MAGNETISM. 617 



n r ith the compass, either in the athwartship line or in the fore-and-aft 

 line, according to circumstances. " The polar-magnet-deviation " may 

 be corrected at any given place by a magnet or magnets, but the mag- 

 nets thus applied at one place will not always correct the deviation in 

 another magnetic latitude. For it appears that this deviation arises 

 partly from a magnetism inherent in the materials of the ship, not 

 changing with the change of magnetic position, and partly from the 

 effect of terrestrial magnetism upon the ship's iron. But the errors 

 arising from both sources may be remedied by adjusting, at a new 

 locality, the positions of the corrective magnets. 



The inherent magnetism of the ship, of which I have spoken, may 

 be much affected by the position in which the ship was built ; and 

 may change from time to time ; for instance, by the effect of the bat- 

 tering of the waves, and other causes. Hence it is called by Mr. Airy 

 " sub-permanent magnetism." 



Another method of "correcting the errors of a ship's compass has 

 been proposed, and is used to %ome extent; namely, by swinging the 

 ship round (in harbor) to all points of azimuth, and thus constructing 

 a Table of Compass Errors for that particular ship. But to this 

 method it is objected that the Table loses its value in a new magnetic 

 latitude much more than the correction by magnets does ; besides the 

 inconveniences of steering a ship by a Table. 



