On the Vibrations or Strings.
A stringed instrument consists essentially of three parts, viz:— the string, catgut, or wire, to be set in vibration ; some means of setting up this vibration; and a sound-board, or other resonant body, by means of which the vibratory movement is to be transmitted to the air.
The means by which strings are set in vibration vary in different instruments. They may be struck by a hammer, as in the pianoforte ; bowed, as in instruments of the violin class; set in motion by a current of air, as in the case of the iEolian harp ; or plucked, like the harp and zither. In this last case, the plucking may be done with the finger tips, as in the case of the harp and guitar, or by means of a quill or plectrum, as in the zither and harpsichord. In the case of bowed instruments, the particles of resin with which the bow is rubbed, catch hold of the portion of the string with which they are in contact, and pull it aside; its own elasticity soon sends it back, but being immediately caught up again by the bow, the vibrations are rendered continuous.
The vibrating string, presenting so small a surface, is capable of transmitting very little of its motion directly to the air. It is necessary that its vibrations should first be communicated to some body, which presents a much larger surface to the air. Thus, in the pianoforte, the vibrations of the wires are first transmitted, by means of the bridge and wrest-pins, to a sound-board; in the harp, the motion of the strings is communicated to the massive framework. In the violin, the vibratory movement of the strings is communicated by means of the bridge to the " belly." The bridge stands on two feet, immediately beneath one of which is the '' sound post," which transmits the motion to the "back " of the instrument, the whole mass of air between the " back" and the "belly" thus being set in vibration.