Page:Physical Geography of the Sea and its Meteorology.djvu/301

Rh there are influences exerted in the formation of clouds and fogs over and near the land which appear not to be felt at sea.

511. The cloudy latitudes.—In the extra-tropical north, the cloud region is high over the land, low over the water; and, as a rule, the farther inland, the dryer the air and the higher the cloud region. In the circum-Antarctic regions, where all is sea, the rising vapours form themselves into clouds low down, and keep the face of the sky almost uninterruptedly obscured. The southern eaves of the cloud plane (§ 509), like the calm belts, vary their latitude as the sun does its declination, though their place is generally found between the parallels of 50° and 70° S.—farther or nearer according to the season; but under this edge, wherever it be, the mariner's heart is seldom made glad by the cheering influences of a clear sky. If not wrapped in mist, or covered with snow, or pelted with hail, or drenched with rain, as he sails through these latitudes, he is dispirited under the influences of the gloomy and murky weather which pervades those regions. His hope in the "brave west winds" and trust in the prowess of a noble ship are then his consolation and his comfort 512. Why there should be less atmosphere in the southern than in the northern hemisphere.—Such are the quantities of Vapour rising up from the engirdling ocean about those austral regions, that it keeps permanently expelled thence a large portion of the atmosphere. The specific gravity of dry air being 1, that of aqueous vapour is 0.6 (§ 252). According to the table (§ 362), the mean height of the barometer at sea, between the equator and 78° 37 north, is 30.01; whilst its mean height in lat. 70° S. is 29.0. To explain the great and grand phenomena of nature by illustrations drawn from the puny contrivances of human device is often a feeble resort, but nevertheless we may, in order to explain this expulsion of air from the watery south, where all is sea, be pardoned for the homely reference. We all know, as the steam or vapour begins to form in the tea-kettle, it expels air thence, and itself occupies the space which the air occupied. If still more heat be applied, as to the boiler of a steam-engine, the air will be entirely expelled, and we have nothing but steam above the water in the boiler. Now at the south, over this great waste of circumfluent waters, we do not have as much heat for evaporation as in the boiler or the tea-kettle; but, as far as it goes, it forms vapour which has proportionally precisely the same tendency that the vapour in the tea-kettle has to drive off the