INTERESTING THINGS ABOUT THE AIR 



9 



Let us consider the aneroid barometer first (fig. 8). 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 rub- 

 ber 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 ar- 

 rangement of levers or 

 gear wheels which will 

 magnify the motion is 

 attached to the box 

 and operates a pointer 

 which moves up or 

 down as the pressure 

 increases or decreases. 

 A still larger move- 

 ment is secured in a 

 compound barometer of this type, which is constructed by 

 joining six or eight of the metal boxes (fig. 11, A). 



The mercurial barometer is quite a different instrument. 

 Indeed, at first it appears to have no resemblance to the 

 aneroid. A simple mercurial barometer may be constructed 

 in the following manner: Secure a glass tube about three 

 feet long, with one end closed. Fill it with mercury, close 

 the open end with the finger, invert the tube, and place the 



FIG. 8. A pocket aneroid barometer 



This type of barometer is graduated so as to 



show both the pressure and the elevation at 



which a reading is taken 



