Piston 



4 ELEMENTS OF GENERAL SCIENCE 



experiences, and many others, teach us that air is a very real 

 substance, even though it is invisible, and that it occupies 

 space as do other substances. This space-occupying property 

 of air is useful to man in many ways. 



4. Some uses of compressed air. As the drinking glass was 

 forced down into the water, as shown in figure 1, the air 

 was slightly compressed. This is shown by the fact that the 

 water rose slightly in the glass. In 

 automobile garages the compressed 

 air is usually stored in strong tanks 

 and when needed is placed in the 

 tires through connecting tubes. The 

 pumps used to inflate footballs and 

 tires on bicycles or automobiles are 

 devised to compress air rapidly and 

 to force it into the tires. 



The inexpensive pump shown in 

 figure 3 can be used to compress air 

 if the amount of compression needed 

 is not large. If the valves of the 

 pump are reversed, the pump ex- 



When the piston A is forced hausts air ******* f Piping it 



downward the valve B is closed into the tire. Most commercial air 

 and c is opened pu mps are similar in principle to the 



one shown in the figure. Air pumps are common in garages, 

 on railway cars, and on street and interurban cars. In trans- 

 portation cars compressed air is used in applying brakes, and 

 everyone is familiar with the shrill noise made by the suddenly 

 released air which has just been used in stopping the cars. 

 Compressed air is also often used in blowing whistles and 

 sirens; in operating compressed-air engines, heavy hammers 

 used in construction work, and drills used in blasting rock 

 and coal ; and for driving the dentist's drill. It is used in 

 operating automatic devices for regulating temperatures in 

 homes, schools, and public buildings. 



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