140 THE FAIRY-LAND OF SCIENCE, 
thousand strings in your ear, which is called Corti's 
organ, vibrates to the air-waves, one thread to one set 
of waves, and another to another, and according to 
the fibre that quivers, will be the sound you hear. 
Here then at last, we see how nature speaks to us. 
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 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 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 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. 
