9'0 Atmosphere, 



CHAP. VI. 



Of the Air and Atmosphere. 



WE have already considered the earth 

 as a planet, or one of the great masses 

 of matter moving about the sun ; we shall 

 now consider it as it is made up of its 

 several parts, abstracting from its diur- 

 nal and annual motions. 



The exterior part of this our habitable 

 world is the air or atmosphere ; a light, 

 thin fluid, or springy body, that incom- 

 passes the solid earth on all sides, and 

 partakes of all its motions., both annual 

 and diurnal. 



The composition of that part of our 

 atmosphere properly called air, was till 

 lately but very little known. Formerly 

 it was supposed to be a simple, homoge- 

 neous, and elementary fluid. But the 

 experiments of Dr. Priestley and others 

 have discovered, that even the purest 

 kind of air, which they call vita'. 



