THE ATMOSPHERE. 



51 



The air is therefore to be considered as a fluid, com- 

 posed of certain particles, separated by the heat dif- 

 fused among them. An interesting illustration of the 

 separation and repulsion of these particles by heat, is 

 shown in the explosion v of gunpowder. This article is 

 composed of nitre, charcoal and sulphur, in definite 

 proportions. Each one of these materials has certain 

 constituents, condensed in the form of a solid. When 

 sufficient heat is applied to overcome their cohesive 

 attraction, these constituents are separated by the par- 

 ticles of heat which enter between them, and which, 

 causing them suddenly to diverge, produce their repul- 

 sive power. 



The atmosphere extends to nearly fifty miles from the 

 surface of the earth, and bears a similar proportion in its 

 height to the diameter of the earth, as a covering of one 

 tenth of an inch in thickness to a sphere of twelve inches. 



By the consideration of the great height of the air, 

 and the materials of which it is formed, we are prepar- 

 ed to commence an examination of its weight. Though 

 its particles are invisible, yet they are attracted to the 

 earth by the same principle as larger perceptible bodies. 

 And consequently, they must exert a specific pressure 

 upon everything on which they rest. No one will 

 deny that a rock or a house has a definite weight. 

 Though these are visible, yet the effect is the same 

 from the pressure of more subtle materials. No one 

 would doubt that the ocean presses with a definite force 

 upon its base. Yet to the eye of the fish, the water is 

 as invisible as the air is to us, and he depends upon his 

 other senses, like ourselves, for evidences of its exis- 

 tence. 



Although the atmosphere extends to nearly fifty miles 

 from the earth, we must not suppose that its density is 

 uniform throughout, any more than we would think a 

 pile of cotton of the same height, as dense in the cen- 

 tre as at the base. The density diminishes as we as- 

 cend, in certain proportions. For on Mont Blanc, which 

 is about three miles and a half above the level of the sea, 

 the pressure is diminished to one half; proving that one half 



