22 PHYSICS. 
distinctly at a great distance at a particular time. Sounds are also better 
heard in a dry atmosphere, as moisture seems to interfere with the 
propagation of sound-waves. Although a simple current of air does not 
of itself produce sound (it only does so when it comes in contact with 
obstacles in its way, as trees, houses, &c., by which vibrations are pro- 
duced), yet it will carry sound-waves in any direction; sounds are 
therefore heard more distinctly when there is a breeze blowing from the 
source of the sound towards the hearer. 
Velocity of Sound—The velocity with which sound-waves are propa- 
gated can be calculated very exactly, owing to the extreme rapidity with 
which light travels. The velocity of light is so great that for any distance 
within the limits to which sound could be heard, its passage may be 
considered as instantaneous. This being the case, the blow, shot, 
explosion, &c. causing any noise, may be seen to take place before the 
noise is heard; and the time that elapses between seeing the cause and 
hearing the noise is the time that it takes for the propagation of the 
sound-waves to the distance of the hearer. If a man be observed at a 
distance striking heavy blows on anything, the blow will be seen to 
descend some time before the sound of it is heard ; so with the firing of 
a gun, the flash is seen before the report is heard. It has been found by 
experiment that sound travels at the rate of about 1142 feet per second, 
or of a mile in about four seconds anda half. The velocity is somewhat 
less, however, when the temperature is lower, being only 1090 feet at the 
freezing-point. In water, sound travels Ane ica times, and in solids, 
from ten to twenty times more quickly than in air. 
Reflection of Sound—Echo—We are now to consider what takes 
place when a sound-wave meets a large obstacle to its progress. The 
vibration is to a small degree comm ed to the solid, just as light is 
transmitted through a new Aa ; but the principal effect is to reflect 
the wave, just as a wave of the sea is thrown back from a rock. Whena 
sound-wave is thrown back in this way from ‘a flat surface, the sound is 
carried back to the point where it was produced, and this return-sound is 
called an echo. It may be produced by a perpendicular wall of rock, 
a wood, or other obstacle. If a sound be produced between two faces 
of rock, for instance, it will first be echoed by both; and then those 
echoed sounds (the sound being now doubled) will be thrown back from 
one to the other again and again, becoming fainter each time ; so much 
so, that a shot in such a situation may be repeated forty or fifty times. 
The best echo is produced by a concave surface, such as the roof of a cave, 
by which the sound-wave is reflected to a focus, as in the case of rays of 
light. If one happen to stand so as to bring one’s ear into the focus, the 
whole of the sound, concentrated at that point, falls upon the ear, and the 
effect is astounding. 
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