CHAPTER XIX 



ATMOSPHERIC PRESSURE 



Introduction. It is a Avell known fact that water has 

 weight and exerts pressure. It is more difficult for us 

 to believe that the air around us is constantly exerting 

 pressure, first, because we do not feel conscious of it; 

 second, because air is invisible. 



Imagine for a moment that extending for over two 

 hundred miles above us there were water. We can 

 easily appreciate that it would have weight. Similarly, 

 the air above us has weight, and hence exerts pressure. 

 Suppose we consider a column of air one inch wide and 

 one inch thick and over two hundred miles high. If 

 this column of air could be placed upon one pan of a 

 balance, it would be found to weigh about 15 pounds. 

 This weight would vary from time to time, and from 

 place to place. If we weighed the column of air extend- 

 ing above one square inch on top of a mountain, its 

 weight would be less than 15 pounds, because the 

 column would not be so high. 



We are practically never conscious of this really 

 enormous pressure of the atmosphere, which is exerted 

 over every inch of qur bodies. We are not conscious of 

 it because the pressure is exerted equally over the 



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