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

Abstract

Captain Wilson being engaged in the prosecution of Mr. Barlow’s inquiries as to the laws of the deflection of a needle by an iron shell, had remarked, while in China, that when a magnetic needle was placed in the equator of an iron shell, though no deviation arose when the compass was in its natural state, yet when one end of the needle was deteriorated, by touching- it with the pole, of its own name, of a magnet, a deviation then arose; and communicating the facts to Mr. Barlow, he proceeded to investigate the laws and amount of the deviation so arising. He distinguishes the action into three several cases:— 1st. When the needle is on any part of the magnetic meridian of the ball. In this case there is no deviation caused by the primary of the shell, nor any secondary deflection produced by deteriorating one pole of the needle.