290 POPULAR LECTURES AND ADDRESSES. 



through its centre of gravity. The other two arrow- 

 heads indicate the " couple" of equal contrary 

 forces of terrestrial magnetism in parallel lines 

 through the centres of gravity of the northern and 

 southern polarities of its two ends, in the oblique 

 directions in which these forces are experienced in 

 the north magnetic hemisphere. In virtue of this 



FIG. 38. 



magnetic couple, the needle would take an inclined 

 position with true south pole down, and true north 

 pole up (as represented in the diagram on page 

 278), if the bearing-thread were precisely in the 

 vertical through the centre of gravity. Hence 

 that the needle may rest horizontally, the point of 

 attachment of the thread must be a little on the 

 northern side of the centre of gravity, as shown in 



