AIR-GUN. A machine in which highly-compressed air is substituted for gunpowder to expel the ball, which will be projected forward with greater or less velocity, according to the state of condensation, and the weight of the body projected. The effect will, therefore, be similar to that of a gun charged with gunpowder, for inflamed gunpowder is nothing more than air very greatly con densed, so that the two forces are exactly similar. The elasticity of the air generated by the inflammation of gunpowder has been estimated by Mr. Robins as equal to about 1000 times that of common air ; it would therefore be requisite to condense air into one-thousandth part of its original bulk to produce the effects of gunpowder. There is, however, this important consideration to be attended to, viz. that the velocities with which balls are impelled are directly proportional to the square root of the forces; • so that if the air in an air-gun be condensed only ten times, the velocity will be ecjual tor of that arising from gunpowder; if condensed twenty times, the velocity would be ; that of gunpowder, and so on. Air-guns, however, project their balls with a much greater velocity than that assigned above, and for this reason, that, as the reservoir or magazine of con densed air is commonly very large in proportion to the tube which contains the ball, its density is very little altered by passing through that narrow tube, and consequently the ball is urged all the way by nearly the same force as at the first instant; whereas the elastic fluid arising from inflamed gunpowder is but very small indeed in proportion to the tube or barrel of the gun, and therefore by dilating into a comparatively large space as it urges the ball along the barrel, its force is proportionally weakened, and it always acts less and less on the ball in the tube. Hence it happens, that air condensed only ten times into a pretty large receiver, will project its ball with a velocity little inferior to that of gunpowder. Having thus explained the principle of the machine, we shall proceed to describe the construction of one : that by Martin is perhaps the best, and is as follows:— It consists of a lock, stock, barrel, ramrod, &c. of about the size and weight of a common fowling-piece. Under the lock at b is screwed on a hollow copper ball c, perfectly air-tight. This ball is fully charged with condensed air, by means of the syringe B, previous to its being applied to the-tube at b. Being charged and screwed on as above stated, if skillet be rammed down in the barrel, and the trigger a be pulled, the pin in b will, by the spring-work in the lock, forcibly strike out into the ball, and thence by pushing it suddenly, a valve within it will let out a portion of the condensed air, which, rushing through the aperture in the lock, will act forcibly against the ball, impelling it to the distance of 60 or 70 yards, or further, if the air be strongly compressed. At every discharge only a portion of the air escapes from the ball ; therefore, by re-cocking the piece another discharge may be made, which may be repeated for a number of times proportioned to the size of the ball. The air in the
copper ball is condensed by the syringe B in the following manner. The ball 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 i i fixed on the side of the barrel of the syringe, when, by moving the barrel B steadily up and down on the rod a, the ball c will become charged with con densed air, and the progress of condensation may be estimated by the increasing difficulty in forcing down the syringe. At the end of the rod k is usually a square hole, that the rod may serve as a key for attaching the ball to either the gun or syringe. In the inside of the ball is fixed a valve and spring, which gives way to the admission of the air, but upon its emission, comes close up to the orifice, shutting out the external air. The piston rod works air-tight by a collar of leather on it, in the barrel B; it is therefore obvious that when the barrel is drawn up, the air will rush in at the hole h ; when it is pushed down, it will have no other way to pass from the pressure of the piston but into the ball c at the top. The barrel being drawn up, the operation is repeated, until the condensation is so great as to resist the action of the piston. If air be very suddenly compressed into a small compass, the heat given out is so considerable, as to be sufficient to ignite inflammable substances. This discovery was made, accidentally, by a French soldier, who, in cleaning his musket with some wadding fastened to the ramrod, found, after thrusting the ramrod suddenly down the piece, that the wadding had ignited. The fact he communicated to the National Institute, and repeated the experiment in their presence. This property has been turned to advantage in an apparatus denominated, "An Instantaneous Light Machine," constructed in a walking stick, which consists of a piston accurately fitted and worked in a cylinder, by the sudden stroke of which the volume of air contained in the cylinder becomes so much compressed as to give out sufficient heat to set fire to a piece of the substance termed German tinder. A patent was also taken out in 1828, by Mr. Newmarch, of Cheltenham, for a similar method of exploding fire-arms, by means of a cylinder and piston, enclosed in the stock behind the breech, which has a small hole, closed by a valve. The piston is forced back, by means of a lever, against a powerful spring, and upon being released, is impelled forward into the cylinder with such force, as to cause the air before it to give out its caloric in the state of sensible heat or fire at the aperture in the breech, which passing the valve, enters the barrel, and instantly ignites the charge of powder.