S. P. Langley — Temperature of the Moon. 423 



newly invented thermopile and galvanometer, first succeeded 

 in getting any certain indications of heat from the moon, 

 though these were of the feeblest kind. It is Lord Rosse, em- 

 ploying the Parsonstown telescope with improved thermopiles 

 and galvanometers, who has the credit of abundantly confirm- 

 ing 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 Rosse, however, as has been 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 im- 

 portance, 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 max- 

 ima 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 tem- 

 perature of the lunar soil has been hitherto supposed to not 

 improbably be. This statement of the first memoir is put for- 

 ward as inferential and probable merely, and not as conclu- 

 sively 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 of 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 posi- 

 tion 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 writer cannot feel that (owing to the 

 extreme experimental difficulty of the subject) the results have 

 obtained a certitude corresponding to the great labor bestowed 

 upon them, he believes that this labor is justified by the fact 



