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Anchor, or lie by to wait the Tide, if you know you are too foon; or what fail to make to fave your Tide, if you fear being too late: And these four Things I fhall handle in this Part, and that in fuch a Manner, as may be intelligible to the meaneft Capacity, and moft ufeful as well as moft eafy to be put in Practice at Sea.

TH

SECTION I.

Of the Variation of the Compass.

HE Way commonly taught for finding the Variation of the Compafs, is by the Sun's Azimuth, or Amplitude; but these Ways not being attainable by any but them that have learnt fomething of Aftronomy; and being alfo treated of in other Books, I fhall not trouble the Learner with them, but proceed to a far easier Way, which is this:

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When you are at Sea, and defire to know the Variation of the Compass, take your Quadrant about 8, 9, or 10 a Clock, when you fuppofe the Sun is near about half up from the Horizon to the Meridian, and take an Obfervation of the Sun's Altitude, as you would do at Noon to find the Latitude of the Place, which being done, lay by your Quadrant (letting the Vanes remain unremoved) and by your Azimuth Compafs (if you have one on board) fet the Sun, and mind what Point of the Compafs the Sun is upon at that Observation. This done, wait till the Afternoon, that the Sun grows almost as low as he was when you observed in the Fo enoon, and then with your Vanes fix'd, as in the firft Obfervation, obferve till the Sun be fo low, as that the Vanes fo fixed will juft take the Sun's Altitude without altering them; which done, obferve immediately (as before) upon what Point of the Compafs the Sun is at M

that

that Obfervation: Then the Space between that Point which the Sun was upon at the first Observation, and that Point upon which the Sun was at the laft Obfervation, divided into two equal Parts, the Middle is the true South Point of the Compafs, and the Distance between that and the South Point of the Card, is the Variation required.

Example.

Suppofe at the Forenoon Obfervation, I find the Sun by the Compass to be South Eaft (it matters not what his Altitude be, fo you mind what it be, or elfe let the Vanes ftand unremoved, till the Afternoon) and fuppofe in the Afternoon I find, when the Sun hath the fame Altitude, that he bears Weft South Weft; now the Distance between South Eaft and Weft South West is ten Points, the Half of that is five Points, which reckononed from South Eaft towards the Weft South Weft, it falls upon the South and by West; therefore I conclude that the South and by West Point of the Compafs points to the true South Point, and the, Diftance between the South and by Weft Point (which is the true South) and the South Point of the Compafs (which we may call falfe South) or (magnetical South) is the Variation of the Compafs; and because the magnetical South is Eaftwards from the true South; therefore the magnetical North is Weftward from the true North. Here I conclude, that the Variation is one Point Westerly, &c.

In this Cafe there is only this Caution to be obferved, viz. that this Obfervation be not made when the Ship is running very faft Northwards or Southwards, which may make fome fmall Error, though fcarce difcernable; for if the Ship ftood ftill, the Sun would have exactly the fame Altitude at 8, 9, or 10, in the Forenoon, that it would have at 2, 3, 4, in the Afternoon; but if the Ship fails very faft to the Southward in North Latitude, or to the Northward in South Latitude, the raiseth the Sun a little, and by Confequence the Sun will be fomewhat higher at 4 in the Afternoon than at 8 in the Morn

ing,

ing, and may cause fome Error, but it is little, and a Thing that feldom happens: And if it do happen that your Courfe be North or South, and the Wind fo fair, you may refer your Obfervation till another Day, (it being not neceffary to fet the Variation every Day) and thereby the Error may be avoided, and yet the Variation exactly found as often as is neceffary.

Note; Í fhall fhew how to find the Variation by the Sun's Azimuth and Amplitude hereafter in the Aftronomical Part.

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How to divide the Log-line, and try a HalfMinute-Glafs.

HEY that take upon them to be Mafter, Mate, or

THEY

in the keeping a Reckoning or Account of the Ship's Way, ought to take Care before they go from the Shore, to be furnifhed with all Things neceffary for that Purpose. For the beft of Scholars, the greateft Artifts, the most profound Mathematicians, or the moft experienced Navigators, may be deceived, and carried into the grofs Errors, by a Defect in their Inftruments, or Means for keeping a Reckoning, as well as the moft ignorant may be by a Defect in their Knowledge; and fo far as they defign that their Account fhall not depend upon their Dead Reckoning, they ought chiefly to be careful in these two Things.

First, the Half-Minute-Glafs, that it be of a due Length; but if it be longer than it should be, it makes the Ship by Eftimation to run fo much more than indeed fhe does, and by that Means perhaps, in a Month or 6 Weeks Sailing, you will expect to arrive at your Port, or make fuch or fuch Land, when indeed you are 50, 60, or 80 Leagues, or more or lefs fhort of it, according as the Error of your Glafs is more or lefs. And altho' it is hard to know a true Half-Minute-Glass,

M 2

yet

yet there are thefe two Ways to prove them, and know whether they are right or wrong.

The firft Way is by an Expedient mentioned by Mr. Henry Philips, in his Advancement of the Art of Navigation, and alfo quoted by Seller, in his Practical Navigation, and 'tis this: Take a Bullet of any competent Weight, it matters not, and make faft to it a Piece of fine Thread, or Silk, of the juft Length of 38 Inches; let there be at Noofe on the End of the Thread, and let the very End of the Noose be juft 38 Inches from the Center of the Bul플 let, (as I faid before) then hang it up by the Noofe upon a fmall Pin, where it may hang at Liberty, and fwing freely, and fo give it Way, and each Swing fhall be a true Second of Time; that is, each Time that it paffes by the Perpendicular let fall from the Pin on which it hangs, fhall be a Second; and every Time of its Return to the Place where it first begun its Motion, is two Seconds of Time, and a Glafs that runs till the faid Bullet hath made 30 Swings fhall be a true Half-Minute-Glafs.

A fecond Way (if it may properly be fo called) is by the Experience of thofe that have had Occasion to use a Glafs in long Voyages; and having a Line rightly divided, by a Glafs of fo true a Length, that their Dead-Reckoning, when carefully kept, hath agreed with the Truth of their Obfervations; and that their making of Land, &c. hath fallen out according to Expectation, by the Dead-Reckoning; I fay fuch a Glass, or another of the fame Length, ought to be preferred before any other, as a true Half-Minute-Glafs.

A fecond Thing neceffary, in order to the Keeping of a true Reckoning, is to take Care that the Log-line be rightly divided; for although the Giafs be true, yet if the Log-line be divided into Knots too long or too fhort, it must needs make an Error in the Reckoning, according to the Proportion of the Error in Divifions of the Line, if you work by a true Half-Minute-Glass. Indeed, if the Divisions of the Logline be too fhort, and the Glafs alfo too fhort, (or if both be too long, which is the fame) then the one Error helps to compenfate the other; but if the Faults in the Line and in the

Glafs

Glafs be contrary, that is, the one too long, and the other too fhort, the Fault is intolerable.

As for the Length of each Knot on the Log-line, or how it fhould be divided, there are different Opinions amongst different Authors and Navigators. Indeed it is an undeniable Truth, and apparent to all Men's Reafon, that oneKnot upon the Log-line fhould be the 120th Part of a Mile; because half a Minute is the 120th Part of an Hour (for as the Whole to the Whole, fo a Part to the Part, &c.) but the Difficulty arifes from the different Opinions, as to how many Feet, Yards, &c. there is in one Degree of a great Circle upon the Earth. Mr. Oughtred, in his Circles of Proportion, will have 66 Miles to answer one Degree upon the Earth, each Mile containing 5280 Feet. Hence there is by his Account, 349800 Feet in one Degree of a great Circle upon the Earth, and 5830 Feet in one Minute, or 60th Part of fuch a Degree, and confequently the 120th Part of a Minute, or Length of one Knot upon the Logline, must be 48 Feet.

But Mr. Norwood, in his Seaman's Practice, P. 43. (relating an Experiment of his for finding the Quuntity of one Degree of a great Circle upon the Earth) faith, that one Degree contains 367200 of our English Feet to a Degree, which Account, without any Allowance would give 51 Feet to one Knot of the Log-line, although, for Reasons there mentioned, he allows i Foot out of the 51, and fo would have one Knot of the Log-line to be juft 50 Feet: But how far that Experiment of his is to be depended upon, (confidering the Unevenness of the Ground, and Crookednefs of the Ways and other Inconveniencies, which he could only give Allowance for according to his Judgment) and alfo how far that one Foot in 51 may compenfate the "Way that the Log makes after the Ship, I fhall not take upon me to determine. In the mean time I fhall, with Submiffion to better Judgments, rather adhere to the Way of dividing the Log-line, that is commonly received, and used by moft Mariners, I mean that of 42 Feet, or 7 Fathom to one Knot. Now it will prefently be objected, that according to that Divifion for Half a Minute, multiplying that

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