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

Abstract

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 corrections recommended by Captain Flinders, who found that in every ship a compass would differ very materially from itself on being removed 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; accordingly, a standard compass was placed in the Isabella exactly amidship 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.