436 S. P. Langley -Temperature of the Moon. 



The following remarks may serve to make the full meaning of 

 this curve clearer. 



The heat is a vanishing quantity at deviation 42° at the left 

 of the scale (42° with a 60° prism at the temperature of 

 +20° Cent., corresponding to a wave-length of ,/ "4S, or that 

 of " blue " radiant energy). Confining our attention to the 

 solid curve, we observe that it reaches a maximum near 

 39° 40 / =l /f "5, corresponding to rays of dark heat which are 

 yet transmitted by glass, and which must be emitted from a 

 source at a very high temperature. The maximum of the 

 solar heat directly observed through the rock-salt train is found 

 to be at the same point. There is no reason to doubt, then, 

 that this maximum is due to the solar heat reflected from the 

 lunar surface, and its actual effect is to produce a deviation of 

 rather less than 20 degrees on the arbitrary scale of the galvano- 

 meter from the small part of the spectrum covered by the 

 bolometer. Continuing to go down the spectrum in the direc- 

 tion of greater wave-lengths, and passing with casual notice a 

 depression at 39° 15' (X=S fl 'l), which, it is probable, would be 

 found in the direct lunar spectrum were there no intervening 

 atmosphere, we come to a very large depression at 38° 30' 

 (A =7'"), due almost beyond doubt to the rays emitted from the 

 lunar soilhaving been here absorbed by our atmosphere. The 

 conclusive evidence that this is due to the atmosphere is 

 derived, first, from the constant appearance of an analogous 

 band in the heat spectrum of the sky away from the moon, 

 and second, from the independent observation of the existence 

 of this band in the invisible spectrum of a terrestrial object 

 after absorption by 100 meters of air. In the latter case it is 

 always found distinctly marked in moist weather and can even 

 be observed under circumstances favorable to its development, 

 in the few meters of air within the length of the observing 

 room. It is important here to remark that the maximum of 

 the unaosorhed radiation of a Leslie cube, at a temperature a 

 little below that of boiling water, is found at the deviation of 

 38° 20' (^=8^), when observed by the same rock-salt train. 



Following the solid curve down the spectrum, we find it 

 rise into its principal maximum just below deviation 37° 30' 

 (X, about 14^) where it attains a height of about 43 degrees of 

 our arbitrary scale.* It is again most important to remark 

 that this point, just below 37° 30', corresponds to the maximum 

 of the unabsorbed radiation of a lamp-blacked surface at a 

 temperature of about —10° Cent. Were it not, then, for 

 atmospheric absorption, we should assert with confidence that, 

 so far as the radiations of a lamp-blacked surface and the 



* It may be interesting to observe that we infer from our bolometric observa- 

 tions that the effect of the total and unconcentrated lunar radiation on a black- 

 ened thermometer would be something like gooff" Cent. 



