XVIII. On the secondary deflections produced in a magnetized needle by an iron shell, in consequence of an unequal distribution of magnetism in its two branches. First noticed by Captain J. P. Wilson, of the Honourable East India Company's Ship Hythe. By Peter Barlow, Esq. F. R. S. Mem. Imp. Sc. Petrop

Abstract

Captain Wilson, in a very early stage of my magnetic experiments, took considerable interest, not only in their application to the purposes of navigation, but in the fundamental laws on which that application was founded; and in pursuit of his own particular views he undertook, in his last voyage, to decide, by actual experiment, some of the points which were not in the beginning universally admitted; amongst which one of the most important, was that relating to the change of position of the ideal magnetic sphere which I had imagined for the purpose of magnetical reference, and for the convenience of reducing the laws of action to their most simple and general form. According to the results which I had obtained, it was presumed, but not confidently asserted, (see page 65, 1st edition “Essay on Magnetic Attractions") that this sphere, in different parts of the world, would take up different positions with regard to the horizon, following in all cases the changes of position of the dipping needle. Captain Wilson proposed to repeat these experiments at different places, where he might have an opportunity, on his outward and homeward voyages; viz. at St.Helena, Bengal, and in China; and he had the satisfaction of finding the sphere nearly inverted in his different experiments, the law in all cases strictly following those which had been predicted from my first series of results. The confirmation however of this fact is now of but secondary interest, because it has been demonstrated, and admitted by all who have taken any part in the enquiry; I shall therefore pass over these experiments, and mention only the curious observations alluded to in the title of this Paper.