4 Dr. Tyndall on the Vibrations and Tones produced by 



" The vibrations never take place betiveen substances of the same 

 natwe.'* 



Let us see whether this law will bear the test of experiment. 



L Iron Rocker. 



Fig. 1 represents a sketch of the rocker; the length AC is 

 6*1 inches; the width AB 1-85, and the length of the stem EF 

 is 12 inches. Fig. 2 is a transverse section of the rocker, showing 



Fig. I. 



Fig. 2. 



Fig. 4. 



Fig. 3. 



the groove underneath ; the depth SN is 0*5 of 

 an inch. The distance rt is 0*35 of an inch, 

 and it is divided into three equal spaces, the 

 central one of which is occupied by the groove, 

 and the other two are the surfaces which rest 

 upon the bearer. Fig. 3 is a plan of the rocker 

 turned upside down. The spaces k (fig. 1) and 

 mnop (fig. 3) will, for the present, be left out of 

 consideration. 



1. The blade of a dinner-knife was fixed in a 

 vice so that the edge was horizontal. The 

 rocker was laid upon the edge, and the stem 

 suitably supported. On stirring the rocker, 

 a loud and musical sound commenced, and con- 

 tinued for a considerable time. 



2. The knife was removed, and a plate of 

 sheet iron :j\yth of an inch in thickness was 

 Jixed in the vice ; the hot rocker was caused to 

 rest upon the edge of the plate. On stirring 

 the rocker, vibrations, accompanied by a musical 

 tone, were set up as before. 







