Temperature of the Moon. 33 



said, concludes that the problem of the moon's temperature is 

 still indeterminate. 



At this point the question is taken up by the writer, who, 

 with the aid of the bolometer, used directly in the lunar 

 image, had already reached, in the first of the memoirs men- 

 tioned above, the following inference among others of less 

 importance, viz. that the sunlit surface of the moon is not 

 far from the freezing temperature. 



This inference resulted both from observations in the direct 

 beam and from a preliminary and partially successful attempt 

 to form a heat spectrum, for this gave indications of two 

 maxima in the heat-curve — the first corresponding to the heat 

 from the solar reflected rays ; the second (indefinitely lower 

 down in the spectrum) corresponding to a greater amount of 

 radiant heat emitted from a source at a far lower temperature, 

 lower at any rate than that of boiling water, above which the 

 temperature of the lunar soil has been hitherto supposed to 

 not improbably be. This statement of the first memoir is put 

 forward as inferential and probable merely, and not as con- 

 clusively proven. 



The second memoir, on " The Solar and the Lunar Spec- 

 trum," is chiefly devoted to the invisible spectrum of the sun, 

 but incidentally describes the progress in the improvement of 

 the apparatus employed so as to better fit it for the delicate 

 task, 



(1) of measuring the already feeble lunar heat when diffused 

 by expansion into a lunar spectrum ; and 



(2) of determining the possible existence and the exact 

 position of the two heat maxima already described in the first 

 memoir. 



We are now prepared to take up the present memoir, and 

 give an abstract of its results. It contains researches pursued 

 through several years with constantly improving instrumental 

 means, and while the w T riter cannot feel that (owing to the 

 extreme experimental difficulty of the subject) the results 

 have obtained a certitude corresponding to the great labour 

 bestowed upon them, he believes that this labour is justified 

 by the fact that it has not been given to a question of merely 

 abstract interest, but that the whole subjects of terrestrial 

 radiation and the conditions of organic life upon our planet 

 are intimately related to the present research. 



Spectroscopy has hitherto dealt almost exclusively with 

 Phil. Mag. S. 5. Vol. 29. No. 176. Jan. 1890. D 



