When it is once understood that a musical sound is caused by the
vibrations more or less frequent which only make the difference we call
pitch, it might at once be inferred, that if we have a body that is
capable of vibrating say a hundred times a second, and it receives a
hundred pulses or pushes a second, it would in this way be made to
vibrate. Suppose, then, that we take two tuning-forks, each capable of
vibrating 256 times a second: if one be struck while the other is left
free, the former one will be giving to the air 256 impulses per second,
which will reach the other fork, each pulse tending to move it a little,
the cumulative result being to make it move perceptibly, that is, to
give out a sound. The principle is just the same as that employed in the
common swing. One push makes the swing to move a little, upon its return
another is given, in like manner a third, and so on until a person may
be swung many feet high. If a glass tumbler be struck, it gives out a
musical sound of a certain pitch, which will set a piano-string sounding
that is tuned to the same pitch, provided that the damper be raised. It
is said that some persons' voices have broken tumblers by singing
powerfully near them the same note which the tumblers could give out,
the vibrations of the tumblers being so great as to overcome cohesion of
the molecules.
There are very many interesting effects due to sympathetic vibrations.
Large trees are sometimes uprooted by wind that comes in gusts timed to
the rate of vibration of the tree. When troops of soldiers are to cross
a bridge, the music ceases, and the ranks are broken, lest the
accumulated strain of timed vibrations should break the structure;
indeed, such accidents have several times occurred. There is not so much
danger to a bridge when it is heavily loaded with men or with cattle, as
when a few men go marching over it. "When the iron bridge at Colebrooke
Dale was building, a fiddler came along, and said to the workmen that he
could fiddle their bridge down. The builders thought this boast a
fiddle-de-dee, and invited the musician to fiddle away to his heart's
content. One note after another was struck upon the strings, until one
was found with which the bridge was in sympathy. When the bridge began
to shake violently, the workmen were alarmed at the unexpected result,
and ordered the fiddler to stop."
Some halls and churches are wretchedly adapted to hear either speaking
or singing in. If wires be stretched across such halls, between the
speaker's stand and the opposite end, they will absorb the passing
sound-waves, and will be made to sympathetically vibrate, thus
preventing in a good degree the interfering echoes. The wire should be
rather fine piano-wire, and it should be stretched so tightly as to give
out a low musical sound when plucked with the fingers. In a large hall
there should be twenty or more such wires.