1869.] 



Heat from the Moon* 



441 



and calculated quantities of heat, the results suggest to us that the law of 

 variation of the moon's heat will probably be found not to differ much from 

 that of the moon's light. It therefore follows that not more than a small 

 part of the moon's heat can come from the first of the three sources already 

 mentioned. 



With the view of ascertaining what proportion of the sun's heat does not 

 leave the moon's surface until after it has been absorbed, some readings of 

 the galvanometer were taken on four different nights near the time of full 

 moon, with a disk of thin plate glass in front of the face of each pile ; and 

 the deviation was about six or eight divisions. 



As the glass screens were examined with care for dew after removal on 

 each night, and none was perceived except on one occasion, the probable 

 percentage of the moon's heat which passes through plate glass is 8, or rather 

 less. 



Few experiments appear to have been made on the absorptive power of 

 glass for the sun's rays ; but, from the best data that I have been able to 

 obtain, I find that probably about 80 per cent, pass through glass. 



The greater part of the moon's heat which reaches the earth appears, 

 therefore, to have been first absorbed by the lunar surface. 



It now appeared desirable to verify this result, as far as possible, by de- 

 termining by direct experiment the proportion which exists between the 

 heat which reaches the earth from the sun and from the moon. 



If we start with the assumption that the sun's heat is composed of two 

 portions, 



the luminous rays, whose amount = L, 



and 



the non-luminous, ,, „ = O, 



also that the moon's light consists of two corresponding portions, L', O', 

 the luminous not being absorbed, and the non-luminous being entirely ab- 

 sorbed in their passage through glass, then 



^ = 8, 1 • - x L ' + Q, -10 

 _LL_ = . 08; f ' ' L' L+O * 



L' + O' J 



Substituting for ^ its generally received value (800,000), we have 

 L 



L' + O' _ 1 m 

 L + O 80,000 } 



Owing to the extremely uncertain state of the weather, only one series of 

 eighteen readings was obtained for the determination of the sun's heat. A 

 beam of sunlight was thrown, by means of a plane mirror, alternately on and 

 off a plate of polished metal with a hole • 1 75 inch in diameter. At a short 

 distance behind this the pile was placed. The deviation thus found was 



