and trigger shut up in a brass case, and the trigger pulled, or the discharge made by pulling the chain. In this form of the machine, there is a round chamber for the condensed air; and it has a valve acting in a similar manner to that of the copper ball.

The AIR-GUN is a pneumatic instrument for the projection of bullets by the condensation of air. The ancients were certainly acquainted with the principles of its construction, as well as the general properties of air. Aristotle speaks of all the elements having weight, with the exception of fire; and Plutarch and Strobœus represent him as having taught, that the weight of air was between that of fire and earth. The last philosopher also quotes Empedocles, as attributing the action of respiration to the pressure of air, by which it insinuates itself into the lungs. Hero, of Alexandria, in his works, Spiritalia, applies the principle of the elasticity of the air to explain various phenomena, and in such a way as sufficiently to prove that he was no stranger to its common properties. Ctesibus adopting the principle of electricity, as well known, constructed wind-guns,' between which, and the modern air-gun, there was only this difference, that in the former the ball was not immediately exposed to the action of the air, but was impelled by the longer arm of a lever, while the air acted on the shorter; but the principle of operation is the same in both, and shows clearly that the elastic property of common air, if it could not be accurately measured, was at least well known at that time. The first modern account of an air-gun which we meet with, is in the Elemens d'Artillerie of David Rivaut, who was preceptor to Louis XIII. He ascribes the invention to Marin, of Lisieux, who presented one to Henry IV. of France.

The elasticity of condensed air has been estimated as equal to about 1000 times that of common air: admitting, therefore, this to be correct, (although there seems to be great reason to suppose it to be under-rated) it would be necessary that air should be condensed 1000 times more than in its natural state, to produce the same effect as gunpowder. There is, however, this important consideration to be attended to, viz. that the velocities with which equal balls are impelled, are directly proportional to the square roots of the forces; so that if the air in an air-gun be condensed only ten times, the velocity with which it will project a ball will be one-tenth of that arising from gunpowder; and if the air were condensed twenty times, it would communicate a velocity of one-seventh of that of gunpowder, and so on.

But air-guns project their balls with a much greater proportion of velocity than that stated above, for this reason, that as the reservoir, or magazine of condensed air, is commonly very large in proportion to the narrow tube which contains the ball, its density is very little altered by expanding through that tube, and, consequently, the ball is urged all the way by nearly the same uniform force as at the first instant; whereas the elastic fluid arising from inflamed gunpowder, is but very small in proportion to the tube or barrel of the gun, occupying, indeed,

but a very small portion of it next the butt-end; and, therefore, by dilating into a comparatively large space as it urges the ball along the barrel, its elastic force is proportionally weakened, and it acts always less and less on the ball in the tube. Whence it happens that air condensed into a pretty large machine only ten times, will project its ball with a velocity but little inferior to that given by gunpowder; and if the valve of communication be suddenly shut again by a spring, after opening it to let some air escape, then the same charge may serve to impel several balls in succession. In all cases where a considerable force is required, and, consequently, a great condensation of air, it will be requisite to have the condensing syringe of a small bore, perhaps not more than half an inch in diameter; otherwise the force requisite to produce the compression will become so great, that the operator cannot work the machine; for, as the pressure against every square inch is about 15 lbs., and against every circular area of an inch diameter, 12 lbs.; if the syringe be an inch in diameter, it will require a force of as many times 12 lbs. as the density of the air in the receiver exceeds that of the common atmosphere; so that when the condensation is ten times, the force required will be 120 lbs.; whereas, with a halfinch bore, it will only amount to 30 lbs.

The common air-gun is made of brass, and has two barrels; the inside barrel A, plate II. fig. 10, which is of a small bore, from whence the bullets are exploded; and a large barrel, ECDR, on the outside of it. There is a syringe, SMP, fixed in the stock of the gun, by which the air is injected into the cavity between the two barrels through the valve EP. The ball K is put down into its place in the small barrel, with the rammer, as in any other gun. At SL is another valve, which, being opened by the trigger O, permits the air to come behind the bullet so as to drive it forwards. If this valve be opened and shut suddenly, one charge of condensed air may be sufficient for several discharges of bullets; but if the whole air be discharged on one single bullet it will drive it out with a greater force. This discharge is effected by means of a lock, fig. 13, placed as in other guns; for the trigger being pulled, the cock will go down and drive the lever O, fig. 10, which will open the valve, and let in the air upon the bullet K.

An improvement of this air-gun has been made by Dr. Ellis, in which the chamber containing the condensed air is not in the stock, but in five or six hollow copper balls, about three inches in diameter. These are fitted to a screw on the lock of the gun, and are so furnished with valves as to retain at pleasure the air forced into their cavities, whereby they may be carried about from place to place, ready charged with condensed air, and the gun itself is rendered as light and portable as the smallest fowling-piece.

The best construction, of modern times, has, however, been suggested by the late Mr. B. Martin, and is represented fig. A is the barrel with the lock, stock, ram-rod, &c. of the size and weight of a common fowling-piece. Under the lock at b, is a round steel tube, having a small moveable pin in the inside, which is pushed out when the

trigger a is pulled, by the spring work within the lock; to this tube, a hollow copper ball c, screws perfectly air-tight. This copper ball is fully charged with condensed air by the syringe, B fig. 9, previous to its being applied to the tube b of fig. 11. It is then evident that if a bullet be rammed down in the barrel, the copper ball screwed fast at b, and the trigger a be pulled, that the pin in b will, by the action of the spring work within the lock, forcibly strike out into the copper ball, and thereby in pushing suddenly a valve within the copper ball, let out a portion of the condensed air; which air will rush up through the aperture of the lock, and forcibly act against the bullet, driving it to the distance of sixty or seventy yards, or further. If the air is strongly condensed, at every discharge only a portion of the air escapes from the ball; therefore, by recocking the piece, another discharge may be made; and thus repeated to the amount of fifteen or sixteen times. An additional barrel is sometimes made, and applied for the discharge of shot, instead of the one above described. The air in the copper ball is condensed by means of the syringe, B, fig. 9, in the following manner. The ball c is screwed quite close on the top of the syringe. At the end of the steel-pointed rod a, is a stout ring through which passes the rod k: upon this rod the feet should be firmly set; then the hands are to be applied to the two handles ii, fixed on the side of the barrel of the syringe. Now by moving the barrel B steadily up and down on the rod a, the ball c will become charged with condensed air; and it may be easily known when the ball is as full as possible, by the irresistible action that the air makes against the piston when you are working the syringe. At the end of the rod k is usually a four-square hole, which, with the rod, serves as a key to fasten the ball c fast on the screw b of the gun and syringe, close to the orifice in the ball c. In the middle is fixed a valve and spring which gives way for the admission of air; but upon its emission comes close up to the orifice, shutting up the internal air. The piston rod works air-tight by a collar of leather on it, in the barrel B; it is therefore plain, when the barrel is drawn up, the air will push in at the hole h. When the barrel is pushed down, the air therein contained will have no other way to pass from the pressure of the piston but into the ball c at top. The barrel being drawn up, the operation is repeated, until the condensation is so strong as to resist the action of the piston. Sometimes the syringe is applied to the end of the barrel C, see fig. 12, the lock and trigger shut up in a brass case d; and the trigger pulled, or discharge made, by pulling the chain b. In this contrivance there is a round chamber for the condensed air at the end of the syringe at e, and it has a valve acting in a similar manner to that of the copper ball. When this instrument is not in use, the brass case d is made to slide off, and the instrument then becomes a walking stick; from which circumstance, and the barrel being made of cane, brass, &c. it has received the appellation of air-cane. The head of the cane unscrews and takes off at a, where the extremity of the piston rod in the barrel is shown: an iron rod is placed in a ring at the end of this,

and the air condensed in the barrel in a similar manner to that of the gun as above; but its force of action is not near so strong and permanent as that of the latter.

M. L. Colbe's magazine air-gun is also an improvement of the common air-gun. By his contrivance ten bullets are so lodged in a cavity; near the place of discharge, that they may be drawn into the shooting barrel, and successively shot so quickly, as to be nearly of the same use with so many different guns; the only motion required, when the air has been previously injected, being that of shutting and opening the hammer, and cocking and pulling the trigger. In fig. 18 is exhibited a section of the gun, as large in every part as the gun itself; and so much of its length is shewn as is necessary to give a complete idea of the stock; G is the end of the injection syringe, with its valve H; opening into the cavity, FFFF between the barrels. KK is the small or shooting barrel, which receives the bullets, one at a time from the magazine, ED, which is a serpentine cavity, wherein the bullets, b, b, &c. are lodged, and closed at the end, D. The circular part, SI sk Mi, is the key of a cock, having a cylindrical hole, I K, through it, equal to the bore of the small barrel, and forming a part of it in the present situation. When the lock is taken off, the several parts, Q, R,T, S, W, &c, come into view, by means of which the discharge is made by pushing up the pin, Pp, which raises and opens a valve, V, to let in the air against the bullets, I, from the cavity, FFF; which valve is immediately shut down again by means of a long spring of brass, NN. This valve V, being a conical piece of brass, ground very true in the part which receives it, will of itself be sufficient to confine the air. To make a discharge, pull the trigger, ZZ, which throws up the seer, yx, and disengages it from the notch, r; upon which the strong spring, W W, moves the tumbler, T, to which the cock is fixed. The end, u, of this tumbler bears down the end, v, of the tumbling lever, R, which, by its other end, m, raises the flat end, l, of the horizontal lever, Q, by which means the pin, Pp, is pushed up, and opening the valve, V, discharges the bullet; all which is evident from a bare view of the figure.

To bring another bullet instantly to succeed I, there is a part, H, called the hammer, represented in fig. 19, and fig. 20, which by a square hole goes upon the square end of the key of the cock, and turns it about, so as to place the cylindric bore of the key I k, in any situation required.

Thus, when the bullet is in the gun, the bore of the key coincides with that of the barrel KK; but when it is discharged, the hammer, H, is instantly brought down to shut the pan of the gun; by which motion the bore of the key is turned into the situation ik, so as to coincide with the orifice of the magazine; and upon lifting the gun upright, the ball next the key tumbles into its cavity, and falling behind two springs, ss, fig. 18, is by them detained. Then lifting the hammer again, the ball is brought into its proper place, near the discharging valve, and the bore of the key again coincides with that of the shooting-barrel. It appears how expeditious >

method this is of charging and discharging a gun; and if the force of condensed air was as great as that of gunpowder, such an air gun would actually answer the end of many guns, and prove the best defence against highwaymen or robbers; because, when there is reason to suspect them, they might then make five or six discharges before the robber could come within pistol-shot.

AIR-JACKET, a sort of jacket made of leather, in which are several bags, or bladders, composed of the same materials, communicating with each other. It is designed for the use of bathers and young swimmers. The bags or bladders are filled with air through a leather tube, having a brass stop-cock accurately ground at the extremity, by which means the air blown in through the tube is confined to the bladders. The jacket must be wet before the air be blown into the bladders, as otherwise, it will immediately escape through the pores of the leather. By the help of these bladders, which are placed near the breast, the person is supported in the water.

The AIR-LAMP is formed by a combination of inflammable air and electricity and although depending upon principles different from the foregoing instruments, is generally classed with pneumatic machines. The construction of it is as follows:

A, fig. 57, is a glass jar, containing inflammable air, B an open glass urn holding water, by the pressure of which, the air is forced out of the jar A, through the brass pipe a; C is the stop-cock, so perforated that the water may descend from B into A, and the air pass out through the pipe a. By turning the bar of the stop-cock into a horizontal position, the communication between the two vessels is cut off, and the passage of the air obstructed; but by placing it in a vertical position, the communication is again opened. The lower jar A is supplied with inflammable air by means of the bladder, fig. 58, and two bladders of this kind are attached to each lamp. The method of using the machine is as follows: Take off the cover D from the lamp, and turn the stop-cock upwards, then pour as much clean water into it as will fill the vessel A up to the pipe a, unscrew this pipe, and put in its place the small brass piece W, and to this screw one of the stop-cocks and bladder, fig. 58.

With the bladder under one arm, and with one hand applied to the cock at C, while the other is directed to the bladder, press the latter, and at the same time open the apertures; by which means, the air will be forced upon the water in A, and driven up the glass pipe through the tube into B. When the vessel A is thus charged with air, the stop-cocks are to be turned, so as to cut off the communication with the external air; and care must be taken that the common atmospheric air does not mix with the inflammable; for, if this takes place, the explosion would be great, and might be attended with unpleasant circumstances.

For lighting the lamp the electric fluid is employed, the apparatus being as follows: The wooden base EF, fig. 59, is a sort of box, about twelve inches square, and five inches deep; and in this is placed an electrophorus, consisting of a

resinous cake c, and metallic plate d, which, by a hinge at its back, admits of being pulled upwards and downwards by the silken string b, connected both with it and with the stop-cock C. When this cake is once excited, its electric effect upon the metallic plate will be continued a long time. A metallic chain G, communicates with a wire and ball, passing through a brass tube below, in the box over the plate; and above, with a fine wire passing through a glass tube. This upper wire is bent to about one-eighth of an inch distance from the flame-pipe. It is evident that when the electrophorus in the box is previously excited, and the stop-cock C turned, the silken string 6 will raise the metallic plate; and this will give an electric spark to the ball and wire above, which will convey it instantly to the flame-pipe, and inflame the air issuing out of the pipe, in consequence of the pressure of the water in its descent into the vessel A. The cock C being turned back, the flame ceases; and turned again, appears; and will serve to light a candle, match, &c. whenever it may be thought proper.

The number of times in which light may be produced will be very great, depending on the quantity of inflammable air in the vessel A. If the cock be not turned back, the flame will continue till the whole of the inflammable air is consumed. The light thus produced will be sufficient for reading a large print in the night, or seeing the hour by a watch. When the electrophorus is to be excited, the silken string b is unhooked from the plate, and the apparatus taken out of the box; and the metallic plate is lifted up, whilst with a silken string, or dry cat-skin rubber, you may briskly rub the surface of the resinous cake. About 20 revolutions in rubbing will be sufficient, so that the plate will give spark to the knuckle about the distance of an inch; and by the strength of the spark the degree of excitation is to be estimated. The silken string and small glass tubes, through which the wire G passes, should always be very dry, that the passage of the electrical spark may be quite perfect. The whole length of this apparatus is about 22 inches; but it may be made of any dimensions. Dr. Ingenhousz used a small apparatus, constructed upon a similar principle, in obtaining light for domestic purposes, both when at home and on his travels. Adams's Lectures, by Jones, vol. ii. p. 99, &c.

AIR-PIPES, are an invention for drawing foul air out of ships, or any other close places, by means of fire, and were first suggested by Mr. Sutton, a brewer in London; from him they received the name of Sutton's air-pipes. The principle on which their operation depends, is this simple one, that air is necessary for the support of fire; and, if it have not access from the places most adjacent, it will not fail to come from those that are more remote. Thus, in a common furnace, the air enters through the ash-hole; but if this be closed up, and a hole made in the side of a furnace, the air will rush in with great violence; if a tube of any length whatever is inserted, the air will rush through the tube into the fire, and of consequence there will be a continued circulation of air in that