Page:Physical Geography of the Sea and its Meteorology.djvu/486

460 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 north pole.—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 atmosphere. 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.—Within 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 precipitation 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