MAGNETISM OF IRON SHIPS. 173 



SECTION X. 



ON THE MAGNETISM OF IRON SHIPS, AS AFFECTING 

 THEIR COMPASSES. 



78. Philosophical and Commercial Importance of 

 this subject. Complication of the Magnetic considera- 

 tions involved in it. 



It is unnecessary to remark on the extent to which 

 at the present time, when so large a portion even of 

 the mercantile navies is built of iron, the interests 

 of commerce are involved in the investigations which 

 alone can make the ship-compasses available. But it 

 may be desirable to point out to what an extent 

 Science may benefit from it. It will be shewn that 

 the principal agent in the disturbance of the compass 

 is subpermanerit magnetism, an element little known 

 before the introduction of iron ships, and whose laws 

 have principally been derived from the examination 

 of iron ships. But another element, whose effects are 

 sensible in all, and very important in some, is transient 



