The Low Barometer of the Antarctic Temperate Zone. 341 



the expulsive effects due to the vapour raised from southern 

 oceans ; and, secondly, the expansive effects due to the 

 liberation of heat as the vapour is condensed. Now I would, 

 in the first place, submit that we cannot assign to the second 

 cause the effects here considered. The amount of heat liberated 

 as the vapours rising from southern oceans are condensed is 

 undoubtedly great, but it cannot be more than the equivalent 

 of the amount of heat rendered latent as the vapours are 

 formed, and therefore the expansive effects due to the liberation 

 of heat cannot be greater than the contrary effects due to the 

 prior imprisonment of heat. It is quite true, and has been 

 accepted as the undoubted explanation of many climatic effects, 

 that if vapour be raised in one place and condensed over 

 another, then the temperature of the air over the latter place 

 is raised. But when we have to consider a phenomenon 

 extending over a zone twenty or thirty degrees in width, we 

 cannot argue in this manner. Nay, it is necessary to the force 

 of Maury's second cause that the condensation of vapour 

 should take place over the very zone in which the vapo- 

 risation is proceeding. To assign similar effects to both 

 processes, is to require that the winding up and the loosening 

 of the spring should take place in the same direction. 



Whatever effects, then, are due to the constant evaporation 

 going on in the southern hemisphere must not be derived 

 from changes of temperature. So far as these are effective at 

 all, they must depend on the excess of evaporation over con- 

 densation (since the excess cannot possibly lie the other way), 

 and therefore represent diminution of heat or increase of 

 pressure, the contrary effect to that we have to account for. 

 We have, therefore, only to consider the first cause mentioned 

 by Maury ; that is the expulsive effects due to the formation 

 of aqueous vapour. 



At first sight, this process of expulsion appears simple 

 enough, and seems further to coincide with many well-known 

 phenomena. The theory supposes that over a wide zone of 

 the southern hemisphere aqueous vapour is continually rising ; 

 that as it rises it displaces in part the heavier air over these 

 regions ; and that equilibrium being thus disturbed, the excess 

 of air flows off continually towards the equator. Now we 

 know that the prevailing surface- winds over that zone of the 

 southern hemisphere in which the barometer exhibits the 

 peculiarity we are considering, blow from the equator ; that is, 

 they tend to sweep the lower strata of the atmosphere towards 

 the south pole. They therefore tend to increase the quantity 

 of humid air in high southern latitudes. We know also that 

 the prevailing upper currents over the southern zone we are 

 considering, blow towards the equator. They tend, therefore, 



