of holes to the outer row and back again. Which row has
the most holes in it? Which makes the highest sound? Hold your
paper against the teeth at the edge of the disk. Is the pitch
higher or lower than before? Blow through a blowpipe against
the different rows of holes while the disk is being whirled.
As the holes make the air vibrate do you get any sound?

This experiment shows that by making the air vibrate you get a sound.

The next experiment will show that when you have sound you are getting
vibrations.

EXPERIMENT 55. Tap a tuning fork against the desk, then
hold the prongs lightly against your lips. Can you feel them
vibrate? Tap it again, and hold the fork close to your ear.
Can you hear the sound?

[Illustration: FIG. 96. An interesting experiment in sound.]

The experiment which follows will show that we usually must have air
to do the vibrating to carry the sound.

EXPERIMENT 56. Make a pad of not less than a dozen thicknesses
of soft cloth so that you can stand an alarm clock on it on
the plate of the air pump. The pad is to keep the vibrations
of the alarm from making the plate vibrate. A still better way
would be to set a tripod on the plate of the air pump and to
suspend the alarm clock from the tripod by a rubber band. Set
the alarm so that it will ring in 3 or 4 minutes, put it under
the bell jar, and pump out the air. Before the alarm goes off,
be sure that the air is almost completely pumped out of the
jar. Can you hear the bell ring? Distinguish between a dull
trilling sound caused by the jarring of the air pump when the
alarm is on, and the actual _ringing_ sound of the bell.

[Illustration: FIG. 97. When the air is pumped out of the jar, you
cannot hear the bell ring.]

The experiment just completed shows how we know there would be no
sound on the moon, since there is practically no air around it. The
next experiment will show you more about the way in which phonographs
work.

EXPERIMENT 57. Put a blank cylinder on the dictaphone, adjust
the recording (cutting) needle and diaphragm at the end of the
tube, start the motor, and talk into the dictaphone. Shut
off the motor, remove the cutting needle, and put on the
reproducing needle (the cutting needle, being sharp, would
spoil the cylinder). Start the reproducing needle where the
recording needle started, turn on the motor, and