40 



AIR AND FIRE 



FIG. 31. 



Filling a Barometer 



Tube and Inverting 



It in Mercury. 



column ought to be raised only about 1 / 13 as high, since 

 mercury is about 13 times as dense as water. 



The apparatus Torricelli used to test his conclusion (Fig. 

 31) was a glass tube about a meter long and closed at one 

 end. He filled the tube entirely with 

 mercury, closed the open end with his 

 finger, and inverted the tube in a vessel 

 of mercury. When he removed his fin- 

 ger, some of the mercury in the tube ran 

 out, but it stopped when the level was 

 about 29 inches (that is, y i8 of 32 ft.) 

 above the mercury in the vessel. It did 

 not fall lower because the pressure of the 

 atmosphere held it up. 



We can also make a barometer out of 

 a tube open at both ends, if we put one 

 of the ends under mercury, and remove the air of the tube 

 by means of an air pump. The mercury will rise about 30 

 inches, but not higher. 



The simple barometer is still made as Torricelli made it. The ver- 

 tical distance between the top of the mercury in the tube and the 

 mercury in the vessel is called the height of the barometer. The 

 barometer height changes as the atmospheric pressure changes. Its 

 average at sea level is about 760 mm. (30 inches) ; hence this is called 

 standard pressure, or a pressure of one atmosphere. The space above 

 the mercury is a vacuum. 



We can readily calculate the atmospheric pressure from the barom- 

 eter height. If the column of mercury in the barometer is 1 square 

 inch in cross section, and 30 inches high, it has a volume of 30 cubic 

 inches. Now, 30 cubic inches of mercury weigh about 15 Ibs. Hence 

 the atmosphere presses down upon every square inch of matter at the 

 earth's surface with a force of 15 Ibs. 



