92 THE WATER-BAROMETER. 



the atmospheric pressure on the mercury at C, which is 

 transmitted from the surface of the mercury in the cistern 

 to the interior of the tube; and as there is no pressure on 

 the surface at P, it is clear that the pressure of the air on C 

 is the force which sustains the column of mercury PB. 



Let a be the density of mercury, and TT the atmospheric 

 pressure at C ; then we have 



7T = <JT<7PB, (1) 



and, since g and a are constant, the height PB may be used 

 as a measure of the atmospheric pressure. 



44. The Mean Barometric Height. The mean 

 height of the barometric column at the level of the sea is 

 found to vary with the latitude, but it is generally between 

 29 and 30 inches. The atmosphere is subject to continual 

 changes, some irregular, others periodical. If the density 

 and consequent elastic force of the air be increased, the col- 

 umn of mercury will rise till it reaches a corresponding 

 increase of weight ; if, on the contrary, the density of the 

 air diminish, the column will fall till its diminished weight 

 is sufficient to restore the equilibrium. The barometric 

 height is therefore subject to continuous variations; during 

 any one day there is an oscillation in the column, and the 

 mean height for one day is itself subject to an annual oscil- 

 lation, independently of irregular and rapid oscillations due 

 to high winds and stormy weather. Usually the height of 

 the column is a maximum about 9 A. M. ; it then descends 

 until 3 P. M., and again attains a maximum at 9 p. M.* 



45. The Water-Barometer. Mercury possesses two 

 great advantages over other liquids, which has led to its 

 being selected above all others for use in barometric instru- 

 ments. The first advantage of mercury is that it does not 

 give off vapor at ordinary temperatures. If it did, the space 



* Bes&nt's Hydrostatics, p. 76. 



