TERRESTRIAL MAGNETISM, ETC. 307 



Archibald Smith's theory as set forth in the 

 Admiralty Manual, are regularly performed at 

 frequent intervals for every ship of the British 

 Navy, with the addition that they would have to 

 be performed not only for the ship upright, but 

 also with a list of 10 or 15 to either side. If the 

 supposition we have made for a moment as to per- 

 fect definiteness of quality of the ship's magnetism 

 were true, the whole of this process could be 

 actually carried out in practice, and the labour 

 required to move loads across the deck of the ship 

 or shift cargo in the hold, so as to give her the 

 requisite list to one side or other, would be well 

 repaid by getting her compasses perfectly corrected 

 once for all. But, alas ! the compass is not to be 

 corrected perfectly once for all by any possible 

 operations or observations, however accurately 

 performed. The ship's permanent magnetism 

 gradually changes, more or less rapidly according 

 to circumstances, and readjustment becomes 

 necessary ; sooner generally in a new ship, but 

 sooner or later in every ship. The labour and 

 expense of " swinging" the ship both upright and 



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