4 
DR. TYNDALL ON THE VIBRATIONS AND TONES PRODUCED BY 
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 continued for a considerable time. 
2. The knife was removed and a plate of sheet iron ^th of an inch in thickness 
was fixed 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. 
3. The experiment was repeated with a second plate of iron y^th °f an inch in 
thickness, and a still better tone was obtained : — when the rocker rested on a block 
of iron the vibrations were not permanent. 
II. Copper Rocker. 
The pieces k and mnop , figs. 1 and .3, are 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 -gyth 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 yjth 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 cylindrical 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 y^th 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. 
