THE HEAT OF THE MOON. 
11 
in the needle itself can be magnified to any extent by increasing 
the distance between the lamp and the scale. This form of 
instrument has been used by Mr. Stone for the purpose of 
measuring the extremely feeble heat radiated from the stars : it 
is possible that but for its capabilities these delicate experi- 
ments would never have been attempted. 
Lord Eosse essayed to determine the proportions of moon- 
heat coming from three sources : 1 . That which it may be sup- 
posed the moon sheds in virtue of its being a globe possessing 
internal or cosmical heat. 2. That solar heat which is reflected 
from the surface with the reflected light. 3. That which, coming 
from the sun, is first absorbed at the lunar surface and then 
radiated as dark heat. The first and third qualities are such 
as we should expect to find but slightly manifested at the 
earth’s surface by reason of the intercepting influence of the 
atmosphere. The second source is that from which we might 
expect the most marked results. Observations were made 
during the winter and spring of 1868-69, on sixteen nights in 
all, on some of which the moon was several days from full. 
At five days old it gave evidence of heating power ; and the 
amount of deviation of the needle increased with the phase, 
showing that the law of variation of the moon’s heat does not 
differ much from that of the moon’s light. The maximum 
of one was found to be coincident in time with the maximum 
of the other ; there was no evidence of greatest heat coming 
at the period of greatest insolation, which would be at about 
third quarter. 
To test the proportion of light to dark heat in the rays 
reaching the mirror, the Earl of Eosse placed screens of plate 
glass before the pile on four nights near the time of full moon, 
and the result was a great interception of heat, manifested by 
a small deviation of the needle. About 8 per cent, was con- 
sidered to be the probable amount that passed through the 
screens, from which Lord Eosse concludes that the greater part 
of the lunar heat is that which has first been absorbed by the 
moon’s surface and then radiated from it.* 
By computation in one case, and by observation of the 
relation of solar and lunar heat to that of a vessel of heated 
water in another case, the relation of solar to lunar radia- 
tion was found to be nearly as 80,000 to 1. And by the use of 
a blackened cistern of hot water, subtending the same angle at 
* This absorbed and radiated heat is presumably dark. How then did it 
get through the atmosphere P There would seem to be heat which passes 
through air, and yet is obstructed by glass. One result of thermoscopic 
investigations upon the light of moon and stars may be to improve our 
knowledge upon such points as this. 
