Now you are all ready to go to work. What are you going to work with?
Books? pencils? paper? Yes, but you have something better than those
and all ready for use. It is that little kit of tools that are
sometimes called our "Five Senses." You remember that we have already
talked about one of them, the sense of touch in the skin. Now which
one are you going to use first this morning? If your teacher talks to
you, I hope it will be the one we call the sense of hearing. Suppose
we try to find out something about this sense of hearing, and begin
with a little experiment.
Take a piece of cork in your hand and lift it up high and then let it
drop into a large basin or tub of water. What happens? The cork
strikes and then goes bob-bob-bobbing up and down on its own waves.
Now watch the little waves all around the cork. Where do they stop?
They don't stop until they touch the edge of the pan; and no matter
how big the pan is, the waves go on and on until they reach the edge.
We can see these waves of water, and so we easily believe that they
are there. Now there are, just as truly, waves of air all around us.
We cannot see the waves, because they are too small and roll too
quickly. But some of these, when they roll against our ears, make us
hear. They make what we call _sound_. You have heard about sending
messages through the air, without telegraph wires. Wireless messages
are often sent to ships out in the middle of the ocean. This is done
by starting tiny electric waves, which travel through the air much as
the waves of water are traveling across the ocean beneath. Of course
there must be a machine, called a _receiver_, to catch the waves and
"hear" the message.
Mother Nature has given each of you two very delicate little receivers
to catch the sound waves and carry them to your brain. You know what
they are--you can name them. But how are these wonderful little
machines made?
You have never seen the whole of your ear. The part on the outside of
the head, of course, you can easily see and feel. Sometimes you notice
a deaf person put his hand behind his ear and press it forward so as
to catch the sound waves better. These waves roll in at the little
hole you can see, and travel along a short passage till they come to a
round _drum_, a piece of very thin skin stretched tight like a
drumhead.
Have you ever beaten a drum with a stick? You felt the drumhead quiver
under the blow, did you not? Well, when the sound waves beat against
the drum in the ear, it quivers and starts little waves inside the
ear. Each little wave in turn beats against a little bone called the
_hammer_; the hammer beats against another called the _anvil_, and
this against a third called the _stirrup_; and the quiver of the
stirrup is passed on to a little window, opening into a little room
with a spiral key-board; and from this, the wave travels along a nerve
to the brain. As the waves reach the brain, the brain hears. In this
way we hear all sorts of sounds, from the tick of a watch to the
whistle of a train.
[Illustration: THE WAY BY WHICH SOUND WAVES REACH THE BRAIN