HEAT AND NOT LIGHT A MOTIVE POWER. 
129 
I have two radiometers exactly alike in make, with the 
-exception that one, which I shall call A, has diamond-shaped 
vanes, of which the reflecting sides are not so effective as the 
absorbing sides; and in the other, B, the vanes are round, 
with very effective reflecting surfaces, but indifferent absorbing 
surfaces. 
When A and B are placed, say, six inches from any 
source of light, as a candle or in daylight, the black discs in 
A are repelled much quicker than those in B — that is to 
say, the rotation is much quicker in A than in B. This 
difference would be partly caused by the difference in black area 
in the two shapes of the vanes, but other reasons for it will 
be mentioned hereafter. I shall class my experiments under 
two sections. Under the first, I will describe those in which 
heat acts immediately upon the globe of the radiometer ; under 
the other, those in which light and heat act indirectly upon it. 
The experiments will make these definitions more intelligible. 
I. I brought radiometer A into a dark room (without a 
fire), and lighted a small benzine lamp, placing it almost 
directly over the top of the globe, and about two feet from it. 
I turned down the flame until the vanes were only just visible. 
The light had no effect on them, and in fact had none when I 
afterwards turned it up to its full height. I then placed one 
finger on the side of the globe, much nearer to a black than to 
a white disc (for convenience I shall call one side of a vane the 
white, the other the black disc). The black was at once 
repelled, but not with sufficient force to allow the white disc 
on the next vane to pass the heated spot, and so bring the next 
black disc under its power. I then allowed the globe to cool, 
and placed my finger as before, only much nearer to a white 
than to a black disc. The white was repelled nearly as much 
as the black had been, but the next vane was not brought past 
the heated spot. In no way could I force a vane through an 
angle of 90°. With two fingers, one on each side of the globe, 
the effects were the same as when I used one finger. I then 
surrounded the globe completely with my warm hands ; no 
rotation was produced. The discs moved backwards and for- 
wards through an angle of about 40°. I then allowed the globe 
to cool — no motion. Bringing radiometer B into the same 
position, I placed my finger as before, nearer to a white disc. 
It was repelled through an angle of about 40°, but the next 
vane was not brought past the heated spot. On removing my 
finger, the black disc gradually approached the heated spot, 
and, as soon as it had passed it, the movement quickened, and 
complete rotation ensued. This backward movement, appa- 
rently resulting from repulsion of the white discs, I shall here- 
after call the 44 reverse motion.” As soon as the discs became 
VOL. XV. — NO. LIX. K 
