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LECTURES TO SCIENCE TEACHERS. 



as double rods vibrating in opposite directions, and thus dis- 

 pensing with a firm fixture ; because one vibration counter- 

 balances the other. A single rod of course transmits its 

 vibration to the support, unless it be very solid, but in a 

 double rod this is not so. When they are struck alone like 

 the bar of steel which I showed you just now, they give very 



FIG. 3. Vibrating Bar. 



high secondary tones. When I hold this and strike it, you do 

 not hear more than a high "ping" though there is here a 

 slight sound of the fundamental note. This we can reinforce 

 till it becomes of considerable value. Tuning-forks are 

 amongst the instruments whose use has extended from sound 

 into other branches of physics, after a pleasant fashion of 



