AERODYNAMICS is that branch of science which treats of air and other gases in motion. It examines first the phenomena of air issuing from a vessel, which correspond in many respects with those of water. See HYDRODYNAMICS. Much depends, as in the case of water. upou the nature of the orifice, whether a mere hole in the side of the vessel, or a tube or adjutage. Another subject of A. is the motion of air in long tubes, where the resistance of friction, etc., has to be ascertained. That resistance is found to be nearly in proportion to the square of the velocity, to the length of the tube, and inversely to its width. A. examines also the velocity of air rushing into a vacuum, of wind, etc. The instrument used for the latter purpose is called an anemometer. See WiNns. Air is found to rush into a void space at the rate of from ft to per second. One of the most important inquiries in A. is the resistance offered to a body moving in air, or—which is the same thing—the pressure exerted by air in motion upon a body at rest. The law may be stated, with sufficient accuracy for practical purposes, as follows: The resistance or pressure Is proportional to the square of the relocity. We might conclude from reason, without experiment. that such would be the ease; for if one body is moving through the air four time faster than another Of the same size, not only will it encounter four times as many particles of air, but it will give each of them four times as great an impulse or shock, and thus encounter 4 X 4, or sixteen times as much resistance.
This resistance is greatly increased by another circumstance, especially with great velocities. The air in front of the moving body becomes accumulated or condensed, and a partial or even entire vacuum is formed behind it. With a velocity of 1700 a. per second, for instance, the resistance is found to be about three times as great as the simple law of the square of the velocity would give. By the operation of these laws of resistance, a heavy body let fall with a parachute attached to it, comes, after a certain time, to move with a velocity approaching more and more nearly to a uniform motion.
At'Il0E, or ARR6E, an island in the Baltic, 14 in. s. of Funen; 14 in. long and 5 wide; it has a port and is well cultivated; pop., 12,400. It belongs to Schleswig Hol stein, and the capital, Anroesjkobing, is a port of some shipping importance.
AtIlOKUN0SCOPE, an instrument to show differences of barometric pressure at remote stations. It consists of a vertical axis 30 ft. high, turning on a pivot, carrying at the top a horizontal arm, of which the inclination can be varied according to the differ ence of barometric pressure at different sides of the station; the amount of dip being indicated by a sliding rod held in position by graded notches at the lower part of the axis, each notch corresponding with one millimetre in pressure. It is used in the weather service.