shape of the piston-rods, and the size and situation of the chains that give them motion, are so contrived, that the vertical axis of the pistons is exactly in the middle of the breadth of the perpendicular part of the chains, and the upper part of the piston-rod taken together. PQ represents one of the two cross bars through the ends of which pass the long handles to which the men apply their hands when they work the engine; these cross bars are fitted on the middle bar at some distance from the sectors. The other parts of this useful engine may be understood by the help of fig. 3, which represents a vertical section taken through the middle line of the hind part of the engine, as also the section of the air-vessel, and that of one of the barrels, and likewise the profiles of the hind sectors, and of several other parts. A B is the section of the bottom of the cistern, and C that of the hindmost axle-tree. DE is the vertical section of a strong piece of cast brass or hard metal so worked as to have a hollow in it, represented by the white part, and fixed to the bottom of the cistern: this reaches from the opening D through the cock W, and afterwards divides itself into two branches, so as to open under the two barrels; one of these branches is exhibited in the figure and the other is exactly behind this. Through this channel, which may be called the sucking-piece, water is conveyed to the pumps by the pressure of the atmosphere, either from the cistern itself, or from any place at a distance, by means of a leathern pipe, F. fig. 4, which screws on to the suckingpiece at D, fig. 3, under the hind trough Z, the grate of which is represented by the horizontal strokes. FG represents the vertical section of another piece of cast brass or hard metal that may be called the communication-piece, having two hollows for conveying the water from under the two pistons to the two openings of the flanch of the air-vessel; one of these hollows appears in the figure; the other lies exactly behind this, though not in a parallel direction. Between the section of the sucking-piece D E, and that of the communication-piece FG, may be observed the section of one of the plates of leather, which makes all tight, and forms one of the two sucking-valves, of which there is another just behind this under the other barrel. RST is the section of the copper air-vessel, and TV that of the conduit-pipe; this vessel is screwed on to the hind part of the communication-piece, and at top is fastened by a collar of iron to a cross piece of

timber.

Between the flanch of the air-vessel and the communication-piece may be observed the section of one of the plates of leather, making all tight, and screwing one of the two forcing valves, of which there is another just behind this, exactly over the other opening of the communication from the air-vessel. These valves are loaded with a lump of cast iron or lead, having a tail or teat let through the flap of the valve and crosspinned under it; and it is to be observed that, though both the valves are represented open in the figure, they are never both open at the same time; for when the engine is not at work they are closed down by the weights on their upper surfaces; and, when the engine works, two are

shut, and the other two are open alternately by the motion of the pistons and the action of the atmosphere, together with the re-action of the air contained in the air-vessel. HI is the section of one of the barrels of the two pumps, which are both sucking and forcing, as is evident from the position of the valves and the structure of the pistons, each of which is composed of two iron plates, of two wooden trenchers, and of two flat pieces of leather turning one up and the other down. LK represents one of the piston-rods edge-wise, behind which is one of the chains, the top screw of which, K, can only be seen. M is the end of the middle bar, and N a section of the hindmost of the two middle stands which support the middle bar.

The principle on which the common engine acts, so as to produce a continued stream, is obvious; the water being driven into the airvessel, 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 always be 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 conduit-pipe, and to play a jet of thirty-two or thirty-three feet high, abating the effect of friction. When the air-vessel is twothirds 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 coinmon air, will project the water with twice the force of the atmosphere, or to the height of sixty-four or sixty-six 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 ninety-six or ninety-nine feet, &c., as in the following table.

more is necessary to ascertain the quantity of water they will deliver, than to calculate the solid or cubical contents of that part of the barrel in which the vacuum is produced, and to reduce this to some standard measure, and then to multiply this by the number of strokes made in a given time; thus, if a pump is nine inches diameter, and makes an effective stroke of about eighteen inches, such cylinder will be found to contain about 1134 cubic inches; and, as 282 cubic inches make a gallon, so four gallons will be equal to 1128 cubic inches, consequently such a barrel will contain and throw out rather more than four gallons at every stroke; and supposing this pump to make ten strokes in a minute, it would yield forty gallons in a minute, or sixty times that quantity in an hour, and so on. This rule applies in every case, whether the water is sent to a small or great elevation, because the piston cannot move without displacing the water in the barrel; but a small allowance must be made for leakage or waste, because some water will constantly pass the piston and escape, or be otherwise lost and wasted.

This mode of calculation, as before observed, only applies to such pumps as have cylindrical working barrels and pistons; but sometimes pumps are otherwise constructed, of which the fire-engine of the late Mr. Bramah, and the executric pump are instances. In the former of these contrivances, the working barrel, instead of being an entire cylinder, is a semi-cylinder, and lies horizontally, while the place of a piston is supplied by a parallelogram of the same radius and length as the semi-cylinder, moving by an iron bar passing through its axis and properly packed at its exterior edges. This parallelogram is made to vibrate through about 170° by its handles, while its outer edges keep in contact with the interior surface and ends of the semicylinder, and two feeding and two delivering valves are placed upon the flat top or covering of the whole. This pump, therefore, in effect is the same as that of M. De la Hire, though quite different in form, and its mode of operation is nearly allied to the excentric pump, a section of which is shown at fig. 5, HYDRAULICS. It consists of a hollow drum or cylinder of metal a d, in the interior of which a solid cylinder b, of the same length, but of only half the diameter or thereabouts, is made to revolve by its axles passing through water-tight stuffing-boxes in the sides of the larger and exterior cylinders. The internal cylinder does not revolve in the centre of the larger, but is so placed that one of its convex exterior edges may come into close contact with some one part of the concave or internal surface of the larger cylinder, as shown in the figure, and the circular exterior surface of the small cylinder is equipped with four metal flaps or valves, c, c, c, c, turning on hinges, and partaking of its own curvature, so that when they are shut down or closed they form no projections, but appear as parts of the same cylinder. These flaps are made to open either by springs placed underneath them, or, what is still better, by two cross wires, sliding through the internal cylinder in such manner that they may cross each other exactly in its centre, by which their operation

will be rendered equable in every part of their revolution. From the formation of this machine, when one of these flaps is brought by the revolution of the internal cylinder between itself and the external one, it will be pressed down close and will shut, but, as the inner cylinder moves, it will be carried into a continually widening space until it arrives at d, opposite to the lastmentioned situation, when the cavity formed between it and the smaller and larger cylinder will have so far increased as to form a vacuum which is filled with water by the feeding pipe e. This cavity is no sooner so increased to its largest dimensions than it is diminished by a continuation of the revolution, in consequence of which the water, being pent up and squeezed into less compass, makes its escape by the delivering pipe f; and, as each of the flaps performs the same operation in its turn, this pump affords a very equable and constant supply of water. The greatest difficulty in its construction is that of keeping the sides of the flaps so packed as to maintain a perfect contact with the sides of the larger cylinder without unnecessary friction, a fault which equally holds good in Mr. Bramah's fire-engine, in all excentric pumps, and in all the rotatory steam-engines that have yet been invented. The excentric pump is of the lift and force variety, since it will deliver water to an indefinite height above its working cylinders. In order to determine the force or power necessary to work a pump of any description, the height to which the water is to be raised must always be taken into account; for this height multiplied into the area of the piston, and reduced to any of the usual denominations of weight, will give the amount of resistance to be overcome (friction of the pump only excepted). The size of the pipe containing the water is quite immaterial, provided it be large enough to prevent friction and an unnatural velocity in the water; and the entire perpendicular height from the surface of the water raised to the point where it is delivered, whether occupied by suction or feeding pipe, or delivering pipe from a forcing pump, must be added together and considered as the height of the lift; so that if a lift and force pump of four inches diameter in the working barrel has ten feet of three-inch suction pipe below its piston, and twenty feet of two-inch delivering pipe (including the length of the working barrel) about it, the column to be lifted will be equal to thirty feet of four-inch pipe filled with water. The contents in gallons of thirty feet of four-inch pipe must therefore be found, and, as each gallon of water weighs about 10-2 pounds, the weight or load upon the pump will be immediately found, to which must be added from one-tenth to onesixth, according to the construction of the pump, for friction. The load upon an excentric or any other pump may be found by the same rule if the effective horizontal area of the piston, or its substitute be found, and this be, in like manner, multiplied into the height of the lift.

The Society of Arts voted a silver medal and twenty guineas to Mr. Furst, in consideration of the utility of a contrivance produced by him, and of which trial was made, for increasing the effect of engines for extinguishing fires: a com

plete model remains in the Repository of the Society, of which the following is a short description:-From a platform rises an upright pole or mast of such height as may be judged necessary; up this pole or mast slides a gaft, and along the upright pole and gaft the leather hose from the engine is conveyed; at the extremity of the gaft the branch of the engine projects; towards this extremity is fixed an iron frame whence hang two chains, and from them ropes serving to give an horizontal direction to the branch, whilst other ropes running through proper pullies, and being thus conveyed down the mast, serve also to communicate a vertical motion to it; by these means the branch or nose pipe of the engine is conveyed into the window of any room where the fire more immediately rages, and the effect of the water discharged therefrom applied in the most efficacious manner to the extinguishing

of it.

Mr. Perkins's method of fastening the seams of hose for fire-engines, and connecting two or more lengths together, consists in rivetting, instead of sewing it; and in connecting the hose with a new modification of the swivel joints, in such a manner as not to contract the water way at the joints. The first idea of rivetting hose belongs to Messrs. Pinnock and Sellers, of Phiadelphia, and has been in successful practice for some years, but without the leathers being overlapped sufficiently. The method of conecting the hose belongs to Mr. Perkins. The advantage of rivetting over sewing is, that the sam lasts much longer, and is much tighter. The rivets, which should be made of copper, vill last four or five times longer than the best thread. If care is taken to have sufficient overap, the pressure of water against the overlap cts as a valve to tighten the seam. It has been found by experience, that the portion of hose text the engine is much the most likely to burst, especially when the water is carried perpendiularly; to obviate this difficulty, the first, tard, or fourth portions are double-rivetted.

When a rivet breaks, it is replaced by making 23 opening in the seam of sufficient size to allow the hand to replace not only the broken rivet, nt the rivets taken out to enlarge the opening. After the rivets are fixed in the holes, they are rivetted by placing them on a flat bar of iron, troduced into the entrance of the hose, and apable of being removed at pleasure. Copper has been found to answer best. It is of such importance that the rivet and burr should be of the same material, that it would not answer well to have the rivets of cast copper (they being an alloy of tin and copper), and the burr of wrought copper. Tin rivets, with copper burrs, will completely destroy the leather in a few months, occasioned, undoubtedly, by the operation of galvanism.

Many have been the attempts to produce Some machine by means of which fires might be more rapidly extinguished than by the common application of water. These machines have been principally constructed with a view to exploding, and thus driving the liquids more forbly to the fire. The first person who attempted this with any tolerable degree of success

was

Zachary Greyl. He contrived certain engines, easily manageable, which he proved to be of sufficient efficacy, and offered to discover the secret by which they were contrived, for a large premium given either from the crown, or raised by a subscription of private persons. The secret was this: a wooden vessel was provided holding a very considerable quantity of water; in the centre of this there was fixed a case made of iron plates, and filled with gunpowder; from this vessel, to the head of the larger vessel containing the water, there proceeded a tube or pipe, which might convey the fire to the gunpowder in the inner vsssel. This tube was filled with a preparation easily taking fire, and quickly burning away; and the manner of using the engine was to convey it into the room or building where the fire was, with the powder in the tube lighted. The consequence of this was, that the powder in the inner case soon took fire, and, with a great explosion, burst the vessel to pieces, and dispersed the water every way: thus was the fire put out in an instant, though the room was flaming before in all parts at once. In our own country a chemist of the name of Godfrey, has brought forward similar machines which he called water-bombs. They were however so very similar to Greyl's as to need no further description, except that instead of water Godfrey used a medicated liquor, probably sal-ammoniac and water. But though these machines will prevent great fires by a timely application, they will not extinguish them after they have reached a frightful height, and several houses are in flames. The floors must be standing, and access to the building safe, otherwise no person can be supposed to approach near enough to apply them in a proper manner. Every fire has its beginning for the most part in some apartment; and, as soon as discovered, the family should immediately apply one or more of these machines, which will then fully answer the intention. In 1761 Mr. Godfrey's experiment for extinguishing fire, was tried in a house erected for that purpose near Mary-le-bone. The then duke of York, prince William Henry, prince Henry Frederic, and a great number of persons of rank, gave their attendance on this singular occasion. The house, which was of brick, consisted of three rooms one above another, a staircase, chimney, lath and plaster ceilings, and a kind of wainscotting round the rooms, of rough deal. Exactly at twelve o'clock the ground room, and that of one pair of stairs, were set on fire, by lighting the faggots and shavings laid in there for that purpose. In about fifteen minutes the wainscot of the under room thought to be sufficiently in flames, and three of the machines were thrown in; which, by almost immediate and sudder explosions instantaneously extinguished the flames, and the very smoke in that apartment in a few minutes totally disappeared. By this time, the firemen, &c., who had the care of throwing in the machines, gave an alarm that the stair-case had taken fire, and that it was necessary directly to go to work upon the next room, which was accordingly done, and with the same effect. The experiment however hitherto did not universally satisfy, in the last

was