Prof. Langley on the Temperature of the Moon. 49 



The final result of the measurements, extending over the 

 three years from 1884 to 1887, is given in fig. 2, in which 

 abscissae correspond to deviations of a rock-salt prism of 60°, 

 vertical ordinates to directly observed heat from radiations, 

 while the dotted curve indicates what seems to be the most 

 probable position and amount of the lower maximum, as it 

 would be observed were there no intervening atmosphere. 

 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 0^48, or that 

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

 solid curve, we observe that it reaches a maximum near 

 39° 40' = 1^5, 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 galvanometer from the small part of the spectrum 

 covered by the bolometer. Continuing to go down the 

 spectrum in the direction of greater wave-lengths, and passing 

 with casual notice a depression at 39° 15' (X=3' A *1), 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' (X = 7 M '), due almost beyond doubt 

 to the rays emitted from the lunar soil having 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 spec- 

 trum 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 metres of air. In the latter case it is always found 

 distinctly marked in moist weather and can even be observed 

 under circumstances favourable to its development, in the 

 few metres of air within the length of the observing-room. 

 It is important here to remark that the maximum of the 

 unabsorhed 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' 



Phil Mag. S. 5. Vol. 29. No. 176. Jan. 1890. E 



