20 HAND-BOOK OF ACOUSTICS.
given distance is thus known. Dividing this distance by the number of seconds, we determine the space through which sound travels per second. This is found to vary considerably with the temperature. At 0° (Centigrade) or 32° (Fahrenheit) the velocity of sound is 1,090 feet per second. M. Wertheim gives the following results at other temperatures:—
The velocity at any temperature may be approximately found by adding on 2 feet for a rise of one degree Centigrade, and 1 foot for a degree Fahrenheit.
The velocity of sound in any medium varies directly as the square root of the elasticity, and inversely as the square root of the density of that medium. Therefore, as all gases have the same elasticity, the velocity of sound in gases varies inversely as the square root of their densities. Thus oxygen is 16 times as heavy as hydrogen, and the velocity of sound in the latter gas is 4 times the velocity in the former.
Sound travels more quickly through water than through gases. Colladon and Sturm proved that the velocity of sound in water is 4,708 feet per second, at 8° Centigrade. Their experiments were conducted on the Lake of Geneva, on the opposite sides of which the observers were stationed in two boats. The sound was emitted under water by striking a bell with a hammer, and after travelling through a known distance was received by a long speaking tube, the larger orifice of which, covered with a vibrating membrane, was sunk beneath the surface. The same movement which gave rise to the sound, also at the same instant ignited some gunpowder, and the number of seconds elapsing between the flash and the sound was determined by a chronometer.
In solids, the velocity of sound is usually greater than in liquids. It may be calculated from the formula
when the elasticity E and density D of the solid are known.