The common air-gun is made of brass, and has two barrels: the inside barrel is of a small bore, from whence the bullets are exploded; and a large barrel on the outside of it. There is likewise a syringe fixed in the stock of the gun by which the air is injected into the cavity between the two barrels through a valve. The ball is put down into its place in the small barrel with the rammer, as in any other gun. Another valve, being opened by the trigger, permits the air to come behind the bullet, so as to drive it out with great force. 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 uncommon force. This discharge is effected by means of a lock placed here, as usual in other guns; for the trigger being pulled, the cock will go down and drive the lever, which will open the valve, and let in the air upon the bullet: but as the expansive power of the condensed air diminishes at each discharge, its force is not determined with sufficient precision for the purposes of war. Hence it has been long out of use among military men. In the air-gun, and all other cases where the air is required to be condensed to a very great degree, it will be necessary to have the syringe of a small bore, viz. not exceeding half an inch in diameter; because the pressure against every square inch is about 15 pounds, and therefore against every circular inch about 12 pounds. If therefore the syringe be one inch in diameter, when the atmosphere is injected, there will be a resistance of 12 pounds against the piston; and when 10 are injected, there will be a force of 120 pounds to be overcome; whereas 10 atmospheres act against the circular half-inch piston (whose area is only ¹⁄₄ part so large) with only a force equal to 30 pounds; or 40 atmospheres may be injected with such a syringe, as well as 10 with the other. In short, the facility of working will be inversely as the squares of the diameter of the syringe. AIR-SHAFTS, in mining. See MINING. ALARM, is a sudden apprehension upon some report, which makes men run to their arms to stand upon their guard; it implies either the apprehension of being suddenly attacked, or the notice given of such an attack being actually made; generally signified by the firing of a cannon, or rocket, the beat of a drum, &c. ALARM-_Post_, in the field, is the ground appointed by the quarter-master general for each regiment to march to, in case of an alarm. ALARM-_Post_, in a garrison, is the place allotted by the governor for the troops to draw up in, on any sudden alarm. _False_-ALARMS, are stratagems of war, frequently made use of to harrass an enemy, by keeping them perpetually under arms. They are often conveyed by false reports, occasioned by a fearful or negligent sentinel. A vigilant officer will sometimes make a false alarm, to try if his guards are strict upon duty.