ACOUSTICS. | 73 
The velocity with which the waves are propagated through the air is 
independent of the velocity of the action of the piston and of the individual 
strata of air; as, however, experiment has shown that the velocity of 
propagation of air waves is independent of the time in which each individual 
part completes its oscillation, and the wave length is the distance by which 
the wave advances while a single layer makes a complete vibration, the 
wave lengths must increase in the same proportion as the time of vibrations 
ef the individual particles of air. Thus, if the piston require triple or 
quadruple the time to make a complete backward and forward motion, the 
wave lengths will be three or four times as great. 
We have thus considered the transmission of air waves in tubes: in the 
open air they must be transmitted in precisely the same manner in all 
directions. 
The impression produced upon the ear in this motion of air waves is very 
different according to their character. If the motion be produced by a 
single blow, and this not repeated, as in a pistol shot, where thus the air is 
suddenly and powerfully condensed, and then advancing as before mentioned, 
we hear a report ; in regularly successive vibrations we hear a tone; and 
if the successive vibrations become more and more irregular, we have a 
noise. ‘The tone itself will be higher as the length of oscillation or the wave 
length is shorter: it becomes stronger or more intense as the amplitude of 
oscillations in the sounding body is greater, for so much the greater is the 
degree of condensation and consequent rarefaction of the air waves. 
The velocity with which tones are transmitted through the air is con- 
stantly the same, whether they be high or low, strong or feeble. Experiments 
were instituted in 1822 by the Bureau des Longitudes, accurately to deter- 
mine this velocity, whence it resulted that sound travelled 340.88 metres, or 
about 1050 Paris feet in a second. During these experiments the thermo- 
meter stood at 60°F., the barometer at 756.5 millimetres, and the hygrometer 
at 78°. Experiments recently performed by Sir John Herschel give 1125 
English feet per second as the rate of transmission at 621°F. Above 621°F., 
each degree adds 1.14 feet to this velocity, and below this temperature the 
velocity is diminished in the same ratio. 
As light travels faster than sound, it will be readily understood why the 
flash of a gun may be seen before hearing the report, and the lightning be 
observed long before the thunder reaches us, the interval depending upon 
the distance at which the phenomenon takes place. But for the numerous 
corrections required by the varying temperature, density, and hygrometric 
condition of the air, it would be an easy matter to determine the distance 
by this interval. 
c. Reflection of Sound. 
Whenever a sound attempts to pass from one medium into another, as 
from air into water, or from one gas to another, it experiences a partial 
reflection ; this, however, is strongest when the sound strikes against a solid 
i247 
