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The attraction of cohesion, which retains together the particles of solid bodies, is the foundation of their strength. It exists in all solids, though in different degrees; and requires, before it can be overcome, the application of force or of art, adapted to the strength and character of the particular body. In some substances, cohesion, when once overcome, cannot be reproduced in its original state. In others, it may be restored by the intervention of fluidity, and in all, its effects may be imitated by mechanical arrangements. The various modes by which bodies may be divided, or united, have an important agency in mechanical constructions, and other processes of art.
MODES OF DIVISION.
Fracture.--The simplest and least artificial mode by which mechanical division is effected, is by breaking. The circumstances which influence the production of fracture by extension, compression, lateral strain, and torsion, have been considered in the third chapter of this work. In general, a force acting suddenly is more liable to occasion fracture, than one which acts more gradually; for in this case, the parts which are first strained may give way, before the stress is proportionally distributed among the remaining parts. A mass of plastic clay, or of warm sealing wax, will bear to be gradually bent, but will break if the motion is sudden. In like manner, percussion occasions fracture more readily than pressure. A crack, or partial fracture, in a body, greatly promotes the separation of the remainder, whenever a lateral force is applied; because the strength of the sound parts tends to throw the strain more immediately upon the weakened points, as explained on page 125. In stone quarries, regular blocks are split out by driving rows of wedges in a straight line, so as to produce a simultaneous strain in the direction where a fracture is desired.
Cutting.--Cutting instruments act, in dividing bodies, upon the same principle as the wedge. The blade of the instrument is in general a thin wedge, but the edge itself is usually much more obtuse. Mr. Nicholson has estimated the angle which is formed ultimately by the finest cutting edge, at about fifty-six degrees. If the edge of an instrument were not angular, but rounded or square, it would still act as a wedge, by pushing before it a wedge-shaped portion of the opposing particles, as is done by obtuse bodies moving in fluids. In general, an oblique motion is more favorable to cutting, than a direct, and this is because the edges of steel instruments are rough with minute asperities, like saw-teeth. This circumstance, however, is of less importance when the material operated upon is very firm and the cutting is deep; for in this case, the friction and compression consume more force, than the actual division. This takes place with axes and chisels, which are necessarily made thick, to secure the requisite strength.
The quality in tools which is called temper, is opposed to brittleness on the one hand, and to flexibility on the other. Independently of the quality of the metal, it appears to be somewhat influenced by temperature, since axes and other tools are liable to break, or gap, in frosty weather, and razors cut best after being immersed in hot water.
The kind of cutting which is performed by scissors, depends upon the process called detrusion, in which the coherent particles are pushed by each other in opposite directions. In this case, the cutting edges require to be angular, but the angle not very acute. The shearing of woollen cloths, the slitting and punching of metals, the cutting of nails, and various other mechanical processes, are performed on this principle.
Cutting Machines.--A variety of fibrous and woody substances, used by druggists and dyers, require to be reduced to a coarse powder like saw-dust, to facilitate the extraction of their soluble matter. This is not easily done in any of the common mills, owing to the toughness of the material. It is sometimes effected by machinery with circular rasps or saws, but a more economical application of a dividing force in these cases, is obtained by the rapid revolutions of a sharp cutting instrument. In a machine for cutting straw, a number of blades revolve upon an axis, with a fly. In Blanchard's ingenious engine for cutting definite forms by a pattern, sharp instruments, of different forms, are made to revolve upon axles, or slide in groves, while the material operated on is put in motion, so as to place itself in the proper position to receive the cut.
Planing machines have been variously constructed, in which the board, or substance to be planed, passes under an edge which cuts away the surface to an even depth. This edge is sometimes stationary, but more frequently a succession of cutting tools revolve with great rapidity, cutting away small successive portions of the surface. The form and direction of the shavings thus removed, is various, according to the direction of the axis about which the cutter revolves. Machines for cutting shingles and laths, are contrived on a variety of principles. One of the most effectual of these consists of a large, upright, revolving iron disc, the anterior surface of which is smooth, and furnished with two knives, or cutters, of different obliquity. A block of wood, previously made soft by steaming, is pressed against the surface of the revolving iron, so that each knife in turn strikes off a wedge-shaped slice, or shingle, of the size of the block.
Penetration.--Bodies are penetrated either by pushing aside a portion of their substance, as in driving a nail; or by removing a portion, as in boring and drilling. In addition to the force of cohesion, the resistance opposed by a solid, or even by a soft substance, to the motion of a body tending to penetrate it, appears to resemble, in some measure, the force of friction, which is nearly uniform, whether the motion be slow or rapid, destroying a certain quantity of momentum in a certain time, whatever the whole velocity may be, or whatever may be the space described. Hence arises an advantage in giving a great velocity to a body which is to penetrate another, since the distance to which a body penetrates will be nearly as the square of its velocity.[A] The same remark applies equally to the action of cutting instruments. The effect of a hammer in driving a nail, depends partly on the influence of velocity in modifying friction, and partly upon the momentum accumulated in the hammer, the effect of which resembles that of a fly wheel.
Boring and Drilling.--The processes of boring and drilling, performed by gimlets, augers, centrebits, drills, &c., is a species of circular cutting, in which a cylindrical portion of the substance is gradually removed. Drills are made to turn rapidly, either in one direction by means of a lathe wheel and pulley, or alternately in opposite directions, by a spiral cord which coils and uncoils itself successively upon the drill, and is aided by a weight or fly. In boring cannon, the tool is at rest, while the cannon revolves, and by this arrangement the bore of the cannon is formed with more accuracy than according to the old method of putting the borer in motion, perhaps because the inertia of so large a mass of matter assists in defining the axis of the revolution with more accuracy. The borer is kept pressed against the cannon by a regular force. Cylinders of steam-engines are cast hollow, and afterwards bored; but in this case the borer revolves, and the cylinder remains at rest. In either case, it is important that the axis of the borer, and that of the cylindrical material, should coincide; for when it is otherwise, if the borer revolves, it will perforate obliquely, and if the material revolves, the perforation will be conical.
Mortising. Square holes, or those having a rectangular outline, are usually cut by mortising with a chisel and mallet. The operation is commonly performed by hand, but it is also executed by various machines, consisting of ingenious combinations of chisels, borers, punches, and saws.
[A] See Young's Natural Philosophy, vol. i. p. 225, and Playfair's, vol. i. p. 97.
Turning.--Turning is an elegant operation, used to produce regular figures the section of which is circular. Like boring, it is a species of circular cutting, and is performed in a well-known machine called a lathe, in which the material to be cut revolves about its axis, while the tool is kept stationary and supported by a rest. Besides circular forms, it may also be used to produce regular curvilinear figures, which may be multiplied indefinitely. The effect of most lathes of complicated construction, depends on a certain degree of motion, of which the axis is capable. If this motion be governed by a frame producing an elliptic curve, any number of ovals having the same centre may be described at once; and if a movable point connected with the work, be pressed by a strong spring against a pattern of any kind, placed at one end of the axis, a copy of the same form may be made at the other end of the axis. Geometrical lathes, governed by eccentric wheels, and capable of describing an indefinite variety of complex figures, upon a metallic plate, are used for bank notes and ornamental designs.
Attrition.--The action of files, rasps, grindstones, and hones, consists in successively cutting or breaking away minute particles from the surface of bodies. They are used chiefly for wearing off portions of hard substances, particularly metals. The surface of grindstones and whetstones, is kept moist with water or oil, the use of which is not so much to obviate the production of heat by friction, as to prevent the adhesion of foreign particles from filling up the insterstices of the grit. In the finer kinds of grinding and polishing, certain hard substances are used in the form of powder, such as emery, tripoli, sand, putty, oxide of iron, &c.
Sawing.--Saw Mill.--A saw, in many respects, resembles a rasp, and acts by cutting or breaking away large particles in the direction of its own plane. The thinner the saw is, the easier is the operation, since a smaller amount, of substance is removed by the teeth. For the sake of this advantage, and for economy of the material, the blades of saws are made thin, and often stretched upon frames, to compensate the want of rigidity. Saw mills erected for cutting logs into boards, consist usually of saws attached to frames, which have a reciprocating motion communicated to them by a crank connected with a water-wheel or steam-engine. A ratchet wheel is connected with the saw by means of a bar and click; so that at every stroke of the saw, the wheel is turned the length of one tooth. The ratchet wheel acts by means of a rack, upon a carriage, which supports the log, causing it slowly to advance, until the whole length of the log has passed the saw.
Circular Saw.--Circular saws, revolving upon an axis, have the advantage that they act continually in the same direction, and no force is lost by a backward stroke. They also are susceptible of much greater velocity than the reciprocating saws, an advantage which enables them to cut more smoothly. Tho size of circular saws, however, is limited; for, if made too large, and of the usual thinness, they are liable to waver, and bend out of their proper plane; and, on the other hand, if made thick enough to secure an adequate degree of strength, they waste both the power and the material, by cutting away too much. Hence, they are not commonly applied to the slitting of large timber, but are nevertheless very useful in smaller works, for cutting off bodies which can be included within a certain distance of the axis, and thus allow the saw to be of small size. Circular saws, however, of large size, are used in cutting thin layers of mahogany for veneering; for in this case the saw can be strengthened by thickening it on one side towards the centre, the flexibility of the layer of wood allowing it to turn aside, as fast as it is sawn off. Circular saws may be rendered more steady by giving them a greater velocity, so that the centrifugal force shall assist in confining the saw to its proper plane.
An ingenious machine has been invented in Maine, for sawing off sheets of wood of an indefinite length, for veneering, by cutting a spiral layer from the surface of a cylindrical log, the layer being turned off like a riband, when unwound from a roller. The sheets of rice paper, mentioned on page 196, note, are said to be cut in the same spiral manner.
Dovetailing machines are made with circular saws, constructed to cut obliquely, and entering in different directions. Or, instead of saws, small wheels are used, with cutters on their circumference. Tenons, or the parts intended to enter mortises, also tongues, rabbets, &c., are cut on similar principles.
The sawing of marble is performed by saws made of soft iron, and without teeth. A quantity of sand and water is kept interposed between them, and the sand, becoming partly imbedded in the iron, serves to grind away the marble. These saws are worked horizontally, for the convenience of retaining the sand, and are moved either by hand, or by reciprocating machinery. The cylindrical blocks, which form the tambours, or frusta, of columns, are sometimes cut out of marble, by perforating the block at the centre, and inserting an iron axis, to the ends of which are attached frames, upon which a narrow, or a concave, saw is stretched parallel to the axis. An alternating motion is then given to the frame, until the saw has cut its way round the axis.
Crushing.--When materials require to be broken into minute parts, or when the texture of vascular substances is to be destroyed, that they may yield their fluid contents, the operation of crushing is resorted to. It is performed either by percussion, with hammers, stampers, and pestles, or by simple pressure, with weights, rollers, and runner stones.
Stamping Mill.--For reducing the ores of metals to powder, a number of heavy vertical bars, called stampers, are alternately raised, and suffered to fall, by the action of cams or wipers, projecting from the arbor of the mill-wheel. The ore is placed in a trough or mortar beneath, where it is acted upon by the stampers, until it is sufficiently comminuted. A stream of water continually runs through the stamping trough, carrying with it the particles, which have become fine enough to pass through a screen provided for the purpose.
Bark Mill.--The bark used by tanners is reduced to a coarse powder in various ways. One of the most common methods, is, to crush the bark by the revolutions of a circular stone, called a runner stone, which resembles the wheel of a carriage, travelling round in a continued circuit. The axis of the stone is connected with a vertical shaft, so that the stone has two motions, one round its own axis, which is horizontal, and the other round the vertical shaft. The bark is raked up into a ridge before the stone, and is crushed or ground up, by the edge of the stone rolling over it. In some more complicated mills, the bark is successively cut with knives, beaten with hammers, and ground with stones, or cylinders.
Oil Mill.--The oleaginous seeds from which oil is expressed, require to have their substance previously broken up by the operations of a mill. In one of the best forms of the oil mill, the seeds are first bruised to the consistence of paste, by the action of runner stones. The paste is received in troughs perforated with holes, through which a portion of the oil drips, and this part is considered the most pure. The paste is then put into strong bags, and subjected to pressure as long as it yields oil. The remaining paste, or oil cake, is next taken out of the bags, broken to pieces, and put into mortars. It is here beaten by the action of heavy stampers, until reduced to a very minute state of subdivision. The oil which is next pressed out from it is inferior in quality to the first, in consequence of its containing more mucilage and farinaceous particles. The seeds are nevertheless subjected to another pressure, after having been exposed to heat, which enables them to yield a quantity more of oil, but of a still poorer quality.
Sugar Mill.--The machine by which sugar canes are crushed, usually consists of three vertical rollers, the middle one of which is turned by a horse, or other power, ^nd turns the remaining two by friction, or by toothed wheels; the latter method being most advantageous. The canes are supplied by attendants, and are drawn in and crushed between the first and second rollers, after which they return and pass between the second and third. The juice, which is pressed out by the same operation, flows into a trough beneath.
Cider Mill.--When the substances to be crushed are so large that they cannot readily be drawn in between smooth cylinders, it is necessary that the rollers should be indented at their circumference. The common cider mill is formed with two indented cylinders, the teeth of one of which enter the indentations of the other. By this arrangement, the fruit to be ground is caught by the projecting parts of the rollers, and regularly carried forward and crushed. Formerly it was the custom to grind apples by runner stones, similar to those used in bark mills. And at the present day cylindrical rasps are sometimes employed, being supposed capable of destroying the texture of the fruit more effectually.
Grinding.--Grinding, in its most limited sense, may be considered as a species of crushing, or breaking, in which the force acts partly in a lateral direction, so as to lacerate, rather than compress, the material acted upon. It is frequently produced, in small mills, by a cylinder or cone, turning within another, which is hollow, the surfaces of both being cut obliquely into teeth. In larger mills, it is commonly performed by one stone moving upon another.
Grist Mill.--The common mill for grinding grain, is constructed with two circular stones placed horizontally. Buhrstone is the best material of which millstones are made, but sienite and granite are frequently used, for Indian corn and rye. The lower stone is fixed, while the upper one revolves with considerable velocity, and is supported by an axis passing through the lower stone, the distance between the two being capable of adjustment, according to the fineness which it is intended to produce in the meal, or flour. When the diameter is five feet, the stone may make about ninety revolutions in a minute, without the flour becoming too much heated. The corn or grain is shaken out of a hopper by means of projections from the revolving axis, which give to its lower part, or feeder, a vibrating motion. The lower stone is slightly convex, and the upper one somewhat more concave, so that the corn, which enters at the middle of the stone, passes outward for a short distance, before it begins to be ground. After being reduced to powder, it is discharged at the circumference, its escape being favored by the centrifugal force, and by the convexity of the lower stone. The surface of the stones is cut into grooves, in order to make them act more readily and effectually on the corn; and these grooves are cut obliquely, that they may assist the escape of the meal, by throwing it outward. The operation of bolting, by which the flour is separated from the bran, or coarser particles, is performed by a cylindrical sieve placed in an inclined position, and turned by machinery. The fineness of flour is said to be greatest when the bran has not been too much subdivided, so that it may be more readily separated by bolting. This takes place when the grinding has been performed more by the action of the particles upon each other, than by the grit of the stone. For this sort of grinding, the buhrstone is peculiarly suited.
Color Mill.--The various coloring substances used by painters, when they are not soluble in oil or water, require to be reduced to an impalpable powder by grinding. This is commonly performed upon a smooth stone slab, by trituration with another stone, called a muller. When the grinding is performed by machinery, a large muller of the shape of a pear, having a groove cut in it for the admission of the paint, is made to revolve in a mortar, the bottom of which is of a corresponding shape. In some color mills, a horizontal stone cylinder revolves in contact with another stone, which is concave, and covers a part of its convex surface. In most cases, the substance to be ground is mixed with oil or water. As some of the substances used for pigments are of a poisonous character, they should be ground in close cavities, or under water.
MODES OF UNION.
Insertion.--The mechanical modes of attaching bodies to each other, usually consist in the insertion of their parts among each other, or in the application of other substances specially adapted for the purpose of connection. Insertion is performed by various modes, the principal of which are, 1. Mortising, in which the projecting extremity of one timber is received into a perforation in another. 2. Scarfing and interlocking, in which the ends of pieces overlay each other, and are indented together, so as to resist longitudinal strain by extension, as in tie beams, and ends of hoops. 3. Tongueing and rabbeting, in which the edges of boards are wholly, or partly, received by channels in each other. 4. Dovetailing, when the parts are connected by wedge-shaped indentations, which permit them to be separated only in one direction. 5. Linking, where the ends of flexible rods are bent over each other. 6. Folding, when the edges of flexible plates are connected in a similar manner. 7. To these may be added the combinations of flexible fibres, by tying, twisting, weaving, &c, in which the permanency of the union depends upon friction.
Interposition.--When two substances are mechanically united by the intervention of a third, the latter, from its smaller size, should be made of the strongest material. Nails are a common connecting medium in wooden structures. The stability of a nail depends upon its friction, or adhesion, and is increased by its roughness, the smallness of the angle made by its sides, and the elasticity of the material into which it is driven.
When the force tending to produce separation is great, nails do not afford an adequate security, In such cases, it is common to employ screws, which are inserted by the force of torsion, and cannot be withdrawn by that of extension, while the material is sound. Where great strength is required, bolts of metal are used, which pass through the substances to be connected, and are secured at their smaller extremity by a nut and screw, or by a transverse key. Rivets are short bolts, the two ends of which are headed, or spread by hammering, after they are inserted.
Binding.--In some cases, the materials to be connected are not perforated, but surrounded by the connecting substance. Hoops and bands of metal, wood, and flexible fibres, are used for this purpose. In cases where it is applicable, binding ordinarily affords the strongest mode of connection, but is attended with the greatest expenditure of the connecting material.
Locking.--For the temporary connection of parts, which requires to be often repeated, latches, bolts, hooks, buttons, and locks are employed. Of these, the lock is the only one whose structure is at all complicated. The principle upon which locks depend, is the application of a lever to an interior bolt, by means of a communication from without. The lever is the key, and the bolt receives from it a progressive motion in either direction. The security of a lock depends upon the number of obstacles which can be interposed between the movement of the bolt, and the action of any instrument except the proper key. The wards of locks are impediments of this kind, and to enable the key to pass them, certain portions of its substance are cut away. Various complicated and difficult locks have been constructed by Messrs. Bramah, Taylor, Spears, and others. In a very ingenious lock invented by Mr. Perkins, twenty-six small blocks of metal, of different sizes, are introduced, corresponding to the letters of the alphabet. Out of these, an indefinite number of combinations may be made. The person locking the door, selects and places the blocks necessary to spell a particular word known only to himself, and no other person, even if in possession of the key, can open the door without a knowledge of the same word.
Cementing.--Cements are, for the most part, soft or semifluid substances, which have the property of becoming hard in time, and cohering with other bodies to which they have been applied. A variety of these substances are used for uniting different materials. The compounds of lime and sand, which constitute the ordinary building cements, have been considered in Chapter II. For uniting pieces of marble, plaster of Paris, dried by heat, and mixed with water, or with rosin and wax, is employed. A cement for iron is made by mixing sulphur and muriate of ammonia with a large quantity of iron chippings. This is used for the joints of iron pipes, and the flanges of steam-engines. Turners, and some other mechanics, confine the material on which they are working, by a cement composed of brick-dust and rosin, or pitch. The cement used by glaziers, under the name of putty, is a mixture of linseed oil and powered chalk. China ware is cemented by common paint, made of white lead and oil, or by resinous substances, such as mastic and shell lac, or by isinglass dissolved in proof spirit or water. Bookbinders, and paper hangers, employ paste, made by boiling flour; and a similar, but more elegant article, under the name of rice glue, is prepared by boiling ground rice in soft water to the consistence of a thin jelly. Wafers are made of flour, isinglass, yeast, and white of eggs, dried in thin strata upon tin plates, and cut by a circular instrument. The color is given by red lead, and other pigments. Sealing wax is composed of shell lac and rosin, and is commonly colored with vermilion.
Glueing.--For uniting wood and similar porous substances, common glue takes precedence of all other cements. It is dissolved by heating it with water, and is applied with a brush to both the surfaces to be united. Glue does not adhere so readily, if the surfaces be in the least oily, or if a coating of old glue is previously upon them, or, indeed, if the pores are filled with any foreign substance. The cementing power of glue depends upon the strength which it possesses when dry, and the hold which it obtains upon the wood, by penetrating its pores. It does not furnish a sufficient bond of union for surfaces which are not porous, as those of metals; and it is not durable when exposed to the action of water.
Welding.--Certain metals, such as iron and platinum, which are exceedingly difficult of fusion, are capable of being united by the process of welding. This consists in hammering them together while they are at a very high temperature. Bar iron cannot be welded without raising it to a heat of nearly sixty degrees of Wedgewood's pyrometer. Cast steel would be melted at this temperature, and therefore in welding iron to steel, the steel is raised only to a common white heat. Care is taken to prevent the surfaces which are to be welded from being oxidized too much, or else to detach the scales when the metal is brought to a welding heat. The union of welded pieces probably depends on an incipient fusion of their surfaces. When properly conducted, the metal is supposed to be as strong in the welded part as in any other.
Soldering.--The process of soldering consists in uniting together parts of the same, or of different metals, by the intervention of a metallic substance employed in a state of fusion. It is necessary that the uniting substance should melt sooner than the substance to be soldered, that it should adhere firmly to its surface, and, as far as practicable, approach to the metal soldered, in hardness and color. Iron is usually soldered with brass, and hence the process is commonly called brazing. An alloy of tin and iron is sometimes used instead of brass, for the same purpose. Copper may be united either by a hard solder made of brass and zinc, or a soft solder composed of zinc and lead. Tin is soldered with pewter made of tin and lead, with sometimes a portion of bismuth. Gold and silver are united with solders made of gold or silver, alloyed with copper or brass. Platinum is soldered with gold. The adhesion of solders depends upon an alloy being formed between the surfaces in contact.
As the oxidation of the surface of metals tends to prevent the adhesion of the solder, it is common to unite with the solder some additional substance, which may obviate this difficulty. In soldering copper, brass, iron, &c, it is common to employ borax, a salt which fuses at the time when the metals would be most liable to oxidate, and, by enveloping the metallic surface, prevents the further action of the oxygen of the atmosphere. Potash, soda, tartar, and various salts are used for the same purpose. Muriate of ammonia has a remarkable effect in freeing the surfaces of metals from oxygen, which it does, apparently, by combining with the metallic oxide, and carrying it off as it sublimes. In soldering the more fusible metals, as tin and lead, a carbonaceous substance is employed, such as rosin, or oil, which tends to cover the surface, and also to reduce the oxide to its metallic state, as fast as it is formed.
Casting.--The process of fusion, or melting, affords, in many substances, the most effectual method both of destroying the cohesion of their particles, and of afterwards restoring it under new arrangements. Many substances, both simple and compound, such as metals, glass, wax, &c, may become liquid and again solid, without essentially changing their physical qualities. On the other hand, many natural bodies, crystallized minerals, and organic combinations, cannot be fused without changing their characteristic properties. Some substances are with difficulty fusible when alone, but become more fusible when combined with another substance, as is the case of sand with an alkali, or iron with carbon. Others again have their fusibility lessened by combination, as happens in metals when they become oxidized.
Fluxes.--The name fluxes has been given to certain substances which assist fusion, either by expediting the process, or by protecting the substance melted from alteration. In separating metals from their ores, fluxes are employed, to render the substances with which the metal is combined, capable of fusion. Thus if the ore abound with silicious earth or stone, an alkaline flux, such as potash, soda, or tartar, has the effect of combining with the silicious substances, and forming with them a vitreous compound, which floats upon the top of the melted metal. Tartar also contains a portion of vegetable matter, the carbon and hydrogen of which serve to deoxidize the metal. Borax, common salt, and many other saline bodies, when melted, prevent the oxidation of metals, by protecting their surface from the atmosphere. Muriate of ammonia, rosin, fatty substances, powdered charcoal, &c, prevent or remove oxidation, by combining either with the oxygen, or with the oxide when formed.
Moulds.--The moulds used for casting melted bodies must be suited to the temperature at which the body melts. For metals which melt at a high heat, as copper, brass, cast iron, &c, the moulds are made of some refractory substance, such as loam, sand, pounded brick with plaster, or clay, &c. Glass is cast in moulds made of copper, but these require to be frequently cooled. Those bodies which melt at temperatures below that of ignition, as tin, lead, wax, &c, may be cast in moulds of any convenient metal, or of wood, and other inflammable materials.
The forms of some bodies may be changed, and their separation or union effected, without the agency of fusion, in various ways. It may be done by mixture with water, as in clay and plaster; by solution in water, as in glue, rice, and gum; and by sublimation, as in camphor, and muriate of ammonia.
WORKS OF REFERENCE.--YOUNG'S Lectures on Natural Philosophy;--GREGORY'S Mechanics;--NICHOLSON'S Operative Mechanic, 8vo.;--GRAY'S Operative Chemist, 8vo. 1828;--REES'S Cyclopedia, and BREWSTER'S Edinburgh Encyclopedia, under various heads.
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