178 ON MAGNETISM. 



correctly. Then, in like manner, if the ship's head 

 is placed east or west, the longitudinal magnetism 

 only disturbs the compass, as the quadrantal deviation 

 vanishes there, and it is to be corrected by a longitudi- 

 nal magnet broadside-on to the compass, tentatively 

 applied. The effects of permanent or subpermanent 

 magnetism are now entirely corrected. In order to 

 correct for the induction-effect which produces quad- 

 rantal deviation, the ship's head must be placed in 

 azimuth 45 (nearly), or 135, or 225, or 315; there 

 will be no difficulty in ascertaining whether the quad- 

 rantal disturbance is such as corresponds to the effect 

 of iron in the direction of the ship's head : and, if so, it 

 must be corrected by iron on one or both sides, shifted 

 by trial till the correction is complete. 



These processes were introduced by the author 

 in 1838, and they are still retained in use without 

 alteration. 



80. Reference to the causes of partial failure in the 

 correction of the compass. 



So long as the ship's magnetism remains unaltered, 

 and so long as she remains in the same region of the 

 earth, her compass will now be perfectly correct. But 

 it is necessary to examine the changes which take 

 place in lapse of time and in change of geographical 

 position. 



The determining circumstances of a ship's subper- 

 manent magnetism, and the effect of time upon it, 

 including the effect of changing the position of the 



