THE THEORY OF TERRESTRIAL MAGNETISM. 407 



4. On a sphere of radius unity, since cos 6 = p, 



dV. j 



--3aaj.( l -rt> 



and V becomes the same as H. 



Collecting the values of the quantities X, we have : 

 When m = 0, ^" = (1 -/*')*, 



When m=l, X^p,, 



