THE ATMOSPHERE 



75 



some of the air exhausted after placing one hand over 

 the glass, some idea may be gained of the extent of this 

 pressure. The experiment may be performed in a slightly 



FIG. 71. Weighing Air. 



FIG. 72. The pressure of the air 

 forces the rubber diaphragm into 

 the bell-jar. 



different way by tying a piece of sheet rubber over the 

 hand glass and then exhausting the air (Figure 72). 



The pressure of the atmosphere is 

 usually measured by the barometer, the 

 most accurate kind being the mercurial 

 barometer (Figure 73), which consists of 

 a strong glass tube about thirty-one 

 inches long and closed at one end. The 

 tube is filled with mercury and inverted 

 in a cup of mercury. At or near the sea 

 level the mercury in the tube will stand 

 at a height of about twenty-nine or 

 thirty inches, leaving a vacuum at the 

 top of the tube. Since this column of FIG. 73. This type 

 mercury is held up by the pressure of 

 the atmosphere on the mercury in the made. 



