THE ANTARCTIC REGIONS A2TD THEIR CLIMATOLOGY. 443 



meter at one pole, a low barometer at the other ; consequently, the 

 difference in the pressm-e of the atmosphere ahoiit the two poles, as 

 shown by the diagram (Plate XYI.), cannot be ascribed to the influ- 

 ence of axial rotation. It is doubtless due to the excess in antarctic 

 regions of aqueous vapour and its latent heat. 



862. The arctic ch^le hes chiefly on the land, the antarctic on 

 PsychrometTT of tho Water. As tho wmds enter one, they are loaded 

 polar winds. ^^-^ vapouT ; but ou their way to the other they are 

 desiccated (§ 826). Northern mountains and the hills wring from 

 tliem water for the great rivers of Siberia and Arctic America. 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 va- 

 pour which is condensed in the act of ascending does not hberate heat 

 enough to produce a rarefaction sufficient to call forth a decided in- 

 draught from a greater distance in the sm-rounding regions than 

 40"^ (§ 852) — 2400 miles ; and the rarefaction bemg not so gxeat, 

 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 baromctor attests it, and the 

 about the north pole, f^^t is consistont with the Kussian theory of a 

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

 a considerable body of comparatively warm water, which observa- 

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

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

 give forth vapour most freely. This vapour, being hghter than air, 

 displaces a certain quantity of atmosphere. Eising up and being 

 condensed, it hberates 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 ^ir which has come over the water for the distance 

 P^^^- 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 vapour, which fills the atmosphere there. Now 

 there must be a place — an immense disc, with irregular outhnes, 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 featm-es in and about 



* Captain M'Clintoek, during liis northern explorations in the schooner "Fox," 

 records the arctic barometer as hio:h as 31 inches. 



