180 THE CHEMISTRY OF CREATION. 



means of the very simple instrument the 

 barometer; to some mention of the phenomena 

 and construction of which we are now conducted. 

 Light and thin though the particles of the 

 atmosphere are individually, they have in the 

 mass a sensible weight, and exert a pressure 

 altogether enormous upon the whole surface of 

 the earth. This pressure is not felt by our 

 bodies, because it is equally distributed both 

 within and without. It might even be very 

 much increased, if very gradually, and yet we 

 should not become sensible of its increase, on 

 this account. But were it possible to disturb 

 this equilibrium, by suddenly exhausting the 

 air from any part of the frame, the lungs, for 

 instance, then immediately the pressure of the 

 air would be painfully felt, and if the exhaus- 

 tion were carried on to any great extent, the 

 sides of the chest would be crushed together 

 by the weight of the external air. The entire 

 amount of its pressure on our globe may be 

 conceived by stating, that could we place at one 

 end of a balance a hollow sphere of mercury, 

 8,000 miles in diameter, and about 30 inches in 

 thickness, it would exactly counterpoise our 

 atmosphere, could it be appended to the other 

 extremity. Upon each square inch of surface 

 at the level of the sea, the air presses with a 

 weight of about 151bs. 



