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On Irregularities observed in the Direction of the Compass Needles of 

 H. M. S. Isabella and Alexander, in their late Voyage of Discovery, 

 and caused by the Attraction of the Iron contained in the Ships. By 

 Captain Edward Sabine, of the Royal Regiment of Artillery, F.R.S. 

 #c. Read February 18, 1819. [Phil. Trans. 1819, p. 112.] 



In this paper Captain Sabine shows in what respect the effects of 

 local attraction in the above-mentioned ships were conformable to 

 observations made in previous voyages ; and how far the errors found 

 to take place on different courses, and under different dips of the 

 magnetic needle, corresponded with those rules for calculating cor- 

 rections recommended by Captain Flinders, who found that in every 

 ship a compass would differ very materially from itself on being re- 

 moved from one place to another, and this was found to be the case 

 in the Isabella and Alexander. 



As the ships ascended Davis's Straits, the binnacle compasses, in 

 consequence of their construction, became nearly useless ; accord- 

 ingly, a standard compass was placed in the Isabella exactly amid- 

 ship between the main and mizen mast, on a stout cross-beam, about 

 nine feet above the deck ; and in the Alexander amidship, on a box 

 of sand five or six feet above deck. Captain Sabine next describes 

 the methods by which the points of no error in these compasses were 

 determined, and which were not in either ship coincident with the 

 magnetic meridian. 



Captain Flinders has shown that the maximum of error in the 

 same compass, and confined to the same spot, is different in different 

 parts of the world ; and by multiplying the observations, and com- 

 paring the series, he was led to trace a connection between the 

 amount of the errors and the dip of the needle, observing that the 

 influence of local attraction on the compass needle increased with 

 the dip. This increase, however, says the author, was a relative 

 one, being in comparison to the directive power of magnetism, the 

 diminution of which is sufficient to account for the effects observed ; 

 as will be evident upon reflecting, that though the magnetic force is 

 greatest at the pole, its directive power must there have ceased : 

 hence the inadequacy of the rule proposed by Captain Flinders, 

 whereby the amount of error under any known dip being ascer- 

 tained, the amount of error for any other dip may be calculated, by 

 using as a multiplier the decimal expression of the proportion which 

 the error in the one ascertained instance may have borne to the dip. 

 In the observations made in the Isabella at Shetland, where the dip 

 is 74 21^', the maximum of error was 5 34' easterly of the true 

 variation, with the ship's head at E.S.E., and 5 40' westerly at 

 W.N.W., making an extreme difference of 11 20'. By Captain 

 Flinders's rule, the common multiplier for this compass would have 

 been about one twelfth, making the extreme difference 15, whereas 

 it was really more than 10. By a similar reference to the obser- 

 vations made by the Alexander in Baffin's Bay, another proof is af- 

 forded of* the inadequacy of Captain Flinders's rule. 



