the Direction of the Compass Needles, cSc. 281 



mer ; and that such a diminution does take place, and in a de- 

 gree which is sufficient to account for all the effects observed, will 

 be evident to every person, who reflects that although the mag- 

 netic force is greatest at the pole, its directive power must theii 

 have whollv ceased, having become less on the horizontal tra- 

 versing of the needle in propt^rtion as the point of attraction has 

 been brought beneath the compass ; indicated by the angle which 

 the dipping needle makes with the horizon. This is doubtless 

 the principal cause of the connection which Captain Flinders was 

 the first to trace. 



It is not designed to say that this cause may not be aided by 

 the increased magnetism of portions of the ship's iron, such as 

 bars and stanchions; which being fixed in an upright position, 

 may receive an addition to their attractive power where the po- 

 sition of the dipping needle is always coincident with theirs; but 

 merely to observe, that a cause is known to exist for the connec- 

 tion, independently of supposition ; which cause, conjointly with 

 experience, shows the inadequacy of the rule proposed by Cap- 

 tain Flinders, whereby the amount of error, under any known 

 dip, being ascertained, the amount may be calculated for any other 

 dip, by using as a multiplier, the decimal expression of the pro- 

 portion 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° 2\\', the maximum of error was 5" 34' easterly of 

 the true variation, with the ship's head at E.S.E. and 5" 46' 

 westerly at W.N.W. making an extreme difference of 1 1° 20'. 



By Captain Flinders's rule, the common multiplier for this com- 

 pass would have been about one twelfth, or '083, which at a dip 

 of 86° 09', which was the greatest observed during the late voy- 

 age, would have given an error of ])etween 7° find 8°, making the 

 extreme fliffcrence 15°; whereas repeated observation showed it 

 to be at that time more nearly 50', if not exceeding that amount. 



The inadequacy of the rule will also appear l)y reference to the 

 observations made by the Alexander in Baffin's Buy. The error 

 at eight points being 6" 46', at a dip of 84" 30'; it ought 

 scarcely to have exceeded 7° at the greatest possible dip, making 

 an extreme difference of less than 1 5°* No o|)portunity occurred 

 indeed of making accurate observations at a greater dip than the 

 above ; but ihe difference in the bearing of objects before and 

 after tacking indicated with sufficient certainty, that the error 

 had increased to an amount very far beyond 15''; fre(iucnt in- 

 stances of an extreme difference of from 3 to 4 points being re- 

 marked, as the ship approached the furthest western longitude to 

 which she attained in a high latitude ; this was in Lancaster's 



Sound 



