122 ON MAGNETISM. 



SECTION VIL 



DISTURBING FORCE PRODUCED ON A SMALL COMPASS- 

 NEEDLE BY A LARGE MAGNET, IN VARIOUS POSITIONS; 

 AND COMPOSITION OF THIS DISTURBING FORCE WITH 

 TERRESTRIAL HORIZONTAL FORCE. 



50. The disturbing magnet is horizontal : its center 

 is broadside-on to the center of the compass : to jind its 

 effect at different distances and elevations. 



In this and subsequent investigations of this Section, 

 we shall consider the dimensions of the compass-needle 

 to be so small, in comparison with other measures, that 

 we may use the lengths of lines measured to the center 

 of the compass-needle instead of those measured to its 

 poles : and we shall investigate the action upon the red 

 end only of the compass-needle, inasmuch as the action 

 on the blue end will be sensibly equal but in the oppo- 

 site direction, and the impressed moment of rotation 

 will be merely doubled. 4 



In Figure 39, the attraction of the blue pole of A 



and the repulsion of the red pole produce in the direc- 



ff ' 



