756 Prof. J. H. Poynting on Prof. Lowell's Method for 



surface has reached an equilibrium between receipt and ex- 

 penditure of radiation. This is a condition to which the 

 surface tends at or soon after noon by day and before dawn 

 at night. We shall suppose that the low temperature radia- 

 tion from the surface is either transmitted or absorbed, so 

 that, using the previous notation, 



^ 1 + a 1 = l and r T —0. 



If R rf is the equilibrium surface radiation reached we 

 suppose about noon 



n nir n 



If R n is the equilibrium surface radiation in the later part 

 of the night we have to omit hS and 



'»■ 





mr n 



To proceed further, we must express R in terms of S. 

 We can only do this by some assumption. Probably it is 



not very far from the truth to assume that R = ^(R rf + R w ), 

 and we shall take this value. It gives us 



t a 



R=i_L^s, 



n 

 and substituting in the values of dav and night radiations 



we get 











iys - 



H7T 



(li 2 71 IT 





71 ' !_"} 



1L 















t a 

 .7 + — 





R„/S = 



nrr 



z nir 

 1 l—^ 1 ' 



Though these formulae are only obtained by making large 

 assumptions, and by neglecting important considerations, 

 they nevertheless show the tendency of the day and night 



