THE SUCTION-PUMP 59 



sides of this box. This is because there is no pressure 

 within the box. These imperceptible movements are mag- 

 nified by a contrivance which records them by movements 

 of a hand on a dial. Since certain pressures correspond to 

 certain altitudes, the hand and dial show the altitude as 

 well as the pressure (see Fig. 25). However, since air- 

 pressure varies with local conditions, as in storms, you can 

 see that an aneroid record of altitude is not always exact. 

 The word aneroid means without fluid. Fluid barometers 

 are also used. A mercurial barom- 

 eter is one in which changes of pres- 

 sure are recorded by the rise or fall 

 of a column of mercury. As you 

 already know, the rise and fall of a 

 column of mercury, in the instru- 

 ment we call the thermometer, re- 

 cords changes of heat. But note 

 this important difference. In a 

 thermometer the mercury is in a FIG. 25. The aneroid barom- 

 closed tube and is not sensitive to 



changes in pressure. But in a mercurial barometer the 

 mercury must be exposed at some point to the air so that 

 it will be sensitive to changes in pressure; also there must 

 be sufficient room for expansion of the mercury outside 

 the tube so that changes in its volume due to heat will not 

 seriously affect the reading of changes due to pressure 

 (see Fig. 26). 



Standard atmospheric pressure is the average pressure 

 at sea-level. You have noted that this is a pressure of 

 fifteen pounds to the square inch. This was determined 

 by measuring at sea- level the height of a column of mer- 

 cury held up in a mercurial barometer; this height is about 



