112 INTRODUCTION TO GENERAL SCIENCE 



the balance, and the flask may be suspended from the other 

 side in a similar manner. Equilibrium may be easily ob- 

 tained, which, when destroyed, shows a loss or gain of weight. 

 The actual amount of loss or gain is of no consequence. 



82. ATMOSPHERIC PRESSURE AND THE BAROMETER 



The pressure of the atmosphere may be measured by an 

 instrument called the 'barometer. It consists of a tube more 

 than 30 inches long, closed at one end, filled with mercury, 

 and inverted in an open dish of mercury. Within the tube 

 the mercury falls until its surface is about 30 inches, or 76 

 centimeters above the open surface. Above the mercury in 

 the tube is a vacuum. If the amount of mercury in the tube 

 is weighed, and its weight is divided by the cross section of 

 the tube, the result will approximate 14.7 pounds per square 

 inch. Since water is only about one-fourteenth as heavy as 

 mercury, a water barometer would have to be about four- 

 teen times as long as a mercury barometer. That is, to be 

 exact, the column of a water barometer would be 34 feet long. 



At sea level the pressure averages 14.7 pounds per square 

 inch, but at any other elevation it varies, being less on moun- 

 tains, and greater in places below sea level. If we ascend 

 and leave some of the atmosphere below us, it would seem nat- 

 ural for that part still remaining above us to have less pres- 

 sure than the whole. Elevations may be roughly deter- 

 mined by means of the barometer. A difference of reading of 

 one millimeter indicates a rise of about twelve meters. This 

 corresponds to one-tenth of an inch for each 90 feet of 

 elevation. 



References : 



1. 1103:73-75. Air Pressure. 



2. 1103:75-77. The Barometer. 



3. 1304:421-422. The Barometer. 



