1890.] 



On the Heat of the Moon and Stars. 



495 



or, at any rate, a large proportion, and as it had a smaller aperture 

 and a greater focal length, than the mirror which he lent me, so that 

 the image of the moon was larger, and on this account also had a 

 smaller heating power, it is not surprising that he did not obtain a 

 large effect from the moon ; but that he obtained none is, in the face 

 of these measures, strong evidence that the deflections observed, and 

 which he attributed to the stars, were spurious. 



Mr. Stone did not observe the moon at all, but he concluded from 

 his observations that Arcturus was in heating power equivalent to a 

 3-inch cube of boiling water at 400 yards. A direct comparison of 

 the candle that I used with a 4-inch cube of boiling water gave as the 

 ratio of the heating powers on a radio-micrometer, of which the tem- 

 perature was 15° C, a figure varying slightly with the state of the 

 flame, but generally slightly over one half. Had the comparison 

 been made with the radio-micrometer at 0° C, as it was when the 

 observations on Arcturus were made, a slightly lower number would, 

 of course, have been obtained. As the face of a 3-inch cube is 

 slightly over one-half the face of a 4-inch cube, the candle and 

 the 3-inch cube may be taken for the purpose of comparison as 

 sensibly equal. Now, the direct and conclusive experiment already 

 described has shown that Arcturus on the meridian is not equal to a 

 candle, and therefore to a 3-inch cube at 3084 yards, so that Arcturus 

 must have a heating power nearly sixty times less than that given by 

 Mr. Stone. I cannot clearly follow the whole of Mr. Stone's reason- 

 ing, more especially that part which relates to the rise of temperature 

 due to the star. By the use of thermometers in contact with the face 

 of his pairs when subject to a much stronger radiation, Mr. Stone 

 determined the amount by which the face must be raised in tempera- 

 ture to produce a deflection of one division, and he concluded from 

 the deflection observed that the face was raised through 1/90° C. 

 Now, on comparing his arrangement with mine, it is evident that, as 

 I have a larger aperture, I have more heat to begin with ; as I have 

 no glass for the rays to pass through, I am free from the large 

 absorption of glass for heat ; and, finally, as my thermo-electric bars 

 have a sectional area about one- twentieth of that of the bars used 

 by Mr. Stone, the heat for a given difference of temperature would 

 have been conducted away at about one-twentieth of the rate ; or, as 

 this is the chief cause of the cooling of the hot junction, for a given 

 rate of radiation my junction would have been heated to nearly 

 twenty times the amount to which Mr. Stone's would be heated. 

 Taking all these things into consideration — if Mr. Stone's figures are 

 correct — my junction should have been warmed through about one- 

 cthird or one-fourth of a degree Centigrade. Now, I have already shown* 



* Cantor Lectures, 1889. 



