40 



GENERAL SCIENCE 



Use of Air Pressure. Air pressure is very useful to us. When 

 we wish to remove the dirt from our carpet, we remove the air from 

 the vacuum cleaner by a motor and fan, and as the air rushes in to 

 fill the place from which the air was removed, it carries the dust 

 and dirt along with it. 



Why is it necessary to have an attachment that fits closely 

 to the carpet? 



Why does the motor push the air into the bag? 



Why is it necessary to have a different attachment for hard- 

 wood floors, one which does not fit as closely to the floor? 



PARTIAL. 

 1^ VACUUM 



FIG. 29. 



FIG. 30. 



Why is it wrong to say the motor sucks the dirt up from the 

 floor? 



Preserving Fruit. When we wish to preserve fruit, we heat the 

 can and drive out the air. The can cover is then put on with a 

 rubber ring to prevent the air getting in. As the can cools off, a 

 partial vacuum is formed in the can,' and the outside pressure is 

 great enough to hold the cover on securely. 



Pumps. A very important use of air pressure is made in the 

 pumps used in wells. As we work the handle up and down, the air 

 is removed from the barrel of the pump. The air pressure on the 

 surface of the water then presses the water up the pipe. 



As the handle in Fig. 31, (4) moves down, why is valve B closed 

 and valve A open? 



When the handle moves up in Fig. 31, (3) why does valve B open 

 and A close? 



Why does the water start to come up from the well? 



Why does water go up through valve A as the handle moves 

 down? 



