olor.

CHAPTER SIX

SOUND

SECTION 28. _What sound is._

What makes a dictaphone or a phonograph repeat your words?

What makes the wind howl when it blows through the branches of
trees?

Why can you hear an approaching train better if you put your
ear to the rail?

If you were to land on the moon tonight, and had with you a tank
containing a supply of air which you could breathe (for there is no
air to speak of on the moon), you might _try_ to speak. But you would
find that you had lost your voice completely. You could not say a
word. You would open and close your mouth and not a sound would come.

Then you might try to make a noise by clapping your hands; but that
would not work. You could not make a sound. "Am I deaf and dumb?" you
might wonder.

The whole trouble would lie in the fact that the moon has practically
no air. And sound is usually a kind of motion of the air,--hundreds of
quick, sharp puffs in a second, so close together that we cannot feel
them with anything less sensitive than the tiny nerves in our ears.

If you can once realize the fact that sound is a series of quick,
sharp puffs of air, or to use a more scientific term, _vibrations_ of
air, it will be easy for you to understand most of the laws of sound.

A phonograph seems almost miraculous. Yet it works on an exceedingly
simple principle. When you talk, the breath passing out of your throat
makes the vocal cords vibrate. These and your tongue and lips make the
air in front of you vibrate.

When you talk into a dictaphone horn, the vibrating air causes the
needle at the small end of the horn to vibrate so that it traces a
wavy line in the soft wax of the cylinder as the cylinder turns. Then
when you run the needle over the line again it follows the identical
track made when you talked into the horn, and it vibrates back and
forth just as at first; this makes the air in the horn vibrate exactly
as when you talked into the horn, and you have the same sound.

All this goes back to the fundamental principle that sound is
vibrations of air; different kinds of sounds are simply different
kinds of vibrations. The next experiments will prove this.

EXPERIMENT 54. Turn the rotator rapidly, holding the corner of
a piece of stiff paper against the holes in the disk. As
you turn faster, does the sound become higher or lower? Keep
turning at a steady rate and move your paper from the inner
row