142 ON ATMOSPHERIC AND BAROMETRIC 



spheric space, as an independent elastic fluid, 

 the upper part of which presses on the lower, 

 the weight of the whole resting on the surface of 

 the globe. When aqueous vapour, one of these, 

 is increased in quantity by evaporation, its weight 

 is increased, and the whole of the vapour presses 

 on the surface with greater force : and a column 

 of mercury exposed to this pressure rises to an 

 extent that is proportioned to the increase in the 

 weight of vapour. But at the same time that 

 this is going on, the gases, the other constituents 

 of the atmosphere, may become warmer and 

 lighter in the part. And it is this lightening of 

 the gases in the morning, that first reduces the 

 strength of the land wind, and finally stops it, 

 whilst the barometer is rising from an increase of 

 vapour pressure. 



The independent actions of the constituents of 

 the atmosphere from four to ten in the afternoon, 

 though the same in their nature as those in the 

 morning, are differently combined. During this 

 time, cloud evaporation over the land cools the 

 gases, and makes them heavier, and vapour pres- 

 sure is increased by cloud evaporation, whilst the 

 small condensation of aqueous vapour that is 

 taking place near the surface in the formation of 



