252 POPULAR LECTURES AND ADDRESSES. 



signify similarly a " centre of gravity " of southern 

 polarity. Thus the action of terrestrial magnetism 

 on a bar magnet is very rigorously the same 

 as that of two forces in dissimilar directions in 

 parallel lines through the two poles, as illustrated 

 in the annexed diagram ; and the result, when 

 the bar is free to turn, is that it can only rest 



FIG. 35. 



with the line joining its poles in the direction of 

 the lines of force. 



Gilbert, in respect to his terella, uses the word 

 pole definitely, to denote either point in which the 

 little indicating needle places itself perpendicular to 

 the surface ; and in this perfectly definite sense the 

 word " pole " is used in the modern science of 

 terrestrial magnetism. The north magnetic pole is 



