ACOTTS'TICS (ante). The velocity of sound has been determined by ascertaining the time intervening between the flash and report of a gun, as observed at a given distance, and dividing the distance by the time. After many experiments in various countries, Van der Kolk assit;ned 1091 ft. 8 in. per second, with a probable error of 3.7 ft. as the velocity of sound in dry air at 32° Fahr. More recent experiments by the astronomer royal at the cape of Good Hope, give 1096 ft. To this velocity may be added 1.11 ft. for each degree Fehr. But air is not a perfect gas, and the variations of elastic force caused by a wave of sound passing through it are not uniform; so these measures, though approximately, may not be absolutely, correct. Furthermore, the rapidity of transmis sion depends upon the loudness of the sound; and caps. Parry found, in the, polar re gions, that the discharge of a cannon at a distance of 2+ m. was heard perceptibly sooner than the word ordering to fire, which, of course, preceded the discharge. There is also a gradual falling off in the speed of sound; and Regnault determined that a sound decreased in speed by 2.2 ft. per second in passing from a distance of 4000 ft. to one of 7500 ft. He also found that the velocity depended upon the pitch, the lower notes traveling faster than the higher ones; thus, the fundamental note of a trumpet travels faster than its harmonies. Sound travels faster in liquids than in air, and faster in solids than in liquids. In the river Seine, at 59' Fahr. the speed was 4714 ft. per second. Through iron, sound travels ten and a half times faster than through air. Experiments on telegraph wire produce almost identical results. Different metals trans mit sound in widely different degrees. Wertheim assigned 16,822 for iron and 4060 for lead, at a temperature of 68' Fahr. Except in a few cases, the loudness of a sound is less as the distance increases between the source of the sound and the ear. In an unlimited and uniform medium, the loudness of the sound proceeding from a very small sounding body varies inversely as the square of the distance. But to verify this fact it would he necessary to make a test at a considerable elevation above the earth's surface, the car and source of sound being separated by air of constant density. As the density of the air diminishes, it would be found that the loudness of a sound at a given distance would decrease. The decay of sound due to this cause is observable in the rarefied air of high mountain regions. De Saussure found that the report of a pistol
at a great elevation appeared no louder than would a small cracker at a lower level. But it must be stated that when air-strata of different densities are interposed between the sound and the ear placed at a given distance, the intensity depends only on the density of the air at the source itself; whence it follows that sounds proceeding from the sur face of the earth may be heard at equal distances as distinctly by a person in a floating balloon as by one situated on the surface itself; whereas any noise originating in the balloon would be heard at the surface as faintly as if the ear were placed in the rarefied air on a level with the balloon. This was exemplified by Glashier, the aeronaut, who, at an elevation of 20,000 ft. heard with great distinctness the whistle of a locomotive passing beneath him. The prolonged roll of thunder, with it•t manifold varieties, is partly to bo ascribed to the reflection of the sound by moutdains, clouds, etc., but is mainly due to the comparatively low rote of transmission through air. The explanation will be more easily understood by noting the case of a volley tired by a long line of troops. A person at a given point in the line would hear the sound of the nearest musket first, and of the others in the order of distance, and the effect would be a pro longed roll, concluded by the musket most remote from the hearer ilwitgh all were fired at the same instant; and the roll would gradually decrease in loudness. If he stood exactly opposite the centre of the line, the reports from either end would reach him simultaneously and the effect would be more nearly a loud crash. if the soldiers formed a circle, the listener in the centre would hear a single explosion, since the report of every gun would reach his ear at the same instant, and the whole explosion would be equal to that of the sum of all the separate discharges. By varying the form of ar ranging the troops, corresponding variations in the sound would be produced. Keep in view, then, the fact that flashes of lightning may be representing lines of troops, at the points and along the ranks of which explosions are generated at the same instant of time; then consider the variety of distance and position relative to the electric discharge of the listener, and we find no difficulty in accounting for the rolling peals of thunder. In a mountainous region this rolling is greatly augmented by rever berations or echoes from the steep declivities.