8 



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The truth of this may be tested in the following manner: 

 A bladder glass (fig. 5) 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 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 was allowed to remain in the glass 

 the pressure upon the two sides was equal. 



5. Measurement of air pressure. The pres- 

 sure of the air is measured by use of instru- 

 ments known as barometers. There are two 

 principal types of barometers the aneroid 

 barometer and the mercurial barometer. 



Let us consider the aneroid first (fig. 6). 

 The part of this instrument which is sensitive 

 to the air pressure is a small, thin-walled 

 metal box shaped somewhat like the case of a 

 watch. The air is exhausted from this metal 

 box and it is sealed air tight. The pressure of 

 the air tends to press the flat top and bottom 

 of the metal box inward, as it did with the 

 rubber top of the bladder glass, but the metal 

 does not yield nearly so much as the rubber 

 did. In fact, the metal bends so little that it 

 is not easy to see the change taking place. 

 For this reason an arrangement of levers or 

 gear wheels which will magnify the motion is 

 attached to the box and operates a pointer 



FIG. 7. A simple 

 barometer 



The weight of the 

 air forces the mer- 

 cury into the hol- 

 low glass tube. 

 Any change in 

 the weight of the 

 air changes the 

 height of the col- 

 umn of mercury 



