8 



ELEMENTS OF GENERAL SCIENCE 



amount of air in the flask. By this means we may ascertain 

 the necessary corrections and discover the true weight of air. 

 This has been done many times. The actual weight of air 

 varies from time to time, because of changes in temperature, 

 moisture, and pressure, but at 60 F. it is about 34.5 grams 

 per cubic foot. 



8. Air pressure. If air has weight, it ought to press down 

 upon the surface of the earth just as other objects do when 

 they rest upon the earth, and it ought to rest upon us and 



other objects on the earth's 

 surface with considerable 

 weight. The truth of this 

 may be tested in the fol- 

 lowing manner: A blad-' 

 der glass (fig. 7) may be 

 placed upon the stand of 

 an air pump, with a sheet 

 of rubber tied over the 

 open top. If there is any 

 pressure upon the top, due 

 to the weight of the air, it 

 does not push the rubber 



downward, because the air within the glass resists, as it does 

 in the football or the tire. If, however, the air is partially 

 removed from the interior of the glass by the use of a pump, 

 the resistance from within is reduced, and the rubber is pressed 

 downward and may even be broken. It is plain that there is 

 pressure upon the upper side of the rubber, but that so long 

 as the air Avas allowed to remain in the glass the pressure 

 upon the two sides was equal. 



9. Measurement of air pressure by barometers. The pres- 

 sure gauge discussed in section 5 is one device for measuring 

 air pressure. It may also be measured by use of instruments 

 known as barometers. There are two principal types of barom- 

 eters the aneroid barometer and the mercurial barometer. 



FIG. 7. Air pressure 



When air is pumped from within the glass 



jar, the sheet of rubher tied over the upper, 



open end of the jar is forced downward by 



the weight, or pressure, of the outside air 



