344 The Low Barometer of the Antarctic Temperate Zone. 



can only take place (on an average) over one half of the 

 southern zone at any one time. 



All these considerations tend to diminish very importantly 

 the efficiency of the cause assigned by Maury. Let us, how- 

 ever, consider what is the maximum value that efficiency could 

 have if all these circumstances were neglected. We shall see* 

 that even in this case, which assigns an efficiency at least three 

 or four times as great as would be consistent with actual facts, 

 we shall still find the cause assigned by Maury inadequate to 

 the production of the phenomenon under consideration. 



The greatest weight of aqueous vapour which is ever 

 present in a given volume of air is equivalent to about one- 

 sixtieth part of the weight of the air. Now, if we suppose 

 the barometer at thirty inches, and the whole column of air 

 above the barometer to be impregnated with air in the above- 

 named proportion — a view very favourable to the theory, since 

 the cold of the upper regions of air largely diminishes the pro- 

 portionate weight of aqueous vapour — it is clear that one- 

 sixtieth part — >or half an inch — of the barometer's height is 

 due to the presence of aqueous vapour. Now, at mean 

 tensions the specific gravity of aqueous vapour is about three- 

 fifths of the specific gravity of air, so that the proportion of 

 one-sixtieth part of weight corresponds to a proportion of 

 one-thirty-sixth part of volume ; in other words, our column 

 of air owes, one-thirty-sixth part of its height to the presence 

 of aqueous vapour. If we suppose this thirty-sixth part to 

 flow off — not from the upper regions only, but in such a 

 manner that one complete thirty-sixth part of the volume of 

 the column should pass off — then, instead of standing at a 

 height of thirty inches, the barometer would stand at a height 

 of 29-J- inches, less by ouly one-third of an inch than the 

 height of 29 £ inches due to the dry air alone. Now* we cannot, 

 in accordance with Maury's theory, legitimately add the 

 five-sixths of an inch of barometric pressure to the height of 

 the barometer under a neighbouring column. For we have no 

 evidence to show that the air assumed to be expelled from the 

 southern temperate zone is heaped over the southern tropical 

 zone ; on the contrary, we have a barometer in the latter zone 

 not quite so high even as the barometer in the corresponding 

 northern zone. Therefore if air is expelled in the manner 

 supposed by Maury, it must be distributed over a very much 

 greater portion of the globe's surface than it had been expelled 

 from. Hence, returning to our imaginary column of air, but 

 a small fraction of the five-sixths of an inch due to over- 

 flow must be added to the barometer under a neighbouring 

 air-column. The latter barometer originally at 29£ may be 

 fairly assumed to rise at most to about 29-f- inches. We 



