PARTIAL FAILURE OF COMPASS-CORRECTIONS. 179 



ship's head while in the same locality, and subjecting 

 her to some degree of violence, were first investigated a 

 few years after the establishment of the principles 

 above described, by a 'Liverpool Committee' appointed 

 by merchants of the City of Liverpool. From their 

 researches it clearly appeared that the magnitude and 

 direction of a ship's subpermanent magnetism are con- 

 nected with the direction in which she is built, exactly 

 according to the law explained in Article 75 ; and thus 

 no doubt remained that the subpermanent magnetism 

 was the effect of terrestrial magnetism acting on the 

 iron during the heavy hammering to which an iron ship 

 is subjected in the operations of building. And it 

 was found that if, after launching, the ship is kept for a 

 time in a different position, and more especially if she 

 is subject to the tremor of steam-navigation, a large 

 proportion of this subpermanent magnetism vanishes: 

 a part however remaining invariable. Now the effect 

 of either making no attempt to correct the subper- 

 manent magnetism by magnets, or of correcting it 

 completely and then finding that, in consequence of the 

 decay of the subpermanent magnetism, it is now over- 

 corrected, may be estimated by reference to the con- 

 struction of Article 55, Figure 45. It is there seen 

 that, with a given change of ship's magnetism, the 

 error produced in the needle's direction will depend on 

 the magnitude of the local terrestrial horizontal force : 

 if the ship is in a part of the earth where horizontal 

 force is large, or where B E is large, the error of the 

 compass will be small : but if the horizontal force is 



122 



