76 



GENERAL SCIENCE 



30 lit 



cup, it must weigh the same as a column of air of equal 

 cross section and extending as high as there is any at- 

 mosphere (Figure 74). 



Torricelli first proved that atmosphere has pressure. 

 He noticed that an ordinary suction pump could not 

 lift water more than thirty-two feet and formulated the 

 theory that the atmosphere was not heavy 

 enough to push water to a higher level. 

 Since mercury is thirteen* times as heavy 

 as water, he reasoned that mercury could 

 only be pumped one thirteenth as high. 

 Upon testing out his conclusions he found 

 them to be correct. 



Torricelli's apparatus was simply a mer- 

 curial barometer similar to the one de- 

 scribed in this chapter. 



In calculating the atmospheric pressure 

 on one square inch of the earth's surface, 

 it is necessary simply to consider the 

 column of mercury in the barometer as 

 having a cross section of one square inch. 

 If the mercury' in the tube stands at 

 twenty-nine and one half inches, it would 

 have a volume of twenty-nine and one 



FIG. 74. Com- 0.1 



merciai Mercurial half cubic inches. Such an amount of 

 mercury weighs about 14.6 pounds, which 

 is approximately the pressure of the atmosphere on each 

 square inch of the earth's surface. 



Changes in Atmospheric Pressure due to Elevation. - 

 If a barometer is carried up the side of a mountain, the 

 height -of the mercury column will decrease. This is 

 because it is only the atmosphere above which exerts 

 pressure, and as we go up the side of a mountain much 



