The broad deep river, on the contrary, makes none of these cascades and commotions. The only places against which it rubs are the banks and the bottom; and here you can sometimes hear it grating the particles of sand against each other if you listen very carefully. But there is another reason why falling water makes a sound, and often even a loud roaring noise in the cataract and in the breaking waves of the sea. You do not only hear the water dashing against the rocky ledges or on the beach, you also hear the bursting of innumerable little bladders of air which are contained in the water. As each of these bladders is dashed on the ground, it explodes and sends sound-waves to your ear. Listen to the sea some day when the waves are high and stormy, and you cannot fail to be struck by the irregular bursts of sound.
The waves, however, do not only roar as they dash on the ground; have you never noticed how they seem to scream as they draw back down the beach? Tennyson calls it,
“The scream of the madden’d beach dragged down by the wave;” and it is caused by the stones grating against each other as the waves drag them down. Dr. Tyndall tells us that it is possible to know the size of the stones by the kind of noise they make. If they are large, it is a confused noise, when smaller, a kind of scream; while a gravelly beach will produce a mere hiss.
Who could be dull by the side of a brook, a waterfall, or the sea, while he can listen for sounds like these, and picture to himself how they are being made? You may discover a number of other causes of sound made by water, if you once pay attention to them.
Nor is it only water that sings to us. Listen to the wind, how sweetly it sighs among the leaves. There we hear it, because it rubs the leaves together, and they produce the sound-waves. But walk against the wind some day and you can hear it whistling in your own ear, striking against the curved cup, and then setting up a succession of waves in the hearing canal of the ear itself.
Why should it sound in one particular tone when all kinds of sound-waves must be surging about in the disturbed air?
This glass jar will answer our question roughly. If I strike my tuning-fork and hold it over the jar, you cannot hear it, because the sound is feeble, but if I fill the jar gently with water, when the water rises to a certain point you will hear a loud clear note, because the waves of air in the jar are exactly the right length to answer to the note of the fork. If I now blow across the mouth of the jar you hear the same note, showing that a cavity of a particular length will only sound to the waves which fit it. do you see now the reason why pan-pipes give different sounds, or even the hole at the end of a common key when you blow across it? Here is a subject you will find very interesting if you will read about it, for I can only just suggest it to you here. But now you will see that the canal of your ear also answers only to certain waves, and so the wind sings in your ear with a real if not a musical note.