OF HEAT FROM THE MOON. 
623 
That is, under the circumstances stated above, a difference of temperature of the black- 
ened tins acting alternately on one pile produces P4933 times the effect that the same 
excess of temperature of the area occupied by the moon’s disk above that of an equal 
area of the neighbouring sky (assumed of the same temperature as the colder tin) would 
do acting on the two piles in their position in the telescope. 
It has been shown (p. 618, note) that the moon at full, or, more strictly speaking, at the 
time of its maximum heat, before its rays traverse our atmosphere, would produce an 
effect on the galvanometer of 513'9 parts of the scale ; therefore the tin at the same excess 
of temperature (the radiating powers of the two surfaces being assumed equal) would 
give 513-9 x 1*4933 = 767-4 parts. 
Therefore, employing Dulong and Petit’s formula for the velocity of cooling'*, 
Y =ma\a t — 1) 
(where $ is the temperature in degrees Centigrade of the colder tin, t-{-Q that of the 
hotter one, a a constant=l - 0077, V the mean difference of consecutive pairs of readings 
of the galvanometer, and m a constant deduced from experiments with the tins = 55 8-0 f), 
a mean value of 0 = 45° Fahr. (7°*22 Cent.) will give V = 767'4 when £=197 0, 5 Fahr. 
(109°- 7 Cent.)$. 
This result, it will be observed, differs much from a rough estimation of the value of 
the scale-readings given at the conclusion of a former communication §. Probably this 
may be caused principally by neglect in distinguishing between the effect of one pile 
and of both piles ; in other words, omitting a factor 2 from the former calculation, the 
* Annales de Ckim. et de Phys. t. vii. This formula is, strictly speaking, only applicable to a radiating body 
in vacuo ; but for the comparatively moderate temperatures here dealt with it is perhaps as correct as any other. 
t These experiments were made on March 25th and 26th, 1872, and are as follows, where Y is the mean of 
10 consecutive differences: — 
Pile A. 
Pile B. 
6 (Cent.). 
t. 
V obs. 
V calc. 
c-o. 
5-67 
21-44 
110-4 
113-9 
+ 3-5 
6-06 
21-83 
100-7 
100-9 
+ 0-2 
7-11 
40-11 
198-8 
200-9 
4-2-1 
7-50 
34-39 
172-3 
169-0 
-3-3 
7-89 
29-78 
145-7 
144-1 
-1-6 
8-33 
24-39 
116-4 
115-9 
-0-5 
5-78 
23-11 
110-4 
116-7 
+ 6-3 
6-17 
20-50 
95-6 
102-8 
+ 7-2 
6-72 
43-22 
241-4 
238-0 
-3-4 
7-33 
36-95 
202-7 
199-4 
-3-3 
7-72 
32-22 
170-2 
171-1 
+ 0-9 
8-00 
28-17 
148-1 
147-5 
-0-6 
8-28 
26-22 
137-1 
136-6 
-0-5 
534-3 
m=581'7 
If Newton’s law (V=C t) be employed, the corresponding excess of temperature of the tin would come ou 
152°-2 Cent. =274° Fahr. 
§ Proceedings of the Royal Society, No. 112 (1869), p. 443. 
