THE ANTARCTIC REGIONS AND THEIR CLIMATOLOGY. 4G3 



one of the most powerful modifiers of climates. It is to the 

 winds precisely what coals are to the steam-ship at sea — the 

 source of motive power. The condensation of vapour is for one 

 what the consumption of fuel is for the other; only with the 

 winds the same heat may be used over and over again, and for 

 many purposes. By simply sending moist air to the top of 

 snow-capped mountains, condensing its moisture, and bringing 

 it down to the surface again, it is made liot. Though by going 

 up the air be cooled, it is expanded, and receives as sensible 

 heat the latent heat of its vapour ; being brought down to the 

 surface again, and compressed by the whole weight of the baro- 

 metric column, it is hotter than it was before by the amount of 

 lieat received from its vapour. That we may form some idea as 

 to the modifying influences upon climate which might arise from 

 this source, let us imagine the air as it impinges upon the ant- 

 arctic continent to be charged with vapour at the temperature of 

 of 40°. In order to arrive at the place of polar calms, it has to 

 cross a mountain range, we will suppose, the summits of which 

 are pushed high up into the regions of perpetual snow. As this 

 air, with its moisture, rises, it expands, cools, and liberates the 

 latent heat of its vapour, which the air receives in the sensible 

 form. Now suppose the expansion due the height of the moun- 

 tain-top to be sufficient to lower the temperature of dry air to 

 — 50^, but, on account of the latent heat which is liberated from 

 the vapour of the moist air, the temperature of the air that has 

 ascended, instead of falling as it crosses the mountain to — 50°, as 

 dry air would do, falls, in consequence of the condensation of its 

 vapour, no lower than — 30°. Thus, in the case supposed, heat 

 enough has been set free to raise the temperature of the newly- 

 arrived air 20°. Consequently, when this air, which, at the 

 temperature of 40°, came from the sea loaded with vapour, passes 

 the mountain, it loses vapour, but receives heat ; descending 

 into the valleys beyond, it is again compressed by the weight of 

 the barometric column, which, let us assume, is the same in the 

 valley as at the sea level on the other side of the mountain. The 

 temperature of this air now, instead of being 40°, will be 60°. A 

 powerful modifier of climate is the latent heat of vapour in the 

 air. 



870. Local variations of climate, how caused. — At one time, as 

 has been shown in Chap. IV., this heat is brought down from 

 the cloud region to scorch the earth ; at another time it causes 



