208 LECTURE XXII. 



sucking up a fluid through a pipe, with the mouth or otherwise, the pres- 

 sure of the air is but partially removed from the upper surface of the fluid, 

 and it becomes capable of ascending to a height which is determined by the 

 difference of the densities of the air within and without the cavity 

 concerned : thus, an exhaustion of one fourth of the air of the cavity would 

 enable us to raise water to the height of 8 feet, and mercury to 7i inches, 

 above the level of the reservoir from which it rises. We 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 difference 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 hydrostatic or pneumatic pres- 

 sure. 



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 surface 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 the tube. If such a tube be placed under the receiver of an air pump, 

 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 the 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 27 and 31 

 inches. This well known instrument, from its use in measuring the weight 

 of the air, is called a barometer. In the same manner a column of water 

 from 30 to 35 feet in height may be sustained in the pipe of a pump ; but 

 if the pipe were longer than this, a vacuum would be produced in the 

 upper part of it, and 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 position, 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 surface 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 

 fully 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 shaken in it for a considerable time, the tube being held in an 

 inverted position ; and where great accuracy is required, the mercury must 

 be boiled in the tube. The reservoir most commonly employed is a flat 

 wooden box, with a bottom of leather ; the cover, which is unscrewejl at 

 pleasure, being cemented to the tube. Sometimes a screw is made to act 

 on the leather, by means of which the surface of the mercury is always 



