ATMOSPHERIC PRESSURE 33 



same way as pressure in liquids, by the simple action 

 of gravity upon its particles, and like liquid pressure, 

 it is felt equally in all directions, upward, downward, 

 and sidewise. 



43. Greatness of Atmospheric Pressure. The pres- 

 sure of the air upon objects at the level of the sea is 

 about fifteen pounds on every square inch of surface. 

 On mountains there is, of course, less air above the sur- 

 face and the pressure is less; the difference is easily 

 noticed. 



This pressure seems very great for the simple weight 

 of air, but we must remember that it reaches far above 

 the earth many miles. Water affords that amount 

 of pressure at a depth of only thirty-three feet, and 

 mercury at thirty inches. 



We do not notice this weight of air because we have 

 always lived under it; moreover, it does not crush us 

 because it is equal on all sides of us, even entering the ' 

 body in the lungs. 



44. The Vacuum. No body is crushed by atmos- 

 pheric pressure for the reason just given ( 43) ; that is, 

 it is felt evenly on all sides and inside as well as out. 

 In a bottle, for example, the pressure of air inside is 

 just the same as that outside ; but remove the air 

 entirely from the bottle, and we have then an unequal 

 condition, no pressure inside to balance fifteen pounds 

 on every square inch outside. Clearly a weak bottle 

 might be crushed by that weight. 



Experiment 30. Draw some of the air from a small bottle 

 by suction, closing its mouth with your tongue. Describe all 



