950 



SCIENCE 



[N. S. Vol. XXXVir. No. 964 



would have a temperature not greatly above 

 — 225° C."= On page 197 of that work, a 

 summary of our observations in the lunar 

 spectrum yields the result that '' the most 

 reliable spectrum comparisons with a black- 

 ened screen show an average ' effective lunar 

 temperature ' of -|- 45° C. near the time of 

 full moon." Although the temperature which 

 I had derived from the spectrum comparisons 

 is thus cited, and is the only authoritative 

 figure mentioned, a lingering predilection for 

 his earlier value prevented more than a half- 

 hearted acquiescence in my result on the part 

 of Mr. Langley, who, as chief spokesman, said :" 

 Contrary to all previous expectations, [the ex- 

 treme infra-red] nevertheless reaches us, thus 

 bringing evidence of the partial transparency of 

 our terrestrial atmosphere even to such rays as are 

 emitted by the soil of our planet. It is probable, 

 as remarked elsewhere, that even of the heat of 

 arctic ice some minute portion escapes by direct 

 radiation into space. If beyond this we can be 

 said to be sure of anything, it is that the actual 

 temperature of the lunar soil is far lower than it 

 is believed to be; but the evidence does not war- 

 rant us in fixing its maximum temperature more 

 nearly than to say it is little above 0° Centigrade. 



The last part of this extract refers to the 

 belief that the moon's maximum temperature 

 exceeds that of boiling water, a belief which 

 rested on the opinion of Sir John Herschel 

 and on some thermopile measures with a very 

 large probable error which had been made of 



-Op. cit., p. 193. Langley 's original statement 

 in regard to the moon's temperature was founded 

 on what he says ("Researches on Solar Heat," 

 p. 213, 1884) concerning the earth's temperature 

 in the absence of a selectively absorbing atmos- 

 phere, namely : ' ' The temperature of the earth 's 

 surface is not due principally to this direct [solar] 

 radiation, but to the quality of selective absorp- 

 tion in our atmosphere, without which the tempera- 

 ture of the soil in the tropics under a vertical sun 

 would probably not rise above ■ — 200° C. ' ' This 

 passage contains a mixture of truth and error. 

 The radiation from a tropical surface at 300° Abs. 

 is 285 times that of a similar surface at 73° Abs. 

 The atmosphere has a protective influence, but not 

 one as extraordinary as this. It must be remem- 

 bered that Stefan 's law was not yet fully accepted. 



^Op. cit., p. 193. 



the total lunar radiation by Lord Rosse, and 

 which, as is well known, had previously beea 

 our most reliable source of information. 



I may perhaps be permitted to say that the 

 preceding citation from our joint work did 

 not represent the opinion of the junior con- 

 tributor. Abbot and Fowle, referring to this 

 memoir, speak of my " most recent revision 

 of the evidence " ;* but this is a misapprehen- 

 sion, inasmuch as I have never revised the 

 material contained in the above memoir. M.^ 

 later publications have been founded on new 

 evidence which is entirely distinct. 



Mr. Abbot also says in his work on " The 

 Sun " (p. 311) : 



Upon the moon there is no atmosphere and by 

 the observations of Lord Eosse, of Langley and 

 of Very, the moon 's sunlit surface falls from about 

 the temperature of boiling water nearly to that 

 of liquid air within the short duration of a total 

 lunar eclipse. 



But the preceding quotations prove that 

 Langley never accepted the doctrine of the 

 " hot moon " while he was director of Alle- 

 gheny Observatory, and that even when 

 pushed by the evidence, he hung back. 



Professor W. H. Pickering in 1902 said:' 



We do not certainly know the temperature of 

 the moon 's surface when exposed to a vertical 

 sun ; but according to Professor Langley, it can 

 not be far from 32° F. 



N. S. Shaler, in a work published by the 

 Smithsonian Institution in 1903 under Secre- 

 tary Langley's supervision,' said: 



The temperature of the moon has been made 

 the matter of numerous experiments. These, for 

 various reasons, have not proved very effective. 

 The most trustworthy, the series undertaken by 

 S. P. Langley, indicate that at no time does the 

 heat attain to that of melting ice. 



Consequently, up to 1903, Langley had not 

 accepted the results which I published in 1898. 



'Annals of the Astropliysical Observatory of the 

 Smithsonian Institution, Vol. 2, p. 174. 



•"la the Moon a Dead Planet?" The Century 

 Magazine, May, 1902, p. 91. 



' " A Comparison of the Features of the Earth 

 and Moon," Smithsonian Contributions to Knowl- 

 edge, No. 1438. Part of Vol. 34, p. 6. 



