§ 513. THE CLOUD REGION, ETC. 275 



being 1, that of aqueous vapor is 0.6 (§ 252). According to the 

 table (§ 862), the mean height of the barometer at sea, between 

 the equator and 78° 87' north, is 80.01 ; while 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 w^^tery south, 

 where all is sea, be pardoned for the homely reference. We all 

 know, as the steam or vapor begins to form in the tea-kettle, it ex- 

 pels air thence, and itself occupies the space which the air occu- 

 pied. If still more heat be applied, as to the boiler of a steam-en- 

 gine, 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 vapor which has proportionally precisely the same 

 tendency that the vapor in the tea-kettle has to drive off the air 

 above and occupy the space it held. Nor is this all. This aus- 

 tral vapor, rising up, is cooled and condensed. Thus a vast 

 amount of heat is liberated in the upper regions, which goes to 

 heat the air there, expand it, and thus, by altering the level, caus- 

 ing it to flow off. This unequal distribution of atmosphere be- 

 tween the two halves of the globe is represented in barometric 

 profile by Plate I. (§ 215) — the shading around the periphery of 

 the circle being intended to represent the relative height, and the 

 scales standing up in it the barometric column. 



518. This part of the southern ocean where the barometer 

 Influence of antarctic shows diminished prcssurc is frequented by ice- 

 ihtl?r%rorauS ^crgs, many of them very large and high, and some 

 regions. ^^ them scudiug up towers, minarets, and steeples, 



which give them the appearance in the distance of beautiful cities 

 afloat. Each one of them is a centre of condensation. Could an 

 eye from aloft look down upon the scene, the upper side of the 

 cloud stratum would present somewhat the appearance of an im- 

 mense caldron, boiling, and bubbling, and intumescing in the upper 

 air. These huge bergs condense the vapor, and the liberated heat 

 causes the air above them to swell out, and to stand like so many 

 curiously-shaped fungi above the general cloud level. And thus, 

 where the icebergs are thick, the clouds are formed low down. 



