TERRESTRIAL MAGNETISM, ETC. 305 



the same direction, and exhibit precisely the 

 same return force when deflected from this direction, 

 as it would were there no iron in the ship. It is 

 only because of the approximate permanence of 

 one part of the ship's magnetism, and the approxi- 

 mate transience of the other, that the compass 

 can be used at all in an iron ship as a guide for 

 her course in the intervals between observations 

 of sun, or moon, or stars. For the sake of simpli- 

 city, and to avoid circumlocutions, I shall first 

 describe the effects on the compass of the ship's 

 magnetism, and explain how they are to be cor- 

 rected on the supposition of perfect permanence 

 and perfect transientness of its two constituents ; 

 and afterwards shortly explain how the mariner 

 must be constantly on his guard to determine 

 and allow for unpredictable irregularities in his 

 compass due to variations of the permanent mag- 

 netism, and to retention of some of the transient 

 magnetism when the inducing influence is past. 



The ship's permanent magnetism produces at 

 the place of the compass a constant force in a 

 direction which is constant relatively to the ship 



VOL. III. X 



