S. P. Langley — Temperature of the Moon. 437 



lunar soil are comparable, the temperature did not exceed 

 — 10° Cent. Below this point, the curve falls off with inter- 

 ruptions- by several cold bands, until evidence of heat disap- 

 pears near deviation .; 3° 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 atmos- 

 pheric 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 temperature, already 

 referred to, supplemented by an estimate of atmospheric, ab- 

 sorption in this region inferred from a comparison of solar and. 

 electric arc radiation, to be presently described ; and this con- 

 structive maximum occurs near deviation 38° 15' which cor- 

 responds to the maximum of unabsorbed radiations from a 

 terrestrial source at a temperature of a little over +50° Cent. 



Direct observation, then, ©f 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 atmos- 

 phere 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 +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 deter- 

 mined, 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 

 memoir for reasons for estimating the total amount of the 

 reflected radiation as little more than \ of that emitted. * 



* Lord Rosse found that 87 per cent of all the solar rays were transmitted by 

 a particular piece of glass which allowed 92 per cent of solar light to pass, and 

 12 per cent of the total lunar beam. He attempted from this to determine the 

 relative amounts of the solar and lunar heat, but felt obliged, in the then state of 

 knowledge, to make the assumption (which our subsequent researches have shown 

 to be erroneous) that the glass absorbed all the invisible rays, or that the lunar 

 radiation contains 12 per cent of luminous rays, instead of less than 5 per cent, 

 which is more nearly the actual case ; but when Lord Rosse's own observations 

 are-reduced with the aid of the facts determined by the writers, and representing 

 the actually large transmissibility by glass of the invisible rays of shorter wave- 

 length in the infra-red, his expression for the relative value (which we will 

 call x) of the emitted part of the lunar radiation (whose transmission by glass 

 is presumed from observations on a Leslie cube to be 1"6 per cent) becomes 



— — t\to, from which x=l'2 times the reflected solar part; so that if 



Lord Rosse could have possessed, at the time his reductions were made, knowl- 

 edge as to the diathermic properties of glass which has only been acquired since, 

 his own observations would have given results for the relative amounts of re- 

 flected and radiated heat in somewhat remarkable accordance with our own. 

 Am. Jour. Sci.— Third Series, Vol XXXVIII, No. 22S.— Dec, 1889. 

 28 



