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Having found the Sun's true Amplitude, or Azimuth, by the preceding Methods, &c. Magnetic Amplitude or Azimuth by Obfervation, it is evident, that when they agree there is no Variation; but when they difagree, then, if the true and obferved Amplitudes be both of the fame Name, that is, both North or both South, their Difference is the Variation; but if the true and obferved Amplitudes be of different Names, that is, one North and the other South, their Sum is the Variation. Again, if the true and observed Azimuths be both on the Eaft, or both on the Weft Side of the Meri-> dian, their Difference is the Variation; but if the true and obferved Azimuths be one on the Eaft and one on the Weft Side of the Meridian, their Sum gives the Variation; and to know whether the Variation is Eafterly or Wefterly, obferve this general Rule, viz.

Let the Obferver's Face be turned to the Sun, then, if the true Amplitude or Azimuth be to the Right Hand of the Magnetic, or Obferved, the Variation is Easterly, but if to the Left Hand, Wefterly.

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Suppofe the Sun's true Amplitude at Rifing is found to be E. 14° 20' N. but by the Compass it is found to be E. 26° 12' N. required the Variation?

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Which is Easterly, because in this Cafe the true Amplitude is to the Right of the Obferved.

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With the Chord of 60 defcribe a Circle to represent the Horizon, through which draw the North, South, Eaft and Weft Lines; take the true Amplitude found by Calculation at Rifing, 26° 12' from the Line of Chords, and fetting it from E. towards N. and likewife the the Magnetic Amplitude 14° 20', and fet from E. towards N. as before, the Difference of these two Angles, or between the true and Magnetic Amplitudes, viz. 11° 52' is the Variation. Now suppose yourself placed at the Centre of the Horizon, reprefented by the Compafs, and looking towards the Magnetic Amplitude at the Sun's Rifing, it is plain that the true Amplitude found by Calculation is towards the left Hand of the Obferved, which fhews the Variation is 11° 52′ W. and must be allowed to the left Hand in all Courses steered, before they can be put into the Traverse Table or Bearings, taking by the Compass.

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Suppose the Sun's true Amplitude at Setting be W. 34°26′ S. and his Magnetic Amplitude W. 23° 13′ S. required the Variation, fince they are both of the fame Name?

From the greater

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Remains the Variation

W. 34° 26'S.

W. 23 13 S.

11 13W.

Which is Wefterly, because the true Amplitude is to the Left of

the Obferved in this Cafe.

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Let N. E. S. and W. reprefent the Horizon, A, B, C, D, an Azimuth Circle, paffing through the Sun's Centre; now an Obferver placed at the Centre G, will fee the Sun at Rifing in the

Line G 1, but when he gets a greater Altitude, and arrives at B, he will fee the Sun in the Line GO 2, and as the Sun alters its Altitude, will be feen as in the Lines GO 3, G4, GO 5, at Length will arrive at its Meridian G z S, and the Figures 2, 3, 4, 5, will reprefent the Magnetic Azimuth; the Difference between these and the true Azimuth found by Calculation is the Variation.

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The Use of the Variation is to correct the Courfe fteered by the Compafs; when the Variation is Eaft, it must be allowed to the Right Hand upon every Courfe fteered quite round the Compass; but when the Variation is Weft, to the Left Hand.

NOTE. The Variation may be eafily found by taking the Sun's Altitude in the Morning, and obferving what Point of the Compass he bears upon; and in the Afternoon when the Altitude is the fame, the Middle Point will be the true Meridian, the Difference between

which and the North or South Points of the Compass is the Variation. If the Altitudes are taken at 5, 6, or 7 o'Clock in the Morning, you will have the fame Altitude at 5, 6, or 7 o'Clock in the Evening, being equally distant from Noon.

The Variation of the Compafs was firft obferved at London, in the Year 1580, to be 11° 15' Easterly; and in the Year 1622, it was 6° o' E. ftill decreafing, and the Needle approaching the true Meridian, until it coincided with it in the Year 1662, fince that Time the Variation ftill continues at London to increase Wefterly, at the Rate of about 11 or 12 Minutes every Year; and is at this Time about 23° 30' Westerly, and in the English Channel about 25° 30' Westerly; but how far it will go that Way, Time and Obfervations will probably be the only Means to discover.

The Variation at Paris in the Year 1640, was 3° E. but in the Year 1681 it was 2° 21′ W. and is now about 22° 20' Wefterly, ftill continuing to go Wefterly.

In fhort, from Obfervations made in different Parts of the World, it appears, that in different Places the Variation differs, both as to its Quantity and Denomination, it being Eaft in one Place, and West in another; the true Cause and Theory of which has not yet been discovered, and therefore in long Voyages it is abfolutely neceffary that the Mariner fhould find the Variation of the Compafs by Obfervation as often as poffible.

One Magnus, a Shepherd, first discovered the wonderful Power of the Loadstone, that gives Polarity to the Needle, by its flicking to the Iron of his Sandals; whence the Name of Magnet was given to the Stone or Magnetic Needle. Gio, of Naples, about 300 Years ago, firft discovered that a Piece of Iron rubbed on it, and then fufpended, had the Property of pointing to the North and South, and thence applied to Navigation. An Author of fome Degree of Popularity, who has drawn a Variation Chart, fays, he has found out the Theory of the Variation of the Magnetic Needle, without favouring the World with the Principles; if his Genius has been able to penetrate into the Bowels of the Earth, to discover this wonderful Phenomenon, it is more than Sir Ifaac Newton's, or Dr. Halley's could do; however, his Veracity may well be doubted, as Mr. Wales, who accompanied Capt. Cook round the World, told me, that his Theory did not reach those Islands which they difcovered.

How to touch the Compafs Needles.

Having two ftrong Magnetical Bars, lay the Compass Needle as nearly North and South as you can, with the intended North and Northwards, join the two Magnets in a Line, confiderably above the Needle, the North End of each being Northward, and bring them down upon the Needle, so that the Place of Junction may be over its Centre, then draw them afunder along each half of the Needle, and continue their Motion till they are eight Inches clear of the Needle's End; by a circular Motion bring them again to the

Centre and join them as before; repeat this Operation fix or feven Times, taking Care pot to put the Magnets out of their Parallelifin, and the Needle will be fufficiently Magnetical.

The Method of Keeping a Journal at Sea.

Y keeping a Journal, is meant keeping fuch an Account of the

Bee way that the Mariner may be able at any Time to

afcertain the Latitude and Longitude the Ship is in; it therefore should be the great Concern of every Perfon who takes upon him the navigating of Ships to remote Parts, to be expert therein, as the Lives and Fortunes of fo many Men are committed to his Charge.

When a Ship is bound from one Place to another, which lies fo far from her, that he is obliged to go out of Sight of Land for any confiderable I ime, as from England to Jamaica; at the Time of her leaving Sight of Land, fhe is faid to take her Departure, and that Part of the Land the then leaves, is faid to be the Place fhe takes her Departure from; fuch as the Land's End, Lizard, &c. and at the Time of taking fuch Departure, the Captain or Mate generally takes the Bearing and Distance of that Land (according to his Judgment) and fets it down on the Log-board, or in the Log--book against the Time it was taken, thus, Land's End N. N. E. Dist. 7 Leagues, or Lizard N. by W. Dift. 5 Leagues, &c.

In the fame Manner may the Departure from any Place be taken, as may be feen in the first Day's Log, of the following Journal, where the Log book is marked in Columns for Hours, Knots, Fathoms, Courfes, Winds, Leeway, Tranfactiors; and under it the Columns for Courses, Distances, Northings, or Southings, Eaftings, or Weftings, the Latitude by Dead Reckoning, Latitude by Obfervation, Meridian Distance, Difference of Longitude, Longitude in, and in the laft, Bearing and Distance of the Land.

Notice must be taken, that in the Column for Courfe, you are always to fet down the Courfe you have made by your Reckoning for that 24 Hours; that is, from the Noon of the Day before to the Noon of the Day you work on, the Sea Account being always kept from Noon to Noon.

In the Columns for Distance you are to fet down the Distance made by your Reckoning for that 24 Hours.

In the Columns of Northing and Southing, you are to fet down the Difference of Latitude made in that 24 H. marking the Column with North, if the Difference of Latitude-be North; and South, if South.

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