e, they act in this
way,--the first telephone changes the sound-waves it receives into
electric impulses which travel along the wire until they reach the
second telephone, here they are changed back to sound-waves exactly
like those received by the first telephone. Accordingly, the listener
in New York seems to hear the very tones of his friend who is speaking
at the other end of the line, say, in Boston.

Still you don't see how.

It is not surprising, for in this description several scientific facts
and principles are involved; and all boys and girls cannot be expected
to know much about the laws of sound and electricity. Perhaps a little
explanation may make it clearer.

The most of you probably know that sound is produced by rapid motion.
Put your finger on a piano wire that is sounding, and you will feel
the motion, or touch your front tooth with a tuning-fork that is
singing; in the last case you will feel very distinctly the raps made
by the vibrating fork. Now, a sounding body will not only jar another
body which touches it, but it will also give its motion to the air
that touches it; and when the air-motions or air-waves strike the
sensitive drums of our ears, these vibrate, and we _hear_ the sound.

You all have heard the windows rattle when it thunders loudly, or when
cannons have been fired near-by. The sound waves in the air fairly
shake the windows; and, sometimes, when the windows are closed, so
that the air-waves cannot pass readily, the windows are shattered by
the shock. Fainter sounds act less violently, yet similarly. Every
time you speak, your voice sets everything around you vibrating in
unison, though ever so faintly.

Thus, from your every-day experience you have proof of two important
facts,--first, sound is caused by rapid motion; second, sound-waves
give rise to corresponding motion. Both these facts are involved in
the speaking telephone, which performs a twofold office,--that of the
ear on the one hand, that of our vocal organs on the other.

To serve as an ear, the telephone must be able to take up quickly and
nicely the sound-waves of the air. A tightened drum-head will do that;
or better, a strip of goldbeaters'-skin drawn tightly over a ring
or the end of a tube. But these would not help Professor Bell, the
inventor of the telephone we shall describe, since he wanted an ear
that would translate the waves of sound into waves of electricity,
which would travel farther and faster tha