6 Dr. Tyndall on the Vibrations and Tones produced by 



IV. Silver Rocker, 



This instrument was formed by attaching a piece of silver tO 

 the brass rocker used in the last experiments, exactly as the 

 piece of copper, mnop, was attached to the iron rocker, fig. 1. 

 The silver partook of the general shape of the under surface of 

 the rocker, being bevelled off on both sides of the groove passing 

 through its centre. 



1. A strip of silver about f^Tydth of an inch in thickness was 

 fixed in the vice, and the silver portion of the rocker was caused 

 to rest upon the edge of the strip. On shaking the rockier, a 

 fine mellow musical tone was obtained. 



2. A new half-crown was fixed in the vice, and the rocker 

 caused to rest upon the milled edge ; no permanent vibrations 

 were obtained. A similar difficulty was encountered with the 

 edge of a shilling. On the edge of a sixpence, a feeble4hough 

 distinct vibration was obtained. 



3. When the edges of the coins were beaten out with a ham- 

 mer, and thus rendered thin, distinct vibrations were obtained 

 with all of them. I do not assert the impossibility of obtaining 

 vibrations on the edge of a half-crown, but merely state that 

 with the same rocker vibrations were obtained upon a thin edge 

 of silver, and not upon a thick oi;e. 



4. The rocker was placed against a block of silver weighing 

 about 10 oz. ; no permanent vibrations were obtained. 



V. Zinc Rocker, 



The instrument is of the same size and shape as the iron 

 rocker, fig. 1, except that the depth, SN fig. 2, is less, and the 

 mass therefore lighter*. 



Placed upon the edge of the thinnest sheet zinc, the edge 

 having been sharpened by a file, distinct musical tones were ob- 

 tained : — on a block of zinc the rocker refused to vibrate per- 

 manently. 



Tin Rocker. 



A cake of tin, formed by pouring the molten metal upon 

 a smooth flat surface, was heated and balanced upon two small 

 protuberances of a second piece of the same metal. Continuous 

 rockings were immediately set up. I met the mass by accident 

 in the laboratory, and having obtained the vibrations without 

 changing its shape, I did not think it necessary to strengthen 

 the action by forming it into a regular rocker. 



* Better results are obtained when the rocker is still further lightened, 

 by scooping away part of its central mass, thus making its upper surface 

 concave instead of flat. 



