ON PNEUMATIC EQUILIBRIUM. 275 



servoir from which it rises. \Ye can draw up a much higher column of mercury 

 by sucking with the muscles of the mouth only, than by inspiring with the 

 chest, and the difference is much more marked than the fiiiference in the 

 forces with which we can blow : for in sucking, the cavity of the mouth is 

 very much contracted by the pressure of the external air, and the same force, 

 exerted on a smaller surface, is capable of counteracting a much greater hy- 

 drostatic or pneumatic pressure. 



When a tube of glass, about three feet long, closed at one end and open at 

 the other, is filled with mercury, and then immersed in a bason of the same 

 fluid, the pressure of the atmosphere is wholly removed from the upper sur- 

 face of the mercury in the tube, while it continues to act on the mercury in 

 the bason, and by its means on the lower surface of the column in tlie tube. 

 If such a tube be placed under the receiver of an air punip, the mercury will 

 subside in the tube, accordingly as the pressure of the atmosphere is diminished; 

 and if the exhaustion be rendered very perfect, it will descend very nearly to 

 the level of the open bason or reservoir. When the air is readmitted, the 

 mercury usually rises, on tl>e level of the sea, to the height of about 30 inches; 

 but the air being lighter at some times than at others, the height varies between 

 the limits of Ti and 3 1 inches. This well known instrument, from its use in mea- 

 suring the weight of the air, is called a barometer. In the same manner a co- 

 lumn of water from 30 to 35 feet in height may be sustained in the pipe of a 

 pump; but if the pipe Avere longer than this, a vacuum would be produced in 

 the upper part of it, aiul the pump would be incapable of acting. 



In order to observe the height of the mercury in the barometer with greater 

 convenience and accuracy, the scale has sometimes been amplified by various 

 methods; either by bending the upper part of the tube into an oblique posi- 

 tion, as in the diagonal barometer, or by making the lower part horizontal, 

 and of much smaller diameter than the upper, or by making the whole tube 

 straight, and narrow, and slightly conical, or by placing a float on the sur- 

 face of the mercury in the reservoir, and causing an axis, which carries an 

 index, to revolve by its motion. But a good simple barometer, about one 

 third of an inch in diameter, furnished with a vernier, is perhaps fulty as 

 accurate as any of these more complicated instruments. In order to exclude 

 the air the more completely from the tube, the mercury must at least be 



