the cock W to the valve under the two pistons; and there is now no communication from the barrel with the cavity of the cistern.

In fig. 276, we have the position of a cock when the handle X Y is turned one quarter of a revolution towards the eye from the last-mentioned situation, in which case there is no communication from the barrels with the outer extremity of the sucking-piece, but the water poured into the fore and hind trough, and passing from thence into the cavity of the cistern, enters the cock sideways at W, and turning at right angles through the cock towards E, proceeds to the barrels of the pumps. Fig. 277* represents the cock W when the handle is placed diametrically opposite to its last situation, in which case there is no communication from the under side of the barrels with the cavity of the cistern or the outward end of the sucking-piece; but this situation affords a communication from the cavity of the cistern with the outside of the engine, and the water left in the cavity of the cistern may by this means be employed when the engine has done working. These engines are made of five or six different sizes.

The principles on which this engine acts, so as to produce a continued stream, are obvious: the water being driven into the air-vessel, as in the operation of common sucking and forcing pumps, will compress the air contained in it, and proportionably increase its spring, since the force of the air's spring will be always inversely as the space which it possesses; therefore when the air-vessel is half filled with water, the spring of the included air, which in its original state counterbalanced the pressure of the atmosphere, being now compressed into half the space, will be equal to twice the pressure of the atmosphere; and by its action on the subjacent water will cause it to rise through the conduitpipe, and play a jet of 32 or 33 feet high, abating the effect of friction. When the air-vessel is two-thirds full of water, the space which the air occupies is only one-third of its first space; therefore its spring being three times as great as that of the common air, will project the water with twice the force of the atmosphere, or to the height of 64 or 66 feet. In the same manner when the air-vessel is three-fourths full of water the air will be compressed into one-fourth of its original space, and cause the water to ascend in air with the force of three atmospheres, or to the height of 96 or 99 feet, &c. as in the following table:

2. The fire-engine by Rowntree is a double force-pump, of a peculiar construction, similar in its action to the beerengine, but as it is on a much larger scale, its constructions. are of course varied.

In this engine, figs. 278 and 279 are two elevations at right angles to each other, of the external part of the engine, mounted on four wheels. Figs. 280 and 281 are two sections perpendicular to each other of the body of the engine or pump; figs. 282 and 283 are parts of the engine. The same letters are used as far as they apply in all the figures; A A A A figs. 280 and 281, is a cast-iron cylinder truly bored, 10 inches diameter and 15 long, and having a flanch at each end whereon to screw two covers, with stuffing-boxes a a, in their centres, through which the spindle, B B, of the engine passes, and being tight packed with hemp round the collar, makes a tight joint; the piston D is affixed to the spindle within the cylinder, and fits it tight all round by means of leathers; at E, fig. 281, a partition, called a saddle, is fixed in the cylinder, and fits against the back of the spindle tight by a leather.

We have now a cylinder, divided by the saddle E and piston into two parts, whose capacity can be increased and diminished by moving the piston, with proper passages and valves to bring and convey the water; this will form a pump. These passages are cast in one piece with the cylinder: one, d, for bringing the water, is square, and extends about one-third round the cylinder; it connects at bottom with a pipe e; at its two upper ends it opens into two large chambers fg, extending near the whole length of the cylinder, and closed by covers, h h, screwed on; ik are square openings (shown by dotted squares in fig. 280) in the cylinder communicating with the chambers; I m in f g are two valves closing the ends of the curved passage d, and preventing any water returning down the passage d; no are two passages from the top of the cylinder to convey away the water; they come out in the top of the cylinder, which, together with the top of the chambers fg, form a flat surface, and are covered by two valves, pq, to retain the water which has passed through them. A chamber, K, is screwed over these valves, and has the air-vessel k, figs. 278 and 279, screwed into its top; from each side of the chamber a pipe, w w, proceeds, to which a hose is screwed, as shown in fig. 280. Levers, xx, are fixed to the spindle at each end, as shown in fig. 279, and carry the handles H H, by which men work the engine. When the piston moves, as shown by the arrow in fig. 281, it produces a vacuum in the chamber ƒ, and that part of the cylinder contiguous to it, the water in the pipe e then opens the valve m, and fills the cylinder.

The same motion forces the water contained in the other part of the cylinder through the valve q, into the chamber K, and thence to the bose through the pipe w; the piston being turned the other way reverses the operation with respect to the valves, though it continues the same in itself, The pipe e is screwed by a flanch to an upright pipe P, fig. 282, connected with another square iron pipe, fastened along the bottom of the chest of the engine; a curved brass tube, G, comes from this pipe through the end of the chest, and is cut into a screw to fit on the suction-hose when it can be used; at other times a close cap is screwed on, and another brass cap at H, within the chest, is screwed upwards on its socket, to open several small holes in it, and allow the water to enter into the pipe; in this case the engine-chest must be kept fuli of water by buckets. The valves are made of brass and turn upon hinges. The principal advantage of the engine is the facility with which it is cleared from any sand, gravel,

or other obstructions, which a fire-engine will always gather when at work.

The chambers fg, being so large, allow sufficient room to lodge a greater quantity of dirt than is likely to be accumulated in the use of the engine at any one fire, and if any of it accidentally falls into the cylinder, it is gently lifted out again into the chambers, by the piston, without being any obstruction to its motion; to clear the engine from the dirt, two circular plates of five inches diameter, are unscrewed from the lids kk, of the chambers ƒ g, and when cleaned are screwed on again; these screw-covers fit perfectly tight without leather, and can be taken out, the engine cleared, and enclosed again in a very short time, even when the engine is in use, if found necessary.

The two upper valves pq, and chamber K, can also be cleaned with equal ease, by screwing out the air-vessel kk, fig. 278, which opens an aperture of five inches, and fits air-tight, without leather, when closed. The valve may be repaired through the same openings. The use of the air-vessel, kk, figs. 278 and 279, is to equalize the jet from the engine during the short intermittance of motion at the return of the piston-stroke; this it does by the elasticity of the compressed air within it, which forces the water out continually, though not supplied quite regularly from the engine.

The engine from which the drawing was taken, was constructed for the Sun Fire Insurance Company, in London, and from some experiments made by their agent, Mr. Samuel Hubert, appears to answer every purpose.

JACKS.

THE jacks which we purpose here to describe are simple machines used for raising heavy weights.

Fig. 341 represents the common or simple hand jack; a block of wood about two feet six inches long, 10 inches broad, and six inches wide, is perforated with a square hole or mortise through it lengthwise for the reception of an iron rack B. This rack is formed with a double claw or horn at its upper end. A small pinion C is made to engage in the teeth of the rack. The axis of the pinion is supported in iron plates bolted to each side of the block, and one end of the axis projects through the side plate, with a square to receive a winch or handle, which, being turned round, the pinion elevates the rack B in the mortise, and raises the claw or horns up to the load to which it is applied. To prevent the weight of the load running the pinion back, the handle is detained by a hook or link a, fastened to the outside of the block.

When a greater power is required than the simple rack and pinion are capable of exerting, a combination of wheel-work is used, as shown in the same figure, where A A is the block of wood, which in this case is made sufficiently wide to contain the cog-wheel F, fixed to the pinion C, which acts in the teeth of the rack B. G is a second pinion of four leaves, working in the wheel F; and the axis of this pinion projects through the side of the block for the winch H to be fixed on it. The block A A is made in two halves, and the recess for the wheel F, and the pinion G, is cut out in one of the halves; the other, being laid flat against it, supports the front pivots

of the wheel and pinions. The two halves are bound together by strong iron hoops b b, driven over the outside. The rack has a claw N, at its lower end, projecting out sideways through an opening or slit cut through in the front half of the block. This claw can be introduced beneath a stone which lies nearly flat upon the ground, and which consequently could not be acted upon by the claw on the top of the rack. To prevent the rack descending when it has a load upon it, the small click a drops into its teeth, but clears it in going up; when it is not required to detain the rack, this click can be turned out of the way sideways.

Fig. 342 is a screw jack. The block of wood A A is perforated nearly its whole length with a hole sufficiently large to allow the screw B to move up and down without touching. The screw passes through a nut n, fixed into the top of the block A; and if the screw is turned round, it must rise up through the nut, and elevate the claw F. This claw is fitted on the top of the screw with a round collar, which allows the screw to turn round without turning the claw; and the claw N, which projects through a groove or opening made in the side of the block, is fitted to the screw with a smaller collar. The bottom of the block has four short points to prevent the machine slipping when used upon hard ground. To give motion to the screw, the lower half of it is formed into a square, and a worm-wheel C is fitted upon the square. The teeth of this wheel are engaged by a worm on the axis of the winch H, and plates of iron, a b, are bolted on each side of the block, near the middle of its height, to carry the ends of the axis of the winch and of the worm which is concealed by the worm-wheel C. When the winch is turned round, it causes the wheel C to revolve by the action of the worm in its teeth; and as the wheel is fitted on the square part of the screw, it compels it to turn with it, but at the same time allows the screw to move up and down. Jacks have been also constructed upon the hydrostatic principle discovered by Pascal, and which has been applied to practice by the late Mr. Bramah, in this and various other useful machines.

CRANES.

CRANES are certain simple machines in which either the wheel and axle, or wheel and pinion, are introduced, to effect the raising of heavy loads, such as the loading or unloading of shipping at the quays or wharfs, or the raising or lowering goods to and from chambers or warehouses.

Various modes have been adopted to turn the wheel, or that part of the machine which is applied to the same purpose, by introducing long staves into the axle, by which it acquires the name of a capstan, or windlass; or by a rope passing over the wheel, and putting it and the axle in motion by friction. Other methods have been adopted, such as forming the wheel hollow, and causing it to revolve by means of labourers inside of it, walking up its sides, which consequently descends beneath their weight; or by forming it into a platform, lying in a slanting direction, and the labourers pushing

against a fixed arm, which forces the platform or wheel round under their feet.

Most of the cranes constructed with the wheel and axle occupy too much space, which is of importance, and consequently, where cranes are in general use, have been superseded by the wheel and pinion, which is of a more compact and convenient construction. The wheel and pinion is generally accompanied with a ratchet-wheel and pall, or some other method of locking the handle, so that should the labourer desist from his exertion, the load may not return to the place whence it has been raised.

The frame-work, or that part of the crane which does not immediately operate to raise the load, is divided into three parts, the post, the jib, and the stay. The post is the upright piece, almost universally made to turn on a centre; the jib is the arm extending from the upper part of the post, and in some cases is horizontal, but more frequently at an angle to the horizon; and the stay is that piece which supports the jib, reaching from the lower part of the post to nearly the extremity of the jib.

The most simple form of the crane is that commonly used in stone and timber wharfs for unloading vessels, for which purpose it is well adapted, its power being very great. It has a frame consisting of a strong beam supported horizontally at 10 or 12 feet from the ground, on the top of several vertical posts very firmly fixed in the ground, and securely braced with stays in every direction. At the extremity of the horizontal beam the upper part of the jib is supported, the lower pivot resting on a post in the ground. The jib, or gibbet, as it is called, from a resemblance to that machine, is a triangular frame of wood, one side being perpendicular, and supported on pivots at the top and bottom, so that the whole moves round on these as a vertical axis of motion. Near the upper end of the perpendicular post, a beam proceeds, forming the upper side of a triangle, while the third side is a brace, extended from the foot of the perpendicular, to support the upper piece. From the extremity of the latter, the burden is suspended by a rope passing over a pulley; the other end of the rope is coiled round a vertical roller, or capstan, turning on pivots, one supported by the horizontal beam first mentioned, and the other on a post in the ground. The capstan is turned round by means of long horizontal levers fixed to it, at which a great number of men may be employed to push them round, or in some cases they are drawn by horses. As the levers admit of a very great