CH. xxx. CHLADNfS SAND-FIGURES. 271 



then, if he scattered sand over a piece of glass and struck 

 the glass so that it gave out a note, the sand ought to be 

 driven from those places which were vibrating, and to 

 collect on the nodes where there was little or no movement. 

 Accordingly he took a round piece of brass belonging 

 to a grinding-machine, and fastening it by its centre in 

 a vice, scattered some sand over it, and drew a violin 

 bow across the edge. As he had expected, the sand 

 moved away from the bow, and from several other points 

 at equal distances round the circle, and arranged itself 



Fig. 50. 

 Plates of brass made to vibrate by a violin bow. 



in the form of a star with several points ; at the same time 

 the metal gave out a very sharp note, showing that the 

 vibrations were very rapid. After many other experi- 

 ments he proved that when the bow was drawn gently, so 

 as to give out the lowest tone of the plate, the sand 

 arranged itself in two lines crossing each other at the 

 centre (see Fig. 50). This experiment is not difficult to 

 try with a plate of metal or glass and a vice ; it will be the 

 more sure to succeed if you place your finger on one of 

 the nodes, that is -|th of the circle away from the bow so 

 as to be sure of getting the lowest note. But you may pro- 

 duce the same effect still more simply by taking a common 

 finger-glass half full of water, and drawing a violin bow 

 gently across the edge at a. Fig. 51. You will then find 



