482 BoEDDiCKER — On Lunar Radiant Heat. 



alternately upon each pile, the efPect sought for was nearly doubled, while 

 extraneous disturbing eiiects tended generally to counteract one another. Still 

 the needle was at all times more or less, and often very unsteady, though by 

 taking means of a long series of alternate readings it was found possible, not 

 only to demonstrate the existence of a very appreciable amount of radiation, but 

 even roughly to estimate its variation with the phase and form, a curve generally 

 representing that variation. The results were published in the Proceedings of the 

 Royal Society, No. 112, 1869. 



During the following season the experiments were pursued with modified 

 apparatus ; and, under the idea that the annoying disturbances of the needle still 

 subsisting were largely due to an inequality of power between the two piles, 

 these were replaced by thermocouples of home manufacture, selected out of a 

 number, so as to be as nearly alike as possible.* They are described in the 

 Proceedings of the Royal Society, No. 122, 1870 ; the results were published in 

 the Proceedings^ No. 123, 1870. In addition to another heat-curve, which, not- 

 withstanding the modifications of the piles, does not give a more precise determi- 

 nation, I was able by the interposition of a sheet of glass to show in the above 

 communication that the Moon's heat differed materially from the Sun's in that it 

 contained a much larger proportion of rays of low refrangibility. From the very 

 rapid diminution of the heat towards New Moon it was probable that at that phase 

 the remaining heat would be scarcely if at all appreciable, and from these two 

 facts I concluded that the heat which I had been attempting to measure was heat 

 received by the Moon from the Sun, but only re-radiated after part of it had been 

 absorbed by the lunar surface, and then emitted as dark rays of heat. At the 

 same time, so far as could be gathered from these rough determinations the 

 maximum of heat did not take place as might, perhaps, have been expected to an 

 appreciable degree after the maximum of light, that is after Full Moon. Some 

 rough determinations of the proportion between lunar and solar radiation were 

 also made. 



At this stage Dr. Copeland, who had taken up the post of Astronomical 

 Assistant in my Observatory, began to devote much labour and patience to the 

 investigation, using the same apparatus, only refitted and adjusted, as I had 

 employed in the season 1869-70. He formed a more concordant series of mean 

 results, and having fully reduced them, and having by long series of observations 

 at largely varying altitudes obtained a curve for the absorption of heat by our 

 atmosphere, he was able to produce a more reliable phase-curve for heat. 



The determinations of the quality of the radiation by interposition of a sheet 



* The thermopiles of bismuth, and an alloy of 12 bismuth to 1 tin were replaced about three and a-haH 

 years ago by thermopiles of iron and German silver, the wire of commerce being used in each case. 



