300 
MR. W. CROOKES ON REPULSION RESULTING FROM RADIATION. 
of a lighted candle causes rotation in the positive direction. But the action is not 
so strong as when the radiometer is cold or no water intervenes. 
334. A four-armed cup-shaped aluminium radiometer, the cups being 10 millims. 
in diameter and the radius of curvature being 6 millims., was sealed on to the 
mercury pump. During exhaustion accurate observations were taken of the number 
of revolutions per minute caused by one or more standard candles 3 inches from the 
eentre of the bulb. At the same time observations of pressure were taken and 
the exhaustion was carried to a very high point. The results are shown in the 
following Table : — 
Pressure in 
millionths of an 
atmosphere. 
Number of 
candles used. 
Number of revolutions per minute. 
Revolutions. 
Reduced to 1 candle. 
577-0 
1 
10 
10 
400-0 
1 
3 1 
31 
309-0 
1 
4-8 
4-8 
219-0 
1 
7-0 
7-0 
159-0 
1 
10-0 
10-0 
102-0 
1 
16-6 
16-6 
69-0 
1 
22-2 
22-2 
41-5 
1 
26-8 
26-8 
27-8 
1 
25-6 
25-6 
24-0 
1 
25-0 
25-0 
19-5 
1 
23-8 
23-8 
14-7 
1 
21-4 
21-4 
9-5 
1 
16-4 
16-4 
8-6 
1 
15-0 
15-0 
6-5 
1 
12-5 
12-5 
3-8 
1 
7-1 
7-1 
2-5 
2 
9-4 
4-7 
1-5 
3 
6-6 
2-2 
0-9 
4 
8-0 
2-0 
0-23 
5 
4-5 
0-9 
0-2 
5 
o-o 
0 0 stopped. 
The first column gives the millionths of an atmosphere* at which the experiment 
was tried. The second column gives the number of candles, 3 inches off, used to 
produce rotation. Up to 3 - 8 millionths of an atmosphere one candle was sufficient, 
beyond that rarefaction a greater number was required. At 0'2 millionth five candles 
ceased to cause rotation. The third column gives the actual number of revolutions 
obtained with one or more candles, each recorded observation being the mean of 
several. The last column gives the revolutions per minute, calculated from the 
third column on the assumption that the number of revolutions per minute is in 
* 0'2 millionth = 0-00015 millim. 
10 millionth = 0 - 00076 „ 
4 0 millionths= 0-00304 ,, 
577-0 milliontlis= 0-43825 „ 
One atmosphere=760'00000 ,, 
