On the Phenomena of the Glacial E'poch. 239 
rarified, rises upwards and flows towards the poles, where 
it mitigates the severity of the polar circles. To the aerial 
currents thus produced, and modified by various causes, the 
prevailing winds that agitate our atmosphere are usually 
ascribed. 
As there is nothing to interrupt those currents, their in- 
fluence continues from age to age unchanged. We cannot, 
therefore, regard them as having any effect in producing the 
phenomena which are peculiar to the glacial epoch. 
II. The second natural agency, by wdiich the heat com- 
municated to the equatorial parts of the globe is distributed 
over the earth, is the air lying above the intertropical seas ; 
which, after being warmed and saturated with moisture, is 
carried, like the air lying above the land, towards the poles. 
In consequence of the water of the equatorial oceans ab- 
sorbing a large portion of the solar influence, the air which 
lies above them has not so large an amount of heat commu- 
nicated to it as that which is imparted to the air which 
covers the land. This is proved by the well-known fact, 
that within the tropics the breeze from the land is warm, 
while that which comes from the sea is comparatively cool. 
When transferred, therefore, to the polar circles, its effect on 
the temperature is less than that of the aerial currents from 
the land, but it is an effect of a precisely similar kind. 
This warm air from the ocean differs also from that 
which is heated by contact with the land, in having a much 
larger quantity of moisture diffused through it. Some have 
conluded that, in consequence' of this admixture, its in- 
fluence on the polar regions is of an altogether different 
kind. The deposition of moisture, more especially in the 
form of hoar-frost or snow, is so generally associated with 
cold, that they have been tempted to regard it as that which 
produces the cold. In doing so, however, they mistake the 
effect for the cause. The fall of the snow is not the origi- 
nating source of the winter's severity ; but is the consequence 
and evidence of the freezing temperature that exists in the 
atmosphere. 
There is no fact in science better established than that 
heat is absorbed when water is transformed into vapour, 
