LAWS OF HEAT. AIR. 103 



the gaseous form of these could bear. Hence, 

 there will be a deficiency of moisture in the lower 

 part of the atmosphere, and if water exist there, 

 it will rise by evaporation, the surface feeling an 

 insufficient tension; and there will thus be a 

 fresh supply of vapour upwards. As, however, 

 the upper regions already contain as much as 

 their temperature will support in the state of gas, 

 a precipitation will now take place, and the fluid 

 thus formed will descend till it arrives in a lower 

 region, where the tension and temperature are 

 again adapted to its evaporation. 



Thus, we can have no equilibrium in such an 

 atmosphere, but a perpetual circulation of vapour 

 between its upper and lower parts. The currents 

 of air which move about in different directions, 

 at different altitudes, will be differently charged 

 with moisture, and as they touch and mingle, 

 lines of cloud are formed, which grow and join, 

 and are spread out in floors, or rolled together in 

 piles. These, again, by an additional accession 

 of humidity, are formed into drops, and descend 

 in showers into the lower regions, and if not 

 evaporated in their fall, reach the surface of the 

 earth. 



The varying occurrences thus produced, tend 

 to multiply and extend their own variety. The 

 ascending streams of vapour carry with them that 

 latent heat belonging to their gaseous state, which, 

 when they are condensed, they give out as sensible 



