Wheeler gift 765. Barlow (1776-1862) was a mathematician, physicist and optician. In this essay he described experiments which led to the invention of a small device - an iron plate - that corrects ships’ compasses with regard to the deviation caused by the iron in their guns. Barlow corrects a rule developed by Captain Matthew Flinders and later modified by Captain Edward Sabine. Sabine had recently returned from John Ross’s expedition in search of a North West Passage, and indeed Barlow reports on experiments made using his new device that Sabine conducted on board the ship Isabella - one of two that took part in the Ross expedition - in Brassa Sound, Shetland. Barlow also reproduces as an appendix Sabine’s article for the Philosophical Transactions, ‘On the irregularities observed in the direction of the compass needles of H.M.S. Isabella and Alexander’, which discusses observations made during Ross’s expedition.