248 Dr. J. Croll on some Controverted Points 
the ocean, say per square mile, in a given time, is greater than 
that absorbed by the land. 
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. The ocean will not reflect the 
heat from the aqueous vapour of the air, but absorbs it, while 
the ground does the opposite. Radiation from the air, there- 
fore, 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 heating 
it." If Professor Newcomb means by this same heat the 
same amount of heat, then 1 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. But 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, how- 
ever, 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 section in full. 
Fourth. — i The aqueous vapour of the air acts as a screen to 
prevent the loss by radiation from water, while it allows 
radiation from the ground to pass more freely into space ; 
the atmosphere over the ocean consequently throws back a 
greater amount of heat than is thrown back by the atmosphere 
over the land. The sea in this case has a much greater 
difficulty than the land has in getting quit of the heat received 
from the sun ; in other words, the land tends to lose its heat 
more rapidly than the sea. The consequence of all these cir- 
cumstances is that the ocean must stand at a higher mean 
temperature than the land. A state of equilibrium is never 
gained until the rate at which a body is receiving heat is 
equal to the rate at which it is losing it ; but as equal surfaces 
of sea and land receive from the sun the same amount of heat, 
it therefore follows that in order that the sea may get quit of 
