ATMOSPHERIC PRESSURE 



133 



water is 34 feet, it has been determined that the weight 

 of the water in that tube is 14.7 pounds. The water 

 is therefore pressing downward with a pressure of 14.7 

 pounds to the square inch. The air is pressing down- 

 ward on the surface of the water with the same force. 

 Column of mercury held by air pressure. If the 

 water were twice as heavy as it is, the column of water 

 which the air would hold up would be only about 17 

 feet. Mercury is 13.6 times as heavy as water, conse- 

 quently the highest column of mercury which can be 

 held up by atmospheric pressure is 1/13.6 of 34 feet. 



of about 2.5 feet, or 30 inches. 



This can easily be demon- 

 strated. Fill a glass tube, a Tor- 

 ricellian tube, about 36 inches 

 long, with mercury. Close the 

 open end of the tube with the 

 finger and then quickly insert the 

 end of the inverted tube into a 

 dish of mercury. When the finger 

 is removed, the mercury falls 

 somewhat. If the height of the 

 mercury is measured, it will be 

 found to be about 30 inches, 

 exactly what we should expect. 

 If tubes of very different diam- 

 eters are taken, it will be found 

 that mercury can be held up to 

 the same vertical height in all of 



30 in. 



Mercury 



Water 



FIG. 48. Air sup- 

 ports a column of 

 mercury 30 inches, 

 a column of water 

 34 feet. 



