432 S. P. Langley — Temperature of the Moon. 



case is 41° 08' 30", or that of the D line in the visible spec- 

 trum. Previously to observation the bolometer has been 

 radiating through the spectroscopic train and mirror to the 

 special screen described, which, on this particular evening, is 

 at the constant temperature of +18° Cent. Under these cir- 

 cumstances, the needle will take up some position representing 

 radiation to the screen, which at this first exposure we will call 

 (A), its position here on our arbitrary scale being at the 215th 

 millimeter. . During this time the siderostat mirror has been 

 so placed as to be sending toward the bolometer radiations 

 from the sky on the east of the moon."- Call the effect of 

 these particular sky radiations (B). They have been inter- 

 cepted by the screen, but now the screen is withdrawn and the 

 bolometer radiating to this eastern adjacent sky receives less 

 heat than the screen gave it (2L3 - 2). The siderostat mirror is 

 moved to throw on the image of the moon. Let us call the 

 resultant deflection (C). The moon appears in this example 

 very slightly warmer than the sky for the image moves on to 

 213"4. ISText by another adjustment of the siderostat mirror, 

 the sky west of moon is thrown on. Call this result (D). The 

 image on the galvanometer scale moves back to 211 "9, indicat- 

 ing cold. Finally the screen is interposed the second time. 

 Call this second interposition (E). In an ideally constant ap- 

 paratus, the second interposition of the screen should give the 

 same reading as the first. Actually 210* divisions is obtained, 

 instead of 215* as before, owing to the so-called "drift" of the 

 needle during observation. The mean of the two readings for 

 sky-east and sky-west is now subtracted from that for the moon 



r> _i_ t\ 



and gives, under the column C , the difference between 



the temperature of the moon and the sky in our arbitrary de- 

 grees. It will be seen from a comparison of all the numbers in 

 this column that there are fairly accordant indications of a max- 

 imum near the prismatic deviation of 39° 30' which corresponds 

 with the wave-length of 2 ,/z L, and approximately with the max- 

 imum of the solar heat curve. There is another maximum of 

 far greater magnitude near 37° 30' (wave-length about M**"), 

 corresponding to the maximum known to exist in the radiations 

 of bodies at a temperature of about 0° Cent., and due, it would 

 seem almost beyond doubt, to radiations from the sun-heated 

 lunar soil. It will be seen also that all numbers in this column 

 have one sign, i. e. the positive, indicating that throughout this 

 series, without exception, the moon has been found warmer 

 than the adjacent sky. This, indeed, is to be expected, since, 

 without the atmosphere, the temperature of this sky would be 

 nearly that of the absolute zero, and at any rate lower than 



* The area of sky observed is virtually the same as that of the moon. 



