The Earl of Rosse on Lunar Heat-Radiation. 



375 



April 1 6th 

 April 20th 



>) 

 u 



Altitude 



of part 



of sky 



examined. 



Calculated 

 difference 

 of tem- 

 perature. 



Tempera- 

 ture of tin 



vessel. 



Apparent 

 tempera- 

 ture of 

 the sky. 



Remarks. 







49 

 49 



5o 

 64 

 64 





 23-9 



327 

 28-3 

 28-3 

 30-1 

 26-2 





 55'5 

 49 

 5 1 

 50-5 



47 

 44 





 31-6 

 16-3 



227 



22 - 2 



i6'9 

 ,7-8 



Sky hazy. 



Sky apparently black 

 ! and transparent ; 

 occasional light 

 j clouds. 



If the temperature of space be really as low as is supposed, this 

 result seems to indicate considerable opacity of onr atmosphere for 

 heat-rays of low refrangibility. 



The ever varying transparency of our atmosphere has been found 

 to be a very serious obstacle ; but the much greater steadiness of 

 the needle daring the later experiments (the mean error of the last 

 few nights' observations having been from two to three and a half 

 percent, only of the whole deviation*) encourages us with the hope 

 that, by taking advantage of favourable moments, and measuring the 

 moon's light simultaneously with her heat, more accurate informa- 

 tion on this subject may soon be acquired. 



The observations were examined with the view of ascertaining how 

 far the heating-power of the moon's rays varies with her altitude. 

 Owing to the interference of clouds, and the limited range of altitude 

 within which the observations were made, it is hardly worth while to 

 give the results in detail ; however, I may just say that the heating- 

 power of the moon's rays appears to diminish with her altitude only 

 about one-third as fast as the intensity of the solar chemical rays as 

 ascertained by Roscoe and Thorpe. 



An attempt was made to ascertain, by comparing two measure- 

 ments of the moon's light at different altitudes with two corresponding 

 measurements of her heat, whether our atmosphere intercepts the 

 keat-rays to a greater extent than the luminous rays. It was found 

 that while the light was diminished with the altitude in the propor- 

 tion of about 3 to 1, the heat was diminished in the proportion of 

 about 5 to 1 . In consequence, however, of much of the moon's light 

 and heat being intercepted by hazy clouds or condensed vapour at 

 the lower altitude, the experiment was inconclusive as to the effect 

 of a transparent atmosphere on the dark rays of heat. 



The accompanying diagram shows the proportion between the 

 amount of lunar heat found on various nights at various ages of the 

 moon. There appears to be a general accordance between the varia- 

 tion of her radiant heat with her phase and the corresponding amount 

 of her light as deduced by calculation. 



* During the experiments of the previous season the mean error varied be- 

 tween 27 per cent, and 85 per cent, or more. 



