§ 803, 8G4. THE ANTARCTIC REGIONS, ETC. 4(31 



Psychrometry of polar ^^^ ^^ their waj to the Other they are desiccated 

 '''°'^'- (§ 826). Korthern mountains and the hills wring 



from them water for the great rivers of Siberia and Arctic Amer- 

 ica. These winds, then, sweep comparatively dry air across the 

 arctic circle ; and when they arrive at the calm disc — the place of 

 ascent there — the vapor which is condensed in the act of ascend- 

 ing does not liberate heat enough to produce a rarefaction sufficient 

 to call forth a decided indraught from a greater distance in the sur- 

 rounding regions than 40° (§ 852) ; and the rarefaction being not 

 so great, the barometer is not so low there as in antarctic regions.*-^ 



863. Nevertheless, there is rarefaction in the arctic regions. 

 . Aerial rarefaction The wiuds show it, the baromctcr attests it, and the 



aboutthenorthpoie. f^^.^ jg cousistcnt with the Eussiau theory of a 

 polynia in polar waters. The presence within the arctic circle of 

 a considerable body of comparatively warm water, which observa- 

 tion has detected going into it as an under current — which induc- 

 tion shows must rise up and flow out as a surface current — would 

 give forth vapor most freely. This vapor, being lighter than air, 

 displaces a certain quantity of atmosphere. Eising up and being 

 condensed, it liberates its latent heat in the cloud region, and so, 

 by raising temperature, causes the moderate degree of rarefaction 

 which the barometer at sea, at Greenwich, at St. Petersburg, and 

 in the arctic ice indicates. 



864. Within the antarctic circle, on the contrary, the winds bring 

 Ditto about the south ^^^ which has come over the water for the distance 

 ^°^^ of hundreds of leagues all around ; consequently, a 

 large portion of atmospheric air is driven away from the austral re- 

 gions by the force of vapor, which fills the atmosphere there. Now 

 there must be a place — an immense disc, with irregular outlines, it 

 may be, and probably is — where these polar winds (§ 855) cease to 

 go forward, rise up, and commence to flow back as an upper current. 

 If the physical aspects — the topographical features in and about 

 this calm place — be such as to produce rapid condensation and 

 heavy precipitation (Chap. XX.), then we shall have, in the latent 

 heat liberated from all this vapor, an agent sufficient not only to 

 produce a low barometer and a powerful indraught, but quite ad- 

 equate also to the mitigation of climate there. 



* Captain M'Clintock, during his arctic explorations in the schooner Fox, records 

 the barometer as high as 31 inches. 



