The Earl of Rosse on the Radiation of Heat from the Moon. 319 



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

 by determining 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, „ ,, =0, 



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

 L', O', the luminous not being absorbed, and the non-luminous being 

 entirely absorbed in their passage through glass, then 



•8, 



L + 

 L' 



= •08: 



l x ^±o;= I o. 



L' L + 



L+0' 



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



L+0 _ 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 connected with that pre- 

 viously found for full moon by using the deviation produced by a 

 vessel of hot water as a term of comparison. 



The relative amount of solar and lunar radiation thus found was 



89819:1, (c) 



which is quite as near that given by (b) as we could expect when we 

 consider the roughness of the data. 



As a further confirmation of the correctness of the two rough 

 approximations to the value of the ratio existing between the sun's 

 and the moon's radiant heat already given, the subject was investi- 

 gated from a purely theoretical point of view. It was assumed 



( 1 ) That the quantity of heat leaving the moon at any instant may 

 without much error be considered the same as that falling on it at 

 that instant. 



(2) That the absorptive power of our atmosphere is the same for 

 lunar and solar heat. 



(3) That, as was already assumed in obtaining formula (a), the 

 moon is a smooth sphere not capable of reflecting light regularly. 

 Then the heat which leaves the moon in all directions = quantity 

 which falls on the moon =y^ of the quantity which falls on the 

 earth from the sun 



= K.f*{(*- 



e) . cos e+ sin e} sin e . de = — .'3t. 



