THE INVERSE SQUARE LAW 



proof that the inverse square is the only law satisfying this 

 condition we may easily show, as follows, that it does satisfy it. - 



S C N 



CD-CE. 



FIG. 162. 



Let m be the strength of each pole and / the distance between 

 them. Let CD = d. 



The intensity due to N is 



*-ff 



and that due to S is 



The result ant is 



in 



Zmld 



Zml 



I 2 



If / is so small compared with d that we may neglect ^-r^ and 

 higher powers, the intensity at D is 



Zml 



The intensity at E is the resultant of -- 2 along NE, and 2 



along ES. By the triangle of forces we may represent these two 



