254 J. Croll— Geological Climatology. 
penetrate to a great depth that the amount of heat stored up 
by the ocean is so great, and it is to this store that its warmth 
during winter is mainly due. The water is diathermanous for 
the rays of the sun, but it is not so, for reasons well known, 
for the rays of water itself. The upper layers of the ocean 
will allow a larger portion of the radiation from the sun +o 
pass freely downward, but they will not allow radiation from the 
layers underneath to pass freely upwards. These upper layers, 
ike the glass of a green-house, act as a trap to the sun’s rays, 
and thus allow the water of the ocean to stand at a higher tem- 
perature than it would otherwise do. Again, the slowness with 
which the ocean thus parts with its heat enables it to maintain 
that comparatively high temperature during the long winter 
months. And again, it is to the mobility of the particles of 
water, the depth to which the heat penetrates, and the rapidity 
with which it is absorbed, that those great currents of warm 
- water become possible. Were the waters of the ocean like the 
land not mobile, and were only a few inches at the surface 
reached by heat from tae sun, there could be no Gulf Stream, 
or any great transference of heat from the Southern to the 
Northern hemisphere, or from equatorial to temperate and 
polar regions, by means of oceanic circulation. 
Second.—‘ The air is probably heated more rapidly by contact 
with the ground than with the ocean; but, on the other hand, it 
is heated far more rapidly by radiation from the ocean than from 
the land. e aqueous vapor of the air is to a great extent dia- 
thermanous to radiation from the ground, while it absorbs the rays 
from water and thus becomes heated.’ 
To this Professor Newcomb objects as follows: “If then 
the air is really heated by contact with the ground more rap- 
idly than by contact with the ocean, it can only be because 
the ground is hotter than the ocean, which is directly contrary 
to the theory Mr. Croll is maintaining.” What I maintained 
was that were it not for certain causes the mean annual tem- 
perature of the ocean would be higher than that of the land, 
During the day and also during the summer the surface of the 
ground is hotter than that of the ocean; and the alr, of 
course, will be heated more rapidly by contact with the former 
than with the latter. *But this does not prove that the air 18 
— more rapidly heated by radiation from the ocean than from 
e 
surprising.” I am surprised that he is not acquainted with 
the fact, and also with its physical explanation. This will 
