246 Dr. J. Croll on some Controverted Points 
These sentences are considered unworthy of criticism. Are 
they really so unworthy ? Let us examine them a little more 
closely. It is in consequence of the sun's rays being able to 
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 diather- 
manous 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 to pass freely downward, but they will not allow radiation 
from the layers underneath to pass freely upwards. These 
upper layers, like the glass of a greenhouse, act as a trap to 
the sun's rays, and thus allow the water of the ocean to stand 
at a higher temperature 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 pene- 
trates, 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 the 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 con- 
tact 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. The aqueous vapour of the air is to a 
great extent diathermanous 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 
rapidly than by contact with the ocean, it can only be because 
the ground is hotter than the ocean, which is directly contraxy 
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 air, of 
course, will be heated more rapidly by contact with the former 
than with the latter. But this does not prove that the air is 
not more rapidly heated by radiation from the ocean than 
from the land. Professor Newcomb says : — " The statement 
that the aqueous vapour of the air is diathermanous to radia- 
