Sometimes, in the mountains, walls of rock rise at some distance one behind another, and then each one will send back its echo a little later than the rock before it, so that the “Ha” which you give will come back as a peal of laughter.  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.  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.

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. 33, we will try to understand roughly our beautiful hearing instrument, the ear.

First, I want you to notice how beautifully the outside shell, or concha as it is called, is curbed round so that any movement of the air coming to it from the front is caught in it and reflected into the hole of the ear.  Put your finger round your ear and feel how the gristly part is curved towards the front of your head.  This concha makes a curve much like the curve a deaf man makes with his hand behind his ear to catch the sound.  Animals often have to raise their ears to catch the sound well, but ours stand always ready.  When the air-waves have passed in at the hole of your ear, they move all the air in the passage, which is called the auditory, or hearing, canal.  This canal is lined with little hairs to keep out insects and dust, and the wax which collects