10 



ON THE SEPAEATE PBESSUBES OF THE AQUEOUS AND 



when it rises, leaving the cooler air from the sea to flow in 

 and supply its place near the surface. In due time, as con- 

 densation proceeds, palpable cloud may be formed over the 

 land, and more heat be there liberated, when the ascending 

 current of air will rise more rapidly, and the sea air blow in 

 stronger to supply its place ; and, the whole operations being 

 carried on with increased energy, thick cloud may be formed, 

 and additional quantities of aqueous matter be borne into the 

 higher regions. 



In the earlier part of this process, and before palpable cloud 

 is formed, the fresh vapour that passes into the air shews 

 itself by its pressure raising the barometer; and this 

 pressure must be sufficient, not only to do that, but also 

 to counteract the influence of the slight heating of the air 

 consequent on the formation of incipient cloud. That such 

 heating really takes place may be inferred from the dimming 

 of the atmosphere by the mist that at the time pervades it ; 

 but this early heating cannot lighten the gases so much as 

 the increase of vapour makes the aqueous portion heavier, 

 because the barometer rises. It is, however, after a certain 

 time, with a diminishing rapidity that it rises, until the 

 two forces become equal, when the rise of the barometer 

 ceases. But this cessation cannot result from any stoppage 

 of the supply of vapour that had previously raised the 

 barometer, as the increasing evaporation which is going on 

 at the surface shews that that supply is more abundant 

 than it was before. The cessation of the rise must therefore 

 be caused by the countervailing influence of the increased 

 quantity of heat that is liberated by the condensation of 

 vapour. 



But soon afterwards, the barometer begins to fall, and this 

 it does although the quantity and consequently the weight of 

 the vapour sent into the atmosphere by evaporation is still 

 increasing, and the whole of it, or of the cloud into which it is 

 converted, is there accumulated. This fall of the barometer 



