of the brakes in customary service may be effected without the operation of the quick-action mechanism of the triple valves; to maintain any reduced train pipe air pressure resulting from an application of the brakes, so that the brakes may be kept applied with the force corresponding to such reduced train pipe pressure; to discharge air from the train pipe to the atmosphere with such rapidity, in emergency applications of the brakes, as shall cause the quick-action mechanism of the triple valves to operate with certainty; and to temporarily supply the train pipe with an unusually high air pressure whenever the brakes are to be released. These various operations are in practice controlled by different positions of a rotary disk valve, the various positions of which are defined and secured by the movement of a handle operated by the engineer.
For detailed descriptions of the operation of the air-brake consult "The Development of the Electro-Pneumatic Brake," and other publications of the Westinghouse Air-Brake Company.
AIRD, SIR JOHN, an English engineer; born in 1833. In his earliest years he exhibited remarkable skill in mechanics and when only 18 years old carried on important engineering work in connection with the Exhibition of 1857. He constructed many large projects in England and elsewhere, but is best known as the builder of the great Assuan Dam. Under his direction the dam was completed twelve months earlier than the contracted time. lie was a member of Parliament from 1887 to 1905. He was created a baronet in 1901. He died in 1911.
AIR-GUN, an instrument designed to propel balls by the elastic force of condensed air. A strong metal globe is formed, furnished with a small hole and a valve opening inward. Into this hole a condensing syringe is screwed. When, by means of this apparatus, the condensation has been brought to the requisite point of intensity, the globe is detached from the syringe and screwed at the breech of a gun, so constructed that the valve may be opened by means of a trigger. A ball is then inserted in the barrel near the breech, so fitting it as to render it air-tight, and, the trigger being pulled, the elasticity of the condensed air impels it with considerable force.
AIR-PUMP, an instrument invented by Otto von Guericke of Magdeburg, in 1650. It was designed to exhaust the air from a receiver. The air-pump now generally in use is a considerable improvement. A bell-formed receiver of glass is made to rest on a horizontal plate of thick glass ground perfectly smooth. In the center of that plate, under the receiver, is an opening into a tube which, passing for some distance horizontally, ultimately branches at right angles into two portions, entering two upright cylinders of glass. The cylinders are firmly cemented to the glass plate, and within them are two pistons fitting them so closely as to be air-tight. Each piston is worked by a rack and pinion, turned by a handle; while each cylinder is fitted with a valve, so contrived that, when the piston is raised, communication is opened between the cylinder and the receiver, which communication is again closed as the piston falls. It is evident that when anyone commences to work the machine, the air in the cylinders will be immediately expelled the first upward motion that they are made to take. The valve will then fly open, and the air from the receiver will fill both the cylinders as well as itself, though, of course, now in a somewhat rarefied state. As the same process is again and again repeated, the air will become increasingly rarefied, though, as stated above, an actual vacuum never can result from the action now described.
AIR-PUMP GAUGE, a gauge for testing the extent to which the air has been exhausted in the receiver of an air-pump. It consists of a glass tube bent like a siphon. One leg is closed, as in a barometer, the other open. It is placed under a small bell jar communicating by a stopcock with the receiver, and the more nearly the mercury stands at the same level the more nearly has a vacuum been produced.
AIR-SHIPS. See AERONAUTICS.
AISNE, a river in France rising in the forest of the Argonne and flowing first N. W. and then W. to a junction with the Oise near Compiègne. It is about 170 miles long and is navigable over one-half its length. Canals connect it with the Meuse and the Marne rivers. The Aisne and the country through which it runs formed the most prominent field of operation during the World War. The northern banks of the river were fortified by the Germans against a failure of their initial attempts to capture Paris in 1914, and following their defeat at the Marne the German armies fell back to these defenses. It constituted during the greater part of the war the northern limit of their possession of French soil. Much of the bitterest fighting during the
