SOUND, REFRACTION OF. Sound moves in straight lines, in spherical-fronted waves; -and any small beam thereof, if unequally retarded or accelerated on either side, bends toward the side of retardation so that the acoustic impulses, always directly radial from the face of the sound wave, vary in their point of available impact, as the " air mirror" of sound turns distorted by disturbance. 1. Sound passing from air into water, or water into air, provides the only example really available, of acoustic refraction by dif ference of elasticity. Sound, encountering. a strong opposition in the density of fluids, and a still greater in solids, avails itself of the enormously disproportioned intensity of resilience or elasticity. '1 he waves of sound arising from an explosion under water are conveyed by the water long distances; but when cast off into the air above the explosion, the few and retarded vibrations diverge so enormously at such a height, by reason of the flat reflector of the water at the surface, and the hollow arc of force beneath generated by the explosion, as to become nearly inaudible,with their low velocity. Guns at sea, fired on a horizontal with an energy far stronger than the inertia of the air, reverse this; and with sound waves compressed between the force of the cannon and reflection from the water, transmit their force to a great distance. 2. The refraction of sound through differences of density has been shown by a convex lens of carbonic acid inclosed in a collodion film, transferring the ticking of a watch to a focus, where it was heard only on the interposition of the lens. The wave front received a concave form on its
entrance, accelerated on its exit at the rim, its lagging center became more concave, flu.; normals converging to a focus. 3. Prof. Stokes in 1857 first suggested the refraction of sound by varying velocities of wind. Of two winds at different levels, the upper, if the faster, will retard an opposing sound wave most, and so elevate the sound focus; if the sound advanced with the wind, the focus would lower, on account of the " drag" of the lower layers. Prof. Henry in 1865, ignorant of the theory, confirmed the above points, observing the audibility of a sound signal against the wind, heard at a ship's mast head, and that the cloud shadows exceeded the ground wind in speed. 4. Prof. Reynolds, 1874, showed first the refraction of sound from difference of temperature. Since the dif ference in velocity of soudid per second is 1 ft. to 1° 'Palm, so when the lower air is warm, the sound beams are tilted up by increased speed below, and when cold, the upper strata bend over and depress the focus; in either case favoring audibility at a distance. Thus is explained the ease of extended hearing in Arctic regions.