THE CLOUD REGION, ETC. 275 



there arc influences exerted in the formation of clonds and fogs 

 ovor and near the land which appear not to be felt at sea. 



511. The cloudy latifudes. — 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 dr3'er the air and the higher the cloud 

 region. In the circiim-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 in- 

 fluences of the gloomy and murk}^ 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 comfoii;. 



512. Why there should he less atmosphere in the southern than in 

 the northe^-n hemisphere. — Such are the quantities of Tapour rising 

 np from the engirdling ocean about those austral regions, that it 

 keeps permanently expelled thence a large portion of the atmo- 

 sphere. The specific gravit}'' of dr}' 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. Kow 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 propoiiionaUy precisely the same 

 tendency that the vapour in the tea-kettle has to drive off the 



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