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AINSWORTH. and directness with which he realized scenes and incidents.

 AINTAB,. A town in the Syrian vilayet of Aleppo, Asiatic Turkey, situated about 65 miles north of the city of Aleppo (Map: Turkey in Asia, G 4). It is an important military post and is well fortified. It carries on an extensive trade in leather and cotton, and lies on the route leading from Aleppo to Armenia. Its population is about 43,000, and consists, to a great extent, of Armenian and Greek Christians.

 AIR,, or. A hilly region in the southern part of Sahara, situated between 17° and 20° northern latitude and 7° and 10° eastern longitude. It is regarded as one of the best populated centres of Sahara, but has been very little explored as yet. The valleys are supposed to be fertile and the climate temperate. There are no permanent rivers, but the numerous ravines fill with water during the rainy season, and it often happens that a large valley is converted into a river in a very short time. The country is ruled by a native Sultan, and the population, estimated at about 100,000, consists chiefly of Tuaregs. Capital, Agades (q.v.).

 AIR (Lat. aër, Gk. ἀήρ, aēr, irom ἂειν, aein, to blow). The mixture of gases forming the atmosphere of the earth. It consists essentially of 79.03 parts of nitrogen and 20.97 parts of oxygen, with varying small quantities of carbonic acid, ammonia, ozone, argon, helium, neon, krypton, and xenon (qq.v.), aqueous vapor. Certain chemical compounds, as common salt, ammonium nitrate, etc., as well as minute particles of animal, vegetable, and mineral matter, are also frequently found in the air. Early chemists called all gases airs. The chief properties of air and the phenomena they give rise to may be found treated under ; ; ; ;, etc.

 AIR BLAD′DER OF FISHES. See.

 AIR BRAKE. A brake worked by compressed air, which is extensively applied to railway cars in the United States, and also to a less extent in other countries. Air brakes are also used on street railway cars. The air brake in its first form was invented by George Westinghouse, Jr., an American engineer, in 1869, and is known as the straight air brake. This brake consisted of an air pump, a main reservoir, and an engineer's valve on the locomotive, and of a train pipe and brake cylinder on each car. The air pump served to keep the main reservoir filled with air under pressure, and the brakes were applied by throwing the engineer's valve so as to allow the air from the main reservoir to enter the train pipe and thence into the brake cylinders on the cars, thus forcing the pistons out and applying the brakes on each car. The train pipe of one car was connected to that of the next by flexible hose, with a coupling between cars. This form of brake had several objections, the more important of which were that the brakes on the forward cars were applied so much sooner than those on the rear cars that the rear cars bunted up against the forward cars, causing shocks and damage; and in case a hose burst or a coupling parted, the air pressure would escape without setting the brakes. These objections to the straight air brake led Mr. Westinghouse to invent, in 1873, the automatic air brake. In this brake each car was equipped with an auxiliary reservoir and a triple valve in addition to the train pipe and brake cylinder. The triple valve was located at the junction of the train pipe and the two pipes leading to the brake cylinder and to the auxiliary reservoir. The principle of operation of this improved brake is as follows: Air pressure is maintained in the auxiliary reservoirs and in the train pipe at all times when the brakes are not applied, the pressure in the train pipe being exactly equal to that in the reservoirs, and there being no pressure in the brake cylinder, owing to the fact that as long as the train pipe and auxiliary reservoir pressures are equal, the triple valve is held in a position closing the air inlet to the brake cylinder. To apply the brakes, the equilibrium between the train pipe and the auxiliary reservoir pressures is disturbed by allowing air to escape from the train pipe; as soon as this is done, the excess air pressure in the auxiliary reservoir throws the triple valve so that it admits pressure from the reservoir into the brake cylinder and applies the brakes. To release the brakes, air pressure is retained in the train pipes by admitting air to it from the main reservoir on the locomotive. This gives an excess pressure in the train pipe above the pressure in the auxiliary reservoir, which throws the triple valve so as to close the inlet to the brake cylinder and open the inlet to the auxiliary reservoir from the train pipe, thus allowing the two to attain equal pressures again. To permit air to escape from the train pipe, and thus apply the brakes, there is the engineer's valve previously mentioned, and also a conductor's valve on each car, the latter being used only in case of emergency. It is evident also that should a break occur in the train pipe, or its hose connections, through any accident, the pressure is relieved and the brakes are applied automatically.

It will readily be appreciated from what has been said that the triple valve is an exceedingly important part of the mechanism of the automatic air brake. It performs three duties: (1) Charges the auxiliary reservoirs; (2) applies the brakes; and (3) releases the brakes. These duties are, moreover, performed automatically, and, as experience has shown, with almost absolute certainty as long as the valve mechanism is kept in good order. The triple valve is, however, not the only automatic feature of the air brake. The operation of the air pump is controlled automatically by a pump governor, which shuts the steam off from the air pump as soon as the pressure in the main reservoir has reached a certain amount, and admits it again when the pressure falls below this amount. There is also an automatic contrivance for closing the ends of the coupling hose when they are disconnected; this valve opens automatically when the hose is coupled. This describes briefly the construction and operation of the plain automatic air brake. It was, as will be obvious to all, a vast improvement over the straight air brake. Its chief objection was that in an emergency application on a long train the forward brakes were applied so much sooner than those in the rear that the slack of the train ran ahead and often did great damage. To remedy this objection Mr. Westinghouse invented, in 1887, the quick action triple valve, by which the application was so much hastened at the rear of the train that the slack had no chance to run ahead. At present the