168 Prof. P. Lowell on a Method for Evaluating 



dioxide. At the earth's surface the transmission in consequence 

 is about 50 per cent. ; at Camp Whitney it was about 59 per 

 cent. We might, therefore, suppose it still greater through 

 the air of Mars, which is very thin, and if we did so, we 

 should find a still larger fraction of solar heat to be received 

 by the planet's surface. So that such a supposition would 

 actually increase the cogency of the present argument. But 

 the very thinness of the air joined to the lesser gravity at 

 the surface of the planet would lower the boiling-point of 

 water, as investigation shows (see later in the paper) to some- 

 thing like 110° F. The sublimation at lower temperatures 

 would be correspondingly increased. Consequently the 

 amount of water-vapour in the Martian air must on that 

 score be relatively greater than in our own. 



By Carbon Dioxide. — Carbon dioxide, because of its greater 

 specific gravity, would also be in relatively greater amount, 

 so far as this cause is considered. For the planet would 

 part, cceteris paribus, with its lighter gases the quickest. 

 Whence as regards both water-vapour and carbon dioxide 

 we have reason to think them in relatively greater quantity 

 than in our own air at corresponding barometric pressure. 

 Since, therefore, thinness of air and its constitution tend to 

 offset each other, we may assume provisionally that the 

 absorption due to both causes is what it is with us at Camp 

 Whitney, or about 40 per cent, of the whole, leaving 60 per 

 cent, of the heat transmitted. 



It is distinctly to be noted not only that this conservative 

 estimate lowers the determination of the heat received at the 

 surface of Mars, but that what is thus lost in reception goes 

 to make the retention of the heat received all the greater. 



Albedoes of Planets. — We turn now to the albedoes. The 

 albedoes of the several planets, according to the determinations 

 latest obtained, those by Midler at Potsdam, together with 

 that found above for the Earth and for the Moon by Zollner,. 

 stand thus : — 



Mercury 0'17 



Venus.* 0-92 



Earth 0*75 (probably a minimal value) 



Moon 0-17 (Zollner) 



Mars . . 0-27 



Jupiter . . ..... . 0*75 



Saturn 0*88 (using Strnve's latest dia- 



Uranus 0*73 metral measures -78) 



Neptune 0-63 



Heat received bxi Earth and Mars. — With these data we sum 



