340 The Low Barometer of the Antarctic Temperate Zone. 



gases which form the earth's atmospheric envelope. The 

 whole question of the circulation of the air is investigated in 

 Maury's interesting work on the Physical Geography of the 

 Sea, and he appears to establish in the most convincing 

 manner, the interchange of air between the northern and 

 southern hemispheres. 



And even if we could assume that the atmospheric covering 

 of any portion of the earth's surface was in any way prevented 

 from passing freely to other regions, yet the cause assigned 

 would be inadequate to account for the difference of barometric 

 pressure actually existing between the two hemispheres. All 

 the land above the sea-level in the northern hemisphere, if 

 uniformly distributed over the surface of that hemisphere 

 would be raised to a height of less than 200 feet above the 

 present sea-level, and the actual difference of level correspond- 

 ing to the observed difference of barometric pressure is more 

 than four times as great. 



Passing over this theory as neither consistent with the 

 known laws regulating the motions of elastic fluids, nor suffi- 

 cient even if the consideration of those laws were neglected, 

 we come to the theory suggested by Captain Maury — a theory 

 deserving of much more attentive consideration. I shall quote 

 his own words, as the fairest method of presenting his theory ; 

 after stating the observed difference of a barometric pressure 

 in the two hemispheres, and mentioning the expulsion of air 

 from the northern hemisphere as the cause of this difference, 

 he writes : — ce 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, that, 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 air be applied, as 

 to the boiler of a steam-engine, the air will be entirely ex- 

 pelled, 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 proportionately precisely the same tendency that the 

 vapour in the tea-kettle has to drive off the air above, and 

 occupy the space it held. Nor is this all. This austral 

 vapour, 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, 

 causes it to flow off." 



The theory thus divides itself into two parts : we have first 



