C 18 6 : 
X. Concerning the Differences in the magnetic Needle , on Board 
the Investigator , arising from an Alteration 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. 
Read March 28, 1805. 
W'hilst 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 diffe- 
rences ; for unless this could be done, many errors must una- 
voidably 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 varia- 
tion of the compass in which the differences are most remark- 
able, and from which I shall beg to point out such inferences 
as I think may be drawn from them. 
