The sound-hoard is a part of the instrument on which the quality of tone in a great degree depends. It is ge nerally made of _Swiss fir, which at one time was believed to be the only wood fit for the purpose. While the Bri tish ports were shut, however, during the revolutionary war, necessity obliged the makers to try the substitution of other kinds, and the American white pine was found frequently to give as good a tone. The sound-board is about one-fifth of an inch in thickness, and in the square. piano-forte occupies about two-fifths of the length of the instrument. In the grand and cabinet it occupies the whole area ; but part of it is deafened by bars of wood glued on the underside.
The quality of tone depends also on the rapidity with which the hammer strikes the string. The mechanism of Schroeter's hammers were simple. The hammer con sisted of a lever, of about 31 inches in length, moving on a pivot with a leather head. The lever rested near the pivot, on a pin with a leather head screwed into the farther end of the finger-key ; and the pin was of such a length, that, when the key was slowly pressed down, the face of the hammer came within about a quarter of an inch of the string ; but when the key was struck smartly, the ham mer, by the rapid motion communicated, was thrown up to give the string a blow, and, instantly recoiling, fell on the leather head of the pin, and left the string free to vi brate. And this form of the mechanism continued for a long while after the piano-forte was in general use. It has subsequently become more complex, for the purpose of attaining more rapidity and smartness in the blow ; and this for the sake of enabling the performer to produce greater contrast of loudness and softness, and greater de licacy in the shades, on which the of the in strument chiefly depends The mechanism consists of an additional lever, placed under that of the proper hammer ; the object of which is, to apply the moving power as near as possible to the pivot of the hammer, which, it is evi dent, increases the rapidity of the blow. The end of the under lever rests on a little piece of mechanism, fixed in the finger-key, called a grasshopper, not unlike, in its ob ject and contrivance, to that of the jack of the harpsi chord. When the key is struck, the upper end of the grasshopper, which is about of an inch in thickness only, is carried past the end of the under lever, which rested on it, but communicates its impulse in passing, and receives the end of the lever on a little block of wood, glued on about a quarter of an inch below. In returning, the grasshopper, which is kept in its upright position by a slight spring of brass wire, yields, and passes the end of the lever again to its original position.
There is a nicety in the structure of the head of the hammer of great importance to the quality of the tone ; and many experiments have been made, and are, we be lieve, daily making, to attain improvement. The head is made of many folds of leather glued over each other. The best outside cover is found to be doe-skin ; and there is even a nicety in the degree with which this is stretched.
In the grand piano-forte the whole key-board is move able towards the left hand ; and, by means of a pedal, the performer has it in his power to make the hammer, which, without the use of the pedal, strikes all the three unisons, at pleasure strike two, or only one of them. The
cabinet piano-forte has a similar pedal, affecting its two unisons. This is an additional source of variety and ex pression.
To each string there is a damper, which is a bit of cloth glued on a little block of wood, and rests on the unisons ; a bit of wire attached to the block passes down, and nearly touches the farther end of the finger-key. When the key is pressed, it raises the damper, and so long as the key is held down, the string has freedom to speak ; when the finger is released, the damper falls down and silences the string. A pedal raises all the dampers at once, and allows, at the performer's pleasure, the sound of the strings to continue after the fingers are removed.
Among the many improvements offered to the public, Clementi and Co. have lately brought forward one which they call the harmonic swell, and which promises to fur nish still additional variety of effect and expression. In the usual construction of the instrument, there is between the bridges on the sound board, and the pins on which the strings are hung, a considerable space ; and it is usual to pass a narrow piece of cloth alternately over and under this portion of the strings, behind the bridges, to prevent any sound being propagated among them. Clementi and Co. have placed an additional bridge on the sound-board between the others and the pins on which the strings are hung, which they call the bridge of reverberation. A general damper lies on all these back strings, which, at the pleasure of the performer, is raised by means of an additional pedal. This new bridge is in a curve some what similar to that of the proper bridge ; but there does not seem to be any principle observed in proportioning the new reverberating string to the length of the proper string to which it belongs.
Messrs. Mott of Pall-Mall have added to the grand piano forte what they call a sostenente, which holds the note while the finger is kept on the key. It resembles the celestina stop long ago applied to the harpsichord ; but the effect is produced, not by drawing a skein of silk over the strings, as in that invention, but on a different and very curious principle. A strong silk thread is stretched across the strings of the grand piano forte, and to each fin ger-key there is a strong silk thread, to which is attached a skein of silk ; which skein passes over a cylinder of about two inches diameter, and is ultimately attached by three threads to the cross thread above mentioned. When the finger-key is pressed, it stretches the skein over the cylinder, and brings the cross thread to press on the string. At the same time the cylinder is turning on its axis, and, being touched by the dust of fiddle rosin, communicates vibration to the string. The patentees say that the sostenente requires no separate tuning. But when the piano-forte is tuned to twelve equal semi-tones, the thirds are so harsh, that they would find it a real im provement to tune the instrument more on the organ prin ciple, that is, to flatten the fifths so as to make the thirds be tter.
For the temperament, or tuning of the piano-forte, see Music, Art. 267.