THE VOICES OF NATURE. 145 
All the movements going on outside, however violent 
and varied they may be, cannot of themselves make 
sound. But here, in the little space behind the drum 
of our ear, the air-waves are sorted and sent on to our 
brain, where they speak to us as sound. 
But why then do we not hear all sounds as music? 
Why are some mere noise, and others clear musical 
notes? This depends entirely upon whether the 
sound-waves come quickly and regularly, or by an 
irregular succession of shocks. 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 them- 
selves; 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 note. This is because the 
rods of the paling are all at equal distances one from 
the other, and so the shocks fall quickly one after an- 
other 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 pro- 
duce a natural musical note. This wheel (Fig. 36) is 
milled at the edge like a quarter of a dollar, and when 
I turn it rapidly so that it strikes against the edge of 
