Air Gun, a pneumatic engine resembling a musket, for the purpose of discharging bullets by means of compressed air. It consists of a lock, stock, barrel, and ramrod. The stock is made hollow, and provided with proper cocks for filling it with compressed air by means of a force pump. The lock is nothing but a valve which lets into the barrel a portion of the air compressed in the stock, when the trigger is pulled. The gun is loaded with wadding and ball in the ordinary way, and the air suddenly introduced from the stock propels it with a velocity proportional to the square root of the degree of compression of the air. There is no doubt that if the discovery of powder had not been made at an early date, these instruments would have reached a point of great effectiveness. The section of the air gun given here represents one of the most practical kind. It has the general form of a musket. The stock A is hollow, and strong enough to withstand the required pressure of the air, being some 50 atmospheres, pumped into it by means of the piston E, moving in the barrel C D; this compressed air is confined in the stock by the spring check valve C. The action of the lock I is such that when pulling the trigger the valve C is for an instant lifted from its seat, which causes a small portion of the air to escape; and as this air is confined under a pressure of some 50 atmospheres or 750 lbs., the impulse given to the ball at the first discharge is almost as great as that of gunpowder.
One charge may fire several balls, but the effect decreases with every firing. Some air guns have, in place of a hollow stock, a separate large hollow metallic ball into which the air is compressed, and which is attached to the side of the lock. These are generally very unsafe. - Arms analogous to air guns have been contrived for producing explosion, with nearly or quite the power of an ordinary musket, by the electrical conversion in the barrel of oxygen and hydrogen, or air and street gas, into steam, and other similar means.