448 THE THEORY OF TERRESTRIAL MAGNETISM. 



If N be the normal terminated by the minor axis, 



N- = af+, y _,.- = n 



The radius of curvature of the meridian is 



If 8S be the elementary area of a belt of the Earth's surface between two 

 parallels of latitude and 8,* be the length of the arc of the meridian, we 

 have 



S/S'= 2irx . 8s = 27rA T sm 6 . p . 80 = 'Zir -A 7 " 4 sin 6 . 80 



,A T4 

 4 



or -T- = - 



and 



Taking the equatorial radius = 1 , we have 



dS 



n + 2 2 2 



. /*j* 



and 



