1870.] The Earl of Rosse on Lunar -Heat Radiation. 9 



a press of other work precluded them from comparison with the instrument 

 (standard) at Kew Observatory. After inspection of the observations, you 

 may be good enough to consider the desirability of sending me a third 

 actinometer of the same kind, due comparison being first made at Kew, 

 whereby the relation hetween the instrument at the Observatory (Kew) 

 and those sent out to me by the Royal Society may be established. 



XV. " On the Radiation of Heat from the Moon.— No. II." By 

 the Earl of Rosse, F.R.S. Received June 14, 1870. 



In a former communication to the Royal Society I gave a short account 

 of some experiments on the radiation of heat from the moon, made with 

 the three-foot reflector at Parsonstown, during the season of 1868-1869. 

 I then showed : 



1st. That the moon's heat can be detected with certainty at any time 

 between the first and last quarter, and that, as far as could be ascertained 

 from so imperfect a series of observations, the increase and decrease of 

 her heat, with her phases, seems to be proportional to the increase and 

 decrease of her light, as deduced by calculation*. 



2ndly. That a much smaller percentage of lunar than of solar rays is 

 transmitted by a plate of glass, and we therefore infer that a large portion 

 of the rays of high refrangibility, which reach the moon from the sun, do 

 not at once leave the moon's surface, but are first absorbed, raise the 

 temperature of the surface, and afterwards leave it as heat-rays of low re- 

 frangibility. 



3rdly. That, neglecting the effect of want of transparency in our atmo- 

 sphere, and assuming, in the absence of any definite information on the 

 subject, that the radiating-power of the moon's surface is equal to that of 

 a blackened tin vessel filled with water, the lunar surface passes through a 

 range of 500° F. of temperature ; consequently the actual range is probably 

 considerably more. 



4thly. The proportion between the intensity of sunlight and moonlight, 

 and between the heat which comes from the sun and from the moon, as 

 deduced from those observations, agreed as nearly as could be expected 

 with the values found by independent methods, and for this reason might 

 be considered the more reliable. 



During the past season these observations have been continued, but much 

 time has been spent in trying various modifications of the apparatus, and 

 a satisfactory comparison of observations made on different nights, under 

 different circumstances, has been impossible ; however, by more numerous 

 and more complete experiments, made alternately with and without an inter- 



* See the Proceedings of the Eoyal Society, No. 112, 1869, page 439. 



