658 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The greater bulk of the depletion in this part of the spectrum comes 

 from the absorption by water-vapor, water itself, or ice and carbon 

 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 frac- 

 tion 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 grav- 

 ity at the surface of the planet would lower the boiling point of water, 

 as investigation shows (see later in the paper) to something like 110° F. 

 The sublimation at lower temperatures would be correspondingly in- 

 creased. Consequently the amount of water-vapor in the Martian 

 air must on that score be relatively greater than in our own. 



Depletion 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, caeteris paribus, with its lighter gases the 

 quickest. Whence, as regards both water- vapor and carbon dioxide we 

 have reason to think them in relatively greater quantity than in our 

 own air at corresponding barometric pressure. We may therefore as- 

 sume provisionally that the absorption due this cause 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 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 the Planets. 



The albedoes of the several planets, according to the determinations 

 latest obtained, those by Miiller at Potsdam, together with that found 

 above for the earth and for the moon by ZoUner, stand thus : 



