X. Concering the differences in magnetic needle, on board the investingator, arising from an alternation in the direction of the ship's head. By Matthew Flinders, Esq. Commander of His Majesty's ship investigator. In a Letter to the Right Hon. Sir Joseph Banks, K. B. P. R. S

Abstract

Whilst surveying along the south coast of New Holland, in 1801 and 1802, I observed a considerable difference in the direction of the magnetic needle, when there was no other apparent cause for it than that of the ship’s head being in a different direction. This occasioned much perplexity in laying down the bearings, and in allowing a proper variation upon them, and put me under the necessity of endeavouring to find out some method of correcting or allowing for these diffe­rences; for unless this could be done, many errors must una­voidably get admission into the chart. I first removed two guns into the hold, which had stood near the compasses, and afterwards fixed the surveying compass exactly a-midships upon the binnacle, for at first it was occasionally shifted to the weather side as the ship went about; but neither of these two arrangements produced any material effect in remedying the disagreements. The following Table contains the observations for the varia­tion of the compass in which the differences are most remark­able, and from which I shall beg to point out such inferences as I think may be drawn from them.