ON COMPASS CORRECTION. 25 



must produce considerable effects, and may even produce that 

 remarkably curious effect, discovered by Archibald Smith. I need 

 not say any more on this point, but I would remind those who have 

 followed the investigations on this subject that the coefficient called by 

 the letter A. in Mr. Smith's theory may be looked for as being of 

 more practical importance in some of the modern ironclads with an 

 unsymmetrical disposition of iron than it ever has been in any other 

 ships hitherto. Before leaving the subject of this particular result, 

 I may say it would be altogether corrected, and permanently, on the 

 same ship, by shifting the needles round a little in the compass card, 

 so that the magnetic axis of the needles should be i?, i, or 2, or 

 whatever it might be on one side or other of the true north. Then 

 that same compass card would always shew precisely the same result 

 as if this peculiar term called A. in Smith's theory did not exist. It 

 has sometimes been supposed that correction in the lubber line would 

 correct this error, but that is a mistake, it can only be done by a cor- 

 rection of the position of the needles in the compass card. 



In respect to the principles of correction pointed out by the 

 Astronomer Royal, in the first place, the actual magnetism of the 

 ship's iron at any moment depends on two influences, the first being 

 the magnetisation which the iron has acquired somehow or other in 

 the process of manufacture. The hammering of bars of iron, if their 

 lengths be in any other direction than perpendicular to the lines of 

 magnetic north, tends to make them magnetic ; hammering masses of 

 iron when they are red-hot, and allowing them to become cold under 

 the hammer, produces this effect to a very marked degree, but it is 

 also produced even in hammering cold iron. The very hammering in 

 of the rivets in fastening the cold plates of the ship's sides shakes the 

 plates themselves in such a manner as to shake in a great deal of 

 magnetism. It shakes up the molecular structure, and causes them 

 to take magnetism due to the position in which they are when the 

 rivets are being hammered in. It appears that there is a great deal 

 of evidence of a very curious kind which has been collected by various 

 writers on this subject, to which I shall refer presently, shewing the 

 influence of the position in which the ship's head was when being 

 built upon the magnetism which she is found to have when launched. 

 Part of the magnetism thus hammered into the ship, as it were, is 



