32 Prof. S. P. Langley on the 



the sea-level. Thus Sir John Herschel says of the moon that 

 " its heat (conformably to what is observed of that of bodies 

 heated below the point of luminosity) is much more readily 

 absorbed in traversing transparent media than direct solar 

 heat, and is extinguished in the upper regions of the atmo- 

 sphere, never reaching the surface of the earth at all/' 



The reader may also be reminded that these statements have 

 remained unchallenged on account of the hitherto insuperable 

 difficulties of experimental investigation, arising from the all 

 but infinitesimal amount of heat which the moon sends us, 

 and the added fact that this heat, small as it is, is necessarily 

 of two essentially different kinds — that which the moon, acting 

 as a mirror, reflects from the sun, and that which directly 

 emanates from the substance of her own sun-heated soil, while 

 it is only by an analysis of each of these two kinds of heat, 

 each in its totality non-existent to the most sensitive ther- 

 mometer, that we can expect to give an experimental answer 

 to the question*. 



It was Melloni who, on Mt. Vesuvius in 1846, by the 

 employment of a polyzonal lens, one metre in diameter, and 

 the newly invented thermopile and galvanometer, first suc- 

 ceeded in getting any certain indications of heat from the 

 moon, though these were of the feeblest kind. It is Lord 

 Posse, employing the Parsonstown telescope with improved 

 thermopiles and galvanometers, who has the credit of abun- 

 dantly confirming Melloni's observation of the fact of the 

 moon's radiant heat being perceptible, and further the great 

 merit of making a preliminary investigation of its character, 

 by showing by its imperfect passage through glass that it is 

 chiefly non-reflected heat. Lord Posse, however, as has been 



the total excess of this lunar radiation over that from the sky by means of 

 that from two blackened vessels, one producing the same effect on his 

 thermopile and galvanometer as the sky, the other as the moon, and finds 

 that the observed galvanometer range is that due to a temperature of 

 excess in the latter vessel, to be computed at 197° "5 F. if Dulong and 

 Petit 's law of cooling is used, or at a still higher one if Newton's is em- 

 ployed. The effective sky temperature was about -f- 20° F., so that if 

 we suppose the result due wholly (instead of mainly) to the emitted 

 heat, this would indicate a temperature of the lunar soil at any rate above 

 that of boiling water. Lord Iiosse, however, in view of the empirical 

 character of the formula emplo} T ed and of other considerations, is careful 

 to state as his conclusion " that the problem of the determination of the 

 lunar temperature is nearly as far as ever removed from our grasp." 



* In an appendix (No. 1) of the memoir referred to, will be found a 

 historical account, believed to be fairly complete, of the labours of previous 

 thinkers and workers in this subject. 



