AN

INTRODUCTION

ΤΟ ΤΗΣ

ART OF NAVIGATION,

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 fountain of heat and light of the whole fyftem, is placed near the common centre of the or bits of feven opaque fpherical bodies, which make their revolutions round it, in lefs or more time, according to their feveral distances from it,

Mercury is neareft to the Sun, and receives its light and heat from it, and revolves round it in ellipfis in two months and twenty-eight days.

Venus is fomewhat higher in the fyftem, and defcribes its ellipfis round the Sun in feven months and fifteen days, and becomes our evening and morning ftar by turns.

The Earth is next to Venus, and describes an ellipfis round the Sun in 365 days, or one year, which being at a greater diftance 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 absence of the Sun, during the winter feafons, in the North and South.

Mars is ftill higher in the System, and takes a larger circuit, revolving round the Sun in I year, 10 months, and 22 days.

Jupiter is the largest of all the planets, and describes 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 revolves round the Sun in 29 years, has 5 Moons which move round him, and is alfo furrounded with a prodigious ring or atmosphere.

The Georgium Sidus is the moft remote of all the planets, and is attended by two Satellites: the firft or nearest of which performs a fynodical revolution in about 8 days and three quarters.

The fecond (which is about half as far again diftant from its primary planet) is about 13 days and a half in performing its synodical revolution.

The fixed ftars are fuppofed to be of the fame matter with the Sun, and made for the fame ends; each of them the centre of its own proper fyftem, having planets moving round them as our Sun has.

Comets are a fort of planets moving round the Sun, in ellipses, 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 purposes 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 the 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.

The Earth is endowed with a wonderful principle of gravitation, whereby all its parts are ftrictly united together; and all bodies that are loose upon it closely adhere to its furface, tending directly to its centre. Hence it is, that ships are able to fail with the fame facility every where (void of impediments) upon the furface of the fea, quite round the Terraqueous Globe, and that, (as to fenfe) there is no fuch thing as an upper or lower part of the Earth; for let the inhabitant be in what part foever, he will there gravitate towards the Earth's center, and imagine himself to be on the highest point of its furface; from whence he will obferve the Heavens like a large vault over his head, and his Antipodes he will imagine to be directly under him, as they will alfo their's, for the like reafon,

According to this law of Gravity, if the Earth were at reft, (and not acted upon by any other power) and its parts loofe, or its furface all over covered with a deep fluid, it would naturally form itself into a true Sphere or Globe. But, admitting the earth revolves about its own axis, with a rapid motion from Weft to Eaft in 24 hours, the gravity towards its centre will thereby be disturbed, and all the parts endeavour to fly off from the axis of the motion;

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and this inclination is greatest to that part of the furface, which is at the greatest diftance from the axis; and, confequently, the gravity towards the centre is there the leaft: whence it will follow, that thofe parts which gravitate the leaft, muft yield or give way. to those that have a greater gravitation, to reftore an equilibrium; and, confequently, here will be formed a Spheroid, whose greatest diameter will be perpendicular to the axis of motion, (commonly called the Earth's axis) and the fhorteft diameter will be the axis itself.

It is demonftrated by the writers on mechanics, that the times of the periodical vibrations of all pendulums of equal lengths are in a certain proportion to the gravity by which they are acted upon; and it has alfo been demonftrated, that gravity acts in a certain proportion to the diftance from its center. Hence, by the help of pendulums, we may find the proportion of gravity upon any part of the earth; and, confequently, the proportional distance of that part to the distance of any other part from the Earth's centre. Now, it has been found by experience, that the degree of gravitation upon the Earth's furface under the equinoctial, is to the fame in any parallel of latitude, in the fame proportion (as near as obfervation could be made) that it would be, if the whole body of the Earth was compofed of a fluid fubftance, and fo formed itself into fuch a figure as above-mentioned. Hence we may infer, that the Earth is a Spheroid; and its greateft diameter (which is under the Equinoctial) is computed to be to the leffer diameter, (which is under the Poles, or the Earth's axis) as 289 to 288; and, consequently, the fpace upon the Earth's furface, anfwering to a degree of a great circle where it is the greateft, (or under the Equinoctial) is to the fpace answering to a degree near the Poles, (where it is least) as 289 to 288; or as 1000 to 996,5 nearly: but this difference is fo fmall, that in all aftronomical and geographical cafes, the figure of the Earth may be efteemed truly fpherical, though the fmall difference from it does fenfibly affect the motions of pendulums.

That the earth is round, or nearly fo, will appear, not only from the circular fhadow it has upon the Moon, when that body happens to be eclipsed by it, but also from the very appearance of the Sea, and the many obfervations made by perfons ftanding upon the fhore, and viewing a fhip departing from the port: they first lose fight of the body of the veffel, whilft they can ftill fee the rigging and uppermoft fails: but as the fhip recedes farther, they lofe fight of these alfo, as if the whole were funk in the deep. Again, in a fhip making towards the land, the mariners firft defcry the tops of fteeples, trees, &c. pointing above the water; next they fee the buildings themfelves; and laftly the fhore, 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; firft by Magellan's