LIBRARY ail oco-oUyvvvU 1 /r'/L '// d/Y. V, H iR A UCSB LIBRARY AN ELEMENTARY COURSE CIVIL ENGINEERING, FOR THE USE OF CADETS OF THE UNITED STATES' MILITARY ACADEI D. H. MAHAN, M. A., PROFESSOR OF MILITARY AND CIVIL ENGINEERING IN THE MILITARY ACADEMY. SIXTH EDITION, WITH LARGE ADDENDA AND MANY NEW CUTS. NEW YOEK : JOHN WILEY, 66 WALKER-STREET. 1861. ENTERED, a:tording to act v Congress, in too yem /848, by WILEY AND PUTNAM, Clerk's Office of the District Court for the Southern District of New York. PREFACE. THE present Edition of this Work, like the two preceding, has been compiled for the use of the ca- dets of the U. S. Military Academy, and comprises that part of the Course of Civil Engineering taught them which the Author deemed would prove the most useful to pupils in other seminaries, studying for the profession of the civil engineer. In preparing this Edition, the Author has found it necessary to recast and rewrite the greater portion of the work ; owing to the considerable additions made to it, and called for by the vast accumulation of important facts since the publication of the former editions. A new form has also been given to the work, in the substitution of wood-cuts in the body of it for the plates in the former editions, as better adapted to its main object as a text-book. From these additions and changes, the Author trusts that the work will be found to contain all of the essential PREFACE. principles and facts respecting those branches of the subject of which it treats; and that it will prove a serviceable aid to instructors and pupils, in opening the way to a more extensive Drosecution oi the studies connected witn the engineer's an. CONTENTS AET. BUILDING MATERIALS. Classification of 3 .. 1 STONE. Classification of Stones 3 . . 1 Silicious Stones 4 . . 2 Argillaceous Stones 17 . . 5 Calcareous Stones 21 .. 6 Durability of Stones 31 . . 10 Hardness of Stones 37 . . 12 LIME. Classification of Limes 38 . . 14 Hydraulic Limes and Cements 42 . . 16 Physical Characters and Tests of Hydraulic Limestones .... 49 . . 18 Calcination of Limestone 54 . . 21 Lime-kilns 59 . . 22 Slaking Lime 67 . . 26 Manner of reducing Hydraulic Cement 78 . . 28 Artificial Hydraulic Limes and Cements 81 . . 29 Puzzolana, &c 87 . . 30 MORTAJU Classification of Mortars 96 . . 32 Sand 100 . . 33 Hydraulic Mortar 107 . . 34 Mortars exposed to Weather 113 . . 35 Manipulations of Mortar 116 . . 36 Setting and Durability of Mortar 119 . . 37 Concrete 128 . . 39 Beton 133 . . 4C Adherence of Mortar 137 . . ib. MASTICS. Bituminous Mastics 149 . . 41 GLOB. Marine Glue.. ... 153 . . 43 6 CONTENTS. ART. BRICK. Uses, &c ...................................... ....... 155 .. 44 Fire Brick ........................................... 167 . . 46 Tiles ............................................ ----- 168 .. ib. Classification of Timbei ................................. 169 . . ib. Felling of Timber ...................................... 172 .. 47 Seasoning of Timber .................................... 175 . . 48 Defects of Timber ..................................... 181 . . 49 Preservation of Timber ................................. 184 . . 5C Durability of Timber ................................... 194 . . 5-2 Trees which furnish Timber ............................. 198 . . 53 METALS. Cast Iron ............................................. 206 . . 55 Wrought Iron .......................................... 220 . . 58 Durability of Iron ...................................... 234 . . 59 Preservation of Iron .................................... 246 . 60 Copper .............................................. . 255 . . 63 Zinc ................................................. 256 . . ib. Tin .................................................. 258 . . 64 Lead ................................................. 259 .. ib. PAINTS AND VARNISHES. Compositions of ........................................ 265 . . 65 Varnish and Paint for Zincked Iron ...................... 267 . . 66 RESULTS OF EXPERIMENTAL RESEARCHES ON THE STRENGTH OF MATERIALS. General Remarks on .................................... 270 . . 67 Strength of Stone ...................................... 281 . . 70 Practical Deductions on the Strength of Stone ............. 288 . . 74 Expansion of Stone by Heat ............................. 289 . . 75 Strength of Mortars .................................... 290 . . 75 Strength of Concrete and Beton .......................... 295 . . 77 Strength of Timber .................................... 296 . . 78 Strength of Cast Iron ................................... 303 . . 81 Strength of Wrought Iron ............................... 324 . . 96 Resistance to Torsion of Wrought and Cast Iron ............ 333 . . 102 Strength of Copper ..................................... 334 . . 103 Strength of other Metals ................................ 336 . . 104 Linear Dilatation of Metals by Heat ...................... 337 . . ib. Adhesion of Iron Spikes to Timber ....................... 338 . . 105 MASONRY. Classification of Masonry 344 . . 107 Cut Stone 345 . . ib. Rubble Stone 358 . . 113 CONTENTS. 7 A RT. PAG* Brie A Masonry 362 . 113 Foundations 366 . . 114 Precautions against Lateral Yielding in Foundations 393 . . 12 Foundations in Water 394 . . 127 Construction of Foundation Courses 406 . . 136 Component Parts of Structures of Masonry 412 . . 138 Walls of Enclosures 413 . . 138 Vertical Supports 414 . . 139 Areas 415 . . ib. Retaining Walls 416 . . ib. Relieving Arches 429 . . 145 Lintel 435 . . 146 Plate-bande 436 . . ib. Arches 437 .. 147 Precautions against Settling , 479 . . 163 Pointing 480 . . 164 Repairs of Masonry 483 . . 165 Effects of Temperature ^ 486 . . 166 FRAMING. General Principles of Framing 490 . . 168 Frames of Timber ; 499 . . 170 Joints of Frames 520 . . 185 Frames of Iron 527 . . 189 Flexible Supports for Frames 534 . . 196 Experiments on the Strength of Frames 549 . . 200 BRIDGES. Classification of Bridges 550 . . 204 Stone Bridges 551 . . ib. Wooden Bridges 583 . . 230 Cast-iron Bridges 606 . . 244 Effects of Temperature on Stone and Cast-iron Bridges .... 611 . . 250 Suspension Bridges 612 . . 251 Moveable Bridges 624 . . 264 Aqueduct Bridges 630 . . 269 ROADS. Reconnaissance 635 . . 277 Survey 636 . . 278 Map and Memoir 637 . . 279 Location of Common Roads 638 . . ib. Kat th-work 646 . . 284 Drainage 653 . Road-coverings 654 . . 292 Pavements 655 . . ib. Broken-stone Road-covering 656 . . 296 Materials and Repairs 661 . 298 Cross Dimensions of Roads .... 662 . 299 CONTENTS. RAILWAYS. ART. PIQB Rails 665 .. 301 Supports 668 . . 303 Chairs 669 .. 304 Ballast 670 . . ib. Railways of WooJ and Iron 671 . . 305 Gauge 672 . . ib. Curves 674 . . 306 Sidings, &c 675 . . 307 Turn-plates 676 . . 308 Street-crossings 677 . . ib. Gradients 678 . . ib. Tunnels.... 680 .. 309 Masonry of Tunnels 682 . . 311 CANALS. Classification of Canals , 687 . . 313 Level Canals 689 . . ib. Canals not on the same Level 690 . . 315 Feeders and Reservoirs 697 . . 321 Lift of Locks 704 . . 326 Levels 705 . . 327 Locks 706 . . ib. Lock-gates 722 . . 333 Accessory Works ; . . 726 . . 335 General Dimensions of Canals 733 . . 337 RIVERS. Natural Features of Rivers 734 . . 340 River Improvements 740 . . 341 Means for protecting River-banks 742 . . 342 Measures against Inundations 743 . . ib. Elbows 744 . . 343 Bars 747 . . 344 Slack- water Navigation 752 . . 346 SEA-COAST IMPROVEMENTS. Classification of, &c 760 . . 349 Roadsteads 765 . . 350 Harbors 766 .. 353 Dikes 777 .. 356 Groins 778 .. 357 779 .. 368 ELEMENTARY COURSE CIVIL ENGINEERING. BUILDING MATERIALS. 1 . A KNOWLEDGE of the properties of building materials rs one of the most important branches of Civil Engineering. An en- gineer, to be enabled to make a judicious selection of materials, and to apply them so that the ends of sound economy and skilful workmanship shall be equally subserved, must know their or- dinary durability under the various circumstances in which they are employed, and the means of increasing it when desirable ; their capacity to sustain, without injury to their physical quali- ties, permanent strains, whether exerted to crush them, tear them asunder, or to break them transversely ; their resistance to rup- ture and wear, from percussion and attrition ; and, finally, the time and expense necessary to convert them to the uses for which they may be required. 2. The materials in general use for civil constructions may be arranged under the three following heads : 1st. Those which constitute the more solid components of structures, as Stone, Brick, Wood, and the Metals. 2d. The cements in general, as Mortar, Mastics, Glue, &c., which are used to unite the more solid parts. 3d. The various mixtures and chemical preparations, as solu- tions of Salts, Paints, Bituminous Substances, &c., employed to coat the more solid parts, and protect them from the chemical and mechanical action of atmospheric changes, and other causes of destructibility. STONE. 3 The term Stone, or Rock, is applied to any aggregation of several mineral substances. Stones, for the convenience of de- scription, may be arranged under three general heads the nli* cious, the argillaceous, and the calcareous. I 2 BUILDING MATERIALS. 4. SILICIOUS STONES. The stones arranged under this head receive their appellation from silex, the principal constituent of the minerals which compose them. They are also frequently desig- nated, either according to the mineral found most abundantly in them, or from the appearance, of the stone, a? feldspathic, quart- zose, arenaceous, &c. 5. The silicious stones generally do not effervesce with acids, and emit sparks when struck with a steel. They possess, in a high degree, the properties of strength, hardness, and durability ; and, although presenting great diversity in the degree of these properties, as well as in their structure, they furnish an ( xtensive yariety of the best stone for the various purposes of the engineer and architect. 6. Sienite, Porphyry, and Green-stone, from the abundance of feldspar which they contain, are often designated as feldspathic rocks. For durability, strength, and hardness, they may be placed in the first rank of silicious stones. 7. Sienite consists of a granular aggregation of feldspar, horn- blende, and quartz. It furnishes one of the most valuable building stones, particularly for structures which require great strength, or are exposed to any very active causes of destructibility, as sea walls, lighthouses, and fortifications. Sienite occurs in exten- sive beds, and may be obtained, from the localities where it is quarried, in blocks of any requisite size. It does not yield easily to the chisel, owing to its great hardness, and when coarse- grained it cannot be wrought to a smooth surface. Like all stones in which feldspar is found, the durability of sienite de- pends essentially upon the composition of this mineral, which, owing to the potash it contains, sometimes decomposes very rap- idly when exposed to the weather. The durability of feldspalhic rocks, however, is very variable, even where their composition is the same ; no pains should therefore be spared to ascertain this property in stone taken from new quarries, before using it for important public works. 8. Porphyry. This stone is usually composed of compact feld- spar, having crystals of the same, and sometimes those of othei minerals, scattered through the mass. Porphyry furnishes stones of various colors and texture ; the usual color being reddish, ap preaching to purple, from which the stone takes its name. One of the most beautiful varieties is a brecciated porphyry, consist ing of angular fragments of the stone united by a cement of com pact feldspar. Porphyry, from its rareness and extreme hardness is seldom applied to any other than ornamental purposes. Th( best known localities of sienite and porphyry are in the neighbor hood of Boston. 9. Green-stone. This stone is a mixture of hornblende wiu STONE. 3 common and compact feldspar, presenting sometimes a granular though usually a compact texture. Its ordinary color, \vhen dry- is some shade of brown ; but, when wet, it becomes green 'sn, from which it takes its name. Green-stone is very hard, and one of the most dui able rocks ; but, occurring in small and iiregular blocks, its uses as a building stone are very restricted. When walls of this stone are built with very white mortar, they present a picturesque appearance, and it is on that account well adapted to rural architecture. Green-stone might also be used as a material for road-making ; large quantities of it are annually taken from the principal locality of this rock in the United States, so well known as the Palisades, on the Hudson, for construct- ing wharves, as it is found to withstand .well the action of .salt water. 1 0. Granite and Gneiss. The constituents of these two stones are the same ; being a granular aggregation of quartz, feldspar, and mica, in variable proportions. They differ only in their structure ; gneiss being a stratified rock, the ingredients of which occur frequently in a more or less laminated state. Gneiss, although less valuable than granite, owing to the effect of ts structure on the size of the blocks which it yields, and from its not splitting as smoothly as granite across its beds of stratifica- tion, furnishes a building stone suitable for most architectural purposes. It is also a good flagging material, when it can be ob- tained in thin slabs. Granite varies greatly in quality, according to its texture and the relative proportions of its constituents. When the quartz is in excess, it renders the stone hard and brittle, and very difficult to be worked with the chisel. An excess of mica usually make? the stone friable. An excess of feldspar gives the stone a white hue, and makes it freer under the chisel. The best granites are those with a fine grain, in which the constituents seem uniformly disseminated through the mass. The color of granite is usually some shade of gray ; when it varies from this, it is owing to the color of the feldspar. One of its varieties, known as Oriental granite, has a fine reddish hue, and is chiefly used for ornamental purposes. Granite is sometimes mistaken for sienite, when il contains but little mica. The quality of granite is affected by the foreign minerals which it may contain ; hornblende is said to render it tough, and schor) makes it quite brittle. The protoxide and sulphurets of iron are the most injurious in their effects on granite ; the former by con version into a peroxide, and the latter by decomposing, destroying the structure of the stone, and causing it to break up and disin- tegrate. Granite, gneiss, and sienite, differ so little in their essential 4 BUILDING MATERIALS. qualities, as a building material, that they may lie used indiffer ently for all structures of a solid and durable character. They are extensively quarried in most of the New England States, in New York, and in some of the other States intersected by the great range of primitive rocks, where the quarries "Je contiguous to tide-water. 1 1 . Mica Slate. The constituents of this stone are quartz and mica ; the latter predominating. It is principally used as a flag- ging stone, and as afire stone, or lining for furnaces. 1 2. Buhr, or Mill-stone. This is a very hard, durable stone, presenting a peculiar, honeycomb appearance. It makes a good building material for common purposes, and is also suitable foi roaJ coverings. 13. Horn-stone. This is a highly silicious and very hard stone. It resembles flint in its structure, and takes its name from its translucent, horn-like appearance. It furnishes a very good road material. 14. Steatite, or Soap-stone. This stone is a partially indura- ted talc. It is a very soft stone, and not suitable for ordinary building purposes. It furnishes a good fire-stone, and is used for the lining of fireplaces. 1 5. Talcose Slate. This stone resembles mica slate, being an aggregation of quartz and talc. It is applied to the same pur- poses as mica slate. 16. Sand-stone. This stone consists of grains of silicious sand, arising from the disintegration of silicious rocks, which are united by some natural cement, generally of an argillaceous or a silicious character. The strength, hardness, and durability of sand-stone vary be- tween very wide limits. Some varieties being little inferior to good granite, as a building stone, others being very soft, friable, and disintegrating rapidly when exposed to the weather. The least durable sand-stones are those which contain the most argil- laceous matter ; those of a feldspathic character are also found not to withstand well the action of weather. Sand-stone is used very extensively as a building stone, for flagging, for road materials, and some of its varieties furnish an 3xcellerit fire-stone. Most of the varieties of sand-stone yield readily under the chisel and saw, and split evenly, and, from these properties, have received from workmen the name of free stone. The colors of sand-stone present also a variety of shades, principally of gray, brown, and red. The formations of sand-stone in the United States are very extensive, and a number of quarries are worked in New England., New York, and the Middle States. These formations, and the character of the stone obtained from them, are minutely desr-ribed STONE. in the Geological Reports of these States, which have been pub- Jished within the last few years. Most of the stone used for the public buildings in Washington, is a sand-stone obtained from quarries on Acquia Creek and the Rappahannock. Much of this stone is feldspathic, possesses but litlle strength, and disintegrates rapidly. The red sand-stones which are used in our large cities, are either from quarries in a formation extending from the Hadson to North Carolina, or from a separate deposite in the valley of the Connecticut. The most durable and hard portions of these formations occur in the neigh borhood of trap dikes. The fine flagging-stone used in our cities is mostly obtained, either from the Connecticut quarries, or from others near the Hudson, in the Catskill group of mountains. Many quarries, which yield an excellent building stone, are worked in the extensive formations along the Appalachian range, which extends through the interior, through New York and Vir- ginia, and the intermediate States. 17. ARGILLACEOUS STONES. The stones arranged under this head are mostly composed of clay, in a more or less indurated state, and presenting a laminated structure. They vary greatly in strength, and are generally not durable, decomposing in some cases very rapidly, from changes in the metallic sulphurets and salts found in most of them. The uses of this class of stones are restricted to roofing and flagging. 18. Roofing Slate. This well-known stone is obtained from a hard, indurated clay, the surfaces of the lamina having a natu- ral polish. The best kinds split into thin, uniform, light slabs ; are free from sulphurets of iron ; give a clear ringing sound when struck ; and absorb but little water. Much of the roofing slate quarried in the United States is of a very inferior quality, and becomes rotten, or decomposes, after a few years' exposure. The durability of the best European slate is about one hundred years ; and it is stated that the material obtained from some of the quar- ries worked in the United States, is not apparently inferior to the best foreign slate brought into our markets. Several quarries of roofing slate are worked in the New England States, New York, and Pennsylvania. 19. Graywacke Slate. The composition of this stone is mostly indurated clay. It has a more earthy appearance than argillaceous slate, and is generally distinctly arenaceous. Its colors are usually dark gray, or red. It is quarried principally for flagging-stone. 20. Hornblende Slate This stone, known also as green-stone slate, properly belongs to the silicious class. It consists mostly of hornblende having a laminated structure. It s chiefly quarried for flagging-stone. 6 BUILDING MATERIALS. 21. CALCAREOUS STONES. Lime is the p:ncipal constituent of this class, the carbonates of which, known as lime-stone and marble, furnish a large amount of ordinary b lilding stone, most of the ornamental stones, and the chief ingredient in the compo- sition of the cements and mortars, used in stone and brick-work. Lime-stone eifervesces copiously with acids ; its texture is de- stroyed by a strong heat, which also drives off its carbonic acid and water, converting it into quick lime. By absorbing water, quick-lime is converted into a hydrate, or slaked lime ; consider- able heat is evolved during this chemical change, and the stone increases in bulk, and gradually crumbles down into a fine powder. The lime-stones present great diversity in their physical prop- erties. Some of them seem as durable as the best silicious stones, and are but little inferior to them in strength and hardness ; others decompose rapidly on exposure to the weather ; and some kinds are so soft that, when first quarried, they can be scratched with the nail, and broken between the fingers. The lime-stones are generally impure carbonates ; and we are indebted to these im- purities for some of the most beautiful, as well as the most inval- uable materials used for constructions. Those which are colored by metallic oxides, or by the presence of other minerals, furnish the large number of colored and variegated marbles ; while those which contain a certain proportion of clay, or of magnesia, yield, on calcination, those cements which, from their possessing the property of hardening under water, have received the various appellations of hydraulic lime, water lime, Roman cement, &c. Lime-stone is divided into two principal classes, granular lime-stone and compact lime-stone. Each of these furnishes both the marbles and ordinary building stone. The varieties not sus- ceptible of receiving a polish, are sometimes called common lime- stone. The granular lime-stones are generally superior to the compact for building purposes. Those which have the finest grain are the best, both for marbles and ordinary building stone. The coarse- grained varieties are frequently friable, and disintegrate rapidly when exposed to the weather. All the varieties, both of the com- pact and granular, work freely under the chisel and grit-saw, and may be obtained in blocks of any suitable dimensions for the heaviest structures. The durability of lime-stone is very materially affected by i.he foreign minerals it may contain ; the presence of clay injures the stone, particularly when, as sometimes happens, it runs through the bed in very minute veins : blocks of stone having this imper- fection, soon separate along these veins on exposure to moisture. The protoxide, the protocarbonate, and the sulphuret of iron, are STONE. also very destructive in their effects ; frequently causing, by then chemical changes, rapid disintegration. Among the varieties of impure carbonates of lime, the magne- sian lime-stones, called dolomites, merit to be particularly no- ticed. They are regarded in Europe as a superior building material ; those being considered the best which are most crys- talline, and are composed of nearly equal proportions of the carbonates of lime and magnesia. Some of the quarries of this stone, which have been opened in New York and Massachusetts, have given a different result ; the stone obtained from them being, in some cases, extremely friable. 22. Marbles. The term marble is now applied exclusively to any lime-stone which will receive a polish. Owing to the cost of preparing marble, it is restricted in its uses to ornamental pur- poses. The marbles present great variety, both in color and ap- pearance, and have generally received some appropriate name descriptive of these accidents. 23. Statuary Marble is of the purest white, finest grain, and free from all foreign minerals. It receives that delicate polish, without glare, which admirably adapts it to the purposes of the sculptor, for whose uses it is mostly reserved. 24. Conglomerate Marble. This consists of two varieties ; the one termed pudding stone, which is composed of rounded pebbles imbedded in compact lime-stone ; the other termed breccia, con- sisting of angular fragments united in a similar manner. The colors of these marbles are generally variegated, forming a very handsome ornamental material. 25. Birds-eye Marble. The name of this stone is descriptive of its appearance, which arises from the cross sections of a pecu- liar fossil (fucoides demissus) contained in the mass, made in sawing or splitting it. 26. Lumachella Marble. This is obtained from a lime-stone having shells imbedded in it, and takes its name from this cir- cumstance. 27. Verd Antique. This is a rare and costly variety, of a beautiful green color, caused by veins and blotches of serpentine diffused through the lime-stone. 28. The terms veined, golden, Italian, Irish, &c., given to the marbles found in our markets, are significant of their appear- ance, or of the localities from which they are procured 2i>. Lime-stone is so extensively diffused throughout the Uni- t?d States, and is quarried, either for building stone or to furnish lime, in so many localities, that it would be impracticable to enu merate all within any moderate compass. One of the most re- markable formations of this stone extends, in an uninterrupted oed, from Canada, through the States of Vermont, Mass., Conn., 8 BUILDING MATERIALS. New York, New Jersey, Perm., and Virg., and, in all probability much farther south. Marbles are quarried in various localities in the United States Among the most noted are the quarries in Berkshire Co., Mass., which furnish both pure and variegated marbles ; those on the Potomac, from which the columns of conglomerate marbles were obtained that are seen in the interior of the Capitol at Washing- ton ; several in New York, which furnish white, the birds-eye, and other variegated kinds ; and some in Conn., which, among other varieties, furnish a verd antique of handsome quality. Lime-stone is burned, either for building or agricultural pur- poses, in almost every locality where deposites of the stone occur. Thomaston, in Maine, has supplied for some years most of the markets on the sea-board with a material which is considered as a superior article for ordinary building purposes. One of the greatest additions to the building resources of our country, was made in the discovery of the hydraulic or water lime-stones of New York. The preparation of this material, so indispensable for all hydraulic works and heavy structures of stone, is carried on extensively at Roundout, on the Delaware and Hudson canal, in Madison Co., and is sent to every part of the United States, being in great demand for all the public works carried on under the superintendence of our civil and military engineers. A not less valuable addition to our building materials has been made by Prof. W. B. Rogers, who, a 1 few years since, directed the atten- tion of engineers to the dolomites, for their good hydraulic prop- srties. From experiments made by Vicat, in France, who first there observed the same properties in the dolomite, and from those in our own country, it appears highly probable that the mag- nesian lime-stones, containing a certain proportion of magnesia, will be found fully equal to the argillaceous, from which hydraulic lime has hitherto been solely obtained. Both of these lime-stones belong to very extensive formations. The hydraulic lime-stones of New York occur in a deposite called the Water-lime Group, in the Geological Survey of New York corresponding to formation VI. of Prof. H. B. Rogers' arrange- ment of the rocks of Penn. This formation is co-extensive with the Helderberg Range as it crosses New York ; it is exposed in many of the valleys of Penn. and Virg., west of the Great Valley. It may be sought for just below or not far beneath the Oriskan} sand-stones of the New York Survey, which correspond to form- ation VII. of Rogers. This sand-stone is easily recognised, being of a yellowish white color, gianular texture, with large cavities left by decayed shells. The lime-stone is usually an earthy drab-colored rock, sometimes a greenish blue, which does no slake after being burned. STONE. The hydraulic magnesian lime-stones belong to the formations [I. and Vl. of Rogers ; the first of these is the same as the Black River, or Mohawk lime-stone of the New York Survey. It is the oldest fossiliferous lime-stone in the United States, and occurs throughout the whole bed, associated with the slates which occu- py formation III. of Rogers, and are called the Hudson River Group in the New York Survey. This extensive bed lies in the great Appalachian Valley, known as the Valley of Lake Cham- plain, Valley of the Hudson, as far as the Highlands, Cumberland Valley, Valley of Virginia, and Valley of East Tennessee. The same stone is found in the deposites of some of the western val- leys of the mountain region of Penn. and Virginia. The importance of hydraulic lime to the security of structures exposed to constant moisture, renders a knowledge of the geo- logical positions of those lime-stones from which it can be ob tained an object of great interest. From the results of the various geological surveys made in the United States, and in Europe, lime-stone, possessing hydraulic properties when calcined, may be looked for among those beds which are found in connection with the shales, or other argillaceous deposites. The celebrated Roman, or Parker's cement, of England, which, from its prompt induration in water, has become an important article of commerce, is manufactured from nodules of a concretionary argillaceous lime-stone, called septaria, from being traversed by veins of sparry carbonate of lime. Nodules of this character are found in Mass., and in some other States ; and it is probable they would yield, if suitably calcined and ground, an article in nowise inferior to that imported. 30. Gypsum, or Plaster of Paris. This stone is a sulphate of lime, and has received its name from the extensive use made of it at Paris, and in its neighborhood, where it is quarried and sent to all parts of the world ; being of a superior quality, owing, it is stated, to a certain portion of carbonate of lime which the stone contains. Gypsum is a very soft stone, and is not used as a building stone. Its chief utility is in furnishing a beautiful material for the orna- mental casts and mouldings in the interior of edifices. For this purpose it is prepared by calcining, or, as the workmen term it, boiling the stone, until it is deprived of its water of crystallization. In this state it is made into a thin paste, and poured into moulds to form the cast, in which it hardens very promptly. Ca Icined piaster of Paris is also used as a cement for stone ; but it is eminently unfit for this purpose ; for when exposed, in any situation, to moisture it absorbs it with avidity, swells, cracks, and exfoliates rapidly. Gypsum is found in various localities in the United States Large quantities of it are quarried in New York, both for build ing and agricultural purposes. 2 10 BUILDING MATERIALS. 31. DURABILITY OF STONE. The most important propertic* of stone, as a building material, are its durability ander the or dinary circumstances of exposure to weather ; its capacity to sustain high degrees of temperature ; and its resistance to the destructive action of fresh and salt water. The wear of stone from ordinary exposure is very variable, depending, not only upon the texture and constituent elements of the stone, but also upon the locality and the position it may oc- cupy in a structure, with respect to the prevailing driving rains. The chemist and geologist have not, thus far, laid down any in- fallible rules to guide the engineer in the selection of a material that may be pronounced durable for the ordinary period allotted to the works of man. In truth, the subject admits of only gen- eral indications ; for stones having the same texture and chemical composition, from causes not fully ascertained, are found to pos- sess very different degrees of duration. This has been particu- larly noted in feldspathic rocks. As a general rule, those stones which are fine-grained, absorb least water, and are of greatest specific gravity, are also most durable under ordinary exposures. The weight of a stone, however, may arise from a large propor- tion of iron in the state of a protoxide, a circumstance generally unfavorable to its durability. Besides, the various chemical com binations of iron, potash and clay, when found in considerable quantities, both in the primary and sedimentary silicious rocks, greatly affect their durability. The potash contained in ftld.-par dissolves, and carrying off a considerable proportion of the yiiica, leaves nothing but aluminous matter behind. The clay, on the other hand, absorbs w r ater, becomes soft, and causes the ^/.one to crumble to pieces. Iron in the form of protoxide, in sorr/i cases only, discolors the stone by its conversion into a peroxide. This discoloration, while it greatly diminishes the value of some Mones, as in white marble, in others is not disagreeable to the eye, pro ducing often a mottled appearance in buildings which p.cW.r. to the picturesque effect. 32. Frost, or rather the alternate actions of freezing &?)d thaw ing, is the most destructive agent of Nature with which the en gineer has to contend. Its effects vary with the texture of stones ; those of a fissile nature usually splitting, while the more porous kinds disintegrate, or exfoliate at the surface. When stone from a new quarry is to be tried, the best indication of its resistance tc frost may be obtained from an examination of any rocks of the same kind, within its vicinity, which are known to have been exposed for a long period. Submitting the stone fresh from the quarry to the direct action of freezing would seem to be the most certain test, were the stone destroyed by the expansive action jone of frost : but besides the uncertainty of this test, it is known STONE. 1 1 lhat some stones, which, when first quarried, are much affected by frost, splitting under its action, become impervious to it afte they have lost the moisture of the quarry, as they do not re-absori near so large an amount as they bring from the quarry. 33. M. Brard, a French chemist, has given a process for as- certaining the effects of frost on stone, which has met with the ap- proval of many French architects and engineers of standing, as ; corresponds with their experience. M. Brard directs that a small cubical block, about two inches on the edge, shaL \e carefully sawed from the stone to be tested, A cold saturate, .olution of sulphate of soda is prepared, placed over a fire, and brought to ihe boiling point. The stone, suspended from a string, is im- mersed in the boiling liquid, and kept there during thirty minutes ; it is then carefully withdrawn ; the liquid is decanted free from sediment into a flat vessel, and the stone is suspended over it in a cool cellar. An efflorescence of the salt soon makes its appear- ance on the stone, when it must be again dipped into the liquid. This should be done once or more frequently during the day, and the process be continued in this way for about a week. The earthy sediment, found at the end of this period in the vessel, is weighed, and its quantity will give an indication of the like effect of frost. This process, with the official statement of a commission of engineers and architects, by whom it was tested, is minutely detailed in vol. 38, Annales de Chimie et de Physique, and the results are such as to commend it to the attention of engineers in submitting new stones to trial. 34. By the absorption of water, stones become softer and more friable. The materials for road coverings should be selected from those stones which absorb least water, and are also hard and not brittle. Granite, and its varieties, lime-stone, and com- mon sand-stone, do not make good road materials of broken stone. All the hornblende rocks, porphyry, compact feldspar, and the quartzose rock associated with graywacke, furnish good, durable road coverings. The fine-grained granites which contain but a small proportion of mica, the fine-grained silicious sand-stones which are free from clay, and carbonate of lime, form a durable material when used in blocks for paving. Mica slate, talcose slate, hornblende slate, some varieties of gneiss, some varieties of sand-stone of a slaty structure, and graywacke slate, yield ex- cellent materials for flag-stone. 35. The influence of locality on *he durability of stone is very marked. Stone is observed to wear more rapidly in cities than m the country : and the stone in those parts of edifices exposed to the prevailing rains and winds, soonest exhibits signs of decay. The disintegration of the stratified stones placed in a wall, is mainly affected by the position which the strata or quarrv W BUILDING receives, with respect to the exposed surface ; proceeding fastct when the faces of the strata are exposed, than in the contrary position. 3f>. Stones which resist a high degree of heat without fusing are used for lining furnaces, and are termed fire-stones. A good fire -stone should not only be infusible, but also not liable to crack or exfoliate from heat. Stones that contain lime, or magnesia, except in the form of silicates, are usually unsuitable for fire- stones. Some porous silicious lime-stones, as well as some gyp- sous silicious rocks, resist moderate degrees of heat. Stones that contain much potash are very fusible under high tempera- tures, running into a glassy substance. Quartz and mica, in various combinations, furnish a good fire-stone ; as, for example, finely granular quartz with thin layers of mica, mica slate of the same structure, and some kinds of gneiss which contain a large proportion of arenaceous quartz. Several varieties of sand-stone make a good lining for furnaces. They are usually those varie ties which are free from feldspar, somewhat porous, and are un crystallized in the mass. Talcose slate likewise furnishes a good fire-stone. 37. Hardness is an essential quality in stone exposed to wear from the attrition of hard bodies. Stones selected for paving, flag- ging, and steps for stairs, should be hard, and of a grain suffi ciently coarse not to admit of becoming very smooth under the action to which they are submitted. As great hardness adds to the difficulty of working stone with the chisel, and to the cost of the prepared material, builders prefer the softer or free-stones^ such as the lime-stones and sand-stones, for most building pur- poses. The following are some of the results, on this point, ob tained from experiment. Table showing the result of experiments made under the direc tion of Mr. Walker, on the wear of different stones in the tram- way on the Commercial Road, London, from 27th March, 1830, to 24*A August, 1831, being a period of seventeen nonths. Transactions of Civil Engineers, vol. 1. Name of stone. Sup. area in feet. Original weight. Loss of weight by wear. Loss per Relative sup. foot. losses. cwt. qrs. ibs. Guernsey . 4.734 7 1 12.75 4.50 0.951 1.000 Herme .... 5.250 7 3 24.25 5.50 1.048 1.102 Budle .... 6.336 9 15.75 7.75 1.223 1.286 Peterhead (blue) . 3.484 4 1 7.50 6.25 1.795 1.887 Heytor . . . 4.313 6 15.25 8.25 1.915 2.014 Aberdeen (red) 5.375 7 2 11.50 11.50 2.139 2.249 Dartmoor . . . 4.500 6 2 25.00 12.50 2.778 2.921 Aberdeen (blue) . ! 4.823 6 2 1C 00 14.75 3.058 3.216 STONE. The v Aftivercial Road stoneway consists of twc parallel lir.es of rectangu'ar tramstones, 18 inches wide by 12 inches deep, and jointed to eath other endwise, for the wheels to travel on, with a common sir^l pavement between for the horses. The follow i\g table gives the results of some experiments on the wear of u line-grained sand-stone pavement, by M. Coriolis, during 8 ya-us, upon the paved road from Paris to Toulouse, the carriage ovir which is about 500 tons daily, published in the Annales des Fonts et Cfiausees, for March and April, 1834 Weight of a cubic foot Volume of water absorbed by the dry stone alter one day's im- mersion, compared to that of the stone. Mean annnal wear. 158 Ibs. 154 " Neglected as insensible. u 0.1023 inch. 0.1063 " 156 " (i 0.1299 " 150 " 148 " T * a in volume. 1 u TT 0.2126 ' 0.2677 " one M. Coriolis remarks, that the weight of water absorbed affords of the best indications of the durability of the fine-grained sand-stones used in France for pavements. An equally good test of the relative durability of stones of the same kind, M. Coriolis states, is the more or less clearness of sound given out by striking the stone with a hammer. The following results are taken from an article by Mr. James Frost, Civ. Engineer, inserted in the Journal of the Franklin Institute for Oct. 1835, on the resistance of various substances to abrasion. The substances were abraded against a piece of white statuary marble, which was taken as a standard, repre- sented by 100, by means of fine emery and sand. The relative resistance was calculated from the weight lost by each substance during the operation. Comparative Resistance to Abrasion. Aberdeen granite Hard Yorkshire paving stone Italian black marble Kilkenny black marble Statuary Marble Old Portland stone . Roman cement stone Fine-grained Newcastle grindstone Stock brick .... Coarse-grained Newcastle grindstone 980 327 260 110 100 79 69 53 34 14 Bath stone 19 14 BUILDING MATERIALS. LIME. * 38. Lime, considered as a building material, is now usually divided into three principal classes ; Common, or Air lime, Hy draulic lime, and Hydraulic, or Water cement. 39. Common, or air lime, is so called because the paste made from it with water will harden only in the air. 40. Hydraulic lime and hydraulic cement both take their name from hardening under water. The former differs from the latter in two essential points. It slakes thoroughly, like common lime, when deprived of its carbonic acid, and it does not harden promptly under water. Hydraulic cement, on the contrary, does not slake, and 'usually hardens very soon. 41. Our nomenclature, with regard to these substances, is still quite defective for scientific arrangement. For the lime-stones which yield hydraulic lime when completely calcined, also give an hydraulic cement when deprived of a portion only of their carbonic acid ; and other lime-stones yield, on calcination, a result which can neither be termed lime nor hydraulic cement, owing to its slaking very imperfectly, and not retaining the hardness which it quickly takes when first placed under water. M. Vicat, whose able researches into the properties of lime and mortars are so well known, has proposed to apply the term cement lime-stones (calcaires a ciment) to those stones which, when com- pletely calcined, yield hydraulic cement, and which under no de- gree of calcination, will give hydraulic lime. For the lime-stones which yield hydraulic lime when completely calcined, and which, when subjected to a degree of heat insufficient to drive off all their carbonic acid, yield hydraulic cement, he proposes to retain the name hydraulic lime-stones ; and to call the cement obtained from their incomplete calcination, under-burnt hydraulic cement, (ciments d'incuits,} to distinguish it from that obtained from the cement stone. With respect to those lime-stones which, by cal- cination, give a result that partakes partly of the properties both of limes and cements, he proposes for them the name of dividing limes, (chaux limites.) The terms fat and meager are also applied to limes ; owing to the difference in the quality of the paste obtained from them with the same quantity of water. The fat limes give a paste which is unctuous both to the sight and touch. The meager limes yield a thin paste. These names were of some importance when first introduced, as they served to distinguish common from hydraulic lime, the former being always fat, the latter meager ; but, later experience having shown that all meager limes are not hydraulic, the terms are no longer of use, except to designate qualities of the paste of limes. LIME. 15 42. Hydraulic Limes and Cements. The lime-stontg which yield these substances are either argillaceous, or magnesian, or argilo-magnesian. The products of their calcination vary con- siderably in their hydraulic properties. Some of the hydraulic limes harden, or set very slowly under water, while others set rap- idly. The hydraulic cements set in a very short time. This diversity in the hydraulic energy of the argillaceous lime-stones arises from the variable proportions in which the lime and clay enter into their composition. 43. M. Petot, a civil engineer in the French service, in an able work entitled Recherches sur la Chauffournerie, gives the follow- ing table, exhibiting these combinations, and the results obtained from their calcination. Lime. Clay. Resulting products. Distinctive characters of the products. 100 Very fat lime. Incapable of hardening in water. 90 10 Lime a little hydraulic. C Slakes like pure lime, when 80 20 do. quite hydraulic. < properly calcined, and hard- 70 30 do. do. f ens under water. 60 40 Plastic, or hydraulic cement. Does not slake under any cir- 50 50 do. cumstances, and hardens un- 40 60 do. der water with rapidity. 30 70 Calcareous puzzolano (brick). Does not slake nor harden un- 20 80 do. do. der water, unless mixed with 10 90 do. do. a fat, or an hydraulic lime. 100 Puzzolano of pure clay do. Same as the preceding. . 44. The most celebrated European hydraulic cements are ob- tained from argillaceous lime-stones, which vary but slightly in their constituent elements and properties. The following table gives the results of analyses to determine the relative proportions of lime and clay in these cements. Table of Foreign Hydraulic Cements, showing the relative pro- portions of Clay and Lime contained in them. LOCALITY. Lime. Clay. English, (commonly known as Parker's, or Roman cement) 55.40 44.60 French, (made from Boulogne pebbles) 54.00 46.00 LV.. (Pouilly) 42.86 57.14 Do. do. 36.37 63.63 Do. (Baye) 21 62 78.38 Russian ......... 62.00 38.00 The hydraulic cements used in England are obtained frofl* 16 BUILDING MATERIALS. various localities; and differ but little in the relative proportion! of lime and clay found in them. Parker's cement, so called from the name of the person who first introduced it, is obtained by calcining nodules of septaria. The composition of these nodules is the same as that of the Boulogne pebbles found on the opposite coast of France. The stones which furnish the English and French hydraulic cements, contain but a very small amount of magnesia. 45. The best known hydraulic cements of the United States, are manufactured in the State of New York. The following analyses of some of the hydraulic lime-stones, from the most noted localities, published in the Geological Report of the State of New York, 1839, are given by Dr. Beck. Analysis of the Manlius Hydraulic Lime-stone. Carbonic acid Lime Magnesia . Silica and alumina Oxide of iron Moisture and loss 39.80 26.24 18.80 13.50 1.25 1.41 100.00 This stone belongs to the same bed which yields the hydraulic cement obtained near Kingston, in Upper Canada. Analysis of the Chittenango Hydraulic Lime-stone, before and after calcination. Unborn t. Burnt. Carbonic acid 39.33 25 00 Carbonic acid and moisture 10.90 39.50 Magnesia . Silira 17.83 11 76 Magnesia .... 22.27 16.56 Alurr.ina 2.73 Alumina and oxide of iron 10.77 Peroxide of irot Moisture 1.50 100.00 100.00 UMh. 17 Analysis of the Hydraulic Lime-stc .e from Ulster Co., ujong the line of the Delaware and Hudson Canal, before and after burning. Carbonic acid Unborn t. H Burnt 34.20 5 Lime 25.50 37.60 Magnesia Silica 12.35 15.37 16. f>5 22.75 Alumina . 9.13 13.40 Oxide of iron 2.25 3.30 Bituminous matter, moisture, and loss 1.20 1.30 100.00 100.00 The hydraulic cement from this last locality has become gen- erally well known, having been successfully used for most of the military and civil public works on the sea-board. From the results of the analyses of all the above limestones, it appears that the proportions of lime and clay contained in them place them under the head of hydraulic cements, according to the classification of M. Petot. They do not slake, and they all set rapidly under water. 46. The discovery of the hydraulic properties of certain mag nesian lime-stones is of recent date, and is due to M. Vicat, who first drew attention to the subject. M. Vicat inclines to the opinion, that magnesia alone, without the presence of some clay, will yield only a feeble hydraulic lime. He states, that he has never been able to obtain any other, from proceeding synthetically with common lime and magnesia ; and that he knows of no well- authenticated instance in which any of the dolomites, either of the primitive or transition formations, have yielded a good hydrau lie lime. The stones from these formations, he states, are devoid of clay ; being very pure crystalline carbonates, or else contain silex only in the state of fine sand. From M. Vicat's experi- ments, it is rendered certain that carbonate of marnesia in combi- nation with carbonate of lime, in the proportion of 4 u parts of the latter to from 30 to 40 of the former, will produce a feebly hy draulic lime, which does not appear to increase in hardness after it has once set ; but that with the same proportions, some hur diedths of clay are requisite to give hydraulic energy to the com- pound. This proportion of clay M. Vicat supposes may cause the formation of triple hydro-silicates of lime, alumina, and mag nesia, having all the characteristic properties of good hydraulic lime. 47. The hydraulic properties of the magnesian lime-stones of 18 BUILDING MATERIALS. the United States were noticed by Professor W. B. Rogers, whi , in his Report of the Geological Survey of Virginia, 1 S38. has given the following analyses of some of the stones from different localities. - No. 1. No. 2. No. 3. No 4. Carbonate of lime . 55 60 53.23 48.20 55.03 Carbonate of magnesia . Alumina and oxide of iron 313.20 1.50 41.00 0.80 35.76 1.20 24.16 2.60 Silica and insoluble matter 2.50 2.80 12.10 15.30 Water .... 0.40 0.40 2.73 1.20 Loss .... 0.60 1.77 0.01 1.71 100.00 100.00 100.00 100.00 The lime-stone No. 1 of the above table is from Sheppardstown on the Potomac, in Virginia ; it is extensively manufactured for hydraulic cement. No. 2 is from the Natural Bridge, and banks of Cedar Creek, Virginia ; it makes a good hydraulic cement. No. 3 is from New York, and is extensively burnt for cement. No. 4 is from Louisville, Kentucky; said to make a good cement. 48. M. Vicat states, that a magnesian lime-stone of France containing the following constituents, lime 40 parts, magnesia 21, and silica 21, yields a good hydraulic cement ; and he gives the following analysis of a stone which gives a good hydraulic lime. Carbonate of lime Carbonate 'of magnesia Silica Alumina Oxide of iron 50.60 42.00 5.00 2.00 0.40 100.00 By comparing the constituents of these two last stones with the analyses of the cement-stones of New York, and the magnesian hydraulic lime-stones of Prof. Rogers, it will be seen that they consist, respectively, of nearly the same combinations of lime, magnesia, and silica. 49. Physical Characters and Tests of Hydraulic Lime-stones. The simple external characters of a lime-stone, as color, texture, fracture, and taste, are insufficient to enable a person to decide whether it belongs to the hydraulic class ; although they assist conjecture, particularly if the rock, from which the specimen is taken, is found in connection with the clay deposites, or if it be- long to a stratum wb >se general level and characteristics are the LIME. 19 same as the argilo-magnesian rocks. These rocks are generally some shade of drab, or of gray, or of a dark grayish blue ; have a compact texture ; fracture even or conchoidal ; with a clayey or earthy smell and taste. Although the hydraulic lime-stones are usually colored, still it may happen that tl> j stone may be of a pure white, arising from the combination of lime with a pure clay. The difficulty of pronouncing upon the class to which a lime- stone belongs, from its physical properties alone, renders it neces- sary to resort to a chemical analysis, and even to direct experiment, to decide the question. 50. In making a complete chemical analysis of alime-stone,more skill in chemical manipulations is requisite than engineers usually possess ; but a person who has the ordinary elementary know- ledge of chemistry, can readily ascertain the quantity of clay or of magnesia contained in a lime-stone, and from these two ele- ments can pronounce, with tolerable certainty, upon its hydraulic properties. To arrive at this conclusion, a small portion of the stone to be tested about five drachms is taken and reduced to a powder ; this is placed in a capsule, or an ordinary watch crystal, and slightly diluted muriatic acid is poured over it until it ceases to effervesce. The capsule is then gently heated, and the liquor evaporated, until the residue in the capsule has acquired the consistence of thin paste. This paste is thrown into a pint of pure water and well shaken up, and the mixture is then fil- tered. The residue left on the filtering paper is thoroughly dried, by bringing it to a red heat ; this being weighed will give the clay, or insoluble matter, contained in the stone. It is important to ascertain the state of mechanical division of the insoluble mat ter thus obtained ; for if it be wholly granular, the stone will not yield hydraulic lime. The granular portion must therefore be carefully separated from the other before the latter is dried and weighed. 51. If the sample tested contains magnesia, an indication of this will be given by the slowness with which the acid acts ; if the quantity of magnesia be but little, the solution will at first proceed rapidly and then become more sluggish. To ascertain the quantity of magnesia, clear lime-water must be added to the filtered solution as long as any precipitate is formed, and this precipitate must be quickly gathered on filtering paper, and then be washed with pure water. The residue from this washing is the magnesia. It must be thoroughly dried before being weighed, to ascertain its proportion to the clay. 52. Having ascertained, by the preceding analysis, the proba- ble hydraulic energy of the stone, a sample of it should also be submitted to di'rect experiment. This may be likewise done or, t small scale. A sample of the stone n- ist be reduced to fras; BUILDING MATERIALS ments about the size of a walnut. A crucible, perforated with holes for the free admission of air, is filled with these fragments, and placed over a fire sufficiently powerful to drive off the car- bonic acid of the stone. The time for effecting this will depend on the intensity of the heat. When the heat has been applied for three or four hours, a small portion of the calcined stone may be tried with an acid, and the degree of the calcination may be judged of by the more or less copiousness of the effervescence that ensues. If no effervescence takes place, the operation may be considered completed. The calcined stone should be tiied soon after it has become cold ; otherwise, it should be kept in a glass jar made as air-tight as practicable until used. 53. When the calcined stone is to be tried, it is first slaked by placing it in a small basket, which is immersed for five or six seconds in pure water. The stone is emptied from the basket so soon as the water has drained off, and is allowed to stand until the slaking is terminated. This process will proceed more or less rapidly, according to the quality of the stone, and the degree of its calcination. In some cases, it will be completed in a few minutes ; in others, portions only of the stone will fall to powder, the rest crumbling into lumps which slake very sluggishly ; while other varieties, as the true cement stones, give no evidence of slak- ing. If the stone slakes either completely or partially, it must be converted into a paste of the consistence of soft putty, being ground up thoroughly, if necessary, in an iron mortar. The paste is made into a cake, and placed on the bottom of an ordinary tum- bler, care being taken to make the diameter of the cake the same as that of the tumbler, which is filled with water, and the time of Immersion noted. ! the lime is only moderately hydraulic, it will have become hard enough at the end of fifteen or twenty days, to resist the pressure of the finger, and will continue to harden slowly, more particularly from the sixth or eighth month after immersion ; and at the end of a year it will have acquired the consistency of hard soap, and wit! dissolve slowly in pure water. A fair hydraulic lime will have hardened so as to resist the pressure of the finger, in about six or eight days after immer- sion, and will continue to grow harder until from six to twelve months after immersion ; it will then have acquired the hardness of the softest calcareous stones, and will be no longer soluble in pure water. When the stone is eminently hydraulic, it will have become hard in from two to four days after immersion, and in one month it will be quite hard and insoluble in pure water ; after six months, its hardness will be abou* equal to the more absorbent calcareous stones ; will splinter from a blow, presenting a slaty fracture. As the hydraulic cements do not slake perceptibly, the burnt LIME. 21 etone must first be reduced to a fine powder before it is made mto a paste. The paste, when kneaded between the fingers, be- comes warm, and will generally set in a few minutes, either in the open air or in water. Hydraulic cement is far more sparingly soluble in pure water than the hydraulic lime ; and the action of pure water upon them ceases, apparently, after a few weeks im- mersion in it. 54. Calcination of Lime-stone. The effect of heat on lime- stones varies with the constituent elements of the stone. The pure lime-stones will stand a high degree of temperature with- out fusing, losing only their carbonic acid and water. The im- pure stones containing silica fuse completely under a great heat, and become more or less vitrified when the temperature much ex- ceeds a red heat. The action of heat on the impure lime-stones, besides driving off their carbonic acid and water, modifies the re- lations of their other chemical constituents. The argillaceous stones, for example, yield an insoluble precipitate when acted on by an acid before calcination, but are perfectly soluble afterwards, unless the silex they contain happens to be in the form of grains. 55. The calcination of the hydraulic lime-stones, from their fusible nature, requires to be conducted with great care ; for, if not pushed far enough, the under-burnt portions will not Slake ; and, if carried too far, the stone becomes dead or sluggish ; slakes very slowly and imperfectly at first ; and, if used in this state for masonry, may do injury by the swelling which accompanies the after-slaking. 56. The more or less facility with which the impure lime-stones can be burned, depends upon several causes ; as the compactness of the stone ; the size of the fragments submitted to heat ; and the presence of a current of air, or of aqueous vapor. The more compact stones yield their carbonic acid less readily than those of an opposite texture. Stones which, when broken into very small lumps, can be calcined under the red heat of an ordinary fire m a few hours, will require a far greater degree of tempera- ture, and for a much longer period, when broken into fragments of six or eight inches in diameter. This is particularly the case with the impure lime-stones, which, when in large lumps, vitrify at the surface before the interior is thoroughly burnt. 57. If a current of vapor is passed over the stone aftei it has commenced to give off its carbonic acid, the remaining portion of the gas which, under ordinary circumstances, is expelled with grnat difficulty, particularly near the end of the process of calci- nation, will be carried off much sooner. This influence of an aqueous current is attributed, by M. Gay-Lussac, purely to a mechanical action, by removing the gas as it is evolved, and his experim' us go to show that a like effect is produced by an at 22 BUILDING MATERIALS. mospheric current. In burning the impure lime-stones, however an aqueous current produces the farther beneficial effect of pre venting the vitrification of the stone, when the temperature haa become too elevated ; but as the vapor, on coming in contact with the heated stone, carries off a large portion of the heat, this, together with the latent heat contained in it, may render its use in some cases, far from economical. 58. Wood, charcoal, peat, the bituminous and anthracite coals are used for fuel in lime-burning. M. Vicat states, that wood is the best fuel for burning hydraulic lime-stones ; that charcoal is inferior to bituminous coal ; and that the results from this last are very uncertain. When wood is used, it should be dry and split up, to burn quickly and give a clear blaze. The common opinion among lime-burners, that, the greener the fuel the better, and that the lime-stone should be watered before it is placed in the kiln, is wrong ; as a large portion of the heat is consumed in converting the water in both cases into vapor. Coal is a more economical fuel than wood, and is therefore generally preferred to it ; but it requires particular care in ascertaining the proper quantity for use. 59. Lime-kilns. Great diversity is met with in the forms and proportions of lime-kilns. Wherever attention has been paid to economy in fuel, the cylindrical, ovoidal, or the inverted conical form has been adopted. The two first being preferred for wood, and the last for coal. 60. The whole of the burnt lime is either drawn from the kiln at once, or else the burning is so regulated, that fresh stone and fuel are added as the calcined portions are withdrawn. The lat- ter method is usually followed when the fuel used is coal. The stone and coal, broken into proper sizes, (Fig. 1,) and in propor- Fig. 1 represents a vertical section through the axis and centre lines of the entrances communicating with the interior of a kiln tor burning lime with coal. A, solid masonry of the kiln, which is built up on the exterior like a square tower, with two arched entrances at 13, B on opposite sides. C, interior of the kiln, lined with fire-brick or stone. D, ash-pit. c, c, openings between B, B and the interior tlirough which the burnt lime is drawn. tions determined by experiment, are placed in the kiln in alternate layers ; the coal is ignited at the bottom of the kiln, and fresh strata are added at the top, as the burnt mass settles down and is partially withdrawn at the bottom. Kilns used in this way are called perpetual kilns ; they are more economical in tne con sumption of fuel than those in which the burning is intermitted anJ which are, on this account, termed intermittent \tl*:*. \Vor LIME. 23 may also be used as fuel in perpetual kiln', but not with such economy as coal ; it moreover presents many inconveniences, in supplying the kiln with fresh stone, and in regulating its dis- charge. The inverted conical-shaped kiln is geneially adopted for coal, and the ovoidal-shaped for wood. 61 . Some care is requisite in filling the kiln with stone when a wood fire is used. A dome (Fig. 2) is formed of the largest blocks Fig. 2 represents a vertical section through the axis and centre line of the entrance of a lime- kiln for wood. A, solid masonry of the kiln. B T arched entrance. C, doorway for drawing kiln and supplying fuel. D, interior of kiln. E, dome of broken stone, shown by the dotted line. of the broken stone, which either rests on the bottom of the kiln or on the ash-grate. The lower diameter of the dome is a few feet less than that of the kiln ; and its interior is made sufficiently capa- cious to receive the fuel which, cut into short lengths, is placed up endwise around the dome. The stone is placed over and around the courses which form the dome, the largest blocks in the centre of the kiln. The management of the fire is a matter of experiment. For the first eight or ten hours it should be care- fully regulated, in order to bring the stone gradually to a red heat. By applying a high heat at first, or by any sudden increase of it until the mass has readied a nearly uniform temperature, the stone is apt to shiver, and choke the kiln, by stopping the voids between the courses of stone which form the dome. After the stone is brought to a red heat, the supply of fuel should be uni- form until the end of the calcination. The practice sometimes adopted, of abating the fire towards the end, is bad, as the last portions of carbonic acid retained by the stone, require a high de- gree of heat for their expulsion. The indications of complete calcination are generally manifested by the diminution which gradually takes place in the mass, and which, at this stage, is about one sixth of the primitive volume ; by the broken appear- ance of the stone which forms the dome, the interstices between which being also choked up by fragments of the burnt stone ; and by the ease with which an iron bar may be forced down through the burnt stone in the kiln. When these indications of complete calcination are observed, the kiln should be closed for ten or twelve hours, to confine the heat and finish the burning of the up- per strata. 62. The form and relative dimensions of a kiln for wood can 4 BUILDING MATERIALS. oe determined only by careful experiment. If too great height be given to the mass, the lower portions may be overburncd be- fore the upper are burned enough. The proportions between the height and mean horizontal section, will depend on the texture of the stone ; the srze of the fragments into which it is broken for burning ; and the more or less facility with which it vitrifies. In the memoir of M. Petot, already cited, it is stated as the results of experiments made at Brest, tint large-sized kilns are more economical, both in the consumption of fuel and in the cost of attendance, than small ones ; but that there is no notable econo- my in fuel when the mean horizontal section of the kiln exceeds sixty square feet. 63. The circular seems the most suitable form for the horizon- tal sections of a kiln, both for strength and for economizing the heat. Were the section the same throughout, or the form of the interior of the kiln cylindrical, the strata of stone, above a certain point, would be very imperfectly burned when the lower were enough so, owing to the rapidity with which the inflamed gases, arising from the combustion, are cooled by coming into contact with the stone. To procure, therefore, a temperature throughout the heated mass which shall be nearly uniform, the horizontal sec- tions of the kiln should gradually decrease from the point where the flame rises, which is near the top of the dome of broken stone, to the top of the kiln. This contraction of the horizontal section, from the bottom upward, should not be made too rapidly, as the draft would be injured, and the capacity of the kiln too much diminished ; and in no case should the area of the top opening be less than about one fourth the area of the section taken near the top of the dome. The best manner of arranging the sides of the kiln, in the plane of the longitudinal section, is to connect the top opening with the horizontal section through the top of the dome, by an arc of a circle whose tangent at the lower point shall be vertical. 64. Lime-kilns are constructed either of brick, or of some of the more refractory stones. The walls of the kiln should be suf- ficiently thick to confine the heat, and, when the locality admits of it, they are built into a side hill ; otherwise, it may be neces- sary to use iron hoops, and vertical bars of iron, to strengthen the brick-work. The interior of the kiln should be faced either with good fire-brick or with fire-stone. 65. M. Petot prefers kilns arranged with fire-grates, and an ash-pit under the dome of broken stone, for the reason that they give the means of better regulating the heat, and of throwing the flame more in the axis of the kiln than can be done in kilns with out them. The action of the flame is thus more uniformly felt through the mass o r st< ne above the top of the dome, while that LIME. 25 of the radiated heat upon the stone around the dome, is also more uniform. 66. M. Petot states, that the height of the mass of stone above the top of the dome should not be greater than from ten to thir- teen feet, depending on the more or less compact texture of the stone, and the more or less ease with which it vitrifies. He pro- poses to use kilns with two stories, (Fig. 3,) for the purpose Fig. 3 represents a vertical section through the axis and centre line of the entrance of a lime-kiln with two stories for wood. A, solid masonry of the kiln. B, dome shown by the dotted line. C, interior of lower story. D, dome of upper story. E, interior of upper story. , arched entrance to kiln. b, receptacle for water to furnish a current of aqueous vapor. c, doorway for drawing kiln, &c., closed by a fire-proof door. tl, ash-pit under fire-grate. e, upper doorway for drawing kiln, &c of economizing the fuel, by using the heat which passes off from the top of the lower story, and would otherwise be lost, to heat the stone in the upper story ; this story being arranged with a side-door, to introduce fuel under its dome of broken stone, and complete the calcination when that of the stone in the lower story is finished. M. Petot gives the following general directions for regulating the relative dimensions of the parts of the kiln. The greatest horizontal section of the kiln is placed rather below the top of the dome of broken stone ; the diameter of this section being 1.82, the diameter of the grate. The height of the dome above the grate is from 3 to 6 feet, according to the quantity of fuel to be con- sumed hourly. The bottom of the kiln, on which the piers of the dome rest, is from 4 to 6 inches above the top of the grate ; the diameter of the kiln at this point being about 2 feet 9 inches greater than that of the grate. The diameter of the horizontal section at top is 0.63, the diameter of the greatest horizontal sec- tion. The horizontal sections of the kiln diminish from the section near the top of the dome to the top and bottom of the kiln ; the sides of the kiln receiving the form shown in Fig. 3 : the object of contracting the kiln towards the bottom being to allow the stone near the bottom of the kiln to be thoroughly burned by the radiated heat The grate is formed of cast-iron bars of the usual form 26 BUILDING rfATBRIALS. tlie area of the spaces between the bars being one fourth the total area of the grate. The bottom of the ash-pit, which may be on the same level as the exterior ground, is placed 18 inches below the grate ; and at the entrance to the ash-pit is placed a reservoir for water, about 1 8 inches in depth, to furnish an aqueous cur- rent. The draft through the grate is regulated by a lateral air- channel to the ash-pit, which can be totally or partially shut by a valve ; the area of the cross section of this channel is one tenth the total area of the grate. A square opening 16 inches wide, the bottom of which is on a level with the bottom of the kiln, leads to the dome for the supply of the fuel. This opening is closed with a fire-proof and air-tight door. In arranging a kiln with two stories, M. Petot states, that the grates of the upper story are so soon destroyed by the heat, that it is better to suppress them, and to place the fuel for completing the calcination of the stone of this story, on the top of the burnt stone of the lower story. 67. Slaking Lime. Quick-lime may be slaked in three dif- ferent ways. By pouring sufficient water on the burnt stone to convert the slaked lime into a thin paste, which is termed drown- ing the lime. By placing the burnt stone in a basket, and im ' mersing it for a few seconds in water, during which time it will imbibe enough water to cause it to fall, by slaking, into a dry powder ; or by sprinkling the burnt stone with a sufficient quan tity of water to produce the same effect. By allowing the stone to slake spontaneously, from the moisture it imbibes from the atmosphere, which is termed air-slaking. 68. Opinion seems to be settled among engineers, that drown- ing is the worst method of slaking lime which is to be used for mortars. When properly done, however, it produces a finer paste than either of the other methods ; and it may therefore be resorted to whenever a paste of this character, or a whitewash is wanted. Some care, however, is requisite to produce this result. The stone should be fresh from the kiln, otherwise it is apt to slake into lumps or fine grit. All the water used should be poured over the stone at once, which should be arranged in a basin or vessel, so that the water surrounding it may be gradually imbibed as the slaking proceeds. If fresh water be added during the slak- ing, it checks the process, and causes a gritty paste to form. 69. In slaking by immersion, or by sprinkling with water, the stone should be reduced to small-sized fragments, otherwise the slaking will not proceed uniformly. The fat limes should be in lumps, about the size of a walnut, for immersion ; and, when withdrawn from the water, should be placed immediately in bins or be covered with sand, to confine the heat and vapor. If left exposed to the air, the lime becomes chilled and separates into a LIME. 87 coarse grit, which takes some time to slake thoroughly when more water is added. Sprinkling the lime is a more convenient process than immersion, and is equally good. To effect the slak ing in this way, the stone should be broken into fragments of a suitable size, which experiment will determine, and be placed in small heaps, surrounded by sufficient sand to cover them up when the slaking is nearly completed. The stone is then sprinkled with about one fourth its bulk of water, poured through the rose of a watering-pot, those lumps which seem to slake most slug- gishly receiving the most water ; when the process seems com- pleted, the heap is carefully covered over with the sand, and allowed to remain a day or two before it is used. 70. Slaking either by immersion or by sprinkling is considered the best. The quantity of water imbibed by lime when slaked by immersion, varies with the nature of the lime ; 100 parts of fat lime will take up only 1 8 parts of water ; and the same quan- tity of meager lime will imbibe from 20 to 35 parts. One volume, in powder, of the burnt stone of rich lime yields from 1 .50 to 1 .70 in volume of powder of slaked lime ; while one volume of meager lime, under like circumstances, will yield from 1 .80 to 2.18 in volume of slaked lime. 71. Quick lime, when exposed to the free action of the air in a dry locality, slakes slowly, by imbibing moisture from the at- mosphere, with a slight disengagement of heat. Opinion seems to be divided with regard to the effect of this method of slaking on fat limes. Some assert, that the mortar made from them is better than that obtained from any other process, and attribute this result to the re-conversion of a portion of the slaked lime into a carbonate ; others state the reverse to obtain, and assign the same cause for it. With regard to hydraulic limes, all agree thai they are greatly injured by air-slaking. 72. Air-slaked fat limes increase two fifths in weight, and for one volume of quick lime yield 3.52 volumes of slaked lime. The meager limes increase one eighth in weight, and for one volume of quick lime yield from 1.75 to 2.25 volumes of slaked lime 7:3. The dry hydrates of lime, when exposed to the atmosphere, gradually absorb carbonic acid and water. This process pro- ceeds very slowly, and the slaked lime never regains all the car- bonic acid which is driven off by the calcination of the lime-stone. When converted into a thick paste, and exposed to the air, the hydrates gradually absorb carbonic acid ; this action first takes place on the surface, and proceeds more slowly from year to year towards the interior of the exposed mass. The absorption of gas proceeds more rapidly in the meager than in the fat limes. Those hydrates which are most thoroughly slaked become hard est. The hydrates of the piue fat limes become in time very 28 BUILDING MATERIALS. hard, while those of the hydraulic limes become only moderately hard. 74. The fat limes, when slaked by drowning, may be pre- served for a long >eriod in the state of paste, if placed in a damp situation and kepi from contact with the air. They may also be preserved for a long time without change, when slaked by im- mersion to a dry powder, if placed in covered vessels. Hydraulic limes, under similar circumstances, will harden if kept in the state of paste, and will deteriorate when in powder, unless kept in perfectly air-tight vessels. 75. The hydrates of fat lime, from air-slaking or immersion, require a smaller quantity of water to reduce them to the state of paste than the others ; but, when immersed in water, they grad- ually imbibe their full dose of water, the paste becoming thicker, but remaining unchanged in volume. -Exposed in this way, the water will in time dissolve out all the lime of the hydrate which has not been re-converted into a sub-carbonate, by the absorption of carbonic acid before immersion ; and if the water contain car- bonic acid, it will also dissolve the carbonated portions. 76. The hydrates of hydraulic lime, when immersed in water in the state of thin pastes, reject a portion of the water from the paste, and become hard in time ; if the paste be very stiff, they imbibe more water, set quickly, and acquire greater hardness in time than the soft pastes. The pastes of the hydrates of hydrau- lic lime, which have hardened in the air, will retain their hardness when placed in water. 77. The pastes of the fat limes shrink very unequally in drying, and the shrinkage increases with the purity of the lime ; on this account it is difficult to apply them alone to any building purposes, except in very thin layers. The pastes of the hydraulic limes can only be used with advantage under water, or where they are constantly exposed to humidity ; and in these situations they are never used alone, as they are found to succeed as well, and to present more economy, when mixed with a portion of sand 78. Manner of Reducing Hydraulic Cement. As the cement Stones will not slake, they must be reduced to a fine powder by some mechanical process, before they can be converted into a hydrate. The methods usually employed for this purpose con- sist in first breaking the burnt stone into small fragments, either under iron cylinders, or in mills suitably formed for this pur- pose, which are next ground between a pair of stones, or eke crushed by an iron roller. The coarser particles are separated from the fine powder by the ordinary processes with sieves. The powder is then carefully uacked in air-tight casks, and kept for use 79. Hydraulic cement, like hydraulic lime, deteriorates by exT)ostirr to the air, and may in time lose all its hydraulic prop. LIME. 29 erties. On this account it should be used when fresh iroiri the kiln ; for, however carefully packed, it cannot be well preserved when transported to any distance. 80. The deterioration of hydraulic cements, from exposure to the air, arises, probably, from a chemical disunion between the constituent elements of the burnt stone, occasioned by the ab- sorption of water and carbonic acid. When injured, their energy can be restored by submitting them to a much slighter degree of heat than that which is requisite to calcine the stone suitably in the first instance. From the experiments of M. Petot, it appears that a red heat, kept up for a short period, is sufficient to restore damaged hydraulic cements. 81. Artificial Hydraulic Limes and Cements. The discovery of the argillaceous character of the stones which yield hydraulic limes and cements, connected with the fact that brick reduced to a fine powder, as well as several substances of volcanic origin having nearly the same constituent elements as ordinary brick when mixed in suitable proportions with common lime, will yield a paste that hardens under water, has led, within a recent period, to artificial methods of producing compounds possessing the prop- erties of natural hydraulic lime-stones. 82. M. Vicat was the first to point out the method of forming an artificial hydraulic lime, by mixing common lime and unburnt clay, in suitable proportions, and then calcining them. The ex- periments of M. Vicat have been repeated by several eminent engineers with complete success, and among others by General Pasley, who, in a recent work by him, Observations on Limes, Calcareous Cements, &c., has given, with minute detail, the results of his experiments ; from which it appears that an hydraulic ce- ment, fully equal in quality to that obtained from natural stones can be made by mixing common lime, either in the state of a carbonate or of a hydrate, with clay, and subjecting the mixture to a suitable degree of heat. In some parts of France, where chalk is found abundantly, the preparation of artificial hydraulic lime has become a branch of manufacture. 83. Different methods have been pursued in preparing this material, the main object being to secure the finest mechanical division of the two ingredients, and their thorough mixture. For this purpose the lime-stone, if soft like chalk or tufa, may be re- duced in a wash-mill, or a rolling-mill, to the state of a soft pulp; it is then incorporated with the clay, by passing them through a pug-miil. The mixture is next moulded into small blocks, or made up into balls between 2 and 3 inches diameter, by hand, and \\ ell dried. The balls are placed in a kiln, suitably calcined, and are finally slaked, or ground down fine for use. 84. If the lime-stone be hard, it must be calcined and slaked 30 BUILDING MATERIALS. in the usual m :nner, before it can be mixed witn the clay. The process for m.xing the ingredients, their calcination, and farthei preparation for use, are the same as in the preceding case. 85. Artificial hydraulic lime, prepared from the hard lime* stones, is more expensive than that made from the soft ; but it is stated to be superior in quality to the latter. 86. As clays are seldom free from carbonate of lime, and as the lime-stones which yield common or fat lime may contain some portion of clay, the proper proportions of the two ingredients, to produce either an hydraulic lime or a cement, must bo determined by experiment in each case, guided by a previous analysis of the two ingredients to be tried. If the lime be pure, and the clay be free from lime, then the combinations in the proportions given in the table of M. Petot will give, by calcination, like results with the same proportions when found naturally combined. 87. Puzzolana, &c. The practice of using brick or tile-dust, or a volcanic substance known by the name of puzzolana, mixed with common lime, to form an hydraulic lime, was known to the Romans, by whom mortars composed of these materials were extensively used in their hydraulic constructions. This practice has been more or less followed by modern engineers, who, until within a few years, either used the puzzolana of Italy, where it is obtained near Mount Vesuvius, in a pulverulent state, or a ma- terial termed Trass, manufactured in Holland, by grinding to a fine powder a volcanic stone obtained near Andernach on the Rhine. Experiments by several eminent chemists have extended the list of natural substances which, when properly burnt and reduced to powder, have the same properties as puzzolana. They mostly belong to the feldspathic and schistose rocks, and are either fine sand, or clays more or less indurated. The following Table gives the results of analyses of Puzzolana, Trass, a Basalt, and a Schistus, lohich, when burnt and pow~ dered, were found to possess the properties of puzzolana. Puzzolana. Trass. Basalt Schistus. 445 570 44.50 46 00 0.150 0.120 16.75 26 00 0.088 0.026 9.50 4 00 Magnesia Oxide of iron 0.047 0.1-20 0.010 0.050 20.00 14.00 Oxide of manganese Potassa 0.014 0.070 2.37 8.00 Soda . 0.030 0.010 2.60 _ Water and loss 0.106 0.144 4.28 2.00 1.000 1.000 100.00 100.00 LIME. 3J 88. All of the? 3 substances, when prepared artificially, are nott generally known by the name of artificial puzzolanas, in contra- distinction to those which occur naturally. 89. General Treussart, of the French Corps of Military Engi- neers, first attempted a systematic investigation of the properties of artificial puzzolanas made from ordinary clay, and of the best manner of preparing them on a large scale. It appears from the results of his experiments, that the plastic clays used for tiles, or pottery, which are unctuous to the touch, the alumina in them being in the proportion of one fifth to one third of the silica, fur- nish the best artificial puzzolanas when suitably burned. The clays which are more meager, and harsher to the touch, yield an inferior article, but are in some cases preferable, from the greater ease with which they can be reduced to a powder. 90. As the clays mostly contain lime, magnesia, some of the metallic oxides, and alkaline salts, General Treussart endeavored to ascertain the influence of these substances upon the qualities of the artificial puzzolanas from clays in which they are found. He states, that the carbonate of potash and the muriate of soda seem to act beneficially ; that magnesia seems to be passive, as well as the oxide of iron, except when the latter is found in a large proportion, when it acts hurtfully ; and that the lime has a mate- rial influence on the degree of heat required to convert the clay into a good artificial puzzolana. 91. The management of the heat, in the preparation of this material, seems of the first consequence ; and General Treussart recommends that, direct experiment be resorted to, as the most certain means of ascertaining the proper point. For this purpose, specimens of the clay to be tried may be kneaded into balls as large as an egg, and the balls, when dry, be submitted to different degrees of heat in a kiln, or furnace, through which a current of air must pass over the balls, as this last circumstance is essential to secure a material possessing the best hydraulic qualities. Some of the balls are withdrawn as soon as their color indicates that they are underburnt ; others when they have the appearance of well-burnt brick ; and others when their color shows that they are overburnt, but before they become vitrified. The burnt balls are reduced to an impalpable powder, and this is mixed with a hydrate of fat lime, in the proportion of two parts of the powder t< :>ne of lime in paste. Water is added, if necessary, to bring the different mixtures to the consistence of a thick pulp ; and they are separately placed in glass vessels, covered with water, and allowed to remain until they harden. The compound which hardens most promptly will inlicate the most suitable degree of heat to be applied. 92. As the arbonates of line, of potash, and of soda, act as 32 BUILDING MATERIALS. fluxes on silica, the presence ot either one of them will modify the degree of heat necessary to convert the clay into a good natu- ral puzzolana. Clay, containing about one tenth of lime, should be brought to about the state of slightly-burnt brick. The ochreous clays require a higher degree of heat to convert them into a good material, and should be burnt until they assume the appearance of well-burnt brick. The more refractory clays will bear a still higher degree of heat ; but the calcination should in no case be carried to the point of incipient vitrification. 93. The quantity of lime contained in the clay can be readily ascertained beforehand, by treating a small portion of the clay, diffused in water, with enough muriatic acid to dissolve out the lime ; and this last might serve as a guide in the preliminary stages of the experiments. 94. General Treussart states, as the results of his experiments, that the mixture of artificial puzzolana and fat lime forms an hy drauiic paste superior in quality to that obtained by M. Vicat's process for making artificial hydraulic lime. M. Curtois, a French civil engineer, in a memoir on these artificial compounds, pub- lished in the Annales des Fonts et Chausstes, 1834, and General Pasley, more recently, adopt the conclusion of General Treussart. M. Vicat's process appears best adapted when chalk, or any very soft lime-stone, which can be readily converted to a soft pulp, is used, as offering more economy, and affording an hydraulic lime which is sufficiently strong for most building purposes. By it General Pasley has succeeded in obtaining an artificial hydraulic cement, which is but little, if at all, inferior to the best natural varieties ; a result which has not been obtained from any com- bination of fat lime with puzzolana, whether natural, or artificial. 95. All the puzzolanas possess the important property of not deteriorating by exposure to the air, which is not the case with any of the hydraulic limes, or cements. This property may ren- der them very serviceable in many localities, where only common, or feebly hydraulic lime can be obtained. MORTAR. 96. Mortar is any mixture of lime in paste with sand. It may be divided into two principal classes ; Hydraulic mortar, which is made of hydraulic lime, and Common mortar, matle of common lime. 97. The term Grout is applied to any mortar in a thin or fluid state ; and the terms Concrete and Beton, to mortars incorporated with gravel and small fragments of stone or brick. 98. Mortar is used for various purposes in building. It servea as a cement to unite blocks of stone, or brick. In concrete and MORTAR. 33 beton, which may be regarded as artificial conglomerate stones it forms the matrix by which the gravel and broken stone are held together ; and it is the principal material with which the ex terior surfaces of walls and the interior of edifices are coated. 99. The quality of mortars, whether used for structures ex- posed to the weather, or for those immersed in water, will depend upon the nature of the materials used ; their proportion ; the manner in which the lime has been converted into a paste to re- ceive the sand ; and the mode employed to mix the ingredients. Upon all of these points experiment is the only unerring guide for the engineer ; for the great diversity in the constituent elements of lime-stones, as well as in the other ingredients of mortars, must necessarily alone give rise to diversities in results ; and w r hen, to these causes of variation, are superadded those resulting from different processes pursued in the manipulations of slaking the lime and mixing the ingredients, no surprise should be felt at the seemingly opposite conclusions at which writers, who have pur sued the subject experimentally, have arrived. From the great mass of facts, however, presented on this subject within a few years, some general rules may be laid down, which the engineer may safely follow, in the absence of the means of making direct experiments. 100. Sand. This material, which forms one of the ingredients of mortar, is the granular product arising from the disintegration of rocks. It may, therefore, like the rocks from which it is de- rived, be divided into three principal varieties the silicious, the calcareous, and the argillaceous. Sand is also named from the locality where it is obtained, as pit-sand, which, is procured from excavations in alluvial, or other deposites of disintegrated rock ; river-sand and sea-sand, which are taken from the shores of the sea, or rivers. Builders again classify sand according to the size of the gram. The term coarse sand is applied when the grain varies between }th and T V th of an inch in diameter ; the term fine sand, when the grain is between y^th and T ' T th of an inch in diameter : and the term mixed sand is used for any mixture of the two prece- ding kinds. 101. The silicious sands, arising from the quartzose rocks, are the most abundant, and are usually preferred by builders. The calcareous sands, from hard calcareous rocks, are more rare, but form a good ingredient for mortar. Some of the argillaceous sand* possess the properties of the less energetic puzzolanas, and are therefore very valuable, as forming, with common lime, an arti- ficial hydraulic lime. 102. The property which some argillaceous sands possess, of forming with common, or slightly hydraulic lime a compound which 5 84 BUILDING MATERIALS. will harden under water, has been long known in France, where these sands are termed arenes. The sands of this nature are usually found in hillocks along river valleys. These hillocks sometimes rest on calcareous rocks, or argillaceous tufas, and are frequently formed of alternate beds of the sand and pebbles. The sand is of various colors, such as yellow, red, and green, and seems to have been formed from the disintegration of clay in a more or less indurated state. The arenes are not as energetic as either natural or artificial puzzolanas ; still they form, with com- mon lime, an excellent mortar for masonry exposed either to the open air, or to humid localities, as the foundations of edifices. 103. Pit-sand has a rougher and more angular grain than river or sea sand ; and, on this account, is generally preferred by build- ers for mortar used for brick, or stone-work. Whether it forms a stronger mortar than the other two is not positively settled, al- though some experiments would lead to the conclusion that it does. 104. River and sea sand are by some preferred for plastering, because they are whiter, and have a finer and more uniform grain lhan pit sand; but as the sands from the shores of tidal water* contain salts, they should not be used, owing to their hygrqmetric properties, before the salts are dissolved out in fresh water by careful washing. 105. Pit-sand is seldom obtained free from a mixture of dirt, or clay ; and these, when found in any notable quantity in it, give a weak and bad mortar. Earthy sands should, therefore, be cleansed from dirt before using them for mortar ; this may be effected by washing the sand in shallow vats, and allowing the turbid water, in which the clay, dust, and other like impurities are held in suspension, to run off. 106. Sand, when pure or well cleansed, may be known by not soiling the ringers when rubbed between them. 107. Hydraulic mortar. This material may be made from the natural hydraulic limes ; from those w r hich are prepared by M. Vicat's process ; or from a mixture of common, or feebly hy- draulic lime, with a natural or artificial puzzolana. All writers, however, agree that it is better to use a natural than an artificial hydraulic lime, when the former can be readily procured. 108. When the lime used is strongly hydraulic, M. Vicat is of opinion that sand alone should be used with it, to form a good hydraulic mortar. General Treussart has drawn the conclusion, from his experiments, that the mortar of all hydraulic limes is improved by an addition of a natural or artificial puzzolana. The quantity of sand used may vary from If to 2 parts of the lime in bulk, when reduced to a thick pulp. 109. For hydraulic mortars, made of common, feeble, or or MQRTAR. 35 dinary hydraulic limes, and artificial puzzolana, M. Vicat state* tbat the puzzolana should be the weaker as the lime is more strongly hydraulic ; using, for example, a very energetic puzzo- lana with a fat, or a feebly hydraulic lime. The proportion of sand which can be incorporated with these ingredients, to form an hydraulic mortar, is stated by General Treussart to be one vol- ume to one of puzzolana, and one of lime in paste. 110. In proportioning the ingredients, the object to which the mortar is to be applied should be regarded. When it is to serve lo unite stone, or brick work, it is better that the hydraulic lime should be rather in excess : when it is used as a matrix for beton, no more lime should be used than is strictly required. No harm will arise from an excess of good hydraulic lime, in any case ; but an excess of common lime is injurious to the quality of the mortar. 1,11. Common and ordinary hydraulic limes, when made into mortar with arenes, give a good material for hydraulic purposes. The proportions in which these have been found to succeed well, are one of lime to three of arenes. 112. Hydraulic cement, from the promptitude with which it hardens, both in the air and under water, is an invaluable mate- rial where this property is essential. Any dose of sand injures its properties as a cement. But hydraulic cement may be added with decided advantage to a mortar of common, or of feebly hy draulic lime and sand. It is in this way that it is generally used in our public works. The French engineers give the preference to a good hydraulic mortar over hydraulic cement, both for uniting stone, or brick work, and for plastering. They find, from their practice, that when used as a stucco, it does not withstand well the effects of weather ; that it swells and cracks in time ; and, when laid on in successive coats, that they become detached from each other. General Pasley, who has paid great attention to the properties of natural and artificial hydraulic cements, does not agree with the French engineers in his conclusions. He states that, when skilfully applied, hydraulic cement is superior to any hydraulic mortar for masonry, but that it must be used only in thin joints : and, when applied as a stucco, that it should be laid on in but one coat ; or, if it be laid on in two, the second must be added long before the first has set, so that, in fact, the two make but one coal. By attending to these precautions, General Pasley stales that a stucco of hydraulic cement and sand will whhsiand per- fectly the effects of frost. 113. Mortars exposed to weather. The French engineers, who have paid great attention to the subject of mortars, coincide in the opinion, that a mortar cannot be made of fat lime and any inert sands, like those of the silicious, or calcareous kinds which 86 BUILDING MATERIALS. will withstand the ordinary exposure of weather ; and that, tc obtain a good mortar for this purpose, either the h) ..Vaulic limes mixed with sand must be employed, or else common lime mixed either with arenes, or witli a puzzolana and sand. 1 14. Any pure sand mixed in proper proportions w r ith hydraulic lime, will give a good mortar for the open air; but the hardness of the mortar will be affected by the size of the grain, particularly when hydraulic lime is used. Fine sand yields the best mortal with good hydraulic lime ; mixed sand with the feebly hydraulic limes ; and coarse sand with fat lime. 115. The proportion which the lime should bear to the sand seems to depend, in some measure, on the manner in which the lime is slaked. M. Vicat states, that the strength of mortar made of a stiff paste of fat lime, slaked in the ordinary way, increases from 0.50 to 2.40 to one of the paste in volume ; and that, when the lime is slaked by immersion, one volume of the like paste will give a mortar that increases in strength from 0.50 to 2.20 parts of sand. For one volume of a paste of hydraulic lime, slaked in the or- dinary way, the strength of the mortar increases from to 1 .80 parts of sand ; and, when slaked by immersion, the morta* of a like paste increases in strength from to 1.70 parts of lime. In every case, when the dose of sand was increased beyond these proportions, the strength of the resulting mortar was found to decrease. 116. Manipulations of Mortar. The quality of hydraulic mor- tar, which is to be immersed in water, is more affected by the manner in which the lime is slaked, and the ingredients mixed, than that of mortar which is to be exposed to the weather ; al- though in both cases the increase of strength, by the best manipu- lations, is sufficient to make" a study of them a matter of some consequence. 117. The results obtained from the ordinary method of slak- ing, by sprinkling, or by immersion, in the case of good hydraulic limes, are nearly the same. Spontaneous, or air-slaking, gives invariably the worst results. For common and slightly hydraulic lime, M. Vicat states that air-slaking yields the best results, and ordinary slaking the worst. 118. The ingredients of mortar are incorporated either by manual labor, or by machinery : the latter method gives results superior to the former. The machines commonly used for mix- ing mortar are either the ordinary pug-mill (Fig. 4) employed by brickmakers for tempering clay, or a grinding-mill, (Fig. 5.) The grinding-mill is the best machine, because it not only re- duces the lumps, which are found in the most carefully burnt tone, after the slaking is apparently complete, but it brings the MORTAR. 37 fime to the state of a uniform stiff r> a ste, wh'ch it should icceive before the sand is incorporated w.th it. Care should be taken Fig. 4 represents a vertical section through the axis of a pug-mill, for mixing of tempering mortar. This mill consists of a hooped vessel, of the form of a co- nical frustum, which receives the in- gredients, and a vertical shaft, to which arms with teeth, resembling an ordi- nary rake, are attached, for the purpose of mixing the ingredients. A, A, section of sides of the vessel. B, vertical shaft to which the arms C are affixed. D, horizontal bar for giving a circular mo- tion to the shaft B. E, sills of timber supporting the mill. F, wrought-iron support through which the upper part of the shaft passes. not to add too much water, particularly when the mortar is to be immersed in water. The mortar-mill, on this account, should be sheltered from rain ; and the quantity of water with which it is Fig. 5 represents a part of a mill for crushing the lime and tempering the mortar. A, heavy wheel of timber, or cast iron. B, horizontal bar passing through the wheel, which at one extremity is fixed to a vertical shaft, and is arranged at the other (C) with the proper gear- ing for a horse. D, a circular trough, with a trapezoidal cross sec- tion which receives the ingredients to be mixed. The trough may be from 20 to 30 feet in diameter ; about 18 inches wide at top, and 12 inches deep; and be built of hard brick, stone, or timber laid on a firm foundation. supplied may vary with the state of the weather. Nothing seems to be gained by carrying the process of mixing, beyond obtaining a uniform mass of the consistence of plastic clay. Mortars of hydraulic lime are injured by long exposure to the air, and fre- quent turnings and mixings with a shovel or spade ; those of common lime, under like circumstances, seem to be improved. Mortar, which has been set aside for a day or two, will become sensibly firmer ; if not allowed to stand too long, it may be again reduced to its clayey consistence, by simply pounding it with a beetle, without any fresh addition of water. 119. Setting and Durability of Mortars. Mortar of common lime, without any addition of puzzolana, will not set in humid situations, like the foundations of edifices, until after a very long lapse of time. They set very soon when exposed to the air, or jo an ai.mosphere of carbonic acid gas. If, after having become 38 BUILDING MATERIALS. liard in the open air, they are placed under water, they in time lose their cohesion and fall to pieces. 120. Common mortars, which have had Lme to harden, resist the action of severe frosts very well, if they are made rather poor, or with an excess of sand. The sand should be over 2.40 parts. in bulk, to one volume of the lime in paste ; and coarse sand is found to give better results than fine sand. 121. Good hydraulic mortars set equally well in damp situa- tions, and in the open air ; and those which have hardened in the air will retain their hardness when immersed in water. They also resist well the action of frost, if they have had time to set before exposure to it ; but, like common mortars, they require to be made with an excess of sand, to withstand well atmospheric changes. 122. The surface of a mass of hydraulic mortar, whether made of a natural hydraulic lime or otherwise, when immersed in water, becomes more or less degraded by the action of the water upon the lime, particularly in a current. When the water is stagnant, a very thin crust of carbonate of lime forms on the surface of the mass, owing to the absorption by the lime of the carbonic acid gas in the water. This crust, if the water be not agitated, will preserve the soft mortar beneath it from the farther action of the water, until it has had time to become hard, when the water will no longer act upon the lime in any perceptible degree. 123. Hydraulic mortars set with more or less promptness, ac- cording to the character of the hydraulic lime, or of the puzzolana v/hich enters into their composition. Artificial hydraulic mortars, with an excess of lime, set more slowly than when the lime is in a just proportion to the other ingredients. 124. The quick-setting hydraulic limes are said to furnish a mortar which, in time, acquires neither as much strength nor hardness as that from the slower-setting hydraulic limes. Ar- tificial hydraulic mortars, on the contrary, which set quickly, gain, in time, more strength and hardness than those which set slowly. 125. The time in which hydraulic mortars, immersed in water, attain their greatest hardness, is not well ascertained. Mortars made of strong hydraulic limes do not show any appreciable in- crease of hardness after the second year of their immersion ; while the best artificial hydraulic mortars continue to harden, in a sen- sible degree, during the third year after their immersion. 126. Theory of Mortars. The paste of a hydrate, either of common or of hydraulic lime, when exposed to the air, absorbs carbonic acid gas from it ; passes to the state of sub-carbonate of lime ; without, however, rejecting the water of the hydrate, and gradually hardens. The time required for the complete satura MORTAR 3S ucn of the mass exposed, will depend on its bulk. The absorp- tion of the gas commences at the surface and proceeds more slowly towards the centre. The hardening of mortars exposed to the atmosphere, is generally attributed to this absorption of the gas, as no chemical action of lime upon quartzose sand, which is the usual kind employed for mortars, has hitherto been detected by the most careful experiments. 127. With regard to hydraulic mortars, it is difficult to account for their hardening, except upon the effect which the silicate of lime may have upon the excess of simple hydrate of uncombined lime contained in the mass. M. Petot supposes, that the parti- cles of silicate of lime form so many centres, around which the uncombined hydrates group themselves in a crystalline form ; becoming thus sufficiently hard to resist the solvent action of water. With respect to the action of quartzose sand in hydraulic mortars, M. Petot thinks that the grains produce the same me- chanical effect as the particles of the silicate of lime, in inducing the aggregation of the uncombined hydrate. 128. Concrete. This term is applied, by English architects and engineers, to a mortar of finely-pulverized quick-lime, sand, and gravel. These materials are first thoroughly mixed in a dry state, sufficient water is added to bring the mass to the ordinary consistence of mortar, and it is then rapidly worked up by a shovel, or else passed through a pug-mill. The concrete is used immediately after the materials are well incorporated, and while the mass is hot. 129. The materials for concrete are compounded in various proportions. The most approved are those in which the limo and sand are in the proper proportions to form a good mortar and the gravel is twice the bulk of the sand. The gravel used should be clean, and any pebbles contained in it larger than an egg, should be broken up before the materials are incorpo- rated. 130. Hot water has in some cases been used in making con crete. It causes the mass to set more rapidly, but is not other- wise of any advantage. 131. The bulk of a mass of concrete, when first made, is found to be about one fifth less than the total bulk of the dry materials But, as the lime slakes, the mass of concrete is found to expand about three eighths of an inch in height, for every foot of the mass in depth. 132. The use of concrete is at present mostly restricted to forming a solid bed, in bad soils, for the foundations of edifices. It has also been used to form blocks of artificial stone, for the walls of buildings and other like purposes ; but experience has shown, that i* possesses neither the durability nor strength requj 40 BUILDING MATERIALS. site for structures of a permanent character, when exposed to the action of water, or of the weather. 133. Beton. The term beton is applied, by French engineers, to any mixture of hydraulic mortar with fragments of brick, stone, or gravel ; and it is now also used by English engineers in the same sense. 134. The proportions of the ingredients used for beton are va- riously stated by different authors. The sole object for which the gravel, or the broken stone is used, being to obtain a more economical material than a like mass of hydraulic mortar alone would yield, the quantity of broken stone should be as great as can be thoroughly united by the mortar. The smallest amount of mortar, therefore, that can be used for this purpose, will be that which will be just equal in volume to the void spaces in any given bulk of the broken stone, or gravel. The proportion which the volume occupied by the void spaces bears to any bulk of a loose material, like broken stone, or gravel, may be readily ascertained by filling a vessel of known capacity with the loose material, and pouring in as much water as the vessel will contain. The vol- ume of water thus found, will be the same as that of the void spaces. 1 35. Beton made of mortar and broken stone, in which the proportions of the ingredients were ascertained by the process just detailed, has been found to give satisfactory results ; but, in order to obviate any defect arising from imperfect manipulation, it is usual to add an excess of mortar above that of the void spaces. The best and most economical beton is made of a mixture of broken stone, or brick, in fragments not larger than a hen's egg, and of coarse and fine gravel mixed in suitable proportions. 136. In making beton, the mortar is first prepared, and then incorporated with the finer gravel ; the resulting mixture is spread out into a cake, 4 or 6 inches in thickness, over which the coarser gravel and broken stone are uniformly strewed and pressed down, the whole mass being finally brought to a homogeneous state with the hoe and shovel. Beton is used for the same purposes as concrete, to which it is superior in every respect, but particularly so for foundations laid under water, or in humid localities. 137. Adherence of Mortar. The force with which mortars in general adhere to other materials, depends on the nature of the material, its texture, and the state of the surface to which the mortar is applied. 138. Mortar adheres most strongly to brick ; and more feebly o wood than to any other material. Among stones, its adhesion to lime-stone is generally greatest ; and to basalt and sand-stones MASTICS. 41 'east , Among stones of the same class, it adheres generally bet- ter .o the porous and coarse-grained, than to the compact and fine-grained. Among surfaces, it adheres more strongly to the rough than to the smooth. 139. The adhesion of common mortar to brick and stone, for the first few years, is greater than the cohesion of its own parti- cles. The force with which hydraulic cement adheres to the same materials, is less than that of the cohesion between its own parti- cles : and. from some recent experiments of Colonel Pasley, on this subject, it would seem that hydraulic cement adheres with nearly the same force to polished surfaces of stone as to rough surfaces. 140. From experiments made by Rondelet, on the adhesion of common mortar to stone, it appears that it required a force vary- ing from 15 to 30 pounds on the square inch, applied perpendicu- lar to the plane of the joint, to separate the mortar and stone after six months union ; whereas, only 5 pounds to the square inch was required to separate the same surfaces, when applied parallel to the plane of the joint. From experiments made by Colonel Pasley, he concludes that the adhesive force of hydraulic cement to stone, may be taken as high as 125 pounds on the square inch, when the joint has had time to harden throughout ; but, he remarks, that as in large joints the exterior part of the joint may have hardened while the interior still remains soft, it is not safe to estimate the adhesive force, in such cases, higher than from 30 to 40 pounds on the square inch. MASTICS. 141. The term Mastic is generally applied to artificial or natu ral combinations of bituminous or resinous substances with other ingredients. They are converted to various uses in constructions, either as cements for other materials, or as coatings, to render them impervious to water. 142. Bituminous Mastic. The knowledge of this material dates back to an early period ; but it is only within, compara- tively speaking, a few years that it has come into common use in Europe and this country. The most usual form in which it is now employed, is a combination of mineral tar and powdered bituminous lime-stone. 143. The localities of each of these substances are very nu- merous ; but they are chiefly brought into the market from several places in Switzerland and France, where these mirerals are found m great abundance ; the most noted being Val-de-Travers in Switzerland, and Seyssel in France. 144. The mineral tar is usually obtained by bo'ling in water a 6 42 BUILDING MATERIALS. soft sand-stone, called by the French molasse, which is stiongly impregnated with the tar. In this process, the tar is disengaged and rises to the surface of the water, or adheres to the sides oi the vessel, and the earthy matter remains at the bottom. An analysis of a rich specimen of the Seyssel bituminous sand-stone gave the following results : Bituminous oil . -086 > Bi Carbon . . . .020 J Quartzy grains ...... .690 Calcareous grains ...... .204 1.000 145. The bituminous lime-stone which, when reduced to a powdered state, is mixed with the mineral tar, is known at the localities mentioned by the name of asphaltum, an appellation which is now usually given to the mastic. This lime-stone occurs in the secondary formations, and is found to contain various pro- portions of bitumen, varying mostly from 3 to 15 per cent., with the other ordinary minerals, as argile, &c., which are met with in this formation. 146. The bituminous mastic is prepared from these two mate- rials by heating the mineral tar in cast-iron or sheet-iron boilers, and stirring in the proper proportion of the powdered lime- stone. This operation, although very simple in its kind, requires great attention and skill on the part of the workmen in managing the fire, as the mastic may be injured by too low, or too high a degree of heat. The best plan appears to be, to apply a brisk fii 3 until the boiling liquid commences to give out a thin whitish vapor. The fire is then moderated and kept at a uniform slate, and the powdered stone is gradually added, and mixed in with the tar by stirring the two well together. When the temperature has been raised too high, the heated mass gives out a yellowish or brownish vapor. In this state it should be stirred rapidly, and be removed at once from the fire. 147. The asphaltic stone may be reduced to powder, either by roasting it in vessels over a fire, or by grinding it down in the or- dinary mortar-mill. For roasting, the stone is first reduced to fragments the size of an egg. These fragments are put into an 'ion vessel ; heat is applied, and the stone is reduced to powder by stirring it and breaking it up with an iron instrument. This process is not only less economical than grinding, but the ma- terial loses a portion of its tar from evaporation, besides being liable to injury from too great a degree of heat. For grinding the stone is first broken as for roasting. Care should be taken, during the process, to stir the mass frequently, otherwise it may GLUE. 4 om the WOOD. 49 exterior layers of the immersed wood. The practice of keeping timber in water, with a view to facilitate its seasoning, has been condemned as of doubtful utility ; particularly immersion in salt water, where the timber is liable to the inroads of those two very destructive inhabitants of our waters, the Limnoria Teiebrans, and Teredo Navalis ; the former of which rapidly destroys the heaviest logs, by gradually eating in between the annual rings ; and the latter, the well-known ship-worm, by converting timber into a perfect honeycomb state by its numerous perforations. 178. Steaming is mostly in use for ship-building, where it is necessary to soften the fibres, for the purpose of bending large pieces of timber. This .is effected by placing the timber in strong steam-tight cylinders, where it is subjected to the action of steam long enough for the object in view ; the period usually allowed, is one hour to each inch in thickness. Steaming slightly impairs the strength of timber, but renders it less subject to decay, and less liable to warp and crack. 179. When timber is used for posts partly imbedded in the ground, it is usual to char the part imbedded, to preserve it from decay. This method is only serviceable when the timber has been previously well seasoned ; but for green timber it is highly inju- rious, as by closing the pores, it prevents the evaporation from the surface, and thus causes fermentation and rapid decay within. 180. The most durable timber is procured from trees of a close compact texture, which, on analysis, yield the largest quantity of carbon. And those which grow in moist and shady localities, furnish timber which is weaker and less durable than that from trees growing in a dry open exposure. 181. Timber is subject to defects, arising either from some peculiarity in the growth of the tree, or from the effects of the weather. Straight-grained timber, free from knots, is superior in strength and quality., as a building material, to that which is the reverse. 182. The action of high winds, or of severe frosts, injures the tree while standing : the former separating the layers from each other, forming what is denominated rolled timber; the latter cracking the timber in several places, from th*- surface to th centre. These defects, as well as those arising from worms, or age, are easily seen by examining a cross section of the trunk. 183. The wet and dry rot are the most serious causes of the decay of timber ; as all the remedies thus far proposed to prevent them, are too expensive to admit of a very general application. Both of these causes have the same origin, fermentation, and consequent putrefaction. The wet rot takes place in wood ex- posed, alternately, to moisture and dryness ; and the dry rot is occasioned by want of a free circulation of air, as in confined 7 50 BUILDING MATERIALS. warm localities, like cellars and the more confined parts of vessels. Trees of rapid growth, which contain a arge portion of sap- wood, and timber of every description, when used green, where there is a want of a free circulation of air, decay very rapidly with the rot. 184. Numberless experiments have been made on the preser- vation of timber, and many processes for this purpose have been patented both in Europe and this country. Several of these processes have yielded the most satisfactory results ; and nearly all have proved more or less efficacious. The means mostly re- sorted to have been the saturation of the timber in the solution of some salt with a metallic, or earthy base, thus forming an in- soluble compound with the soluble matter of the timber. The salts which have been most generally tried, are the sulphate of iron, or copper, and the chloride of mercury, zinc, or calcium. The results obtained from the chlorides have been more satisfac- tory than those from the sulphates ; the latter class of salts with metallic bases possess undoubted antiseptic properties ; but it is stated that the freed sulphuric acid, arising from the chemical action of the salt on the wood, impairs the woody fibre, and changes it into a substance resembling carbon. 185. The processes which have come into most general use, are those of Mr. Kyan, and of Sir W. Burnett, called after the patentees kyanizing and burnetizing. Kyan's process is to sat- urate the timber with a solution of chloride of mercury ; using, for the solution, one pound of the salt to five gallons of water Burnett uses a solution of chloride of zinc, in the proportion of one pound of the salt to ten gallons of water, for common pur- poses ; and a more highly concentrated solution when the object is also to render the wood incombustible. 186. As timber under the ordinary circumstances of immer- sion imbibes the solutions very slowly, a more expeditious, as well as more perfect means of saturation has been used of late, which consists in placing the wood to be prepared in strong wrought-iron cylinders, lined with felt and boards, to protect the iron from the action of the solution, where, first by exhausting the cylinders of air, and then applying a strong pressure by means of a force-pump, the liquid is forced into the sap and air-vessels, and penetrates to the very centre of the timber. 187. Among the patented processes in our country, that of Mr. Earle has received most notice. This consists in boiling the timber in a solution of the sulphates of copper and iron. Opinion seems to be divided as to the efficacy of this method. It has been tried for the preservation of timber for artillery carriages, but not with satisfactory results WOOD. 51 188. M. Boucherie, ., whose able researches on this subject reference has been made, noticing the slowness with which aqueous solutions were imbibed by wood, when simply im- mersed in them, conceived the ingenious idea of rendering the vital action of the sap-vessels subservient to a thorough impreg- nation of every part of the trunk where there was this vitality To effect this, he first immersed the butt end of a freshly-felled tree in a liquid, and found that it was diffused throughout all parts of the tree, in a few days, by the action in question. But, find ng it difficult to manage trees of some size when felled, M. Boucherie next attempted to saturate them before felling ; for which purpose he bored an auger-hole through the trunk, and made a saw-cut from the auger-hole outwards, on each side, to within a few inches of the exterior, leaving enough of the fibres untouched to support the tree. One end of the auger-hole was then stopped, as well as all of the saw-cut on the exterior, and the liquid was introduced by a tube inserted into the open end of the auger-hole. This method was found equally efficacious with the first, and more convenient. 189. After examining the action of the various neutral salts on the soluble matter contained in wood, M. Boucherie was led to try the impure pyrolignite of iron, both from its chemical compo- sition and its cheapness. The results of this experiment were perfectly satisfactory. The pyrolignite of iron, in the proportion of one fiftieth in weight of the green wood, was found not only to preserve the wood from decay, but to harden it to a very high degree. 190. Observing that the pliability and elasticity of wood de- pended, in a great measure, on the moisture contained in it, M. Boucherie next directed his attention to the means of improving these properties. For this purpose, he tried solutions of various deliquescent salts, which were found to answer the end proposed. Among these solutions, he gives the preference to that of chloride of calcium, which also, when concentrated, renders the wood in-, combustible. He also recommends for like purposes the mother water of salt-marshes, as cheaper than the solution of the chloride of calcium. Timber prepared in this way is not only improved in elasticity and pliability, but is prevented from warping and cracking ; the timber, however, is subject to greater variations in weight than when seasoned naturally. 191. M. Boucherie is of opinion that the earthy chlorides will also act as preservatives, but to ensure this he recommends that they be mixed with one fifth of pyrolignite of iron. 192. From other experiments of M. Boucherie, it appears thai the sap may be expelled from any freshly- felled timber by the pressure of a liquid, and the timber be impregnated as thoroughly 5% BUILDING MATERIALS. as by the preceding processes. To effect this, the piece to b, principal results of ex- periments made by Mr. G. Rennie, and published in the Philo- sophical Transactions of 1818. The stones tried were in small cubes, measuring one and a half inches on the edge. The table gives the pressure, in tons, borne by each superficial inch of the stone at the moment of crushing. DESCRIPTION OF STONE. Spec, gravity. Crushing w'ght. Granites. Aberdeen, (blue) ...... Peterhead ....... 2.625 4.83 3.70 Cornwall ....... 2.662 2.83 Sand-stones. Dundee ....... 2.530 2.96 Do. 2.506 2.70 Derby, (red and friable} 2.316 1.40 Lime-stones. Marble, (white-veined Italian) Do. (white Brabant) Limerick, (black compact) Devonshire, (red marble) Portland stone, (fine-grained oolite) 2.726 2.697 2.598 2.428 4.32 4.11 3.95 3.31 2.04 The following results are taken from a series of experiments made under the direction of Messrs. Bramah & Sons, and pub- lished in Vol. 1, Transactions of the Institution of Civil En- gineers. The first column of numbers gives the weights, in tons, borne by each superficial inch when the stones commenced to fracture ; the second column gives the crushing weight, in tons, on the same surface. 72 BUILDING MATERIAL. DESCRIPTION Or STONE. Aver, weight pro- ducing fractures. Average crashing weight. Granites. Herme ... ... 4.77 6.64 Aberdeen, (blue) 4.13 4.64 Heytor ... . . 3.94 6.19 Dartmoor . . 3.52 5.48 Peterhead, (red) 2.88 4.88 Peterhead, (blue gray) 2.86 4.36 Sand-stones. Yorkshire .... 2.87 3.94 Craigleith 1.89 2.97 Humble ...... 1.69 2.06 Whitby 1.00 1.06 The following Table is taken from one published in Vol. 2, Civil Engineer and Architect's Journal, which forms a part of the Report on the subject of selecting stone for the New Houses of Parliament. The specimens submitted to experiment were cubical blocks measuring two inches on an edge. DESCRIPTION- OF STONE. Specific gravity. Weight produ- cing fracture. Crushing w'ght. Sand-stones. Craigleith .... 2.232 1.89 3.5 Darley Dale .... 2.628 2.75 3.1 2.229 0.82 1.75 Kenton ..... 2.247 1.51 2.21 Mansfield .... 2.338 0.88 1.64 Magnesian Lime-stones. Bolsover ..... 2.316 2.21 3.75 Huddlestone ... 2.147 1.03 1.92 Roach Abbey .... 2.134 0.75 1.73 Park Nook .... 2.138 0.32 1.92 Oolites. Ancaster ... 2.182 0.75 1.04 Bath Box 1.839 0.56 0.66 Portland 2.145 0.95 1.75 Ketton 2.045 0.69 1.18 Lime-stones. Barnack ..... 2.090 0.50 0.79 Chilmark, (si.icious) 2.481 1.32 3.19 i Hamhill 2.260 0.69 1.80 The numbers of the first column give the specific gravities STRENGTH OF MATERIALS. 72 those in the second column the weight in tons on a sq lare inch, when the stone commenced to fracture ; and those in the third the crushing weight on a square inch. The following Table exhibits the results of experiments on the resistance of stone to a transverse strain, made by Colonel Pasley. on prisms 4 inches long, the cross section being a square of 2 inches on a side ; the distance between the points of support 3 inches. DESCRIPTION Or STONE. Weight of stone per cubic foot in Ibs. Average breaking weight in Ibs. 1. Kentish Rag 165.69 4581 2. Yorkshire Landing 147.67 2887 3. Cornish granite . 172.24 2808 4. Portland 148.08 2682 5. Craigleith . . . 144.47 1896 6. Bath ... . 122.58 666 7. Well-burned bricks 91.71 752 8. Inferior bricks - 329 284. The conductors of the experiments on the stone for the New Houses of Parliament, Messrs. Daniell and Wheatstone, who also made a chemical analysis of the stones, and applied to them Brard's process for testing their resistance to frost, have appended the following conclusions from their experiments : " If the stones be divided into classes, according to their chemical composition, it will be found that in all stones of the same class there exists generally a close relation between their various phy- sical qualities. Thus it will be observed that the specimen which has the greatest specific gravity possesses the greatest cohesive strength, absorbs the least quantity of water, and disintegrates the least by the process which imitates the effects of weather. A. comparison of all the experiments shows this to be the general rule, though it is liable to individual exceptions." " But this will not enable us to compare stones of different classes together. The sand-stones absorb the least quantity of water, but they disintegrate more than the magnesian lime-stones, which, considering their compactness, absorb a great deal." 285. Rondelet, from a numerous series of experiments on the same subject, published in his work, Art de B&tir, has arrived at like conclusions with regard to the relations between the specific gravity and strength of stones belonging to the same class. 2S6. Among the results of the more recent experiments on this subject, those obtained by Mr. Hodgkinson, showing the relation 10 74 BUILDING MATERIALS. between the crushing, the tensile, and the transverse strength of stone, have already been given. M. Vicat, in a memoir on the same subject, published in the Annales des Fonts et Chaussees, 1 833, has arrived at an opposite conclusion from Mr. Hodgkinson, stating, as the results of his experiments, that no constant relation exists between the crush- ing and tensile strength of stone in general, and that there is no other means of determining these two forces, but by direct ex- periment in each case. 287. The influence of form on the strength of stone, and the circumstances attending the rupture of hard and soft stones, have been made the subject of particular experiments by Rondelet and Vicat. Their experiments agree in establishing the points that the crushing weight is in proportion to the area of the base. Vicat states, more generally, that the permanent weights borne by similar solids of stone, under like circumstances, will be as me squares of their homologous sides. These two authors agree on the point that the circular form of the base is the most favor- able to strength. They differ on most other points, and particu- larly on the manner in which the different kinds of stone yield by rupture. 288. Practical Deductions. Were stones placed under the same circumstances in structures as in the experiments made to ascertain their strength, there would be no difficulty in assigning what fractional part of the weight which, in the comparatively short period usually given to an experiment, will crush them, could be borne by them permanently with safety. But, in- dependently of the accidental causes of destruction to whicl structures are exposed, imperfections in the material itself, as well as careless workmanship, from which it is often placed in the most unfavorable circumstances of resistance, require to be guarded against. M Vicat, in the memoir before-mentioned, states that a permanent .strain of fW f t ne crushing force of ex- periment, may be borne by stone without danger of impairing its cohesive strength, provided it be placed under the most favorable circumstances of resistance. This fraction of the crushing weight of experiment is greater than ordinary circumstances would jus- tify, and it is recommended in practice not to submit any stone 'o a greater permanent strain than one tenth of the crushing weight of experiments made on small cubes measuring about two inches on an edge. The following Table shows the permanent strain, and crushing weight, for a square foot of the stones in some of the most re markable structures in Europe. STRENGTH OF MATERIALS. 75 r Permanent Crushing strain. weight. Pillars of the dome of St. Peter's, (Rome) 33330 536000 Do. St. Paul's, (London) 39450 537000 Do. St. Genevieve, (Paris) 60000 456000 Do. Church of Toussaint, (Angers) . 90000 900000 Lower courses of the piers of the Bridge of iNeuilly 3600 570000 The stone employed in all the structures enumerated in the Table, is some variety of lime-stone. 289. Expansion of Stone from Heat. Experiments have been made in this country by Prof. Bartlett, and in England by Mr. Adie, to ascertain the expansion of stone for every degree of Fahrenheit. The experiments of Prof. Bartlett give the follow- ing results : Granite expands for every degree Marble " " Sand-stone " " .000004825 .000005668 .000009532 Table of the Expansion of Stone, <$-c.,from the Eocperiments oj Alexander J. Adie, Civil Engineer, Edinburgh. DESCRIPTION OF STONE. Decimal of tin inch oa S3 inchw for 180" F. Decimal of length for 180" . Decimal of J-fjh for ! Remarkf. 1. Roman cement . .0330043 .0014349 .00000750 2. Sicilian white marble . j .0325392 .0253946 .0014147 .00110411 .00000780 .00000613 I One experiment, (moist.) ( Mean of three, (dry.) 3. Carrara marble . . ] .0274344 .0150405 .0011928 .0006539 .00000602 .00000363 j One experiment, (moist.) ( Mean of two, (dry.) 4. Sand-stone, (Craigleith) 5. Slate, (Welch) . . . .0270093 .02384559 .0011743 .0010376 .000001352 .00000576 Mean of four experiments. Mean of three do. 6. Red pranite, (Peterkead) j .0220416 .02U6266 .0009583 .0008968 .00000532 .00000498 SOne experiment, (moist.) Mean of two. (dry.) 7. Arbroath pavement .0200652 .ooorttes .ooo<*Hy9 Mean of four experiments. 8. Caithness pavement .0205788 .0008947 .00000497 Mean of three do. 9. Green-stone, (Ratho) . .0186043 .0008089 .00000449 Mean of three do 10. Crty frranite, (Aberdeen) .01815695 .00078943 .00000438 Mean of two do. 11. Ftet stock brick .0126542 .0005502 .00000306 Mean of two do. 12. Fire brick .... .0113334 .0004928 .00000274 Mean of two do. 13. liluck marble, (Galway) .0102394 .00044519 .00000247 Mean of three do. 290. STRENGTH OF MORTARS. A very wide range of experi- ments has been made, within a few years back, by engineers both at home and abroad, upon the resistance offered by mortars to a transversal strain, with a view to compare their qualities, both as regards their constituent elements and the processes followed in their manipulation. As might naturally have been anticipated these experiments have presented very diversified, and, in many instances, contradictory results. The general conclusions, how- ever, drawn from them, have been nearly the same in the majority 76 BUILDING MATERIALS. of cases ; and they furnish the engineer with the most reliable guides in this important branch of his art. 291. The usual method of conducting these experiments has been to subject small rectangular prisms of mortar, resting on points of support at their extremities, to a transversal strain ap- plied at the centre point between the bearings. This, perhaps, is as unexceptionable and convenient a method as can be followed for testing the comparative strength of mortars. 292. M. Vicat, in the work already cited, gives the following as the average resistances on the square inch offered by mortars to a force of traction ; the deductions being drawn from experi- ments on the resistance to a transversal strain. Mortars of very strong hydraulic lime 170 pounds " ordinary do. 140 " medium do. 100 " common lime . 40 " " do. (bad quality) 10 " These experiments were made upon prisms a year old, which had been exposed to the ordinary changes of weather. With re- gard to the best hydraulic mortars of the same age which had been, during that same period, either immersed in water, or buried in a damp position, M. Vicat states that their average tenacity may be estimated at 140 pounds on the square inch. 293. General Treussart, in his work on hydraulic and common mortars, has given in detail a large number of experiments on the transversal strength of artificial hydraulic mortars, made by sub- mitting small rectangular parallelepipeds of mortar six inches in length, and two inches square, to a transversal strain applied at the centre point between the bearings, which were four inches apart. From these experiments he deduces the following prac- tical conclusions. That when the parallelepipeds sustain a transversal strain vary- ing between 220 and 330 pounds, the corresponding mortar will be suitable for common gross masonry ; but that for important hydraulic works the parallelepipeds should sustain, before yield ' ing, from 330 to 440 pounds. 294. The only published experiments on this subject made in this country are those of Colonel Totten, appended to his transla- tion of General Treussart.'s work. The results of these experi- ments are of peculiar value to the American engineer, as they were made upon materials in very general use on the public works throughout the 'country. From these experiments Colonel Totten deduces the following general results : 1st. That mortar of hydraulic cement and sand is the strongei and harder as the quantity of sand is less. STRENGTH OF MATERIALS. 77 2d. That common mortar is the stronger and harder as the quantity of sand is less. 3d. That any addition of common lime to a mortar of hydraulic cement and sand weakens the mortar, but that a little lime ma}i be added without any considerable diminution of the strength of the mortar, and with a saving of expense. 4th. The strength of common mortars is considerably improved by the addition of an artificial puzzolana, but more so by the ad- dition of an hydraulic cement. 5th. Fine sand generally gives a stronger mortar than coarse sand. 6th. Lime slaked by sprinkling gave better results than lime slaked by drowning. A few experiments made on air-slaked lime were unfavorable to that mode of slaking. 7th. Both hydraulic and common mortar yielded better results when made with a small quantity of water than when made thin. 8th. Mortar made in the mortar-mill was found to be superior to that mixed in the usual way with a hoe. 9th. Fresh water gave better results than salt water. 295. STRENGTH OF CONCRETE AND BETON. From experiments made on concrete, prepared according to the most approved pro- cess in England, by Colonel Pasley, it appears that this material is very inferior in strength to good brick, and the weaker kinds of natural stones. From experiments made by Colonel Totten on beton, the fol- lowing conclusions are drawn : That beton made of a mortar composed of hydraulic cement, common lime, and sand, or of a mortar of hydraulic cement and sand, without lime, was the stronger as the quantity of sand was the smaller. But there may be 0.50 of sand, and 0.25 of com- mon lime, without sensible deterioration ; and as much as 1 .00 of sand, and 0.25 of lime, without great loss of strength. Beton made with just sufficient mortar to fill the void spaces between the fragments of stone was found to be less strong than that made with double this bulk of mortar. But Colonel Totten remarks, that this result is perhaps attributable to the difficulty of causing so small a quantity of mortar to penetrate the voids, and unite all the fragments perfectly, in experiments made on a small scale. The strongest beton was obtained by using quite small frag- ments of brick, and the weakest from small, rounded, stone gravel. A beton formed by pouring grout among fragments of stone, 01 brick, was inferior in strength to that made in the usual way with mortar. Comparing the strength of the batons on which the experi- ments were made, which were eight months old when tried, with 78 BUILDING MATERIALS. that of a sample of sound red sand stone of good quality, it ap pears that the strongest prisms of bcton were only half as strong as the sand-stone. 296. STRENGTH OF TIMBER. A wide range of experiments has been made on the resistance of timber to compression, ex- tension, and a transverse strain, presenting very variable results Among the most recent, and which command the greatest confi- dence from the ability of their authors, are those of Professor Barlow and Mr. Hodgkinson : the former on the resistance to extension and a transverse strain ; the latter on that to com- pression. 297. Resistance to Extension. The following Table exhibits the specific gravity, and the mean resistance per square inch of various kinds of timber, from the experiments of Prof. Barlow. DESCRIPTION OF TIMBER. Spec. grav. Mean strength of cohesion per square inch. Ash, (English) 0.76C 17000 Beech, do. ...... 0.7M) 11500 Box 1.000 20000 Deal, (Christiana) C.680 11000 Do. (Meniel 0.590 11000 Elm 0.540 5780 Fir, (New England) 0.550 12000 Do. (Riga) 0.750 12600 Do. (Mar Forest) 0.700 12000 Larch, (Scotch) ...... 0.540 7000 Locust ....... 0.950 20580 Mahogany ....... 0.637 8000 Norway spars ...... 0.580 12000 Oak, (English) . . . . | t ^ om 0.700 0.900 9000 15000 Do. (African) 0.980 14400 Do. (Adriatic) 0.990 14000 Do. (Canadian ...... 0.872 12000 Do. '(Dantzic) ...... 0.760 14500 Pear 0.646 9800 Poon 0.600 14000 Pine, (pitch) - . 0.660 10500 Do. (red) 0.660 10000 Teak 0.750 15000 298. But few direct experiments have been made upon the elongations of timber from a strain in the direction of the fibres From some made in France by MM. Minard and Desormes, it would appear that bars of oak having a sectional area of one square i 7 ich, will be elongated .001 176 of their length by a strain of onf con. STRENGTH OF MATERIALS. 299. Resistance to Compression. The following Table ex. hibits the results obtained by Mr. Hodgkinson from experiments on short cylinders of timber with flat ends. The diameter of each cylinder was one inch, and its height two inches. The re- sults, in the first column, are a mean from about three experiments on timber moderately dry, being such as is used for making models for castings ; those in the second column were obtained, in a like manner, from similar specimens, which were turned and kept dry in a warm place two months longer. A comparison of the results in the two columns, shows the effect of drying on the strength of timber ; wet timber not having half the strength of the same when dry. The circumstances of rupture were the same as already stated in the general observations under this head ; the height of the wedge which would slide off in tim- ber being about half the diameter, or thickness of the specimen crushed. Strength per square inch in Ibs. Alder . 6831 6960 Ash 8683 9363 Baywood ........ 7518 7518 Beech ........ 7733 19363 Birch, (American) 3297 11663 Do. (English) 3297 6402 Cedar 5674 5863 Crab 6499 7148 5748 6586 White deal 6781 7293 Elder 7451 9973 Elm 10331 Fir, (spruce) 6499 6819 Hornbeam ........ 4533 7289 Mahogany ........ 8198 8198 4231 5982 Do (English) 6484 10058 Do. (Dantzic, very dry) ..... 7731 Pine, (pitch) 6790 6790 Do. (yellow, full >>f turpentine) .... 5375 5445 Do. (red) . 5395 7518 Poplar ........ 3107 5124 Plum, (wet) ....... 3654 - Do. (dry) 8241 to 1049 Sycamore ........ 7082 - Teak 12101 Larch, (fallen two months) 3201 5568 Walnut ..... . 6063 7227 Willow 2898 6128 300. Resistance of Square Pillars. Mr. Hodgkinscn has 80 BUILDING MATERIALS. made a number of invaluable experiments on the strengtn of pillars of timber, and of columns of iron and steel, and from them has deduced formulae for calculating the pressure which they will support before breaking. The experiments on timber were made on pillars with flat ends. The following are the for- mulae from which their strength may be estimated. Calling the breaking weight in Ibs. W. " the side of the square base in inches d. " the length of the pillar in feet I. Then for long columns of oak, in which the side of the squai base is less than /^th the height of the column ; d* W =24542-3-. and for red deal, W= 17511^. For shorter pillars, where the ratio between their thickness and height is such that they still yield by bending, the strength is es- timated by the following formula : Calling the weight calculated from either of the preceding for- mulae, W. Calling the crushing weight, as estimated from the preceding Table, W. Calling the breaking weight in Ibs., W". Then the formula for the strength is WW' W" = In each of the preceding formulae d must be taken in inches, and I in feet. 301. Resistance to Transverse Strains. As timber, from the purposes to which it is applied, is for the most part exposed to a transverse strain, the far greater number of experiments have been made to ascertain the relations between the strain, the deflection caused by it, and the linear dimensions of the piece subjected to the strain. These relations have been made the subject of mathematical investigations, founded upon data derived from ex- periment, which will be given in the APPENDIX. The following Table exhibits the results of experiments made upon beams having a rectangular sectional area, which were laid horizontally upon supports at their ends, and subjected to a strain applied at the middle point between the supports, in a vertical direction. For a more convenient application of the formulae to the results of the experiments, the notation adopted in the preceding Art will be here given. STRENGTH OF MATERIALS. 81 Ca.l the transverse force necessary to break th 3 beam in Ibs., W " the distance between the supports in inches, I. " the horizontal breadth of the sectional area in inches, 6. " the vertical depth " d. " the deflection arising from a weight w in inches,/. Table of Experiments with the foregoing Notation. DISCRIPTION OF WOOD. ,^g^? Values 5? Values of b. Value of d. Value of Value of to. Value of W. Authors of ex. peri meats. ! Inche.. Inches. Inches. Inches. It... Ibs. Oak, (English) . . . .934 84 2 2 1.280 200 637 Prof. Barlow. Do. (Canadian) , . .872 84 2 2 1.080 225 673 " Pine, (American) . . I - 84 2 2 0.931 150 " Oak, (English] ... 30 1 1 0.5 137 Tredgold. White spruce, (Canadian) .465 24 1 1 0.5 180 285 " White pine, (American) . .455 85.2 2.75 5.55 0.177 777 5189 Lieut. Brown. Black spruce, do. . .490 85.2 2.75 5.55 0.177 892 5646 " Southern pine, do. . .872 85.2 2.75 5.54 0.177 1175 9237 302. Resistance to Detrusion. From the experiments of Prof. Barlow, it appears that the resistance offered by the lateral adhe- sion of the fibres of fir, to a force acting in a direction parallel to the fibres, may be estimated at 592 Ibs. per square inch. Mr. Tredgold gives the following as the results of experiments on the resistance offered by adhesion to a force applied perpen- dicularly to the fibres to tear them asunder. Oak . . 2316 Ibs. per square inch. Poplar . . 1782 " " Larch, 970 to 1700 " " 303. STRENGTH OF CAST IRON. The most recent experiments on the' strength of this material are those of Mr. Hodgkinson. Those, particularly, made by him on the subject of the strength of columns, and the most suitable form of cast-iron beams to sus- tain a transversal strain, have supplied the engineer and architect with the most valuable guide in adapting this material to the various purposes of structures. 304. Resistance to Extension. From a few experiments made by Mr. Rennie and Captain Brown, the tensile strength of cast iron varies from 7 to 9 tons per square inch. The experiments of Mr. Hodgkinson upon both hot and cold blast iron give the tensile strength from 6 to 9 tons per square inch. From some experiments made on American cast iron, under the direction of the Franklin Institute, the mean tensile strength is 20834 Ibs., or 9| tons per square inch. 305. Resistance to Compression. The general circumstances attending the rupture of this material by compression, drawn frorr 11 BUILDING MATERIALS. he experiments of Mr. Hodgkinson, have already been given The angle of the wedge resulting from the rupture is about 55. The mean crushing weight derived from experiments upon short cylinders of hot blast iron was 121,685 Ibs., or 54 tons 6 cwt. per square inch. That on short prisms of the same, with square bases, 100,738 Ibs., or 44 tons 19 j cwt. per square inch. That on short cylinders of cold blast iron was 1 25,403 Ibs., or 55 tons 19| cwt. per square inch. That on short prisms of the same, having other regular figures for their bases, was 100,631 Ibs., or 44 tons 18| cwt. per square inch. Mr. Hodgkinson remarks with respect to the forms of base differing from the circle : "In the other forms the difference of strength is but little ; and therefore we may perhaps admit that difference of form of section has no influence upon the power of a short prism to bear a crushing force." In remarking on the circumstances attending the rupture, Mr. Hodgkinson farther observes : " We may assume, therefore, without assignable error, that in the crushing of short iron prisms of various forms, longer than the wedge, the angle of fracture will be the same. This simple assumption, if admitted, would prove at once, not only in this material, but in others which break in the same manner, me proportionality of the crushing force in different forms to the area ; since the area of fracture would always be equal to the direct transverse area multiplied by a constant quan- tity dependent upon the material." Table exhibiting the Ratio of the Tensile to the Compressive Forces in Cast Iron, from Mr. Hodgkinson 's Experiments. DESCRIPTION OF METAL. Compressive force per square inch. Tensile force per square inch. Ratio. Devon iron, No. 3. Hot blast 145,435 21,907 6.638 1 Buffery iron, No. 1. Hot blast 86,397 13,434 6.431 1 Do. " Cold blast J3,385 17,466 5.346 1 Coed-Taleniron,No. 2. Hot blast 82,734 16,676 4.961 J Do. " Cold blast 81,770 18,855 4.337 1 Carron iron, No. 2. Hot blast 108,540 13,505 8.037 1 Do. " Cold blast 106,375 16,683 6.376 1 Canon iron, No. 3. Hot blast 133,440 17,755 7.515 i Do. " Cold blast 115,442 14,200 8.129 1 306. Resistance of Cylindrical Columns. The experiments under this head were made upon solid and hollow columns, both ends of which were either flat or rounded, fixed or loose, or one STRENGTH OF MATERIALS 83 end flat and the other rounded. In the case of columns with rounded ends, the pressure was applied in the direction of the axis of the column. The following extracts are made from Mr. Hodgkinson's paper on this subject, published in the Report of the British Association 0/1840. " 1st. In all long pillars of the same dimensions, the resistance to crushing by flexure is about three times greater when the ends of the pillars are flat, than when they are rounded. " 2d. The strength of a pillar, with one end rounded and the other flat, is the arithmetical mean between that of a pillar of the same dimensions with both ends round, and one with 'both ends flat. Thus, of three cylindrical pillars, all of the same length md diameter, the first having both its ends rounded, the second with one end rounded and one flat, and the third with both ends flat, the strengths are as 1, 2, 3, nearly. "3d. A long, uniform, cast-iron pillar, with its ends firmly fixed, whether by means of discs or otherwise, has the same power to resist breaking as a pillar of the same diameter, and half the length, with the ends rounded or turned so that the force \vould pass through the axis. " 4th. The experiments show that some additional strength is given to a pillar by enlarging its diameter in the middle part ; this increase does not, however, appear to be more than one seventh, or one eighth of the breaking weight. " 5th. The index of the power of the diameter to which the strength of long pillars with rounded ends is proportional, is 3.76 nearly, and 3.55 in those with flat ends, as appeared from the re- sults of a great number of experiments ; or the, strength of both may be taken as the 3.6 power of the diameter nearly. " 6th. In pillars of the same thickness, the strength is inversely proportional to the 1 .7 power of the length nearly. " Thus the strength of a solid pillar with rounded ends, the d?- 6 diameter of which is d, and the length I, is as -^-7." " The absolute strength of solid pillars, as appeared from the experiments, are nearly as below. In pillars with rounded ends, d?- 6 Strength in tons = 14.9 -^-y. In pillars with flat ends, d?- 6 Strength in tons = 44.16 -j^. L In hollow pillars nearly the same laws were found to obtain ; )ms, if D and d be the external and internal diameters of a pillai 84 BUILDING MATERIALS. whose length is I, the strength of a hollow cylinder of which the ends were moveable (as in the connecting rod of a steam-engine) Would be expressed by the formula below. jy.t _ ^3.6 Strength in tons = 13 - - . In hollow pillars, whose ends are flat, we had from experimen as before, Strength in tons = 44.3 - ^ -- The formulae above apply to all pillars whose length is not less than about thirty times the external diameter; for pillars shorter than this, it is necessary to have recourse to the ' for- mula,' given under the head of STRENGTH OF TIMBER, for short pillars of timber, substituting for W and W in that formula, the proper values applicable to cast iron." 307. Similar Pillars. " In similar pillars, or those whose length is to the diameter in a constant proportion, the strength is nearly as the square of the diameter, or of any other linear di- mension ; or, in other words, the strength is nearly as the area of the transverse section." " In hollow pillars, of greater diameter at one end than the other, or in the middle than at the ends, it was not found that any additional strength was obtained over that of cylindrical pillars." " The strength of a pillar, in the form of the connecting rod of a steam-engine," (that is, the transverse section presenting the figure of a cross -f ,) " was found to be very small, perhaps not half the strength that the same metal would have given if cast in the form of a uniform hollow cylinder." " A pillar irregularly fixed, so that the pressure would be in the direction of the diagonal, is reduced to one third of its strength. Pillars fixed at one end and moveable at the other, as in those flat at one end and rounded at the other, break at one third the length from the moveable end ; therefore, to economize the metal, they should be rendered stronger there than in other parts." 308. Long-continued Pressure on Pillars. " To determine the effect of a load lying constantly on a pillar, Mr. Fairbairn had, at the writer's suggestion, four pillars cast, all of the same length and diameter. The first was loaded with 4 cwt., the second with 7 cwt., the third with 10 cwt., and the fourth with 13 cwt. ; this last load was T 9 / r of what had previously broken a pillar of the same dimensions, when the weight was carefully laid on with- out loss of time. The pillar loaded with 1 3 cwt. bore the weight between five and six months, and then broke." 309. General Properties of Pillars. " In pillars of wrought STRENGTH OF MATERIALS. 85 iron, steel, and timber, the same laws, with respect to rounded and flat ends, were found to obtain, as had been shown to exist in cast iron." " Of rectangular pillars of timber, it was proved experimental y that the pillar of greatest strength of the same material is a square." 310. Comparative Strengths of Cast Iron, Wrought Iron, Sttel, and Timber. " It resulted from the experiments upon pillars of the same dimensions but of different materials, that if we call the strength of cast iron 1000, we shall have for wrought iron 1745, cast steel 2518, Dantzic oak 108.8, red deal 78.5." 311. Resistance to Transverse Strains. The following Tables and deductions are drawn from the experiments of Messrs. Hodg- kinson and Fairbairn, on hot and cold blast iron, as published in their Reports to the British Association in 1837. Table exhibiting the results of experiments by Mr. HodgJcinson on bars of hot blast iron 5 feet long, with a rectangular sec- tional area ; the bars resting horizontally on props 4 feet 6 inches apart ; the weight being applied at the middle of the bar. EXPERIMENT 1. EXPERIMENT 13. EXPERIMENT 14. Rectangular bar, 1.00 inch broad, Rectangular bar, 1.03 inches broad, Rectangular bar, 1.02 inches broad, 1.00 " deep. 3.00 " deep. 4.98 " deep. Weight of bar, 15 Ibs. 2 oz. Weight 78 Ibs. _e s e j; *!< s 1 e c I jt *3 .2 S c S | ~ . .r i Jj . O o 9 3.1 "S H C .2f If 5 i*~ c * J9 r' s 5 > I" 1 I~ i 16 .037 visible 1474 _ .001 5867 .127 _ 23 .052 increased 1605 .130 .003 6798 .153 .01 30 .070 .oon 1866 .156 .006 7730 .177 - 56 .132 .002 2126 .185 .010 8661 .207 - 112 .271 .008 2388 .212 .012 9593 .235 - 2^4 .588 .037 '2649 .243 .017 10524 .275 .03 336 .940 .087 2910 .272 .022 1J387 broke - 448 1.360 .181 3172 .307 .030 - - - 469 broke - 3433 .340 .038 - - - _ _ _ 3694 .378 .050 - - - - - 3956 broke - - - - Ultimate deflection Ultimate deflection Ultimate deflection 1.444 inches. .416 inch. .299 inch. BUILDING MATERIALS. Results of experiments, by the same, on the transverse strength of cold blast iron ; length of bars, and distance between the pointi of support the same as in the preceding Table. EXPERIMENT 1 EXPERIMENT 12. EXPERIMENT 13. Rectangular bar, 1.025 inch deep, 1.002 " broad. Weight, 15 Ibs. 6 oz. Rectangular bar, 3.00 inches deep, 1.02 " broad. Weight, 46 Ibs. 8 oz. Rectangular bar, 4.98 inches deep, 1.03 " broad Weight, 78 Ibs. Weight in Ibs. Deflection in inches. -1 tJ2 Weight in Ibs. Defluction in inches. a i r ~ Weight in Ibs. Deflection in inches. si \ o 16 .033 visible 1082 .091 .003 4936 .110 .013 30 .062 increased 1343 .111 .006 5867 .130 _ 56 .120 .002 1605 .138 .008 6798 .153 .020 112 .240 .007 1886 .164 .010 7730 .179 .025 168 .370 .014 2126 .190 .012 8662 .195 _ 224 .510 .028 2388 .220 .015 9593 .219 .034 280 .649 .041 2649 .250 .019 10525 .250 .042 336 .798 ,061 2910 .281 .026 10588 broke _ 392 .953 .084 3172 .310 .031 _ _ _ 448 1.120 .120 3433 .345 .037 - - - 504 1.310 .170 3694 .378 .046 - - _ 514 it bore - 3825 broke - - - - 518 broke - - - - - - Ultimate deflection 1.36 inch. Ultimate deflection 0.395 inch. Ultimate deflection 0.252. 312. The following remarks are extracted from the same Re- port : "I had remarked, in some of the experiments, that the elasticity of the bars was injured much earlier than is generally conceived ; and that instead of its remaining perfect till one third, or upwards, of the breaking weight was laid on, as is generally admitted by writers, it was evident that ilh, or less, produced in some cases a considerable set or defect ot elasticity ; and judging from its slow increase afterwards, I was persuaded that it had not come on by a sudden change, but had existed, though in a less degree, from a very early period." " From what has been stated above, deduced from experiments made with great care, it is evident that the maxim of loading bodies within the elastic limit, has no foundation in nature ; but it will be considered as a compensating fact, that materials will bear for an indefinite period a much greater load than has hitherto oeen conceived." 313. "We may admit," from the mean results, "tint the strength of rectangular bars is as the square of the depth." STRENGTH OF MATERIALS. 87 314. Effects of time upon the deflections caused by. a perma- nent load on the middle of horizontal bars. The following Table exhibits the results of Mr. Fairbairn's ex- periments on this point. The experiments were made on bars 5 feet long, 1 .05 inch deep ; the one of cold blast iron, 1 .03 inch nroad ; the other of hot blast, 1.01 broad; distance between the points of support 4 feet 6 inches. The constant weight sus- pended at the centre o r the bars was 280 Ibs. This weight re mained on from March llth, 1837, to June 23d, 1838. Cold Wast iron. Deflection in inches. Date of observation. Temp. Hot blast iron. Deflection in inches. Ratio of increase of deflections. .930 .963 March llth, 1837, June 23d, 1838, 78 1.064 1.107 .033 Increase, - .043 1000 : 1303 315. Mr. Fairbairn in his Report remarks on the above and like results : " The hot blast bar in these experiments being more deflected than the cold blast, indicates that the particles are more extended and compressed in the former iron, with the same weight, than in the latter. This excess of deflection may in some degree account for the rapidity of increase, which it will be observe^ is considerably greater in the hot than in the cold blast bar." " It appears from the present state of the bars, (which indicate a slow but progressive increase in the deflections,) that we must at some period arrive at a point beyond their bearing powers ; or otherwise to that position which indicates a correct adjustment of the particles in equilibrium with the load. Which of the two points we have in this instance attained is difficult to determine : sufficient data, however, are adduced to show that the weights are considerably beyond the elastic limit, and that cast iron will support loads to an extent beyond what has usually been consid- ered safe, or beyond that point where a permanent set takes place." 316. Effects of Temperature. Mr. Fairbairn remarks : " The infusion of heat into a metallic substance may render it more ductile, and probably less rigid in its nature ; and I apprehend it will be found weaker, and less secure under the effects of heavy strain. This is observable to a considerable extent in the experi- ments" on transverse strength "ranging from 26 up to 190 Fahr." * The cold blast at 20 and 190, is in strength as 874 : 743, The hot blast at 26" and 190, is in strength as 811 : 731, oeuig a diminution ir strength as 100 : 85 for the cold blast, arid 100 to 90 for the hot blast, or 15 per cent, loss of strength in the cold blast, and 10 per cent, in the hot blast." 4< A number of the experiments made on No. 3 iron have giver 68 BUILDING MATERIALS. extraordinary and not unfrequently unexpected results. Ge^ier ally speaking, it is an iron of an irregular character, and presents less uniformity in its texture than either the first or second quali- ties ; in other respects it is more retentive, and is often used for giving strength and tenacity to the finer metals." " At 212 we have in the No. 3 a much greater weight sus- tained than what is indicated by the No. 2 at 190 ; and at 600 there appears in both hot and cold blast the anomaly of increased strength as the temperature is advanced from boiling water to melted lead, arising from the greater strength of the No. 3 iron. 5 * 317. Influence of Form in Cast Iron upon the Transverse Strength of Beams. Upon no point, respecting the strength of cast iron, have the experiments of Mr. Hodgkinson led to more valuable results to the engineer and architect, than upon the one under this head. The following Tables give the results of experi- ments on bars of a uniform cross section, (thus "T">) cast from hot and cold blast iron. The bars were 7 feet long, and placed, for breaking, on supports 6 feet 6 inches asunder. Table exhibiting the results of experiments on bars of hot b last iron of the form of cross section as above. EXPERIMENT 4. EXPERIMENT 5. Bar broken as shown Bar broken as shown with the rib downward. with the rib upward. o _o o s o J3 . y j . u-g - J " 1 II & 1 IM 1 .S 7 .015 visible 7 _ not visible 14 .032 .001 14 .025 visible 21 .046 .002 21 .045 .002 28 .064 .004 28 .065 .003 56 .130 .005 56 .134 .005 112 .273 .020 112 .270 .015 168 .444 .035 224 .580 .058 224 .618 .058 336 .895 .101 280 .813 .093 448 1.224 .155 336 1.030 .130 560 1.585 .235 364 broke - 672 I 1.985 .330 - - - 784 2.410 .490 - - - 896 3.450 .722 - - - 1008 4.140 1.040 1064 - _ - - - 1120 broke - Ultimate deflection 1.138 inches. Fracture caused by a wedge 2.92 inches long and 1.05 deep, of this form ,. flying out. ^^ >^ Ultimate deflection 4.830. STRENGTH OF MATERIALS. 8S Note. The annexed diagram shows the A form of the uniform cross section of the bars. The linear dimensions of the cross section in the two experiments were as fol- lows : Length of parallelogram A B 5 inches"! Depth " AB 0.30 " 1 Total depth of bar . CD 1.55" f Breadth of rib . . . DE 0.36 " J . 4 " 5 inches'] 0.30 " I 1.56 " ( 0.365" J Table exhibiting results of experiments on bars of cold blast iron 5 feet long, of the same form of cross section as in preceding Table. EXPERIMENT 4. EXPERIMENT 5. Bar broken with rib Bar broken with rib downward. upward. Weight In Ibs. o II 1 0.10 " English ~ ~* m 35.81 Te.ford. Table exhibiting the Mean Strength of Boiler Iron, per square inch in Ibs., cut from plates with shears. Process of manufacture. Rough edge bar. Edges filed uni- formly. Notches filed into bar on each edge. Piled iron .... 53,045 56,081 63,266 Hammered plate 47,506 55,584 58,447 Puddled iron 52,341 51,039 62,420 It is remarked in the Report of the Sub-committee, " that the inherent irregularities of the metal, even in the best specimen*, STRENGTH OF MATERIALS. 9? wi ether of rolled or hammered iron, seldom fall short of 10 or 15 per cent, of the mean strength." 3'rom the same series of experiments, it appears that th fe'-rtngir of rolled plate lengthwise is about 6 per cent, greater than r*. strength crosswise. In the Tenth Report of the British Association in 1840, Mr. Fairbairn has given the results of experiments on plate iron by Mr. Hodgkinson, from which it appears that the mean strength jf iron plates lengthwise is 22.52 tons. Crosswise " 23.04 " Single-riveted plates " 18,590 Ibs. Double-riveted plates " 22,258 "' Representing the strength of the plate by 100. The double-riveted plates will be . . 70. The single " " 56. 325. Professor Barlow, in his Report to the Directors of the London and Birmingham Railroad, (Journal of Franklin Insti- tute, July, 1835,) states, as the results of his experiments, that a car of malleable iron one inch square is elongated the ipooth part of its length by a strain of one ton ; that good iron is elongated the txth part by a strain of 10 tons, and is injured by this strain, while indifferent, or bad iron is injured by a strain of 8 tons. From the Report made to the Franklin Institute, it appears that the first set, or permanent elongation may take place under very different strains, varying with the character of the material. The most ductile iron yields permanently to a low degree of strain. The extremes by which a permanent set is given vary between the 0.416 and 0.872 of the ultimate strength; the mean of thir- teen comparisons being 0.641. 326. Resistance to Compression. But few experiments have been published on the resistance of this material to compression. Rondelet states that it commences to yield under a pressure of about 70,800 Ibs. per square inch, and that when the altitude of the specimen tried is greater than three times the diameter of the base it yields by bending. Mr. Hodgkinson states that the circumstances of its rupture from crushing indicate a law simi- lar to what obtains in cast iron. 327. Resistance to a Transverse Strain. The following Ta- bles exhibit the circumstances of deflection from a transverse strain on bars laid on horizontal supports ; the weight being ap- plied at the middle of the bar. The Table I. gives the results on bars 2 inches square, laid on supports 33 inches asunder; Table II. the results on bars 2 inches deep, 1 .9 in. broad, bearing as in Table I. 13 98 BUILDING MATERIALS. TABLE I. TABLE II. Weight In tons. Deflections in inches for each half ton. Weight in ton*. Deflections in inches for eae. half ton .75 .020 .250 _ 1.00 .020 .50 .016 1.50 .020 1.00 .022 2.00 .030 1.50 .020 2.50 .020 2.00 .026 3.00 Set 2.25 .018 _ _ 2.50 .026 - - 2.75 .038 - 3.00 .092 The above experiments were made by Professor Barlow, an: published in his Report already cited. He remarks on the re- sults in Table II., that the elasticity was injured by 2.50 I us and destroyed by 3.00 tons. 328. Trials were made to ascertain mechanically the position of the neutral axis on the cross section. Professor Barlow re- marks on these trials, that " the measurements obtained in these experiments being tension 1.6, compression 0.4, giving exactly the ratio of 1 to 4 in rectangular bars. These results seem the most positive of any hitherto obtained ; still there can be little doubt this ratio varies in iron of different qualities ; but looking to the preceding experiments, it is probably always from 1 to 3, to 1 to ft." 329. Effects of time on the elongation of Wrought Iron from a constant strain of extension. M. Vicat has given, in the An- nales de Chimie et de Physique, vol. 54, some experiments on this point, made on iron wires which had not been annealed, by subjecting four wires, respectively, to strains amounting to the i, the i, the , and f of their tensile strength, during a period of 33 months. From the results of these experiments it appears, that each wire, immediately upon the application of the strain to which it was subjected, received a certain amount of extension. The first wire, which was subjected to a strain of ith its ten- sile strength, was found at the end of the time in question not to have acquired any increase of extension. The second, submitted to d its tensile strength, was elongated 0.027 in. per foot, independently of the elongation it at first re- ceived. The third, subjected under like circumstances to a strain oi th its tensile strength, was elongated 0.40 in. per foot, besides its first elongation. STRENGTH OF MATERIALS. 99 The fourtn, similarly subjected to fths the tensile st/ength, was elongated 0.061, bi .sides its first elongation. From observations made during the experiments, it was found '.hat, reckoning from the time when the first elongations took place, .he rapidity of the subsequent elongations was nearly proportional to the times ; and that the elongations from strains greater than I th the tensile strength are, after equal times, nearly proportionav to the strains. 330. M. Vicat remarks ir substance upon the results of these experiments, that iron wire, when not annealed, commences to exhibit a permanent set when subjected to a strain between the -{ and i of its tensile strength, and that therefore it is rendered probable that the wire ropes of a suspension bridge, which should be subjected to a like strain, would, when the vibratory motion to which such structures are liable is considered, yield constantly from year to year, until they entirely gave way. M. V r icat farther remarks, in substance, that the measure of the resistance offered by materials to strains exerted only some minutes, or hours, is entirely relative to the duration of the experiments. To ascertain the absolute measure of this resistance, which should serve as a guide to the engineer, the materials ought to be sub- jected for some months to strains ; while observations should be made during this period, with accurate instruments, upon the manner in which they yield under these strains. 331. Effects of Temperature on the Tensile Strength of Wrought Iron The experiments made under the direction of the Franklin Institute, already noticed, have developed some very curious facts of an anomalous character, with respect to the effect of an increase of temperature upon the strength of wrought iron. Jt was found that at high degrees of heat the tensile strength was greater up to a certain point than was exhibited by the same iron at ordinary temperatures. The Sub-committee in their Report remark : " This circumstance was noted at 212, 392, and 572, rising by steps of 1 80 each from 32, at which last, point some trials have been made in melting ice. At the highest of these points, however, it was perceived that some specimens of the metal exhibited but little, if any, superiority of strength over that which they had possessed when cold, while others allowed of being heated nearly to the boiling point of mercury, before they manifested any decided indications of a weakening effect from in- crease of temperature." " It hence became apparent that any law, taking for a basis the strength of iron in its ordinary condition, and at common temperatures, must be liable to great uncertainty, in regard to its application to different specimens of the rnetal. It was evident .hat the anomaly above referred to must be only apparent, and 100 BUIiDING MATERIALS. that the tenacity actually exhibited at 572, as well as that which prevails while the iron is in the state in which it was left by forging, or rolling, must be below its maximum tenacity." From the experiments made upon several bars of the same iron, it appeared that their "maximum tenacity was 15 17 pel cent, greater than their mean strength when tried cold." Calculating the maximum tenacity in other experiments from this standard, the Sub-committee have drawn up the following Table exhibiting the relations between diminutions from the max- imum tenacity and the degrees of temperature by which they are caused, from which the curve representing the law of these rela- tions can be constructed. TABLE, No. of the com- parison. Observed tem- peratures. Observed tem- peratures 80". Observed dimi- nution of te- nacity. Power of the temperature which represents the diminution of tenaciiy at each point. 1 520 440 .0738 2.25 2 570 490 .0869 2.17 3 596 516 .0899 2.38 4 662 582 .1155 2.67 5 770 690 .1627 2.85 6 824 744 .2010 2.94 7 932 852 .3324 2.97 8 1030 950 .4478 2.92 9 1111 1031 .5514 2.63 10 1155 1075 .6000 2.60 11 1237 1157 .6622 2.41 12 1317 1237 .7001 2.14 Mean 2.58 The Sub-committee remark on ths construction of the above Table "As some of the experiment which furnished the stand- ards of comparison for strength at ordinary temperatures, were made at 80, and as at this point small variations with respect to heal appear to affect but very slightly the tenacity of iron, it was conceived that for practical purposes, at least, the calculations might be commenced from that point." " It will be found that with the exception of a slight anomaly between 520 and 570, amounting to .08, the numbers express- .ng the ratios between the elevations of temperature, and the diminutions of tenacity, constantly increase until we reach 932, at which it is 2.97, and that from this point the ratio of diminu- tion decreases to the limits of our range of trials, 1317, where it is 2.14. It will also be observed, that thf diminution of tenacity at 932, where the law changes from an ir ;reasing to a decreasing STRENGTH OF MATERIALS. 101 rate of diminution, is almost precisely one third of the total, or maximum strength of the iron at ordinary temperatures." From the mean of all the rates in the above Table the follow- ing rule is deduced : " the thirteenth power of the temperature above 80 is proportionate to the fifth power of the diminution froir. the, maximum tenacity" Professor W. R. Johnson, a member of the sub-committee, has since applied the results developed in the preceding experi- ments to practical purposes, in increasing the tenacity of wrought iron by subjecting it to tension under a high degree of tempera- ture, before using it for purposes in which it will have to undergo considerable strains, as, for example, in chain cables, &c. This subject was brought by Prof. Johnson before the Board of Navy Commissioners in 1841 ; subsequently, experiments were made by him under direction of the Navy Department, the results of which, as exhibited in the following Table, were published in the Senate Public Documents, ( 1 ) 28th Congress, 2d Session, p. 641. Dec. 3, 1844. Table of the effects of Thermo-tension on the Tenacity and Elongation of Wrought Iron. Strength af- Gain of < KIND OF IRON. Strength ofcolQ. ter treating with Ther- Gain of length. strength by the treat- Total gain of value. mo-tension. ment. Tredegar, No. 1, round iron 60 71.4 6.51 19.00 25.51 Do. do. 60 72.0 6.51 20.00 26.51 Tredegar, square bar iron 60 67.2 6.77 12.00 18.77 Tredegar, No. 3, round iron 58 68.4 5.263 17.93 23.19 Salisbury, round, (Ames') 105.87 121.0 3.73 14.29 18.02 Mean, . 5.75 16.64 22.40 Prof. Johnson in his letter remarks : " It will be observed that in these experiments the temperature has, with a view to economy of time, been limited to 400, whereas the best effects of the pro- cess have generally been obtained heretofore when the heat has been as high as 575." 332. Resistance of Iron Wire to Impact. The following Ta- ble of experiments gives the results obtaine ; by Mr. Hodgkinson, by suspending an iron ball at the end of a w_re, (diameter No. 17,) nnd letting another iron ball impinge upon it from different alti- tudes. The suspended and impinging balls had holes drilled through them, through which the wire passed. A disc of lead was placed on the suspended ball to receive the blow, and lesser the recoil froii elasticity. 102 BUILDING MATERIALS. TABLE. Lenrth cf wire. Weight of linking ball. Weight of suspended ball and lead. Htght fallen through by striking Wire broke with ball fall- ing through. Rcflarka, ft. in. Ibs. oz. Ibs. oz. 25 5 14 9 2, 24, 3, 34, 4, (repeated) 24, 3,3 i, 4, 44, ?S 4 * No lead. 24 6 10 1 7, 74 j. j (repeated with fresh wire.) 6, 64 I 7 The wire usually 44 1, 2, 3, 4, 5, 6, 6A, 7, 74 broke near the point - - 80 8 89 125 6, 64, 7, 7.4, 8, 8 \, 9, 8, 8,, 9, 94, 10, 104, 8, 8l, 9, 94, 10, 94 11 104 1 of impact, and it Vwas adjusted to it* I length, if frh wire were not used by a 40 10 1 3, 4 inches, 5 inches reserve at the top. 80 8 2, 3, 4, 5, 6 inches, 7 d*. J 89 4, 5 inches, 6 do. Broke one inch 24 8 85 44 2 inches, 3 do. from top. The following observations are made by Mr. Hodgkinson : " To ascertain the strength and extensibility of this wire, it was broken in a very careful experiment with 252| Ibs., suspended at its lower end, and laid gradually on. And to obtain the incre- ment of a portion of the wire (length 24 ft. 8 in.) when loaded by a certain weight, it had 139 Ibs. hung at the bottom, and when 89 Ibs. were taken off the load, the wire decreased in length .39 inch. " Should it be suggested that the wire by being frequently im- pinged upon would perhaps be much weakened, the author would beg to refer to a paper of his on Chain Bridges, Manchester Me- moirs, 2d series, vol. 5, where it is shown that an iron wire broken by pressure several times in succession is very little weakened, and will nearly bear the same weight as at first." " The first of the preceding experiments on wires are the only ones from which the maximum can, with any approach to cer- tainty, be inferred ; and we see from them that the wire resisted the impulsion with the greatest effect when it was loaded at bot- tom with a weight, which, added to that of the striking body, was a little more than one third of the weight that would break the wire by pressure." " From these experiments generally, it appears that the wire was weak to bear a blow when lightly loaded." " These last experiments and remarks, and some of the prece- ding ones," (on horizontal impact,) " show clearly the benefit of giving considerable weight to elast' t structures subject to impact and vibration." 333. Resistance to Torsion of Wrought and Cast Iron. The following Table exhibits the results of experiments made by Mr. Dunlop, at Glasgow, on round bars of wrought iron. The twisting weights were applied with an arm of lever 14 feet 2 inches. STRENGTH OF MATERIALS. 103 Length of bars in inches. Diameter of bars in inches. Weight in Ibs. pro- ducing rapture. 2? 21 3 2 21 24 250 384 408 3 4 5 2? 31 700 1170 1240 5 s| 1662 5 4 1938 6 41 2158 Table of eocperiments made by Mr. G. Rennie upon Cast and Wrought Iron. Weight applied at an arm of lever of 2 feet. MATERIAL. Length of blocks iu inches. Size of sectional area. Mean break- ing weight in Ibs. Iron cast horizontally .... i Ibs. oz. 9 15 vertically .... i 10 10 horizontally .... 1 i 7 3 u i 8 1 (( 1 * 8 8 vertically .... j> * 10 1 u 3 i 8 9 M 1 i 8 5 u 6 i 9 12 horizontally .... i 93 12 " ..... I 74 u 10 52 Wrought iron, (English) .... i 10 2 '' (Swedish) .... * 9 8 334. STRENGTH OF COPPER. The various uses to which cop- per is applied in constructions, render a knowledge of its resist- ance under various circumstances a matter of great interest to the engineer. Resistance to Extension. The resistance of cast copper on the square inch, from the experiments of Mr. G. Rennie, is 8.51 tons, that of wrought copper reduced per hammer at 15.08 tons. Copper wire is stated to bear 27.30 tons on the square inch. From the experiments made under the direction of the Franklin Institute, already cited, the mean strength of rolled sheet copper is stated at 14.35 tons per square inch. Resistance to Compression. Mr. Rennie's experiments on cubes of one fourth of an inch on the edge, give for the crushing 104 BUILDING MATERIALS. weight of cube of cast copper 7318 Ibs., and of wrought coppei 6440 Ibs. 335. Effects of Temperature on Tensile Strength. The ex- periments already cited of the Franklin Institute, show that the difference in strength at the lower temperatures, as between 60 and 90, is scarcely greater than what arises from irregularities in the structure of the metal at ordinary temperatures. At 550 Fahr. copper loses one fourth of its tenacity at ordinary tempera- tures, at 817 precisely one half, and at 1000 two thirds. Representing the results of experiments by a curve of which the ordinates represent the temperatures above 32, and the ab- scissas the diminutions of tenacity arising from increase of tern perature, the relations between the two will be thus expressed : the squares of the diminutions are as the cubes of the tempera- tures. 336. STRENGTH OF OTHER METALS. Mr. Rennie states the tenacity of cast tin at 2.11 tons per square inch ; and the resist- ance to compression of a small cube of of an inch on an edge at 966 Ibs. In the same experiments, the tenacity of cast lead is stated at 0.81 tons per square inch ; and the resistance of a small cube of same size as in preceding paragraph at 483 Ibs. In the same experiments, the tenacity of hard gun-metal is stated at 16.23 tons ; that of fine yellow brass at 8.01 tons. The resistance to compression of a cube of brass the same as before- mentioned, is stated at 10304 Ibs. 337. Linear Dilatation of Metals by Heat. The following Table is taken from results of experiments on the dilatation of solids, by Professor Daniell, published in the Philosophical Transactions, 1831. Table of Dimensions which a bar takes whose length at 62 is 1.0000. Iron, (wrought) Iron, (cast) Zinc At 212, (150.) At 662, (600.) At point of fusion. 1.000984 1.000893 1.002480 1.004483 1.003943 1.008527 1.018378* 1.016389 1.012621 Copper Lead 1.001430 1.002323 1.006347 1.024376 1.009072 Tin . 1.001472 1.003798 Brass, (zinc ) Bronze, (tin |) Pewter, (tin ) 1.001787 1.001541 1.001696 1.007207 1.007053 1.021841 1.016336 1.003776 * \tfusingpointofcastiron STRENGTH OF MATERIALS. 105 338. Adhesion of Iron Spikes to Timber. The following Tables and results are taken from an article, by Professor Walter R. Johnson, published in the Journal of the Franklin Institute, vol. 19, 1837, giving the details of experiments made by him on spikes of various forms driven into different kinds of timber. 339. The first series of experiments was made with Burden's plain square spike, the flanched, grooved, and swell spike, and the grooved and swelled spike. The timber was seasoned Jersey yellow pine, and seasoned white oak. From these experiments it results, that the grooved and swelled form is about 5 per cent, less advantageous than the plain, in yel- low pine, and about 18 per cent, superior to the plain in oak. The advantage of seasoned oak over the seasoned pine, for re- taining plain spikes, is as 1 to 1 .9, and for grooved spikes as 1 to 2.37. 340. The second series of experiments, in which the timber was soaked in water after the spikes were driven, gave the fol- lowing results. For swelled and grooved spikes, the order of retentiveness was, 1 locust ; 2 white oak ; 3 hemlock ; 4 unseasoned chesnut ; 5 yellow pine. For grooved spike without swell, the like order is 1 unsea- soned chesnut ; 2 yellow pine ; 3 hemlock. The swelled and grooved spike was, in all cases, found to be inferior to the same spike with the swell filed off. 341. The third series of experiments gave the following results. Thoroughly seasoned oak is twice, and thoroughly seasoned locust 2f times as retentive as unseasoned chesnut. The forces required to extract spikes are more nearly propor tional to the breadths than to either the thickness or the weights of the spikes. And, in some cases, a diminution of thickness with the same breadth of spike afforded a gain in retentiveness. " In the softer and more spongy kinds of wood the fibres, in- stead of being forced back longitudinally and condensed upon themselves, are, by driving a thick, and especially a rather ob cusely-pointed spike, folded in masses backward and downward so as to leave, in certain parts, the faces of the grain of the timber in contact with the surface of the metal." " Hence it appears to be necessary, in order to obtain the greatest, effect, that the fibres of the wood should press the faces as nearly as possible in their longitudinal direction, and with equal intensities throughout the whole length of the spike." The following is the order of superiority of the spikes from thai of the ratio of their weights and extracting forces respeo lively. 14 106 BUILDING MATERIALS. 1. Narrow flat . . . 7.049 ratio of weight to exl racting forca 2. Wide flat ... 5.712 " 3. Grooved but not swelled 5.662 4. Grooved and not notched 5.300 5. Grooved and swelled . 4.624 6. Burden's patent . . 4.509 7. Square hammered . . 4.129 8. Plain cylindrical . . 3.200 " " All the experiments prove that when a spike is once started, the force required for its final extraction is much less than thai which produced the first movement." " When a bar of iron is spiked upon wood, if the spike be driven until the bar compresses the wood to a great degree, the recoil of the latter may become so great as to start back the spike for a short distance after the last blow has been given." 342. From the fourth series of experiments it appears, that the spike tapering gradually towards the cutting edge, gives better results than those with more obtuse ends. That beyond a certain limit the ratio of the weight of the spike to the extracting force begins to diminish ; " showing that it would be more economical to increase the number rather than the length of the spikes for producing a given effect." " That the absolute retaining power of unseasoned chesnut on square or flat spikes of from two to four inches in length, is a little more than 800 Ibs. for every squrre inch of their two faces which condense longitudinally the fibres of the timber." MASONRY. 107 MASONRY. 343. MASONRY is the art of raising structures, in stone, brick, and mortar. 344. Masonry is classified either from the nature of the mate- .rial, as stone masonry, brick masonry, and mixed, or that which is composed of stone and brick ; or from the manner in which the material is prepared, as cut stone or ashlar masonry, rubble stone or rough masonry, and hammered stone masonry; or, finally, from the form of the material, as regular masonry, and irregular masonry. 345. Cut Stone. Masonry of cut stone, when carefully made, is stronger and more solid than that of any other class ; but, owing to the labor required in dressing, or preparing the stone, it is also the most expensive. It is, therefore, mostly restricted to those works where a certain architectural effect is to be produced by the regularity of the masses, or where great strength is indispen- sable. 346. Before explaining the means to be used to obtain the greatest strength in cut stone, it will be necessary to give a few definitions to render the subject clearer. In a wall of masonry, the term face is usually applied to the front of the wall, and the term back to the inside ; the stone which forms the front, is termed the facing ; that of the back, the backing ; and the interior, ihejilling. If the front, or back of the wall, has a uniform slope from the top to the bottom, this slope is termed the batter, or batir. The term course is applied to each horizontal layer of stone in the wall : if the stones of each layer are of equal thickness throughout, it is termed regular coursing ; if the thicknesses are unequal, the term random, or irregular coursing, is applied. The divisions between the stones, in the courses, are termed the joints ; the upper surface of the stones of each course is also, sometimes, termed the bed, or build. The arrangement of the different stones of each course, or of contiguous courses, is termed the bond. 347. The strength of a mass of cut stone masonry will depend on the size of the blocks in each course ; on the accuracy of the dressing ; and on the bond used. 348. The size of the blocks varies with the kind of stone, and the nature of the quarry. From some quarries the stone may be obtained of any required dimensions ; others, owing to some pe- culiarity in the formation of the stone, only furnish blocks of small 1 08 MASONRY. size. Again, tie strength of some stones is so great as to admit of their being used in blocks of any size, without danger to the stability of the structure, arising from their breaking ; others can only be used with safety, when the length, breadth, and thickness of the block bear certain relations to each other. No fixed rule can be laid down on this point : that usually followed by builders, is to make, with ordinary stone, the breadth at least equ?l to the thickness, and seldom greater than twice this dimension, and to limit the length to within three times the thickness. When the breadth or the length is considerable, in comparison with the thickness, there is danger that the block may break, if any un- equal settling, or unequal pressure should take place. As to the absolute dimensions, the thickness is generally not less than one foot, nor greater than two ; stones of this thickness, with the rel- ative dimensions just laid down, will weigh from 1000 to 8000 pounds, allowing, on an average, 160 pounds to the cubic foot. With these dimensions, therefore, the weight of each block will require a very considerable power, both of machinery and men, to se*t it on its bed. 349. For the coping and top courses of a wall, the same ob- jections do not apply to excess in length : but this excess may, on the contrary, prove favorable ; because the number of top joints being thus diminished, the mass beneath the coping will be better protected, being exposed only at the joints, which cannot be made water-tight, owing to the mortar being crushed, by the expansion of the blocks in warm weather, and, when they contract, being washed out by the rain. 350. The closeness with which the blocks fit is solely depen- dent on the accuracy with which the surfaces in contact, are wrought or dressed ; if this part of the work is done in a slovenly manner, the mass will not only present open joints from any in- equality in the settling ; but, from the courses not fitting accurately on their beds, the blocks will be liable to crack from the unequal pressure on the different points of the block. 351. The surfaces of one set of joints should, as a prime con- dition, be perpendicular to the direction of the pressure : by this arrangement, there will be no tendency in any of the blocks to slip. In a vertical wall, for example, the pressure being down- ward, the surfaces of one set of joints, which are the beds, must be horizontal. The surfaces 'of the other set must be perpen- dicular to these, and, at the same lime, perpendicular to the face, or to the back of the wall, according to the position of the stones in the mass ; two essential points will thus be attained ; the an- gles of the blocks, at the top and bottom of the course, and at the face or back, will be right angles, and the block will therefore be as strong as the nature of the stone will admit. The principles MASONRY. 109 here applied to a vertical wall, are applicable in all caseh what- ever may be the direction of the pressure and the form of the ex- terior surfaces, whether pane or curved. 352. A modification of this principle, however, may in some cases be requisite, arising from the strength of the stone. It is laid down as a rule, drawn from the experience of builders, that no stone work with angles less than 60 will offer sufficient strength and durability to resist accidents, and the effects of the weather. If, therefore, the batter of a wall should be greater than 60, which is about 7 perpendicular to 4 base, the horizontal joints (Fig. 6) must not be carried out in the same plane, to the Fig. 6 Represents the arrangement of stone with abutting, or elbow joints for very inclined sur- faces. A, face of the block. c, elbow joint. B, buttress block, termed a newell stone. face or back, but be broken off at right angles to it, so as to form a small abutting joint of about 4 inches in thickness. As the batter of walls is seldom so great as this, except in some cases of sustaining walls for the side slopes of earthen embankments, this modification in the joints will not often occur; for, in a greater batter, it will generally be more economical, and the construction will be stronger, to place the stones of the exterior in offsets, the exterior stone of one course, being placed within the exterior one of the course below it, so as to give the required general direction of the batter. The arrangement with offsets has the farther advantage in its favor of not allowing the rain water to lodge in the joint, if the offset be slightly bevelled off. 353. Workmen, unless narrowly watched, seldom take the pains necessary to dress the beds and joints accurately ; on the con- trary, to obtain what are termed close joints, they dress the ioints Fig 7 Represents a section of a wall in which th face is of cut stone, with the tails of the block* thinned off, and the backing of rubble A, section of face block. 3, rubble backing. with accuracy a few inches only from the outward surface, and then chip away the stone towards the back, or tail, (Fig. 7,) so 110 MASONRY that, when the blocV. is set, it will be in contact with the adjacent stones, only through nit this very small extent of bearing surface. This practice is objectionable under every point of view ; for, in the first place, it gives an extent of bearing surface, which, being generally inadequate to resist the pressure thrown on it, causes the block to splinter off at the joint ; and in the second place, to give the block its proper set, it has to be propped be- neath by small bits of stone, or wooden wedges, an operation termed pinning-up, or under-pinning, and these props, causing the pressure on the block to be thrown on a few points of the lower surface, instead of being equally diffused over it, expose the stone to crack. 354. When the facing is of cut stone, and the backing of rub- ble, the method of thinning off the block may be allowed for the purpose of forming a better bond between the rubble and ashlar ; but, even in this case, the block should be dressed true on each ioint, to at least one foot back from the face. If there exists any cause, which would give a tendency to an outward thrust from the back, then, instead of thinning off all the blocks towards the tail, it will be preferable to leave the tails of some thicker than the parts which are dressed. 355. Various methods are used by builders for the bond of cut stone. The system, termed headers and stretchers, in which the vertical joints of the blocks of each course alternate with the ver- tical joints of the courses above and below it, or as it is termed breakpoints with them, is the most simple, and offers, in most cases, all requisite solidity. In this system, (Fig. 8,) the blocks of each course are laid alternately with their greatest and least dimensions to the face of the wall ; those which present the longest dimen- A B n rn i Fig. 8 Represents an elevation A, end view B, and plan C, of a wall arranged as head- ers and stretchers. a, stretchers. 6, headers. ion along the face, are termed stretchers ; the others, headers If the header reaches from the face to the back of the wall, it is MASONRY. Ill termed a through ; if it only reaches part of the distance, it ia termed a binder. The vertical joints of one course are either just over the middle of the blocks of the next course below, or else, at least four inches on one side or the other of the vertica 1 joints of that course ; and the headers of one course rest as nearly as practicable on the middle of the stretchers of the course be- neath. If the backing is of rubble, and the facing of cut stone, a system of throughs or binders, similar to what has just been ex- plained, must be used. By the arrangement here described, the facing and backing of each course are well connected ; and, if any unequal settling takes place, the vertical joints cannot open, as would be the case were they in a continued line from the top to the bottom of the mass ; as each block of one course confines the ends of the two blocks on which it rests in the course beneath. 356. In masses of cut stone exposed to violent shocks, as those of which light-houses, and sea-walls in very exposed positions are formed, the blocks of each course require to be not only very firmly united with each other, but also with the courses above and below them. To effect this, various means have been used. The beds of one course are sometimes arranged with projections (Fig. 9,) which fit into corresponding indentations of the next course. Iron cramps in the form of the letter S, or in any other Fig. 9 Represents an elevation A, plan B, and per- spective views C and D of two of the blocks of a wall in which the blocks are fitted with in- dents, and connect- ed with bolts and cramps of metal. shape that will answer the purpose of giving them a firm hold on the blocks, are let into the top of two blocks of the same course at a vertical joint, and are firmly set with melted lead, or with bolts, so as to confine the two blocks together. Holes are, in some cases, drilled through several courses, and the blocks of these courses are connected by strong iron bolts fitted to the holes. The the bond most noted examples of these methods of strengthening id of cut stone, are to be found in the works of the Romans 112 MASONRY. which have been preserved to our time, and in uvo celebrated modern structures, the Eddy-stone and Bell-rock light-houses ir. Great Britain. (Fig. 10.) Fig. 10 Represents the manner of arranging stone* of the same course by dove-tail joints and joggling, taken from a horizontal section of the masonry 01 the Bell-rock light-house. 357. The manner of dressing stone belongs to the stonecutter's art, but the engineer should not be inattentive either to the accu- racy with which the dressing is performed, or the means employed to effect it. The tools chiefly used by the workman are the chisel, axe, and hammer for knotting. The usual manner of dress- ing a surface, is to cut draughts around and across the stone with the chisel, and then to use the chisel, the axe with a serrated edge, or the knotting hammer, to work down the intermediate portions into the same surface with the draughts. In performing this last operation, the chisel and axe should alone be used for soft stones, as the grooves on the surface of the hammer are liable to become choked by a soft material, and the stone may in consequence be materially injured by the repeated blows of the workman. In hard stones this need not be apprehended. In large blocks which require to be raised by machinery, a hole, of the shape of an inverted truncated wedge, is cut to receive Fig- 11 Represents a perspective view A of a block of stone with draughts around the edges of its faces, and the intermediate space axed, or knotted, and its tackling for hoisting: also the common iron lewis B with its tackling. a, draughts around edge of block. 6, knotted part between draughts. c, iron bolts with eyes let into obliqui holes cut in the block. d and e, chain and rope tackling. n, n, side pieces of the lewis. o, centre piece of lewis with ey fastened to n, n by a bolt. p, iron ring for attaching tackling. a small iron instrument termed a lewis, (Fig. 11,) to which the rope is attached for suspending the block ; or else two holes arc MASONRY. 113 cut obliquely into the block to receive bolls with eyes for the same purpose. When a block of cut stone is to be laid, the first point to be attended to, is to examine the dressing, which is done by placing the block on its bed, and seeing that the joints fit close, and the face is in its proper plane. If it be found that the fit is not accu- -ate, the inaccuracies are marked, and the requisite changes made. The bed of the course, on which the block is to be laid, is then ihoroughly cleansed from dust, &c., and well moistened, a bed of thin mortar is laid evenly over it, and the block, the lower sur- face of which is first cleansed and moistened, is laid on the mor- tar-bed, and well settled by striking it with a wooden mallet. When the block is laid against another of the same course, the joint between them is prepared w.'th mortar in the same manner as the bed. 358. Rubble Stone Masonry. With good mortar, rubble work, when carefully executed, possesses all the strength and durability required in structures of an ordinary character ; and it is much less expensive than cut stone. 359. The stone used for this work should be prepared simply by knocking off all the sharp, weak angles of the block ; it is then cleansed from dust, &c., and moistened, before placing it on its bed. This bed is prepared by spreading over the top of the lower course an ample quantity of good ordinary-tempered mortar, into which the stone is firmly imbedded. The interstices between the larger masses of stone are filled in, by thrusting small fragments, or chippings of stone, into the mortar. Finally, the whole course may be carefully grouted before another is commenced, in order to fill up any voids left between the full mortar and stone. 360. To connect the parts well together, and to strengthen the weak points, throughs or binders should be used in all the courses ; and the angles should be constructed of cut or hammered stone. In heavy walls of rubble masonry, the precaution, moreover, should be observed, to lay the stones on their quarry-bed; that is, to give them the same position, in the mass of masonry, that they had in the quarry ; as stone is found to offer more resistance to pressure in a direction perpendicular to the fiarry-bed, than in any other. The directions of the lamina in stratified stones, show the position of the quairy-bed. 361. Hammered stone, or dressed rubble, is stone roughly fashioned into regular masses with the hammer. The same pre cautions must be taken in laying this kind of masonry, as in the two preceding. 362. Brick Masonry. With good brick and mortar, this ma sonry offers great strength and durability, arising from the strong adhesion between the mortar and brick. 15 114 MASONRY. 363. The bond used in brick work is very various, depending on the character of the structure. The most usual kinds are Known as the English and Flemish. The first consists in ar- ranging the courses alternately, entirely as headers or stretchers, the bricks through the course breaking joints. In the second the bricks are laid as headers and stretchers in each course. The first is stated to give a stronger bond than the last, the bricks of which, owing to the difficulty of preventing continuous joints, either in the same or different courses, are liable to separate, causing the face or the back to bulge outward. The Flemish bond presents the finer architectural appearance, and is therefore preferred for the fronts of edifices. 364. Timber and iron have both been used to strengthen the bond of brick masonry. Among the. most remarkable example." of their uses are the well, faced in brick, forming an entrance to the Thames Tunnel, the celebrated work of Mr. Brunei, and his experimental arch of brick, a description of which is given in the Civil Engineer and Architect's Journal, No. 6, vol. I. In both these structures Mr. Brunei used pantile laths and hoop iron, in the interior of the horizontal courses, to connect two contiguous courses throughout their length. The efficacy of this method lias been farther fully tested by Mr. Brunei, in experiments made on the resistance to a transversal strain of a brick beam bonded with hoop iron, accounts of which, and of experiments of a like kind, are given by Colonel Pasley in his work on Limes, Calca- reous Cements, f an accidental cha racter. It should, in all cases, be placed so far below the surface of the soil on which it rests, that it will not be liable to be un- covered, or exposed ; and its surface should not only be normal to the resultant of the efforts which it sustains, but this resultant should intersect the base of the bed so far within it, that the por- tion of the soil between this point of intersection and the outward edge of the base shall be*broad enough to prevent its yielding from the pressure thrown on it. 368. The first preparatory step to be taken, in determining the kind of bed required, is to ascertain the nature of the subsoil on which the structure is to be raised. This may be done, in or- dinary cases, by sinking a pit ; but where the subsoil is composed of various strata, and the structure demands extraordinary pre- caution, borings must be made with the tools usually employed for this purpose. 369. With respect to foundations, soils are usually divided into three classes : The 1st class consists of soils which are incompressible, or, at least, so slightly compressible, as not to affect the stability of the heaviest masses laid upon them, and which, at the same time, do not yield in a lateral direction. Solid rock, some tufas, compact stony soils, hard clay which yields only to the pick, or to blast- ing, belong to this class. The 2d class consists of soils which are incompressible, but require to be confined laterally, to prevent them from spreading out. Pure gravel and sand belong to this class. The 3d class consists of all the varieties of compressible soils ; under which head may be arranged ordinary clay, the common earths, and marshy soils. Some of this class are found in a more or less compact state, and are compressible only to a certain ex- tent, as most of the varieties of clay and common earth ; others are found in an almost fluid state, and yield, with facility, in every direction. 370. To prepare the bed for a foundation on rock, the thick- ness of the stratum of rock should first be ascertained, if there are any doubts respecting it : and if there is any reason to suppose that the- stratum has not sufficient strength to bear the weight of the structure, it should be tested by a trial weight, at least twice as great as the one it will have to bear permanently. The rock is next properly prepared to receive the foundation courses, by levelling its surface, which is effected by breaking down all pro- ecting points, and rilling up cavities, either with rubble masonry or with beton , and by carefully removing any portions of the up per stratum which present indications of having been injured b^ the weather. The surface, prepared in this manner, should, more 116 MASONRY. over, be perpendicular to the direction of the pressure ; if this ii vertical, the surface should be horizontal, and so for any othei direction of the pressure. Should there, however, be any diffi- culty in so arranging the surface as to have it normal to the re- sultant of the pressure, it may receive a position such that one component of the resultant shall be perpendicular to it, and the other parallel ; the latter being counteracted by the friction and adhesion between the base of the bed nd the surface of the rock. If, owing to a great declivity of the surface, the whole cannot be brought to the same level, the rock must be broken into steps, in order that the bottom courses of the foundation throughout, may rest on a surface perpendicular to the direction of the pressure. If fissures or cavities are met with, of so great an extent as to render the filling them with masonry too expensive, an arch must then be formed, resting on the two sides of the fissure, to support that part of the structure above it. The slaty rocks require most care in preparing them to receive a foundation, as their top stratum will generally be found injured to a greater or less depth by the action of frost. 371. In stony earths and hard clay, the bed is prepared by digging a trench wide enough to receive the foundation, and deep enough to reach the compact soil which has not been injured by the action of frost : a trench from 4 to 6 feet, will generally be deep enough for this purpose. 372. In compact gravel, and sand, where there is no liability to lateral yielding, either from the action of rain or any other cause, the bed may be prepared as in the case of stony earths. If there is danger from lateral yielding, the part on which the foundation is to rest must be secured by confining it laterally by means of sheeting piles, or in any other way that will offer suffi- cient security. 373. In laying foundations on firm sand, a further precaution is sometimes resorted to, of placing a platform on the bottom of the trench, for the purpose of distributing the whole weight more uniformly over it. This, however, seems to be unnecessary ; for if the bottom courses of the masonry are well settled in their bed, there is no good reason to apprehend any unequal settling from the effect of the superincumbent weight : whereas, if the wocd of the platform should, by any accident, give way, it would leave a part of the foundation without any support. When the sand under the bed is liable to injury from springs they must be cut off, and a platform, or, still better, an area of beton should compose the bed, acid this should be confined on all sides between walls of stone, or beton sunk below the bottom of the bed. 374. If, in opening -. trench in sand, water is found at a slight ForNDATIONS. 117 depth, and in such quantity as to impede the labors of the work- men, and the trench cannot, be kept dry by the use of pumps 01 scoops, a row of sheeting piles must be driven on each side of the space occupied by it, somewhat below the bottom of the bed, the sand on the outside of the sheeting piles be thrown out, and its place filled with a puddling of clay, to form a water-tight en- closure round the trench. The excavation for the bed is then commenced ; but if it be found that the water still makes rapidly at the bo torn, only a small portion of the trench must be opened, and after *he lower courses are laid in this portion, the excavation will be g rdually effected, as fast as the workmen can execute the work *vithout difficulty from the water. 375. TK- beds of foundations in compressible soils require pe- culiar care "irticularly when the soil is not homogeneous, pre- senting mor<) vp.sistance to pressure in one point than in another ; for, in that c^?*, it will be very difficult to guard against unequal settling. 376. In ordi.'-x-:-' ''lay, or earth, a trench is dug of the proper width, and deep eno-^h to reach a stratum, beyond the action of frost ; the bottom o f fcc trench is then levelled off to receive the foundation. This mav ho laid immediately on the bottom, or else upon a grillage an A j^a+form. In the first case, the stones forming the lowest cour<;, should be firmly settled in their beds, by ramming them with a ~ery heavy beetle. In the second a timber grating, termed. a gi'MaTO (Fig. 12,) which is formed of a course of heavy beams laid Hicrthwise in the trench, ano connected firmly by cross pieces ir*u.i which they are notched, is firmly settled in the bed, and the earth i-- solidly packed between the longitudinal and cross pieces ; a Scoring of thick planks, termed a platform, is then laid on tue gr0ige, to receive the lowest course of the foundation. The obje-^c >. the grillage, and Fig. 1-2 Represents the arrangemei platform fitted on piles. A, masonry. aa, piles. b, string pieces. c, cross pieces, a, capping piece. e, plattorm of pi auk. foliage and platform, is to dirfuse the weight more uniformly o face of the trench, to prevent any part from yielding. 118. MASONK1T. 377. Repeated failures in grillages and platfoTns, arising eithei from the compression of the woody fibre, or from a transversal strain occasioned by the subsoil offering an unequal resistance, have impaired confidence in their efficacy. Engineers now pre fer beds formed of an area of beton, as offering more security than any bed of timber, either in a uniformly, or unequally compressi- ble soil. 378. The preparation of an area of beton for the bed of a foundation, will depend on the circumstances of the case. In ordinary cases the beton is thrown into the trench, and carefully rammed in layers of 6 or 9 inches, until the mortar collects in a semi-fluid state on the top of the layer. If the base of the bed is to be broader than the top, its sides must be confined by boards suitably arranged for this purpose. Whenever a layer is left in- complete at one end, and another is laid upon it, an offset should be left at the unfinished extremity, for the purpose of connecting the two layers more firmly when the work on the unfinished part is resumed. Considerable economy may be effected, in the quantity of be- ton required for the bed, by using large blocks of stone which should be so distributed throughout the layer, that the beetle will meet with no difficulty in settling the beton between and around the blocks. When springs rise through the soil over which the beton is to be sjaread, the water from them must either be conveyed off by artificial channels, which will prevent it rising through the mass of beton and washing out the lime ; or else strong cloth, prepared so as to be impermeable to water, may be laid over the surface of the soil to receive the bed of beton. The artificial channels for conveying off the water may be formed either of stone blocks with semi-cylindrical channels cut in them, or of semi-cylinders of iron, or tiles, as may be most convenient. A sufficient number of these channels should be formed to give an outlet to the water as fast as it rises. An impermeable cloth may be formed of stout canvass, pre- pared with bituminous pitch and a drying oil. It is well to have the cloth doubled on each side with ordinary canvass to prevent accidents. The manner of settling the cloth on the surface of the soil must depend on the circumstances of the case. Each of the foregoing expedients for preventing the action of springs on an area of beton, has been tried with success. When artificial channels are used, they may be completely choked sub- sequently, by injecting into them a semi-fluid hydraulic cement, and the action of the springs be thus destroyed. Foundation beds of beton are frequently made without exhaust- ing the water from the area on which they are laid. For this FOUNDATIONS. 119 purpose the beton is thrown in layers over the area, by using either a wooden conduit reaching nearly to the position of the layer, or else by placing the beton (Fig 13) in a box by which it is lowered to the position of the layer, and from which it is de- posited so as not to permit the water to separate the lime from the other ingredients. Fig. 13 Represents an end view A of a semi-cylindri- cal box for lowering beton in water, and B the same view of the box when open- ed to let the beton fall through. o, hinge around which the halves of the box open. a, rope tackling for lowering box. , pin, or catch to fasten the two parts of the box. cord to detach the pin and open the box. Should it be found that springs boil up at the bottom, it must be covered with an impermeable cloth. 379. In marshy soils, the principal difficulty consists in form- ing a bed sufficiently firm to give stability to the structure, owing to the yielding nature of the soil in all directions. The following are some of the dispositions that have been tried with success in this case. Short piles from 6 to 12 feet long, and from 6 to 9 inches in diameter, are driven into the soil as close together as they can be crowded, over an area considerably greater than that which the structure is to occupy. The heads of the piles are accurately brought to a level to receive a grillage and platform ; or else a layer of clay, from 4 to 6 feet thick, is laid over the area thus prepared with piles, and is either solidly rammed in layers of a foot thick, or submitted to a very heavy pressure for some time before commencing the foundations. The object of preparing the bed in this manner, is to give the up- per stratum of the soil all the firmness possible, by subjecting it to a strong compression from the piles ; and when this has been effected, to procure a firm bed for the lowest course of the foundation by the grillage, or clay bed ; by these means the whole pressure will be uniformly distributed throughout the en- tire area. This case s also one in which a bed of beton would replace, with great advantage, either the one of clay, or the grillage. The purposes to which the short piles are applied in this case is different from the object to be attained usually in ihe employ 120 MASONRY. ment of piles for foundations ; which is to transmit the weight of the structure that rests on the piles, to a firm incompressible soil, overlaid by a compressible one, that does not offer sufficient firmness for the bed of the foundation. 380. When a firm soil is overlaid by one of a compressible character, and its distance below the surface is such that it can be reached by piles of ordinary dimensions, they should be used in preference to any other plan, when they can be rendered durable, on account of their economy and the security they afford. To prepare the bed to receive the foundations in this case, strong piles are driven at equal distances apart, over the entire area on which the structure is to rest. These piles are driven, until they meet with a firm stratum below the compressible one, which offers sufficient resistance to prevent them from penetra- ting farther. ' 381. Piles are generally from 9 to 18 inches in diameter, with a length not above 20 times the diameter, in order that they may not bend under the stroke of the ram. They are prepared for driving, by stripping them of their bark, and paring down the knots, so that the friction, in driving, may be reduced as much as possible. The head of the pile is usually encircled by a strong hoop of wrought iron, to prevent the pile from being split by the action of the ram. The foot of the pile may receive a shoe formed of ordinary boiler iron, well fitted and spiked on ; or a cast-iron shoe of a suitable form for penetrating the soil may be cast around a wrought-iron bolt, by means of which it is fastened to the pile. Fig. 14 Represents a section through the axis of a cast-iron shoe and wrought-iron bolt for a pile 382. A machine, termed a pile engine, is used for driving piles. It consists essentially of two uprights firmly connected at top by a cross piece, and of a ram, or monkey of cast iron, for driving the pile by a force of percussion. Two kinds of en- gines are in use ; the one termed a crab engine, from the ma- chinery used to hoist the ram to the height from which it is to fall on the pile ; the other the ringing engine, from the monkey being raised by the sudden pull of several men upon a rope, by which the ram is drawn i,p a few feet to descend on the pile. The crab engine is by far the more powerful machine, but on FOUNDATIONS. 121 this account is inapplicable in some cases, as in the driving of cast-iron piles, where the force of the blow might destroy the pile ; also in long slender piles it may cause the pile to spring so much as to prevent it from entering the subsoil. The manner of driving piles, and the extent to which they may be forced into the subsoil, will depend on local circumstances. It sometimes happens that a heavy blow will effect less than several slighter blows, and that piles after an interval between successive volleys of blows, can with difficulty be started at first. In some cases the pile breaks below the surface, and continues to yield to the blows, by the fibres of the lower extremity being crushed. These difficulties require careful attention on the part of the en- gineer. Piles should be driven to an unyielding subsoil. The French civil engineers have, however, adopted a rule to stop the driving when the pile has arrived at its absolute stoppage, this being measured by the farther penetration into the subsoil of about fVths of an inch, caused by a volley of thirty blows from a ram of 800 Ibs., falling from a height of 5 feet at each blow. 383. If the head of a pile has to be driven below the level to which the ram descends, another pile, termed a punch, is used for the purpose. A cast-iron socket of a suitable form embraces the head of the pile and the foot of the punch, and the effect of the blow is thus transmitted through the punch to the pile. 384. When a pile from breaking, or any other cause, has to be drawn out, it is done by using a long beam as a lever for the pur- pose ; the pile being attached to the lever by a chain, or rope suitably adjusted. 385. The number of piles required, will be regulated by the weight of the structure. An allowance of 1000 pounds on each square inch will ensure safety. The least distance apart, at which the piles can be driven with ease, is about 2^ feet between their centres. If they are more crowded than this, they may force each other up, as they are successively driven. When this is found to take place, the driving should be commenced at the centre of the , rea, and the pile should be driven with the butt end downward. 386. From experiments carefully made in France, it appears that piles which resist only in virtue of the friction arising from the compression of the soil, cannot be subjected with safety to a load greater than one fifth of that which piles of the same dimen- sions will safely support when driven into a firm soil. 387. After the piles are driven, they are sawed off to a level, to receive a grillage and platform for the foundation. A large beam, termed a capping, is first placed on the heads of the out- liie row of piles, to which it is fastened by means of wooden 16 122 MASONRY. pins, or tree-naus driven into an auger-hole, made through the cap into the head of each pile. After the cap is fitted, longitudi nal beams, termed string pieces, are laid lengthwise on the heads of each row, and rest at each extremity on the cap, to which they are fastened by a dove-tail joint and a wooden pin. Another series of beams, termed cross pieces, are laid crosswise on the string pieces, over the heads of each row of piles. The cross and string pieces are connected by a notch cut into each, so that, when put together, their upper surfaces may be on the same level, and they are fastened to the heads of the piles in the same manner as the capping. The extremities of the cross pieces rest on the capping, and are connected with it, like the string pieces. The platform is of thick planks laid over the grillage, with the extremity of each plank resting on the capping, to which, and to the string and cross pieces, the planks are fastened by nails. The capping is usually thicker than the cross and string pieces by the thickness of the plank ; when this is -the case, a rabate, about four inches wide, must be made on the inner edge of the capping, to receive the ends of the planks. 388. An objection is made to the platform as a bed for the foundation, owing to the want of adhesion between wood and mortar ; from which, if any unequal settling should take place, fhe foundations would be exposed to slide off the platform. To obviate this, it has been proposed to replace the grillage and plat- form by a layer of beton resting partly on the heads of the piles, and partly on the soil between them. This means would furnish a firm bed for the masonry of the foundations, devoid of the ob- jections made to the one of timber. To counteract any tendency to sliding, the platform may be inclined if there is a lateral pressure, as in the case, for example, of the abutments of an arch. 389. In soils of alluvial formation, it is common to meet with a stratum of clay on the surface, underlaid with soft mud, in which case, the driving of short piles would be injurious, as the tenacity of the stratum of clay would be destroyed by the oper- ation. It would be better not to disturb the upper stratum in this case, but to give it as much firmness as possible, by ramming it with a heavy beetle, or by submitting it to a heavy pressure. 390. Piles and sheeting piles of cast iron have been used with complete success in England, both for the ordinary purposes of cofferdams, and for permanent structures for wharfing. The piles have been cast of a variety of forms ; in some cases they have been cast hollow for the purpose of excavating the soii within the pile as it was driven, and thus facilitate its penetration FOUNDATIONS. 123 *nto the subsoil. Fig. 15 represents a cross section of one of the more recent arrangements of iron piles and sheeting piles. Fig. i5 Represents a horizontal section of an arrangement of piles and sheeting piles of cast iron. a, sheeting pile with a lap e to cover the joint between it and the next sheeting pile. b, piles with a lap on each side. c, sheeting pile lapped by pile and sheeting pile next it. a. , ribs of piles and sheeting piles. 391. Sand has also been used with advantage to form a bed for foundations in a very compressible soil. For this purpose a trench is (Fig. 16) excavated, and filled with sand ; the sand being spread in layers of about 9 inches, and each layer being firmly settled by a heavy beetle, before laying the next. If water Fig. 16 Represents a section of a sand foun- dation bed and the masonry upon it. A, sand bed in a trench. B, masonry. should make rapidly in the trench, it would not be practicable to pack the sand in layers. Instead, therefore, of opening a trench, Fig. 17 Represents a section of a foun- dation bed made by filling holes with sand. A, holes filled with sand. B, masonry. holes about 6 feet deep, and 6 inches in diameter, (Fig. 17,) 124 MASONRY. should be made, by means of a short pile, as close together at practicable ; when the pile is withdrawn from the hole, it is im- mediately filled with sar.d. To cause the sand to pack firmly, it should be slightly moistened before placing it in the holes, or trench. Sand, when used in this way, possesses the valuable property of assuming a new position of equilibrium and stability, should, the soil on which it is laid yield at any of its points. Not only does this take place along the base of the sand bed, but also along the edges, or sides, when these are enclosed by the sides of the trench made to receive the bed. This last point offers also some additional security against yielding in a lateral direction. The bed of sand must, in all cases, receive sufficient thickness to cause the pressure on its upper surface to be distributed over the entire base. 392. When, from the fluidity of the soil, the vertical pressure of the structure causes the soil to rise around the bed, this action may be counteracted, either by scooping out the soil to some depth around the bed and replacing it by another of a more compact nature, well rammed in layers, or with any rubbish of a solid character ; or else a mass of loose stone may be placed over the surface exterior to the bed, whenever the character of the struc- ture will warrant the expense. 393. Precautions against Lateral Yielding. The soils which have been termed compressible, strictly speaking, yield only by the displacement of their particles either in a lateral direction, or upward around the structure laid upon them. Where this action arises from the effect of a vertical weight, uniformly distributed over the base of the bed, the preceding methods for giving per- manent stability to structure, present all requisite security. But when the structure is subjected also to a lateral pressure, as for example, that which would arise from the action of a bank of earth resting against the back of a wall, additional means of secu- rity are demanded. One of the most obvious expedients in this case, is to drive a row of strong square piles in juxtaposition immediately in contact with the exterior edges of the bed. This expedient is, however, only of service where the p'ies attain either an incompressible soil, or one at least firmer than that on which the bed imme- diately rests. For otherwise, as is obvious, the piles only serve to transmit the pressure to the yielding soil in contact with them. But where they are driven into a firm soil below, they gain a fixed point of resistance, and the only insecurity they offer is either by the rupture of the piles, from the cross strain upon them, or from the yielding of the firm subsoil, from the same cause FOUNDATIONS. 125 In case the piles reach a firm subsoil, it wil oe oest to scoop out the upper yielding soil before driving the piles, and to fill in between and around them with loose broken stone, (Fig. 18.1 This will give the piles greater stiffness, and effectually prevent them from spreading at top. Fig. 18 Represents the manner of using loose stone to sustain piles and prevent them from yielding laterally. A, section of the masonry. B, loose stone thrown around the piles a When the piles cannot be secured by attaining a firm subsoil, :t will be better to drive them around the area at some distance from the bed, and, as a farther precaution, to place horizontal buttresses of masonry at regular intervals from the bed to the piles. By this arrangement, some additional security is gained from the counter-pressure of the soil enclosed between the bed and the wall of piles. But it is obvious that unless the piles in this case are driven into a firmer soil than that on which the struc- ture rests, there will still be danger of yielding. In using horizontal buttresses, the stone of which they are con- structed should be dressed with care ; their extremities near the wall of piles should be connected by horizontal arches, (Fig. 19,) to distribute the pressure more uniformly ; and where there is an upward pressure of the soil around the structure, arising from its weight, the buttresses ought to be in the form of reversed arches. In buttresses of this kind, as likewise in broad areas resting on a very yielding soil, since as much danger is to be apprehended from their breaking by their own weight as from any other cause, it must be carefully guarded against. Something may be done for this purpose by ramming the earth around the structure with a heavy beetle, when it can be made more compact by this means ; or else a part of the upper soil may be removed, and be replaced by one of n more compact nature which n.vy be rammed ID layers 126 MASONRY. Fig. 19 Represents the manner of pre- venting a sustaining wall from yielding laterally to a thrust behind it, by using horizontal buttresses of reversed arches abutting against vertical counter arches. A, vertical section of wall, buttresses, and counter arches. B, plan of wall, buttresses, and counter arches. a, plan of wall. b, section of do c, buttresses. a, counter arches. The following methods, where they can be resorted to, and where the character of the structure will justify the expense, have been found to offer the best security in the case in question. When the bed can be buttressed in front with an embankment, a low counter-wall (Fig. 20) may be built parallel to the edge of the bed, and some 10 or 12 feet from it ; between this wall and the bed a reversed arch connecting the two may be built, and a surcharge of earth of a compact character and well rammed, may be placed against the counter wall to act by its counter pressure against the lateral pressure upon the bed. Fig. 20 Represents the man- ner of buttressing a sustain- ing wall in front by the ac- tion of a counter pressure oi" earth transmitted to the wall by a reversed arch. a, section of sustaining wall. b, section of sustaining waM of embankment d. c, section of reversed arch. d, section of embankment from which counter pressure comes. e, section of embankment be- hind sustaining wall. When the bed cannot be buttressed in front, as in quay walls, a grillage and platform supported on piles (Fig. 21) may be built to the rear from the back of the wall, for the purpose of support- ing the embankment against the back of the wall, and preventing the effect which its pressure on the subsoil might have in thrust- ing forward the bed of the foundation. In addition to these means, land ties of iron will give great ad FOUNDATIONS 127 ditional security, when a fixed point in rear of the wall can he found to attach them firmly. Fig. 21 Represents the manner of re- lieving a sustaining wall from the lateral action caused by the pressure of an embankment on the subsoil by means of a platform built behind the wall. A, section of the wall. B, section of embankment. a, piles supporting the grillage and plat- form of A . b, loose stone forming a firm bed un- der the platforms. c, piles supporting the platfonn d be- hind the wall 394. Foundations in Water. In laying foundations in water, two difficulties have to be overcome, both of which require great resources and care on the part of the engineer. The first is found in the means to be used in preparing the bed of the foundation ; and the second, in securing the bed from the action of the water, to ensure the safety of the foundations. The last is, generally, the more difficult problem of the two ; for a current of water will gradually wear away, not only every variety of loose soils, but also the more tender rocks, such as most varieties of sand-stone, and the calcareous and argillaceous rocks, particularly when they are stratified, or are of a loose texture. 395. To prepare the bed of a foundation in stagnant water, the only difficulty that presents itself is to exclude the water from the area on which the structure is to rest. If the depth of water is not over 4 feet, this is done by surrounding the area with an ordinary \vater-tight dam of clay, or of some other binding earth. For this purpose, a shallow trench is formed around the area, by removing the soft, or loose straium on the bottom ; the foundation of the dam is commenced by filling this trench with the clay, and the dam is made by spreading successive layers of clay about one foot thick, and pressing each Hyer as it is spread, to render it more compact. When the dam s completed, the water is pumped out from the enclosed area, and he bed for the foundation is pre- pared as on dry land. 396. When the depth of stagnant water is ove/ 4 feet, and in running water, of any depth, the ordinary dam must be replaced by the coffer-dam. This construction consists of two rows of 128 MASONRY. plank, termed sheeting piles, driven into the soil vertically, form- ing thus a coffer work, between which clay or binding earth, termed the puddling, is filled in, to form a water-tight dam to ex- clude the water from the area enclosed. The arrangement, construction, and dimensions of coffer dams depend on their specific object, the depth of water, and the nature of the subsoil on which the coffer-dam rests. With regard to the first point, the width of the dam between the sheeting piles should be so regulated as to serve as a scaffold- ing for the machinery and materials required about the work. This is peculiarly requisite where the coffer-dam encloses an isola- ted position removed from the shore. The interior space enclosed by the dam should have the requisite capacity for receiving the bed of the foundations, and such materials and machinery as may be required within the dam. The width, or thickness of the coffer-dam, by which is under- stood the distance between the sheeting piles, should be sufficient not only to be impermeable to water, but to form, by the weight of the puddling, in combination with the resistance of the timber work, a wall of sufficient strength to resist the horizontal pressure of the water on t/.e exterior, when the interior space is pumped dry. The resistance offered by the weight of the puddling to the pressure of the water can be easily calculated ; that offered by the timber work will depend upon the manner in which the framing is arranged, and the means taken to stay, or buttress the dam from the enclosed space. The most simple and the usual construction of a coffer-dam Fig. 22 Represents a sec- tion of the ordinary cof- fer-dam. , main piles. b, wale, or string pieces. c. cross pieces. a , sheeting piles. e, guide string pieces foi sheeting piles. A, puddlingi B, interior space. (Fig. 22) consists in driving a row of ordinary straight piles round the area to be enclosed, placing their centre lines about 4 FOUNDATIONS. 129 feet asunder. A second row is driven parallel to the first, the respective piles being the same distance apart ; the distance be tween the centre lines of the two rows being so regulated as to leave the requisite thickness between the sheeting piles for the dam. The piles of each row are connected by a horizontal beam of square timber, termed a string or wale piece, placed a foot or two above the highest water line, and notched and bolted to each pile. The string pieces of the inner row of piles is placed on the side next to the area enclosed, and those of the outer row on the outside. Cross beams of square timber connect the string pieces of the two rows, upon which they are notched, serving both to prevent the rows of piles from spreading from the pressure that may be thrown on them, and as a joisting for the scaffolding. On the opposite sides of the rows, interior string pieces are placed, about the same level with the exterior, for the purpose of serving both as guides and supports for the sheeting piles. The sheeting piles being well jointed, are driven in juxtaposition, and against the interior string pieces. A third course of string, or ribbon pieces of smaller scantling confine, by means of large spikes, the sheeting piles against the interior string pieces. As has been stated, the thickness of the dam and the dimen- sions of the timber of which the coffer work is made, will depend upon the pressure due to the head of water, when the interior space is pumped dry. For extraordinary depths, the engineer would not act prudently were he to neglect to verify by calcula- tion the equilibrium between the pressure and resistance ; but for ordinary depths under 10 feet, a rule followed is to make the thickness of the dam 10 feet ; and for depths over 10 feet to give an additional thickness of one foot for every additional depth of three feet. This rule will give every security against filtrations through the body of the dam, but it might not give sufficient strength unless the scantling of the coffer work were suitably in- creased in dimensions. In very deep tidal water, coffer-dams have been made in off- sets, by using three rows of sheeting piles for the purpose of giving greater thickness to the dam below the low-water level. In such cases strong square piles closely jointed and tongued and grooved, shouM be used in place of the ordinary sheeting piles. Besides providing against the pressure of the head of water, suitable dimensions must be given to the sheeting piles, in order that they may sustain the pressure arising from the puddling when the interior space is emptied of water. This pressure against the hiterior sheeting piles may be farther increased by that of the ex lerior water upon the exterior sheeting piles, should the pressure of the latter be greater than the former. To provide more se- curely against the effect of these pressures, intermediate string 17 130 MASONRY. pieces may be placed against the interior row of piles before th sheeting piles are driven ; and the opposite sides of the dam on the interior may be buttressed by cross pieces reaching across the top string pieces, and by horizontal beams placed at intermediate points between the top and bottom of the dam. The main inconvenience met with in coffer-dams arises from the difficulty of preventing leakage under the dam. In all cases the piles must be driven into a firm stratum, and the sheeting piles should equally have a firm footing in a tenacious compact sub-stratum. When an excavation is requisite on the interior, to uncover the subsoil on which the bed of the foundation is to be laid, the sheeting piles should be driven at least as deep as this point, and somewhat below it if the resistance offered to the driving does not prevent it. The puddling should be formed of a mixture of tenacious clay and sand, as this mixture settles better than pure clay alone. Before placing the puddling, all the soft mud and loose soil be- tween the sheeting piles should be carefully extracted ; the pud- dling should be placed in and compressed in layers, care being taken to agitate the water as little as practicable. With requisite care coffer-dams may be used for foundations in any depth of water, provided a water-tight bottoming can be found for the puddling. Sandy bottoms offer the greatest difficulty in this respect, and when the depth of water is over 5 feet, extraor- dinary precautions are requisite to prevent leakage under the puddling. When the depth of water is over 10 feet, particularly where the bottom is composed of several feet of soft mud, or of loose soil, below which it will be necessary to excavate to obtain a firm stratum for the bed of the foundation, additional precautions will be requisite to give sufficient support to the interior sheeting pilts against the pressure of the puddling, to provide against leakage under the puddling, and to strengthen the dam against the pres- sure of the exterior water, when the interior space is pumped dry and excavated. The best means for these ends, when the local- ity will admit of their application, is to form the exterior of the dam, as has already been described, by using piles and sheeting piles, giving to the latter additional points of support, by interme- diate string pieces between the one at top and the bottom of the water ; and to form a strong framing of timber for a support to the interior sheeting piles, giving to it the dimensions of the area to be enclosed. The frame-work (Fig. 23) may be composed of upright square beams, placed at suitable distances apart, depend- ing on the strength required, upon which square string pieces are bolted at suitable distances from the top to the bottom, the bottom string resting on the surface of the mud. The string pieces* FOUNDATIONS. 131 sen ing as supports for the sheeting piles, must be on Jie sides of the uprights towards the puddling, and their faces in the same Fig. 23 Represents a section of the cof- fer-dam used for the Potomac aque duct. rt, main exterior piles. b, strong square beams correspond- ing to a on which the wales n. n are notched and bolt- ed. c. sheeting piles. a, top wale on main piles. e, cross pieces. i, guide and support- ing string pieces for sheeting piles. oo. horizontal shores buttressing opposite sides of dam. A, puddling. B, interior space. C, mud, &c. D, rock bottom. vertical plane. Between each pair of opposite uprights, horizon- tal shores may be placed at the points opposite the position of the string pieces, to increase the resistance of the dam to the pressure of the water; the top shores extending entirely across the dam, and being notched on the top string pieces. The interior shores must be so arranged that they can be readily taken out as the masonry on the interior is built up, replacing them by other shores resting against the masonry itself. 397. When the bed of a river presents a rocky surface, or rock covered with but a few feet of mud, or loose soil, cases may occur in which it will be more economical and equally safe to lay a bed of beton without exhausting the water from the area to be built on ; enclosing the area, before throwing in the beton, by a simple coffer work formed of a strong frame work of uprights and hori- zontal beams and sheeting piles. The frame work (Fig. 24) in this case is composed of uprights connected by string pieces in pairs ; each pair is notched and bolted to the uprights, a sufficient interval being left between them for the insertion of the sheeting piles. To secure the frame work to the rock, it may be re- quisite to drill holes in the rock to receive the foot of each up- right. The holes may be drilled by means of a long iron bar, termed a. jumper, which is used for this purpose, or else the or- dinary diving bell may be employed. This machine is very ser- viceable in all similar constructions where an examination of work under water is requisite, or in cases where it is necessary to lay 132 MASONRY. masonry under water. The frame work is put together on land, floated to its position, and settled upon tl .e rock ; the sheeting Fig. 524 Represents a coffer work for confining beton. A, section of coffer work and beton. B, plan of coffer work. a, a', square uprights connected by horizontal beams I, b bolted to them in pairs. c, c, sheeting piles inserted between the beams b, b' anc the uprights a, a'. d, d', iron rods connecting opposite sides of coffer work. piles are then driven into close contact with the surface of the rock. 398. The convenience ana economy resulting from the use of beton for the beds of structures raised in water, have led General Treussart to propose a plan for laying beds of this material, and then to take advantage of their strength and impermeability to con- struct a coffer-dam upon them, in order to carry on the super- structure with more care. To effect this, the space to be occupied by the bed (Fig. 25) is first enclosed by square piles, driven in juxtaposition and secured at top by a string piece. The mud and loose soil are then scooped from the enclosed area to the firm soil on which the bed of beton is to be laid. The bed of beton is next laid with the usual precautions, and while it is still soft a second row of square piles is driven into it, also in juxtaposition, and at a suitable distance from the first for the thickness of the dam FOUNDATIONS. 133 hese are also secured at top by a string piece. Cross pieces are Fig. 25 Represents a section of Gen eral Treussart's dam. A, bed of beton, B, puddling of dam. C, masonry of structure. a, square piles. b, wale pieces. c, cross pieces. notched upon the string pieces, to secure the rows of piles and form a scaffolding. An ordinary puddling is placed in between the rows of piles, and the interior space is pumped dry. Should the soil under the bed of beton be permeable, the pres - sure of the water on the base of the bed may be sufficient to raise the bed and the dam upon it, when the water is taken from the interior space. A proper calculation will show whether this dan- ger is to be apprehended, and should it be, a provisional weight must be placed on the dam, or the bed of beton, before exhaust- ing the interior. 399. When the depth of water is great, or when, from the permeability of the soil at the bottom, it is difficult to prevent leakage, a coffer-dam may be a less economical method of laying foundations than the caisson. The caisson (Fig. 26) is a strong water-tight vessel having a bottom of solid heavy timber, and vertical sides so arranged that they can be readily detached from the bottom. The following is the usual arrangement of the cais- son, it, like the coffer-dam, being subject to changes to suit it to the locality. The bottom of the .caisson, serving as a platform for the foundation course of the masonry, is made level and of heavy timber laid in juxtaposition, tiie ends of the beams being confined by tenons and screw-bolts to longitudinal capping pieces of larger dimensions. The sides of the box are usually vertical, The sides are formed of upright pieces of scantling covered with thick plank, the seams being carefully calked to make the cais- son water-tight. The lower ends of the uprights are inserted into shallow mortises made in the capping. The arrangemeal 134 MASONRY. for detaching the sides, is effected in the following manner Strong hooks of wrought iron are fixed to the bottom of the Fig. 26 Represents a cross sec- tion and interior end view of a caisson. The boards in this Fig. are represented as let into grooves in the vertica. pieces, instead of being nailed to them on the exterior. a, bottom beams let iutc grooves in the capping. b, square uprights to sustain _, the boards. = c. cross pieces resting on b. ==2 ^' ' ron roc ' 8 fitted to hooks at bottom and nuts at top. = e, longitudinal beams to stay ~: the cross pieces c. . '-- A, section of the masonry. caisson at the sides of the capping piece, corresponding to the points where the uprights of the sides are inserted into this piece rieces of strong scantling are laid across the top of the caisson, resting on the opposite uprights, upon which they are notched. These cross pieces project beyond the sides, and the projecting parts are perforated by an auger-hole, large enough to receive a bolt of two inches in diameter. The object of these cross pieces is twofold ; the first is to buttress the sides of the caisson at top against the exterior pressure of the water ; and the second is to serve as a point of support for a long bolt, or rod of iron, with an eye at the lower end, into which the hook on the capping piece is inserted, and a screw at top, to which a nut, or female screw is fitted, and which, resting on the cross pieces at a point of sup- port, draws the bolt tight, and, in that way, attaches the sides and bottom of the caisson firmly together. A bed is prepared to receive the bottom of the caisson, by lev- elling the soil on which the structure is to rest, if it be of a suit- able character to receive directly the foundation ; or by driving large piles through the upper compressible strata of the soil to the firm stratum beneath. The heads of the piles are sawed off on a level to receive the bottom of the caisson. To settle the caisson on its bed, it is floated to and moored over it ; and the rrasonry of the structure is commenced and carried up, until the weight grounds the caisson. The caisson should be so contrived, that it can be grounded, and afterwards raised, in case that the bed is found not to be accurately levelled To effect this, a sma'l sliding gate should be placed in the side of the caisson, for the purpose of filling it with water at pleasure FOUNDATIONS. 135 By means of this gate, the caisson can be filled and grounded, and, by closing the gate and pumping out the water, it can be set afloat. After the caisson is settled on its bed, and the masonry of the structure is raised above the surface of the water, the sides are detached, by first unscrewing the nuts and detaching the rods and then taking off the top cross pieces. By first filling the cais- son with water, this operation of detaching the sides can be more easily performed. 400. To adjust the piles before they are driven, and to prevent them from spreading outward by the operation of driving, a strong grating of heavy timber, formed by notching cross and longitudi- nal pieces on each other, and fastening them firmly together, may be resorted to. This grating is arranged in a similar manner to a grillage ; only the square compartments, between the cross and string pieces, are larger, so that they may enclose an area for 4 or 9 piles ; and, instead of a single row of cross pieces, the grating is made with a double row, one at top, the other at the bottom, embracing the string pieces on which they are notched. The grating may be fixed in its position at any depth under water, by a few provisional piles, to which it can be attached. 401. Where the area occupied by a structure is very consider- able, and the depth of water great, the methods which have thus far been explained cannot be used. In such cases, a firm bed is made for the structure, by forming an artificial island of loose heavy blocks of stone, which are spread over the area, and receive a batter of from one perpendicular to one base, to one perpen- dicular and six base, according to the exposure of the bed to the effects of waves. This bed is raised several feet above the sur- face of the water, according to the nature of the structure, and the foundation is commenced upon it. 402. It is important to observe, that, where such heavy masses are laid upon an untried soil, the structure should not be com- menced before the bed appears entirely to have settled ; nor even then, if there be any danger of further settling taking place from the additional weight of the structure. Should any doubts arise on this point, the bed should be loaded with a provisional weight, somewhat greater than that of the contemplated structure, and this weight may be gradually removed, if composed of other materials than those required for the structure, as the work pro- gresses. 403. To give perfect security to foundations in running water, the soil around the bed must be protected to some extent from the action of the current. The most ordinary method of effect- ing this, is by throwing in loose masses of broken stone of suffi- cient size to resist the force of the current. This method will 136 MASONRY. give all required security, where the soil is not of a shifting cha racter, like sand and gravel. To secure a soil of this last nature, it will, in some cases, be necessary to scoop out the bottom around the bed to a depth of from 3 to 6 feet, and to fill this excavated part with beton, the surface of which may be protected from the wear arising from the action of the pebbles carried over it by the current, by covering it with broad flat flagging stones. 404. When the bottom is composed of soft mud to any great depth, it may be protected by enclosing the area with sheeting piles, and then filling in the enclosed space with fragments of loose stone. If the mud is very soft, it would be advisable, in the first place, to cover the area with a grillage, or with a layer of brushwood laid compactly, to serve as a bed for the loose stone, and thus form a more stable and solid mass. CONSTRUCTION OF MASONRY. 405. Under this head will be comprised whatever relates to the manner of determining the forms and dimensions of the most im- portant elementary components of structures of masonry, together with the practical details of their construction. 406. Foundation Courses. As the object of the foundations is to give greater stability to the structure by diffusing its weight over a broad surface, their breadth, or spread, should be propor- tioned both to the weight of the structure and to the resistance offered by the subsoil. In a perfectly unyielding soil, like hard rock, there would be no increase of stability by augmenting the base of the structure beyond what would be strictly necessary to its stability in a lateral direction ; whereas in a very compressible soil, like soft mud, it would be necessary to make the base of the foundation very broad, so that by diffusing the weight over a great surface, the subsoil may offer sufficient resistance, and any un- equal settling be obviated. 407. The thickness of the foundation course will depend on the spread ; the base is made broader than the top from motives of economy. This diminution of the volume (Fig. 27) is made Fig. 27 Section of foundation courses and superstruc- ture. A, batter. B, offsets. C, superstructure. either in steps, termed offsets, or else by giving a uniform batter from the base to the top. The latter method is now generally CONSTRUCTION OF MASONRY. 137 used ; it presents equal stability with the former with a smaller volume. When the foundation has to resist only a vertical pressure, an equal batter is given to it on each side ; but if it has to resist also a lateral effort, the spread should be greater on the side opposed to this effort, in order to resist its tendency, which would be to cause a yielding on that side. 408. The bottom course of the foundations is usually formed of the largest sized blocks, roughly dressed off with the hammer ; but if the bed is compressible, or the surfaces of the blocks are winding, it is preferable to use blocks of a small size for the bot- tom course ; because these small blocks can be firmly settled, by means of a heavy beetle, into close contact with the bed, which cannot be done with large sized blocks, particularly if their undei surface is not perfectly plane. The next course above the bottom one should be of large blocks, to bind in a firm manner the smaller blocks of the bottom course, and to diffuse the weight more uni formly over them. 409. When a foundation for a structure rests on isolated sup- ports, like the pillars, or columns of an edifice, an inverted or counter-arch, (Fig. 28,) should connect the top course of the foundation under the base of each isolated support, so that the pressure on any two adjacent ones may be distributed over the bed of the foundation in the interval between them. This precau- tion is obviously necessary only in compressible soils. In incom- pressible soils it would be alone requisite to carry up the courses immediately below each support with great care, to present a stable bed tor the base of the support. Fig. 28 Section of vertical supports on reversed arch- es. A, reversed arch. B, vertical supports. C, bed of stone. The reversed arch is also used to give greater breadth to the foundations of a wall with counterforts, and in cases where an upward pressure from water, or a semi-fluid soil requires to be counteracted. In the former case the reversed arches are turned under the counterforts ; in the latter they form the points of sup- poit of the walls of the structure. 410. The angles of the foundations should be formed of the most massive blocks. The courses should be carried up uni 18 138 MASONRY formlv throughout the foundation, to prevent unequ^ ettling ii the mass. The stones of the top course of the foundation shoii i be suir) ciently large to allow the course of the superstructure next abcr r to rest on the exterior stones of the top course. 411. Hydraulic mortar should be used for the foundations and die upper courses of the structure should not be commence until the mortar has partially set throughout the entire found i tion. 412. Component parts of Structures of Masonry These m \ be divided into several classes, according to the efforts they sm tain ; their forms and dimensions depending on these efforts. 1st. Those which sustain only their own weight, and are nc liable to any cross strain upon the blocks of which they ar formed, as the walls of enclosures. 2d. Those which, besides their own weight, sustain a vertica pressure arising from a weight borne by them, as the walls of ed fices, columns, the piers of arches, &c. 3d. Those which sustain lateral pressures, and cross strair upon the blocks, arising from the action of earth, water, frames or arches. 4th. Those which sustain a vertical upward, or downward pressure, and a cross strain, as areas, lintels, &c. 5th. Those which transfer the pressure they directly receive to lateral points of support, as arches. 413. Walls of Enclosures. Walls for these purposes may be built of brick, rubble, or dry stone. Brick walls are usually built vertically upon the two faces ; their thickness cannot be less than that of one brick. A wall of one brick and a half thick will serve for any length, provided the height be not over 1 5 or 20 feet. Rubble stone walls should never receive a thickness less than 18 inches when the two faces are vertical. Rondelet, in his work TArt de Batir, lays down a rule that the mean thickness of both rubble and brick walls should be T ' F of their height. Dry stone walls shoula not receive a less thickness than two feet. When their height exceeds 12 feet, their mean thickness should be of the height. Stone walls are usually built with sloping faces. The batter should not be greater, when the stones are cemented with mor- tar, than one base to six perpendicular, in order that the rain may run rapidly from the surface, and that the wall be not too much exposed to decay from the germination of seeds which may lodge in the joints. The batter is arranged either by building the wall in offsets from top to bottom, or by a uniform surface. In either case, the CONSTRUCTION OP MASONRY. 139 thickness of the wall at top should not be less than from 8 to 1 2 inches. When a wall is built with an equal batter on each face, and the thickness at the top and the mean thickness are fixed, the base of the wall, or its thickness at the bottom, will be found by subtract- ing the thickness at top from twice the mean thickness. This rule evidently makes the batter of the wall depend upon the two preceding dimensions. The mean thickness of long walls may be advantageously diminished by placing counterforts, or buttresses, upon each face at equal distances along the line of the wall. These are spurs of masonry projecting some length from the wall, and are firmly connected with it by a suitable bond. The horizontal section of the counterforts may be rectangular ; their height should be the same as that of the wall. In rubble wall the counterforts may be made of hammered, or cut stone. In addition to this means of strengthening walls, hori- zontal courses, or chains of dressed stone may be advantageously used from distance to distance, from the bottom of the wall up- ward. 414. Vertical Supports. These consist of walls, columns, or pillars, according to circumstances. The dimensions of the courses of masonry which compose the supports should be regu- lated by the weight borne. If, as in the walls of edifices, the resultant of the efforts sustained by the wall should not be verti- cal, it must not intersect the base of the wall so near the outer edge, that the stone forming the lowest course would be in danger of being crushed. In broad enclosed spaces covered at top, the dimensions of the wall may be calculated as in the case of ordinary enclosures, and the dimensions thus obtained be increased in proportion to the weight to be borne. Cross walls between the exterior walls, as the partition walls of edifices, should be regarded as counterforts which strengthen the main walls. 415. Areas. The term area is applied to a mass of masonry, usually of a uniform thickness, laid over the ground enclosed by the foundations of walls. It seldom happens that areas have an upward pressure to sustain. Whenever this occurs, as in the case of the bottoms of cellars in communication with a head of water which causes an upward pressure, the thickness and ar- rangement of the area should be regulated to resist this pressure. When the pressure is considerable, an area of uniform thickness may not be sufficiently strong to ensure safety ; in this case an inverted arch must be used. 416. Retaining, or Sustaining Walls. These terms are ap- 140 MASONRY. pliea to walls which sustain a lateral pressure from an embank ment, or a head of water. 417. Retaining walls may yield by sliding either along th base of the foundation courses, or along one of the horizontal joints, or by rotation about the exterior edge of some one of the horizontal joints. 418. The determination of the form and dimensions of a re- taining wall for an embankment of earth is a problem of consider- able intricacy, and the mathematical solutions which have been given of it have genera^y been confined to particular cases, for which approximate results alone have been obtained ; these, how- ever, present sufficient accuracy for all practical purposes within the limits to which the solutions are applicable. Among the many solutions of this problem, those given by M. Poncelet of the Corps of French Military Engineers, in a Memoir on this subject, pub- lished in the Memorial de VOfficier du Genie, No. 10, present a degree of research and completeness which peculiarly charac- terize all the writings of this gentleman, and have given to his productions a claim to the fullest confidence of practical men. The following formula, applicable to cases of rotation abo*ut the exterior edge of the lowest horizontal joint, are taken from the memoir above cited. Calling H, the height BC (Fig. 29) of a wall of uniform thick- ness, the face and back being vertical. N Fig. 29 Represents a section O of a retaining wall with the face and back vertical. P, section of the embankment above the wall. h, the mean height CG of the embankment, retained by the wall, above the top of the wall. :, the berm DI, or distance between the foot of the embankrr em and the outer edge of the top of the wall. a, the angle between the line of the natural slope BN of the earth of the embankment and the vertical BG. f = cot. , the co-efficient of friction of the earth of the embank ment. CONSTRUCTION OF MASONRY. 141 , the weight of a cubic foot of the earth. a, 1 ', the weight of a cubic foot of the masonry of the wall. />, the base AB, or thickness of the wall at bottom. Then, 6=0.74tan . |a V , -0.6^4-0.25). The above formula gives the value of the base of a wall with vertical faces, within a near degree of approximation to the true result, only when the values of the quantities which enter into it are confined within certain limits. These limits are as follows : for h, between and H ; c, between and |H ; f, between 0.6 and 1.4, which correspond to values of a of 70 and 35, being in the one case the angle which the line of the natural slppe of very fine dry sand assumes, and in the other of heavy clayey earth : and for w, between w', and f w/. Besides these limits, the formula also rests on the assumption that the excess of stabil- ity of the wall over that of a strict equilibrium is represented by 0.912 ; or, in other words, that the moment of the pressure against the wall is taken 0.912 greater than the moment of strict equi- librium between it and the wall. This excess of stability given to the wall supposes an excess of resistance above the pressure against it equal to what obtains in the retaining walls of Vauban, for fortifications which have now stood the test of more than a century with security. 419. Having by the preceding formula calculated the value of 6 for a vertical wall, the base b' of another wall, presenting equal stability, but having a batter on the face, the back being vertical, Fig. 30-^Represents a section O of a retaining wall with a sloping lace AD. P, section of the embankment, which is the usual form of the cross section of retaining walls, can be calculated from the following notation and formula. Calling (Fig. 30) 6' the base of the sloping wall. 142 MASONRY. Ad n = =r-j, the batter, or ratio of me base of the slope o the per pendicular, or height of the wall. Then, 420. With regard to sliding either on the base of the founda- tion courses, or on the bed of any of the horizontal joints of the wall, M. Poncelet shows, in the memoir cited, by a comparison of the results obtained from calculations made under the suppo- sitions both of rotation and sliding, that no danger need be appre- hended from the latter, when the dimensions are calculated to conform to the former, so long as the limits of h are taken between and 4H ; particularly if the precaution be taken to allow the mortar of the masonry to set firmly before forming the embank- ment behind the wall. 421. Form of Section of Retaining Walls. Retaining walls have been built with a variety of forms of cross section. The more usual form of cross section is that in which the back of the wall is built vertically, and the face with a batter varying between one base to six perpendicular, and one base to twenty-four perpen- dicular. The former limit having been adopted, for the reasons already assigned, to secure the joints from the effects of weather ; and the latter because a wall having a face more nearly vertical, is liable in time to yield to the effects of the pressure, and lean forward. 422. The most advantageous form of cross section for econo- my of masonry is the one (Fig. 31) termed a leaning retaining Fig. 31 Represents a section O of a leaning retaining wall with a eloping face AD and the back BC coun- ter-eloped. wall. The counter slope, or reversed batter of the back of the wall, should not be less than six perpendicular to one base. In this case strength requires that the perpendicular let fall from the centre of gravity of the section upon the base, should fall so far within the inner edge of the base, that the stone of the bottom CO.VSTRUCriON OF MASONRY. course of the foundation may present sufficient surface to cear the pressure upon it. 423. Walls with a curved batter (Fig. 32) both upon the face and back, have been used in England, by some engineers, for quays. They present no peculiar advantages in strength over Fig. 32 Represents a section A of a wall with a curved face and back, and an elevation B of th counterforts. C, water. D, embankment behind the wall, a, fender beams of timber walls with plane faces and backs, and require particular care in arranging the bond, and fitting the stones or bricks of the face. 424. Measures for increasing the Strength of Retaining Walls. These consist in the addition of counterforts, in the use of relieving arches, and in the modes of forming the embank- ment. 425. Counterforts give additional strength to a retaining Avail m several ways. By dividing the whole line of the wall into shorter lengths between each pair of counterforts, they prevent the horizontal courses of the wall from yielding to the pressure of the earth, and bulging outward between the extremities of the walls ; by receiving the pressure of the earth on the back of the counterfort, instead of on the corresponding portion of the back of the wall, its effect in producing rotation about the exterior foot of the wall is diminished ; the sides of the counterforts acting as abutments to the mass of earth between them may, in the case of sand, or like soil, cause the portion of the wall between the coun- terforts to be relieved from a part of the pressure of the earth nehind it owing to the manner in which the particles of sand be- come buttressed against each other when confined laterally, and offer a resistance to pressure. 426. The horizontal section of counterforts may be either rectangular, or trapezoidal. Wher placed against the back of a wall, the rectangular form offers the greater stability in the case of rotation, and is more economical in construction ; the trape 144 MASONRY. eoidal form gives a broader and therefore a firmer connection be- tween the wall and counterfort than the rectangular, a point ot some consideration where, from the character of the materials, the strength of this connection must mainly depend upon the strength of the mortar used for the masonry. 427. Counterforts have been chiefly used by military engineers for the retaining walls of fortifications, termed revetements. In regulating their form and dimensions, the practice of Vauban has been generally followed, which is to make the horizontal section of the counterfort trapezoidal, making the height of the trapezoid ef, (Fig. 33,) which corresponds to the length of the counterfort, two tenths of the height of the wall added to two feet, the base of the trapezoid ab corresponding to the junction of the counterfort and back of the wall, one tenth of the height added to two feet, and the side cd which corresponds to the back of the counterfort equal to two thirds of the base ab. The counterforts are placed - Fig. 33 Represents a section A 1 , and plan D of a wall, and an elevation B, and plan E of a trapezoidal counterfort. ?" all, from 15 to 18 feet from centre to centre along the back of the wall, according to the strength required, 428. In adding counterforts to walls, the practice has generally been to regard them only as giving additional stability to the wall, and not as a means of diminishing its volume 01 masonry of which the addition of the counterforts ought to admit. Considered in this last point of view, the problem for determining both the suitable dimensions of the counterforts and the thickness of the corresponding wall, is one of very considerable mathematical difficulty, whose solution must repose upon assumptions made as CONSTRUCTION OF MASONRY. 145 co the manner in which the portions of the wall between the counterforts would be likely to yield to the pressure upon them, the support which they receive from the two counterforts at their extremities, and the stability which the counterforts add to the entire system in preventing rotation. 429. Relieving Arches are so termed from their preventing a portion of the embankment from resting against the back of the wall, and thus relieving it from a part of the pressure. They consist (Fig. 34) of one or more tiers of brick arches 'built upon counterforts, which act as the piers of the arches. M Fig. 34 Represents a section M and an ele- vation N of a wall and relieving arches in three tiers. A, section of the wall. c, c, c, sections of the arches through their crowns. d , d, interior elevations of counterforts serv- ing as piers of the arches. e, e, interior end elevations of arches. In arranging a combination of relieving arches and their piers, the latter, like ordinary counterforts, are placed about 18 feet apart between their centre lines ; their length should be so regu- lated that the earth behind them resting on the arches, and falling under them with the natural slope, shall not reach the wall be- tween the arch and the foot of the back of the wall below the arch. The thickness of the arches, as well as that of the counterforts, will depend upon the weight which the arches sustain. The dimensions of the wall will be regulated by the decreased pres sure against it caused by the action of the arches, and the point at which this pressure acts. 430. Whenever it becomes necessary to form the embankment before the mortar of the retaining wall has had time to set firmly, various expedients may be employed to relieve the wall from the pressure which the embankment, if formed of loose earth, would throw upon it. The portion of the embankment next to the wall may be of a compact binding earth placed in layers inclining downward from the back of the w r all, and well rammed ; or of a stiff mortar made either of clay, or sand, with about ^Vtfi i n bulk of lime. Instead of bringing the embankment directly against the back of the wall, dry stone, or fascines may be laid in to a suitable depth back from the wall for the same purpose. The precaution, however, of allowing the mortar to set firmly before Forming the embankment, should never be omitted except in cases of extreme urgency, and then the bond of the masonry should be 19 1 46 MASONRY. arranged with peculiar care, to prevent disjunction aloi.g any ol the horizontal joints. 431. Walls built to sustain a pressure of water should be regu- lated in form and dimensions like the retaining walls of embank- ments. The problem in this case is one of less difficulty thai! in the other, from the greater simplicity of the mathematical formula for the pressure of water. The buoyant effort of the water must be taken into account in this calculation, whenever the masonry is so placed as to be partially immersed in the water. 432. Heavy walls, and even those of ordinary dimensions, when exposed, to moisture, should be laid in hydraulic mortar. Grout has been tried in laying heavy rubble walls, but with de- cided want of success, the successive drenchings of the stone causing the sand to separate from the lime, leaving when dry a weak porous mortar. When the stone is laid in full mortar, grout may be used with advantage over each course, to fill any voids left in the mass. 433. Beton has frequently been used as a filling between the back and facing of water-tight walls ; it presents no advantage over walls of cut, or rubble stone laid in hydraulic mortar, and causes unequal settling in the parts, unless great care is taken in the construction 434. When a wejght, arising from a mass of masonry or earth, rests upon two or more isolated supports, that portion of it which is distributed over the space, or bearing between any two of the supports, may be borne by a block of stone, termed a lintel, laid horizontally upon the supports, by a combination of blocks termed a plate-bande, so arranged as to resist, without disjunction, the pressure upon them ; or by an arch. 435. Lintel. Owing to the slight resistance of stone to a cross strain, and to shocks, lintels of ordinary dimensions cannot be used alone with safety, for bearings over six feet. For wider bearings, a slight brick arch is thrown across the bearing above the lintel, and thus relieves it from the pressure of the parts above. 436. Plate-bande. The plate-bande is a combination of blocks cut in the form of truncated wedges. From the form of the blocks, the pressure thrown upon them causes a lateral pressure which must be sustained either by the supports, or by some other arrangement, (Fig. 35.) The plate-bande should be used only for narrow bearings, as the upper edges of the blocks at the acute angles are liable to splinter from the pressure. If the bearing exceeds 10 feet, the plate-bande should be relieved from the pressure by a brick arch above it. Additional means of strengthening the plate-bande are ometimes used by forming a broken joint between the blocks, or CONSTRUCTION OF MASONRY. 147 by a projection made on the face of one block to fit into a cor- responding indent in the adjacent one, or by connecting the blocks with iron bolts. c Fig. 35 Represents a crow section of a plate-bande, showing the manner in which the voussoirs A, A and B are cut and con- nected by meta! cramps. ab, tie of wrought iron for the plate-bande fastened to the bolts cd, let into the piers of the plate bande. When, from any cause, the supports cannot be made suffi- ciently strong to resist the lateral pressure of the plate-bande, the extreme blocks must be united by an iron bar, termed a tie, suit- ably arranged to keep the blocks from yielding. 437. Arches. The arch is a combination of wedge-shaped blocks, termed arch stones, or voussoirs, truncated towards the angle of the wedges by a curved surface which is usually normal to the surfaces of the joints between the blocks. This inferior surface of the arch is termed the intrados, or soffit. The upper, or outer surface of the arch is termed the extrados, or back, .'Fig. 36.) M N J L Fig. 36 Represents an elevation 31 of the head of a right cylindrical arch, and a section N through the crown of the arch A, with an elevation B ot the soffit and the face C of the abutment. ab, span of the arch. dc rise. aco\ curve of the intrados. e, e, voussoirs forming ring courses of heads. f, key stone. g, cushion stone of abutment. mn, crown of the arch. op, springing line. 438. The extreme blocks of the arch rest against lateral sup- ports, termed abutments, which sustain both the vertical pressure arising from the weight of the arch stones, and the weight of whatever lies upon them ; also the lateral pressure caused by the tction of the arch. 439. In a range, or series f -.rches placed side by side, the 148 MASONRY. extreme supports are termed the abutments, the inte.mediale sup ports which sustain the intermediate arches and the halves of the two extreme ones are termed piers. When the size of the arches is the same, and their springing lines are in the same horizontal plane, the piers receive no other pressure but that arising from the weight of the arches. 440. Arches are classified, from the form of the soffit, intd cylindrical, conical, conoidal, warped annular, groined, clois* tered, and domes. They are also termed right, oblique, or askew, and rampant, from their direction with respect to a vertical, or horizontal plane. 441. Cylindrical Arch. This is the most usual and the sim- plest form of arch. The soffit consists of a portion of a cylindri- cal surface. When the section of the cylinder perpendicular to the axis of the arch, termed a right section, cuts from the surface a semi-circle, the arch is termed a full centre arch ; when the section is an arc less than a semi-circle, it is termed a segment arch; when the section gives a semi-ellipse, it is termed an elliptical arch; when the section gives a curve resembling a semi-ellipse, formed of arcs of circles tangent to each other, the arch is termed an oval, (Fig. 37,) or basket handle, and is called Fig. 37 Represents an oval curve ol three centres, the arcs of which are each 60. DB, span of the curve. -' AC, rise. P, O, and R, centres of the arcs of 60. O a curve of three, five, &c. centres, according to the number of arcs, which must be odd to obtain a curve symmetrical with respect to the vertical line bisecting it ; when the section is that of two arcs of circles intersecting at the middle point of the curve, it is termed a pointed, or an obtuse or surbased arch, (Figs. 3S and 39,) according as the angle between the arcs at their inter- section is acute, or obtuse. A cylindrical arch is denominated a right arch when it is ter minated by two planes, termed the heaJs of the arch, perpendicu CONSTRUCTION OF MASONRY. 149 iar to the axis of the arch ; oblique, or askew, when the heads are oblique to the axis ; and rampant when the axis of the arch is oblique to the horizontal plane. Fig. 38 Represents the half of a pointed ciun of four centres. ab, half span. be, rise, m and n, centres of the half curve ac. \ \ \ Fig. 39 Represents the half of an obtuse or based curve of four centres. ab, half span. be, rise. m and n, centres of the half curve ac. 'in 442. The chord of the curve of right section (see Fig. 36) is termed the span of the arch, its versed sine the rise of the arch. When the heads of the arch are oblique to the axis, the chord of the oblique section made by the plane of the heads is termed the span of the askew section. The lines of the soffit corresponding to the extremities of the span are termed the springing lines of the arch ; the top portion, or line of the soffit, is termed the crown. The ti > stone of the crown the key stone. The line drawn through the middle point of the span at the extremities of the arch, is termed the axis of the arch.* 443. The form of right section will depend upon the purposes which the arch is to serve, the locality, and the style of architec- ture employed. When the rise is less than half the span, the arch is weaker than in cither the full centre, or where the rise i * See Xote C., Appendix. 150 MASONRY. greater than half the span. The methods of Describing the various curves of right section will be explained in the Appendix. 444. The same general principle is followed in arranging the joints and bond of the masonry of arches, as in other structures of cut stone. The surfaces of the joints should be normal to the surface of the soffit, and the surfaces of any two systems of joints should be normal to each other at their lines of intersection. These conditions, with respect to the joints, will be satisfied by tracing upon the soffit its lines of least and greatest curvature, and taking the edges of one series of joints to correspond with one of these systems of lines, and the edges of the other series with the other system, the surfaces of the joints being formed by the surfaces normal to the soffit along the respective lines in question. When- ever the surface of the soffit belongs to any of the families of geometrical surfaces, the joints will be thus either plane, or de- velopable surfaces. In the cylindrical arch, for example, the edges of one series of joints will correspond to the elements of the cylindrical surface, while those of the other will correspond to the curves of right section, the former answering to the line of least, and the latter of greatest curvature. The surfaces of the joints will all be plane surfaces, and, being normal to the soffit along the lines in question, will be normal also to each other. 445. In full centre and segment arches, the voussoirs are usually made of the same breadth, estimated along the curve of right section. The planes of the joints of each course of vous soirs between the heads of the arch are made continuous, (see Fig. 36,) each of these courses being termed a string course, and their joints coursing joints. The planes of the joints along the curves of right section are not continuous, but break joints ; the stones which correspond to two consecutive series of these joints being termed a ring course, and its joints heading joints. By this combination of the ring and string courses, the fitting of the blocks, the settling of the courses, and the bond are arranged in the best manner. 440. In the other forms of right section of cylindrical arches, it may not. in many cases, be practicable to give the voussoirs the sime breadth, owing to the variable curvature of the right sec- tion ; but the same arrangement is followed for the ring and string courses. 447. Tr oblique cylindrical aiJies, when the obliquity is but slight. T o change will be required in the arrangement of the courses and joints ; but when the angle between the heads and the axi? i? considerably less than a right angle, the ring courses at the e-ftr amities of the ai.ii would have what is termed a false bearing, that is, the pressure upon their coursing joints would CONSTRUCTION OF MASONRY 151 not be transmitted in the direction of the pressure to the fixed lateral supports, and therefore these portions of the arch would be insecure. To obviate these defects, as well as the unequal bearing upon the lateral supports in such case, arrangements of the coursing and heading joints have been devised, by which a better bond is obtained, and the total pressure from the voussoirs thrown upon the abutments. One method for this purpose has been mostly used in England, and consists in placing the edges of the heading and coursing joints along spira. lines of the cylindrical soffit which intersect each other at right angles. The directing spirals for the heading joints (Fig. 40) being taken parallel to the one which is drawn con Fig. 40 Represents an ele- vation A of the head and of a part of the soffit B of an oblique cylindrical arch with spiral joints. a, voussoirs of cut stone. c, c, bottom course of stone voussoirs cut to receive the brick courses. C, face of the abutment. D, ends of the abutments. necting the extreme points of the askew curve of the head ; those for the coursing being traced perpendicular to the former. The joints being normal to the soffit along the spirals, will be helicoi- dal surfaces. This method palliates only to some extent the weakness of the bond in the courses near the heads, and giving a considerable dip to the coursing joints at the extremities of the abutments which make an acute angle with their faces, it presents here also a weak point. It possesses an important advantage, however, in permitting the soffit ends of the string courses to be of equal breadth throughout, and therefore allows the method tc be adapted as well to brick as cut stone. To bring the coursing joints to correspond exactly with the divisions of the ring courses of the heads, it may be necessary, in some cases, to shift the spirals of the coursing joints slightly, in making the drawings for the arch. The end blocks of the string courses which rest upon the abutment, or else the top course of the abutment, must be suitably cut to correspond to the direction of the heading joints and that of the horizontal courses of the abutment. 448. A second method, in use among the French engineers, consists in n aking the heading joints plane surfaces and parallel to the heads of the arch, and in taking for the edges of the coursing joints (Fig. 41) the trajectories traced on the soffit perpendicular to the edges of the heading joints. The surfaces of the coursing 152 MASONRY". joints are made normal to the soffit. B\ nis plan sjine of the defects of the former are remedied, but it has the disadvantage ' Fig. 41 Represents an elevation of the head and a portion of the soffit of an oblique cylindrical arch, with the edges of the cours- ing joints forming trajectories at right angles to the edges of beading joints parallel to the curves of the heads of the arch. The letters refer to same parts as in Fig. 40. of giving an unequal breadth to the soffit ends of the voussoirs, and therefore is inapplicable to brick arches. The curves of the trajectories and the coursing joints are of more difficult construc- tion than in the first method. 449. Cylindrical, groined, and cloistered arches are formed by the intersections of two or more cylindrical arches. The span of the arches may be different, but the rise is the same in each. The axes of the cylinders will be in the same plane, and they may intersect under any angle. The groined arch (Fig. 42) is formed by removing those por- i s M 4 r Fig. 42 Represents the plan of the soffit and the right sections'M and N of the cylinders forming a groined arch. aa, pillars supporting the arch. be, groins of the sottit. am, mn, edges of coursing joints. A, key stone of the two arches formed of one block. B, B, groin stones of one block below the key stone forming a part of each arch iions of each cylinder which lie under the other and between their common curves of intersection ; thus forming a projecting, or salient edge on the soffit along these curves. The cloistered arch (Fig. 43) is formed by removing those por- tions of each cylinder which are above the other and exterior to theii common intersection, forming thus re-entering angles along the same lines. 450. The planes of the joints in both of these arches are placed in the same manner as in the simple cylindrical arch. The inne? CONSTRUCTION OF MASONRY. 15? edges of the correspond'ng course of voussoirs in each arch are placed in the same plane parallel to that of the axes of the cvlin- M Fig. 43 Represents a section M of the vciiseoirs and an elevation of the soffit of a cloistered arcli, with a plan N of the soffit. A, A, voussoirs. mn, edge of coursing joint. a, o, edges of heading joints H, B, abutments of the arches. acb, curve of the groin. C, C, groin stones of one block. ders. The portions of the soffit in each cylinder, corresponding to each course of voussoirs, which form either the groin m the one case, or the re-entering angle in the other, are cut from a single stone, to present no joint along the common intersection of the arches, and to give them a firmer bond. 451. Conical arches are of rare application. When used, the same general principles with respect to the joints and bond apply to them. The surfaces of one set of joints will be planes passed through the elements of the cone and normal to the soffit ; the other will be conical, or other surfaces, likewise normal to the soffit and passing through the curves of least curvature. 452. When the spans at the two ends of an arch are unequal, but the rise is the same, then the soffit of the arch is made of a c.onoidal surface. The curves of right section at the two ends may be of any figure, but are usually taken from some variety of the elliptical, or oval curves. The soffit is formed by moving a line upon the two curves, and parallel to the plane containing their spans. The conoidal arch belongs to the class with warped soffits. A variety of warped surfaces may be used for soffits according to circumstances ; the joints and the bond depending on the gener- ation of the surface. 453. In arranging the joints in conoidal arches, the heading joints are contained in planes perpendicular to the axis of the arch. The coursing joints are also formed of plane surfaces, so 20 154 MASONRY. arranged that the portion of the joint corresponding to each block is formed by a plane normal to the conoid at the middle point of the lower edge of the block. In this way the joints of the string course will not be formed of continuous surfaces. To make them so, it would be necessary to give them the form of warped surfaces, which present more difficulty in their mechanical exe cution, and not sufficient advantages over the method just ex- plained to compensate for having them continuous. 454. The annular arch is formed by revolving the plane of a semi-circle, or semi-oval, or other curve, about a line drawn with- out the figure and parallel to the rise of the arch, (Fig. 44 ) One Fig. 44 Repre- sents a plan M of the abut- ments A and B, and the soffit C of an annular arch. N, right section of the arch. a, position of vertical axis around which the section Ps T IB revolved. series of joints in this arch will be formed by conical surfaces passing through the inner edges of the stones which correspond to the string courses ; and the other series will be planes passed through the axis about which the semi-circle is revolved. This last series should break joints with each other. 455. The soffit of a dome is usually formed by revolving the quadrant of one of the usual curves of cylindrical arches around the rise of the curve ; or else by revolving the semi-curve about the line of the span, and taking the half of the surface thus gen erated for the soffit of the dome. In the first of these cases the hori^pntal section of the dome at the springing line will be a cir- cle ; in the second the entire curve of the semi-curve by which the soffit is generated. The plan of domes may also be of regu- lar polygonal figure? , in which case the soffit will be a polygonal- CONSTRUCT :ON OF MASONRY. 155 cloistered arch formed of equal sections of cylinders, (Fig. 45 ) The joints and the bond are determined in the same manner as in other arches.' M Fig. 45 Represents a section M of the voussoirs and an elevation of the soffit, with a plan N of the soffit of an octagonal-cloistered dome. The letters refer to the same parts as in Fig. 43. 456. The voussoirs which form the ring course of the heads, in ordinary cylindrical arches, are usually terminated by plane surfaces at top and on the sides, for the purpose of connecting them with the horizontal courses of the head which lie above and on each side of the arch, (Figs. 46 and 47.) This connection Fig. 46 Represents a manner of connecting the voussoirs and horizontal courses in an oval arch. o, o, are examples of voussoirs with elbow joints. Fig. 47 Represents a mode of arranging the voussoira and horizontal courses in flat segment arches may be arranged in a variety of ways. The two points to be kept in view are, to form a good bond between the voussoirs and horizontal courses, and to give a pleasing architectural effect by the arrangement. This connection should always give a sym- metrical appearance to the halves of the structure on each side of the crown. To effect these several objects it may be neces- sary, in cases of oval arches, to make the breadth of the voussoirs unequal, diminishing usually those near the springing lines. 457. In small arches the voussoirs near the springing line are ao cut as to form a part also of the horizontal course, (see Fig. 46,) forming what is termed an elbow joint. This plan is objec- 156 MASONRY. tionable, both because there is a waste of material in forming a joint of this kind, and the stone is liable to crack when the arch settles. 458. The forms and dimensions of the voussoirs should be de- termined both by geometrical drawings and numerical calcula- tion, whenever the arch is important, or presents any complication of form. The drawings should, in the first place, be made to a scale sufficiently large to determine the parts with accuracy, and from these, pattern drawings giving the parts in their true size may be made for the use of the mason. To make the pattern drawings, the side of a vertical wall, or a firm horizontal area may be prepared, with a thin coating of mortar, to receive a thin smooth coat of plaster of Paris. The drawing may be made on this surface in the usual manner, by describing the curve either by points from its calculated abscissas and ordinates, or, where it is formed of circular arcs, by using the ordinary instrument for describing such arcs when the centres fall within the limits of the prepared surface. In ovals the positions of the extreme radi ; should be accurately drawn either from calculation, or construc- tion. To construct the intermediate normals, whenever the cen- tres of the arcs do not fall on the surface, an arc with a chord of about one foot, may be set off on each side of the point through which the normal is to be drawn, and the chord of the whole arc, thus set off, be bisected by a perpendicular. This construction will generally give a sufficiently accurate practical result, for elliptical and other curves of a large size. 459. The masonry of arches may be either of dressed stone, rubble, or brick. In wide spans, particularly for oval and other flat arches, cut stone should alone be used. The joints should be dressed with extreme accuracy. As the voussoirs have to be supported by a framing of timber, termed a centre, until the arch is completed, and as this structure is liable to yield, both from the elasticity of the materials and the number of joints in. the frame, an allowance for the settling in the arch, arising from these causes, is some- times made, in cutting the joints of the voussoirs false, that is, not according to the true position of the normal, but from 7 he supposed position the joints will take when the arch has settled thoroughly. The object of this is to bring the surfaces of the joints into perfect contact when the arch has assumed its perma- nent state of equilibrium, and thus prevent the voussoirs from breaking by unequal pressures on their coursing joints. This is a problem of considerable difficulty, and it will generally be better to cut the joints true, and guard against settling and its effects by giving great stiffness to the centres, and by placing be- tween the joints of those voussoirs, where the principal movement CONSI1.UCTION OF MASONRY. 157 takes place in arches, sheets of lead suitably hammered lo fit the joint and yield to any pressure. 460. The manner of laying the voussoirs demands peculiar care, particularly in those which form the heads of the arch. The positions of the inner t dges of the voussoirs are determined by fixed lines, marked on the abutments, or some other immovea- ble object, and the calculated distances of the edges from these lines. These distances can be readily set off by means of the level and plumb-line. The angle of each joint can be fixed by a quadrant of a circle, connected with a plumb-line, on which the position of each joint is marked. 461. Rubble stone is used only for very small arches which do not sustain much weight, or as a filling between a network of ring and string courses in large arches which sustain only their own weight. In each case the blocks of rubble should be roughly dressed with the hammer, and be laid in good hydraulic mortar. 462. Brick may be used alone, or in combination with cut stone, for arches of considerable size. When the thickness of a brick arch exceeds a brick and a half, the bond from ihe soffit outward presents some difficulties. If the bricks are laid in con- centric layers, or shells, a continuous joint will be formed parallel to the surface of the soffit, which will probably yield when the arch settles, causing the shells to separate, (Fig. 48.) If the Fig. 48 Represent* >n end view M of a brie* arch built with blocks C, and shells A, and B. N, represents the manner of arranging the courses of brick forming the crown of the arch. bricks are laid like ordinary string courses, forming continuous joints from the soffit outward, these joints, from the form of the bricks, will be very open at the back, and, from the yielding of the mortar, the arch will be liable to injury in settling from this cause. To obviate both of these defects, the arch may be built partly by the first plan and partly by the second, or as it is termed, in shells and blocks. The crown, or key of the arch should be laid in a block, increasing the breadth of the block by two bricks for each course from the soffit outward. These bricks should be 158 MASONRY. iaid in hydraulic cement, and be well wedged with pieces of thin hard slate between the joints. 463. When a combination of brick and cut stone is used, the ring courses of the heads, with some intermediate ring courses, the bottom string courses, the key-stone course, and a few inter- mediate string courses, are made of cut stone, (Fig. 49,) the Fig. 49 Repre- sents a cross sec- tion of a stone segment arch capped with brick and beton. A, stone voussoirs. B and D, brick and beton capping. C, abutment. E, cushion stone. intermediate spaces being filled in with brick. The brick poi tions of the soffit may, if necessary, be thrown within the stone portions, forming plain caissons. 464. The centres of small arches are not removed, or struck until the mortar has become hard ; in large arches, the centres should not be struck until the whole of the mortar has set firmly. In the joints near the springing lines the mortar will have become hard, in the ordinary progress of building an arch, before that in the higher joints will have had time to set, unless hydraulic mor- tar of a quick set be used. After the centres are struck, the arch is allowed to assume its permanent state of equilibrium, before any of the superstructure is laid. 465. When the heads of the arch form a part of an exterior surface, as the faces of a wall, or the outer portions of a bridge, the voussoirs of the head ring courses are connected with the horizontal courses, as has been explained ; the top surface of the voussoirs of the intermediate ring courses are usually left in a roughly dressed state to receive the courses of masonry termed the capping, (see Fig. 49,) which rests upon the arch between the walls of the head. Before laying the capping, the joints of the voussoirs on the back of the arch should be carefully exam- ined, and, wherever they are found to be open from the settling of the arch, they should be filled up with soft-tempered mortar, and by driving in pieces of hard slate. The capping may be va- riously formed of rubble, brick, or beton. Where the arches are exposed to the filtration of rain water, as in those used for bridges, and the casemates of fortifications, the capping should be of beton CONSTRUCTION OF MASONRY. 159 (aid in layers, and well rammed with the usual precautions for obtaining a solid homogeneous mass. 466. The difficulty of forming water-tight cappings of mason- ry has led engineers, within a few years back, to try a coating of asphalte upon the surface of beton. The surface of the beton capping is made uniform and smooth by the trowel, or float, and the mass is allowed to become thoroughly :"jy before the asphalte is laid. Asphalte is usually laid on in twc layers. Before apply- ing the first, the surface of the beton should be thoroughly cleansed of dust, and receive a coating of mineral tar applied hot with a swab. This application of hot mineral tar is said to pre- vent the formation of air bubbles in the layers of asphalte which, when present, permit the water to percolate through the masonry. The first layer of asphalte is laid on in squares, or thin blocks, care being taken to form a perfect union between the edges of the squares by pouring the hot liquid along them in forming each new one. The surface of the first layer is made uniform, and rubbed until it becomes smooth and hard with an ordinary wooden float. In laying the second layer, the same precautions are taken as for the first, the squares breaking joints with those of the first. Fine sand is strewed over the surface of the top layer, and pressed into the asphalte before it becomes hard. Coverings of asphalte have been used both in Europe and in our military structures for some years back with decided success. There have been failures, in some instances, arising in all prob- ability either from using a bad material, or from some fault of workmanship. 467. In a range of arches, like those of bridges, or casemates, the capping of each arch is shaped with two inclined surfaces, like a common roof. The bottom of these surfaces, by their junction, form gutters where the water collects, and from which it is conveyed off in conduits, formed either of iron pipes, or of vertical openings made through the masonry of the piers which communicate with horizontal covered drains. A small arch of sufficient width to admit a man to examine its interior, or a square culvert, is formed over the gutter. When the spaces between the head walls above the capping is filled in with earth, a series of drains running from the top, or ridge of the capping, and leading into the main gutter drain, should be formed of brick. They may be best made by using dry brick laid flat, and with intervals left for the drains, these being covered by other courses of dry brick with the joints in some degree open. The earth is filled in upon the upper course of bricks, which should be so laid as to form a uniform surface. 468. When the space above the capping is not filled in with a colid mass, for the purpose of receiving the -veight borne by the 160 MASONRY. arches, walls of a requisite height may be built parallel to the head walls, and these may serve either as the piers of small arches, (Fig. 50,) upon which the weight borne directly rests, or Fig. 50 Represents a section thiough a pier and the heads of an arch, sliowing the manner in which small arches are built on piers C, C, paral- lel to the head walls B, to sustain the load above the arch else be covered by strong flat stones to effect the same object In this last case (Fig. 51) the walls may be made lighter by form Fig. 51 Represents a cross section of the parts of two arches, and the pier A, showing the manner in which walls B, with arched open- ings C. C through them are built parallel to the heads, to receive the flat stones a, a which support the load above the arches. me arched openings through them, or else a system of small right cylindrical groined arches may be used. All of these methods are in use in bridge building for sustaining the roadway, and also in roofing arched edifices. They throw less weight upon the abut- ments and piers of the arches than would a filling of solid ma- terial. 469. From observations taken on the manner in which large cylindrical arches settle, and experiments made on a small scale, it appears that in all cases of arches where the rise is equal to or less than the half span they yield (Fig. 52) by the crown of the arch falling inward, and thrusting outward the lower portions, presenting five joints of rupture, one at the key stone, one on each side of it which limit the portions that fall inward, and one on each side near the springing lines which limit the parts thrust CONSTRUCTION OF MASONRY. 161 outward. In pointed arches, or those in which the rise is greater Fie:. 52 Represents the manner in which flat arches yield By rupture o, joint of rupture at the key stone. m, ?, .joints of rupture below the key stone. n n, , joints of rupture at springing lines. than the half span, the tendency to yielding is, in some cases, different ; here the lower parts may fall inward, (Fig. 53,) and thrust upward and outward the parts near the crown. Fig. 53 Represents the manner in which pointed arches may yield. The letters refer to same points as in Fig. 52. 470. From this movement in arches a pressure arises against the key stone, termed the horizontal thrust of the arch, the tendency of which is to crush the stone at the key, and to over- turn the abutments of the arch, causing them to rotate about the exterior edge of some one of their horizontal joints. 471. The joints of rupture below the key stone vary in arch- es of different forms, and in the same arch with the weight it sus- tains. From experiments, it appears that in full centre arches the joints in question make an angle of about 27 with the horizon ; in segment arches of arcs less than 120 they are at the springing lines ; and in oval arches of three centres they are found about the angle of 45 of the small arc which forms the extremity of the curve at the springing line. 472. T^he calculation of the joints of rupture, the consequent horizontal thrust, and its effects in crushing the stone at the key and in overturning the abutment are problems of conside- rable mathematical intricacy. When the joints of rupture are given the problem assumes a more simple form, being one of statical equilibrium between the moments of the horizontal thrust and the weight of the arch and its abutments. The problem for finding the joints of rupture by calculation, and the consequent thickness of the abutments necessary to preserve the arch from yielding, has been solved by a number of writers on the theory of the equilibrium of arches, and tables for effecting the necessary numerical calculations have been drawn up from their results to abridge the labor in each case. 473. The connection between the top of the abutment, term- ed the impost of the arch, and the bottom courses of the arch, 21 162 MASONBT. requires peculiar care in segmental, askew, and rampant arches. In the first, the thrust of the arch being very great, it will be well, in heavy arches, to make the joints of the interior courses of the abutment, for some courses at least below the impost, ob- lique to the horizon to counteract any danger from sliding. The top stone of the abutment, termed the cushion stone of the arch, should be well bonded with the stones of the backing, and its bed, or bottom joint should be so far below the impost joint, that the stone shall offer sufficient strength to resist the pressure on it. In the askew arch the abutments are not uniformly loaded, and the entire thrust of the arch will not be received by the abutments if the arch is constructed in the usual manner. Each of these points requires peculiar attention ; the first demanding the thickness of the abutment to be suitably regulated ; the se- cond that the arch be so built that the thrust may be thrown, as nearly as practicable, parallel to the planes of the heads. To effect this last point, the portion of the arch above the upper joints of rupture (Fig. 54,) must be divided into several zones, each of these zones being built without any connection with the two adjacent to it, but with their ends so arranged that this connec- tion may be formed, and the arch made continuous after the Fig. 54 Represents the development of half of the soffit of an oblique cylindrical arch with helicoidal joints, showing the divisions of the soffit into zones A, B, C, D by a series of heading joints mn laid open without mortar. acb, development of curve of oblique section. c, one of the edgos of the coursing joints perpendicu- lar to the right line db. ad, springing line of arch. centres are struck. By this plan the settling will take place after uncentring without causing cracks, and the thrust will be thrown on the abutments in the direction desired. In rampant arches, the impost joint being oblique to the ho- rizon, care must be taken, if this obliquity be not less than the angle of friction of the stone used, either to cut the impost into steps, or else to use some suitable bond, or iron cramps and bolts to prevent disjunction between the arch and abutment. 474. The abutments of right and of slightly oblique cylindri- cal arches are made of uniform dimensions : but when the ob CONSTRUCTION' OF MASONRY. 16S hquity is considerable, it may be necessary to increase the thick- ness of a portion of each abutment where there is the greatest pressure. In conical and conoidal arches the abutments will in like man ner vary in dimensions with the span. 475. In cloistered arches the abutments will be less than in an ordinary cylindrical arch of the same length ; and in groined arches, in calculating the resistance offered by the abutments, the counter resistance offered by the weight of one portion in resisting the thrust of the other, must be taken into consideration. 476. When abutments, as in the case of edifices, require to be of considerable height, and therefore would demand extraordinary thickness, if used alone to sustain the thrust of the arch, they may be strengthened by the addition to their weight made in carrying them up above the imposts like the battlements and pinnacles in Gothic architecture ; by adding to them ordinary, full, or arched buttresses, termed flying buttresses ; or by using ties of iron con- necting the voussoirs near the joints of rupture below the key stone. The employment of these different expedients, their forms and dimensions, will depend on the character of the structure arid the kind of arch. The iron tie, for example, cannot be hid- den from view except in the plate-bande, or in very flat segment arches, and wherever its appearance would be unsightly some other expedient must be tried. Circular rings of iron have been used to strengthen the abut- ments of domes, by confining the lower courses of the dome and relieving the abutment from the thrust. 477. When abutments sustain several arches above each other, like relieving arches in tiers, their dimensions must be calculated to sustain the united thrusts of the arches ; and tiie several por- tions between each tier must be strong enough to resist the thrust of their corresponding arches. 478. In a range of arches of unequal size, the piers will have to sustain a lateral pressure occasioned by the unequal horizontal thrust of the arches. In arranging the form and dimensions of the piers this inequality of thrust must be estimated for, taking also into consideration the position of the imposts of the unequal arches. 479. Precautions against Settling. One of the most difficult and important problems in the construction of masonry, is that of preventing unequal settling in parts which require to be con- nected but sustain unequal weights, and the consequent ruptures in the masses arising from this cause. To obviate this difficulty requires on the part of the engineer no small degree of practical tact. Several precautions must be taken to diminish as far as practicable the danger from unequal settling. Walls sustaining 164 MASOMU . heavy vertical pressures should be built up uniformly, aid with great attention to the bond and co'rrect titling of the courses. The materials should be uniform in quality and size ; hydraulic mor- tar should alone be used ; and the permanent weight not be laid on the wall until the season after the masonry is laid. As a far- ther precaution, when practicable, a trial weight may be laid upon the wall before loading it with the permanent one. Where the heads of arches are built into a wall, particularly if they are designed to bear a heavy permanent weight, as an embankment of earth, the wall should not be carrried up higher than the imposts of the arches until the settling of the latter has reached its final term ; and as there will be danger of disjunction between the piers of the arches and the wall at the head, from the same cause, these should be carried up independently, but sc arranged that their after-union may be conveniently eifected. It would moreover be always well to suspend the building of the arches until the season following that in which the piers are finished, and not to place the permanent weight upon the arches until the season following their completion. 480. Pointing. The mortar in the joints near the surfaces of walls exposed to the weather should be of the best hydraulic lime, or cement, and as this part of the joint always requires to be carefully attended to, it is usually filled, or as it is termed pointed, some time after the other work is finished. The period at which pointing should be done is a disputed subject among builders, some preferring to point while the mortar in the joint i& still fresh, or green, and others not until it has become hard. The latter is the more usual and better plan. The mortar for pointing should be poor, that is, have rather an excess of sand ; the sand should be of a fine uniform grain, and but little water be used in tempering the mortar. Before applying the pointing, the joint should be well cleansed by scraping and brushing out the loose matter, and then be well moistened. The mortar is applied with a suitable tool for pressing it into the joint, and its surface is rubbed smooth with an iron tool. The practice among our military engineers is to use the ordinary tools for calking in applying pointing ; tp calk the joint with the mortar in the usual way, and to rub the surface of the pointing until it becomes hard. To obtain pointing that will withstand the vicissitudes of our cli- mate is not the least of the difficulties of the builder's art. The contraction and expansion of the stone either causes the pointing to crack, or else to separate from the stone, and the surface water penetrating into the cracks thus made, when acted upon by frost throws out the pointing. Some have tried to meet this difficulty by giving the surface of the pointing such a shape, and so ar ranging it with respect to the surfaces of the stones forming the CONSTRUCTION OF MASONRY. 165 joint, that the water shall trickle over the pointing without enter- ing the crack which is usually between the bed of the stone and the pointing. 481. The term flash pointing is sometimes applied to a coat- ing of hydraulic mortar laid over the face, or back of a wall, tc preserve either the mortar joints, or the stone itself from the action of moisture, or the effects of the atmosphere. Mortar for flash pointing should also be made poor, and when it is used as a stucco to protect masonry from atmospheric action, it should be made of coarse sand, and be applied in a single uniform coat over the sur- face, which should be prepared to receive the stucco by having the joints thoroughly cleansed from dust and loose mortar, and being well moistened. No pointing of mortar has been found to withstand the effects of weather in our climate on a long line of coping. Within a few years a pointing of asphalte has been tried on some of our mili- tary works, and has given thus far promise of a successful issue. 482. Stucco exposed to weather is sometimes covered with paint, or other mixtures, to give it durability. Coal tar has been tried, but without success in our climate. M. Raucourt de Charleville, in his work Traite des Mortiers, gives the following compositions for protecting exposed stuccoes, which he states to succeed well in all climates. For important work, three parts of linseed oil boiled with one sixth of its weight of litharge, and one part of wax. For common works, one part of linseed oil, one tenth of its weight of litharge, and two or three parts of resin. The surfaces must be thoroughly dry before applying the compositions, which should be laid on hot with a brush. 483. Repairs of Masonry. In effecting repairs in masonry, when new work is to be connected with old, the mortar of the old should be thoroughly cleaned off wherever it is injured along the surface where the junction is effected. The bond and other ar- rangements will depend upon the circumstances of the case ; the surfaces connected should be fitted as accurately as practicable, so that by using but little mortar, no disunion may take place from settling. 484. An expedient, very fertile in its applications to hydraulic constructions, has been for some years in use among the French engineers, for stopping leaks in walls and renewing the beds of foundations which have yielded, or have been otherwise removed by the action of water. It consists in injecting hydraulic cement into the parts to be filled, through holes drilled through the ma- sonry, by means of a strong syringe. The instruments used for this purpose (Fig. 55) are usually cylinders of wood, or of cast iron ; the bore uniform, except at the end which is terminated with a nozle c f the usual conical form ; the piston is of wood 166 MASONRY. and is duven down by a heavy mallet. In asing the synnge i; is adjusted to the hole ; the hydraulic cement in a semi-flu it Fig. 55 Represents the arrangements for in jeeting hydraulic cement under a wall. A, section of the wall with vertical holes c, c drilled through it. B, syringe and piston for injecting the cement into the space C under the wall. state poured into it ; a wad of tow, or a disk of leather being in- troduced on top before inserting the piston. The cement is forced in by repeated blows on the piston. 485. A mortar of hydraulic lime and fine sand has been used for the same purpose ; the lime being ground fresh from the kiln, and used before slaking, in order that by the increase of volume which takes place from slaking, it might fill more compactly all interior voids. The use of unslaked lime has received several ingenious applications of this character ; its after expansion may prove injurious when confined. The use of sand in mortar for injections has by some engineers been condemned, as from the tate of fluidity in which the mortar must be used, it settles to the bottom of the syringe, and thus prevents the formation of a homogeneous mass. 486. Effects of Temperature on Masonry. Frost is the most powerful destructive agent against \vhich the engineer has to guard in constructions 01 masonry. During severe winters in the northern parts of our country, it has been ascertained, by obser- vation, that the frost will penetrate earth in contact with walls to depths exceeding ten feet ; it therefore becomes a matter of the first imporcance to use every practicable means to drain thoroughly all the ground in contact with masonry, to whatever depths the foundations may be sunk below the surface ; for if this precau- tion be not taken, accidents of the most serious nature may hap- pen to the foundations from the action of the frost. If watei collects in any quantity in the earth around the foundations, it CONSTRUCTION OF MASONRY. 167 may be necessary to make small covered drains under them to convey it off, and to place a stratum of loose stone between the sides of the foundations and the surrounding earth to give it a free downward passage. Tt may be laid down as a maxim in building, that mortar which '.s exposed to the action of frost before it has set, will be so much damaged as to impair entirely its properties. This fact places in a stronger light what has already been remarked, on the necessity of laying the foundations and the structure resting on them in hy- draulic mortar, to a height of at least three feet above the ground ; for, although the mortar of the foundations might be protected from the action of the frost by the earth around them, the parts immediately above would be exposed to it, and as those parts at- tract the moisture from the ground, the mortar, if of common lime, would not set in time to prevent the action of the frosts of winter. In heavy walls the mortar in the interior will usually be se- em ed from the action of the frost, and masonry of this character might be carried on until freezing weather commences ; but still in all important works it will be by far the safer course to sus- pend the construction of masonry several weeks before the or- dinary period of frost. During the heats of summer, the mortar is injured by a too rapid drying. To prevent this the stone, or brick, should be thoroughly moistened before being laid ; and afterwards, if the weather is very hot, the masonry should be kept wet until the mortar gives indications of setting. The top course should al- ways be well moistened by the workmen on quitting their work for any short period during very warm weather. The effects produced by a high or low temperature on mortar in a green state are similar. In the one case the freezing of the water prevents a union between the particles of the lime and sand ; and in the other the same arises from the water being rapidly evaporated. In both cases the mortar when it has set is weak and pulverulent. 168 FRAMING FRAMING. 487. FRAMING is the art of arranging beams of solid material* for the various purposes to which they are applied in structures. A frame is any arrangement of beams made for sustaining strains. 488. That branch of framing which relates to the combinations of beams of timber is denominated Carpentry. 489. Timber and iron are the only materials in common use for frames, as they are equally suitable to resist the various strains to be met with in structures. Iron, independently of offering greater resistance to strains than timber, possesses the farther advantage of being susceptible of receiving the most suit- able forms for strength without injury to the material ; while tim- ber, if wrought into the best forms for the object in view may, in some cases, be greatly injured in strength. 490. The object to be attained in framing is to give, by a suit- able combination of beams, the requisite degree of strength and stiffness demanded by the character of the structure, united with a lightness and an economy of material of which an arrangement of a massive kind is not susceptible. To attain this end, the beams of the frame must be of such forms, and be so combined that they shall not only offer the greatest resistance to the efforts they may have to sustain, but shall not change their relative po- sitions from the effect of these efforts. 491. The forms of the beams will depend upon the kind of material used, and the nature of the strain to which it may be subjected, whether of tension, compression, or a cross strain. 492. The general shape given to the frame, and the combina tions of the beams for this purpose, will depend upon the objeci of the frame and the directions in which the efforts act upon it. In frames of timber, for example, the cross sections of eacn beam are generally uniform throughout, these sections being either circular, or rectangular, as these are the only simple forms which a beam can receive without injury to its strength. In frames of cast iron, each beam may be cast into the most suitable form for the strength required, and the economy of the material. 493. In combining the beams, whatever may be the general shape of the frame, the parts which compose it must, as far as practicable, present triangular figures, each side of the triangles being formed of a single beam ; the connection of the beams at the angular points, termed the joints, being so arranged thai no yielding can take place. In all combinations, therefore, in which FRAMING. 169 the principal beams form polygonal figures, secondary beams must be added, either in the directions of the diagonals of the polygon, or so as to connect each pair of beams forming an angle of the polygon, for the purpose of preventing any change of form of the figure, and of giving the frame the requisite stiffness. These secondary pieces receive the general appellation of braces. When they sustain a strain of compression they are termed struts; when one of extension, ties. 494. As one of the objects of a frame is to transmit the strain it directly receives to firm points of support, the beams of which it is formed should be so combined that this may be done in the way which shall have the least tendency to change the shape of the frame, and to fracture the beams. These conditions will be best satisfied by giving the principal beams of the frame a position such that the strains they receive shall be transmitted through the axes of the beams to the fixed supports ; in this man- ner there can be no tendency to change the shape of the frame, ex- cept so far as this may arise from the contractions, or elongations of the beams, caused by the strains ; and as all unnecessary transversal strains will in like manner be avoided, the resistances offered by the beams will be the greatest practicable. 495. Whenever these conditions cannot be satisfied, the strains un the frame should be so combined that those which are not transmitted to the points of support shall balance, or destroy each other ; and those beams which, from being subjected to a cross strain, might be either in danger of rupture, or of being deflected to so great a degree as to injure the stability of the frame, should be supported by struts abutting either against fixed supports, or against points of the frame where the pressure thrown upon the strut would have no effect in changing the shape of the frame. 496. The points of support of a frame may be either above, or below it. In the first case, the frame will consist of a suspended system, in which the polygon will assume a position of stable equilibrium, its sides being subjected to a strain of extension. In the second case the frame, if of a polygonal form, must satisfy the essential conditions already enumerated, in order that its state of equilibrium shall be stable. 497. The strength of the frame and that of its parts, and their consequent dimensions, must be regulated by the strains to which they are subjected. When the form of the frame and the direc- tion and amount of the strain borne by it are given, the direction and amount of the strain which the different parts sustain can be ascertained by the ordinary laws of statics, and, from these data, the requisite dimensions and forms of the parts. 498. The object of the structure will necessarily decide the general shape of the frame, as well as the direction of ihe strains 22 170 FRAMING. to which it will be subjected. An examination, therefore, of the frames adapted to some of the more usual structures will be the best course for illustrating both the preceding general principles and the more ordinary combinations of the beams and joints. 499. Frames of Timber. These are composed either entirely of straight beams, or of a combination of straight beams and of arches formed by bending straight beams. Pieces of crooked timber are used either where the form of the parts requires them, or else where a strong connection is necessary between straight pieces that form an angle between them. 500. As has already been stated, the cross section of each beam is generally uniform and rectangular. This will, in some cases, give more strength than the character of the strain resisted may demand ; and will, also, throw a greater amount of pressure on the points of support, than if beams of a form more strictly adapted to the object in view were used : but it avoids cutting the fibres across the grain, or making, as it is termed, grain-cm beams, and thereby materially injuring the strength of the piece. This objection, however, is only applicable to the parts of a frame formed of single beams. Wherever several thicknesses of beams are required in the arrangement of any part, the advantage may be taken of giving the combination the most suitable form for strength and lightness combined. 501. Frames for Cross Strains. The parts of a frame which receive a cross strain may be horizontal, as the beams, or joists of a floor; or inclined, as the beams, or rafters which form the inclined sides of the frame of a roof. The pressure producing the cross strain may either be uniformly distributed over the beams, as in the cases just cited, arising from the flooring boards in the one case, and the roof covering in the other ; or it may act only at one point, as in the case of a weight laid upon the beam. In all of these cases the extremities of the beam must be firmly fixed against immoveable points of support ; the longer side of the rectangular section of the beam should be parallel to the di- rection of the strain, on account of placing the beam in the best position for strength. If the distance between the points of support, or the bearing, be not great, the framing may consist simply of a row of parallel beams of such dimensions, and placed so far asunder as the strain borne may require. When the beams are narrow, or the depth Fig. 56 Represents a cross section ol' horizontal beams a braced by diagonal battens 6. of the rectangle considerably greater than the breadth, (Fig. FRAMING. 171 chort struts of battens may be placed at intervals betw een each pair of beams, in a diagonal direction, uniting the bottom of the one with the top of the other, to prevent the beams from twisting, or yielding laterally. When the bearing and strain are so great that a single beam will not present sufficient strength and stiffness, a combination of beams, termed a built beam, which may be solid, consisting of several layers of limber laid in juxtaposition, and firmly con- nected together by iron bolts and straps, or open, being formed of two beams, with an interval between them, so connected by cross and diagonal pieces, that a strain upon either the upper or lower beam will be transmitted to the other, and the whole system act under the effect of the strain like a solid beam. 502. Solid built Beams. In framing solid built beams, the pieces in each course (Fig. 57) are laid abutting end to end with Fig. 57 Represents a solid built beam of three courses, the pieces of each course breaking joints and confined by iron hoops. a square joint between them, the courses breaking joints to fonn a strong bond between them. The courses are firmly connected either by iron bolts, formed with a screw and nut at one end to bring the courses into close contact, or else by iron bands driven on tight, or by iron stirrups (Fig. 58) suitably arranged with screw ends and nuts for the same purpose. iL k r Fig. 58 Represents an iron stirrup, or hoop a with nuts or femaJ* screws c which confine the cross piece of tiie stirrup b When the strain is of such a character that the courses would be liable to work loose and slide along their joints, the beams of the different courses may be made with shallow indentations, (Figs. 59, 60,) accurately fitting into each other ; or shallow rec- Fig. 59 Represents a solid built beam of three courses arranged witli indents and confined by iron hoops. FIR. 60 -Represents a solid built beam, the top part being of two pieces b, b which abut against a broad flat iron bolt a, termed 'a king bolt. vangnlar notches (Fig. 61) may be cut across each beam, being 172 FRAMING. BO placed as to receive blocks, or keys of hard wood. The key t Fig. 61 Represents a solid biili beam witli keys b, b of hard T-OO between the courses. are sometimes made of two wedge-shaped pieces, (Fig. 62,) for I I Fig. 62 Represents the keys in the form of double, or, folding wedges a, b let into a shal- low notch in the beam c. the purpose of causing them to fit the notches more closely, and to admit of being driven tight upon any shrinkage of the woody fibre. The joints between the courses may be left slightly open without impairing in an appreciable degree the strength of the combination. This is a good method in beams exposed to mois- ture, as it allows of evaporation from the free circulation of the air through the joints. Felt, or stout paper saturated with min- eral tar, has been recommended to secure the joints from the action of moisture. The prepared material is so placed as to occupy the entire surface of the joint, and the whole is well screwed together. 503. Open built Beams. In framing open built beams, the principal point to be kept in view is to form such a connection between the upper and lower solid beams, that they shall be strained uniformly by the action of a strain at any point between the bearings. This may be effected in various ways, (Fig. 63.) Fisr. o:i Represents an open built beam ; A and B are the top and bottom rails or strings ; 11, a, cross pieces, either single or in pairs; b, diagonal braces in pairs; c, single diagonal braces. The upper and lower beams may consist either of single beams, or of solid built beams ; these are connected at regular intervals by pieces at right angles to them, between which diagonal pieces are placed. By this arrangement the relative position of all the parts of the frame will be preserved, and the strain at any point will be brought to bear upon the intermediate points. Two of the best known applications of this combination, when limber alone is used, are those of Colonel Long, of the U. S. Topographical Engineers, and of the late Mr. Town. 504. That of Colonel Long (Fig. 64) consists in forming both the upper and lower beams, termed by the inventor the strings, FRAMING 173 of three parallel beams, sufficient space being left between the one in the centre and the other two to insert the cross pieces, Fig. 64 Represents a panel of Long's truss. A and B, top and bottom strings of three courses. C, C, posts in pairs. D, braces in pairs. E, counter brace single. a. a, mortises where jibs and keys are inserted F, jib and key of hard wood. termed the posts ; the posts consist of beams in pairs placed at suitable intervals along the strings, with which they are connected by wedge blocks, termed jibs and keys, which are inserted into rectangular holes made through the strings, and fitting a corre- sponding shallow notch cut into each post. A diagonal piece, termed a brace, connects the top of one post with the foot of the out ad- jacent by a suitable joint. Another diagonal piece, termed the counter-brace, is placed crosswise between the two braces and their posts, with its ends abutting against the centre beam of the upper and lower strings. The counter-braces are connected with the posts and braces by wooden pins, termed tree-nails. In wide bearings, the strings will require to be made of several beams abutting end to end ; in this case the beams must break FRAMfNG. oints, and short beams must be inserted between the centre anr; exterior beams wherever the joints occur, to strengthen them. The beams in this combination are all of uniform cross section, the joints and fastenings are of the simplest kind, and the parts are well distributed to call into play the strength of the strings, and to produce uniform stiffness and strain. 505. The combination of Mr. Town (Fig. 65) consists in two B A Fie. 65 Represents an elevation A, and end view B, of a portion of Town's truss. a, a, top strings. b, b, bottom strings. c, c, diagonal braces main strings, each formed of two or three parallel beams of two thicknesses breaking joints. Between the parallel beams are in- serted a series of diagonal beams crossing each other. These diagonals are connected with the strings and with each other by tree-nails. When the strings are formed of three parallel beams, diagonal pieces are placed between the centre and exterior beams, and two intermediate strings are placed between the two courses of diagonals. This combination, commonly known as the lattice truss, is of very easy mechanical execution, the beams being of a uniform cross section and length. The strains upon it are borne by the tree-nails, and when used for structures subjected to variable strains and jars, it loses its stiffness and sags between the points of support. It is more recommendable for its simplicity than scientific combination. 506. A third method, called after the patentee, How's tru,.s, has within a few years come into general notice. It consists of (Fig. 66) an upper and lower string, each formed of several thick- nesses of beams placed side by side and breaking joints. On the upper side of the lower string and the lower side of the upper, blocks of hard wood are inserted into shallow notches ; the blocks are bevelled off on each side to form a suitable point of support, or step for the diagonal pieces. One series of the diagonal pieces are arranged in pairs, the others are single and placed between those in pairs. Two strong bolts of iron, which pass through the blocks, connect the upper and lower strings, and are arranged with a screw cut on one end and a nut to draw the parts closely together. This combination presents a judicious arrangement of the parts The blocks give abutting surfaces for the braces superior to those FRAMING 175 obtained by the ordinary forms of joint for this purpose. The bolts replace advantageously the timber posts, and in case of the Fig. 66 Represent! an elevation of a portion of Howe's truss. a, top string. b, bottom strings. c, c, diagonal Ibracee in pairs. d, single braces. e, e, steps of hard wood for braces. /,/, iron rods with nuts and screws -IS"' rrame working loose and sagging, their arrangement for tighten- ing up the parts is simple and efficacious. The timber of each string is not combined to give as great strength as its cross sec- tion is susceptible of, and the lower string, upon which a strain of tension is brought, against which timber offers the greatest resistance, has received a greater cross section than that of the upper. The preceding combinations have been applied .generally in our country to bridges. In this application, the timber support- ing the roadway of the bridge is usually placed on the iowei strings ; two, three, or four built beams being used, as the case may require, for supporting the transverse beams under the road- way, the centre beams leaving an equal width of roadway between them and the exterior beams. 507. Framing for intermediate Supports. Beams of ordinary dimensions may be used for wide bearings when intermediate supports can be procured between the extreme points. The simplest and most obvious method of effecting this is to ij. ace upright beams, termed props, or shores, at suitable intervals under the supported beam. When the props would interfere with some other arrangement, and points of support can be procured at the extremities below ihose on which the beam rests, inclined struts (Fig. 67) may be u.-'.ed. The struts must have a suitably formed step at the foot, li be connected at top with the beam by a suitable joint. In some cases the bearing may be diminished by placing OB 176 R \MING. Fig. 67 Represents a horizontal beam C snp ported near the middle by inclined strut! A, A the points of suppr rt si ort pieces, termed corbels, (Fig. 68,) md supporting these near tle-r ends by struts. Fig. 68 Represents a horizontal beam c sup- ported by vertical post a, , with corbel pie ces d, d and inclined struts e, e to diminish the bearing. In other cases a portion of the beam, at the middle, may be strengthened by placing under it a short beam, called a straining beam, (Fig. 69,) against the ends of which the struts abut. Fig. 69 Represents a horizontal beam c, strengthened by a straining beam / and inclined struts e, e. Whenever the bearing may require it the two preceding ar- rangements (Fig. 70) may be used in connection. F g. 70 Re- presents a combination of Figs. 68 and 69 In all combinations with struts, a lateral thrust will be thrown on the point of support where the foot of the strut rests. This strain must be provided for in arranging the strength of the sup- ports. 508. When intermediate supports can be procured only above the beam, an arrangement must be made which shall answer the purpose of sustaining the beam at its intermediate points by sus- pension. The combination will depend upon the number of in- termediate points required. FRAMING. 177 When the beam requires to be supported only at the middle, it may be done by placing two inclined pieces, resting on the beam" at its extremities, and meeting under an angle above it, from which the middle of the beam can be suspended by a rod of iron, or by another beam. If the suspending piece be of iron, it must be arranged at one end with a screw and nut. When the support is of timber, a single beam, called a Icing post, (Fig. 71,) Fig. 71 Represents a hori- zontal beam c supported in its middle by & king post g suspended from the struts e, e. may be used, against the head of which the two inclined pieces may abut ; the foot of the post is connected with the beam by a bolt, an iron stirrup, or a suitable joint. Instead of the ordinary king post, two beams maybe used; these are placed opposite to each other and bolted together, embracing between them the sup- ported beam and the heads of the inclined beams which fit into shallow notches cut into the supporting beams. Pieces arranged in this manner for suspending portions of a frame receive the name of suspension pieces, or oridle pieces. When two intermediate points of support are required, they may be obtained by two inclined pieces resting on the ends of the beam and abutting against the extremities of a short horizontal straining beam, (Fig. 72.) The suspension pieces in this case Fig. 72 Represents a beam supported at two points by posts g, g suspended from the * struts , e and straining beam m may be either posts, termed queen posts, arranged like a king post, iron rods, or bridle pieces. This combination may be used for very wide bearings, (Fig. 73,) by suitably increasing the num- ber of inclined pieces and straining beams. Some of the preceding combinations maybe used for support- ing one end of a beam subjected to a cross strain when the other has a fixed point of support. This may be done either by an in- clined strut beneath, or an inclined tie above the beam. When a wooden tie is used it should consist of two pieces bolted to- gether and embracing the beam. 23 178 FEAMIXG. Fig. 78 Represents a beam c suspended from a combination of struts and strain- ing beams by posts g, g. 509. The classifications under the two preceding heads repre- sent the principal combinations of straight beams applied to the purposes of framing. The frame of an ordinary roof presents one of the simplest combinations by which the action of the different parts of a frame may be illustrated. A roof of the ordinary form consists of two equally inclined sides of metal, slate, or other material, which is attached to a covering of boards that rests upon the frame of the roof. The frame consists of several vertical frames, termed the trusses of the roof, which are placed parallel to and at suitable intervals from each other ; these receive horizontal beams termedpurlins, which rest upon them and are placed at suitable intervals apart, and upon the purlins are placed inclined pieces termed the long rafters, to which the boards are attached. The truss of a roof, for ordinary bearings, consists (Fig. 74) Fig. 74 Represents a roof truss for medium spans. a. tie beam of truss. &, 6, principal rafters framed into tie beam and the king post c, and confined at their foot by an iron strap. !<>. Wooden Arches. A wooden arch may be formed by bending a single beam (Fig. 76) and confining its extremities to Fig. 76 Represents a horizontal beam o supported at its middle point by * bent beam 6. prevent it from resuming its original shape. A beam in this state presents greater resistance to a cross strain than when straight, and may be used with advantage where great stiffness is required, provided the points of support are of sufficient strength to resist the lateral thrust of the beam. This method can be resorted to only in narrow bearings. For wide arches a curved built beam must be adopted ; and for this purpose a solid, (Figs. 77 and 78,) or an open built beam may be used, depending on the bearing to be spanned by the arch. In either case the curved beams are built in the same manner as straight beams, the pieces of whicli they are formed being suitably bent to conform to the curvature of the arch, which may be done either by steaming the pieces, by mechanical power, or by the usual method of softening the woody fibres by keeping the pieces wet while subjected to the heat of a light blaze. 180 FRAMING. Fig. 77 Represents & wooden arch A formed of a solid built beam of three courses which support the beams c, e by the posts g, g which are formed of pieces in pairs. b, 6, inclined struts to strengthen the arch by relieving it of a part of the load on the beams c, c. Fig. 78 Represents a wooden arch of a solid built beam A which supports the horizontal beam B by means of the posts a. a. The arch is let into the beam B which acts as a tie to confine Its extremities. Wooden arches may also be formed by fastening together sev- eral courses of boards, giving the frame a polygonal form, (Fig. 79,) corresponding to the desired curvature, and then shaping the Fig. 79 Represents an elevation A of a wooden arch formed of short pieces a, b which abut end to end and break joints. B represents a perspective view of this combina- tion, showing the manner in which the parts are keyed together. outer and inner edges of the arch to the proper curve. Er\ch course is formed of boards cut into sharp lengths, depending on the curvature required ; these pieces abut end to end, the joints being in the direction of the radii of curvature, and the pieces composing the different courses break joints with each other. The courses may be connected either by jibs and keys of hard wood, or by iron bolts. This method is very suitable for all light frame work where the presure borne is not great. "Wooden arches are chiefly used for bridges and roofs. They FRAMING. 181 serve as intermediate points of support for the framing on which the roadway rests in the one ease, and the roof covering in the other. In bridges the roadway may lie either above the arch, or below it ; in either case vertical posts, iron rods, or bridles connect the horizontal beams with the arch. 511. The greatest strain in wooden arches takes place between the crown and springing line ; this part should, therefore, when practicable, be relieved of the pressure that it would directly receive from the beams above it by inclined struts, so arranged as to throw this pressure upon the lateral supports of the arch. The pieces which compose a wooden arch may be bent into any curve. The one, however, usually adopted is an arc of a circle, as the most simple for the mechanical construction of the framing, and presenting all desirable strength. 612. Centres. The wooden frame with which the voussoirs of an arch are supported while the arch is in progress of con- struction is termed a centre. A centre, like the frame of a roof, consists of a number of vertical frames (Figs. 80, 81, 82, 83,) termed trusses, or ribs, upon which horizontal beams, termed bolsters, are placed to re- ceive the voussoirs of the arch. The curved, or back pieces of a centre on which the bolsters rest consist of beams cut into suitable lengths and shaped to the proper curvature ; these pieces abut end to end, the joints be- tween them being in the direction of the radii of curvature ; the joints are usually secured by short pieces, or blocks placed un- cler the abutting ends to which the back pieces are bolted. The blocks form abutting surfaces for shores, or inclined struts seated against firm points of support below the back pieces. To pre- vent the shores, or the struts from bending, braces, or bridles, which are usually formed of two pieces, each with shallow notches cut into them, are added, and embrace between them the shores, or struts, the whole being firmly connected with iron bolts. The combinations used for the frames of centres will depend up- on the position of the points of support and the size of the arches. Fig. 80 Represents the rib of a centre for light arches. n, u, rib formed as in Fig. 79. &, 6, bolster piece* which receive the masonry. 513. For small light arches (Fig. 80) the ribs may be formed 182 FBAMINO. of two or more thicknesses of short boards, firmly nai ed togeth- er ; the boards in each course abutting end to end by a joint in the direction of the radius of curvature of the arch, ana breaking joints with those of the other course. The ribs are shaped to the form of the intrados of the arch, to receive the bolsters, which are of battens cut to suitable lengths and nailed to the ribs. 514. For heavy arches with wide spans, when firm interme- diate points of support can be procured between the abutments, the back pieces (Fig. 81) may be supported by shores placed Fig. 81 Represents the rib of a centre with in- termediate points of sup- port a, back pieces of the rib which receive the bol- sters/ b, 6, struts which support the back pieces. e, e, braces. c, solid beam resting on the intermediate gup- ports d, d, which re- ceive the ends of the struts 6, b. under the blocks in the direction of the radii of curvature of the arch, or of inclined struts (Fig. 82) resting on the points of Fig. 82 Represents a part of the rib of Orosvenor Bridge over the Dee at Chester. Span 200 feet. A, A, intermediate points of support. a, a, a, struts resting upon cast iron sockets on the supports A. 6, 6, two courses of plank each 4X inches thick bent over the struts a, a, to the form of the arch, the courses breaking joints. c, c, folding wedges laid upon the back pieces b of each rib to receive the bolsters on which the voussolrs are laid. support. The shores, or struts, are prevented from bending by traces suitably placed for the purpose. FRAMING. 183 515. If intermediate points of support cannot be obtained, a broad framed support; must be made at each abutment to receive the extremities of the struts that sustain the back pieces. The framed support (Fig. 83) consists of a heavy beam laid either FJg. 83 Represents a part of a rib of Waterloo Bridge over the Thames. a, a, and b, three heavy beams, forming: the striking plates, which with the shores A, \ form the framed support for the struts of the centre. c, c, struts abutting against the blocks fir, g placed under the joints of the back pieces// rf, d, bridk- or radial pieces in pairs which are confined at top and bottom between the horizontal ties >i. 11 of the ribs, also in pairs. e, , cast iron .sockets. m, m, bolsters of the centre resting on the back pieces / horizontally, or inclined, and is placed at that joint of the arch, (the one which makes an angle of about 30 with the horizon,) where the voussoirs, if unsupported beneath, would slide on their beds. This beam is borne by shores which find firm points of support on the foundations of the abutment. The back pieces of the centre (Fig. 83) may be supported by inclined struts which rest immediately upon the framed support, one of the two struts under each block resting upon one of the framed supports, the other on the one on the opposite side, the two struts being so placed as to make equal angles with the radius of curvature of the arch drawn through the middle point of the block. Bridle pieces, placed in the direction of the radius of curvature, embrace the blocks and struts in the usual manner, 184 FRAMING. and prevent the latter from sagging. This combination presents a figure of invariable form, as the strain at any one point is received by the struts and transmitted directly to the fixed points of support. It has the disadvantage of requiring beams of great length when the span of the arch is considerable, and of present- ing frequent crossing of the struts where notches will be re- quisite, and the strength of the beams thereby diminished. The centre of Waterloo Bridge over the Thames (Fig. 83) was framed on this principle. To avoid the inconveniences re- sulting from the crossing of the struts, and of building beams of sufficient length where the struts could not be procured from a single beam, me device was imagined in this work of receiv- ing the ends of several struts at the points of crossing into a large cast-iron socket suspended by a bridle piece. 516. When the preceding combination cannot be employed, a strong truss, (Fig. 84,) consisting of two inclined struts resting L Fie. 84 Represents a frame for a rib in which the two Inclined struts >, ft and the straining beams c form inter- mediate supports for some of the struts that support the back pieces a, , shoulders, or ribs to strengthen the flanches against lateral strains. c, tie plate between the ribs. /, (Fig. B) side view of the rinc cf the tie-plate fitted to the interior of the tube. d, cL, (Figs. A and B) saddle pieces to receive the open beams of a form similar to Fig 94. whloh rest on the tubular ribs. , cross section of the rib through the saddle piece. FRAMING. 193 States by Major Delafield of the U. S. Corps of Engineers, in an arch for a bridge of 80 feet span. Each rib was formed of nine segments ; each segment (Fig. 95) being cast in one piece, the cross section of which is an elliptical ring of uniform thick- ness, the transverse axis of the ellipse being in the direction of the radius of curvature of the rib. A broad elliptical flanch with ribs, or stays, is cast on each end of the segment, to connect the parts with each other ; and three chairs, or saddle pieces^ with grooves in them, are cast upon the entrados of each seg- ment, and at equal intervals apart, to receive -the open beam which rests on the curved rib. The ribs are connected by an open tie plate, (Fig. 95.) Raised elliptical projections are cast on each face of the tie plate, where it is connected with the segments, which are adjusted accurately to the interior surface of each pair of segments, between which the tie plate is embraced. The segments and plate are fastened by screw bolts passed through the end flanches of the segments. The tie plates form the only connection between the curved ribs ; the broad ribbed flanches of the segments, and the raised rims of the tie plates inserted into the ends of the tubes, giving all the advantages and stiffness of diagonal pieces. 532. Tubular ribs with an elliptical cross section have been used in France for many of their bridges. They were first intro- duced but a/ew years back by M. Polonceau, after whose Fig. 96 Represents a side view A and a cross section and end view B through a joint of M. Folonceau's tubular arch. a, a, top flanch, ft, b bottom flanch of the semi-segments united along the vertical joint cd through the axis of the rib. gh, side view of the joint between the flanches e, e of two semi-segments. TO, inner side of the flanc! c. cross section of a sonii-si-frment and top and bottom flanches. // thfn wedges of wrought iron placed between the end flanches of the semi-segments to bring the parts to their proper bearing. 25 194: FRAMING. designs the greater part of these structures have been built. According to M. Polonceau's plan, each rib consists of two symmetrical parts divided lengthwise by a vertical joint Each half of the rib is composed of a number of segments so distribut- ed as to break joints, in order that when the segments are put together there shall be no continuous cross joint through the ribs. The segments (Fig. 96) are cast with a top and bottom flanch and one also at each end. The halves of the rib are connected by bolts through the upper and lower flanches, and the segments by bolts through the end flanches. For the purposes of adjusting the segments and bringing the rib to a suitable degree of tension, flat pieces of wrought iron of a wedge shape are driven into the joints between the segments, and are confined in the joints by the bolts which fasten the segments and which also pass through these wedges. To connect the ribs with each other, iron tubular pieces are Fie. 97 Represents the half of a trass of wrought Iron for the new Houses of Parliament, England. The pieces of this truss are formed of bars of a rectangular section. The joints are secured by cast iron sockets, within which the ends of the hars are secured by screw bolts. FRAMING. 195 placed between them, the ends of the tubes being suitably ad- justed to the sides of the ribs. Wrought iron rods which serve as ties pass through the tubes and ribs, being arranged with screws and nuts to draw the ribs firmly against the tubular pieces. Diagonal pieces of a suitable form are placed between the ribs to give them the requisite degree of stiffness. In the bridges constructed by Mr. Polonceau according to this plan, he supports the longitudinal beams of the roadway by cast iron rings which are fastened to the ribs and to each other, and bear a chair of suitable form to receive the beams. 533. Iron roof Trusses. Frames of iron for roofs have been made either entirely of wrought iron, or of a combination of wrought and cast iron, or of these two last materials combined with timber. The combinations for the trusses of roofs of iron are in all respects the same as in those for timber trusses. The parts of the truss subjected to a cross strain, or to one of corn- Fig. 98 Represents the half of a trass for the same building composed of wrorfght and cast iron. o, a, feathered struts of cast iron. 6, b, suspension bars in pairs. *, n, tie and straining bars. e, , and// cross sections of beams resting in the cast iron sockets connected with the suspen- sion bars. 196 FKA311JNG. pression, are arranged to give the most suitable forms for strength, and to adapt them to the object in view. The parts subjected to a strain of extension, as the tie-beam and king and queen posts, are made either of wrought iron or of timber, as may be found best adapted to the particular end proposed. The joints are in some cases arranged by inserting the ends of the beams, or bars, in cast iron sockets, or shoes of a suitable form ; in others the beams are united by joints arranged like those for timber frames, the joints in all cases being secured by wrought iron bolts and keys. (Figs. 9T, 98, and 99.) Fig. 99 Represents the arrange- ments of the parts at the joint c in Fig. 98. A, side view of the pieces and joint a, principal rafter of the cross section B. &, common rafter of the cross section C. c, cross section of purlins and joint for fastening the com- mon rafters to the purlins. d, cast iron socket arranged to confine the pieces a, 2>, e,e. 534. Flexible Supports for Frames. Chains and ropes may frequently be substituted with advantage, for rigid materials, as intermediate points of support for frames, forming systems of suspension in which the parts supported are suspended from the flexible supports, or else rest upon them either directly, or through the intermedium of rigid beams. 535. All systems of suspension are based upon the property which the catenary curve in a state of equilibrium possesses of converting vertical pressures upon it into tensions in the di- rection of the curve. These systems therefore offer the advan- tages of presenting the materials of which they are composed in the best manner for calling into action the greatest amount of resistance of which they are capable, and of allowing the dimensions of the parts to be adapted to the strain thrown upon them more accurately than can be done in rigid systems ; thus avoiding much of the unproductive weight necessarily intro- duced into structures of stone, wood, and cast iron. They offer also the farther advantages that in their construction the parts of which they are composed can be readily adjusted, put together, FRAMING. 197 and taken apart for repairs. They present the disadvai. tages of changing both their form and dimensions from the action of the weather and variations of temperature, and of being liable to grave accidents from undulations and vertical vibrations caused by high winds, or inoveable loads. The require, therefore, that the fixed points of support of the system should be very firm and durable, and that constant attention should be given to keep the system in a thorough state of repair. 536. A chain or rope, when fastened at each extremity to fixed points of support, will, from the action of gravity, assume the form of a catenary in a state of equilibrium, whether the two extremities be on the same, or different levels. The rela- tive height of the fixed supports may therefore be made to conform to the locality. 537. The ratio of the versed sine of the arc to its chord, or span, will also depend, for the most part, on local circumstances and the object of the suspended structure. The wider the span, or chord, for the same versed sine, the greater will be the tension along the curve, and the more strength will therefore be required in all the parts. The reverse will obtain for an increase of versed sine for the same span ; but there will be an increase in the length of the curve. 538. The chains may either be attached at the extremities of the curve to the fixed supports, or piers ; or they may rest upon them, (Fig. 100, 101,) being fixed into anchoring masses, or Fig. 100 Represents a chain arch dbcde, resting upon two piers f, f and anchored at the points a and e, from which a horizontal beam mn is suspended by vertical chains, or rods. Fig. 101 Represents the manner in which the system may be arranged whon :i siii^li- pier is placed between the extreme points of the bearing. abutments, at some distance beyond the piers. Local circum- stances will determine which of the two methods will be the more suitable. The latter is generally adopted, particularly if the piers require to be high, since the strain upon them from the tension might, from the leverage, cause rupture in the pier near the bottom, and because, moreover, it remedies in some 198 FRAMING. degree the inconveniences arising from variations of tensior caused either by a moveable load, or 'changes of temperature. Piers of wood, or of cast iron moveable around a joint at their base, have been used instead of fixed piers, with the object of remedying the same inconveniences. 539. When the chains pass over the piers and are anchored at some distance beyond them, they may either rest upon saddle pieces of cast iron, or upon pulleys placed on the piers. 540. The position of the anchoring points will depend upon local circumstances. The two branches of the chain may either make equal angles with the axis of the pier, thus assuming the same curvature on each side of it, or else the extremity of the chain may be anchored at a point nearer to the base of the pier. In the former case the resultant of the tensions and weights will be vertical and in the direction of the axis of the pier, in the latter it will be oblique to the axis, and should pass so far within the base that the material will be secure from crushing. 541. The anchoring points are usually masses of masonry of a suitable form to resist the strain to wnich they are subjected. They may be placed either above or below the surface of the ground, as the locality may demand. The kind of resistance offered by them to the tension on the chain will depend upon the position of the chain. If the two branches of the chain make equal angles with the axis of the pier, the resistance offered by the abutments will mainly depend upon the strength of the material of which they are formed. If the branches of the chain make unequal angles with the axis of the pier, the branch fixed to the anchoring mass is usually deflected in a vertical direction, and so secured that the weight of the abutment may act in resisting the tension on the chain. In this plan fixed pulleys placed on very firm supports will be required at the point of deflection of the chain to resist the pressure arising from the tension at these points. Whenever it is practicable the abutment and pier should be suit- ably connected to increase the resistance offered by the former. The connection between the chains and abutments should be so arranged that the parts can be readily examined. The chains at these points are sometimes imbedded in a paste of fat lime to preserve them from oxidation. 542. The chains may be placed either above or below the structure to be supported. The former gives a system of more stability than the latter, owing to the position of the centre <>\' gravity, but it usually requires high piers, and the chain cannot generally be so well arranged as in the latter to subserve the re- quired purposes. The curves may consist of one or more chains. Several are usually preferred to a single one, as for the same FRAMING. 199 amount of metal they offer more resistance, can be more accu- rately manufactured, are less liable to accidents, and can be more easily put up and replaced than a single chain. The chains of the curve may be placed either side by side, or above each other, according to circumstances. 543. The curves may be formed either of chains, of wire ca- bles, or of bands of hoop iron. Each of these methods has found its respective advocates among engineers. Those who prefer wire cables to chains urge that the latter are more liable to accidents than the former, that their strength is less uniform and less in proportion to their weight than that of wire cables, that iron bare are more liable to contain concealed defects than wire, that the proofs to which chains are subjected may increase without, in all cases, exposing these defects, and that the con- struction and putting up of chains is more expensive and diffi- cult than for wire cables. The opponents of wire cables state that they are open to the same objections as those urged against chains, that they offer a greater amount of surface to oxidation than the same volume of bar iron would, and that no precau- tion can prevent the moisture from penetrating into a wire cable and causing rapid oxidation. That in this, as in all like discussions, an exaggerated degree of importance should have been attached to the objections urged on each side was but natural. Experience, however, derived from existing works, has shown that each method may be ap- plied with safety to structures of the boldest character, and that wherever failures have been met with in either method, they were attributable to those faults of workmanship, or to defects in the material used, which can hardly be anticipated and avoided in any novel application of a like character. Time alone can definitively decide upon the comparative merits of the two methods, and how far either of them may be used with advantage in the place of structures of more rigid materials. 544. The chains of the curves may be formed of either round, square, or flat bars. Chains of flat bars have been most gene- rally used. These are formed in long links which are connected by short plates and bolts. Each link consists of several bars of the same length, each of which is perforated with a hole at each end to receive the connecting bolts. The bars of each link are placed side by side, and the links are connected by the plates which form a short link, and the bolts. The links of the portions of the chain which rest upon the piers may either be bent, or else be made shorter than the others to accommodate the chain to the curved form of the sur- face on which it rests. 545. The vertical suspension bars may be either of round 01 200 FRAMING. square bars. They are usually made with one or more articu- lations, to admit of their yielding with less strain to the bar to any motion of vibration, or of oscillation. They may be sus- pended from the connecting bolts of the links, but the prefera- ble method is to attach them to a suitable saddle piece which is fitted to the top of the chain and thus distributes the strain upon the bar more uniformly over the bolts and links. The lower end of the bar is suitably arranged to connect it with the part suspended from it. 546. The wire cables used for curves are composed of wires laid side by side, which are brought to a cylindrical shape and confined by a spiral wrapping of wire. To form the cable seve- ral equal sized ropes, or yarns, are first made. This may be done by cutting all the wires of the length required for the yarn, or by uniting end to end the requisite number of wires for the yarn, and then winding them around two pieces of wrought or of cast iron, of a horse-shoe shape, with a suitable gorge to re- ceive the wires, which are placed as far asunder as the required length of the yarn. The yarn is firmly attached at its two ends to the iron pieces, or cruppers, and the wires are temporarily con- fined at intermediate points by a spiral lashing of wire. Whichever of the two methods be adopted, great care must be taken to give to every wire of the yarn the same degree of tension by a suitable mechanism. The cable is completed after the yarns are placed upon the piers and secured to the anchoring ropes or chains ; for this purpose the temporary lashings of the yarns are undone, and all the yarns are united and brought to a cylindrical shape and secured throughout the extent of the cable, to within a short distance of each pier, by a continuous spiral lashing of wire. The part of the cable which rests upon the pier is not, bound with wire, but is spread over the saddle piece with a uniform thickness. 547. The suspension ropes are formed in the same way as the cables ; they are usually arranged with a loop at each end, form- ed around an iron crupper, to connect them with the cables, to which they are attached, and to the parts of the structure suspended from them by .suitable saddle pieces. 548. To secure the cabh-s from oxidation the iron wires are coated with varnish before they are made into yarns, and after the cables are completed they are either coated with the usual paints for securing iron from the effects of moisture, or else covered with some impermeable material. 549. Experiments on the Strength of Frames. Experimental researches on this point have been mostly restricted to those made with models on a comparatively small scale, owing to the expense and difficulty attendant upon experiments on frames FRAMING. 201 having the form and dimensions of those employed in ordinary structures. Among the most remarkable experiments on a large scale are those made by order of the French government at Lorient, under the direction of M. Riebell, the superintending engineer of the port, and published in the Annales Maritime^ et Colo niale^ Feb. and Not., 1837. The experiments were made by first setting up the frame to be tried, and, after it had settled under the action of its own weight, suspending from the back of it, by ropes placed at equal intervals apart, equal weights to represent a load uni- formly distributed over the back of the frame. The results contained in the following table are from experi- ments on a truss (Fig. 102) for the roof of a ship shed. The truss consisted of two rafters and a tie beam, with suspension Fig. 102. pieces in pairs, and diagonal iron bolts which were added be- cause it was necessary to scarf the tie beam. The span of the truss was 65^ feet ; the rafters had a slope of 1 perpendicular to 4 base. The thickness of the beams, measured horizontally, was about 2 inches, their depth about 18 inches. The amount of the settling at each rope was ascertained by fixed graduated vertical rods, the measures being taken below a horizontal line marked 0. Amount of settling on the right of the ridge below the horizontal 0, in inches. 3 o o a | o ft WEIGHTS BOBNE BT TUB TRUSS. * t ~ = I <8 ?ij C be ""H 2 o . *a 2 71. The entire spandrel courses of the heads are usually not laid until the arches have been uncentred, and have settled, in order that the joints of these courses may not be subject to any other cause of displacement than what mav arise from the effects of variations of temperature upon the arches. The thickness of 8TOXE BRIDGES. 217 the head-walls will depend upon the method adopted for support- ing the roadway. If this be by a filling of earth between the head-walls, then their thickness must be calculated not only to resist the pressure of the earth which they sustain, but allowance must also be made for the effects of the shocks of floating bodies in weakening the bond, and separating the blocks from their mor- tar-bed. The more approved methods of supporting the roadway, and which are now generally practised, except for very flat seg- ment arches, are to lay the road materials either upon broad flag- ging stones (Fig. 120, 121,) which rest upon thin brick walls built Fig. 121 Represents a profile of Fig. 120 through the centre of the pier, showing the arrangement of the roadway and its drainage, &c. A, section of masonry of pier and spandrel. 6, Z>, sections of walls parallel to head- wall, which support the flagging stone on which the roadway is laid. c, section of head-wall and buttress above the starling d. , footpath. f, recess for seats over the buttress. o, cornice and parapet. , vertical conduit in the pier com- municating with two others under the roadway from the side chan- nels. parallel to the head- walls, and supported by the piers and arches; or by small arches, (Fig. 122,) for which these walls serve as piers ; or by a system of small groined arches supported by pillars resting upon the piers and main arches. When either of these methods is used, the head-walls may receive a mean thickness of one fifth of their height above the solid spandrel. 572. Superstructure. The superstructure of a bridge consists of a cornice, the roadway and footpaths, &c., and a parapet. The object of the cornice is to shelter the face of the head- walls from rain. To subserve this purpose, its projection beyond the surface to be sheltered should be the greater as the altitude of the sheltered part is the more considerable. This rule will require a cornice with supporting blocks, (Fig. 123,) termed modiUions, below it, whenever the projecting part would be actually, or might seem insecure from its weight. The height of the cornice, including its supports, should generally be equal to its projections ; this will often require more or less of detail in the profile of the cornice, in order that it may not appear heavy. The top surface of the cornice should be a little above 28 218 BRIDGES, ETC. that of the footpath, or roadway, and be slightly sloped out- ward ; the bottom should be arranged with a situable larmier^ jj-jc, 122 Represents a section through the axis of a pier of bridge built of stone with brick fiiling, showing the arrangement for supporting the roadway on small arches. or drip-, to prevent the water from finding a passage along ite under surface to the face of the wall. Fig. 128 Represents a section through the crown of an arch, showing the cornice a, modillion 6, para- pet c, and footpath d. A, key-stones. B, side elevation of soffit 5T3. The parapet surmounts the cornice, and should be high enough to secure vehicles and foot-passengers from accidents, without however intercepting the view from the bridge. The parapet is usually a plain low wall of cut stone, surmounted by a copine slightly rounded on its top surface. In bridges which STONE BRIDGES. 219 have a character of lightness, like those with flat segment arches, the parapet may consist of alternate panels of plain wall and balustrades, provided this arrangement be otherwise in keeping with the locality. The exterior face of the parapet should not project beyond that of the heads. The blocks of which it is formed, and particularly those of tb.e coping, should be firmly secured with copper or iron cramps. 57-t. The width of the roadway and of the footpaths will be regulated by the locality ; being greatest where the thoroughfares connected by the bridge are most frequented. They are made either of broken, or of paving stone. They should be so arranged that the surface-water from rain shall run quickly into the side channels left to receive it, and be conducted from thence by pipes which lead to vertical conduits (Fig. 121) in the piers that have their outlets in one of the faces of the piers, and below the lowest water-level. 575. Strong and durable stone, dressed with the chisel, or hammer, should alone be used for the masonry of bridges where the span of the arch exceeds fifty feet. The interior of the piers, and the backing of the abutments and head-walls may, for economy, be of good rubble, provided great attention be bestowed upon the bond and workmanship. For medium and small spans a mixed masonry of dressed stone and rubble, or brick, may be used ; and, in some cases, brick alone. In all these cases (Figs. {'2-2, 124) the starlings, the foundation courses, the impost stone, the ring courses, at least of the heads, and the key- stone, should be of good dressed stone. The remainder may be of coursed rubble, or of the best brick, for the facing, with good rubble or brick for the fillings and backings. In a mixed masonry of this character the courses of dressed stone may project slight- ly beyond the surfaces of the rest of the structure. The archi- tectural effect of this arrangement is in some degree pleasing, particularly when the joints are chamfered ; and the method is obviously useful in structures of this kind, as protection is af- forded by it to the surfaces which, from the nature of the mate- rial, or the character of the work, offer the least resistance to the destructive action of floating bodies. Hydraulic mortar should alone l>e used in every part of the masonry of bridges. ~>7*i. Approaches. The arrangement of the approaches will depend upon the number and direction of the avenues leading to the bridge, the width of the avenues, and their position above or below the natural surface of the ground, and the locality. The principal points to be kept in view in their arrangement are to procure an easy and safe access to the bridge for vehicles, and not to obstruct unnecessarily the channels, for purposes of navi- gation, which may be requisite under the extreme arches. 220 BRIDGES, ETC. J L Fig. 124 Ri-p'f sents an elevation of a pier, a portion of two arches, and the centre of the bridge of which Fig. 122 is the section. A, face of starling;. B, hood. C, voussoirs with chamfered joints. When the avenue to the bridge is, by an embankment, in the same line as its axis, and the roadway and bridge are of the same width, the head-walls of the bridge (Fig. 125) may be prolonged sufficiently far to allow the foot of the embankment slope to fall within a few feet of the crest of the slope of the water-course ; this portion of the embankment slope being shaped into the form of a quarter of a cone, and reveted with dry stone or sods, to pre- serve its surface from the action of rain. "When several avenues meet at a bridge, or where the width of the roadway of a direct avenue is greater than that of the STONE BRIDGES. 221 bridge, the approaches are made by gradually widening the out let from the bridge, until it attains the requisite width, by means ol Fig. 125 Klcvation Mand plan N, showing the manner of arranging the embankments of the npl>roarhes, when the head-walls of the bridge are simply prolonged, a, ', side slope of embankment. 6, 6', dry stone facing of the embankment where its end is rounded off, forming a quarter of a cone finish. f,f\ flight of steps for foot-passengers to ascend 1 the embankment. c, c', embankment arranged as above, but simply sodded. d, d\ facing of dry stone for the side slopes of th banks. t, e', facing of Ae'bottom of the stream. wing-walls of any of the usual forms that may suit the locality. The form of wing-wall (Fig. 126) presenting a concave surface outward is usually preferred when suited to the locality, both Fig. 126 Represent an elevation M and plan N of a curved face wing-wall. A, front view of wing-wall. B, I! , slope of embankment. for its architectural effect and its strength. When made ot dressed stone it is of more difficult construction and more ex- pensive than the plane surface wall. 222 BRIDGES, ETC. In order that the approaches may not obstruct the com- munications along the banks for the purposes of navigatu n, an arched passage-way will, in most cases, be requisite under the roadway of the approach and behind the abutment of the ex- treme arch, for horses, and, if necessary, vehicles. "When the form of the arch will admit of it, as in flat segment arches, a roadway, projecting beyond the face of the abutment, may be made under the arch for the same purpose. 577. Water-wings. To secure the natural banks near the bridge, and the foundations of the abutments from the action of the current,- a facing of dry stone, or of masonry, should be laid upon the slope of the banks, which should be properly prepared to receive it, and the foot of the facing must be secured by a mass of loose stone blocks spread over the bed around it, in ad- dition to which a line of square-jointed piles may be previously driven along the foot. When the face of the abutment projects beyond the natural banks, an embankment faced with stone should be formed connecting the face with points on the natural banks above and below the bridge. By this arrangement, termed the water-wings, the natural water-way will be gradu- ally contracted to conform to that left by the bridge. 578. Enlargement of Water-way. In the full centre and oval arches, when the springing line's are placed low, the spandrels present a considerable surface and obstruction to the current during the higher stages of the water. This not only endangers the safety of the bridge, by the accumulation of drift-wood and ice which it occasions, but, during these epochs, gives a heavy appearance to the structure. To remedy these defects the solid angle, formed by the heads and the soffit of the arch, may be truncated, the base of the cuneiform-shaped mass taken away being near the springing lines of the arch, and its apex near the crown. The form of the detached mass may be variously ar- ranged. In the bridge of o^euilly, which is one of the first where this expedient was resorted to, the surface, marked F, (Figs. 113, 114,) left by detaching the mass in question, is warped, and lies between two plane curves, the one an arc of a circle n o, traced on the head of the bridge, the other an oval m o op, traced on the soffit of the arch. This affords a funnel-shaped water-way to each arch, and, during high water, gives a light appearance to the structure, as the voussoirs of the head ring-course have then the appearance of belonging to a flat segmental arch. 579. Centres. The framing of centres, ana the arrangement for striking them, having been already fully explained under the article Framing, with illustrations taken from some of the most celebrated recent structures, nothing further need be here added than to point out the necessity of great care both in the combi - STONE BRIDGES. 223 nation of the frame, and in its mechanical execution, in order to prevent any change in the form of the arch while under con- struction. The English engineers have generally been more successful in this respect than the French. The latter, in several of their finest bridges, used a form of centre composed of seve ral polygonal frames, with short sides, so inscribed within each other that the angles of the one corresponded to the middle of the sides of the other. The sides of each frame were united by joints, and the series of frames secured in their respective posi tions by radial pieces, in pairs, notched upon and bolted to the frames, which they clamped between them. A combination of this character can preserve its form only under an equable pressure distributed over the back of the exterior polygon. When applied to the ordinary circumstances attending the con- struction of an arch, it is found to undergo successive changes of shape, as the voussoirs are laid on it ; rising first at the crown, then yielding at the same point when the key-stone and the ad- jacent voussoirs are laid on. The English engineers have gen- erally selected those combinations in which, the pressures being transmitted directly to fixed points of support, no change of form can take place in the centre but what arises from the contraction or elongation of the parts of the frame. 580. General Remarks. The architecture of stone bridges has, within a somewhat recent period, been carried to a very high de- gree of perfection, both in design and in mechanical execution. France, in this respect, has givenan example to the world, and has found worthy rivals in the rest of Europe, and particularly in Great Britain. Her territory is dotted over with innumerable fine monuments of this character, which attest her solicitude as well for the public welfare as for the advancement of the in- dustrial and liberal arts. For her progress in this branch of architecture, France is mainly indebted to her School and her Corps of Ponts et Chaussees: institutions which, from the time of her celebrated engineer Perronet, have supplied her with a long line of names, alike eminent in the sciences and art- which pertain to the profession of the engineer. England, although on somepoints of mechanical skill pertain- ing to the engineer's art the superior of Franco, holds the second rank to her in the science of her engineers. Without establish- ments for professional training corresponding to those of France, the English engineers, as a body, have, until within a few years, labored under me disadvantage of having none of those institu- tions which, by creating a common bond of union, serve not only to diffuse science throughout the whole body, but to raise merit to its proper level, and frown down alike, through an enlightened esprit de corps, the assumptions of ignorant pretension, and the 224: BRIDGES, ETC. . malevolence of petty jealousies. Although, as a body, less ad vantageously placed, in these respects, than their more thorough- bred brethren of France, the engineers of England can point, with a just feeling of pride, not only to the monuments of their skill, but to individual names among them which, achieved under the peculiar obstacles ever attendant upon self-education, yet stand in the first rank of those by whose genius the industrial arts have been advanced and ennobled. The other European States have also contributed largely to bridge architecture, although their efforts in this line are less widely known through their publications than those of France andEngland. Among the many bridges belonging to Italy, may be justly cited the far-famed Rialto the bridge of Santa Trinita at Florence, the curve of whose iutrados was" so long a mathe- matical puzzle; and the recent single arch over the Dora Riparia near Turin. In the United States, the pressing immediate wants of a young people, who are still without that accumulated capital by which alone great and lasting public monuments can be raised, have pre- vented much being done, in bridge building, except of a temporary character. The bridges, viaducts, and aqueducts of stone in our country, almost without an exception, have been built of rustic work through economical considerations. The selection of this kind of masonry, independently of its cheapness, has the merit of ap- propriateness, when taken in connection with the natural features of the localities where most of the sestructures are placed. Among the works of this class, may be cited the railroad bridge, called the Thomas Viaduct, over the Patapsco, on the line of the Baltimore and Washington railroad, designed and builtby Mr. B. H.Latrobe, the engineer of the road. This is one of the few existing bridge structures with a curved axis. The engineer has very happily met the double difficulty before him, of being obliged to adopt a curved axis, and of the want of workmen sufficiently conversant with the application of working drawings of a rather compli- cated character, by placing full centre cylindrical arches upon jjiers with a trapezoidal horizontal section. This structure, with ine exception of some minor details in rather questionable taste, as the slight iron parapet railing, for example, presents an impo- sing aspect, and does great credit to the intelligence and skill of the engineer, at the time of its construction, but recently launched in a new career. The fine single arch, known as the Carrolton Viaduct, on the Baltimore and (3hio railroad, is also highly credit- able to the science and skill of the engineer and mechanics under whom it was raisd. One of the largest bridges in the United States, designed and partly executed in stone, is the Potomac Aqueduct at Georgetown, where the Chesapeake and Ohio canal STONE BRIDGES. 225 intersects the Potomac river. This work, to which a wooden superstructure has been made, was built under the superintend- ence of Captain Turnbull of the U. S. Topographical Engineers. In the published narrative of the progress of this work, a very full account is given of all the operations, in which, while the Ye- sources and skill of the engineer, in a very difficult and, to him, untried application of his art, are left to be gathered by the reader from the successful termination of the undertaking, his failures are stated with a candor alike creditable to the man, and worthy of imitation by every engineer who prizes the advancement of his art above that personal reputation which a less truthful course may place in prospect before him. 581 . The following table contains a summary of the principal details of some l5f the more noted stone bridges of Europe. NJkME OF BRIDGE. River. Form of Arch. Numb, of arche.. Span of wideat .pan. Rise. SSS?'*^ the 161 head.. Date. Name of engineer Vieille-Brionde Allier. Segment. 1 178 69 5.3 1454 Grenicr & Estone. Rialto . . " 1 98.6 23 - 1578 Michel Angelo. Claix . . Drac. " 1 150 54 3.1 1611 Neuilly (A) Seine. Elliptical. 5 127.9 31.9 5.3 47.9 1774 Perronet. \ pout. * 1 160.5 65 10.9 1775 Saget. Saint-Maxence (B) Oise. Segment. 3 76.7 6 5 41.5 1784 Perronet. Gignac . . Erault. Elliptical. 1 160 44 6.5 - 1793 Garipny. Jena (C) . Seine. Segment. 5 91.8 10.8 4.6 43.7 1811 Lamande. Rouen . . Seine. " 5 101.7 i:i.? 4.6 49.2 1813 Lamande. Waterloo (D) (Thames. Elliptical. 9 120 35 4.9 45 1816 Rennie. Gloucester (E Severn. " 1 150 54 4.5 35 18i!7 Telford. London (F) Thames. " 5 152 37.8 5 56 1831 Rennie. Turin (G) . Dora Riparia Segment. 1 147.6 18 4.9 40 Mosca. Grosvenor (H) Dee. 1 200 42 4 1833 Hartley. (A.) This fine structure, designed and built by the celebrated Perronet, forms an epoch in bridge architecture, from the bold- ness of its design, its skilful mechanical execution, and the simple but appropriate character of its architectural details. The curve of the intrados is an oval of eleven centres, the radius of the arc at the spring being 20.9 feet, and that of the arc at the crown 159.1 feet. The engineer conceived the idea of giving to the soffit a funnel shape, by widening it at the heads, from the crown to the springing line. This he effected by connecting the soffit of each arch and the heads by a warped surface, which passed, on the one hand, through a flat circular arc, described upon the heads through the points of the crown and the top of the two ad- jacent starlings, and, on the other, through two curves on the soffit, cut out by two vertical planes, obliqu6 to the axis, passed through the highest point of the curve on the heads, and through points on the two respective springing lines of the arch. The ob ject of this arrangement was twofold ; first, as the springing lines were placed at the low-water level, the bridge, during the eeasoni 29 226 BRIDGES, ETC. of high water, would have appeared rather heavy, as the greater part of the solfit, at this period, would have been under water, it gave the bridge a lighter appearance during the epochs of high water ; and, second, as the obstruction to the free flow of the water from the spandrels would be very considerable at the same periods, the funnel form given to the soffit at the heads partially remedied this inconvenience. The axis of the roadway, the cornice, and all the correspond- ing architectural lines were made horizontal, a feature in bridge architecture which the reputation of Perronet has since rendered classical ; and to obtain which points truly essential conditions have in some more recent structures been sacrificed. The abutments are 32 feet thick at the springing lines, and the piers but 13.8 feet at the same point, giving an -example of judi- cious boldness combined with adequate strength, on scientific principles, which had been partially lost sight of by preceding en- gineers in designing this part of bridges. The centres of the Neuilly bridge were designed upon the faulty principle of concentric polygonal frames. Perronet was aware of the inconveniences of this combination, and in no part of the construction of the bridge than in this was more sagacious forethought displayed by him, in providing for foreseen contingen- cies, nor greater resources and skill in remedying those which could not have been anticipated. An oversight, rather more serious in its consequences, was committed in widening the natu- ral water-way of the river where the bridge was erected ; the effect of this has been a gradual deposition near the bridge, and an obstruction of the navigable channels. The bridge of Neuilly is a noble monument of the genius and practical skill of its engineer. The style of its architecture, both as a whole and in its several parts, is imposing and in the best taste. (B) This bridge was built after the designs of Perronet. Se- duced by a thorough knowledge of the capabilities of his art, the engineer was led, in planning this structure, into the error of sacrificing apparent strength, for the purpose of producing great boldness and lightness of design. This he effected by placing very flat segment arches upon piers formed of four columns ; the two, forming the starlings, being united to the two adjacent by a connecting wall, an interval being left between the two centre columns. The diameters of the columns are 9.6 feet, with the same interval between them. The engineer who constructed the bridge, apprehensive appa- rently for its safety, introduced into the courses of the piers and of the arches a large quantity of iron ties and cramping pieces, a measure of precaution which, if necessary, ought to have con- STONE BRIDGES. 227 iemned the original designs, although supported by the high authority of Perronet, and caused others to be substituted foi them. (C) This bridge, now designated as the Pont de FEcole Mili taire, from its locality, and the bridge of Rouen, are built upon nearly the same designs. The former is a model of architectural taste and of skilful workmanship. Its horizontal architectural lines, its fine cornice, copied from that of the temple of Mars the Avenger, and the sculptured wreath on its spandrels, form a whole of singular beauty. (D) This bridge, designated when first built as the Strand Bridge, is worthy of the great metropolis in which it is placed. The engineer, influenced perhaps by other examples of the same character in the vicinity of this structure, has placed small col- umns upon the starlings, which support recesses with seats for foot-passengers, and has thus, in no inconsiderable degree, de- prived the bridge of that imposing character which its massive- ness, and the excellent material of which it is built, could not otherwise have failed to produce. (E) This fine elliptical arch is, in some respects, built in imi- tation of the Neuilly bridge, with a funnel-shaped soffit. Its gen- eral architectural effect is heavy, and its mere ornamental parts are in questionable taste. The details of its construction are alike monuments of the eminent professional skill, and of the truthfulness of character of the great engineer who planned and superintended it. In his narrative of the work, Mr. Telford takes blame to himself for oversights and unanticipated results, in which the scrupulous care that he conscientiously brought to every un- dertaking committed to him is unwittingly thrown into bolder relief, by the very confession of his failures ; and a lesson of in- struction is conveyed, more pregnant with important consequences to the advancement of his profession than the recording of hun- dreds of successful instances only could have furnished. (F) This noble work of Sir John Rennie must ever rank among the master-pieces of bridge architecture, in every point by which this class of structures should be distinguished. For boldness, strength, simplicity, massiveness without heaviness, and a happy adaptation of design to the locality, it stands unrivalled. The beauty which is generally recognised in a level bridge has, in this, been judiciously sacrificed to a well-judged economy ; and the artificial approaches have thus been accommodated to the existing, by decreasing the dimensions of the arches from the centre to the two extremities. The square plain buttresses, which rise above the starlings and support the recesses for seats, are of farther obvious utility in strengthening the head-walls, which, at these points are of considerable height ; and they alac 828 BRIDGES, ETC. prodi ce, in this case, a not unpleasing architei tural effect, in separating the unequal arches, without impairing the unity of the general design. (G) This is the boldest single arch of stone now standing, and is a splendid example of architectural design and skilful workman- ship. The soffit oi the arch is made slightly funnel-shaped, which gives the bridge an air of almost too great boldness. The cornice, which is copied from the same model as that of the bridge of Jena ; the convex cylindrical-shaped wing-walls, which give an approach of 144 feet between the parapets ; with the other archi- tectural accessories, have made this bridge a model of good taste for imitation under like circumstances. From the omission of a usual architectural member, there is perhaps a slight feeling of nakedness produced on the mind of the rigid connoisseur in art, on first seeing this structure, and its beauty is in some degree marred by this want. The abutments of this bridge are 40 feet thick at the founda- tions, and, besides the wing-walls, are strengthened by two coun- terforts 20 feet long and 10 feel wide. (H) The span of this arch is the widest on record. For architectural effect this bridge presents but little to the eye that is commendable ; for this the engineer who superintended it is hardly responsible, except so far as, from professional sympathy and respect for a deceased member of the profession, he was led to adopt the designs of another. The abutments form a continua- tion of the arch ; and the other details of the construction through- out exhibit that thorough acquaintance with their art for which the Hartleys, father and sr>n t are well known to the profession. 582. The practice of bridge building is now generally the same throughout the civilized world. In France, the method of laying the foundations by caissons has, in most of their later works, been preferred by her engineers to that of coffer-dams ; and in the su- perstructure of their bridges the French engineers have generally filled in, between the arches and the roadway, with solid material. In some of these bridges, as in that of Bordeaux, where appre- hension was felt for the stability of the piling, a mixed masonry of stone and brick was used, and the roadway was supported by a system of light-groined arches of brick. Among the recent French bridges, presenting some interesting features in their con- struction, may be cited that of Souillac over the Dordogne. The river at this place having a torrent-like character, and the bed being of lime-stone rock with a very uneven surface, and occa skmal deep fissures filled with sand and gravel, the obstacle tf using either the caisson, or the ordinary coffer-dam for the foun- dations, was very great. The engineer, M. Vicat, so well known y his researches upon mortar, &c., devised, to obviate these STONE BRIDGES. 221 difficulties, the plan of enclosing the area of each pier by a coffer- work accurately fitted to the surface of the bed, and of filling this with beton to form a bed for the foundation courses. This he effected, by first forming a frame-work of heavy timber, so ar- ranged that thick sheeting-piles could be driven close to the bot- tom, between its horizontal pieces, and form a well-jointed vessel to contain the semi-fluid material for the bed. After this coffer- work was placed, the loose sand and gravel was scooped from the bottom, the asperities of the surface levelled, and the fissures were voided, and refilled with fragments of a soft stone, which it was found could be more compactly settled, by ramming, in the fissures, than a looser and rounder material like gravel. On this prepared surface, the bed of beton, which was from 12 to 15 feet in thickness, was gradually raised, by successive layers, to with- in a few feet of the low-water level, and the stone superstructure then laid upon it, by using an ordinary coffer-dam that rested on the frame-work around the bed. In this bridge, as in that of Bordeaux, a provisional trial-weight, greater than the permanent load, was laid upon the bed, before commencing the superstruc- ture. To give greate: security to their foundations, the French usually surround them with a mass of loose stone blocks thrown in and allowed to find their own bed. Where piles are used and pro- iect some height above the bottom, they, in some cases, use, be- sides the loose stone, a grating of heavy timber, whica lies between and encloses the piling, to give it greater stiffness and prevent outward spreading. In streams of a torrent character, where the bed is liable to be worn away, or shifted, an artificial covering, or apron of stone laid in mortar, has, in some cases, been used, both under the arches and above and below the bridge, as far as the bed seemed to require this protection. At the bridge of Bor- deaux loose stone was spread over the river-bed between the piers, and it has been found to answer perfectly the object of the jngineer, the blocks having, in a few years, become united into a firm mass by the clayey sediment of the river deposited in their interstices. At the elegant cast-iron bridge, built over the Lary near Plymouth, resort was had to a similar plan for securing the bed, which is of shifting sand. The engineer, Mr. Rendel, here laid, in the first place, a bed of compact clay upon the sand bed between the piers, and imbedded in it loose stone. This method, which for its economy is worthy of note, has fully answered the expectations of the engineer. The English engineers have greatly improved the method of centrng, and, in their boldest arches, any settling approaching that which the French engineers usually counted upon, on striking their centres, would now be regarded as an evidence of great de 230 BRIDGES, ETC. feet in the design, or of very unskilful workmanship. They have generally, in their recent bridges, supported their roadway either upon flat stones, resting on light walls built parallel to the heads, or else upon light cylindrical arches laid upon piers having the same direction. In the preparation for laying the beds of their foundations, they have generally preferred the coffer-dam to any other plan, although in many localities the most expensive, on account of the greater facility and security offered by it for carry- ing on the work. They have not, until recently, made as exten- sive an application of beton a? the French for hydraulic purposes, and, from having mostly usec. what is known as concrete among their architects, have met with some signal failures in its employ- ment for these purposes. WOODEN BRIDGES. 583. A wooden bridge consists of three essential parts : 1st, the abutments and piers which form the points of support for the bridge frame ; 2d, the bridge frame which supports the su- perstructure between the piers and abutment ; 3d, the super- structure, consisting of the roadway, parapets, roofing, &c. 584. The abutments and piers may be either of stone, or of timber. Stone supports are preferable to those of timber, both on account of the superior durability of stone, and of its offering more security than frames of timber against the accidents to which the piers of bridges are liable from freshets, ice, &c. 585. The forms, dimensions, and construction of stone abut- ments and piers for wooden bridges will depend, like those for stone bridges, upon local circumstances, and the kind of bridge- frame adopted. If the bridge-frame is so arranged that no lateral thrust is received from it by the piers, the dimensions of the latter should be regulated to support the weight of the bridge-frame and its superstructure, and to resist any action arising from acci- dental causes, as freshets, ice, &c. The forms and dimen- sions of the abutments, under the like circumstances, will be mainly regulated by the pressure upon them from the embank- ments of the approaches. 586. If the bridge-frame is of a form that exerts a lateral pressure, the dimensions of the abutments and piers must be suit ably adapted to resist this action, and secure the supports from being overturned. Abutment-piers may be used with advantage in this case, as offering more security to the structure than sim- ple piers, when a frame between any two supports may require to be taken out for repairs. The starlings should in all cases be carried above the line of the highest water-level, and the portior of the pier above this line, which supports the roadway bearers may be built with plane faces and ends. WOODEN BRIDGES. 231 587. Wooden abutments may be formed by constructing whal is termed a crib-work, which consists of large pieces of square timber laid horizontally upon each other, to form the upright, or sloping faces of the abutment. These pieces are halved into each other at the angles, and are otherwise firmly connected togethei by diagonal ties and iron bolts. The space enclosed by the crib- work, which is usually built up in the manner just described, only on three sides, is filled with earth carefully rammed, or with dry stone, as circumstances may seem to require. A wooden abutment of a more economical construction may be made, by partly imbedding large beams of timber placed in a vertical or an inclined position, at intervals of a few feet from each other, and forming a facing of thick plank to sustain the earth behind the abutment. Wooden piers may also be made according to either of the methods here laid down, and be filled with loose stone, to give them sufficient stability to resist the forces to which they may be exposed ; but the method is clumsy, and inferior, under every point of view, to stone piers, or to the methods which are about to be explained. 588. The simplest arrangement of a wooden pier consists (Fig. 127) in driving heavy square or round piles in a single -ow, placing them from two to four feet apart. These upright B Fig. 127 Elevation of a wooden pier. a, a, piles of substructure. b, b, capping of piles arranged to receive the ends of the uprights , e, which support the string pieces t, i. d, upper fender beam. e, lower fender beam. /, horizontal ties bolted in pairs on the uprights. f, ff, diagonai braces bolted in pairs on trie uprights. , capping of the uprights placed under the string pieces. A, roadway B, parapet. 232 BRIDGES, ETC. pieces are sawed off level, and connected at top by a horizontal oeam, termed a cap, which is either mortised to receive a tenon made in each upright, or else is fastened to the uprights by bolts or pins. Other pieces, which are notched and bolted in pairs on the sides of the uprights, are placed in an inclined, or diagonal position, to brace the whole system firmly. The several uprights of the pier are placed in the direction of the thread of the current. If thought necessary, two horizontal beams, arranged like the diagonal pieces, may be added to the system just below the lowest water-level. In a pier of this kind, the place of the starlings is supplied by two inclined beams on the same line with the up- rights, which are termed fender-beams. 589. A strong objection to the system just described, arises from the difficulty of replacing the uprights when in a state of decay. To remedy this defect, it has been proposed to drive large piles in the positions to be occupied by the uprights, (Fig. 128,) to connect these piles below the low-water level by four Fig. 128 Plan O, elevation M, and cross section N, showing the arrangement of the capping of the foundation piles with the uprights. a, piles. b, capping of four beams bolted together. c, uprights. horizontal beams, firmly fastened to the heads of the piles, which are sawed off at a proper height to receive the horizontal beams. The two top beams have large square mortises to re- ceive the ends of the uprights, which rest on those of the piles. The rest of the system may be constructed as in the former case. By this arrangement the uprights, when decayed, can be readily replaced, and they rest on a solid substructure not subject to de- cay ; shorter timber also can be used for the piers than when the uprights are driven into the bed of the stream. 590. In deep water, and especially in a rapid current, a single row of piles might prove insufficient to give stability to the up rights ; and it has therefore been proposed to give a sufficient spread to the substructure to admit of bracing the uprights by struts on the two sides. To effect this, three piles (Fig. 129) should be driven for each upright ; one just under its position, and the other two on each side of this, on a line perpendicular to that of the pier. The distance between the three piles will depend on he inclination and ength that it may be deemed necessary to WOODEN BRIDGES. 233 give the struts. The heads of the three piles are sawed off leve! and co.ni ected by two horizontal clamping pieces below the low bJ Fig. 129 Elevation of the arrangement of a wid foundation for a wooden pier. a, upright. b, b, piles of the foundation. c, c. capping of the piles. rights. est water. A square mortise is left in these two pieces, over the middle pile, to receive the uprights. The uprights are fastened together at the bottom by two clamping pieces, which rest on those that clamp the heads of the piles, and are rendered firmer by the two struts. 591. In localities where piles cannot be driven, the uprights of the piers may be secured to the bottom by means of a grating, arranged in a suitable manner to receive the ends of the uprights. The bed, on which the grating is to rest, having been suitably prepared, it is floated to its position, and sunk either before or after the uprights are fastened to it, as may be found most con- venient. The grating is retained in its place by loose stone. As a farther security for the piers, the uprights may be covered by a sheathing of boards, and the spaces between the sheathing be filled in with gravel. Wooden piers may also be constructed, if necessary, of two parallel rows of uprights placed a few fe&\ apart, and connected by cross and diagonal ties and braces. 592. As wooden piers are not of a suitable form to resist heavy shocks, ice-breakers should be placed in the stream, opposite to each pier, and at some distance from it. In streams with a gen- Fig. 130 Elevation M and plan N of a simple : breaker. _ a, a, foundation piles. :: b, b, capping of piles. c. c, uprights. a, inclined fender-beam shod with iron. tie current, a simple inclined beam (Fig. 1 30) covered with thick uheet iron, and supported by uprights and diagonal pieces, will 30 234 BRIDGES, ETC. be all that is necessary for an ice-breaker. But in rapid currents a crib-work, having the form of a triangular pyramid, (Fig. 131, I Fig . 131 Elevation M and plan N of th frame of an ice-breaker to be filled iii with broken stone the up-stream edge of which is covered with iron, will be re- quired, to offer sufficient resistance to shocks. ' The crib-work may be filled in, if it be deemed advisable, with blocks of stone. 593. The width of the bays in wooden bridges will depend on the local circumstances. As a general rule, the bays may be wider, and in bridge-frames of curved timber the rise less, than in stone bridges. In arranging this point, the engineer must take into consideration the fact that wooden bridges require more fre- quent repairs than those of stone, arising from the decay of the material, and from the effects of shrinking and vibrations upon the joints of the frames, and that the difficulty of replacing de- cayed parts, and readjusting the frame-work, increases rapidly wiih the span. 594. Bridge-frames may be divided into two general classes. To the one belong all those combinations, whether of straight or of curved timber, that exert a lateral pressure upon the abutments and piers, and in which the superstructure is generally above the bridge-frame. To the other, those combinations which exert no lateral pressure upon the points of support, and in which the road way, &c. may be said to be suspended from the bridge-frame. 595. Any of the combinations, whether of straight, or of curved timber, described under the head of Framing, may be used for bridges, according to the width of bay selected. A preference, within late years, has been generally given by engineers to com- binations of straight timber over curved frames, from the greater simplicity and facility of their construction, as well as their greater economy ; as curved frames require much moie iron in WOODEN BRIDGES. 235 the form of bolts, ties, &c., than frames of straight timber, and more costly mechanical contrivances for putting the parts together, and setting the frame upon its supports. 590. The number of ribs in the bridge-frame will depend on the general strength required by the object of the structure, and upon the class of frame adopted. In the first class, in which the roadway is usually above the frames, any requisite number of ribs may be used, and they may be placed at equal intervals apart, or else be so placed as to give the best support to the loads which pass over the bridge. In the second class, as the frame usually lies entirely, or projects partly above the roadway, &c., if more than two ribs are required, they are so arranged that one or two, as circumstances may demand, form each head of the bridge, and one or two more are placed midway between the heads, so as to leave a sufficient width of roadway between the centre and adja- cent ribs. The footpaths are usually, in this case, either placed between the two centre ribs, or, when there are two exterior ribs, between them. 597. The manner of constructing the ribs, and of connecting them by cross ties and diagonal braces, is the same for bridge frames as for other wooden structures ; care being taken to ob- tain the strength and stiffness which are peculiarly requisite in wooden bridges, to preserve them from the causes of destructi bility to which they are liable. In frames which exert a lateral pressure against the abutments and piers, the lowest points of the frame-work should be so placed as to be above the ordinary high-water level ; and plates of some metal should be inserted at those points, both of the frame and of the supports, where the effect of the pressure might cause injury to the woody fibre. 59b. The roadway usually consists of a simple flooring formed of cross joists, termed the roadway-bearers, or floor-girders, and flooring-boards, upon which a road-covering either of wood, or stone is laid. A more common and better arrangement of the roadway, now in use, consists in laying longitudinal joists of smaller scantling upon the roadway-bearers, to support the flooring-boards. This method preserves more effectually than the other the roadway-bearers from moisture. Besides, in bridges which, from the position of the roadway, do not admit of vertical diagonal braces to stiffen the frame-work, the only means, in most cases, of effecting this object is in placing hori- zontal diagonal braces between each pair of roadway-bearers For like reasons, ston. road-coverings for wooden bridges are generally rejected, and one of plank used, which, for a horse track, should be of two thicknesses, so that, in case of repairs, arising from the wear and tear of travel, the boards resting upon the flooring-joists may not require to be removed The footpath* 236 BRIDGES, ETC. consist simply of a slight flooring of sufficient width, which r't usually detached from and raised a few inches above the roadwa> surface. 599. When the bridge-frame is beneath the roadway, a distinct parapet will be requisite for the safety of passengers. This may be formed either of wood, of iron, or of the two combined. It is most generally made of timber, and consists of a hand and foot rail connected by upright posts and stiffened by diagonal braces A wooden parapet, besides the security it gives to passengers, may be made to add both to the strength and stiffness of the bridge, by constructing it of timber of a suitable size, and con- necting it firmly with the exterior ribs. 600. In bridge frames in which the ribs are above the roadway, a timber sheathing of thin boards will be requisite on the sides, and a roof above, to protect the structure from the weather. The tie-beams of the roof-trusses may serve also as ties for the ribs at top, and may receive horizontal diagonal braces to stiffen the structure, like those of the roadway-bearers. The rafters, in the case in which there is no centre rib, and the bearing, or distance between the exterior ribs, is so great that the roadway-bearers re- quire to be supported in the middle, may serve as points of sup- port for suspension pieces of wood, or of iron, to which the middle point of the roadway-bearers may be attached. 601. When the bridge-frame is beneath the roadway, the floor- ing, if sufficient projection be given it beyond the head, will pro- tect it from the weather, if the depth of the ribs be not very great. In the contrary case a side sheathing of boards may be requisite. 602. The frame and other main timbers of a wooden bridge will not require to be coated with paint, or any like composition, to preserve them from decay when they are roofed and boarded in to keep them dry. When this is not the case, the ordinary preservatives against atmospheric action may be used for them. The under surface and joints of the planks of the roadway may be coated with bituminous mastic when used for a horse-track ; in railroad bridges a metallic covering may be suitably used when the bridge is not traversed by horses. 603. Wooden bridges can produce but little other architectural effect than that which naturally springs up in the mind of an educated spectator in regarding any judiciously-contrived struc- ture. When the roadway and parapet are above the bridge- frame, a very simple cornice may be formed by a proper combi- nation of the roadway-timbers and flooring, which, with the para- pet, will present not only a pleasing pppearance to the eye, but will be of obvious utility in covering trie parts beneath from the weather. In covered bridges, the most that can be done will be to paint them with a uniform coat of some subdued tint. A WOODEN BRIDGES. 237 best, from their want of height as compared with their length, covered wooden bridges must, for the most part, be only unsightly and also apparently insecure structures when looked at from such a point of view as to embrace all the parts in the field of vision : and any attempt, therefore, to disguise their true character, and to give them by painting the appearance of houses, or of stone arches, while it must fail to deceive even the most ignorant, will only be- tray the bad taste of the architect to the more enlightened judge. 604. The art of erecting wooden bridges has been carried to great perfection in almost every part of the world where timber has, at any period, been the principal building material at the disposal of the architect. The more modern wooden bridges of Switzerland and Germany occupy in Europe the first rank, for boldness of design and scientific combination in their arrangement and construction. These fine foreign structures have been even surpassed in the United States, and our wooden bridges and the skill of our engineers and carpenters, as shown through them, have become deservedly celebrated throughout the scientific world. The more recent structures of this class are peculiarly characterized for simplicity of arrangement, perfection in the me- chanical execution, and baldness of design. If they are open to the charge of any fault, it is to that of too great boldness of de- sign, in spanning very wide bays with ribs of open-built beams either unsupported, or but imperfectly so, at intermediate points, by any combination of struts and corbels, or straining beams. The want of these additions is more or less apparent in the great vibratory motion felt on some of the more recent railroad and other bridges, and in a consequent disposition in the frame to work loose at the joints and sag. 605. The following Table contains the principal dimensions of some of the most celebrated American and European wooden bridges. NAME, ETC. OF BRIDGE. Number of bays. Width of widest bay. Rise or depth of rib. Bridge of Schaff hausen, (A) Bridge of Kandel, (B) . Bridge of Bamberg, ) x/-,v Bridge of Freysingen, J Essex bridge, (D) 2 1 1 2 1 1 3 5 29 19 193 f 166 208 153 250 340 195 200 200 153 180 t. 16.9 ft- 11.6 " 20 " 12 " 27 " 15.4 " 18 Upper Schuylkill bridge, (E) Market-street bridge, (F) Trenton bridge, (G) i Columbia bridge, (H) Richmond bridge, (I) Springfield bridge, (K) . 238 BRIDGES, ETC. (A) This celebrated Swiss bridge, built by John Ulrich Gra benmann, a carpenter, consisted of two bays, the one 193 and the other 172 feet. The bridge-frame was formed of two ribs with a roadway between them. Each rib was framed, in some respects, on th'e same principle as an open-built beam, the uppei string being supported by a number of inclined struts which rested against the abutments and pier, and the lower string, upon which the roadway timbers were laid, being suspended from tht upper by suspension pieces. The whole structure was consoli- dated and braced by bolts, stays, and straps of iron. Remarkable in its day, yet the drawings extant of the bridge of Schaff hausen, while they attest the ingenuity and practical skill of the builder, present it in singular contrast with the equally bold and less com- plicated structures of the like nature recently erected in the United States. (B) This is also a Swiss bridge, built over the torrent of Kan- del in the canton of Berne. Its ribs are formed of solid-built beams which gradually decrease in depth from the centre to the extremities ; this decrease being made by offsets, the built beams presenting the appearance of a number of straining beams placed below each other, against the ends of which abut inclined struts that rest against the faces of the abutments. The roadway rests upon the built beams. (C) These two bridges are selected from among a number of the like character constructed in various parts of Germany by Wiebeking. The bridge-frame in all of them consists of several ribs of curved solid-built beams upon which the roadway timbers are laid. This method of constructing bridge-frames combines great strength and stiffness. It is more expensive than frames of straight timber, as it requires a larger amount of iron, and more complicated mechanical means for its construction than the latter, and the ribs, although stiffer, are impaired in strength by the operation of bending them. (D) This is a very remarkable structure built over the river Merrimack near Newburyport. The ribs consist of curved open- built beams, each of which is composed of three concentric solid- built beams, connected, at intervals along the rib, by two radial pieces of hard wood which fit into mortises made through the centre of each solid beam, and by a long wedge of hard wood in- serted, in the direction of the radius of curvature, between each pair of radial pieces. Each of the solid-built beams of the rib is formed of two thicknesses of scantling, about 12 or 15 feet in length, which abut end to end, breaking joints, and are connected by keys of hard wood inserted into mortises made through the two thicknesses. By these arrangements the architect has sought to preserve both the curved shape and the parallelism of the solid WOODEN BRIDGES. 239 beams forming the nb, and also to connect all the parts firmly, The combination is an ingenious attempt at constructing an arch of wood on similar principles to one of stone, but is inferior tc the more simple and usual methods of forming curved open-built beams by using radial and diagonal pieces. (E) This bridge, designed and built by L. Wernwag, has the widest span on record. The bridge-frame (Fig. 132) consists Fig. 132 Represents a side view of a portion of the open curved rib of the bridge over the Schuylkill at Philadelphia. A, lower curved solid beam. B, top beam. a, a, posts. c, c, diagonal braces. o, o, iron diagonal ties. m, m, iron stays anchored in the abutment C. of five ribs. Each rib is an open-built beam formed of a bottom, curved solid-built beam and of a single top beam, which are con- nected by radial pieces, diagonal braces, and inclined iron stays. The bottom curved beam is composed of three concentric solid- built beams slightly separated from each other, each of which has seven courses of curved scantling in it, each course 6 inches thick by 13 inches in breadth ; the courses, as well as the con- centiic beams, being firmly united by iron bolts, &c. A road vay that rests upon the bottom curved ribs is left on each side .f the centre rib, and a footpath between each of the two exterior ribs. The bridge was covered in by a roof and a sheathing on the sides. (F) This is also one of the many bridges designed and built by Wernwag in the States of Pennsylvania and New Jersey. The bridge-frame consists of three ribs placed so far apart as to leave space between them for a carriage-way and a footpath on each side of the centre rib. Each rib is an open-built beam, consisting of a bottom curved solid-built beam, with mortises at intervals to receive radial pieces, which are connected at top by a single beam, also mortised to receive tenons on the heads of the radial pieces. A single diagonal brace . placed between each 240 BRIDGES. ETC. pair of radial pieces. Longitudinal beams reach from the crown of the curved rib of one bay to that of the next, and on these the roadway-bearers are laid transversely. (G) In this fine structure, the roadway-bearers are suspended from curved solid-built beams by iron-bar chains and suspension rods. The span of the centre bay is 200 feet, that of the two adjacent 180 feet, and that of the extreme bays 160 feet. The bridge-frame (Fig. 133) consists of five ribs, having the same Fig. 133 Represents a side view of a portion of a rib of Trenton bridgs. A, solid curved beam. a, a, a, cross girders suspended from A by the iron chains b, b. c, c. roadway-bearers. d, d, diagonal braces. B, portion of pier. C, frame work of roof-covering over the piers. arrangement for the roadway and footpaths as in the upper Schuyl- kill bridge. Each of the central ribs is formed of 8 courses of curved scantling, each course being 4 inches thick, and 1 3 inches broad. The two exterior ribs have 9 courses of scantling of the same dimensions. Inclined timber braces connect the curved beam. and roadway timbers. The ribs are tied at top by cross pieces, and stiffened by diagonal braces. The bridge-frame is braced on the exterior by vertical and horizontal timber-stays which abut against the top of the piers. The roadway is of plank laid upon longitudinal joists that rest on the roadway bearers. The roadway-bearers are stiffened by diagpnal braces. The abutments and piers are of stone, the latter being 20 feet thick at the impost. (H) This, like most of the more recent bridges for railroads, aqueducts, &c., in Pennsylvania, is built upon Burr's plan, which {Fig. 134) consists in forming each rib of an open-built beam of straight timber, and connecting with it a curved solid-built beam formed of two or more thicknesses of scantling, between which the frame-work of the open-built beam is clamped. The open- built beam consists of a horizontal bottom beam of two thicknesses of scantling, termed the chords, which clamp uprights, termed the WOODEN BRIDGES. 241 queen posts, between them, of a single top beam, termed the plat', of the side frame, which rests upon the uprights, with which it i ? Fig. 134 Represents a side view of a portion of a rib of Burr's bridge. a, a, arch timbers. d, d, queen-posts. 6, b, braces. c, c, chords. e, e, plate of the side frame. 0, o, floor girders on which the flooring joists and flooring boards rest. n, n, check braces. 1 , t , tie beams of roof. A, portion of pier. connected by a mortise and tenon joint, and of diagonal braces and other smaller braces, termed check braces, placed between the uprights. The curved-built beam, termed the arch-timbers, is bolted upon the timbers of the open-built beam. The bridge- frame may consist of two or more ribs, which are connected and stiffened by cross ties and diagonal braces. The roadway-floor- ing (Fig. 135) is laid upon cross pieces, termed the floor girders, which may either rest upon the chords, or else be attached at any intermediate point between them and the top beam. The road way and footpaths may be placed in any position between the several ribs. There is great similarity between the combination adopted by Burr and those of the two bridge-frames just described. The main difference consists in the application by Burr of wL-it he terms the arch-timbers, to strengthen and stiffen an open-bunt beam. It may be remarked from the Figs. 134, 135, that the framing of the open-built beam is faulty, in that the top beam, or plate, is not only of less dimensions than the bottom beam, or chord, but is weakened by mortises, and moreover affords no other support to the queen posts, or uprights, which act as sus- pension pieces for the chord, than that of the pin which confines the tenon in the mortise. From the manner in which the anh- timbers are formed arid connected with the parts of the open-built 31 242 BRIDGES, ETC. ,'oeam, they add but little if any more strength and stiffness tha would be given by straight timbers reaching from the springing point of the arch timbers to their crown ; and they are certainly Fig. 135 Represents the half of a cross section of Fig. 134 through the crown of the arch timbers, in which the letters designate the same parts as in the preceding Fig. f, rafters of roof truss. , A, diagonal braces of bridge frama B, side view of the pier. less efficacious in subserving their end than would be inclined struts, occupying a like position at bottom, and abutting against a straining beam, placed either under the centre part of the chord, where the locality would permit it, or under the centre portion of the plate. In localities whei^e fine timber is less abundant than in those in which the most of Burr's bridges have been built, a ju- dicious regard to economy would undoubtedly have suggested a selection of forms for the secondary parts of the frame, which would have prevented these parts from being as much cut to waste as the Figs, show they must have been in the example taken to illustrate this system. (I) This structure, constructed under the superintendence of Moncure Robinson, Esq., is upon Town's plan. The width of the bays varies from 130 to 153 feet. It consists of two ribs, each of which is formed of a double lattice, with two chords at bottom and one at top. The roadway, for rails, rests on the top girders. The ribs are braced by vertical diagonal braces, and by horizontal diagonal braces between each pair of the top and bot- tom girders. The />iers are of rustic work ; they are 40 feet above the low-water level, and 4 feet thick at top. The exam- ple here selected for illustration (Fig. 136) is taken from another bridge, of nearly the same width of bay, erected subsequently M the Richmond bridge, by the same engineer, in which the top WOODEN BRIDGES. chord also i& doubled, as the former bridge was found to be nthei weak. Fig. 136 Represents a cross section of a railroad bridge with Town's truss, in which the raL's of the road are placed on a flooring on the top of the truss. a, a, double lattice. c, c, top and bottom chords doubled. b, b, vertical diagonal braces of the two ribs. d, d, top and bottom girders. e, e, flooring joists. n, cross bearers of the rails. o, o, corbels or support timbers on the piers on which the ribs rest. /, sheathing, or weather-boarding. r (K f This bridge is constructed on Howe's plan. It consist? (Fig. i W 7) of two ribs which are connected at top and bottom, in the usuu manner, with cross ties and diagonal braces. The roadway flooring rests upon the cross girders at bottom. The bridge is not roofed, as is usually the case, the ribs being covered in on thf. sides and at top by a sheathing of boards, and the flooring-boards by a metallic covering. The bridges constructed according to Colonel Long's plan have been mostly applied to medium spans. In the printed de- scription of the different improvements of this system patented by Colonel Long, he very judiciously introduces struts, which he terms arch braces, either below the top or the bottom string, as the locality may demand, for the purpose of preventing sag- ging, which must necessarily take place in time in all open-built oeams of considerable span, if not strengthened in this way. 44 URIDGES, ETC. IS? 1 V] '.' i ^ R J H F <-? - n H f ! Hr rrr) Fig. 137 Represents a side view of the truss and an end view of the pier, M ; section of the truss and side view of the pier, N ; and a top view O, of the the railroad bridge at Springfield. A, inclined plane of the ice-breaker of the up-stream starling, a, a, iron side-stays of the ribs anchored into the piers at top. a cross pier of CAST-IRON BRIDGES. 606. Bridges of cast iron admit of even greater boldness of design than those of timber, owing to the superiority, both in strength and durability, of the former over the latter material ; and they may therefore be resorted to under circumstances very nearly the same in which a wooden structure would be suitable. 607. The abutments and piers of cast-iron bridges should be built of stone, as the corrosive action of salt water, or even of fresh water when impure, would in time render iron supports of this character insecure ; and timber, when exposed to the same destructive agents, is still less Jurable than cast iron. The forms and dimensions ~f the stone abutments and piers are regulated on the same principles as the like parts in wooden bridges with curved frames. The piers may be either built up high enough to receive the roadway-bearers, or else they may be terminated just above the springing plates of the bridge-frame, CAST-IRON BRIDGES. 245 and form supports for cast-iron standards upon which tLe roadway bearers may be laid. 608. The curved ribs of cast-iron bridge-frames have under gone various modifications and improvements. In the earliei bridges, they were formed of several concentric arcs, or curved beams, placed at some disSu.ce asunder, and anited by radial pieces ; the spandrels being filled either by contiguous rings, or by vertical pieces of cast iron upon which the roadway bearers were laid. In the next stage of progress towards improvement, the curved ribs were made less deep, and were each formed of several seg- ments, or panels cast separately in one piece, each panel con- sisting of three concentric arcs connected by radial pieces, and having flanches, with other suitable arrangements, for connecting them firmly by wrought-iron keys, screw-bolts, &c. ; the entire rib thus presenting the appearance of three concentric arcs con- nected by radial pieces. The spandrels were filled either with panels formed like those of the curved ribs, with iron rings, or with a lozenge-shaped reticulated combination. The ribs were connected by cast-iron plates and wrought-iron diagonal ties. In the more recent structures, the ribs have been composed of voussoir-shaped panels, each formed of a solid thin plate with flanches around the edges ; or else of a curved tubular rib, formed like those of Polonceau, or of Delafield, described under the head of Framing. The spandrel-filling is either a reticulated combi- nation, or one of contiguous iron rings. The ribs are usually united by cast-iron tie-plates, and braced by diagonal ties of cast and wrought iron. 609. The roadway-bearers and flooring may be formed either of timber, or of cast iron. In the more recent structures in Eng- land, they have been made of the latter material ; the roadway- bearers being cast of a suitable form for strength, and for their connection with the ribs ; and the flooring-plates being of cast iron. The roadway and footpaths, formed in the usual manner, rest upon the flooring-plates. The parapet consists, in most cases, of a light combination of cast or wrought iron, in keeping with the general style of the structure. 610. The English engineers have taken the lead in this branch o f architecture, and, in their more recent structures, have carried it to a high degree of mechanical perfection and architectura 1 elegance. Among the more celebrated cast-iron bridges in Eng land, that of Coalbrookdale belongs to the first epoch above men tioned ; those of Staines and Sunderland to the second ; and tc the third, the bridge of Southward, at. London ; that of Tewkes 846 BRIDGES. ETC. bury over the Severn ; that over the La.y near Plymouth, and & number of others in various parts of the United Kingdom. The French engineers have not only followed the lead set them by the English, but have taken a new step, in the tubular-shaped ribs of M. Polonceau. The Pont des Arts at Paris, a very ligh bridge for foot-passengers only, and which is a combination ol cast and wrought iron, belongs to their earliest essays in this line ; the Pont d'Austerlitz, also at Paris, which is a combination simi- lar to those of Staines and Sunderland, belongs to their second epoch ; and the Pont du Carrousel, in the same city, built upon Polonceau's system, with several others on the same plan, belong to the last. In the United States a commencement can hardly be said to have been made in this branch of bridge architecture ; the bridge of eighty feet span, with tubular ribs, constructed by Major Dela- field at Brownsville, stands almost alone, and is a step contem- porary with that of Polonceau in France. The following Table contains a summary description of some of the most noted European cast-iron bridges. NAME OF BRIDGE. River. Numb, of archei. Span in leet. Rise in feet. Numb, of ribs. Date. Eiigiiuer. Coalhrookdale, (A) Severn. 1 100.5 40 5 1779 _ Wearmouth, (B) Wear. 1 240 30 6 17% Bunion. Staines, (C) ] 181 16.5 1802 Austerlitz, (D) . Seine. 5 106.6 10.6 7 1805 Lamande. Vanxhall, (E) . Thames. 9 78 9 1816 Walker. Southwark, (F) Thames. 3 240 24 8 1818 Rennie. Tewkesbnry, (G) Severn. 1 170 17 6 - Tel lord. Lary. (H) . . I,ary. 5 100 14.5 5 1827 Rendel. Carrousel, (1) Seine. 3 130 16 5 1838 Polonceau. (A) This is the first cast-iron bridge erected in England. The curved rib is nearly a semicircle in shape, and is composed of three concentric arcs, which are connected at intervals by short columnar pieces, in the direction of the radii of the curve. (B) This structure, which connects Wearmouth and Sunder- land, has a remarkably bold appearance, both from its great span, and its height, which is 100 feet between the high water-level and the intrados of the arch at the crown. The entire rib pre- sents the appearance of an open-built beam, composed of three concentric arcs united by radial pieces. The spandrel-filling is formed of contiguous iron rings, of increasing diameters from the crown to the springing line, which rest upon the back of the curved rib, and support the roadway-bearers. (C) Slaines bridge was designed on the same plan as Wear- mouth ; but from a defect in the strength of its abutments, they successively yielded to the horiz .intal thrust, which in so flat ac arch was very considerable. CAST-IRON BRIDGES. 24"? (D) The bridge of Austerlitz is constructed on the same prin :.IUIP as the two. last, and produces a light and pleasing architec- tural effect. Each curved rib consists of 21 voussoir-shaped panels, about 4 feet in depth. The spandrel-fillings present the appearance of a continuation of the curved rib outwards, to form a support for the roadway-bearers. The piers are terminated at the springing lines of the curved rib, and are at this point 13 feet thick ; the roadway above them being supported by the ribs con tinueci up to its level. The roadway is on a level, the roadway nearers and flooring being of timber. (E) In this structure the curved rib is formed of solid panels The spandrel-fillings consist of vertical shafts united by cross pieces. The piers are built, up to support the roadway-bearers ; they are 13 feet thick at the springing line. The entire width of the bridge is 36 feet, the carriage-way occupying 25 feet. (F) In this bold structure, the width of each of the two extreme bays is 210 feet. The curved rib is composed of thirteen solirl panels, each of which is 2f inches thick, and has a rim, or flanch around it about 4 inches broad. The rib is 6 feet deep at the crown and 8 feet at the spring. The spandrel-filling is composed of lozonge-shaped panels with vertical joints ; they are secured to the back of the curved rib and support the roadway-plates. The curved ribs are connected by tie-piates inserted between the jc-ints of the voussoirs ; and they are braced by feathered diago- nal braces. The piers are 24 feet thick at the springing line, and are built up to the level of the roadway-plates. The width of the carriage-way is 25 feet, and that of each of the footpaths 7 feet. (G) This bridge presents a very light and elegant appearance ; the panels of the curved rib being cast with open curvilinear spaces, which divide the panel into several rectangular-shaped figures, with solid sides and diagonals. Each rib consists of twelve panels. The depth of the ribs is 3 feet. The thickness of the two exterior ribs is 2^ inches, that of the four interior 2 inches. The ribs are connected by grated tie-plates between the panel-joints, and they abut against springing plates which in- 3 feet wide and 4 inches thick. The roadway-bearers and road-plates are of cast iron. The spandrel-filling is composed of lozenge-shaped panels, the sides of the lozenges being fea- thered, and tapering from the middle to the extremities. The ribs of the bridge-frame are connected and braced in the usual manner. The road-bearers are laid lengthwise upon the ribs, to which they are firmly secured, and they are covered with iron road-plates, upon which the road-covering rests. The free road space is 24 feet. (H) In this structure, (Figs. .3!*, 139,) the engineer has de 248 BRIDGES, ETC. ana parted from the usual form of a circulai segment aict adopted an elliptical segment. The following summary ei main-chain. . upper and lower main-chains ; to the upper by a saddle-piece and bolts, and to the coupling-bolt of the lower by an arrange- ment of articulations, which allows an easy play to the rods ; at bottom (Fig. 142) they are connected by a joint with a bolt that fastens firmly the roadway-timbers. Fig. 142 Shows an elevation of the roadway-timbers. a, bottom longitudinal beam. b, b, road way- bearers in pairs. c, piatform. , d, top longitudinal beam forming the bottom rail of the para- pet. e, Dolt, with a forked head to receive the end of the suspending- rod, which is keyed beneath and secures the beams, &c. g, wrought-iron horizontal diagonal ties. TT The roadway-timbers consist of a strong longitudinal bottom beam, upon which the roadway-bearers are notched ; these last pieces are in pairs, the two being so far apart that the bolts con- necting with the suspending-rods by a forked head can pass be tween them ; the flooring-plank is laid upon the roadway-bearers ; and a top longitudinal beam, which forms the bottom rail of the parapet, is secured to the bottom beam by the connecting bolt Wrought-iron diagonal ties are placed horizontally below the flooring, to brace the whole of the timbers beneath. The roadway is 14 feet wide. It slopes from the centre poiu SUSPENSION BRIDGES. 263 along the axis to the extremities, being 4 feet higher in the centre than at the two last points. The piers are in the form of towers, resembling the Italian belfry. They are of brick, 80 feet high, and so constructed and combined with the top saddles, that they have to sustain no othei strain than the vertical pressure from the main-chains. The whole weight of the structure, with an additional load of 100 Ibs. per square foot of the roadway, would throw about 100C Ions on each pier. The tension on the chains from this load is calculated at about 1480 tons ; while the strain they can beai without impairing their strength is about 5000 tons. Monongahela wire Bridge. This bridge, erected at Pittsburgh, Penn., upon plans, and under the superintendence of Mr. Roe- bling, has 8 bays, varying between 188 and 190 feet in width. It is one of the more recent of these structures in the United States. The roadway of each bay is supported by two wire cables, of 4 inches in diameter, and by diagonal stays of wire rope, at tached to the same point of suspension as the cables, and con necting with different points of the roadway-timbers. The ends of the cables of each bay arc attached to pendulum-bars, by means of two oblique arms, which are united by joints to the pendulum-bars. These bars are suspended from the top of 4 cast-iron columns, inclining inwards at top, which are there firmly united to each other ; and, at bottom, anchored to the top of a stone pier built up to the level of the roadway-timbers. The side columns of each frame are connected throughout by an open lozenge-work of cast iron. The front columns have a like con- nection, leaving a sufficient height **f passage-way for foot-pas- sengers. *- The frame-work of 4 columns on each side is firmly connected at top by cast-iron beams, in the form of an entablature. A car- riage-way is left between the two frames, and a footpath between the two columns forming the fronts of each frame. The points of suspension of the cables are over the centre line of the footpaths ; and the cables are inclined so far inward that the centre point of the curve is attached just outside of the car- riage-way. The suspending-ropes have a like inward inclination, the object in both cases being to add stiffness to the system, and diminish lateral oscillations. The roadway consists of a carriage-way 22 feet wide, and two footpaths each 5 feet wide. The roadway-bearers are transversal beams in pairs, 35 feet long, 15 inches deep, and 4| inches wide They are attached to the suspending-ropes. The flooring con sists of 21 inch plank, laid longitudinally over the entire roadway- surface ; and of a second thickness of 2| inch oak plank laid transversely o - er the carriage-way. 264 BRIDGES, ETC. The parapet, which is on the principle of Town's lattice, ex tends so far below the roadway-bearers that they rest and arc notched on the lowest chord of the lattice. A second chord em braces them on top, and finally a third chord completes the lattice at top. The object of adopting tins form of parapet was to in- crease the resistance of the roadway to vindications. MOVE ABLE BRIDGES 624. The term moveable bridge is commonly applied to a platform supported by a frame-work of timber, or of cast iron, by means of which a communication can be formed or inter- rupted at pleasure, between any two points of a fixed bridge, or over any narrow water-way. These bridges are generally de- nominated draw-bridges, but this term is now, for the most part, confined to those moveable bridges which can be raised or low- ered by means of a horizontal axis, placed either at one extremity of the platform, or at some intermediate point between the two ends, and a counterpoise which is so connected with the platform in either case, that the bridge can be easily manoeuvred by a small power acting through the intermedium of some suitable mechanism applied to the counterpoise. The term turning or swinging bridge is used when the bridge is arranged to turn horizontally around a vertical axis placed at a point between its two ends, so that the parts on each side of the axis balance each other ; and the term rolling bridge is applied when the bridge resting upon rollers can be shoved forward or backward horizon- tally, to open or interrupt the passage. To the above may be added another class of moveable bridges, used for the same purpose, which consist of a platform supported by a boat, or other buoyant body, which can be placed in or withdrawn from the water-way, as circumstances may require. 625. Local circumstances will, in all cases, determine what description of moveable bridge will be best. If the width of the water-way is not over 24 feet, a single bridge may be used ; but for greater widths the bridge must consist of two symmetrical parts. 626. Draw-bridges. When the horizontal axis of this de- scription of bridge is placed at the extremity of the platform, the bridge is manoeuvred by attaching a chain to the other extremity, which is connected with a counterpoise and a suitable mechanism, by which the slight additional power required for raising the bridge can be applied. A number of ingenious contrivances have been put in practice for these purposes. They consist usually either of a counter- poise of invariable weight, connected with additional animal mo MOVEABLE BRIDGES. 265 tive power, which acts with constant intensity but with a variable arm of lever ; or of a counterpoise of variable weight, which is assisted by animal motive power acting with an invariable arm of lever. In some cases the bridge is worked with a less compli- cated combination, by dispensing with a counterpoise, and ap- plying animal motive power, of variable intensity, acting with a constant or a variable arm of lever. Among the combinations of the first kind, the most simple consists in placing a framed lever (Fig. 143) revolving on a hori Fig. 143 Shows the man- ner of manoeuvring a draw- bridge either by a framed lever, or by a counterpoise suspended from a spiral eccentric. A, abutment. a, section of the platform b, framed lever. c, chain attached tj the ends of the lever and the platform. a, strut moveable around its lower end. e, bar with an articulation at each end that confines the strut to the platform. /, spiral eccentric connect- ed with the counterpoise g by a chain passing over the gorge of the eccentric. h, chain for raising the bridge, one end of which is attached to the extre- mity of the platform, and the other to the axle of the eccentric. t, fixed pulley over which the chain A is passed. m, Wheel fixed to the axle of the eccentric for the purpose of turning it by means of animal power applied to the endless chain . zontal axis above the platform. The anterior part of the frame is connected with the moveable extremity of the platform by two chains. The posterior portion, which forms the counterpoise, has chains attached to it by which the lever can be worked by men. When the locality does not admit of this arrangement, the chain attached to the moveable end of the platform may be con nected with a horizontal axle above the platform, to which is also attached a fixed eccentric of a spiral shape, (Fig. 143,) connected with a chain that passes over its gorge and sustains a counter- poise of invariable weight. Upon the same axle an ordinary wheel is hung, over the gorge of which passes an endless chain u manoeuvre the bridge by animal power. Of the combinations of variable counterpoises the mechanism 34 266 BRIDGES, ETC. of M. Poncelet, which has been successfully applied in many instances in France for the draw-bridges of military works, is one of the most simple in its arrangement and construction. The moveable end of the platform (Fig. 144) is connected by a corn- Fig. 144-Shows the ar- rangement of a d raw- bridge with a varia- ble counter|)oise. A and B, abutments. g, variable counter- poise formed of a chain with rial links, one end of which is attached to a lixed point, and the other to the chain r attach- ed to the moveable end of the platform. i, fixed pulley over which the chain c passes to the small wheel k fixed on a horizontal shaft, tc which is also attach- ed the wheel m and the endless chain n for manoeuvring the bridge. mon chain, that passes over the gorge of a wheel hung upon r horizontal shaft above the platform, with another chain of variable breadth, formed of flat bar links, which forms the counterpoise. The chain counterpoise is attached at its other extremity to a fixed point in such a way, that when the platform ascends, a por- tion of the weight of the chain is borne by this fixed point ; and thus the weight of the counterpoise decreases as the platform rises. The system is manoeuvred by an endless chain passed over the gorge of a wheel hung upon the horizontal shaft. For light platforms a counterpoise may be dispensed with, and the bridge may be manoeuvred by connecting the chain attached to the moveable end of the platform to a horizontal shaft, which is turned by the usual tooth-work combinations. When the locality does not admit of manoeuvring the bridge by UP^ Fig. 145 Shows the ar- rangement of a draw- bridge where the coun- terpoise is formed by prolonging back the platform. A, abutment. Bj well of a suitable form for manoeuvring the bridge. a, chain-stay to keep the platform firm when the bridge is down. a chain connected with some point above the frame-work of the platform, Fig. 145 is continued back, from two thirds to three MOVEABLE BRIDGES. 267 fifths its length, from the face of the abutment, to form a coun- terpoise for the platform of the bridge. The horizontal axis of the bridge is placed near the face of the abutment, and a well of a suitable shape to receive the posterior portion of the platform that forms the counterpoise is formed behind the abutment. The mechanism for working the bridge may consist of i chain and capstan below the platform-counterpoise, or of a suitable combination of tooth-work. In bridges of a single platform, the moveable extremity, whe;> the bridge is lowered, rests on the opposite abutment, and no intermediate support will be required for the structure if the frame-work be of sufficient strength ; but when a double bridge, consisting of two platforms, is used, the platforms (Fig. 143) should be supported near their moveable ends, when the bridge is down, by struts moveable around the joint by which they are connected with the face of the abutments. These struts are so connected with the bridge that they are detached from it and drawn up when it is raised, and fall back into their places, abutting against blocks near the moveable end of the platform, when the bridge is down. By these arrangements the chains for working the bridge are relieved from a portion of the strain when the bridge is down, and it is also rendered more firm. When the counterpoise is formed by the rear part of the plat form, additional security may be given to the bridge when down by attaching two chains beneath the platform, and securing them to anchoring-points at. the bottom of the well. In some cases a heavy bar, fitted to staples beneath connected with the timbers of the platform, is used for the same purpose. In double bridges the two platforms when lowered should abut against each other, giving a slight elevation to the centre of the bridge. This not only gives greater stiffness, but is favorable to detaching the platforms when the bridge is to be raised. For draw, and every kind of moveable bridge, temporary bar- riers should be erected on each side at the entrance upon the bridge, to prevent accidents by persons attempting to cross the bridge before it is properly secured when lowered. (ix!7. Turning-bridges. These bridges revolve horizontally upon a vertical shaft, or gudgeon below the platform, which is usually thrown far enough back from the face of the abutment to place the side of the bridge, when brought round, just within this face. The weights of the parts of the bridge around the shaft should balance each other. To support and manoeuvre the bridge (Fig. 146) a circular ring of iron, or roller-way, of less diameter than the breadth of Jie bridge, and concentric with the venical shaft, is firmly im- oedded in masonry. Fixed rollers, in the shaoe of truncated 268 BRIDGES, ETC. cones, are attached at equal distances apart to the frame-work of the platform beneath, and rest upon the roller-way. The bridge Fig. 146 Represents the arrangement of a turning-bridge. a, platform ot the bridge. b, vertical posts to which the iron stays , n are attached. c, vertical shaft or gudgeon on which the bridge turns. o, o, conical rollers. is worked by a suitably arranged tooth-work, or by a chain and capstan. In some cases cast-iron balls, resting on a grooved roller-way and fitting into one of corresponding shape fixed be- neath the platform, have been used for manoeuvring the bridge. The ends of the bridge are cut in the shape of circular arcs tc fit recesses of a corresponding form in the abutments, so arranged as not to impede the play of the bridge. In double turning-bridges the two ends of the platforms which come together should be of a curved shape. The platforms should be sustained from beneath by struts, like those used for draw-bridges, which can be detached and drawn into recesses when the passage is interrupted ; or else they may be arranged with a ball-and-socket joint at their lower extremity, so as to be brought round with the bridge. For the purpose of giving addi- tional strength and security to the bridge, iron stays are, in some cases, attached on each side of the platform near the extremities, and connected with vertical posts placed in a line with the verti cal shaft. Turning-bridges may be made either of timber, or of cast iron ; the latter material is the more suitable, as admitting of more ac- curacy of workmanship, and not being liable to the derangements caused by the shrinking or warping of frame-work of timber. 628. Rolling-bridges. These bridges are placed upon fixed rollers, so that they can be moved forward or backward, to inter- rupt, or open the communication across the water-way. The part of the bridge that rests upon the rollers, when the passage is closed, must form a counterpoise to the other. The mechan- ism usually employed for manoeuvring these bridges consists of tooth-work, and may be so arranged that it can be worked by one or more persons standing on the bridge. Instead of fixed rollers turning on axl :-s, iron balls resting in a grooved roller-way AQUEDUCT-BRIDGES. 269 may be jsed, a similar roller-way being affixed to the fi . me-work beneath. 629. Boat-bridge. A moveable bridge of this kind may be made by placing a platform to form a roadway upon a boat, or a water-tight box of a suitable shape. This bridge is placed in, or withdrawn from the water-way, as circumstances may require, a suitable recess or mooring being arranged for it near the water way when it is left open. A bridge of this character cannot be conveniently used in tidal waters, except at certain stages of the water. It may be em ployed with advantage on canals in positions where a fixed bridge could not be placed. AQUEDUCT-BRIDGES. 630. In aqueducts and aqueduct-bridges of masonry, for sup- plying reservoirs for the wants of a city, or for any other purpose, the volume of water conveyed being, generally speaking, small, the structure will present no peculiar difficulties beyond affording a water-tight channel. This may be made either of masonry, 01 of cast-iron pipes, according to the quantity of water to be deliv- ered. If formed of masonry, the sides and bottom of the channel should be laid in the most careful manner with hydraulic cement, and the surface in contact with the water should receive a coating of the same material, particularly if the stone or brick used be of a porous nature. This part of the structure should not be commenced until the arches have been uncentred and the heavier parts of the structure have been carried up and have had time to settle. The interior spandrel -filling, to the level of the masonry which forms the bottom of the water-way, may either be formed of solid material, of good rubble laid in hydraulic cement, or of belon well settled in layers ; or a system of interior walls, like those used in common bridges for the support of the roadway, may be used in this case for the masonry of the water-way to rest on. 631. In canal aqueduct-bridges of masonry, as the volume of \vater required for the purposes of navigation is much greater than in the case of ordinary aqueducts, and as the structure has to be traversed by horses, every precaution should be taken to procure great solidity, and secure the work from accidents. Segment arches of medium span will generally be found most suitable for works of this character. The section of the water- way is generally of a trapezoidal form, the bottom line being horizontal, and the two sides receiving a slight batir ; its dimen- sions are usually restricted to allow the passage of a single boat at a time. On one side of the water-way a horse or tow path if 270 BRIDGES, E'lv,. placed, and on the other a narrow footpath. The water-way should be faced with a hard cut-stone masonry, well bonded to secure it from damage from the passage of the boats. The space between the facing of the water-way, termed the trunk of the aqueduct, and the head-walls, is filled 'n with solid material, either of rubble or of beton. A parapet-wall of the ordinary form and dimensions surmounts the tow and footpaths. The approach to an aqueduct-bridge from a canal is made by gradually increasing the width of the trunk between the wings, which, for this purpose, usually receives a curved shape, and narrowing the water-way of the canal so as to form a convenient access to the aqueduct. Great care should be taken to form a perfectly water-tight junction between the two works. 632. When cast iron or timber is used for the trunk of an aqueduct-bridge, the abutments and piers should be built of stone. The trunk, which, if of cast iron, is formed of plates with flanches to connect them, or, if of timber, consists of one or two thick- nesses of plank supported on the outside by a framing of scant- ling, may be supported by a bridge-frame of cast iron, or of tim- ber, or be suspended from chains or wire cables. The tow-path may be placed either within the water-way, or, as is most usually done, without. It generally consists of a sim- ple flooring of plank laid on cross-joists supported from beneath by suitably arranged frame-work. 633. The following succinct descriptions of some of the aque- duct-bridges of the United States and of Europe are derived from authentic sources. Chirk Aqueduct-bridge over the Ceriog. This work, built by Telford, consists of 10 full centre arches of masonry, of 40 feet span each. The water-way is only 1 1 feet wide and 5 feel deep. The tow-path 6 feet wide. The piers of this work, which in some places are over 100 feet i 1 height, are built hollow for some distance below the top ; the xcing being connected by cross-walls upon which the bottom jf the water-way, formed of broad iron-flanched plates, and the masonry of the sides rest. Pont-y-Cy stile Aqueduct-bridge over the Dee. This is also one of Telford's early works. The trunk is of cast-iron plates connected by flanches. These rest upon stone piers and upon a bridge-frame of cast iron consisting of four ribs of solid panels The span of the ribs is 45 feet and the rise 7| feet. The breadth of the water-way is 11 feet 10 inches. The tow- path is 4 feet 8 inches wide, and is placed within the water -way, resting upon cast-iron uprights. The canal aqueduct-bridges at Gut tin over the Allier, and al AQUEDUCT-BRIDGES. 271 Digoin upon the Loire, are among the more recent structures of this character in France. They are both built upon the same plan, and of mixed masonry. The first has eighteen arches ; the second eleven. The span of each arch is 52| feet, and the rise about 23 feet. The piers are about 10 feet thick at the im- post. The breadth of the aqueduct between the heads is 31 feet, and that of the water-way about 16 feet. Rochester Canal Aqueduct-bridge. This is the most recent and the largest aqueduct-bridge built entirely of masonry in the United States. It consists of seven segment arches. Its water way is of sufficient width for the passage of two boats, and is adapted to the enlargement of the Erie canal. The span of each arch is 52 feet; the rise 10 feet. The key-stone is 2 feet 6 inches in depth, and the top of it is on a level with the bottom of the trunk. The piers are 10 feet thick at the impost. The water-way is 9 feet in depth, the masonry of the sides receiving a batir of 2 inches in one foot. The depth of water is 7 feet, and the width at the water-line 45 feet. The sides of the water- way, the top surface of which forms the tow-paths, are 1 1 feet in width at top, including the projection of the coping. The trunk at each extremity is gradually enlarged, in a curved shape, to the width of 55 feet, where it unites with the slopes of the water-way of the canal. This work is built throughout in a very strong and superior manner, of heavy blocks of gray lime-stone laid in hydraulic mortar. Potomac Canal Aqueduct-bridge. This work, originally in- tended to be of stone throughout, was to have consisted of twelve oval arches of eleven centres, the span of each being 100 feet, and the rise 25 feet. Every third pier forms an abutment-pier, and is 21 feet thick at the impost ; the others are only 12 feet thick at the same level. The piers have been built upon the original design, but a wooden superstructure, consisting of the trunk of the aqueduct, a tow-path, and the frame-work for their support, has been substituted for the stone arches. The trunk (Fig. 147) is formed of a frame consisting of two parallel open-built beams, connected at bottom by parallel cross- joists and horizontal diagonal braces, which are sheathed on the interior with plank to form the water-way. Each of the open-built beams is composed of a top and bottom string, connected by uprights that project above and below the strings, and by single diagor al braces placed between each pair of uprights. The tow-path is placed on the outside of the trunk, and con sists of a flooring laid upon cross-jcfsts placed between one of the built beams of the trunk and a thir- 1 parallel to it. 272 BRIDGES, ETC. The exterior-built beam of the tow-path is framed of smaller scantling than the other two. It is connected with the built 1 B ..a d Fig. 147 Represents a cross section of the trunk and tow-path of the Potomac canal aqueduct-bridge. A, interior of trunk. B, tow-path. a, a, uprights of the open-built beams on the sides of the trunk. b, upright of the open-built beam of the tow-path. c, lower strings of the built beams. a, upper string. e, cross-joists on which the sheathing of the bottom of the trunk rests. n, eross-joists of the tow-path. m, vertical diagonal braces between the cross joists. /, parapet. beam of the trunk by every fourth cross-joist of the trunk, by the top cross-joists of the flooring, and by vertical diagonal braces placed between each pair of top and bottom cross-joists. The uprights of the exterior-built beam of the tow-path pro- ject sufficiently high above the flooring to form a parapet. The frame-work of the trunk and tow-path is supported at intermediate points from beneath by inclined struts which abut against the faces of the piers at a point above the high-water level. The section of the water-way is rectangular. The interior width is 17 feet; the height of the sheathing 8 feet 4 inches within ; and the depth of water 4 feet 4 inches. The surface of the tow-path is 6 feet wide between the uprights of the built beams, and is on a level with the top of the sheathing. The exterior parapet is 3 feet 10 inches above the level of the tow-path, and an interior parapet, 2 feet above the same level, is formed by a capping on the uprights of the built beam, making the height of the capping on each side of the trunk 10 feet 4 inches above the sheathing of the bottom. The lrame-w r ork of this structure is simple in its combinations and well arranged both for strength and stiffness. Wire Suspension Canal Aqueduct-bridge over the Alleghany river at Pittsburgh. This novel work (Fig. 148) was planned AQUEDUCT-BRIDGES. 273 Fig. 148 shows in elevation a portion of the stone supports, and a era* section of the trunk, &c., of the Alleghany canal aqueduct-bridge. A, piers. B, Supports of masonry on the piers for the wire cables. C, interior of a portion of the trunk. a, cross-joists suspended from the cables in by the bent susptnding-bare r, on which the bottom e of trunk rests. b, inclined struts in pairs connected with the pieces c to support the sides d of the trunk. D, tow-path. , cross-joists of the tow-path. r, inclined supports of s. t and v, parapets. A, sleepers on top of the piers on which the cross-joists a rest. and constructed by Mr. Roebling, through whom the following detailed description was obtained : " This work is formed of seven spans of 160 feet each from centre to centre of pier. The trunk is of wood and 1140 feet long, 14 feet wide at bottom, 16^ feet wide on top ; the sides 8| feet deep. These as well as the bottom are composed of a double course of 2^ inch white-pine plank laid diagonally, the two courses crossing each other at right angles, so as to form a solid lattice-work of great strength and stiffness, sufficient to bear its own weight and resist the effects of the most violent storms. The bottom of the trunk rests upon transverse beams, arranged in pairs 4 feet apart ; between these the posts which support the sides of the trunk are let i with dove-tailed tenons, secured by bolts. The outside posts which support the side-walk and tow- path incline outwards and are connected with the beams in a similar manner. Each trunk-post is held by two braces 2^x10 inches, and connected with the outside posts by a double joist of 2^x10. The trunk-posts are 7 inches square at the top and 35 274 BRIDGES, ETC. 7x14 at the heel. The transverse beams are 27 feet long 16 inches deep, and 6 inches wide ; the space between the two ad- joining is 4 inches. It will be observed that all parts of the frame, with the exception of the posts, are double, so as to admit the suspension-rods. Each pair of beams is supported on each side of the trunk by double suspending-rods of 1| inch round bar-iron, bent in the shape of a stirrup, and mounted on a small cast-iron saddle, which rests on the cable. These saddles are on top of the cables connected by links, which diminish in size from the pier towards the centre. The sides of the trunk rest solid against the bodies of masonry, which are erected on each pier and abutment as bases for the pyramids which support the cables. These pyramids, which are constructed of three blocks or courses of a durable coarse-grained hard mountain sand-stone, rise 5 feet above the level of the side-walk and tow-path, and measure 3x5 feet on top, and 4x6^ feet in base. The side-walk and tow-path being 7 feet wide, leave 3 feet space outside for the passage of he pyramids ; the ample width of the tow and footpath is there- 'ore contracted on every pier ; but this arrangement proves no inconvenience, and was necessary for the suspension of the cables next to the trunk. " As the caps which cover the saddles and cables on the pyra- mids rise 3 feet above the inside, or trunk-railing, they would obstruct the passage of the tow-line ; this however is obviated by a slide-rod of round iron, which passes over the top of the cap and forms a gradual slope down to the railing on each side of the pyramid. " The wire cables, which are the main support of the structure, are suspended next to the trunk, one on each side. Each of these two cables is exactly 7 inches in diameter, perfectly solid and compact, and constructed in one piece from shore to shore, 1175 feel long ; it is composed of 1900 wires of | inch diameter, which are laid parallel to each other. Great care has been taken to insure an equal tension of the wires. The oxidation of the wires is guarded against by a varnish applied to each separately. The preservation of the cables is insured by a close, compact, and continuous wrapping, made of annealed wire and laid on by machinery in the most perfect manner " The extremities of the cables on the aqueduct do not extend below ground, but connect with anchor-chains, which in a curved line pass through large masses of masonry, the last links occupy- ing a vertical position. The bars composing these chains aver- age 1^x4 inches, and are from 4 to 12 feet long; they are manufactured of boiler-scrap, and forged in one piece wit! out a Weld. The extreme links are anchored to heavy cast-iron plates trf 6 feet square, which are held down by the foundations, upor AQUEDUCT-BRIDGES. 276 which the weight of 700 perches of masonry rests. The stability of this pa:t of the structure is fully insured, as the resistance of the anchorage is twice as great as the greatest strain to which the chains can ever be subjected. " The plan of anchorage adopted on the aqueduct varies mate- rially from those methods usually applied to suspension bridges, where an open channel is formed under ground for the passage of the chains. The chains below ground are imbedded and com- pletely surrounded by cement. In the construction of the ma- sonry this material and common lime-mortar have been abundantly applied. The bars are painted with red lead : their preservation is rendered certain by the known quality of calcareous cements tc prevent oxidation. If moisture should find its way to the chains, it will be saturated with lime, and add another calcareous coat- ing to the iron. This portion of the work has been executed with scrupulous care, so as to render it unnecessary, on the part of those who exercise a surveillance over the structure, to examine it. The repainting of the cables every two or three years will insure their duration for a long period. " Where the cables rest on the saddles, their size is increased at two points, by introducing short wires and forming swells which fit into corresponding recesses of the casting. Between these swells the cable is forcibly held down by three sets of strong iron wedges, driven through openings which are cast in the sides of the saddle. During the raising of the frame-work, the several arches were frequently subjected to very unequal and considerable forces, which never disturbed the balance, and proved the correctness of previous calculations. The woodwork in any of the arches, separately, may be removed and substituted by new material, without affecting the equilibrium of the next one. " The original idea upon which the plan has been perfected, was to form a wooden trunk, strong enough to support, its own weight, and stiff enough for an aqueduct, or bridge, and to com- bine this structure with wire cables, of a sufficient strength tc bear safely the great weight of water. " Table of Quantities on Aqueduct. Length of aqueduct without extensions Length of cables .... Length of cables and chains Diameter of cables Aggregate weight of both cables . Section of 4 feet of water in trunk . Total weight of water in aqueduct . Do. do. in one span . Weight of one span including all Aggregate number of wires in both cables 11 40 feet. 1175 " 1283 " 7 inches. 110 tons. 59 superf. feel 2100 tons. 295 " 420 " 3800 276 BRIDGES, ETC. Aggregate solid sectior of both cables Do. do. anchor-chains Deflection of cables .... Elevation from top of pyramids to top of piers Weight of water in one span between piers Tension of cables resulting from this weight Tension of one single wire Average ultimate strength of one wire Ultimate strength of cables Tension resulting from weight of water upon square inch of wire cable Tension resulting from veight of water upon 1 inch of anchor-chains Pressure resulting from water upon a pyramid Do. upon one superficial foot 1 solid square 53 superf. inck 72 " 14 feet 6 incV 16 " 6 Ci 275 tons. 392 " 206 Ibs. 1100 " 2090 tons. 14800 Ibs. 11000 " 137i tons 18400"lbs. M See Votf A., Appendix. ROADS. 27" ROADS. 634. IN establishing a line of internal communication of any character, whether it be an ordinary road, railroad, or canal, the main considerations to which the attention of the engineer must be directed in the outset are 1, the probable character and amount of traffic over the line ; 2, the wants of the community in the neighborhood of the line ; 3, the natural features of the country, between the points of arrival and departure, as regards their adaptation to the proposed communication. As the last point alone comes exclusively within the province of the engineer's art, and within the limits prescribed to this work, attention will be confined solely to its consideration. 635. Reconnaissance. A thorough examination and study of the ground by the eye, termed a reconnaissance, is an indis- pensable preliminary to any more accurate and minute survey by instruments, to avoid loss of time, as by this more rapid ope- ration any ground unsuitable for the proposed line will be as cer- tainly detected by a person of some experience, as it could be by the slow process of an instrumental survey. Before however pro- ceeding to make a reconnaissance, a careful inspection of the general maps of that portion of the country through which the communication is to pass, will facilitate, and may considerably abridge, the labors of the engineer ; as from the natural features laid down upon them, particularly the direction of the water- courses, he will at once be able to detect those points which will be favorable, or otherwise, to the general direction selected for the line. This will be sufficiently evident when it is considered 1, that the water-courses are necessarily the lowest lines of the valleys through which they flow, and that their direction must also be that of the lines of greatest declivity of their respective valleys ; 2, that from the position of the water-courses the position also of the high grounds by which they are separated naturally follows, as well as the approximate position at least of the ridges, or highest lines of the high grounds, which separate their opposite slopes, and which are at the same time the lines of greatest de- clivity common to these slopes, as the water-courses are the cor responding lines of the slopes that form the valleys. Keeping these facts (which are susceptible of rigid mathemati cal demonstration) in view, it will be practicable, from a careful examination of an ordinary general map, if accurately constructed, not only to trace, with consideraDle accuracy, the general direc 278 ROADS. tion of the ridges from having that of the water-courses, but alsc to detect those depressions in them which will be favorable to the passage of a communication intended to connect two main or two secondary valleys. The following illustrations may serve to place this subject in a clearer aspect. If, for example, it be found that on any portion of a map the water-courses seem to diverge from or converge towards one point, it will be evident that the ground in the first case must be the common source or supply of the water-courses, and therefore the highest ; and in the second case that it is the lowest, and forms their common recipient. If tw r o w'ater-courses flow in opposite directions from a common point, it will show that this is the point from which they derive their common supply, at the head of their respective valleys, and that it must be fed by the slopes of high grounds above this point ; or, in other words, that the valleys of the two water-courses are separated by a chain of high grounds,- which, at the point where it crosses them, presents a depression in its ridge, which would be the natural position for a communication connecting the two valleys. If two water-courses flow in the same direction and parallel to each other, it will simply indicate a general inclination of the ridge between them, in the same direction as that of the water- courses. The ridge, however, may present in its course eleva- tions and depressions, w^hich will be indicated by the points in which the water-courses of the secondary valleys, on each side of it, intersect each other on it ; and these will be the lowest points at which lines of communication, through the secondary valleys, connecting the main water-courses, would cross the divi- ding ridge. If two water-courses flow in the same direction, and parallel to each other, and then at a certain point assume divergent direc- tions, it will indicate that this is the lowest point of the ridge be- tweeen them. If two water-courses flow in parallel but opposite directions, depressions in the ridge between them will be shown by the meeting of the water-courses of the secondary valleys on the ridge ; or by an approach towards each other, at any point, of the two principal water-courses. Furnished with the data obtained from the maps, the charactei of the ground should be carefully studied both ways by the en gineer, first from the point of departure to that of arrival, and then returning from the latter tc the former, as without this double traverse natural features of essential importance might escape he eye. 636. Surveys. From the results of the reconnaissance, the ROADS. 279 engineer will be able to direct understanumgly the requisite sur veys, which consist in measuring the lengths, determining the directions, and ascertaining both the longitudinal and cross levels of the different routes, or, as they are termed, trial lines, witli sufficient accuracy to enable him to make a comparative estimate both of their practicability and cost. As the expense of making the requisite surveys is usually but a small item compared with that of constructing the communication, no labor should be spared in running every practicable line, as otherwise natural features might be overlooked which might have an important influence on the cost of construction. 637. Map and Memoir. The results of the surveys are ac- curately embodied in a map exhibiting minutely the topographical features and sections of the different trial lines, and in a memoir which should contain a particular description of those features of the ground that cannot be shown on a map, with all such infor- mation on other points that may be regarded as favorable, or otherwise, to the proposed communication ; as, for example, the nature of the soil, that of the water-courses met with, &c., &c. 638. Location of common Roads. In selecting among the different trial-lines of the survey the one most suitable to a com- mon road, the engineer is less restricted, from the nature of the conveyance used, than in any other kind of communication. The main points to which his attention should be confined are 1, to connect the points of arrival and departure by the most direct, or shortest line ; 2, to avoid unnecessary ascents and descents, or, in oilier words, to reduce the ascents and descents to the smallest practicable limit ; 3, to adopt such suitable slopes, or gradients, for the axis, or centre line of the road, as the nature of the con- veyance may demand ; 4, to give the axis such a position, with re- gard to the surface of the ground and the natural obstacles to be overcome, that the cost of construction for the excavations and. embankments required by the gradients, and for the bridges and other accessories, shall be reduced to the lowest amount. 639. Deviations from the right line drawn on the map, between the points of arrival and departure, will be often demanded by the natural features of the ground. In passing the dividing ridges of main, or secondary valleys, for example, it will frequently be found more advantageous, both for the most suitable gradients, and to diminish the amount of excavation and embankment, to cross the ridge at a lower point than the one in which it is inter- sected by the right line, deviating from the right line either towards the head, or upper part of the valley, or towards its out let, according to the advantages presented by the natural features of the ground, both for reducing the gradients and the amount of excavation and embankment. ROADS. Where the right line intersects either a marsh, or water-course, it may be found less expensive to change the direction, avoiding the marsh, or intersecting the water-course at a point where the cost of construction of a bridge, or of the approaches to it, wiF be moifc favorable than the one in which it is intersected by the right line. Changes from the direction of the right line may also be fa- vorable lor the purpose of avoiding the intersection of secondary water-courses ; of gaining a better soil for the roadway ; of giv- ing a better exposure of its surface to the sun and wind ; or of procuring better materials for the road-covering. By a careful comparison of the advantages presented by these different features, the engineer will be enabled to decide how far the general direction of the right line may be departed from with advantage to the location. By choosing a more sinuous course the length of the line will often not be increased to any very consider able degree, while the cost of construction may be greatly re- duced, either in obtaining more favorable gradients, or in lessening the amount of excavation and embankment. 640. When the points of arrival and departure are upon dif- ferent levels, as is usually the case, it will seldom be practicable to connect them by a continual ascent. The most that can be done will be to cross the dividing ridges at their lowest points, and to avoid, as far as practicable, the intersection of considerable secondary valleys which might require any considerable ascent on one side and descent on the other. 641. The gradients upon common roads will depend upon the kind of material used for the road-covering, and upon the state in which the road-surface is kept. The gradient in all cases should be less than the angle of repose, or of that inclination of the axis of the road in which the ordinary vehicles for transporta tion would remain at a state of rest, or, if placed in motion, would descend by the action of gravity with uniform velocity. The gradients corresponding to the angle of repose have been ascertained by experiments made upon the various road-coverings in ordinary use, by allowing a vehicle to descend along a road of variable inclination until it was brought to a state of rest by the retarding force of friction ; also, b)^ ascertaining the amount of force, termed the force of traction, requisite to put in motion a vehicle with a given load on a level road. The following are the results of experiments made by Mr. Macneill, in England, to determine the force of traction for one ion upon level roads. No. 1. Good pavement, the force of traction is . 33 Ibs " 2. Broken stone surface laid on an old flint road 65 " " 3. Gravel road . . 147 " ROADS. 281 No. 4. Broken-stone surface on a rough pave nent bottom ....... 16 Ibs. " 5. Broken-stone surface on a bottom of beton . 46 " From this it appears that the angle of repose in the first case is represented by ^ffo, or -^-g nearly; and that the slope of the road should therefore not be greater than one perpendicular to sixty-eight in length ; or that the height to be overcome must not be greater than one sixty-eighth of the distance between the two points measured along the road, in order that the force of friction may counteract that of gravity in the direction of the road. A similar calculation will show that the angle of repose in the other cases will be as follows : " No. 2, . . . . 1 to . . .35 nearly. "3, . . . . 1 to . . . 15 " ; 4 and 5, . 1 to . . .49 " These numbers, which give the angle of repose between ^ J T and ? V for the kinds of road-covering Nos. 2 and 4 in most or- dinary use, and corresponding to a road-surface in good order, may be somewhat increased, to from ^ to ^, for the ordinary state of the surface of a well-kept road, without there being any necessity for applying a brake to the wheels in descending, or going out of a trot ir ascending. The steepest gradient that can be allowed on roads with a broken-stone covering is about ^ as this, from experience, is found to be about the angle of repose upon roads of this character in the state in which they are usually kept. Upon a road with this inclination, a horse can draw at a walk his usual load for a level without requiring the assistance of an extra horse ; and experience has farther shown that a horse at the usual walking pace will attain, with less apparent fatigue, the summit of a gradient of ^V in nearly the same time that he would require to reach the same point on a trot over a gradient of aV. A road on a dead level, or one with a continued and uniform ascent between the points of arrival and departure, where they lie upon different levels, is not the most favorable to the draft of the horse. Each of these seems to fatigue him more than a line of alternate ascents and descents of slight gradients ; as, for exam- pie, gradients of T ^, upon which a horse will draw as heavy a load with the same speed as upon a horizontal road. The gradients should in all cases be reduced as far as prac- ticable, as the extra exertion that a horse must put forth in over- coming heavy gradients is very considerable ; they should as a general rule, therefore, be kept as low at least as ^, wherever the ground will admit of it. This can generally be effected, even iii ascending steep hill-sides, by giving the axis of the road a zig- 36 ROADS. zag direction, connecting the straight portions of the zigzag* 0? circular arcs. The grad ents of the curved portions of the zig. zags should be rec'uced, and the roadway also at these points be widened, for the safety of vehicles descending rapidly. The width of the roadway may be increased about one fourth, when the angle between the straight portions of the zigzags is from 120 to 90; and the increase should be nearly one half where the angle is from 90 to 60. 642. Having laid down upon the map the approximate location of the axis of the road, a comparison can then be made between the solid contents of the excavations and embankments, which should be so adjusted that 'hey shall balance each other, or, in other words, the necessary excavations shall furnish sufficient earth to form the embankments. To effect this, it will frequently be necessary to alter the first location, by shifting the position of the axis to the right or left of the position first assumed, and also by changing the gradients within the prescribed limits. This is a problem of very considerable intricacy, whose solution can only be arrived at by successive approximations. For this pur- pose, the line must be subdivided into several portions, in each of which the equalization should be attempted independently of the rest, instead of trying a general equalization for the whole line at once. In the calculations of solid contents required in balancing the excavations and embankments, the most accurate method consists in subdividing the different solids into others of the most simple geometrical forms, as prisms, prismoids, wedges, and pyramids, whose solidities are readily determined by the ordinary rules for the mensuration of solids. As this process, however, is frequently long and tedious, other methods requiring less time but not so accurate, are generally preferred, as their results give an approx- imation sufficiently near the true for most practical purposes. They consist in taking a number of equidistant profiles, and cal- culating the solid contents between each pair, either by multiply- ing the half sum of their areas by the distance between them, or else by taking the profile at the middle point between each pair, and multiplying its area by the same length as before. The latter method is the more expeditious ; it gives less than the true solid contents, but a nearer approximation than the former, which -gives more than the true solid contents, whatever may be the form of the ground between each pair of cross profiles. In calculating the solid contents, allowance must be made for the difference in bulk between the different kinds of earth when occupying their natural bed and when made into embankment From some careful experiments on this point made by Mr. Elwood Morris, a civil ergineer, and published in the Franklin Journai ROADS. 283 it appears that light smdy earth occupies the same space both in excavation and embankment ; clayey earth about one tenth less in embankment than in its natural bed ; gravelly earth also about one twelfth less ; rock in large fragments about five twelfths more, and in small fragments about six tenths more. 643. Another problem connected with the one in question, is that of determining the lead, or the mean distance to which the earth taken from the excavations must be carried to form the embankments. From the manner in which the earth is usually transported from the one to the other, this distance is usually that between the centie of gravity of the solid of excavation and that of its corresponding embankment. Whatever disposition may be made of the solids of excavation, it is important, so far as the cost of their removal is concerned, that the lead should be the least possible. The solution of the problem under this point of view will frequently be extremely intricate, and demand the application of all the resources of the higher analysis. One gen- eral principle however is to be observed in all cases, in order to obtain an approximate solution, which is, that in the removal of the different portions of the solid of excavation to their corre- sponding positions on that of the embankment, the paths passed over by their respective centres of gravity shall not cross each other either in a horizontal, or vertical direction. This may in most cases be effected by intersecting the solids of excavation and embankment by vertical planes in the direction of the re- moval, and by removing the partial solids between the planes within the boundaries marked out by them. 644. The definitive location having been settled by again going over the line, and comparing the features of the ground with the results furnished by the preceding operations, general and de- tailed maps of the different divisions of the definitive location are prepared, which should give, with the utmost accuracy, the lon- gitudinal and cross sections of the natural ground, and of the ex- cavations and embankments, with the horizontal and vertical measurements carefully written upon them, so that the superin- tending engineer may have no difficulty in setting out the work from them on the ground. In addition to these maps, which are mainly intended to guide the engineer in regulating the earth-work, detailed drawings of the road-covering, of the masonry and carpentry of the bridges, cul- verts, &c., accompanied by written specifications of the manner in which the various kind of work is to be performed, should be prepared for the guidance both of the engineer and workmen. 645. With the data furnished by the maps and drawings, the engineer can proceed to set out the line on the ground. The axis of the road is determined by placing stout stakes, or picket* 284 ROADS. at equal intervals apart, which are nuii.fcered to correspond with the same points on the map. The width of the roadway and the lines on the ground corresponding to the side slopes of the exca- vations and embankments, are laid out in the same manner, by stakes placed along the lines of the cross profiles. Besides the numbers marked on the stakes, to indicate their position on the map, other numbers, showing the depth of the excavations, or the height of the embankments from the surface of the ground, accompanied by the letters Cut. Fill, to indicate a cutting, or a. filling, as the case may be, are also added to guide the workmen in their operations. The positions of the stakes on the ground, which show the principal points of the axis of the road, should, moreover, be laid down on the map with great ac- curacy, by ascertaining their bearings and distances from any fixed and marked objects in their vicinity, in order that the points may be readily found should the stakes be subsequently misplaced. 646. Earth-work. This term is applied to whatever relates to the construction of the excavations and embankments, to prepare them for receiving the road-covering. 647. In forming the excavations, the inclination of the side slopes demands peculiar attention. This inclination will depend on the nature of the soil, and the action of the atmosphere and internal moisture upon it. In common soils, as ordinary garden earth formed of a mixture of clay and sand, compact clay, and compact stony soils, although the side slopes would withstand very well the effects of the weather with a greater inclination, it is feest to give them two base to one perpendicular ; as the sur- face of the roadway will, by this arrangement, be well exposed, to the action of the sun and air, which will cause a rapid evapo- ration of the moisture on the surface. Pure sand and gravel may require a greater slope, according to circumstances. In all cases where the depth of the excavation is great, the base of the slope should be increased. It is not usual to use any artificial means to protect the surface of the side slopes from the action of the weather ; but it is a precaution which, in the end, will save much labor and expense in keeping the roadway in good order. The simplest means which can be used for this purpose, consist in cov- ering the slopes with good sods, (Fig. 149,) or else with a layer Fig. 149 Cross section of a roac in excavation. A, road-surface. B, side slopes. C, top surface-drain. of vegetable mould about four inches thick, carefully laid and sown* with grass seed. These means will be amply sufficient to protect the side slopes from injury when they are not exposed to ROADS. 285 any other causes of deterioration than the wash of the rain, and the action of frost on the ordinary moisture retained by the soil. The side slopes form usually an unbroken surface from the foot to the top. But in deep excavations, and particularly in soils liable to slips, they are sometimes formed with horizontal offsets, termed benches, which are made a few feet wide and have a ditch on the inner side to receive the surface-water from the portion of the side slope above them. These benches catch and retain the earth that may fall from the portion of the side slope above. When the side slopes are not protected, it will be well, in lo- calities where stone is plenty, to raise a small wall of dry stone at the foot of the slopes, to prevent the wash of the slopes from being carried into the roadway. A covering of brush wood, or a thatch of straw, may also be used with good effect ; but, from their perishable nature, they will require frequent renewal and repairs. In excavations through solid rock, which does not disintegrate on exposure to the atmosphere, the side slopes might be made perpendicular ; but as this would exclude, in a great degree, the action of the sun and air, which is essential to keeping the road- surface dry and in good order, it will be necessary to make the side slopes with an inclination, varying from one base to one perpendicular, to one base to two perpendicular, or even greater, according to the locality ; the inclination of the slope on the south side in northern latitudes being greatest, to expose better the road-surface to the sun's rays. The slaty rocks generally decompose rapidly on the surface, when exposed to moisture and the action of frost. The side slopes in rocks of this character maybe cut into steps, (Fig. 150,) and then be covered by a layer of vegetable mould sown in grass seed, or else the earth may be sodded in the usual way. 648. The stratified soils and rocks, in which the strata have a dip, or inclination to the horizon, are liable to slips, or to give way by one stratum becoming detached and sliding on another which is caused either from the action of frost, or from the pres- sure of water, which insinuates itself between the strata. The worst soils of this character are those formed of alternate strata of clay and sand ; particularly if the clay is of a nature to become semi- fluid when mixed with water. The best preventives that can be 886 ROADS. resorted to in these cases, are to adopt a thorough system of drainage, to prevent the surface-water of the ground from run ning down the side slopes, and to cut off all springs which rur. towards the roadway from the side slopes. The surface-watei may be cut off by means of a single ditch (Fig. 149) made on the up-hill side of the road, to catch the water before it reaches the slope of the excavation, and convey it off to the natural water-courses most convenient ; as, in almost every case, it wiP be found that the side slope on the down-hill side is, compara lively speaking, but slightly affected by the surface-water. Where slips occur from the action of springs, it frequently becomes a very difficult task to secure the side slopes. If the sources can be easily reached by excavating into the side slopes, drains formed of layers of fascines, or brush-wood, may be placed to give an outlet to the water, and prevent its action upon the side slopes. The fascines may be covered on top with good sods laid with the grass side beneath, and the excavation made to place the drain be filled in with good earth well rammed. Drains formed of broken stone, covered in like manner on top with a layer of sod to prevent the drain from becoming choked with earth, maybe used under the same circumstances as fascine drains. Where the sources are not isolated, and the whole mass of the soil forming the side slopes appears saturated, the drainage may be effected by excavating trenches a few feet wide at inter- vals to the depth of some feet into the side slopes, and filling them with broken stone, or else a general drain of broken stone may be made throughout the whole extent of the side slope by excavating into it. When this is deemed necessary, it will be well to arrange the drain like an inclined retaining-wall, with buttresses at intervals projecting into the earth farther than the general mass of the drain. The front face of the drain should, in this case, also be covered with a layer of sods with the grass side beneath, and upon this a layer of good earth should be compactly laid to form the face of the side slopes. The drain need only be carried high enough above the foot of the side slope to tap all the sources ; and it should be sunk sufficiently below the roadway- surface to give it a secure footing. The drainage has been effected, in some cases, by sinking .wells or shafts at some distance behind the side slopes, from the top surface to the level of the bottom of the excavation, and lead- ing the water which collects in them by pipes into drains at the foot of the side slopes. In others a narrow trench has been ex- cavated, parallel to the axis of the road, from the top surface to a sufficient depth to tap all the sources which flow towards the side slope, and a drain formed either by filling the trench wholly with broken stone, or else by arranging an open conduit at the ROADS 28" bottom to receive the water collected, over wnich a layer of brushwood is laid, the remainder of the trench being filled with broken stone. In some recent instances in England, the side slopes of very bad soils have been secured by a facing of brick arranged in a manner very similar to the method resorted to for securing the perpendicular sides of narrow deep trenches by a timber-facing. The plan pursued is to place, at intervals along the excavation, strong buttresses of brick on each side, opposite to each other, and to connect them at bottom by a reversed arch. Between these buttresses are placed, at suitable heights, one or more brick beams, formed at bottom with a flat segment arch, and at top with a like inverted arch. The buttresses, secured in this way, serve as piers for vertical cylindrical arches, which form the facing and support the pressure of the earth between the but- tresses. 649. In forming the embankments, (Fig. 151,) the side slopes should be made with a less inclination than that which the earth natunilly assumes ; for the purpose of giving them greater dura- bility, and to prevent the width of the top surface, along which the roadway is made, from diminishing by every change in the side slopes, as it would were they made with the natural slope To protect the side slopes more effectually, they should be sod ded, or sown in grass seed ; and the surface-water of the lop should not be allowed to run down them, as it would soon wash them into gullies, and destroy the embankment. In localities where stone is plenty, a sustaining wall of dry stone may be ad- vantageously substituted for the side slopes. To prevent, as far as possible, the settling which takes place in embankments, they should be formed with great care; the earth being laid in successive layers of about four feet in thick- ness, and each layer well settled with rammers. As this method is very expensive, it is seldom resorted to except in works which require great care, and are of trifling extent. For extensive works, the method usually followed on account of economy, is* to embank out from one end, carrying forward the work on a level with the top surface. In this case, as there must be a want of compactness in the mass, it would be best to form the outsidea of the embankment first, and to gradually fill in towards the cen- tre, in order that the earth may arrange itself in layers with a dip from the sides inwards : this will in a great measure counteract 288 ROADS. any tendency to slips outward. The foot of the slopes should be secured by buttressing them either by a low stone wall, 01 by forming a slight excavation for the same purpose. 650. When the axis of the roadway is laid out on the side slope of a hill, and the road-surface is formed partly by excava- ting and partly by embanking out, the usual and most simple method is to extend out the embankment gradually along the whole line of excavation. This method is insecure, and no pains therefore should be spared to give the embankment a good foot- ing on the natural surface upon which it rests, particularly at the foot of the slope. For this purpose the natural surface (Fig. 152) should be cut into steps, or oifsets, and the foot of the slope be secured by buttressing it against a low stone wall, or a small terrace of carefully rammed earth. In side-formings along a natural surface of great inclination, the method of construction just explained will not be sufficiently secure ; sustaining-walls must be substituted for the side slopes, both of the excavations and embankments. These walls may be made simply of dry stone, when the stone can be procured in blocks of sufficient size to render this kind of construction of sufficient stability to resist the pressure of the earth. But when the blocks of stone do not offer this security, they must Fig. 153 Cross section of a road in steep side-forming. A, filling. B, sustaining- wall of filling. C, breast-wall of cutting. D, parapet-wall of footpath. oe laid in mortar, (Fig. 153,) and hydraulic mortar is the onlf ROADS. 289 Kind which will form a safe construction. The wall which sup plie^i the slope of the excavation should be carried up as high as the natural surface of the ground ; the one that sustains the em- bankment should be built up to the surface of the roadway ; and a parapet-wall should be raised upon it, to secure vehicles from accidents in deviating from the line of the roadway. A road may be constructed partly in excavation and partly in embankment along a rocky ledge, by blasting the rock, when the inclination of the natural surface is not greater than one perpen- dicular to two base ; but with a greater inclination than this, the whole should be in excavation. 651. There are examples of road constructions, i: .ocalities like the last, supported on a frame-work, consisting of horizontal pieces, which are firmly fixed at one end by being let into holes drilled in the rock, and are sustained at the other by an inclined strut underneath, which rests against the rock in a shoulder formed to receive it. 652. When the excavations do not furnish sufficient earth for the embankments, it is obtained from excavations, termed side- cuttings, made some place in the vicinity of the embankment, from which the earth can be obtained with the most economy. If the excavations furnish more earth than is required for the embankment, it is deposited in what is termed spoil-bank, on the side of the excavation. The spoil-bank should be made at some distance back from the side slope of the excavation, and on the down-hill side of the top surface ; and suitable drains should bf arranged to carry off any water that might collect near it and af feet the side slope of the excavation. The forms to be given to side-cuttings and spoil-banks wiU depend, in a great degree, upon the locality : they should, as far as practicable, be such that the cost, of removal of the earth shal' oe east possible 653. Drainage. A system of thorough drainage, by whicn .ne water that filters through the ground will be cut off from the loil beneath the roadway, to a depth of at least three feet below "lie bottom of the road-covering, and by which that which falls upon the surface will be speedily conveyed off, before it can filter through the road-covering, is essential to the good condition of a road. The surface-water is conveyed off by giving the surface of the roadway a slight transverse convexity, from the middle to the sides, where the water is received into the gutters, or side chan- nels, from which it is conveyed by underground aqueducts, termed culverts, built of stone or brick and usually arched at top, into he main drains that communicate with the natural water-courses. This convexity is regulated by making the figure of the profile 37 290 ROADS. an ellipse, of which the semi-transverse axis is 15 feet, and the semi-conjugate axis 9 inches ; thus placing the middle of the roadway nine inches above the bottom of the side channels. This convexity, which is as great as should be given, will not be suffi cient in a flat country to keep the road-surface dry ; and in such localities, if a slight longitudinal slope cannot be given to the road, it should be raised, when practicable, three or four feet above the general level ; both on account of conveying off speedily the surface-water, and exposing the surface better to the action of the wind. To drain the soil beneath the roadway in a level country, ditches, termed open side drains, (Fig. 154,) are made parallel Fig. 154 Cross section of broken-stone road-covering. A, road-surface. B, side channels. C, footpath. D, covered drains, or culverts, leading from side channels to the side drains t,. to the road, and at some feet from it. on each side. The bottom of the side drains should be at least three feet below the road- covering ; their size will depend on the nature of the soil to be drained. In a cultivated country the side drains should be on the field side of the fences. As open drains would be soon filled along the parts of a road in excavation, by the washings from the side slopes, covered drains, built either of brick or stone, must be substituted for them. These drains (Fig. 155) consist simply of a flooring of Fig. 155 Cross section of a covered drain. A, drain. a, a, side walls. b, top stones. r, bottom stones. . . (3). The flight, on one side, is thus left full after the passage of the first boat, and on the other side, empty. If a second boat, then, follows directly after the first, the prism of lift must be drawn from the lowest lock to admit the boat, this prism is then supplied from the lock next above, and so on to the summit lev- el ; so that but one prism of lift will be drawn off for the ascent of this boat, and it will require one of lift, and two of draught, to carry it down the opposite flight. If, therefore, the total number of boats which follow in this order, including the first, be represented by m, the total expenditure will be represent- ed by (n + 1) L + (n + 1) D + (m - 1) 2L + (m 1) 2v. . (4). If the second boat, instead of following the first, arrives in the opposite direction, or alternates with it, the expenditure for its ascent will be represented by the formula (1), and for its de- scent it will be nothing, since it finds the opposite flight filled, as left by the first boat ; but if the locks had been emptied, then the passage of the second boat would have taken place under the same circumstances as that of the first. It will be unnecessary here to go farther into these calcula- tions for the various cases that may occur, under the different circumstances of passage of the boats or of empty or full flights ; the preceding gives the spirit of the method, and will give the means for entering upon a calculation to allow for the loss or gain by the passage of freighted or of empty boats, following any prescribed order of passage. These refinements are, for the most part, more curious than useful; and ihe engineer should confine himself to making an ample allowance for the most un- favorable cases, both as regards the order of passage and the number of boats. 697. Feeders and Reservoirs. Having ascertained, from the preceding considerations, the probable supply which should be collected at the summit level, the engineer will next direct his attention to the sources from which it may be procured. Theo- retically considered, all the water that drains from the ground adjacent to the summit level, and above it, might be collected for its supply ; but it is found in practice that channels for the con- veyance of water must have certain slopes, and that these slopes, moreover, will regulate the supply furnished in a certain time, all other things being equal. In making, however, the survey of the country, from which the water is to be supplied to the summit level, all the gruiind above it should be examined, leav- 41 322 CANALS. ing the determination of the slopes for after considerations. The survey for this object consists in making an accurate delineation of all the water-courses above the summit level, and in ascer- taining the quantity of water which can be furnished by each in a given time. This survey, as well as the measurement of the quantity of water furnished by each stream, which is termed the gauging, should be made in the driest season of the year, in or- der to ascertain the minimum supply. 698. The usual method of collecting the water of the sources, and conveying it to the summit level, is by feeders and reser- voirs. The feeder is a canal of a small cross section, which is traced on the surface of the ground with a suitable slope, to convey the water either into the reservoir, or direct to the summit level. The dimensions of the cross section, and the longitudinal slope of the feeder, should bear certain relations to each other, in order that it shall deliver a certain supply in a given lime. The smaller the slope given to the feeder, the lower will be the points at which it will intersect the sources of supply, and therefore the greater will be the quantity of water which it will receive. This slope, however, has a practical limit, which is laid down at four inches in 1000 yards, or nine thousand base to one altitude ; and the greatest slope should not exceed that which would give the current a greater mean velocity than thir- teen inches per second, in order that the bed of the feeder may not be injured. Feeders are furnished, like ordinary canals, with contrivances to let off a part, or the whole, of the water in them, in cases of heavy rains, or for making repairs. But a small proportion of the water collected by the feeders is delivered at the reservoir ; the loss from various causes being much greater in them than in canals. From observations made on some of the feeders of canals in France, which have been in use for a long period, it appears that the feeder of the Briare canal delivers only about one fourth of the water it gathers from its sources of supply ; and that the annual loss of the two feed- ers of the Languedoc canal, amounts to 100 times the quantity of water which they can contain. 699. A reservoir is a large pond, or body of water, held in resei ve for the necessary supply of the summit level. A reser- voir is usually formed by choosing a suitable site in a deep and narrow valley, which lies above the summit level, and erecting a dam of earth, or of masonry, across the outlet of the valley, or at some more suitable point, to confine the water to be collected. The object to be attained, in this case, is to embody the greatest volume of water, and at the same time present the smallest evaporating suiface, at the smallest cost for the construction of the dam CANALS. 323 It is generally deemed best to have two reservoirs for the sup- ply, one to contain the greater quantity of water, and the other, which is termed the distributing reservoir, to regulate the sup- ply to the summit level. If, however, the summit level is very capacious, it may be used as the distributing reservoir. The proportion between the quantity of water that falls upon a given surface, and that which can be collected from it for the supply of a reservoir, varies considerably with the latitude, the season of the year, and the natural features of the locality. The drainage is greatest in high latitudes, and in the winter and spring seasons ; with respect to the natural features, a wooded surface with narrow and deep valleys will yield a larger amount than an open flat country. But few observations have been made on this point by engi- neers. From some by Mr. J. B. Jervis, in reference to the reservoirs for the Chenango canal, in the state of New York, it appears that in that locality about two fifths of the quantity of rain may be collected for the supply of a reservoir. The pro- portion usually adopted by engineers is one third. The loss of water from the reservoir by evaporation, filtration, and other causes, will depend upon the nature of the soil, and the exposure of the water surface. From observations made upon some of the old reservoirs in England and France, it ap- pears that the daily loss averages about half an inch in depth. 700. The dams of reservoirs have been variously constructed: in some cases they have been made entirely of earth, (Fig. 167;) B Fig. 107 Represents the section of a dam with three discharging culverts. A, body of the dam. B, pond. a, a, a, culverts, with valves at their inlets, which discharge into the vertical well b. r, c, c, grooves, in tlie faces of the side-walls, which form the entrance to the culverts, for stop-plank d, stop-plank dam across the outlet of the bottom culvert, to dam back the water into the vertical well. f , parapet wall on top of the dam. in others, entirely of masonry ; and in others, of earth packed in between several parallel stone walls. It is now thought best to 324 CANALS. use either earth or masonry alone, according to the circum stances of the case ; the comparative expense of the two meth ods being carefully considered. Earthen dams should DC made with extreme care, of the best binding earth, well freed from every thing that might cause fil- trations. A wide trench should be excavated to the firm soil, tc receive the base of the dam ; and the earth should be carefully spread and rammed in layers not over a foot thick. As a farther precaution, it has in some instances been thought necessary to place a stratum of the best clay puddling in the centre of the dam, reaching from the top to three or four feet below the base. The dam may be from fifteen to twenty feet thick at top. The slope of the dam towards the pond should be from three to six base to one perpendicular ; the reverse slope need only be some- what greater than the natural slope of the earth. The slope of dams exposed to the water is usually faced with dry stone, to protect the dam from the action of the surface ripple. This kind of facing has not been found to withstand well the action of the water when agitated by high winds. Upon some of the more recent earthen dams erected in France, a facing of stone laid in hydraulic mortar has been substituted for the one of dry stone. TThe plan adopted for this facing (Fig. 166) con- Fig. 168 Represents the method of facing the pond slop* of a dam, with low walls placed in offsets. A, body of the dam. a, a. , low walls, the faces of which are built in offsets. b, b, top surface of the offsets be- tween the walls, covered with stone slabs laid in mortar. c, top of dam faced like the off- sets b. d, parapet wall. sists in placing a series of low walls, in offsets above each other, along the slope of the dam, covering the exposed surface of each offset, between the top of one wall and the foot of the next, with a coating of slab-stone laid in mortar. The walls are from five to six feet high. They are carried up in small offsets upon the face, and are made either vertical, or leaning, on the back. The width of the'offsets of the dam, between the top of one wall and the foot of the next, is from two to three feet. An arched culvert, or a large cast-iron pipe, placed at some suitable point of the base of the dam, which can be closed or opened by a valve, will serve for drawing off the requisite supply of water, and for draining the reservoir in case of re- pairs. The culvert should be strongly constructed, and the earth CANALS. 323 around it be well puddled and rammed, to prevent filiations. Its size should be sufficient for a man to enter it with ease. The valves may be placed either at the entrance of the culvert, or at some intermediate point between the two ends. Great care should be taken in their arrangement, to secure them from accidents. When the depth of water in a reservoir is considerable, several culverts should be constructed, (Fig. 167,) to draw off the water at different levels, as the pressure upon the lower valves in this case would be very great when the reservoir is full. They may be placed at intervals of about twelve feet above each other, and be arranged to discharge their water in a common vertical shaft. In this case it will be well to place a dam of timber at the outlet of the bottom culvert, in order to keep it filled with water, to prevent the injury which the bottom of it might receive from the water discharged from the upper culverts. The side walls which retain the earth at the entrance to the culverts, should be arranged with grooves to receive pieces of scantling laid horizontally between the walls, termed stop-planks, to *brm a temporary dam, and cut off the water of the reser- voir, in case of repairs to the culverts, or to the face of the dam. The valves are small sliding gates, which are raised and low- ered by a rack and pinion, or by a square screw. The cross section of the culvert is contracted by a partition, either of ma- sonry or timber, at the point where the valve is placed. 701. Dams of masonry are water-tight walls, of suitable forms and dimensions to prevent filtration, and resist the pressure of water in the reservoir. The most suitable cross section is that of a trapezoid, the face towards the water being vertical, and the exterior face inclined with a suitable batter to give the wall sufficient stability. The wall should be at least four feet thick at the water line, to prevent filtration, and this thickness may be increased as circumstances may seem to require. Buttresses should be added to the exterior facing, to give the wall greater stability. 702. Suitable dispositions should be made to relieve the dam from all surplus water during wet seasons. For this purpose arrangements should be made for cutting off the sources of sup- ply from the reservoir ; and a cut, termed a waste-weir, (Fig. 169,) of suitable width and depth should be made at some point along the lop of the dam, and be faced with stone, or wood, to give an outlet to the water over the dam. In high dams the total fall of the water should be divided into several partial falls, by dividing the exterior surface over which the water runs into offsets. To break the shock of the water upon the horizontal 326 CiNALS. surface of the offset, it should be kept covered with a sheet of water retained by a dam placed across its outlet. Fig. 169 Represents a section of a waste-weir divided into two falls. A, body of the dam. a, top of the waste-weir. b, pool, formed by a stop-plank dam at c, to break the fall of the water. d, covering of loose stone to break the fall of the water from the pool above. 703. In extensive reservoirs, in which a large surface is ex- posed to the action of the winds, waves might be forced ovei the top of the dam, and subject it to danger ; in such cases the precaution should be taken of placing a parapet wall towards the outer edge of the top of the dam, and facing the top through- out with flat stones laid in mortar. 704. Lift of locks. From the preceding observations on the expenditure of water for the service of the navigation, it appears that isolated locks are more favorable under this point of view than locks in flights. The engineer is not, however, always left free to select between the two systems ; for the form of the natural surface of the ground may compel him to adopt a flight of locks at certain points. As to the comparative expense of the two methods, a flight is *in most cases cheaper than the same number of single locks, as there are certain parts of the masonry which can be suppressed. There is also an economy in the suppression of the small gates, which are not needed in flights. It is, however, more difficult to secure the foundations of com- bined than of single locks from the effects of the water, which forces its way from the upper to the lower level under the locks. Where an active trade is carried on, a double flight is sometimes arranged ; one for the ascending, the other for the descending boats. In this case the water which fills one flight may, after *,he passage of the boat, be partly used for the other, by an arrangement of valves made in the side wall separating the locks. The lift of locks is a subject of importance, both as regards the consumption of water for the navigation, and the economy of construction. Locks with great lifts, as may be seen from the remarks on the passage of boats, consume more water than those with small lifts. They require also more care in thcii CANALS 327 construction, to preserve them from accidents, owing to the gresu pressure of water against their sides. The expense of construc- tion is otherwise in their favor; that is, the expense will increase with the total number of locks, the height to be ascended being the sam?. The smallest lifts are seldom less than five feet, and the greatest, for ordinary canals, not over twelve ; medium lifts of seven or eight feet are considered the best under every point cf view. This is a point, however, which cannot be settled arbitrarily, as the nature of the foundations, the materials used, the embankments around the locks, the changes in the direction of the canal, caused by varying the lifts, are so many modifying causes, which should be carefully weighed before adopting a definitive plan. The lifts of a flight should be the same throughout; but in isolated locks the lifts may vary according to circumstances. If the supply of water from the summit level requires to be econo- mized with care, the lifts of locks which are furnished from it may be less than those lower down. 705. Levels. The position and the dimensions of the levels must be mainly determined by the form of the natural surface. Those points are naturally chosen to pass from one level to another, or as the positions for the locks, where there is an ab- rupt change in the surface. A level, by a suitable modification of its cross section, can be made as short as may be deemed desirable ; there being but one point to be attended to in this, winch is, that a boat passing be- tween the two locks, at the ends of the level, will have time to enter either lock before it can ground, on the supposition, that the water drawn off to fill the lower lock, while the boat is tra- versing the level, will just reduce the depth to the draught of the boat. 706. Locks. A lock (Fig. 170) may be divided into three distinct parts : 1st. The pait included between the two gates, which is termed the chamber. 2d. The part above the uppei gates, termed the fore, or head-bay. 3d. The part below the lower gates, termed the aft, or tail-bay. 707. The lock chamber must be wide enough to allow an easy ingress and egress to the boats commonly used on the ca- nal; a surplus width of one foot over the width of the boat across the beam is usually deemed sufficient for this purpose. The length of the chamber should be also regulated by that of the boats ; it should be such, that when the boat enters the lock from the lower level, the tail-gates may be shut without requiring the boat to unship its rudder. The plan of the chamber is usually rectangular, as this form w, in every respect, superior to all others. In the cross section Fig. 170 Represents a plan 'M, and a section N, through the axis of a single lock laid on a be- ton foundation. A, lock-chamber. B, fore-bay. C, tail-bay, a, a, chamber-walls. 6, i re- cesses or chambers in the side walls for upper-gates. e,e, lower-gate chambers, d, d, lift wall and upper mitre sill. e,e, lower mitre sill. A, A, tail walls. 0,0, head walls, m, m, upper wing, or return wiills. n, n, lower wing walls. D, body of masonry under the fore-bajv CANALS. 32S of the chamber, (Fig. 171,) the sides receive generally a slight Fig. 171 Represents a section of Fig. 170, through the chamber. A, A, chamber walls. B, chamber formed with an inverted-arch bottom batter ; as when so arranged they are found to give greater fa- cility to the passage of the boat than when vertical. The bot- tom of the chamber is either flat or curved ; more water will be required to fill the flat-bottom,ed chamber than the curved, but it will require less masonry in its construction. 708. The chamber is terminated just within the head gates by a vertical wall, the plan of which is usually curved. As this wall separates the upper from the lower level, it is termed the lift-wall ; it is usually of the same height as the lift of the lev- els. The top of the lift-wall is formed of cut stone, the vertical joints of which are normal to the curved face of the wall ; this top course projects from six to nine inches above the bottom of the upper level, presenting an angular point, for the bottom of the head-gates, when shut, to rest against. This is termed the mitre-sill. Various degrees of opening have been given to the angle between the two branches of the mitre-sill ; it is, however, generally so determined, that the perpendicular of the isosceles triangle, formed by the two branches, shall vary between on& fifth and one sixth of the base. As stone mitre-sills are liable to injury from the shock of the gate, they are now usually constructed of timber, (Fig. 172,) by Fig. 172 Represents a plan of a wooden mitre- sill, and a horizontal section of a lock-gate (Fig. 173) closed. a, a, mitre-sill framed with the pieces b and c, and firmly fastened to the side walls A, A. d, section of quoin posts of lock-gate. e, section of mitre posts. framing two strong beams with the proper angle for the gate when closed, and securing them firmly upon the top of the lift- wall. It will be well to place the top of the mitre-sill on the lift-wall a little lower than the bottom of the canal, to preserve it from being struck by the keel of the boat on entering, or leaving the lock. 709. The cross section of the chamber walls is usually trape- zoidal ; the facing receives a slight batter. The chamber wall* 42 830 CANALS ire exposed to two opposite efforts ; the watei in the JOCA un one side, and the embankment against the wall on the other. The pressure of the embankment is the greater as well as the more permanent effort of the two. The dimensions of the wall must be regulated by this pressure. The usual manner of doing this, is to make the wall four feet thick at the water line of the upper level, to secure it against filtration ; and then to determine the base of the batter, so that the mass of masonry snail present sufficient stability to counteract the tendency of the pressure. The spread, and other dimensions of the foundations, will be regulated according to the nature of the soil, in the same way as in other structures. 710. The bottom of the chamber, as has been stated, may be either flat or curved. The flat bottom is suitable to very firm soils, which will neither yield to the vertical pressure of the chamber walls, nor admit the water to filter from the upper level under the bottom of the lock. In either of the contrary cases, the bottom should be made with an inverted arch, as this form will oppose greater resistance to the upward pressure of the water under the bottom, and will serve to distribute the weight of the walls over the portion of the foundation under the arch. The thickness of the masonry of the bottom will depend on the width of the chamber, and the nature of the soil. Were the soil a solid rock, no bottoming would be requisite ; if it is of soft mud, a very solid bottoming, from three to six feet in thickness, might be requisite. 711. The principal danger to the foundations arises from the water which may filter from the upper to the lower level, under the bottom of the lock. One preventive for this, but not an ef- fectual one, is to drive sheeting piles across the canal at the o.nd of the head-bay ; another, which is more expensive, but more certain in its effects, consists in forming a deep trench of two or three feet in width, just under the head-bay, and filling it with beton, which unites at top with the masonry of the head-bay. Similar trenches might be placed under the chamber were it considered necessary. 712. The lift-wall usually receives the same thickness as the chamber walls ; but, unless the soil is very firm, it would be more prudent to form a general mass of masonry under the en- tire head-bay, to a level with the base of the chamber founda- tions, of which mass the lift-wall should form a part. 713. The head-bay is enclosed between two parallel walls, which form a part of the side walls of the lock. They are ter- minated by two wing walls, which it will be found most eco- nomical to run back at right angles with the side walls. A re- cess, termed the gate-chamber, is made ,h the wall of the head- CANALS. 331 bay : the depth of this recess should be sufficient to allow the gate, when open, to fall two or three inches within the facing of the wall, so that it may be out of the way when a boat is pass- ing ; the length of the recess should be a few inches more than the width of the gate. That part of the recess where the gate turns on its pivot is termed the hollow quoin ; it receives what is termed the heel, or quoin-post of the gate, which is made of a suitable form to fit the hollow quoin. The distance between the hollow quoins and the face of the lift-wall will depend on the pressure against the mitre-sill, and the strength of the stone ; eighteen inches will generally be found amply sufficient. The side walls need not extend more than twelve inches be yond the other end of the gate-chamber. The wing walls may be extended back to the total width of the canal, but it will be more economical to narrow the canal near the lock, and to ex- tend the wing walls only about two feet into the banks, or sides. The dimensions of the side and wing walls of the head-bay are regulated in the same way as the chamber walls. The bottom of the head-bay is flat, and on the same level with the bottom of the canal ; the exterior course of stones at the en- trance to the lock should be so jointed as not to work loose. 714. The gate-chambers for the lower gates are made in the chamber walls ; and it is to be observed, that the bottom of the chamber, where the gates swing back, should be flat, or be oth- erwise arranged not to impede the play of the gates. 715. The side walls of the tail-bay are also a part of the gen- eral side walls, and their thickness is regulated as in the prece- ding cases. Their length will depend chiefly on the pressure which the lower gates throw against them when the lock is full; and partly on the space required by the lock-men in opening and shutting gates manoeuvred by the balance beam. A calculation must be made for each particular case, to ascertain the most suitable length. The side walls are also terminated by wing walls, similarly arranged to those of the head-bay. The points of junction between the wing and side walls should, in both cases, either be curved, or the stones at the angles be rounded off. One or two perpendicular grooves are sometimes made in the side walls of the tail-bay, to receive stop-planks, when a temporary dam is needed, to shut off the water of the lower level from the chamber, in case of repairs, &c. Similar arrangements might l>e made at the head-bay, but they are not indispensable in either case. The strain on the walls at the holl w quoins is greater than ai any other points, owing to the pressure at those points from the gates, when they are shut, and to the action of the gates whec in motion ; to counteract this, and strengthen the walls, bu*.- 332 CANALS. tresses should be placed at the back of the walls, in the most favorable position behind the quoins to subserve the object in view. The bottom of the tail-bay is arranged, in all respects, like that of the head-bay. 716. The top of the side walls of the lock may be from one to two feet above the general level of the water in the upper reach ; the top course of the masonry being of heavy large blocks of cut stone, although this kind of coping is not indis- pensable, as smaller masses have been found to suit the same purpose, but they are less durable. As to the masonry of the lock, in general, it is only necessary to observe, that those parts alone need be of cut stone where there is great wear and tear from any cause, as at the angles generally ; or where an accu- rate finish is indispensable, as at the hollow quoins. The other parts may be of brick, rubble, beton, &c., but every part should be laid in the best hydraulic mortar. 717. The filling and emptying the lock chamber have given rise to various discussions and experiments, all of which have been reduced to the comparative advantages of letting the watei in and off by valves made in the gates themselves, or by culverts in the side walls, which are opened and shut by valves. When the water is let in through valves in the gates, its effects on the sides and bottom of the chamber are found to be very injurious, particularly in high lift-walls ; besides the inconvenience result- ing from the agitation of the boat in the lock. To obviate this, in some degree, it has been proposed to give the lift-wall the form of an inclined curved surface, along which the water might descend without producing a shock on the bottom. 718. The side culverts are small arched conduits, of a circu- lar, or an elliptical cross section, which are made in the ?nass of masonry of the side-walls, to convey the water from the up- per level to the chamber. These culverts, in some cases, run the entire length of the side walls, on a level with the bottom of the chamber, from the lift-wall to the end of the tail-wall, and have several outlets leading to the chamber. They are arranged with two valves, one to close the mouth of the cuJvert, at the upper level, the other to close the outlet from the chamber, to the lower level. This is, perhaps, one of the best arrangements for side culverts. They all present the same difficulty in making repairs when oui of 01 ier, and they are moreover very subject to accidents. They are therefore on these accounts inferior to valves in the gates. 719. It has also been proposed, to avoid the inconveniences of culverts, and the disadvantages of lift-walls, by suppressing the latter, and gradually increasing the depth of the upper level CANALS 333 to the bottom of the chamber. This method presents a saving in the mass of masonry, but the gates will cost more, as the head and tail gates must be of the same height. It would en- tirely remove the objection to valves in the gates, as the current through them, in this case, would not be sufficiently strong to injure the masonry. 720. The bottom of the canal below the lock should be pro- tected by what is termed an apron, which is a covering of plank laid on a grillage, or else one of brush-wood and dry stone. The sides should also be faced with timber or dry stone. The length of this facing will depend on the strength of the current ; gene- rally not more than from fifteen to thirty feet from the lock will require it. The entrance to the head-bay is, in some cases, similarly protected, but this is unnecessary, as the current has but a very slight effect at that point. 721. Locks constructed of timber and dry stcne, termed com- posite-locks, are to be met with on several of the canals of the United States. The side walls are formed of dry stone carefully laid; the sides of the chamber being faced with plank nailed to horizontal and upright timbers, which are firmly secured to the dry stone walls. The walls rest upon a platform laid upon heavy beams placed transversely to the axis of the lock. The bottom of the chamber usually receives a double thickness of plank The quoin-posts and mitre-sills are formed of heavy beams. 722. Lock Gates. A lock gate (Fig. 173) is composed of two Fig. 173 Repre- sents the eleva- tion of a lock- gate closed. a, a, quoin-posts. b, mitre-posts. c, c, cross pieces framed into a and b and firmly connected with them by wrought iron plates. o, plank or sheath- ing of the gate. d, valve. m. m, balance- beam. leaves, each leaf consisting of a solid frame-work covered on the side towards the water with thick plank made water-tight. The frame usually consists of two uprights, of several horizon- tal cross pieces let into the uprights, and sometimes a diagonal piece, or brace, intended to keep the frame of an invariable 334 CANALS. form, is added. The upright, around which the leaf turns, termed the quoin or heel-post, is rounded off on the back to fit in the hollow quoin ; it is made slightly eccentric with it, so that it may turn easily without rubbing against the quoin ; its lower end rests on an iron gudgeon, to which it is fitted by a corresponding in dentation in an iron socket on the end ; the upper extremity is secured to the side walls by an iron collar, within which the posi turns. The collar is so arranged that it can be easily fastened to, or loosened from two iron bars, termed anchor-irons, which are firmly attached by bolts, or a lead sealing, to the top course of the walls. One of the anchor-irons is placed in a line with the leaf when shu, the other in a line with it when open, to re sist most effectually the strain in those two positions of the gate. The opposite upright, termed the mitre-post, has one edge bev- elled off, to fit against the mitre-post of the other leaf of the gate. 723. A long heavy beam, termed a balance beam, from its partially balancing the weight of the leaf, rests on the quoin post, to which it is secured, and is mortised with the mitre post. The balance beam should be about four feet above the top of the lock, to be readily manoeuvred ; its principal use being to open and shut the leaf. 724. The top cross piece of the gate should be about on a level with the top of the lock ; the bottom cross piece should swing clear of the bottom of the lock. The position of the in- termediate cross pieces may be made to depend on their dimen- sions : if they are of the same dimensions, they should be placed nearer together at the bottom, as the pressure of the water is there greatest ; but, by making them of unequal dimensions, they may be placed at equal distances apart ; this, however, is not of much importance except for large gales, and considerable depths of water. The plank may be arranged either parallel to the uprights, or parallel to the diagonal brace ; in the latter position they will act with the brace to preserve the form of the frame. 725. A wide board supported on brackets, is often affixed to the gates, both for the manoeuvre of the machinery of the valves, and to serve as a foot bridge across the lock. The valves are small gates which are arranged to close the openings made in the gates for letting in, or drawing off the water. They are ar- ranged to slide up and down in grooves, by the aid of a rack and pinion, or a square screw ; or they may be made to open or shut by turning on a vertical axis, in which case they are termed pad- iLle gates. The openings in the upper gates are made between the two lowest cross pieces. In the lower gates the openings are placed just below the surface of the water in the reach. The CANALS. 335 *1Z6 of the opening will depend on the time in which it is r quired to fill the lock. 720. Accessory Works. Under this head are classed those constructions which are not a part of the canal proper, although generally found necessary on all canals : as the culverts for con- veymg off the water courses which intersect the line of the canal; the inlets of feeders for the supply of water ; aqueduct bridges, &c. &e. 727. Culverts. The disposition to be made of water courses intersecting the line of the canal will depend on their size, the character of their current, and the relative positions of the canal and stream. Small biooks which lie lower than the canal may be conveyed under it through an ordinary culvert. If the level of the canal and brook is naarly the same, it will then be necessary to make the culvert in the shape of an inverted syphon, and it is therefore termed a broken-back culvert. If the water of the brook is generally limpid, and its current gentle, it may, in the last case, be received into the canal. The communication of the brook, or feeder, with the canal, should be so arranged that the water may be shut off, or let in at pleasure, in any quantity desired. For this purpose a cut is made through the side of the canal, and the sides and bottom of the cut are faced with masonry laid in hy- draulic mortar. A sliding gate, lilted into two grooves made in the side walls, is manoeuvred by a rack rnd pinion, so as to reg- ulate the quantity of water to be Jet in. The water of the feeder, or brook, should first be received in a baein, or reservoir, near the canal, where it may deposite its sediment before it is drawn off. In cases where the line of the canal is crossed by a torrent, which brings down a large quantity of sand, pebbles, &c., it may be necessary to make a permanent structure over the canal, form- ing a channel for the torrent ; but if the discharge of the torrent is only periodical, a moveable chpnn.^1 may be arranged, for the same purpose, by constructing a boat with a deck and sides to form the water-way of the torreni. The boat is kept in a recess in the canal near the point where it is used, and is floated to its position, and sunk when wanted. 728. Aqueducts, fyc. When the line of the canai is intersect- ed by a wide water-course, the communication between the two shores must be effected either by a canal pqueduct bridge, or by the boats descending from the canal into the stream. As thr construction of aqueduct bridges has alrredy been considered nothing farther on this point need here be added. The expe dient of crossing the stream by the boats may be tended many grave inconveniences in water courses 'bhl" f o f or to considerable variations of ?evel at differed 336 CANALS. these cases locks must be so arranged on each side, where the canal enters the stream, that boats may pass from the one to the other under all circumstances of difference of level between the two. The locks and the portions of the canal which join the stream must be secured against damage from freshets by suita- ble embankments ; and, when the summer water of the stream is so low that the navigation would be impeded, a dam across the stream will be requisite to secure an adequate depth of water during this epoch. 729. Canal Bridges. Bridges for roads over a canal, termed canal-bridges, are constructed like other structures of the same kind. In planning them the engineer should endeavor to give sufficient height to the bridge to prevent those accidents, of but too frequent, occurrence, from persons standing upright on the deck of the passage-boat while passing under a bridge. 730. Waste-Wier. Waste-wiers must be made along the levels to let off the surplus water. The best position for them is at points where they can discharge into natural water courses. The best arrangement for a waste-wier is to make a cut through the side of the canal to a level with the bottom of it, so that, in case of necessity, the waste-wier may also serve for draining the level. The sides and bottom of the cut must be faced with ma- sonry, and have grooves left in them to receive stop-plank, or a sliding gate, over which the surplus water is allowed to flow, under the usual circumstances, but which can be removed, if it be found necessary, either to let off a larger amount of water, or to drain the level completely. 731. Temporary Dams. In long levels an accident happen- ing at any one point might cause serious injury to the navigation, besides a great loss of water. To prevent this, in some meas- ure, the width of the canal may be diminished, at several points of a long level, to the width of a lock, and the sides, at these points, may be faced with masonry, arranged with grooves and stop-planks, to form a temporary dam for shutting off the water on either side. 732. Tide, or Guard Lock. The point at which a canal en- ters a river requires to be selected with judgment. Generally speaking, a bar will be found in the principal water course at, or below, the points where it receives its affluents. When the canal, therefore, follows the valley of an affluent, its outlet should be placed below the bar, tu render its navigation perma- r.ently secure from obstruction. A large basin is usually formed at the outlet, for the convenience of commerce ; and the entrance from this basin to the canal, or from the river to the basin, is ef- fected by means of a lock with double gates, so arranged that a boat can be passed either way, according as the level in the one CANALS. 337 is higher or lower than that in the other. A lock so arranged ia termed a tide, or guard lock, from its uses. The position of the tail of this lock is not indifferent in all cases where it forms the outlet to the river ; for were the tail placed up stream, it would be more difficult to pass in or out, than if it were down stream. 733. The general dimensions of canals and their locks in this country and in Europe, with occasional exceptions, do not differ in any considerable degree. English Canals. Two classes of canals are to be met with in England, differing materially in their dimensions. The following are the usual dimensions of the cross section of the largest size, and those of their locks : Width of section at the water level, from 36 to 40 feet. Width at bottom, . . . . 24 " Depth, 5 " Length of lock between mitre-sills, 75 to 85 " Width of chamber, . . . . 15 ' The Caledonian canal, in Scotland, which connects Loch-Eil on the Western sea with Murray Firth on the Eastern, is re- markable for its size, which will admit of the passage of frigates of the second class. The following are the principal dimensions of the cross section of the canal and its locks : Width of canal at the water level, . 110 feet. Width at bottom, . . . . 50 " Depth of water, .... 20 " Width of berm, 6 " Length of lock between mitre-sills, . 180 " Width of chamber at top, . . . 40 '* Lift of lock, 8 " The side walls of the locks are built with a curved batter , they are of the uniform thickness of 6 feet, and are strengthened by counterforts, placed about 15 feet apart, which are 4 feet wide and of the same thickness. The bottom of the chamber is form ed with an inverted arch. French Canals. In France the following uniform system has been established for the dimensions of canals and their locks - Width of canal at water level, . . 52 feet. Width at bottom, . . . . 33 to 36 " Depth of water, ..... 5 " Length of lock between mitre-sills, . 115 " Width of lock, ... 17 " The boats adapted to these dimensions are from 105 to 108 feet long, 16 feet across the beam, and have a draught of 4 feet. 43 838 CANALS. The English and French canals usually have but one tow-path which is from 9 to 1 2 feet wide, and about 2 feet above the wa ter level. The side of the tow-path embankment next to the water-way is usually faced either with dry stone, masonry, o) planks retained by short piles. Canals of the United States and Canada. The original di mensions of the New- York Erie canal and its locks, have been generally adopted for similar works subsequently constructed in most of the other states. The dimensions of this canal and its locks are as follows : Width of canal at top, .... 40 feet. Width at bottom, . . . . 28 " Depth of water, ..... 4 " Width of tow-path, . . . . 9 to 12 " Length of locks between mitre-sills, . 90 " Width of locks, . . . . 15 " For the enlargement of the Erie canal, the following dimen- sions have been adopted : Width of canal at top, .... 70 feet. Width at bottom, . . . . 42 " Depth of water, 7 " Width of tow-path, .... 14 " Length of locks between mitre-sills, . 110 " Width of lock at top, . . . 18.8" Width of lock at bottom, . . . 14.6" Lift of locks, 8 " Between the double locks a culvert is placed, which allows the water to flow from the level above the lock to the one below, when there is a surplus of water in the former. A well, or pit, is left between the lift-wall of the lock and the cross wall which retains the earth at the head of the lock to the level of the bottom of the canal. This pit, receiving the deposite of sand and gravel brought down by the current, prevents it from obstructing the play of the gates. On the Chesapeake and Ohio canal, the cross section of the canal below Harper's Ferry has received the following dimen- sions : Width of canal at top, .... 60 feet. Width at bottom, . . . . 42 " Depth of water, 6 " Length of locks between mitre-sills, . 90 " Width of locks, 15 " The following dimensions have been adopted on the Jamet lirer canal, in Virginia : CANALS. J^9 Width of canal at top, .... 50 ftet. Width at bottom, .... 30 " Depth of water, 5 " Length of locks, . . . . 100 " Width of locks 15 " The Rideau canal, which connects Lake Ontario with the River Ottawa, is arranged for steam navigation. A considerable portion of this line consists of slack-water navigation, formed by connecting the natural water-courses between the outlets of the canal. The length of the locks on this canal is 134 feet between the mitre-sills, and their width 33 feet. The Welland canal, between lakes Erie and Ontario, as origin- ally constructed, received the following dimensions : Width of canal at top, .... 56 feet. Width at bottom, .... 24 " Depth of water, 8 " Length of locks betwen mitre-sills, . 110 " Width of locks, 22 " The canals and locks made to avoid the dangerous rapids oi the St. Lawrence are in all respects among the largest in the world. The following are the dimensions of the portion of the canal and the locks between Long Sault and Cornwall : , Width of canal at top, .... 132 feet. Width at bottom, . . . . 100 " Depth of water, 8 " Width of tow-path, . . . . 12 " Length of locks between mitre-sills, . 200 " Width of locks at top, . . . 56.6 " Width of locks at bottom, . . . 43 " A berm of 5 feet is left on each side between the water way and the foot of the interior slope of the tow-path. The height of the tow-path is 6 feet above the berm. By increasing the depth of water in the canal to 10 feet, the water line at top can be increased to 150 feet RIVERS. RIVERS. 734. Natural features of Rivers. All rivers present the same natural features and phenomena, which are more or less strongly marked and diversified by the character of the region through which they flow. Taking their rise in the highlands, and gradu- ally descending thence to some lake, or sea, their beds are mod- ified by the nature of the soil of the valleys in which they lie, and the velocities of their currents are affected by the same causes. Near their sources, their beds are usually rocky, irregular, narrow, and steep, and their currents are rapid. Approaching their outlets, the beds become wider and more regular, the de- clivity less, and the current more gentle and uniform. In the upper portions of the beds, their direction is more direct, and the obstructions met with are usually of a permanent character, aris- ing from the inequalities of the bottom. In the lower portions, the beds assume a more tortuous course, winding through their valleys, and forming those abrupt bends, termed elbows, which seem subject to no fixed laws ; and here are found those ob- structions, of a more changeable character, termed bars, which, are caused by deposites in the bed, arising from the wear of the banks by the current. 735. The relations which are found to exist between the cross section of a river, its longitudinal slope, the nature of its bed, and its volume of water, are termed the regimen of the river. When these relations remain permanently invariable, or change insensibly with time, the river is said to have a.Jixed regimen. 736. Most rivers acquire in time a fixed regimen, although periodically, and sometimes accidentally, subject to changes from freshets caused by the melting of snow, and heavy falls of rain. These variations in the volume of water thrown into the bed, cause corresponding changes in the velocity of the current, and in the form and dimensions of the bed. These changes will depend on the character of the soil, and the width of the valley. In narrow valleys, where the banks do not readily yield to the action of the current, the effects of any variation of velocity will only be temporarily to deepen the bed. In wide valleys, where the soil of the banks is more easily worn by the current than the bottom, any increase in the volume of water will widen the bed ; and if one bank yields more than the other, an elbow will be formed, and the position of the bed will be gradually shifted to- wards the concave side of the elbow. RIVERS. 341 737. The formation of elbows occasions also variations in the depth and velocity of the water. The greatest depth is found at the concave side. At the straight portions which connect two elbows, the depth is found to decrease, and the velocity of the current to increase. The bottom of the bed thus presents a se- ries of undulations, forming shallows and deep pools, with rapid tnd gentle currents. 738. Bars are formed at those points, where from any cause the velocity of the current receives a sudden check. The particles suspended in the water, or borne along over the bottom of the bed by the current, are deposited at these points, and con- tinue to accumulate, until, by the gradual filling of the bed, the water acquires sufficient velocity to bear farther on the particles that reach the bar, when the river at this point acquires and re- tains a fixed regimen, until disturbed by some new cause. 739. The points at which these changes of velocity usually take place, and near which bars are found, are at the junction of a river with its affluents, at those points where the bed of the river receives a considerable increase in width, at the straight portions of the bed between elbows, and at the outlet of the river to the sea. The character of the bars will depend upon that of the SOL of the banks, and the velocity of the current. Generally speak- ing, the bars in the upper portions of the bed will be composed of particles which are larger than those by which they are formed lower down. These accumulations at the mouths of large rivers form in time extensive shallows, and great obstructions to the discharge of the water during the seasons of freshets. The river then, not finding a sufficient outlet by the ordinary channel, ex- cavates for itself others through the most yielding parts of the deposites. In this manner are formed those features which char- acterize the outlets of many large rivers, and which are termed delta, after the name given to the peculiar shape of the outlets of the Nile. 740. River Improvements. There is no subject that falls with- in the province of the engineer's art, that presents greater diffi- culties and more uncertain issues than the improvement of rivers. Ever subject to important changes in their regimen, as the re- gions by which they are fed are cleared of their forests and brought under cultivation, one century sees them deep, flowing with an equable current, and liable only to a gradual increase in volume during the seasons of freshets ; while the next finds their beds a prey to sudden and great freshets, which leave them, after their violent passage, obstructed by ever shifting bars and elbows. Besides these revolutions brought about in the course of years, every obstruction temporarily placed in the way of the current every attempt to guard one point from its action by any artificial 312 RIVERS means, inevitably produces some coi responding change at another, which can seldom be foreseen, and for which the remedy applied may prove but a new cause of harm. Thus, a bar removed from one point is found gradually to form lower down ; one bank pro- tected from the current's force transfers its action to the opposite one, on any increase of volume from freshets, widening the bed, and frequently giving a new direction to the channel. Owing to these ever varying causes of change, the best weighed plans of river improvement sometimes result in complete failure. 741. In forming a plan for a river improvement, the principal objects to be considered by the engineer, are, 1st, The means to be taken to protect the banks from the action of the current. 2d, Those to prevent inundations of the surrounding country. 3d, The removal of bars, elbows, and other natural obstructions to navigation. 4th, The means to be resorted to for obtaining a suitable depth of water for boats, of a proper tonnage, for the trade on the river. 742. Means for protecting the banks. To protect the banks, either the velocity of the current in-shore must be decreased so as to lessen its action on the soil ; or else a facing of some ma- terial sufficiently durable to resist its action must be employed. The former method may be used when the banks are low and have a gentle declivity. The simplest plan for this purpose con- sists either in planting such shrubbery on the declivity as will thrive near water ; or by driving down short pickets and interla- cing them with twigs, forming a kind of wicker-work. These corv stnictions break t. l ie force of the current, and diminish its velocity near the shore, and thus cause the water to deposite its finer par- ticles, which gradually fill out and strengthen the banks. If the banks are high, and are subject to cave in from the action of the current on their base, they may be either cut down to a gentle declivity, as in the last case ; or else they may receive a slope of nearly 45, and be faced with dry stone, care being taken to secure the base by blocks of loose stone, or by a facing of brush and stone laid in alternate layers. 743. Measures against inundations. At the points in the course of a river whe*e inundations are to be apprehended, the water-way, if practicable, should be increased ; all obstructions to the free discharge of the water below the point should be re- moved ; and dikes of earth, usually termed levees, should be raised on each side of the river. By increasing the water-way a temporary improvement only will be effected ; for, except in the season of freshets, the velocity of the current at this point will be so much decreased as to form deposites, which, at some future day, may prove a cause of damage. In confining the water be- tween levees, two methods have been tried ; the one consists in RIVERS. 34.- leaving a water-way strictly necessary for the discharge of fresh ets ; the other in giving the stream a wide bed. The Po in Italy and the Mississippi present examples of the -ormer method ; the effect of which in both cases has been to laise the bed of the stream so much that in many parts the water is habitually above the natural surface of the country, leaving it exposed to serious : nundations should the levees give way. The other method nas been tried on the Loire in France, and observation has proved that the general level of the bed has not sensibly risen for a long series of years ; but it has been found that the bars, which are formed after each freshet, are shifted constantly by the next, so that when the waters have subsided to their ordinary state, the navigation is extremely intricate from this cause. Other means have been tried, such as opening new channels at the ex- posed points, or building dams above them to keep the water back ; but they have all been found to afford only a temporary relief. 744. Elbows. The constant wear of the bank, and shifting of the channel towards the cDncave side of elbows, have led to various plans for removing the inconveniences which they pre- sent to navigation. The method which has been most generally tried for this purpose consists in building out dikes, termed wing- di.ms, from the concave side into the stream, placing them either ct right angles to the thread of the current, or obliquely down *tr".am, so as to deflect the current towards the opposite shore Fig. 174 Kepie-cnts ii sect i. ii i in the limber win^-damson the I'o, formed of plank nailed on the inclined pieces ;' tie- riUs. ab and br, inclined I'MCCS of the dam, the first making an angle of 63, and of -J.'i" with the horizon. d and e, pieces of the rib. f and g, horizontal pieces connecting the libs. 344 RIVERS. Wing-dams are usually constructed either of blocks of stone if crib-work formed of heavy timbers filled in with broken stone or of alternate layers of gravel and fascines. Within a few years back, wing-dams, consisting simply of a series of vertical frames, or ribs, (Fig. 174,) strongly connected together, and covered on the up-stream side by thick plank, which present a broken in- clined plane to the current, the lower part of which is less steep than the upper, have been used upon the Po, with, it is stated, complete success, for arresting the wear of a bank by the cur- rent. These dams are placed at some distance above the point to be protected, and their plan is slightly convex on the up-stream side. Wing-dams of the ordinary form and construction are now regarded, from the experience of a long series of years on the Rhine, and some other rivers in Europe, as little serviceable, if not positively hurtful, as a river improvement, and the abandon- ment of their use has been strongly urged by engineers in France. The action of the current against the side of the dam causes whirls and counter-currents, which are found to undermine the base of the dam, and the bank adjacent to it. Shallows and bars are formed in the bed of the stream, near the dam, by the debris borne along by the current after it passes the dam, giving very frequently a more tortuous course to the channel than it had na- turally assumed in the elbow. The best method yet found of arresting the progress of an elbow is to protect the concave bank by a facing of dry stone, formed by throwing in loose blocks of stone along the foot of the bank, and giving them the slope they naturally assume when thus thrown in. 745. Elbows upon most rivers finally reach that state of de- velopment in which the wear upon the concave side, from the action of the current, will be entirely suspended, and the regi- men of the river at these points will remain stable. This state will depend upon the nature of the soil of the banks and bed, and the character of the freshets. From observations made upon the Rhine, it is stated that elbows, with a radius of curvature of nearly 3000 yards, preserve a fixed regimen ; and that the banks of those which have a radius of about 1500 yards are seldom injured if properly faced. 746. Attempts have, in some cases, been made to shorten and straighten the course of a river, by cutting across the tongue of land that forms the convex bank of the elbow, and turning the water into a new channel. It has generally been found that the stream in time forms for itself a new bed of nearly the same char acter as it originally had. 747. Bars. To obtain a sufficient depth of water over bars, the deposite must either be scooped up by machinery, and b RIVERS. 345 conveyed away, or be removed by giving an increased velocity to the current. When the latter plan is preferred, an artificial channel is formed, by contracting the natural way, confining it between two low dikes, which should rise only a little above the ordinary level of low water, so that a sufficient outlet maybe left for the water during the season of freshets, by allowing it to flow over the dams. If the river separates into several channels at the bar, dams should be built across all except the main channel, so that by throwing the whole of the water into it the effects of the current may be greater upon the bed. The longitudinal dikes, between which the main channel is confined, should be placed as nearly as practicable in the direc tion which the channel has naturally assumed. If it be deemed advisable to change the position of the channel, it should be shift- ed to that side of the bed which will yield most readily to the action of the current. 748. In situations where large reservoirs can be formed near the bar, the water from them may be used for removing it. Foi this purpose an outlet is made from the reservoir, in the direction of the bar, which is closed by a gate that turns upon a vertical axis, and is so arranged that it can be suddenly thrown open to let off the water. The chase of water formed in this way sweep- ing over the bar will prevent the accumulation of deposites upon it. This plan is frequently resorted to in Europe for the removal of deposites that accumulate at the mouth of harbors in those lo- calities where, from the height to which the tide rises, a great head of water can be obtained in the reservoirs. 749. In the improvement of the mouths of rivers which empty into the sea through several channels, no obstruction should be placed to the free ingress of the tides through all the channels. K the main channel is subject to obstmctions from deposites. dams should be built across the secondary channels, which may be so arranged with cuts through them closed by gates, that the flood-tide will meet with no obstruction from the gates, while the ebb-tide, causing the gates to close, will be forced to recede through the main channel, which, in this way, will be daily scoured, and freed from deposites by the ebb current. The same object may be effected by building dams without inlets across the secondary channels, giving them such a height that at a cer- tain stage of the flood-tide, the water will flow over them, and fill the channels above the dams. The portion of water thus . dammed in will be forced through the main channel at the ebb. 750. When the bed is obstructed by rocks, it may be deepened \TJ blasting the rocks, and removing the fragments with the as- sistance of the diving-bell, and other machinery, 44 843 RIVERS. 751. In some of our rivers, obstructions of a very dangerous character to boats are met with, in the trunks of large trees which are imbedded in the bottom at one end, while the other is near the surface ; they are termed snags and sawyers by the boatmen. These obstructions have been very successfully re moved, within late years, by means of machinery, and by pro- pelling two heavy boats, moved by steam, which are connected by a etrong beam across their bows, so that the beam will strike the snag, and either break it off near the bottom, or uproot it Other obstructions, termed rafts, formed by the accumulation of drift wood at points of a river's course, are also found in some of our western rivers. These are also in process of removal, by cutting through them by various means which have been found successful. 752. Slack-Water Navigation. When the general depth of water in a river is insufficient for the draught of boats of the most suitable size for the trade on it, an improvement, termed slack-water, or lock and dam navigation, is resorted to. This consists in dividing the course into several suitable ponds, by forming dams to keep the water in the pond at a constant head ; and by passing from one pond to another by locks at the ends of the dams. 753. The position of the dams, and the number requisite, will depend upon the locality. In streams subject to heavy freshets, it will generally be advisable to place the dams at the widest parts of the bed, to obtain the greatest outlet for the water over the dam. The dams may be built either in a straight line be- tween the banks and perpendicular to the thread of the current, or they may be in a straight line oblique to the current, or their plan may be convex, the convex surface being up stream, or it may be a broken line presenting an angle up stream. The three last forms offer a greater outlet than the first to the water that flows over the dam, but are more liable to cause injury to the bed below the stream, from the oblique direction which the cur- rent nuv receive, arising from the form of the dam at top. 75 \ . The cross section of a dam is usually trapezoidal, the face up-stream being inclined, and the one down-stream either vertical or inclined. When the down stream face is vertical, the velocity of the water which flows over the dam is destroyed by the shock against the water of the pond below the dam, but whirls are formed which are more destructive to the bed than would be the action of the current upon it along the inclined face of a dam. In all cases the sides and bed of the stream, for some distance below the dam, should be protected from the action of the current by a facing of dry stone, timber, or any other con- struction of sufficient durability for the object in view. RIVERS. 347 755. The dams should receive a sufficient height only to maintain the requisite depth of water in the ponds for the pui ooses of navigation. Any material at hand, offering sufficient durability against the action of the water, may be resorted to in .heir construction. Dams of alternate layers of brush and gravel, ,vith a facing of plank, fascines, or dry stone, answer very well in gentle currents. If the dam is exposed to heavy freshets, to shocks of ice, and other heavy floating bodies, as drift-wood, it would be more prudent to form it of dry stone entirely, or of crib-work filled with stone ; or, if the last material cannot be ob- tained, of a solid crib- work alone. If the dam is to be made water-tight, sand and gravel in sufficient quantity may be thrown in against it in the upper pond. The points where the dam joins the banks, which are termed the roots of the dam, require par- ticular attention to prevent the water from filtering around them. The ordinary precaution for this is to build the dam some dis tance back into the banks. 756. The safest means of communication between the ponds is by an ordinary lock. It should be placed at one extremity of the dam, an excavation in the bank being made for it, to secure it from damage by floating bodies brought down by the current. The sides of the lock and a portion of the dam near it should be aised sufficiently high to prevent them from being overflowed by the heaviest freshets. When the height to which the freshets rise is great, the leaves of the head gates should be formed of two parts, as a single leaf would, from its size, be too unwieldy, the lower portion being of a suitable height for the ordinary man osuvres of the lock ; the upper, being used only during the fresh- ets, are so arranged that their bottom cross pieces shall rest, when the gates are closed, against the top of the lower portions. An arrangement somewhat similar to this may be made for the tail gates, when the lifts of the locks are great, to avoid the dim* culty of manoeuvring very high gates, by permanently closing the upper part of the entrance to the lock at the tail gates, either by a wall built between the side walls, or by a permanent frame- work, below which a sufficient height is left for the boats to pass. 757. A common, but unsafe method of passing from one pond to another, is that which is termed flashing ; it consists of a sluice in the dam, which is opened and closed by means of a gate revolving on a vertical axis, which is so arranged thai it can be manoeuvred with ease. One plan for this purpose is to divide the gale into two unequal parts by an axis, and to place a valve of such dimensions in the greater, that when opened the surface against which the water presses shall be less than that of the smaller part. The play of the gate is thus rendered very simple ; when the valve is shut, the pressure of water on the larger sur 348 RIVERS. face closes it against the sides of the sluice ; when the v?'ve is opened, the gate swings round and takes a position in the direc- tion of the current. Various other plans for flashing, on similir principles, are to be met with. 758. When the obstruction in a river cannot be overcome by any of the preceding means, as for example in those considerable descents in the bed known as rapids, where the water acquires a velocity w) great that a boat can neither ascend nor descend with safety, resort must be had to a canal for the purpose of uniting its navigable parts above and below the obstruction. The general direction of the canal will be parallel to the bed of the river. In some cases it may occupy a part of the bed bv forming a dike in the bed parallel to the bank, and sufficiently far from it to give the requisite width to the canal. Whatever posi- tion the canal may occupy, every precaution should be taken to secure it from damage by freshets. 759. A lock will usually be necessary at each extremity of the canal where it joins the river. The positions for the extreme locks should be carefully chosen, so that the boats can at all times en- ter them with ease and safety. The locks should be secured by guard gates and other suitable means from freshets ; and if they are liable to be obstructed by deposites, arrangements should be made for their removal either by a chase of water, or by ma- chinery. If the river should not present a sufficient depth of water at all seasons for entering the canal from it, a dam will be required at some point near the lock to obtain the depth requisite. It may be advisable in some cases, instead of placing the ex- treme locks at the outlets of the canal to the river, to form a ca- pacious basin at each extremity of the canal between the lock and river, where the boats can lie in safety. The outlets from the basins to the rivers may either be left open at all times, or else guard gates may be placed at them to shut off the water during freshets. 8EACOAST IMPROVEMENTS. 349 SEACOAST IMPROVEMENTS. 760. THE following subdivisions may be made of the works belonging to this class of improvements. 1st. Artificial Road- steads. 2d. The works required for natural and artificial Har- bors. 3d. The works for the protection of the seacoast against the action of the sea. 761. Before adopting any definitive plan for the improvement of the seacoast at any point, the action of the tides, currents, and waves at that point must be ascertained. 762. The theory of tides is well understood ; their rise and duration, caused by the attraction of the sun and moon, are also de- pendent on the strength and direction of the wind, and the confor- mation of the shore. Along our own seaboard, the highest tides vary greatly between the most southern and northern parts. At Eastport, Me., the highest tides, when not affected by the wind, vary between twenty-five and thirty feet above the ordinary low water. At Boston they rise from eleven to twelve feet above the same point, under similar circumstances ; and from New- York, following the line of the seaboard to Florida, they seldom rise above five feet. 763. Currents arc principally caused by the tides, assisted, in some cases, by the wind. The theory of their action is simple. From the main current, which sweeps along the coast, secondary currents proceed into the bays, or indentations, in a line more or less direct, until they strike some point of the shore, from which they are deflected, and frequently separate into several others, the main branch following the general direction which it had when it struck the shore, and the others not unfrequently taking an opposite direction, forming what are termed counter currents, and, at points where the opposite currents meet, that rotary mo- tion of the water known as whirlpools. The action of currents on the coast is to wear it away at those points against which they directly impinge, and to transport the debris to other points, thus forming, and sometimes removing, natural obstructions to navigation. These continual changes, caused by currents, make it extremely difficult to foresee their effects, and to foretell the consequences which will arise from any change in the direction, or the intensity of a current, occasioned by artificial obstacles. 764. A good theory of waves, which shall satisfactorily ex- plain all their phenomena, is still a desideratum in science. It is known that they are produced by vinds acting on the surface 350 SEACOAST IMPROVEMENTS. of the sea ; bait how far this action extends below the surface and what are its effects at various depths, are questions that re- main to be answered. The most commonly received theory is, that a wave is a simple oscillation of the water, in which each particle rises and falls, in a vertical line, a certain distance during each oscillation, without receiving any motion of translation in a aorizontal direction. It has been objected to this theory that it ("ails to explain many phenomena observed in connection with waves. In a recent French work on this subject, its author, Colonel Emy, an engineer of high standing, combats the received theory ; and contends that the particles of water receive also a motion of translation horizontally, which, with that of ascension, causes the particles to assume an orbicular motion, each particle de- scribing an orbit, which he supposes to be elliptical. He farther contends, that in this manner the particles at the surface com- municate their motion to those just below them, and these again to the next, and so on downward, the intensity decreasing from the surface, without however becoming insensible at even very considerable depths ; and that, in this way, owing to the reaction from the bottom, an immense volume of water is propelled along the bottom itself, with a motion of translation so powerful as to overthrow obstacles of the greatest strength if directly opposed to it. From this he argues that walls built to resist the shock of the waves should receive a very great batir at the base, and that this batir should be gradually decreased upward, until, towards the top, the wall should project over, thus presenting a concave surface at top to throw the water back. By adopting this form, he contends that the mass of water, which is rolled forward, as it were, on the bottom, when it strikes the face of the wall, will ascend along it, and thus gradually lose its momentum. These views of Colonel Emy have been attacked by other engineers, who have had opportunities to observe the same phenomena, on the ground that they are not supported by facts ; and the question still remains undecided. It is certain, from experiments made by the author quoted upon walls of the form here described, that they seem to answer fully their intended purpose. 765. Roadsteads. The term roadstead is applied to an in- dentation of the coast, where vessels may ride securely at an- chor under all circumstances of weather. If the indentation is covered by natural projections of the land, or capes, from the action of the winds and waves, it is said to be land-locked ; in the contrary case, it is termed an open roadstead. The anchorage of open roadsteads is often insecure, owing to violent winds setting into them from the sea, and occasioning high waves, which, are very straining to the moorings. The 8EACOAST IMPROVEMENTS. 351 remedy applied in this case is to place an obstruction, near the entrance of the roadstead, to break the force of the waves froir the sea. These obstructions, termed breakwaters, are artificial islands of greater or less extent, and of variable form, according to the nature of the case, made by throwing heavy blocks ol stone into the sea, and allowing them to take their own bed. The first great work of this kind undertaken in modern times, was the one at Cherbourg in France, to cover the roadstead in front of that town. After some trials to break the effects of the waves on the roadstead by placing large conical shaped struc- tures of timber filled with stones across it, which resulted in failure, as these vessels were completely destroyed by subsequent storms, the plan was adopted of forming a breakwater by throw- ing in loose blocks of stone, and allowing the mass to assume the form produced by the action of the waves upon its surface. The subsequent experience of many years, during which this work has been exposed to the most violent tempests, has shown that the action of the sea on the exposed surface is not very sensible at this locality at a depth of about 20 feet below the water level of the lowest tides, as the blocks of stone forming this part of the breakwater, some of which do not average over 40 Ibs. in weight, have not been displaced from the slope the mass first assumed, which was somewhat less than one perpendicular to one base. From this point upwards, and particularly between the levels of high and low water, the action of the waves has been very powerful at times, during violent gales, displacing blocks of several tons weight, throwing them over the top of the breakwater upon the slope towards the shore. Wherever this part of the surface has been exposed the blocks of stone have been gradually worn down by the action of the waves, and the slope has become less and less steep, from year to year, until finally the surface assumed a slightly concave slope, which, at some points, was as great as ten base to one perpendicular. The experience acquired at this work has conclusively shown that breakwaters, formed of the heaviest blocks of loose stone, are always liable to damage in heavy gales when the sea breaks over them, and that the only means of securing them is by cov ering the exposed surface with a facing of heavy blocks of ham mered stone carefully set in hydraulic cement. As the Cherbourg breakwater is intended also as a military construction, for the protection of the roadstead against an ene- my's fleet, the cross section shown in (Fig. 175) has been adopt- ed for it. Profiting by the experience of many years' observation, > was decided to construct the work that forms the cannon battery of solid masonry laid on a thick and broad bed of beton. The top surface of the breakwater is covered with heaw loose block* 35.C SEACOAST IMPR JVEMENTS. of stone, and the foot of the wall on the face is protected by large blocks of artificial stone formed of beton. The top of the battery is about 12 feet above the highest water level. Fig. 175 Represents a section of the Cherbourg breakwater. A, mass of stone. B, battery of masonry. The next work of the kind was built to cover the roadstead of Plymouth in England. Its cross section was, at first, made with an interior slope of one and a half base to one perpendicular, and an exterior slope of only three base to one perpendicular ; but from the damage it sustained in the severe tempests in the winter of 1816-17, it is thought that its exterior slope was too abrupt. A work of the same kind is still in process of construction on our coast, off the mouth of the Delaware. The same cross sec- tion has been adopted for it as in the one at Cherbourg. All of these works were made in the same way, discharging the stone on the spot, from vessels, and allowing it to take its own bed, except for the facing, where, when practicable, the blocks were carefully laid, so as to present a uniform surface to the waves. The interior of the mass, in each case, has been formed of stone in small blocks, and the facing of very large blocks. It is thought, however, that it would be more prudent to form the whole of large blocks, because, were the exterior to suffer damage, and experience shows that the heaviest blocks yet used have at times been displaced by the shock of the waves, the interior would still present a great obstacle. From the foregoing details, respecting the cross sections of breakwaters, which from experiment have been found to answer, the proper form and dimensions of the cross section in similar cases may be arranged. As to the plan of such works, it must depend on the locality. The position of the breakwater should be chosen with regard to the direction of the heaviest swells from the sea. .nto the roadstead, the action of the current, and that of the waves. The part of the roadstead which it covers should afford a proper depth of water, and secure anchorage for vessels of the largest class, during the most severe storms ; and vessels should be able to double the breakwater under all circumstances SEACOAST IMPROVEMENTS. 353 of wind and tide. Such a position should, moreover, be chosen that there will be no liability to obstructions being formed within the roadstead, or at any of its outlets, from the change in the current which may be made by the breakwater. 766. The difficulty of obtaining very heavy blocks of stone, u well as their great cost, has led to the suggestion of substitu ting for them Mocks of artificial stone, formed of concrete, which can be made of any shape and size desirable. This plan has been tried with success in several instances, particularly in a jetty or mole, at Algiers, constructed by the French government. The beton for a portion of this work was placed in large boxes, the sides of which were of wood, shaped at bottom to correspond to the irregularities of the bottom on which the beton was to be spread. The bottom of the box was made of strong canvass tar- red. These boxes were first sunk in the position for which they were constructed, and then filled with the beton. 767. Harbors. The term harbor is applied to a secure an- chorage of a more limited capacity than a roadstead, and there- fore offering a safer refuge during boisterous weather. Harbors are either natural, or artificial. 768. An artificial harbor is usually formed by enclosing a space on the coast between two arms, or dikes of stone, or of wood, termed jetties, which project into the sea from the shore, in such a way as to cover the harbor from the action of the wind and waves. 769. The plan of each jetty is curved, and the space enclosed by the two will depend on the number of vessels which it may be supposed will be in the harbor at the same time. The dis- tance between the ends, or heads, of the jetties, which forms the mouth of the harbor, will also depend on local circumstances ; it should seldom be less than one hundred yards, and generally need not be more than five hundred. There are certain winds at every point of a coast, which are more unfavorable than others to vessels entering and quitting the harbor, and to the tranquil- lity of its water. One of the jetties should, on this account, be longer than the other, and be so placed that it will both break the force of the heaviest swells from the sea into the mouth of the harbor, and facilitate the ingress and egress of vessels, by preventing them from being driven by the winds on the other jetty, just as they are entering or quitting the mouth. 770. The cross section, and construction of a stone jetty differ in nothing from those of a breakwater, except that the jetty is usually wider on top, thirty feet being allowed, as it serves foi a wharf in unloading vessels. The head of the jetty is usually made circular, and considerably broader than the other parts, as : t, in some instances, receives a lighthouse, and a battery of csu- 45 854 8EACOAST IMPROVEMENTS. non. It should be made with great care, of large blocks of stone well united by iron, or copper cramps, and the exterior courses should moreover be protected by fender beams of heavy timber, to receive the shock of floating bodies. 771. Wooden jetties are formed of an open framework of heavy timber, the sides of which are covered on the interior by a strong sheeting of thick plank. Each rib of the frame (Fig. 176) consists of two inclined pieces, which form the sides Fig. 17C Represents a cross section of a wooden jetty. a, foundation piles. b, inclined side pieces. . c. middle upright. a, cross pieces bolted in pairs, c, struts. m, longitudinal pieces bolted in pairs. o, parapet. of an upright centre piece, and of horizontal clamping pieces, which are notched and bolted in pairs on the inclined and upright pieces ; the inclined pieces are farther strengthened by struts, which abut against them and the upright. The ribs are con- nected by large string-pieces, laid horizontally, which are notched and bolted on the inclined pieces, the uprights, and the clamping pieces, at their points of junction. The foundation, on which this framework rests, consists usually of three rows of large piles driven under the foot of the inclined pieces and the uprights. The rows of piles are firmly connected by cross and longitudinal beams notched and bolted on them ; and they are, moreover, firmly united to the framework in a similar manner. The inte- rior sheeting does not, in all cases, extend the entire length of the sides, but open spaces, termed clear-ways, are often ]eft, t SEACOA8T IMPROVEMENTS. 355 give a free passage and spread to the waves confined between the jetties, for the purpose of forming smooth water in the chan nel. If the jetties are covered at their back with earth, the cleat ways receive the form of inclined planes. The foundation of the jetties requires particular care, espe- cially when the channel between them is very narrow. Loose stone thrown around the piles is the ordinary construction used for this purpose ; and, if it be deemed necessary, the bottom of the entire channel may be protected by an apron of brush and loose stone. The top of the jetties is covered with a flooring of thick plank, which serves as a wharf. A strong hand railing should be placed on each side of the flooring as a protection against acci- dents. The sides of jetties have been variously inclined ; the more usual inclination varies between three and four perpendicu- lar to one base. 772. Jetties are sometimes built out to form a passage to a natural harbor, which is either very much exposed, or subject to bars at its mouth. By narrowing the passage to the harbor be- tween the jetties, great velocity is given to the current caused by the tide, and this alone will free the greater part of the chan- nel from deposites. But at the head of the jetties a bar will, in almost every case, be found to accumulate, from the current along shore, which is broken by the jetties, and from the dimin- ished velocity of the ebbing tides at this point. To remove these bars resort may be had, in localities where they are left nearly dry at low water, to reservoirs, and sluices, arranged with turn- ing gates, like those adverted to for river improvements. The reservoirs are formed by excavating a large basin in-shore, at some suitable point from which the collected water can be di- rected, with its full force, on the bar. The basin will be filled at flood-tide, and when the ebb commences the sluice gates will be kept closed until dead low water, when they should all be opened at once to give a strong water chase 773. In harbors where vessels cannot be safely and conve- niently moored alongside of the quays, large basins, termed wet- docks, are formed, in which the water can be kept at a constant level. A wet-dock may be made either by an in-shore excavation, or by enclosing a part of the harbor with strong water-tight walls ; the tirst is the more usual plan. The entrance to the basin may be by a simple sluice, closed by ordinary lock gates, or by means of an ordinary lock. With the first method vessels can enter the basin only at high tide ; by the last they may be entered or passed out at any period of the tide. The outlet of the lock should be provided with a pair of guard gates, to be shut against Tory high tides, or in cases of danger from storms. 356 SEACOAST IMPROVEMENTS. 774. The construction of the locks for basins differs in nothirg in principle, from that pursued in canal locks. The greatest care will necessarily be taken to form a strong mass free from all danger of accidents. The gates of a basin-lock are made convex towards the head of water, to give them more strength to resist the great pressure upon them. They are hung and manoeuvreci differently from ordinary lock gates ; the quoin-post is attached to the side walls in the usual way : but at the foot of the mitre- post an iron or brass roller is attached, which runs on an iron roller way, and thus supports that end of the leaf, relieving the collar of the quoin-post from the strain that would be otherwise thrown on it, besides giving the leaf an easy play. Chains are attached to each mitre-post near the centre of pressure of the water, and the gate is opened, or closed, by means of windlasses to which the other ends of the chains are fastened. 775. The quays of wet-docks are usually built of masonry. Both brick and stone have been used ; the facing at least should be of dressed stone. Large fender-beams may be attached to the face of the wall, to prevent it from being brought in contact with the sides of the vessels. The cioss section of quay-walls should be fixed on the same principles as that of other sustaining walls. It might be prudent to add buttresses to the back of the wall to strengthen it against the shocks of the vessels. 776. Quay-walls with us are ordinarily made either by form- ing a facing of heavy round or square piles driven in juxtaposition, which are connected by horizontal pieces, and secured from the pressure of the earth filled in behind them by land-ties ; or, by placing the pieces horizontally upon each other, and securing them by iron bolts. Land-ties are used to counteract the pres- sure of the earth or rubbish which is thrown in behind them to form the surface of the quay. Another mode of construction, which is found to be strong and durable, is in use in our Eastern seaports. It consists in making a kind of crib-work of large blocks of granite, and filling in with earth and stone rubbish. The bottom course of the crib may be laid on the bed of the river, if it is firm and horizontal ; in the contrary case a strong grillage, termed a cradle, must be made, and be sunk to receive the stone work. The top of the cradle should be horizontal, and the bottom should receive the same slope as that of the bed, in order that when the stones are laid they may settle horizontally. 777. Dikes. To protect the lowlands bordering the ocean from inundations, dikes, constructed of ordinary earth, and faced to- "wards the sea with some material which will resist the action of the current, are usually resorted to. The Dutch dikes, by means of which a large extent of country has been reclaimed and protected from the sea, are the most re ZEACOAST IMPROVEME - T TS 351 niarkable structires of this kind in existence. The cross section of those dikes is of a trapezoidal form, the width at top averaging from four to six feet, the interior slope being the same as the na- tural slope of the earth, and the exterior slope varying, according to circumstances, between three and twelve base to one perpendic- ular. The top of the dike, for perfect safety, should be about six feet above the level of the highest spring tides, although, in many places, they are only two or three above this level. The earth for these dikes is taken from a ditch in-shore, be- tween which and the foot of the dike a space of about twenty feet is left, which answers for a road. The exterior slope is va- riously faced, according to the means at hand, and the character of the current and waves at the point. In some cases, a strong straw thatch is put on, and firmly secured by pickets, or other means ; in others, a layer of fascines is spread over the thatch, and is strongly picketed to it, the ends of the pickets being al- lowed to project out about eighteen inches, so that they can re- ceive a wicker-work formed by interlacing them with twigs ; the spaces between this wicker-work being filled with broken stone ; this forms a very durable and strong facing, which resists not only the action of the current, but, by its elasticity, the shocks of the heaviest waves. The foot of the exterior slope requires peculiar care for it 3 protection ; the shore, for this purpose, is in some places cover- ed with a thick apron of brush and gravel in alternate layers, to a distance of one hundred yards into the water from the foot of the slope. On some parts of the coast of France, where it has been found necessary to protect it from encroachments of the sea, a cross section has been given to the dikes towards the sea, of the same form as the one which the shore naturally takes from the action of the waves. The dikes in other respects are constructed and faced after the manner which has been so long in practice in Holland. 778. Groins. Constructions, termed groins, are used when- ever it becomes necessary to check the effect of the current along the shore, and cause deposites to be formed. These are artificial ridges which rise a few feet only above the surface of the beach, and are built out in a direction either perpendiculai to that of the shore, or oblique to it. They are constructed ei- ther of clay, which is well rammed and protected on the surface by a facing of fascines or stones ; or of layers of fascines ; or of ono or two rows of short piles driven in juxtaposition ; or any other means that the locality may furnish may be resorted to ; the object being to interpose an obstacle, which, breaking the force of the current, will occasion a deposite near it, and thus gradually cause the shore to gain upon the sea. 358 SEACOAST IMPROVEMENTS. 779. Sea-walls. When the sea encroaches upon the land forming a steep bluff, the face of which is gradually worn away a wall of masonry is the only means that will afford a permanent protection against this action of the waves. Walls made for this object are termed sea-walls. The face of a sea-wall should be constructed of the most durable stone in large blocks. The backing may be of rubble or of becoi. The whole work shou.d be laid with hydraulic mortar. APPENDIX. Note A to Arts. Framing and Bridges. Tubular Frames of Wrought Iron. Except for the obvious application to ateam boilers, sheet iron had not been considered as suitable for structures demanding' great strength, from its apparent deficiency in rigidity ; and although the principle of gaining strength by a proper distribution of the material, and of giving any desirable rigidity by combinations adapted to the object in view, were at every moment acted upon, from the ever-increasing demands of the art, engineers seem not to have looked upon sheet iron as suited to such purposes, until an extraordinary case occurred which seemed about to baffle all the means hitherto employed. The occasion arose when it became a question to throw a bridge of rigid material, for a railroad, across the Menai Straits; suspension systems, from their flexibility, and some actual failures, being, in the opinion of the ablest European engineers, unsuitable for this kind of communication. Robert Stephenson, who for some years back has held the highest rank among English engineers, appears, from undisputed testimony, to have been the first to entertain the novel and bold idea of spanning the Strait by a tube of sheet iron, supported on piers, of sufficient dimensions for the passage within it of the usual trains of railroads. The preliminary experiments for testing the practicability of this conception, and the working out the details of its execution, were left chiefly in the hands of Mr. William Fairbairn, to whom the profession owes many valuable papers and facts on professional topics. This gentleman, who, to a thorough acquaintance with the mode of conducting such experiments, united great zeal and judgment, carried through the task committed to him ; proceeding step by step, until conviction so firm took the place of apprehension, that he rejected all suggestions for the use of any auxiliary means, and urged, from his crowning experiment, reliance upon the tube alone as equal to the end to be attained. Numerous experiments were made by him upon tubes of circular, elliptical, and rectangular cross section. The object chiefly kept in view in these experiments was, to determine the form of cross section which, when the tub* 360 APPENDIX. vas submitted to a cross strain, would present an equality of resistance in the parts brought into compression and extension. It was shown, at an early stage of the operations, that the circular and elliptical forms were too weak in Jie parts submitted to compression, but that the elliptical was the stronger of he two; and that, whatever form might be adopted, extraordinary means would be requisite to prevent the parts submitted to compression from yielding, by " puckering " and doubling. To meet this last difficulty, the fortunate expedient was hit upon of making the part of the main tube, upon which the strain of compression was brought, of a series of smaller tubes, or cells of a curved or a rectangular cross section. The latter form of section was adopted definitively for the main tube, as having yielded the most satisfactory results as to resist- ance ; and also for the smaller tubes, or cells, as most easy of construction and repair. As a detail of each of these experiments would occupy more space than can be given in this work, that alone of the tube which gave results that led to the forms and dimensions adopted for the tubular bridges subsequently constructed, will be given in this place. Model Tube, The total length of the tube was 78 ft. The distance, or bearing between the points of support, on which it was placed to test its strength, was 75 ft. Total depth of the tube at the middle, 4 ft. 6 in. Depth at each extremity, 4 ft. Breadth, 2 ft. 8 in. The top of the tube was composed of a top and bottom plate, formed of pieces of sheet iron, abutting end to end, and connected by narrow strips riveted to them over the joints. These plates were 2 ft. 11^ in. wide. They were 6 in. apart, and connected by two vertical side plates, and five interior division plates, with which they were strongly joined by angle irons, riveted to the division plates, and to the top and bottom plates where they joined. Each cell, between two division plates and the top and bottom plates, was nearly 6 in. wide. The sides of the tube were made of plates of sheet iron similarly connected ; their depth was 3 ft. 6 in. A strip of angle iron, bent i0 a curved shape, and running from the bottom of each end of the tube to the top just below the cellular part, was riveted to each side to give it stiifness. Besides this, precautions were finally taken to stiffen the tube by diagonal braces within it. The bottom of the tube was formed of sheets, abutting end to end, and secured to each other like the top plates; a continuous joint, running the entire length of the tube along the centre line of the bottom, was secured by a continuous strip of iron on the under side, riveted to the plates on each side of the joint. The entire width of the bottom was 2 ft. 11 in. The sheet iron composing the top cellular portion was 0'147 in. thick ; that of the sides 0'099 in. thick. The bottom of the tube at the final experiments, to a distance of 20 ft. on each side of the centre, was composed of two thick- nesses of sheet iron, each - 25 in. thick, the joints being secured by strips ab )ve and below them riveted to the sheets ; the remainder, to the end of the tube, was formed of sheets 0'156 in. thick. The total area of sheets composing the top cellular portion was 24'024 in. j that of the bottom plates at the centre portion, 22*450 in. APPENDIX. 361 The general dimensions at the tube were one sixth those of the proposed structure. Its weight at the final experiment, 13,020 Ibs. The experiments, as already stated, were conducted with a view to obtain an equality between the resistances of the parts strained by compression and those extended ; with this object, at the end of each experiment, the parts torn asunder at the bottom were replaced by additional pieces of increased strength. The following table exhibits the results of the final experiments. No. of Experiments. Weight in Ibs. Deflection in inches 1 . . 20,006 . . 0-55 2 . . 35,776 . . 0-78 3 . . 48,878 . . 1-12 4 . . 62,274 . . T48 5 . . 77,534 . . 1-78 6 . . 92,299 . . 2-12 7 . . 103,550 . . 2-38 8 . 114,660 . . 2-70 9 . . 132,209 . . 3-05 10 . . 138,060 . . 3-23 11 . . 143,742 . . 3-40 12 . . 148,443 . . 3-58 13 . . 153,027 . . 3-70 14 . . 157,728 . . 3-78 15 . . 161,886 . . 3-88 16 . . 164,741 . . 3-98 17 . . 167,614 . . 4-10 18 . . 171,144 . . 4-23 19 . . 173,912 . . 4-33 20 . . 177,088 . . 4-47 21 . . 180,017 . . 4-55 22 . . 183,779 . . 4'62 23 . . 186,477 . . 4'72 24 . . 189,170 . . 4'81 25 . 192,892 The tube broke with the weight in the 25th experiment ; the cellular tor yielding by puckering at about 2 ft. from the point where the weight was applied. The bottom and sides remained uninjured. The ultimate deflection was 4'89 in. Britannia Tubular Bridge. Nothing further than a succinct description of this marvel of engineering will be attempted here, and only with a view of showing the arrangement of the parts for the attainment of the proposed end. Tt differs in its general structure from the model tube, chiefly in having the bottom formed like the top, of rectangular cells, and in the means taken for giving stiffness to the sides. 302 APPENDIX. The total distance spanned by the bridge is 1489 ft. This is livided into four bays, the two in the centre being each 460 ft., and the one at each end 230 ft each. The tube is 1524 ft. long. Its bearing on the centre pier is 45 ft ; that on the two intermediate 32 ft. ; and that on each abutment 17 ft. 6 in. The height of the tube at the centre pier is 30 ft. ; at the intermediate piers 27 ft ; and at the ends S3 ft This gives to the top of the tube the shape of 9 parabolic curve. Fig. 1 Represents a vertical cross section of the Britannia Bridge. A, interior of bridge. It, cells of top cellular beam. C, cells of bottom cellular beam. a, top plates of top and bottom beams. 6, bottom plates of top and bottom beam*. c, division plates of top and bottom beams. d and e, strips riveted over the joints of top and bottonr plates. o, angle irons riveted to a, 6, and c. f, plates of sides of the tube A. exterior T irons riveted over vertical joints of g. t, interior T irons riveted over vertical joints of g, md bent at the angles of A, beyond the second cell of the top beam, and be} .mil the first of the bottom beam. , triangular piece* on each side oft, and riveted to them APPENDIX. 363 The cellular top (Fig. 1) is divided into eight cells B, by division plates c, connected with the top a, and bottom b, by angle irons o, riveted to the plates connected. The different sheets composing the plates a and b abnt end to end lengthwise the tube ; and the joints are secured by the strips d and p riveted to the sheets by rivets that pass through the interior angle irons. The sheets of which this portion is composed are each 6 ft. longhand 1 ft. 9 in. wide ; those at the centre of the tube are jjths of an inch thick : they decrease in thickness towards the piers, where they are }?ths of an inch thick. The division plates are of the same thickness at the centre, and decrease in the same manner towards the piers. The rivets are 1 in. thick, and are placed 3 in. apart from centre to centre. The cells are 1 ft. 9 in. by 1 ft. 9 in., so as to admit a man for painting and repairs. The cellular bottom is divided into six cells C, each of which is 2 ft. 4 in. wide by 1 ft. 9 in. in height. To diminish, as far as practicable, the number of joints, the sheets for the sides of the cells were made 12 ft. long. To give sufficient strength to resist the great tensile strain, the top and bottom plates of this part are composed of two thicknesses of sheet iron, the one layer breaking joint with the other. The joints over the division plates are secured by angle irons o. in the same manner as in the cellular top. The joints between the sheets are secured by sheets 2 ft. 8 in. long placed over them, which are fastened by rivets that pass through the triple thickness of sheets at these points. The rivets, for attaining greater strength at these points, are in lines lengthwise of the cell. The sheets forming the top and bottom plates of the cells are ," f ths of an inch at the centre of the tube, and decrease to i' 4 ths at the ends. The division plates are ,*ths in the middle, and *,ths at the ends of the tube. The rivets of the top and bottom plates are Ij in. in diameter. Fig. 2 Fig 2 Represents a horizontal cross section of the T irons and side plates. D, cross section near centre of bridge. >', cross section near the piers. i.'. plates of the sides. h, exterior T irons. ', interior T irons. The sides of the tube (Fig. 2) between the cellular top and bottom are formed of sheets g, 2 ft. wide ; the lengths of which are so arranged that tnere are alternately three and four plates in each pannel, the sheets of each pannel abutting end to end, and forming a contir uous vertical joint between the adjacent pannels. These vertical joints are secured by strips of iron, h and t, of the "T cross section, placed over each side of the joint, and 364 APPENDIX. elamping the sheets of the adjacent pannels between them. The T iront within and without are firmly riveted together with 1 in. rivets, placed at 3 In. between their centres. Over the joints, between the ends of the sheets in each pannel, pieces of sheet iron are placed on each side, and connected by rivets. The sheets of the pannels at the centre of the tube are * 6 ths of an inch thick; they increase to ijjths to within about 10 ft. of the piers, where their thickness is again increased ; and the T" irons are here also increased in thickness, being composed of a strip of thick sheet iron, clamped between strips of angle iron which extend from the top to the bottom of the joints. Tae object of this increase of thickness, in the pannels and ~T irons at the piers, is to give sufficient rigidity and strength to resist the crushing strain at these points. The T irons on the interior are bent at top and bottom, and extended as far as the third cell from the sides at top, and to the second at bottom. The projecting rib of each in the angles is clamped between two pieces, n, of sheet iron, to which it is secured by rivets, to give greater stiffness at the angles of the tube. The arrangement of the ordinary T irons and sheets of the pannels is shown in cross section by D, Fig. 2 ; and that of the like parts near the piers by E, same Fig. For the purpose of giving greater stiffness to the bottom, and to secure fastenings for the wooden cross sleepers that support the longitudinal beams on which the rails lie, cross plates of sheet iron, half an inch thick, and 1 in. in depth, are laid on the bottom of the tube, from side to side, at every fourth rib of the "T iron, or 6 ft. apar.t. These cross plates are secured to the bottom by angle iron, and are riveted also to the T iron. The tube is firmly fixed to the central pier, but at the intermediate piers and the abutments it rests upon saddles supported on rollers and balls, to allow of the play from contraction and expansion by changes of temperature. The following tabular statements give the details of the dimensions, weights, fee., of the Britannia Bridge. , Feet. Plates Angle iron. T iron. Rivet iron. Cast iron Total, top;. 689 2067 1400 4200 J07 107 2000 1524 3048 4l)(l 30 27 23 14| 882,000 tons. 450 1350 965 2895 64 64 tons. 109 327 188 564 26 26 tons. 70 210 139 417 10 10 tons. 60 180 108 324 7 7 tons. 2000 Extreme width of tabes " 3 tubes 274 ft. long " 3 tubes 472 ft long 1 tube over pier 33ft. long .... 5788 1240 856 686 2000 10,570 1 1 APPENDIX. 365 Formula for reducing the Breaking Weight of Wrought Iron Tubes. Representing by A, the total area in inches of the cross section of the metal, " " d, the total depth in inches of the tube. " " /, the length in inches between the points of support. " " C, a constant to be determined by experiment. " " W, the breaking weight in tons. Then the relations between these elements, in tubes of cylindrical, elliptical and rectangular cross section, will be expressed by = The mean value for C for cylindrical tubes, deduced from several experi- ments, was found to be 13-03; that for elliptical tubes, 15'3; and that for rectangular tubes, 21 '5. 366 APPENDIX. Note B to Art. Reads. Plank-Roads. A road covering, consisting of thick boards, or planks, resting on longitudinal beams, or sleepers, and known as Plank-Roads, has, within the past few years, been introduced among us; and from its adaptation to our uncleared forest districts, its superior economy to the ordinary road coverings in such localities, and its intrinsic merits, as fulfilling the requisites of a good road covering, is rapidly coming into extensive use throughout all parts of our country. Fig A C fig. ^Represents a cross-section. Fig. B A plan of a plank road. a a, board surface. A 6, sills. c, summer road. d d, side surface drains. The method most generally adopted in constructing plank-roads consists ic laying a flooring, or track, eight feet wide, composed of boards from nine to twelve inches in width, and three inches in thickness, which rest upon two parallel rows of sleepers, or sills, laid lengthwise of the road, and having their centre lines about four feet apart, or two feet from the axis of the road. Sills of various sized scantling have been used, but experience seems in favor of scantling about twelve inches in width, four inches in thickness, and in lengths of not less than fifteen to twenty feet. Sills of these dimensions, laid flatwise, and firmly embedded, present a firm and uniform bearing to the boards, and distribute the pressure they receive over so great a surface, that, if the soil npon which they rest is compact and kept well drained, there can be but little APPENDIX. 367 settling and displacement of the road surface, from the usual loads passing over it. The better to secure this uniform distribution of the pressure, the sills of one row are so laid as to break joints with the other; and to prevent the ends of the sills from yielding the usua. precaution is taken to place short sills at the joints, either beneath the main sills, or on the same level with them. The boards are laid perpendicular to the axis of the road, experience having shown that this position is as favorable to their wear and tear as any other, and is otherwise the most economical. Their ends are not in an unbroken line, but so arranged that the ends of every three or four project alternately, on each side of the axis of the road, three or four inches beyond those next to them, for the purpose of presenting a short shoulder to the wheels of vehicles, to facilitate their coming upon the plank surface, when from any cause they may have turned aside. On some roads the boards have been spiked to the sills; but this is, at present, regarded as unnecessary, the stability of the boards being best secured by well packing the earth between and around the sills, so as to present, with them, a uniform bearing surface to the boards, and by adopting the usual precautions for keeping the subsoil well drained, and preventing any accumulation of rain water on the surface. The boards for plank-roads should be selected from timber free from the usual defects, such as knots, shakes, &c., which would render it unsuitable for ordinary building purposes ; as durability is an essential element in the economy of this class of structures. So far as experience has furnished data, boards of three inches in thickness offer all the requisites of strength and durability that can be obtained from timber in its ordinary state, in which it is used for plank-roads. Besides the wooden track of eight feet, an earthen track of twelve feet in width is made, which serves as a summer road for light vehicles, and as a turn out for loaded ones ; this, with the wooden track, gives a clear road surface of twenty feet, the least that can be well allowed for a frequented road. It is recommended to lay the wooden track on the right hand side of the approach of a road to a town, or village, for the proper convenience of the rural traffic, as the heavy trade is to the town. The surface of this track receives a cross slope from the side towards the axis of the road outwards of 1 in 32. The surface of the summer road receives a cross slope in the opposite direction of 1 in 16. These slopes are given for the purpose of facilitating a rapid surface drainage. The side drains are placed for this purpose parallel to the axis of the road, and connected with the road surface in a suitable slope. Where, from the character of the soil, good summer roads cannot be had, it will be necessary to make wooden turn outs, from space to space, to prevent the inconvenience and delay of miry roads. This it is proposed to do by laying, at these points, a wooden track of double width, to enable vehicles meeting to pass each ot\ er. It is recommended to lay these turn outs on four or five sills, to spring the boards slightly at the centre, and spike their ends to the exterior sills. The angle of repose, by which the grade of plank-roads should be regu- 368 APPENDIX. lated, has not yet been determined by experiment ; but as the wooden surface is covered with a layer of clean sand, fine gravel, or tan bark, before it is thrown open to vehicles, and as it in time becomes covered with a permanent stratum of dust, &c., it is probable that this angle will not materially diffei from that on a road with a broken stone surface, like the one of McAdam, Or of Telford, when kept in a thorough state of repair. In some of the earlier plank-roads made in Canada, a width of sixteen feet was given to the wooden track, the boards of which were laid upon four or five rows of sills ; experience soon demonstrated that this was by no means an economical plan, as it was found that vehicles kept the centre of the wooden surface, which was soon worn into a beaten track, whilst the remainder was but slightly impaired. This led to the abandonment of the wide track for the one now usually adopted, which answers all the ends of the wants of travel, and is much more economical, both in the first outlay and for subsequent renewals. The great advantages of plank-roads over every other kind, in a densely wooded country, for the rural traffic, are so obvious, that, did not experience teach us by what mere accidents, apparently, improvements of the most important kind have been suggested and carried into effect, it might be a subject of astonishment that they had not been among the first to be intro- duced, after a trial of the old corduroy road, so generally resorted to in the early stages of road-making in this country. APPENDIX. 300 Note C to Arts. 441 and 442. Methods of describing Curies composed of Arcs of Circles. Tl * span and rise of an arch being given, together with the directions of the tangents to the curve at the springing lines and crown, an infinite numoer of curves, composed of arcs of circles, can be determined, which shall satisfy the conditions of form- ing a continuous curve, or one in which the arcs shall be consecutively tangent to each other, and such that those at the springing lines and the erown shall be tangent to the assumed directions of the tangents to the curve at those points. To give a determinate character to the problem, in each particular case, certain other conditions must be imposed, upon which the solution will depend. When the tangents to the curve at the springing lines and crown are respectively perpendicular to the span and rise, the curve satisfying the above general conditions will belong to the class of oval or basket-handle curves; wken the tangents at the springing lines are perpendicular to the span, and those at the crown are oblique to the rise, the curves will belong to the class of pointed or obtuse curves. In the class of ovals, when the rise is not less than one third of the span, the oval of three centres will generally give a curve of a more pleasing form to the eye than one of a greater number of centres ; when the rise is less than a third of the span, a curve of five, seven, or a greater odd number of centres will give, under this point of view, a more satisfactory solution. In the pointed and obtuse curves the number of centres is even, and is usually restricted to four. Three Centre Curves. To obtain a determinate solution in this case it will be necessary to impose one more condition, which shall be compatible with the two general ones of having the directions of the tangents at the springing lines and crown fixed. One of the most simple, and at the same time admitting of a greater variety of curves to choose from, is to assume the radius of the curve at the springing lines. In order that this condition shall be com- patible with the other two, the length assumed for this must lie between zero and the rise of the arch ; for were it zero there would be but one centre, and If taken equal to the rise the radius of the curve at the crown would be infinite. Let A D (fig. A) be the half span, and A C the rise. Having prolonged C .4 indefinitely, take any distance less than A C, and set it off from D to R, along AD; and from C to P, along A C. Join R and P, which distance bisect by a perpendicular. Prolong the perpendicular, to intersect the indefinite prolong- ation of C A. Through this point of intersection S, and the point R, draw an indefinite line. From R, as a centre, with the radius R D, describe an arc, which prolong to Q to intersect S R prolonged. From S, as a centre, with the radius S Q, describe an arc, which, from the construction, must pass through 870 APPENDIX. the point C, and be tangent to the first arc at Q The centres R and S, thus determined, and the curve D D C deduced from them, will satisfy the imposed conditions. Fig. A. The two following constructions, from their simplicity and the agreeable form of curve which they produce, ape in frequent use. The first consists in imposing the condition that each of the three arcs shall be of 60; the second, R that the ratio between the radii of the arcs at the crown and springing line shall be a minimum, To construct the curve satisfying the former condition, let A B be the half span, and A C the rise. With the radius A B describe A a of 90, set off on it B b = 60, draw the lines a b, b B and A b, from C draw a parallel to a b, and mark its intersection c with b B, from c draw a parallel to A b, and mark its intersections N and O with A B, and C A prolonged. From N with the radius N B describe the are B c, from O with the radius O c describe the arc C c. The curve B c C will be the half of the one satisfying the given conditions ; and N and O two of the centres. To construct the curve satisfying the second condition, or e? ( ) = o. Let A D be the half span, A C the rise. Draw D C, and from C set off on it C d = C a, equal to the difference between the half span and rise. Bisect the distance D d by a perpendicular, which prolong to intersect D A, and C A prolonged, at R and S, from these points, as centres, with the radii R D and S Q, describe the arcs D Q and Q C ; and the curve D Q C will be the half of the one required. The analysis, from which the ab ve result is obtained, is of a very simple character ; for designating by R S C the greater radius, by r = R D the APPENDIX. 371 esser, by a = A D the half span, and by b = A C the rise, there result^ from the right angled triangle SAR, 8R> = AS* + AR\ or from which is obtained R a" f 0' - 2ar r (26 2r)r Differen*iati.T; this expression, and placing its first differential co d() efficient equal to zero, or making - - = 0, there results, after the terms are reduced, . , || _ 4 I ! f T\ I I* / Q I I .1 * Q i JO / TV Qi "T" u ~ (fl 0) yd ~^~ yd T* D ^ d + [(t ~ w) 2a ence the given construc- va a -f // J = /)C, and va 3 -j- 6* (a 6) = tion for the centres required. By comparing the two methods just explained, for the same span and rise, it will be seen that the former gives a curve in which the lengths of the arcs differ less than in the latter, and which is therefore more agreeable to the eye. Obtuse and Pointed Curves of Four Centres. Let A B be the half span, A C the rise of the required curve, and C D ihe direction of the tangent to it at the crown. At C draw a perpendicular to C D. Take any point R on A B, such that R B shall be less than the perpendicular R b, from R upon the tangent C D. From C, on the perpendicular to C D, set off C d, equal to tto assumed distance R B, draw R d, and bisect it by a perpendicular, which prolong to intersect the on from C at the point S, through 8 and R draw a line, from i?, with the radius R B, describe an arc, which prolong to Q, to intersect the line through S and R, from S with the j, f j_ radius S Q, describe an arc, which will be tangent to the first at Q, and pass through C. The curve B Q C will b the half of the one required to satisfy the given conditions. The analogy between this instruction and the one first given for three 372 APPENDIX. centre curves will be readily seen by comparing the constructions for the two Fhe Centre Oval Curves, <3fc. When the rise is less than one third of the span, it is found that oval curves of a pleasing shape cannot be obtained by using only three centres, and five, or a greater odd number of centres must bo resorted to. Besides the two general conditions common to all ovals, a greater number of particular ones must be imposed, as the number of centres is increased, restricting them within the limits of compatibility with each other and with the two common to all. By imposing, for example, on the oval of five centres, the conditions that the radii of the two consecutive arcs from the springing line shall be assumed as the particular conditions, a very simple construction, analogous to the one for ovals of thiee centres, will show the limits within which these must be restricted, not to interfere with the others that are common to all. Without stopping to illustrate this by an example, which will present no difficulty to any one tolerably conversant with the elements of geometry to make out alone,* more general nethod will be given, applicable alike to all curves of this class. The half span and rise being given, let it be required to determine an oval of five centres with the particular conditions, that the radii of the consecutive arcs, from the springing line towards the crown, shall be in an increasing geometrical progression, in which case the curvatures of the arcs will be in a decreasing geometrical pro- gression and the lengths of the consecutive arcs shall increase in a given ratio. Desig- nate the half span AB by p (Fig. C), the rise by q, the ratio of the radii by m, the ratio of the arcs by n, and the number of degrees in the arc at the springing line by a. Suppose the centres O, P and Q found, and draw PS perpendicular to AB, and PR perpendicular to BC produced. The radii OA, PE and QD will be re- presented respectively by r, rm, and rm 9 , and the angles AOE, EPD, and DQC, between them by now, from the properties of the figure, the following equations are obtained Pig. C. a + o-= m m- (A) + OS+ PR . n' (PS-f Qfl), (B) (C) APPENDIX. From the right angle triangle OPS, and PQR, there results, OS = OP cos. a = (rm r) cos. a ; PS = OP sin. a = (rm r) sin. a ; PR = PQ cos. (a + a- f- (rm 4 rm) cos. (a + a ) ; QR = PQ sin. (a+ a~ \ = (rm 9 rm) sin. ('a + a\ ; \ m / \ m / by substituting these values in equations (B) and (C), there results, '" p = r | 1 + (m 1) cos. a + (m a m) cos. / '"\ a I (E) a = r } m s (m 1) sin. a (m* m) sin. f - / a f ' ^ and by reduction, equation (A) becomes, The equations (), (F) and (G) express, therefore, the relations which sub- sist between the six quantities p, , corresponding to the displacement A x'", from which point it will decrease to the point b', the new position of equilibrium of the molecules. From what has been thus far stated, a clear idea may be formed of the elastic resistance offered by the two molecules to any force which tends to dis- place them from their state of natural equilibrium, and the law between this resistance and the corresponding displacement within the range of elasticity. For let any extraneous force be applied to separate or bring together the two molecules, the intensity of this force being less than the maximum resistance yiii z/, its effect will be to change the distance between the molecules, until 378 APPENDIX. they have gained a new position, where all the forces will be again in equi- librium. Let this position be the one corresponding to A x' for example ; novj o long as the extraneous force acts, the molecules will retain their respective positions apart, A and x 1 ; if the extraneous force be suddenly withdrawn the molecules will approach each other to regain their primitive distance apart A a, with a certain velocity, which velocity, or rather the living force accu- mulated, will cause the molecules to approach nearer to each other than the distance A a, passing which the force of repulsion will be brought into play, and oy its resistance, having destroyed the living force gained, will cause the molecules to recede from each other, creating in turn a certain amount ol living force, and they will thus continue to oscillate, between their primitive positions until they are finally brought to rest with respect to each other in it by extraneous resistances. If a tangent be drawn to each of the curves y, y, &c., and z, z', &c., at their common point b, these tangents will, like the curves, intersect at />, and will each coincide with its corresponding curve, for a greater or smaller distance on each side of the point b. Now, if any distances a x 1 , a x", &c., be taken on each side of a, and be considered infinitely small with respect to Acz, the primitive distance apart of the molecules, the portions of the ordinates to the curves as n 1 rf, n" o", at these points, intercepted between the tangents, will be equal to the portions of the same ordinates intercepted between the curves, as the curves and their tangents are taken as coinciding along the portions corresponding to a x , a x", &c. The intercepted portions of the ordinates, with the portions of the tangents, as b n', b n", intercepted between them and the point 6, will form similar triangles, from which is readily deduced that the portions of the intercepted ordinates are proportional respectively to the cor- responding distances a a:', a x'\ &c. ; or, in other words, that the forces with which the molecules attract, or repel each other, for infinitely small displace- ments, as compared with the primitive distance of natural equilibrium, are Tl o proportional to the displacements. But since the ratio - = -, between ax' a x" the portions of the ordinates intercepted between the tangents and the cor- responding displacement is constant, it may be taken to express the numerical value of the resistance offered by the molecules, in their position of natural equilibrium, to the infinitely small displacements in which the tangents coin- cide with the elements of the curve ; which amounts to saying, that, for infinitely small displacements of the molecules of a body, the value of the elastic force remains sensibly constant. It will be readily seen, from an examination of Fig. A, that, in proportion as the two curves approach more nearly to coincide with the ordinate a b, the angle between the tangents will be the smaller, and the distances a x', a 3/> &c., will also be the smaller, as compared with the corresponding parts of the ordinates n' o 1 , n" o", &c., intercepted between the tangents ; and the elastic resistance, or rigidity of the Tiolecules, measured by the constant ratio - , will be the greater. ax APPENDIX. 37C Relation between the Eh ".gation, or Compression, and the Force or Strain, by which it is caused, in the use of a rod, or bar of a given cross section, the force acting in the direction of the length of the bar. Let the original length of the bar be represented by L; the area of its cross section by A ; by W the force acting in the direction of the length of the bar ; which force, regarding the weight of the bar as inconsiderable with respect to W, may be considered as a weight suspended from the lower end of the bar ; and by / the elongation of L due to W. Now whether the bar be supposed to consist of as many parallel fibres as there are equidistant molecules in the section A, each fibre being of the length L, or whether it be supposed divided into a number of infinitely thin slices of the same thickness, to the extremities of each of which a force W is so applied that its effort will be uniformly distributed over each element of the area A of the slice ; it will be apparent, from a moment's consideration, that the resistance offered by the bar to elongation will be independent of its length, and proportional to the number of fibres, or to the area A ; that the elongations of the different portions /W\ of the length of the bar, arising from the action of W, will be directly proportional to their original lengths, so that the total elongation will be proportional to the total length of the bar ; and the resistance arising from elasticity will be measured, as in the case of the dis- placement of two molecules, by the ratio between the very small and propor- tional displacement of two molecules of the same fibre, and the force by which this displacement is produced. As L is the original length, and I the total elongation, the proportional elongation, or that which takes place for each foot, or other unit in which L is expressed, is represented by the fraction = X, and is the same for every fibre of the bar. The measure of the elastic resistance therefore will be W . Representing by E the measure of the elastic resistance on a unit of the A surface A, the measure of the total resistance on this surface will be ExA. From this is obtained the relation W Z =EX A;orW = EAA=EA4- A Li from which W or I may be found when the other is known. By making A equal to unity of area, and 1= L, the above relation become* W E. In other words, E is the force which applied to a bar, the area of the cross st-ctions of which being unity, would elongate the bar a quantity equal to its original length. This quantity E is termed by writers the coefficient, or the modulus of elasticity. The reasoning here used for the circumstances of elongation will equaJy apply, from what precedes, to the case of the shortening of a bar by a force of compression. 380 APPENDIX. To find the relations between the elongation and strain when the weight oftM bar is taken into consideration. Represent by L the original length, before elongation, of a bar A B (Fig. B) ; by x the length A C of any portion of it, estimated from A ; by da; an element of the part x ; by W the weight suspended at B ; and by w the unit of weight of the material of the bar. The weight of the portion of the bar B C, and which lends to elongate the part A C above it, will be expressed by (L x) JoA; the total force, or strain, acting at the point C, will therefore be expressed by W+(L *>wA; and the effect of this strain on the 'MPtiPHt, represented in length by dx will be, from the preceding propoF''.K i, J o produM an elongation expressed by W-f 'L- *) A "EA- ~ dx > the total length of the el^m^D' d t/Kr elongation will therefore be vVJ.(L-~;*A " B A~~ Integrating this expr^*'.op o^vf'^c r : the limits x = o and x = L, there obtains V, T, -(- $ a, L' A ~^A~" for the total lerg* \ f'. t' <} ',ar after elongation. Relation* bf-.iw,er t ih". F/rce, or Strain applied to a bar, or nod, of a given cross section^ ad :h f LP-I^'A, cf-c., of the bar when rupture ensues ; the strain being parallel l r t>t ' 7 .irl'on of the length of the bar. The principal results of experiments on the resistance offered by materials 'jo rupture from a strain acting either to compress, or tear asunder the par- icles, thus calling into play the tenacity, or their resistance to compression, lave been so fully given, that but little remains to be said here farther than to how the effect produced by the weight of the material itself, in modifying the strain arising from any external force ; also the manner in which the form of ihe bar may be so modified as to make it most suitable to resist the strain wising from this external force and its own weight combined. Suppose a bar of uniform cross section thoughout (Fig. B), the area of which is expressed by A, and its length A B by L, submi ,ted to a strain arising from a weight W, suspended from B, and that of its own weight, and Vet w represent the unit of weight of the material of the bar. Representing by R, the coefficient of rupture of the given material, that is the strain that would tear asunder a bar of the same material, the area of the cross section of which is unity, then the resistance offered by the bar A B will be expressed by RX A. fbe weight of the bar itself will be expressed by LA w. APPENDIX. 381 It is evident that the greatest strain on the bar, arising from the combined action of W and its own weight, will be at the point A, and will therefore be expressed by W+LAw>; and as the resistance offered by the tenacity of the bar must be equal to this strain, there obtains to express the required relations. To show the manner in which the form of a bar may be so modified that the area of its cross section, at any point, shall be just sufficient to resist the strain brought upon the material at that point; let a b (Fig. C) be such a bar the length of which is expressed by L ; let any portion of the length, as b c, be expressed by x ; let w represent the unit of weight of the material of the bar ; W a weight suspended at its lower end; let the cross section of the bar, for example, at any point be a circle ; let r designate the radius as c d of the cross section at the point c ; r" the radius a m of the top section ; r' the radius b n of the bottom section ; and d x the length of an element of the bar. The area of the cross section at c is * r 8 , and as this area is supposed to vary from point to point the weight of a portion of the bar of which the length is x will be expressed by r*'dx-, Fig. C. c I and the strain upon the section at c, arising from this weight and that of the suspended weight W, will be expressed by >dx + W; but as this strain must be just equal to the tenacity of the bar at c, and ft* R X * r* represents its measure, there obtains dr differentiating this expression, there results W yt T d x - hence d r w which integrated gives Log. r = x + C, which shows that the curve n d m, cut from the bar by a plane througn itt centre, is a logarithmic curve. Without passing from the preceding logarithmic expression to the equivalent numbers, equation 382 APPENDIX. */** ran be placed under ti e form by making * r 3 = A ; differentiating this as before, there result* u,A L , R /W R / K \ e .*r-W( -l); which is the value of the weight of the entire bar. APPENDIX. 383 Relations between a force producing the rupture oj a solid body by a cross strain on its fibres, and the resistances of compression and extension of the fibres pro- duced by the action of the force. The effect of a cross strain upon the fibres of a solid body, as a bar, or rod, caused by the action of a force, of which the line of direction is either perpen- dicular, or oblique to that of the fibres, is to deflect the solid, bringing a strain of extension upon the fibres towards the convex side, and one of compression on those towards the concave side of the solid. Separating the fibres which are elongated by the cross strain from those which are compressed, it ia generally assumed that there exists a layer of fibres which is not affected by the cross strain, and which, on that account, has received from writers on this subject the name of the neutral line, or neutral axis of the solid. It is also generally assumed that when the deflection is inconsiderable, the elongations and diminutions in length of the extended and compressed fibres which are at equal distances on each side of the neutral axis are equal, and that these changes in the original lengths of the fibres are proportional to the distances of the fibres from the neutral axis. It therefore follows from what precedes, that so long as the elasticity of the fibres remains unimpaired under the action of the force, the resistances offered to elongation, or compression, will be pro- portional to the distances of the fibres from the neutral axis. In the state of a solid immediately bordering on rupture from the effects of a cross strain, it is probable that the elastic limits of the fibres which are farthest from thu neutral line, both on the convex and concave sides, are exceeded before they are reached by fibres lying nearer to the neutral line, and that the resistances therefore are no longer strictly proportional to the distances of the fibres from the neutral axis. But as the hypothesis, that the elasticity of the fibres remains unimpaired up to the moment of rupture, approaches more nearly the actual state of the question than any other, it has been assumed by writers on this subject as the basis of the theory from which the formulas, showing the relations between the forces, are obtained, and a correction for the imperfec- tion of the results thus arrived at has been sought, by compar- ing them with those obtained by direct experiment, and, by means of this comparison, deducing formulas more in accordance with the actual state of the case, and more suitable to practical applications. Let A B C D (Fig. D) repre- sent a longitudinal section of a bar firmly fastened at the end D, and acted upon by a force W, at the end BC, which tends to deflect the bar. Let EF be the position of the neutral axis, supposed to be xt.own, and OP the line along which rupture is 384 APPENDIX. about to take place from the effect of W. Let the area comprised within the curved line (Fig E) bfl the cross section of the bar at O P. Let A X repie- sent the position of the neutral axis on this cross section, and let this line, with the one A Y drawn perpendicular to it at the point A, be taken as the coordinate axes to which all points of the cross section are referred. Represent by b, the breadth of the cross section estimated on AX; d, the distance from AX and above it of the extreme fibre of the cross section, or the one which is most elongated ; d", the distance from AX of the one most compressed ; x and y, the coordinates of any fibre, as o ; R, the coefficient of rupture. The area of any fibre will be expressed by dx X dy ; and the resistance which the extreme fibre from AX offers at the instant of rupture will be expressed by R X dx dy. Now as the resistances offered by the fibres are proportional to their elonga- tions, or compressions, and as these last are proportional to the distances of the fibres from the neutral axis, it follows that d being the distance of the extreme fibre from AX, and y that of any other fibre, as o, from the same, the resistance offered by o will be expressed by -^- R X dx dv. d The total resistance offered by all the fibres will therefore be expressed by the integral of this last expression, or by In like manner the total resistance offered by the compressed fibres is expressed by / dx ( y dy Now assuming, at the instant of rupture, that the deflection of the solid is inconsiderable, and that the force VV which causes it is perpendicular to the direction of the fibres, it follows that the conditions of equilibrium require that the algebraic sum of the forces in the direction of the fibres shall be equal to zero, and that the sum of the moments of all the forces with respect to the neutral line across the section at OP shall also be equal to zero. The first of these conditions will be expressed by I dx I y dy j dx I y dy = o (A) APPENDIX. 385 as the resistance to elongation offered by any fibre at the distance y from the R R neutral line is dx dy y, the moment of this resistance will be dx dv t/" d d and the sum of the moments of all the resistances to elongation will be expressed by In like manner the sum of the moments of the resistances to compression R Representing by z the perpendicular from OP upon the line of direction of W, the moment of VV with respect to the neutral line across OP will be Wz. To express therefore the second condition of equilibrium there obtains R. = o. (B) Representing by dA.dxdy the section of a fibre, or an element of the cross section corresponding to it, the eq. (A) will take the form which expresses the condition that the neutral line AX (Fig. E) drawn through the section of rupture passes through its centre of gravity. When the neutral line therefore divides the section of rupture symmetrically, eq. (B) will take the form ; ,. R 2 fdxff ily = Wz (C) The sum of the two integrals in eq. (B), and the integral which forms the first member of eq. (C), have received the name of the moment of rupture. The integration being affected by the usual rules for integrals of this form, the limits of x being taken between x = o and x = b, and y being either constant, a function of x, depending on the figure of the section of rupture, and its limits in the last case being y = n and y=f(x), the resulting equation will express the relations between the dimensions of the solid and the force pro- ducing rupture, when the value of R has been suitably ascertained by experiment. Before proceeding farther it will be well to effect the integrations for the moment of rupture, as expressed in eq. (C) for some of the more usual cases where the cross section has uni- form dimensions throughout the entire length of the solid. Taking the case of a rectangular cross section (Fig. F) the breadth of which is represented by b, and the depth by d = 2 d 1 , the expression for the moment of APPE.WIX. In the case of a circa ar cross section, representing by r the radios of the circle, the expression becomes R r r ry = r 3 ^n r r 2 dx fdy=2 IdxKS r J -r-J r J , r R , /"t** 4 Rrrr 2 X 2 I dx sin. 4 . .r = r J o 3 4 In the case of a tube with a rectangular cross section (Fig. G), representing by b and d the breadth and depth of the exte- rior rectangle, and by b' and d' the like parts of the interior rectangle which forms the hollow of the tube, the moment of rupture will be l b'd R 6d Fig. G. and in that of a tube with a circular cross section, r and r 1 being the radii of the exterior and interior circles, the expression becomes R 4r In the case of a uniform cross section like (Fig H), representing by b the entire breadth of the solid portion, and by d its depth : by b' the sum of the breadths of the rectangular voids on each side, and by d their depth, the expression becomes, as in the case of a tube, The foregoing examples will serve to show the method of obtaining the moment of rupture in all like cases of solid bars, or tubes with uniform cross sections where the neutral line divides the cross section symmetrically. Resuming the general expression (C), which shows the relations between W and the other quantities, and applying it to the case of a beam with a rectangular i-ross section of uniform dimensions, the length of which AB, beyond the point where it is firmly fastened, is repre- sented by I, the expression becomes Vig.E (D) APPENDIX. 387 supposing 1 the rupture to take place, as is evident from the eq., it will in such a case, at AD. From this eq. there obtains for the weight which the beam will bear at the moment of rupture. If a beam is laid horizontally on two props, and a weight is placed upon it at the middle point between the props, the tendency of the beam to rupture will be at this point ; and the strain upon the beam from W will evidently bo the same as if the beam were firmly fixed at the middle point, and a force equal to W were applied at the point where the beam rests upon one of the props, and in a direction contrary to that of W. Representing by I the dis- tance between the props, b the breadth, and d the depth of the rectangular cross section, d being estimated in the same direction as W, the preceding eq. ^D) will take the form aonsequently It is from experiments made upon beams of a rectangular section, laid hori- zontally upon two props, and broken by a weight placed upon them at the middle point between the props, that the quantity R has been determined for the different kinds of materials. Having determined R from a number of such experiments the value of W can be determined by calculation when the quantities b, d, and I are known. If instead of a weight acting at a single point, a beam of rectangular cross section is strained by a weight uniformly distributed over its top surface, the conditions of equilibrium from which the relations between the weight and the dimensions of the beam are established will be expressed as follows Representing by ir the weight on each unit of length of the beam, by dz (Fig. I) an elementary length of the solid at the distance z from the point where the solid is firmly fastened, and where the rupture will take place ; then the weight distributed over the element dz will be wdz, and its moment with respect to the point of rupture will be wdz x z. Calling the entire length of the solid Z, the moment of the entire weight distributed over its length will be expressed by dz z; c I Tig. I. 883 APPENDIX. and this, from the conditions of equilibrium, mu of the solid, and by x' its abscissa PB ; by w the weight borne by each linear unit of AB ; by dx any elementary portion of AB, at the distance x from B ; then to dx will be the weight distributed over the element dx. To express the conditions of equilibrium between the moment of rupture of the cross section at PM, and that of the weight of any portion of the solid of the length J?, there obtains, R = I w dx (x 1 x} = $wx (2x f a?). Representing by Z = AB the length of the solid from DC where it is firmly fixed, and by b= AC, the foregoing expression becomes bd 3 R = 6 hence b = Since for the cross section at AC, x = x' = I, and y' = b. The preceding expressions show that the curve CMB is a parabola, of which C is the vertex, and the line CA the axis. APPENDIX. 391 From the preceding examples, the relations between the moment of lapture and the strain caused by a force acting in a given manner on a solid of any form nf cross section, whether constant or variable, may be established when the dimensions of the cross section are connected by any geometrical law. In the most important cases in structures, these relations are usually expressed in the most simple terms for convenient arithmetical calculation, the constant, represented by R in the preceding equations, being determined from experi- ments made on the solids of the form to which the formulas are applicable. Examples of formulas derived in this manner, and adapted to arithmetical cal- culation, are given in Art. 320, page 92, and at the end of Note A on tubular bridges. , A large number of experiments have been made to ascertain the value, of R, by submitting solids of a rectangular cross section to cross strains producing ruptuiv. As might have been anticipated, the values of R so determined vary considerably. The mean value for timber is usually taken at 12,000, and that for cast iron at 40,000. In practical applications, the value for timber is reduced to the one-tenth, or 1200, and that for cast iron to the fourth, or 10,000, of that determined by experiment, when the formulas are used to fix ihe strain to which the material can be subjected in safety in the ordinary cases jf structures. Manner of estimating the strains on frames of straight teams of rectangular cross section, and the relations between the strain on each piece and its dimensions. The problems presented under this head consist of two parts : the first, to And the directions and intensities of the strains on the component parts of a frame, composed of straight beams with rectangular cross sections, arising from a given strain acting at any assumed point of the frame ; the second, so to pro- portion the dimensions of each part that it shall resist, without danger of rupture, the strain thrown upon it. The first part depends for its solution on the two fundamental propositions of statics ; the parallellogram of forces and the theo- rem of moments. The second is but an application of the preceding formulas in this note. Solid Built Beams. The resistance offered by this class of beams to a cross strain will depend on the manner in which the pieces are placed together, and the connexion between their surfaces of contact. In beams like (Fig. 57), p. 171, art. 502, in which each course is formed of two or more beams abutting end to end, and all the courses are firmly secured to each other, so that the surfaces in contact cannot slide on each other, we may regard the strength of the beam, supposing the courses of equal thickness, as equal to that of a solid beam having a depth equal to that of the built beam diminished by the thickness of one course. Where each course consists of a single beam, and the whole are united so as to prevent sliding (Figs. 59, &c.), then the resistance offered may be regarded the same as that of a solid beam of equal depth. Adopting the same notation, as in the preceding part of the note, for beams of rectangular cross section , the formulas for a solid beam, on pp. 386-7-8, may 392 APPENDIX. be used in these cases of built beams, under like circumstances of the position of the beam and the point of application and direction of the strain. The formula at the bottom of p. 389 is applicable to this class when sub- mitted to a strain acting at any point between the centre of the beam and its points of support. In all such cases, however, the liability of the beam to break at one point rathe! than another, owing to the manner in which it is built, must be carefully con- sidered, and a suitable modification of the formula be made, if requisite. The quantity R also, which enters into the formulas, and which for solid timber it ia stated should be reduced to 1200lbs. in cases of practice, may be further reduced one-third in such cases, to be on the side of safety. Open Built Beams. In this class of beams (Fig. 63), p. 172, in which the breadth and depth of the solid parts at top and bottom is the same, the relations between the weight that the beam will bear with safety applied at the centre pouit between the supports, and the other quantities will be bd' b d' 3 w = **'Tl -- in which R' is the reduced value of R. Solid Beams. From the formula (D), p. 386, by substituting R' for R, there obtains which gives the relations which must exist between the weight and dimensions of the beam, in order that the strain on the unit of surface of the fibres shall not exceed the quantity R'. If, in addition to the strain arising from W applied per- pendicularly to the axis of the beam, there was another W acting parallel to the axis, so as either to compress or extend the fibres, this would cause a strain on the unit of surface of the cross section expressed by W 7 Td" In order, therefore, that the total strain arising from W and W, applied as hee supposed, shall not exceed that which the unit of cross section can be submitted to with safety, there must obtain WZ W R ~ 6 Td? + 'bd If we now suppose a beam (Fig. O), fast- ened at one extremity, to be subjected to a ^ strain applied at the other obliquely to the cu ............ .^jfr'' axis, we can regard the oblique strain as replaced by its two components, the one per- e * - .......... -J**. pendicular the other parallel to the axis of the beam. Representing the perpendicular com- ponent by W, and the parallel one by W. the APPENDIX. a Fig. P. ame rations will evidently obtain between these components and the other quantities as in the preceding case, in order that the strain on the unit of area of the cross section shall not exceed R'. When a pressure, or a force of extension is applied parallel to the axis of a beam (Fig. P.), but not in the same line with it, its effects will be to compress, or extend the beam in the direction of the axis, and to produce a cross strain on the fibres, owing to the tendency to bend the beam. Supposing a vertical beam A B, fastened at the point A, and a weight W applied at the extremity of another beam B c, firmly connected with the first, and perpendicular to its axis. The compression of A B, in the direction of the axis will be equal to the entire weight W, and that on the unit of area will be W bd' in which b is the breadth, and d the thickness of A B, the latter estimated in the plane through the axis in which the force acts. Representing by h the horizontal distance B c, from the axis at which W acts, its moment will be equal to the moment of rupture of the fibres, or Wh= ^R'bd* In order, then, that the strain on the unit of area shall not exceed the prescribed limit there obtains Wh W R = 6 1 bd" bd Let the case now be supposed of an in- clined beam, A B, which rests on a horizon- tal surface A c, whilst the upper end lies against a vertical surface B c, the beam having a weight W, suspended from its middle point o. Represent by I the length A B, of the beam, and by a, the angle between its axis, and the vertical at o. Leaving out of consideration the Motion between the foot 'of the beam and the horizontal surface on which it rests, it is evident that the reaction of the vertical Fig. Q. surface against the upper end would cause the beam to slide along A c, and that to prevent this, the end, A, must be confined. The tendency of W, there- fore, will be to turn the beam around the point A, and this will be counteracted 394 APPENDIX. by the reaction of B, acting horizontally. Designating Jus foi je of reaction at B, by P, there obtains from the theorem of moments, PXAD=WXAE; or, substituting for A D and A E their respective values, I cos. a, and $ I sin. a P = W tan. a. By confining the foot of the beam, an equal horizontal force is called into play at A, whilst the total weight, W, is supported at the same point. The total pressure at A will, therefore, be the resultant of these two forces, which are perpendicular to each other, and will be expressed by y/ W 1 + 4 W 1 tan. 'a = W J\ + 4 tan. 'a. Or representing by A b and A c the directions and intensities of the two forces, W, and 4 W tan. a, the diagonal, A d, of the rectangle will represent the direction and intensity of the resultant. Having thus found the magnitude, direction, and point of application of the two forces, there is next to be considered the strains to which they subject the fibres. To do this, the beam may be regarded as confined at the point o, the lower portion, o A, being acted upon by the two forces, A b and A c, at the point A ; the upper portion, o B, by a horizontal force equal to A b, at the point B Decomposing the force A b=i W tan. a, perpendicular and parallel to the axis A o, the components will be represented respectively by 4 W tan. a cos. a ; and J W tan. a sin. a. The components of W in the same directions are W sin. a ; and W cos. a. *t will be observed that the components perpendicular to the axis, and which oroduce a cross strain, act in opposite directions with an intensity equal to their difference expressed by W sin. a 4 W tan. a cos. a=4 W sin. a. The components parallel to the axis act in the same direction, compressing the fibres with an intensity equal to their sum, and expressed by W cos. a + i W tan. a sin. a = W cos. a (1 + tan. a a). Representing by b the breadth, and d the depth, of the beam estimated in tho plane of the forces, there obtains, as in the preceding case, to represent the limit of the strain on the unit of area T>, _ a W sin, a I W cos, a. (1 + jtan.*a) ~ 2 bd* bd Considering next the upper half o B, which is acted on only by the horizontal force at B, equal to 4 W tan. a ; the components of this force, perpendicular and parallel to the axis, are i W tan. a cos. a ; and | W tan. a. sin. a. APPENDIX. 393 The perpendicular component producing a cross strain, and the parallel one a strain of compression. To express the limit of the strain on the unit of arc* for this portion, there obtains W sin. a I Flg.fi. W tan. a. sin. a To prevent the flexure of a horizonta beam, as A B (Fig. R), confined at one end, and sustaining a weight at the other, an inclined strut D c is placed beneath it, abutting against some fixed point as D, and against the beam at some point between A and B ; or else a tie is used leading from a point c, to some fixed point, as E above. Represent by I the distance A c, by I' that c B ; by a the angle that the axis of the strut makes with the vertical. As the point c is fixed, the weight W, act- ing at B, will tend to turn the beam around this point, and this will call into play a ver- tical force at the point A which, from the theorem of moments, is expressed by the fixed point c, therefore, sustains a vertical effort which is equal to the sum of this force and W, and is expressed by f 1 4- /' w + w-j- = w- As the point c is supported by the strut, the vertical force at c may be decomposed into components in the direction of the axes of the strut and the beam. That along the strut will be expressed by I cos. a and will compress it in the direction c D. The one along the axis of the beam is I and will produce a "train of extension on the part A c of the beam. This por- tion A c is, therefore, subjected to a cross strain, which tends to rupture it at c, from the force N it ihe point A, and to extension from the force acting along its axis. .There obtains, therefore, to express the limit of the strain on tno unit of area S96 APPENDIX. p/ _6Wr W (I + O tan. a " ~~^ IS "4" For the strut there obtains, bd* R' = /. bd I cos. a 6' d' b' and of' being the two sides of its rectangular cross section. If the beam A B (Fig. S) is supported by a vertica. post A E firmly fastened at A, the strains on the strut and the part A c will be estimated as in the case just examined. As to the strains on the post, it will be observed that the force acting along the strut, and which is represented by I cos. a will be transmitted to the point D. This will be equi- valent to a horizontal force represented by Fig.S. and a vertical one represented by W 1 acting at c. From the connexion formed by the strut between the horizontal beam and the post, the effect of these two components will be to cause a pressure on the part D E of the post, which will be equal to W; and to produce a cross strain by the horizontal force, the moment of this force being expressed by W : tan. a x tan. a = w To express, therefore, the limit of the strain on the unit of area of this part D K, there obtains 6 W (I + Q W b d* b d R' = In like manner, the part A D will be subjected to a cross strain from the hori- zontal force above-mentioned, which tends to cause rupture at the point D, and to a strain of extension, acting upwards along A D, from a vertical force at A expressed by The limit of the strain, therefore, on this portion will be wr w APPENDIX. 397 By comparing the two expressions (X) and (Y), it will be seen that they will be equal when 1=1', in which case the port will have an equal tendency to rupture at D, or at any point of the portion D E. When 1 < 1' then the tend- ency to rupture is rather hi the part D E, than at D. The discussion of the strains on (Figs. R A n . n , A/ and S) naturally leads to a consideration of combinations of beams represented in (Figs. R' and S'). In combinations of this kind, the simplest jy and safest plan consists in supposing the frame resolved into parts, such that either alone will be strong enough for the object in view, and to regard their combination as affording an excess of strength sufficient to in- sure perfect safety from rupture. In (Fig. R'), which consists of a horizontal beam, resting on the points of support A, A', and supported at the intermediate points c, c', by struts D c, D' c', firmly con- nected with the beam, and at the fixed points, D, D' ; the beam, in the first place, may be regarded as alone supporting the weight, W. at its middle point B, and the area of its cross section be so determined, that the strain on the unit of area shall not be greater than the limit fixed on. Having calculated the dimensions of the horizontal beam in this manner, the weight may, in the second place, be regarded as supported by the portion c c' of the beam, and the two struts D c and D' c', in which case the strains on these parts, and their limits on the unit of area of their cross section will be determined by considering, that at the points 3 and c' a vertical force 4 W is acting, the components of which in the direction C D and c B are respectively W Representing by V and d' the sides of the cross section of the strut, there obtains to express the limit of the strain for it E'- 2 cos. a V d'' For the portion of the beam c c', it may be regarded as confined at its middle point, where W acts, and subjected to the two forces i W, and $ W tan. a, the first producing a cross strain, and the second one of compression in the direction of its axis. Representing the distance B c therefore by f, there obtains tc express the limit Representing by I the distance A c, the-e obtains to express the limit, when the beam A A' alone sustains the weight, 398 \PPE1TDIX. - 2bd ' ' Now, comparing the two values of R' in the expressions (X') and (V), it irlL oe seen that the combination of the struts and t&e piece cc' alone wiL be rtronger than the beam A A' alone, when er tan. a < j-. In like manner in combinations like (Fig. S') in which the points of support are two posts, A E, and A' E', firmly fastened at B and E', we may regard, in the first place, the beam A A and the two posts as alone sustaining the en tire weight W, appb'ed at B, the centre point of A A', and ascertain the limits of the strains to which these pieces can be respectively sub- jected. Then the weight may be considered as sustained by the portion c c alone, the two struts, c D, and c' D', and the posts. Having ascertained, as in (Fig. S), the strains on thes parts, and the limits to which they can be sub- jected, if it be found that either of these combinations alone will be sufficiently strong, then will the entire combination have an excess of strength. As the beam A A' is firmly connected with the posts at A and A', a strain of extension will be thrown on the portion A c, from the pressure at D acting W Fig. '. through the strut c D, which pressure is represented by - This again 2 cos. a may be regarded as equivalent to a horizontal and a vertical force respectively represented by Wtan. a and W Representing by A, and h' the distance E D, and r A, the horizontal force will cause a strain at A, acting in the direction A c to extend this part represented by Wfr'tan.a 2 (h + h~' and one at the fixed point E represented by W h tan, a 2 (h + h') ' These two components of the horizontal force at D will cause a cross strain OP She post. We may, therefore, regard the post as confined at the point D, and *e portions on each side of D subjected to a c'oss strain from the horizonta components, the moments of which components will be the same, and respeO hrely represented by APPENDIX. W ft V tan, a and to a strain in the direction of the axis represented by .~. The limit there fore, of the strain to which the. post can be subjected will be R' = W 6 W h h> tan - a ~ 2b'd' 2 b' d'* (h + h') in which b' and d f are the sides of the rectangular cross section of the post. Straining and Tie Beams. If the struts c D, c' D' (Figs. R', S'), instead of being connected with the beam A A', abutted against a straining beam placet beneath the portion c c', as shown by the dotted line, then the horizontal strain on the beam A A', from the action of the struts, would be borne by the strain- ing beam alone. This portion of the frame, composed of this beam and the por- tion c c' of the beam A A', may be regarded as a single beam submitted to a cross strain from the weight W, and to a strain of compression on the strain- ing beams alone from the horizontal component of the pressure at the point c or c', and the relations between the limits of the strain on the unit of area and the dimensions of the beams 'be found as in the previous cases. If the struts, instead of abutting at D and B' against the wall, or posts, were confined by a tie beam shown by the dotted lines D D', this beam would relieve ihese points from the horizontal pressure, and would itself be subjected to a strain of extension equal in amount to the horizontal strain on the straining beam. In the preceding cases, the frame has been considered as subjected to the action of a strain acting at one point alone ; if, in addition to this, the beam A A' was subjected to a strain uniformly distributed along it, the vertical pressures at c c' and A A' would have to be increased. Representing by w the weight uni- formly distributed on the unit of length of A A', the total weight on the portion A c will be represented by iv Z, and that on the portion B c by w I' ; the verti- cal pressure at c arising from these two will, therefore, be represented by Iwl + vl'i and this must be added to the vertical component of W at the same point, in all the preceding formulas, to make them applicable to this case. Combinations, like (Fig. T), com- posed of two inclined pieces, A c and B c, abutting against each other at A, and confined at B and c, either by being inserted into a horizontal tie beam, or abutting against fixed points, may be arranged either for sustaining a vertical strain at the point A, as that arising from a weight suspended at A; or, M in 400 APPENDIX. he case of the rafters of a roof truss, to sustain a pressure uniformly distri buted over the two inclined pieces. Representing by p and q, the angles between the inclined pieces, and the vertical through A, the components of W, in the directions A B and A c, wiL be respectively represented by sin.? . __ sin. (p + q)' and the horizontal pressure at A, which is the same as the horizontal strain at the points B and c, is represented by sin, p sin, g. sin. (p + q) Each of tnese beams may, therefore, be regarded as confined at their lower points, and subjected at their upper ones to the strains in the direction of their axes just determined ; and from these the relations between the limits of the strain on the unit of area and the dimensions of the beams can be determined as in the preceding examples. When the angle p is equal to q, the strains in the direction of each beam will be expressed by W 2cos. p' and the horizontal strain by i W tan. p. Were the beams in this last case, instead of supporting a vertical pressure at tne point A, each subjected to a vertical strain applied at any point between A and the foot of each beam, it is obvious that each would be in the same condi- tion as the one (Fig. Q,), and that the horizontal strains at the points A, B, and c, those in the direction of the axes of each beam, and the total pressure at B and c, would be found as in (Fig. Q) ; and from these the relations between the dimensions of the beams and the limit of the strain on the unit of area be in like manner determined. The applications of the preceding problems to the analogous cases of frames composed of rigid materials, as timber and iron, will be readily seen on a com- parison of the forms in Art. 507, and the following, with those which have just been the subjects of examination. The main difficulty in each case will be in determining those strains both in intensity and direction, which arise from the reciprocal action of the component parts of the frame, and this will be most easily overcome by supposing the parts pressed against removed, and their resist- ance replaced by forces acting in the same directions as the resistances ; as, fo example, in the case of (Fig. Q), where a horizontal force at B might be made bo replace the resistance of the wall against which the beam rests, &c. In all cases, moreover, whare a strict equilibrium does no* obtain, either I the strains being transmitted to fixed points, or by equal counteracting forces, the stability of the framing will depend on the joints, or connexions of the beams ; and wherever there is a tendency to the rotation of any one of the beams around a joint, it must be prevented by the introduction of a strut or tie, so placed as to hold the beam, liable to this movement, in a fixed position. w o n K s O F PROF. D H. MAHAN, LL.D An Elementary Course of Civil Engineering, For the Use of the Cadets of the United States Military Academy. 1 vol. 8vo. with numerous wood-cuts. New Edition, with large Addenda, . This Work is used as the Text-book on this subject in the U. S. Military Academy. It is designed Ifto fur use in other institutions. The body of the Work is routined to a succinct statement of th :snd principle* of each subject contained in il.. The Appendix consists of the mathematical demonstrations of principles found in the tei with Notes on any new facts that from time to tim appear II. A Treatise on field Fortification : Containing Instructions on the Methods of Laying Out, Constructing, Defending, and At tack ins; Intrenehmente, \Vith the General Outlines, also, of the Arrangement, the Attack and Defence of Permanent Fortifications. Third Edition, revised and enlarged. 1 vol., full cloth, with steel plates. $1 00. This Work is the Text-book on this subject used in the U. S. Military Academy. It is also desi Ljned as a practical work for Officers, to lie used in the field in planning and throwing up entrench- ments. III. AX ELEMENTARY TREATISE ON Advanced-Guard, Out-post, and Detachment Service of Troops, And the Manner of Posting and Handling them in the presence of an Enemy. With a Historical Sketch of the Rise and Progress of Tactic?, fc?., ' The df- -u teach (.'< ninrtiiral Drawing, as applicable to all industrial pursuits n it x .. ti manner, to persons even who have made no attainments in Elementary Alathe malics. For this purpose the method recommended is the oral one, in whicti each operatiom will b i- rt'iirmcd |,y HIH 'IVaeher bef,.rc I he t-jcs of the pupil, by whom 111 turn it will be repeated. It u that the Work will also be found useful to all who are preparing themselves for any of th industrial pursuits in which Geometrical Drawing is required. V. Treatise on Permanent Fortifications. Tins work is a Lithographed volmin-, with a numlier of Plates, and is used as the Text-book on this branch in the U. S. Military Academy. Sot being a sub- ject of general interest, the sale of it would not, warrant its being printi published in the usual form. Price *3 00. 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