9 



Fig. 1. 



r 



the instrument (which height is called its neutral point) : 

 when the mercury rises in the tube, a portion equal to 

 that rise leaves the cistern, and the surface- 

 line falls towards the dotted line e-, and 

 being lower than the surface from which its 

 neutral point was measured, the actual varia- 

 tion in the atmosphere is indicated too little : 

 turn the screw / until the lines on the float h 

 coincide, and the mercury then records the 

 exact change : when depressions occur, the 

 mercury sinking from the tube into the cis- 

 tern raises the surface-line to wards g ; in this 

 case the screw f must be unscrewed until the 

 leather at the bottom of the cistern be suffi- 

 ciently loosened to allow the mercury to as- 

 sume its proper level at the surface D. 



When there is not a gauge to the baro- 

 meter, the relative capacities of the cistern 

 and tube are ascertained by experiment, in 

 the construction of the instrument, and 

 marked thereon ; as is also its neutral point. 

 In this case, when the mercury in the tube 

 is above the neutral point, the difference be- 

 tween it and the neutral point is to be divided by the 

 capacity , and the quotient added to the observed height 

 will give the correct height; if the mercury be below the 

 neutral point, the difference is to be divided as before, 

 and the quotient subtracted from the observed height 

 will give the correct height. 



Let capacity for every inch of elevation of the mer- 



B5 



