Chap, xxiii.] ATMOSPHERIC PRESSURE. 275 



extent of the atmospheric layer is supposed to be 

 between 50 and 100 miles. The pressure, then, on a 

 square inch of the earth's surface, let us say, will be 

 equal to the weight of the column of air which it 

 supports ; it is about 14 '7 pounds. 



The effects of the atmospheric pressure are easily 

 made manifest. Let a glass cylinder be covered over 

 at one end with a piece of bladder. Place the open 

 end with greased edges on the plate of an air pump ; 

 let it be pressed close on the plate to prevent the 

 passage of air. After working the pump a little, the 

 air will be exhausted from within the cylinder, and 

 the bladder will be bearing the full weight of the 

 atmosphere on the outside without any counterbalan- 

 cing force within. It will yield, become concave, and 

 finally burst with a loud report. Let the same 

 cylinder be put on the plate of the air pump, but not 

 over the pipe by which the exhaustion is made, and 

 let it be covered by a globe. On exhausting the 

 globe of air, the cylinder containing air will be in a 

 space devoid of it, and the air by its elastic force will 

 cause the bladder to bulge outwards. 



The pressure of the atmosphere might be estimated 

 by the height of the column of water which it would 

 support. If a long glass tube closed at one end were 

 exhausted of air, and the open end plunged into a 

 vessel of water, which was open to the air, the sur- 

 face of the water would bear the atmospheric pressure, 

 while the surface within the glass tube would be under 

 no pressure, the tube being free of air. Consequently 

 the water would rise in the tube until the height of 

 the column of water above the level of the water in 

 the vessel produced a pressure equal to the atmo- 

 spheric pressure, when equilibrium would be restored. 

 The height of such a column would be thirty-four 

 feet. Suppose mercury were used instead of water, 

 then since the density of mercury is to the density of 



