HEAT AND NOT LIGHT A MOTIVE POWER. 
131 
per minute ; this motion, gradually decreasing, continued for 
nearly ten minutes. These and many other similar experiments 
seem to point to the conclusion that when a radiometer is cooler 
than its surroundings, or, in other words, is receiving heat, the 
black discs are repelled ; but if, on the other hahd, it be 
warmer than its surroundings, and is giving out heat, the white 
will be repelled, or the black attracted. 
By covering the globe with hot and cold glass shades alter- 
nately, I have made the discs revolve in whichever direction 
I wish. I have no doubt that all radiometers with reflecting 
and absorbing discs may be made to rotate in either direction, 
provided they are properly treated, some requiring greater 
differences in temperature than others. I find that my A, in 
order to obtain reverse motion, must be subjected to a change 
of nearly 40°, while B requires very much less. The latter 
is so sensitive that, when rotating under the influence of sun- 
light, reverse motion sets in when the window-blind is drawn 
down. The effect of hot or cold breath upon the glass is 
similar to the above ; so one can blow the vanes round in either 
direction. 
When A and B were placed on the chimney-piece, both 
the black discs were repelled under the influence of diffused 
sunlight ; A with a velocity equal to three revolutions per 
minute, and B seven. Both were then covered with equally 
cold glass shades (at about freezing point). In five seconds B 
stopped and reversed motion (five revolutions per minute), 
stopping in about 1-J minute, and then again repelling the 
black. A simply decreased in velocity for a minute or 
two, and then moved as quickly as before being covered with 
the shade. When a candle was placed seven inches from B, 
the black discs were repelled at the rate of eleven revolutions 
per minute. I then covered it with a cold glass shade. 
Motion ceased in thirty seconds, and reverse motion set in 
(against the candle-light), and continued for forty-five seconds, 
when it again stopped, and original motion ensued. I then 
placed A 10 J inches from the same candle, which caused 
the black to be repelled with the same velocity (eleven revolu- 
tions per minute). When covered with the same glass shade, 
re-chilled to the same temperature, the rotation decreased to 
six revolutions per minute, and then gradually quickened as 
the shade became warmer. To cause the black to be repelled 
six revolutions per minute, I had to place the candle 1 5% inches 
from the globe. 
I must now pass on to section II. The last-mentioned experi- 
ment comes partly under one section and partly under the other. 
When a candle or any source of light is placed, say, 6 
inches from the globe, the heat, having passed through 6 
x 2 
11 
