40 CHELTENHAM COLLEGE NATURAL HISTORY SOCIETY. 
“rocker as soon as it comes into contact with its support heats that 
support just underneath where it rests upon it and consequently 
expands it ; this expansion jerks the rocker up again and maintains 
the vibration. The conditions which had been found to be neces- 
sary for the successful maintenance of the vibrations were quite in 
accordance with this view of the way in which they arose. Thus it 
was found that, except under peculiar conditions, to be noticed 
soon, the rocker would not vibrate upon a block made of the same 
material as itself. A little consideration would show that this should 
be so. For it must be noted that the block was heated at the ex- 
pense of the heat of the rocker. Consequently coincident with the 
expansion of the block by heating there must be contraction of the 
rocker through cooling. Now, if the rocker and block were of the 
same material the expansion of the block must be exactly equal to 
the contraction of the rocker, and as the contraction must act to 
damp the vibration, the transference of heat would on the whole 
have no re-inforcing effect upon the vibrations, which could not 
therefore in this case be maintained. In fact, the point of support 
must rise by expansion more than the point of the rocker, which 
touched the block, shrunk into the block by cooling. This would 
generally be the case if the block were made of material more ex- 
pansible by heat than that of which the rocker is made. This was 
found to be the case. There were other circumstances which had 
considerable influence upon the vibrations. Thus the smaller the 
conducting power of the block and the greater the conducting 
power of the rocker, the more readily would vibrations occur. For 
if the block be of material which conducts heat badly, more of the 
heat which passes into the block will be confined to that part of the 
block directly under the point of contact of the rocker than if it 
were a good conductor, and it was this portion only of the heat 
which was effective in jerking the rocker up. A like consideration 
would shew that the rocker should be of good conducting material. 
It was for these reasons that copper and brass formed good mate- 
rials for a rocker, and rock salt and lead good materials for a block. 
He had already said that under certain conditions the rocker and 
block might be of the same material. If the rocker be made to 
rock upon thin sheets or projecting points of metal of the same 
material as itself, vibrations, it was found, could be maintained. 
This clearly arose from the fact that, in these cases the heat which 
flowed into the support was necessarily confined to places directly 
below the point of contact of the rocker, while this heat had much 
