1870.] 



The Earl of Rosse on Lunar-Heat Radiation. 



11 



slight and imperceptible film of moisture on the surface of the glass, which 

 was much more unlikely to form during the much shorter period* of ex- 

 posure to the night air in the later observations. 



The experiment made during the previous season to determine the ratio 

 between the heating-power of the moon and of the sun was repeated with 

 more care, and the value found, taking what appeared to be the most pro- 

 bable mean heating-power of full moon, as determined on various nights, 

 was 



Sun's total heat „ „ nn 



^ r— - m — r = 82600. 



Moon s total heat 



Takingf the percentage of light transmitted by glass = 92 



Do. do. of sun's heat =87 



Do. do. of moon's heat =12 



Do. do. of heat from a body at 180° F. = 1-6 



If and ^ represent respectively the percentage of dark and lu- 



0' V 



minous rays present in the moon's radiant heat, and ri — - and 



0' + /' 0' + / 



the corresponding quantities for the sun's radiant heat, we have 

 0X-016 + JX-92 



and 



£+0 



0'x-016 + Z'x-92 

 I' + O 1 



= •12, 

 = •87; 



••4=o^ x ^ 82600x ?S= 678300 - 



In all the foregoing experiments on lunar radiation the quantity measured 

 by the thermopile was the difference between the radiation from the circle 

 of sky containing the moon's disk and that from a circle of sky of equal 

 diameter not containing the moon's disk ; we have obtained no information 

 in reference to the absolute temperature of either the moon or the sky. 



The following experiment was therefore made with the view of trying to 

 connect the radiation of the sky with that of a body of known temperature, 

 the deviation due to each degree (Fahrenheit) difference of temperature 

 between a blackened tin vessel containing hot water and subtending a given 

 angle at the pile and a similar vessel containing colder water was first 

 ascertained ; then a similar determination of that due to the difference of 

 radiation from one of these vessels, and from a portion of sky of equal 

 diameter, was made. The following was the result : — 



* About 12 minutes in place of 30 to 60 minutes. 



f All these values, except the first, were determined by experiment for the specimen 

 of glass employed. 



