THE ATMOSPHERE. 43 



Tt produces a prossiiro \\\um every spot equal to the 

 weiu'lit of the wliole cohunu above. This pressure 

 aniounls to al)out hl'tecu pounds upon every S(piar(! 

 ineli. 



The jiressure is not simply upon Uu' lop of an object, 

 but upon the sides and undei-neath as welL It is not 

 like one solid body restinjj,- upon another. The particles 

 of g'ascs 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 l)alanced upon all 

 sides, Ave are not conscious of it. 



Upon the outstretched liand there rests a column of 

 air that would weitili two or three hundi-ed jiounds. We 

 are not conscious of it, because tliere 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 rubl)ed 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 moutli by 

 expanding the lungs, the pressure of air enitside 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 aljove 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 tul)e sealed 

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

 or Ijag of mercury. The pressure of air u[)on the mer- 



