75-1 Prof. J. H. Poynting on Prof. Lowell' 's Method for 



hard to see bow the temperature of Mars can be raised to 

 anything like the value obtained by Professor Lowell. 

 Perhaps the data are quite wrong. It is conceivable that 

 Mars has a quite peculiar atmosphere practically opaque to 

 radiations from the cold surface. Those who believe that 

 there is good evidence for the existence of intelligent beings 

 on that planet, should find no difficulty in supposing that they 

 have been sufficiently intelligent to cover the planet with a 

 glass roof or its equivalent. Then we might easily have 



H- ^ =0*77 and t x + ~ =05, and then the temperature might 



be raised to 281° A. or 8° 0. Indeed, if the glass were of 

 such kind as to transmit solar radiation, and if it were 

 quite opaque to dark radiation va hile still reflecting a con- 

 siderable proportion, the temperature might easily be raised 

 far above this. 



An Attempt to represent the Effect of Day and Night 

 on the Temperature of the Earth. 



The " greenhouse " formula, which has been used in the 

 foregoing discussion, would hold only if all the conditions 

 were steady. But in reality the alternations of day and 

 night prevent a steady state, and we can only hope that the 

 neglect of these alternations does not greatly affect the ratios 

 of the tempeiatures found for different planets or for different 

 elevations on the same planet. 



I shall now attempt to represent the effect of the diurnal 

 variation in the supply of solar heat to the Earth, or rather 

 to an abstract Earth. For even if we could represent the 

 actual conditions we should obtain differential equations so 

 complicated that they would be useless for practical 

 purposes. 



To simplify matters, let us suppose that we are dealing 

 with the equatorial region of the earth at the equinox, that 

 the air is still, that the surface is solid and black, and that 

 the sky is clear. 



The temperature of the air except near the surface can 

 change but little during 24 hours. For over each square 

 centimetre at sea-level we have 1000 gms. of air with specific 

 heat 0*2375, and therefore with heat capacity 237*5. Con- 

 sider a band of the atmosphere 1 cm. wide round the equator. 

 A stream of solar radiation of length equal to the diameter 

 2r of the earth enters a band of air of length equal, to half 

 the circumference. If the solar constant is 3 the, average 



