LOW BAROMETER OF ANTARCTIC ZONE. 301 



southern hemisphere, nor (2) do they prevail over the 

 whole of that hemisphere. 



Lastly, we must remember that the process conceived 

 by Maury must be wholly or principally a diurnal pro- 

 cess, and so 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 im- 

 portantly the efficiency of the cause assigned by Maury, 

 Let us, however, 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 proportionate 

 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 



