50 Prof. S. P. Langley on the 



(A,, 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 imabsorbed radiation of a lampblacked 

 surface at a temperature of about — 10° Cent. Were it not, 

 then, for atmospheric absorption, we should assert with confi- 

 dence that, so far as the radiations of a lampblacked surface 

 and the lunar soil are comparable, the temperature did not 

 exceed — 10° Cent. Below this point the curve falls off with 

 interruptions by several cold bands, until evidence of heat 

 disappears near deviation 33° of our rock-salt train ; but of 

 this latter portion of the solid curve we will not pause here 

 to speak. The dotted line is an attempted reconstruction of 

 the original curve of lunar heat as it would appear before 

 atmospheric absorption. It is made by allowing for the 

 amount of absorption directly observed in the sky radiation, 

 and in the radiation from terrestrial objects at a low tem- 

 perature, already referred to, supplemented by an estimate of 

 atmospheric absorption in this region inferred from a com- 

 parison of solar and electric arc radiation, to be presently 

 described ; and this constructive maximum occurs near 

 deviation 38° 15', which corresponds to the maximum of 

 unabsorbed radiations from a terrestrial source at a tempera- 

 ture of a little over -f 50° Cent. 



Direct observation, then, of the lunar heat-curve indicates 

 that the probable temperature of the lunar soil is between 0° 

 and —20° Cent. This is subject to the effect of our atmo- 

 sphere, which probably is to displace this maximum in some 

 degree towards the position of greater cold ; but the highest 

 temperature we can assign by an allowance for this is -f- 50° 

 Cent. Between these points, we believe it probable that the 

 temperature of most of the lunar sunlit soil must lie. The 

 temperature of the lunar poles has not been specifically 

 determined, but direct observation indicates that it is still 

 lower. 



The relative amounts of the reflected solar and the emitted 

 heat could evidently be obtained with satisfactory accuracy 

 by measurements within the respective portions of the solid 

 curve, were it not for the distorting action of the terrestrial 

 atmosphere already mentioned. We must refer to the original 



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

 observations that the effect of the total and imconcentrated lunar radia- 

 tion on a blackened thermometer would be something like -^Vo Cent. 



