THE RADIOMETER. 
173 
reached, and left it at the higher temperature.” The uncompen- 
sated excess of pressure is, therefore, 0*000,011,5 of a gramme 
per square centimetre; Mr. Crookes making it = 0*000,01. 
It is obvious that all needed to substantiate the truth of this 
explanation was experimental evidence of a reactive force on the 
case of the radiometer, corresponding to the action on the vanes. 
This was immediately supplied in a paper read before the 
Royal Society on March 23, 1876, by Mr. Arthur Schuster, of 
Owens College. “Whenever,” he says, “we observe a force 
tending to drive a body in a certain direction, we are sure to 
find a force equal in amount acting in the opposite direction on 
the body from which the force emanates. If the force is 
directly due to radiation, the reaction will be on the radiating 
body ; if, on the other hand, it is due to any interior action, 
such as the one suggested by Professor Reynolds, the reaction 
will be on the enclosure of the moving bodies.” 
This proposition was easy to test by experiment. A radio- 
meter was suspended on the bifilar principle, by two cocoon fibres, 
from the top of a vessel which could be exhausted, and its 
slightest movement detected by an attached mirror. The light of 
an oxy hydrogen lamp was concentrated on the vanes, which 
then revolved about 200 times a minute. The light being cut 
off by a screen, the position of the case was read by means of 
the dot of light on a scale. The screen was then suddenly 
removed, and in every case a large deflection was observed ; 
the vessel being deflected in the opposite direction to that in 
which the vanes turned. When the velocity of these had 
become constant, the vessel returned to its original position. 
On suddenly cutting off the light, the vessel was again deflected, 
but in the opposite direction to that on starting the experiment. 
These experiments are easily explained, on the assumption that 
the force acting on the vessel enclosing the vanes is exactly 
equal and opposite to that acting on them. While the velocity 
of the latter is increasing, a force acts in the opposite direction 
on the vessel. When the light is cut off, the reaction of resist- 
ance will act on the enclosure, and the enclosure will turn in 
the same direction as the vanes. 
“By means of the reaction of the enclosure,” concludes 
Mr. Schuster, “ I have been able to calculate the strength of 
the force ; and I have found that the pressure on a surface on 
which light of equal intensity to that used in my experiments 
falls, is equal to that produced by the weight of a film of water 
on a horizontal surface equal in thickness to the length of a 
wave of violet light.” 
A paper by Professor O. Reynolds was read on the same 
evening, commenting on Mr. Schuster’s experiment, and some- 
what intensifying Mr. Crookes’s differences of view ; describing 
