AIRD, ard, Thomas, Scottish poet who has won praise from high critics, but little popular acceptance: b. Roxburghshire 1802; d. 25 April 1876. He studied in the University of Edin burgh, and formed a lifelong intimacy with Carlyle; contributed to '131acicwood's,' and won the warm good will of Wilson; edited the The piston of this syringe is worked by an ap paratus which passes through to the exterior of the gun, 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 the muzzle, as ordinary muskets or fowling-pieces. The trigger opens the valve; the highly compressed air rushes forth and propels 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 air gun contains several bullets in a receptacle or channel under the barrel; by the movement of a cock or lever one of these bullets can readily be shifted into the barrel; and thus several suc cessive discharges can be made after one load ing—on a principle somewhat analogous to that of the revolving pistol. Some varieties of air-gun 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 be hind the bullet in the barrel. Those air-guns which present the external, appearance of stout walking-sticks, and are hence called air-canes, have a chamber within the handle for contain ing condensed air, which can be unscrewed and subjected to the action of the condensing synnge. One inventor has devised a form of
air-gun 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 bf elastic springs with that of compressed air in an air-gun; springs of gutta-percha, or of vulcanized india-rubber, are employed in substitution of or in co-operation with a condensing syringe. No form of air-gun hitherto made has had power enough to propel a bullet to any con siderable distance, and therefore the instrument is scarcely available in war. The air-gun was known in France over two centuries ago; but the ancients had 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.
or a fault in a casing, due to the presence of a bubble of air or other gas.
an air-tight chamber used in tunneling, when the tunnel has to be kept filled with compressed air to prevent the en trance of water. The air-lock communicates with the tunnel by one door, and with the out side air by another. It serves the double pur pose of permitting the workmen to enter and leave the tunnel without undue loss of air, and of partially mitigating the physiological effects of a too •sudden transition from the high pres sure in the tunnel to lower pressure outside.