162 Prof. P. Lowell on a Method for Evaluating 



the resulting temperature, he seems to have assumed either 

 with Newton that a body radiates heat in direct proportion 

 to its temperature, which would give for the mean temperature 

 of Mars 223°*6 abs. (—230° F.), or Dulong and Petit's law, 

 which would make it 363° abs. or —96° F., for he entertains 

 the possibility that the polar caps may be composed of solid 

 carbonic acid, which freezes only at —109° F. 



A closer determination has recently been made by Moulton, 

 by taking Stefan's law of radiation, that of the fourth power 

 of the temperature. On this basis the mean temperature 

 comes out —35° F. ? the reasoning being this : — If a body 

 remain at the same temperature it must radiate as much 

 heat as it receives, and, reversely, receive as much as it 

 radiates. Consequently the temperature is as the fourth root 

 of the amount received. Absolute zero is minus 459° F. 

 The mean temperature of the Earth is usually taken at 60° F. 

 Therefore, to determine the mean temperature of Mars we 

 have, calling x its temperature on the absolute scale, the 

 following equation : — 



x : 519° abs. : : ^4 : ^ 7 9, 



which gives 35° F. for the mean temperature of the planet. 



Now in these and similar determinations, the only thing- 

 considered is the distance from the sun, as if the surfaces of 

 the planets were immaterial points in space, and the whole 

 heat arriving there went to warm the bodies. Bat such is 

 far from being the fact. The actual amount of heat received 

 is not at all what mere distance would lead one to infer, quite 

 apart from the blanketing effect of air. 



The following solution of the problem is general. After 

 developing it we shall apply it to the Earth and Mars. 



Division of Radiant Energy. — So soon as a radiant ray 

 strikes matter it suffers division of its energy. Part of it is 

 reflected, part absorbed, and part transmitted. What is 

 reflected is sent off again into space, performing no work in 

 the way of heating the body. Now the amount reflected is 

 not the same in all cases, depending for its proportion upon 

 the character of the matter the ray strikes. 



If the surface of a planet be itself exposed unblanketed by 

 air, the absorbed and transmitted portions go to heat the planet, 

 directly or indirectly. 



If the planet be surrounded by air, the portion transmitted 

 by this air, plus what is radiated or reflected from it to the 

 solid surface, must first be considered. Then upon this as a 

 basis must secondly be determined how much the surface in 



