THE ATMOSPHERE. 43 



It produces a pressure upon every spot equal to the 

 weight of the whole column above. This pressure 

 amounts to about fifteen pounds upon every square 

 inch. 



The pressure is not simply upon the top of an object, 

 but upon the sides and underneath as well. It is not 

 like one solid body resting upon another. The particles 

 of gases and liquids move about so freely that the pres- 

 sure upon any object contained in them is evenly distrib- 

 uted in every direction. 



As the pressure of the air is evenly balanced upon all 

 sides, we are not conscious of it. 



Upon the outstretched hand there rests a column of 

 air that would weigh two or three hundred pounds. We 

 are not conscious of it, because there is an equivalent 

 pressure underneath the hand to support it. When the 

 pressure underneath is removed by placing the hand 

 upon the receiver of an air-pump, and exhausting the 

 air, the pressure upon the top becomes painful. If the 

 moistened palms are rubbed closely together, so as par- 

 tially to remove the air, some force is required to pull 

 them apart. If the air is removed from the mouth by 

 expanding the lungs, the pressure of air outside forces 

 the cheeks inward. 



The atmosphere near the earth is more compressed 

 and heavy than at some distance above it, because there 

 is more air above to press down. There is a marked 

 difference between the pressure and density of air in 

 a valley and on a high mountain. 



The barometer is an instrument for determining the 

 pressure of the air. It really consists of a tube sealed 

 at one end, filled with mercury, and inverted into a cup 

 or bag of mercury. The pressure of air upon the mer- 



