Although such a calculation as the above may afford us a good approximation to the rate of loss of heat by Mars with its very scanty atmosphere, we have now good evidence that in the case of the moon the loss is much more rapid.  Two independent workers have investigated this subject with very accordant results—­Dr. Boeddicker, with Lord Rosse’s 3-foot reflector and a Thermopile to measure the heat, and Mr. Frank Very, with a glass reflector of 12 inches diameter and the Bolometer invented by Mr. Langley.  The very striking and unexpected fact in which these observers agree is the sudden disappearance of much of the stored-up heat during the comparatively short duration of a total eclipse of the moon—­less than two hours of complete darkness, and about twice that period of partial obscuration.

Dr. Boeddicker was unable to detect any appreciable heat at the period of greatest obscuration; but, owing to the extreme sensitiveness of the Bolometer, Mr. Very ascertained that those parts of the surface which had been longest in the shadow still emitted heat “to the amount of one per cent. of the heat to be expected from the full moon.”  This however is the amount of radiation measured by the Bolometer, and to get the temperature of the radiating surface we must apply Stefan’s law of the 4th power.  Hence the temperature of the moon’s dark surface will be the [fourth root of (1 over 100)] = 1 over 3.2 [A] of the highest temperature (which we may take at the freezing-point, 491 deg.  F. abs.), or 154 deg.  F. abs.,
 just below the liquefaction point of air.  This is about 50 deg. lower than the amount found by calculation from our most rapid radiation; and as this amount is produced in a few hours, it is not too much to expect that, when continued for more than two weeks (the lunar night), it might reach a temperature sufficient to liquefy hydrogen (60 deg.  F. abs.), or perhaps even below it.

[Note A:  LaTex markup $\root 4 \of {1 \over 100} = {1 \over 3.2}$ ]

Theory of the Moon’s Origin.

This extremely rapid loss of heat by radiation, at first sight so improbable as to be almost incredible, may perhaps be to some extent explained by the physical constitution of the moon’s surface, which, from a theoretical point of view, does not appear to have received the attention it deserves.  It is clear that our satellite has been long subjected to volcanic eruptions over its whole visible face, and these have evidently been of an explosive nature, so as to build up the very lofty cones and craters, as well as thousands of smaller ones, which, owing to the absence of any degrading or denuding agencies, have remained piled up as they were first formed.