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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    ( ) tabulated his Obfervations thereupon, together with Bon-George Juan's, another eminent Mathematician and affociate in this Ex¬ pedition : Both thefe he has aranged and compared with Variations extradled from our former Chart, of fimilar Latitude and Longi¬ tude ; all which agree to his Satisfaction, as appears by his own particular Account thereof, given in Tom i„ page 14, &c. of his Book publifhed at Paris, in the Year 1752: But had he made fome adequate Allowance for the Mutation of the Variation, between the Years 1735, and the Date of our Chart, (which we prefume he has not) it feems evident from his own Tables, that they would have been much more concurrent. This Gentleman further recites the excellency and ufe of thefe Variation Charts in the Practice of Navigation, and ftrongly re¬ commends to the Mariners, that due Attention be given in the choice of their Compares, and not only that they be well exe¬ cuted, but alfo that they be tried and approved upon a Meridian Line, by Perfons of Knowledge and Judgment. In Page 18, he has the following Paffage, viz. “ I;*y encore 44 dans la Navigation une erreur, a quoi les Pilotes font fort Su- 44 jets, quoique moins par leur propre faute : c’eft de governer de 44 Vaiffeau par une Aiguille, et d’ obferver la Variation par une 44 autre, &V. which may be tranflated into Englijh, as here fol- 44 loweth, vm. 44 There is yet again another Error in Navigation, to which the 44 Mariners are excreamly fubjeCt, altho’ it be not quite fo much 44 their own fault; and that is, to Beer the Ship by one Com- <c pafs, and obferve the Variation by another ; For although they 44 have been compared, and due Notice taken wherein they differ, 44 as their Motions are not alike, were there only at the Beginning 44 of aVoyage, fewDegrees of Difference between them, theMoti- 44 on, which the former makes continually upon the Pivot, renders 44 it more inactive than the other, which is only made Ufe of for 44 Obfervations, and at all other Times preferved with Care j Hence 44 it comes to pafs, that there remains a Difference in the Altera- 44 tions. To re&ify which, it would be neceffary that all Corn- 44 paffes intended for Sea-Service, fhould alfo be equally fitted for 44 Obfervations of the Variation, and that thefe Observations fhould 44 be made with the fame Needle of Compafs, by which the Ship’s 44 Courfe is dire&ed •, And, in order to have the full Advantage of V the Variation Charts, it would be highly necefTry that the Needles 44 fhould all be touched by one and the fame Manner and Method, 44 and adjufted to the Meridian of the Place precifely, with the 44 Variation which is there known to be ftri&ly true. If this Me- 45 thod
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