2 THE POPULAR SCIENCE MONTHLY. 



duction of noise, the thrills of matter are transient and irregular, but, 

 when prolonged and regular, they give rise to musical sounds. Vibra- 

 tion depends upon elasticity, and bodies which are capable of the pro- 

 tracted and measured pulsations of music must, of course, be highly 

 elastic. We have said that bodies vibrate differently, and this de- 

 pends 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 differ- 

 ent 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 length- 

 wise 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. 



Fig. 1. 





A String in Different Phases of Vibration. 



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 col- 

 lected in other parts, forming regular geometrical figures. The sand 

 collects at the lines of rest, which are called nodal lines. Fig. 3 rep- 

 resents 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. 



