428 S. P. Langley — Terryperatwe of the Moon. 



ent parts of the eclipsed moon, on the night of September 23, 

 1885. 



The diameter of the lunar image was 28'3 mm , and of this 

 only a limited portion (0*08 of the whole), fell upon the bolom- 

 eter. As the penumbra came on, the diminution of heat 

 was marked, being measured by the bolometer even before the 

 eye had detected any appearance of shadow. The heat contin- 

 ued to diminish rapidly with the progress of the immersion in 

 the penumbra, but at no time did the lunar radiation from the 

 part in full shadow entirely vanish. At one hour before the 

 middle of the total eclipse, the deflection in the umbra was 3*8 

 divisions. Fifty minutes after the middle of the eclipse it had 

 diminished to approximately 1 "3 divisions, less than one per cent 

 of the heat from a similar portion of the uneclipsed moon, a 

 deflection so small that its significance may be somewhat doubtful. 

 It need hardly be stated that this heat from the eclipsed moon 

 was almost absolutely cut off by the interposition of glass. 

 The rise of the temperature after the passage of the umbra 

 was apparently nearly as rapid as the previous fall. The vicis- 

 situdes of the lunar climate indicated by these observations in 

 the short time of a few hours, must exceed the change from 

 our torrid zone to the greatest cold of an arctic winter. 



In this connection it should be stated that repeated observ- 

 ations on the dark side of the moon, have given only the same 

 heat-spectrum as shown by the sky away from the moon, the 

 conclusion being that, so far as our present observation carries 

 us, the moon has no internal heat sensible at the surface, so 

 that the radiations from the lunar soil, already spoken of, are 

 to be understood to be due purely to solar heat which has 

 been absorbed and almost immediately re-radiated. 



The principal method employed in the present research for 

 determining the temperature of the surface of the moon is 

 founded on the fact, already experimentally established by the 

 writer, that the position of the maximum in a curve, represent- 

 ing invisible radiant heat, furnishes a reliable criterion as to 

 the temperature of the radiating (solid) body,* and on the fur- 

 ther fact, established by Mr. F. W. Very and the writer, that 

 two distinct heat maxima are observable in the lunar spectrum, 

 one corresponding to the radiation reflected from the soil, the 

 other to that emitted by it. It at first seemed, in accordance 

 with what has just been said, that the accurate determination 

 of the wave length of this latter maximum would give a cor- 

 respondingly accurate determination of the temperature of the 

 sunlit surface of the moon ; and, accordingly, to this object the 

 main portion of the observations were given. We may antici- 

 pate what follows by here saying that by this method, a per- 



* Proc. Am. Assoc, for Adv. of ScL. 1 885 ; also this Journal, xxxi, Jan., 1886. 



