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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    Capt. George Snow, fumifhed a confiderable Number of Obfer¬ vations, made with great Care and Accuracy by himfelf, in feveral fucceflive Voyages to, and from Barbadoes and Virginia, together with feveral Remarks upon the Subject. Mr. Mungo Murray, Author of a Treatife on Ship Building, prefented us with feveral Obfervations taken on Board the Prime Edward and Chejierfield Indiamen, and His Majefty’s Ship the Neptune. All which Affiftance is gratefully acknowledged. Among the Journals which we examined, it was found. That fome contained no Obfervations of the Variation. That in fome wherein fuch Obfervations were found, no Ac¬ count of Difference of Longitude was kept. That few contained Obfervations made near at Home (as fup- pofmg the Variation there generally known) in any Harbour, or even within fight of Land. 1 hat, where the Account of the Difference of Longitude was kept, feveral Infiances have been obferved in Journals of identical Voyages, wherein the Ship’s Place, at the fame Time, has been differently determined; And, Some Journals kept on Board Ships in the fame Fleet, have differed more confiderably. Obfervations, therefore, of the Variation, made under any of the above Difadvantages (however ferviceable they might have been) were rejected •, a confiderable Number occuring, which were made by thole, whofe Accounts were as corred: as the Practice of Navigation will permit. But the Difference of Longitude made between two Ports, by different Ships, or by the fame Ship in different Voyages, frequent¬ ly disagreed with each other, and with the Chart. Therefore in order to apply Obfervations made under fuch Cir- cumftances we were obliged to reduce the Difference of Longitude by Account, to the Standard of the Chart; for Lnftance, let us fuppofe a Ship’s Journal makes the Difference of Longitude from the Weft-End of Madeira to Barbadoes, to be 44 deg. 44 min. and that according to the Chart \ it is but 40 deg. 40 min. fince it can¬ not be granted, that the Error of 4 deg. 4 min. can a rife at once; ’twill therefore be more rational to fuppofe it diffufed through the whole Voyage ; and then to rectify it, there mu ft be a Decreafe of one Degiee in eleven; now if an Obfervation of the Variation was made, when according to Account, the Ship had made 30 deg. 48 min. Difference of Longitude from Madeira; if the eleventh Part, thereof, {viz. 2 deg. 48 min.) be taken therefrom, the Re¬ mainder