u 



ON THE SEPARATE PBESSUBE8 OF THE AQTTEOUS AND 



like the atmosphere, which tends so strongly to establish an 

 equilibrium. 



In doing this, it will be necessary to bear in mind the 

 causes that are presumed to be in operation to produce the 

 result*?, namely, varying quantities of aqueous matter in the 

 atmosphere, and local alterations of gaseous temperature; and 

 these causes are considered sufficiently powerful to produce 

 all the eflfects exhibited in the different latitudes; taking 

 the parts about the equator first, with a dew-point of SO*^, 

 indicating, according to common assumption, a quantity of 

 vapour in the atmospheric column equal to an inch of mer- 

 cury, which would leave 29 inches of gaseous pressure to 

 make up the total, supposed to be 30 inches. But with the 

 high gaseous pressure which is admitted to exist on each side 

 of this part of the globe, and with such winds blowing from 

 each side towards it as the north and south-eastern trade 

 winds, how can we imagine that gaseous pressure can be so 

 low in this locality as is thus represented, — only 29 inches f 

 Direct solar heating of the mass of the atmosphere, as has 

 been shewn, is never considerable, it being confined mostly 

 to the portion near the surface ; and that heating would tend 

 to heap up the gases within the tropics to enable them to 

 flow over north and south towards the polar regions; it is, 

 therefore, difficult even to conceive that the direct influence 

 of the sun could make the gaseous pressure here so small, as 

 it must be if an inch of mercury is to be allowed for vapour 

 pressure. 



For reasons already given, however, the real quantity of 

 aqueous matter in the part, in the two forms of vapour and 

 globules of water, cannot be considered to be much less than 

 that named, that is, a quantity equal in weight to an inch of 

 mercury. Because, although there is certainly much less 

 vapour in the upper part of the atmosphere than has been 

 supposed, there is here much water floating in it, as is seen 

 in the thick stratum of mist that constantly fills the air in the 



