AIRE, or a t. of the dep. of Pas-de-Calais, France, on the Lys, 30 m. s.e. from Calais. It is a fortress of the third class; the t. well built, but its situation low and marshy. The barracks are capable of containing 6000 men. There are manu factures of woolen stuffs, linen yarn, thread, hats, starch, Dutch tiles and soap; also some trade in grain. Osier-work is carried on to some extent. Pop. '76, 5058.
See CALORIC ENGINE.
an instrument for bullets or other projectiles, by the force of com pressed air instead of gunpowder. Various forms of construction have 'been adopted. The most usual plan. is to insert a condensing syringe in the, stock of the gun. The piston of this syringe is worked by an apparatus which passes through to the exterior of the gull; and this working causes a small body of air to be condensed into a chamber. The chamber has a valve opening into the barrel, just behind the place where the bullet is lodged. The gun is loaded from themuzzle, as ordinary muskets or fowling-pieces; and there is at that time just behind it a small body of highly compressed air, ready to rush out at any opening. This opportunity is afforded by a movement of the trigger, which opens the valve; the air rushes forth with such impetuosity as to propel the bullet. By a certain management of the trigger, two or three bullets, successively and separately introduced, can be fired off—if firing it can be called-,-by one mass of condensed air. Another form of A. contains several bullets in a receptacle or channel under the barrel; by the movement of a cock or level, one of these bullets can readily be shifted into the barrel; and thus several successive discharges can be made after one loading—on a prin ciple somewhat analogous to that orthe revolving pistol. Some varieties of A. have the condensing syringe detached, by which means a more powerful condensation of air may be produced; this done, the air-chamber is replaced in its proper position beLind the bullet in the barrel. Those air-guns which present the external appearance of stout walking sticks, and are thence called air-canes, have a chamber within the handle for containing condensed air, which can be unscrewed, and subjected to the action of the condensing syringe. One inventor has devised a form of A. with two barrels—one of small bore for the reception of the bullets, and another of larger bore for the reservoir of condensed air; the condensing syringe being within the stock of the gun. An attempt has more recently been made to combine the action of elastic springs with that of compressed air, in an A.• ' springs of gutta-percha. or of vulcanized india-rubber, are employed. in substi tution of or in cooperation with a condensing syringe. No form of A. hitherto made has had power enough to propel a bullet to any considerable distance, and therefore the instrument is scarcely available in war ; there are, however, circumstances in which such an arm may be useful—seeing that there is no expense for gunpowder, no noise, no smoke, no unpleasant odor. The A. was known in France more than two centuries
ago; but the ancients were acquainted with some kind of apparatus, by which air was made to act upon the shorter arm of a lever, while the larger arm impelled a bullet.
an instrument for removing the air from a vessel. The essential part is a hollow brass or glass cylinder, in which an air-tight piston is made to move up and down by a rod. From the bottom of the cylinder a connecting tube leads to the space which is to be exhausted, which is usually formed by placing a bell-glass, called the receiver, with edges ground smooth and smeared with bird, on a flat, smooth plate or table. When the piston is at the bottom of the barrel, and is then di awn up, it lifts out the air from the barrel, and a portion of the air under,,the receiver, by its own expansive force, passes through the connecting tube and occupies the space below the piston, which would otherwise be a vacuum. The air in the receiver and barrel is thus rarefied. The piston is now forced down and the effect of this is to close a valve placed at the mouth of the connecting tube and opening inwards into the barrel. The air in the barrel is thus cut off from returning into the receiver, and, as it becomes condensed, forces up a valve in the piston, which opens outwards, and thus escapes into the atmosphere. When the piston reaches the bottom and begins to ascend again, this valve closes; and the same process is repeated as at the first ascent. Each stroke thus diminishes the quantity of air in the receiver; but from the nature of the process it is evident that the exhaustion can never be complete. Even theoretically, there must always be a portion left, though that portion may be rendered less than any assignable quantity; and practically the process isiimited by the elastic force of the remaining air being no longer sufficient to open the valves. The degree of rarefaction is indicated by a gauge on the principle of the barometer. By means of the partial vacuum formed by the A., a great many inter esting experiments can be performed, illustrating the effects of atmospheric pressure and other mechanical properties of gases.—The A. was invented by Otto Guericke (q.v.), 1654; and though many improvements and varieties of structure have been since devised, the principle of all is the same. Two barrels are generally used, so as to double the effect of one stroke. In some A., stop-cocks turned by the hand take the place of valves, and in others the entrance of the connecting tube into the cylinder is so contrived that the valve through the piston is not required.