166 ON MAGNETISM. 



the angle of deviation of the needle in a deflexion-ex- 

 periment at the second station, the direction of the 

 magnet A will be such, that its blue end will deviate 

 by 90 (j> from the north meridian. The horizontal 

 force which produces induction in A is E x sin <p. 

 Now we found that when the inducing force was E, 

 the magnet-power produced by the induction was 



tan 1(0 -<9 2 ) 

 A x cotan. original local dip x - i ta . Q~\ 



Hence at the second station the induced magnet-power 

 in the experiment of deviating the needle will be 



jE"sin( A 



~X4 x cotan. original local dip x 



or the efficient magnet-power will be 



But (see Article 27) E' sin (f> is sensibly equal to 2 A. 

 Also, if be the deviation observed in the same manner 



at the original station, E sin 6 2A . Hence - ' ^ 

 = sin 0. And the efficient magnet power is 



A x \ 1 sin 6 x cotan. original local dip x ^\\Q~ Q\ \ '> 



where, after once making the necessary experiments, the 

 quantity within the bracket is constant for all stations. 

 This formula, it will be remarked, applies to the 



