THE CONTACT OF BODIES HAVING DIFFERENT TEMPERATURES. 
5 
3. The experiment was repeated with a plate twice the thickness of the former ; 
distinct vibrations were obtained. It was found in this case also that the thinner the 
plate, within its limits of rigidity, the more decided were the effects : — when the rocker 
was laid upon a block of brass there was no permanent vibration. 
4. The rocker was turned upside down, its flat surface resting upon the points of 
two common brass pins ; a constant rocking was the consequence. 
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 To^th 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 rocker 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 feeble, though 
distinct vibration was obtained. 
3. When the edges of the coins were beaten out with a hammer, and thus rendered 
thin, distinct vibrations were obtained with all of them. I do not assert the impos- 
sibility 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 one. 
4. The rocker was placed against a block of silver weighing about ten ounces ; 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*. 
1. Placed upon the edge of the thinnest sheet zinc, the edge having been sharpened 
by a file, distinct musical tones were obtained : — on a block of zinc the rocker refused 
to vibrate permanently. 
Tin Rocker. 
1. 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 
* 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. 
