B 



ELEMENTS OF GENERAL SCIENCE 



barometers were the first used, and usually we express the 

 ;iir pressure by stating the height of the column in a mercury 

 barometer. These barometers are therefore graduated in inches 

 or centimeters, and a scale of this sort will be found upon any 

 practical instrument for measuring air pressure (fig. 8). An 

 aneroid is commonly given a scale so graduated that the 



numbers correspond 

 with the readings of 

 the mercury barometer. 

 The readings of the 

 two are therefore 

 identical. 



Since the air pressure 

 decreases with increase 

 of elevation, barom- 

 eters may also be 

 graduated to read in 

 terms of the elevation 

 above sea leveL Aner- 

 oids are frequently so 

 graduated, since their 

 small size makes it 

 possible to carry them 

 conveniently (fig. 9). 

 FIG. 9. A pocket aneroid barometer They may be used to 



This pocket type of barometer is graduated so ascertain the approx- 

 as to show both the pressure and the elevation . , . , , ,. 



at which a reading is taken imate height of moun- 



tains and the elevations 



reached by balloons or aeroplanes. Some aneroids are so con- 

 structed that a pen attached to the pointer is caused to write 

 on a piece of paper which is kept moving by means of a clock 

 in such a manner as to make a record of the readings. Such 

 an instrument is called a barograph (fig. 6). 



7. Altitude and air pressure. It was stated in the preceding 

 paragraph that the pressure of the air is due to the weight of 



