152 ON MAGNETISM. 



In x or towards the magnetic north, 



- HM+ HP . cos 2 A + VN. cos A. 

 In y or towards the magnetic east, 



+ HP . sin 2A + VN. sin A. 

 In z or vertically downwards, 

 -VQ+HN.cosA. 



These are the forces which act on the red end of the 

 compass-needle. Those which act on the blue end are 

 of the same magnitude but opposite signs, and therefore 

 merely double the power which produces deviation of 

 the needle. 



69. Examination of the physical meaning of the 

 different terms of this disturbing force. 



First. If we compound together the terms VN. cos A 

 towards the north, and VN. sin A towards the east, we 

 find that they produce a term VN directed in the 

 azimuth A, that is, directed to the head of the ship. 

 This term therefore resembles in all respects a permanent 

 magnetism of the ship, so long as the ship remains in 

 one place. But it vanishes when V vanishes (that is, at 

 the magnetic equator) : and it changes sign when V 

 changes sign (that is, in the south magnetic hemisphere): 

 and this circumstance will give facility for determining 

 the influence of this term, and correcting it by a magnet 

 at each place of the ship. It will be seen from, the ex- 

 pression for N that if the whole mass of iron is either 



