342 The Loiu Barometer of the Antarctic Temperate Zone. 



to carry the drier portion of the air towards the equator. 

 All this seems in accordance with Maury's theory, and, indeed, 

 if the prevailing upper and lower currents flowed in directions 

 contrary to those indicated, the theory would fall at ouce. 



Again, although we find no evidence in barometric pressure 

 over the south tropical zone of that increase which Maury's 

 theory would lead us to expect (since the surplus air is carried 

 first to this zone), yet it might be argued that the surplus is so 

 distributed, as to appear in another way.. It is evident that if 

 the atmospheric envelope normally appertaining to the southern 

 hemisphere were, through the effects of the causes assigned by 

 Maury, increased in extent, this increase might show itself, not 

 in an increase of pressure over the south tropical zone — that 

 is, not in an increase of height there — but in the extension of 

 the surplus atmosphere into the northern hemisphere. This 

 would be shown by the extension of the southern trade-winds 

 to or beyond the equator, so that the (so-called) equatorial 

 zone of calms should lie north of the equator. As this is 

 really the position occupied by the belt of calms, Maury's 

 theoiy appears to gain additional force by the coincidence. 



Another argument may be drawn from the analogy of the 

 low barometer in moist weather. In fact, it is well known that 

 Deluc explained this phenomenon in a manner precisely 

 accordant with the views expressed by Maury. 



Despite the apparent force of these arguments, and others 

 that might be adduced, it will not be difficult, I think, to show 

 that neither is Maury's theory consistent with known physical 

 laws, nor (passing over this objection) is the theory sufficient 

 to account for the grand phenomenon under consideration. 



It is quite true that a volume of aqueous vapour weighs less 

 than an equal volume of air ; it is equally true that a volume 

 of moist air weighs less than an equal volume of dry air at the 

 same tension. But water, quietly evaporating in the open air, 

 does not displace the air, but penetrates into its interstices, 

 according to the well-established law regulating the mixture of 

 vapours. The aqueous vapour which thus intimately mixes 

 itself with the air produces no effect whatever, either by its 

 •weight, or by its elasticity, on the movements of the atmosphere. 

 The experiments of Gay-Lussac, Dalton, and others, have long* 

 since proved that the actual effects of the quiet evaporation of 

 water are those here described. It is on this account that 

 Deluc' s hypothesis in explanation of the fall of the barometer 

 when the air is moist is now no longer accepted. It has been 

 shown that the observed fall is not due to the moistness of the 

 air, but to increase of temperature. Hot winds bring (in 

 Europe) moist air, and thus moist air and a low barometer are 

 found to be coexistent phenomena. But they are not in the 



