The Phenomena of the Atmosphere. 93 



You need not be told,. I. presume, that clouds 

 are water in a suspended state, and so is the 

 common smoke which ascends from our chim- 

 neys, the columns of which, in fact, are so 

 many clouds. Vapour is water expanded by 

 heat or fire to the state of an elastic fluid, 

 and it rises in the atmosphere*, because va- 

 pour is lighter or less dense than our common 

 air (it is, in fact, fourteen hundred times lighter 

 than the water of which it is composed, whereas 

 common air is only about nine hundred times 

 lighter than water) ; and it is a rule in philosophy, 

 depending on the principle of gravitation, that 

 when two fluids of different densities are brought 

 together, the lighter will always rise to the sur- 

 face. It is, however, only near the surface of 

 the earth that the air is denser and more heavy 



* There is a constant process of evaporation going on 

 from all bodies on the surface of the earth which contain 

 moisture. In a dry atmosphere the evaporation from the 

 human body is very considerable, but the heat which that 

 carries ofi" is continually recruited by the vital principle, 

 which is wonderfully adapted to resist, to a certain extent, 

 the eflecti both of a hot and a cold medium, keeping the 

 blood in either, very nearly at the same temperature. 

 When, however, this principle is roused by exercise, and 

 a warm and moist air, or a spasm on the skin obstructs the 

 free passage of the perspirable matter, the blopd becomes 

 over-heated, and we feel oppressed. On the other hand, 

 exposure to a keen dry wind, without sufficient exercise, 

 endangers delicate persons, from the too great cooling of 

 the blood. 



