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



A planetary atmosphere no doubt; acts in some such way 

 as the greenhouse glass. Let us, for the sake of comparison 

 with Prof. Lowell's results, assume, as he has done, that we 

 have a steady state, with the incident radiation normal to 

 the surface. I do not see how to estimate the distribution 

 of the radiation from the air between the upward stream 

 into space and the downward stream to the surface. Since 

 the lower layers of air are warmer than the upper pro- 

 bably more than half comes down, and the truth probably 

 lies between the assumptions that the atmospheric radiation 

 is ^(aS-fa 1 R)as it is with the greenhouse, and that it is 

 <xS + «iH when all the radiation would be downwards. Let 



us suppose that (aS + t^R) comes downward. 



The albedos of the surfaces of both the Earth and Mars 

 average, according to Lowell, 0*1 for visible radiation. They 

 must be much less for the whole spectrum. Where all the data 

 are uncertain the effect of small albedo may be neglected, 

 and indeed in our ignorance of the dependence of tempera- 

 ture on radiation with a partially reflecting surface, it is safer 

 to neglect it. If 6 S is the actual surlace temperature under 

 a vertical sun, and 6 is the temperature which the surface 

 would have without atmo>phere, it is easily found that 



4 / t + a/n * 



Earth. — If we use Lowell's figures for the Earth under a 

 clear sky, 



* = Q-42, a = 0-5x0-65 = 0'325. 



t { =0'5, cilice of the invisible radiation half is transmitted, 

 a x = 0*5, very little is reflected, 



We shall suppose in succession that 



(a) half of the radiation is downwards or that w=2. 



(6) two-thirds. n — ^. 



(c) all n = l. 



We then find 



(«) y =0-94; (i) J* =0-99; (c) J-M2. 



For the case of a cloud-covered earth the data are very 

 uncertain. Lowell takes t = 0'2 of 0-42 = 0*084, assuming 

 that the atmosphere has already reflected and absorbed 0*58 

 before the cloud is reached, surely an overestimate, since 

 the cloud-surface is in the higher air. Let us guess that 



