how is the 
 rise and fall of the mercury indicated ? Why was the wheel barometer invented 1 
 
156 
 
 BAROMETER. 
 
 578. The whole length of the tube of the 
 wheel barometer, Fig. 132, from C to A, is 34 
 or 35 inches, and it is filled with mercury, as 
 usual. The mercury rises in the short leg to 
 the point o, where there is a small piece of 
 glass floating on its surface, to which there 
 is attached a silk string, passing over th.e pul- 
 ley p. To the axis of the pulley is fixed an 
 index, or hand, and behind this is a graduated 
 circle, as seen in the figure. It is obviouS, that 
 a very slight variation in the height of the 
 mercury at o, will be indicated by a considera- 
 ble motion of the" index, and thus changes in 
 the weight of the atmosphere, hardly percepti- 
 ble by the common barometer, will become 
 quite apparent by this. 
 
 579. Heights Measured by the Barometer. 
 The mercury in the barometer tube being sus- 
 tained by the pressure of the atmosphere, and 
 its medium altitude at the surface of the earth 
 
 being 29 to 30 inches, it might be expected that if the instru- 
 ment was carried to a height from the earth's surface, the mer- 
 cury would suffer a proportionate fall, because the pressure must 
 be less at a distance from the earth, than at its surface, and ex- 
 periment proves 'this to be the case. When, therefore, this 
 instrument is elevated to any considerable height, the descent 
 of the mercury becomes perceptible. Even when it is carried 
 to the top of a hill, or high tower, there is a sensible depression 
 of the fluid, so that the barometer is employed to measure the 
 height of mountains and the elevation to which balloons ascend 
 from the surface of the earth. On the top of Mont Blanc, 
 which is about 16,000 feet above the level of the sea, the me- 
 dium elevation of the mercury in the tube is only 14 inches, 
 while on the surface of the earth, as above stated, it is 29 to 
 30 inches. 
 
 580. Diminution of Density. The following table shows at 
 what rate the atmosphere decreases in density, as indicated by 
 the barometer. A part of these numbers are from actual ob- 
 
 Wheel Barometer. 
 
 578. Explain Fig. 132, and describe the construction of the wheel barometer. 5~9. 
 What is stated to be the medium ranjre of the barometer at the surface of the earth 1 
 Suppose the instrument is elevated from the earth, what is the effect on the 
 mercury? How does the barometer indicate the height of mountains? 580. Explain 
 by the table the correspondence between the height, the fall of the mercury, and the 
 temperature. 
 
BAROMETER. 
 
 servations made from ascents in balloons, and a part from esti- 
 mates. The medium pressure of the atmosphere, at the level 
 of the sea, is estimated at 30 inches, and is expressed by 0. 
 
 HEIGHT IN MILES. 
 
 PRESSURE. 
 
 TEMPERATURE. 
 
 
 Inches. 
 
 Fahr. 
 
 
 
 30.00 
 
 50.0 
 
 1 
 
 24.61 
 
 35.0 
 
 2 
 
 20.07 
 
 19.5 
 
 3 
 
 16.35 
 
 3.4 
 
 4 
 
 13.06 
 
 13.3 
 
 5 
 
 10.41 
 
 30.6 
 
 10 
 
 2.81 
 
 126.4 
 
 15 
 
 .45 
 
 240.6 
 
 Thus, according to this estimate, at the height of fifteen miles, 
 the mercury falls to less than half an inch, while the cold is 
 equal to 240 degrees below the zero of Fahrenheit. 
 
 581. Principles of the Barometer applied to the Water 
 Pump. As the efficacy of the pump depends on the pressure 
 of the atmosphere, the barometer will always indicate the height 
 to which it can be effectual at any given place. Thus, on Mont 
 Blanc, where the barometer stands at only 14 inches, being less 
 than one-half its height on the sea level, the water pump would 
 only raise the fluid about 1 5 feet. Hence, engineers and others, 
 who visit elevated countries, should calculate by the barometer, 
 from what depth they can raise water by aerial pressure, before 
 they erect works for this purpose. 
 
 At the city of Mexico and at Quito, for instance, the suction 
 tube can only act to the depth of 22 or 20 feet, while on the 
 Himalay mountains its rise will be only about 8 or 10 feet. 
 
 582. USE AS A WEATHER GLASS. While the barometer 
 stands in the same place, near the level of the sea, the mercury 
 seldom or never falls below" 28 inches, or rises above 31 inches; 
 its whole ran^e, while stationary, being only about 3 inches. 
 
 583. These changes in the weight of the atmosphere, indf- 
 cate corresponding changes in the weather, for it is found, by 
 watching these variations in the height of the mercury, that 
 when it falls, cloudy or falling weather ensues, and that when 
 
 581. How hish will the pump raise water on Mont Blanc? To what height in 
 Mexico and Quito ? 582. How many inches does a fixed barometer vary in height 1 
 5S3 When the mercury falls, what kind of weather is indicated ? When the mer- 
 cury r s~s. what kind of weather may be expected ? When fog and smoke descenj 
 toward the ground, is it a sign of a light or heavy atmosphere 1 
 
158 BAROMETER. 
 
 it rises, fine clear weather may be expected. During the time 
 when the weather is damp and lowering, and the smoke of 
 chimneys descends toward the ground, the mercury remains de- 
 pressed, indicating that the weight of the atmosphere, during 
 such weather, is less than it is when the sky is clear. This con- 
 tradicts the common opinion, that the air is the heaviest when 
 it contains the greatest quantity of fog and smoke, and that it 
 is the uncommon weight of the atmosphere which presses these 
 vapors toward the ground. 
 
 584. A little consideration will show, that in this case the 
 popular belief is erroneous, for not only the barometer, but all 
 the experiments we have detailed on the subject of specific grav- 
 ity, tend to show that the lighter any fluid is, the deeper any 
 substance of a given weight will sink in it. Common observa- 
 tion ought, therefore, to correct this error, for every body knows 
 that a heavy body will sink in water, while a light one will 
 swim, and by the same kind of reasoning ought to consider, 
 that the particles of vapor would descend through a light atmos- 
 phere, while they would be pressed up into the higher regions 
 by a heavier air. 
 
 585. The following indications of the barometer with respect 
 to the weather, may be depended on as correct, being tested by 
 the observations of the author : 
 
 I. In calm weather, when the wind, clouds, or sun, indicate 
 approaching rain, the mercury in the barometer is low. 
 
 II. In serene, fine, settled weather, the mercury is high, and 
 often remains so for days. 
 
 III. Before great winds, and during their continuance, from 
 whatever quarter they come, the mercury sinks lowest, and 
 especially if they come from the south. 
 
 IV. During the coldest, clear days, when a gentle wind from 
 the north or west prevails, the mercury stands highest. 
 
 V. After great storms, when the mercury has been lowest, it 
 rises most rapidly. 
 
 VI. It often requires considerable time for the mercury tc 
 gain its wonted elevation after a storm ; and on the contrary, 
 it sometimes rains without the usual corresponding change in 
 its altitude. 
 
 VII. In general, whether there are any appearances of change 
 in the horizon or not, we may prognosticate rain whenever the 
 mercury sinks during fine weather. 
 
 584. By what analogy is it shown that the air is lightest when filled with vapor 1 
 685. Mention the indications of the barometer concerning the weather. 
 
LIB/ 
 
 WATER PUMPS. JT ^ V ^ffcr TEE 
 
 [. When it rains with the mercury high, w$ may be sure 
 that it will soon be fair. 
 
 586. USE AT SEA. The principal use of the baroi 
 board of ships, where it is employed to indicate the aj 
 of storms, and thus to give an opportunity of preparing accord- 
 ingly ; and it is found that the mercury suffers a most remark- 
 able depression before the approach of violent winds, or hurri- 
 canes. The watchful captain, particularly in southern latitudes, 
 is always attentive to this monitor, and when he observes the 
 mercury to sink suddenly, 'takes his measures without delay to 
 meet the tempest. During a violent storm, we have seen the 
 wheel barometer sink a hundred degrees in a few hours. 
 
 58.7. Preservation by the Barometer. But we can not illus- 
 trate the use of this instrument at sea better than to give the 
 following extract from Dr. Arnot, who was himself present at 
 the time. " It was," he says, " in a southern latitude. The sun 
 had just set with a placid appearance, closing a beautiful after- 
 noon, and the usual mirth of the evening watch proceeded, 
 when the Captain's orders came to prepare with all haste for a 
 storm. The barometer had begun to fall with appalling ra- 
 pidity. As yet, the oldest sailors had not perceived even a 
 threatening in the sky, and were surprised at the extent and 
 hurry of the preparations ; but the required measures were not 
 completed, when a more awful hurricane burst upon them than 
 the most experienced had ever braved. Nothing could with- 
 stand it ; the sails, already furled, and closely bound to the 
 yards, were riven into tatters ; even the bare yards and masts 
 were in a great measure disabled ; and at one time the whole 
 rigging had nearly fallen by the board. Such, for a few hours, 
 was the mingled roar of the hurricane above, of the waves 
 around, and the incessant peals of thunder, that no human voice 
 could be heard, and amidst the general consternation, even the 
 trumpet sounded in vain. On that awful night, but for a little 
 tube of mercury which had given the warning, neither the 
 strength of the noble ship, nor the skill and energies of her 
 commander, could have saved one man to tell the tale." 
 
 WATER PUMPS. 
 
 588. The efficacy of the common pump in raising water, de- 
 pends upon the force of atmospheric pressure, the principles of 
 
 586. Of what use is the barometer on board of ships! When does the mercury 
 suffer the most remarkable depression 1 5S7. What remarkable instance is stated, 
 where a ship seemed to be saved by the use of the barometer 1 583. On what does 
 th efficacy of the common pump depend ] 
 
160 
 
 WATER PUMPS. 
 
 FIG. 133. 
 
 which have been fully illustrated under the articles, Air Pump 
 and Barometer. 
 
 589. An experiment, of jvhich few are ignorant, and which 
 all can make, shows the principle of the pump in a very strik- 
 ing manner. If one end of a straw be dipped into a vessel of 
 liquid, and the other end be sucked, the liquid will rise into the 
 mouth, and may be swallowed. 
 
 The principles which this experiment involves are exactly the 
 same as those concerned in raising water by the pump. The 
 vessel of liquid answers to the well, the straw to the pump log, 
 and the mouth acts as the piston, by which the air is removed. 
 
 Water pumps are of three kinds, namely, the sucking, or com- 
 mon pump, the lifting pump, arid the forcing pump. 
 
 590. COMMON METALLIC 
 PUMP. This (Fig. 133,) 
 consists of a brass or iron bar- 
 rel, A, containing at its up- 
 per part a hollow piston and 
 valve, opening upward. Be- 
 low this there is another 
 valve, also opening upward. 
 The pipe and stop-cock C, 
 are for the purpose of letting 
 the water from the barrel to 
 the tube, which descends into 
 the well. 
 
 The action of this pump 
 depends on the pressure of 
 the atmosphere, and will be 
 readily understood by the 
 pupil who has learned what 
 is said under the articles air 
 pump and barometer. 
 
 591. On raising the lever, 
 D, the piston, A, descends 
 down the barrel, the lower 
 valve, B, at the same mo- 
 ment Closing by the weight Common Metallic Pump. 
 
 of the water, while the up- 
 
 5S9. What experiment is stated, as illustrating the principle of the common pump 1 
 How many kinds of pumps are mentioned 1 590. Which kind is the common 1 De- 
 scribe the common pump. Explain how the common pump acts. 591. When the 
 ever is raised, what takes place in the pump-barrell When it is depressed, what 
 ,akes place ? 
 
WATER PUMPS. 
 
 161 
 
 FIG. 134. 
 
 per one opens and lets the water through. Then, on depress- 
 ing the lever, the piston rises, its valve closing, and elevating 
 the water above it. By this action a vacuum would be formed 
 between the two valves, did not the lower one open and admit 
 the water through the pipe above it. The lever again being 
 worked, the same process is repeated, and the water is elevated 
 to the spout in an interrupted stieam. 
 
 The tube, with the stop-cock C, leading from the barrel to 
 the pipe, is added for the purpose of letting the water escape 
 from the former in cold weather, and thus prevent its freezing. 
 
 592. Although, in common language, this is called the suc- 
 tion pump, still it will be observed that the water is elevated 
 by suction, or, in more philosophical terms, by atmospheric 
 pressure, only above the valve A, after which it is raised by lift- 
 ing up to the spout. The water, therefore, is pressed into the 
 pump-barrel by the atmosphere, and thrown out by the power 
 of the lever. 
 
 593. LIFTING PTJMP. The 
 lifting pump, properly so called, 
 has the piston in the lower end 
 of the barfll, and raises the 
 water through the whole dis- 
 tance, by forcing it upward, 
 without the agency of the at- 
 mosphere. 
 
 In the suction pump, the 
 pressure of the atmosphere will 
 raise the water 33 or 34 feet, 
 and no more, after which it 
 may be lifted to any height re- 
 quired. 
 
 594. FORCING PUMP. The 
 forcing pump differs from both 
 these, in having its piston solid, 
 or without a valve, and also in 
 having a side pipe, through 
 which the water is forced, in- 
 stead of rising in a perpendicu- 
 lar direction, as in the others. Forcing Pump. 
 
 595. The forcing pump is 
 
 represented by Fig. 134, where A is a solid piston, working 
 
 592. How far is the water raised by atmospheric pressure, and how far by lifting? 
 593. How does the lifting pump differ from the common pump ? 594. How does the 
 forcing pump differ from the common pump ? 
 
162 WATER PUMPS. 
 
 air-tight in its barrel. The tube, C, leads from the barrel to 
 the air-vessel, D. Through the pipe, P, the. water is thrown 
 into the open air. G is a guage, by which the pressure of the. 
 water in the air-vessel is ascertained. Through the pipe I, the 
 water ascends into the barrel, its upper end being furnished 
 with a valve opening upward. 
 
 To explain the action of this pump, suppose the piston to be 
 down to the bottom of the barrel, and then to be raised upward 
 by the lever L ; the tendency to form a vacuum in the barrel, 
 will bring the water up through the pipe I, by the pressure of 
 the atmosphere. Then, on depressing the piston, the valve at 
 the bottom of the barrel will be closed, and the water, not find- 
 ing admittance through the pipe, whence it came, will be forced 
 through the pipe C, and opening the valve at its upper end, 
 will enter into the air-vessel D, and be discharged through the 
 pipe P, into the open air. 
 
 The water is therefore elevated to the piston-barrel by the 
 pressure of the atmosphere, and afterward thrown out by the 
 force of the piston. It is obvious, that by this arrangement, 
 the height to which this fluid may be thrown, wijl depend on 
 the power applied to the lever, and the strength witn which the 
 pump is made. 
 
 596. The air-vessel D contains air in its upper part only, the 
 lower part, as we have already seen, being filled with water. 
 The pipe P, called the discharging pipe, passes down into the 
 water, so that the air can not escape. The air is therefore com- 
 pressed, as the water is forced into the lower part of the vessel, 
 and reacting upon the fluid by its elasticity, throws it out of the 
 pipe in a continued stream. The constant stream which is 
 emitted from the direction pipe of the fire-engine, is entirely 
 owing to the compression and elasticity of the air in its air-ves- 
 sel. In pumps, without such a vessel, as the water is forced 
 upward only while the piston is acting upon it, there must be 
 an interruption of the stream while the piston is ascending, as 
 in the common pump. The air-vessel is a remedy for this de- 
 fect, and is found also to render the labor of drawing the water 
 more easy, because the force with which the air in the vessel 
 acts on the water, is always in addition to that given by the 
 force of the piston. 
 
 595. Explain Fift. 134, and show in what manner the water is brought up through 
 the pine I, and afterward thrown out at the pipe P. 596. Why does not the air es- 
 cape from the air-vessel in this pump 1 What effect does the air-vessel have on the 
 stream discharged 1 Why does the air-vessel render the labor of raising the water 
 more easy 1 
 
WATER PUMPS. 
 
 163 
 
 FIG. 135. 
 
 597. ATMOSPHERIC AND FORCING PUMP. A curious com 
 bi nation of the atmospheric and forcing pumps, is the following 
 Fig. 135. 
 
 The atmospheric, is furnished with a rod 
 and piston, with the valve C, opening in the 
 usual manner. The forcing piston B, is of 
 solid metal, working water-tight in its bar- 
 rel. The barrels are joined below the valve 
 D, their pistons being also connected by a 
 cross-bar, A, between the rods, so that they 
 rise and fall together. 
 
 Now when the lever is depressed, and the 
 pistons raised, the water above the valve C 
 is discharged at the spout in the manner of 
 the common suction pump, and the space 
 is filled by atmospheric pressure through 
 the lower valve D, by the suction pipe. 
 When the pistons descend, this valve closes, 
 and the solid piston B, drives the water 
 through the valve C, and above that piston 
 and to the spout. Thus one piston operates 
 when the lever rises, and the other when it 
 falls, producing in effect a constant stream 
 of water from the spout. 
 
 In the construction of this pump, it should 
 be considered that as both cylinders are 
 filled at the same time, the suction pipe 
 ought to be large in proportion. 
 
 598. STOMACH PUMP. The design of 
 
 this pump, of which there are several varieties, is to throw a 
 fluid into the stomach, and again to withdraw it without chang- 
 ing the apparatus, but only its position. In cases of poisoning, 
 the contents of the stomach may thus be diluted and withdrawn, 
 including the deleterious matter, and thus the life of the indi- 
 vidual be saved. 
 
 599. That here described is from the Journal of the Franklin 
 Institute* It consists of a common metallic syringe, A, Fig. 
 136, screwed to a cylindrical valve-box, B, which contains two 
 ovoid cavities, in each of which there is a loose, spherical me- 
 tallic valve. The ends of the cavities are pierced, and the valves 
 
 597. What is the difference between the pump, ^isr. 135, and the common atmos. 
 pheric and forcing pump 7 598. What is the use of tne stomach pump 1 599. De- 
 scribe the stomach pump, and shgw the reason why it acts in opposite ways on being 
 turned over. 
 
 Atm 
 
 'spheric and Forc- 
 ing Pump. 
 
164 WATER PUMPS. 
 
 FIG. 136. 
 
 Stomach Pump. 
 
 fit exactly, either of the orifices. Thus it makes no difference 
 which end of the valve-box is upturned, the valve falling down 
 and closing the orifices in either direction. The flexible India 
 rubber tubes, C D, are attached to the opposite ends of the 
 cavities. 
 
 Now suppose the then upper tube is introduced into the 
 stomach, and the lower one into a basin of warm water ; in this 
 position, on working the syringe the liquid would be injected 
 into the stomach, and the poison diluted ; then on reversing the 
 position, by turning the syringe in the hand, without withdraw- 
 ing the tube from the stomach, the valves drop on the other 
 orifices, and the water is pumped from the stomach into the 
 basin, as represented by the figure. 
 
 This is an interesting and beautiful invention", and no doubt 
 has been the instrument of saving many human lives in cases 
 of poisoning. 
 
 600. FIRE ENGINE. The fire engine is a modification of the 
 forcing pump. It consists of two such pumps, the pistons of 
 which are moved by a lever with equal arms, the common ful- 
 crum being at C, Fig. 137. While the piston A is descending, 
 the other piston, B, is ascending. The water is forced by the 
 pressure of the atmosphere, through the common pipe P, and 
 then dividing, ascends into the working barrels of each piston, 
 where the valves, on both sides, prevent its return. By the 
 alternate depression of the pistons, it is then forced into the air- 
 box D, and then, by the direction pipe E, is thrown where it is 
 
 600. Explain Fig. 137, and describe the action of the fire engine. What causes the 
 continued stream from the direction pipe of this engine ? 
 
WATER PUMPS. 
 
 165 
 
 FIG. 137. 
 
 FIG. 13& 
 
 Fire Engine. 
 
 wanted. This machine acts precisely like the forcing pump, 
 only that its power is doubled, by having two pistons instead 
 of one. 
 
 601. ROTARY PUMP. This is an ancient invention, though 
 more than once re-invented and constructed in various forms in 
 modern times. That here represented, Fig. 138, according to 
 Mr. Ewbank, from whom the cut is taken, is one of the oldest, 
 as well as best, ever constructed. 
 
 The design is to produce a continued stream, by simply turn- 
 ing a crank, thus converting the piston into cog-wheels, and the 
 vertical motion into a rotary one. 
 
 Its construction is as follows : Two metallic cog-wheels, with 
 obtuse teeth, are inclosed in a metallic case, so nicely fitted to 
 each other that the water can not escape between them. The 
 teeth also work so accurately between each other as to retain 
 the fluid. The axle of one of the wheels is continued through 
 one side of the case to receive the crank by which it is turned, 
 the joint being secured by a collar of leather. 
 
 One side of the case being removed in the figure to show the 
 construction, it will be observed that the motion of one wheel. 
 
 . 
 by the figure. 
 
 said of the antiquity of the rotary pump 1 Explain its construction 
 What objection to this pump is stated ? 
 
166 
 
 WATER PUMPS. 
 
 will turn the other in the opposite direction, the arrows show- 
 ing the course of the water. 
 
 Now the wheels being water-ti^ht between themselves and 
 both sides of the case, the only vacant spaces for the water are 
 those between the cogs, as they revolve, and the diameter of 
 the case. 
 
 The machine being put in motion, the water enters the case 
 by the suction pipe B, is carried up by the cogs in succession, 
 and these being always in contact, it can not escape except at 
 the forcing pipe A, where it issues in a continued stream. This, 
 therefore, is a suction and forcing pump in one. 
 
 But the friction is such between the metallic surfaces that the 
 machine remains perfect only for a short time, nor does it ap- 
 pear that the recent improvements in this sort of pump have 
 been such as to bring it into general use, and the defects of the 
 plan seem to be insuperable. 
 
 602. FOUNTAIN OF HIERO. There FIG. 139. 
 
 is a beautiful fountain, called the 
 fountain of Hiero, which acts by 
 the elasticity of the air, and on the 
 principle of hydrostatic pressure. Its 
 construction will be understood by 
 Fig. 139, but its form may be varied 
 according to the dictates of fancy or 
 taste. The boxes A and B, together 
 with the two tubes, are made air- 
 tight, and strong, in proportion to 
 the height it is desired the fountain 
 should play. 
 
 To prepare the fountain for action, 
 fill the box A through the spouting 
 tube, nearly full of water. The tube 
 0, reaching nearly to the top of the 
 box, will prevent the water from 
 passing downward, while the spout- 
 ing pipe will prevent the air from mero's Fountain. 
 escaping upward, after the vessel is 
 
 about half filled with water. Next, shut the stop-cook of the 
 spouting pipe, and pour water into the open vessel D. This 
 will descend into the vessel B, through the tube E, which nearly 
 reaches its bottom, so that after a few inches of water are poured 
 
 602. How is the fountain of Hiero constructed ? On w^at will the height of the jet 
 from Hiero'g fountain depend 7 
 
DISTRIBUTION OF HEAT. 167 
 
 in, no air can escape, except by the tube C, up into the vessel 
 A. The air will then be compressed by the weight of the 
 column of water in the tube E, and therefore the force of the 
 water from the jet-pipe will be in proportion to the height of 
 this tube. If this tube is 20 or 30 feet high, on turning the 
 stop-cock, a jet of water will spout from the pipe that will amuse 
 and astonish those who have never before seen such an experiment. 
 
 CHAPTER VIII. 
 
 HEAT, AND THE LAWS OF ITS ACTION. 
 
 603. In respect to the laws of incidence and reflection, and 
 in many other respects, the phenomena of light and heat are the 
 same. But in respect to transmission, radiation, distribution, 
 effects on other substances, both chemical and mechanical, and 
 the manner in which it affects our senses, there are, it is well 
 known, great differences. 
 
 DISTRIBUTION OF IIEAT. 
 
 604. The rays of heat falling on a body are disposed of in 
 three ways. First, they may be reflected, or rebound from the 
 surface ; second, they may be absorbed or received into the sub- 
 stance of the body ; or third, they may be transmitted, or pass 
 through its substance. 
 
 605. Reflection. Radiant heat, that is, heat flowing from 
 any hot body, is like light reflected from polished surfaces, and 
 as in light, the angle of reflection is equal to that of incidence. 
 Those surfaces, however, which reflect light most perfectly, are 
 not always the best reflectors of heat. Thus, polished metals 
 are the best reflectors of heat, while glass, which reflects light 
 most perfectly, is a very imperfect reflector of heat ; thus tin 
 plate reflects about eight times as much heat as a glass mirror. 
 
 606. Absorption. Radiant heat is absorbed with very differ- 
 ent facilities by bodies and surfaces of different kinds. Those 
 
 603. In what respects are action of heat and light the same ? In -what respects are 
 their phenomena dissimilar ? 604. In what ways are the rays of light diffused 1 605 
 What is meant by rellection of heat 7 606. What by absorption ? What by radia 
 tion 1 What surfaces reflect heat best 7 Give examples. What surfaces possess the 
 greatest absorbing powers 7 Examples. 
 
168 DISTRIBUTION OF HEAT. 
 
 surfaces which radiate most readily, absorb heat with the great- 
 est, facility, and on the contrary, those surfaces which radiate 
 feebly, do not readily absorb heat. Thus a plate of tin, if 
 painted black, will both absorb, and radiate perfectly ; while, if 
 the surface retains its bright metallic polish, it will neither ab- 
 sorb nor radiate. Hold the black surface near the fire, and the 
 metal will soon become too hot for the fingers ; while the bright 
 surface will not become even warm, by the same exposure 
 There is also a difference between culinary heat and that of the 
 sun, with respect to absorption, for if a piece of plate glass be 
 held before the fire it soon becomes hot, while every window 
 shows by the temperature of the glass, that it does not absorb 
 the heat of the sun. 
 
 607. Transmission. Most transparent substances transmit 
 heat, that is, allow it to pass through their pores, with more or 
 less facility; in this respect, however, experiment proves that 
 there are great differencesjn bodies, where from external ap- 
 pearance, little or none might be expected. Thus, rock-crystal 
 transmits heat very perfectly, while alum, though equally trans- 
 parent, admits few of the calorific rays, to pass through it. This 
 difference is so great, that a piece of smoky, brown rock-crystal, 
 which was fifty-eight times thicker than a transparent plate of 
 alum, transmitted 19 rays, while the alum transmitted only 6. 
 The cause of this remarkable difference is unknown, though 
 probably it depends on the crystaline structure of the two 
 substances. 
 
 608. Operation of these Laws. The general diffusion of heat 
 seems to depend on the operation of the above described natural 
 laws, and hence it is, that in the same vicinity, two thermome- 
 ters graduated alike, and equally exposed, always indicate the 
 same temperature. 
 
 609. When the sun, that universal source of heat, as well as 
 of light, radiates his rays upon the earth, they are absorbed by 
 some bodies, and transmitted or reflected by others, according 
 to their several powers, or natures. But the great means of the 
 general and equal diffusion of heat, is the earth itself, arid the 
 atmosphere with which it is surrounded. Having absorbed the 
 radiant heat of the sun, the ground becomes in its turn, a radiant 
 source to all surrounding objects, while the atmosphere acts as 
 a perpetual absorbent, rising up from the earth, in proportion 
 
 607. Give examples of the transmission of heat through substances. 608. What ar? 
 the means of the general diffusion of heat 1 609. By what means is it said this law is 
 illustrated in rooms? 
 
THERMOMETER. 169 
 
 *o the quantity of heat it obtains, and again sinking down, in 
 cooler places. Thus there is a constant interchange among the 
 warmer, and cooler strata of the atmosphere, while currents in 
 the form of wind, tend to mix these with each other, making 
 the temperature, at the same distance from the earth and in the 
 sane vicinity every where the same. This law of equal distri- 
 bution is strikingly illustrated in rooms warmed by the admis- 
 sion of hot air from beneath, for although the register, or place 
 of admission may be in one corner, or through the partition, still 
 the temperature m in every part of the room, with the exception 
 of over the register, is the same. Even rooms, 30 or 40 feet iu 
 length, and when the air is admitted through only one register, 
 and this in a corner, are made equally comfortable throughout, 
 by this admirable method. 
 
 THERMOMETER. 
 
 610. Did not the heat diffuse itself as above described, the 
 thermometer would be entirely itseless, since several in the same 
 vicinity, though graduated exactly alike, would indicate different 
 temperatures. 
 
 611. The term thermometer comes from two Greek words, 
 signifying heat measurer ; and its use strictly corresponds to the 
 name, being an instrument for comparing the degrees of free 
 heat existing in other bodies. This it does by the expansion 
 and contraction of a fine thread of mercury, confined in a glass 
 tube, having a small reservoir of the same metal at the lower 
 end, called the bulb. ' 
 
 612. Mercury is employed for this purpose for several rea- 
 sons ; one is that fluids, as alcohol, occupy too much space ; 
 another, that this metal is more uniform in expanding and con- 
 tracting than any other substance ; and lastly, it is not liable to 
 vaporize in the vacuum in which it is placed, and thus, like 
 liquids, to interfere with its own variation in the stem. 
 
 613. ALCOHOLIC THERMOMETER. Although mercury, or 
 quicksilver, is the best substance known for the construction of 
 thermometers, and is that universally employed in temperate 
 climates, yet it is objectionable in extreme, or polar latitudes, 
 on account of its liability to freeze. In Siberia, and other north- 
 ern inhabited regons, where the cold is often down to 40 of 
 Fahrenheit's scale, alcoholic thermometers are of necessity em- 
 
 610. What issaid of the use of the thermometer without an equal diffusion of h at T 
 611. What does thermometer mean ? 612. Why is mercury used in thermometers "n 
 preference to liquids ? 613. Why are alcoholic thermometers used 1 
 
 8 
 
170 THERMOMETER. 
 
 ployed, since at that point mercury becomes solid by freezing, 
 and therefore useless. These thermometer tubes are much 
 longer than ordinary, since alcohol expands in a greater propor- 
 tion than mercury by the same increment of heat. 
 
 614. Different Mercurial Thermometers. There are three 
 thermometers in general use, namely, Fahrenheit^, which is 
 ir^ed in England, and in this country ; the Centigrade, con- 
 
 tructed by Celsius, which is generally used in France ; and 
 Reaumur's thermometer, adopted in Germany.' 
 
 615. Fahrenheit, (Fah.) In this the intermediate space be- 
 tween the freezing and boiling points is divided into 180 de- 
 grees ; the freezing being marked 32, and the boiling 212. 
 This scale was invented by Fahrenheit, from an erroneous belief 
 that 32 of these divisions below the freezing point of water, 
 which is therefore on -the scale, indicated the zero, or greatest 
 degree of cold. But he afterward discovered his error, and his 
 instrument being in use, corrected it as far as possible, by add- 
 ing a series of descending degrees below his zero, prefixing to 
 them the sign , or minus, that is, below zero. 
 
 616. Centigrade, (Cent.) It is also sometimes indicated by 
 Cel, for the name of the inventor. It consists of an arrange- 
 ment of the scale, in which the freezing point is marked 0, or 
 zero, and the boiling point is marked 100. This is a more 
 convenient scale than the other, the freezing and boiling points 
 being even numbers, and all below the former minus. 
 
 617. Reaumur, (Beau.) In this the freezing point, as in 
 the laSt, is marked 0, while the boiling f>oint, instead of being 
 100, is marked 80. The degrees are continued both above 
 and below these points, those below being negative or minus, 
 as in the others. 
 
 These Thermometers Compared. In books of foreign travels, 
 where the author adopts the thermometer of the country he de- 
 scribes, the reader is often perplexed to know what degrees of 
 temperature are indicated according to his own scale, by what 
 he reads. Figures are therefore added of each, Fig. 140, to- 
 gether with a table showing the correspondence of the three, 
 and the rules for converting one scale into the others. 
 
 618. Thus the Centigrade scale is reduced to that of Fahren- 
 heit, by multiplying by 9 and dividing by 5, and that of Reau- 
 
 614 What are the names of the mercurial thermometers? 615. What are the 
 div sions of Fahrenheit's scale? 616. What are those of the Centigrade 1 617. Wha' 
 are those of Reaumur 7 618. Ho v is the Centigrade reduced to that of Fahrenheit t 
 Itow is that of Reaumur reduced 'o that of Fahrenheit 7 
 
THERMOMETER 
 FIG. 140. 
 
 171 
 
 Fahrenheit. 
 
 Centigrade. 
 
 Reaumur. 
 
 mur to that of Fahrenheit, by multiplying by 9 and dividing by 
 4 ; or that of Fahrenheit to either of the others by reversing 
 these processes. Examples : 
 
 Cent. 100 X 9 = 900-4-5 = 180+32 = 212 Fah. 
 Reau. 80-' X 9 = 720-4-4 = 180 + 32 = 212 Fah. 
 Fah. 212 32 = 180x5 = 900-4- 9 = 100 Cent. 
 Fah. 212 32 = 180 X4 = 720^- 9= 80 Reau. 
 
 The following Table from Prof. Hoblyn's Dictionary of Science, 
 shows at a single view the correspondence between these ther- 
 mometers, from the zero to the boiling point of Fahrenheit. 
 
 Fahrenheit. Centigrade. Reaumur. 
 
 BOILING. 212 100 80 
 
 200 93.33 74.66 
 
 190 87.77 70.22 
 
 180 82.22 65.77 
 
 170 7G.C6 61.33 
 
 160 71.11 56.88 
 
 150 65.55 52.33 
 
 140 60 48 
 
 130 55.55 33.55 
 
 120 . 48.88 . . 39.11 
 
172 
 
 THERMOMETER. 
 
 Fahrenheit. Centigrade. 
 
 110 43.33 . 
 100 37.77 . 
 90 32.22 . 
 80 26.^>fi 
 
 Reaumur. 
 . . . .34.66 
 .... 30.22 
 .... 25.77 
 21 33 
 
 FREEZING* 
 ZERO. 
 
 70 
 
 21*11 
 
 16 88 
 
 60 
 50 
 
 15.55 . 
 10 
 
 .... 12.44 
 . . 8 
 
 40 
 
 4 44 
 
 3 35 
 
 32 
 
 
 
 . . . . 
 
 20 
 
 6 66 . 
 
 5 33 
 
 10 
 
 . 
 
 12.22 . 
 
 17.77 . 
 
 . . . . 9.77 
 
 . 14.22 
 
 619. Rutherford's Register Thermometer. By this, the high- 
 est and lowest temperatures which occur within a given time 
 are indicated, and made to register themselves. This instru- 
 ment consists of two thermometers fastened to the same plate 
 with their tubes in a horizontal position, as shown by Fig. 141. 
 
 FIG. 141. 
 
 Rutherford's Register Thermometer. 
 
 One of these, A, contains alcohol ; the other, B, contains mer- 
 cury. In the stem of B, a small piece of iron wire acts the part 
 of an index, being propelled forward as the mercury expands, 
 and being left at the point of the greatest expansion when the 
 mercury contracts, thus indicating the highest temperature to 
 which it had been exposed. In the stem of the other, a small 
 piece of ivory, A, is immersed in the alcohol, and by a slight 
 inclination of the instrument, is brought to the surface of the 
 liquid. When the temperature falls, the ivory, by adhering to 
 the liquid, is drawn back with it ; but when it rises, the spirit 
 only advances, leaving the ivory behind, thus indicating the 
 lowest temperature which had occurred since the last observa- 
 
 619. What are the indications made by Rutherford's thermometer? Describe the 
 construction of this instrument. What are the peculiar advantages of this in 
 strument 1 
 
HYGROMETER. 
 
 173 
 
 1FIG - 142 - 
 
 tion. By inverting the instrument, the particle of ivory is again 
 brought to its place for a new observation. This is a very con- 
 venient thermometer on many accounts. Thus the highest 
 temperature during the day or the week, can be told without 
 watching the instrument, and at a single inspections If it is re- 
 quired to obtain the degree of heat at the bottom of a deep 
 well, or in the depths of the sea, this can be done accurately by 
 letting down the instrument, while the common thermometer 
 would change while drawing it up. 
 
 620. DIFFERENTIAL THERMOMETER. 
 This instrument is shown by Fig. 142. 
 It consists of two thin glass bulbs of an 
 inch in diameter, contacted by a glass 
 tube bent at right angles, as the figure 
 shows. This tube is partly filled with col- 
 ored alcohol. Now when one of the bulbs 
 is heated more than the other, the air in 
 it expands, and drives the liquid into the 
 other bulb. 
 
 621. It does not, therefore, indicate the 
 temperature of the atmosphere, as the 
 
 same degree of heat on both bulbs at the same time produces no 
 change, its design being merely to show the difference of tem- 
 perature to which the bulbs are exposed. 
 
 HYGROMETER. FIG. 143. 
 
 622. The name of this instrument, 
 from the Greek, signifies " moisture 
 measurer." Its use is, to show the state 
 of moisture in the atmosphere. Many 
 inventions for this purpose have been 
 tried, but that represented by Fig. 143, 
 is at present considered the best. 
 
 It is called DanieVs dew-point hygrorz- 
 eter. It consists of two balls, connected 
 together by a bent tube, as shown by the 
 figure, the whole being of glass. The 
 ball B, contains a small quantity of 
 ether, by the boiling of which, the air 
 has been expelled from the tube. In it 
 a small thermometer is placed, with its 
 bulb in the ball. The lower part of this 
 ball is gilded, that the deposited dew __ - 
 
 may be visible. The other ball, A, is Hygrometer. 
 
174 STEAM ENGINE. 
 
 covered with muslin, and is kept moist with ether, the evapora 
 tion of which produces cold, which gradually, by the evaporation 
 of the ether in the other ball, reduces the temperature in that 
 to the dew-point, which is indicated by the deposition of moisture 
 on the gilded ball. 
 
 The degree of temperature at which this deposition takes 
 place, is shown by the thermometer in the tube, and this degree 
 is called the dew-point, and this is effected at a higher or lower 
 degree, according to the moisture in the atmosphere. The ther- 
 mometer on the stem indicates the temperature of the air at tha 
 time when the observations are made. 
 
 CHAPTER IX. 
 
 STEAM ENGINE. 
 
 NOTE. The following description of the steam engine is 
 taken from Prof. Hoblyn's Edition of the Author's Natural 
 Philosophy, published by Adam Scott, Charter-house Square, 
 London. 
 
 We have however omitted, in this edition, the ingenious ma- 
 chines of Hero, Branca, and Savery, contained in former copies, 
 as merely showing the progress of invention, and being quite 
 unnecessary for the comprehension of the engine, as it exists 
 at the present day. This omission will be found replaced by 
 some of the most important inventions of the present day. 
 
 The description of Newcomen's engine has been retained, as 
 containing some parts, leading to the explanation of Watt's en- 
 gine, by which it was succeeded. 
 
 What is meant by the double-action of Watt's engine, con- 
 sisted in the application of steam alternately on each side of the 
 piston, and by which it was moved both up and down, while 
 that of Newcomen was moved only in one direction by the 
 
 620. What is the construction of the differential thermometer? 621. What is the 
 use of this instrument ? 622. What is the meaning of the term hygrometer ( What 
 B that here described called 1 Explain its principle, and the manner of using it. 
 
BTEAM ENGINE. 175 
 
 steam, and in the other by the pressure of the atmosphere over 
 a vacuum. The importance of Watt's invention can hardly be 
 appreciated, since on it is founded the action of all steam engines 
 to this day. 
 
 623. NEWCOMEN'S ATMOSPHERIC ENGINE. The drainage of 
 deep mines was a matter of great importance, and the failure 
 of Silvery 's engine in this respect, paved the way to further ex- 
 periment. In 1705, Thomas Newcomen, a smith of Dartmouth, 
 obtained letters patent for the construction of a new kind of 
 steam engine, in which he availed himself of the atmospheric 
 pressure in a different way from that adopted by Savery. 
 
 624. The novelty of this plan consists in the admission of 
 steam beneath an air-tight piston, and the condensation of the 
 steam by the injection of cold water into the interior of the cyl- 
 inder. The use of a cylinder and piston may be easily ex- 
 plained. In order that the pressure of steam may be rendered 
 available in machinery, the steam must be confined within an 
 air-tight cavity, so constructed that its dimensions, or capacity, 
 may be altered without altering its tightness. When the steam 
 enters such a vessel, it enlarges its actual cavity, by causing 
 some movable part to recede before it, and from this movable 
 part motion is communicated to machinery. A hollow cylinder, 
 having a movable piston accurately fitted to its bore, constitutes 
 a vessel of this kind ; the piston, thus employed, has an alternate 
 or reciprocating vertical motion, which may be converted into 
 a circular motion by appropriate machinery. The engine em- 
 ployed by Newcomen, in its most improved state, was as fol- 
 lows. Over a boiler a is fixed a cylinder c, containing a piston 
 r, the rod of which is connected with one of the arched extrem- 
 ities of a lever-beam working on a pivot ; to the other extremity 
 of the beam is attached a chain connected with the pump-rod. 
 
 625. Such is the simple outline of the atmospheric engine. 
 Its mode of operation is as follows : Steam is admitted from 
 the boiler into the cylinder, through the tube I, by means of a 
 regulating cock, e, which is worked by a handle outside the 
 boiler ; the pressure of the atmosphere above the piston being 
 thus balanced by the force of the steam beneath it, the extremity 
 of the lever-beam to which the piston is attached is elevated by 
 proportionate weights, w, attached to the purnp-rod, and the 
 piston is drawn to the top of the cylinder, the other extremity 
 of the beam being depressed. 
 
 623. What was Newcomen's engine called 1 624. What is said to have been the 
 novelty of Newcomeivs plan 7 How can the cavity of a vessel be enlarged by steam 
 and still be tight? 625. Describe this machine by the figure. 
 
176 
 
 STEAM ENGINE. 
 FIG. 144. 
 
 Newcomen's Engine, 
 
 626. In order to effect the descent of the piston, the steam in 
 the cylinder must now be condensed. The regulating cock e is 
 accordingly closed, and the further admission of steam pre- 
 vented ; another cock, called the condensing cock, p, is now 
 opened, and a jet of cold water is admitted through a tube from 
 the cistern m, which is placed at a sufficient height to insure a 
 forcible injection ; the steam in the cylinder is instantly con- 
 densed, a vacuum is formed, and the pressure of the atmosphere 
 forces the piston to the bottom of the cylinder, while the pump- 
 rod on the other end of the beam is raised. Such is the gc'i- 
 eral operation of Newcomen's atmospheric engine, which is 
 merely a pump worked by steam. 
 
 627. WATT'S DOUBLE-ACTING ENGINE. In considering the 
 applicability of the steam engine to manufactures generally, it 
 
 I 626. Uow was the steam condensed 1 627. What was Watt's great improvement 
 in the steam engine ? 
 
STEAM ENGINE. IVY 
 
 occurred to Watt, that if lie could contrive to admit steam 
 alternately above and below the piston, and, at the same time, 
 produce a vacuum alternately below and above the piston, a 
 double-acting cylinder would be produced, an impulse thus be 
 communicated by the ascent, as well as by the descent of the 
 piston, and a uniform continuous action be effected. It was de- 
 sirable, also, to convert this reciprocating action into a circulai 
 one. 
 
 628. On this subject "Watt observes: "Baring made my 
 single reciprocating engines very regular in their movements, I 
 considered how to produce rotative motions from them in the 
 best manner; and among various schemes which were sub- 
 jected to trial, or which passed through my mind, none appeared 
 so likely to answer the purpose as the application of the crank, 
 in the manner of the common turning lathe ; but as the rota- 
 tive motion is produced in that machine by impulse given to 
 the crank in the descent of the foot only, it requires to be con- 
 tinued in its ascent by the energy of the wheel, which acts as a 
 
 fly- 
 
 629. "Being unwilling to load my engine with a fly-wheel 
 heavy enough to continue the motion during the ascent of the 
 piston (or with a fly-wheel heavy enough to equalize the mo- 
 tion, even if a counter-weight were employed to act during the 
 ascent,) I proposed to employ two engines, acting upon two 
 cranks fixed on the same axis, at an angle of 120 to one an- 
 other, and a weight placed upon the circumference of the fly- 
 wheel at the same angle to each of the cranks, by which means 
 the motion might be rendered nearly equal, and only a very- 
 light fly-wheel would be requisite." In following out this plan, 
 some very important changes were introduced into the ma- 
 chinery of the steam engine: the principal of these are the 
 double-acting cylinder, the parallel motion, the crank, the fly- 
 wheel, and the governor. Each of these will first be severally 
 
 ( described ; and their operation in the double-acting engine be 
 afterward pointed out. 
 
 630. Double-acting Cylinder. The first alteration to be no- 
 ticed in the double-acting engine is that of the cylinder. To 
 insure its double action, it is necessary to provide, at each end 
 of the cylinder, a means of admission of steam from the boiler, 
 and of escape for the steam to the condenser. Hence the 
 double action, which means that the piston is both raised and 
 depressed by the force of steam. 
 
 030. What is meant bv the double-acting cylinder ? 
 
 "8* 
 
178 
 
 STEAM ENGINE. 
 
 Double-acting 
 
 Cylinder. 
 
 631. For this purpose, a steam-box is fixed 
 to eacli end of the cylinder, communicating, in 
 the one case with the upper, in the other with 
 the lower, surface of the piston. In Fig. 145, 
 B is the upper, and B' the lower, steam-box. 
 Each of these boxes is furnished with two 
 valves. 
 
 632. I. In the upper steam-box, the up- 
 per, or steam vaWe, S, admits steam from the 
 boiler through a tube, the mouth of which is 
 seen immediately above the valve ; the lower, 
 or exhausting valve, C, permits the escape of 
 the steam from the cylinder to the condenser, 
 
 through a tube opening immediately below the valve. In this 
 figure, the piston is at the top of the cylinder ; the exhausting 
 valve is therefore represented as closed, and the steam valve as 
 open, for the admission of steam, which rushes through the 
 passage D to the top of the cylinder, in order to force the piston 
 to the bottom. 
 
 633. II. In the lower steam-box, a corresponding mechan- 
 ism is observed, and its valves must be worked at the same mo- 
 ment as those of the upper box, but upon an exactly opposite 
 principle. The cylinder is full of steam, and the piston at the 
 top ; the steam valve S' must therefore be closed, and the ex- 
 hausting valve C' opened, in order that the steam may rush 
 out at the passage D', and a vacuum be formed beneath the pis- 
 ton, to give effect to the steam which is now entering above it. 
 
 634. In Fig. 146, the piston is at the bot- 
 tom of the cylinder. 1. In the upper steam- 
 box, the steam valve S is accordingly closed, 
 and the exhausting valve C opened, to admit 
 of the escape of the steam from above the 
 cylinder through the passage D into the con- 
 denser, and thus to produce a vacuum above 
 the piston. 2. In the lower 'steam-box, the 
 exhausting valve C' is closed, and the steam 
 valve S opened, in order that steam may rush 
 in by the passage D', and force the piston to 
 the top of the cylinder. 
 
 From the preceding description, it is evi- 
 dent that the alternate motions of the piston depend on the 
 opening and closing of the valves, alternately, in pairs. When 
 
 FIG. 146. 
 
 Double-acting 
 Cylinder. 
 
 631. Explain the double-acting cylinder by Figs. 145 and 146. 
 
STEAM ENGINE. 
 
 179 
 
 the piston is at the top of the cylinder, the upper steam valve 
 and the lower exhausting valve are to be opened, while the 
 lower steam valve and the upper exhausting valve are to be 
 closed. When the piston is at the bottom of the cylinder, this 
 process is reversed. 
 
 635. PARALLEL MOTION. In the double-acting engine, the 
 pressure of the steam acts alternately on both sides of the pis- 
 ton, which must therefore be pushed upward as well as pulled 
 downward; the connection between the piston-rod and the 
 beam by any flexible medium is, therefore, obviously inadmissi- 
 ble ; a chain can not communicate an upward impulse from the 
 piston to the beam. 
 
 The difficulty was, to adjust the rectilinear motion of the pis- 
 ton-rod to the circular motion of the beam ; without such ad- 
 justment, it is evident that either the piston-rod, being forced 
 to the right and left alternately, at each motion of ascent and 
 of descent, would be broken or bent ; or that the stuffing-box 
 would be so injured by these derangements of action, as to cease 
 
 FIG. 147. 
 
 Parallel Motion. 
 
 to be air and steam-tight. The contrivance by which these 
 difficulties were removed by Watt, is one of the most happy 
 inventions ever introduced into machinery. It has been termed 
 the parallel motion ; and its mechanism may be understood by 
 means of the subjoined figure, where B represents the end of 
 
 635. Explain by Fig. 147, how parallel motion is effected. 
 
180 STEAM ENGINE. 
 
 the beam, which is pulled downward, and pushed upward, by 
 the motion of the piston-rod R P ; the motion of B is in the 
 direction of the dotted curve ; that of R P is rectilinear. 
 
 636. To adjust these counteracting motions, a series of bars 
 are introduced, which are movable on pivots, and which by the 
 balance of their action prevent the piston from deviating to any 
 injurious extent from the straight line. Two fixed points of 
 support are taken, the one at F, as near as possible to the line 
 in which the piston-rod moves ; the other at C, the center of 
 the working beam. Two perpendicular bars, B R and E II, 
 are attached to the beam at B and E ; and two transverse bars, 
 R H and F H, are added, the former connecting the lower ex- 
 tremities of the two vertical bars, the latter connecting the lower 
 extremity of the vertical bar E H with the fixed point F ; all 
 the bars move freely on pivots at all their points of attachment. 
 The head of the piston-rod is connected with the pivot at R. 
 The smaller diagram, Fig. 147, relates to paragraph 639. 
 
 637. The action of this machinery is as follows : 1. Let us 
 imagine the end of the beam B to descend in the direction of 
 the dotted curve. During its progress to the horizontal posi- 
 tion, indicated by the dotted line k C, it is continually pushing 
 the perpendicular bar B R outward ; and this effect, if not coun- 
 teracted, would disturb the rectilinear course of the piston-rod. 
 But this outward push of the bar B R is counteracted by an 
 inward pull by the rod R II upon the point R ; the end II of 
 the rod R II is preserved at a proper distance from the 
 line of motion of the piston-rod by means of the rod called 
 the radius rod, H F, which is attached to the fixed point 
 F; and the rod H F, being thus fixed, describes, with its ex- 
 tremity H, the curve d a, which is directed inwardly, and coun- 
 teracts the outward direction of the curve described by B. 
 Hence it follows, that the top of the piston-rod R moves in a 
 direction almost vertical. It is correct to say almost, for it is 
 not strictly so ; the deviation, however, from the vertical motion 
 involves a minute calculation, and it is of comparatively little 
 importance in practical operation. 
 
 638. 2. As the beam quits the horizontal position in completing 
 its descent, it is continually pushing the bar B R inward ; tut 
 this inward push of ;the bar B R is now counteracted by the 
 outward pull of the bar H F, which now completes the curve 
 g o, and, by means of the transverse connecting bar II R, main- 
 tains the piston-rod in its nearly vertical direction. 3. It is ob- 
 
 636. Explain Watt's engine by means of Fig. 117. 
 
STEAM ENGINE. 181 
 
 vious, that during the ascent of the beam, the same movements 
 of the bars will secure the vertical ascent of the piston-rod. 
 This beautiful contrivance represents, in fact, a kind of jointed 
 parallelogram, three of the angles of which describe curves, 
 while the fourth, which is connected with the piston-rod, moves 
 nearly in a straight line. 
 
 639. MOTION OF THE AIR-PUMP ROD. The same machinery 
 which regulates the motion of the piston-rod of the cylinder, 
 also regulates those of the pump-rod. In the preceding Fig. 
 147, the upper part of the air-pump rod is represented at A 
 K ; it is connected at the top to the middle of the bar E H, 
 where it works freely on a pivot A. This machinery may be 
 readily understood by means of the smaller figure, in which the 
 bars composing it are separated from the beam, the letters be- 
 ing preserved precisely as in Fig. 147. C E and F H are two 
 bars, working on pivots at the fixed points C and F, and de- 
 scribing curves at their free extremities. The bar E H con- 
 nects these free extremities, upon which it moves by pivots. 
 From the antagonizing action of the two transverse bars, it fol- 
 lows, that the point A, the head of the air-pump rod, will move 
 in a nearly vertical direction. 
 
 640. NATURE OF THE CRANK. It has been shown that the 
 alternate motions of the piston-rod, determined by the double- 
 acting cylinder, are communicated to the working end of the 
 beam, to the curved motion of which they are adjusted by the 
 contrivance of the parallel motion. The next object was to con- 
 vert the rectilinear motion, thus produced, into a rotatory 
 motion. 
 
 641. So long as the force of steam was employed for the 
 mere purpose of raising water, no such motion was wanted ; 
 but when its application was required for the purpose of turn- 
 ing the wheels of mills of giving effect to the machinery of 
 cotton manufactures and printing presses of propelling- steam 
 vessels and other locomotive engines it became necessary to 
 impart a new direction to its operation. To obtain this object 
 the crank was introduced. 
 
 642. The simplest idea of a crank is that of the handle to a 
 wheel; its. action is familiarly illustrated in the process of draw- 
 ing water from a well ; the bent handle attached to the wheel 
 is first pushed out, then pulled in the opposite direction, and 
 thus a continued rotatory motion is produced upon an axle. 
 
 639. How is the motion of the air-pump effected ? 640. What is the cranic, and 
 how does it act 1 
 
182 
 
 STEAM ENGINE. 
 
 FIG. 148. 
 
 The application of this principle to the steam engine, and the 
 variations of pressure on the crank of a steam engine, may be 
 conveniently illustrated by curves. 
 
 643. This will be readily perceived by Fig. 148, which rep- 
 resents the lower portion of the connecting-rod, which works at 
 its upper extremity on a pivot connected with the working ex- 
 tremity of the beam. 
 
 The lower extremity of the rod 
 is connected by a movable joint at 
 I, with the lever I K. The center 
 or axis to which the rotatory mo- 
 tion is to be communicated, is indi- 
 cated by the letter K. Hence it 
 would appear, that as the connect- 
 ing-rod moves upward and down- 
 ward, it would carry the lever I K 
 round the center K, so as to oc- 
 cupy successively the positions de- 
 noted in the figure by the dotted 
 shadows of the lever ; and thus a 
 continued rotatory motion would . 
 be communicated to the axis. 
 
 644. Irregular Action of the 
 Crank. On considering more 
 closely the action of the crank, it 
 
 will be found to be by no means continuous in its motion. 
 There are two positions which the crank assumes in its circuit, 
 in which the moving power has positively no effect whatever in 
 communicating a rotatory motion to it. 
 
 645. I. When the piston is at the bottom of the cylinder, 
 the crank will be in the position denoted in the preceding figure; 
 the joint I will be in a perpendicular line between the upper 
 end of the connecting-rod and the center K. It is obvious, that 
 as the piston ascends in the cylinder, the connecting-rod will 
 tend to push the joint I, not to the right nor to the left of the 
 dotted circle, but dirctly downward upon the axis K. 
 
 646. II. When the piston is at the top of the cylinder, the 
 crank will have performed half a revolution, and the joint I will 
 be in a perpendipular line below the center K. As the piston 
 descends, the connecting-rod will tend to pull the joint I, not to 
 the right nor to the left of the dotted circle, but directly up- 
 
 643. What are the dead points in the motion of the crank 1 Explain this by 
 Fig. 148. 
 
STEAM ENGINE. 183 
 
 ward upon the axis K. It is evident, that if in either of these 
 positions, the action of the crank were for a moment to cease, 
 it would be out of the power of the piston to put it again into 
 motion. 
 
 647. 111. Another difficulty connected with the crank, is the 
 inequality of its motion. In two positions, it has been shown 
 to be actually stationary. There are also two positions, in which 
 its action is most energetic ; and it becomes feebler in propor- 
 tion as the crank moves from these points toward the two sta- 
 tionary positions above described. 
 
 Let the reader once more direct his attention to the process 
 of drawing water from a well ; let him imagine his own arm to 
 be the connecting-rod ; and the handle of the wheel the crank ; 
 he will find that his force is most effective, when the angle de- 
 scribed by his arm upon the crank is a right angle ; and that 
 his force will become less effective, as the angle of leverage be- 
 comes smaller or greater. The application of this simple illus- 
 tration to the crank of the steam engine is obvious ; and the 
 result of it is a variable, instead of a uniform, unremitting ac- 
 tion. In the following paragraph, a remedy for these incon- 
 veniences will be described. 
 
 648. NATURE OF A FLY-WHEEL. In impelling machinery 
 by force, it is frequently necessary that the force should be reg- 
 ulated. Jhis necessity may arise from several causes. There 
 may be a want of uniformity in the f,rst moving power, as in 
 the single-acting engine of James Watt, in which the descent 
 of the piston is effected by the pressure of steam, while its ascent 
 is effected by a totally different means. Or, there may be a 
 want of uniformity in the resistance which the force has to over- 
 come, as in the crank described in the preceding paragraph. 
 
 To regulate these inconveniences and equalize the motion, a 
 large heavy wheel, called a fy-wheel, is connected with the ma- 
 chinery, so as to receive its motion from the impelling power, 
 to keep up the motion by its own inertia, and distribute it 
 equally in all parts of its revolution. If the moving power 
 slackens, the fly-wheel impels the machine forward ; if the power 
 tends to impel the machine too fast, the fly-wheel slackens it. 
 The object of the fly-wheel, therefore, is to absorb, as it were, 
 the surplus force at one part of the action of the machine, and 
 to give it out when the action of the machine is deficient ; by 
 Leslie it was well compared to a " reservoir which collects the 
 intermittent currents, and sends forth a regular stream." 
 
 648. How does the fly-wheel eontinue the motion of the crank ? 
 
184 STEAM ENGINE. 
 
 649. Connection of the Fly- Wheel with the Crank. In or- 
 .der to equalize the motion of the crank, Watt attached a fly- 
 wheel to its axis. This wheel is constructed of large diameter, 
 in order that its circumference may revolve rapidly : it is of 
 great weight, being made of iron, that it may acquire consider- 
 able momentum so as to render the motion as uriform as pos- 
 sible; and it is so nicely placed upon the axis, as to be almost 
 free from friction, and thus enabled to communicate its motion 
 to the axis, when this is required from the irregular action of 
 the crank. 
 
 The objects of the fly-wheel in the steam engine, as here de- 
 scribed, are obviously twofold : first, to extricate the machine 
 from the mechanical difficulties which occur at the two station- 
 ary positions of the crank ; and, secondly, to equalize the effects 
 of the varying leverage by which the first mover acts on the 
 crank. But besides the irregularity in the action of the crank, 
 there are other causes which, in the absence of a fly-wheel, 
 would disturb the uniform velocity of the engine : there are 
 variations of resistance, and of power. 
 
 The resistance which an engine has to overcome, particularly 
 in manufactures, is continually liable to vary. When the re- 
 sistance is diminished, the quantity of steam admitted through 
 the valves into the cylinder, is increased or diminished, as the 
 case may be. 
 
 When the resistance is increased, or the moving power dimin- 
 ished, the momentum accumulated in the fly-wheel continues 
 the motion with little diminution of its own velocity. It is not, 
 however, pretended that the equalization of force produced by 
 the fly-wheel, is perfect ; but it is sufficient for ordinary pur- 
 poses ; and its efficiency will be proportioned to the mass of 
 matter in the circumference of the wheel and to the square 
 of the wheel's velocity. The next step in the progress of im- 
 provement was to regulate the velocity of the fly-wheel. 
 
 650. THE GOVERNOR. Of all the contrivances for regulating 
 the motion of machinery, this is said to be the most effectual. 
 It will be readily understood by the following description of 
 Fig. 150. It consists of two heavy iron balls, ft, attached to 
 the extremities of the two rods, b e. These rods play on a 
 joint at <?, passing through a mortice in the vertical stem d d. 
 At /, these pieces are united, by joints, to the two short rods, 
 
 649. Why must the fly-wheel be of larre diameter and great weieht ? Does the 
 fly-wheel completely equalize the motion of machinery? 650. What is the gov- 
 ernor 1 How does the governor operate to equalize the motion of machinery J 
 
STEAM ENGINE. 
 
 185 
 
 The Governor. 
 
 f A, -which, at their upper 
 ends, are again connected by 
 joints at A, to a ring which 
 slides upon the vertical stem 
 d d. Now it will be appa- 
 rent that when these balls 
 are thrown outward, the 
 lower links connected at /", 
 will^ be made to diverge, in 
 consequence of which the up- 
 per links will be drawn down 
 the ring with which they are 
 
 connected at h. With this ring at ?', is connected a lever hav- 
 ing its axis at g, and to- the other extremity of which, at k, is 
 fastened a vertical piece, which is connected by a joint to the 
 valve v. To the lower part of the vertical spindle d^s attached 
 a grooved wheel w, around which a strap passes, which is con- 
 nected with the axis of the fly-wheel. 
 
 Now when .it so happens that the quantity of steam is too 
 great, the motion of the fly-wheel will give a proportionate ve- 
 locity to the spindle d d, by means of "the strap around w, and 
 by which the balls, by their centrifugal force, will be widely 
 separated ; in consequence of which the ring A, will be drawn 
 down. This will elevate the arm of the lever &, and by which 
 the end z, of the short lever, connected with the valve v, in the 
 steam pipe, will be raised, and thus the valve turned so as to 
 diminish the quantity of steam admitted to the piston. When 
 the motion of the engine is slow, a contrary effect will be pro- 
 duced, and the valve turned so that more steam will be admit- 
 ted to the engine. 
 
 651. Connected View of the Double-acting Engine. We are 
 now in a condition to understand the relation which the several 
 parts of the engine, already separately described, bear to each 
 other. In Its general construction it resembles the single-acting 
 engine of Watt not described in the present work, but it differs 
 in several important features. Among these are, its capability 
 of performing twice the amount of work in the same time, frcm 
 the simultaneous action of the pressure, and of the condensation 
 of steam, at each ascent and descent of the piston ; its near ap- 
 proximation to uniformity of power ; its economy of heat, and 
 consequently of fuel, by the diminution of cooling surface; and 
 its reduced bulk. In the following engraving, taken from the 
 valuable work of Tredgold, a section of this engine is illustra- 
 
186 STEAM ENGINE. 
 
 ted ; a few additional remarks to those which have already been 
 made on its separate details, will serve to explain its general 
 operation. 
 
 652. At the right is seen (Fig. 151) the great horizontal 
 steam tube S, which admits steam into the cylinder through the 
 throttle valve, which appears near S in the form of a disc. The 
 boiler is omitted in the plate, but its connection with the tube, 
 and the means by which it is supplied with warm water, may 
 be inferred from descriptions already given. 
 
 653. The double-acting cylinder C, its two steam boxes and 
 four valves, and the apparatus for working the valves,. are the 
 next objects which claim attention. These are explained by 
 Figs. 145 and 146. The piston is at the top of the cylinder. 
 The upper steam valve a is, therefore, represented as open for 
 the admission of steam, the upper exhausting valve c as closed ; 
 the conditio^i of the two lower valves is reversed. The operation 
 of opening and closing these four valves is effected by a series 
 of levers, terminating in one handle or spanner, which is worked 
 by two pegs attached to the pump-rod R. 
 
 Before the piston arrives at the bottom of the cylinder, the 
 upper peg strikes the handle of the levers downward, and in a 
 moment reverses the condition of the four valves. The steam 
 from above the piston th'en rushes down through the perpen- 
 dicular tube S, issues at the lower steam valve c?, which will 
 now be open, and forces up the piston ; but, before the piston 
 arrives at the top of the cylinder, the lower peg strikes the 
 handle of the levers upward, the condition of the valves is again 
 reversed, the steam below the cylinder rushes through the lower 
 exhausting valve 6 into the condenser B, and the stroke of the 
 engine is repeated. 
 
 654. In the condenser B, the steam meets with a continual 
 jet of cold water. In the double-acting engine, condensation 
 goes on equally during the descent and ascent of the piston, and 
 the condensing jet is therefore incessantly at play; Engines 
 with a condenser are called low pressure engines. The .varia- 
 tions which occur in the velocity of the piston, and the conse- 
 quent variations in the quantity of steam dischaFged -into the 
 condenser, require corresponding variations in the quantity of 
 condensing water ; its amount is, therefore, regulated by the in- 
 jection cock, which is worked by a lever and handle, I. The 
 water produced by condensation of the steam is removed by 
 
 652. In Fig. 151, where is the steam pipe 7 653. Which is the cylinder 7 654. Which 
 is the condenser? 
 
STEAM ENGINE. 
 FIG. 151.* 
 
 187 
 
 Modern Steam Engine. 
 
188 HIGH PRESSURE ENGINE. 
 
 the air-pump A, and carried into the warm cistern, from which 
 a portion of it is drawn by the pump L, and conveyed to the 
 boiler. The cistern containing the condenser, the air-pump, and 
 the injection cock, is supplied with water by the pump N, on 
 the left side of the beam. 
 
 On the extreme left is the fly-wheel, a part of which is seen 
 at P, and to the axle of which is fixed the crank, this being 
 moved by the connecting-rod attached to the end of the work- 
 ing-beam. To the fly -wheel is also attached the governor, but 
 these parts having already been explained, and being unneces- 
 sary to the understanding of the whole, are omitted in the 
 drawing. 
 
 On the right extremity of the beam is seen the apparatus 
 which produces the parallel motion. The moving parallelo- 
 gram is represented at/, 6, d, g ; the rod d c is the radius rod: 
 it terminates the arc of the circle through which the point d 
 travels. At e is seen the extremity of the pump-rod R, which 
 is worked by the same machinery as that of the parallel motion. 
 
 655. Returning to the left side of the beam, we find the 
 pumping apparatus. D represents the barrel of the pump, and 
 M is the pump-rod, which is connected with the beam by me- 
 chanism similar to that of the parallel motion, already described. 
 When the piston of the pump descends, the water is forced up- 
 ward through the pipe G, and conveyed by appropriate chan- 
 nels to a distance and height proportional to the power of the 
 engine. The barrel of the pump is filled through the pipe F by 
 means of machinery adapted to this purpose below ; and, when 
 the piston of the pump ascends, the valve at the left of the bar- 
 rel opens, and the water rushes through in the same direction 
 as that from the pipe G. The supply for the descent of the 
 piston will rush in at the bottom valve from F, and be raised 
 through the pipe G, as before. The valves with which the pis- 
 ton of the air-pump is furnished are termed clacks. 
 
 HIGH PRESSURE ENGINE. 
 
 656. Tn the high pressure engines, the piston is pressed up 
 and down by the force of the steam alone, and without the 
 assistance of a vacuum. The additional power of steam re- 
 quired for this purpose is very considerable, being equal to the 
 entire pressure of the atmosphere on the surface of the piston. 
 We have already had occasion to show that on a piston of 13 
 
 655. Which is the air-pump 7 Explain the water pump. 656. What is the differ- 
 ence between the high and low pressure engines? 
 
HORSE POWER. 189 
 
 inches in diameter, the pressure of the atmosphere amounts to 
 nearly two tons. 
 
 657. Now in the low pressure engine, in which a vacuum is 
 formed on one side of the piston, the force of steam required to 
 move it is diminished by the amount of atmospheric pressure 
 nearly equal to the size of the piston. 
 
 65*8. But in the high pressure engine, the piston works in 
 both directions against the weight of the atmosphere, and hence 
 requires an additional power of steam equal to the weight of 
 the atmosphere on the piston. 
 
 659. These engines are, however, much more simple and cheap 
 than the low pressure, since the condenser, cold water pump, 
 air-pump, and cold water cistern, are dispensed with ; nothing 
 more being necessary than the boiler, cylinder, piston, and 
 valves. Hence for railroads, and all locomotive purposes, the 
 high pressure engines are, and must be used. 
 
 With respect to engines used on board of steamboats the low 
 pressure are universally employed by the English, and it is well 
 known, that few accidents from the bursting of machinery have 
 ever happened in that country. In most of their boats two en- 
 gines are used, each of which turns a crank, and thus the neces- 
 sity of a fly-wheel is avoided. 
 
 In this country high pressure engines are in common use for 
 boats, though they are not universally employed. In some, two 
 engines are worked, and the fly-wheel dispensed with, as in 
 England. 
 
 660. Accidents. The great number of accidents which have 
 happened in this country, whether on board of low or high 
 pressure boats, must be attributed in a great measure, to the 
 eagerness of our countrymen to be transported from place to 
 place with the greatest possible speed, all thoughts of safety 
 being absorbed in this passion. It is, however, true, from 
 the very nature of the case, that there is far greater danger 
 from the bursting of the machinery in the high, than in the 
 low pressure engines, since not only the cylinder, but the boiler 
 and steam pipes must sustain a much higher pressure in order 
 to gain the same speed, other circumstances being equal. 
 
 HORSE POWER. 
 
 661. When steam engines were first introduced, they were 
 employed to work pumps for draining the English coal mines, 
 
 657. What constitutes a low pressure engine ? 658. How much more force of 
 steam is required in high than in low pressure engines ? 659. What parts are dis- 
 pensed with in high pressure engines ? 
 
190 HCmSE POWER. 
 
 thus taking the places of horses, which from the earliest times 
 of using coal had performed this service. 
 
 662. It being therefore already known how many horses were 
 required to raise a certain amount of coal from a given depth, 
 the powers of these engines were very naturally compared to 
 those of horses, and thus an engine which would perform the 
 work of ten horses, was called an engine of ten horse power. 
 To this day the same term is used, with the same meaning, 
 though very few appear to know either the origin of the term, 
 or the amount of power it implies. 
 
 Several engineers, after the term was thus used, made exper- 
 iments, for the purpose of ascertaining the average strength of 
 horses, with a view of fixing a standard of mechanical force 
 which should be indicated by the term horse power. 
 
 This was done by means which it is not necessary here to 
 describe. 
 
 663. Smeaton, a celebrated mechanical philosopher, estima- 
 ted that the average power of the horse, working eight hours a 
 dey, was equal to the raising of 23,000 pounds at the rate of 
 one foot per minute. 
 
 664. Messrs. Bolton and "Watt caused experiments to be 
 made with the horses used in the breweries of London, said to 
 be the strongest in the world, and from the result they estima- 
 ted that 33,000 pounds raised at the rate of one foot per min- 
 ute, was the value of a horse's power, and this is the estimate 
 now generally adopted. When, therefore, an engine is said to 
 be so many horses' power, it is meant that it is capable of over- 
 coming a resistance equal to so many times 33,000 pounds 
 raised at the rate of one foot per minute. Thus an engine of 
 ten horse power is one capable of raising a load of 330,000 
 pounds one foot per minute, and so at this rate, whether the 
 power be more or less. 
 
 665. POWER OF STEAM. Experiment has proved that an 
 ounce of water converted into steam will raise a weight of 2,160 
 pounds one foot. 
 
 666. A cubic foot of water contains 1,728 cubic inches, and 
 the power, therefore, of a cubic foot of water, when converted 
 
 669. What is said of accidents from steam in our country 1 661. Where did the steam 
 engine first take the place of horses? 662. What is the orijrm of the term horse 
 power? 663 What was Smeaton's estimate of a horse's power ? 664. What was 
 Watt and Boltou's estimate of a horse's power ? What is meant by a horse's power 
 at the present time? How many horses would raise 33.000 poundsone foot per min- 
 ute ? 665. What is the power of a square inch of water converted into steam ? 666. 
 What is the power of a cubic foot of water converted into steam 7 How much power 
 is lost in acting upon the engine ? 
 
HORSE POWER. 191 
 
 into steam, will be equal to 2,160 multiplied by 1,728, equal to 
 3,732,480 pounds. This, then, expresses the number of pounds 
 weight which a cubic foot of water would raise one foot when 
 converted into steam, supposing that its entire mechanical force 
 could be rendered available. But in practice, it is estimated 
 that the friction, and weight of the machinery in action, require 
 about four-tenths of the whole force, while six-tenths only re- 
 main as an actual mechanical power. 
 
 667. Quantity of Water required for each Horse Power. 
 One horse power, as already explained, is equal to a force which 
 will raise 33,000 pounds one foot high per minute. This being 
 multiplied by 60 will show the force required to raise the same 
 weight at the rate of one foot per hour, namely, 33,000 X 60 = 
 1,980,000 pounds. 
 
 668. Now the quantity of water required for this effect, will 
 be found by considering, as already shown, that a cubic inch of 
 water in the form of steam, is equal to a force raising 2,160 
 pounds a foot. If we divide 1,980,000, therefore, by 2,160, we 
 shall have the number of cubic inches of water required to pro- 
 duce a one horse power, namely, 9.160. But we have already 
 shown that only 6 parts out of 1 of the force of steam can be 
 calculated on as a moving power, 4 parts being expended on 
 the action of the enjrine. To find, then, the amount of wa>te 
 in 916 cubic inches of water, we must divide that number by 6, 
 and multiply the result by 4, when we shall have 610 as the 
 number of cubic inches of water wasted. The total quantity of 
 water, therefore, which is turned into steam per hour, to pro- 
 duce a one horse power, is equal to 610 added to 916, namely, 
 1,526 cubic inches. Hence we see the necessity of the immense 
 capacities of the boilers of large steamboats. 
 
 669. Amount of Mechanical Virtue in Coal. For more than 
 thirty years, the engineers of many of the English coal mines 
 have published annual accounts of their experiments with the 
 steam engines under their care, for the purpose of ascertaining 
 the exact amount of coal required to perform certain duties. 
 The results of these experiments are among the most curious and 
 instructive facts which the lights of science at the present day, 
 have thrown upon the manufacturing arts. They were entirely 
 unexpected to the owners of the mines, and equally so to men 
 of science. 
 
 667. How many cubic inches of water is required to produce a one horse power? 
 668. How do you find how many cubic inches of water there are in a one horse 
 power 1 669. What amount of weight is it said a bushel of coal will raise by means 
 of steam! What was the weight raised by (he second trial 1 
 
192 LOCOMOTIVE. 
 
 In the report of the engineers thus employed, for 1835, it 
 was announced that a steam engine employed at a copper mine 
 in Cornwall, had raised, as its average work, 95 millions of 
 pounds a foot high, with a single bushel of bituminous coal. 
 
 This mechanical effect was so enormous and so unexpected, 
 that the best judges of the subject considered it beyond the 
 bounds of credulity ; the proprietors, therefore, agreed that an- 
 other trial should be made in the presence of competent wit- 
 nesses : when, to the astonishment of all, the result exceeded 
 the former report by 30 millions of pounds. In this experi- 
 ment, for every bushel of coal consumed under the boiler, the 
 engine raised 125-i- millions of pounds one foot high. 
 
 670. On this subject, Dr. Lardner, in his treatise on the steam 
 engine, has made the following calculations : 
 
 A bushel of coal weighs 84 pounds, and can lift 56,027 tons 
 a foot high, therefore, a pound of coal would raise 667 tons to 
 the same height ; and an ounce would raise 42 tons one foot 
 high, or it would lift 18 pounds a mile high. 
 
 Since a force .of 18 pounds is capable of drawing two tons 
 upon a railway, it follows that an ounce of coal would draw 2 
 tons a mile, or 1 ton two miles. (In the common engines, how- 
 ever, the actual consumption of coal is equal to about 8 ounces 
 per ton for every mile.) 
 
 The great Egyptian pyramid has a base of 700 feet each way, 
 and is 500 feet high ; its weight amounting to 12,760,000,000 
 pounds. To construct it, is said to have cost the labor of 100,- 
 000 men for 20 years. Yet according to the above calculations, 
 its materials could have been raised from the ground to their 
 present positions by the combustion of 479 tons of coal. 
 
 LOCOMOTIVE. 
 
 671. This word, from the Latin, means "moving from place 
 to place," and is applied to steam engines used on railroads. 
 
 Our limits will only allow a short description of this wonder- 
 working machine, which, during the last quarter of a century, 
 has been the means, 'with respect to locomotion, of converting 
 days into hours, and weeks into days. 
 
 The principal external parts of a locomotive are indicated by 
 the letters on Fig. 152. 
 
 670. What weight will a pound of coal raise ? How great a force mav an ounce of 
 coal be made to produce! What is the size and weight of the great pyramid of 
 Egypt 1 What weight of coal would be required to raise its materials to their present 
 elevation ? 
 
LOCOMOTIVE. 
 
 193 
 
 672. The truck wheels, A A, are of cast iron, about two and 
 a half feet in diameter, all Of them connected by an iron frame, 
 in the center of which, the end of the boiler rests on a pivot, so 
 as to allow a revolving motion, in order to accommodate the 
 engine to short curves in the road. 
 
 FIG. 152. 
 
 673. The boiler B, which makes the chief bulk of the loco- 
 motive, is of rolled iron, about 12 feet long, of great weight, and 
 strength, to resist the pressure of the steam. It is put together 
 by iron bolt*, only an inch or two apart, so as to be perfectly 
 steam-tight under the greatest force. 
 
 672. What are the truck wheels of a locomotive, and why do they revolve on a cen- 
 fw? 673. What forms the chief bulk of the locomotive 1 
 
194 LOCOMOTIVE. 
 
 Above R is the fire-box, with a door, not shown, for admis- 
 sion of the wood. The interior of the boiler is composed of 
 about 100 copper tubes, through which the smoke and heat 
 pass to the chimney. These tubes are entirely surrounded by 
 water, which the heat emitted by the tubes, as they pass through 
 it, converts into steam. 
 
 674. The pump P, supplies the boiler with water, which it 
 takes from the tender, not shown, but which is connected with 
 the locomotive, and on which the fuel is carried. In cold 
 weather, the waste steam heats this water before it is admitted 
 to the toiler. 
 
 675. The steam cylinder 0, communicates with the boiler by 
 a short pipe, for admission of the steam. In this cylinder works 
 the piston, which gives motion to the engine. 
 
 The cylinder is externally of brass, kept polished in order to 
 prevent the radiation of the heat. Its diameter is about 12 
 inches, and the movement of the piston 20 inches. This is 
 furnished with valves, working in the same manner as those 
 already described for the steam engine. 
 
 The alternate, horizontal motion of the piston, is so connected 
 with the driving wheels, as to give them a rotatory motion, by 
 which the engine is moved. 
 
 676. This is done by means of the connecting-rods R R, which 
 are jointed to the spokes of the drivers at one end, and to the 
 piston rod I, at the other, thus connecting the force of the steam 
 with that part of the engine by which the whole is actuated. 
 
 The immense force which the steam exerts, is shown by the 
 power required to draw 20 or 30 cars, loaded with hundreds 
 of tons, at the rate of 20 or 30 miles an hour. And yet a 
 single locomotive'will draw such a load even up an inclined 
 plane. 
 
 677. The driving wheels D D, by which the locomotive is 
 moved, are of cast iron, with strong wrought iron tire, so as to 
 withstand any shock which it is considered possible to happen, 
 since on the strength of these, the lives of hundreds of passen- 
 gers may depend, as the fracture of one of them may cast the 
 engine and entire train from the rails. In diameter, they are 
 from 5 to 6 feet. 
 
 678. These four wheels are connected together, not only by 
 the connecting-rods, but also by a strong iron frame, and by the 
 
 675. Describe the steam cylinder, and tell its use. 676. What are the connecting- 
 rods? 677. What are the wheels which give motion to the locomotive 1 Why are 
 the driving wheels made of great strength ? 678. Why are these fotir wheels con- 
 nected 7 On what principle do these wheels apt 1 
 
LOCOMOTIVE. N 195 
 
 axle-trees which revolve with them, so that the greatest amount 
 of adhesion to the rails is obtained. This is a most important 
 point in the construction of the engine, since by this means all 
 the wheels must act together, thus forming by their adhesion 
 to the rails a single fulcrum, acting as a lever of the third kind, 
 (322,) of which the spoke of the wheel is the lever, and the pis- 
 ton, through the connecting-rods, the power. It^ therefore, one 
 of the wheels slips on the rail, they must all slip, it being this 
 connection by which locomotives draw such enormous loads 
 over inclined planes. 
 
 679. The lever L, opens the throttle valve, by which the 
 steam is admitted to the cylinder, from the boiler. When the 
 engine is to be started, the engineer opens this communication ; 
 when the piston begins its alternate motion ; the drivers their 
 revolutions, and the engine and train their progress. 
 
 680. The reversing handle H, acts on machinery for that 
 purpose, in such a manner as to reverse the motion of the driv- 
 ing wheels, giving them a backward instead of a forward action, 
 in a moment. It is used whenever there is danger of a colli- 
 sion, or when it is desired to give the engine a reverse move- 
 ment on any occasion. 
 
 The spring balance N, is connected with a graduated scale by 
 which the pressure of the steam is indicated. 
 
 681. The safety valve lever S, is connected with a valve, so 
 constructed .as to open when the pressure is above a certain 
 amount, and thus allow the steam to escape. When properly 
 adjusted, this may be the means of saving the engine from one 
 of the most fearful of accidents, that of bursting the boiler. 
 
 682. The smoke pipe M, is connected with the fire-box, by 
 means of the copper tubes running through the boiler, already 
 mentioned. Various contrivances have been invented to arrest 
 the sparks which are emitted with the smoke, and which have 
 often set fire to bridges and other buildings. For this purpose 
 a wire gauze placed across the mouth of the pipe, has been the 
 most efficient. 
 
 The engine frame F, is made of wrought iron, strongly con- 
 nected by rivets, and to which all parts of the locomotive are 
 attciched, and by which they are combined into a single instru- 
 ment, to be moved forward as a great power, by means of which 
 hundreds of tons are to follow. 
 
 679. Describe the manner of starting the engine. 680. What is the use of the re- 
 Tersing handle. 681. How does the safety valve act, and for what purpose ? 682. 
 What are the means of arresting sparks from the smoke pipe ? 
 
. - 
 196 THE RAILS. 
 
 The valve box V, contains the valves of the cylinder, which 
 have already been described while treating of the steam engine. 
 
 The steam whistle U, is composed of a cylinder, with peculiar 
 internal arrangements, on which the steam from the boiler be- 
 ing admitted, by a valve, makes a well-known sound, heard at 
 the distance of many miles. By its report, the degrees of press- 
 ure of the steam are indicated. 
 
 The slide valve rod K, works the valve by which the steam 
 is admitted from the boiler to the cylinder, and by which the 
 piston is moved. 
 
 683. Springs of the Boiler. The boiler rests on steel springs, 
 composed of many flat pieces of different lengths, laid one on 
 the other, forming a pyramidal pile six or eight inches high, 
 and' of sufficient strength to bear many tons. By the slight 
 motion of these springs, the concussion between the engine and 
 the rails is prevented, and without which neither would preserve 
 its integrity for an hour, under the tremendous shocks, the 
 weight and motion of the engine sometimes give. 
 
 ADHESION TO THE RAILS. 
 
 684. We have already noticed the necessity of so combining 
 the action of the driving wheels, as to make them form an in- 
 dividual fulcrum, by their adhesion to the rails. 
 
 On this the motion of the engine, and consequently of the 
 whole train depends, and hence the necessity of the enormous 
 weight of the locomotive. On roads, through hilly sections of 
 the country, the weight of the engine is made to correspond to 
 the inclination of the grade. Were this not the case, as the 
 adhesion depends on the weight, the wheels would revolve with- 
 out advancing, and thus the whole train would remain motion- 
 less, because the weight, with the inclination, required a greater 
 force than the power of the engine. On such roads where 
 heavy freight trains are to be drawn, the weight of the engine 
 sometimes amounts to 40 or 50,000 pounds. 
 
 685. In all cases, the invariable condition must be, that the 
 force be greater than the resistance, otherwise no progress will 
 be effected ; and as we have already seen, the force depends on 
 the adhesion, and this on the weight, so it is obvious that a 
 
 683. What is said of the springs on which the boiler rests? 684. Why are the 
 driving wheels so connected as to form an individual fulcrum ? What is the weight 
 of some engines'? 685. What is said of the proportion between the weight of the en- 
 gine and the grade of the road? What must be the condition with respect to the 
 weight and force ? What is the estimate between the force of adhesion and -the 
 weight of the drivers! 
 
THE RAILS. 
 
 197 
 
 FIG. 153. 
 
 ponderous engine only, will draw a heavy train over a rapid 
 inclination. It is estimated that the force of adhesion amounts 
 to one-sixth of the weight of the drivers on the rails. 
 
 686. Section of the Boiler. It has been noticed above, that 
 locomotive boilers are furnished with copper tubes, passing from 
 the fire-box to the chimney. The ordinary number of these 
 tubes is 120, and their diameters about two inches. If larger 
 than this, they are liable to collapse by the pressure of the 
 steam, and if smaller, they soon become clogged by the soot. 
 
 The end of such a boiler is repre- 
 sented by Fig. 153. The fire-box 
 with the grate for fuel, is seen at B, 
 above which are the ends of the tubes. 
 In Fig. 152, these parts are above R. 
 A shows the dome, above the fire-box, 
 and which forms a part of the boiler, 
 being open and containing the steam 
 as it is formed. 
 
 687. The steam is conveyed to the 
 cylinders from the large pipe, seen at 
 the upper part of the dome, the two 
 arrows showing that it is admitted 
 from all directions. The mouth of 
 this pipe is thus elevated, in order to 
 avoid the admission of the water when 
 in the state of the greatest ebullition. 
 
 The fire-box is made of thick, rolled 
 iron, with double walls, about three 
 
 inches apart, the space between them being filled with water, 
 so that the fire is surrounded with water, except at the door 
 where the fuel is admitted. 
 
 688. The water is pumped into the side of the fire-box at C, 
 whTch opens into the boiler. 
 
 The boiler is only about half filled with water, the upper part 
 being devoted to steam. 
 
 The boiler is made of thiok, rolled iron, strongly riveted to- 
 gether, and in the form of a cylinder, being that which best 
 resists the pressure of the steam. 
 
 In order to confine the heat, or prevent radiation, boilers are 
 covered with wood, in the form of narrow strips of board, over 
 which there is a covering of sheet iron. 
 
 686. Show by Fig. 153, the situations of the fire-box, steam pipe, tubes and grate. 
 687. Why is the mouth of the steam pipe so high in the dome 7 688. Where is the 
 water admitted to the boiler. 
 
 Fire-Box and Boiler. 
 
198 
 
 ACOUSTICS. 
 
 689. Locomotive engines are always on the high pressure 
 principle, because such engines are more simple in structure 
 than those of low pressure, the former not requiring the con- 
 densing apparatus which is indispensable in the latter. 
 
 CHAPTER X. 
 
 ACOUSTICS. 
 
 690. Acoustics is that branch of natural philosophy which 
 treats of the origin, propagation, and effects of sound. 
 
 691. Vibration of Solids. When a sonorous, or sounding 
 body is struck, it is thrown into a tremulous or vibrating mo- 
 tion. This motion is communicated to the air which surrounds 
 us, and by the air is conveyed to our ear drums, which also 
 undergo a vibratory motion, and this last motion throwing the 
 auditory nerves into action, we thereby gain the sensation of 
 sound. 
 
 If any sounding body, of considerable size, is suspended in 
 the air and struck, this tremulous motion is distinctly visible 
 to the eye, and while the eye perceives its motion, the ear per- 
 ceives the sound. 
 
 692. Proof by the Air-Pump, That 
 sound is conveyed to the ear by the motion 
 which the sounding body communicates to 
 the air, is proved by an interesting experi- 
 ment with the air-pump. 
 
 693. This is done by a little piece of me- 
 chanism shown by Fig. 154. It consists 
 of a block of lead weighing a pound or two, 
 into which is inserted the standard of the 
 bell A. A piece of wire, also fixed to the 
 lead, is bent over the bell at B, to which is 
 jointed the handle of a small hammer. At 
 half an inch from the joint, the handle 
 passes through the end of the sliding rod 
 
 689. Why are locomotives on the high pressure principle ? 690. What is acoustics 7 
 691. When a sonorous body is struck within hearing, in what manner do we gain 
 from it the sensation of sound 1 692. How is it proved that sound is conveyed to the 
 ear by the medium of the air 7 693. Describe the mechanism, Fig. 154, by which 
 this is proved. 
 
 FIG. 154. 
 
 About Sound. 
 
DIVING BELL. 199 
 
 C, which passes air-tight through the stuffed collar of the glass 
 receiver D. 
 
 Now it is obvious by the figure, that on working the sliding- 
 rod by its handle, the hammer will strike the bell, the sound of 
 which may be heard to a considerable distance. But if the re- 
 .ceiver be set on the plate of an air-pump, and the air exhausted, 
 its sound will become less and less audible, until a vacuum is 
 formed, when, although the hammer is made to strike the bell, 
 no sound will be heard. The lead should be placed on a piece 
 of cotton batting, so as not to transmit the sound through the 
 solid on which it stands. 
 
 DIVING BELL. 
 
 694. On the contrary, when the air is more dense than or- 
 dinary, or when a greater quantity is contained in a vessel, than 
 in the same space in the open air, the effect of sound on the 
 ear is increased. This is illustrated by the use of the diving 
 bell 
 
 The diving bell is a large vessel, open at the bottom, under 
 which men descend to the beds of rivers, for the purpose of ob- 
 taining articles from the wrecks of vessels. When this machine 
 is sunk to any considerable depth, the water above, by its press- 
 ure, condenses the air under it with greafr force* In this situa- 
 tion, a 'whisper is as loud as a common voice in the open air, 
 and an ordinary voice becomes painful to the ear. 
 
 695. Effects in high Places. Again, on the tops of high 
 mountains where the pressure, or density of the air is much less 
 than on the surface of the earth, the report of a pistol is Eeard 
 only a few rods, and the human voice is so weak as to be in- 
 audible at ordinary distances. 
 
 Thus, the atmosphere which surrounds us, is the medium by 
 which sounds are conveyed to our ears, and to its vibrations we 
 are indebted for the sense of hearing, as well as for all we enjoy 
 from the charms of music. 
 
 696. Solids conduct Sound. The atmosphere, -though the 
 most common, is not, however, the only, or the best conductor 
 of sound. Solid bodies conduct sound better than elastic riuids. 
 Hence, if a person lay his ear on a long stick of timber, the 
 scratch of a pin may be heard from the other end, which could 
 not be perceived through the air. 
 
 694. When the air is more dense than ordinary, how does it affect sound? 695. 
 What is said of the effects of sound on the tops of high mountains 1 696. Which ar 
 the best conductors of sound, solid or elastic substances 1 
 
200 ACOUSTICS. 
 
 697. The earth conducts loud rumbling sounds made below 
 its surface to great distances. Thus, it is said, that in countries 
 where volcanoes exist, the rumbling noise which generally pre- 
 cedes an eruption, is heard first by the beasts of the field, be- 
 cause their ears are commonly near the ground, and that by 
 their agitation and alarm, they give warning of its approach to- 
 the inhabitants. , 
 
 698. The Indians of our country, by laying their ears on the 
 ground, will discover the approach of horses or men when they 
 are at such distances as not to be heard in any other manner. 
 
 699. Velocity of Sound. Sound is propagated through the 
 air at the rate of 1,142 feet in a second of time. When com- 
 pared with the velocity of light, it therefore moves but slowly. 
 Any one may be convinced of this by watching the discharge 
 of cannon at a distance. The flash is seen apparently at the 
 instant the gi-nner touches fire to the powder; the whizzing of 
 the ball, if the ear is in its direction, is next heard, and lastly, 
 the report. 
 
 700. Blot's Experiment. Solid substances convey sounds 
 with greater velocity than air, as is proved by the following ex- 
 periment, made at Paris, by M. Biot: 
 
 At the extremity of a cylindrical tube, upward of 3,000 feet 
 long, a ring of metal, was placed, of the same diameter as the 
 aperture of the tube ; and in the center of this ring, in the 
 mouth of the tube, was suspended a clock-bell and hammer. 
 The hammer was made to strike the ring and the bell at the 
 same instant, so that the sound of the ring would be transmit- 
 ted to the remote end of the tube, through the conducting 
 power of the tube itself, while the sound of the bell would be 
 transmitted through the medium of the air inclosed in the tube. 
 The ear being then placed at the remote end of the tube, the 
 sound of the ring, transmitted by the metal of the tube, was 
 first heard distinctly, and after a short interval had elapsed, the 
 sound of the bell transmitted by the air in the tube, was heard. 
 The result of several experiments was, that the metal conducted 
 the sound at the rate of about 11,865 feet per second, which is 
 about ten and a half times the velocity with which it is con- 
 ducted by the air. 
 
 701. Sound moves forward in straight lines, and in this re- 
 
 697. What is said of the earth as a conductor of sounds'? 608. Ho\v is it said that 
 the Indians discover the approach of horses 1 699. How fast does sound pass through 
 t'he air ] What is said of the firing of cannon with respect to sound 7 700. Which 
 convey sounds with the greatest velocity, solid substances, or air 1 
 
ACOUSTICS. 
 
 201 
 
 spect follows the same laws as moving bodies, and light. It 
 also follows the same laws in being reflected, or thrown back, 
 when it strikes a solid, or reflecting surface. 
 
 7U2. ECHO. If the surface be smooth, and of t considerable 
 dimensions, the sound will be reflected, and an echo will be 
 heard ; but if the surface is very irregular, soft, or small, no 
 such effect will be produced. 
 
 ]n order to hear the echo, the ear must be placed in a certain 
 direction, in respect to the point where the sound is produced, 
 and the reflecting surface. 
 
 If a sound be produced at A, Fig. 155, and strike the plane 
 surface B, it will be reflected back in the same line, and the 
 echo will be heard at C or A. That is, the angle under which 
 it approaches the reflecting surface, and that under which it 
 leaves it, will be equal. 
 
 FIG. 157. 
 
 A. 
 
 Echo. 
 
 Reverberation. 
 
 FIG. 156. 
 
 Reflection of Sound. 
 
 703. Whether the sound strikes the reflecting surface at right 
 angles, or obliquely, the angle of approach, and the angle of re- 
 flection, will always be the same, and equal. 
 
 700. Describe the experiment, proving that sound is conducted by a metal with 
 greater velocity than by the air. 701. In what lines does sound move? 703. Explain 
 Fijr. 156, and show in what direction sound approaches and leaves a reflecting 
 surface. r>* 
 
202 
 
 ACOUSTICS. 
 
 Tliis is illustrated by Fig. 156, where suppose a pistol to be 
 fired at A, while the reflecting surface is at C ; then the echo will 
 be heard at B, the angles 2 and 1 being equal io each other. 
 
 704. Reverberation of Sound. If a sound be emitted be- 
 tween two reflecting surfaces, parallel to each other, it will rever- 
 berate, or be answered backward and forward several times. 
 
 Thus, if the sound be made at A, Fig. 157, it will not only 
 rebound back again to A, but will also be reflected from the 
 points C and D, and were such reflecting surfaces placed at 
 every point around a circle from A, the sound would be thrown 
 back from them all, at the same instant, and would meet again 
 at the point A. 
 
 We shall see, under the article Optics, that light observes 
 exactly the same law in respect to its reflection from plane sur- 
 faces, and that the angle at which it strikes, is called the angle 
 of incidence, and that under which it leaves the reflecting sur- 
 face, is called the angle of reflection. The same terms are em- 
 ployeM in respect to sound. 
 
 705. Reflection in a Circle. In a circle, sound is reflected 
 from every plane surface placed around it, and hence, if the 
 sound is emitted from the center of a circle, this center will be 
 the point at which the echo will be most distinct. 
 
 Suppose the ear to be placed at 
 the point A, Fig. 158, in the cen- 
 ter of a circle ; and let a sound be 
 produced at the same point, then 
 it will move along the line A E, 
 and be reflected from the plane sur- 
 face, back on the same line to A ; 
 and this will take place from all the 
 plane surfaces placed around the 
 circumference of a circle ; and as 
 all these surfaces are at the same 
 distance from the center, so the re- 
 flected sound will arrive at the point 
 A, at the same instant; and the 
 echo will be loud, in proportion to the number and perfection 
 of these reflecting surfaces. 
 
 706. WHISPERING GALLERY. It is apparent that the audi- 
 tor, in this case, must be placed in the center from which the 
 
 704. What is the angle under which sound strikes a reflecting surface called? 
 What is the angle under which it leaves a reflecting surface called? Is there any 
 difference in the quantity of these two angles? 705. Suppose a pistol to be fired in 
 the center of a circular room, where would be the echo ? 
 
 FIG 158. 
 
 Reflection in a Circle. 
 
ACOUSTICS. 203 
 
 s 
 
 sound proceeds, to receive the greatest effect. But if the shape 
 of th room be oval, or elliptical, the sound may be made in 
 one part, and the echo will be heard in another part, because 
 the ellipse has two points, called foci, at ^ne of which, the sound 
 being produced, it will be concentrated in the other. 
 
 Suppose a sound to be produced at 
 A, Fig. 159, it will be reflected from FIG. 159. 
 
 the sides of the room, the angles of in- 
 cidence being equal to those of reflection, 
 and will be concentrated at B. Hence, 
 a hearer standing at B, will be affected 
 by the united rays of sound from differ- 
 ent parts of the room, so that a whisper 
 at A, will become audible at B, when it 
 would not be heard in any other part of 
 the room. Were the sides of the room 
 lined with a polished metal, the rays of 
 light or heat would be concentrated in 
 
 the same manner. Whispering Gallery. 
 
 The reason of this will be understood, 
 
 when we consider that an ear, placed at C, will receive only one 
 ray of the sound proceeding from A, while if placed at B, it 
 will receive the rays from all parts of the room. Such a room, 
 whether constructed by design or accident, would be a whisper- 
 ing gallery. 
 
 7 07. Successive Reflections of Sound. "Several reflecting 
 surfaces may be so situated in respect to distance and direction, 
 that a sound proceeding from a certain point, will be reflected, 
 first from one surface, and then from another, at a little dis- 
 tance, afterward from a third, and so on ; or it may be reflected 
 from the first surface to the second, and from the second to the 
 third, and from this to a fourth, and so on, even it is said, to 
 the number of eight or ten." 
 
 708. According to the distance at which the speaker stands, 
 a reflecting surface will return the echo of several, or of fewer 
 syllables ; for in order to avoid confusion, all the syllables must 
 be uttered before the echo of the first syllable reaches the ear. 
 In a moderate way of speaking, about 3^- syllables are pro- 
 nounced in one second, or seven syllables in two seconds. 
 
 706. Explain Fig. 159, and give the reason.- Suppose a sound to be produced in 
 one. of the foci of an ellipse, where then might it be most distinctly heard ? 707. 
 What number of echoes are said to happen from one sound ? 708- How many sylla- 
 bles are pronounced in a second 1 When an echo repeats seven syllables, how far 
 off is the reflecting surface ? Explain this. 
 
204 ACOUSTICS. 
 
 Therefore when an echo repeats seven syllables, the reflecting 
 surface is 1,142 feet distant; for sound travels at the rate of 
 1,142 feet per second, and the distance from the speaker to the 
 reflecting object, and ^again from the latter to the former, is 
 twice 1,142 feet. When the echo returns 14 syllables, the re- 
 flecting object must be 2,284 feet distant, and so on. 
 
 709. It is stated that a famous echo in Woodstock, (Eng- 
 land,) repeats seventeen syllables in the day, and twenty in the 
 night, and en the north side of Shepley church in Sussex, it is 
 said that an echo repeats distinctly, under favorable circum- 
 stances, twenty-one syllables. 
 
 710. Effects of Surface. On a smooth surface, the rays, or 
 pulses of sound, will pass with less impediment than on a rough 
 one. For this reason, persons can talk to each other on the 
 opposite sides of a river, when they could not be understood at 
 the same distance over the land. The report of a cannon at 
 sea, when the water is smooth, may be heard at a great dis- 
 tance, but if the sea is rough, even without wind, the sound will 
 be broken, and will reach only half as far. 
 
 711. MUSICAL INSTRUMENTS. The strings of musical instru- 
 ments are elastic cords, which being fixed at each end, produce 
 sounds by vibrating in the middle. 
 
 The string of a violin or piano, when pulled to one side by 
 its middle, and let go, vibrates backward and forward, like a 
 pendulum, and striking rapidly against the air, produces tones, 
 which are grave, or acute, according to its tension, size, or 
 length. 
 
 712. The manner FIG - 1GO - 
 
 in which such a ^^~ - 
 
 string vibrates is ^^^-~~~ 
 shown by Fig. 160. 
 
 If pulled from E 
 to A, it will not stop 
 again at E, but in Musical string. 
 
 passing from A to 
 
 E, it will gain a momentum, which will carry it to C, and in 
 returning, its momentum will again carry it to D, and so on, 
 backward arid forward, like a pendulum, until its tension, arid 
 the resistance of the air, will finally bring it to rest. 
 
 713. Tones depend on Size and Tension. The grave or 
 
 709. How many syllables is it said some echoes repeat ? 710. Why is it that per- 
 eons can converse on the opposite sides of a river, when they could not hear each 
 other at the same distance over the land 7 711. How do the strings of musical instru- 
 ments produce sounds 7 712. Explain Fig. 160. 
 
ACOUSTICS. 205 
 
 sharp tones of the same string, depend on its different degrees 
 of tension ; hence, if a string be struck, and while vibrating, its 
 tension be increased, its tone will be changed from a lower to a 
 higher pitch. 
 
 Strings of the same length'are made to vibrate slow, or quick, 
 and consequently to produce a variety of sounds, by making 
 some larger than others, and giving them different degrees of 
 tension. The violin and bass viol are familiar examples of this. 
 The low, or bass strings, are covered with metallic wire, in order 
 to make their magnitude and weight prevent their vibration 
 from being too rapid, and thus they are made to give deep or 
 grave tones. The other strings are diminished in thickness, and 
 increased in tension, so as to make them produce a greater 
 number of vibrations in a given time, and thus their tones be- 
 come sharp or acute in proportion. 
 
 714. JEoiJAx HARP. Under certain circumstances, a long 
 string will divide itself into halves, thirds, or quarters, without 
 depressing any part of it, and thus give several harmonious 
 tones at the same time. 
 
 The fairy tones of the u^Eolian harp are produced in this man- 
 ner. This instrument consists of a simple box of wood, with 
 four or five strings, two or three feet long, fastened at each end. 
 These are tuned in unison, so that when made to vibrate with 
 force, they produce the same tones. But when suspended in a 
 gentle breeze, each string, according to the manner or force in 
 which it receives the blast, either sounds, as a whole, of is 
 divided into several parts, as above described. The result 
 of which, is the production of the most pleasing combination 
 and succession of sounds, that the ear ever listened to or fancv 
 perhaps conceived. After a pause, this fairy harp is often heard 
 beginning with a low and solemn note, like the bass of distant 
 music in the sky ; the sound then swells as if approaching, and 
 other tones break forth, mingling with the first, and with each 
 other. 
 
 715. The manner in which a string vibrates in parts, will be 
 understood by Fig. 161. 
 
 Suppose the whole length of the string to be from A to B, 
 and that it is fixed at these two points. The portion from B to 
 C vibrates as though it was fixed at -C, and its tone differs from 
 those of the other parts of the string. The same happens from 
 
 713. On what do the grave or acute tones of the same strin? depend ? Why ate 
 the ba=s strings ofmstruments covered with metallic wire ? 714. Why is there a va* 
 riety of tones in the ^Eolian harp, since ail the strings are tuned in unison 1 715. Ex- 
 plain Fig 161s showing the manner in which strings vibrate in parts. 
 
206 
 
 ACOUSTICS. 
 
 C to D, and from D to A. While a string is thus vibrating, if 
 a small piece of paper be laid on the part C, or D, it will re- 
 main, but -if plactd on any other part of the string, it will bo 
 shaken off. 
 
 FIG. 161. 
 
 JEolian Harp. 
 
 716. MONOCHORD. An instrument called monocJwrd "single 
 string," or sonometer " sound measurer," is used to determine 
 the number and theory of musical vibrations, as applied to 
 stringed instruments. It consists of a wooden box, several feet 
 in length, 1, 2, Fig. 162. At A, a catgut or metallic string is 
 fastened, which passing over the bridges B and C, and then 
 over the roller D, has a weight suspended for its tension at E. 
 
 FIG. 162. 
 
 Monochord. 
 
 The bridge C is attached to a scale, on which it moves, 
 so that the string can be shortened at pleasure. There is also 
 provided a number of leaden weights, having slits to the center, 
 to be slipped on the string, and by which its tension can be in- 
 creased or diminished. 
 
 717. By means of the monochord, many curious and important 
 inferences, with respect to stringed instruments have been drawn. 
 We extract from Muller a few of the most important of these 
 laws. 
 
 716. What is the meaning of monochord, and what its use? Describe the mono 
 chord * 
 
WIND INSTRUMENTS. 207 
 
 718. The number of vibrations of a string, is inversely as its 
 length. 
 
 If the string of any instrument makes a given number of vi- 
 brations in a certain time, it would make in the same time, 2, 
 3, or 4 times as many vibrations, if, with the same tension, we 
 let only , i, or -J- of its length vibrate, and so in these propor- 
 tions, whether it be made longer or shorter. 
 
 719. The number of vibrations of a string is proportional to 
 the square root of its stretching weight, or its tension. 
 
 Thus, if the tension of a given length of string be equal to 4, 
 9, or 16, then the velocity of its vibrations will be 2, 3, or 4 
 times as great. 
 
 720. The number of vibrations of different strings, of the 
 same substance, is inversely as their thickness. 
 
 If we take two steel wires of equal length, whose diameters 
 are as 1 and 2, then with the same tension, 1 will make twice 
 as many vibrations as 2 in the same time. 
 
 721. Capacity of the Human Ear. From Prof. Hoblyn's, 
 London, edition of this work, we add the following : " The ca- 
 pacity of the human ear for appreciating the vibrations of a son- 
 orous body, is restricted within certain limits. It has been 
 proved by experiment, that the lowest note we are capable of 
 perceiving, is that produced by a body performing 32 half vi- 
 brations, or 16 impulses, in one second of time ; and the highest, 
 that which is performed by 16,000 impulses in the same time. 
 It is stated, however, that a finely attuned ear is capable of ap- 
 preciating, as a distinct sound, a kind of hissing noise, occasioned 
 by 48,000 half vibrations, or 24,000 impulses in a second of 
 time." 
 
 WIND INSTRUMENTS. 
 
 722. In stringed instruments, we have seen that the sounds 
 are produced by the vibration of stretched cords on the air. 
 
 In musical pipes the tones are produced, in part, by the pass- 
 age of the air through apertures of various forms, and in part 
 by the vibration of the pipes themselves. 
 
 723. Organ Pipes. The most complicated, important, and 
 costly instrument is the organ. This, indeed, embraces in its 
 structure nearly every known wind instrument, and therefore 
 may be considered as a collection of such instruments, each of 
 
 718. To what is the number of vibrationsof a string proportioned ? 719. How docs 
 tension affect the vibrations 1 720. How does thickness affect vibrations? 721. What 
 is said of the capacity of the ear to appreciate sounds? 722. How are the tones in 
 musical pipes produced ? 723. What instruments does the organ embrace? 
 
208 
 
 WIND INSTRUMENTS. 
 
 FIG. 163. 
 
 which may be played separately, or, when great power is re- 
 quired, several may be played in unison. 
 
 724. Stops. A stop consists of a rank of pipes on a uniform 
 model. Some are only treble, and others only bass stops. In 
 general, however, a stop includes the pipes belonging to each 
 instrument, as the Flute, Trumpet, Hautboy, and'Dulciana 
 stops. 
 
 725. The Diapason (which means through all) 
 is the principal stop, and on this all the other 
 stops are founded, or are made to correspond. 
 
 726. Flue and Reed Stops. This is the great 
 division of the whole organ, and depends on the 
 mechanism by which the tones are produced, 
 every organ in this respect, having only two 
 stops, or sorts of pipes, however numerous the 
 individual stops may be. 
 
 727. flue Pipes. These consist of the body 
 or tube B, Fig. 163, and the foot P, between 
 which there is a diaphragm or partition, having 
 a narrow, transverse aperture to emit the wind 
 from the bellows, as shown by the figure. Over 
 this aperture is a sharp edge called the upper lip, 
 against which the wind is forced, and by which 
 the sound is produced, and which is" modified by 
 the size and form of the pipe. 
 
 728. Chestnut Whistle. The chestnut or wil- 
 low whistle, made by every lad in the country, is 
 a good illustration of the flue organ pipe, the 
 construction of both, being precisely on the same 
 principle. 
 
 729. Reed Pipes. These differ from the above, 
 in having a piece of thin brass, or other metal, 
 placed in the mouth of the pipe, and called the 
 tongue, the vibration of which produces the sound. 
 The tongue is fastened to a cylindrical piece of 
 metal between C C, Fig. 164, which is called the 
 block. The dotted lines C C, show the tuning 
 wire, which passes through the block, and by the 
 
 sliding of which, up and down, the tones are Reed Pipe. 
 
 Flue Pipe. 
 
 FIG. 164. 
 
 724. What is meant by a stop ? 725. What is said of the diapason stop ? 726. Into 
 what stops is the entire organ divided? 727. Show by Fig. 163, the construction of 
 the flue pipes. 728. What is said to be a good illustration of these pipes 1 729. De- 
 scribe the parts of a reed pipe by Fig. 164. 
 
WIND INSTRUMENTS. 200 
 
 varied, the pitch becoming flat or sharp, as the tongue is made 
 long or short. 
 
 the reed pipes are generally of metal, the body of which is 
 shown by A B. 
 
 730. Structure of the Pipes. The large pipes are commonly 
 made of wood, and are square in form, though some wood 
 pipes are only a few inches long. The largest of these pipes 
 are 32 feet long and 15 inches in diameter. 
 
 731. The metal pipes are in the form of a cone or cylinder, most 
 of the smaller ones being of these forms and substance. In a 
 few instances, metallic pipes of immense size and weight have 
 been constructed. 
 
 The largest ever made, is at Birmingham, England, which 
 is 32 feet Jong and 24 inches in diameter. It is of zinc, in 
 form of a cylinder, standing in front of all the other pipes. 
 
 732. Tuning the Organ. The pipes are tuned by various 
 means, depending on frheir forms ; the substance of which they 
 are made, and whether they are open or stopped. 
 
 733. Stopped wooden pipes are tuned by a pompion, or stop- 
 per, which is of wood, covered with leather, exactly fitting the 
 end, and which is drawn up, or pushed down, to make the tones 
 more grave or sharp. 
 
 The stopped metal pipes, have a cap on the top, and by the 
 movement of which, they are tuned on the same principle as 
 those of wood. In some cases, stopped metallic pipes are tuned 
 by means of ears on each side of the mouth, by the bending of 
 which, the tones are varied. 
 
 Open metal pipes are tuned by a wooden instrument, one 
 end of which is a solid cone* and the other a hollow cone. By 
 this, the tops of the pipes are expanded by introducing the solid 
 end, to make the . pitch sharper, and- contracted by the hollow 
 cone, to make the pitch fatter. 
 
 The reed pipes, as already noticed, are tuned by the motions 
 of the tuning wire. 
 
 Reed Pipes vary with the Temperature. The tongues of 
 these pipes vary in length by heat and cold, and hence their 
 tones change, for the same reason that the clock goes faster in 
 winter than in summer, as explained, 283. It is probably on 
 this account that organists find difficulty in keeping these stops 
 in tune. 
 
 730. What is said of the structure of these pipes ? 731. Of what size are the largest 
 organ pipes 1 732. How are the pipes tuned 1 733. How are the stooped nines tuned 1 
 How are the open p, P es tuned ? What is said of the influence ofTemperTuSe on the 
 
210 LARGE ORGANS. 
 
 734. ANTIQUITY OF THE ORGAN. The earliest account of 
 any instrument similar to the organ, occurs in the Tenth Book 
 of Vitruvius, a Greek writer, who lived a century before the 
 Christian era. This was moved by water, and hence was called 
 a hydraulicon. 
 
 The first organ spoken of in France, was of Greek construc- 
 tion, and was sent to King Pepin, the father of Charlemagne, 
 by the Emperor Constantine, about A. D. 757. This was 
 moved by wind. 
 
 The first of any size known in England, was that of Winches- 
 ter Cathedra], in 951. This had 26 pairs of bellows, which it 
 required 70 men to work. It had 10 keys, and 40 pipes to 
 each key. 
 
 Notwithstanding the antiquity of this invention, it was not 
 until after the Reformation that any great improvements were 
 made in this instrument. Even so late as 1660, only four organ 
 builders were to be found in Great Britain. 
 
 This instrument, in our country, was unknown to the common 
 people a century ago ; and at the time of our revolution, com- 
 paratively few persons, except in large cities, had ever heard an 
 organ. It is hardly necessary to add, that the organ, as it now 
 exists, is an entirely different instrument from that so called 
 only fifty years ago, and that, at present, no village having a 
 church of any pretensions, is without an organ. 
 
 LARGE ORGANS. 
 
 735. Perhaps we can not gratify our readers more than to 
 add short notices of a few of the largest organs in the world. 
 
 Haarlem Organ. This has long been the most celebrated of 
 organs. It was built in 1638, at the cost of $60,000. The case 
 is 108 feet high by 50 feet wide. It has 60 stops ; 12 pair of 
 bellows; 4 rows of keys; 5000 pipes, of which two are 32 feet 
 long and 15 inches wide. The fee for hearing the whole is $5. 
 
 Freyburg Organ, in Switzerland. It is said that no instru- 
 ment ever was, or ever will be built like this ; the artist, Moser, 
 refusing to build another, and no one being allowed to see the 
 interior. The wonder, and the secret, with respect to this organ, 
 is in its having a stop, the tones of which are so exactly like 
 those of the. human voice, that visitors mistake it for a large choir 
 of singers. It has 68 stops and 4 rows of keys. 
 
 Music Hall, Edinburgh, Organ. This immense instrument 
 
 734. What is said of the antiquity of the organ 7 
 
HARMONICON. 
 
 211 
 
 has 82 stops, 4 rows of keys, 1 wooden pipe of 32 feet, and 
 several of metal of 16 feet in length. 
 
 Hamburgh Organ. This organ, in St. Michael's Church, was 
 biiilt in 1762, and cost more than $20,000. It has 4 rows of 
 keys; three pipes of 32 feet, and nine of 16 feet; 10 wind 
 chests and 10 pair of bellows. The pipes of the large pedal 
 stop, are of pure tin highly polished, and placed in front 
 
 The Weingarten Organ. This is in the Benedictine Monas- 
 tery, in Suabia, and was built about 1750. It has 4 rows of 
 keys ; 3 pipes of 32 feet; 4 of 16 feet, and 4 unisons. It has 
 in the whole, 6,666 pipes ; namely, in the great organ, 2,176 ; 
 in the choir, 1,176 ; in the third organ, 1,274 ; in the echo or- 
 gan, 1,225, and in the pedal organ, 815. 
 
 Berlin Organ. This organ, at Berlin, Prussia, was designed 
 to be the largest in the world, and to contain 150 stops and 6 
 rows of keys, besides the pedals, but it remains unfinished. 
 
 Baltimore Cathedral Organ. This is said to be the largest 
 in the United States. It has 36 stops and 2,213 pipes, the 
 largest being 32 feet long. 
 
 HARMONICON. 
 
 This is a musical instrument invented by Dr. Franklin, though 
 i* has been much improved since his day. 
 
 FIG. 165. 
 
 o 
 O O 
 
 Harmonicon. 
 
 It consists of a number of glass goblets of different sizes, and 
 so attuned to each other as to form the harmonic scale. 
 
212 WIND. 
 
 * 
 
 They are firmly fastened to the bottom of a box, their tones 
 being so nicely adapted to the scale, by the artist who con- 
 structs them, as to need no tuning, though one or two of them 
 contain water as a convenience. 
 
 They are played by touching the edges with the wet finger, 
 and their tones may be prolonged, and made to swell or dimin- 
 ish, like .those of the violin. 
 
 Perhaps no music to which the human ear has ever listened, 
 is equal in sweetness, delicacy, and smoothness to this. No one 
 can hear it without a thrill of delight, nor for the first time, 
 without astonishment. It is indeed an .JSolian harp under 
 command of the artist. 
 
 The arrangement and comparative sizes of the goblets, are 
 shown by Fig. 165, which presents the natural key, or C major. 
 
 The goblets hold from a quart to half a pint, and their tones 
 depend, in part, upon their capacity, and in part upon the 
 weight or thickness. 
 
 The instrument here represented, is capable of producing all 
 the tones of the most common and simple melodies. 
 
 We are told that Mr. Francis H. Smith, of Baltimore, furn- 
 ishes Harmonicons, put up in boxes, at various prices, from 18 
 to 85 dollars. 
 
 ATMOSPHERIC PHENOMENA. 
 
 736. The term atmosphere is from two Greek words, which 
 signify vapor and sphere. It is the air which surrounds the 
 earth to the height of forty-five miles, and is essential to the 
 lives of all animals, and the production of all vegetables. 
 
 All meteorological phenomena, with which we are acquainted, 
 depend chiefly, if not entirely, on the influence of the atmos- 
 phere. Fogs, winds, rain, dew, hail, snow, thunder, lightning, 
 electricity, sound, and a variety of other phenomena of daily 
 occurrence, belong to the atmosphere. We have, however, only 
 room for the most common result of atmospheric changes. 
 Wind and Rain. 
 
 WIND 
 
 737. Wind is nothing more than^ air in motion. The use 
 of a fan, in warm weather, only serves to move the air, and thus 
 to make a little breeze about the person using it. 
 
 736. What is the atmosphere? How high does the atmosphere extend? What 
 phenomena mentioned, depend on the atmosphere? 737. What is wind? As a 
 natural phenomenon, how is wind produced ; or, what is the cause of wind? How 
 is this illustrated 7 
 
WIND. 213 
 
 As a natural phenomenon, that motion of the air which we 
 call wind, is produced in consequence of there being a greater 
 degree of heat in one place than in another. The air thus 
 heated, rises upward, while that which surrounds this, moves 
 forward to restore the equilibrium. 
 
 The truth of this is illustrated by the fact, that during the 
 burning of a house in a calm night, the motion of the air to- 
 ward the place where it is thus rarefied, makes the wind blow 
 from every point toward the flame. 
 
 738. Sea and Land Breeze. On islands, situated in hot 
 climates, this principle is charmingly illustrated. The land, 
 during the day time, being under the rays of a tropical sun, 
 becomes heated in a greater degree than the surrounding ocean, 
 and, consequently, there rises from the land a stream of warm 
 air, during the day, while the cooler air from the surface of the 
 water, moving forward to supply this partial vacancy, produces 
 a cool breeze setting inland on all sides of the island. This 
 constitutes the sea breeze, which is so delightful to the inhabit- 
 ants of those hot countries, and without which men could hardly 
 exist in some of the most luxuriant islands between the tropics. 
 
 During the night, the motion of the air is reversed, because 
 the earth being heated superficially, soon cools when the sun ia 
 absent, while the water, being warmed several feet below its 
 surface, retains its heat longer. 
 
 Consequently, toward morning, the earth becomes colder than 
 the water, and the air sinking down upon it, seeks an equilib- 
 rium, by flowing outward, like rays from a center, and thus 
 the land breeze is produced. 
 
 The wind then continues to blow from the land until the 
 equilibrium is restored, or until the morning sun makes the land 
 of the same temperature as the water, when for a time there 
 will be a dead calm. Then again the land becoming warmer 
 than the water, the sea breeze returns as before, and thus the 
 inhabitants of those sultry climates are constantly refreshed dur- 
 ing the summer season, with alternate land and sea breezes. 
 
 739. TRADE WINDS. At the equator, which is a part of the 
 earth continually under the heat of a burning sun, the air is 
 expanded, and ascends upward, so as to produce currents from 
 the north and south, which move forward to supply the place 
 of the heated air as it rises. 
 
 738. In the islands of hot climates, why does the wind blow inland during the day 
 and off the land during the night ? What are these breezes called ? 739. What is 
 said of the ascent of heated air at the equator ? What is the consequence on the air 
 toward the north and south 7 How are the trade winds formed 1 
 
214 WIND. 
 
 These two currents, coming from latitudes where the daily- 
 motion of the earth is less than at the equator, do not obtain 
 its full rate of motion, and therefore, when they approach the 
 equator, do not move so fast eastward as that portion of the 
 earth, by the difference between the equator's velocity, and that 
 of the latitudes from which they come. This wind, therefore, 
 falls behind the earth in her diurnal motion, and consequently 
 has a relative motion toward the west. This constant breeze 
 toward the west is called the trade wind, because a large por- 
 tion of the commerce of nations comes within its influence. 
 
 740. Counter Currents. While the air in the lower regions 
 of the atmosphere is thus constantly flowing from the north and 
 south toward the equator, and forming the trade winds between 
 the tropics, the heated air from these regions as perpetually 
 rises, and forms a counter current through the higher regions, 
 toward the north and south from the tropics, thus restoring the 
 equilibrium. 
 
 This counter motion of the air in the upper and lower regions 
 is illustrated by a very simple experiment. Open a door a few 
 inches, leading into a heated room, and hold a lighted candle 
 at the top of the passage ; the current of air, as indicated by 
 the direction of the flame, will be out of the room. Then set 
 the candle on the floor, and it will sjiow that the current is 
 there into the room. Thus, while the heated air rises and passes 
 out of the room, at the same time that which is colder flows in, 
 along the floor, to take its place. 
 
 This explains the reason why our feet are apt to suffer with 
 the cold, in a room moderately heated, while the other parts of 
 the body are comfortable. It also explains why those who sit 
 in the gallery of a church are sufficiently warm, while those 
 who sit below may be shivering with the cold. 
 
 741. From such facts, showing the tendency of heated air to 
 ascend, while that which is colder moves forward to supply its 
 place, it is easy to account for the reason why the wind blows 
 perpetually from the north and south toward the tropics ; for 
 the air being heated, as stated above, it ascends, and then flows 
 north and south toward the poles, until, growing cold, it sinks 
 down and again flows toward the equator. 
 
 740. While the air in the lower regions flows from the north and south toward the 
 equator, in what direction does it flow in the higher regions ? How is this counter 
 current in lower and upper regions illustrated by a simple experiment? 741. What 
 common fact does this experiment illustrate? Explain Fig. 166, and show which 
 way the air passes. 
 
WIND. 215 
 
 Perhaps these opposite motions of the two 
 better understood by the sketch, Fig. 166. 
 
 Opposite Currents of Air. 
 
 Suppose A B to represent a portion of the earth's surface, 
 A being toward the north pole, C toward the south pole, and 
 B the equator. The currents of air are supposed to pass in the 
 direction of the arrows. The wind, therefore, from A to B 
 would blow on the surface of the earth, from north to south, 
 while from E to A, the upper current would pass from south to 
 north, until it came to A, when it would change its direction 
 toward the south. The currents in the southern hemisphere 
 being governed by the same laws, would assume similar di- 
 rections. 
 
 VELOCITY OF WIND. 
 
 742. The velocity of aerial movements amount, according to 
 authors, from to upwards of 100 miles an hour ; but the max- 
 imum is variously stated by different experiments, nor do we 
 see how any great degree of accuracy can be attained on this 
 point. The best method is, to deduce the velocity, by the force 
 of wind ; which is done by an instrument invented for that pur- 
 pose by Dr. Lind, a figure of which we here insert. 
 
 743. ANEMOMETER, OR WIND MEASURER. It consists of a 
 glass tube, Fig. 167, bent into the form of the letter U, and 
 open at both ends. The upper end of B is bent to the hori- 
 zontal direction, and is widened at the mouth for the purpose 
 of receiving the wind. The tube being partly filled with water, 
 and exposed to a current of air, the fluid is depressed in that, 
 and of course rises in the other leg of the tube. As the water 
 
 743. What is the name of the instrument which measures the force of wind? How 
 is it constructed ? 
 
216 
 
 RAIN. 
 
 is on a level in both branches when the air is FIG. 167. 
 
 still, if it is depressed to B on one side, it 
 must rise to C on the other, the amount of 
 rise, and consequently the degree of force, be- 
 ing measured by a graduated scale. Now as 
 the pressure of water is as its height, the rise 
 in the tube will not be in direct proportion to 
 the force of the wind, but the velocity of the 
 wind will be in the ratio of the square root of 
 the resistance. The tube is diminished at the 
 base to check the undulations of the water. 
 
 744. By this instrument it is found that Anemometer. 
 
 ' the following popular expressions with respect 
 to aerial currents, are indicated on the scale as here expressed. 
 
 Velocity of the Wind in miles per hour. Com. appellation of the force of Wind. 
 
 1 . Hardly perceptible. 
 
 4 Gentle breeze. 
 
 6 Pleasant Wind. 
 
 10 Brisk wind. 
 
 15 Very brisk wind. 
 
 20 High wind. 
 
 30 Very high wind. 
 
 40 A storm. 
 
 50 A hard storm. 
 
 60 A great storm. 
 
 80 A hurricane. 
 
 100 A violent hurricane. 
 
 RAIN. 
 
 745. Rain is falling water in the form of drops. It appears 
 to result from the meeting of two clouds of different tempera- 
 tures. 
 
 In explaining the theory of rain, it must be understood, that 
 warm air has a greater capacity for moisture than cold. It is 
 also ascertained, that this capacity increases at a much faster 
 ratio than the increase of temperature itself, and h'ence it fol- 
 lows that if two clouds at different temperatures, completely 
 saturated, meet and mingle together, a precipitation of moisture 
 
 744. What correspondence is there between the velocity of wind, and common ex- 
 pressions? 745. What is rain ? What is said of the ratio of capacity for moisture 
 increasing faster than the temperature in clouds? Explain the reason why, when 
 two clouds meet of different temperatures, rain is the result. 
 
RAIN. 17 
 
 must take place in consequence of the mixture. This would 
 result from the fact that the warmest cloud contained a greater 
 portion of moisture than is indicated by its temperature, as 
 stated above, while the mixture would form a mean tempera- 
 ture, but the mean quantity of vapor could not be retained, 
 since the sum of their capacities for vapor would thus be di- 
 minished. 
 
 746. Suppose for example, that at the temperature of 15 de- 
 grees, air can hold 200 parts of moisture ; then at 30 degrees 
 it would hold 400 parts, and at 45 degrees 800 parts. Now 
 let two equal bulks of this air, one at 15, and the other at 45 
 degrees be mixed, the compound would then contain 200 and 
 800 parts of moisture = 1000, that is, 500 each, and the tem- 
 perature of the mixture would be 30 degrees. But at this 
 temperature, air is saturated with 400 parts of vapor, therefore, 
 100 parts is rejected and falls in the form of rain. 
 
 This is Dr. Button's theory of rain, and observation has 
 seemed to prove its truth. 
 
 747. RAIN GAUGE. This is an instrument designed to 
 measure the quantity of rain which falls at any given time and 
 place. 
 
 748. A variety of forms, some quite compli- FIG. J68. 
 cated, have been invented for this purpose. The 
 
 most simple and convenient, for common pur- 
 poses, is that represented by Fig. 168. It may 
 be two feet high, round in form, and made of 
 tin, or copper, well painted. It is furnished with 
 a small metallic faucet for drawing off the water, 
 and into the stem of this, is inserted a glass tube, 
 as a scale, divided into inches and tenths of 
 inches. This may be done by means of paper, 
 pasted on and then varnished. 
 
 The water will stand at the same height in the Rain Gau se 
 glass scale that it does in the cylinder, and being 
 on the outside, the quantity may be known at a glance. If the 
 funnel, or top, is twice the size of the cylinder, then, an inch in 
 the scale will indicate half an inch received into the gauge, or 
 these proportions may be a tenth, when much accuracy is 
 required. 
 
 746. What is the design of the rain ffaurei " 747. What are the forms and materials 
 of this instrumenl 1 748. Describe the scale, and what it indicates with respect to 
 the size of the funnel and cylinder ? 
 
 10 
 
CHAPTER XL 
 
 OPTICS. 
 
 1. This term, derived from the Greek, signifies seeing, or to 
 see. It is that science which treats of vision, and the laws, 
 properties, and phenomena of light. 
 
 2. It admits of two divisions, viz., Dioptrics, or a discourse on 
 the laws of refracted light, and Catoptrics, a treatise on reflected 
 light. 
 
 This science involves some of the most elegant and import- 
 ant branches of natural philosophy. It presents us with exper- 
 iments which are attractive by their beauty, and which astonish 
 us by their novelty ; and, at the same time, it investigates the 
 principles of some of the most useful among the articles of 
 common life. 
 
 3. There are two opinions concerning the nature of light. 
 Some maintain that it is composed of material particles, which 
 are constantly thrown off from the luminous body ; while others 
 suppose that it is a fluid, diffused through all nature, and that 
 the luminous, or burning body, occasions waves or undulations 
 in this fluid, by which the light is propagated in the same man- 
 ner as sound is conveyed through the air. 
 
 4. The most probable opinion, however, is that light is com- 
 posed of exceedingly minute particles of matter. But whatever 
 may be the nature or cause of light, it has certain general prop- 
 erties or effects which we can investigate. Thus, by experiment, 
 we can determine the laws by which it is governed in its pass- 
 age through different transparent substances, and also those by 
 which it is governed when it strikes a substance through which 
 it can not pass. We can likewise test its nature to a certain 
 degree, by decomposing or dividing it into its elementary parts, 
 as the chemist decomposes any substance he wishes to analyze. 
 
 5. Definitions. To understand the science of optics, it is 
 necessary to define several terms, which, although some of them 
 may be in common use, have a technical meaning, when ap- 
 plied to this science. 
 
 1. What is the meaning of optics? 2. What are the meaning of dioptrics and cat- 
 optrics? What is said of the elegance and importance of this science? 3. What ar^ 
 the two opinions concerning the nature of light 1 4. What is the most probable opin- 
 ion ? 5. What is light 1 
 
OPTICS. 219 
 
 Light is that principle, or substance, which enables us to see 
 any body from which it proceeds. If a luminous substance, as 
 a burning candle, be carried into a dark room, the objects in 
 the room become visible, because they reflect the light of the 
 candle to our eyes. 
 
 6. Luminous bodies are such as emit light from their own 
 substance. The sun, fire, and phosphorus are luminous bodies. 
 The moon, and the other planets, are not luminous, since they 
 borrow their light from the sun. 
 
 7. Transparent bodies are such as permit the rays of light to 
 pass freely through them. Air and some of the gases are per- 
 fectly transparent, since they transmit light without being visible 
 themselves. Glass and water are also considered transparent, 
 but they are not perfectly so, sines they are themselves visible, 
 and therefore do not suffer the light to pass through them with- 
 out interruption. 
 
 8. Translucent bodies are such as permit the light to pass, 
 but not in sufficient quantity to render objects distinct, when 
 seen through them. 
 
 9. Opaque is the reverse of transparent. Any body which 
 permits none of the rays of light to pass through it, is opaque. 
 
 10. Illuminated, enlightened. Any thing is illuminated 
 when the light shines upon it so as to make it visible. Every 
 object exposed to the sun is illuminated. A lamp illuminates a 
 room, and every thing in it. 
 
 A Ray is a single line of light, as it coirues from a luminous 
 body. 
 
 A Beam of light is a body of parallel rays. 
 
 A Pencil of light is a body of diverging or converging rays. 
 
 Divergent rays are such as come from a point, and contin- 
 ually separate wider apart as they proceed. 
 
 Convergent rays are those which approach each other, so as 
 to meet at a common point. 
 
 Luminous bodies emit rays, or pencils of light, in every di- 
 rection, so that the space through which they are visible, is 
 filled with them at every possible point. 
 
 Thus, the sun illuminates every point of space, within the 
 whole solar system. A light, as that of a light-house, which 
 
 6. What is a luminous body? 7. What is a transparent body ? Are glass and wa 
 ter perfectly transparent ? How is it proved that air is perfectly transparent? 8 
 What are trans ucent bodies? 9. What are opaque bodies? 10. What is meant by* 
 illuminated } What is a ray of light ? What is a beam .' What a pencil 1 What are 
 divergent rays? What are convergent rays ? In what direction do luminous bodies 
 emit light ? How is it proved that a luminous body fills every point within a certain 
 distance with light 7 
 
220 OPTICS. 
 
 can be seen from the distance of ten miles in one direction, fills 
 every point in a circuit of ten miles from it, with light. Were 
 this not the case, the light from it could not be seen from every 
 point within that circumference. 
 
 11. Motion of light. The rays of lighj, move forward in 
 straight lines from the luminous body, and are never turned out 
 of their course, except by some obstacle. 
 
 FIG. 169. 
 
 Motion of Light. 
 
 Let A, Fig. 169, be a beam of light from the sun passing 
 through a small orifice in the window shutter, B. The sun 
 can not be seen through the crooked tube C, because the beam 
 passing in a straight line, strikes the side of the tube, and there- 
 fore does not pass through it. 
 
 12. All illuminated bodies, w r hether natural or artificial, throw 
 off light in every direction of the same color as themselves, 
 though the light with which they are illuminated is white or 
 without color. 
 
 This fact is obvious to all who are endowed with sight. Thus 
 the light proceeding from grass is green, while that proceeding 
 from a rose is red, and so of every other color. 
 
 We shall be convinced, in another place, that the white light 
 with which things are illuminated, is really composed of several 
 colors, and that bodies reflect only the rays of their own color, 
 while they absorb all the other rays. 
 
 13. Velocity of Light. Light moves with the amazing ra- 
 pidity of about 95 millions of miles in 8- minutes, since it is 
 proved by certain astronomical observations, that the light of 
 the sun comes to the earth in that time. This velocity is so 
 great, that to any distance at which an artificial light can be 
 seen, it seems to be transmitted instantaneously. 
 
 11. Why can not a beam of light be seen through a bent tube 1 12. What is the 
 eolor of the light which different bodies throw off? If grass throws off green light, 
 what becomes of the other rays 7 13. What is the rate of velocity with which light 
 moves? Can we perceive any difference in the time which it takes an artificial 
 light to pass to u from a great or small distance ? 
 
REFRACTION OF LIGHT. 221 
 
 If a ton of gunpowder were exploded on the top of a moun- 
 tain, where its light could be seen a hundred miles, no percept- 
 ible difference would be observed in the time of its appearance 
 on the spot, and at -the distance of a hundred miles. 
 
 DIOPTRICS, OR THE REFRACTION OF LIGHT. 
 
 14. Although a ray of light will pass in a straight line, when 
 not interrupted, yet when it passes obliquely from one transpar- 
 ent body into another, of a different density, it leaves its linear 
 direction, and is bent, or refracted more or less, out of its former 
 course. 
 
 This change in the direction of light, FIG. 170. 
 
 seems to arise from a certain power, 
 or quality, which transparent bodies 
 possess in different degrees ; for some 
 substances bend the rays of light much 
 more obliquely than others. 
 
 The manner in which the rays of 
 light are refracted, may be readily un- 
 derstood by Fig. 170. 
 
 Let A be a ray of the sun's light, 
 proceeding obliquely toward the sur- Refraction. 
 
 face of the water C D, and let E be 
 the point which it would strike, if moving only through the air. 
 Now, instead of passing through the water in the line A E, it 
 will be bent or refracted, on entering the water, from O to 
 N, and having passed through the fluid it is again refracted in 
 a contrary direction on passing out of the water, and then pro- 
 ceeds onward in a straight line as before. 
 
 15. Cup and Shilling. The refraction of water is beauti 
 fully proved by the following simple experiment. Place an 
 empty cup, Fig. 171, with a shilling on the bottom, in such a 
 position that the side of the cup will just hide the piece of 
 money from the eye. Then let another person fill the cup with 
 water, keeping the eye in the same position as before. As the 
 water is poured in, the shilling will become visible, appearing 
 to rise with the water. ' The effect of the water is to bend the 
 ray of light coming from the shilling, so as to make it meet the 
 eye below the point where it otherwise would. Thus the eye 
 
 14. What is meant by the refraction of light 7 Do all transparent bodies refract 
 1'ght equally? Explain Fig. 170, and show how the ray is refracted in passing into, 
 and out of the water. 15. Explain Fig. 171, and state the reason why the shilling 
 seems to be raised up by pouring in the water. 
 
222 REFRACTION OF LIGHT. 
 
 could not see the shilling in ^J mo - ITL 
 
 the direction of C, since the 
 line of vision toward A and 
 C is hidden by the side of 
 the cup. But the refraction 
 of the water bends the ray 
 downward,- producing the 
 same effect as though the ob- 
 ject had been raised upward, 
 and hence it becomes visible. c 
 
 1 6. Refraction by Several Cup and shilling. 
 
 Media. Any transparent 
 
 body through which light passes, is called a medium, and it is 
 found in all cases, " that where a ray of light passes obliquely 
 from one medium into another of a different density, it is re- 
 fracted, or turned out of its former course" This is illustrated 
 in the above examples, the water being a more dense medium 
 than air. The refraction takes place at the surface of the me- 
 dium, and the ray is refracted in its passage out of the refract- 
 ing substance as well as into it. 
 
 1*7. If the ray, after having passed through' the water, then 
 strikes upon a still more dense medium, as a pane of glass, it 
 will again be refracted. It is understood, that in all cases, the 
 ray must fall upon the refracting medium obliquely, in order to 
 be refracted, for if it proceeds from one medium to another per- 
 pendicularly to their surfaces, it will pass straight through them 
 all, and no refraction will take place. 
 
 18. Thus, in Fig. 172, let A represent air, B water, and C a 
 piece of glass. The ray D, striking each medium in a perpen- 
 dicular direction, passes through them all in a straight line. 
 The oblique ray passes through the air in the direction of C, 
 but meeting the water, is refracted in the direction of O ; then 
 falling upon the glass, it is again refracted in the direction of 
 P, nearly parallel with the perpendicular line D. 
 
 19. In all cases where the ray passes out of a rarer into a 
 denser medium, it is refracted toward a perpendicular line, 
 raised from the surface of the denser medium, and so, when it 
 
 passes out of a denser, into a rarer medium, it is refracted 
 
 from the same perpendicular. 
 
 16. What is a medium ? In what direction must a ray of light pass from one me- 
 dium to another, to be refracted 1 17. Will a ray falling perpendicularly on a medium 
 be refracted ? 18. Explain Fig. 172. and show how the ray E is refracted. 19. When 
 it passes out of a rarer into a denser medium, in what direction is it refracted ? When 
 it passes out of a denser into a rarer medium, in what direction is the refraction * 
 Explain this by Fig. 173. 
 
BEFRACTIQN OF LIGHT. 
 
 223 
 
 Let the medium B, Fig. 173, be glass, and the medium C, 
 water. The ray A, as it falls upon the medium B, is refracted 
 toward the perpendicular line E D ; but when it enters the wa- 
 ter, whose refractive power is less than that of glass, it is not 
 bent so near the perpendicular as before, and hence it is re- 
 fracted /rom, instead of toward the perpendicular line, and ap- 
 proaches the original direction of the ray A G, when passing 
 through the air. 
 
 FIG. 173. 
 
 Water. 
 
 Glass and Water. 
 
 Air, Water, and Glass. 
 
 20. The cause of refraction appears to be the power of at- 
 traction, which the denser medium exerts on the passing ray; 
 and in all cases the attracting force acts in the direction of a 
 perpendicular to the refracting surface. 
 
 21. Refraction by Water. The refraction of the rays of 
 light, as they fall upon the surface of the water, is the reason 
 why a straight rod, with one end in the water, and the other 
 end rising above it, appears to be broken, or bent, and also to 
 be shortened. 
 
 Suppose the rod A, Fig. 174, to be set with one half of its 
 length below the surface of the water, and the other half above 
 it. The eye being placed in an oblique direction, will see the 
 lower end apparently at the point 0, while the real termination 
 of the rod would be at N ; the refraction will therefore make 
 the rod appear shorter by the distance from O to N, or one- 
 fourth shorter than the part below the water really is. The 
 reason why the rod appears distorted, or broken, is, that we 
 
 20. What is the cause of refraction 1 21. What is the reason that a rod, with one 
 end in the water, appears distorted and shorter than it really is? Why does the wa- 
 ter in a pond appear less deep than it really is 7 
 
224 DOUBLE REFRACTION. 
 
 judge of the direction of the part which is under the water, by 
 that which is above it, and the refraction of the rays coining 
 from below the surface of the water, give them a different direc- 
 tion, when compared with those coming from that part of the 
 rod which is above it. Hence, when the whole rod is below 
 the water, no such distorted appearance is observed, because 
 then all me rays are refracted equally. 
 
 For the.reason just explained, persons are often deceived in 
 respect to the depth of water, the refraction making it appear 
 much more shallow than it really is ; and there is no doubt but 
 the most serious accidents have often happened to those who 
 have gone into the water under such deception ; for a pond 
 which is really six feet deep, will appear to the eye only a little 
 more than four feet deep. 
 
 DOUBLE REFRACTION. 
 
 22. By double refraction is meant that property in certain 
 native minerals, by which they transmit two images of a single 
 object. 
 
 This property is most perfect in specimens of carbonate of 
 lime, usually called Iceland spar ; the latter name being form- 
 erly given to the fine specimens from that country. At present, 
 these' rhomboids are found in most primitive limestone countries. 
 
 A perfect piece, two inches in diameter, will show the lines 
 about a quarter of an inch apart, the greater the thickness the 
 more distant will be the images presented. Sometimes two or 
 three pieces of different sizes, are wanted by the experimenters. 
 
 If a piece of this spar be laid over a black line, and then be 
 made to revolve slowly, it will be observed that the doubly re- 
 fractive power increases in proportion as the acute angles of the 
 rhomb correspond to the direction of the line, when the refrac- 
 tion is greatest, or the two lines are widest apart. On the con- 
 trary, if the crystal is turned in either direction beyond this 
 point, the refracted lines approach each other, until the short 
 diagonal or obtuse angles, correspond with the line, when the 
 double refraction ceases entirely, and only a single line appears. 
 
 22. What is meant by double refraction 7 Explain its cause. 
 
REFLECTION Of LIGHT. 
 
 225 
 
 Explanation. The cause of this difference is, that when the 
 acute angles of the rhomb correspond to the black line, the re- 
 fracted ray is most widely separated from the common ray, 
 which depends on the thickness of the crystal, but when the 
 position is reversed, the common ray is brought into the exact 
 line of the refracted one, thus forming only a single line. 
 
 CATOPTRICS, OR THE REFLECTION OF LIGHT. 
 
 23. If a boy throws his ball against the side of a house swiftly, 
 and in a perpendicular direction, it will bound back nearly in 
 the line in which it was thrown, and he will be able to catch it 
 with his hands ; but if the ball be thrown obliquely to the right, 
 or left, it will bound away from the side of the house in the 
 same relative direction in which it was thrown. 
 
 The reflection of light, so far as regards the line of approach, 
 and the line of leaving a reflecting surface, is governed by the 
 same law. 
 
 Thus, if a sunbeam, Fig. 175, passing through a small aper- 
 ture in the window-shutter A, be permitted to fall upon the 
 plane mirror, or looking-glass, C, D, at right-angles, it will be 
 reflected back at right-angles with the mirror, and therefore 
 will pass back again in exactly the same direction in which it 
 approached. 
 
 FIG. 175. 
 
 FIG. 176. 
 
 FIG. 177. 
 
 Reflection of Light. 
 
 24. But if the ray strikes the mirror in an oblique direction, 
 it will also be thrown off in an oblique direction, opposite to 
 that from which it came. 
 
 23. Suppose a sunbeam falls upon a plane mirror, at right angles with its surface, 
 \.i what direction will it be reflected ? 24. Suppose the ray falls obliquely on its sur- 
 face in what direction will it then be reflected 1 
 
 10* 
 
226 MIRRORS. 
 
 Let a ray pass toward a mirror in the line A C, Fig. 176, it 
 will be reflected off in the direction C D, making the angles 1 
 and 2 exactly equal. 
 
 The ray A C, is called the incident ray, and the ray C D, 
 the reflected ray ; and it is found, in all cases, that "whatever 
 angle the ray of incidence makes with the reflecting surface, or 
 with a perpendicular line drawn from the reflecting surface, ex- 
 actly the same angle is made by the reflected ray. 
 
 25. From these facts, arises the general law in optics, that 
 the angle of reflection is equal to the angle of incidence. 
 
 The ray A C, Fig. 177, is the ray of incidence, and that 
 from G to D, is the ray of reflection. The angles which A C, 
 make with the perpendicular line, and with the plane of the 
 mirror, are exactly equal to those made by C D, with the same 
 perpendicular, and the same plane surface. 
 
 26. Mirrors are of three kinds, namely, plane, convex, and 
 concave. They are made of polished metal, or of glass covered 
 on the back with an amalgam of tin and quicksilver. 
 
 PLANE MIRROR. The common looking-glass is a plane mir- 
 ror, and consists of a plate of ground glass so highly polished 
 as to permit the rays of light to pass through it with little in- 
 terruption. On the back of this plate is placed the reflecting 
 surface, which consists of a mixture of tin and mercury. The 
 glass plate, therefore, only answers the purpose of sustaining the 
 metallic surface* in its place, of admitting the rays of light to 
 and from it, and of preventing its surface from tarnishing, by 
 excluding the air. Could the metallic surface, however, be re- 
 tained in its place, and not exposed to the air, without the glass 
 plate, these mirrors would be much more perfect than they are, 
 since, in practice, glass can not be made so perfect as to trans- 
 mit all the rays of light which fall on its surface. 
 
 27. When applied to the plane mirror, the angles of incidence 
 and of reflection are equal, as already stated ; and it therefore 
 follows, that when the rays of light fall upon it obliquely in one 
 direction, they are thrown off under the same angle in the op- 
 posite direction. 
 
 What is an incident ray cf light 1 What is a reflected ray of light? 25. What gen- 
 eral law in optics results from observations on the incident and reflected rays 1 26. 
 How many kinds of mirrors are there 1 What kind of mirror is the common look- 
 ing-glass 7 Of what use is the glass plate in the construction ot this mirror 1 27. Ex- 
 plain Fig 178, and show how the image of an object can be seen in a plane mirror 
 when the real object is invisible. 
 
MIRRORS. 
 
 227 
 
 Eyuai Angles. 
 
 This is the reason why the images of FIG - I7a 
 
 objects can be seen when the objects 
 themselves are not visible. 
 
 Suppose the mirror, A B, Fig. 1*78, 
 to be placed on the side of a room, and 
 a lamp to be set in another room, but 
 so situated as that its light would shine 
 upon the glass. The lamp itself could 
 not be seen by the eye placed at E, be- 
 cause the partition D is between them ; 
 but its image would be visible at E, be- B 
 cause the angle of the incident ray, 
 coming from the light, 'and that of the 
 reflected ray which reaches the eye, are equal. 
 
 28. An image from a plane mirror appears to be just as far 
 behind the mirror, as the object is before it, so that when a per- 
 son approaches this mirror, his image seems to come forward to 
 meet him ; and when he withdraws from it, his image appears 
 to be moving backward at the sam-e rate. 
 
 If, for instance, one end of a rod, two feet long, ^e made to 
 touch the surface of such a mirror, this end of the rod, and its 
 image, will seem nearly to touch each other, there being only 
 the thickness of the glass between them ; while the other end 
 of the rod, and the other end of its image, will appear to be 
 equally distant from the point of contact. 
 
 29. The reason of this is ex- 
 plained on the principle that the 
 angle of incidence and that of 
 reflection are equal. 
 
 Suppose the arrow A to be 
 the object reflected by the mir- 
 ror D C, Fig. 179 ; the incident 
 rays A, flowing from the end of 
 the arrow, being thrown back 
 by reflection, will meet the eye 
 in the same state of divergence 
 that they would do, if they pro- 
 ceed to the same distance be- 
 hind the mirror, that the eye is 
 
 before it, as at 0. Therefore, by the same law, the reflected 
 rays, where they meet the eye at E, appear to diverge from a 
 
 FIG. 179. 
 
 Plane Mirror. 
 
 28. The image of an object appears just as far behind a plane mirror, 
 Is before it. 29. Explain Fig. 1/9, and show why this is the case. 
 
 the object 
 
228 MIRRORS. 
 
 point H, just as far behind the mirror as A is before it, and 
 consequently the end of the arrow most remote from the glass 
 will appear to be at H, or the point where the approaching 
 rays would meet, were they continued onward behind the glass. 
 The rays flowing from every other part of the arrow follow the 
 same law ; and thus every part of the image seems to be at 
 the same distance behind the mirror that the object really is 
 before it. 
 
 30. In a plane mirror, a person may see his whole image, 
 when the mirror is only half as long as himself, let him stand 
 at any distance from it whatever. 
 
 This is also explained by the law, that the angles of incidence 
 and reflection are equal. If the mirror be elevated so that the 
 ray of light from the eye falls perpendicularly upon the mirror, 
 this ray will be thrown back by reflection in the same direction, 
 so that the incident and reflected ray by which the image of 
 the eyes and face are formed, will be nearly parallel, while the 
 ray flawing from his feet will fall on the mirror obliquely, and 
 will be rejected as obliquely in the contrary direction, and so 
 of all the other rays by which the image of the different parts 
 of the person is formed. 
 
 This will be under- 
 stood by Fig. 180, 
 where the ray of light 
 A B, proceeding from 
 the eye, falling perpen- 
 dicularly on the plane 
 mirror IB D, will be re- 
 flected back in the same 
 
 line; but the ray C D Mirror half the Length of the Object, 
 
 coming from the feet, 
 
 which falls obliquely on the mirror, will be reflected back under 
 the same angle in the line D A ; and since we see objects in 
 the direction of the reflected rays, and the image appears at the 
 same distance behind the mirror that the object is before it, 
 (28,) we must continue the line A D to the feet, E, and for the 
 same reason, the rays A B, from the eye, must be prolonged to 
 F, as far behind the mirror as the line E extends, where the 
 whole image will be represented. 
 
 Now, the line D E, behind the mirror, is just equal to D A 
 
 30. What must be the comparative length of a plane mirror in which a person may 
 see his whole image 1 Explain, by the lines in Fig. 180, why it is that a lady may see 
 herself in a mirror one half her length. 
 
MIRRORS. 
 
 229 
 
 FIG. 181. 
 
 Convex Mirror. 
 
 before it ; and the distance of A C is just twice that of B D ; 
 therefore, the whole person is seen in a mirror of half its length, 
 the image being as far behind the reflector as the object is 
 before it. 
 
 31. A shorter mirror would not show the whole person, be- 
 cause the rays coming from the feet would fall so obliquely upon 
 it as to be reflected above his head, and thus could not be seen ; 
 but another placed there might see the whole image, though 
 the owner could not. 
 
 32. CONVEX MIRRO'R. A con- 
 vex mirror is a part of a sphere, 
 or globe, reflecting from the out- 
 side. 
 
 Suppose Fig. 181 to be a 
 sphere, then the part from A to O, 
 would be a section of the sphere. 
 Any part of a hollow ball of glass, 
 with an amalgam of tin and quick- 
 silver spread on the inside, or any 
 part of a metallic globe polished 
 on the outside, would form a con- 
 vex mirror. 
 
 The axis of a convex mirror, is a line, as C B, passing through 
 its center. 
 
 33. Divergent and Convergent FIG. 182. 
 Rays. Rays of light are said to 
 
 diverge, when they proceed from 
 the same point, and constantly re- 
 cede from each other, as from the 
 point A, Fig. 182. Rays of light 
 are said to converge, when they 
 approach in such a direction as 
 finally to meet at a point, as at B, Fig. 182. 
 
 The image formed by a plane mirror, as we have already 
 seen, is of the same size as the object, but the image reflected 
 from the convex mirror is always smaller than the object. 
 
 The law which governs the passage of light with respect to 
 the angles of incidence and reflection, to and from the convex 
 mirror, is the same as already stated, for the plane mirror. 
 
 34. From the surface of a plane mirror, parallel rays are re- 
 si. Why can not a person see his whole figure in a mirror less than half his length? 
 
 32. What is a convex mirror 1 What is the axis of a convex mirror 7 33. What are 
 diverging rays 1 What are converging rays! What law governs the passage of light 
 from and to the convex mirror 7 
 
 Rays of Light. 
 
230 
 
 MIRRORS. 
 
 fleeted parallel ; but the convex mirror causes parallel rays fall- 
 ing on its surface to diverge, by reflection. 
 
 To make this understood, let 1, 
 2, 3, Fig. 183, be parallel rays, FIG. 183. 
 
 falling on the surface of the convex 
 reflector, of which A would be the 
 center, were the reflector a whole 
 sphere. The ray 2 is perpendicu- 
 lar to the surface of the mirror, for 
 when continued in the same direc- 
 tion, it strikes the axis, or center 
 of the circle A. The two rays, 1 
 and 3, being parallel to this, all 
 three would fall on a plane mirror 
 in a perpendicular direction, and 
 consequently would be reflected in 
 the lines of their incidence. But 
 the obliquity of the convex surface, Divergent Rays. 
 
 it is obvious, will render the direc- 
 tion of the rays 1 and 3 oblique to that surface, for the same 
 reason that 2 is perpendicular to that part of the circle on which 
 it falls. Rays falling on any part of this mirror, in a direction 
 which, if continued through the circumference, would strike the 
 center, are perpendicular to the side where they fall. Thus, the 
 dotted lines, C A and D A, are perpendicular to the surface, as 
 well as 2. 
 
 Now the reflection of the ray 2, will be back in the line of 
 its incidence, but the rays 1 and 3, falling obliquely, are reflected 
 under the same angles as those at which they fell, and there- 
 fore their lines of reflection will be as far without the perpen- 
 dicular lines C A and D A, as the lines of their incident rays, 
 1 and 3, are within them, and consequently they will diverge 
 in the direction of E and O ; and since we always see the image 
 in the direction of the reflected ray, an object placed at one, would 
 appear behind the surface of the mirror, at N, or in the direc- 
 tion of the line O N. 
 
 35. Plane Surfaces. Perhaps the subject of the convex 
 mirror will be better understood, by considering its surface to 
 be formed of a number of plane faces, indefinitely small. In 
 this case, each point from which a ray is reflected, would act in 
 
 34. Are parallel rays falling on a convex mirror, reflected parallel? Explain Fig. 
 183. 35. How is the action of the convex mirror illustrated by a number of piano 
 mirrors ? Explain Figs. 184 and 185. 
 
MIRRORS. 231 
 
 the same manner as a plane mirror, and the whole, in the man- 
 ner of a number of minute mirrors inclined from each other. 
 
 Suppose A and B, Fig. 184, to be the points on a convex 
 mirror, from which the two parallel rays, C and D, are reflected. 
 Now, from the surface of a plane mirror, the reflected rays 
 would be parallel, whenever the incident ones are so, because each 
 will fall upon the surface under the same angles. But it is ob- 
 vious, in the present case, that these rays fall upon the surfaces, 
 A and B, under different angles, as respects the surfaces, C ap- 
 proaching in a more oblique direction than D ; consequently C 
 is reflected more obliquely than D, and the two reflected rays, 
 instead of being parallel as before, diverge in the direction of N 
 andO. * 
 
 FIG. 184. FIG. 185. 
 
 Again, the two converging rays A and B, Fig. 185, without 
 the interposition of the reflecting surfaces, would meet at C, but 
 because the angles of reflection are equal to those of incidence, 
 and because the surfaces of reflection are inclined from each 
 other, these rays are reflected less convergent, and instead of 
 meeting at the same distance before the mirror that C is behind 
 it, are sent off in the direction of D, at which point they meet. 
 
 36. " Thus parallel rays falling on a convex mirror, are ren- 
 dered divergent by reflection ; converging rays are made less 
 convergent, or parallel, and diverging rays more divergent. 
 
 The effect of the convex mirror, therefore, is to disperse the 
 
 36. What effect does the convex mirror have upon parallel rays by reflection ? 
 What is its effect en con verging rays 1 What is its effect on diverffing rays? Do the 
 rays oflight proceed only from the extremities of objects, as represented in figures, 
 or from all their parts ? Do all the rays of light proceeding from an object enter the 
 eye, or only a few of them 1 
 
232 MIRRORS. 
 
 rays of light in all directions ; and it is proper here to remind 
 the pupil, that although the rays of light are represented on 
 paper by single lines, there are, in fact, probably millions of 
 rays, proceeding from every point of all visible bodies. Only a 
 comparatively small number of these rays, it is true, can enter 
 the eye, for it is only by those which proceed in straight lines 
 from the different parts of the object, and enter the pupil, that 
 the sense of vision is excited. 
 
 37. When, therefore, it is said, that the convex mirror dis- 
 perses the rays of light which fall upon it from any object, and 
 when the direction of these reflected rays are shown only by 
 single lines, it must be remembered, that each line represent* 
 pencils of rays, and that the ligh* not only flows from the parts 
 of the object thus designated, but from all the other parts. 
 Were this not the case, the object would be visible only at cer- 
 tain points. 
 
 38. CURVED IMAGES. The images of objects reflected from 
 the convex mirror, appear curved, because their different parts 
 are not equally distant from its surface. 
 
 If the object A be placed 
 
 obliquely before the convex FIG< 186 - 
 
 mirror, Fig. 186, then the 
 converging rays from its two 
 extremities falling obliquely 
 on its surface, would, were 
 they prolonged through the 
 mirror, meet at the point C, 
 behind it. But instead of 
 being thus continued, they Curved image. 
 
 are thrown back by the mir- 
 ror in less convergent lines, which meet the eye at E, it being, 
 as we have seen, one of the properties of this mirror, to reflect 
 converging rays less convergent than before. 
 
 The image being always seen in the direction from which the 
 rays approach the eye, it appears behind the mirror at D. If 
 the eye be kept in the same position, and the object, A, be 
 moved further from the mirror, its image will appear smaller, 
 in a proportion inversely to the distance to which it is removed. 
 Consequently, by the same law, the two ends of a straight ob- 
 ject will appear smaller than its middle, because they are further 
 
 37. What would be the consequence, if the rays of light proceeded only from the 
 parts of an object shown in diagrams? 38. Why do the images of objects reflected 
 from convex mirrors appear curved? Why do the features of the face appear out 
 of proportion by this mirror? 
 
MIRRORS. 
 
 233 
 
 from the reflecting surface of the mirror. Thus, the images of 
 straight objects, held before a convex mirror, appear curved, and 
 for the same reason, the features of the face appear out of pro- 
 portion, the nose being too large, and the cheeks too small, or 
 narrow. 
 
 39. Why Objects appear Large or Small. Objects appear to 
 us large or small, in proportion to the angle which the "rays of 
 light, proceeding from their extreme parts, form, when they 
 meet at the eye. For it is plain that the half of any object will 
 appear under a less angle than the whole, and the quarter 
 under a less angle still. Therefore the smaller an object is, the 
 smaller will be the angle under which it will appear at a given 
 distance. Hence the image of an object, when reflected from the 
 convex mirror, appears smaller than the object itself. This will 
 be understood by Fig. 1&7. 
 
 Suppose the rays flowing from the extremities of the object 
 A, to be reflected back to C, under the same degrees of con- 
 vergence at which they strike the mirror; then, as in the plane 
 mirror, the image D would appear of the same'size as the ob- 
 ject A; for if the rays from A were prolonged behind the 
 mirror, they would meet at B, but forming the same angle, by 
 reflection, that they would do, if thus prolonged, the object seen 
 from B, and its image from C, would appear of the same 
 dimensions. 
 
 FIG. 187. 
 
 FIG. 188. 
 
 Object Diminished. 
 
 Convex Mirror. 
 
 But instead of this, the rays from the arrow A, being rendered 
 less convergent by reflection, are continued onward, and meet 
 the eye under a more acute angle than at C, the angle under 
 which they actually meet, being represented at E, consequently 
 
 39. Why does an image reflected from a convex surface appear smaller than the 
 object 1 Why does the half of an object appear to the eye smaller than the whole ? 
 
234 MIRRORS. 
 
 the image of the object is shortened in proportion to the acute- 
 ness of this angle, and the object appears diminished as repre- 
 sented at O. 
 
 40. The image of an object appears less, as the object is re- 
 moved to a greater distance from a convex mirror. 
 
 To explain this, let us suppose that the arrow A, Fig. 188, 
 is diminished by reflection from the convex surface, so that its 
 image appearing at D, with the eye at C, shall seem as much 
 smaller in proportion to the object, as D is less than A. Now, 
 keeping the eye at the same distance from the mirror, withdraw 
 the object, so that it shall be equally distant with the eye, and 
 the image will gradually diminish, as the arrow is removed. 
 
 The reason will be made plain by the next figure ; for as the 
 arrow is moved backward, the angle at C, Fig. 189, must be 
 diminished, because the rays flowing from the extremities of the 
 object fall a greater distance before they reach the surface of 
 the mirror ; and as the angles of the reflected rays bear a pro- 
 portion to those of the incident ones, so the angle of vision will 
 become less in proportion, as the object is withdrawn. The 
 effect, therefore, of withdrawing the object, is first to lessen the 
 distance between the converging rays, flowing from it, at the 
 point where they strike the mirror, and as a consequence, to 
 diminish the angle under which the reflected rays convey its 
 image to the eye. 
 
 41. Why the Image seems near the Surface. In the plane 
 mirror, as already shown, the image appears exactly as far be- 
 hind the mirror as the object is before it, but the convex mirror 
 shows the image just under the surface, or, when the object is 
 removed to a distance,, a little way behind it. To understand 
 the reason of this difference, it must be remembered, that the 
 plane mirror makes the image seem as far behind, as the object 
 is before it, because the rays are reflected in the same relative 
 position at which they fall upon its surface. Thus parallel rays 
 are reflected parallel; divergent rays equally divergent, and 
 convergent rays equally convergent. But the convex mirror, as 
 also above shown, (36,) reflects convergent rays less convergent, 
 and divergent rays more divergent, and it is from this property 
 of the convex mirror that the image appears near its surface, 
 
 40. jflfow is the image affected when the object is withdrawn from the surface of a 
 convex mirror 1 Explain Figs. 187 and 188, and show the reason why the images are 
 diminished when the objects are removed from the convex mirror. What is said to 
 be the effect of withdrawing the object from a convex surface, and what the conse- 
 quence on the angle of reflected rays ? 41. Explain the reason why the image ap- 
 pears near the surface of the convex mirro*. 
 
MIRRORS. 235 
 
 FIG. 189. 
 
 ''I-' 
 
 and not as far behind it as the object is before it, as in the 
 plane mirror. 
 
 Let us suppose that A, Fig. 190, is a luminous point, from 
 which a pencil of diverging rays falls upon a convex mirror. 
 These rays, as already demonstrated, will be reflected more 
 divergent, and consequently will meet the eye at E, in a wider 
 state of dispersion than they fell upon the mirror at O. Now, 
 as the image will appear at the point where the diverging rays 
 would converge to a focus in a contrary direction, were they 
 prolonged behind the mirror, so it can not appear as far behind 
 the reflecting surface as the object is before it, for the more 
 widely the rays meeting at the eye are separated, the shorter 
 will be the distance at which they will come to a point. The 
 image will, therefore, appear at N, instead of appearing at an 
 equal distance behind the mirror that the object A is before it. 
 
 CONCAVE MIRROR. 
 
 42. The reflection of the concave mirror takes "place from its 
 inside, or concave surface, while that of the convex mirror is 
 from the outside, or convex surface. Thus the section of a 
 metallic sphere, polished on both sides, is both a concave and 
 convex mirror, as one or the other side in employed for reflection. 
 
 The effects and phenomena of this mirror will therefore be, in 
 many respects, directly the contrary from those already detailed 
 in reference to the convex mirror. 
 
 From the plane mirror, the relation of the incident rays is 
 not changed by reflection ; from the convex mirror they are dis- 
 
 42. What is the shape of the concave mirror, and in what respect dees it differ 
 from the convex mirror ? How may convex and concave mirrors be united in the 
 tame instrument? What is the difference of effect between the concave, convex 
 ind plane mirrers, on the reflected rays 1 
 
236 MIRRORS. 
 
 persed ; but the concave mirror renders the rays reflected from 
 it more convergent, and tends to concentrate them into a focus. 
 
 The surface of the concave mirror, like that of the convex, 
 may be considered as a great number of minute plane mirrors, 
 inclined to, instead of from, each other at certain angles, in pro- 
 portion to its concavity. 
 
 The laws of incidence and reflection are the same, when ap- 
 plied to the concave mirror as those already explained in refer- 
 ence to the other mirrors. 
 
 43. Plane Mirrors Inclined. In refer- PIG> 191 - 
 ence to the concave mirror, let us, in the 
 
 first place, examine the effect of two plane 
 mirrors inclined to each other, as in Fig. 
 191, on parallel rays of light. The inci- 
 dent rays, A and B, being parallel before 
 they reach the reflectors, are thrown off .at 
 unequal angles in respect to each other, for 
 B falls on the mirror more obliquely than 
 A, and consequently is thrown off more ob- / / 
 
 liquely in a contrary direction, therefore, the 
 angles of reflection being equal to those of incidence, the two 
 rays meet at C. 
 
 Thus we see that the effect of two plane mirrors inclined to 
 each other, is to make parallel rays converge and meet in a 
 focus. 
 
 The effect of this mirror, as we. have seen, being to render 
 parallel rays convergent, the same principle will render diverg- 
 ing rays parallel, and converging rays still more convergent. 
 
 44. Focus of a Concave Mirror. The focus of a concave 
 mirror is the* point where the rays are brought together by re- 
 flection. The center of concavity in a concave mirror, is the 
 center of the sphere, of which the mirror is a part. In all con- 
 cave mirrors, the focus of parallel rays, or rays falling directly 
 from the sun, is at the distance of half the semi-diameter of the 
 sphere, or globe, of which the reflector is a part. 
 
 Thus, the parallel rays 1, 2, 3, <fcc., Fig. 192, all meet at the 
 point O, which is half the distance between the center A, of the 
 whole sphere, and the surface of the reflector, and therefore one 
 quarter the diameter of the whole sphere, of which the mirror 
 is a part. 
 
 43. In what respect may the concave mirror be considered as a number of plane 
 mirrors? 44. What is the focus of a concave mirror! At what distance from its 
 surface is the focus of parallel rays in this mirror? 
 
MIRRORS. 
 
 237 
 
 45. Principal Focus. 
 In concave mirrors, of 
 all dimensions, the re- 
 flected rays follow the 
 same law ; that is, par- 
 allel rays meet and cross 
 each other at the dis- 
 tance of one-fourth the 
 diameter of the sphere 
 of which they are sec- 
 tions. This point is call- 
 ed the principal focus 
 of the reflector. 
 
 But if the incident 
 rays are divergent, the 
 focus will be removed 
 to a greater distance from the surface of the mirror, than when 
 they are parallel, in proportion to their divergence. 
 
 FIG. 193. 
 
 FIG. 194 
 
 Divergent Rays. 
 
 Convergent Rays. 
 
 This might be inferred from the general laws of incidence and 
 reflection, but will be made obvious by Fig. 193, where the 
 diverging rays 1, 2, 3, 4, form a focus at the point O, whereas, 
 had they been parallel, their focus would have been at A. That 
 is, the actual focus is at the center of the sphere, instead of be- 
 ing half way between the center and circumference, as is the 
 case when the incident rays are parallel. The real focus, there- 
 fore, is beyond, or without, the principal focus of the mirror. 
 
 45. What is the principal focus of a concave mirror? If the incident rays are di 
 vergent. where will be the focus ? If the incident rays are convergent, where will be 
 the focus 7 
 
238 
 
 MIRRORS. 
 
 By the same law, converging rays will form a point within 
 the principal focus of the mirror. 
 
 Thus, were the fays falling on the mirror, Fig. 194, parallel, 
 the focus would be at A ; but in consequence of their previous 
 convergence, they are brought together at a less distance than 
 the principal focus, and meet at O. 
 
 46. OBJECTS WITHIN THE Focus. 'The concave mirror, when 
 the object is nearer to it than the principal focus, presents the 
 image larger than the object, erect, and behind the mirror. 
 
 To explain this, let us 
 
 suppose the object A, FIG. 195. 
 
 Fig. 195, to be placed 
 before the mirror, and 
 nearer to it than the prin- 
 cipal focus. Then the 
 rays proceeding from the 
 extremities of the object 
 without interruption, 
 would continue to diverge 
 in the lines O and N, as 
 seen behind the mirror ; 
 but, by reflection they 
 are made to diverge less 
 than before, and conse- 
 quently to make the an- 
 gle under which they meet more obtuse at the eye B, than it 
 would be if they continued onward to E, where they would 
 have met without reflec- 
 tion. The result, there- 
 fore, is to render the image 
 H, upon the eye, as much 
 larger than the object A, 
 as the angle at the eye 
 is more obtuse than the 
 angle at C. 
 
 47. Magnified Human 
 Face. A more striking 
 illustration of this princi- 
 ple is seen at Fig. 196. 
 When the concave mir- 
 
 Object within the Focus. 
 
 Magnified Human Face. 
 
 46. When will the image from a concave mirror oe larger than the object, erect, 
 and behind the mirror 1 Explain Fig. 195, and show why the image is larger than 
 the object. 47. What is the effect when the face is seen within the principal focus of 
 this mirror 1 
 
MIRRORS. 
 
 239 
 
 ror is large, say six inches in diameter and eight or ten inches 
 focal distance, it exhibits the human face of enormous bulk, the 
 spectator being frightened at the size and coarseness of his own 
 features. Thus, if the face be presented within the principal 
 focus of the mirror, as at B, the magnified image will be seen 
 as far behind the mirror as the face is before it, as at A, and 
 will appear two or three times the size of the face, according* to 
 the power of the reflector; the reason of which has already 
 been explained and illustrated by Fig. 196. 
 
 48. Curious Deceptions by Concave Mirrors. From the 
 property of the concave mirror to form an inverted image of 
 the object suspended in the air, many curious and surprising 
 deceptions may be produced. Thus y when the mirror, the ob- 
 ject, and the light, are placed so that they can not be seen, 
 (which may be done by placing a screen before the light, and 
 permitting the reflected rays to pass through a small aperture 
 in another screen,) the person mistakes the image of the object 
 for its reality, and not understanding the deception, thinks he 
 sees persons walking with heads downward, and cups of water 
 turned bottom upward, without spilling a drop. Again, he 
 sees clusters of delicious fruit, and when invited to help himself, 
 on reaching out his hand for that purpose, he finds that the ob- 
 ject either suddenly vanishes from his sight, owing to bis having 
 moved his eye out of the proper range, or that it is intangible. 
 
 49. One method of ef- 
 fecting such deceptions, is 
 shown by Fig. 197, where 
 A is a large concave mir- 
 ror, six or eight inches in 
 diameter, placed on the 
 back part of a dark box ; 
 the performer D, is con- 
 cealed from the spectators 
 by the partition C; the 
 strong light E, is also con- 
 cealed by the partition I, 
 but is thrown upon the 
 actor, or any thing he 
 holds in his hand. If he 
 holds a book, as shown by 
 
 FIG. 197. 
 
 Deceptions by Concave Mirrors. 
 
 48. What property has the concave mirror, by which singular deceptions may be 
 produced? What are these deceptions? 49. Explain Fig. 197, and show how th 
 deception is produced. 
 
240 MIRRORS. 
 
 the figure, the light reflected from A, will pass between the par- 
 titions, G and I, to the mirror, and will reflect the image of the 
 book to Z, where it will appear so distinct and tangible, that a 
 person looking through the opening at X, will have no doubt 
 that it is a real book, and will be much astonished to find, when 
 be puts out his hand to take it, that it -has no substance, and 
 that his hand will pass through it, as though it was nothing 
 but a shadow, which he can not at first be made to believe is 
 the case. 
 
 50. Heat Produced by a Concave Mirror. The concave 
 mirror having the property of converging the rays of light, is 
 equally efficient in concentrating the rays of heat, either separ- 
 ately or with the light. When, therefore, such a mirror is pre- 
 sented to the rays of the sun, it brings them to a focus, so as 
 to produce degrees of heat in proportion, to the extent and per- 
 fection of its reflecting surface. A metallic mirror of this kind, 
 of only four or six inches in diameter, will fuse metals, set wood 
 on fire, &c. 
 
 51. Experiment with a Hot Ball. As the parallel rays of 
 heat or light are brought to a focus at the distance of one quar- 
 ter of the diameter of the sphere, of which the reflector is a 
 section, so if a luminous or heated body be placed at this point, 
 the rays from such body passing to the mirror will be reflected 
 from all parts of its surface, in parallel lines ; and the rays so 
 reflected by the same law, will be brought to a focus by an- 
 other mirror standing opposite to this. 
 
 52. Suppose a red-hot ball to be placed in the principal focus 
 of the mirror A, Fig. 198, the rays of heat and light proceed- 
 ing from it will be reflected in the parallel lines 1, 2, 3, &c. 
 The reason of this is the same as that .which causes parallel 
 rays, when falling on the mirror, to be converged to a focus. 
 The angles of incidence being equal to those of reflection, it 
 makes no difference in this respect, whether the rays pass to or 
 from the focus. In one case, parallel incident rays from the 
 sun, are concentrated by reflection ; and in the other, incident 
 diverging rays, from the heated ball, are made parallel by 
 reflection. 
 
 The rays, therefore, flowing from the hot ball to the mirror 
 
 50. Will the concave mirror concentrate the rays of heat, as well as those of light ? 
 51. Suppose a luminous body be placed in the focus of a concave mirror, in what di- 
 rection will its rays be reflected] 52. Explain Fig 198, and show why the rays 
 from the focus of A are concentrated in the focus B. What curious experiments may 
 be made by two concave mirrors placed opposite to each other by a hot ball? How 
 may it be shown that heat and light are distinct principles ? 
 
MIRRORS. 
 
 FIG. 198. 
 
 
 
 241 
 
 Reflection of Heat. 
 
 A, are thrown into parallel lines by reflection, and these re- 
 flected rays, in respect to the mirror B, become the rays of inci- 
 dence, which are again brought to a focus by reflection. 
 
 Thus the heat of the ball, by being placed in the focus of one 
 mirror, is brought to a focus by the reflection of the other 
 mirror. 
 
 To show that heat and light are separate principles, place a 
 piece of phosphorus in the focus of B, and when the ball is so 
 cool as not to be luminous, remove the screen, and the phos- 
 phorus will instantly inflame. 
 
 53. Deception by Several Mirrors. The wonderful feat of 
 reading through a brick, sometimes exhibited in the streets, at 
 a penny a head, is explained by Fig. 199. 
 
 Reading through a Brick. 
 
 The apparatus consists of five short tubes jointed at right-an- 
 gles, and containing four mirrors placed at the angle of 45 to 
 
 53. Explain by Fig. 199, how a candle can apparently be seen through a brick. 
 11 
 
243 REFRACTION BY LENSES. 
 
 the incident rays of light, flowing, first, from the object to be 
 seen, and then from one mirror to the other. Thus the rays 
 from tho object P, pass to the mirror L, through the tube D C, 
 and from L are reflected to H, anc\ from H, horizontally, to G, 
 and from G, vertically, to K, and lastly, from K horizontally to 
 the eye. 
 
 Now the angle of reflection, being equal to that of incidence, 
 each mirror has the effect to change the direction of the rays of 
 light equal to that of a quarter of a circle, and the four mirrors, 
 therefore, produce a change equal to that of an entire circle. 
 
 REFRACTION BY LENSES. 
 
 54. A Lens is a transparent body, generally made of glass, 
 and so shaped that the rays of light in passing through it are 
 either collected together or dispersed. Lens is a Latin word, 
 which comes from lentile, a small flat bean. 
 
 It has already been shown, that when the rays of light pass 
 from a rarer to a denser medium, they are refracted, or bent 
 out of their former course, except when they happen to fall per- 
 pendicularly on the surface of the medium. (19.) 
 
 - The point where no refraction is produced on perpendicular 
 rays, is called the axis of the lens, which is a right line passing 
 through its center, and perpendicular to both its surfaces, as op. 
 
 In every beam of light the middle ray is called its axis. 
 
 Rays of light are said to fall directly upon a lens, when their 
 axes coincide with the axis of the lens ; otherwise they are said 
 to fall obliquely. 
 
 The point at which the rays of the sun are collected, by pass- 
 ing through a lens, is called the principal focus of that lens. 
 
 55. Lenses are of various kinds, and have received certain 
 names, depending on their shapes. The different kinds are 
 shown at Fig. 200. 
 
 A prism, seen at A, has two plane surfaces, A R, and A S, 
 inclined to each other. 
 
 A plane glass, shown at B, has two plane surfaces, parallel to 
 each other. 
 
 A spherical lens, C, is a ball of glass, and has every part of 
 its surface at an equal distance from the center. 
 
 A double-convex lens, D, is bounded by two convex surfaces, 
 opposite to each other. 
 
 54. What is a lens? What is the axis of a lens? In what part of a lens IP no re- 
 fraction produced 7 Where is the axis of a beam of light ? When are rays of light 
 said to fall directly upon a lens? How many kinds of lenses are mentioned? 55. 
 What are the names and shapes of each 1 
 
 - 
 
CONVEX LENfl. 
 
 243 
 
 Lenses of Various Forma. 
 
 A plano-convex lens, E, is bounded by a convex surface on 
 one side, and a plane one on the other. 
 
 A double-concave lens, F, is bounded by two concave spher- 
 ical surfaces, opposite to each other. 
 
 A plano-concave lens, G, is bounded by a plane surface on 
 one side and a concave one on the other. 
 
 A meniscus, H, is bounded by one concave, and one convex 
 spherical surface, which two surfaces meet at the edge of the 
 lens. 
 
 A concavo-convex lens, I, is bounded by a concave, and con- 
 vex surface, but which diverge from each other, if continued. 
 
 The effects of the prism on the rays of light will be shown in 
 another place. The refraction of the plane glass bends the 
 parallel rays of light equally toward the perpendicular, as already 
 shown. The sphere is not often employed as a lens, since it is 
 inconvenient in use. 
 
 CONVEX LENS. 
 
 56. The effect of the convex lens, by increasing the vimal 
 angle, is to magnify all objects seen through it. 
 
 Focal Distance. The 
 
 focal distances of convex FIG> 
 
 lenses, depend on their de- 
 grees of convexity. The 
 focal distance of a single, 
 or plano-convex lens, is 
 the diameter of a sphere, 
 of which it is a section. 
 
 If the whole circle, 
 Fig. 201, be considered 
 the circumference of a 
 sphere, of which the pla- 
 no-convex lens B A, is a 
 
 Piano- Convex Lent. 
 
244 
 
 
 CONVEX LENS. 
 
 section, then the focus of parallel rays, or the principal focus, 
 will be at the opposite side of the sphere, or at C. 
 
 57. The focal distance of a double-convex lens, is the radius, 
 or half the diameter of the sphere, of which it is a part. Hence 
 the plano-convex lens, being one half of the double-convex lens, 
 the latter has twice the refractive power of the former ; for the 
 rays suffer the same degree of refraction in passing out of the 
 one convex surface, that they do in passing into the other. 
 
 58. Double-Convex Lens. The shape of the double-convex 
 lens, D C, Fig. 202, is that of two plano-convex lenses, placed 
 with their plane surfaces 
 
 in contact, and conse- 
 qiiently the focal distance 
 of this lens is nearly the 
 center of the sphere of 
 which one of its surfaces 
 is a part. If parallel 
 rays fall on a convex lens, 
 it is evident that the ray 
 only, which penetrates 
 the axis and passes to- 
 ward the center of the 
 
 sphere, will proceed with- Double-Convex Lens. 
 
 out refraction, as shown 
 
 in the above figures. All the others will be refracted so as to 
 meet the perpendicular ray at a greater or less distance, de- 
 pending on the convexity of the lens. 
 
 59. DIVERGING RAYS ON A CONVEX LENS. If diverging 
 rays fall on the surface of this lens, they will, by refraction, be 
 rendered less divergent, parallel, or convergent, according to the 
 degrees of their divergence, and the convexity of the surface of 
 the lens. 
 
 Thus, the diverging rays 1, 2, <fec., Fig. 203, are refracted 
 by the lens A 0, in a degree just sufficient to render them par- 
 allel, and therefore, would pass off in right lines, indefinitely, or 
 without ever forming a focus. 
 
 It is obvious by the same law, that were the rays less diverg- 
 ent, or were the surface of the lens more convex, the rays in 
 
 56. What is the effect of the convex lens? On what do the focal distances of con- 
 vex lenses depend 1 57. What is the focal distance of any plano-convex lens ? What 
 is the focal distance of the double-convex lens? 58. What is l he shape of the double- 
 convex lens 7 59. How are divergent rays affected bypassing through a convex 
 lens 7 What is its effect on parallel rays? What is its effect on converging rays1 
 What kind of leases are spectacle glasses for old people ? 
 
CONVEX LEWS. 245 
 
 Fig. 203, would be* nG ** 
 
 come convergent, 
 instead of parallel, 
 because the same 
 retractive power 
 which would render 
 divergent rays par- 
 allel, would make 
 parallel rays con- 
 vergent, and con- o 
 
 verging rays Still Diverging Rays. 
 
 more convergent. 
 
 Thus the pencils of converging rays, Fig. 204, are rendered 
 still more convergent by their passage through the lens, and 
 are therefore brought to a focus nearer the lens, in proportion 
 to their previous convergence. 
 
 The eyeglasses of spectacles for old people are double-convex 
 lenses, more or less spherical, according to the age of the person, 
 or the magnifying power required. 
 
 60. BURNING GLASS. The common burning glasses, which 
 are used for lighting cigars, and sometimes for kindling fires, 
 are also convex lenses. Their effect is to concentrate to a focus, 
 or point, the heat of the sun which falls on their whole surface; 
 and hence the intensity of their effects is in proportion to the 
 extent of their surfaces, and their focal lengths. 
 
 61. VISUAL ANGLE. It has been explained, that the reason 
 why the convex mirror diminishes the images of objects is, that 
 the rays which come to the eye from the extreme parts of the 
 object are rendered less convergent by reflection, from the con- 
 vex surface, and that in consequence, the angle of vision is madft 
 more acute. (41.) 
 
 62. Now, the refractive power of the convex lens has exactly 
 the contrary effect, since by converging the rays flowing from 
 the extremities of an object, the visual angle is rendered more 
 obtuse, and therefore all objects seen through it appear magnified. 
 
 63. Suppose the object A, Fig. 205, appears to the naked 
 eye of the length represented in the drawing. Now, as the 
 rays coming from each end of the object, form by their con- 
 vergence at the eye, the visual angle, or the angle under which 
 the object is seen, and we call objects large or small in propor- 
 
 60. What kind of a lens is a burning glass! 61. What is the visual angle? 62. 
 What is the effect of the convex lens on the visual angle ? 63. Why does the same 
 object, when at a distance, appear smaller than when near ? Why does an object 
 appear larger through the convex lens than otherwise 1 
 
246 
 
 CONVEX LENS. 
 
 FIG. 204. 
 
 FIG. 208. 
 
 Converging Ray. 
 
 FIG. 205. 
 
 Visual Angle. 
 
 Visual Angle. 
 
 tion as this angle is obtuse or acute, if, therefore, the object A. 
 be withdrawn further from the .eye, it is apparent that the rays 
 O O, proceeding from its extremities, will enter the eye under a 
 more acute angle, and therefore that the object will appear di- 
 minished in proportion. This is the reason why things at a 
 distance appear smaller than when near us. When near, the 
 visual angle is increased, and when at a distance it is diminished. 
 
 The effect of the convex lens is to increase the visual angle, 
 by bending the rays of light coming from the object, so as to 
 make them meet at the eye more obtusely ; and hence it has 
 the same effect, in respect to the visual angle, as bringing 
 the object nearer the eye. This is shown by Fig. 206, where 
 it is obvious, that did the rays flowing from the extremities of 
 the arrow meet the eye without refraction, the visual angle 
 would be less, and therefore the object would appear shorter. 
 Another effect of the convex lens, is to enable us to see objects 
 nearer the eye than without it, as will be explained under the 
 article Vision. 
 
 Now, as the rays of light flow from all parts of a visible ob- 
 ject of whatever shape, so the breadth, as well as the length, is 
 increased by the convex lens, and thus the whole object appears 
 magnified. 
 
 64. CONCAVE LENS. The effect of the concave lens is di- 
 
 64. What is the effect of the concave lens 7 
 
CONVEX LENS. 
 
 247 
 
 rectly opposite to that of the convex. In other terras, by a con- 
 cave lens, parallel rays are rendered diverging, converging rays 
 have their convergence diminished, and diverging rays have their 
 divergence increased, according to the concavity of the lens. 
 
 These glasses, therefore, exhibit things smaller than they 
 really are, for by diminishing the convergence of the rays com- 
 ing from the extreme points of an object, the visual angle is 
 rendered more acute, and hence the object appears diminished 
 by this lens, for the opposite reason, that it is increased by the 
 convex lens. This will be made plain by the two following 
 diagrams. 
 
 Suppose the object A B, Fig. 207, to be placed at such a 
 distance from the eye, as to give the rays flowing from it, the 
 degrees of convergence represented in the figure, and suppose 
 that the rays enter the eye under such an angle as to make the 
 object appear two feet in length. 
 
 FIG. 207. 
 
 FIG. 203. 
 
 Natural Vision. 
 
 Piano- Concave Lent. 
 
 Now, the length of the same object, seen through the con- 
 cave lens, Fig. 208, will appear diminished, because the rays 
 coming from it are bent outward, or made less convergent by 
 refraction, as seen in the figure, and consequently the visual 
 angle is more acute than when the same object is seen by the 
 naked eye. Its length, therefore, will appear less in proportion 
 as the rays are rendered less convergent. 
 
 The spectacle glasses of short-sighted people are concave 
 lenses, by which the images of objects are formed further back 
 in the eye than otherwise, as will be explained under the next 
 article. 
 
 What effect does this lens have upon parallel, diverging, and converging rays? 
 Why do objects appear smaller through this glass than they do to the naked eye? 
 Explain Figs. 207 and 203. and show the reason why the same object appears smaller 
 through 208. What defect in the eye requires concave leases? 
 
248 
 
 VISION. 
 
 Human Eye. 
 
 VISION. 
 
 65. In the application of the principles of optics to the ex- 
 plonation of natural phenomena, it is necessary to give a descrip- 
 tion of the most perfect of all optical instruments, the eye. 
 
 Human Eye. Fig. 
 
 209 is a vertical section FIG - 209 - 
 
 of the human eye. Its 
 form is nearly globular, 
 with a slight projection 
 or elongation in front. 
 It consists of four coats, 
 or membranes ; namely, 
 the sclerotic, the cornea, 
 the choroid, and the 
 retina. It has two fluids 
 confined within these 
 membranes, called the 
 aqueous, and the vitre- 
 ous humors, and one lens, called the crystaline. The sclerotic 
 coat is the outer and strongest membrane, and its anterior part 
 is well known as the white of the eye. This coat is marked in 
 the figure a a a a. It is joined to the corpea b b, which is 
 the transparent membrane in front of the eye, through which 
 we see. The choroid coat is a thin, delicate membrane, which 
 lines the sclerotic coat on the inside. On the inside of this lies 
 the retina, d d d d, which is the innermost coat of all, and is 
 an expansion, or continuation of the optic nerve o. This ex- 
 pansion of the optic nerve is the immediate seat of vision. The 
 iris, o o, is seen through the cornea, and is a thin membrane, 
 or curtain, of different colors in different persons, and therefore 
 gives color to the eyes. In black-eyed persons it is black, in 
 blue-eyed persons it is blue, &c. Through the iris, is a circular 
 opening, called the pupil, which expands or enlarges when the 
 light is faint, and contracts when it is too strong. The space 
 between the iris and the cornea is called the anterior chamber 
 of the eye, and is filled with the aqueous humor, so called from 
 its resemblance to water. Behind the pupil and iris is situated 
 
 65. What is the most perfect of all optical instruments ? What is the form of the 
 numan eye ?. How many coats or membranes has the eye 1 What are they called? 
 How maiiy fluids has the eye, and what are they called 7 What is the lens of the eye 
 called ? What coat forms the white of the eye? Describe wher.e the several coats 
 and humors are situated. What is the iris] What is the retinal Where is the 
 sense of vision ? What is the design of Fig. 210 J What is said concerning the small 
 number of the rays which enter the eye from a visible object 1 
 

 VISION. 
 
 249 
 
 FIG. 210. 
 
 the crystaline lens e, which is a firm and perfectly transparent 
 body, through which the rays of light pass from the pupil to 
 the retina. Behind the lens is situated the posterior chamber 
 of the eye, which is filled with the vitreous humor, v v. This 
 humor occupies much the largest portion of the whole eye, and 
 on it depends the shape and permanence of the organ. 
 
 From the above description of the eye it will be easy to trace 
 the progress of the rays of light through its several parts, and 
 to explain in what manner vision is performed. 
 
 In doing this, we must keep in mind that the rays of light 
 proceed from every part and point of a visible object, as here- 
 tofore stated, and that it is necessary only for a few of the rays, 
 when compared with the whole number, to enter the eye, in 
 order to make the object visible. 
 
 Thus, the object A B, Fig. 
 210, being placed in the 
 li^ht, sends pencils of rays in 
 all possible directions, some 
 of which will strike the eye 
 in any position where it is vis- 
 ible. These pencils of rays 
 not only flow from the points 
 designated in the figure, but 
 in the same manner from 
 every other point on the sur- 
 face of a visible object. To 
 render an object visible, there- 
 fore, it is only necessary that 
 the eye should collect and 
 concentrate a sufficient num- 
 ber of these rays on the retina, 
 to form its image there, and 
 from this image the sensation 
 of vision is excited. 
 
 66. From the luminous body L, Fig. 211, the pencils of rays 
 flow in all directions, but it is only by those which enter the 
 pupil, that we gain any knowledge of its existence; and even 
 these would convey to the mind no distinct idea of the object, 
 unless they were refracted by the humors of the eye, for did 
 these rays proceed in their natural state of divergence to the 
 
 Pencils of Rays. 
 
 66. Explain the design of Fig. 211. Why would not the ravs of light give a distinct 
 Idea of the object, without refraction by the humors of tbe eye 1 
 
 11* 
 
250 
 
 Refraction by the Eye. 
 
 retina, the image there formed would be too extensive, and con- 
 sequently too feeble to give a distinct sensation of the object. 
 
 It is, therefore, by the refracting power of the aqueous hu- 
 mor, and of the crystaline lens, that the pencils of rays are so 
 concentrated as to form a perfect picture of the object on the 
 retina. 
 
 67. Inverted Image on the Retina. We have already seen, 
 that when the rays of light are made to cross each other by re- 
 flection from the concave mirror, the image of the object is in- 
 verted ; the same happens when the rays are made to cross 
 each other by refraction through a convex lens. This, indeed, 
 must be a necessary consequence of the intersection of the rays ; 
 for as light proceeds in straight lines, those rays which come 
 from the lower part of an object, on crossing those which come 
 from its upper part, will represent this part of the picture on 
 the upper half of the retina, and, for the same reason, the up- 
 per part of the object will be painted on the lower part of the 
 retina. 
 
 Now, all objects are represented on the retina in an inverted 
 position ; that is, what we call the upper end of a vertical ob- 
 ject, is the lower end of its picture on the retina, and so the 
 contrary. 
 
 68. Eye of an Ox. This is readily proved by taking the eye 
 of an ox, and cutting away the sclerotic coat, so as to make it 
 transparent on the back part, next the vitreous humor. If now 
 a piece of white paper be placed on this part of the eye, the 
 images of objects will appear figured on the paper in an in- 
 verted position. The same effect will be produced on looking 
 at things through an eye thus prepared ; they will appear 
 inverted. 
 
 67. Explain how it is that the images of objects are inverted on the retina. 6S 
 What experiment proves that th images of objects are inverted on the retina 7 Ex 
 riiain Fitf. 212. 
 
251 
 
 Inversion of Objects. 
 
 The actual position of the vertical object A, Fty. 212, as 
 painted on the retina, is therefore such as is represented by the 
 figure. The rays from its upper extremity, coming in divergent 
 lines, are converged by the crystaline lens, and fall on the retina 
 at O ; while 'those from its lower extremity, by the same law, 
 fall on the retina at C, the rays crossing each other as they pass 
 the humors of the eye. . 
 
 69. In order that vision may be perfect, it is necessary that 
 the images of objects should be formed precisely on the retina, 
 and consequently, if the refractive power of the eye be too small, 
 or too great, the image will not fall exactly on the seat of vision, 
 but will be formed either before, or tend to form behind it. In 
 both cases, perhaps, an outline of the object may be visible, but 
 it will be confusrd and indistinct. 
 
 70. Cornea too Prominent. If the cornea is too convex, or 
 prominent, the image will be formed before it reaches the retina, 
 for the same reason, that of two lenses, that which is most con- 
 vex will have the least focal distance. Such is the def ct in the 
 eyes of persons who are short-sighted, and honce the necessity 
 of their bringing objects as near the eye as possible, so as to 
 make the rays converge at the greatest distance behind the 
 crystaline lens. 
 
 The effect of uncommon convexity in the cornea on the rays 
 of light, is shown at Fig. 213, where it will be observed that 
 the image, instead of being formed on the retina R, is suspended 
 in the vitreous humor, in consequence of there being too gn at 
 a refractive power in the eye. It is hardly necessary to say, 
 that in this case, vision must be very imperfectly performed. 
 
 This defect of sight is remedied by spectacles, the glasses of 
 
 69. Suppose the refractive power of the eye is too great, or too little, why will TI'SJOD 
 be imperfect ? 70 If the cornea is too convex, where wift the image be formed* 
 How is the sight improved, when the cornea is too convex 1 How do such lenses act 
 to improve the Bight 1 
 
252 
 
 Cornea too Convex. 
 
 which are concave lenses. Such glasses, by rendering the rays 
 of light less convergent, before they reach the eye, counteract 
 the too great convergent power of the cornea and lens, and thus 
 throw the image on the retina. 
 
 71. Cornea too Flat. If, on the contrary, the humors of the 
 eye, in consequence of age, or any other cause, have become 
 less in quantity than ordinary, the eyeball will not be suffi- 
 ciently distended, and the cornea will become too flat, or not suf- 
 ficiently convex, to make the rays of light meet at the proper 
 place, and the image will therefore tend to be formed beyond 
 the retina, instead of before it, as in the other case. Hence, 
 aged people, who labor under this defect of vision, can not see 
 distinctly at ordinary distances, but are obliged to remove the 
 object as far from the eye as possible, so as to make its refrac- 
 tive power bring the image within the seat of vision. 
 
 G. 214. 
 
 Cornea too Flat. 
 
 The defect arising from this cause is represented by Fig. 214, 
 
 71. Where do the rays tend to meet when the cornea is not sufficiently convex? 
 How is vision assisted when the eye wants convexity? How do convex lenses help 
 the sight of aged persons ? 
 
VISION. 253 
 
 where it will be observed that the image is formed behind the 
 retina, showing that the convexity of the cornea is not sufficient 
 to bring the image within the seat of distinct vision. This im- 
 perfection of sight is common to aged persons, and is corrected 
 in a greater or less degree by double-convex lenses, such as the 
 common spectacle glasses. Such glasses, by causing the rays 
 of light to converge, before they meet the eye, assist the refrac- 
 tive power of the crystaline lens, and thus bring the focus, or 
 image, within the sphere of vision. 
 
 72. Why we see Objects Erect. It has been considered dif- 
 ficult to account for the reason why we see objects erect, when 
 they are painted on the retina inverted, and many learned theo- 
 ries have been written to explain this fact. But it is most 
 probable that this is owing to habit, and that the image, at the 
 bottom of the eye, has no relation to the terms above and be- 
 low, but to the position of our bodies, and other things which 
 surround us. The term perpendicular, and the idea which it 
 conveys to the mind, is merely relative ; but when applied to 
 an object supported by the earth, and extending toward the 
 skies, we call the body erect, because it coincides with the posi- 
 tion of our own bodies, and we see it erect for the same reason. 
 Had we been taught to read by turning our books upside down, 
 what we now call the tfpper part of the book would have been 
 its under part, and that reading would have been as easy in 
 that position as in any other, is plain from the fact that printers 
 read their type, when set up, as readily as they do its impres- 
 sions on paper. 
 
 ANGLE OF VISION. 
 
 73. This subject, partly explained, needs further illustration. 
 The angle under which the rays of light, coming from the 
 
 extremities of an object, cross each other at the eye, bears a pro- 
 portion directly to the length,. and inversely to the distance of 
 the object. 
 
 Suppose the object A B, Fig. 215, to be four feet long, and 
 to be placed ten feet from the eye, then the rays flowing from 
 its extremities, would intersect each other at the eye, under a 
 given angle, which will always be the same when the object is 
 at the same distance. If the object be gradually moved toward 
 
 72. Why do we see things erect, when the images are inverted on the retina? 73. 
 What is the visual angle 1 How may the visual angle of the same object be increased 
 or diminished 1 When do objects of different magnitudes form the same visual an 
 gle 1 Explain Fig. 215. 
 
254 
 
 VISION. 
 FIG. 215. 
 
 Angle of Vision. 
 
 the eye, to the place C D, then the angle will be gradually in- 
 creased in quantity, and the object will appear larger, since its 
 image on the retina will be increased in length in the propor- 
 tion as the lines I I, are wider apart than O O. On the con- 
 trary, were A B removed to a greater distance from the first 
 position, it is obvious that the angle would be diminished in 
 proportion. 
 
 The lines thus proceeding from the extremities of an object, 
 and representing the rays of light, form an angle at the eye, 
 which is called the visual angle, or the angle under which things 
 are seen. The lines A N B, therefore, form, one visual angfe, 
 and the lines C N D another visual angle. 
 
 We see from this investigation, that the apparent magnitude 
 of objects depending on the angles of vision, will vary according 
 to their distances from the eye, and that these magnitudes di- 
 minish in a proportion inversely as their distances increase. 
 
 74. How WE JUDGE OF MAGNITUDES. In the apparent mag- 
 nitude of objects seen through a lens, or when their images 
 reach the eye by reflection from a mirror, our senses are chiefly, 
 if not entirely, guided by the angle of vision. In forming our 
 judgment of the sizes of distant objects, whose magnitudes were 
 before unknown, we are also guided more or less by the visual 
 angle, though in this case we do not depend entirely on the 
 sense of vision. Thus, if we see two balloons floating in the 
 air, one of which is larger than the other, we judge of their 
 comparative magnitudes by the difference in their visual angles, 
 and of their real magnitudes by the same angles, and the dis- 
 tance we suppose them to be from us. 
 
 74. How do we judge of the magnitudes of distant objects ? Under what circum< 
 etantes IB our riense of vieltfn guided entirely By the visual angle t 
 
VISION. 255 
 
 75. But when the object is near us, and seen with the naked 
 eye, we then judge of the magnitude by our experience, and not 
 entirely by the visual angle. Thus, the 't^ree arrows, A E M, 
 Fig. 215, all of them make the same angle on the eye, and yet 
 we know, by further examination, that they are all of different 
 lengths. And so the two arrows, A B, and C D, though seen 
 under different visual angles, will appear of the same size, be- 
 cause experience has taught us that this difference depends only 
 on the comparative distance of the two objects. 
 
 76. As the visual angle diminishes inversely in proportion as 
 the distance of the object increases, so when the distance is so 
 great as to make the angle too minute to be perceptible to the 
 eye, then the object becomes invisible. Thus, when we watch 
 an eagle flying from us, the angle of vision is gradually dimin- 
 ished, Until the rays proceeding from the bird form an image on 
 the retina too small to excite sensation, and then we say the 
 eagle has flown out of sight. 
 
 The same principle holds with respect to objects which are 
 near the eye, but are too small to form an image on the retina 
 which is perceptible to the senses. Such objects to the naked 
 eye, are of course invisible, but when the- visual angle is en- 
 larged, by means of the convex lens, they become visible ; that 
 is, their images on the retina excite sensation. 
 
 77. SIZE OF THE IMAGE ON THE RETINA. The actual size 
 of an image on the retina, capable of exciting sensation, and 
 consequently of producing vision, may be too small for us to 
 appreciate by any of our other senses ; for when we consider 
 how much smaller the image must be than the object, and that 
 a human hair can be distinguished by the naked eye at the dis- 
 tance of twenty or thirty feet, we must suppose that the retina 
 is endowed with the most delicate sensibility, to be excited by 
 a cause so minute. It has been estimated that the image of a 
 man, on the retina, seen at the distance of mile, is not more 
 than the five thousandth part of an inch in length. 
 
 78. INDISTINCT VISION. On the contrary, if the object be 
 brought too near the eye, its image becomes confused and in- 
 distinct, because the rays flowing from it, fall on the crystaline- 
 lens in a state too divergent to be refracted to a focus'on the 
 retina. 
 
 75. How do we judge of the comparative size of objects near us ? 76. When does 
 a retreat n? object become invis ble to the eye 1 How does a convex lens act to make 
 us see objects which are invisible without it) 77. What is said uf the actual size of 
 an image on the retina ? 78. Why are objects indistinct when brought too near the 
 eye? 
 
256 
 
 OPTICAL INSTRUMENTS. 
 
 Indistinct Vision. 
 
 This will be apparent by 
 Fig. 216, where we sup- 
 pose that the object "A, is 
 brought within an inch or 
 two of the eye, and that 
 the rays proceeding from 
 it enter the pupil so ob- 
 liquely as not to be refract- 
 ed by the lens, so as to form 
 a distinct image. 
 
 Could we see objects dis- 
 tinctly at the shortest distance, we should be able to examine 
 things that are now invisible, since the visual angle would then 
 be increased, and consequently the image on the retina enlarged, 
 in proportion as objects were brought near the eye. 
 
 79. This is proved by intercepting the most divergent rays; 
 in which case an object may be brought near the eye, and will 
 then appear greatly magnified. Make a small orifice, as a pin- 
 hole, through a piece of dark-colored paper, and then look 
 through the orifice at small objects, such as the letters of a 
 printed book. The letters will appear much magnified. The 
 rays, in this case, are refracted to a focus, on the retina, because 
 the small orifice prevents those which are most divergent from 
 entering the eye, so that notwithstanding the nearness of the 
 object, the rays which form the image are nearly-parallel. 
 
 OPTICAL INSTRUMENTS. 
 
 80. SINGLE MICROSCOPE. The principle of the single micro- 
 scope, or convex lens, will be readily understood, if the pupil 
 will remember what has been said on the refraction of lenses, in 
 connection with the facts just stated. For, the reason why ob- 
 jects appear magnified through a convex lens, is not only be- 
 cause the visual angle is increased, but because when brought 
 near the eye, the diverging rays from the object are rendered 
 parallel by the lens, and are thus thrown into a condition to be 
 brought to a focus in the proper place by the humors of the 
 eye. 
 
 Let A, Fig. 217, be the distance at which an object can bo 
 
 Suppose objects could be seen distinctly within an inch or two of the eye, how 
 would their dimensions be affected 1 79. How is it proved that objects placed near 
 Jie eye are magnified 7 How does a small orifice enable us to see an object distinctly 
 near "the eye? 80. Why does a convex lens make an object distinct when near the 
 eve ( Explain Fig. 217. How are the most powerful single microscopes made? 
 
OPTICAL INSTRUMENTS. 257 
 
 FIG. 217. 
 
 Single Microscope. 
 
 seen distinctly, and B, the distance at which the same object is 
 seen through the lens, and suppose the distance of A from the 
 eye, be twice that of B. Then, because the object is at half the 
 distance that it was before, it will appear twice as large ; and 
 had it been seen one- third, one-fourth, or one-tenth its former 
 distance, it would have been magnified three, four, or ten times, 
 and consequently its surface would be increased 9, 16, or 100 
 times. 
 
 The most powerful single microscopes are made of minute 
 globules of glass, which are formed by melting the ends of a 
 few threads of spun glass in a flame of alcohol. Small globules 
 of water placed in an orifice through a piece of tin, or other 
 thin substance, will also make very powerful microscopes. In 
 these minute lenses, the focal distance is only a tenth or twelfth 
 part of an inch from the lens, and therefore the eye, as well 
 as the object to be magnified, must be brought very near the 
 instrument. 
 
 81. COMPOUND MICROSCOPE. This consists of two convex 
 lenses, by one of which the image is formed within the tube of 
 the instrument, and by the other this image is magnified as 
 seen by the eye ; so that by this instrument the object itself is 
 not seen, as with the single microscope, but we see only its 
 magnified image. 
 
 The small lens placed near the object, and by which its image 
 is formed within the tube, is called the object glass, while the 
 larger one, through which the image is seen, is called the 
 eyeglass. 
 
 This arrangement is represented at Fig. 218. The object A 
 is placed a little beyond the focus of the object glass B, by which 
 an inverted and enlarged image of it is formed within the in- 
 
 81. How many lenses form the compound microscope 1 Which is the object, and 
 which the eyeglass? Is the object seen with this instrument, or only its image? 
 Explain Fig. 218, and show where the image is formed in this tube. 
 
258 
 
 OPTICAL INSTRUMENTS. 
 
 Compound Microscope. 
 
 strument at C. This image is seen through the eyeglass D, by 
 which it is again magnified, and it is at last figured on the retina 
 in its original position. 
 
 These glasses are set in a case of brass, the object glass being 
 made to take out, so that others of different magnifying powers 
 may be used, as occasion requires. 
 
 82. SOLAR MICROSCOPE. This consists of two lenses, one of 
 which is called the condenser, because it is employed to con- 
 centrate the rays of the sun, in order to illuminate more strongly 
 the object to be magnified. The other is a double-convex lens, 
 of considerable magnifying power, by which the image is formed. 
 In addition to these lenses, there is a plain mirror, or piece of 
 common looking-glass, which can be moved in any direction, 
 and which reflects the rays of the sun on the condenser. 
 
 FIG. 219. 
 
 Solar Microscope. 
 
 The object A, Fig. 219, being placed nearly in the focus of 
 
 82. How many lenses has the solar microscope? Why is one of the lenses of the 
 solar microscope called the condenser ? Describe the uses of the two lenses and the 
 reflector. Is the object, or only the shadow, seen by this instrument 1 
 
TELESCOPE. 
 
 the condenser B, is strongly illuminated, in consequence of the 
 rays of the sun being thrown on B, by the mirror C. The ob- 
 ject is not placed exactly in the focus of the condenser, because, 
 in most cases, it would be soon destroyed by its heat, and be- 
 cause the focal point would illuminate only a small extent of 
 surface, but may be exactly in the focus of the small lens D, by 
 which no such accident can happen. The lines O O, represent 
 the incident rays of the sun, which are reflected on the condenser. 
 
 When the solar microscope is used, the room is darkened, 
 the only light admitted being that which is thrown on the ob- 
 ject by the condenser, which light passing through the small 
 lens, gives the magnified shadow E, of the small object A, on 
 the wall of the room, or on a screen. The tube containing the 
 two lenses is passed through the window of the room, the re- 
 flector remaining outside. 
 
 In the ordinary use of this instrument, the object itself is not 
 seen, but only its shadow on the screen, and it is not designed 
 for the examination of opaque objects. 
 
 When the small lens of the solar microscope is of great mag- 
 nifying power, it presents some of the most striking and curious 
 of optical phenomena. The shadows of mites from cheese, or 
 figs, appear nearly two feet in length, presenting an appearance 
 exceedingly formidable and disgusting ; and the insects from 
 common vinegar appear eight or ten feet long, and in perpetual 
 motion, resembling so many huge serpents. 
 
 TELESCOPE. 
 
 83. The Telescope is an optical instrument, employed to view 
 distant bodies, and, in effect, to bring them nearer the eye, by in- 
 creasing the apparent angles under which such objects are seen. 
 
 These instruments are of two kinds, namely, refracting and 
 reflecting telescopes. In the first kind, the image of the object 
 is seen with the eye directed toward it ; in the second kind, the 
 image is seen by reflection from a mirror, while the back is to- 
 ward the object, or by a double reflection, with the face toward 
 the object. 
 
 The telescope is the most important of all optical instruments, 
 since it unfolds the wonders of other worlds, and gives us the 
 means of calculating the distances of the heavenly bodies, and 
 
 83. What is a telescope ? How many kinds of telescopes are mentioned ? What 
 ia the difference between them 1 In. what respect does the refracting telescope differ 
 from the compound microscope ? 
 
260 TELESCOPE. 
 
 of explaining their phenomena for astronomical and nautical 
 purposes. 
 
 The principle of the telescope will be readily comprehended 
 after what has been said concerning the compound microscope, 
 for the two instruments differ chiefly in respect to the place of 
 the object lens, that of the microscope having a short, while 
 that of the telescope has a long, focal distance. 
 
 84. REFRACTING TELESCOPE. The most simple refracting 
 telescope consists of a tube, containing two convex lenses, the 
 one having a long, and the other a short, focal distance. (The 
 focal distance of a double-convex lens, it will be remembered, is 
 nearly the center of the sphere, of which it is a part. 56.) 
 These two lenses are placed in the tube, at a distance from each 
 other equal to the sums of their two focal distances. 
 
 FIG. 220. 
 
 Principle of the Telescope. 
 
 Thus, if the focus of the object glass, A, Fig. 220, be eight 
 inches, and that of the eyeglass B, two inches, then the distance 
 of the sums of the foci will be ten inches, and, therefore, the 
 two lenses must be placed ten inches apart ; and the same rule 
 is observed, whatever may be the focal lengths of any two 
 lenses. 
 
 Now, to understand the "effect of this arrangement, suppose 
 the rays of light, C D, coming from a distant object, as a star, 
 to fall on the object glass, A, in parallel lines, and to be re- 
 fracted by the lens to a focus at E, where the image of the star 
 will be represented. The image is then magnified by the eye- 
 glass B, and thus, in effect, is brought near the eye. 
 
 All that is effected by the telescope, therefore, is to form an 
 image of a distant object, by means of the object Jens, and then 
 to assist the eye in viewing this image as nearly as possible by 
 the eye lens. 
 
 84. How is the most simple refracting telescope formed ? Which is rhe object, and 
 which the eye lens, in Fig. 220 1 What is the rule by which the distance of the two 
 glasses apart \s found 1 How do the two glasses act, to bring an object near the eye ? 
 Explain Fig. 221, and show how the object comes to be inverted by the two lenses. 
 
TELESCOPE. 261 
 
 It is, however, necessary here to state, that by the last figure, 
 the principle only of the telescope is intended to be explained, 
 for in the common instrument, with only two glasses the image 
 appears to the eye inverted. 
 
 The reason of this will be seen by the next figure, where the 
 direction of the rays of light will show the position of the image. 
 
 FIG. 5221. 
 
 Principle of the Telescope. 
 
 Suppose A, Fig. 221, to be a distinct object, from which pen- 
 cils of rays flow from every point toward the object lens B. 
 The image of A, in consequence of the refraction of the rays by 
 the object lens, is inverted at C. which is the focus of the eye- 
 glass D, and through which the image is then seen, still inverted. 
 
 85. Spyglass. The inversion of the object is of little conse- 
 quence when the instrument is employed for astronomical pur- 
 poses, for since the forms of the heavenly bodies are spherical, 
 their positions, in this respect, do not affect their general ap- 
 pearance. But for terrestrial purposes, this is manifestly a great 
 defect, and therefore those constructed for such purposes, as 
 ship, or spyglasses, have two additional lenses, by means of 
 which, the images are made to appear in the same position as 
 the objects. These are called double telescopes. 
 
 Such a telescope is represented at Fig. 222, and consists of 
 an object glass A, and three eyeglasses, B C and D. The eye- 
 glasses are placed at equal distances from each other, so that 
 the focus of one may meet that of the other, and thus the 
 image formed" by the object lens, will be transmitted through 
 the other three lenses to the eye. The rays coming from the 
 object 0, cross each other at the focus of the object lens, and 
 thus form an inverted image at F. This image being also in 
 the focus of the first eyeglass, B, the rays having passed through 
 this glass become parallel, for we have seen in another place, 
 
 85. How is the inversion of the object corrected ? Explain Fig. 222, and show why 
 the two additional lenses make the image of the object erect. Does the addition of 
 these two lenses make any difference with the apparent magnitude of the object ? 
 
262 
 
 Refracting Telescope. 
 
 that diverging rays are rendered parallel by refraction through a 
 convex lens. The rays, therefore, pass parallel to the next lens 
 C, by which they are made to converge, and cross each other, 
 and thus the image is inverted, and made to assume the original 
 position of the object O. Lastly, this image, being in the focus 
 of the eyeglass D, is seen in the natural position. 
 
 The apparent magnitude of the object is not changed by these 
 two additional glasses, but depends, as ia Fig. 220, on the mag- 
 nifying power of the eye and object lenses ; these two glasses 
 being added merely for the purpose of making the image ap- 
 pear erect. 
 
 86. REFLECTING TELESCOPE. The common reflecting tele- 
 scope consists of a large tube, containing two concave reflecting 
 mirrors, of different sizes, and two eyeglasses. The object is 
 first reflected from the large mirror to the small one, and from 
 the small one, through the two eyeglasses, where it is then seen. 
 
 In comparing the advantages of the two instruments, it need 
 only be stated, that the refracting telescope with a focal length 
 of a thousand feet, if it could be used, would not magnify dis- 
 tinctly more than a thousand times, while a reflecting telescope, 
 only eight or nine feet long, will magnify with distinctness 
 twelve hundred times. 
 
 The principle and construction of the reflecting telescope will 
 be understood by Fig. 223. Suppose the object to be at 
 such a distance, that the rays of light from it pass in parallel 
 lines, P P, to the great reflector, R R. This reflector being 
 concave, the rays are converged by reflection, and cross each 
 other at A, by which the image is inverted. The rays then 
 pass to the small mirror, B, which being also concave, they are 
 thrown back in nearly parallel lines, and having passed the 
 
 86. How many lenses and mirrors form the reflecting telescope ? What are the 
 advantages of the reflectins over the refracting telescope ? Explain Fig. 223. and 
 show the course of the rays from the object to the eye. Why is the small mirror in 
 this instrument made to move by means of a screw ? 
 
TELESCOPB. 
 FIG. 223. 
 
 Reflecting Teletcope. 
 
 aperture in the center of the great mirror, fall on the plano- 
 convex lens E. By this lens they are refracted to a focus, and 
 cross each other between E and D, and thus the image is again 
 inverted, and brought to its original position, or in the position 
 of the object. The rays then passing the second eyeglass, form 
 the ima^e of the object on the retina. 
 
 The large mirror in this instrument is fixed, but the small 
 one moves backward and forward, by means of a screw, so as to 
 adjust the image to the eyes of different persons. Both mirrors 
 are made of a composition, consisting of several metals melted 
 together. 
 
 &7. One great advantage which the reflecting telescope pos- 
 sesses over the refracting, appears to be, that it admits of an 
 eyeglass of shorter focal distance, and, consequently, of greater 
 magnifying power. The convex object glass of the refracting 
 instrument, does not form a perfect image of the object, since 
 some of the rays are dispersed, and others" colored by refraction. 
 This difficulty does not occur in the reflected image from the 
 metallic mirror of the reflecting telescope, and consequently it 
 may be distinctly seen, when more highly magnified. 
 
 The instrument just described is called " Gregory's telescope," 
 because some parts of the arrangement were invented by Dr. 
 Gregory. 
 
 88. fferschers Telescope. In Dr. Herschel's grand telescope, 
 the largest then constructed, the reflector was 48 inches in 
 diameter, and had a focal distance of 40 feet. This reflector 
 was three and a half inches thick, and weighed 2000 pounds. 
 Now, since the focus of a concave mirror is at the- distance of 
 one-half the semi-diameter of the sphere, of which it is a sec- 
 
 87. \Vhat IP the advantage of the reflecting- telescope in respect to the eyeglass' 
 88. What was the focal distance and diameter of the mirror in Dr. Hersche'i's great 
 telescope 1 Where is the largest Herschers telescope now in existence 7 What ia 
 the diameter and focal distance of the reflector of this telescope ? 
 
264 TELESCOPE. 
 
 tion, Dr. Herschel's reflector having a focal distance of 40 feet, 
 formed a part of a sphere of 160 feet in diameter. 
 
 This great instrument was begun in 1785, and finished four 
 years afterward. The frame by which this wonder to all astron- 
 omers was supported, having decayed, it was taken down in 
 1822, and another of 20 feet focus, with a reflector of 18 inches 
 in diameter, erected in its place, by Herschel's son. 
 
 The largest Herschel's telescope now in existence is that of 
 Greenwich observatory, in England. This has a concave re- 
 flector of 15 inches in diameter, with a focal length of 25 feet, 
 and was erected in 1820. 
 
 89. LORD ROSSE'S TELESCOPE. Dr. Herschel's telescope was 
 the largest ever constructed until recently, when a young no- 
 bleman of fortune in Ireland, Lord Rosse", being led by an in- 
 ventive genius, and having, it appears, a degree of enterprise 
 not to be deterred by difficulties, projected the plan of building 
 a telescope of a size and power hitherto unknown in the world. 
 
 The following account of the " Monster Telescope," as it has 
 been called, is taken from that of Thomas Dick, LL. D., con- 
 tained in his works. 
 
 It appears that the possibility of casting a speculum, or re- 
 flector for a telescope, of six feet in diameter, was entertained by 
 his lordship in 1840, though others considered such an under- 
 taking in the light of a chimera. But the trial being made 
 through the perseverance and large expenditures of the pro- 
 jector, complete success crowned the experiment, a nearly per- 
 fect casting of a speculum 72 inches in diameter being the result. 
 Thus the difficulty of constructing an instrument one-third larger 
 than Herschel's, was at once surmounted. 
 
 90. Composition and Casting. The composition of this 
 speculum is copper and tin united very nearly in their atomic 
 proportions, namely : copper 1 26.4, to tin 58.9 parts. A foundry 
 was constructed expressly for this great casting, the chimney of 
 which was 18 feet high, and 16- feet square at the foundation. 
 The crucibles for containing the fused alloy were of cast iron, 2 
 feet in diameter, and 2 feet deep. Iron baskets, suspended by 
 cranes, were so contrived as to receive the crucibles and their 
 melted contents, and swing them to the mold into which, one 
 after the other, they were poured. The mold, 6 feet in diam- 
 eter and 5- inches deep, was arranged in an exact horizontal 
 
 89. Who constructed the largest telescope in the world ? What is the size of the 
 speculum? 90. What is its composition ? How much larger is this instrument than 
 Herschel's } 
 
TELESCOPE. 265 
 
 position by means of spirit levels. The crucibles were 10 hours 
 in the furnace before the metal was sufficiently fluid to cast. 
 The speculum weighed 3 tons lost one-eighth of an inch in 
 thickness by grinding. 
 
 Grinding. The grinding was conducted under Vater, the 
 moving power being a steam-engine of 3 horse power. The 
 grinder is of cast iron with grooves in its face to retain the emery, 
 and the two faces having a mutual motion, both became per- 
 fect, whatever might have been their inequalities. The polish- 
 ing was done by means of a thin layer of pitch spread on the 
 grinder, on which rouge was smeared in the form of paste with 
 water. This process took six hours. 
 
 Construction of the Tube. The tube is 56 feet long, made 
 of boards and hooped with iron. On the inside at intervals of 
 8 feet, are iron rings to support the boards. Its diameter is 7 
 feet, the whole being easily moved in any direction by means 
 of pulleys and levers, a universal joint at the lower end being 
 designed for this purpose. 
 
 Wall of Support. At a distance of 12 feet, on each side of 
 the instrument is a brick wall, 72 feet long, 48 high on the out- 
 side, and 56 on the inside, ranging exactly on the mfcridianal 
 line. These walls have rods of iron and wood passing from one 
 to the other, for the support of the telescope, as it is turned in 
 different directions. 
 
 The weight of the speculum and tube, including that of the 
 bed on which it is sustained, is about 15 tons. 
 
 This being a reflecting telescope, the observer stands in a 
 gallery at the upper end, and looks into the side of the great 
 tube, where the observations are made by means of a reflecting 
 surface of 4,071 square inches, while Herschel's great reflector 
 had a surface of only 1,811 square inches. 
 
 The cost of this wonder of the age is 60,000 dollars. 
 Description of the Figure. The following description of a 
 section of Lord Rosse's telescope, Fig. 224, though not so per- 
 fect as could be desired, is the best we could obtain. It ex- 
 Libits a view of the inside of the eastern wall, with the tube, 
 and machinery by which it is moved. A is the mason- work on 
 the ground ; B the universal joint, which allows the tube to 
 turn in all directions ; C the speculum in the tube ; E the eye- 
 piece through which the observer looks; F a pulley by which 
 the tube is moved ; H a chain attached to the pulley, and to 
 the side of the tube ; I a chain running to K, the counterpoise; 
 L a lever connecting the chain M with the tube ; Z another 
 
 12 
 
Lord Rosse's Telescope. 
 
 cliain which passes from the upper part of the tube over a pul- 
 ley at W, (not seen,) and crosses to the opposite wall ; X a 
 railroad on which the speculum is drawn either to or from the 
 tube. The dotted line H, shows th*e course of the weight R, as 
 the tube rises or falls. The tube is moved from wall to wall 
 by a ratchet wheel at R, which is turned by the lever O, on the 
 circle N, the ends of which are fixed in the two walls. 
 
 91. CAMERA OBSCURA. Camera obscura strictly signifies a 
 darkened chamber, because the room must be darkened, in order 
 to observe its effects. 
 
 To witness the phenomena of this instrument, let a room be 
 closed in every direction, so as to exclude the light. Then from 
 an aperture, say of an inch in diameter, admit a single beam of 
 light, and the images of external things, such as the trees and 
 houses, and persons walking the streets, will be seen inverted 
 on the wall opp6site to where the light is admitted, or on a 
 screen of white paper, placed before the aperture. 
 
 92. The reason why the image is inverted will be obvious, 
 when it is remembered that the rays proceeding from the ex- 
 tremities of the object must converge in order to pass through 
 the small aperture ; and as the rays of light always proceed in 
 
 91 \A r liat dops camera obscura mean? Describe the phenomena of the camera 
 pbscura. 92. Why is the imajre formed by the camera obscura inverted! How may 
 an outline of the image formed by the camera obscura be taken 1 Describe the re- 
 volving camera obscura. 
 
TELESCOPE. 
 
 267 
 
 Principle of the Camera. 
 
 FIG. 226. 
 
 straight lines, they HQ - 
 
 must cross each 
 
 other at the point 
 
 of admission, as 
 
 explained under 
 
 the article Vision. 
 
 Thus the pencil 
 A, Fig. 225, com- 
 ing from the up- 
 per part of the 
 tower, and pro- 
 ceeding straight, 
 will represent the 
 
 image of that part at B, while the lower part C, for the same 
 reason, will be represented at D. If a convex lens, with a short 
 tube, be placed in the aperture through which the light passes 
 into the room, the images of things will be much more perfect, 
 and their colors more brilliant. 
 
 This instrument is sometimes 
 employed by painters, in order 
 to obtain an exact delineation 
 of a landscape, an outline of the 
 image being easily taken with 
 a pencil, when the image is 
 thrown on a sheet of paper. 
 
 There are several modifica- 
 tions of this machine, and among 
 them the revolving camera ob- 
 scura is the most interesting. 
 
 It consists of a small house, 
 Fly. 226, with a plane reflector 
 A B, and a double-convex lens 
 C B, placed at its top. The re- 
 flector is fixed at an angle of 
 
 45 degrees with the horizon, so Camera obscura. 
 
 as to reflect the rays of light 
 
 perpendicularly downward, and is made to revolve quite around, 
 in either direction, by pulling a string. 
 
 Now suppose the small house to be placed in the open air 
 with the mirror, A B, turned toward the east, then the rays of 
 light flowing from the objects in that direction, will strike the 
 mirror in the direction of the lines 0, and be reflected down 
 through the convex lens C B, to the table E E, where they will 
 
268 
 
 MAGIC LANTERN. 
 
 form in miniature a most perfect and beautiful picture of the 
 landscape in that direction. Then, by making the reflector re- 
 volve, another portion of the landscape may be seen, and thus 
 the objects, in all directions, can be viewed at K without chang- 
 ing the place of the instrument. 
 
 MAGIC LANTERN. 
 
 93. The Magic Lantern is a microscope on the same principle 
 as the solar microscope. But instead of being used to magnify 
 natural objects, it is commonly employed for amusement, by 
 casting the shadows of small transparent paintings done on glass, 
 upon a screen placed at a proper distance. 
 
 FIG. 227. 
 
 Magic Lantern. 
 
 Let a candle C, Fig. 227, be placed on the inside of a box or 
 tabe, so that its light may pass through the plano-convex lens 
 N, and strongly illuminate the object O. This object is gen- 
 erally a small transparent painting on a slip of glass, which 
 slides through an opening in the tube. In order to show the 
 figures in the erect position, these paintings are inverted, since 
 their shadows are again inverted by the refraction of the convex 
 lens M. 
 
 In some of these instruments there is a concave mirror, D, by 
 which the object O, is more strongly illuminated than it would 
 be by the lamp alone. The object is magnified by the double- 
 convex lens, M, which is movable in the tube by a screw, so 
 
 93. What is the magic lantern 1 For what purpose is this instrument emplojed 7 
 Describe the construction and effect of the magic lantern. 
 
CHROMATICS. 
 
 269 
 
 that its focus can be adjusted to the required distance. Lastly, 
 there is a screen of white cloth, placed at the proper distance, 
 on which the image or shadow of the picture, is seen greatly 
 magniti d. 
 
 The pictures being of various colors, and so transparent, that 
 the light of the lamp shines through them, the shadows arc 
 also of various colors, and thus soldiers and horsemen are repre- 
 sented in their proper costume. 
 
 CHROMATICS, OR THE PHILOSOPHY OF COLORS. 
 
 94. We have thus far considered light as a simple body, and 
 have supposed that all its parts were equally refracted, in its 
 passage, through the several lenses described. But it will now 
 be shown that light is a compound body, and that each of its 
 rays, which to us appear white, is composed of several colors, 
 and that each color suffers a different degree of refraction, when 
 the rays of light pass through a piece of glass, of a certain shape. 
 This was a discovery of Sir Isaac Newton. 
 
 95. SOLAR SPECTRUM. If a ray, proceeding from the sun, 
 be admitted into a darkened chamber, through an aperture in 
 the window shutter, and allowed to pass through a triangular 
 shaped piece of glass, called a prism, the light will be decom- 
 posed, and instead of a spot of white, there will be seen, on the 
 opposite wall, a most brilliant display of colors, including all 
 those seen in the rainbow. 
 
 FIG. 
 
 M 
 
 1/hUe <j>-" C 
 
 Solar Spectrum. 
 
 Suppose S, Fig. 228, to be a ray from the sun, admitted 
 through the window shutter A, in such a direction as to fall on 
 
 94. Who made the discovery, that l.<tht is a compound substance 1 95. In what 
 manner, and by what means, is light decomposed 1 
 
270 CHROMATICS. 
 
 the floor at C, where it would form a round, white spot. Now, 
 on interposing the prism P, the ray will be refracted, and at the 
 same time decomposed, and will form on the screen M N, an 
 oblong figure, containing seven colors, which will be situated in 
 respect to each other, as named on the figure. 
 
 It may be observed, that of all the colors, the red is least re- 
 fracted, or is thrown the smallest distance from the direction of 
 the original sunbeam, and that the violet is most refracted, or 
 bent out of that direction. 
 
 This oblong image containing the colored rays, is called the 
 solar or prismatic spectrum. 
 
 96. decomposition of White Light. That the rays of the 
 sun are composed of the several colors above named, is suffi- 
 ciently evident by the fact, that such a ray is divided into these 
 several colors by passing through the prism, but in addition to 
 this proof, it is found by experiment, that if these several colors 
 be blended or mixed together, white will be the result. 
 
 This may be done by mixing together seven powders whose 
 colors represent the prismatic colors, and whose quantities are 
 to each other, as the spaces occupied by each color in the spec- 
 trum. When this is done, it will be found that the resulting 
 color will be a grayish white. A still more satisfactory proof 
 that these seven colors form white, when united, is obtained by 
 causing the solar spectrum to pass through a lens, by which 
 they are brought to a focus, when it is found that the focus will 
 be the same color as it would be from the original rays of the 
 sun. 
 
 97. Other Means of Decomposing Light. The prism is not 
 the only instrument by which light can be decomposed. A 
 soap bubble blown up in the sun will display most of the pris- 
 matic colors. This is accounted for by supposing that the sides 
 of the bubble vary in thickness, and that the rays of light are 
 decomposed by these variations. The unequal surface of mother 
 of pearl, and many other shells, send forth colored rays on the 
 same principle. 
 
 Two surfaces of polished glass, when pressed together, will 
 also decompose the light. Rings of colored light will be ob- 
 
 What are the prismatic colors, and how do they succeed each other in the spec- 
 trum'? Which color is refracted most and which least? 90. When the several 
 prismatic colors are blended, what color is the result ? When the solar spectrum is 
 made to pass through a lens, what is the color of the focus? How do we learn that 
 each colored ray has a refractive power of its own ? 97. By what other means be- 
 side the prism, can the rays of light be decomposed? How may light be decomposed 
 by two pieces of glass 7 
 
CHROMATICS. 271 
 
 served around the point of contact between the two surfaces, 
 and their number may be increased or diminished by the de- 
 grees of pressure. Two pieces of common looking-glass, pressed 
 together with the fingers, will display most of the prismatic 
 colors. 
 
 98. A variety of substances, when thrown into the form of 
 the triangular prism, will decompose the rays of light, as well 
 as a prism of glass. A very common instrument for this pur- 
 pose is made by putting together three pieces of plate glass, in 
 form of a prism. The ends may be made of wood, and the 
 edges cemented with putty, so as to make the whole water-tight. 
 When this is filled with water, and held before a sunbeam, the 
 solar spectrum will be formed, displaying the same colors, and 
 in the same order, as that above "described. 
 
 99. decomposition of Li<jh{ by a Circle. On this subject, a 
 curious and satisfactory experiment may be made by means of 
 a dark center, and circle, with 
 
 divisions between them, repre- FIG. 229. 
 
 senting the proportions of the 
 prismatic rays, and colored to 
 imitate them. The letters, 
 Fig. 229, show the different 
 colors of the seven rays, and 
 the spaces they severally oc- 
 cupy. 
 
 100. Now if this card, thus 
 colored, be placed on a spindle 
 and made to turn rapidlv. the 
 seven colors will entirely dis- 
 appear, and a dull white only 
 
 will be presented to the eye, Recmtipositim of Light. 
 
 instead of them. 
 
 101. Explanation. Any color remains for an instant on the 
 eye after it is covered, or removed, and hence a fire-brand 
 whirled rapidly, appears a circle of fire. Two revolving colors 
 will thus be so blended as to seem a medium between them ; 
 thus if all the colors on the card are covered, except the yellow 
 and red, they will appear orange. And if all the prismatic 
 tints are blended, whether in the form of powders, or propor- 
 tionate, colored, revolving surfaces, they produce white. 
 
 98. Of what substances may prisms be formed, besides glass ? 99. For what ar 
 the divisions of the circle, Fig. 229, intended? What is said to be the effect when 
 this card is revolved on a spindle ? 100. What color is said to be produced when all 
 the prismatic tints are mixed or revolved 7 101. What is the explanation 1 
 
272 
 
 RAINBOW. 
 
 THE RAINBOW. 
 
 102. The rainbow was a phenomenon, for which the ancienU 
 were entirely unable to account ; but after the discovery thai 
 light is a compound principle, and that its colors may be separ- 
 ated by various substances, the solution of this phenomenon 
 became easy. 
 
 Sir Isaac Newton, after his great discovery of the compound 
 nature of light, and the different refrangibility of the colored 
 rays, was able to explain the rainbow on optical principles. 
 
 103. If a glass globe be suspended in a room, where the rays 
 of the sun can fall upon it, the light will be decomposed, or 
 separated into several colored rays, in the same manner as is 
 done by the prism. A well defined spectrum will not, however, 
 be formed by the globe, because its shape is such as to disperse 
 some of the rays, and converge others ; but the eye, by taking 
 different positions in respect to the globe, will observe the va- 
 rious prismatic colors. Transparent bodies, such as glass and 
 water, reflect the rays of light from both their surfaces, but 
 chiefly from the second surface. That is, if a plate of naked 
 glass be placed so as to reflect the image of the sun, or of a 
 lamp, to the eye, the most distinct image will come from the 
 second surface, or that most distant from the eye. The great 
 brilliancy of the diamond is owing to this cause. It will be un- 
 derstood directly, how this principle applies to the explanation 
 of the rainbow. 
 
 104. How the Bow 
 is Formed. Sup- 
 pose the circle A B 
 C, Fig. 230, to rep- 
 resent a globe, or a 
 drop of rain, for each 
 drop of rain, as it 
 falls through the air, 
 is a small globe of 
 water. Suppose also, 
 that the sun is at S, 
 and the eye of the 
 spectator at E. Now, 
 it has already been 
 
 102. What discovery preceded the explanation of the rainbow? Who first ex- 
 planed the rainbow on optical principles .' 103. Why does not a glass globe form a 
 well defined spectrum! From which surface do transparent bodies chiefly reflect 
 the light 1 
 
 How the Bow is Formed. 
 
RAINBOW. 273 
 
 stated, (103,) that from a single globe, the whole solar spectrum 
 is not seen in the same position, but that the different colors are 
 seen from different places. Suppose, then, that a ray of light 
 from the sun S, on entering the globe at A is separated into its 
 primary colors, and at the same time the red ray, which is the 
 least refrangible, is refracted in the line from A to B. From 
 the second, or inner surface of the drop, it would be reflected to 
 C, the angle of reflection being equal to the angle of incidence. 
 On passing out of the drop, its refraction at C, would be just 
 equal to the refraction of the incident ray at A, and therefore 
 the red ray would fall on the eye at E. All the other colored 
 rays would follow the same law, but because the angles of inci- 
 dence and those of reflection are equal, and because the colored 
 rays are separated from each other by unequal refraction, it is 
 obvious, that if the red ray enters the eye at E, none of the 
 other colored rays could be seen from the same point. 
 
 105. From this it is evident, that if the eye of the spectator 
 is moved to another position, he will not see the red ray com- 
 ing from the same drop of rain, but only the blue, and if to an- 
 other position, the green, and so of all the others. But in a 
 shower of rain, there are drops at all heights and distances, and 
 though they perpetually change their places, in respect to the 
 sun and the eye, as they fall, still there will be many which 
 will be in such a position as to reflect the red rays to the eye, 
 and as many more to reflect the yellow rays, and so of all the 
 other colors. 
 
 106. This will be made obvious by Fig. 231, where, to avoid 
 confusion, we will suppose that only three drops of rain, and, 
 consequently, only three colors, are to be seen. 
 
 The numbers 1, 2, 3, are the rays of the sun, proceeding to 
 the drops ABC, and from which these rays are reflected to the 
 eye, making different angles with the horizontal line H, be- 
 cause one colored ray is refracted more than another. Now, 
 suppose the red ray only reaches the eye from the drop A, the 
 green from the drop B, and the violet from the drop C, then the 
 spectator would see a minute rainbow of three colors. But 
 during a shower of rain, all the drops which are in the position 
 of A, in respect to the eye, would send forth red rays, and no 
 
 104. Explain Fig. 230, anrl show the different refractions, and the reflection con- 
 cerned in forming the rainbow. In the case supposed, why will only the red ray 
 meet the eye? 105. Suppose a person looking at the rainbow moves his eye, will he 
 see the same colors from the same drop of rain ? 106. Explain Fig. 231, and show 
 why \ve see different colors from different drops of rain. Do several persons see the 
 same rainbow at the same time ) Explain the reason of this. 
 
 12* 
 
274 
 
 COLORS OF OBJECTS. 
 FIG. 231. 
 
 Rainbow. 
 
 other, while those in the position of B, would emit green rays, 
 and no other, and those in the position of C, violet rays ; and 
 so of all the other prismatic colors. Each circle of colors, of 
 which the rainbow is formed, is therefore composed of reflections 
 from a vast number of different drops of rain, and the reason 
 why these colors are distinct to our senses, is, that we see only 
 one color from a single drop, with the eye in the same position. 
 It follows, then, that if we change our position, while looking at 
 a rainbow, we still see a bow, but not the same as before, and 
 hence, if there are many spectators, they will all see a different 
 rainbow, though it appears to be the same. 
 
 COLORS OF OBJECTS. 
 
 10 7. Color Depends on Absorption and Reflection. It ap- 
 pears that the colors of all bodies depend on some peculiar 
 property of their surfaces, in consequence of which, they absorb 
 some of the colored rays, and reflect the others. Had the sur- 
 faces of all bodies the property of reflecting the same ray only, 
 all nature would display the monotony of a single color, and 
 our senses, would never have known the charms of that variety 
 which we now behold. 
 
 107, On what do the colors of bodies depend? Suppose all bodies reflected the 
 same ray, what would be the consequence in regard to color ? 
 
COLORS OF OBJECTS. 275 
 
 108. To account for such a variety of colors as we see in dif- 
 ferent bodies, it is supposed that all substances, when made 
 sufficiently thin, are transparent, and consequently, that they 
 transmit through their surfaces, or absorb, certain rays of light, 
 while other rays are thrown back, or reflected. Gold, for ex- 
 ample, may be beat so thin as to transmit some of the rays of 
 light, and the same is true of several of the other metals, which 
 are capable of being hammered into thin leaves, It is there- 
 fore most probable, that all the metals, could they be made suf- 
 ficiently thin, would permit the rays of light to pass through 
 them. Most, if not quite all mineral substances, though in the 
 mass they may seem quite opaque, admit the light through 
 their edges, when broken, and almost every kind of wood, when 
 made no thinner than writing paper, becomes translucent. 
 Thus we may safely conclude, that every substance with which 
 we are acquainted, will admit the rays of light, when made suf- 
 ficiently thin. 
 
 109. Transparent Substances. Transparent, colorless sub- 
 stances, whether solid or fluid, such as glass, water, or mica, 
 reflect and m transmit light of the same color ; that is, the light 
 seen through these bodies, and reflected from their surfaces, is 
 white. This is true of all transparent substances under ordinary 
 circumstances ; but if their thickness be diminished to a certain 
 extent, these substances will both reflect and transmit colored 
 light of various hues, according to their thickness. Thus, the 
 thin plates of mica, which are left on the fingers after handling 
 that substance will reflect prismatic rays of various colors. 
 
 110. From such phenomena. Sir -Isaac Newton concluded, 
 that air, when below the thickness of ha/fa millionth of an inch, 
 ceases to reflect light ; and also, that water, when below the 
 thickness of three-eighths of a millionth of an inch, ceases to re- 
 flect light. But that both air and water, when their thickness 
 is in a certain degree above these limits, reflect all the colored 
 rays of the spectrum. 
 
 111. From a great variety of experiments on this subject, Sir 
 Isaac Newton concludes that the transparent parts of bodies, 
 according to the sizes of their transparent pores, reflect rays of 
 one color, and transmit those of another, for the same reason 
 
 108. How is the variety of colors accounted for, by considering all bodies trans- 
 parent 1' 109 What is said of the reflection of colored light by transparent sub- 
 stances ? What substance is mentioned, as illustrating this fact ? 11U. What is the 
 conclusion of Sir-Isaac Newton, concerning the tenuity at which water and air cease 
 to reflect light 1 111. What is said of the porous nature of the solid bodies 7 
 
276 ASTRONOMY. 
 
 that thin plates, or minute particles of air, water, and some 
 other substances, reflect certain rays, and absorb or transmit 
 others, and that this is the cause of all their colors. 
 
 CHAPTER XII. 
 
 _ ASTRONOMY. 
 
 112. THIS term is compounded of the Greek astra, the stars, 
 and nomos a law ; and hence signifies the laws of the celestial 
 bodies. 
 
 Astronomy is that science which treats of the motions and 
 appearance of the heavenly bodies ; accounts for the phenomena 
 which these bodies exhibit to us ; and explains the laws by which 
 their motions, or apparent motions, are regulated. 
 
 Astronomy is divided into Descriptive, Physical, and Prac- 
 tical. 
 
 Descriptive astronomy demonstrates the magnitudes, distances, 
 and densities of the heavenly bodies, and explains the phenom- 
 ena dependent on their motions, such as the change of seasons, 
 and the vicissitudes of day and night. 
 
 Physical astronomy explains the theory of planetary motion, 
 and the laws by which this motion is regulated and sustained. 
 
 Practical astronomy details the description and use of astro- 
 nomical instruments, and develops the nature and application 
 of astronomical calculations. 
 
 The heavenly bodies are divided into three distinct classes, or 
 systems, namely, the solar system, consisting of the sun, moon, 
 and planets ; the system of the fixed stars / and the system of 
 the comets. 
 
 THE SOLAR SYSTEM. 
 
 113. The Solar System consists of the Sun, and forty-two 
 other bodies, including the satellites, which revolve around him 
 at various distances, and in various periods of time. 
 
 112. What is astronomy? How is astronomy divided 7 What does descriptive 
 astronomy teach ] What is the object of physical astronomy? What is practical 
 astronomy? How are the heavenly bodies divided? 113. O'f what does the solar 
 system consist ? 
 
ASTRONOMY. 
 
 277 
 
 These bodies, being perpetually in motion, are called planets, 
 from a Greek word signifying wanderers, and they are distin- 
 guished with reference to their centers of revolution, into 
 primary and secondary. 
 
 114. The Primary planets are those which revolve around 
 the sun as their proper center. These are twelve in number ; 
 that nearest the sun being Mercury, the others follow in succes- 
 sion, thus: Venus, Earth, Mars, Vesta, Ceres, Pallas, Juno, 
 Jupiter, Saturn, Herschel or Uranus, and Neptune. 
 
 115. The Secondary planets are those which move round the 
 primaries, as these move round the sun. Of these, there are 
 nineteen, called also moons, pr satellites. These, as we shall 
 see, like their primaries, complete their revolutions at various 
 periods of time. 
 
 PRIMARY PLANETS. 
 
 116. The following tabular view of the primary planets ex- 
 hibits their respective diameters ; their distances from the sun; 
 the periods of their revolutions round the sun ; the periods of 
 their revolutions round their axes, where this is known ; and 
 their hourly motion through their several orbits. 
 
 Names 
 of 
 the Planets. 
 
 Diameter 
 in English 
 mites. 
 
 Distances 
 from the Sun in 
 English miles. 
 
 Revolution 
 round 
 the Sun. 
 
 Periods of rerolu- 
 lion on their own 
 axes. 
 
 Hourly 
 motion in 
 miles. 
 
 
 
 
 Days. 
 
 Dars. Hrs. M. 
 
 
 Mercury, 
 
 3,22t 
 
 37.000,000 
 
 88 
 
 15 5 
 
 110,000 
 
 Venus. 
 The Earth. 
 
 7,6S7 
 7.912 
 
 6S.OOO.OOO 
 95.000.000 
 
 it! 
 
 23 21 
 1 
 
 80.000 
 68,000 
 
 Mars, 
 
 4,189 
 
 144,000.000 
 
 687 
 
 1 39 
 
 55.000 
 
 Vesta, 
 
 238 
 
 225.000,000 
 
 1.335 
 
 ) 
 
 45.000 
 
 Ceres, 
 
 163 
 
 260, 00.000 
 
 1,631 
 
 T > Unknown. 
 
 41.000 
 
 Pallas, 
 
 80 
 
 266.000.000 
 
 1.680 
 
 s 
 
 41,000 
 
 Juno, 
 
 1.425 
 
 275,000.000 
 
 2,008 
 
 1 3 
 
 45.000 
 
 Jupiter, 
 
 89.170 
 
 490.000000 
 
 4.330* 
 
 9 56 
 
 36.000- 
 
 Saturn. 
 
 79.042 
 
 9QO.OOO;000 
 
 10.746| 
 
 10 16 
 
 22,000 
 
 Herschel, 
 
 3.3.112 
 
 l.SOO.O'O.OOO 
 
 30.fi37i 
 
 070 
 
 15,000 
 
 Neptune, 
 
 35.000 
 
 2,850,000,000 
 
 166 ys. 
 
 Unknown. 
 
 Unk'wn. 
 
 NOTE. The above table, taken from the last London (Prof. Hoblyn's) edition of 
 our Philosophy, is believed to be correct, according to the most recent observations. 
 It will be seen that in the descriptions of the planets, round numbers are generally 
 employed, as being more easily remembered also that the periodic revolutions ol 
 the planets are given in years, days and hours, instead of days only, as in the table. 
 
 117. A Year, what. A year consists of the time which it 
 takes a planet to perform one complete revolution through its 
 
 114. What are the bodies called, which revolve around the Sun as a center 1 ? 115 
 What are those planets called which revolve around these primaries as a center? 
 116. In what order are the several planets situated in respect to the Sun? How long 
 does it take each planet to make its revolution around the Sun ? 117. What is a year 1 
 
278 
 
 ASTRONOMY. 
 
 orbit, or to pass once around the Sun. Our Earth performs 
 this revolution in about 365 days, and therefore this is the 
 period of our year. Mercury completes his revolution in 88 
 days, and therefore his year is no longer than 88 of our days. 
 But the planet Herschel is situated at such a distance from the 
 Sun, that his revolution is not completed in less than about 84 
 of our years. The other planets complete their revolutions in 
 various periods of time, between these ; so that the time of these 
 periods is generally in proportion to the distance of each planet 
 from the Sun. 
 
 118. Besides the above enumerated primary planets, our sys- 
 tem contains nineteen secondary planets, or moons. Of these, 
 our Earth has one moon, Jupiter four, Saturn eight, and Her- 
 schel six. None of these moons, except our own, and one or 
 two of Saturn's, can be seen without a telescope. The seven 
 other planets, so far as has been discovered, are entirely with- 
 out moons. 
 
 119. All the planets move around the Sun from west to east, 
 and in the same direction do the moons revolve around their 
 primaries, with the exception of those of Herschel, which appear 
 to revolve in a contrary direction. 
 
 NEW PLANETS AND ASTEROIDS. 
 
 120. The following table contains the names of the new 
 planets and asteroids, with the date, place of discovery, and the 
 name of the discoverer. 
 
 Name. 
 
 When discovered. 
 
 By whom. 
 
 Where. 
 
 Uranus, . 
 
 March 13, 1781. 
 
 Herschel, 
 
 Slough. 
 
 Ceres,. . . . 
 
 Jan. 1, 1801. 
 
 Piazzi, 
 
 Palermo. 
 
 Pallas, . . . 
 
 March 28, 1802. 
 
 Gibers, 
 
 Bremen. 
 
 Juno, .... 
 
 Sept. 1, 1804. 
 
 Harding, 
 
 Lilienthral. 
 
 Vesta, . . . 
 
 March 29, 1807. 
 
 Gibers, 
 
 Bremen. 
 
 Astraea, . . . 
 
 Dec. 8, 1845. 
 
 Hencke, 
 
 Driessen. 
 
 Neptune, . 
 
 Sept. 23, 1846. 
 
 Galle, 
 
 Berlin. 
 
 Hebe,. . . . 
 
 July 1, 1847. 
 
 Hencke, 
 
 Driessen. 
 
 Iris, .... 
 
 Aug. 13, 1847. 
 
 Hind, 
 
 London. 
 
 Flora,. . . . 
 
 Oct. 18, 1847. 
 
 Hind, 
 
 London. 
 
 Metis, . . . 
 
 . April 25, 1848. 
 
 Graham, 
 
 Marknee. 
 
 Hygeia, . . . 
 
 April 12, 1849. 
 
 Gasparis, 
 
 Naples. 
 
 Parthenope, . 
 
 May 13, 1850. 
 
 Gasparis, 
 
 Naples. 
 
 Clio, .... 
 
 Sept. 13, 1850. 
 
 Hind, 
 
 London. 
 
 Egeria, . . . 
 
 Nov. 2, 1850. 
 
 Gasparis, 
 
 Naples. 
 
 Irene, .... 
 
 May 20, 1850. 
 
 Hind, 
 
 London. 
 
 New Planet, . 
 
 July 29, 1851. 
 
 Gasparis, 
 
 London. 
 
ASTRONOMY. 
 
 The preceding table is taken from the American Almanac 
 for 1852. 
 
 With the exception of Uranus, or Herschel, and Neptune, 
 these planets are called Asteroids, meaning star-like, or more 
 recently Planetoids, planet-like, on account of their diminutive 
 sizes, and in order to distinguish them from the larger planets. 
 
 121. Mr. Hind proposed Victoria, or Clio, for the name of 
 the planet which he discovered on the 13th of September, and 
 at first the name of the Queen was adopted by many foreign 
 astronomers. But it seems that the scientific world have .long 
 since refused to name planets after their discoverers, or their 
 patrons, or indeed after any mortal individual, choosing to adopt 
 for them the names of heathen deities, thus following the 
 ancient custom in this respect. 
 
 122. Number of New, or Recently Discovered Celestial 
 Bodies. In our former edition, the solar system was stated to 
 consist of the Sun, and twenty-nine bodies revolving around 
 him. At the present time, the number has increased to forty- 
 one, namely, the planet Neptune, and eleven Asteroids, the 
 names and dates of discovery of which, are contained in the 
 preceding table. It has been stated also, that an eighth satel- 
 lite of Saturn has been discovered, but of this, we have obtained 
 no certain account. 
 
 The power and perfection of new astronomical instruments, 
 will probably lead to further celestial discoveries, of which the 
 world at present can have no conception. 
 
 123. The following table contains the distances of the Aste- 
 roids, or what recently have been called the Planetoids, from the 
 Sun. 
 
 The radius of the Earth's orbit, in these computations, is as- 
 sumed to be 95,000,000 of miles. 
 
 Names. Distances from the Sun in Miles. 
 
 1. Flora, 209,160,265 
 
 2. Clio, 221,813,220 
 
 3. Vesta, 224,302,695 
 
 4. Iris 226,159,280 
 
 5. Metis, '. . 226,632,665 
 
 118. How many moons does our system contain 7 Which of the planets are at- 
 tended by mocns. and how many has each ? 119. In what direction do the planets 
 move around the Sun 7 121. What is said with respect to the names of the planets? 
 1:22. What number of revolving celestial bodies were formerly known 7 How many 
 have recently been discovered, and what are they called 1 123. In the above table 
 what is the estimated distance of the Sun 7 
 
280 ASTRONOMY. 
 
 Names. Distances from the Sun in Mile*. 
 
 6. New Planet, 227,946,800 
 
 7. Hebe, 230,449,670 
 
 8. Parthenope, .* . 232,829,135 
 
 9. Egeria, 243,206,650 
 
 10. Irene, 242,468,785 
 
 11. Astrsea, 244,819,465 
 
 12. Juno, 253,729,515 
 
 13. Ceres, 262,964,845 
 
 14. Pallas, 263,421,510 
 
 15. Hygeia, 299,255,700 
 
 Observations. The periods of the revolutions of many of the 
 recently discovered Asteroids have not been determined. We 
 have, therefore, allowed the old ones to remain in the table with 
 the Planets, as in the former edition. It will be observed that 
 there is a difference between the numbers expressing the dis- 
 tances of these Asteroids from the Sun, in the above, and in the 
 former table. In that the sums are given in the nearest round 
 numbers, while in this, the fractions are detailed. 
 
 124. ORBITS OF THE PLANETS. The paths in which the 
 planets move round the Sun, and in which the moons move 
 round their primaries, are called their orbits. These orbits are 
 not exactly circular, as they are commonly represented on pa- 
 per, but are elliptical, or oval, so that all the planets are nearer 
 the Sun, when in one part of their orbits than when in another. 
 
 In addition to their annual revolutions, some of the planets 
 are known to have diurnal, or daily revolutions, like our Earth. 
 The periods of these daily revolutions have been ascertained, in 
 several of the planets, by spots on their surfaces. But where 
 no such mark is discernible, it can not be ascertained whether 
 the planet has a daily revolution or not, though this has been 
 found to be the case in every instance where spots are seen, 
 and, therefore, there is little doubt but all have a daily as well 
 as a yearly motion. 
 
 125. The axis of a planet is an imaginary line passing through 
 its center, and about which its diurnal revolution is performed. 
 The poles of the planets are the extremities of this axis. 
 
 The orbits of Mercury and Venus are within that of the 
 
 124. What i? the orbit of a planet? What revolutions have the planets, besides 
 their yearly revolutions? Have all the planets diurnal revolutions? How is it 
 known that the njanets have daily revolutions? 125. What is the axis of a planet? 
 What is the pole of a planet? Which are the superior, and which the inferior 
 planets 1 
 
ASTRONOMY. 
 
 281 
 
 Earth, and consequently they are called inferior planets. The 
 orbits of all the other planets are without, or exterior to that of 
 the Earth, and these are called superior planets. 
 
 126. That the orbits of Mercury and Venus are within that 
 of the Earth, is evident from the circumstance that they are 
 never seen in opposition to the Sun, that is, they never appear 
 in the west when the Sun is in the east. ' On the contrary, the 
 orbits of all the other planets are proved to be outside of the 
 Earth's, since these planets are sometimes seen in opposition to 
 the Sun. 
 
 This will be understood by Fig. 232, where suppose S to be 
 the Sun, M the orbit of Mercury or Yenus, E the orbit of the 
 Earth, and J that of Jupiter. Now, it is evident, that if a 
 spectator be placed any where on the Earth's orbit, as at E, he 
 may sometimes see Jupiter in opposition to the Sun, as at J, 
 because then the spectator would be between Jupiter and the 
 Sun. But the orbit of Venus, being surrounded by that of the 
 Earth, she never can come in opposition to the Sun, or in that 
 part of the heavens opposite to him, as seen by us, because our 
 Earth never passes between her and the Sun. 
 
 FIG. 232. 
 
 E 
 EUiptical Orbit. 
 
 Orbits of the Planets. 
 
 127. Orbits Elliptical. It has already been stated, that the 
 orbits of the planets are elliptical, (124,) and that, consequently, 
 these bodies are sometimes nearer the Sun than at others. An- 
 
 126. How is it proved that the inferior planets are within the Earth's orbit, and the 
 superior ones without it? Explain Fis. 232, and show why the inferior planets 
 never can be in opposition to the Sun. 127. What are the shap'es of the planetary or 
 bits ? What is meant by perihelion 7 What by aphelion ? 
 
282 ASTRONOMY. 
 
 ellipse, or oval, has two foci, and the Sun, instead of being in 
 the common center, is always in the lower focus of their orbits. 
 
 The orbit of a planet is represented by Fig. 233, where A D 
 B E is an ellipse, with its two foci, S and 0, the Sun being in 
 the focus S, which is called the lower focus. 
 
 When the Earth, or any other planet, revolving around the 
 Sun, is in that part of its orbit nearest the Sun, as at A, it is 
 said to be in its perihelion ; and when in that which is at the 
 greatest distance from the Sun, as at B, it is said to be in its 
 aphelion. The line S D, is the mean, or average distance of a 
 planet's orbit from the Sun. 
 
 128. ECLIPTIC. The planes of the orbits of all the planets 
 pass through the center of the Sun. The plane of an orbit is 
 an imaginary surface, passing from one extremity, or side of the 
 orbit, to the other. If the rim of a drum head be considered the 
 orbit, its plane would be the parchment extended across it, on 
 which the drum is beaten. 
 
 Let us suppose the Earth's orbit to be such a plane, cutting 
 the Sun through his center, and extending out on every side to 
 the starry heavens ; the great circle so made, would mark the 
 line of the ecliptic, or the Sun's apparent path through the 
 heavens. 
 
 The circle is called the Sun's apparent path, because the rev- 
 olution of the Earth gives the Sun the appearance of passing 
 through it. It is called the ecliptic, because eclipses happen 
 when the Moon is in, or near, this apparent path. 
 
 129. ZODIAC. The Zodiac is an imaginary belt, or broad 
 circle, extending quite around the heavens. The ecliptic divides 
 the zodiac into two equal parts, the zodiac extending 8 degrees 
 on each side of the ecliptic, and therefore is 16 degrees wide. 
 The zodiac is divided into 12 equal parts, called the signs of the 
 zodiac. 
 
 130. The sun appears every year to pass around the ^great 
 circle of the ecliptic, and consequently, through the 12 constel- 
 lations, or signs of the zodiac. But it will be seen, in another 
 place, that the Sun, in respect to the Earth, stands still, and 
 that his apparent yearly course through the heavens is caused 
 by the annual revolution of the Earth around its orbit. 
 
 To understand the cause of this deception, let us suppose that 
 
 128. What isthe plane of an orbit 7 Explnin what is meant by the ecliptic. Why 
 is the ecliptic called the Sun's apparent path 7 129. What is the zodiac ? How doea 
 the ecliptic divide the zodiac 7 How far does the zodiac extend on each side of the 
 ecliptic 1 130. Explain Fig. 234, and show why the Sun seems to pass through the 
 ecliptic, when the Earth only revolves around the Sun 1 
 
ASTRONOMY. 
 
 283 
 
 S, Fig. 234, is the Sun, A B, a part no. 234. 
 
 of the circle of the ecliptic, and C 
 D, a part of the Earth's orbit. Now 
 if a spectator be placed at C, he will 
 see the Sun in that part of the eclip- 
 tic marked by B, but when the 
 Earth moves in her annual revolu- 
 tion to D, the spectator will see the 
 Sun in that part of the heavens 
 marked by A ; so that the motion 
 of the Earth in one direction, will 
 give the Sun an apparent motion in 
 the contrary direction. 
 
 131. CONSTELLATIONS. A sign 
 or constellation, is a collection of 
 fixed stars, and as we have already 
 seen, the Sun appears to move 
 
 through the twelve signs of the zodiac every year. Now, the 
 Sun's place in the heavens, or zodiac, is found by his apparent 
 conjunction, or nearness to any particular star in the constella- 
 tion. Suppose a spectator at C, Fig. 234, observes the Sun to 
 be nearly in a line with the star at B, then the Sun would be 
 near a particular star in a certain constellation. When the 
 Earth moves to D, the Sun's place would assume another direc- 
 tion, and he would seem to have moved into another constellation, 
 and near the star A. 
 
 132. Each of the 12 signs of the zodiac is divided into 30 
 smaller parts, called degrees ; each degree into 60 equal parts, 
 called minutes, and each minute into 60 parts, called seconds. 
 
 The division of the zodiac into signs, is of very ancient date, 
 each sign having also received the name of some animal, or 
 tiling, which the constellation, forming that sign, was supposed 
 to resemble. It is hardly necessary to say, that this is chiefly 
 the result of imagination, since the figures made by the places 
 of the stars, never mark the outlines of the figures of animals, 
 or other things. This is, however, found to be the most con- 
 venient method of finding any particular star at this day, for 
 among astronomers, any star, in each constellation, may be de- 
 signated by describing the part of the animal in which it is 
 
 131. What is a constellation, or sign? How is the Sun's apparent place in the 
 heavens found ? 132. Into how many parts are the signs of the zodiac divided, and 
 what are these parts called? Is there any resemblance between the places of the 
 Ftary. and the figures of the animals after which they are called ? Explain why this 
 is a convenient method of finding any particular star in a sign. 
 
284 
 
 ASTRONOMY. 
 
 situated. Thus, by knowing how many stars belong to the con- 
 stellation Leo, or the Lion, we readily know what star is meant 
 by that which is situated on the Lion's ear or tail. 
 
 133. Names of the Signs. The names of the 12 signs of 
 the zodiac are, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, 
 Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. The 
 common names, or meaning of these words, in the same order, 
 are, the Ram, the Bull, the Twins, the Crab, the Lion, the Vir- 
 gin, the Scales, the Scorpion, the Archer, the Goat, the Waterer, 
 and the Fishes. 
 
 FIG. 235. 
 
 Signs of the Zodiac. 
 
 134. The twelve signs of the zodiac, together with the Sun, 
 and the earth revolving around him, are represented at Fig. 
 235. When the Earth is at A, the Sun will appear to be just 
 
 133. What are the names of the twelve signs 7 134. Explain why the Sun will be 
 in the beginning of Aries when the Earth is at A, Fig. 235. 
 
ASTRONOMY. 285 
 
 entering the sign Aries, because then, when seen from the Earth, 
 he ranges toward certain stars at the beginning of that constel- 
 lation. When the Earth is at C, the Sun will appear in the 
 opposite part of the heavens, and therefore in the beginning of 
 Libra. The middle line, dividing the circle of the zodiac into 
 equal parts, is the line of the ecliptic. 
 
 135. DENSITY OF THE PLANETS. Astronomers have no 
 means of ascertaining whether the planets are composed of the 
 same kind of matter as our Earth, or whether their surfaces are 
 clothed with vegetables and forests, or not. They have, how- 
 ever, been able to ascertain the densities of several of them, by 
 observations on their mutual attraction. By density, is meant 
 compactness, or the quantity of matter in a given space. (72.) 
 When two bodies are of equal bulk, that which weighs most, 
 has the greatest density. It was shown, while treating of the 
 properties of bodies, that substances attract each other in pro- 
 portion to the quantities of matter they contain. (132.) If, 
 therefore, we know the dimensions of several bodies, and can 
 ascertain the proportion in which they attract each other, their 
 quantities of matter, or densities, are easily found. 
 
 136. Thus, when the planets pass each other in their circuits 
 through the heavens, they are often drawn a little out of the 
 lines of their orbits by mutual attraction. As bodies attract in 
 proportion to their quantities of matter, it is obvious that the 
 small planets, if of the same density, will suffer greater disturb- 
 ance from this cause, than the large ones. But suppose two 
 planets, of the same dimensions, pass each other, and it is found 
 that one of them is attracted twice as far out of its orbit as the 
 other, then, by the known laws of gravity, it would be inferred, 
 that one of them contained twice the quantity of matter that 
 the other did, and therefore that the density of the one was 
 twice that of the other. 
 
 By calculations of this kind, it has been found, that the 
 density of the Sun is but a little greater than that of water, while 
 Mercury is more than nine times as dense as water, having a 
 specific gravity nearly equal to that of lead. The Earth has 
 a density about five times greater than that of the Sun, and a 
 little less than half that of Mercury. The densities of the othei 
 planets seem to diminish in proportion as their distances from 
 
 135. How has the density of the planets been ascertained ? What is meant by dens 
 ty ? In what proportion do bodies attract each other 1 136. How are the densities 
 of the planets ascertained? What is the density of the Sun, of Mercury, and of the 
 Earth ? la what preportions do the densities of the planets appear to diminish 7 
 
286 THE SUN. 
 
 the Sun increase, the density of Saturn, one of the most remote 
 of planets, being only about one-third that of water. 
 
 137. The Sun is the center of the solar system, and the grea.t 
 dispenser of heat and light to all the planets. Around the Sun 
 all the planets revolvers around a common center, he being the 
 largest body in our system, and, so far as we know, the largest 
 in the universe. 
 
 Distance of the Sun. The distance of the Sun from the 
 Earth is 95 millions of miles, and his diameter is estimated at 
 887,000 miles. Our globe, when compared with the magnitude 
 of the Sun, is a mere point, for his bulk is about thirteen hun- 
 dred thousand times greater than that of the Earth. Were the 
 Sun's center placed in the center of the Moon's orbit, his cir- 
 cumference would reach two hundred thousand miles beyond 
 her orbit in every direction, thus filling the whole space be- 
 tween us and the moon, and extending nearly as far beyond 
 her as she is from us. A traveler, who should go at the rate 
 of 90 miles a day, would perform a journey of nearly 33,000 
 miles in a year, and yet it would take such a traveler more 
 than 80 years to go round the circumference of the Sun. A 
 body of such mighty dimensions, hanging on nothing, it is cer- 
 tain, must have emanated from an Almighty power. 
 
 The Sun appeal's to move around the Earth every 24 hours, 
 rising in the east, and setting in the west. This motion, as will 
 be proved in another place, is only apparent, and arises from 
 the diurnal revolution of the Earth. 
 
 138. Diurnal Revolution of the Sun. The Sun, although 
 he does not, like the planets, revolve in an orbit, is, however, 
 not without motion, having a revolution around his own axis, 
 once in 25 days and 10 hours. Both the fact that he has such 
 a motion, and the time in which it is performed, have been as- 
 certained by the spots on his surface. If a spot is seen, on a 
 revolving body, in a certain direction, it is obvious, that when 
 the same spot is again seen, in the same direction, that the body 
 has made one revolution. By such spots the diurnal revolutions 
 of the planets, as well as the Sun, have been determined. 
 
 137. Where is the place of the Sun in the solar system 7 What is the distance of th 
 Sun from the Earth / What is the diameter of the Sun ? Suppo.se the center of the 
 Sun and that of the Moon's orbit to be coincident, how far would the Sun extend le- 
 yond the Moon's orbit ? 138. How is it proved that the Sun has a motion around hi* 
 own axis? How often does the Sun revolve 7 
 
MERCURY. 287 
 
 139. SPOTS ON THE SUN. Spots on the Sun, seem first to 
 have been observed in the year 1611, since which time they 
 have constantly attracted attention, and have been the subject 
 of investigation among astronomers. These spots change their 
 appearance as the Sun revolves on his axis, and become greater 
 or less^ to an observer on the Earth, as they are turned to, or 
 from him ; they also change in respect to real magnitude and 
 numl)er ; one spot, seen by Dr. Herschel, was estimated to t>e 
 more than six times the size of our Earth, being 50,000 miles in 
 diameter. Sometimes forty or fifty spots may be seen at the 
 same time, and sometimes only one. They are often so large 
 as to be seen with the naked eye ; this was the case in 1816. 
 
 140. Nature and Design of these Spots. In respect to the 
 nature and design of these spots, almost every astronomer has 
 formed a different theory. Some have supposed them to be 
 solid opaque masses of scoriae, floating in the liquid fire of the 
 Sun ; others, as satellites, revolving round him, and hiding his 
 light from us ; others, as immense masses, which have fallen on 
 his disc, -and which are dark-colored, because they have not yet 
 become sufficiently heated. From these various theories we 
 may infer that, at present, nothing certain is known of the na- 
 ture and design of these spots. 
 
 141. Mercury, the planet nearest the Sun, is about 3,000 
 miles in diameter, and revolves around him at the distance of 
 37 millions of miles. The period of his annual revolution is 88 
 days, and he turns on his axis once in about 15 hours. 
 
 No signs of an atmosphere have been observed in this planet. 
 The Sun's heat at Mercury is about seven times greater than it 
 is on the Earth, so that water, if nature follows the same laws 
 there that she does here, can not exist at Mercury, except in the 
 state of steam. 
 
 The nearness of this planet to the Sun, prevents his being 
 often seen. He may, however, sometimes be observed just be- 
 fore the rising, and a little after the setting of the Sun. When 
 seen after sunset, he appears a brilliant, twinkling star, showing 
 
 139. When were the spots on the Sun first observed 1 What has been the differ- 
 ence in the number of spots observed ? What was the size of the spots seen by Dr. 
 Herschel? 110. What lias been advanced concerning the nature of these spotsl 
 H;ive they been accounted for satisfactorily 1 141. What is the diameter of Mercury, 
 and what are his periods of annual and diurnal revolution ? How great is the Sun's 
 heat at Mercury? At what times is Mercury to be seen? What is a transit of 
 Mercury 1 
 
288 VENUS. 
 
 a white light, which, however, is much obscured by the glare 
 of twilight. When seen in the morning, before the rising of 
 the Sun, his light is also obscured by the Sun's rays. 
 
 Mercury sometimes crosses the disc of the Sun, or comes be- 
 tween the Earth and that luminary, so as to appear like a small 
 dark spot passing over the Sun's face. This is called the transit 
 of Mercury. 
 
 142. Venus is the other planet, whose orbit is within that of 
 the Earth. Her diameter is about 8,000 miles, being somewhat 
 larger than the Earth. 
 
 Her revolution around the Sun is performed in 224 days, at 
 the distance of 68 millions of miles from him. She turns on 
 her axis once in 23 hours, so that her day is a little shorter 
 than ours. Her hourly motion in her orbit, is 80,000 miles. 
 
 Venus, as seen from the Earth, is the most brilliant of all the 
 primary planets, and is better known than any nocturnal lumin- 
 arv except the Moon. When seen through a telescope, she ex- 
 hibits the phases or horned appearance of the moon, and her 
 face is sometimes variegated with dark spots. 
 
 143. This planet may often be seen in the day time, even 
 when she is in the vicinity of the blazing light of the Sun. A 
 luminous appearance around this planet, seen at certain times, 
 proves that she has an atmosphere. Some of her mountains 
 are several times more elevated than, any on our globe, being 
 from 10 to 22 miles high. 
 
 144. Venus sometimes makes a transit across the Sun's disc, 
 in the same manner as Mercury, already described. The transits 
 of Venus occur only at distant periods from each other. The 
 last transit was in 1769, and the next will not happen until 
 1874. These transits have been observed by astronomers with 
 the greatest care and accuracy, since it is by observations on 
 them that the true distances of the Earth and planets from the 
 Sun are determined. 
 
 145. Motions and Phenomena of Venus. Sometimes Venus 
 appears to recede from the Sun, and then approach him, and 
 as her orbit is within that of the earth, her distance from us 
 
 142. Where is the orbit of Venus, in respect to that of the Earth? What is the 
 time of Venus' revolution round the Sun? How often does she turn on her axis? 
 143. What is said of the height of the mountains in Venus? 144. On what account 
 are the transits of Venus observed with great care ? 145. What is the least and great- 
 est distance of Venus from the Earth ? When is she in her inferior, and when in her 
 tUferior conjunction with the Sun ? Why is she invisible in those two positions 7 
 
V EXITS. 
 
 289 
 
 FIG. 236. 
 
 varies from 27,000.000 to 163,000,000 of miles. When near- 
 est the earth she forms her inferior conjunction with the Sun ; 
 that is, she is between us and him, and hence being overpow- 
 ered with his light, is invisible. When at the greatest distance 
 from us, she forms her superior conjunction with that luminary, 
 and for the same reason again becomes invisible to us. 
 
 These phenomena will 
 be understood by the fol- 
 io wing explanations in con- 
 nection with Fig. 236, 
 which we quote from Dr. 
 Dick. 
 
 146. Let the earth be 
 supposed at K, then when 
 Venus is in the position 
 marked A, it is in a line 
 with the Sun as seen from 
 the earth, and is then in 
 its superior conjunction, 
 being in the remotest part 
 of its orbit. When in this 
 position, the whole of its 
 enlightened hemisphere is 
 toward the earth, but is invisible on account of the Sun's light. 
 As it moves from A to B, being from west to east, which is 
 called its direct motion, it begins to appear after sunset as the 
 evening star. When at B, it appears among the stars at L, 
 when it appears in a gibbous shape, nearly half its disc being ( 
 luminous. When at C, it appears among the stars at M, nearly' 
 in the form of a half moon. At D, being at the point of its 
 greatest elongation, it has the form of a half moon, and is seen 
 among the stars at N. It now appears, for some time to be 
 stationary, because moving nearly in a straight line toward the 
 earth, its motion is not seen ; when it again appears to move 
 rapidly, but in a contrary direction from before, or, from east 
 to west, during which it presents, the form of a crescent. It 
 now gradually becomes so overpowered by the Sun's rays as 
 again to be invisible to the naked eye, and when arrived at E, 
 forms her inferior conjunction with the Sun, and her nearest 
 approach to the Earth. . 
 
 147. In this position, Venus is 36,000,000 miles nearer the 
 
 Phases of Venus. 
 
 146. Describe by means of Fig. 236, the phases of Venus, from her superior lo her 
 inferior conjunction with the Sun. 
 
 13 
 
290 THE EARTH. 
 
 Earth than when in her superior conjunction, and hence the 
 great difference in her apparent size, and the luster with which 
 she shines upon us. When near her superior conjunction, 
 almost her entire disc is enlightened to us, and yet she appears 
 like a faint star when compared with her luster when near her 
 inferior conjunction, and when only her small crescent is turned 
 toward us. 
 
 Having passed her inferior conjunction, her light becomes 
 less and less until she again becomes invisible, as she again ap- 
 proaches her superior conjunction, as before. 
 
 148. When Venus is in that part of her orbit which gives 
 her the appearance of being west of the Sun, she rises before 
 him, and is then called the morning star ; and when she ap- 
 pears east of the Sun, she is behind him in her course, and is 
 then called the evening star. These periods do not agree, either 
 with the yearly revolution of the Earth, or of Venus, for she is 
 alternately 290 days the morning star, and 290 days the even- 
 ing star. The reason of this is, that the Earth and Venus 
 move round the Sun in the same direction, and hence her rela- 
 tive motion, in respect to the Earth, is much slower than her 
 absolute motion in her orbit. If the Earth had no yearly mo- 
 tion, Venus would be the morning star one half of the year, 
 and the evening star the other half. 
 
 THE EARTH. 
 
 149. The next planet in our system, nearest the Sun, is the 
 t Earth. Her diameter is 8,000 miles. This planet revolves 
 around him in 365 days, 5 hours, and 48 minutes ; and at the 
 distance of 95 millions of miles. It turns round its own axis 
 once in 24 hours, making a day and a night. The Earth's rev^ 
 olution around the Sun is called its annual or yearly motion, 
 because it is performed in a year ; while the revolution around 
 its own axis, is called the diurnal or daily motion, because it 
 takes place every day. The earth's motion in her orbit is at 
 the rate of 68,000 miles per hour. The figure of the Earth, 
 with the phenomena connected with her motion, will be ex- 
 plained in another place. 
 
 147. Why is the light of Verms to us so much less at some times than at others? 
 How much nearer the e^rth is this planet at h^r inferior than at her superior con- 
 junction ? 148. When is Venus the morning, and when the evening star? How long 
 is Venus the morning and how long the evening star? 149. How long does it take 
 the Earth to revolve round the Sun ? What is meant by the Earth's annual revolu- 
 tion, and what by her diurnal revolution ? 
 
MARS. 291 
 
 THE MOON. 
 
 150. The Moon, next to the Sun, is, to us, the most brilliant 
 and interesting of all the celestial bodies. Being the nearest to 
 us of any of the heavenly orbs, and apparently designed for our 
 use, she has been observed with great attention, and many of 
 the phenomena which she presents, are therefore better under- 
 stood and explained, than those of the other planets. 
 
 While the Earth revolves round the sun in a year, it is at- 
 tended by the Moon, which makes a revolution round the Earth 
 once in 27 days, 7 hours, and 43 minutes. The distance of 
 the Moon from the Earth is 240,000 miles, and her diameter 
 about 2,000 miles. 
 
 Her surface, when seen through a telescope, appears diversi- 
 fied with hills, mountains, valleys, rocks, and plains, presenting 
 a most interesting and curious aspect : but the explanation of 
 these phenomena are reserved for another section. 
 
 151. The next planet in the solar system, is Mars, his orbit 
 surrounding that of the Earth. The diameter of this planet is 
 upward of 4,000 miles, being about half that of the Earth. 
 The revolution of Mars around the Sun is performed in nearly 
 687 days, or in somewhat less than two of our years, and he 
 turns on his axis once in 24 hours and 40 minutes. His mean 
 distance from the Sun is 144,000,000 of miles, so that he moves 
 in his orbit at the rate of about 55,000 miles in an hour. The 
 days and nights at this planet, and the different seasons of the 
 y^ar, bear a considerable resemblance to those of the Earth. 
 The density of Mars is less than that of the Earth, being only 
 three times that of water. 
 
 152. Telescopic View of Mars. This planet, to the naked 
 eye, reflects a yellowish, or dull red light, by which he may be 
 distinguished from all the others. His telescopic appearance is 
 quite peculiar, and often interesting, on account of the changes 
 his face presents, being sometimes spotted, then striped, then 
 clouded, and so on ; and sometimes all these figures appear at 
 
 150. Why are the phenomena of the Moon better explained than those of the other 
 planets? in what time is H revolution of the Moon about the Earth performed? 
 What is the distance of the Moon from the Earth? 151. What is the d : amf-ter of 
 Mars i How much lo -.^cr is a year at Mars than our year? What is his= rate of mo- 
 tion in his orbit \ 152. What is his appearance through the telescope? How is u 
 proved that Mars has an atmosphere of great density ? Why does Mars sometimes 
 appear to us larger than at others 1 How great is the Sun's heat at Mars 1 
 
292 JUPITER. 
 
 the same time, presenting a great variety of aspects, some of 
 which are represented by Fig. 237. It is difficult to account 
 for these appearances, though they are attributed to dense vapor 
 in the atmosphere of the planet. 
 
 FIG. 237. 
 
 Telescopic Phases of Mars. 
 
 Mars has an atmosphere of great density and extent, as i 
 proved by the dim appearance of the fixed stars, when seen 
 through it. When any of the stars are seen nearly in a line 
 with this planet, they give a faint, obscure light, and the nearer 
 they approach the line of his disc, the fainter is their light, until 
 the star is entirely obscured from the sight. 
 
 This planet sometimes appears much larger to us than at 
 Dthers, and this is readily accounted for by his greater or less 
 distance. At his nearest approach to the Earth, his distance is 
 only 50 millions of miles, while his greatest distance is 240 
 millions of miles; making a difference in his distance of 190 
 millions of miles, or the diameter of the Earth's orbit. 
 
 The Sun's heat at this planet is less than half that which we 
 enjoy. 
 
 To the inhabitants of Mars, our planet appears alternately as 
 the morning and evening star, as Venus does to us. 
 
 JUPITER. 
 
 153. Jupiter is 89,000 miles in diameter, and performs his 
 annual revolution once in about 11 of our years, at the distance 
 of 490 millions of miles from the Sun. This is the largest 
 planet in the solar system, being about 1,400 times larger than 
 
 153. What is the diameter of Jupiter 1 What is his distance from the Sun ? What 
 is the period of Jupiter's diurnal revolution ? What is the Sun's heat and light at 
 Jupiter, when compared with that of the Earth ? For what is Jupiter particularly 
 distinguished 1 Is the appearance of Jupiter's belts always the same, or do they 
 change ? What is said of the cause of Jupiter's belted appearance? 
 
JUPITER. 293 
 
 the Earth. His diurnal revolution is performed in nine hours 
 and titty -six minutes, giving his surface, at the equator, a mo- 
 tion of 28,000 miles per hour. This motion is about twenty 
 times more rapid than that of our Earth at the equator. 
 
 Jupiter, next to Venus, is the most brilliant of the planets, 
 though the light and heat of the Sun on him is nearly 25 times 
 less than on the earth. 
 
 This planet is distinguished from all the others, by an ap ^ 
 pearance resembling bands, which extend across his disc. These/ 
 are termed belts, and are variable, both in respect to number* 
 and appearance. Sometimes seven or eight are seen, several of 
 which extend quite across his face, while others appear broken, 
 or interrupted. 
 
 FIG. 238. 
 
 Belts of Jupiter. 
 
 These bands, or belts, when the planet is observed through a 
 telescope, appear as represented in Fig. 238. This appearance 
 is much the most common, the belts running quite across the 
 face of the planet in parallel lines. Sometimes, however, his 
 aspect is quite different from this, for in 1780, Dr. Herscjiel 
 saw the whole disc of Jupiter covered with small curved lines, 
 each of which appeared broken, or interrupted, the whole hav- 
 ing a parallel direction across his disc, as in Fig. 239. 
 
 154. Jupiter has four satellites, or moons, two of which are 
 sometimes seen with the naked eye. They move round, and 
 attend him in his yearly revolution, as the Moon does our Earth. 
 They complete their revolutions at different periods, the shortest 
 of which is less than two clays, and the longest seventeen days. 
 
 154. How many moons has Jupiter, and what are the periods of their revolutions? 
 
294 
 
 Occasional Views of Jupiter. 
 
 155. Eclipses of Jupiter* s Moons. These satellites often fall 
 into the shadow of their primary, in consequence of which they 
 are eclipsed, as seen from the Earth. The eclipses of Jupiter's 
 moons have been observed with great care by astronomers, be- 
 cause they have been the means of determining the exact longi- 
 tude of places, and the velocity with which light moves through 
 space. How longitude is determined by these eclipses, can not 
 be explained or understood at this place, but the method by 
 which they become the means of ascertaining the velocity of 
 light, may be readily comprehended. An eclipse of one of these 
 satellites appears, by calculation, to take place sixteen minutes 
 sooner, when the Earth is in that part of her orbit nearest to 
 Jupiter, than it does when the Earth is in that part of her orbit 
 at the greatest distance from him. Hence, light is found to be 
 sixteen minutes in crossing the Earth's orbit, and as the Sun is 
 in the center of this orbit, or nearly so, it must take about eight 
 minutes for the light to come from him to us. Light, there- 
 fore, passes at the velocity of 95 millions of miles, our distance 
 from the Sun, in about eight minutes, which is nearly 200,000 
 miles in a second." 
 
 156. The planet Saturn revolves round the Sun in a period 
 of about 30 of our years, and at the distance from him of 900 
 
 155. What occasions the eclipses of Jupiter's moons? Of what uses are these 
 eclipses to astronomers 1 How is the velocity of light ascertained by the eclipses of 
 Jupiter's satellites? 156. What is the time of 'Saturn's periodic revolution round the 
 Sun ? What is his distance from the Sun 1 What his diameter .' What is the pe- 
 riod of his diurnal revolution'? How many days make a year at Saturn? How 
 many moons has Saturn 1 
 
SATURN. 295 
 
 millions of miles. His diameter fs 79,000 miles, making his 
 bulk nearly nine hundred times greater than that of the Earth, 
 but notwithstanding this vast size, he revolves on his axis once 
 in about ten hours. Saturn, therefore, performs upward of 
 25,000 diurnal revolutions in one of his years, and hence his 
 year consists of more than 25,000 days ; a period of time equal 
 to more than 10,000 of our days. On account of the remote 
 distance of Saturn from the Sun, he receives only about a 90th 
 part of the heat and light which we enjoy on the Earth. But 
 to compensate, in some degree, for this vast distance from the 
 Sun^ Saturn has seven moons, which revolve round him at dif- 
 ferent distances, and at various periods, from 1 to 80 days. 
 
 157. Rings of Saturn. Saturn is distinguished from the 
 other planets by his ring, as Jupiter is by his belt. When this 
 planet is viewed 
 
 through a tele- FIG 
 
 scope, he ap- 
 pears surround- 
 ed by an im- 
 mense luminous 
 circle, which is 
 represented by 
 Fig. 240. 
 
 There are in- 
 deed IwO lumill- Saturn and his Ring . 
 
 ous circles, or 
 
 rings, one within the other, with a dark space bei ween them, so 
 that they do not appear to touch each other. N< ither does the 
 inner ring touch the body of the planet, there bei ng, by estima- 
 tion, about the distance of thirty thousand miles between them. 
 
 The external circumference of the outer ring is 630,000 miles, 
 and its breadth from the outer to the inner circumference, 7,200 
 miles, or nearly the diameter of our Earth. The dark space, 
 between the two rings, or the interval between the inner and 
 the outer ring, is 2,800 miles. 
 
 A third ring, interior to those heretofore known, was discov- 
 ered in 1851, by Mr. Bond, of Cambridge, Mass. 
 
 This immense appendage revolves round the Sun with the 
 planet, performs daily revolutions with it, and, according to 
 
 157. How is Saturn particularly distinguished from all the other planets? What 
 distance is there between the body of Saturn and his inner rin?? What distance is 
 there between his inner and outer ring 7 What is the circumference of the outer 
 
 ring"? 
 
206 
 
 IIERSCHEL. 
 
 FIG. 241. 
 
 Dr. Ilerschel, is a solid substance, equal in density to the body 
 of the planet itself. 
 
 The design of Saturn's ring, an appendage so vast, and so 
 different from any thing presented by the other planets, has 
 always been a matter of speculation and inquiry among astron- 
 omers. One of its most obvious uses appears to be that of re- 
 flecting the light of the Sun on the body of the planet, and 
 possibly it may reflect the heat also, so as in some degree to 
 soften the rigor of so inhospitable a climate. 
 
 158. As this planet revolves around the Sun, one of its sides 
 is illuminated during one half of the year, and the other side 
 during the other half; so that, as Saturn's year is equal to 
 thirty of our years, one of his sides will be enlightened and 
 darkened, alternately, every fifteen years, as the poles of our 
 Earth are alternately in thg light and dark every year. 
 
 Fig. 241 represents 
 Saturn as seen by an 
 eye, placed at right-an- 
 gles to the plane of his 
 ring. When seen from 
 the Earth, his position is 
 always oblique, as repre- 
 sented by Fig. 240. 
 
 The inner white circle 
 represents the body of 
 the planet, enlightened 
 by the Sun. The dark 
 circle next to this, is the 
 unenlightened space be- 
 tween the body of the 
 planet and the inner 
 ring, being the dark ex- 
 panse of the heavens be- 
 yond the planet. The two white circles are the rings of the 
 planet, with the dark space between them, which also is the 
 dark expanse of the heavens. 
 
 The eighth satellite of this planet, was discovered by Mr. 
 Bond, the discoverer of this third ring, as above stated. 
 
 HERSCHEL. 
 
 159. In consequence of some inequalities in the motions of 
 
 Direct View of Saturn. 
 
 158 How long is one of Saturn's sides alternately in the light and darkl In what 
 position is Saturn represented by Fig. 210 ) 
 
. NEPTUNJE. 297 
 
 Jupiter and Saturn, in their orbits, several astronomers had sus- 
 pected that there existed another planet beyond the orbit of 
 Saturn, by whose attractive influence these irregularities were 
 produced. This conjecture was confirmed by Dr. Herschel, in 
 1781, who in that year discovered the planet, which is now 
 generally known by the name of its discoverer, though called 
 by him Georgium Sidus. The orbit of Herschel is beyond 
 that of Saturn, and at the distance of 1,800 millions of miles 
 from the Sun. To the naked eye, this planet appears like a 
 star of the sixth magnitude, being, with the exception of some 
 of the comets and Neptune, the most remote body, so far ra 
 known in the solar system. 
 
 160. Herschel completes his revolution round the Sun in 
 nearly 84 of our years, moving in his orbit at the rate of 15,000 
 miles in an hour. His diameter is 35,000 miles, so that his 
 bulk is about eighty times that of the Earth. The light and 
 heat of fhe Sun at Herschel, is about 360 times less than it is 
 at the Earth, and yet it has been found, by calculation, that 
 this light is equal to 248 of our full Moons ; a striking proof of 
 the inconceivable quantity of light emitted by the Sun. 
 
 This planet has six satellites, which revolve round him at 
 various distances, and in different times. The periods of some 
 of these have been ascertained, while those of the others remain 
 unknown. 
 
 NEPTUNE. 
 
 161. The discovery of this planet is a signal instance of the 
 power of mathematical calculations, applied to the motions of 
 the celestial bodies. 
 
 Astronomers, for more than half a century, had observed 
 from various parts of the world, certain secular perturbations in 
 several of the most remote members of the solar system, espe- 
 cially in Saturn and Uranus. 
 
 These irregular motions, due to the law of attraction, could 
 not be explained by the influence of any known body, circula- 
 ting around the Sun, and hence the inference that there existed 
 in that region, another planet not yet seen by mortal eyes. 
 
 162. Two young astronomers, Adams, an Englishman, and 
 
 159. What circumstance led to the discovery of Herschel ? In what year, and by 
 whom was Herschel discovered ? What is the distance of Herscnel from the Sunl 
 ^160. In what period is his revolution round the Sun performed ? What is the diam- 
 *eter of Herschel 1 What is the quantity of light and heat at Herschel. when com 
 p.-ir^d \v : !h that of the Earth 1 162. Hv whom, and in what manner was Neptune 
 dsruvr-.l? jg* 
 
298 
 
 NEPTUNE. 
 
 Leverrier, a citizen of France, unknown to each other, pursuing 
 this suggestion, both demonstrated, not only the existence of 
 this undiscovered body, but showed within a few degrees the 
 point in the heavens where it would be found, and where in 
 truth the discovery was made. Dr. Galle, of Berlin, sweeping 
 the heavens with his telescope, according to the directions of 
 these demonstrators, first saw the planet now called Neptune, on 
 the 26th of September, 1846. Still Leverrier and Adams, by 
 the common consent of astronomers and the scientific world, 
 must have the honor of this discovery, "so that the discovery 
 of Neptune, has happily crowned two heads with laurels." 
 
 FIG. 242. 
 
 Herschel 
 
 Relative Distance of the Planets. 
 
 163. This planet was first called Leverrier, but it seems that 
 astronomers have long since determined that new ones shall 
 
 163. What is said about calling new planets after their discoverers? 
 
NIPTUNE 299 
 
 not receive the names of their discoverers, but of some heathen 
 divinity, and hence Herschel, the name of the discoverer, has 
 been changed to Uranus, and Leverrier into Neptune. 
 
 164. Distance of Neptune. It is stated in the table of the 
 planets, that the distance of Neptune from the Sun, is 2,850 
 millions of millions of miles. On this subject, a curious calcu- 
 lator says, " Had Adam and Eve started on a railway, to go 
 from Neptune to the Sun, at the rate of fifty miles an hour, 
 they would not yet have arrived there, for this planet, at the 
 above rate, is more than 6000 years from the center of our system." 
 
 And yet this orb was discovered by the science of man. 
 
 RELATIVE SITUATIONS OF THE PLANETS. Having now given 
 a short account of each planet composing the solar system, the 
 relative situations of their several orbits, with the exception of 
 those of the Asteroids, are shown by Fig. 242. 
 
 In this figure, the orbits are marked by the signs of each 
 planet, of which the first, or that nearest the Sun, is Mercury, 
 the next Venus, the third the Earth, the fourth Mars ; then 
 come those of the Asteroids, then Jupiter, then Saturn, and 
 lastly Herschel. 
 
 FIG. 243. 
 
 Relative Sizes of the Planets. 
 
 COMPARATIVE DIMENSIONS OF THE PLANETS. The compara- 
 tive dimensions of the planets are delineated at Fig. 243. 
 
 164. How long would it take to go from our system to Neptune, at the rate of fifty 
 miles an hour 7 
 
300 MOTIONS OF THE PLANETS. 
 
 MOTIONS OF THE PLANETS. 
 
 It is said, that when Sir Isaac Newton was near demonstra- 
 ting the great truth, that gravity is the cause which keeps the 
 heavenly bodies in. their orbits, he became so agitated with the 
 thoughts of the magnitude and consequences of this discovery, 
 as to be unable to proceed with his demonstrations, and desired 
 a friend to finish what the intensity of his feelings would not 
 allow him to complete. 
 
 We have seen, in a former part of this work, (183,) that all 
 undisturbed motion is straight forward, and that a body pro- 
 jected into open space, would continue, perpetually, to move in 
 a right line, unless retarded or drawn out of this course by some 
 external cause. 
 
 To account for the motions of the planets in their orbits, we 
 will suppose that the Earth, at the time of its creation, was 
 thrown by the hand of the Creator into open space, the Sun 
 having been before created and fixed in his present place. 
 
 165. Circular Motion of the Planets. Under Compound 
 Motion, (190,) it has been shown, that when a body is acted on 
 by two forces perpendicular to each other, its motion will be in 
 a diagonal between the direction of the two forces. 
 
 But we will again here sup- 
 pose that a ball is moving in the FIG ' 244 * 
 line M X, Fig. 244, with a given 
 force, and that another force half 
 as great should strike it in the 
 
 direction of N, the ball would 
 then describe the diagonal of a 
 
 -M 
 
 parallelogram, whose length- 
 would be just equal to twice its Diagonal Motion. 
 
 bread tli, and the line of the ball 
 
 would be straight, because it would obey the impulse and direc- 
 tion of these two forces only. 
 
 Now let A. Fig. 245, represent the Earth, and S the Sun ; 
 and suppose the Earth to be moving forward, in the line from 
 A to B, and to have arrived at A, with a velocity sufficient, in 
 a given time, and without olisturbance, to have carried it to B. 
 But at the point A, the Sun, S, acts upon the Earth with his 
 attractive power, and with a force which would draw it to C, 
 
 165. Suppose a body to hp acted on by two forces perpendicular to each other, in 
 what direction will it move? Why does the ball. Fig. 244. move in a straight line? 
 Why does the Earth. Fig 245, move in a curved line? Explain Fig. 245. and show 
 now the two forces act to produce a circular line of motion 1 
 
MOTIONS OF THE PLANETS. 
 
 301 
 
 Circular Motion of the Planets. 
 
 in the same space of time 
 that it would otherwise 
 have gone to 13. Then the 
 Earth, instead of passing to 
 B, in a straight line,, would 
 be drawn down to D, the 
 diagonal of the parallelo- 
 gram, AB DC. The line 
 of direction, in Fig. 244, 
 is straight, because the 
 body moved, obeys only 
 the direction of the two 
 forces, but it is curved from 
 A to D, Fig. 245, in con- 
 sequence of the continued force of the Sun's attraction, which 
 produces a constant deviation from a right line. 
 
 When the Earth arrives at D, still retaining its projectile or 
 centrifugal force, its line of direction would be toward N, but 
 while it would pass along to N without disturbance, the attract- 
 ing force of the Sim is again sufficient to bring it to E, in a 
 straight line, so that, in obedience to the two impulses, it again 
 describes the curve to O. 
 
 It must be remembered, in order to account for the circular 
 motions of the planets, that the attractive force of the Sun is 
 not exerted at once, or by a single impulse, as is the case with 
 the cross forces, producing a straight line, but that this force is 
 imparted by degrees, and is constant. It therefore acts equally 
 on the Earth, in all parts of the course from A to D, and from 
 D to O, Fig. 245. From O, the Earth having the same im- 
 pulses as before, it moves in the same curved or circular direc- 
 tion, and thus its motion is continued perpetually. 
 
 166. The tendency of the Earth to move forward in a straight 
 line, is called the centrifugal force, and the attraction of the 
 Sun, by which it is drawn downward, or toward a center, is 
 called its centripetal force, and it is by these two forces that the 
 planets are made to perform their constant revolutions around 
 the Sun, (197.) 
 
 167. Elliptical Orbits. In the above explanation, it has 
 been supposed that the Sun's attraction, which constitutes the 
 Earth's gravity, was at all times equal, or that the Earth was 
 at an equal distance from the. Sun, in all parts of its orbit. 
 
 166. What is the projectile force" of the. Earth called ? What is the attractive fore* 
 of the Sun. which draws the Earth toward him, called? 
 
302 
 
 MOTIONS OF THE PLANETS, 
 
 FIG. 246. 
 
 But, as heretofore explained, the orbits of all the planets are 
 elliptical, the Sun being placed in the lower focus of the ellipse. 
 The Sun's attraction is, therefore, stronger in some parts of their 
 orbits than in others, and for this reason their velocities are 
 greater at some periods of their revolutions .than at others. 
 
 The Earth, therefore, in its journey round the Sun, moves at 
 very unequal velocities, sometimes being retarded, and then 
 again accelerated, by the Sun's attraction. 
 
 168. Planets Pass Equal Areas in Equal Times. It is an 
 interesting circumstance, respecting the motions of the planets, 
 that if the contents of their orbits be divided into unequal tri- 
 angles, the acute angles of which center at the Sun, with the 
 line of the orbit for their bases, the center of the planet will 
 pass through each of these bases in equal times. 
 
 This will be understood by 
 Fig. 246, the elliptical circle 
 being supposed to be the 
 Earth's orbit, with the Sun, 
 in one of the foci. 
 
 Now the spaces, 1, 2, 3, 
 &c., though of different shapes, 
 are of the same dimensions, 
 or contain the same quantity 
 of surface. The Earth, we 
 have already seen, in its jour- 
 ney round the Sun, describes 
 an ellipse, and moves more 
 rapidly in one part of its orbit 
 than in another. But what- 
 ever may be its actual ve- 
 locity, its comparative motion * 
 
 is through equal areas in equal Elliptical Orbits. 
 
 times. Thus its center passes 
 
 from A to B, and from B to D, in the same period of time, and 
 so of all the other divisions marked in the figure. If the figure, 
 therefore, be considered the plane of the Earth's orbit, divided 
 into 12 equal areas, answering to the 12 months of the year, 
 the Earth will pass through the same areas in every month, but 
 the spaces through which it passes will be increased, during 
 every month, for one half the year, and diminished, during 
 every month, for the other half. 
 
 168. What is meant by a planet's passing through equal spaces in equal times 7 
 
THE EARTH. 303 
 
 169. Wliy the Planets do not Fall to the Sun. The reason 
 why the planets, when they approach near the Sun, do not fall 
 to him, in consequence of his increased attraction, and why 
 they do not fly off into open space, when they recede to the 
 greatest distance from him, may be thus explained. 
 
 Taking the Earth as an example, we have shown that when 
 in the part of her orbit nearest the Sun, her velocity is greatly 
 increased by his attraction, and that consequently the Earth's 
 centrifugal force is increased in proportion. 
 
 170. Now, the velocity of the earth increases in an inverse 
 proportion, as its distance from the Sun diminishes, and in pro- 
 portion to the increase of velocity is its centrifugal force in- 
 creased ; so that, in any other part of its orbit, except when 
 nearest the Sun, this increase of velocity would carry the Earth 
 away from its center of attraction. But this increase of the 
 Earth's velocity is caused by it near approach to the Sun, and 
 consequently the Sun's attraction is increased, as well as the 
 Earth's velocity. In other terms, when the centrifugal force is 
 increased, the centripetal force is increased in proportion, and 
 thus, while the centrifugal force prevents the Earth from falling 
 to the Sun, the centripetal force prevents it from moving off in 
 a straight line. 
 
 THE EARTH. 
 
 1 71. Proofs of the Earth's Diurnal Revolution. It is almost 
 universally believed, at the present day, that the apparent daily 
 motion of the heavenly bodies from east to west, is caused by 
 the real motion of the Earth from west to east, and yet there 
 are comparatively few who have examined the evidence on 
 which this belief is founded. For this reason, we will here state 
 the most obvious, and to a common observer, the most convinc- 
 ing proofs of the Earth's revolution. These are, first, the in- 
 conceivable velocity of the heavenly bodies, and particularly the 
 fixed stars, around the Earth, if she stands still. Second, the 
 fact that all astronomers of the present age agree, that every 
 phenomenon which the heavens present, can be best accounted 
 for, by supposing the Earth to revolve. Third, the analogy to 
 be drawn from many of the other planets, which are known 
 
 169. How is it shown, that, if the motion of a revolving body is increased, its pro- 
 iectile force is also increased ? 170. By what force is the Earth's velocity increased 
 as it approaches the Sun 1 When the Earth is nearest the Sun, why does it. not fall 
 to him? When the Earth's centrifugal force is greatest, what prevents its flying 
 from the Sun ? 171. What are the most obvious and convincing proofs that the Earth 
 revolves on its axis 1 
 
304 THE EARTH. 
 
 to revolve on their axes ; and fourth, the different lengths of 
 days and nights at the different planets, for did the Sun revolve 
 about the solar system, the days and nights at many of the 
 planets must be of similar lengths. 
 
 172. The distance of the Sun from the Earth being 95 mil- 
 lions of miles, the diameter of the Earth's orbit is twice its dis- 
 tance from the Sun, and, therefore, 190,000,000 of miles. 
 Now, the diameter of the Earth's orbit, when seen from the 
 nearest fixed star, is a mere point, and were the orbit a solid 
 mass of dark matter, it could not be seen, with such eyes as 
 ours, from such a distance. This is known by the fact, that 
 these stars appear no larger to us, even when our sight is assisted 
 by the best telescopes, when the Earth is in that part of her 
 orbit nearest them, than when at the greatest distance, or in the 
 opposite part of her orbit. The approach, therefore, of 190 
 millions of miles toward the fixed stars, is so small a part of 
 their whole distance from us, that it makes no perceptible dif- 
 ference in their appearance. 
 
 173. Now, if the Earth does not turn on her axis once in 24 
 hours, these fixed stars must revolve around the Earth at this 
 amazing distance once in 24 hours. If the Sun passes around 
 the Earth in 24 hours, he must travel at the rate of nearly 
 400,000 miles in a minute ; but the fixed stars are at least 
 400,000 times as far beyond the Sun, as the Sun is from us, 
 and, therefore, if they revolve around the Earth, must go at the 
 rate of 400,000 times 400,000 miles, that is, at the rate of 
 160,000,000,000, or 160 billions of miles in a minute; a ve- 
 locity of which we can have no more conception than of infinity 
 or eternity. 
 
 174. In respect to the analogy to be drawn from the known 
 revolutions of the other planets, and the different lengths of 
 days and nights among them, it is sufficient to state, that to 
 the inhabitants of Jupiter, the heavens appear to make a revo- 
 lution in about 10 hours, while to those of Venus, they appear 
 to revolve once in 23 hours, and to the inhabitants of the other 
 planets a similar difference seems to take place, depending on 
 the periods of their diurnal revolutions. 
 
 175. Now, there is no more reason to suppose that tho 
 heavens revolve round us, than there is to suppose that they re- 
 
 172. Were the Earth's orbit a solid mass, could it be seen by us at the distance ot 
 the fixed stars? 173. Suppose the Earth stood still, how fast must the Sun move to 
 co round it in 24 hours ? At what rate must the fixed stars move to go round it in 24 
 hours? 174 If the heavens appear to revolve every 10 hours at Jupiter, and every 
 24 hours at the Earth, hew can this difference be accounted for, if they revolve at all 7 
 
HORIZON. 305 
 
 volve around any of the other planets, since the same apparent 
 revolution is common to them all ; and as we know that the 
 other planets, at least many of them, turn on their axes, and as 
 all the phenomena presented by the Earth, can be accounted 
 for by such a revolution, it is folly to conclude otherwise. 
 
 HORIZON. 
 
 176. The horizon is distinguished into the sensible and ra- 
 tional. The sensible horizon^ is tljat portion of the surface of 
 the Earth which bounds our vision, or the circle around us, 
 where the sky seems to meet the Earth. When the Sun rises, 
 he appears above the sensible horizon, and when he sets, he 
 sinks below it. The rational horizon is an imaginary line pass- 
 ing through the center of the Earth, and dividing it into two 
 equal parts. 
 
 177. DIRECTION OF THE ECLIPTIC. The ecliptic, (128,) w*e 
 have already seen, is divided into 360 equal parts, called de- 
 grees. All circles, however large or small, are divided into 
 degrees, minutes, and seconds, in the same manner as the 
 ecliptic. 
 
 The axis of the ecliptic is an imaginary line passing through 
 its center and perpendicular to its plane. The extremities of 
 this perpendicular line, are called the poles of the ecliptic. 
 
 If the ecliptic, or great plane of the earth's orbit, be consid- 
 ered on the horizon, or parallel with it, and the line of the 
 Earth's axis be inclined to the axis of this 'plane, or the axis of 
 the ecliptic, at an angle of 2 3 degrees, it will represent the 
 relative positions of the orbit, and the axis of the Earth. 
 
 These positions are, however, merely relative, for if the posi- 
 tion of the Earth's axis be represented perpendicular to the 
 equator, then the ecliptic will cross this plane obliquely. But 
 when the Earth's orbit is considered as having no inclination, 
 its axis of course will have an inclination to the axis of the 
 ecliptic, of 23- degrees. 
 
 As the orbits of all the other planets are inclined to the 
 ecliptic, perhaps it is the most natural and convenient method 
 to consider this as a horizontal plane, with the equator inclined 
 
 175. Is there any more reason to believe that the Sun revolves round the Earth than 
 round any of 'the other planets ? How can all the phenomena of the heavens be ac- 
 counted for if the planets do not revolve? 176 How is the sensible horizon distin- 
 guished from the rational? 177. How are circles divided? VVnat is the axis of the 
 ecliptic ? What are the poles of the ecliptic 1 How manv degrees is the a^is of the 
 Earth inclined to that of the ecliptic ? What is said concerning the relative position* 
 of the Earth's aiis and the plane of the ecliptie ? 
 
306 HORIZON. 
 
 ' to it, instead of considering the equator on the plane of the 
 horizon, as is sometimes done. 
 
 178. INCLINATION OF THE EARTH'S Axis. The inclination 
 of the Earth's axis to the axis of its orbit never varies, but 
 always makes an angle with it of 23i degrees, as it. moves 
 round the Sun. The axis of the Earth is therefore always par- 
 allel with itself. That is, if a line be drawn through the center 
 of the Earth, in the direction of its axis, and extended north and 
 south, beyond the Earth's diameter, the line so produced will 
 always be. parallel to the same line, or any number of lines, so 
 drawn, when the Earth is in different parts of its orbit. 
 
 Suppose a rod to be fixed into the flat surface of a table, and 
 so inclined as to make an angle with a perpendicular from the 
 table of 23 degrees. Let this rod represent the axis of- the 
 Earth, and the surface of the table, the ecliptic. Now place on 
 th^e table a lamp, and round the lamp hold a wire circle three 
 or four feet in diameter, so that it shall be parallel with the 
 plane of the table, and as high above it as the flame of the 
 lamp. Having prepared a small terrestrial globe, by passing a 
 wire through it for an axis, and letting it project a few inches 
 
 Inclination of the Earth's Axis. 
 
 . 178. Are the orbits of the other planets parallel to the Earth's orbit, or inclined to 
 it? What is meant by the Earth's axis being parallel to itself? 
 
DAY AND NIGHT. 307 
 
 each way, for the poles, take hold of the north pole, and carry 
 it round the circle with the poles constantly parallel to the rod 
 rising above the table. -The rod being inclined 23- degrees 
 from a perpendicular, the poles and axis will be inclined in the 
 same decree, and thus the axis of the earth will be inclined to 
 that of the ecliptic every where in the same degree, and lines 
 drawn in the direction of the Earth's axis will be parallel to 
 each other in any part of its orbit. 
 
 179. This will be understood by Fig. 247, where it will be 
 seen, that the poles of the Earth, in the several positions of A 
 B C and D, being equally inclined, are parallel to each other. 
 Supposing the lamp to represent the Sun, and the wire circle 
 the Earth's orbit, the actual position of the Earth, during its 
 annual revolution around the Sun, will be comprehended, and 
 if the globe be turned on its axis, while passing round the lamp, 
 the diurnal, or daily revolutions of the Earth will also be 
 represented. 
 
 DAY AND NIGHT. 
 
 180. Were the direction of the Earth's axis perpendicular to 
 the plane of its orbit, the days and nights would be of equal 
 length all the year, for then just one half of the Earth, from 
 pole to pole, would be enlightened, and at the same time the 
 other half would be in darkness. 
 
 FIG. 
 
 Day and Night. 
 
 Suppose the line S o, Fig. 248, from the Sun to the Earth, 
 to be the plane of the Earth's orbit, and that N S is the axis of 
 the Earth perpendicular to it, then it is obvious, that exactly 
 
 179. How does it appear by Frr. 247, that the axis of the Earth is parallel to itself, 
 in ail parts of its orb ; t ? ISO. How are the annual and diurnal revolurons of the 
 Earth illustrated by Fig 248 7 Explain, by Fg. 248, why the days and nights would 
 every where be equal, were the axis of the Earth perpendicular to the plane of his 
 orbit. 
 
308 SEASONS OF THE YEAK. 
 
 the same points on the Earth would constantly pass through 
 the alternate vicissitudes of day and night; for all who live on 
 the meridian line between N and S, which line crosses the 
 equator at o, would see the Sun at the same time, and conse- 
 quently, as the Earth revolves, would pass into the dark hem- 
 isphere at the same time. Hence, in all parts of the globe, the 
 days and nights would be of equal length, at any given place. 
 
 181. Now it is the inclination of the Earth's axis, as above 
 described, which causes the lengths of the days and nights to 
 differ at the same place at different seasons of the year ; for on 
 reviewing the position of the globe at A, Fig. 247, it will be 
 observed that the line formed by the enlightened and dark 
 hemispheres, does not cojncide with the line of the axis and 
 pole, as in Fig. 248, but that the line formed by the darkness 
 arid the light, extends obliquely across the line of the Earth's 
 axis, so that the north pole is in the light while the south is in 
 the dark. In the position A, therefore, an observer at the north 
 pole would see the sun constantly, while another at the south 
 pole would not see it at all. Hence those living in the north 
 temperate zone, at the season of the year when the earth is at 
 A, or in the Summer, would have long days and short nights, 
 in proportion as they approached the polar circle ; while those 
 who live in the south temperate zone, at the same time, and 
 when it would be Winter there, would have long nights and 
 short days in the same proportion. 
 
 SEASONS OF THE YEAR. 
 
 182. The vicissitudes of the seasons are caused by the annual 
 revolution of the Earth round the Sun, together with the in- 
 clination of its axis to the.pla.ne of its orbit. 
 
 It has already been explained, that the ecliptic is the plane 
 of the Earth's orbit, and is supposed to be placed on a level 
 with the Earth's horizon, and hence, that this plane is consid- 
 ered the standard, by which the inclination of the lines crossing 
 the Earth, and the obliquity of the orbits of the other planets, 
 are to be estimated. 
 
 183. The Solstices. The solstices are the points where the 
 ecliptic and the equator are at the greatest distance from each 
 
 181. What is the cause of'the unequal lengths of the days and nights in different 
 parts of the world ? 182 What are the causes which produce the seasons of the 
 year? 183. What are the solstices? When the Sun enters the Summer solstice, 
 what is said of the length of the days and nights? When does the Sun enter the 
 Winter solstice, and what is the proportion between the length of the days and 
 nights 1 
 
REVOLUTIONS OF THE EARTH. 
 
 809 
 
 other. The Earth, in its yearly revolution, passes through each 
 of these points. One is called the Summer, and the other the 
 Winter solstice. The Sun is said to enter the Summer solstice 
 on the 21st of June; and at this time, in our hemisphere, the 
 days are longest and the nights shortest. On the 21st of De- 
 cember, he enters his Winter solstice, when the length of the 
 days and nights are reversed from what they were in June be- 
 fore, the days being shortest, and the nights longest. 
 
 Having learned these explanations, the student will be able 
 to understand in what order the seasons succeed each other, 
 and the reason why such changes are the effect of the Earth's 
 revolution. 
 
 REVOLUTIONS OF THE EARTH. 
 
 184. Suppose the Earth, Fig. 24P, to be in her Summer 
 solstice, which takes place on the 21st of June. At this period 
 4he will be at A, having her north pole, N so inclined toward 
 
 FIG. 249. 
 
 Seasons of the Tear. 
 
 the Sun, that the whole arctic circle will be illuminated, and 
 consequently the Sun's rays will extend 23^- degrees, the breadth 
 of the polar circle, beyond the north pole. The diurnal revolu- 
 tion, therefore, when the Earth is at A, causes no succession of 
 day and night at the pole, since the whole frigid zone is within 
 
 184. At what season of the year ia the whole arctic circle illuminated ? 
 
310 REVOLUTIONS OF THE EARTH. 
 
 the reach of his rays. The people who live within the arctic 
 circle will, consequently, at this time, enjoy perpetual day. 
 
 185. During this period, just the same proportion of tho 
 earth that is enlightened in the northern hemisphere, will be in 
 total darkness in the opposite region of the southern hemisphere ; 
 so that while the people of the north are blessed with perpetual 
 day, those of the south are groping in perpetual night. Those 
 who live near the arctic circle in the north temperate zone, will, 
 during the Summer, come, for a few hours, within the regions 
 of night, by the Earth's diurnal revolution ; and the greater the 
 distance from the. circle, the longer will be their nights, and the 
 shorter their days. 
 
 186. Hence, at this season, the days will be longer than the 
 nights every where between the equator and the arctic circle. 
 At the equator, the days and nights will be equal, and between 
 the equator and the south polar circle, the nights will be longer 
 than the days, in the same proportion as the days are longer 
 than the nights, from the equator to the arctic circle. 
 
 187. The Sun always Shines on 180 Degrees of the Earth. 
 It will be observed by a careful perusal of the above explana- 
 tion of the seasons, and a close inspection of the figure by which 
 it is illustrated, that the Sun constantly shines on a portion of 
 the Earth equal to 90 degrees north, and 90 degrees south, 
 from his place in the heavens, and consequently, that he always 
 enlightens 1 80 degrees, or one half of the Earth. If, therefore, 
 the axis of the Earth were perpendicular to the plane of its 
 orbit, the days and nights would every where be equal, for as 
 the Earth performs its diurnal revolutions, there would be 12 
 hours day, and 12 hours night. But since the inclination of its 
 axis is 23 degrees, the light of the Sun is thrown 23 degrees 
 farther in that direction, when the north pole is turned toward 
 the Sun, than it would, had the Earth's axis no inclination. 
 Now, as the Sun's light reaches only 90 degrees north or south 
 of his place in the heavens, so when the arctic circle is enlight- 
 ened, the antarctic circle must be in the dark ; for if the light 
 reaches 23 degrees beyond the north pole, it must fall 23 de- 
 grees short of the south pole. 
 
 188. As the Earth travels round the Sun, in his yearly cir- 
 cuit, this inclination of the poles is alternately toward and from 
 
 185. At what season is the whole antarctic circle in the rlark ? While the people 
 near the north pole enjoy perpetual day, what is the situation of those near the south 
 pole ? 186. At what season will the days be longer than the niglits every where be- 
 tween the equator and the arctic circle? 187. How many degrees does the Sun's 
 light reach, north and south of him, on the Earth? 
 
REVOLUTIONS OF THE EARTH. 311 
 
 him. During our "Winter, the north polar region is thrown be- 
 yond the rays of the Sun, while a corresponding portion around 
 the south pole enjoys the Sun's light. And thus, at the poles, 
 there are alternately six months of darkness and Winter, and 
 six months of sunshine and Summer. 
 
 189. While we, in the northern hemisphere, are chilled by 
 the cold blasts of Winter, the inhabitants of the southern hem- 
 isphere are enjoying all the delights of Summer; and while 
 we are scorched by the rays of a vertical Sun in June and July, 
 our southern neighbors are shivering with the rigors of mid- 
 Winter. 
 
 190. At the equator, no such changes take place. The rays 
 of the Sun, as the Earth passes round him, are vertical twice a 
 year at every place between the tropics. Hence, at the equator, 
 there are two Summers and no Winter, and as the Sun there 
 constantly shines on the same half of the Earth in succession, 
 the days and nights are always equal, there being 12 hours of 
 light and 12 of darkness. 
 
 191. VELOCITY OF THE EARTH. The motion of the Earth 
 round the Sun, is at the rate of 68,000 miles in an hour, while 
 its motion on its own axis, at the equator, is at the rate of about 
 1,042 miles in the hour. The equator being that part of the 
 Earth most distant from its axis, the motion there is more rapid 
 than toward the poles, in proportion to its greater distance from 
 the axis of motion. 
 
 192. The method of ascertaining the velocity of the Earth's 
 motion, both in its orbit and round its axis, is simple and easily 
 understood ; for by knowing the diameter of the Earth's orbit, 
 its circumference is readily found, and as we know how long it 
 takes the Earth to perform her yearly circuit, we have only to 
 calculate what part of her journey she goes through in an hour. 
 By the same principle, the hourly rotation of the Earth is as 
 readily ascertained. 
 
 193. We are insensible to these motions, because not only 
 the Earth, but the atmosphere, and all terrestrial things, partake 
 of the same motion, and there is no change in the relation of 
 objects in consequence of it. 
 
 1S8. Purin? our Winter, is the north pole turned to or - from the Sun ? At th* 
 poles, how many days and nights are there in the year? 189. When it is Winter in 
 the northern hemisphere, what is the season in the southern hemisphere ? 190. At 
 what rate does the Earth move around the Pun ? What are the seasons at ihe equa- 
 tor? 191. How fast does it move around its axis at the equator? 192. How is the 
 velocity of the Earth ascerlained? 193. Why are we insensible of the Earth's 
 motion ? 
 
312 HEAT .AND COLD. 
 
 CAUSES OF THE HEAT AND COLD OF THE SEASONS. 
 
 194. We have seen that the Earth revolves round the Sun in 
 an elliptical orbit, of which the Sun is one of the foci, and con- 
 sequently that the Earth is nearer him, in one part of her orbit 
 than in another. From the great difference we experience be- 
 tween the heat of Summer and that of Winter, we should be 
 led to suppose that the Earth must be much nearer the Sun in 
 the hot season than in the cold. But when we come to inquire 
 into this subject, and to ascertain the distance of the Sun at dif- 
 ferent seasons of the year, we find that the great source of heat 
 an'd light is nearest us during the cold of Winter, and at the 
 greatest distance during the heat of Summer. 
 
 195. It has been explained, under the article Optics, (39,) 
 that the angle of vision depends on the distance at which a 
 body of given dimensions is seen. Now, on measuring the an- 
 gular dimensions of the Sun, with accurate instruments, at dif- 
 ferent seasons of the year, it has been found that his dimensions 
 increase and diminish, and that these variations correspond ex- 
 actly with the supposition that the Earth moves in an elliptical 
 orbit. 
 
 196. If, for instance, his apparent diameter be taken in 
 March, and then again in July, it will be found to have dimin- 
 ished, which diminution is only to be accounted for, by suppos- 
 ing that he is at a greater distance from the observer in July 
 than in March. From July, his angular diameter gradually in- 
 creases, till January, when it again diminishes, and continues to 
 diminish, until July. By many observations, it is found, that 
 the greatest apparent diameter of the Sun, and therefore his 
 least distance from us, is in January, and his least diameter, and 
 therefore his greatest distance, is in July. 
 
 197. The actual difference is about three millions of miles, 
 the Sun being that distance further from the Earth in July than 
 in January. This, however, is only about one-sixtieth of his 
 mean distance from us; and the difference we should experience 
 in his heat, in consequence of this difference of distance, 
 will therefore be very small. Perhaps the effect of his prox- 
 imity to the Earth may diminish, in some small degree, the 
 severity of Winter. 
 
 194. At what season of the year is the Sun at the greatest, and at what season the 
 least d'stance from the Earth'? 195. How is it ascertained that the Earth moves in 
 an elliptical orbit, by the appearance of the Sun } 196. When does the Sun apnpar 
 under the greatest apparent diameter, and when under the least! 197. How much 
 further is the Sun from us in July than in January 1 What effect does this difference 
 produce on the Earth? 
 
HEAT AND COLD. 
 
 313 
 
 198. Temperature of Summer and Winter. The heat of 
 Summer,' and the cold of Winter, must therefore arise from the 
 difference in the meridian altitude of the Sun, and in the time 
 of his continuance above the horizon. In Summer, the sglar 
 ra^s fall on the Earth, in nearly a perpendicular direction, and 
 his powerful heat is then constantly accumulated by the long 
 days and short nights of the season. 
 
 199. In Winter, on the contrary, the solar rays fall so ob- 
 liquely on the Earth, as to produce little warmth, and the small 
 effect they do produce during the short days of that season, is 
 almost entirely destroyed by the long nights which succeed. 
 The difference between the effects of perpendicular and oblique 
 rays, seems to depend, in a great measure, on the different ex- 
 tent of surface over which they are spread. 
 
 200. When the rays of the Sun are made to pass through a 
 convex lens, the heat is increased, because the number of rays 
 which naturally cover a large surface, are then made to cover a 
 smaller one, so that the power of the glass depends on the num- 
 ber of rays thus brought to a focus. If, on the contrary, the 
 rays of the Sun are suffered to pass through a concave lens, 
 their natural heating power is diminished, because they are dis- 
 persed, or spread over a wider surface than before. 
 
 201. Summer and 
 Winter Rays. Now 
 to apply these differ- 
 ent effects to the Sum- 
 mer and Winter rays 
 of the Sun, let us sup- 
 pose that the rays fall- 
 ing perpendicularly on 
 a given extent of sur- 
 face, impart to it a 
 certain degree of heat, 
 then it is obvious, that 
 if the same number of 
 rays be spread over 
 twice that extent of 
 surface, their heating 
 power would be di- 
 minished in propor- 
 
 FIG. 250. 
 
 Summer and Winter Rays. 
 
 193 How is the heat of Summer, and the cold of Winter, accounted for? 199 
 Why do the perpendicular rays of Summer produce greater effects than the oblique 
 rays of Winter ? 200. How is this illustrated by the convex and concave lenses ' 
 
 14 
 
814 FIGURE OP THE EARTH. 
 
 tion, and that only half the heat would be imparted. This is 
 the effect produced by the Sun's rays in the Winter. They 
 fall so obliquely on the Earth, as to occupy nearly double the 
 spa^ce that the same number of rays do in the Summer. 
 
 This is illustrated by Fig. 250, where the number of rays, 
 both in Winter and Summer, are supposed to be the same. 
 But, it will be observed, that the Winter rays, owing to their 
 oblique direction, are spread over nearly twice as much surface 
 as those of Summer. 
 
 202. It may, however, be remarked, that the hottest season 
 is not usually at the exact time of the year, when the Sun is 
 most vertical, and the days the longest, as is the case toward the 
 end of June, but some time afterward, as in July and August. 
 
 203. To account for this, it must be remembered, that when 
 the Sun is nearly vertical, the Earth accumulates more heat by 
 day than it gives out at night,' and that this accumulation con- 
 tinues to increase after the days begin to shorten, and, conse- 
 quently, the greatest elevation of temperature is some time after 
 the longest days. For the same reason, the thermometer gen- 
 erally indicates the greatest degree of heat at two or three 
 o'clock on each day, and not at twelve o'clock, when the Sun's 
 rays are most powerful. 
 
 FIGURE OF THE EARTH. 
 
 204. Astronomers have proved that all the planets, together 
 with their satellites, have the shape of the sphere, or globe, and 
 hence, by analogy, there was every reason to suppose that the 
 Earth would be found of the same shape ; and several phe- 
 nomena tend to prove, beyond all doubt, that this is its form. 
 The figure of the Earth is not, however, exactly that of a globe, 
 or ball, because its diameter is about 34 miles less from pole to 
 pole, than it is at the equator. But that its general figure is 
 that of a sphere, or ball, is proved by many circumstances. 
 
 205. When one is at sea, or standing on the sea-shore, the 
 first part of a ship seen at a distance, is its mast. As the vessel 
 advances, the mast rises higher and higher above the horizon, 
 and finally the hull, and whole ship, 'become visible. Now, 
 were the Earth's surface an exact plane, no such appearance 
 
 201. How is the actual difference of the Summer and Winter rays shown ? 202. 
 Why is not the hottest season of the year at the period when the days are longest, 
 and the Sun most vertical 1 203. How is this accounted for ? 204. What is the gen- 
 eral figure of the Earth 1 How much less is the diameter of the Earth at the poles 
 than at the equator ? 205. How is the convexity of the Earth proved, by the ap 
 proach of a ship at sea 1 Explain Fig. 251. 
 
TIGURE OF THE EARTH. 815 
 
 FIG. 251. 
 Tht Earth's Convexity 
 
 Spheroidal Fbrm of the Earth. 
 
 would take place, for we should then see the hull long before 
 the mast or rigging, because it is much the largest object. 
 
 It will be plain by Fig. 251, that were the ship, A, elevated 
 so that the hull should be on a horizontal line with the eye, the 
 whole ship would be visible, instead of the topmast, there being 
 no reason, except the convexity of the earth, why the whole 
 ship should not be visible at A, as well as at B. 
 
 206. We know, for the same reason, that in passing over a 
 hill, the tops of the trees are seen, before we can discover the 
 ground on which they stand ; and that when a man approaches 
 from the opposite side of a hill, his head is seen before his feet. 
 
 It is a well known fact also, that navigators have set out from 
 a particular port, and by sailing continually westward, have 
 passed around the Earth, and again reached the port from which 
 they sailed. This could never happen, were the Earth an ex- 
 tended plain, since then the longer the navigator sailed in one 
 direction, the further he would be from home. 
 
 Another proof of the spheroidal form of the Earth, is the 
 figure of its shadow on the Moon, during eclipses, which shadow 
 is always bounded by a circular line. 
 
 These circumstances prove beyond all doubt, that the form of 
 the Earth is globular, but that it is not an exact sphere ; and 
 that it is depressed or flattened at the poles, is shown by the 
 difference in the lengths of pendulums vibrating seconds at the 
 poles, and at the equator. 
 
 207. The compression of the Earth at the poles, and the 
 consequent accumulation of matter at the equator, is considered 
 the effect of its diurnal revolution, while it was in a soft or 
 plastic state. If a ball of soft clay, or putty, be made to revolve 
 
 206. What other proofs of the globular shape of the Earth are mentioned 7 207. 
 How is the form of the Earth illustrated by experiment ? Explain the reason why a 
 plastic ball will swell at the equator, when made to revolve. 
 
316 
 
 TIGURE OF THE EARTH. 
 
 rapidly, by means of a stick passing through its center, as an 
 axis, it will swell out in the middle, or equator, and be de- 
 pressed at the poles, assuming the precise figure of the Earth. 
 
 208. Centrifugal Force. The effects of centrifugal force are 
 very satisfactorily illustrated in the following manner : 
 
 Two hoops of thin 
 
 iron are placed upon FIG. 252. 
 
 an axis which passes 
 through their poles, as 
 shown by Fig. 252. 
 The two poles of each 
 hoop cross each other 
 at right-angles, and 
 are fastened .together, 
 and to the axis at the 
 bottom. At the up- 
 per end they slide up 
 and down on the axis, 
 which is turned rap- 
 idly by wheel-work as 
 represented. These 
 hoops, before the mo- 
 tion begins, have an 
 oval form, but when 
 turned rapidly, the 
 centrifugal force occa- 
 sions them to expand, 
 or swell at the equator, while they are depressed at the poles, the 
 two polar regions becoming no more distant than A and B. 
 
 209. The weight of a body at the poles is found to be greater 
 than at the equator, not only because the poles are nearer the 
 center of the Earth than the equator, but because the centrifu- 
 gal force there tends to lessen its gravity. The wheels of ma- 
 chines, which revolve with the greatest rapidity, are made in 
 the strongest manner, otherwise they will fly in pieces, the cen- 
 trifugal force not only overcoming the gravity, but the cohesion 
 of their parts. 
 
 210. It has been found, by calculation, that if the Earth 
 turned over once in 84 minutes and 43 seconds, the centrifugal 
 
 Depressions of the Poles. 
 
 208. Explain Fig. 252, and show how it illustrates the form of the earth. 209. What 
 two causes render the weights of bodies less at the equator than at the poles ? 210. 
 What would be the consequence on the weights of bodies at the equator, did the 
 Earth turn over onee in 84 minutes and 43 seconds ? 
 
SOLAR AND SIDERIAL TIME. 31 Y 
 
 force at the equator would be equal to the power of gravity 
 there, and that bodies would entirely lose their weight. If the 
 Earth revolved more rapidly than this, all the buildings, rocks, 
 mountains, and men, at the equator, would not only lose their 
 weight, but would fly away, and leave the Earth, as the water 
 does from a revolving grindstone. 
 
 SOLAR AND SIDERIAL TIME. 
 
 211. The stars appear to go round the Earth in 23 hours, 
 56 minutes, and 4 seconds, while the Sun appears to perform 
 the same revolution in 24 hours, so that the stars gain 3 minutes 
 ajid 56 seconds upon the Sun every day. In a year, this 
 amounts to a day, or to the time taken by the Earth to per- 
 form one diurnal revolution. It therefore happens, that when 
 time is measured by the stars, there are 366 days in the year, 
 or 366 diurnal revolutions of the Earth ; while, if measured by 
 the Sun from one meridian to another, there are only 365 
 whole days in the year. The former are called the siderial, 
 and the latter solar days. 
 
 212. If the Earth had only a diurnal motion, her revolution, 
 in respect to the Sun, would coincide exactly with the. same 
 revolution in respect to the stars ; but while she is making one 
 revolution on her axis toward the east, she advances in the 
 same direction about one degree in her orbit, so that to bring 
 the same meridian toward the Sun, she must make a little more 
 than one entire revolution. 
 
 213. Thus, the Earth must complete one revolution, and a 
 portion of a second revolution, equal to the space she has ad- 
 vanced in her orbit, in order to bring the same meridian back 
 again to the Sun. This small portion of a second revolution 
 amounts daily to the 365th part of her circumference, and 
 therefore, at the end of the year, to one entire rotation, and 
 hence, in 365 days, the Earth actually turns on her axis 366 
 times. Thus, as one complete rotation forms a siderial day, 
 there must, in the year, be one siderial, more than there are 
 solar days, one rotation of the Earth, with respect to the Sun, 
 being lost, by the Earth's yearly revolution. The same loss of 
 
 211. The stars appear to move round the Earth in less time than the Sun ; what 
 does the difference amount to in a year 1 What is the year measured by a star 
 called? What is that measured by the Sun called? 212." Had the Earth 'only a 
 diurnal revolution, would the siderial and solar time agree? 213. How many times 
 does the Earth turn on her axis in a vear 7 Why does sh turn more times than 
 there are days in the year? 
 
318 TIME. 
 
 a day happens to a traveler, who, in passing round the Earth 
 to the west, reckons his time by the rising and setting of the 
 Sun. If he passes round toward the east, he will gain a day 
 for the same reason. 
 
 EQUATION OF TIME. 
 
 214. 'As the motion of the Earth about its axis is perfectly 
 uniform, the siderial days are exactly of the same length, in all 
 parts of the year. But as the orbit of the Earth, or the appa- 
 rent path of the Sun, is inclined to the Earth's axis, and as the 
 Earth moves with different velocities in different parts of its 
 orbit, the solar, or natural days, are sometimes greater and 
 sometimes less than 24 hours, as shown by an accurate clock. 
 The consequence is, that a true sun-dial, or noon mark, and a 
 true time-piece, agree with each other only a few times in a 
 year. The difference between the sun-dial and clock, thus 
 shown, is called the equation of time. 
 
 215. The difference between the Sun and a well regulated 
 clock, thus arises from two causes, the inclination of the Earth's 
 axis to the ecliptic, and the elliptical form of the Earth's orbit. 
 
 That the Earth moves in an ellipse, and that its motion is 
 more rapid sometimes than at others, as well as that the Earth's 
 axis is inclined to the ecliptic, have already been explained and 
 illustrated. It remains, therefore, to show how these two com- 
 bined causes, the elliptical form of the orbit, and the inclination 
 of the axis, produce the disagreement between the Sun and 
 clock. 
 
 MEAN TIME. 
 
 216. Equal, or mean time, is that which is reckoned by a 
 clock, supposed to indicate exactly 24 hours, from 12 o'clock on 
 one day, to 12 o'clock on the next day. Apparent time, is 
 that which is measured by the apparent motion of the Sun in 
 the heavens, as indicated by a meridian line, or sun-dial. 
 
 217. Were the Earth's orbit a perfect circle, and her axis 
 perpendicular to the plane of this orbit, the days would be of a 
 uniform length, and there would be no difference between the 
 clock and the Sun ; both would indicate 12 o'clock at the same 
 
 214. Why are the solar days sometimes greater, and sometimes less, than 24 
 hours 1 What is the difference between the time of a sun-dial and clock called ? 215. 
 What are the causes of the difference between the Sun and clock. 216. What is 
 meant by equal, or mean time? 217. Were the Earth's orbit a perfect circle, and 
 aer axis perpendicular to its plane, what would be the effect on tims 1 
 
MOON. 319 
 
 time, on every day in the year. But on account of the inclina- 
 tion of the Earth's axis to the ecliptic, unequal portions of the 
 Sun's apparent path through the heavens will pass any meridian 
 in equal times. 
 
 218. Thus the elliptical form of the Earth's orbit, her unequal 
 motions and the inclination of her axis, would prevent the 
 agreement of the Sun and clock, except when the Earth is at 
 the greatest distance from the Sun, which is on the 1st of July, 
 and when she is at the least distance, which is on the 1st of 
 January. From these causes the Sun would be faster than the 
 clock, from the 1st of July to the 1st of January, and then 
 slower than the clock, from the 1st of January to the 1st of 
 July. 
 
 Now these two causes, which result from sources which can 
 not be here explained, counteract each other, so that the Sun 
 and clock agree only when they coincide, or balance each other, 
 which takes place, on, or about the loth of April, the 15th of 
 June, the 31st of August and the 24th of December. 
 
 On these days the Sun and clock, keeping exact time, coin- 
 cide, or as the Almanac says, are even. 
 
 219. The greatest differences between the Sun and clock, are 
 on the 1st of November, when the clock is 16 minutes too 
 fast, and on the 10th of February, when it is 14 minutes too 
 slow. 
 
 THE MOON. 
 
 220. While the Earth revolves round the Sun, the Moon 
 revolves round the Earth, completing her revolution once in 2*7 
 days, 7 hours and 43 minutes, and at the distance of 240,000 
 miles from the Earth. The period of the Moon's change, that 
 is, from new Moon to new Moon again y is 29 days, 12 hours y 
 and 44 minutes. 
 
 221. The time of the Moon's revolution round the Earth is 
 called her periodical month ; and the time from change to 
 change is called her synodical month. If the Earth had no an- 
 nual motion, these two periods would be equal, but because the 
 Earth goes forward in her orbit, while the Moon goes round 
 the Earth, the Moon must go as much further, from change to 
 change, to make these periods equal, as the Earth goes forward 
 
 218. What prevents the agreement of the Sun and clock? When do the Snn and 
 clock agree ? 219. Wtien do they differ most ] 220. What is the period of the Moon's 
 revolution round the Earth ? What is the period from new Moon to new Moon 
 again ? 221. What are these two periods called 7 Why are uot the periodical and 
 ty nodical months equal 1 
 
320 MOON. 
 
 during that time, which is more than the twelfth part of her 
 orbit, there being more than twelve lunar periods in the year. 
 
 222. Illustration by the Hands of a Watch. These two 
 revolutions maybe familiarly illustrated by the motions of the 
 hour and minute hands of a watch. Let us suppose the 12 
 hours marked on the dial plate of a watch to represent the 12 
 signs of the zodiac through which the Sun seems to pass in his 
 yearly revolution, while the hour hand of the watch represents 
 the Sun, and the minute hand the Moon. Then, as the hour 
 hand goes around the dial plate once in 12 hours, so the Sun 
 apparently goes around the zodiac once in twelve months ; and 
 as the minute hand makes 12 revolutions to one of the hour 
 hand, so the Moon makes 12 revolutions to one of the Sun. 
 But the Moon, or minute hand, must go more than once round, 
 from any point on the circle, where it last came in conjunction 
 with the Sun, or hour hand, to overtake it again, since the hour 
 hand will have moved forward of the place where it was last 
 overtaken, and consequently the next conjunction must be for- 
 ward of the place where the last happened. During an hour, 
 the hour hand describes the twelfth part of the circle, but the 
 minute hand has not only to go round the whole circle in an 
 hour, but also such a portion of it as the hour hand has moved 
 forward since they last met. Thus, at 12 o'clock, the hands are 
 in conjunction; the next conjunction is 5 minutes 27 seconds 
 past I o'clock; the next, 10 min. 54 sec. past II o'clock; the 
 third, 16 min. 21 sec. past III; the 4th, 21 min. 49 sec. past 
 IV; the 5th, 27 min. 10 sec. past V; the 6th, 32 min. 43 sec. 
 past VI;- the 7th, 38 min. 10 sec. past VII ; the 8th, 43 min. 
 38 sec. past VIII; the 9th, 49 min. 5 sec. past IX; the 10th, 
 54 min. 32 sec. past X ; and the next conjunction is at XII. 
 
 223. The same principle is true in respect to the Moon ; for 
 as the Earth advances in. its orbit, it takes the Moon 2 days, 5 
 hours and 1 minute longer to come again in conjunction with 
 the Sun, than it does to make her monthly revolution round 
 the Earth ; and this 2 days 5 hours and 1 minute being added 
 to 27 days 7 hours and 43 minutes, the time of the periodical 
 revolution, makes 29 days 12 hours and 44 minutes, the period 
 of her synodical revolution. 
 
 224. We only see one Side of the Moon. The Moon always 
 
 222. How are these two revolutions of the Moon illustrated by the two hands of a 
 watch? Mention the time of several conjunctions between the two hands of a 
 watch. 223. How much longer does it take the Moon to comg again in conjunction 
 with the Sun, than it does to perform her periodical revolution? 224. Plow is it 
 proved that the Moon makes but one revolution on her axis, as she passes around the 
 Earth 7 
 
MOOIC. 321 
 
 presents the same side, or face, toward the Earth, and hence it 
 is evident that she turns on her axis but once, while she is per- 
 forming one revolution round the Earth, so that the inhabitants 
 of the Moon have but one day and night in the course of a 
 lunar month. 
 
 225. One half of the Moon is never in the dark, because 
 when this half is not enlightened by the Sun, a strong light is 
 reflected to her from the Earth, during file Sun's absence. 
 The other half of the Moon enjoys alternately two weeks of the 
 Sun's light, and two weeks of total darkness. 
 
 PHASES OF THE MOON. One of the most interesting circum- 
 stances to us, respecting the Moon, is the constant changes 
 which she undergoes, in her passage around the Earth. When 
 she first appears, a day or two after her change, we can see 
 only a small portion of her enlightened side, which is in the 
 form of a crescent ; and at this time she is commonly called 
 new Moon. From this period she goes on increasing, or show- 
 ing more and more of her face, every evening, until at last she 
 becomes round, and her face is fully illuminated. She then 
 begins again to decrease, by apparently losing a small section 
 of her face, and the next evening another small section from 
 the same part, and so on, decreasing a little every day, until she 
 entirely disappears ; and having been absent a day or two, re- 
 appears in the form of a crescent, or_new Moon, as* before. 
 
 226. When the Moon disappears7 she is said to be in con- 
 junction, that is, she is in the same direction from us with the 
 Sun. When she is full, she is said to be in opposition, that is, 
 she is in that part of the heavens opposite to the Sun, as seen 
 by us. 
 
 227. The different appearances of the Moon from new iofull, 
 and from full to change, are owing to her presenting different 
 portions of her enlightened surface toward us at different times. 
 These appearances are called phases of the Moon, and are easily 
 accounted for, and understood by the following figure. 
 
 228. Let S, Fig. 253, be the Sun, E the Earth, and A, B, C, 
 D, F, the Moon in different parts of her orbit. Now when the 
 Moon changes, or is in conjunction with the Sun, as at A, her dark 
 side is turned toward the Earth, and she is invisible, as repre- 
 
 225. One half of the Moon is never in the dark ; explain why this is so. How long 
 is the day and night at thf other half? How is it shown that the Moon shines only 
 by reflected light ? 2C6. When is the Moon said to be in conjunction with the Sun, 
 and when in opposition to the Sun 1 227. What are the phases of the Moon 1 228 
 D-scr b>v Fijf. *>3, and show how the Moon passes from change to full, and from 
 full to change. -l* 
 
'322 
 
 Phases of the Moon. 
 
 sented at a. The Sun always shines on one half of the Moon, 
 in every direction, as represented at A and B, on the inner 
 circle ; but we at the Earth can see only such portions of the 
 enlightened part as are turned toward us. After her change, 
 when she has moved from A to B, a small part of her illumi- 
 nated side comes in sight, and she appears horned, as at 6, and 
 is then called the new Moon. When she arrives at C, several 
 days afterward, one half of her disc is visible, and she appears 
 as at c, her appearance being the same in both circles. At this 
 point she is said to be in her first quarter, because she has 
 passed through a quarter of her orbit, and is 90 degrees from 
 the place of her conjunction with the Sun. At D, she shows us 
 still more of her enlightened side, and is then said to appear 
 gibbous, as at d. When she comes to F, her whole enlightened 
 side is turned toward the Earth, and she appears in all the 
 splendor of the full Moon. During the other half of her revo- 
 lution she daily shows less and less of her illuminated side, 
 until she again becomes invisible by her conjunction with the 
 Sun. Thus in passing from her conjunction a, to her full <?, the 
 Moon appears every day to increase, while in going from her 
 full to her conjunction again, she appears to us constantly to 
 decrease, but as seen from the Sun, she appears always full. 
 
 229. How the Earth appears at the Moon. The earth, seen 
 by the inhabitants of the Moon, exhibits the same phases that 
 
 229. What is Paid concerning the phases of the Earth, as seen from the Moon 1 
 When does the Earth appear full at the Moon 1 
 
ECLIPSES. 328 
 
 the Moon does to us, but in a contrary order. When the Moon 
 is in her conjunction, and consequently invisible to us, the 
 Earth appears full to the people of the Moon, and when the 
 Moon is full to us, the Earth is dark to them. 
 
 230. The Earth shines upon the Moon. That the Earth 
 shines upon the Moon, as the Moon does upon us, is proved by 
 the fact that the outline of her disc may be^seen, when only a 
 part of it is enlightened by the Sun. Thus when the sky is 
 clear, and the Moon only two or three days old, it is not un- 
 common to see the brilliant new Moon, with her horns enlight- 
 ened by the Sun, and at the same time the old Moon faintly 
 illuminated by reflection from the Earth. This phenomenon is 
 sometimes called " the old Moon in the new Moon's arms." 
 
 231. It was a disputed point among former astronomers, 
 whether the Moon has an atmosphere ; but the more recent 
 discoveries have decided that she has an atmosphere, though 
 there is reason to believe that it is much less dense than ours. 
 
 232. Surface of the Moon. When the Moon's surface is 
 examined through a telescope, it is found to be wonderfully 
 diversified, for besides the dark spots perceptible to the naked 
 eye, there are seen extensive valleys, and long ridges of highly 
 elevated mountains. 
 
 Some of these mountains, according to Dr. Herschel, are 4 
 miles high, while hollows more than three miles deep, and 
 almost exactly circular, appear excavated on the plains. As- 
 tronomers have been at vast labor to enumerate, figure, and de- 
 scribe the mountains and spots on the surface of the Moon, so 
 that the latitude and longitude of about 100 spots have been 
 ascertained, and their names, shapes, and relative positions given. 
 A still greater number of mountains have been named, and 
 their heights and the lengths of their bases detailed. 
 
 233. The deep caverns, and broken appearance of the Moon's 
 surface, long since induced astronomers to believe that such 
 effects were produced by volcanoes, and more recent discoveries 
 have seemed to prove that this suggestion was not without 
 foundation. 
 
 234. Every planet and satellite in the Solar System, is il- 
 luminated by the Sun, and hence they cast shadows in the di- 
 
 230. How is it known that the Earth shines upon the Moon, as the Moon does upon 
 us! 231. What is said concerning the Moon's atmosphere'? 232. How high are 
 some of the mountains, and how deep the caverns of the Moon 1 233. What is said 
 concerning the volcanoes of the Moon 7 234. What is a lunar, and what a solar 
 eclipse 1 
 
324 LUNAR ECLIPSES. 
 
 rection opposite to him, just as the shadow of a man reaches 
 from the Sun. 
 
 235. Eclipses, what. Eclipses are of two kinds, namely 
 Lunar, an eclipse of the Moon, and Solar, an eclipse of the 
 Sun. The first is occasioned by the shadow of the Earth on 
 the Moon, and the second by the shadow of the Moon on the 
 Earth. 
 
 Hence, in both cases, the two planets and the Sun must be in 
 nearly a straight line with respect to each other. In eclipses 
 of the Moon, the Earth is between the Sun and Moon ; and in 
 eclipses of the Sun, the Moon is between the Earth and Sun. 
 
 LUNAR ECLIPSES. 
 
 236. When the Moon falls into the shadow of the Earth, 
 the rays of the Sun are intercepted, or hid from her, and she 
 then becomes eclipsed. 
 
 When the Earth's shadow covers only a part of her face, as 
 seen by us, she suffers only a partial eclipse, one part of her 
 disc being obscured, while the other part reflects the Sun's light. 
 But when her whole surface is obscured by the Earth's shadow, 
 she then suffers a total eclipse, and of a duration proportionate 
 to the distance she passes through the Earth's shadow. 
 
 FIG. 254. 
 
 Eclipse of the Moon. 
 
 237. Fig. 254 represents a total lunar eclipse; the Moon 
 being in the midst of the Earth's shadow. Now it will be ap- 
 parent that in the situation of the Sun, Earth, and Moon, as 
 represented in the figure, this eclipse will be visible from all 
 parts of that hemisphere of the Earth which is next the Moon, 
 and that the Moon's disc will be equally obscured, from what- 
 ever point it is seen. When the moon passes through only a 
 
 235. What occasions the lunar, and what the solar eclipse? 236. What is meanf 
 DY a partial, and what by a total eclipse ? 237. Why is the same eclipse total at one 
 place, and only partial at another 1 
 
SOLAR ECLIPSES. 325 
 
 part of the Earth's shadow, then she suffers only a partial 
 eclipse, but this is also visible from the whole hemisphere next 
 the Moon. It will be remembered that lunar eclipses happen- 
 only at full Moon, the Sun and Moon being in opposition, and 
 the Earth between them. 
 
 SOLAR ECLIPSES. 
 
 238. When the Moon passes between the Earth and Sun, 
 there happens an eclipse of the Sun, because then the Moon's 
 shadow falls upon the Earth. 
 
 239. A total eclipse of the Sun happens often, but when it 
 occurs, the total obscurity is confined to a small part of the 
 Earth ; since the dark portion of the Moon's shadow never ex- 
 ceeds 200 miles in diameter on the Earth. But the Moon's 
 partial shadow, or penumbra, may cover a space on the Earth 
 of more than 4,000 miles in diameter, within all which space 
 the Sun will be more or less eclipsed. When the penumbra 
 first touches the Earth, the eclipse begins at that place, and 
 ends when the penumbra leaves it. But the eclipse will be 
 total only where the dark shadow of the Moon touches the 
 Earth. 
 
 FIG. 255. 
 
 Eclipse of the Sun. 
 
 Fig. 255, represents an eclipse of the Sun, without regard to 
 the penumbra, that it may be observed how small a part of the 
 Earth the dark shadow of the Mooji covers. To those who 
 live within the limits of this shadow, the eclipse will be total, 
 while to those who live in any direction around it, and within 
 reach of the penumbra, it will be only partial. 
 
 240. Solar eclipses are called annular, from annulus, a ring, 
 
 238. Why is a total eclipse of the Snn confined to so small a part of the Earth * 
 239. What is meant by penumbra? What will be the difference in the aspect of the 
 eclipse, whether the observer stands within the dark shadow, or only within the pe- 
 numbra? 240. What is meant by annular eclipses ] Are annular eclipses ever total 
 in any part of the Earth ? In annular eclipses, what part of the Moon's shadow 
 reaches the Earth ? 
 
326 SOLAR ECLIPSES. 
 
 when the Moon passes across the center of the Sun, hiding all 
 his light, with the exception of a ring on his outer edge, which 
 the Moon is too small to cover from the position in which it is 
 seen. 
 
 241. Umbra and Penumbra. A solar eclipse, with the pe- 
 numbra, or light and shadow, D C, and the umbra, or dark 
 shadow, O, is seen in Fig. 256. 
 
 When the Moon is at its greatest distance from the Earth, its 
 shadow M 0, sometimes terminates before it reaches the Earth, 
 and then an observer standing directly under the point O, will 
 see the outer edge of the Sun, forming a bright ring around the 
 circumference of the Moon, thus forming an annular eclipse. 
 
 FIG. 256. 
 
 Umbra and Penumbra. 
 
 The penumbra D C, is only a partial interception of the Sun'a 
 rays, and in annular eclipses it is this partial^hadow only which 
 reaches the Earth, while the umbra, or dark shadow, terminates 
 in the air. Hence annular eclipses are never total in any part 
 of the Earth. The penumbra, as already stated, may cover 
 more than 4,000 miles of space, while the umbra never covers 
 more than 200 miles in diameter ; hence partial eclipses of the 
 Sun may be seen by a vast number of inhabitants, while com- 
 paratively few will witness the total eclipse. 
 
 242. When there happens a total solar eclipse to us, we are 
 eclipsed to the Moon, and wJien the Moon is eclipsed to us, an 
 eclipse of the Sun happens to the Moon. To the Moon, an 
 eclipse of the Earth can never be total, since her shadow cover? 
 only a small portion of the Earth's surface. Such an eclipse, 
 therefore, at the Moon, appears only as a dark spot on the face 
 of the Earth ; but when the Moon is eclipsed to us, the Sun is 
 partially eclipsed to the Moon for several hours longer than the 
 Moon is eclipsed to us. 
 
 241 . What do penumbra, and umbra, mean 1 242. What is said concerning eclipses 
 of the Earth, as seen from the Moon 7 
 
TIDES. 327 
 
 THE TIDES. 
 
 243. The ebbing and flowing of the sea, which regularly 
 takes place twice in 24 hours, are called the tides. 
 
 244. The cause of the tides, is the attraction of the Sun and 
 Moon, but chiefly of the Moon, on the waters of the ocean. In 
 virtue of the universal principle of gravitation, heretofore ex- 
 plained, the Moon, by her attraction, draws, or raises the water 
 f ~rd her, but because the power of attraction diminishes as 
 uie squares of the distances increase, the waters, on the oppo- 
 site side of the Earth, are not so much attracted as they are on 
 the side nearest the Moon. 
 
 245. The want of attraction, together with the gfeater cen- 
 trifugal force of the Earth on its opposite side, produced in con- 
 sequence of its greater distance from the common center of 
 gravity, between the Earth and Moon, causes the waters to rise 
 on the opposite side, at the same time that they are raised by 
 direct attraction on the side nearest the Moon. 
 
 Thus the waters are constancy elevated on the sides of the 
 Earth opposite to each other above their common level, and 
 consequently depressed at opposite points equally distant from 
 these elevations. 
 
 FIG. 257. 
 
 Illustration of the Tides. 
 
 246. LetM, Fig. 257, be the Moon, and E the Earth, covered 
 with water. As the Moon passes round the Earth, its solid 
 and fluid parts are equally attracted by her influence according 
 to their densities ; but while the solid parts are at liberty to 
 move only as a whole, the water obeys the slightest impulse, 
 and thus tends toward the Moon where her attraction is the 
 strongest. Consequently, the waters are perpetually elevated 
 immediately under the Moon. 
 
 247. If, therefore, the Earth stood still, the influence of the 
 
 243. What are the tides ? 244. What is the cause of the tides? 245. What causes 
 the tide to ri.se on the side of the Earth opposite to the Moon ? 246. Explain Fig. 257. 
 247. If the Earth stood still, the tides would rise only as the Moon passes round the 
 Earth ; what then causes the tides to rise twice in 24 hours ? 
 
328 TIDES. 
 
 Moon's attraction would raise the tides only as she passed round 
 the Earth. But as the Earth turns on her axis every 24 hours, 
 and as the waters nearest the Moon, as at A, are constantly 
 elevated, they will, in the course of 24 hours, move round the 
 whole Earth, and consequently from this cause there will be 
 high water at every place once in 24 hours. As the elevation 
 of the waters under the Moon causes their depression at 90 de- 
 gress distance on the opposite sides of the Earth, D and C, the 
 point C will come to the same place, by the Earth's diurnal 
 revolution, six hours after the point A, because C is one quarter 
 of the circumference of the Earth from the point A, and there- 
 fore, there will be low water at any given place six hours after 
 it was high "water at that place. 
 
 248. But while it is high water under the Moon, in conse- 
 quence of her direct attraction, it is also high water on the op- 
 posite side of the Earth in consequence of her diminished 
 attraction, and the Earth's centrifugal motion, and therefore it 
 will be high water from this cause twelve hours after it was high 
 water from the former cause, and six hours after it was low 
 water from both causes. 
 
 249. But while the Earth turns on her axis, the Moon 
 advances in her orbit, and consequently any given point on the 
 Earth will not come under the Moon on one day so soon as it 
 did on the day 'before. For this reason, high or low water at 
 any place comes about fifty minutes later on one day than it 
 did the day before. 
 
 Thus far we have considered no other attractive influence ex- 
 cept that of the Moon, as affecting the waters of the ocean. 
 But the Sun, as already observed, has an effect upon the tides, 
 though on account of his great distance, his influence is small 
 when compared with that of the Moon. 
 
 250. When the Sun and Moon are in conjunction, as repre- 
 sented in Fig. 257, which takes place at her change, or when 
 they are in opposition, which takes place at full Moon, then 
 their forces are united, or act on the waters in the same direc- 
 tion, and consequently the tides are elevated higher than usual, 
 and on this account are called spring tides. 
 
 251. Neap Tides. But when the Moon is in her quadra- 
 tures, or quarters, the attraction of the Sun tends to counteract 
 
 248. When it is high water under the Moon by her attraction, what is the cause of 
 high water on the opposite side of the Earth, at the same time 1 249 Why are the 
 tides about fifty minutes later every day? 250. What produces spriiig tides? 
 Where must the Moon be in respect to the Bun, to produce spring tides? 251 
 What is the occasion of neap tides ? 
 
LATITUDE AND LONGITUDE. 329 
 
 that of the Moon, and although his attraction does not elevate 
 the waters and produce tides, his influence diminishes that of 
 the Moon, and consequently the elevation of the waters are less 
 when the Sun and Moon are so situated in respect to each other, 
 than when they are in conjunction or opposition. 
 
 This effect is represented by Fig. 258, where the elevation 
 of the tides at C and D is produced by the causes already ex- 
 plained ; but their elevation is not so great as in Fig. 257, since 
 the influence of the Sun acting in the direction A B, tends to 
 counteract the Moon's attractive influence. These small tides 
 are called neap tides, and happen only when the Moon is in her 
 quadratures. 
 
 FIG. 258. 
 
 Small, or Neap Tides. 
 
 The tides are not at their greatest heights at the time when 
 the Moon is at its meridian, but some time afterward, because 
 the water, having a motion forward, continues to advance by its 
 own inertia, some time after the direct influence of the Moon 
 has ceased to affect it. 
 
 LATITUDE AND LONGITUDE. 
 
 252. Latitude is the distance from the equator in a direct 
 line, north or south, measured in degrees and minutes. 
 
 The number of degrees is 90 north, and as many south, each 
 line on which these degrees are reckoned running from the 
 equator to the poles. Places at the north of the equator are in 
 north latitude, and those south of the equator are in south lat- 
 itude. The parallels of latitude are imaginary lines drawn 
 parallel to the equator, either north or south, and hence every 
 place situated on the same parallel, is in the same latitude be- 
 
 252. What is latitude ? How many degrees of latitude are there ? How far do the 
 lines of latitude extend ? What is meant by north and south latitude ? What are 
 the parallels of latitude 7 
 
330 
 
 LATITUDE AND LONGITUDE. 
 
 cause every such place must be at the same distance from the 
 equator. The length of a degree of latitude is 60 geographical 
 mile*. 
 
 253. Longitude is the distance measured in degrees and min- 
 utes, either east or west, from any given point on the equator, or 
 on any parallel of latitude. Hence the lines, or meridians of 
 longitude, cross those of latitude at right-angles. The degrees 
 of longitude are 180 in number, its lines extending half a circle 
 to the east, and half a circle to the west, from any given me- 
 ridian, so as to include the whole circumference of the Earth. 
 A degree of longitude, at the equator, is of the same length as 
 a degree of latitude, but as the poles are approached, the de- 
 grees of longitude diminish in length, because the Earth grows 
 smaller in circumference from the equator toward the poles ; 
 hence the lines surrounding it become less and less. This will 
 be made obvious by Fig. 259. 
 
 Let this figure represent the 
 Earth, N being the north pole, 
 S the south pole, and E W the 
 equator. The lines 10, 20, 30, 
 and so on, are the parallels of 
 latitude, and the lines N a S, N 
 b S, &c., are meridian lines, or 
 those of longitude. 
 
 The latitude of any place on 
 the globe, is the number of de- 
 grees between that place and 
 the equator, measured on a me- 
 ridian line; thus, x is irik' lati- 
 tude 40 degrees, because lie x Parallel* of Longitude, 
 q part of the meridian contains 
 
 40 degrees. The longitude of a place is the number of degrees 
 it is situated east or west from any meridian line ; thus, v is 20 
 degrees west longitude from x, and # is 20 degrees east longi- 
 tude from v. 
 
 254. As the equator divides the Earth into two equal parts, 
 or hemispheres, there seems to be a natural reason why the de- 
 grees of latitude should be reckoned from this great circle. But 
 
 253. What is longitude 7 How many degrees of longitude are there, east or west ? 
 What is the latitude of any place ? What is the longitude of a place 1 254. Why are 
 the degrees of latitude reckoned from the equator? What is said concerning the 
 places from which the degrees of longitude have been reckoned 1 What is the in- 
 convenience of estimating longitude from a place in each country 7 From what 
 place is longitude reckoned in Europe and America 1 
 
LATITUDE AND LONGITUDE. 331 
 
 from east to west there is no natural division of the Earth, each 
 meridian line being a great circle, dividing the Earth into two 
 hemispheres, and hence there is no natural reason why longi- 
 tude should be reckoned from one meridian any more than an- 
 other. It has, therefore, been customary for writers and mar- 
 iners to reckon longitude from the capital of their own country ; 
 as the English from London, the French from Paris, and the 
 Americans from Washington. But this mode, it is apparent, 
 must occasion much confusion, since each writer of a different 
 nation would be obliged to correct the longitude of all other 
 countries, to make it agree with his own. More recently, there- 
 fore, the writers of Europe and America have selected the royal 
 observatory, at Greenwich, near London, as the first meridian, 
 and on most maps and charts lately published, longitude is 
 reckoned from that place. 
 
 255. How Latitude is Found. The latitude of any place is 
 determined, by taking the altitude of the Sun at mid-day, and 
 then subtracting this from 90 degrees, making proper allow- 
 ances for the Sun's place in the heavens. The reason of this 
 will be understood, when it is considered that the whole num- 
 ber of degrees from the zenith to the horizon is 90, and there- 
 fore if we ascertain the Sun's distance from the horizon, that is, 
 his altitude, by allowing for the Sun's declination north or south 
 of the equator, and subtracting this from the whole number, the 
 latitude of the place will be found. Thus, suppose that on the 
 20th of March, when the Sun is at the equator, his altitude 
 from any place north of the equator should be found to be 48 
 degrees above the horizon ; this, subtracted from 90, the whole 
 number of the degrees of latitude, leaves 42, which will be the 
 latitude of the place where the observation was made. 
 
 256. If the Sun, at the time of observation, has a declination 
 north or south of the equator, this declination must be added 
 to, or subtracted from, the meridian altitude, as the case may 
 be. For instance, another observation being taken at the place 
 where the latitude was found to be 42, when the Sun had a 
 declination of 8 degrees north, then his altitude would be 8 de- 
 grees greater than before, and therefore 56, instead of 48. 
 Now, subtracting this 8, the Sun's declination, from 56, and the 
 remainder from 90, and the latitude of the place will be found 
 42, as before. If the Sun's declination be south of the equator, 
 
 255. How is the latitude of a place determined'? Give an example of the method 
 of finding the latitude of the same place at different seasons of the year? 256. When 
 must the Sun's declination from the equator be added to, and when subtracted from, 
 his meridian altitude ? 
 
332 LATITUDE AND LONGITUDE. 
 
 and the latitude of the place north, his declination must be 
 added to the meridian altitude instead of being subtracted from 
 it. The same result may be obtained by taking the meridian 
 altitude of any of the fixed stars, whose declinations are known, 
 nstead of the Sun's, and proceeding as above directed. 
 
 257. How Longitude is Found. There is more difficulty in 
 ascertaining the degrees of longitude, than those of latitude, 
 because, as above stated, there is no fixed point, like that of the 
 equator, from which its degrees are reckoned. The degrees of 
 longitude are therefore estimated from Greenwich, and are as- 
 certained by the following methods : 
 
 258. When the Sun comes to the meridian of any place, it 
 is noon, or 12 o'clock, at that place, and therefore, since the 
 equator is divided into 360 equal parts, or degrees, and since 
 the Earth turns on its axis once in 24 hours, 15 degrees of the 
 equator will correspond with one hour of time, for 360 degrees 
 being divided by 24 hours, will give 15. The Earth, therefore, 
 moves in her daily revolution, at the rate of 15 degrees for 
 every hour of time. Now, as the apparent course of the Sun is 
 from east to west, it is obvious that he will come to any me- 
 ridian lying east of a given place, sooner than to one lying west 
 of that place, and therefore it will be 12 o'clock to the east of 
 any place, sooner than at that place, or to the west of it. 
 
 259. When, therefore, it is noon at any one place, it will be 
 1 o'clock at all places 15 degrees to the east of it, because the 
 Sun was at the meridian of such places an hour before ; and so, 
 on the contrary, it will be 11 o'clock, 15 degrees west of tlie 
 same place, because the Sun has still an hour to travel before 
 he reaches the meridian of that place. It makes no difference, 
 then, where the observer is placed, since, if it is 12 o'clock 
 where he is-, it will be 1 o'clock 15 degrees to the east of him, 
 and 11 o'clock 15 degrees to the west of him, and so in this 
 proportion, let the time be more or less. Now, if any celestial 
 phenomenon should happen, such as an eclipse of the Moon, or 
 of Jupiter's satellites, the difference of longitude between two 
 places where it is observed, may be determined by the differ- 
 ence of the times at which it appeared to take place. Thus, if 
 the Moon enters the Earth's shadow at 6 o'clock in the evening, 
 as seen at Philadelphia, and at half past 6 o'clock at another 
 
 257. Why is there more difficulty in ascertaining the degrees of longitude than of 
 latitude? 258. How many degree's of longitude does the surface of the Earth pass 
 through in an hour? 259. Suppose it is noon at any given place, what o'clock will 
 it be fifteen degrees to the east of that place 1 Explain the reason. How may longi- 
 tude be determined by an eclipse? 
 
FIXED STARS. 333 
 
 place, then this place is half an hour, or 7 degrees, to the east 
 of Philadelphia, because 1% degrees of longitude are equal to 
 half an hour of time. To apply these observations practically, 
 it is only necessary that it should be known exactly at what 
 time the eclipse takes place at a given point on the Earth. 
 
 260. USE OF THE CHRONOMETER. Suppose two chronome- 
 ters, which are known to go at exactly the same rate, are made 
 to indicate 12 o'clock by the meridian line at Greenwich, and 
 the one be taken to sea, while the other remains at Greenwich. 
 Then suppose the captain, who takes his chronometer to sea, has 
 occasion to know his longitude. In the first place, he ascer- 
 tains, by an observation of the Sun, when it is 12 o'clock at 
 the place where he is, and then by his time-piece, when it is 
 12 o'clock at Greenwich, and by allowing 15 degrees for every 
 hour of the difference in time, he will know his precise longi- 
 tude in any part of the world. 
 
 261. For example, suppose the captain sails with his 'chro- 
 nometer for America, and after being several weeks at sea, finds 
 by observation that it is 1 2 o'clock by the Sun, and at the same 
 time finds by his chronometer, that it is 4 o'clock at Greenwich. 
 Then, because it is noon at his place of observation after it is 
 noon at Greenwich, he knows that his longitude is west from 
 Greenwich, and by allowing 15 degrees for every hour of the 
 difference, his longitude is ascertained. Thus, 15 degrees, mul- 
 tiplied by 4 hours, give 60 degrees of west longitude from 
 Greenwich. If it is noon at the place of observation, before it 
 is noon at Greenwich, then the captain knows that his longitude 
 is east, and his true place is found in the same manner. 
 
 FIXED STARS. 
 
 262. The stars are called fixed, because they have been ob- 
 served not to change their places with respect to each other. 
 They may be distinguished by the naked eye from the planets 
 of our system by their scintillations, or twinkling. The stars 
 are divided into classes, according to their magnitudes, and are 
 called stars of the first, second, and so on to the sixth magni- 
 
 260. Explain the principles on which longitude is determined by the chronometer. 
 261. Suppose the captain finds by his chronometer that it is 12 o'clock where he is 
 6 hours later than at Greenwich, what then would be his longitude? Suppose he 
 finds it to be 12 o'clock 4 hours earlier where he is. than at Greenwich, what then 
 would be his longitude'? 262. Why are the stars called fixed ? How may the stars 
 be distinguished from the planets? The stars are divided into classes, according to 
 their magnitudes ; how many classes are there ? How manv stars may be seen 
 with the naked eye in the whol firmament? 
 
334 FIXED STARS. 
 
 tude. About 2,000 stars may be seen with the naked eye in 
 the whole vault of the heavens, though only about 1,000 are 
 above the horizon at the same time. Of these, about 1Y are of 
 the first magnitude, 50 of the second magnitude, and 150 of the 
 third magnitude. The others are of the fourth, fifth, and sixth 
 magnitudes, the last of which are the smallest that can be dis- 
 tinguished with the naked eye. 
 
 263. It might seem incredible, that on a clear night only 
 about 1,000 stars are visible, when on a single glance at the 
 different parts of the firmament, their numbers appear innumer- 
 able. But this deception arises from the confused and hasty 
 manner in which they are viewed ; for if we look steadily on a 
 particular portion of sky, and count the stars contained within 
 certain limits, we shall be surprised to find their number so few. 
 
 264. The nearest fixed stare to our system, from the most 
 accurate astronomical calculations, can not be nearer than 
 20,000,000,000,000, or 20 trillions of miles from the Earth, a 
 distance so immense, that light can not pass through^ it in less 
 than three years. Hence, were these stars annihilated at the 
 present time, their light would continue to flow toward us, and 
 they would appear to be in the same situation to us, three years 
 hence, that they do now. 
 
 265. Our Sun, seen from the distance of the nearest fixed 
 stars, would appear no larger than a star of the first magnitude 
 does to us. These stars appear no larger to us, when the Earth 
 is in that part of her orbit nearest to them, than they do, when 
 she is in the opposite part of her orbit ; and as our distance 
 from the Sun is 95,000,000 of miles, we must be twice this 
 distance, or the whole diameter of the Earth's orbit, nearer a 
 given fixed star at one period of the year than at another. The 
 difference, therefore, of 190,000,000 of miles, bears so small a 
 proportion to the whole distance between us and the fixed stars, 
 as to make no appreciable difference in their sizes, even when 
 assisted by the most powerful telescopes. 
 
 266. The amazing distances of the fixed stars may also be 
 inferred from the return of comets to our system, after an ab- 
 sence of several hundred years. 
 
 The velocity with which some of these bodies move, when 
 
 2C3. Why does there appear to be more stars than there really are ? 264. What is 
 the computed distance of the nearest fixed stars from the Earth 1 How long would 
 it take liiiht to reach us from the fixed stars? 265. How large would our Sun ap- 
 
 ear at the distance of the fixed stars? What is said concerning the difference of the 
 istance between the Earth and the fixed stars at. different seasons of the year, and 
 of their different appearance in consequence? 266. How may the distances of the 
 fixed stars be inferred, by the long absence and return of comets 1 
 
COMETS. 335 
 
 nearest the Sun, has been computed at nearly a million of miles 
 in an hour, and although their velocities must be perpetually 
 retarded, as they recede from the Sun, still, in 250 years of 
 time, they must move through a space which to us would be 
 infinite. The periodical return of one comet is known to be 
 upward of 500 years, making more than 250 years in perform- 
 ing its journey to the most remote part of its orbit, and as 
 many in returning back to our system ; and that it must still 
 always be nearer our system than the fixed stars, is proved by 
 its return ; for by the laws of gravitation, did it approach nearer 
 another system it would never again return to ours. 
 
 267. From such proofs of the vast distances of the fixed stars, 
 there can be no doubt that they shine with their own light, 
 like our Sun, and hence the conclusion that they are Suns to 
 other worlds, which move around them, as the planets do around 
 our Sun. Their distances will, however, prevent our ever 
 knowing, except by conjecture, whether this is the case or not, 
 since, were they millions of times nearer us than they are, we 
 should not be able to discover the reflected light of their planets, 
 
 COMETS. 
 
 268. Besides the planets, which move round the Sim in reg- 
 ular order and in nearly circular orbits, there belongs to th& 
 solar system an unknown number of bodies called Comets, which 
 move round the Sun in orbits exceedingly eccentric, or elliptical, 
 and whose appearance among our heavenly bodies is only occa- 
 sional. Comets, to the naked eye, have no visible disc, but 
 shine with a faint, glimmering light, and are accompanied by a 
 train or tail, turned from the Sun, and which is sometimes of 
 immense length. They appear in every region of the heavens^ 
 and move in every possible direction. 
 
 269. Number and Periods of Comets. The number of 
 comets is unknown, though some astronomers suppose that 
 there are nearly 500 belonging to our system. Ferguson, who 
 wrote in about 1760, supposed that there were less than 30 
 comets which made us occasional visits ; but since that period 
 the elements of the orbits of nearly 100 of these bodies have 
 l>een computed. 
 
 Of these, however, there are only three whose periods of re- 
 turn among us are known with any degree of certainty. The 
 
 267. On what grounds is it supposed that the fixed stars are suns to other worlds! 
 269. What number of comets are supposed to belong to our system 1 
 
336 COMETS. 
 
 first of these has a period ! I?;,f!!: 
 
 of 75 years ; the second a 
 period of 129 years; and 
 the third a period of 575 
 years. The third appeared 
 in 1680, and therefore can 
 hot be expected again until 
 the year 2225. This 
 comet, Fig. 260, in 1680, 
 excited the most intense 
 interest among the astronomers of Europe, on account of its 
 great apparent size and near approach to our system. In the 
 most remote part of its orbit, its distance from the Sun was es- 
 timated at about 11,200,000,000 of miles. At its nearest ap- 
 proach to the Sun, which was only about 50,000 miles, its 
 velocity, according to Sir Isaac Newton, was 880,000 miles in 
 an hour ; and supposing it to have retained the Sun's heat, like 
 other solid bodies, its temperature must have been about 2,000 
 times that of red hot iron. The tail of this comet was at least 
 100,000,000 of miles long. 
 
 270. In the Edinburgh Encyclopedia, article Astronomy, there 
 is the most complete table of comets yet published. This table 
 contains the elements of 97 comets, calculated by different as 
 tronomers, down to the year 1808. 
 
 From this table it appears that 24 comets have passed be- 
 tween the Sun and the orbit of Mercury ; 33 between the orbits 
 of Venus and the Earth; 15 between the orbits of the Earth 
 and Mars ; 3 between the orbits of Mars and Ceres ; and 1 be- 
 tween the orbits of Ceres and Jupiter. It also appears by this 
 table that 49 comets have moved round the Sun from west to 
 east, and 48 from east to west. 
 
 271. Nature of Comets. Of the nature of these wandering 
 planets very little is known. When examined by a telescope, 
 they appear like a mass of vapors surrounding a dark nucleus. 
 When the comet is at its perihelion, or nearest the Sun, its coloi 
 seems to be heightened by the intense light or heat of that 
 luminary, and it then often shines with more brilliancy than 
 the planets. At this time the tail or train, which is always 
 directly opposite to the Sun, appears at its greatest length, but 
 is commonly so transparent as to permit the fixed stars to be 
 
 270. How many have had the elements of their orbits estimated by astronomers? 
 How many are there whose periods of return are known? 271. What is said of the 
 tomet of 16SO? 
 
PARALLAX. 
 
 837 
 
 seen through it. A variety of opinions have been advanced by 
 astronomers concerning the nature and causes of these trains, 
 but no satisfactory theory has been offered. 
 
 A new comet was discovered by Miss Maria Mitchell, of 
 Kantucket, in October, 1847, for which she received the gold 
 medal of the king of Denmark, offered for the first discovery of 
 a new cornet in any country. 
 
 PARALLAX. 
 
 272. Parallax is the difference between the true and apparent 
 place of a celestial body. The apparent place is that in which 
 the body seems to be when viewed fror i the surface of the 
 Earth, the true place being that in which it would appear if 
 seen from the center of the earth. 
 
 This will be understood 
 by Fig. 261, where if we FIG 
 
 suppose a spectator placed 
 at G, in the Earth's center, 
 he would see the moon E, 
 among the stars at I, whereas 
 without changing the posi- 
 tion of the moon, if that 
 body is seen from A, on the 
 surface of the Earth, it 
 would appear among the 
 stars at K. Now I is the 
 true and K the apparent 
 place of the moon, the space 
 between them, being the 
 Moon's parallax. 
 
 The parallax of a body is greatest when on the sensible hor- 
 izon, (170,) or at the moment when it becomes visible to the 
 eye. From this point it diminishes until it reaches the zenith, 
 or the highest place in the heavens, when its parallax ceases 
 entirely. Thus it will be seen by the figure, that the parallax 
 of the moon is less when at D, than it was at E, and that when 
 it arrives at the zenith, Z, its position is the same whether seen 
 from the center of the Earth, G, or from its surface, A. 
 
 The greater the distance of the heavenly body from the spec- 
 tator, the less is its parallax. 
 
 Diurnal Parallax. 
 
 272. What is parallax ? What is the apparent place of a celestial body ? What \* 
 the true place of such a body ? Explain Fig. 261, and show why there is no parallax 
 when the body is in the zenith ? 
 
 15 
 
338 PARALLAX. 
 
 Thus were the Moon at e instead of at E, her parallax would 
 be only equal to p K, instead of I K. Hence the Moon, being 
 the nearest celestial body, has the greatest parallax, the differ- 
 ence of her place among the stars, when seen from the surface 
 of the Earth, A, and the center G, being about 4,000 miles. 
 
 273. Parallax of the Stars. The stars are at such immense 
 distances from the Earth, that the difference of station between 
 the center and surface of the Earth makes no perceptible change 
 in their places, and hence they have no parallax. 
 
 274. Diurnal Parallax. This applies to the solar system, 
 and takes place every day in the apparent rotation of the planets 
 around the Earth. The Moon, as above shown, has a parallax 
 when she rises, which diminishes until she reaches the zenith, 
 when it ceases entirely ; the same is the case with the Sun and 
 planets, which have sensible parallaxes. 
 
 275. Annual Parallax. This is the difference in the appa- 
 rent places of the celestial bodies, as seen from the Earth at the 
 opposite points of her orbit, during her annual revolution round 
 the Sun. 
 
 Suppose A, Fig. 262, 
 to be a stationary ce- 
 lestial object, then as 
 the Earth makes her an- 
 nual revolution around 
 the Sun S, this object 
 at one time will appear 
 among the stars at E, 
 but six months after, 
 when the Earth comes 
 
 to the Opposite point Annual Parallax. 
 
 in her orbit, the same 
 
 object will be seen at C, the space from C to E being the an- 
 nual parallax of the object A. But the distances of the stars 
 are so great that the diameter of the Earth's orbit, or 190,000,000 
 of miles make no difference in their apparent places. Were the 
 fixed stars within 19,000,000,000,000, or 19 trillions of miles, 
 their distance could be told by their parallaxes. 
 
 But since, as above stated, these celestial points have no sens- 
 ible parallaxes, their distances must be greater than this, but 
 how much is unknown. 
 
 273. Why have the stars no parallaxes ? 274. What is diurnal parallax 1 275. What 
 is annual parallax') 
 
ELECTRICITY. 339 
 
 CHAPTER XIII. 
 
 ELECTRICITY. 
 
 276. THE science of Electricity, which now ranks as an im- 
 portant branch of Natural Philosophy, is wholly of modern date. 
 The ancients were acquainted with a few detached facts de- 
 pendent on the agency of electrical influence, but they never 
 imagined that it was extensively concerned in the operations of 
 nature, or that it pervaded material substances generally. The 
 term electricity is derived from electron, the Greek name of 
 amber, because it was known to the ancients, that when that 
 substance was rubbed or excited, it attracted or repelled small 
 light bodies, but it was then unknown that other substances 
 when excited, would do the same. 
 
 When a piece of glass, sealing-wax, or amber, is rubbed with 
 a dry hand, and held toward small and light bodies, such as 
 threads, hairs, feathers, or straws, these bodies will fly toward 
 the surface thus rubbed, and adhere to it for a short time. The 
 influence by which these small substances are drawn, is called 
 electrical attraction ; the surface having this attractive powei 
 is said to be excited ; and the substances susceptible of this ex- 
 citation, are called electrics. Substances not having this attrac- 
 tive power when rubbed, are called non-electrics. 
 
 277. The principal electrics are amber, resin, sulphur, glass, 
 the precious stones, sealing-wax, and the fur of quadrupeds. 
 But the metals, and many other bodies, may be excited when 
 insulated and treated in a certain manner. 
 
 After the light substances which had been attracted by the 
 excited surface, have remained in contact with it a short time, 
 the force which brought them together ceases to act, or acts in 
 a contrary direction, and the light bodies are repelled, or thrown 
 away from the excited surface. Two bodies, also, which have 
 been in contact with the excited surface, mutually repel each 
 other. 
 
 278. Electroscope. Various modes have been devised for 
 exhibiting distinctly the attractive and repulsive agencies of 
 
 276. From what is the term electricity derived 1 What is electrical attraction 1 ? 
 What are electrics 7 What are non-electrics'? 277. What are the principal dec- 
 tries 1 What is meant by electrical repulsion ? 278. What is an electroscope 1 
 
340 
 
 ELECTRICITY. 
 
 electricity, and for obtaining indications of its presence, when it 
 exists only in a feeble degree. Instruments for this purpose are 
 termed Electroscopes. 
 
 One of the simplest instruments of this kind consists of a me- 
 tallic needle, terminated at each end by a light pith-ball, which 
 is covered with gold leaf, and supported horizontally at its center 
 by a fine point, Fig. 263. When a stick of sealing-wax, or a 
 glass tube, is excited, and then presented to one of these balls, 
 the motion of the needle on its pivot will indicate the electrical 
 influence. 
 
 FIG. 263. 
 
 FIG. 264. 
 
 Electroscope. 
 
 Electrical Attraction. 
 
 279. If an excited substance be brought near a ball made of 
 pith, or cork, suspended by a silk thread, the ball will, in the 
 first place, approach the electric, as at A, Fig. 264, indicating 
 an attraction toward it, and if the position of the electric will 
 allow, the ball will come into contact with the electric, and ad- 
 here to it for a short time, and will then recede from it, show- 
 ing that it is repelled, as at B. If, now, the ball which had 
 touched the electric, be brought near another ball, which has 
 had no communication with an excited substance, these two 
 balls will attract each other, and come into contact ; after which 
 they will repel each other, as in the former case. 
 
 It appears, therefore, that the excited body, as the stick of 
 sealing-wax, imparts a portion of its electricity to the ball, and 
 that when the ball is also electrified, a mutual repulsion then 
 takes place between them. Afterwards, the ball, being electri- 
 fied by contact with the electric, when brought near another 
 ball not electrified, transfers a part of its electrical influence to 
 that, after which these two balls repel each other, as in the 
 former instance. 
 
 280. Thus, when one substance has a greater or less quan- 
 
 279. When do two electrified bodies attract, and when do they repel each other? 
 280. How will two bodies act, one having more, and the other less, than the natural 
 quantity of electricity, when brought near each other 1 How will they act when 
 both have more or Use than their natural quantity 1 
 
841 
 
 tity of electricity than another, it will attract the other sub- 
 stance, and when they are in contact will impart to it a portion 
 of this superabundance ; but when they are both equally elec- 
 trified, both having more or less than their natural quantity of 
 electricity, they will repel each other. 
 
 ELECTRICAL THEORIES. To account for these phenomena, 
 two theories have been advanced, one by Dr. Franklin, who 
 supposes there is only one electrical fluid, and the other by 
 Du Fay, who supposes that there are two distinct fluids. 
 
 281. Franklin's Theory. Dr. Franklin supposed that all 
 terrestrial substances were pervaded with the electrical fluid, and 
 that by exciting, an electric, the equilibrium of this fluid was 
 destroyed, so that one part of the excited body contained more 
 than its natural quantity of electricity, and the other part less. 
 If in this state a conductor of electricity, as a piece of metal, be 
 brought near the excited part, the accumulated electricity would 
 be imparted to it, and then this conductor would receive more 
 than its natural quantity of the electric fluid. This he called 
 positive electricity. But if a conductor be connected with that 
 part which has less than its ordinary share of the fluid, then 
 the conductor parts with a share of its own, and therefore will 
 then contain less than its natural quantity. This he called 
 negative electricity. When one body positively and another 
 negatively electrified, are connected by a conducting substance, 
 the fluid rushes from the positive to the negative body, and the 
 equilibrium is restored. Thus, bodies which are said to be pos- 
 itively electrified, contain more than their natural quantity of 
 electricity, while those which are negatively electrified, contain 
 less than their natural quantity. 
 
 282. Du Fay's Theory. The other theory is explained thus. 
 When a piece of glass is excited and made to touch a pith-ball, 
 as above stated, then that ball will attract another ball, after 
 which they will mutually repel each other, and the same will 
 happen if a piece of sealing-wax be used instead of the glass. 
 But if a piece of excited glass, and another of wax, be made to 
 touch two separate balls, they will attract each other ; that is, 
 the ball which received its electricity from the wax will attract 
 that which received its electricity from the glass, and will be 
 
 281. Explain Dr. Franklin's theory of electricity. What is meant by positive, and 
 wnat by negative electricity ? What is the consequence, when a positive and a neg- 
 ative body are connected by a conductor? 282. Explain Du Fay's theory. When 
 two balls are electrified, one with glass and the other with wax, will they attract or 
 repel each other 1 What are the two electricities called 7 From what substances 
 are the two electricities obtained? 
 
942 
 
 ELECTRICITY. 
 
 attracted by it. Hence Du Fay concludes that electricity con- 
 sists of two distinct fluids, which exist together in all bodies 
 that they have a mutual attraction for each other that they 
 are separated by the excitation of electrics, and that when thus 
 separated, and transferred to non-electrics, as to the pith-balls, 
 their mutual attraction causes the balls to rush toward each 
 other. These two principles he called vitreous and resinous 
 electricity. The vitreous being obtained from glass, and the 
 resinous from wax and other resinous substances. 
 
 Dr. Franklin's theory is by far the most simple, and will ac- 
 count for most of the electrical phenomena equally well with 
 that of Du Fay, and therefore has been adopted by the most 
 able and recent electricians. 
 
 283. It is found that some substances conduct the electric 
 fluid from a positive to a negative surface with great facility, 
 while others conduct it with difficulty, and others not at all. 
 Substances of the first kind are called conductors, and those of 
 the last non-conductors. The electrics, or such substances as 
 being excited, communicate electricity, are all non-conductors, 
 while the non-electrics, or such substances as do not communi- 
 cate electricity on being merely excited, are conductors. The 
 conductors are the metals, charcoal, water, and other fluids, ex- 
 cept the oils ; also smoke, steam, ice, and snow. The best con- 
 ductors are gold, silver, platina, brass, and iron. 
 
 The electrics, or non-conductors, are glass, amber, sulphur, 
 resin, wax, silk, most hard stones, and the furs of some animals. 
 
 A body is said to be insulated, when it is supported or sur- 
 rounded by an electric. Thus, a stool standing on glass legs, 
 is insulated, and a plate of metal laid on a plate of glass, is 
 insulated. 
 
 284. Electrical Machines. When large quantities of the 
 electric fluid are wanted for experiment, or for other purposes, 
 it is procured by an electrical machine. These machines are of 
 various forms, but all consist of an electric substance of consid- 
 erable dimensions ; the rubber by which this is excited ; the 
 prime conductor, on which the electric matter is accumulated ; 
 the insulator, which prevents the fluid from escaping ; and ma- 
 chinery, by which the electric is set in motion. 
 
 Formerly a glass cylinder was employed as an electric, but 
 
 283. What are conductors? What are non-conductors 7 What substances are 
 conductors 1 What substances are the best conductors 1 What substances are elec- 
 trics, or non-conductors? When is a body eaid to be insulated? 284. What are 
 the several parts of an electrical machine ? 
 
ELECTRICITY. 843 
 
 more recently, round, flat plates of glass, called plate machines, 
 are used instead of cylinders. This is a great improvement, 
 since both sides of the plate are exposed to electrical friction, 
 while in the cylinder, the outside only could be excited. 
 
 This machine is represented by Fig. 265, and consists of a 
 circular plate of glass, from one to two or three feet in diameter, 
 turning on an axis of wood which passes through its center. 
 The plate is rubbed as it revolves, by two leather cushions, A 
 
 FIG. 265. 
 
 Plate Electrical Machine. 
 
 and B, fixed at opposite points of its circumference, and by 
 means of elastic slips of wood adjusted by screws, made to press 
 on its surface. On the opposite side are two other cushions 
 not seen, the plate revolving between them. A hollow brass 
 prime conductor, C, supported by a glass standard D, is attached 
 to the frame of the machine. On each side of the conductor 
 are branches of the same metal, at the ends of which are sharp 
 wires nearly touching the glass plate, and by means of which, 
 the electric fluid is collected and conveyed to the conductor. 
 
 285. Mode of Action. The manner in which this machine 
 acts is easily understood. The friction of the cushions against 
 the glass plate, transfers the electrical fluid from the cushions 
 to the glass, so that while the glass becomes positive, the cush- 
 ions become negative. Meantime, the fluid, which adheres to 
 the surface of the glass, is attracted by the metallic points and 
 
 Describe the electrical machine, Fig. 265. 285. Whence comes the electricity, when 
 the plate is turned 1 Why is it necessary to throw the chain on the ground to obtain 
 more electricity 1 
 
344 ELECTRICITY. 
 
 conveyed to the prime conductor, which being insulated by the 
 glass standard, the electricity is there accumulated in quantities 
 proportionate to the surface of the conductor. 
 
 If the cushions are insulated, the quantity of electricity ob- 
 tained is limited, consisting of that, merely, which the cushions 
 contained, and when this is transferred to the plate, no more 
 can be obtained. It is then necessary to make the cushions 
 communicate with the ground, the great reservoir of electricity, 
 by laying the chain attached to the cushions on the floor or 
 table, when on again turning the machine, more of the fluid 
 will be conveyed to the conductor. 
 
 286. If a person who is insulated takes the chain in his 
 hand,*the electric fluid will be drawn from him, along the chain, 
 to the cushion, and from the cushion will be transferred to the 
 prime conductor, and thus the person will become negatively 
 electrified. If, then, another person, standing on the floor, hold 
 his knuckle near him who is insulated, a spark of electric fire 
 will pass between them, with a crackling noise, and the equili- 
 brium will be restored ; that is, the electric fluid will pass from 
 him who stands on the floor, to him who stands on the stool. 
 But if the insulated person takes hold of a chain, connected 
 with the prime conductor, he may be considered as forming a 
 part of the^conductor, and therefore the electric fluid will be 
 accumulated all over his surface, and he will be positively elec- 
 trified, or will obtain more than his natural quantity of electricity. 
 If now a person standing on the floor touch this person, he will 
 receive a spark of electrical fire from him, and the equilibrium 
 will again be restored. 
 
 287. If two persons stand on two insulated stools, or if they 
 both stand on a plate of glass, or a cake of wax, the one person 
 being connected by the chain with the prime conductor, and 
 the other with the cushion, then, after working the machine, if 
 they touch each other, a much stronger shock will be felt than 
 in either of the other cases, because the difference between their 
 electrical states will be greater, the one having more and the 
 other less than his natural quantity of electricity. But if the 
 two insulated persons both take hold of the chain connected 
 with the prime conductor, or with that connected with the 
 
 286. If an insulated person takes the chain, connected with the cushion, in his 
 nand, what change will be produced in his natural quantity of electricity 1 If the in- 
 eulatod person takes hold of the chain connected with the prime conductor, and the 
 machine be worked, what then will be the change produced in his electrical state 7 
 387. If two insulated persons take hold of the two chains, one connected with the 
 prime conductor, and the other with the cushion, what changes will be produced 7 
 If an insulated person takes the chain, what effect will it produce on him 1 
 
ELECTRICITY. 
 
 345 
 
 cushion, no spark will pass between them, on touching each 
 other, because they will then both be in the same electrical 
 state. 
 
 288. We have seen, Fig. 264, that the pith-ball is first at- 
 tracted and then repelled, by the excited electric, and that the 
 ball so repelled will attract, or be attracted by other substances 
 in its vicinity, in consequence of having received from the ex- 
 cited body more than its ordinary quantity of electricity. 
 
 These alternate movements are amusingly exhibited by plac- 
 ing some small light bodies, such as the figures of men and 
 women, made of pith, or paper, between two metallic plates, the 
 one placed over the other, as in Fig. 266, the upper plate com- 
 municating with the prime conductor, and the other with the 
 ground. When the electricity is communicated to the upper 
 plate, the little figures, being attracted by the electricity, will 
 jump up and strike their heads against it, and having received 
 a portion of the fluid, are instantly repelled, and again attracted 
 by the lower plate, to which they impart their electricity, and 
 
 FIG. 266. 
 
 FIG. 267, 
 
 Attraction and Repulsion. 
 
 then are again attracted, and so fetch and carry the electric 
 fluid from one to the other, as long as the upper plate contains 
 more than the lower one. In the same manner, a tumbler, if 
 electrified on the inside, and placed over light substances, as 
 pith-balls, will cause them to dance for a considerable time. 
 
 288. Explain the reason why the little images dance between the two metallic 
 plates, Fig. 206. 
 
 15* ' 
 
846 ELECTRICITY. 
 
 289. Electrometer. Instruments designed to measure' the 
 intensity of electric action, are called Electrometers. Such an 
 instrument is represented by Fig. 267. It consists of a slender 
 rod of light wood, A, terminated by a pith-ball, which serves as 
 an index. This is suspended at the upper part of the wooden 
 stem, B, so as to play easily backward and forward. The ivory 
 semicircle C, is affixed to the stem, having its center coinciding 
 with the axis of motion of the rod, so as to measure the angle 
 of deviation from the perpendicular, which the repulsion of the 
 ball from the stem produces on the index. 
 
 When this instrument is used, the lower end of the stem is 
 set into an aperture in the prime conductor, and the intensity 
 of the electric action is indicated by the number of degrees the 
 index is repelled from the perpendicular. 
 
 .The passage of the electric fluid through a perfect conductor 
 is never attended with light, or the crackling noise which is 
 heard when it is transmitted through the air, or along the sur- 
 face of an electric. 
 
 290. Several curious experiments illustrate this principle, for 
 if fragments of tin foil, or other metal, be pasted on a piece of 
 glass, so near each other that the electric fluid can pass between 
 them, the whole line thus formed with the pieces- of metal, will 
 be illuminated by the passage of the electricity from one to the 
 other. 
 
 FIG. 268. 
 
 Franklin. 
 
 In this manner figures or words may be formed, as in Fig. 
 268, which, by connecting one of its ends with the prime con- 
 ductor, and the other with the ground, will, when the electric 
 fluid is passed through the whole, in the dark, appear one con- 
 tinuous and vivid line of fire. 
 
 289. What is an electrometer ? Describe that represented in Fig. 267, together 
 with the mode of using it. When the electric fluid passes along a perfect conductor 
 is it attended with light or not ? When it passes along an electric, or through the 
 air, what phenomena does it exhibit? 290. Describe the experiment, Fig. 268, in- 
 tended to illustrate this principle. 
 
5P rLI 2& 
 
 ELECTRICITY. J7 V- V OF TH^47 
 
 291. Electrical Light. Electrical ligfct seems not to differ 
 in any respect, from the light of the. Sun. or of a burning lamp. 
 Dr. Wollaston observed, that when this light was seen through 
 a prism, the ordinary, colors arising from the decomposition of 
 light were obvious. 
 
 292. When the electric fluid is discharged from a point, it is 
 always accompanied by a current of air, whether the electricity 
 be positive or negative. The reason of this appears to be, that 
 the instant a particle of air becomes electrified, it repels, and is 
 repelled, by the point from which it received tie electricity. 
 
 Several curious little experiments are 
 made on this principle. Thus, let two FIG - 269< 
 
 cross wires, as in Fig. 269, be suspended 
 on a pivot, each having its point bent in a 
 contrary direction, and electrified by being 
 placed on the prime conductor of a ma- 
 chine. These points, so long as the machine 
 is in action, will give oft' streams of elec- 
 tricity ; and as the particles of air repel the 
 points by which they are electrified, the 
 little machine will turn round rapidly, in the direction contrary 
 to that of the stream of electricity. Perhaps, also, the*eaction 
 of the atmosphere against the current of air given off by the 
 points, assists in giving it motion. 
 
 293. Leyden Vials. When one part or side of an electric is 
 positively, the other part or side is negatively electrified. Thus, 
 if a plate of glass be positively electrified on one side, it will be 
 negatively electrified on the other, and if the inside of a glass 
 vessel be positive, the outside will be negative. 
 
 Advantage of this circumstance is taken, in the construction 
 of electrical jars, called from the place where they were first 
 made, Leyden vials. 
 
 The most common form of this jar is represented by Fig. 
 270. It consists of a glass vessel, coated on both sides up to 
 A, with tin foil ; the upper part being left naked, so as to pre- 
 vent a spontaneous discharge, or the passage of the electric fluid 
 from one coating to the other. A metallic rod, rising two 01 
 three inches above the jar, and terminated at the top with a 
 
 291. What is the appearance of electrical light through a prism? 292. Describe 
 Fig. 269. and explain the principle on which its motion depends. 293. Suppose one 
 part or side of an electric is positive, what will be the electrical state of the other side 
 or parti What part of the electrical apparatus is constructed on this principle 1 
 How is the Leyden vial constructed ? Why is not the whole surface of this vial cov- 
 ered with the tin foil ) 
 
348 ELECTRICITY. 
 
 brass ball, which is called the knob of the jar, is made to de- 
 scend through the cover, till it touches the interior coating. It 
 is along this rod that the charge of electricity is conveyed to 
 the inner coating, while the outer coating is made to communi- 
 cate with the ground. 
 
 FIG. 270. FIG. 271. 
 
 Leyden Jars. 
 
 294. When a chain is passed from the prime conductor of an 
 electrical machine to this rod, the electricity is accumulated on 
 the tin foil coating, while the glass above the tin foil prevents 
 its escape, and thus the jar becomes charged. By connecting 
 togetl 1 jr a sufficient number of these jars, any quantity of the 
 electric fluid may be accumulated. For this purpose, all the 
 inferior coatings of the jars are made to communicate with each 
 other, by metallic rods passing between them, and finally ter- 
 minating in a single rod. A similar union is also established, 
 by connecting the external coats with each other. When thus 
 arranged, the whole series may be charged, as if they formed 
 but one jar, and the whole series may be discharged at the 
 same instant. Such a combination of jars is termed an electri- 
 cal battery. 
 
 295. For the purpose of making a direct communication be- 
 tween the inner and outer coating of a single jar, or battery, by 
 which a discharge is effected, an instrument called a discharg- 
 ing-rod is employed. It consists of two bent metallic rods, 
 terminated at one end by brass balls, and at the other end con- 
 nected by a joint. This joint is fixed to the end of a glass 
 
 294. How is a Leyden vial charged ? In what manner may a number of these vials 
 be charged ? What is an electrical battf ry '! 295. Explain the design of Fig. 271, and 
 show how an equilibrium is produced by the discharging-rod. 
 
ELECTRfCITY. 349 
 
 handle, and the rods being movable at the joint, the balls can 
 be separated or brought near each other, as occasion requires. 
 When opened to a proper distance, one ball is made to touch 
 the tin foil on the outside of the jar, and then the other is 
 brought into contact with the knob of the jar, as seen in Fig. 
 271. In this manner a discharge is effected, or an equilibrium 
 produced between the positive and negative sides of the jar. 
 
 296. When it is desired to pass the charge through any sub- 
 stance for experiment, then an electrical circuit must be estab- 
 lished, of which the substance to be experimented upon must 
 form a part. That is, the substance must be placed between 
 the ends of two metallic conductors, one of which communicates 
 with the positive, and the other with the negative side of the 
 jar, or battery. 
 
 297. When a person takes the electrical shock in the usual 
 manner, he merely takes hold of the chain connected with the 
 outside coating, and the battery being charged, touches the 
 knob with his finger, or with a metallic rod. On making this 
 circuit, the fluid passes through the person from the positive to 
 the negative side. 
 
 Any number of persons may receive the electrical shock, by 
 taking hold of each other's hand, the first person touching the 
 knob, while the last takes hold of a chain connected with the 
 external coating. In this manner, hundreds, or, perhaps, thou- 
 sands of persons, will feel the shock at the same instant, there 
 being no perceptible interval in the time when the first and the 
 last person in the circle feels the sensation excited by the passage 
 of the electric fluid. 
 
 298. Atmospheric Electricity. The atmosphere always con- 
 tains more or less electricity, which is sometimes positive, and 
 at others negative. It is, however, most commonly positive, 
 and always so when the sky is clear or free from clouds or fogs. 
 It is always stronger in winter than in summer, and during the 
 day than during the night. It is also stronger at some hours 
 of the day than at others ; being strongest about 9 o'clock in 
 the morning, and weakest about the middle of the afternoon. 
 These different electrical states are ascertained by means of long 
 
 296. When it is desired to pass the electrical fluid through any substance, where 
 must it be placed in respect to the two sides of the battery ? 297. Suppose the bat- 
 tery is charged, what must a person do to take the shock? What circumstance ii 
 related, which shows the surprising velocity with which electricity is transmitted 1 
 293. Is the electricity of the atmosphere positive or negative? At what times doea 
 the atmosphere cont'ain most electricity? How are the different electrical states of 
 the atmosphere ascertained 1 
 
350 ELECTRICITY. 
 
 metallic wires extending from one building to another, and con- 
 nected with electrometers. 
 
 299. It was .proved by Dr. Franklin, that the electric fluid 
 and lightning are the same substance, and this identity has 
 been confirmed by subsequent writers on this subject. 
 
 If the properties and phenomena of lightning be compared 
 with those of electricity, it will be found that they differ only in 
 respect to degree. Thus, lightning passes in irregular lines 
 through the air ; the discharge of an electrical battery has the 
 same appearance. Lightning strikes the highest pointed ob- 
 jects takes in its course the best conductors sets fire to non- 
 conductors, or rends them in pieces, and destroys animal life ; 
 all of which phenomena are caused by the electric fluid. 
 
 300. LIGHTNING RODS. Buildings may be secured from the 
 effects of lightning, by fixing to them a metallic rod, which is 
 elevated above any part of the edifice and continued to the 
 moist ground, or to the nearest water. Copper, for this pur- 
 pose, is better than iron, not only because it is less liable to rust, 
 but because it is a better conductor of the electric fluid. The 
 upper part of the rod should end in several fine points, which 
 must be covered with some metal not liable to rust, such as 
 gold, platina, or silver. 
 
 301. No protection is afforded by the conductor, unless it is 
 continued without interruption from the top to the bottom of 
 the building, and it can not be relied on as a protector, unless 
 it reaches the moist earth, or ends in loater connected with the 
 earth. Conductors of copper may be three-fourths of an inch 
 in diameter, but those of iron should be at least an inch in 
 diameter. In large buildings, complete protection requires 
 many lightning rods, or that they should be elevated to a 
 height above the building in proportion to the smallness of their 
 numbers, for modern experiments have proved that a rod only 
 protects a circle around it, the radius of which is equal to twice 
 its length above the building. 
 
 302. Thus a rod 20 feet above the building, will protect a 
 space of 40 feet from it in all directions. 
 
 299. Who first discovered that electricity and lightning are the same ? What phe- 
 nomena are mentioned which belong in common to electricity and lightning? 300. 
 How may buildings be protected from the effects of lightning? Which is the best 
 conductor, iron or copper? 301. What circumstances are necessary, that the rod 
 mav be relied on as a protector! 302. What diameter will a rod 20 feet above th 
 building protect ? 
 
MAGNETISM. 851 
 
 -CHAPTER XIV. 
 
 MAGNETISM. 
 
 303. The native Magnet, or Loadstone, is an ore of iron, 
 which is found in various parts of the world. Its color is iron 
 black ; its specific gravity from 4 to 5, and it is sometimes 
 found in crystals. 
 
 This substance, without any preparation, attracts iron and 
 steel, and when suspended by a string, will turn one of its sides 
 toward the north, and another toward the south. 
 
 It appears that an examination of the properties of this 
 species of iron ore, led to the important discovery of the mag- 
 netic needle, and subsequently laid the foundation for the 
 science of magnetism ; though at the present day magnets are 
 made without this article. 
 
 304. The whole science of magnetism is founded on the fact, 
 that pieces of iron or steel, after being treated in a certain man- 
 ner, and then suspended, will constantly turn one of their ends 
 toward the north, and consequently the other toward the south. 
 The same property has been more recently proved to belong to 
 the metals nickel and cobalt, though with much less intensity. 
 
 305. Still more recently, it has been found by Prof. Faraday, 
 that when a strong electro-magnet is employed, the following 
 metals are acted upon with varying intensity, and therefore 
 must be added to the list of magnetic metals, viz., manganese, 
 chromium, cerium, titanium, palladium, platinum, and osmium. 
 
 306. The poles of a magnet are those parts which possess the 
 greatest power, or in which the magnetic virtue seems to be 
 concentrated. One of the poles points north, and the other 
 south. The magnetic meridian is a vertical circle in the heavens, 
 which intersects the horizon at the points to which the magnetic 
 needle, when at rest, directs itself. 
 
 307. The axis of a magnet, is a right line which passes from 
 one of its poles to the other. 
 
 The equator of a magnet, is a line perpendicular to its axis, 
 arid is at the center between the two poles. 
 
 303. What is the native magnet or loadstone ? What are the properties of the 
 loadstone \ 304. On what is tlie whole subject of magnetism founded ? What other 
 metals besides iron possess the magnetic property? 305. What metals besides iron, 
 nickel, and cobalt, are magnetic ? 306. What are the poles of a magnet 7 307. What 
 is the axis of a magnet 1 What is the equator of a magnet 7 
 
352 MAGNETISM. 
 
 308. Leading Properties. The leading properties of the 
 magnet are the following : It attracts iron and steel, and when 
 suspended so as to move freely, it arranges itself so as to point 
 north and south ; this is called the polarity of the magnet. 
 When the south pole of one magnet is presented to the north 
 pole of another, they will attract each other ; this is called mag- 
 netic attraction. But if the two north, or two south poles be 
 brought together, they will repel each other, and this is called 
 magne tic rep u Ision. 
 
 309. When a magnet is left to move freely, it does not lie in 
 a horizontal direction, but one pole inclines downward, and con- 
 sequently the other is elevated above the line of the horizon. 
 This is called the dipping, or inclination of the magnetic needle. 
 Any magnet is capable of communicating its own properties to 
 iron or steel, and this, again, will impart its magnetic virtue to 
 another piece of steel, and so on indefinitely. 
 
 310. If a piece of iron or steel be brought near one of the 
 poles of a magnet, they will attract each other, and if suffered 
 to come into contact, will adhere so as to require force to sep- 
 arate them. This attraction is mutual ; for the iron attracts the 
 magnet with the same force that the magnet attracts the iron. 
 This may be proved, by placing the iron and magnet on pieces 
 of wood floating on water, when they will be seen to approach 
 each other mutually. 
 
 311. force of Attraction. The force of magnetic attraction 
 varies with the distance in the same ratio as the force of gravity ; 
 the attracting force being inversely as the square of the distance 
 between the magnet and the iron. 
 
 312. The magnetic force is not sensibly affected by the in- 
 terposition of any substance except those containing iron, or 
 steel. Thus, if two magnets, or a magnet and piece of iron, 
 attract each other with a certain force, this force will be the 
 same if a plate of glass, wood, or paper, be placed between 
 them. Neither will the force be altered, by placing the two 
 attracting bodies under water, or in the exhausted receiver of 
 an air-pump. This proves that the magnetic influence passes 
 equally well through air, glass, wood, paper, water, and a 
 vacuum. 
 
 308. What is meant by the polarity of a magnet ? When do two magnets attract, 
 and when repel each other? 309. What is understood by the dipping of the mag- 
 netic needle 1 310. How is it proved that the iron attracts the magnet with the same 
 force that the majrnet attracts the iron 1 31 1. How does the force of magnetic attrac- 
 tion vary with the distance ? 312. Does the magnetic force vary with the interposi- 
 tion of any substance between the attracting bodies J 
 
MAGNETISM. 353 
 
 313. Destroyed by Heat. Heat weakens the attractive power 
 of the magnet, and a white heat entirely destroys it. Electricity 
 will change the poles of the magnetic needle, and the explosion 
 of a small quantity of gunpowder on one of the poles, will have 
 the same effect. 
 
 314. The attractive power of the magnet may be increased 
 by permitting a piece of steel to adhere to it, and then suspend- 
 ing to the steel a little additional weight every day, for it will 
 sustain, to a certain limit, a little more weight on one day than 
 it would on the day before. 
 
 315. Small natural magnets will sustain more than large 
 ones in proportion to their weight. It is rare to find a natural 
 magnet, weighing 20 or 30 grains, which will lift more than 
 thirty or forty times its own weight. But a minute piece of 
 natural magnet, worn by Sir Isaac Newton, in a ring, which 
 weighed only three grains, is said to have been capable of lifting 
 746 grains, or nearly 250 times its own weight. 
 
 316. Artificial Magnets. The magnetic property may be 
 communicated from the loadstone, or artificial magnet, in the 
 following manner, it being understood that the north pole of 
 one of the magnets employed, must always be drawn toward the 
 south pole of the new magnet, and that the south pole of the 
 other magnet employed, is to be drawn in the contrary direc- 
 tion. The north poles of magnetic bars are usually marked 
 with a line across them, so as to distinguish this end from the 
 other. 
 
 Place two magnetic bars 
 A and B, Fig. 272, so that 
 the north end of one may 
 be nearest the south end 
 of the other, and at such 
 a distance that the ends of _,_______ 
 
 the steel bar to be touched, \MwJ c 
 
 may rest upon them. Hav- Artificial Magnets. 
 
 ing thus arranged them, 
 as shown in the figure, take the two magnetic bars, D and E, 
 and apply the south end of E, and the north end of D, to the 
 middle of the bar C, elevating their ends as seen in the figure. 
 Next separate the bars E and D, by drawing them in opposite 
 directions along the surface of C, still preserving the elevation 
 
 313. What is the effect of heat on the magnet 1 What is the effect of electricity, or 
 the explosion of gunpowder on it 1 314. How may the power of a magnet be in- 
 creased ? 315. What is said concerning the comparative powers of great and small 
 magnets 1 316. Explain Fig. 272, and describe the mode of making magnet. 
 
354 MAGNETISM. 
 
 of their ends ; then removing the bars D and E, to the distance 
 of a foot or more from the bar C, bring their north and south 
 poles into contact, and then having again placed them on the 
 middle, C, draw them in contrary directions, as before. The 
 same process must be repeated many times on each side of the 
 bar, C, when it will be found to have acquired a strong and per- 
 manent magnetism. 
 
 317. If a bar of iron be placed, for a long period of time, 
 in a north and south direction, or in a perpendicular position, 
 it will often acquire a strong magnetic power. Old tongs, 
 pokers, and fire shovels, almost always possess more or less 
 magnetic virtue ; and the same is found to be the case with the 
 iron window bars of ancient houses, whenever they have hap- 
 pened to be placed in the direction of the magnetic line. 
 
 318. A magnetic needle, such as is employed in the mariner's 
 and surveyor's compass, may be made by fixing a piece of steel 
 on a board, and then drawing two magnets from the center 
 toward each end, as directed at Fig. 272. Some magnetic 
 needles, in time, lose their virtue, and require again to be mag- 
 netized. This may be done by placing the needle still suspend- 
 ed on its pivot, between the opposite poles of two magnetic 
 bars. While it is receiving the magnetism, it will be agitated, 
 moving backward and forward, as though it were animated ; 
 but when it has become perfectly magnetized, it will remain 
 quiescent. 
 
 FIG. 273. 
 
 Magnetic Rotation. 
 
 319. Magnetic Rotation. It is quite interesting to observe 
 the different" directions the needle of a small magnetic compass 
 
 317. In what positions do bars of iron become magnetic spontaneously? 
 318. How may a needle he magneti/.ed without removing from its pivot 1 
 
MAGNETISM. 
 
 355 
 
 will assume when moved round a bar magnet. If the latter be 
 laid on the table, and the former carried slowly around it, from 
 S, or south, to N, or north, and so back again on the other 
 side, the needle will alternately take all the positions shown 
 by Fig. 273. 
 
 320. DIP OF THE MAGNET. The dip, or inclination of the 
 magnetic needle, is its deviation from its horizontal position, as 
 already mentioned. A piece of steel, or a needle which will 
 rest on its center, in a direction parallel to the horizon, before 
 it is magnetized, will afterward incline one of its ends toward 
 the earth. This property of the magnetic needle was discov- 
 ered by a compass-maker, who, having finished his needles 
 before they were magnetized, found that immediately after- 
 ward, their north ends inclined toward the earth, so that he 
 was obliged -to add small weights to their south poles, in order 
 to make them balance, as before. 
 
 321. The dip of the magnetic needle is measured, by a grad- 
 uated circle, placed in the vertical position, with the needle 
 suspended by its side. Its inclination from a horizontal line, 
 marked across the face of this circle, is the 
 
 measure of its dip. The circle, as usual, FIG. 274. 
 
 is divided into 360 degrees, and these into 
 minutes and seconds. 
 
 322. Dipping Needle. Fig. 274 is said 
 to represent a convenient form of the dip- 
 ping needle. It is a strongly magnetized 
 steel needle, turning on the center of grav- 
 ity A B, in a brass frame which is suspend- 
 ed by a thread. Thus the needle has 
 universal motion. The scale is omitted as 
 unnecessary for the present purpose. 
 
 323. The dip of the needle does not 
 vary materially at the same place, but dif- 
 fers in different latitudes, increasing as it is 
 carried toward the north, and diminishing 
 as it is carried toward the south. At 
 London, the dip for many years has varied 
 little from 72 degrees. In the latitude of 
 80 degrees north, the dip, according to the 
 observations of Captain Parry was 88 de- Dipping Needle. 
 grees. 
 
 320. How was the dip of the magnetic needle first discovered? 321. In what 
 manner is the dip measured 7 
 
356 
 
 MAGNETISM. 
 
 324. VARIATION OF THE MAGNET. Although, in general 
 terms, the magnetic needle is said to point north and south, yet 
 this is very seldom strictly true, there being a variation in its 
 direction, which differs in degree at different times and places. 
 This is called the variation, or declination, of the magnetic 
 needle. 
 
 325. This variation is determined at sea, by observing the 
 different points of the compass at which the sun rises, or sets, 
 and comparing them with the true points of the sun's rising or 
 setting, according to astronomical tables. By such observa- 
 tions it has been ascertained that the magnetic needle is contin- 
 ually declining alternately to the east or west from due north, 
 and that this variation differs in different parts of the world at 
 the same time and at the same place at different times. 
 
 326. The annexed table shows at once, the dip, or inclina- 
 tion, and the variation or declination of the needle, for a series 
 of years. It was formed from observations made at Brussels, 
 and by it there appears to be a gradual, but constant diminu- 
 tion of the angle, both of inclination and declination, in Europe. 
 
 Month. 
 
 Year. 
 
 Inclination. 
 
 Declination. 
 
 October, . . 
 
 1827, 
 
 * 680, 56', 5" 
 
 220, 28', 8" 
 
 March, . ... 
 
 1830, 
 
 680, 52', 6" 
 
 220, 25', 3" 
 
 March, . . 
 
 1832, 
 
 680, 49/, i// 
 
 220, 19/, 0" 
 
 March, . . 
 
 1833, 
 
 680, 42/, 8" 
 
 220, is/, 4// 
 
 April, . . 
 
 1834, 
 
 680, 38/, 4" 
 
 22, 15', 2" 
 
 March, . . . 
 
 1835, 
 
 680, ss/ 0" 
 
 220, 6', 7" 
 
 March, . . 
 
 1836, 
 
 680, 32/, 2" 
 
 220, 7/ ; 6" 
 
 March, . . . 
 
 1837, 
 
 680, 28', 8" 
 
 220, 4/, 3// 
 
 March, . . 
 
 1838, 
 
 680, 26', 1" 
 
 220, 3/,7/x 
 
 March, . . . 
 
 1839, 
 
 680, 22', 4" 
 
 210, 53/ 6'' 
 
 327. The difference in the declination, which may be of 
 much importance, as on it may depend the safety of ships at 
 sea, is very material in different countries, and at different 
 periods. Thus at present it is about 24 west, at London. 
 At Paris, 22 west. At New York, 5 25' west, and at Hart- 
 ford, about 6 west. 
 
 Before 1660, the variation at London, was toward the east, 
 and on that year the needle pointed due north. From that 
 
 323. What circumstance increases or diminishes the dip of the needle 7 324. What 
 is meant hy the declination of the magnetic needle? 326. What changes does the 
 above table indicate 7 327. Why is the difference of declination of importance to 
 ships 1 i 
 
ELECTRO-MA GXETISM. 
 
 357 
 
 time to the present, it has gained from two to six degrees to- 
 ward the west every year. 
 
 The greatest variation of the magnetic needle, recorded, was 
 that observed by Capt. Cook, which was about 43 west. This 
 was in S. lat. 60, and E. longitude, 92 36'. 
 
 CHAPTER XV. 
 
 ELECTRO. MAGNETISM. 
 
 328. When two metals, one of which is more easily oxyda- 
 ted than the other, are placed in acidulated water, and the two 
 metals are made to touch each other, or a metallic communica- 
 tion is made between them, there is excited an electrical or gal- 
 vanic current, which passes from the metal most easily oxy dated, 
 through the water, to the other metal, and from the other metal 
 through the water around to the first metal again, and so in a 
 perpetual circuit. 
 
 FIG. 275. 
 
 FIG. 276. 
 
 Galvanic Current. 
 
 Galvanic Battery. 
 
 o2 v J. If we take, for example, one slip of zinc, and another 
 of copper, and place them in a cup of diluted sulphuric acid, 
 Fig. 275, tiieir upper ends in contact and above the water, and 
 
 328. What conditions aie necessary to excite the galvanic action ? From which 
 metal does the galvanism proceed 7 329. Describe the circuit by Fig. 275. 
 
358 
 
 ELECTRO-MAGNETISM. 
 
 their lower ends separated, then there will be constituted a 
 galvanic circle, of the simplest form, consisting of three ele- 
 ments, zinc, acid, copper. The galvanic influence being excited 
 by the acid, will pass from the zinc Z, the metal most easily 
 oxydated, through the acid, to the copper C, and from the cop- 
 per to the zinc again, and so on continually, until one or the 
 other of the elements is destroyed, or ceases to act. 
 
 The same effect \vill be produced, if instead of allowing the 
 metallic plates to come in contact, a communication between 
 them be made by means of wires, as shown by Fig. 276. In 
 this case, as well as in the former, the electricity proceeds from 
 the zinc Z, which is the positive side, to the copper C, being 
 conducted by the wires in the direction shown by the arrows. 
 
 330. The completion of the circuit by means of wires enables 
 us to make experiments on different substances by passing the 
 galvanic influence through them, this being the method em- 
 ployed to exhibit the effects of galvanic batteries, and by which 
 the most intense heat may be produced. 
 
 When the two poles of a battery are connected by means of 
 a copper wire of a yard or two in length, the two parts being 
 supported on a table in a north and south direction, for some 
 of the experiments, but in others the 'direction must be changed 
 as will be seen. This wire, it will be remembered, is called the 
 uniting wire. 
 
 331. Theory. In theory, the 
 
 r'tive electricity is produced 
 the mutual action of the 
 acid, water, and zinc ; the water, 
 in small quantity, being decom- 
 posed. If this action is too vio- 
 lent, that is, if the acid is too 
 strong and the hydrogen pro- 
 duced in too large quantity, the 
 electrical current is diminished, 
 or ceases almost entirely. 
 
 332. GALVANIC BATTERY. 
 One of the most convenient 
 forms of a galvanic battery for ex- 
 periments described in this work 
 is represented by Fig. 277. It 
 consists of a cylinder of sheet 
 copper, within which is another 
 
 U. How is positive dectrieity produced 1 
 
 FIG. 277. 
 
ELECTRO-MAGNETISM. 359 
 
 of zinc. The zinc has for its bottom a piece of sheep-skin, or 
 bladder, tied on with a string, and is suspended an inch or two 
 from the bottom of the copper cylinder. Or, the whole inner 
 cylinder may be made of leather with a slip of zinc within it. 
 This is done to prevent the fluid which the inner cylinder contains 
 from mixing with that contained between the two ; and still, the 
 leather being porous, the water it contains conducts the galvanic 
 influence from one cell to the other, as already stated. The 
 diameter of the outer cup may be five or six inches, and the 
 inner one three or four. The zinc may be suspended by making 
 two holes near the top and tying on a piece of glass tube or a 
 slip of wood. This part has often to be removed and cleaned, 
 by scraping off the black oxyd, which, if it remains, will pre- 
 vent the action of the battery. The action will be sustained 
 much longer if the zinc is amalgamated by spreading on it a 
 little mercury before it is used, and while the surface is bright. 
 
 The cups P N", are the positive and negative poles. They may 
 be made of percussion caps, soldered to the ends of two copper 
 wires ; the other ends being connected by soldering, or other- 
 wise, one with the zinc, and the other with the copper, cylinder. 
 
 The inner cup is to be filled with water, mixed with about a 
 twentieth part of sulphuric acid, while the cell between the two 
 contains a saturated solution of sulphate of copper, or blue 
 vitriol. In order to keep the solution saturated, especially when 
 casts are to be taken, some of the solid vitriol is to be tied in a 
 rag and suspended in it. 
 
 This battery, it will be seen, differs materially from that 
 hereafter to be described under the name of Grove's battery, 
 but for common purposes it is equally useful ; is much more 
 readily made, and costs only a tenth as much. 
 
 GROVE'S BATTERY. 
 
 333. This is the most powerful arrangement, according to its 
 size and cost, which has been proposed, and is that generally used 
 for telegraphic purposes. Fig. 278 shows a battery of twelve 
 cups, each of which consists of a cylinder of amalgamated 
 zinc, within which is a cup of unglazed clay ; these being placed 
 within an outer cup of glass. To the zinc is attached a con- 
 ducting arm of the same metal, which reaches to the next 
 series of cups, and at the end of which is attached a thin piece of 
 platina, which dips into the porous cup, as shown by the figure. 
 
 332. Explain Fisr. 277, and show the action ef the battery. 333. Describe the prin- 
 ciple of Grove's battery, Fig, 278, 
 
360 
 
 ELECTRO-MAGNETISM. 
 FIG. 278. 
 
 &rave'a Twevle-Cup Battery. 
 
 The battery is charged,, by filling the clay cup with nitric 
 acid, and the space within and around the zinc, which is open 
 at the bottom and side, with sulphuric acid, diluted with 30 
 parts of water. The action is strong, and requires very little 
 expense. 
 
 334. Tobacco Pipe Battery. For telegraphic batteries the 
 vessels are about four inches high, but for common experiments 
 any one may make a miniature battery in the following man- 
 ner, and at a very trifling expense. 
 
 Procure six toy tumblers, an inch and a half high. Cut 
 from sheet zinc, strips of such 
 size as to form cylinders to FIG. 279. 
 
 go within these tumblers. Cut 
 one end of each strip nearly off, 
 and a quarter of an inch wide, 
 as shown at A, Fig. 279, and 
 turn it up so as to make a con- 
 necting arm with the next cup. 
 At the end of this arm cut a slit 
 B, into which put a little slip of 
 platina foil, half an inch wide, and 
 an inch long. In this manner the 
 whole can be made without sold- 
 ering the arm to the CUp, which, Tobacco Pipe Battery. 
 when amalgamated, will drop off. 
 
 Next take six tobacco pipes, and breaking off the stems, stop 
 the orifices of the bowls with sealing-wax, and the elements of 
 your little battery is finished. 
 
 Now take a little mercury in a bowl, and touching the zinc 
 
 334. How is the tobacco pipe battery made? 
 
ELECTRO-MAGNETISM. 
 
 361 
 
 cylinders to it, a little will adhere to the metal, and may be 
 spread over its surface with a wisp of cotton. The action is 
 thus much increased. 
 
 335. Lastly, put the bowls within the zinc cylinders, and these 
 into the tumblers, and then fill the bowls with nitric acid, and 
 the tumblers with sulphuric acid diluted with 30 parts of water, 
 fixing the arms so that the platina will dip into the bowls, and 
 the action will commence instantly. 
 
 AVith this little battery, which any one of ordinary ingenuity 
 can make, all the common experiment with a galvanic battery 
 may be performed. 
 
 336. Circular Motion of Electro- Magnetism. In conse- 
 quence of the circular magnetic currents which seem to 
 emanate from the regular influence 
 
 of the battery, the fluid may be FIG. 280. 
 
 made to act so as to produce a 
 continued rotation of the conducting 
 wire, or the magnet. 
 
 Magnet Revolving Around the 
 Conducting Wire. The arrangement 
 shown by Fig. 280, and which causes 
 the magnet to revolve around the 
 conducting wire, consists of the mag- 
 net N S, having an angular bend in 
 the middle, where it becomes hori- 
 zontal, while the extremities are vert- 
 ical. To the north pole, or lower 
 end of the magnet N, is attached a 
 piece of brass, at a right-angle with 
 the magnet, which has a little pro- 
 jection, forming a pivot, which rests 
 in an agate cup, fixed to the stand. 
 A wire loop attached to the upper 
 pole of the magnet S, encircles the 
 conducting wire, and thus keeps the 
 magnet in its place. The galvanic 
 current is conveyed by this wire, the 
 lower end of which dips into a little 
 cup of mercury on the horizontal Revolving Magnet. 
 
 portion of the magnet. 
 
 337. The wire has a brass cup at A, containing mercury, and 
 into which the pole of the battery is inserted. From this cup 
 
 335. With what is this battery charged ? 
 
 16 
 
362 
 
 ELECTRO-MAGNETISM. 
 
 FIG. 281. 
 
 projects a bent wire, as seen in the figure, the end of which 
 dips into a circular cistern of mercury, contained in a brass 
 cup, and through which the magnet revolves. A wire passes 
 through the side of the cistern to the mercury, and terminates 
 in the screw-cup B, into which the other pole of the battery is 
 placed. 
 
 Now on making the connection, the current flows down by 
 the side of the upper pole of the magnet to the middle, and 
 then takes the direction of the cup B, so as not to act on the 
 lower pole, the galvanic force being between the mercury in 
 the cistern and the bent wire, and by the attraction of which 
 the magnet revolves rapidly around the conducting wire. On 
 changing the poles the rotation will be in a contrary direction. 
 
 338. REVOLVING SPUR-WHEEL. Many curious experiments 
 are made by combining the action of electricity with that of 
 magnetism. Such a combination is shown by Fig. 281 where 
 W is a copper wheel cut into points, and made to revolve be- 
 tween the legs of a U magnet fixed in an upright position. 
 The axis of the wheel is supported by strips of brass fastened 
 to the magnetic poles N and S. 
 
 The trough T may be of brass or 
 wood, and is placed between the 
 bifurcation of the magnet. This 
 contains a little mercury, into 
 which the teeth of the wheel just 
 dip, as they revolve. 
 
 339. On the platform or stand, 
 to which the magnet is fastened, 
 are two screw-cups to which the 
 opposite poles of the battery are 
 fastened. One of these cups is 
 connected with the magnet, and 
 through that, with the axis of the 
 wheel, and the other with the 
 mercury in the trough. Now 
 on making the connection be- 
 tween the poles of the battery, 
 the wheel begins to move, in con- 
 sequence of the attraction be- 
 tween the points of the wheel 
 
 and the mercury, and if the cur- Revolving wheel. 
 
 rent is strong the wheel turns 
 
 with great velocity, snapping and striking fire as the point* 
 
VIBRATING WIRE. 
 
 363 
 
 approach the fluid metal. The points of the wheel should be 
 amalgamated to make the experiment succeed well. 
 
 CLOCK-WORK VIBRATING WIRE. 
 
 340. This is a curious and singular arrangement, and will 
 quite astonish those who are not conversant with motions com- 
 municated by galvanic influence. 
 
 The cut, Pig. 282, shows a connection between the spiral 
 ribbon, A, and the single Grove's battery, B, by means of a 
 
 FIG. 282. 
 
 Clock-work Vibrating Wire. 
 
 copper wire. The bent wire C C, suspended in the middle, is 
 set in motion by a spring below the milled-head F, and is made 
 to vibrate rapidly by clock-work, the ends of the wire dipping 
 alternately in the glass cups C C, containing mercury. The 
 spring is wound up by turning the milled-head. 
 
 The glass cups are open at the bottom to allow the mercury 
 to come in contact with the brass pillars on which they stand. 
 
 Both of these pillars are connected with one of the screw -cups 
 D D, while the other cup is connected with the middle brass 
 pillar E, on which is a brass cup of mercury. From the latter 
 cup ascends a vertical wire, attached to the vibrating wire, as 
 the figure shows. 
 
 341. Such a quantity of mercury is put into the brass cup 
 as to keep the end of the vertical wire covered, and enough 
 into the glass cups C C, to allow one end of the vibrating wire 
 to leave the mercury in the cup, before the other end dips into 
 that metal. 
 
 342. The spiral ribbon is made by cutting strips of sheet 
 copper, an inch wide, into lengths, and soldering them together. 
 
864 BELL ENGINE. 
 
 Then having covered the whole with cotton cloth, and rolled it 
 into a spiral, like a watch-spring, the article in question is formed. 
 At each end, the ribbon being sometimes 100 feet long, there is 
 fixed a screw-cup to contain mercury for the poles of the bat- 
 tery. In the above, one end is connected with the battery, and 
 the other with the screw-cup D, and so to E, on the platform. 
 
 The current must be transmitted through the two instru- 
 ments in succession, by connecting one of the screw-cups with 
 one of those attached to the spiral wire, and the other with 
 the pole of the battery ; the remaining cup on the spiral being 
 made to communicate with the other pole of the battery. 
 
 343. Action. On making the connection with the spiral, 
 as shown, and turning the milled-head to put the vibrating 
 wire in motion, a brilliant spark will be seen, and a loud snap 
 heard, at the alternate rupture of the contact between the ends 
 of the wire and the mercury in the cups C C. 
 
 With a battery of a few pairs of large sized plates, the size 
 of the spark will be greatly increased. 
 
 A strong shock may also be given, especially when the mer- 
 cury in the cups C C are covered with a little oil. 
 
 [The author is indebted for the above, as well as for several 
 other cuts of the same kind, to Davis's "Manual of Magnetism," 
 Boston, 1850. 
 
 This work contains the most complete and extensive set of 
 figures, and their descriptions, on the subjects of Magnetism 
 and Electricity, ever published in this country. Price $1,00. 
 The number of figures, 184.] 
 
 REVOLVING BELL ENGINE. 
 
 S44. This curious arrangement is the invention of Mr. Page. 
 It consists of a U shaped magnet, the north and south poles, 
 N S, being fixed in the base board. Between these is a small 
 electro-magnet of iron, surrounded with insulated copper wire. 
 This is fixed to a revolving axis, or wire, the upper end of 
 which is confined in the bend of the large magnet, and the 
 lower one running in a support below the electro-magnet. 
 On the outside of the U magnet are the connecting screws for 
 the opposite poles of the battery, by which the machine is oper- 
 ated. On the axis, and connected with the notches of the 
 wheel, is an endless screw, and with this is connected the ham- 
 mer, which strikes the bell, seen as a crown on the figure. 
 
 345. Action. The operation, or motion, of this curious little 
 
BELL ENGINE. 
 
 365 
 
 engine, depends on the alternate FIG. 283. 
 
 attraction and repulson of the poles 
 of the U magnet, and those of the 
 small electro-magnet bet ween them. 
 The magnetism of the latter de- 
 pends on the influence of the bat- 
 tery with which it is connected, 
 and therefore ceases when this 
 connection is broken. The revo- 
 lution is therefore caused by the 
 mutual repulsion, and then the 
 mutual attraction between the two 
 opposite poles of the two magnets, 
 as the connection is broken and the 
 poles of the electro-magnet are 
 reversed. 
 
 The hammer is made to strike 
 by a pin on the wheel, moved by 
 the endless screw, and which press- 
 es back the handle until it is re- 
 leased, when a spiral spring on the 
 handle impels it against the bell. 
 
 346. If the wheel has 100 teeth, 
 as in the cut, the electro-magnet 
 must revolre 100 times in order to 
 
 produce one revolution of the wheel, and consequently one 
 stroke on the bell. The velocity of the electro -magnet in this 
 machine, as shown by the striking of the hammer, is some- 
 times equal to 6000 revolutions in a minute. 
 
 347. VIBRATION OF A WIRE. A conducting copper wire 
 W, Fig. 284, is suspended by a loop from a hook of the same 
 metal, which passes through the arm of metal or wood, as seen 
 in the cut. The upper end of the hook terminates in the cup 
 P to contain mercury. The lower end of the copper wire just 
 touches the mercury, Q, contained in a little trough about an 
 inch long, formed in the wood on which the horseshoe magnet, 
 M, is laid, the mercury being equally distant from the two poles. 
 
 The cup, N, has a stem of wire which passes through the 
 wood of the platform into the mercury, this end of the wire 
 being tinned, or amalgamated, so as to form a perfect contact. 
 
 Bell Engine. 
 
 347. Explain Fig. 2S4, and describe the course of the electric fluid from one cup to 
 the other. How must the points of the vibrating wire be adjusted in order to act 7 
 
366 
 
 BELL ENGINE. 
 
 Vibration of a Wire. 
 
 348. Having thus prepared 
 the apparatus, put a little mer- 
 cury into the cups P and N, and 
 then form the galvanic circuit 
 by placing the poles of the bat- 
 tery in the two cups, and if every 
 thing is as it should be, the 
 wire will begin to vibrate, being 
 thrown with considerable force 
 either toward M or Q, accord- 
 ing to the position of the mag- 
 netic poles, or the direction of 
 the current, as already explained. 
 In either case it is thrown out 
 of the mercury, and the galvanic 
 circuit being thus broken, the 
 effect ceases until the wire falls 
 back again by its own weight, 
 and touches the mercury, when 
 the current being again perfect- 
 ed, the same influence is repeated, and the wire is again thrown 
 away from the mercury, and thus the vibratory motion becomes 
 constant. 
 
 This forms an easy and beautiful electro-magnetic experi- 
 ment, and may be made by any one of common ingenuity, 
 who possesses a galvanic battery, even of small power, and a 
 good magnet. 
 
 The platform may be nothing more than a piece of pine 
 board, eight inches long and six wide, with two sticks of the 
 same wood, forming a standard and arm for suspending the 
 vibrating wire. The cups may be made of percussion caps, 
 exploded, and soldered to the ends of pieces of copper bell 
 wire. 
 
 The wire must be nicely adjusted with respect to the mer- 
 cury, for if it strikes too deep or is too far from the surface, no 
 vibrations will take place. It ought to come so near the mer- 
 cury as to produce a spark of electrical fire, as it passes the 
 surface, at every vibration, in which case it may be known that 
 the whole apparatus is well arranged. The vibrating wire must 
 be pointed and amalgamated, and may be of any length, from 
 a few inches to a foot or two. 
 
 349. ROTATION OF A WHEEL, similar to, but more simple, than 
 Fig. 281. The same force which throws the wire away from 
 
BELL ENGINE. 
 
 367 
 
 Rotation of a Whed. 
 
 the mercury, will cause the ro- FIG. 255. 
 
 tation of a spur-wheel. For this 
 purpose the conducting wire, in- 
 stead of being suspended, as in 
 the former experiment, must be 
 fixed firmly to the arm, as shown 
 by Fig. 285. A support for the 
 axis of the wheel may be made 
 by soldering a short piece to the 
 side of the conducting wire, so as 
 to make the form of a fork, the 
 lower end of which must be flat- 
 tened with a hammer, and pierced 
 with fine orifices, to receive the 
 ends of the axis. 
 
 The apparatus for a revolving- 
 wheel is, in every respect, like 
 that already described for the vi- 
 brating wire, except in that above 
 noticed, the wheel may be made 
 
 of brass or copper, but must be thin and light, and so suspended 
 as to move freely and easily The points of the notches must 
 be amalgamated, which is done in a few minutes, by placing 
 the wheel on a flat surface, and rubbing them with mercury 
 by means of a cork. A little diluted acid from the gal- 
 vanic batteiy will facilitate the process. The wheel may be 
 from half an inch to several inches in diameter. A cent ham- 
 mered thin, which may be done by heating it two or three 
 times during the process, and then made perfectly round, and 
 its diameter cut into notches with a file, will answer every 
 purpose. 
 
 This affords a striking and novel experiment; for when every 
 thing is properly adjusted, the wheel instantly begins to revolve 
 on touching with one of the wires of the battery the mercury 
 in the cup P, the other pole being in N. 
 
 When the poles of the magnet, or those of the battery, are 
 changed, the wheel instantly revolves in a contrary direction 
 from what it did before. 
 
 It is. however, not absolutely necessary to divide the wheel 
 into notches, or rays, in order to make it revolve, though the 
 
 349. Explain Fig. 285. In what manner may the points of the spur-wheel be amal- 
 gamated 1 If the motion of the fluid is changed, what effect does it have on the 
 wheel 7 
 
368 BELL ENGINE. 
 
 motion is more rapid, and the experiment succeeds much better 
 by doing so. 
 
 350. ELECTRO-MAGNETIC INDUCTION. Experiment proves 
 that the passage of the galvanic current through a copper wire 
 renders iron magnetic when in the vicinity of the current. 
 This is called magnetic induction. 
 
 The apparatus for this 
 
 purpose is represented p FIG - 286 - 
 
 by Fig. 286, and con- ^ ^ 
 
 sists of a copper wire 
 coiled, by winding it 
 around a piece of wood. 
 The turns of the wire 
 should be close together 
 for actual experiment, Electrical Helix. 
 
 they being parted in the 
 
 figure to show the place of the iron to be magnetized. The best 
 method is, to place the coiled wire, which is called an electrical 
 helix, in a glass tube, the two ends of the wire, of course, pro- 
 jecting. Then placing the body to be magnetized within the 
 folds, send the galvanic influence through the whole by placing 
 the poles of the battery in the cups. 
 
 351. Steel thus becomes permanently magnetic, the poles, 
 Kowever, changing as often as the fluid is sent through it in a 
 contrary direction. A piece of watch-spring placed in the helix, 
 and then suspended, will exhibit polarity, but if its position be 
 reversed in the helix, and the current again sent through it, the 
 north pole will become south. If one blade of a knife be put 
 into one end of the helix, it will repel the north pole of a mag- 
 netic needle, and attract the south ; and if the other blade be 
 placed in the opposite end of the helix, it will attract the north 
 pole, and repel the south, of the needle. 
 
 352. TEMPORARY MAGNETS. Temporary magnets, of almost 
 any power, may be made by winding a thick piece of soft iron 
 with many coils of insulated copper wire 1 and passing the gal- 
 vanic influence through it. 
 
 The best form of a magnet for this purpose is that of a horse- 
 shoe, and which may be made in a few minutes by heating and 
 bending a piece of cylinder iron, an inch or two in diameter, 
 into this form. 
 
 350. What is meant by magnetic induction 1 Explain Fig. 286. What is the figure 
 called ? 351. Does any substance become permanently magnetic by the electrical 
 helix ? How may the poles of a magnet be changed by the helix 1 352. How may 
 temporary magnets be made I 
 
THERMO-ELECTRICITY. 
 
 369 
 
 The copper wire (bell wire) may be insulated by winding it 
 with cotton thread. If this can not be procured, common bon- 
 net wire will do, though it makes less powerful magnets than 
 copper. 
 
 353. The coils of wire FIG. 287. 
 may begin near one pole 
 
 of the magnet and term- 
 inate near the other, as 
 represented by Fig. 287, 
 or the wire may consist 
 of shorter pieces wound 
 over each other, on any 
 part of the magnet. In 
 either case, the ends of 
 the wire, where several 
 pieces are used, must be 
 soldered to two strips of 
 tinned sheet copper, for 
 the combined positive 
 and negative poles of the 
 
 wires. To form the mag- Temporary Magnet. 
 
 net, these pieces of cop- 
 per are made to communicate with the poles of the battery, by 
 means of cups containing mercury, as shown in the figure, 01 
 by any other method. 
 
 354. The effect is surprising, for on completing the circuit 
 with a piece of iron an inch in diameter, in the proper form, and 
 properly wound, a man will find it difficult to pull off the arma- 
 ture from the poles ; but on displacing one of the galvanic poles, 
 the attraction ceases instantly, and the man, if not careful, will 
 fall backward, taking the armature with him. Magnets have 
 been constructed in this manner, which would suspend ten 
 thousand pounds. 
 
 THERMO-ELECTRICITY. 
 
 355. This means electricity by heat, and its principles will 
 be understood, when it is stated that if any two metals of dif- 
 ferent kinds be joined together and then heated, a current of 
 electricity will pass from one to the other. Thus, if two wires 
 of a few inches in length, German-silver and brass, have their 
 ends soldered together, and the junction heated with an alcohol 
 
 353. For what purpose dire the ends of the wires to be soldered to pieces of cop- 
 >r 1 355. What is meant by thermo-electricity 1 
 
 16* 
 
 perl 
 
570 ELECTROTYPE. 
 
 lamp, or by other means, a current of electricity will flow from 
 the silver to the brass, which may be detected by the gal- 
 vanometer, or by the common electrical needle. 
 
 356. Composition of German-silver. As this alloy is cheap, 
 and is much used for electrical purposes, we give its proportions. 
 In 100 parts, it consists of copper 50, zinc 30, and nickel 20. 
 This alloy is a positive electric to all other metals except bis- 
 muth, to which it is negative. 
 
 FIG. ssa 
 
 yhermo- Electricity. 
 
 Writers give a great variety of combinations of different 
 metals, with the amount of electrical influence indicated by each. 
 Among these, that shown by Fig. 288, is among the most easily 
 constructed and most powerful. It consists of ten strips of 
 German -silver, and as many of brass, rolled thin and laid on 
 each other with their alternate ends soldered together. Strips 
 of pasteboard are placed between the adjacent metals, so that 
 they touch only at the ends where they are soldered. Now by 
 heating the end opposite the poles with a spirit lamp, and bring- 
 ing the poles in contact, an electrical current will flow from one 
 side or pole, to the other, in the direction of the arrows. 
 
 ELECTROTYPE. 
 
 35 Y. The art of covering the 'base metals, as copper, and the 
 alloys of zinc, tin, &c., with gold and silver, as also of copying 
 medals, by means of the electrical current, is called electrotype 
 or voltatype. 
 
 This new art is founded on the simple fact, that when the 
 galvanic influence is passed through a metallic solution, under 
 certain conditions, decomposition takes place, and the metal is 
 deposited in its pure form on the negative pole of the battery. 
 
 The theory by which this effect is explained is, that the 
 
 356. Explain by Fig. 288, how thermo-electricity is developed ? 357. What is elec- 
 trotype 1 On what fact is it said this art is founded? On which pole is the metal 
 deposited 7 What is the theory by which this effect is explained 1 
 
ELECTROTYPE. 37l 
 
 hydrogen evolved by the action of the acid on the positive pole 
 of the battery combines with the oxygen of the dissolved metal 
 foiming water, while the metal itself thus set free, is deposited 
 at the negative side of the battery. 
 
 Many of the base metals, as copper, the alloys of zinc, and 
 tin, may by such means be covered with gold, or silver, and thus 
 a cheap and easy method of gilding and plating is effected. 
 
 This art, now only a few years old, has excited great interest, 
 not only among men of science, but among mechanics, so that 
 in England many hundreds, and perhaps thousands of hands 
 are already employed in silvering, gilding, and coppering, taking 
 impressions of medals and of copperplates, for printing, and of 
 performing such other work as the art is capable of. Volumes 
 have been written to explain the different processes to which 
 this art is applicable, and considering its recent discovery and 
 the variety of uses to which it is already applied, no doubt can 
 exist that it will finally become of great importance to the world. 
 
 In this short treatise we can only introduce the pupil to the 
 subject, by describing a few of the most simple processes of the 
 art in question, and this we hope to do in so plain a manner, 
 that any one of common iigenuity can gild, silver,, or copper, 
 and take impressions of medals at his leisure. 
 
 358. COPYING OF MEDALS. This new art has been applied 
 very extensively in the copying of ancient coins and medals, 
 which it does in the utmost perfection, giving every letter, and 
 feature, and even an accidental scratch, ex-actly like the original. 
 When the coin is a cameo, the figures or letters ^eing raised, it 
 is obvious that if the metal be cast directly upon */, the medal 
 will be reversed, that is, the figures will be indented, and the 
 copy will be an intaglio instead of a cameo. To remedy this, a 
 cast, or impression must first be taken of the medal, on which 
 the electrotype process is to act, when the copy will, in all re- 
 spects, imitate the original. 
 
 There is a variety of ways of making such casts, according to 
 the substance used for the purpose. We shall only mention 
 plaster of Paris, wax, and fusible metal. 
 
 359. PLASTER CASTS. When plaster is used, it must be, 
 what is termed baked, that is, heated, so as to deprive it of all 
 mofeture. This is the preparation of which stereotype casts are 
 made. The dry powder being mixed with water to the con- 
 sistence of cream, is placed on the medal with a knife to the 
 thickness of a quarter or half an inch, according to its size. In 
 a few minutes the plaster sets, as it is termed, or becomes hard. 
 
372 ELECTROTYPE. 
 
 To insure its easy detachment, the medal is rubbed over with a 
 little oil. 
 
 The cast thus formed is first to be coated with boiled linseed 
 oil, and then its face covered with fine pulverized black lead, 
 taking care that the indented parts are not filled, nor the raised 
 parts left naked. The lead answers the purpose of a metallic 
 surface, on which the copper is deposited by the galvanic current. 
 This is a curious and very convenient discovery, since wood cuts, 
 engraved stones, and copies in sealing-wax, can thus be copied. 
 
 To insure contact between the black lead on the face of the 
 cast and the wire-conductor, the cast is to be pierced with an 
 awl, on one of its edges, and the sharp point of the wire passed 
 to the face, taking care, after this is done, to rub on more lead, 
 so that it shall touch the point of the wire, and thus communi- 
 cate with the whole face of the medal. 
 
 360. WAX CASTS. To copy medallions of plaster of Paris, 
 place the cast in warm water, so that the whole may be satura- 
 ted with the water, but keeping the face above it. When the 
 cast has become warm and moist, remove, and having put a 
 slip of paper around its rim, immediately pour into the cup thus 
 formed bees wax, ready melted for this purpose. In this way 
 copies may be taken, not only from^)laster casts, but from those 
 of other substances. 
 
 To rer ler the surface of the wax a conductor of electricity, it 
 is to he covered with black lead in the manner directed for 
 plaster casts. This is put on with a soft brush, until it becomes 
 black and shining. 
 
 The electrical conductor is now to be heated and pressed upon 
 the edge of the wax, taking care that a little of its surface is left 
 naked, on, and around which the black lead is again to be 
 rubbed, to insure contact with the whole surface. 
 
 Both of the above preparations require considerable ingenuity 
 and attention, in order to make them succeed in receiving the 
 copper. If the black lead does not communicate with the pole, 
 and does not entirely cover the surface, or if it happens to be a 
 poor quality, which is common, the process will not succeed; 
 but patience, and repeated trials, with attention to the above 
 descriptions, will insure final success. 
 
 361. FUSIBLE METAL CASTS. This alloy is composed of 8 
 parts of bismuth, 5 of lead, and 3 of tin, melted together. It 
 melts at about the heat of boiling water, and hence may be 
 used in taking casts from engraved stones, coins, or such other 
 substances as a small degree of heat will not injure. 
 
ELECTROTYPE. . 373 
 
 To take a cast with this alloy, surround the edge of the medal 
 to be copied, with a slfp of paper, by means of paste, so as to 
 form a shallow cup, the medal being the bottom. Then hav- 
 ing melted the alloy in a spoon, over an alcohol lamp, pour it 
 in, giving it a sudden blow on the table, or a shake, in order to 
 detach any air, which may adhere to the medal. In a minute 
 or two it will be cool, and ready for the process. 
 
 Another method is, to attach the medal to a stick, with seal- 
 ing-wax, and having poured a proper quantity of the fused alloy 
 on a smooth board, and drawn the edge of a card over it, to 
 take off the dross, place the medal on it, and with a steady 
 hand let it remain until the cast cools. 
 
 Next, having the end of the copper wire for the zinc pole 
 clean, heat it over a lamp, and touch the edge of the cast there- 
 with, so that they shall adhere, and the cast will now be ready 
 for the galvanic current. 
 
 To those who have had no experience in the electrotype art, 
 this is much the best, and most easy method of taking copies, 
 as it is not liable to failure like those requiring the surfaces of 
 the molds to be black leaded, as above described. 
 
 362. GALVANIC ARRANGEMENT. Having prepared the molds, 
 as above directed, these are next to be placed in a solution ot 
 the sulphate- of copper, (blue vitriol) and subjected to the elec- 
 trical current. For this purpose only a very simple battery is 
 required, especially where the object is merely a matter of 
 curiosity. 
 
 For small experiments, a glass jar holding a pint, or a pitcher, 
 or even a tumbler will answer, to hold the solution. Provide 
 also a cylinder of glass two inches in diameter, and stop the 
 bottom with some moist plaster of Paris, or instead thereof, tie 
 around it a piece of bladder, or thin leather, or the whole cylin- 
 der may be made of leather, with the edges sewed nicely to- 
 gether, and stopped with a cork, so that it will not leak. The 
 object of this part of the arrangement is, to keep the dilute sul- 
 phuric acid which this contains, from mixing with the solution of 
 sulphate of copper, which surrounds it, still having the texture 
 of this vessel so spongy as to allow the galvanic current to pass 
 through the moisture which it absorbs, water being a good con- 
 ductor of electricity. 
 
 Provide also a piece of zinc in form of a bar, or cylinder, or 
 slip, of such size as to pass freely into the above described 
 cylinder. 
 
 Having now the materials, the arrangement will readily 
 
374 
 
 SMEE'S BATTERY, 
 
 FIG. 289. 
 
 be understood by Fig. 289, where c is the ves- 
 sel containing the solution of sulphate of cop- 
 per ; a, the cylinder of leather, or glass ; 2, the 
 zinc, to which a piece of copper wire is fastened, 
 and at the other end of which, is the cast m, to be 
 copied. The proportions for the vessel, a, are 
 about 1 part sulphuric acid to 16 of water by 
 measure. The solution of copper for c, may be in 
 the proportions of 2 ounces of the salt to 4 ounces 
 of water. The voltaic current passes from the 
 positive zinc to the negative amalgam cast, where 
 the pure copper is deposited. 
 
 In order to keep the solution saturated, a little 
 sulphate of copper is tied in a rag, and suspended in the solu- 
 tion. In 24 or 36 hours, the copper, (if all is right,) will be 
 sufficiently thick on the cast, the back and edges of which should 
 be covered with varnish to prevent its deposition except on the 
 face. 
 
 If the copper covers the edges, a file or knife will remove it, 
 when by inserting the edge of the knife between the two metals, 
 the copy will be separated, and will be found an exact copy of 
 the original. 
 
 If the acid in the inner cylinder is too strong, the process is 
 often too vigorous, and the deposition, instead of being a film 
 of solid copper on the cast, will be in the form of small grains 
 on the lower end of the wire. The weakest power consistent 
 with precipitation should therefore be applied. 
 
 BMEE 7 S BATTERY. 
 
 363. This is an improved method of copying casts, or molds, 
 in copper. It consists of two glass vessels, each holding a pint, 
 or less, one of which holds the battery, and the other the de- 
 positing apparatus. These arrangements will be understood 
 by Fig. 290, of which 1 is a little mercury on the bottom of 
 the vessel, containing the battery. Just above this is a piece 
 of platinum foil, suspended in the center. A piece of zinc, 4, 
 rests against the side of the vessel. A curved copper wire, 3, 
 descends through the liquid, insulated by a glass tube. This 
 wire, by the mercury, connects the zinc plate with the metallic 
 cup on the top of the jar, and by the wire, 2, with the other 
 jar. The wire, 5, descends from the screw-cup into the depos- 
 iting cell, to the end of which the cast, 6, is suspended. The 
 plate 7, is a piece of copper suspended in the solution of sulphate 
 
ELECTRO-MAGNETISM. 
 
 375 
 
 of copper, in order to keep 
 it always of the same 
 strength, a portion being 
 dissolved, while another 
 portion is deposited on 
 the cast. 
 
 The liquid in the bat- 
 tery is composed of one 
 part sulphuric acid, and 
 20 or 30 of water. That 
 in the depositing side, is 
 composed of 2 ounces of 
 sulphate of copper, 1 ounce 
 
 of sulphuric acid, and 15 Smee's Battery. 
 
 ounces of water. 
 
 The general directions for obtaining casts have been given 
 above, and need not be repeated. 
 
 MAGNETISM BY ELECTRO-MAGNETISM. 
 
 364. The apparatus, Fig. 291, is designed to communicate 
 strong and permanent magnetism to steel. It consists of a 
 small Smee's battery, with its opposite poles connected with the 
 horizontal U magnet, which is closely wound with insulated 
 copper wire. Of course the wires convey the electrical influence 
 from the positive to the negative sides of the battery. 
 
 FIG. 291. 
 
 Magnetism by Electro-Magnetism. 
 
 The cut represents a U magnet in the process of being mag- 
 netized. This is done by drawing it from the bend, across the 
 electro -magnet to the poles, and repeating this on both its sides, 
 taking care to do it in the same direction. A steel bar may be 
 
376 ELECTRO-GILDING. 
 
 magnetized by the same process, or, if a short one, by applying 
 it as an armature to the poles of the electro-magnet ; the north 
 pole becoming the south pole of the new magnet. 
 
 365. To remove the magnetism of a steel magnet of the U 
 form, it is only required to reverse the process, that is, to place 
 one of its poles on each pole of the electro-magnet, and draw it 
 over them, in the direction contrary to the indication of the 
 arrow seen in the figure. 
 
 In the vertical magnet, the letters N S, indicate its north and 
 south poles. 
 
 ELECTRO-GILDING. 
 
 366. Gilding without a Battery. After the solution is pre- 
 pared, the process of electrotype-gilding is quite simple, and may 
 be performed by any one of common ingenuity. 
 
 The solution for this purpose is cyanide of gold dissolved in 
 pure water. This is prepared by dissolving the metal in aqua- 
 regia, composed of one part nitric, and two of muriatic acid. 
 Ten or fifteen grains of gold, to an ounce and a half of the 
 aqua-regia may be the proportions. The acid being evaporated, 
 the salt which is called the chloride of gold is dissolved in a 
 solution, made by mixing an ounce of the cyanuret of potash 
 with a pint of pure water. The cyanuret of potash is decom- 
 posed and a cyanide of gold remains in solution. About 20 
 grains of the chloride of gold is a proper quantity for a pint of 
 the solution. The cyanuret of potash, and the chloride, or 
 oxyd of gold, may be bought at the apothecaries. 
 
 Having prepared the solution, the most simple method of 
 gilding is to pour a quantity of it into a glass jar, or a tumbler, 
 and place in it the silver, copper, or German-silver to be gilded, 
 in contact with a piece of bright zinc, and the process will im- 
 mediately begin. No other battery, except that formed by the 
 zinc, and metal which receives the gold, is required. The zinc 
 at the point of contact must be bright and well fastened to the 
 other metal by a string or otherwise. The process will be 
 hastened by warmth, which may be applied by placing the jar 
 and its contents in a vessel of warm water. So far as the author 
 knows, this simple process originated with himself, and answers 
 admirably as an experiment in the electrotype art. The gold, 
 however, is apt to settle upon the zinc, but which may be pre- 
 vented by a little shellac varnish rubbed on it, except at the 
 point of contact. The handles of scissors, silver spectacles, pen- 
 cils, &c., may be handsomely gilt by this process. 
 
ELECTRO-PLATING. 37 7 
 
 367. GILDING WITH A BATTERY. If the operator desires to 
 extend his experiments in the art of electro -gilding, a small bat- 
 tery must be employed, of which, there are many varieties. 
 The best for more extensive operations, is that composed of 
 platinized silver, and amalgamated zinc. 
 
 For this purpose the platina is first dissolved in aqua-regia, 
 in proportion of 10 grains to the ounce, and then precipitated 
 on the silver. The silver is in sheets, such as is used for plating, 
 no thicker than thin writing paper. This may be obtained of 
 the silver- platers, and being well cleaned, is ready for the process. 
 
 These plates being covered with platina, are insoluble in the 
 acid employed, and hence they will last many years. The amal- 
 gamated plates are also durable, and do not require cleaning. 
 
 368. These platinized sheets ar^ confined between two plates 
 of amalgamated zinc. The process of amalgamation consists in 
 rubbing mercury, with a little mass of cotton wool held in the 
 fingers, on the clean zinc. These plates may be fixed half an 
 inch apart by means of little pieces of wood, with the sheets be- 
 tween them, but not touching each other. The plates, having 
 a metallic connection, form the positive side of the battery, 
 while a copper wire soldered to the silver sheet makes the nega- 
 tive side. The dimensions of these plates may be four or five 
 inches long, and three or four wide. 
 
 For experimental purposes, however, a less expensive battery 
 may be used, that represented by Fig. 289, made of copper and 
 zinc, being sufficient. 
 
 To gild by means of a battery, place the solution, made as 
 above described, in a glass vessel, and connect the article to be 
 gilded with the pole coming from the zinc side of the battery, 
 letting the other wire, which should be tipped with a little piece 
 of gold, dip into the solution. The gilding process will imme- 
 diately begin, and in three or four hours a good coat of gold 
 will be deposited on the article immersed. 
 
 To keep the solution quite pure, the tips of the poles where 
 they dip into the fluid should be of gold. If they are of copper, 
 a portion of the metal will be dissolved and injure the result. 
 
 ELECTRO-PLATINO. 
 
 369. The process of silvering copper, or the alloys of the 
 metals, such as German-silver, is done on the same principle as 
 that described for gilding, but there seems to be more difficulty 
 in making the process succeed to the satisfaction of the artist 
 than there is in depositing gold. 
 
378 ELECTRO -PLATING. 
 
 The following is the method employed by Mr. Sumner Smith, 
 of this city, the most experienced electrotype artist within our 
 acquaintance. It will succeed perfectly in the hands of those 
 who will follow the directions. 
 
 Make a solution of cyanuret of potash in pure water, in the 
 proportion of an ounce to a pint. Having placed it in a glass 
 vessel, prepare the battery for action as usual. Then attach to 
 the pole of the silver, or copper side of the battery, a thin plate 
 of silver, and immerse this in the cyanuret solution. The pole 
 from the zinc side being now dipped into the fluid, the electro- 
 chemical action on the silver plate instantly begins, and a rapid 
 decomposition of the metal is effected, and in a short time the 
 solution will be saturated with the silver, as will be indicated 
 by the deposition of the metal on the end of the copper pole 
 coming from the zinc side of the battery. The solution is now 
 ready for use, but the remains of the silver, still undissolved, 
 must not be removed before immersing the articles to be plated, 
 since the solution is thus kept saturated. 
 
 This solution is much better than that prepared by dissolving 
 the silver separately in an acid, and then re-dissolving in the 
 cyanuret of potash as is usually done, for in the latter case the 
 silver is apt to be deposited on German-silver, brass, iron, and 
 other metals, without the galvanic action, in which case it does 
 not adhere well, whereas the solution made as above directed 
 is not liable to this imperfection. 
 
 During the preparation of the fluid, only a very small copper 
 wire should be employed on the zinc side of the battery. 
 
 The articles to be plated must be well cleaned before immer- 
 sion. To effect this, dip them into dilute sulphuric acid for a 
 few minutes, then rub them with sand or whiting, and rinse in 
 pure water. 
 
 Now having exchanged the small copper pole of the zinc side 
 of the battery, for a larger one of the same metal, tipped with 
 silver, connect the article to be plated with this, the other pole 
 with the silver plate attached being still immersed in the solution. 
 
 The process must now be watched, and the silver attached to 
 the copper side raised nearly out of the fluid, in case bubbles 
 of hydrogen are observed to rise from the pole on the other 
 side, or the articles attached to it. The greater the surface of 
 silver in the fluid, the more energetic will be the action, short 
 of the evolution of hydrogen from the other pole, but when this 
 is observed, the decomposing silver must be raised so far out of 
 the fluid as to stop its evolution. 
 
ERICSSON'S CALORIC ENGINE. 
 
 379 
 
 By this method a thick and durable coat of silver may be 
 placed on old copper tea-pots, candlesticks, or other vessels of 
 this sort, where the silvering has been worn off by long use. 
 
 ERICSSON'S CALORIC ENGINE. 
 
 Description of the plates. The following description of the 
 plates, and explanation of the working of the caloric engine, 
 was furnished for Appleton's Magazine by Capt. Ericsson himself, 
 from which this is abridged. The plates are an exact working 
 drawing of the stationary test engine, that being more available 
 for illustration than the engines of the ship. 
 
 a, Air receiver, b 6, supply cylinder ; e', self-acting valve for 
 letting air into, and e' e', self-acting valve for letting air out of 
 the same, c, supply piston ; c', piston-rod of the same, con- 
 
 FIG. 292. 
 
 Caloric Engine. 
 
 nected to the working beam of the engine, d d, working cylin- 
 der ; d' d', holes at the junction of the two cylinders, through 
 which the atmospheric air passes in and out freely, e e, work- 
 ing piston ; d" d", rods connecting the two pistons together. 
 e", air-tight vessel suspended below the working piston, filled 
 with clay and charcoal, to prevent transmission of heat from 
 below. //, regenerator ; /', disks of wire-net placed vertically 
 in the regenerator box. y, valve worked by the engine for ad- 
 
S80 ERICSSON'S CALORIC ENGINE. 
 
 mitting air into the regenerator and working cylinder ; A, valve 
 for letting the air out of the same, i i, pipe, open to the at- 
 mosphere, for carrying off the air after having passed through 
 the engine ; k, fire-place. I m n, thermometers inserted to 
 ascertain the temperature within, o, working beam, p, connect- 
 ing-rod. <7, crank, s, pipe from cylinder to receiver. 
 
 Operation. A slow fire being kept up at k for about two 
 hours, until the parts within the brick-work shall have become 
 moderately heated, the air receiver is charged with air by means 
 of a hand-pump ; as soon as the internal pressure shall have 
 reached about six pounds to the square inch, effected in less 
 than two minutes, the hand-pump is stopped, and the valve g 
 opened by a starting lever ; the compressed air from the re- 
 ceiver, thus admitted under the valve #, rushes through the 
 partially heated wires /' into the working cylinder, forcing its 
 piston e upwards, as also the supply piston c, by means of the 
 connecting-rods d" d" ; the atmospheric air contained in the 
 upper part of b will, by this upward movement of the supply 
 piston, be forced through the valve e" into the air receiver ; 
 when the working piston has reached three-fourths of the full 
 up-stroke, the valve g is closed by the engine ; and when the 
 piston has arrived at the full up-stroke, the valve h is opened. 
 A free communication with the atmosphere being thereby es- 
 tablished, by means of the open pipe i i, the air under the 
 working piston passes off, and, owing to the removal of pressure 
 under the working piston, it will instantly begin to descend by 
 its own weight. 
 
 The heated air from under the working piston, in passing off 
 through the wires /', gives out its caloric to the same so effect- 
 ually, that, on reaching the thermometer m, the temperature 
 never exceeds that of the entering air at I by more than 30 ; 
 on the other hand, the cold air from the receiver, in circulating 
 through the meshes of wires in its passage to the working cyl- 
 inder, becomes so effectually heated that, on passing n, its tem- 
 perature is increased to upwards of 450, when the machine is 
 in full operation. 
 
 During the descent of the supply piston c, the outlet valve e" 
 remains closed by the pressure from the receiver, whilst the in- 
 let valve e' is kept open by suction, and hence that & fresh quan- 
 tity of air enters the supply cylinder at each down stroke of its 
 piston, and by the up-stroke is forced into the receiver. The 
 regenerator measures 26 inches in height and width ; each disk 
 of wire-net contains 676 superficial inches, and the net has 10 
 meshes to the inch ; each superficial inch therefore contains 100 
 
CALORIC ENGINE. 381 
 
 meshes, which, multiplied by 676, gives 67,600 meshes to each 
 disk ; 200 disks being employed, it follows that each regenerator 
 contains 13,520,000 meshes, and, consequently, if we consider 
 
 FIG 
 
 Caloric Engine. 
 
 that there are as many small spaces between the disks as there 
 are meshes, we shall find that the air within the regenerator is 
 distributed in 27,000,000 of minute cells. 
 
 In explanation of the wonderful efficiency of the regenerator, 
 it may be stated that each disk contains 1140 feet of wire in 
 length, and each regenerator 228,000 feet, or41 miles of wire; 
 
382 DAGUERREOTYPE. 
 
 the superficial measurement of which is 2014 square feet, which 
 is equal to the entire surface of four steam-boilers, forty feet long 
 and four feet diameter ; and yet the regenerator displaying that 
 great amount of heating surface is only a two foot cube, less 
 than -iViTo" of the bulk of said boilers. 
 
 DAGUERREOTYPE. 
 
 3*74. This branch of photography was the invention of M. 
 Daguerre, an ingenious French artist, and is entirely independ 
 ent of the art of taking impressions on paper, as above de- 
 scribed. In that the pictures are reversed, in this they are in 
 the natural position, and instead of paper, the picture is on 
 silver. 
 
 As an art, this is one of the most curious and wonderful 
 discoveries of the present age ; for when we witness the variety 
 of means necessary to the result, it would appear equally im- 
 probable that either accident or design could possibly have 
 produced such an end by means so vario*us and complicated, 
 and to which no other art, (save in the use of the camera 
 obscura,) has the least analogy in the manner in which the ob- 
 ject is accomplished. 
 
 This being a subject of considerable public interest, and, 
 withal, a strictly philosophical art, we shall here describe all 
 the manipulations as they succeed each other in producing the 
 result, a human likeness. 
 
 The whole process may conveniently be divided into eight 
 distinct operations. 1st. Polishing the plate. 2d. Exposing it 
 to the vapor of iodine. 3d. Exposing it to the vapor of bromine. 
 4th. Adjusting the plate in the camera obscura. 5th. Exposing 
 it to the vapor of mercury. 6th. Removing the sensitive 
 coating. 7th. Gilding the picture. 8th. Coloring the picture. 
 
 1. Polishing the Plate. The plates are made of thin sheets 
 of silver, plated on copper. It is said that for some unknown 
 reason the photographic impression takes more readily on these 
 plates, than on entire silver. The silver is only thick enough 
 to prevent reaching the copper in the process of scouring and 
 polishing. 
 
 The polishing is considered one of the most difficult and im- 
 portant manipulations in the art, and hence hundreds of pages 
 have been written to describe the various methods devised and 
 employed by different artists or amateurs. 
 
 We can only state here, that the plate is first scoured with 
 emery to take off the impressions of the hammer in plenish- 
 ing ; then pumice, finely powdered, is used, with alcohol, to 
 
DAGUERREOTYPE. 383 
 
 remove all oily matter, and after several other operations, it is 
 finally given the last finish by means of a velvet cushion cov- 
 ered with rouge. 
 
 2. Iodizing the Plate. After the plate is polished, it is in- 
 stantly covered from the breath, the light, and the air, nor 
 must it be touched, even on the edges, with the naked hand ; 
 but being placed on a little frame, with the face down, it is 
 carried to a box containing iodine, over which it is placed as a 
 cover. Here it remains for a moment or two in a darkened 
 room, being often examined by the artist, whose eye decides 
 by the yellowish color to which the silver changes, the instant 
 when the metal has combined with the proper quantity of 
 iodine. This is a very critical part of the process, and requires 
 a good eye and much experience. The vapor of iodine forms 
 a film of the iodid of silver on the metal, and it is this which 
 makes it sensible to the light of the camera, by which the pic- 
 ture is formed. If the film of iodine is too thick, the picture 
 will be too deep, and dark ; if too thin, either a light impres- 
 sion, or none at all, will be made. 
 
 3. Exposure of the Vapor of Bromine. Bromine is a pe-- 
 culiar substance, in the liquid form, of a deep red color, ex- 
 ceedingly volatile, very poisonous, and having an odor like 
 chlorine and iodine, combined. It is extracted from sea water, 
 and the ashes of marine vegetables. 
 
 This the photographic artists call an accelerating substance, 
 because it diminishes the time required to take the picture in 
 the camera obscura. 
 
 The iodized plate will receive the picture without it, but the 
 sitter has to remain without motion before the camera for sev- 
 eral minutes, whereas by using the bromine, the impression is 
 given, in a minute, or in a minute and a quarter. Now as the 
 least motion in the sitter spoils the likeness, it is obvious that 
 bromine is of much importance to the art, especially to nervous 
 people and children. 
 
 The bromine is contained in a glass vessel closely covered, 
 and is applied by sliding the plate over it for a few seconds. 
 
 4. Adjusting the Plate in the Camera. The plate is now 
 ready for the photographic impression by means of the camera. 
 If a likeness of a person is to be taken, he is already placed 
 before the instrument, in a posture which the artist thinks will 
 give the most striking picture, and is told that the only motion 
 he can make for a half a minute to a minute, is winking. 
 
 The artist now takes the plate from a dark box, and undej 
 
884 DAGUERREOTYPE. 
 
 cover of a black cloth fixes it in the focus of the lens. This is 
 done in a light room, with the rays of the sun diffused by 
 means of white curtains. 
 
 The artist having left the sitter for the specified time, returns, 
 and removes the plate for the next operation. Still, not the 
 least visible change has taken place on the bright surface of the 
 silver. If examined ever so nicely, no sign of a human face is 
 to be seen, and the sitter who sees the plate, and knows nothing 
 of the art, wonders what next is to be done. 
 
 5. Exposure to the Fumes of Mercury. The plate is next 
 exposed to the fumes of mercury. This is contained in an 
 iron box in a darkened room, and is heated by means of an 
 alcohol lamp, to about 180 degrees, Fah. The cover of the 
 box being removed, the plate is laid on, with the silver side 
 down, in its stead. 
 
 After a few minutes, the artist examines it, and by a faint 
 light now sees that the desired picture begins to appear. It is 
 again returned for a few minutes longer, until the likeness is 
 fully developed. 
 
 If too long exposed to the mercury, the surface of the silver 
 turns to a dark ashy hue, and the picture is ruined ; if re- 
 moved too soon, the impression is too faint to be distinct to the 
 eye. 
 
 6. Removal of the Sensitive Coating. The next operation 
 consists in the removal of the iodine, which not only gives the 
 silver a yellowish tinge, but if suffered to remain, would darken, 
 and finally ruin the picture. Formerly this was done by a 
 solution of common salt, but experiment has shown that the 
 peculiar chemical compound called hyposulphate of soda, an- 
 swers the purpose far better. This is a beautiful transparent 
 crystalized salt, prepared by chemists for the express purpose. 
 
 A solution of this is poured on the plate until the iodine is 
 entirely removed, and now the picture, for the first time, may 
 be exposed to the light of the sun without injury, but the plate 
 has still to be washed in pure water, to remove all remains of 
 the hyposulphate, and then heated and dried over an alcohol 
 lamp. 
 
 7. Gilding the Picture. This is called, fixing, by the chlo- 
 ride of gold. 
 
 Having washed the picture thoroughly, it is then to be placed 
 on the fixing stand, which is to be adjusted previously, to a 
 perfect level, and as much solution of chloride of gold as the 
 plate can retain, poured on. The alcohol lamp is then held 
 
MORSE'S ELECTRO-MAGNETIC TELEGRAPH. 385 
 
 under all parts of it successively. At first the image assumes 
 a dark color, but in a few minutes grows light, and acquires an 
 intense and beautiful appearance. 
 
 The lamp is now removed, and the plate is again well washed 
 in pure water, and then dried by heat. 
 
 Before gilding, the impression may be removed by repolish- 
 ing the plate, when it is perfectly restored ; but after gilding, no 
 polishing or scouring will so obliterate the picture, as to make 
 it answer for a second impression. Such plates are either sold 
 for the silver they contain, or are re-plated by the electrotype 
 process. 
 
 8. Coloring the Picture. Coloring daguerreotype pictures 
 is an American invention, and has been considered a secret, 
 though at the present time it is done with more or less success 
 by most artists. 
 
 The color consists of the oxyds of several metals, ground to 
 an impalpable powder. They are laid on in a dry state, with 
 soft camel-hair pencils, after the process of gilding. The plate 
 is then heated, by which they are fixed. This is a very deli- 
 cate part of the art, and should not be undertaken by those 
 who have not a good eye, and a light hand. 
 
 The author is indebted to Mr. N. G. Burgess, of 192 Broad- 
 way, New York, for much of the information contained in tho 
 above account of the daguerreotype art. Mr. B. is an experi- 
 enced and expert artist in this line. 
 
 MORSE'S ELECTRO-MAGNETIC TELEGRAPH. 
 
 375. The means by which Mr. Morse has produced his won- 
 der-working and important machine, is the production of a 
 temporary magnet, by the influence of the galvanic fluid. 
 
 We have already described the method of making tempo- 
 rary magnets of soft iron, by covering the latter with insulated 
 copper wire, to each end of which the poles of a small gal- 
 vanic battery is applied. 
 
 The description of Fig. 2 87, with what is said before on the 
 subject, will inform the student how the power is obtained by 
 which the philosopher in question has brought before the world 
 such wonderful and unexpected effects. 
 
 The machine itself is sufficiently simple, and will be compre- 
 hended at once, by those who have made electro-magnetic 
 experiments, by the annexed diagram and description. 
 
 The temporary magnet A, Fig. 2 94, enveloped with its insu- 
 lated copper wire, is fastened to the wooden frame B G, by 
 means of cords or otherwise. 
 
 17 
 
386 MORSE'S ELECTRO-MAGNETIC TELEGRAPH. 
 
 FIG. 294. 
 
 Principle of Morse's Telegraph. 
 
 This frame also supports the standard H, which sustains the 
 revolving drum F, on which the paper to receive the emblem- 
 atical alphabet is fixed, M being the edge of the paper. 
 
 To the arm G, is appended the lever C, of wood, which has 
 a slight vertical motion, in one direction by the steel spring D, 
 and in the other, by the armature of soft iron E. 
 
 The two poles of the magnet rest in two little cups of mer- 
 cury, into which are also to be plunged the poles of the mag- 
 netic battery, (not shown in the drawing,) of which P is the 
 positive, and N the negative. The steel point I, attached to the 
 lever, is designed to mark the telegraphic alphabet on the paper. 
 
 Having thus explained the mechanism, we will now show in 
 what manner this machine acts to convey intelligence from one 
 part of the country to another. 
 
 It has already been explained that when a bar of soft iron 
 surrounded by insulated copper wire, as shown at A, has its two 
 poles connected with the poles of a galvanic battery, the iron 
 instantly becomes a magnet, but returns to its former state, 
 or ceases to be magnetic, the instant the connection between 
 them ceases. 
 
 To break the connection, it is not necessary that both of the 
 poles should be detached, the circuit being broken by the sepa- 
 ration of one only. 
 
 Supposing then, that N and P are the poles of such a bat- 
 tery, on placing N into the cup of mercury, the wires from the 
 soft iron being already there, the armature E is instantly at- 
 
VELOCITY OF ELECTRICITY. 387 
 
 tracted, which brings the point I against the paper on the re- 
 volving wheel F. If N is instantly detached after the point 
 strikes the paper, then only a dot will be made, for the mag- 
 netic power ceasing with the breaking of the circuit, the spring 
 D withdraws the point from the paper the instant the pole 
 is removed. 
 
 If a line is required in the telegraphic alphabet, then the 
 pole is kept longer in the vessel of mercury, and as the alphabet 
 consists of dots, and lines of different lengths, it is obvious that 
 writing in this manner can not be difficult The understanding 
 of the alphabet is another matter, though we are informed that 
 this may be done with facility. 
 
 The marks of the point I, are made by indenting the paper, 
 the roller on which it is fixed being made of steel in which a 
 groove is turned, into which the paper is forced by the point. 
 The paper is therefore raised on the under side like the printing 
 for the blind. 
 
 The roller F is moved by means of clock-work, having an 
 uniform motion, consequently the dots and lines depending 
 on the time the point is made to touch the paper, are always 
 uniform. 
 
 Now with respect to the distance apart at which the tem- 
 porary magnet and writing apparatus, and the battery are 
 placed, experiment shows that it makes little difference with 
 respect to time. Thus, suppose the battery is in Hartford, and 
 the magnet in New York, with copper or iron wires reaching 
 from one to the other. Then the telegraphic writer at Hartford, 
 giving the signal by means of an.alarm bell, that he is ready to 
 communicate, draws the attention of the person at New York to 
 the apparatus there the galvanic action being previously broken 
 by taking one of the poles from the battery at Hartford. 
 
 If now we suppose the letter A is signified by a single dot, 
 he at Hartford dips the pole in the cup of the battery, and in- 
 stantly at New York the soft iron becomes a magnet, and a dot 
 is made on the paper, and so, the rest of the alphabet. 
 . The wires are carried through the air by being wound around 
 glass caps supported by iron L shaped arms, which are driven 
 into wooden posts about 20 feet from the ground. These posts 
 are erected for this purpose chiefly on the railway lines from 50 
 to 100 feet apart. 
 
 VELOCITY OF ELECTRICITY. 
 
 376. The long experience of the officers of the United States 
 government on the coast survey, with telegraphic lines, have 
 
388 
 
 HOUSE'S PRINTING TELEGRAPH. 
 
 enabled them to measure the velocity of the galvanic current 
 with uncommon accuracy. From experiments and calculations 
 thus made, it appears that its velocity is about fifteen thousand 
 four hundred miles per second. 
 
 The period of its transit between Boston and Bangor, was re- 
 cently measured, and the result was, that the time occupied in 
 its passage, was the one hundred and sixtieth of a second. Ac- 
 cording to this experiment the velocity is at the rate of 16,000 
 miles per second, which it appears is about 600 miles per second 
 more than the estimates made on the coast survey. Annual 
 Scientific Discoveries. 
 
 Telegraphs in the Country. According to a recent estimate, 
 the length of telegraphic lines in the country, in actual opera- 
 tion, is not far from 15,000 miles. 
 
 The most remote points in communication are Quebec and 
 New Orleans ; their distances apart, following the circuitous 
 routes of the wires, being about 3,000 miles. 
 
 Number of Stations. The number of towns and villages ac- 
 commodated with stations, and from which, therefore, intelli- 
 gence by telegraph, from one to the other, or from one to all 
 the others can be interchanged, are between 450 and 500. 
 
 MORSE'S TELEGRAPHIC ALPHABET. 
 
 Alphabet. 
 
 A - 
 
 B 
 
 C -- - 
 
 D 
 
 E - 
 
 F 
 
 G 
 
 H 
 
 I -- 
 
 K 
 
 L 
 
 M 
 
 Alphabet. 
 
 N 
 
 o 
 p 
 
 Q 
 R 
 8 
 T 
 U 
 V 
 
 w 
 
 X 
 T 
 Z 
 
 Alphabet. 
 
 &- --- 
 
 Numerals. 
 
 HOUSE'S PRINTING TELEGRAPH. 
 
 377. This instrument, one of the wonders of our time, prints 
 all communications in Roman capitals, and that much more 
 rapidly than the most expert compositor. 
 
 To go into a description of all its parts would probably so 
 confuse the mind of the reader, that in the end none of it would 
 be understood. We shall, therefore, describe only such portions 
 
r\ 
 
 HOUSE'S PRINTING 
 
 of the machinery as are necessary to show 'how the result is 
 produced. 
 
 In the first place, when a communication is to be made from 
 one city to another, notice is given, by an electrical current on 
 the wires, which occasions a vibration of a part of the ma- 
 chinery, and by which the attendant knows that a message is to 
 be sent. At every station there is an electrical battery, con- 
 sisting of 12 or 14 cups, the power most commonly used being 
 that known as Grove's battery, a description of which may be 
 seen in another place. 
 
 The forms of all visible parts of the instrument are shown by 
 Fig. 295. That portion by which the printing is performed 
 consists of a soft iron, or electro-magnet contained in the cylin- 
 der A, of an escapement B, moved by condensed air, by means 
 of the pump G, above which is seen the band ' by which that 
 part of the machinery is turned ; D is the printing apparatus, 
 the projecting portion being the lever ; E is the inking band, 
 by which the type are inked for printing ; F is a strip of paper 
 for printing. 
 
 FIG. 235 
 
 House's Printing Telegraph. 
 
 This engine is moved by a boy, who turns the wheel by the 
 lever shown, and by which air is condensed by the pump G, and 
 by the force of which, the printing portion of the machinery is 
 actuated. 
 
890 PRINTING PRESS. 
 
 The action of the electricity in this telegraph is merely to 
 produce a correspondence of motion in the machinery at the 
 different ends of the line. All the mechanical results are pro- 
 duced by local, mechanical power, connected with the printing 
 apparatus at each station, where manual force is employed for 
 this purpose. 
 
 378. The letters on the keys, moving by the touch like those 
 of the piano, are the instruments by which the different letters 
 are, one by one, printed from one station to the next. Thus 
 one letter of the 26, on the different keys, will be printed at. the 
 other end of the line, when that letter is depressed. This is 
 done by converting a piece of soft iron into a magnet at the 
 next station, on the principle already explained and illustrated, 
 in the description of Morse's telegraph, only that the letter, 
 instead of the point, is made to act or advance. 
 
 This 'is a most complicated machine as a whole, though its 
 different parts are sufficiently simple. The effect, though the 
 means are so difficult to understand, is highly curious and inter- 
 esting, as it prints Roman capitals at the rate of 150 or 200 in 
 a minute. This is done on strips of paper an inch wide ; and 
 when in operation, any one may print a sentence, as his own 
 name, by touching the keys on which the letters are placed, 
 which spells the sentence. 
 
 PRINTING PRESS. 
 
 3*78. It is said that the Chinese printed from blocks of wood, 
 with letters engraved on them, before the Christian era. 
 
 But the first printing on metallic type, was executed on the 
 celebrated Mentz Bible, in about 1450. The next specimen of 
 printing known was the Psalter, done in Germany, m 1457. 
 
 It is said that these books are printed in such a style of 
 beauty and finish, as to command the astonishment of all 
 printers who behold them, and that even at the present day, 
 with all our boasted inventions and improvements in the arts, 
 it is difficult to imitate, and hardly possible to excel, these as 
 specimens of work in the art of printing. 
 
 Of the mechanical means by which printing has been, and 
 still is performed, many singular and curious examples might 
 be described, but our limits will only admit descriptions of two 
 figures, representing Ramage's press and the cylinder press. 
 
 379. Ramage's Press. This press was that most commonly 
 used on both sides of the Atlantic, until within the last 20 years. 
 In addition to this, the Stanhope and Smith presses were used 
 
INKING BALLS. 391 
 
 in England, and the Clymer and Washington in this country. 
 These may be considered as varieties of the Ramage ; and their 
 description would possess no interest, except to the antiquated 
 printer who had worked at them with the inking balls, now 
 long since disused, as we shall see. 
 
 The' Ramage press is repre- FIG. 296. 
 
 sented by Fig. 296, and will be 
 understood by the following de- 
 scription : The cheeks A A, are 
 the sides of the wooden frame 
 which supports the other parts, 
 and sustains the force of the 
 screw by which the impression 
 is made. The bed B, is that 
 
 part on which the type are laid 
 
 for printing. The ball C is seen Ramage's Press. 
 
 on a little shelf, called the rack, 
 
 made for that purpose. [This will be described hereafter.] 
 The frisket, F, turns down, and confines the sheet on the tym- 
 pan. The bar or lever L, turns the screw by which the force 
 is given and the impression on the type made. The platen, P, 
 is fastened to the lower end of the screw, being the part by 
 which the impression is made. It is of cast iron, about two 
 feet square, thick at the center, and strong, so as to give a 
 heavy force. The tympan, T, is covered w r ith parchment to re- 
 ceive the sheet, confined by the frisket, and then run under the 
 platen to be printed. 
 
 Action. The type being ^et, and locked firmly in an iron 
 frame, called a form, this is laid on the bed, and the type inked 
 by the balls ; the sheet is next laid on the tympan, and covered 
 by the frisket, which has open spaces for the pages, as seen in 
 the figure. The type and sheet spread over them, are then 
 moved under the platen by the action of a lever, connected 
 with a wooden cylinder, surrounded by leather straps, and called 
 the rounce. The impression is then made by pulling the lever, by 
 the action of which, on the screw, the platen is forced upon the 
 paper, and this on the type. The bed is then " run out," the type 
 again inked by dabbing with the balls, and the whole is again 
 ready to be run in for another impression, and so on to the 
 end. 
 
 INKING BALLS. 
 
 380. The former method of distributing the printing ink on 
 the type, consisted iu the use of a pair of balls, represented by 
 
392 
 
 INKING ROLLER. 
 
 FIG. 297. 
 
 Inking Balls. 
 
 Fig. 297. These were made of 
 sheeps' skin, undressed, and tech- 
 nically called pelts were six or 
 eight inches in diameter, stuffed 
 with wool, and furnished with wood- 
 en handles. 
 
 One of these being struck on the 
 board where the ink, a little thicker 
 than cream, was spread, took up a 
 small quantity, which, by turning 
 the balls skillfully on each other, 
 was equally spread over both. They 
 were then taken, one in each hand, and dabbed, or rapidly 
 struck on the type, until the ink was nicely distributed over 
 their faces, and thus they were made ready to give an impres* 
 sion. This was a critical and laborious operation, requiring 
 much experience and a strong arm, like that of a blacksmith, in 
 order to cover the type speedily and equally with the ink. 
 [Printer's ink is made of oil and lampblack.] 
 
 381. Invention of the Roller. The ancient method of inking 
 the type, as above described, was destined to give place to an 
 improvement, which, among printers, formed an era long to be 
 remembered. 
 
 FIG. 298. 
 
 Inking Roller. 
 
 Ihis was the invention of the roller which is composed of 
 molasses, glue, and tar, intimately mixed and combined by heat. 
 This composition has all the qualities to be desired for this pur- 
 pose, namely, softness, elasticity, and readiness to receive and 
 impart the ink. This being cast into a cylinder, on a wooden 
 support, and fitted to an iron frame, with handles, as shown by 
 Fig. 298, form the important instrument in question. 
 
 Rollers have also been made of India rubber. 
 
 As the ends of the support revolve easily in the frame, all 
 that it is necessary to do to spread the ink on the type, is first 
 
CYLINDER PRESS. 393 
 
 to pass the roller a few times over the board on which the ink 
 is spread, and then revolve it over the type two or three 
 times. 
 
 This invention completely obviated the most laborious and 
 unpleasant portion of the art of printing by hand ; and in ma- 
 chine printing, these rollers are so absolutely indispensable, that 
 without them that mode of printing, without which the world 
 would now remain in comparative ignorance, would have to be 
 relinquished. These rollers are from two to eight inches in 
 diameter; and for machine work, from three to six feet in 
 length. 
 
 DOUBLE CYLINDER PRINTING MACHINE. 
 
 382. This printing press, when compared with the ancient 
 or former one of Ramage, already described, will be seen to 
 present an entirely new invention, or series of inventions ; for 
 many years were consumed in devising and adapting its several 
 parts to each other, and bringing it to the state of perfection in 
 which it now exists. 
 
 Instead of printing, as did the hand presses in old times, 
 2,000 copies a day, by means of ten hour's hard labor of two 
 men, this engine, driven by steam, will, with the help of two 
 boys to fix the sheets in their places, print from 3,000 to 6,000 
 sheets per hour, or from 30,000 to 60,000 copies per day. 
 Such are the improvements in printing machines within the last 
 twenty years. 
 
 283. Description. This is a length, or side view of the ma- 
 chine ; the length of the printing cylinders and inking rollers 
 being about four feet. The length here shown of the whole 
 machine, is from 8 to 10 feet, and the height to the upper 
 cylinder 4 feet. 
 
 The ink, about the consistence of cream, is taken from the 
 trough, which is of the length of the small, rapidly revolving 
 roller, by which it is taken up, and from it is taken by adhe- 
 sion to another and larger roller, from which it is derived by 
 the type, over which it passes with a reciprocating motion. 
 
 At, or during each impression, the ink on the type is re- 
 newed by the continually revolving rollers. Thus, while this 
 engine is in action, being generally moved by steam, nothing 
 more is necessary than to supply the ink by putting it in the 
 trough, and to place the ends of the sheets under the revolving 
 cylinders, which latter work is done by two boys, as shown by 
 the cut. 
 
 17* 
 
SHARP'S RIFLE. 395 
 
 384. The parts of the press shown by the Fig. 299, are 
 marked as follows : The bed A, on the left, corresponds to the 
 same part in the hand press already explained. This has a re- 
 ciprocating or in and out motion ; the type which rest on it, 
 being alternately run out to be inked, and run in to be printed. 
 The revolving cylinders B B, receive the paper and press it 
 upon the type, by which it is almost instantly printed. The 
 cam C moves the flies D D, by which the printed sheets are 
 carefully laid away in a pile. This movement is communicated 
 by the cam to the flies, by the long iron bar seen oh the left. 
 The pulley E, moved by a .strap connected with the steam 
 power, gives motion to the entire machine by-means of gearing. 
 The revolving wheels G G, give motion to the cylinders and 
 inking rollers. The tape wheels, so called, H H, are the wheels 
 over which run tape bands, not shown, which convey the printed 
 sheets from the form to the flies. The printed sheets shown at 
 I, have been laid off by the flies, and are ready for circulation, 
 or the bindery, as the case may be. 
 
 [The author has thus tried his best to give an idea of print- 
 ing presses to those who never saw them ; but he would advise 
 all those who desire to know how printing is done, especially by 
 a cylinder press, to go and see with their own eyes, which they 
 can do now in nearly every village in the country.] 
 
 SHARP S RIFLE. 
 
 385. This is undoubtedly for the purpose designed, the most 
 perfect and efficient single instrument of destruction ever in- 
 vented ; and of which, we here propose to give such a descrip- 
 tion, with illustrations, as to make all its peculiarities readily 
 understood. 
 
 The barrel is about 22 inches long, and the bore of the size 
 to admit round balls of 32 to the pound ; but being elongated, 
 or acorn-shaped, the number is only 18 to the pound. 
 
 This rifle loads at the breech, the form of the ball inclosed in 
 its cartridge being shown at A, Fig. 300, introduced into its 
 place. 
 
 The slide B, which takes the place of the breech pin in other 
 guns, is a solid piece of steel, represented depressed for the in- 
 troduction of the ball. The cone E, is that part on which the 
 percussion cap, or its substitute, is exploded, and which in- 
 flames the charge in the gun. 
 
 The manner in which the breech slide is depressed, will be 
 understood by the section, Fig. 301, where D is the lever by 
 
396 
 
 FIG. SCO. 
 
 Sharp's Rifle. 
 
 which it is drawn down for the introduction of the ball, and 
 then elevated preparatory to the discharge. The upper and 
 anterior portion of the slide, has a cutting edge, seen above B, 
 Fig. 298, which separates the end of the paper cartridge, thus 
 exposing the powder to the action of the percussion priming, by 
 which it is inflamed and the gun discharged. 
 
 386. The Priming. The former mode of discharging this 
 rifle was by means of Maynard's patent priming, which con- 
 sisted of kernels of percussion powder, inclosed in varnished 
 paper. But this mode the inventor of the rifle found objec- 
 tionable on several accounts, and especially as it became useless 
 on exposure to moisture. 
 
 He therefore invented a new, and an entirely original mode 
 of prming, which has been recently patented, and which he 
 has allowed the author to figure, and explain for the use of this 
 work. 
 
 This consists of the tube A, Fig. 300, of iron, about the one- 
 fifth of an inch in diameter and two inches long, called the 
 magazine. In the lower part of this is a spring, above which 
 are the priming discs, consisting of thin, round envelopes of cop- 
 per, containing the percussion powder, completely protected 
 from moisture, so that they may remain under water for hours, 
 or weeks, without damage. 
 
 Each tube holds 60 of these primers, one of which is forced 
 up against the slide C by the spring. When the hammer is 
 drawn to the back notch, the slit B, working on the arm of the 
 slide C, which is fastened to the plate of the lock, draws it 
 back from over the tube A, and admits one of the percussion 
 discs in front of the slide at C, and by which, when the trigger 
 is pulled, it is thrown forward, between the face of the hammer 
 
397 
 
 FIG. 301. 
 
 Sharp's Rifle. 
 
 and the cone, where it is instantly exploded, and the rifle dis- 
 charged. 
 
 One of the most singular and curious results of this mechan- 
 ism, is, that the percussion disc is struck, as it were, " on ' the 
 wing," or while it is flying between the hammer and the cone; 
 and yet it never fails to explode in the proper place and dis- 
 charge the gun, let its position be vertical or horizontal. 
 
 387. Practical effects of this Rifle. We have seen this arm 
 fired at a target at the several distances of 300, 500, 600, and 
 700jards, being respectively 900, 1,500, 1,800 and 2,100 feet. 
 
 The target was a pine board 30 inches square, and by the 
 inventor was hit on the average, twice out of three shots. 
 
 By experiments and calculations lately made in France, it 
 was found that a man, at the distance of 1,638 feet, appears to 
 the naked eye only one-fifth his real size, and therefore, by this 
 estimate, a target of 30 inches in diameter, at the distance of 
 2,100 feet, appears less than six inches square, a small object 
 truly in practice, and requiring an accuracy of aim so minute, 
 that the tenth of an inch in the direction of the sight, would 
 carry the ball far aside of such a mark, and yet it was pierced 
 twice out of three shots. 
 
 388. Adjusted Sight. This rifle has an adjusting sight, 
 which is elevated, or depressed and fixed, according to the dis- 
 tance of the mark. All the shots were made, with the gun in 
 the hands, or without a rest, and also, with the striking pecu- 
 liarity of being placed on the left shoulder. 
 
 at the Distance of a Mile. Although in the above 
 
398 SHARP'S RIFLE. 
 
 trial, the distance was only 700 yards, the inventor has proved 
 by experiment, that this rifle throws its ball with a force equal 
 to the destruction of human life to the distance of a mile. 
 
 In battle, therefore, the approaching enemy can be effectually 
 assailed with this arm, at that distance, the aim, of course, being 
 more and more sure, as the distance diminishes. 
 
 Number of Balls Thrown. The rapidity with which this arm 
 may be loaded and fired is such, that if one ball be sent along 
 the surface of water, another may be made to follow before the 
 first ceases its motion. 
 
 The inventor loads and fires it ten times in a minute ; but 
 estimating that in battle the number of balls fired by each sol- 
 dier would be only six in a minute, then 1,000 men would dis- 
 charge 6,000 in a minute, or 360,000 in an hour. 
 
 389. Invention of Gunpowder. In Europe, the invention of 
 gunpowder is attributed to Roger Bacon, who died in 1292 ; 
 but it seems to have been known to the Chinese long before 
 that period. 
 
 The first account of its use in European war, was at the bat- 
 tle of Cressy, in 1346, and from that time it superseded, chiefly, 
 all other means of destruction on the battle-field. 
 
 Effects of this Invention. There is no doubt but this inven- 
 tion has proved a humane a merciful discovery in the art of 
 war. 
 
 Before its use, the instruments of death in battle were the 
 barbed arrow, the halbert and spear, various kinds of swords, 
 and the war-club. 
 
 The combatants fought hand to hand, each one trying to in- 
 flict the most cruel tortures on the other; and, indeed, the in- 
 struments employed, were much better calculated for this pur- 
 pose, than for the infliction of sudden and quiet death. 
 
 On the contrary, gun-shot wounds, when not instantly fatal, 
 afford a prospect of recovery, while those made by the barbed 
 arrow and spear, more commonly portend a miserable death, 
 after protracted agony. 
 
 Besides, if we examine the accounts of ancient battles, we 
 shall find, that including the carnage on the field, and the num- 
 ber who died of their wounds afterward, the destruction of 
 human life, where an equal number were engaged, was much 
 greater than it was, after the invention and use of gunpowder. 
 
 390. Conclusion. Although there is no doubt but the use 
 of fire-arms, in warfare, has heretofore diminished the horrors 
 of the battle-field, this circumstance, as history informs us, has 
 
SHARP'S RIFLE. 399 
 
 had no influence on our species, except to foster an increasing 
 desire to render the instruments of death more and more per- 
 fect, so that in the day of battle, the carnage should be as sud- 
 den and as great as possible. And hence, within a few years, 
 great improvements have been made on fire-arms in France, 
 England, Germany, and America, all tending of course, to the 
 increase of their destructive effects. 
 
 The inventors of these improvements in the arts of human 
 destruction, are by no means considered by political, or even 
 by moral and religious writers, as enemies of the human race, 
 but are viewed, at least, by many such, as the pioneers of uni- 
 versal peace, if, indeed, fallen man should ever cease to learn 
 and practice the art of war. 
 
 391. Settlement by Arbitration Improbable. The history of 
 man affords no foundation* for the belief that national quarrels 
 will be settled by the intervention, or arbitration of other nations, 
 and hence, there can be little doubt, if the moral and political 
 condition of the world remain as heretofore, that " nation will 
 continue to lift up its arm against nation," and that the time 
 when " man shall learn war no more," is not at hand, unless 
 indeed, it should be by the approach of the millenium. 
 
 Under this view of the case, the only prospect of universal 
 and permanent peace, is in such a degree of perfection in the 
 art of war, that certain death awaits at least five out of six of all 
 who enter as combatants on the field of battle ; and in naval war- 
 fare, an equal proportion of ships shall as certainly be buried 
 in the ocean. 
 
 Until such a state of things exist, men will continue to en- 
 gage each other in mortal strife ; and it is on this account 
 that moralists of the present day, look with favor on the im- 
 provements in fire-arms, knowing that the paramount design of 
 all such inventions is to render escape more difficult, and death 
 more sudden and certain on the battle ground. 
 
 Nor is it probable that the time is far distant, when at least 
 ten will foil on the field, where with an equal number of com- 
 batants, only one fell 30 years ago ; the result being solely from 
 the more deadly power of the fire-arms employed. 
 
 The author having served as surgeon on the frontier, in the 
 U. S. Army, in the war of 1812-15, is able to appreciate, in a 
 measure, the difference between the destructive power of the 
 fire-arms then furnished by the government, and those now to 
 be introduced into the Army of the United States. 
 
 392. Contrast between Old and New Arms. -To those who 
 
400 
 
 have examined this subject, and are acquainted with the arms 
 employed formerly, and those now going into general use, it 
 will not be considered an over estimate to suppose, that 100 
 men armed with Sharp's rifle, and Colt's revolvers, would com- 
 mit greater carnage, on the battle ground, than 1,000 men 
 could do, with the flint lock, government muskets, in former 
 use. 
 
 393. Who indeed will enlist into a military service when he 
 knows that his enemy will oppose him with messengers of death, 
 at the rate of 600 per minute, or 36,000 per hour, for every 
 100 men, and this at the distance of a mile, or less; and with 
 the same number of such messengers, in half that time, when 
 within any distance between 20 yards, and the reach of the 
 bayonet ; which will be the case when armies are supplied with 
 Sharp's rifles, and Colt's revolvers. 
 
 From such sources only, according to the present aspect of 
 the nations of the earth, can we look for permanent peace. 
 
 COLT'S REPEATING PISTOL. 
 
 394. This celebrated fire-arm has been brought to its present 
 degree of perfection, only after years of experience, trial, and 
 invention, by the original patentee Col. Samuel Colt, of Hart- 
 ford, Conn. 
 
 An account of this weapon is introduced here, as an inven- 
 tion with which all the civilized nations of the earth, are now, 
 or are soon, to become acquainted. 
 
 As Americans, therefore, we are bound to know something, 
 at least, of the history and mechanism of so important an 
 invention. 
 
 The examination and trial of Colt's revolvers at the World's 
 Fair, and the award passed upon them there by the best judges, 
 and the most experienced military men of the age, are ample, 
 and sufficient proofs that this, for the purpose for which it is 
 designed, is the most efficient and perfect fire-arm ever invented. 
 
 The immense demand for the article in foreign countries, as 
 well as in our own country, evinces, also, that no substitute 
 exists for this weapon. 
 
 About 400 artificers, we understand, are employed in their 
 manufacture, which number, it is stated, is to be increased to 
 1000, in order to supply the demand. 
 
 The United States government have adopted Colt's repeat- 
 ing pistol, as the best weapon known, for mounted men, both 
 for offensive and defensive use. And in the Mexican war, no 
 
COLT'S PISTOL. 401 
 
 officer who could obtain a revolver, ever went a day without 
 one, and those who could not, often considered their lives in 
 peril, in consequence. 
 
 395. In 1851, the President of the United States in a mes- 
 sage to the Senate, states, that 
 
 " Such is the favorable opinion entertained of the value of this 
 arm, particularly for mounted corps, that the secretary of war 
 has contracted with Mr. Colt, for four thousand of his pistols," 
 without waiting a special appropriation of Congress. This 
 contract, of course, was confirmed by the Senate. 
 
 Such is considered the importance of this arm as a weapon 
 of defence, that the military committee of the House of Rep- 
 resentatives, recommend that it should be furnished to emi- 
 grants, as the following shows. 
 
 " We, the undersigned, members of the military committee 
 of the House of Representatives, understanding that an appli- 
 cation is pending before your committee, favorably commended 
 by the ordnance department, for the purchase of a suitable 
 number of Colt's pistols, and authorizing the department to 
 furnish the same, to emigrants at government prices, and to de- 
 liver the same to the States, under the act of 1808, for arming 
 the militia, recommend the same to your favorable considera- 
 tion, and believe that such a clause in the army bill would be 
 desirable and proper." 
 
 Signed by the committee, nine in number, January, 1851. 
 
 396. Description of Colt's Pistol. Why these arms are 
 called revolvers, and by what means they are made the most 
 efficient of fire-arms, for certain purposes, will be understood 
 by Fig. 3 02, and the following explanation. 
 
 The letters by 
 
 which the principal FIG - 302 - 
 
 parts of the pistol 
 are denoted, are the 
 following, as seen on 
 the cut. The barrel 
 B, is from three, to 
 eight inches in length 
 according to the size cwfa Pistol. 
 
 and design of the 
 
 pistol. The cylinder C, is the part which revolves, and from 
 which the arm takes its distinctive name. The mechanism by 
 which the rotary motion is performed, can not be shown by a 
 single figure. The cylinder is pierced with six apertures, called 
 
402 MC CORMICK'S REAPER. 
 
 chambers, each of which, when ready for action, contains a 
 charge of powder and a ball. Caps are then put on the tubes, 
 corresponding to each charge, and now the arm is ready for the 
 discharge of six balls, as rapidly as the hammer can be drawn 
 to the back notch and the trigger pulled. 
 
 The hammer H, being drawn back to where it now stands, is 
 made to strike, with its face," the cap on the tube, by which it 
 is exploded, and the pistol discharged. Then on drawing the 
 hammer back for another discharge, the mechanism makes the 
 cylinder revolve one notch, by which the next cap is brought 
 under the hammer, and by pulling the trigger is discharged, 
 and so of all the other charges. The trigger requires no expla- 
 nation, being in all tire-arms the same. The ramrod R, is con- 
 nected with the lever L, by the united action of which, the ball 
 is pushed down the chamber to the powder. 
 
 397. Having explained the references, we will give tho 
 inventors own directions for loading, &c. 
 
 " Draw back the hammer to the half notch, which allows the 
 cylinders to be rotated ; a charge of powder is then placed in 
 each chamber, and the balls, without wadding, or patch, are 
 put, one at a time, upon the mouths of the chambers, turned 
 under the rammer, and forced down with the lever below the 
 mouth of the chamber. This is repeated until all the cham- 
 bers are loaded. Percussion caps are then placed on the tubes, 
 when, by drawing back the hammer to the full catch, the arm 
 is in condition for a discharge by pulling the trigger ; a repeti- 
 tion of the same motion produces like results." 
 
 When this arm is prepared, therefore, all that is required in 
 defence, or in action, is to draw back the hammer, and pull the 
 trigger, until the six balls are discharged, which is done in less 
 than half a minute. 
 
 MC CORMICK'S REAPER. 
 
 398. The principal, or cutting apparatus of this famous ma- 
 chine, is shown by Fig. 303. The entire machinery, consisting 
 not only of the four wheels on which the whole rests, but also 
 of bands, cranks, cog-wheels, driver's seat, and platform for the 
 grain the whole being connected and supported by braces in 
 all directions ; it is obvious, is too complicated an engine for 
 the purposes of a school book. 
 
 Nor are these parts necessary to show the mystery, in which 
 the public are chiefly interested, viz., how it is possible that a 
 
MO CORMICK S REAPER. 
 FIG. 803. 
 
 403 
 
 McCormick's Reaper. 
 
 machine drawn by horses, can do what only the hands of man 
 have heretofore performed with the sickle and cradle. 
 
 The above drawing is designed merely to illustrate and ex- 
 plain this wonder. 
 
 The angular pointed projections, marked by numbers 1, 2, 3, 
 4, and 5, are called the fingers. They are firmly driven into a 
 beam of wood, at the distance of 4- inches from the center of 
 one to that of the other, and their length is about the same 
 number of inches. They are of cast iron, without cutting edges. 
 
 At the base of the fingers, and between their angles, are seen 
 the sickles, angular in form, and composed of sections of steel 
 plate riveted to an iron strap, about an inch wide, which strap 
 is movable to the right and left on the beam of wood into which 
 the fingers are driven. 
 
 399. The sickles have thin cutting edges, which are finely 
 serrated, similar to a common sickle, the teeth standing right 
 and left from the center or angle of each. 
 
 While the fingers are fixed to the beam, the sickles have a 
 reciprocating motion of about 4 inches, alternately to right and 
 left by means of a crank, turned by the force of the wheels, by 
 which the whole machine is moved. 
 
 This is the effective or cutting portion of McCormick's reaper. 
 All the other parts are adjuvants to this, being the means by 
 which this is moved and actuated. 
 
 The divider A, is a piece of iron which extends forward of 
 the fingers, and is designed to separate the grain to be cut from 
 that which is to be left standing. This, as its shape indicates, 
 
404 
 
 bends the grain to be cut inwards, leaving that which remains 
 in a well-defined and perfect line, until the return of the reaper. 
 
 The strips of wood, B, fastened to the beam on which the 
 sickles work, show where the force, by means of horses, is ap- 
 plied, and by which the whole is drawn on four wheels of mod- 
 erate size. 
 
 400. Action of the Reaper. It will be observed that the 
 effective or cutting portion of the machine, extends to the right 
 of the place where the moving force is applied, and hence that 
 the horses work on the side of the standing grain. 
 
 The grain, therefore, is cut by beginning on the outside and 
 going around the field, the horses passing that which has been 
 cut, while the sickles extend about six feet into that which is 
 standing. 
 
 The cutting is performed by the alternate, or reciprocating 
 motions of the sickles against the grain, which is kept from re- 
 ceding by the oblique, angular form of the fingers, as shown 
 by the figure, after the inspection of which, no further descrip- 
 tion will be required to show how the operation is performed. 
 
 As the grain is cut, it falls upon a platform, where a man 
 stands, with a rake, to gather and remove it to the outside of 
 the machine, and where it is bound by men who follow for this 
 purpose. Thus the way is cleared for the return of the horses 
 and reaper. 
 
 It is stated that the fields of wheat thus cut, present a very 
 smooth and, to the eyes of the farmer and others, beautiful ap- 
 pearance the stubble presenting a level and even surface 
 throughout. 
 
 The inventor of this reaper not only received the highest pre- 
 mium at the World's Fair, in England, but also the gold medal, 
 in the States of Ohio and Illinois, for the most complete, and 
 best working machine of this kind presented. 
 
 It is stated that such is the demand for this reaper, that sev- 
 eral thousands will probably be sold in the course of the present 
 year. The price is about $120. 
 
 The inventor also constructs mowing machines, on the same 
 principle as the reaper. 
 
INDEX. 
 
 A. 
 
 Ball, cannon, velocity 
 of, 58,60 
 
 Colt's revolver, 400 
 
 Comets, 335 
 
 ACOUSTICS, 198 
 Accidents 189 
 
 path of, 58 
 Barkers' mill, 131 
 Battery, galvanic, 358 
 tobacco pipe, 360 
 Biot, on sound, 200 
 Bodies, properties of, . . 7 
 fall of li"ht, 35 
 
 Concave mirror, effects 
 of, 239 
 
 to the rails, 196 
 
 Counter currents, 214 
 Convex lens, 243 
 Condenser, 148 
 Cornea too convex, .... 251 
 Cornea, too flat, 252 
 
 Action and reaction, . . 40 
 ^Eolian harp 205 
 
 Air, elasticity of, 139 
 
 expansion of, 140 
 in every crevice, . . 138 
 compression of, ... 139 
 weight of, 140 
 
 
 n S> 
 
 Constellations, 283 
 
 Body, definition of, .... 7 
 
 Crank, nature of, 181 
 
 Artificial magnet 353 
 Ascent of bodies, 32 
 Air-ffun, 149 
 
 Boat', and bellows, 41 
 Battery, galvanic, 358 
 Grove's 360 
 
 Cup and shilling ... 2ii2 
 
 Cylinder steam . . . 178 
 
 D. 
 
 pump, 1** 
 
 
 double-acting, 143 
 experiments with, . 14 
 Atmospheric pump, . . . 160 
 Atmosphere, pressure of, 146 
 phenomena of, 140 
 conveys sound, 198 
 Atmospheric electricity, 349 
 Anemometer, 215 
 
 construction of, ... 150 
 use of, at sea, 159 
 wheel 155 
 
 Day and night,' 310 
 
 weather glass, 157 
 
 Decomposition, 10 
 Definition, 172 
 
 measures heights, . 156 
 
 Differential thermometer. 173 
 Density, 20 
 
 
 diminution of, .... 157 
 of the planets 285 
 Dick Thomas .. 264 
 
 Ascent of bodies, 32 
 
 Brittleness,' 21 
 
 Astraea, **a 
 Attraction in general,.. 12 
 of built 16 
 
 Brick, reading through a, 241 
 Burning glass 240 
 
 Divisibility of matter, . . 9 
 Dioptrics, 221 
 Diving bell 199 
 
 capillary 13, J 7 
 of cohesion, 13 
 chemical 18 
 
 C. 
 
 Caloric engine, 379 
 Camera obscura, . . 266, 380 
 Cask, bursting of, 107 
 
 Ductility, .' 22 
 Double refraction, 224 
 Dipping needle,. . . 352, 355 
 
 E. 
 
 Tort I, 200 
 
 of gravitation, 15 
 electrical, 20 
 
 magnetic, .... 19, 352 
 proportionate to mat- 
 ter, 30 
 
 Capstan, 82 
 
 Card machine, 100 
 
 axis of, 290 
 
 Angle of vision,... 245,254 
 
 Cannon ball, velocity of, 60 
 fall of 58 
 
 distance from sun,. 290 
 diurnal motion of,. 290 
 revolution of, 290 
 falling to Sun 29 
 form of, 314 
 
 definition of, 276 
 physical, 276 
 
 revolution of, 59 
 
 practical, 276 
 
 Ceres ' 278 
 
 Archimedes' screw, 128 
 Asteroids 277 
 
 Centrifugal force, . 46, 316 
 Centripetal force, 46 
 Center of gravity, 48 
 
 velocity of, 311 
 
 Ecliptic 282 
 
 
 
 Atwoods' machine, 28 
 Axis of a planet, 280 
 of the earth, 290 
 
 lunar, 325 
 
 Chronomete'r 332 
 
 solar, 328 
 
 
 Echo 325 
 
 B. 
 Balance, 117 
 
 Chromatics, 269 
 Clock, common, 66 
 Clio 278 
 
 Egeria, 278 
 Electricity 339 
 
 theories of, 341 
 
 Circus rider, 45 
 
 Electrical machine, .... 342 
 
 Ball, movement of,. . 11, 50 
 revolution of, 59 
 
 Coal, power of, 191 
 Colors of objects, 272 
 
 attraction, 21 
 battery, 348,356 
 helix. ... . . 368 
 
406 
 
 INDEX. 
 
 Elasticity, 20 
 
 Electrical telegraph,... 386 
 
 bodies, :..... 339 
 
 Electro-magnetism. . . . 357 
 
 laws of; 357 
 
 motion of, 361 
 
 Electroscope, 361 
 
 Electrotype, 370 
 
 Electrometer, 346 
 
 Electro-gilding, 376 
 
 plating, 377 
 
 Engine, steam, 174 
 
 Engine, atmospheric, . . 175 
 
 caloric, 379 
 
 fire, 165 
 
 Equal forces, 32 
 
 Equation of time, 318 
 
 Equilibrium, 55 
 
 Erect image, 250 
 
 Extension, 8 
 
 Eye, human 248 
 
 of an ox, 250 
 
 F. 
 
 Falling bodies,... . 26 
 
 light 35 
 
 direction of, 25 
 
 velocity of, 26 
 
 Figure of bodies, 8 
 
 of the earth 315 
 
 Fire engine, 165 
 
 Five mechanical pow- 
 ers 101 
 
 Flora, 278 
 
 Fluids, what 103 
 
 discharge of, 124 
 
 Focus, principal, 237 
 
 Form, change of, 21 
 
 Fly-wheel, 184 
 
 Focal distance, ... 232, 243 
 
 'Force, what, 71 
 
 equal, 31 
 
 not created, 86 
 
 of gravity, 12 
 
 Fountain of Hiero, 166 
 
 expansion, 147 
 
 Friction of machinery,. 86 
 
 of fluids, 126 
 
 Fulcrum, 72 
 
 G. 
 
 Galvanism, 357 
 
 Gallery, whispering, ... 202 
 
 Galvanic battery, 358 
 
 Grove's, 360 
 
 Globular form, 14 
 
 Gold leaf, 9 
 
 G overnor, steam, 185 
 
 Gravity terrestrial, .... 24 
 
 force of, 24 
 
 Gravitation, 15 
 
 Gravity, center of, 48 
 
 in man 52 
 
 how taken, 48 
 
 specific, .- 116 
 
 table of, 118 
 
 Gregory's telescope, . . . 263 
 
 Gun, air, 149 
 
 
 59 
 
 H. 
 
 
 Harmonicon, 
 
 211 
 
 Hay, load of, 
 
 52 
 
 Heat, absorption of, ... 
 
 167 
 
 distribution of, . . . . 
 
 167 
 
 by concave mirror, 
 
 240 
 
 radiation of, 
 
 167 
 
 reflection of, 
 
 167 
 
 transmission of, ... 
 
 168 
 
 
 20 
 
 Harp, ajoliuu 
 
 205 
 
 Hebe 
 
 278 
 
 Herschel, planet, 
 
 296 
 
 his telescope, 
 
 2G3 
 
 Hiero's fountain, 
 
 166 
 
 Helix, electrical, 
 
 368 
 
 High pressure engine,.. 
 Hoblyn, Prof., 
 
 188 
 174 
 
 Horizon, 
 
 189 
 308 
 
 Horology, 
 Hydraulics, 
 
 65 
 123 
 
 Hydrostatic bellows, . . . 
 
 110 
 
 press, 
 
 110 
 
 Human face magnified, 
 
 238 
 
 Hydrometer, 
 
 119 
 
 Hydrostatics, 
 
 136 
 102 
 
 Hydrostatic paradox, . . 
 
 108 
 
 Hygrometer, 
 
 173 
 
 itygeia, 
 
 278 
 
 I. 
 
 
 Impenetrability, 
 Inertia, center of, 
 
 7 
 54 
 
 Imp, bottle, 
 Inclined plane, 
 
 150 
 93 
 
 motion on, 
 
 93 
 
 
 256 
 
 Indestructibility, 
 Instruments, musical,.. 
 
 9 
 
 204 
 
 Irene 
 
 278 
 
 Iria 
 
 278 
 
 3. 
 
 
 Juno, 
 
 278 
 
 Jupiter, 
 
 292 
 
 
 293 
 
 distance of, 
 
 292 
 
 K. 
 
 
 
 80 
 
 L. 
 
 
 
 268 
 
 Latitude, what, 
 
 329 
 
 how found, 
 
 330 
 
 Lenses, what, 
 
 243 
 
 forms of, 
 
 243 
 
 refraction by, 
 Leyden jar, 
 
 242 
 348 
 
 
 51 
 
 Lens, concave, 
 
 245 
 
 convex, 
 
 243 
 
 double-convex,.... 
 
 244 
 
 Level, water 115 
 
 spirit, 116 
 
 Lever, what 72 
 
 simple, 72 
 
 compound, 78 
 
 compared, 78 
 
 knee, 80 
 
 Lightning rods, 350 
 
 Light, convergent rays, 237 
 
 diverging rays of,. . 244 
 
 refraction of, 221 
 
 reflection of, 225 
 
 decomposition of, . 269 
 
 motion of, 220 
 
 re-composition of, . 270 
 
 velocity of, 220 
 
 Locomotive, 192 
 
 boiler, 197 
 
 Locomotive, described, 193 
 
 Longitude, what, 330 
 
 how found, 332 
 
 Lunar, eclipses, 324 
 
 M. 
 
 Machine, what, 70 
 
 for raising water, . . 128 
 
 Mars 291 
 
 Malleability, 22 
 
 Magic lantern 268 
 
 Machinery, use of, .... 70 
 
 Magnetism, 351 
 
 Magnitudes, judged of,. 254 
 Magnets, artificial, .... 353 
 
 temporary, 355 
 
 Magnetic needle, 355 
 
 rotation, 354 
 
 dip of, 287 
 
 Magdeburg hemispheres, 146 
 
 Mechanics, 70 
 
 Mercury, 383 
 
 Metronome, 69 
 
 McCormick's reaper, . . . 402 
 
 Metis 278 
 
 Microscope, simple, 256 
 
 compound 257 
 
 solar, 258 
 
 Momentum, 39 
 
 Mountain, rupture of,. . 112 
 Mechanical powers, ... 72 
 
 Mirrors, what, 226 
 
 concave, 235 
 
 focus of, 237 
 
 convex, 229 
 
 metallic, 240 
 
 plane, 226 
 
 plane inclined, 235 
 
 Morse's telegraph, 383 
 
 Moon, 291 
 
 fall to earth, 29 
 
 phases of, 321 
 
 surface of, 323 
 
 eclipses of, 384 
 
 Motion, what, 36 
 
 absolute, 37 
 
 axis of, 43 
 
 center of, 46 
 
 compound 43 
 
 circular, 45 
 
INDEX. 
 
 407 
 
 Motion, crank, 182 
 
 Perkins' experiments, . . 103 
 
 Sound, propagation of, 200 
 reflection of 201 
 
 curvilinear 55 
 diagonal, 44, 64 
 parallel 179 
 reflected, 41 
 
 cylinder, 393 
 
 Prismatic spectrum, . . . 269 
 Properties of bodies, ... 7 
 Projectiles . 60 
 
 reverberation of, . . 200 
 velocity of, 200 
 Solar spectrum, 269 
 system, 276 
 
 relative, 37 
 resultant, 63 
 Motion, retarded, 38 
 
 Pump, air, 142 
 atmospheric, 160 
 chain 132 
 
 Spectacles, 247, 245 
 Summer and winter,. . 313 
 Specific gravity, 116 
 Spring, intermitting,.. 122 
 System of pulleys, 89 
 Steel-yard, 75 
 
 uniform, 3 
 
 
 forcin" ' 163 
 
 vertical, 33 
 
 lifting? 161 
 
 String, vibration of, . . 204 
 
 Musical strings, 204 
 instruments, 204 
 Monochord 206 
 
 stomach, 16 
 
 Steam cylinder,... 178,186 
 
 water,' 159 
 
 N. 
 Neptune, 297 
 
 
 engine, 174 
 modern, 187 
 
 
 low pressure, 188 
 high pressure, 188 
 
 White's,'- 92 
 
 New plant, 278 
 Needle dipping 355 
 
 R. 
 
 Rails, adhesion to, .... 196 
 Rain 216 
 
 Watt's, 176 
 Newcomen's, 176 
 
 0. 
 
 Objects seen erect, 253 
 
 btwna pump, .. 286 
 
 distance of, 286 
 eclipses of, 328 
 
 Optics, 218 
 Optics, definition of, ... 218 
 Optical instruments,... 256 
 definitions, 219 
 
 guage, 217 
 Ravs, convergent, 229 
 'divergent, 230 
 Rainbow, 272 
 
 revolution of, 286 
 
 Siphon 121 
 
 Orbit, what 280 
 elliptical " 281 
 
 Rariety, 20 
 Reaper, McCormick's, . 402 
 Ret .. .37 
 
 T. 
 
 Table of velocities, 38 
 Temporary magnets, ... 259 
 TeleJcope,.. 258 
 
 Organ 207 
 
 construction of, . . 208 
 invention of, 210 
 ant quity of, 210 
 larae, .. 210 
 
 Revolver, Colt's 400 
 Revolving bell engine, . 364 
 
 whiel 36-2 
 
 pipV... 208 
 P. 
 
 Revolution of wheels, . . 47 
 oftheearth, 309 
 
 principle of, 260 
 
 refracting 260 
 
 of planets, 59 
 
 381 
 
 Pallas 278 
 Paradox, hydrostatic, . . 109 
 
 Reflection bv mirrors,. . 225 
 of sound 201 
 
 Telescope, reflecting, . . 262 
 Rosse's, 264 
 
 Telegraph . . . 386 
 
 . annual, 338 
 diurnal, 338 
 
 Refraction, what, 221 
 oflight, 223 
 laws of, 222 
 
 House's 383 
 
 Pipes, organ 208, 209 
 Piaster, casts of, 374 
 Plane, inclined, 93 
 Planetoids, 279 
 
 double, 224 
 by glass, 223 
 by water 222 
 
 ofwood 22 
 of metals, 23 
 Thermo-electricity, .... 369 
 solar . 317 
 
 Retina, 248,250 
 
 distances of, 277 
 density of 285 
 
 Recapitulation, 23 
 Rifle. Sharp's, 395 
 Rotation of a wheel, . . 4"t 
 River, currents in, 12"* 
 Rosse's telescope, 264 
 
 3. 
 Saturn, 294 
 
 alcoholic, 169 
 comparison of, .... 170 
 Rutherford's, 172 
 Leslie's, 172 
 
 motion of, 277 
 
 situation of . 279 
 
 table of 278 
 
 Tides, 327 
 
 
 
 u , . . 
 
 Trade wind, 212 
 
 Penumbra, 326 
 
 U. 
 
 Umbra, 326 
 
 Phenomena, atmospher- 
 
 Scales 75 
 Seasons 310 
 
 ic, 212 
 379 
 
 Philosophy defined,.... 7 
 Piledriver 40 
 
 heat and cold of, . . 312 
 Screw 96 
 
 V. 
 
 Variation magnetic,... 356 
 
 Archimedes', 12t 
 perpetual, 9J 
 
 Power, what, 71 
 
 power of, 97 
 
 Shepherds of Landes, . . 5J 
 
 table of, 356 
 
 Velocity of fall ing bodies, 2t 
 accelerated, 38 
 
 varying, 84 Dmae U8llcrjr "* 
 
408 
 
 INDEX, 
 
 VeJbcity of a ball,, 
 retarded . . 
 
 .... 60 
 .... 38 
 
 Visual angle, 
 
 254 
 206 
 196 
 347 
 
 84 
 103 
 103 
 
 106 
 112 
 128 
 114 
 223 
 128 
 127 
 159 
 114 
 117 
 136 
 
 Water wheels, .... 
 
 . 133 
 
 Vibration of cords, .... 
 of solids, 
 
 Weight, what, 
 
 24,47 
 .. 85 
 .. 47 
 
 of the earth 
 
 . 311 
 
 Wheels, system of, . 
 Wheel, revolving, . . 
 
 of light 
 
 220 
 
 Vial, Leyden, 
 
 
 37 
 
 W. 
 Watch-work 
 
 flv 
 
 184 
 
 of certain bodi 
 table of, , ., 
 of wind 
 
 es... 38 
 38 
 216 
 
 
 134 
 
 breast, ... 
 
 135 
 .. 130 
 
 of electricity, 
 V enus 
 
 .. 337 
 288 
 
 Water, what, 
 elasticity of, 
 equal pressure of, 
 104, 
 
 Wheel and axle 
 
 .. 85 
 .. 47 
 
 
 289 
 
 Wed^e 
 
 96 
 
 evening star,.. 
 
 .... 290 
 
 Whispering gallery, . 
 Wind instruments, . . 
 Wind, what, 
 
 .. 202 
 .. 207 
 .. 2J2 
 213 
 
 morning star, . 
 Vibrating wire, . . . 
 
 ....290 
 .... 363 
 245, 253 
 242 
 .... 251 
 
 friction of, 
 
 level, 
 
 angle of, 
 perfect, 
 
 refraction by, 
 raising of, 
 
 velocity of, 
 Windlass, 
 
 215 
 .. 83 
 
 indistinct, 
 Vertical motion, . . 
 
 Vrtn 
 
 251 
 256 
 33 
 
 278 
 
 pumps, 
 
 table of pressure, . . 
 weighing in, 
 
 Z. 
 Zodiac, 
 
 .. 283 
 
 
 7b ram, 
 
 
 ERRATA, Astronomy, p. 114, insert the Asteroids as in p. 120. In p. 
 122, omit the last clause, and look to Saturn for the correction. The rela 
 tive size of Neptune can not be shown on the page. 
 
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