AIR < ° ) A L C

arid has two barrels : the inside barrel isof a small bore, from whence the bulletsare exploded; and a large barrel on theoutside of it. There is likewise a sy-ringe fixed in the stock of the gun, bywhich the air is injected into the cavitybetween the two barrels through a valve.The ball is put down into its place inthe small barrel with the rammer, as inany other gun. Another valve, beingopened by the trigger, permits the airto come behind the bullet, so as to driveit out with great force. If this valve beopened ami shut suddenly, one chargeof condensed air may be sufficient forseveral discharges of bullets; but if thewhole air be discharged on one singlebullet* it will drive it out with uncom-mon force. This discharge is effected bymeans of a lock placed here, as usualin other guns; for the trigger beingpulled, the cock will go down and drivethe lever, which will open the valve, andlet in the air upon the bullet: but as theexpansive power of the condensed airdiminishes at each discharge, its force isnot determined with sufficient precisionfor the purposes of war. Hence it hasbeen long out of use among militarymen.

In the air-gun, and all other caseswhere the air is required to be condensedto a very great degree* it will be neces-sary to have the syringe of a small bore,viz. not exceeding half an inch in dia-meter ; because the pressure against everysquare inch is about 15 pounds, andtherefore against every circular inchabout 12 pounds. If therefore the sy-ringe he one inch in diameter, when oneatmosphere is injected, there will be aresistance of 12 pounds against the pis-ton ; and when ten are injected, therewill be a force of 120 pounds to be over-come ; whereas ten atmospheres actagainst the circular half-inch piston(whose area is only x part big) withonly a force equal to 30 pounds ; or 40atmospheres may be injected with sucha syringe, as well as 10 with the other.Tn short, the facility of working will beinversely as the squares of the diameterof the syringe.

AIR-SHAFTS, in mining. SeeMining.

AIRE, TV. any smooth or even spotof ground upon which one treads.

Aire, Fr. in geometry, the area orinside of any geometrical figure.

Aire, TV. in architecture, the spacebetween the walls in a building.

AIREE, Fr. a barn-floor.

AIRIER, Fr. to fumigate.

AIS, TV. board, plank.

Ais (Tenlrevoux , Fr. boards or plankswhich cover the space between the raft-ers, or beams, in a building.

AISCEAU, Fr. a chip-axe, or onehanded plane axe, witli which carpentershew their timber smooth.

AISCETTE, TV. a small planing axe.

AISSE, Fr. a linch pin.

AISSIEU, TV. axle-tree, axis. It isalso called tympan or tambour , roundwhich a rope may be wound for the pur-pose of drawing up any load affixed toit.

AJUTAGE, (ajutage , Fr.) in hy-draulics, part of the apparatus of an ar-tificial fountain; beinga sort of jet d y cau,or kind of tube fitted to the mouth oraperture of a vessel, through which thewater is to be played, and thrown into aparticular form or figure.

AJUTAGES* TV. pipes for water-works.

ALAISE, Fr. in carpentry, a thinpiece of wood which is used to finish thewooden pannels of a door. It is alsowritten alese.

ALARM is a sudden apprehension upon some report, which makes men runto their arms to stand upon their guard;it implies either the apprehension of be-ing suddenly attacked, or the notice givenof sucli an attack being actually made ;generally signified by the firing of a can-non, the beat of a drum, &c.

A larm- Post, in the field, is theground appointed by the quarter-mastergeneral for each regiment to march to,in case, of an alarm.

Alarm -Post, in a garrison, is theplace allotted by the governor for thetroops to draw up in, on any suddenalarm.

False-A larms, are stratagems of war,frequently made use of to harass anenemvjby keeping them perpetually un-der arms. They are often conveyed byfalse reports, occasioned by a fearful ornegligent sentinel. A vigilant officer willsometimes make a false alarm, to try itbis guards are strict upon duty.

Alarm-Uc//, the bell rung upon anysudden emergency, as a fire, mutiny, ap-proach of an enemy, or the like, calledby the French , Tocsin.

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