ON COMPASS CORRECTION. 35 



to sub-permanent magnetism, varying, and to the different induction 

 produced by the different amount of the vertical component of the 

 earth's magnetic force in different parts of the world may also be 

 corrected. The rule will simply be to keep the compass correct on 

 whatever course you are steering, and once every three or four days, 

 or a week, or a month, according to circumstances, put the ship on 

 some other course, keep her on that course for a few minutes, correct 

 the error on that other course, and then the quadrantal error having 

 been once corrected originally, you may be perfectly certain that the 

 compass will be correct, and will remain correct until either the sub- 

 permanent magnetism of the ship changes or until she goes to. a 

 different magnetic latitude. It may be supposed that it is using too 

 strong a term to say it will be perfectly correct, and I admit that mathe- 

 matically there is no such thing as perfection. I mean as correct as 

 it is possible for a compass to be with the correctors which we have 

 in the ship. How small in fact can we get the practical error by 

 applying the table of corrections ? how near can we be sure of our 

 result ? Can we apply the table with any accuracy as supplied by the 

 Admiralty? Can we do it within a quarter of a degree, within i, or 

 within 2 ? We can sometimes and sometimes we cannot, according 

 to circumstances. Then we can be neither more nor less near, and 

 neither more nor less sure, of the degree of nearness with which we 

 obtain our result by the application of the correctors on the 

 system which I am advocating, as by the application of the table of 

 errors on the principles laid down by Mr. Archibald Smith, and 

 practised thoroughly in the Admiralty, with the aid of the magnetic 

 correctors since introduced by Captain Evans. I must apologise for 

 having detained you so long, but* I thought the importance of the 

 subject warranted me in bringing it before you. 



The PRESIDENT : 1 am sure we arc all much obliged to Professor 

 Thomson for the able manner in which he has brought this important 

 subject before us, and I have no doubt it is well known to many of you 

 that he is a bold British navigator himself, and therefore competent 

 to speak on both the theory and practice of the matter ; I beg to offer 

 our best thanks to Professor Thomson for his communication. 



CAPTAIN EVANS : In case an erroneous impression should be left by 

 Sir \V. Thomson's very able remarks upon this new error which hc-hai 



