For example, when a
load of stones is being shot out of a cart, you hear only a long,
continuous noise, because the stones fall irregularly, some
quicker, some slower, here a number together, and there two or
three stragglers by themselves; each of these different shocks
comes to your ear and makes a confused, noisy sound. But if you
run a stick very quickly along a paling, you will hear a sound
very like a musical not. This is because the rods of the paling
are all at equal distances one from another, and so the shocks
fall quickly one after another at regular intervals upon your
ear. Any quick and regular succession of sounds makes a note,
even though it may be an ugly one. The squeak of a slate pencil
along a slate, and the shriek of a railway whistle are not
pleasant, but they are real notes which you could copy on a
violin.
I have here a simple apparatus which I have had made to show you
that rapid and regular shocks produce a natural musical note.
This wheel (Fig.
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