256 J. Croll—Geological Climatology. 
Third.—* The air radiates back a considerable portion of its 
heat, and the ocean absorbs this radiation from the air more 
readily than the ground does. e ocean will not reflect the heat 
from the aqueous vapor of the air, but absorbs it, while the 
ground does the e. Radiation from the air, therefore, 
tends more readily to heat the ocean than it does the land.’ 
“Here we have,” he says, ‘‘the air giving back to the 
ocean the same heat which it absorbs from it, and thus heat- 
ing it.” If Professor Newcomb means by this same heat the 
same amount of heat, then I believe in no such thing. But if 
his meaning be that here we have the air giving back to the 
ocean a quantity of the heat which it absorbed from it, then 
he is certainly correct in supposing that this is affirmed by me. 
ut this is a conclusion which no physicist could for a moment 
doubt. To deny this would be to contradict Prevost’s well- 
known theory of exchanges. Did the air throw back to the 
ocean none of the heat which it derives from it the entire 
waters of the ocean would soon become solid ice. In fact, as 
we have seen, mercury would not remain fluid and every living 
thing on the face of the globe would perish. 
He states that reason fourth seems to be little more than a 
repetition of reason second in a different form. It is, however, 
much more than that. It is a demonstration that were it not 
for the causes to which I have alluded the mean temperature 
of the water hemisphere ought to be higher than that of the 
land hemisphere, and for this reason I shall here give the sec- 
tion in full. 
urth.—‘ The aqueous vapor of the air acts as a screen to p 
vent the loss by radiation from water, while it allows radiation 
equal surfaces of sea and land receive from the sun the same 
amount of heat, it’ therefore follows that in order that the sea 
