SCIENCE AND PRACTICE. 329 



quantitively determined with the aid of a drop of mercury 

 sucked into one end, and its length measured at intervals 

 along the whole length. The effect which conicalness has 

 upon the resistance was calculated 

 as follows : The tube is regarded 

 as a truncated cone, A, B, c, D 

 (Fig. 153), whose parallel bottom 

 and top have the radii n and r, and 



whose length is L At the distance, 



x, from the top, suppose a disc, M N, 



of the thickness d x, and radius z, 



to be drawn ; then if W is the re- c F* 153 



sistance of the cone, in the direction 



of its axis, and d W the resistance of the disc M N, in 



the same direction, 



and 



This value of z differentiated gives 



R r 



which we must insert in the first equation to obtain the 

 resistance of the differential disc. 



And this integrated gives W, the resistance of the cone. 



f 

 I 



L - r) TT s 2 (R-r) TT \ r R 

 t/r 

 or 



W = . . .' . . (L 

 R r TT v 



