If the hammer is hard, and impelled with force, the string breaks into
shorter sections, and the discordant upper partials of the string, thus
brought into prominence, make the tone harsh. If the hammer is soft, and
the force employed is moderated, the harmonious partials of the longer
sections strike the ear, and the tone is full and round. By the
frequency of vibration, that is to say, the number of times a string
runs through its complete changes one way and the other, say, for
measurement, in a second of time, we determine the pitch, or relative
acuteness of the tone as distinguished by the ear.
We know, with less exactness, that the sound-board follows similar laws.
The formation of nodes is helped by the barring of the sound-board,
a ribbing crosswise to the grain of the wood, which promotes the
elasticity, and has been called the "soul" of stringed musical
instruments. The sound-board itself is made of most carefully chosen
pine; in Europe of the _Abies excelsa_, the spruce fir, which, when well
grown, and of light, even grain, is the best of all woods for resonance.
The pulsations of the strings are communicated to the sound-board by the
bridge, a thick rail of close-grained beech, curved so as to determine
their vibrating lengths, and attached to the sound-board by dowels.
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