TERRES TRIA L MA GNE TISM, E 7'C. 289 



steering on any other course than due east or 

 due west. 



It has generally been considered that the greater 

 the magnetic moment 1 of the needles the better the 

 compass ; it is not generally known that the greater 

 the magnetic moment, other things the same, the 

 more unsteady will the compass be when the ship is 

 rolling on ocean wave slopes. 



Froude's theory of the rolling of ships, according 

 to which he finds that the longer the vibrational 

 period of the ship when set a-rolling in still water 

 by men running from side to side, the steadier she 

 will be in a seaway, is also applicable to the oscilla- 

 tions of the compass produced by the rolling of the 

 ship. The cause of these oscillations will be 

 readily understood by looking at the diagram on 

 page 290, which shows a magnetized needle hung 

 by a single vertical thread. The arrow-head in the 

 vertical line through its middle indicates the down- 

 ward resultant force of its weight or gravity 



1 "Magnetic moment" is the proper expression for what in 

 common language is often called "power," or "strength, ' of the 

 needles. 



VOL. III. U 



