460 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



distance in the surrounding regions than 40^ (§ 852)— 2400 miles ; 

 and the rarefaction being not so great, the barometer is not so 

 low there as in antarctic regions.* 



863. Aerial rarefaction about the woriA ^oZe.— Nevertheless, there 

 is rarefaction in the arctic regions. The winds show it, the 

 barometer attests it, and the fact is consistent with the Russian 

 theory of a polynia in polar waters. The presence within the 

 arctic circle of a considerable body of comparatively warm water, 

 which observation has detected going into it as an under current 

 —which induction shows must rise up and flow out as a surface 

 current— would give forth vapour most freely. This vapour, 

 being lighter than air, displaces a certain quantity of atmo- 

 sphere. Rising 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. Ditto about the south pole.— ^Yiihm the antarctic circle, on 

 the contrary, the winds bring air 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 

 regions by the force of vapour, 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 vapour, 

 an agent sufficient not only to produce a low barometer and a 

 powerful indraught, but quite adequate also to the mitigation of 

 climate there. 



865. Influences favourable to heavy precipitation,— Mere altitude, 

 with its consequent refrigeration, does not seem as favourable as 

 mountain peaks and solid surfaces to the condensation and pre- 

 cipitation of vapour in the air. In the trade-wind regions out at 

 sea it seldom rains ; but let an island rise never so little above 

 the water, and the precipitation upon it becomes copious. In 

 Colonel Sykes' (§ 299) rain-fall at Cherraponjie, we have an 



* Captain M'Clintock, during liis northern explorations in the schooner 

 lox, records the arctic barometer as high as 31 inches. 



