26 ON MAGNETISM. 



tending to turn it in the direction opposite to the mo- 

 tion of watch-hands. Similarly, the momentum obtain- 

 ed from the action of the two poles of A upon the blue 

 pole of B is 



-m-l 



in the same direction as the former. And the whole 

 angular momentum, opposite to watch-hands, is 



-m-l 



It is a great convenience, connected with the assump- 

 tion that c is large in proportion to a or b, that we can 

 at once proceed to expand these expressions in terms 

 with progressive powers of c in the denominator, stop- 

 ping at a definite power of c. It will be found sufficient, 

 for future use, to stop with c" 2 in the expansions of the 

 brackets. Thus we shall have 



24 



