The Transmission of Sound.
We have seen in the preceding chapter, that all sounds originate in the vibratory movements of bodies. This vibratory motion is capable of being communicated to, and transmitted by, almost all substances, to a greater or less extent. Wood, glass, water, brass, iron, and metals in general may be taken as examples of good conductors of sound. But the substance, which in the vast majority of cases transmits to our ears the vibratory motion, which gives rise to the sensation, sound, is the air.
A very little reflection is sufficient to show us that some medium is absolutely necessary for the transmission of sound; for inasmuch as sound is caused by vibratory motion, it is plain that this motion cannot pass through a vacuum. This fact can, however, be easily proved experimentally. Under the receiver of an air-pump a bell is suspended. We shake the pump, and the bell may be heard ringing ; for its vibrations are communicated by the air to the glass of the receiver, and by the latter to the outer air, and so to our ears. We now pump as much air as possible out of the receiver, and again ring the bell. It can still be heard faintly, for we cannot remove all the air. We now allow dry hydrogen to pass into the receiver, and on again ringing the bell, there is very little increase of sound, this gas being so very light—only aboutas heavy as air. If we now exhaust the receiver, we shall be able still further to attenuate the atmosphere within it, and then, although we may violently shake the apparatus, no sound will be heard.
As the air is generally the medium, by means of which the vibratory motion reaches our ears, we shall now have to carefully study the manner in which this transmission takes place. When we stand on the sea shore, or better still, on a cliff near it, and watch the waves rolling in from afar, our first idea is, that the