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XIII. Oii the Errors in the Course of Vessels, occasioned by Local Attraction ; 
with some Remarks on the recent Loss of His Majesty s ship Thetis. By 
Peter Barlow, Esq. F.R.S. Cor. Mem. of the Inst. France, Imp. Acad. 
Sciences St. Petersburgh, Acad. Sciences Brussels, 8$c. 8$c. 
Read April 21, 1831. 
On presenting the following remarks to the attention of the Royal Society, 
I feel it necessary, first, to apologize for their not possessing that degree of 
scientific novelty which is generally expected in such communications, — and 
secondly, that in enforcing my argument, I may perhaps seem to give more 
value to my own investigations than is consistent with good taste. I must, 
however, either in some measure do this, or leave the evil untouched, which 
I feel it my duty to endeavour to remove ; and therefore, trusting to a liberal 
interpretation of my motives, I shall state without reserve such facts as 
appear necessary to establish the object I have in view, and it is hoped the 
want of novelty will be compensated by the importance of the subject under 
consideration. 
That a ship’s compass is subject to a deviation from its true direction in 
consequence of the attraction of the iron used in the construction and appoint- 
ments of the vessel, is now too generally admitted to require any argument, 
although I believe there may be still some few officers who are sceptical on this 
point ; I have at least been seriously assured by one of rank and long standing, 
(t that there certainly was no local attraction when he was at sea.” Now there 
is really more in this observation than one would imagine, for there can be no 
question that forty years back the error arising from this disturbing force was 
very inconsiderable to what it is at present ; every year in fact increasing 
the amount, and rendering a correction of the error more and more necessary. 
This increase is occasioned by the immense quantity of iron now employed in 
the construction of a ship of war and its appointments. At the period above 
