ME. W. CEOOKES OUST EEPTJLSION EESULTINGr EEOM EADIATION. 343 
150. From the construction of the radiometer it is evident that the position of the 
lio-ht in the horizontal plane is of no consequence, provided the distance is not altered. 
This was tested during the last-described experiment. When a candle had to be removed, 
it was found to make no difference from what part of the circle it was taken. The 
following experiments were tried to further verify this result, the radiometer being a 
different one from that last used : — 
seconds. 
1 candle, placed 1 foot from centre of radiometer, gave 1 revolution in 
78 seconds (78x1=) 78 
2 candles, placed 1 foot from centre of radiometer and put close together, 
gave 1 revolution in 39-5 seconds (39-5x2 = ) 79 
2 candles, placed opposite to each other, gave 1 revolution in 39 seconds 
(39 X 2=) 78 
3 candles, close together, gave 1 revolution in 26-5 seconds (26-5x3 = ) . 79-5 
3 candles, spread round circumference, gave 1 revolution in 26 seconds 
(26x3=) 78 
4 candles, close together, gave 1 revolution in 19 seconds (19x4 = ) . . 76 
5 candles, close together, gave 1 revolution in 16 seconds (16x5=) . . 80 
5 candles, spread round circumference, gave 1 revolution in 15-5 seconds 
(15-5x5=) 77-5 
151. I wished now to ascertain what would be the effect of bringing a radiometer 
into a uniformly lighted space, so that there should be no difference of action on 
any side. 
A radiometer was covered oyer the top with a large sheet of paper, and the light 
from an argand gas-burner was reflected vertically downwards on to the paper. The 
apparatus was arranged so that, as near as possible, exactly the same amount of light 
should illuminate the instrument all round. The arms revolved at a uniform speed 
of one revolution in six seconds, and kept on at this rate as long as the experiment 
lasted. 
A radiometer was taken on to the roof of the house, where there was an almost unin- 
terrupted view all round. The sky was of a uniform dull leaden colour, a cold north- 
east wind was blowing, and, as far as the eye could judge, there was no difference in the 
amount of light received from any quarter of the sky. The radiometer was covered 
with a white handkerchief, to still further diffuse the light. In this condition the arms 
made one revolution in an average of 1-9 second. On shading the light from the south 
the time of revolution was 2-7 seconds. On shading it from the north the time was one 
revolution in 2‘9 seconds. With the west shaded off it was one in 2 - 3 seconds; and 
w 7 ith the east shaded off, the rate was one in 2*9 seconds. 
The same radiometer, exposed near a south-east window in a room in the afternoon 
of the same dull April day, revolved once in 16 seconds. 
[The radiometer shows a striking difference between heat and light, as commonly 
3 b 2 
