PUMPS 



41 



Height 



n Miles 



35 



15 



Baro- 

 meter 



riches 



41. Changes in Atmospheric Pressure. If you were 

 at the bottom of a haystack, trying to work your way up 

 through it, you would have the hardest time at the bot- 

 tom, and the task would become easier as you approached 

 the top. The reason is that the hay is most compact, or 

 dense, in the lowest layers, since these layers have to bear 

 the weight of all the hay above them. So it is with the 

 atmosphere : its lower layers are crowded together by the 

 weight of the air they support. 



That this is so is proved by the bwhavior of a barometer as we carry 

 it up a mountain or up in a balloon. The mercury column falls more 

 and more, showing that the density and the pressure of the air grow 

 smaller as we ascend (Fig. 32). At a height of less than 4 miles the 

 barometer height is about 

 380 millimeters, or 15 inches; 

 hence half of the atmosphere 

 is within four miles of the 

 earth's surface. In a famous 

 balloon ascent the barometer 

 fell to 7 inches. The balloon 

 was then about 7 miles above 

 sea level, and had left more 

 than three fourths of the air 

 behind. When a barometer is 

 carried down into a deep mine, 

 the mercury column rises, 

 because the density and the 

 pressure of the air are greater 

 there than at the surface. FlG - 32 - 



The density of the atmosphere grows less as we 

 ascend. 



42. Pumps. Pumps 



were used by man at least 2,000 years before Torricelli 

 showed that their action is due to the pressure of the 

 atmosphere. 



