person can find some tone of
the voice that will seem to meet some response from the room. Some short
tunnels will from certain positions yield very powerful, responsive,
resonant tones. There is certainly one such in Central Park, New York.
It is forty or fifty feet long. To a person standing in the middle of
this, and speaking or making any kind of a noise on a certain pitch, the
resonance is almost deafening. It is easy to understand. When a column
of air enclosed in a tube is made to vibrate by any sound whose
wave-length is twice the length of the tube, we have such column of air
now filled with the condensed part of the wave, and now with the
rarefied part; and as these motions cannot be conducted laterally, but
must move in the direction of the length of the tube, the air has a
very great amplitude of motion, and the sound is very loud. If one end
of the tube be closed, then the length must be but one-fourth of the
wave-length of the sound. Take a tuning-fork of any convenient pitch,
say a C of 512 vibrations per second: hold it while vibrating over a
vertical test-tube about eight inches long. No response will be heard;
but, if a little water be carefully poured into the tube to the depth of
about two inches, the tube will respond loudly, so that it might be
heard over a large hall. In this case the length of the air-column that
was responding, being one-fourth the wave-length, would give twenty-four
inches as the wave-length of that fork.

It is easy in this way to measure approximately the number of vibrations
made by a fork.

Letting _l_ = depth of tube,

_d_ = diameter of tube,

_v_ = velocity of sound reduced for temperature,

_N_ = number of vibrations,

Then _N_ = _v_
------------
(4(_l_+_d_)).

When a vibrating tuning-fork is placed opposite the embouchure of an
organ-pipe of the same pitch, the pipe will resound to it, giving quite
a volume of sound. In 1872 it occurred to me, that the action of an
organ-pipe might be quite like that of a vibrating reed in front of the
embouchure. As the air is driven past it from the bellows, the form of
the escaping air will evidently be like a thin, elastic strip; and,
having considerable velocity, it will carry off by friction a little of
the air in the tube: this will of course rarefy the air in the tube
somewhat, and a wave of condensation will travel down th