the Contact of Bodies having different Temperatures, 5 



3. The experiment was repeated with a second plate of iron 

 y J^dth of an inch in thickness, and a still better tone was ob- 

 tained : — when the rocker rested on a block of iron, the vibra- 

 tions were not permanent, 



II. Copper Rocker. 



The pieces k and mnop (figs. 1 and 3) arc plates of copper 

 screwed tightly on to the surface of the iron. In this way a 

 single rocker is made to do the duty of two. 



1 . A plate of copper ^^q th of an inch in thickness was fixed in 

 the vice, as in the former cases; and the copper portion of the 

 rocker was caused to rest upon it. A slight shock, imparted to 

 the rocker, immediately excited a strong and durable tone. 



2. A bit of copper foil was fixed in the vice ; it was almost as 

 flexible as stout foolscap paper, but to give it rigidity the height 

 of it which projected above the vice was very minute. With a 

 little care I obtained tones stronger and more musical than in 

 the foregoing instance. 



3. When the rocker was laid upon a block of copper, no tone 

 was obtained ; and it was found that the difficulty of obtaining 

 a tone increased as the plate made use of became thicker. 



4. Instead of the plate, two wires of copper ^ jth of an inch 

 thick, and pointed with a file, were fixed in the vice at about 

 one-eighth of an inch apart. The rocker was turned upside 

 down, so that the flat surface of the copper k rested on the wires 

 as in fig. 4 : — forcible vibrations were obtained in this way. 



III. Brass Rocker. 



1. A piece of brass tube was fixed in the vice, and its cylin- 

 drical surface rendered clean by a fine file. A brass rocker, of 

 the same dimensions as that represented in fig. 1, was caused to 

 rest upon the tube ; on stirring the rocker, continuous vibrations 

 succeeded. 



2. A plate of brass -j-i^dth of an inch in thickness was fixed 

 in the vice. The rocker being laid upon the edge of the plate 

 and stirred, stronger and more durable vibrations were obtained 

 than in the case of the tube. 



3. The experiment was repeated with a plate twice the thick- 

 ness 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 eff^ects : — 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. 



