AN

INTRODUCTION

TO THE ART OF

NAVIGATIO N.

BEFORE

EFORE we begin Navigation, it may not be improper to give the Learner fome Idea of the Syftem of the Universe, commonly called the Solar, or Copernican Syftem, which is as follows: The Sun, that immenfe and amazing Globe of Fire, the Fountain of the Heat and Light of the whole Syftem, is placed near the common Center of the Orbits of fix opaque Spherical Bodies, which make their Revolutions round it in lefs or more Time, according to their several Distances from it.

Mercury is nearest to the Sun, and receives its Light and Heat from it, and revolves round it in an Ellipfis in two Months and twenty-eight Days.

Venus is fomewhat higher in the Syftem, and describes its Ellipfis round the Sun in 7 Months and 15 Days,, and becomes our Evening and Morning Star by Turns.

The Earth is next to Venus, and defcribes an Ellipfis round the Sun in 365 Days, or one Year, which being at a greater Distance from the Sun than the former Planets, and therefore receiving lefs of its Light and Heat, to make up the Deficiency, the wife Author of Nature has caufed a fecondary Planet, called the Moon, to move round it in 27 Days, 12 Hours, and 44 Minutes; it receives its Light and Heat from the Sun, and reflects it upon the Earth, which in fome Measure compenfates for the Abfence of the Sun, during the Winter Seasons, in the North and South.

Mars is ftill higher in the Syftem and takes a larger Circuit, revolving round the Sun in 1 Year, 10 Months, and 22 Days.

Jupiter is the largest of all the Planets, and defcribes a large Ellipfis round the Sun in 11 Years, 10 Months, 27 Days; there are four Satellites or Moons moving round it; they receive their Light from the Sun, and reflect it upon their primary Planet, as the Moon does upon the Earth.

Saturn is the moft remote of all the Planets, revolving round the Sun in 29 Years, has 5 Moons which move round him, and is alfo furrounded with a prodigious Ring or Atmosphere.

The Fixed Stars are supposed to be of the fame Matter with the Sun, and made for the fame Ends; each of them the Center of its

2

own

own proper System, having Planéts moving round it as our Sun has.

Comets are a Sort of Planets moving round the Sun in Ellipfis fo very oblong, that their vifible Parts feem to be in a Manner Parabolical, but have fuch vaft Atmospheres about them, and Tails derived from the fame, efpecially when they come near the Sun, as imply them defigned for very different purpofes from the other Planets.

Having given a curfory View of the Syftem of the Universe, we fhall now confider the Earth a little more particularly; a perfect Knowledge of the Figure and Motion of which, with various real and imaginary Lines upon it, is abfolutely neceffary in the Art of Navigation.

The Land and Water of this Earth or Planet upon which we live, make a Compofition of a Spherical Form, or rather an Oblate Figure, called the Terraqueous Globe, which by turning round its Axis every 24 Hours, from Weft to Eaft, caufe all the heavenly Bodies to revolve apparently from Eaft to Weft in the fame Time, making the Viciffitudes of Day and Night; and this Earth, together with its Moon, by moving round the Sun in 1 Year, or in 365 Days, 6 Hours nearly, produce the Seasons of the Year, viz. Winter, Summer, Autumn, and Spring.

That the Earth is round, or nearly fo, will appear, not only from the circular Shadow it has upon the Moon when that Body happens to be eclipfed by it, but alfo from the very Appearance of the Sea, and the many Obfervations made by Perfons ftanding upon the Shore, and viewing a Ship departing from the Fort; they first lose Sight of the Body of the Veffel, whilft they can ftill fee the Rigging and uppermoft Sails; but as the Ship recedes farther, they lofe Sight of these alfo, as if the Whole were funk into the Deep. Again, in a Ship making towards land the Mariners first descry the Tops of Steeples, Trees, &c. pointing above the Water; next, they fee the Buildings themselves; and laftly the Shore, which can only be the Effects of the Earth's Rotundity.

Its being a Globe, is alfo confirmed by the many Voyages which have been made round it, from Eaft to Weft: First by Magellan's Ship in the Years 1519, 1520, 1521, in 1124 Days; by Sir Francis Drake in the Years 1577, 1578, 1579, 1580, in 1056 Days; by the late Lord Anfon, in 4 Years; and lately by the Captains Byron, Carteret, Cook, and Clark, accompanied with feveral able Mathematicians and Naturalifts, whofe Obfervations and Discoveries do Honour to this Nation, as well as greatly contribute to the Improvement of Geography and Navigation: they have difcovered many Iflands in the South Seas, which formerly were unknown to Europeans.

The little Unevenneffes of the Earth's Surface, arifing from the Hills and Vales, is no material Objection to its being confidered as

rouud, fince the higheft Hill or Mountain bears not fo great a Proportion to the Bulk of the Earth itfelf, as the little Rifings upon the Coat of an Orange, bear to the Bignefs of that Fruit.

In order to defcribe the Pofition of Places, Geographers have found it neceffary to imagine certain Circles drawn upon the Surface of the Earth; to which they have given the Names of Equator, Meridian, Horizon, Parallels of Latitude, &c.

1. The Axis is a ftraight Line, imagined to pass through the Centre of the Earth; the extreme Points are the Poles, on which the Earth is fuppofed to move, one called the Artic or North Pole, and the other the Antartic, or South Pole.

II. The Equator is a great Circle under the Equinoctial Line in the Heavens, compaffing the Earth in the Middle, between the two Poles, and divides it into two equal Parts, called the Northern and Southern Hemifpheres: From it the Latitude of Places is reckoned either North or South; and on it are counted the Degrees of Longitude from Eaft to Weft. This Circle is called Equator, because when the Sun comes to it, which is twice a Year, viz. about the 21st of March, at his Entrance into Aries, and again into Libra about the 23d of September; then, making equal Day and Night throughout the World.

III. The Meridians are Circles which pafs through the Poles of the Earth, the Zenith and Nadir; croffing the Equator at Right Angles, and divide the Earth into two equal Parts, one Eaft and the other Weft; and is fo called, becaufe, when the Sun comes to the Meridian of any Place, it is then Noon or Mid-day. They are infinite in Number, for all Places from Eaft to Weft have their feveral Meridians; of thefe one is called the firft or chief Meridian, from which the Longitude of Places is reckoned; it is of fpecial Note and Ufe, but varioufly placed by Geographers, fome placing it at London, others at Paris, Teneriff, &c. and is divided into twice 90 Degrees, numbered from the Equator towards each Pole ending in 90 Degrees. And fince the Earth turns once round its Axis in 24 Hours, every Point upon its Surface defcribing a Circle of 360 Degrees in that Time, therefore, any Place lying 15 Degrees to the Eaft of us, has the Sun upon its Meridian one Hour fooner; or it is Twelve o'clock with the Eaftermoft, when it is Eleven with us; and any Place 15 Degrees to the Weftward of us, has the Sun one Hour after

us.

IV. Latitude is the nearest Distance of any Place from the Equator; it is meafured on an Arch of the Meridian intercepted between the Place and the Equator, and therefore can never exceed 90 Degrees. It takes its Name according as the Place is fituated, either North or South of the Equator; therefore, all Places that lie at the fame Distance from, and on the faine Side of the Equator, are faid to be under the fame Parallel of Latitude. Whence it follows, that if a Ship fails from a North Latitude directly North, or in a

South Latitude directly South, the encreases her Latitude equal to the Distance failed; and if a Ship fails in North Latitude Southerly, or in South Latitude Northerly, the decreases her Latitude, because fhe fails nearer to the Equator; from whence the Latitude is counted upon the Meridian: but if a Ship fails from a South Latitude into a Northerly one, or from a North Latitude into a Southerly one, from the Difference of Latitude fubtract the Latitude left, and the Remainder will be the Latitude come to, and of a different Name with the Latitude left.

Parallels of Latitude are Circles parallel to the Equator.

The Difference of Latitude is an Arch of the Meridian contained between the two Parallels of Latitude; or it is the leaft Distance of the Parallels of Latitude of two Places: fhewing how far one of them is to the Northward or Southward of the other, and can never exceed 180 Degrees.

V. The Longitude of any Place on the Earth is expreffed by an Arch of the Equator, fhewing the Eaft or Weft Distance of the Meridian of that Place, from fome fixed Meridian, where Longitude is reckoned to begin.

Difference of Longitude is an Arch of the Equator, intercepted between the Meridians of two Places, fhewing how far one of them is to the Eastward or Weftward of the otherr.

As Longitude begins at the Meridian of fome Place, and is counted from thence both Eastward and Westward, till it meets at the fame Meridian on the oppofite Point, therefore the Difference of Longitude can never exceed 180 Degrees.

VI. The Horizon is that apparent Circle which limits or bounds the View of a Spectator on the Sea, or an extended Plain; the Eye of the Spectator being always fuppofed the Centre of his Horizon. Every Part of this Circle is 90 Degrees from the Centre of it over our Heads, which Point is called the Zenith; and the Point of the Heavens oppofite to it, or under our Feet, is called the Nadir.

When the Sun or Stars come above the Eastermoft Part of the Horizon, they are faid to rife; and when they defcend the Western Part they are faid to fet.

When a Ship is under the Equator, both the Poles appear in the Horizon, and, in Proportion as the fails towards either, or increases her Latitude, that Pole is feen proportionably above the Horizon, and the other difappears as much; but when a Ship is failing towards the Equator, or decreases her Latitude, fhe depreffes the elevated Pole; that is, its Distance from the Horizon decreases : confequently the Latitude of a Place is always equal to the Elevation of the Pole above the Horizon.

This Circle is reprefented by the Mariner's Compafs, divided int 32 Points or Rhumbs, each 11° 15'.