THE HEAT OF THE MOON. 
13 
the value of his galvanometer scale in terms of the centigrade, 
he has been able to give the actual observed warmth upon each 
occasion of observation. The following is his table of results : 
Date 1869 
Age of 
Moon 
Mean Solar 
Time (Paris) 
Mean Devia- 
tion of Needle 
Value in parts of Centi- 
grade Degree 
days 
h. m. 
d. 
O 
Oct. 9 
4 
7-32 
1-3 
000017 
„ 10 
5 
7-46 
1-0 
0-00013 
„ 12 
7 
8-45 
5*8 
0-00075 
„ 12 
7 
912 
2-2 
000029 
„ 17 
12 
8-39 
20-0 
000260 
„ 20 
15 
10-11 
221 
0-00287 
The second series of observations on the 12th were made 
with the moon approaching a foggy horizon : the result shows 
the enfeebling influence of a low altitude or a misty atmo- 
sphere. The object-glass here used concentrated the light 
247 times, so that the deviation on the full-moon night, 
ascribable to direct influence, corresponds to twelve-millionths 
of a degree.* 
Collecting the comparable results, we have for the following 
determinations of the directly measurable lunar warmth in de- 
cimal parts of a centigrade degree — 
Forbes, less than 0 0000033 
Smyth (on Teneriffe) 0-00075 
Marie-Davy 0-000012 
These are perhaps as consistent as we can expect such dif- 
ficultly determinable figures to be. They agree in showing the 
available lunar heat at the earth’s surface to be a quantity only 
expressible by a number in the fourth or fifth decimal sub- 
division of a degree centigrade. Such a quantity can have no 
effect upon terrestrial temperatures : whatever traces of lunar 
influence are shown upon thermometer registers, and we have 
seen that such are strongly manifested, must be those of dark 
heat acting indirectly upon our lower atmosphere. 
But the subject is by no means exhausted. What Lord Rosse 
has done he regards as initiatory, and what Marie-Davy has 
done he on his part puts forth as merely an instalment of 
results at which he hopes to arrive by isolating and evaluating 
if possible the dark as well as the luminous calorific rays of the 
moon. This paper, it is hoped, having brought up the subject 
to the present time, may prepare interested minds for whatever 
may follow. 
P.S. — Since the foregoing was written, M. Marie-Davy has 
given to the French Academy the results of a new series of ob- 
servations made in November with a mirror of eight inches 
* “ Comptes rendus,” lxix. 922. 
