300 LIGHT SCIENCE FOR LEISURE HOURS. 



a process to account for the great phenomenon we are 

 dealing with. 



It must be remembered, in the first place, that the 

 theory requires that there should be a greater volume 

 of mixed air and vapour over the southern temperate 

 zone than there is in the corresponding northern zone, 

 otherwise there would not be that continual overflow 

 towards the equator which is required by the theory. 

 So far as it goes, this increment of volume implies an 

 increment of weight. The increase of volume is more 

 than compensated (in theory) by diminution of specific 

 gravity, but it must be held in mind that the increase 

 of volume has to be accounted for by the theory as well 

 as the difference in barometric pressure. 



Again, the theory requires that the upper regions of 

 air should be dry, for it is the upper air that is carried 

 towards the equator ; and if this air were moist, we 

 should no longer have the different proportions of moist 

 and dry air which are required by the theory. We must 

 have an aggregation of moist air in high southern lati- 

 tudes, and of dry air towards the equator. 



Again, we must call to mind that one-half of the 

 northern hemisphere is covered by water, and a part of 

 the southern hemisphere is not so covered, so that the 

 effects suggested by Maury are (1) not peculiar to the 



with those metereologists who consider that the notion of any appreci- 

 able uplifting of the air by the rising vapour of water is a mistaken one. 

 But whether it be so or not, it is evident that Herschel's view would re- 

 quire a regular increase of pressure from the equator to the antarctic 

 pole, and therefore is opposed to Maury's explanation. 



