viii ATMOSPHERIC PRESSURE AND BOYLE'S LAW 111 



this height above the mercury in the cup (Fig. 49, c) the 

 mercury fills it completely ; and if the tube is less than 30 inches 

 long, it is also filled by the mercury (Fig. 49, .). On an average 

 the atmosphere at sea-level will balance a column of mercury 

 30 inches in length. No matter if the closed tube is 30 feet 

 long, the top of the mercury column will only be about 30 

 inches above the level of the mercury in the cistern. 



EXPT. 109. Weigh the column of mercury sustained in the 

 tube and measure the diameter of the tube. The area of the 

 bore can then 

 be found (area of 

 circle = radius 2 

 x 31). Calcu- 

 late from these 

 observations the 

 weight of a co- 

 lumn of air on 

 any given area. 



If the tube had 

 a bore with a sec- 

 tional area of ex- 

 actly one square - 

 inch, there would 

 be 30 cubic inches 

 of mercury in a 

 column 30 inches 

 long ; and since 

 a cubic inch of 

 mercury weighs 

 about half a 

 pound, the whole 

 column would 



weigh 15 Ibs. This column balances a column of air of the same 

 area, so that we find that the weight of a column of air upon 

 an area of one inch is 15 Ibs. when the barometer stands at 

 30 inches. 



Mercury a Convenient Liquid for Barometers. The use of 

 mercury for barometers is a matter of convenience. Since the 

 column of mercury which the atmosphere is able to support is 

 30 inches high, it is clear that, as water, e.g., is 13 '6 times as 



FIG. 40. Height of Mercury in Barometer, 



