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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    [ * 6 ] Hydographer, dedicated to Willyam Borough, Comptroller of her Majefties Navie, printed by Thomas Kingfton, Anno 1581, the following Obfervations were communicated by John Hyde, Efq; viz. In Page 15 he fays, North Pole dips under the Horizon 7id. 50m. Annext is upon the Variation of the Compafs by W. B. In 3d Chapter he fhews the Variation at Limehouje, the 16 Ottober, 1580. in Latitude 5id. 32m, North, was one Point Eafterly.— n Chap, at Antwerp it was qd. Eaft. — By Mercator, 12 Chap, at Ratijbon, in Latitude 48d. 52m. North, was iid. 45m. Eaft. — By Ger¬ ard Mercator at Waygatz, in Latitude 7od. 00m. North, was 7d. Weft.—The Variations here fox London, Antwerp, and Waygatz, are the fame as in Wright's Tables; it is therefore more than probable that he took them from Borough's, as Wright's firft Edition was not publifn’d until the Year 1599 ; and what adds to this Pro¬ bability, may be gathered from the fame in Page 72, containing Obfervations made by himfelf. “ Thefe Obfervations (fays he) made in the Town of Fayall, “ and the former alfo at St. Cruz in Flores, were taken by Mr. “ William Borows's Inftrument of Variation (publifhed in his Book “ of the Variation of the Compafs) and by a Quadrant v/hofe Se- “ midiameter was almoft three Foot.” In Page 84 he makes the Inclination or Dip of the Magnetic Needle in the Latitude of Lon¬ don 5id. 32m. North, to be about 73d. by Calculation. The following are Dr. Bradley's Obfervations made at the Royal Obfervatory at Greenwich. 1749 Jan. 7 1750 June 24 1752 Aug. 28 1754- 7une 2 5 1756 Nov. 16 1757 AuZ' 5 Variation Weft. Dip. 17d. 22-|m. ■-- 73d . 30m. 17 25 73 30 3 7 55 - 73 25 — 18 03 -* - 73 2 5 — 18 33 -* - -■ 73 25 ~ 18 30 to 34- - 73 15 to 25 Hence, by comparing the foregoing ancient Obfervations with thofe of Dr. Bradley's, and the prelent Chart for the fame Places, may not only be difcovered what fmall Alteration is made in the Inclination or Dip, during the Space of about 170 Years laft paft ; but alfo the very confiderable and furprizing Mutation of the Va¬ riation, both in Quantity and Quality. FINIS.