Sweet Home," for example, a bass voice may start with a
note vibrating only 132 times a second; while a tenor may start at a
higher pitch, with a note vibrating 198 times per second, and a
soprano would probably take a much higher range still, with an initial
frequency of 528 vibrations per second. But no matter where the voices
start, the intervals are always identical. The air as sung by the bass
voice would be represented by _A_. The air as sung by the tenor voice
would be represented by _B_. The air as sung by the soprano voice
would be represented by _C_.

[Illustration: FIG. 178.--A song as sung by three voices of different
pitch.]

CHAPTER XXVIII

MUSICAL INSTRUMENTS

265. Musical instruments maybe divided into three groups according
to the different ways in which their tones are produced:--

_First._ The stringed instruments in which sound is produced by the
vibration of stretched strings, as in the piano, violin, guitar,
mandolin.

_Second._ The wind instruments in which sound is produced by the
vibrations of definite columns of air, as in the organ, flute, cornet,
trombone.

_Third._ The percussion instruments, in which sound is produced by the
motion of stretched membranes, as in the drum, or by the motion of
metal disks, as in the tambourines and cymbals.

266. Stringed Instruments. If the lid of a piano is opened, numerous
wires are seen within; some long, some short, some coarse, some fine.
Beneath each wire is a small felt hammer connected with the keys in
such a way that when a key is pressed, a string is struck by a hammer
and is thrown into vibration, thereby producing a tone.

If we press the lowest key, that is, the key giving forth the lowest
pitch, we see that the longest wire is struck and set into vibration;
if we press the highest key, that is, the key giving the highest
pitch, we see that the shortest wire is struck. In addition, it is
seen that the short wires which produce the high tones are fine,
while the long wires which produce the low tones are coarse. The
shorter and finer the wire, the higher the pitch of the tone produced.
The longer and coarser the wire, the lower the pitch of the tone
produced.

[Illustration: FIG. 179.--Piano wires seen from the back.]

The constant striking of the hammers against the strings stretches and
loosens them and alters their pitch; for this reason each string is
fastened to a screw which can be turned so as to tighten the string or