COMMUNICATION OF HEAT BETWEEN A SURFACE AND A GAS. 
729 
For, in the first place, if the force acting on the vanes arose from an external source, 
then the vanes in turning, owing to the friction of the pivot and the friction of the air, 
must tend to drag the envelope round with the mill ; consequently, on the light being 
turned on, the envelope would have turned in the same direction as the vanes, and con- 
tinued to do so until the torsion of its suspension had restrained its further motion : it 
would then have remained steady until the light was turned off, when it would have 
come back to its former position. 
Whereas, on the other hand, if the force on the vanes arises entirely within the vessel, 
if the air is, as it were, the fulcrum against which the force acts, then, in order to over- 
come the inertia of the vanes and set them in motion, the air must itself move in the 
opposite direction, just as when a steamboat starts it sends a stream of water backwards. 
This motion of the air will be communicated by friction to the vessel, and the effect 
will be that on the light being turned on the envelope must turn in the opposite 
direction to the vanes ; that when the mill has acquired its full speed then, as in the 
case of a steamboat, the backward motion given to the fluid by the propellers will just 
balance the forward motion imparted by the resistance of the ship, and the resultant 
force will be nothing. When, therefore, the mill has acquired its full speed the 
envelope will come back to its normal position, where it will remain until the light 
is turned off, when the friction acting alone will tend to drag the internal fluid and 
hence the envelope forward. 
This was the view of the case which I took when Dr. Schuster first suggested his 
experiment to me ; and when it came to be performed the results, as may be seen, were 
in strict accordance with the second supposition, namely, that the force acts entirely 
between the vanes and the air within the mill. 
This experiment of Dr. Schuster’s also afforded a means of arriving approximately at 
The Magnitude of the Force. 
The weight of the mill and the envelope, considered in conjunction with its manner 
of suspension, gave the moment of the torsional force necessary to turn it through an 
angle of ‘06 as -00000002641b., or one forty-millionth part of a pound acting on a lever 
a foot long. To cause this deviation the light had to be such as would cause the vanes 
to make 240 revolutions per minute. Hence, when making 240 revolutions per minute, 
we have a measure of the force which causes the motion and the resistance which opposes 
it. Now considering that the centres of the vanes are f inch from the axis, the whole 
force acting on the vanes will be 16 X '0000000264 of a pound, — that is -00000042, or 
one two-million-five-hundred-thousandth part of a pound ; this distributed over the vanes 
(whose joint area is 1 sq. inch) is -000000421b., or one two-million-five-hundred -thousandth 
part of a pound on the square inch. And assuming that the tension of gas within the 
mill is -0005 lb., or one two-thousandth part of a pound on the square inch (the tension 
of a toricellian vacuum at 60° F.), then we see that the difference of pressure on the two 
sides of the vanes is "0008 of the pressure within the mill, or less than one thousandth part. 
