On the Phenomena of the Glacial Epoch, 239 



rarified, rises upwards arid flows towards the poles, where 

 it mitigates the severity of the polar cireles. 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 which 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 bettor established than that 

 neat is absorbed when water is transformed into vapour, 



