Aire-sur-l'Adour (ār-su˙r-la˙-dör), a small but ancient town of France, department of Landes, the see of a bishop. Pop. 3000.
Aire-sur-la-Lys (ār-su˙r-la˙-lē), an old fortified town of France, department of Pas de Calais, 10 miles south-east of St. Omer. Pop. 5000.
Air-gun, a gun from which the bullet is propelled by means of compressed air. Until about the middle of the nineteenth century air-guns were made with a metal reservoir in the butt; this reservoir was charged with air by means of a pump, and although one pumping put in enough air for six or seven shots, the process of loading was awkward and laborious. The well-known 'Gem' air-gun was worked by means of a spring, which compressed the air; the great defect of this gun was that the barrel was used as a cocking-lever, and so was apt to become bent and inaccurate. The 'Gem' was a smooth-bore gun, and early attempts at rifled air-guns failed, as the pellet was apt to stick in the barrel, owing to the low velocity not allowing it to take the grooves. The 'Quackenbush' air-gun made an attempt to get over this difficulty; its slugs were felted, and the felt took the rifling and greatly increased the accuracy of the weapon, but, of course, the ammunition was much more expensive than ordinary air-gun pellets. The B.S.A. air-rifle is an excellent weapon which has overcome all the early difficulties of construction. It has a fixed barrel, a separate cocking-lever, and a rotating breech-plug, and the muzzle velocity of its 16-grain pellet is 600 feet per second, which compares not unfavourably with the 1000 feet per second of the 40-grain bullet of a .22 long-rifle cartridge. An air-gun is a splendid weapon for practising markmanship, as it is almost noiseless, and as its ammunition costs little. It does not need to be elaborately cleaned, as a miniature rifle does; an occasional oiling is all that it requires to keep it in order, and with care it should fire an indefinite number of shots without losing its accuracy.
Airolo (a˙-i-rō′lō), a small town of Switzerland, canton Ticino, at the southern end of the St. Gothard Tunnel, and the first place on this route at which Italian is spoken. Pop. 2000.
Air-plants, or Epiphytes, are plants that grow upon other plants or trees, apparently without receiving any nutriment otherwise than from the air. The name is restricted to flowering plants (mosses or lichens being excluded) and is suitably applied to many species of orchids. The conditions necessary to the growth of such plants are excessive heat and moisture, and hence their chief localities are the damp and shady tropical forests of Africa, Asia, and America. They are particularly abundant in Java and tropical America.
Air-pump, an apparatus by means of which air or other gas may be removed from or compressed into an enclosed space. It was invented by Otto von Guericke of Magdeburg about the year 1654, and described in 1657 by Gaspar Schott. An ordinary suction-pump for water is on the same principle as the air-pump; indeed, before water reaches the top of the pipe the air has been pumped out by the same machinery which pumps the water. An ordinary air-pump (see fig. 1) consists essentially of a cylinder or barrel with a piston and valves. The barrel is connected to the vessel from which the air is to be pumped. A is the vessel to be exhausted, C the air-pump cylinder, P the piston, VV valves in the piston, and O the connection to the vessel A. When the piston moves downwards from the position shown, it cuts off the connection with A by passing over O. The length L is made long enough so that O is kept covered up during the downstroke. The air filling the space D is compressed, and so lifts the valves VV and passes out through them. This goes on till the end of the downward stroke, when the volume is very small indeed. When the upward motion begins, the valves VV close, and the piston rises and creates a vacuum in D. When the piston rises sufficiently to uncover O (as in figure), air rushes from A into the highly-exhausted space D and fills it. The process is repeated indefinitely, and A is gradually exhausted.
Air-pumps for compressing air are constructed on the same principle, but the valves act the reverse way. The bicycle pump is a well-known example of this form of pump. In the Fleuss or Geryk pump greater efficiency is attained by having layers of oil in the barrel and above the piston. In nearly all pumps for producing the high vacua necessary, e.g. for the electric glow-lamp and the X-ray tube, mercury is employed. In one form, the Toepler pump, a reservoir containing mercury is connected by a flexible tube to the receiver. (See fig 2. T tube connecting pump to vessel to be exhausted; R, reservoir, raised above A to drive air in B and C through D and out into open air; R is then lowered, and B and C fill with air from receiver. Process then repeated.) By alternately lowering and raising the reservoir, gas is first withdrawn from the receiver and then expelled through D, which also acts as a barometer. The process is repeated until the desired degree of exhaustion is reached. In a second type, the Sprengel pump, a stream of mercury from a reservoir situated above the vessel to be exhausted falls in drops through a narrow vertical tube which communicates with the vessel. (See fig. 3. A, reservoir; B, tube leading to vessel to be exhausted; C, bubbles of air carried down by mercury.) The air is entrapped between the falling drops of mercury,