CORRECTION OF THE COMPASS IN IRON SHIPS. 177 



tudinal values. It was certain therefore that there was 

 permanent or subpermanent magnetism transversal 

 to the keel (because no induction could account 

 for it) ; but it was difficult to say how much of the 

 longitudinal part was subpermanent, how much due 

 to the first term in Article 69, and how much to the 

 iron stern-post acting as a vertical magnet (Articles 

 60 and 53). 



The laws thus ascertained were verified by placing 

 below the compass a magnet, in the position opposite 

 to the ship's magnetism, and at a distance which (as 

 had been ascertained by experiment) would enable 

 the magnet to produce an effect equal to the ship's 

 effect; and also applying a mass of iron at one side 

 of the compass to correct the quadrantal deviation 

 (Article 62). Then, in swinging the ship round, the 

 compass was found to be correct in every position of 

 the ship. 



In the next experiments, a change of great value 

 was made in the practical operations; founded on 

 the following theoretical considerations. Conceive the 

 ship's magnetism to be resolved into two parts, one 

 transversal to the ship, one longitudinal. When the 

 ship's head is placed north or south, the transversal force 

 alone disturbs the compass, and the quadrantal disturb- 

 ance vanishes (Articles 65 and 66) ; and the transversal 

 magnetic part can be corrected by an opposite transver- 

 sal magnet broadside-on to the compass, whose distance 

 is determined without any calculation, simply by trying 

 its effect at different distances till the needle points 



12 



