TEMPERATURE OF THE SURFACE OF THE MOON. 41 



(2) Comparison ofmoon''s heat with that of Leslie cube. — If we may draw any infi^reiice from 

 tbis class of observations it is that the sunlit surface of the moou is not far from the freezing tem- 

 perature, but not so far below as we might expect to find that of an absolutely airless j)lanet. 



(l?) Transmission of lunar heat by the eartKs atmosphere. — Our observations indicate a not ma- 

 terially greater coetlicient of transmission for lunar heat than for solar; and though their limited 

 number and the uncertainty of the correction for change of heat with phase render more certainty 

 as to the fact desirable, we may (accepting them as probable) reason thus. 



Previous observations both at Allegheny and Mount Whitney have shown that the solar rays 

 are transmitted with greater and greater facility (except for cold bands) as the wave-length 

 increases up to the point (near /. = 3" ) where they suddenly disappear altogether. This shows either 

 that (1) the solar heat, which according to the customary assumption exists to an unlimited wave- 

 length before absorption, has here been cutoff hy a suddenly absorbent action, like that of a cold 

 baud extending indetinitely below 3*^, or (2) that, either through a precedent absorption of such 

 rays in the sun's own atmosphere or their non-existence, no solar rays below S** present themselves 

 to our atmosphere for admission. 



The flrst view is that which I have treated as most in accordance with received opinion. It 

 is not, however, the only one, since the second is not to be absolutely rejected, considering our 

 experimental ignorance of the laws of radiation from gaseous bodies for great wave-lengths. Of these 

 two hypotheses we see that, accoiding to the first, our atmosphere is quite opaque to all heat below 

 S**, and the writer's (unpublished) experiments show that heat above this point must come almost 

 wholly from a source nuch above 100° C. In this view, then (unless we agree that the radiations 

 from the lunar soil correspond to a source much above 100° C), we conclude that sensibly none of 

 them pass our atmosphere, but that what we receive is diffused and reflected heat coming within 

 the range of the known solar energj' spectrum, and transmitted with nearly the same facility as 

 solar heat, or if with a little greater, because lowered in wave-length by selective reflection at the 

 lunar surface, not by absorption and reradiatiou from the lunar soil. 



In the second view, for anything we have absolutely known to the contrary, our atmosphere 

 may be permeable to radiations of any wave-length below Si^, and we could draw no certain infer- 

 ence, even if the lunar radiation were more distinctly difierent in transmissibility than it is. 



As a matter of fact, with the actually limited difl'erence in the character of its transmissibility, 

 a difl'erence which, as so far determined, is of the same order as that of the error of observation, 

 we have no ground then from this present class of observation (i e., class 3) for any absolute con- 

 clusion one way or the other. But we repeat it seems to be a probable inference from our whole 

 work that the earth's atmosphere is more diathermanous to heat of extremely low refrangibility 

 than has heretofore been supposed. 



(■4) Comparative transmission of glass for lunar and solar heat. — The evidence here, which 

 at flrst seems to so directly support the view of a sensible radiation from the surface of the moon, 

 proves on examination to be subject to other interpretation, for the observed effect is almost cer- 

 tainly due in part to a degradation of wave-length by selective reflection from the lunar soil. 



We can draw no absolute conclusion, then, from this evidence at first in appearance so prom- 

 ising, though we may say that it certainly indicates an increased probability for the view that 

 radiations from the lunar soil may be transmissible by our atmosphere. 



(5) Observations during a lunar eclipse. — If our own observations in this respect are imperfect, 

 those of Lord Rosse before cited are on the other hand clear. They appear to bear but one inter- 

 pretation, that all heat from the moon disajipears immediately that it passes into the earth's shadow, 

 and there is no evidence of any being retained, for any sensible time, more than if it were reflected. 



It is so difficult to conceive that while the moon has been storing heat during many days of 

 sunshine, it can part with it instantly, so that the temi^erature of the whole earthwaid surface of 

 the i)lanet disapi)ears in an inappreciable interval, that most will see in this observation an argu- 

 ment against the existence of any such heat sensible to us at any time whatever. 



(G) Formation of a lunar heat spectrum. — The observations made here with the lunar heat spec- 

 trum are as yet incomplete. With improving experience and apparatus, we hope to make others 

 which shall give information of a character no other means can furnish. (See note, infra.) 

 S. Mis. 69 6 



