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I stop the vibrating fork; but the sound has not ceased.
The second fork has taken up the vibrations of its neigh-
bor, and is now sounding in its turn. I dismount one of
the forks, and permit the other to remain upon its stand.
I throw the dismounted fork into strong vibration, but you
cannot hear it sound. Detached from its stand the amount
of motion which it can communicate to the air is too small
to make itself sensible to the ear at any distance. I now
bring the dismounted fork close to the mounted one, but
not into actual contact with it. Out of the silence rises a
mellow sound. Whence comes it ? From the vibrations
which have been transferred from the dismounted fork to
the mounted one.

That motion should thus transfer itself through the air
it is necessary that the two forks should be in perfect unison.
If I place on one of the forks a morsel of wax not larger
than a pea, it is rendered thereby powerless to affect, or to
be affected by, the other. It is easy to understand this
experiment. The pulses of the one fork can affect the other,
because they are perfectly timed. A single pulse causes
the prong of the silent fork to vibrate through an infinitesi-
mal space. But just as it has completed this small vibra-
tion, another pulse is ready to strike it. Thus, the small
impulses add themselves together. In the five seconds
during which the forks were held near each other, the vi-
brating fork sent 1,280 waves against its neighbor, and
those 1,280 shocks, all delivered at the proper moment, all,
as I have said, perfectly timed, have given such strength
to the vibrations of the mounted fork as to render them
audible to you all.

Let me give you one other curious illustration of the
influence of synchronism on musical vibrations. Here are
three small gas-flames inserted in three glass tubes of dif-
ferent lengths. Each of these flames can be caused to emit
a musical note, the pitch of which is determined by the

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