There is an echo in
Woodstock Park which repeats the word twenty times. Again
sometimes, as in the Alps, the sound-waves coming 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 able 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 you eye as it goes on its way undisturbed by the
rest.
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