734 
PROF. 0. REYNOLDS ON THE FORCES CAUSED BY THE 
we may now look upon the motion caused by light and heat as a direct proof of the 
kinetic or molecular theory of gas. 
A new Light-Mill. 
Although the proofs against the forces in the light-mills being directly referable to 
radiation are already more than sufficient, I will venture to suggest one more test, which 
the difficulty of obtaining the instrument has as yet prevented me applying. If a 
“ light-mill ” were made unlike those which have hitherto been constructed, inasmuch 
that, instead of its vanes being perpendicular to the direction of motion, and having one 
side black and the other white, it has vanes arranged like the sails of a wind-mill or the 
screw of a ship, all inclined to the direction of motion, and of the same colour on both 
sides ; then if this mill turned, it would show that the force is not influenced by the 
direction from which the light and heat come, but that, like the wind on a wind-mill, it 
acts perpendicularly to the surface of the vanes*. 
It seems to me that, inasmuch as the vanes of such a mill would be continuously 
acted upon, and would experience the full and not merely the differentiated effect of 
light, it would be much more sensitive than those at present constructed. 
Appendix (March 7, 1877). 
Vanes fixed in the Envelope. 
In the discussion which followed the reading of this paper, it was stated by Mr. 
Crookes that he had suspended his instruments upside down by a single fibre, and 
floated them upside down in water, and had then found, when the vanes could not turn 
in the envelope, that the whole envelope rotated very slowly under the action of light, 
steadily and continuously in the same direction as that in which the vanes would have 
turned had they been free. And at the Meeting on March 30th, subsequent to the 
reading of this paper, Mr. Crookes described how the case of one of his instruments 
floating in water revolved at a rate of about 1 revolution an hour when the vanes were 
free to turn. Comparing this effect with that which was caused when the vanes were 
fixed by the magnet one revolution in 2 minutes, it appears that the force turning the 
envelope with the vanes free was ^th that turning the vanes ; for the resistance of 
the water at such small velocities would be proportional to the velocity. 
As no such effect to turn the envelope had been observed during Dr. Schuster’s 
experiment, in which I took part, and as it was difficult to conceive any method of 
* In the discussion which followed the reading of the paper, Mr. Crookes mentioned that he had already 
constructed mills with inclined vanes, and found them answer ; and I am informed that he exhibited one at the 
next meeting of the Society. I may mention here that I have received a mill from Dr. Geisseer, which I had 
previously ordered. This instrument, although damaged in transit, is sufficiently sensitive to prove that the 
action of heat is altogether independent of the direction from which the heat comes. — July 31, 1876. 
