TERRESTRIAL MAGNETISM. (A PP. A.} 327 



rangement, which constitutes, as it were, a jointed 

 horse-shoe magnet, adjustable to greater or less 

 magnetic moment by increasing or diminishing 

 the distance between its poles through the action 

 of the screw, is so supported on its sole-plate that, 



FIG. 42. DD, the gimballed nuts ; CC, the right and left handed screws; a, a 

 divided micrometer circle to aid when very minute measurement of the 

 distance between the poles is wanted ; ABA'. ABA', the two frames jointed 

 round an axis through BB of the first diagram, and perpendicular to the plane 

 of the second diagram through its central point B ; NS, the effective true 

 north poles and true south poles; HUH, the scale indicating the distance 

 between them ; EG, the glass of the compass-bowl ; K, the foot resting in the 

 central conical hollow ; L, one of the other feet ; F, the spring to keep pres- 

 sure on the feet LL. When the screw is turned so as to bring DD nearer one 

 another the distance between S and N is diminished, and the axis BB rises 

 with its ends B, B, guided by two vertical slots, of which both are seen in 

 plane in the first figure, and one in elevation in the second figure. 



when this is properly placed on the glass of the 

 compass-bowl, the effective poles move to and fro 

 horizontally about half an inch above the glass on 



