of determining Stream Lines and Equipotentials. 253 
r + 8r is proportional to 8r X (magnitude of flux) x (peri- 
meter of the trace of the cone), and by the above this is pro- 
portional simply to Sr. Therefore the polar axis must be a 
Fig-. 4. 
line source of uniform strength and similarly for any other 
source or sink when the guiding lines are straights passing 
through a common point — the strength must be independent 
of the radius. 
Case (b). — Greneral method for conduction in a thin shell 
of any shape, the thickness and conductivity being any given 
Phil. Mag. S. 6. Vol. 15. No. $6. Feb. 1908. T 
