38 Prof. S. P. Langley on the 



(9) On other measurements giving the means of estimating 



the total lunar radiation in terms of solar ; 

 but for these and many more subsidiary ones, the reader must 

 again be referred to the original memoir. 



The only one of these subsidiary researches which needs 

 further mention here is the measurement of the heat from 

 different parts of the eclipsed moon, on the night of September 

 23, 1885. 



The diameter of the lunar image was 28*3 millim., and of 

 this only a limited portion (0*08 of the whole) fell upon the 

 bolometer. 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 

 continued to diminish rapidly with the progress of the im- 

 mersion in the penumbra, but at no time did the lunar radia- 

 tion from the part in full shadow entirely vanish. At one 

 hour before the middle of the total eclipse, the deflexion 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 deflexion 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 vicissitudes of the lunar climate in- 

 dicated 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 connexion it should be stated that repeated obser- 

 vations 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 obser- 

 vation 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 as 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, 

 representing invisible radiant heat, furnishes a reliable crite- 

 rion as to the temperature of the radiating (solid) body *, 



* Proc. Am. Assoc, for Adv. of Sci., 1885 ; also Phil. Mag. xxi. Mav 

 1836. 



