le, however, and so the space
available for the remaining air soon again becomes so crowded with air
molecules that the pressure is again sufficient to open the membranes.
Another puff of air escapes.

This happens over and over again while one is speaking or singing.
Hundreds of times a second the vocal cords vibrate back and forth. The
frequency with which they do so determines the note or pitch of the
speaker's voice.

What determines the significance of the sounds which he utters? This is
a most interesting question and one deserving of much more time than I
propose to devote to it. To give you enough of an answer for your study
of radio-telephony I am going to tell you first about vibrating strings
for they are easier to picture than membranes like the vocal cords.

Suppose you have a stretched string, a piece of rubber band or a wire
will do. You pluck it, that is pull it to one side. When you let go it
flies back. Because it has inertia[7] it doesn't stop when it gets to
its old position but goes on through until it bows out almost as far on
the other side.

[Illustration: Pl. VII.--Photographs of Vibrating Strings.]

It took some work to pluck this string, not much perhaps; but all the
work which you did in deforming it, goes to the string and becomes its
energy, its ability to do work. This work it does in pushing the air
molecules ahead of it as it vibrates. In this way it uses up its energy
and so finally comes again to rest. Its vibrations "damp out," as we
say, that is die down. Each swing carries it a smaller distance away
from its original position. We say that the "amplitude," meaning the
size, of its vibration decreases. The frequency does not. It takes just
as long for a small-sized vibration as for the larger. Of course, for
the vibration of large amplitude the string must move faster but it has
to move farther so that the time required for a vibration is not
changed.

First the string crowds against each other the air molecules which are
in its way and so leads to crowding further away, just as fast as these
molecules can pass along the shove they are receiving. That takes place
at the rate of about 1100 feet a second. When the string swings back it
pushes away the molecules which are behind it and so lets those that
were being crowded follow it. You know that they will. Air molecules
will always go where there is the least crowding. Following the shove,
therefore, there is a chance for the mo