THE VOICES OF NATURE. 135 
back rebound from mountain to mountain and are 
driven backwards and forwards, becoming fainter and 
fainter till they die away ; these echoes are very 
beautiful. 
If you are now*a.ble to picture to yourselves one set 
of waves going to the wall, and another set returning 
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 
beautiful 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. 
