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zon, he at length intirely drop'd this Method alfo, as almoft unneceffary, and as ufually impracticable. However, a great Part of his Differtation upon this whole Propofal was read before the RoyalSociety, Feb. 20, 1728-9. But then,

On or a little before the Year 1730, he light upon a fourth Method for the Discovery of the Longitude, much better than any of the former; and that is, to enable Seamen to view the Eclipfes, Occultations, and Conjunctions of Jupiter's Planets on Shipboard: Which Eclipfes, Occultations, and Conjunctions are well known, by Aftronomers, to be the very best Ways of finding the Longitude, both at Land and Sea; could the Concuffion of the Ship be avoided, or could fuch a Telescope be made, as, notwithstanding that Concuffion, would ftill render thofe Phænomena visible.

Our Projector's first Thought was this: That in cafe the Tube of a reflecting Telescope, of 9 or 10 Foot long, could be made fo far tapering, or conical, as to bear a good Number of Object Glaffes, ground on the fame or equal Tool, and by confequence of the fame focal Length; while the Eye-Glafs was very large, and but a fingle one alfo; the Concuffion of the Ship would only make any lucid Point apparently to move from one Object-glass to another; but fo that the Point would still come to the Eye in the very fame Manner, as if there were, as ufual, but one Object-glafs only. This, upon Trial, proved to be Fact. A Telescope of this Conftruction, having feven Object-glaffes and one Eye-glafs, he exhibited before the Royal-Society, April 22, 1730, the very fame Evening, when Mercury was expected to touch or pass over the Disk of the Sun. After this, Mr.Whiston, in Company with others, took a View therewith of Jupiter and his Planets, which afforded them an entertaining Sight: For whereas, at firft, they faw but two of these Planets, they, in a little

time, faw three. One of the other parting itself into two; as having been in Conjunction, and now feparating one from another: Which is the only apparent corporal Conjunction of two of thofe Satellites that Mr. Whiston has yet heard of, and fuch as Mr. Lynn himself, a diligent Obferver, in full 30 Years, confeffes he never faw.


We come now to Mr. Whifton's fifth and laft, and, as he believes, a better and eafier Method than the former, of difcovering the Longitude at Sea, by Telescopes; and that ftill by the Means of the faid Eclipfes, Occultations, and Conjunctions; and this is by a reflecting Telescope, of his own Invention also. Some Intimation was given of this Method, in a Memorial laid before the Board of Admiralty, July 15, 1737. And the actual Discovery foon followed that Intimation. In the Memorial of July 15, Mr. Whifton tells the Lords of the Admiralty, that "One of the main Reasons of his long delaying "to produce his refracting Telescope (abovementioned) before their Board or the Publick, was, because he hoped a much fhorter reflecting Telescope, with one large reflecting Speculum, "and many fmall Eye-glaffes, might invert the "Problem, might be fo fitted as to perform the fame "equally with the other much longer refracting Te"lescope. This, he adds, he pitched upon, as a "ftill better Method, A. D. 1737, though he had "not hitherto been fo happy as to bring that great


Improvement to Perfection." In a Memorial, laid before the Board of Admiralty, August 12, 1737. he tells the Lords, that " Since the concave Speculum of a reflecting Telescope, of Mr. Gregory's Form, whofe leffer Reflecter is round, broad and plain, and has its focal Distances 4 or { 6, or 8, or 10 Foot, requires a Tube of no "more than 2 or 3, or 4, or 5 Foot long; and



the Eye-glafs need be but one, only not [very


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much broader than the Hole in the Speculum; this reflecting Telescope will perfect the great Improvement before wanted; and will be eafily "manageable on Ship board; and indeed feems to "him, by far, the best and fureft of all the aftro

nomical Methods for difcovering the Longitude "there: Which he accordingly proposed to that honourable Board, and to the Commiffioners for the Discovery of the Longitude, as what would "bring that Discovery to its utmost Perfection, and would be very cheap, and very readily. "practised at Sea by ordinary Seamen.

Upon the whole, Mr. Whifton now looks upon the Longitude in the open Sea to be difcoverable by two fure Methods of his own Invention, viz. by the refracting Telefcope, with many Objectglaffes, and by the double reflecting Telescope, with its one large concave Reflecter, and one fmall flat Reflecter; befides the Eye-glafs, which is of Course a Refracter in both Inftruments. And this he means over and above his Discovery of the Place of a Ship near the Shores, or within 80 geographical Miles of the Coafts, by Signals of Light or Fire'; to which, half the respective Rewards are allotted by the Act of Parliament; and over and above the Hope there is, that Mr. Harrison's curious Clock may be able to weather Storms at Sea, and to go evenly through them, from Europe to America: Which, if it fucceed, will be a third, or rather a fourth very valuable Method for discovering that Longitude alfo. But to proceed with the Hiftory.


At the latter End of the Year 1739, Mr. Whiffon was very defirous to make fome farther Progrefs in this Affair: And as he knew that Dr. Halley's Chart of the Variation of the magnetic Needle, made for 1700, was excellent in its kind, and would be still of very great Advantage to Navigation, if it were renewed, and fitted to the prefent Time; that Va


For JANUARY, 1742.2 riation being altered, even in the British Channel, about eight Degrees in thefe laft 40 Years; And as he knew that both Mr. Harrifon's Clock, and his own Telescopes, difcovering the Longitude in the open Seas, would be of little or no Advantage to Navigation till the Coafts were better determined; he defired Sir Charles Wager, firft Commiffioner of the Admiralty, to call the Commiffioners of the Longitude together, that he might lay thefe Matters before them: Which he readily did, and the Majority of the whole Number, then alive, being prefent at the Admiralty-Board, Nov. 24, 1739, he laid the following Memorial before them, and added large Explications of feveral of its Parts by Word of Mouth, for their more intire Information. The Memoral fets forth:



That the Perfection of the Art of Naviga tion depends on the frequent Discovery of the Longitude and Latitude of Ships at Sea; and "on the Knowledge of the Variation of the mag

netic Needle, in all Places where Ships do fail : "For want of the due Allowance for which Va "riation many Ships are believed to have been loft ic at Sea, especially in our British Channel.

"That before the Discovery of the Longitude stat Sea can be thoroughly ufeful to Navigation, "the Knowledge of the Longitude of the Coafts,

Ports, and Havens, is highly neceffary; and "that, in many Places, to a Degree far exceeding

what is hitherto known; and particularly in thofe " of Great Britain and Ireland, and the Planta

tions to them belonging; In which the French Aftronomers and Navigators are known to have "greatly outdone thofe of our Nation, altho' our "Skill in aftronomical Obfervations be not inferior, and our Trade and Navigation far fuperior to theirs.


"That the Longitude of fuch Places can almost "only be difcovered by Obfervations made fome"times of the Appulfes of the Moon to fixed Stars,

and more frequently of Jupiter's Planets, when "Jupiter is fufficiently remote from the Sun, and "chiefly when he is in northern Signs; which

two Characters excellently agree to the next five "Months, and to about eight Months in each of "the next five following Years.


"That there are feverál Inftruments relating to << the Discovery of the Longitude at Sea, and which promife great Advantages to Navigation, which ought to be thoroughly tried in the Ports "on Ship-board; and may be easily tried there

at the fame time that the Longitudes and Lati"tudes of fuch Ports are determined; fuch as Mr. "Hadley's and Mr. Smith's Catadioptric Quadrants, "Mr. Barfton's Quadrant, Mr. Whifton's refracting and reflecting Telescopes, Captain Elton's Quadrant, &c.

That, at the fame time, new and accurate Charts may be made of our Sea-coafts, for the Advantage both of Geography and Navigation. "That Dr. Halley made a moft ufeful Map of the Variation of the magnetic Needle, fitted to the "Year 1700. which Variation is yet fo greatly altered

fince that time, that the fame may now greatly "mislead fuch Mariners as do not carefully allow

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for that Alteration; which Variation may be a"new exactly observed on this occafion at all our

Ports, when their Longitude and Latitude are taken: And other obfervations, made fince 1700 by Capt. Middleton, Capt. Hoxton, and many others, may be compared, in order to the forming "a new Map of that Variation for the Year 1740. "That by Reafon of this Variation of the Variation, it is very fit that a new Map of fuch Variation be made at leaft every ten Years, and



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