86 TERRESTRIAL ADAPTATIONS. 



expansive force of this vapour increases rapidly 

 as the heat increases ; so that when we reach the 

 heat of boiling water, it operates in a far more 

 striking manner than when it is colder ; but in 

 all cases the surface of water is covered with an 

 atmosphere of aqueous vapour, the pressure or 

 tension of which is limited by the temperature of 

 the water. To each degree of pressure in steam 

 there is a constituent temperature corresponding. 

 If the surface of water is not pressed by vapour 

 with the force thus corresponding to its tempera- 

 ture, an immediate evaporation will supply the 

 deficiency. We can compare the tension of such 

 vapour with that of our common atmosphere ; the 

 pressure of the latter is measured by the baro- 

 metrical column, about thirty inches of mercury; 

 that of watery vapour is equal to one inch of 

 mercury at the constituent temperature of 80 

 degrees, and to one-fifth of an inch, at the tem- 

 perature of 32 degrees. 



Hence, if that part of the atmosphere which 

 consists of common air were annihilated, there 

 would still remain an atmosphere of aqueous 

 vapour, arising from the waters and moist parts 

 of the earth ; and in the existing state of things 

 this vapour rises in the atmosphere of dry air. 

 Its distribution and effects are materially in- 

 fluenced by the vehicle in which it is thus car- 

 ried, as we shall hereafter notice ; but at present 

 we have to observe the exceeding utility of water 



