36 



FLUID PRESSURE 



46. The Barometer. The barometer is a device for 

 measuring the pressure of the atmosphere. This may 

 be done by allowing the atmosphere to hold up as high 

 a column of mercury as it will, and then weighing the 

 mercury. If the column had a cross section of just 



one square inch, its weight 

 would show us, in pounds, 

 the pressure of the air upon 

 one square inch. 



Experiment 36. To make 

 a barometer, take a glass tube 

 about 32 inches long, closed at 

 one end ; fill this with mercury, 

 closing the open end with the 

 finger, as in A (Fig. 21). Invert 

 it into a cup of mercury, as in J5, 

 being careful to keep the end 

 tightly closed until it is under 

 the surface. Now remove the 

 finger ; a little mercury runs out 

 into the cup, leaving a column 

 about 30 inches long which is 

 held up by atmospheric pressure on its lower end. 



Since the tube was full and the mercury in falling from the top 

 allowed no air to enter, it is clear that a vacuum is formed in the 

 tube above the liquid. Thus there is no pressure on its upper 

 end, so that the column is as high as the atmospheric pressure 

 can force it. 



As the air varies in weight from day to day it pushes 

 the column higher or lower. Therefore the higher the 

 column of mercury, the heavier the air. Changes in 

 atmospheric pressure often attend weather changes, so 

 that a change of weather may sometimes be foretold by 



PIG. 21 



