32 



FLUID PRESSURE 



SECTION II 

 ATMOSPHERIC PRESSURE 



42. Cause of Atmospheric Pressure. The atmosphere 

 is a mixture of gases, commonly called air. It is 

 known to extend many miles -above us (probably over 

 one hundred), though the greater part of it is within five 

 or six miles of the earth's surface. Now we have found 

 that air is matter, for it occupies space ( 3), and we 



know that all mat- 

 ter is acted upon 

 by gravity ; there- 

 fore we see that the 

 atmosphere must 

 have weight. 



Experiment 29. If 



possible, secure some 

 vessel from which the 

 air may be removed 

 and kept out like the 

 globe in Fig. 17. Re- 

 move the air by using 

 an air pump, and close 



the stopcock c. Balance the globe with weights on the scale beam 

 at a. Then open the stopcock, letting air into the globe b again. 

 Is the globe now balanced by the weights at a? How do you 

 account for this? Add weights until both sides balance each 

 other. What do the added weights represent? 



If the air has weight, then, however little it may be, 

 the total amount above us is so great that it causes a 

 considerable pressure at the surface of the earth. So 

 we see that atmospheric pressure is produced in just the 



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