ATMOSPHERIC PRESSURE 53 



into the space immediately behind a swiftly moving train ? 

 It is the pressure of the atmosphere that causes air to 



Galloway 



Why does this rapidly moving train pick up dust and papers directly behind the 



observation car? 



move into a place from which any object has been re- 

 moved, and moving air can move other bodies with it. 

 If we remove the air from a closed space, the air will try 

 to get back in, and failing, will perhaps push the walls 

 together. When the air is pumped out of an ordinary 

 rubber tube which has one end closed, the tube is pressed 

 until it is flat, just as if a heavy weight had been laid upon 

 it. A ten-pound block of stone resting on the table presses 

 down upon the table because the force called gravity is 

 pulling on the stone. Any body of matter which has 

 weight will in a similar way exert force upon anything 

 that is under it. This leads us to ask, "Does air have 

 weight ?" 



Many years ago Galileo, who was the first man in the 

 world to make extensive use of experiments to answer his 

 questions, was the first to weigh air. While he used a 

 copper globe as a container, you could do it in your own 



