[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 to loosen it if necessary.  The tuning of the piano is the adjustment of the strings so that each shall produce a tone of the right pitch.  When the strings are tightened, the pitch rises; when the strings are loosened, the pitch falls.

What has been said of the piano applies as well to the violin, guitar, and mandolin.  In the latter instruments the strings are few in number, generally four, as against eighty-eight in the piano; the hammer of the piano is replaced in the violin by the bow, and in the guitar by the fingers; varying pitches on any one string are obtained by sliding a finger of the left hand along the wire, and thus altering its length.

Frequent tuning is necessary, because the fine adjustments are easily disturbed.  The piano is the best protected of all the stringed instruments, being inclosed by a heavy framework, even when in use.

[Illustration:  FIG. 180.—­Front view of an open piano.]

267.  Strings and their Tones.  Fasten a violin string to a wooden frame or box, as shown in Figure 181, stretching it by means of some convenient weight; then lay a yardstick along the box in order that the lengths may be determined accurately.  If the stretched string is plucked with the fingers or bowed with the violin bow, a clear musical sound of definite pitch will be produced.  Now divide the string into two equal parts by inserting the bridge midway between the two ends; and pluck either half as before.  The note given forth is of a decidedly higher pitch, and if by means of the siren we compare the pitches in the two cases, we find that the note sounded by the half wire is the octave of the note sounded by the entire wire; the frequency has been doubled by halving the length.  If now the bridge is placed so that the string is divided into two unequal portions such as 1:3 and 2:3, and the shorter portion is plucked, the pitch will be still higher; the shorter the length plucked, the higher the pitch produced.  This movable bridge corresponds to the finger of the violinist; the finger slides back and forth along the string, thus changing the length of the bowed portion and producing variations in pitch.

[Illustration:  FIG. 181.—­The length of a string influences the pitch.]

[Illustration:  FIG. 182.—­Only one half of the string is bowed, but both halves vibrate.]

If there were but one string, only one pitch could be sounded at any one time; the additional strings of the violin allow of the simultaneous production of several tones.

268.  The Freedom of a String.  Some stringed instruments give forth tones which are clear and sweet, but withal thin and lacking in richness and fullness.  The tones sounded by two different strings may agree in pitch and loudness and yet produce quite different effects on the ear, because in one case the tone may be much more pleasing than in the other.  The explanation of this is, that a string may vibrate in a number of different ways.