1890.] 



On the Heat of the Moon and Stars. * 



481 



in kind to that produced by a heating of the exposed face of the 

 other. Under these conditions a change in the sky which would act 

 on both faces alike, or nearly so, would not disturb the galvanometer, 

 whereas a star made to shine first on one face and then on the other 

 would cause a deflection first in one direction and then in the other. 

 This arrangement had previously been employed by Lord Rosse, in 

 his experiments on the heat of the Moon.* The pairs used by 

 Mr. Stone were about 31 mm. long and had a sectional area of 

 about 4 square mm., that is, each bar was about 31 X 2 x 1 mm. 

 Wires were taken to a distant galvanometer and the telescope was set 

 with the image of the star alternately on the two faces. About 

 10 minutes were allowed before a reading was taken. The rays of 

 Arcturns concentrated by the 12f-inch object glass produced devia- 

 tions of from 20 — 30 divisions of the scale, while a 3-inch cube of 

 boiling water at two feet from the faces produced a deflection of 

 about 150 divisions. Mr. Stone concluded that the face of the pile 

 was heated through about 1/oOth of a degree Farenheit. With these 

 figures before me, I had no doubt that the radio-micrometer, which in 

 sensibility vastly exceeds the thermopile, while unlike the thermopile 

 and galvanometer it is free from disturbing effects of magnetism 

 and outside changes of temperature and has the further advantage 

 — and for astronomical work this perhaps is even more important — 

 that a measure can be made in five seconds instead of several minutes 

 which are necessary with the older apparatus, would be capable not 

 only of making good and exact measures of the heat of the brighter 

 stars, but I went so far as to hope that even faint stars would pro- 

 duce an appreciable effect and thai most interesting results might be 

 derived from an examination of planets, comets, nebulae, and the red 

 stars. 



I therefore determined to put the radio-micrometer to a severe 

 test, and one which promised not only to show its suitability for such 

 delicate work, but at the same time to give much valuable informa- 

 tion. The Royal Society gave me, out of the Government Grant, a 

 sum of £50, which, thanks to the advice which I received, espe- 

 cially from Mr. W. H. Massey, and Mr. A. A. Common, in the 

 matter of design and construction, was nearly sufficient to meet all 

 the expenses which I have incurred. I should say also that 

 Mr. Paxman, of Colchester, who made the steel tube, Avhich is a 

 beautiful example of miniature boiler construction, kindly presented 

 this in the cause of science ; that I have been able to use some few 

 pieces of apparatus belonging to the Physical Laboratory, at South 

 Kensington, such as the large magnet of about 25 lbs. for the radio- 

 micrometer, some of the lime-light apparatus, and the finder ; and, 



* ' Eoy. Soc. Proc.,' vol. 17, p. 436. 



