TERRESTRIAL MAGNETISM, ETC. 291 



the diagram ; and similarly we see that when the 

 needle is supported by a cup on a point, as shown 

 in subsequent diagrams, it will rest with the centre 

 of gravity of the needle and fly-card a little to the 

 south of the vertical through the bearing-point in 

 the northern magnetic hemisphere, and a little to 

 the north of this vertical in the southern magnetic 

 hemisphere. Hence (except at the magnetic 

 equator, where the needle rests with its centre of 

 gravity exactly under the bearing-point), if the 

 bearing-point be moved to and fro in the east and 

 west horizontal direction, the centre of gravity of 

 the card will tend to lag and again to shoot 

 forward when the motion of the bearing-point is 

 alternately being accelerated and being retarded. 

 This is just what happens through the rolling of 

 the ship when sailing on a north or south magnetic 

 course, as the axis round which the ship is rolling 

 is always below the position of the compass. The 

 same action is experienced, though to a less degree, 

 on any course not due east or due west. When a 

 ship is sailing due east or due west, it is only 

 through pitching that the needle can be thus 



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