140 THE FAIRY-LAND OF SCIENCE. 
and crossing them, you will be ready to understand 
something of that very difficult question, How is it 
that we can hear many different sounds at one time 
and tell them apart ? 
Have you ever watched the sea when its surface is 
much ruffled, and noticed how, besides the big waves 
of the tide, there are numberless smaller ripples made 
by the wind blowing the surface of the water, or the 
oars of a boat dipping in it, or even rain-drops falling? 
If you have done this you will have seen that all 
these waves and ripples cross each other, and you can 
follow any one ripple with your eye as it goes on its 
way undisturbed by the rest. Or you may make beau- 
tiful crossing and recrossing ripples on a pond by 
throwing in two stones at a little distance from each 
other, and here too you can follow any one wave on 
to the edge of the pond. 
Now just in this way the waves of sound, in their 
manner of moving, cross and recross each other. You 
will remember too, that different sounds make waves 
of different lengths, just as the tide makes a long wave 
and the rain-drops tiny ones. Therefore each sound 
falls with its own peculiar wave upon your ear, and 
you can listen to that particular wave just as you look 
at one particular ripple, and then the sound becomes 
clear to you. 
All this is what is going on outside your ear, but 
what is happening in your ear itself? How do these 
blows of the air speak to your brain? By means of 
the following diagram, Fig. 35, we will try to under- 
stand roughly our beautiful hearing instrument, the 
ear. 
