SCIENCE AND PRACTICE. 331 



1 + 



g 



(R 2 \ 

 -J5J, and not a 



number, is given by the mercury thread, measured in cali- 

 brating the tube. The thread is longest where the section 

 is smallest, and vice versa, or 



L : I = r 2 TT : R 2 TT. 

 E 2 f 



^ _ . ff 



~^~ L ' 



When great care is taken in the manipulation, a cor- 

 responding degree of precision may, unquestionably, be 

 attained. 



When a tube is very slightly conical, the coefficient* 



1 

 1 _j_ v/cT-l- ^/ a approaches so very nearly to the value, one, 



~~3~~ 



that it may be neglected, and the resistance of the tube cal- 

 culated by the formula 



in which the only measurements necessary are the length of 

 the tube and the weight of its contents of mercury. 



A still nearer approximation to the true resistance, when 

 the tube consists of a number of small obtuse cones of various 

 lengths and proportions, is to calculate the resistance of each 

 separate little cone, and the sum of all these several subordi- 

 nate resistances will be found to give very nearly the same 



* In his reproduction of the mercury unit Dr. Matthiessen has used, 



n% 

 instead of the one given above, the coefficient _ /A ._.. in which n is the 



number of measurements, and X the length of the column of mercury in any 

 position. This coefficient, however, is not more exact, as the tubes are taken 

 into account as being series of different cylinders instead of cones. 



