278 PHYSIOLOGICAL PHYSICS. [Chap. xxm. 



means of the float and thread move the wheel, which 

 has attached to it a hand travelling over a dial, and 

 indicating the variations. In the aneroid barometers 

 mercury columns are discarded, and a metallic box, 

 partly exhausted of air, is employed. Variations of 

 pressure cause movements of the top of the box, which 

 are transmitted to levers, and move an indicator. 

 The position of the indicator is determined for 

 different pressures by means of the mercury column, 

 and these positions are then marked on a dial over 

 which the indicator moves. 



Effects of atmospheric pressure. It has 

 been seen that into a tube from which the air is ex- 

 hausted, and in which the atmospheric pressure, there- 

 fore, is reduced to zero, a column of mercury will rise 

 to 30 inches, and a column of water to 34 feet. Other 

 fluids will also rise to a height in the inverse ratio of 

 their density. It is obvious that there is thus afforded 

 a means of raising water or other liquid from a low 

 level to a higher one. It is equally obvious that there 

 is a limit to the height to which the liquid 

 can be raised by exhaustion of the air ; 

 that, in fact, it will rise only to the height 

 sufficient to produce a downward pressure 

 equal to the upward pressure of the 

 atmosphere, a height which, as already 

 said, varies with the density of the liquid. 



The suction pump is an application 

 of these facts. It consists essentially of a 

 barrel or cylinder fitted with a piston 



(Fig. 120). The lower part of the barrel is 



Fi<* 120. continued into a tube which dips into the 



fraction water to be pumped. When the piston is 



pulled from the lower end of the barrel to 



the upper, the space it leaves below is devoid of air, and 



the water rises in the tube, filling the barrel, and closely 



following the piston upwards. When the piston 



