needle of a phonograph. The chief
difference is that you have struck the tuning fork to make it and
the needle vibrate, instead of making it vibrate by air waves set in
motion by your talking. It is because these vibrations of the tuning
fork are more powerful and larger than are those of the recording
needle of a phonograph that you can see the record on the recording
drum, while you cannot see it clearly on the phonograph cylinder.

[Illustration: FIG. 100. How the apparatus is set up.]

In all ordinary circumstances, sound is the vibration of _air_. But
in swimming we can hear with our ears under water, and fishes hear
without any air. So, to be accurate, we should say that sound is
vibrations of any kind of matter. And the vibrations travel better in
most other kinds of matter than they do in air. Vibrations move rather
slowly in air, compared with the speed at which they travel in other
substances. It takes sound about 5 seconds to go a mile in air; in
other words, it would go 12 miles while an express train went one.
But it travels faster in water and still faster in anything hard like
steel. That is why you can hear the noise of an approaching train
better if you put your ear to the rail.

[Illustration: FIG. 101. When the tuning fork vibrates, the glass
needle makes a wavy line on the smoked paper on the drum.]

WHY WE SEE STEAM RISE BEFORE WE HEAR A WHISTLE BLOW. But even through
steel, sound does not travel with anything like the speed of light.
In the time that it takes sound to go a mile, light goes hundreds of
thousands of miles, easily coming all the way from the moon to the
earth. That is why we see the steam rise from the whistle of a train
or a boat before we hear the sound. The sound and the light start
together; but the light that shows us the steam is in our eyes almost
at the instant when the steam leaves the whistle; the sound lags
behind, and we hear it later.

_APPLICATION 42._ Explain why a bell rung in a vacuum makes
no noise; why the clicking of two stones under water sounds
louder if your head is under water, than the clicking of the
two stones in the air sounds if your head is in the air; why
you hear a buzzing sound when a bee or a fly comes near you;
how a phonograph can reproduce sounds.

INFERENCE EXERCISE

Explain the following:

251. The paint on woodwork blisters when hot.

252. You can screw a nut on a bolt very much tighter with a