in Geological Climatology. 247 
tion from land, but not to that from water, is quite new to us, 
and very surprising." I am surprised that he is not acquain- 
ted with the fact, and also with its physical explanation. This 
will help to account for his inability to perceive how radiation 
from the ocean may heat the air more rapidly than radiation 
from the land, even though the surface of the latter may be at 
a higher temperature than that of the former. 
He says : — " The rapidity with which the heating process 
goes on depends on the difference of temperature, no matter 
whether the heat passes by conduction or by radiation." This 
statement will hardly harmonize with recent researches into 
radiant heat. It is found that the rapidity with which a body 
is heated by radiation depends upon the absorbing power of 
the body ; and the absorbing power again depends upon the 
quality of the heat-rays. Professor Tyndall, for example, 
found that in the case of vapours, as a rule, absorption dimi- 
nishes as the temperature rises. With a platinum spiral 
heated till it was barely visible, the absorption of the vapour 
of bisulphide of carbon was 6 '5, but when the spiral was raised 
to a white heat the absorption was reduced to 2*9. A similar 
result took place in the case of chloroform, formic ether, 
acetic ether, and other vapours. The physical cause of this is 
well known. 
If the aqueous vapour of the air, he says, be more diather- 
manous to radiation from land than from water, as I have 
stated, then I assigned directly contrary effects to the same 
cause. For, " reasoning as in (1), he, Mr. Croll, would have 
said that the air over the land, owing to its transparency for 
the heat-rays from the land, becomes heated to a greater 
height rapidly, while the air over the ocean, not being trans- 
parent, can acquire heat from the ocean only by the slow 
process of convection."" I would have said no such thing. 
Eadiation from the surface of the land will, no doubt, pene- 
trate more freely through the aqueous vapour than radiation 
from the ocean ; but the aqueous vapour will not absorb the 
radiation of the land so rapidly as that of the ocean, for the 
ocean gives off that quality of rays which aqueous vapour 
absorbs most rapidly. 
This is not in opposition to what I have stated in reason (1); 
for if the ground were transparent to the sun's rays like water, 
evidently the total quantity of heat absorbed by it would be 
greater than that by the ocean. But radiation from the sun 
heats only the surface of the ground, all below the surface 
depends for its supply on the slow process of conduction, 
whereas the ocean is heated by direct radiation to great 
depths. Consequently the total quantity of heat absorbed by 
