956 



SCIENCE 



[N. S. Vol. XXXVII. No. 964 



The language of these quotations implies 

 that I have not considered the radiating power 

 of the actual lunar substance, but have as- 

 sumed an ideal moon ; whereas the truth is that 

 extensive observations were made on the radia- 

 tions from heated silicates and other sub- 

 stances of which the moon's surface is liable 

 to be composed, and comparisons were insti- 

 tuted with these, and not with an ideal radia- 

 tor, before reaching a final conclusion. The 

 result of this comparison of the radiant be- 

 havior of various materials is important, since 

 I find"° that, given a sufficient duration of 

 insolation to enable a steady state to be 

 reached, there is not much difference in the 

 emission from various materials. Thus a 

 very poor radiator, such as rock salt, radiates 

 from a great depth of its interior substance, 

 and the summation of radiation from many 

 interior layers compensates for the small emis- 

 sion from any one layer. Hence it is not cor- 

 rect in such cases to state emissivity as a sur- 

 face function. The complete statement of 

 emissivity must be a volumetric one and must 

 include the subsurface thermal gradient. 



Ferrel showed on theoretical grounds that 

 the law connecting temperature and radiation 

 for the moon may be expressed as an equation 

 of condition, where, if the coefficients of radia- 

 tion and absorption of the same substance are 

 always equal, it makes no difference what the 

 substances are; all will eventually reach the 

 same temperature. Some exception to the 

 theory must be made for such substances as 

 ice which are kept cool by melting and evapo- 

 ration in sunshine. In his actual illustration, 

 Ferrel used the law of Dulong and Petit, but 

 any other formula may be substituted as far 

 as the principle in question is concerned. The 

 important point is that 



the same results would be obtained sensibly with 

 any ordinary conductivity for heat if the same 

 side of the moon were permanently exposed to the 

 sun, for the temperature gradient by which the 

 heat would be conducted inward would soon be- 



-' ' ' The Probable Range of Temperature on the 

 Moon," I., Asirophysical Journal, Vol. 8, pp. 199- 

 217, November, 1898. 



come so small, in this case, that the rate by which 

 heat would be conducted inward would be insen- 

 sible, as in the case in which heat is conducted 

 outward from the interior of the earth."" 



In a different category from the books and 

 articles already cited come such works as that 

 by Fauth." A footnote on page 26 of this 

 book refers to my writings on the moon, but 

 the author does not appear to have read them 

 carefully. On page 139 he says: 



Lord Eosse was enabled by his measurements to 

 appreciate the differences in temperature on the 

 moon 's surface during full radiation and by night, 

 and found them to be over 300° C. But the tem- 

 perature can not be determined with any accuracy. 

 Lord Rosse 's results have often been questioned, 

 but they are supported by the recent investigations 

 of Very. Very believes that at the moon's equa- 

 tor, when the sun is at its highest, the ground 

 increases its temperature by more than 100° C. 

 (which would be —173° C). 



This is pretty nearly a hopeless case. I 

 am sure that neither Lord Rosse nor I could 

 recognize our own work in the conclusions at- 

 tributed to us, where absolute temperatures 

 and temperatures on the centigrade scale are 

 mixed up indiscriminately, in spite of care- 

 fully guarded language in the original 

 sources, and where the opinion is hazarded 

 that the maria are frozen oceans — a supposi- 

 tion which is completely overthrown by the 

 thermal measures. 



The selection of these quotations for special 

 mention does not imply that there are not 

 others equally objectionable in the literature 

 of the subject. 



My chief reason for wishing to call attention 

 to the imperfect conceptions of one whose 

 splendid contributions to science condone all 

 minor imperfections is because Langley's 

 early and gradually changing opinions on the 

 subject of lunar temperature still act as a bar- 

 rier against the acceptance of conclusions 

 which are founded on reliable observations. 

 This will be quite evident from the passages 



"■' William Ferrel, Science, Vol. 6, p. 542. 



"' ' ' The Moon in Modern Astronomy, ' ' by Ph. 

 Pauth, with an introduction by J. E. Gore, 

 P.E.A.S. 



