vibrations of the inclosing case, or of the sounding board.
If a vibrating tuning fork is held in the hand, the sound will be
inaudible except to those quite near; if, however, the base of the
fork is held against the table, the sound is greatly intensified and
becomes plainly audible throughout the room.

The vibrations of the fork are transmitted to the table top and throw
it into vibrations similar to its own, and these additional vibrations
intensify the original sound. Any fork, no matter what its frequency,
can force the surface of the table into vibration, and hence the sound
of any fork will be intensified by contact with a table or box.

This is equally true of strings; if stretched between two posts and
bowed, the sound given out by a string is feeble, but if stretched
over a sounding board, as in the piano, or over a wooden shell, as in
the violin, the sound is intensified. Any note of the instrument will
force the sounding body to vibrate, thus reenforcing the volume of
sound, but some tones, or modes of vibration, do this more easily than
others, and while the sounding board or shell always responds, it
responds in varying degree. Here again we have not only enrichment of
sound but also individuality of instruments.

271. The Kinds of Stringed Instruments. Stringed instruments may be
grouped in the following three classes:--

_a_. Instruments in which the strings are set into motion by
hammers--piano.

_b_. Instruments in which the strings are set into motion by
bowing--violin, viola, violoncello, double bass.

_c_. Instruments in which the strings are set into motion by
plucking--harp, guitar, mandolin.

[Illustration: FIG. 186.--1, violin; 2, viola; 3, violoncello; 4,
double bass.]

_a_. The piano is too well known to need comment. In
passing, it may be mentioned that in the construction of the
modern concert piano approximately 40,000 separate pieces of
material are used. The large number of pieces is due,
partly, to the fact that the single string corresponding to
any one key is usually replaced by no less than three or
four similar strings in order that greater volume of sound
may be obtained. The hammer connected to a key strikes
four or more strings instead of one, and hence produces a
greater volume of tone.

_b_. The viola is larger than the violin, has heavier and
thicker strings, and is pitched to a lower key; in all other