s. 



y 



'/ 



Sect. IX.] 



METEOROLOGY 



"7 



( 



the Tuaker. Suppose tlie capacity to be tV, marked 

 thus on the instrument, " Capacity rV :" this indicates 

 that for every inch of variation of the mercury in the 

 tube, that m the cistern will vary contrariwise A th of an 



When the mercury in the tube is ahove the neutral 

 point, the difference between it and the neutral point is 

 to be reduced in the proportion expressed 



incl 



the 



/ 



I 



the quotient added to the observed height ; if helow, sub- 

 tracted from it. In barometers furnished with a fiducial 

 point for adjusting the lower level, this correction is 



superfluous, and must not be applied. 



The second correction required is for the capillary 

 action of the tube, the effect of which is always to depress 

 the mercury in the tube by a certain quantity inversely 



proportioned to the diameter of the tube. Tliis quantity 

 should be experimentally determined during the con- 

 struction of the instrument, and its amount marked upon 

 it by the maker, and is always to be odded to the height 

 of the mercurial column, previously corrected as before. 

 For the convenience of those who may have barometers, 

 the capillary action of which has not been determined, a 

 table of corrections for tubes of different diametors is 

 placed in the Appendix, Table I. 



The next correction, and in some respects the most 

 important of all, is that due to the temperatur- of the 



mercury in the barometer tube at the time of observa- 

 tion, and to the expansion of the scale. Table II. of 

 the Appendix gives for every degree of the thermometer 

 and every half-inch of the barometer, the proper quantity 

 to be added or subtracted for the reduction of the ob- 



t ■ 

 IP 



^-a=- 



-^'r. 



