156 Mr. C. S. Whitehead on the Effect of 



by the North Sand Head Lightship off the Goodwin Sands, then 



If 



Uq 



= •21, and 79 per cent, is lost. 

 ?7=1000, jt>=1885, 



and 54 per cent, is lost. 



^=•46 

 u 



The above value of o~ is from experiments on a sample of 

 sea-water from the North Sea made by Mr. S. Evershed. 



(3) Case I. 



Take the centre of the shell for origin, and axis of the 

 circuit for axis of z. 



Let D be the centre of the circuit, C any point in it, 

 OA = external radius of the shell = a. 

 OB = internal radius of the shell = b. 

 OC = c, zDOC = «. 



a — v = r}. 



Let P, Q, Ii be the components of the electromotive in- 

 tensity parallel to the axes of x, y, z respectively, a the 

 specific resistance of the shell, fi the permeability of the shell, 

 li that oi the dielectric, K' the specific inductive capacity of 

 the dielectric, r, 6, § the polar coordinates of any point, 

 6 being the colatitude, 6 the longitude. 



Let the current in the circuit be represented by the real 

 part of ye ipt , p = 27r x frequency, i = ( — 1) * . 



By symmetry the induced currents in the shell will flow 

 in circles parallel to the plane of the circuit. 



Let <E> be the electromotive intensity tangential to one of 

 these circles; 



