374 
ME. W. CEOOKES ON EEPTJLSION EESULTING EEOM EADIATION. 
therefore 12 inches from the surface of the pith. The screens were removed, and the 
reflected rays of the candle were allowed to fall perpendicularly on to the pith surface. 
The pith instantly sank to the bottom of the tube, and the torsion-handle had to be 
turned through 442° to restore equilibrium. 
The yjjo of a grain required 10,073° of torsion to restore equilibrium (214); and the 
ratio between the weights being the same as that between the degrees of torsion, the 
mechanical force of the radiation from the candle is easily calculated : — 
10,073° : 0-01 grain : : 442° : 0-0004387 grain. 
Other experiments were tried, the candle being blown out and the apparatus allowed 
to cool in the dark between each. The following Table gives the results : — 
Mechanical Force of Radiation from a candle on 2 square inches of blackened pith. 
Distance of candle 
from pith. 
Degrees of 
Torsion. 
Mechanical Pressure. 
Difference of last column 
from mean. 
12 inches 
442° ' 
0-000438 grain. 
— 0-000006 grain. 
449 
0-000445 „ 
+ 0-000001 „ 
» » 
452 
0-000448 „ 
+ 0-000004 „ 
Mean 
1 448 
0-000444 „ 
6 inches 
1815 
j 0-001801 „ 
+ 0-000029 „ 
1800 
0-001787 „ 
+ 0-000015 „ 
» ” 
1740 
| 0-001727 „ 
-0 000045 „ 
Mean 
1785 
0-001772 „ 
6 ins., with water- 
cell interposed . 
235 
0-000233 „ 
The pressure at 12 inches off is 0-000444 grain, whilst that at 6 inches is 0-001772 
grain. At half the distance the pressure should be four times, or 0-001776 grain. The 
difference between theory and experiment, being only 4 millionths of a grain, is a 
sufficient proof that the indications of this instrument, like those of the previously 
described apparatus, follow the law of inverse squares. 
The last column in this Table, the “ difference from mean,” shows that my estimate 
of the sensitiveness of this balance is not excessive, and that in practice it will safely 
indicate the millionth of a grain. 
217. I have tried in vain to get a good observation with sunlight. On the very rare 
occasions within the last few months on which the sun has been shining brightly my 
balance has not been in adjustment, or I have been away from home. I was able, 
however, on December 13th last to get a few observations. The sun was obscured by 
thin clouds and haze, and its rays were but faint. The pressure of its radiation was, on 
this occasion, only equal to that of 10-2 candles, six inches off, pressing dowq the pith, 
therefore, with a weight of 0-Q18074 grain. 
