Page:The New International Encyclopædia 1st ed. v. 01.djvu/294

AIR GUN. arm of which was used to propel a bullet. In 1886 Lieutenant E. L. Zalinski of the United States Army invented a pneumatic gun for throwing projectiles filled with dynamite; and later the Vesuvius was built for the United States Navy and equipped with three of these guns. This vessel was used during the blockade of Santiago Harbor in the Spanish-American War of 1898. During the Brazilian civil war of 1893 the Nietheroy was equipped with a pneumatic gun 50 feet long and of 15 inches calibre. The conclusions of experience with both sets of guns was that the range of the gun was too small and the accuracy of its fire insufficient to make it a serviceable weapon on shipboard. The Sims-Dudley pneumatic gun used in the last Cuban rebellion is a field piece having a range of from 2600 to 3600 yards. It consists of a lower, or combustion, tube 7 feet long and 4½ inches in diameter, and an upper tube, or barrel. 20 feet long and 2½ inches in diameter, mounted on a regular field gun carriage. A cartridge inserted into the breech of the combustion chamber, and containing a 7 to 9 ounce charge of smokeless powder, is fired; this compresses the air in the lower chamber so that it passes into the upper tube or barrel behind the projectile and forces it out. The projectile is a light casing filled with explosive gelatine, which is fired by a time fuse, or by a contact fuse upon striking.

AIROLO, i-ro'lij (In German, Eriels). A village in Switzerland, in the canton of Ticino, on the upper Ticino, 3755 feet above the sea, 66 miles south of Lucerne, at the south end of the St. Gothard Pass and of the St. Gothard Rail- way Tunnel (q.v.) (Map: Switzerland C 2). On September 17, 1877, it was two-thirds burned, but later rebuilt in stone. December 27-28, 1898, it was partially destroyed by an avalanche. The inscription, "Suvarov Victor," that was carved in the rocks to commemorate the victory here of the Russians over the French, September 13, 1799, is now obliterated. Pop., less than 2000.

AIR PLANT. See.

AIR PORT. See.

AIR PUMP. An instrument for removing the air from a vessel. These pumps may be divided into two classes, mechanical air pumps and mercurial air pumps. The mechanical air pump was invented by Otto von Guericke about 1654, and a specimen of his early apparatus is shown in Fig. 1 of the accompanying page illustration. In Fig. 2 is illustrated a modern simple air pump whose essential part is a hollow brass or glass cylinder, in which an air-tight piston is made to move up and down by a rod. From the bottom of the cylinder a connecting tube leads to the space which is to be exhausted, which is usually formed by placing a bell-glass, called the receiver, with edges ground smooth and smeared with lard, on a flat, smooth plate or table. When the piston is at the bottom of the barrel and is then drawn up. it lifts out the air from the barrel, and a portion of the air under the receiver, by its own expansive force, passes through the connecting tube and occupies the space below the piston, which would otherwise be a vacuum. The air in the receiver and barrel is thus rarefied. The piston is now forced down, and the effect of this is to close a valve placed at the mouth of the connecting tube and opening inward into the barrel. The air in the barrel is thus cut off from returning into the receiver, and as it becomes condensed forces up a valve in the piston, which opens outward, and thus escapes into the atmosphere. When the piston reaches the bottom and begins to ascend again this valve closes; and the same process is repeated as at the first ascent. Each stroke thus diminishes the quantity of air in the receiver; but from the nature of the process it is evident that the exhaustion can never be complete. Even theoretically there must always be a portion left, though that portion may be rendered less than any assignable quantity; and practically the process is limited by the elastic force of the remaining air being no longer sufficient to open the valves. The degree of rarefaction is indicated by a gauge, on the principle of the barometer. As this air pump only withdraws the air at the rate of one cylinder full for a double stroke of the piston, pumps with two barrels are frequently used, in which case the pistons are each attached to the same handle but each moves in an opposite direction to the other, the object being to double the work done at each stroke of the handle. Such a pump is illustrated in Fig. 5 of the page plate. A large number of modifications of this type of pump have been invented, all of which are the same in general principles. There are several reasons why such pumps do not continue the process of rarefaction indefinitely, but after a certain stage their effects cease and the tension of the air undergoes no further change. Leakage at various joints in the pump is one limiting cause to the action of the machine. It is impossible to prevent leakage entirely, and at the beginning of the operation the quantity of air which enters the receiver through leakage is very small in comparison with the amount pumped out. But as the exhaustion proceeds the leakage is faster on account of the reduced pressure in the receiver, and finally a limiting point is reached when the inflow and outflow are equal and no reduction in the tension of the air takes place. Another limit to the action of this machine is caused by the fact that there must always be some space between the bottom of the piston and the lower end of the cylinder, which is untraversed by the piston. At the beginning of the operation this space contains air at atmospheric pressure, which is rarefied at each stroke of the piston: but some tension always remains there, and when the air of the receiver reaches the same tension no further effect will be produced by the pump. Perhaps the most important trouble, however, with this type of air pump, as well as the most difficult one to remedy, is the absorption of air by the oil used for lubricating the pistons. This oil finds its way in a greater or less quantity to the bottom of the cylinder, where its absorbed air is partially given up at the moment the piston begins to rise. This class of pumps is not good enough for the manufacture of incandescent lamps and vacuum tubes, and recourse is had to the mercurial air pump, by means of which a much greater degree of exhaustion is obtained. The principle of the mercurial air pump was first known in the seventeenth century, when Torricelli showed how to produce a vacuum by filling a tube over 30 inches long and closed at one end, with mercury, and then inverting the tube, with the open end temporarily closed, in a vessel containing the same liquid. The mercury