An account of the methods used to describe lines, on Dr. Halley's chart of the terraqueous globe ; shewing the variation of the magnetic needle about the year 1756, in all the known seas ; their application and use in correcting the longitude at sea ; with some occasional observations relating thereto / [William Mountaine].

  • Mountaine, William, active 1736-1778.
Date:
[1757]
Catalogue details

Licence: Public Domain Mark

Credit: An account of the methods used to describe lines, on Dr. Halley's chart of the terraqueous globe ; shewing the variation of the magnetic needle about the year 1756, in all the known seas ; their application and use in correcting the longitude at sea ; with some occasional observations relating thereto / [William Mountaine]. Source: Wellcome Collection.

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    ed not of being able to draw Lines reprefenting the Variation at the Four different Periods of 1711, 1722, 1733, and 1744, and thence by Companion to have performed the fame for 1755; which if it could have been executed, would have prolonged the Ufe of thefe Undertakings confiderably. But the impracticability of this Scheme, will readily appear to thofe who will give themfelves the Trouble to examine the Situation of the Variation Lines in Dr. HALLEY’s Chart, and thofe laid down for the Year 1744, as alfo thofe in the prefent, in and near the Bay of Bengal: All that is neceffary to befaid concerning them, is to allure the Public, that no Lines on the Chart are defcribed on better Evidence than thofe \ and that the Delineations for the Periods before mentioned, altho’ they in a great meafure contribute to confirm the laft, yet would not have given any Thing near Information fufficient, to have formed fuch kind of Lines as now appear, without Obfervations made about the refpeClive times of ConftruCtions •, the fame may be faid at, and near about the Ifland Jamaica, where the Variation feems to have been almoit Stationary for 56 Years laft paft. Experience having thus pointed out to us the irregular Mutation of the Variation, any Expectations before retained, of re-con- ftruCting a Set of Lines by Analogy, now vaniihed, and we have been obliged to purfue our former tedious, but more fafe and juftifiable Method of proceeding, which was by collecting the greateft Number of Obfervations poftible. For which Purpofe, The Honourable the Commiflloners of the N AV T, obliged us with an Order of free Accefs to all their Matters Log-Books and Journals. The DireClors of the Honourable the Eaft-lndia Company, in¬ dulged us with the like Privilege. The Committee of the Honourable Hudfotf s-Bay Company, gave us a Set of Obfervations made in the Tracks of their Trade, and tabulated by fome of their Captains. Dr. James Bradley, Regius Profeffor of Aftronomy, and F.R.S. favoured us with feveral Obfervations made at the Royal Obfer- vatory at Greenwich. . • John Hyde, Efq-, F.R.S. communicated a Set of ufeful Obfer¬ vations, extracted from two Journals, kept on board the Triton and Britannia Faft-lndiamen. A correCt Journal kept on board the Delawar Eajl-lndiamany was handed to us by a Gentleman unknown. Cap