394 
MESSRS. W. E. WILSON AND P. L. GRAY 
Let 
= radiation due to sun falling on unit area of receiving surface; 
and R ^,2 = respective radiations due to platinum, also on unit area, when 
giving heat—(1) to the upper surface; (2) to the lower. will, of course, 
be the same in the two positions; 
= effective area of upper surface ; 
5 , lower 
using the word “ efiective ” to cover any slight difference of absorptive power, &c. 
Then, if we suppose. First, the radiation due to the sun falling on the upper surface, 
the lower being sheltered from the platinum, we should have a deflection 6 -^, and as 
deflections may be taken proportional to received radiation, then 
R^ = mOi 
where m is a constant. 
Secondly, let the radiation from the platinum fall on the lower circuit, the sun being 
now cut off from the upper ; we shall have 
But if both effects are allowed to be produced together, at the moment of balance 
$1 and 6 . 2 , will be equal and opposite, and therefore 
rq Rj — rq 
Similarly, with the sun and platinum reversed as regards the upper and lower 
surfaces, while R^ remains the same, becomes Pt^^, and we have 
which gives immediately 
Id — fq 
lifi. Ifj 
R.o = 
V ’ 
J.t5 
or 
R^ — 
from which the reason for taking the geometrical mean of the corresponding tempera¬ 
tures follows directly. 
The final result, therefore, arrived at, is only given to the nearest 100 ; it is 
6200° C. 
In conclusion, we may point out that this method would probably give excellent 
results, if a series of observations were tmdertaken to settle the question of how, or if, the 
solar temperature varies during a sun-spot cycle. The instrument should, of course, be 
