duction of noise, the thrills of matter are transient and irregular, but, when prolonged and regular, they give rise to musical sounds. Vibration depends upon elasticity, and bodies which are capable of the protracted and measured pulsations of music must, of course, be highly elastic. We have said that bodies vibrate differently, and this depends upon the nature, form, and magnitude of the mass in motion. The vibrations of bells differ with their sizes and the metals and alloys which compose them; while wooden and metallic tubes, strained strings, and stretched membranes, illustrate the same thing. If a tense wire be plucked aside, it executes lateral vibrations which differ with its varying length, strain, and density. It may vibrate as a whole (1), Fig. 1, while, by relaxing the tension, or by touching or damping it at different points, it may be made to break up into different systems of vibration as shown in (2), (3), (4), Fig. 1. The points of rest in such cases are called nodes. Rods and tubes of wood or glass may be made to vibrate longitudinally by rubbing them lengthwise with the rosined fingers or a damp cloth. Fig. 2 represents a glass tube, six feet long and two inches in diameter, which, by being vigorously rubbed in this way, was set into such violent vibration that it went to pieces.
If thin plates of glass or metal be clamped in the centre, and fine sand scattered over the surface, they may be set into vibration, and the sand will be tossed away from certain parts of the surface and collected in other parts, forming regular geometrical figures. The sand collects at the lines of rest, which are called nodal lines. Fig. 3 represents this experiment, the vibration being produced by a fiddle-bow, while the application of the fingers at different points determines the lines of rest and the geometrical figures. Fig. 4 represents a number of the beautiful patterns that were obtained by Chladni, who first drew attention to this interesting phenomenon.