c6 SECTION PHYSICS. 



held loosely, and is shaken out again, but part of it remains as long as 

 the plates remain attached to the frames, and the ship for twenty, 

 thirty, or fifty years, or whatever may be the life of an iron ship, 

 retains a very large part of the magnetism originally imparted to her 

 by terrestrial magnetic influence under the blows and shocks which 

 the metal experienced in the building. But besides the magnetism 

 which I have thus referred to, and which has been called by the 

 Astronomer Royal permanent magnetism and sub-permanent mag- 

 netism, and which has been called by Dr. Scoresby retentive magnet- 

 ism, which differs from induced magnetism which comes and goes 

 with the inducing force, there is a certain amount of magnetism 

 depending upon the position of the ship at the moment. The mag- 

 netic influence of the earth may be resolved into two components, a 

 vertical and a horizontal component. Whichever way the ship turns 

 the vertical component remains unchanged, and hence, as long as she 

 remains in one neighbourhood, or does not go to a different part of 

 the world, she experiences a magnetisation due to the constant vertical 

 component of the earth's magnetism. But, besides that, there is a 

 variable magnetism due to the horizontal component which differs 

 according to the position of the ship. Thus, when the ship's bow is 

 pointing to the magnetic north, magnetism becomes induced through- 

 out the whole of the ship, which makes the bow become a true south 

 pole and the stern a true north pole. When she is turned broadside 

 to the magnetic north, so as to point east and west, she experiences 

 magnetism in which the side next the magnetic north has a true south 

 magnetic polarity. Whatever permanent and sub-permanent mag- 

 netism there may be, this inductive magnetism comes and goes so far 

 as we know independently of it, so that the change of magnetism due 

 to turning the ship round is sensibly the same as if she had no per- 

 manent magnetism. It seems to me this is a subject which wants 

 experimenting upon, and one reason for my wishing to bring this 

 question before you is, that it seems to be one in which an impulse is 

 just now wanting for experimental investigation. Among other points, 

 it is very important to find whether the inductive magnetism is quite 

 independent of the permanent magnetism retained by the position of 

 the metal in question to find whether a compass will, for instance, 

 experience the same inductive magnetism when it is turned into 



