422 S. P. LaiKjley — Temperature of the Moon. 



moon, would be excessively high. Thus Sir John Herschel, in 

 his latest " Outlines of Astronomy," says : " The surface of the 

 full moon exposed to us must necessarily be very much heated, 

 jpossibly to a degree much exceeding that of boiling water." 

 The only experimental evidence obtainable appeared to lend 

 support to this view, for though Lord Rosse did not undertake 

 to directly determine the lunar temperature (as he is often 

 supposed to have done) any inference which can be drawn from 

 his experiments appears to support the above views of Her- 

 schel, which he also cites.* 



It has also been almost universally supposed that our atmos- 

 phere was nearly impervious to the lunar radiant heat, so that, 

 if any existed, it could still not be perceived by us at 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 atmosphere, 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 ema- 

 nates 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 thermometer, that 

 we can expect to give an experimental answer to the ques- 

 tion, f 



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

 ployment of a polyzonal lens, one meter in diameter, and the 



* See Proc. Royal Society, xvii, 1869, p. 443; xix, 1870, p. 12; Transactions, 

 clxiii, pp. 622-4. Having elsewhere shown that the lunar radiation consists of a 

 small quantity of reflected heat and a comparatively large quantity of heat emitted 

 from its soil, Lord Rosse compares the total effect of this lunar radiation over that 

 from the sky with 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 employed. The effective sky tem- 

 perature was about + 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 Rosse, however, in view of the 

 empirical character of the formula employed, and of other considerations, is care- 

 ful 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 labors of previous thinkers and 

 workers in this subject. 



