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

The fixed stars are supposed to be of the same matter with the Sun, and made for the same ends; each of them the centre of its own proper system, having planets moving round them, as our Sun has.

Comets are a sort of planets moving round the Sun, in ellipses, so very oblong, that their visible parts seem to be, in a manner, parabolical, but have such vast atmospheres about them, and tails derived from the same, especially when they come near the Sun, as imply them designed for very different purposes from the other planets.

Having given a cursory View of the System of the Universe, we shall now consider 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 absolutely necessary in the Art of Navigation.

The land and water of this Earth, or Planet upon which we live, make a composition of a spherical form, or rather an oblate figure, called the Terraqueous Globe, which, by turning round its axis every 24 hours, from West to East, causes all the heavenly bodies to revolve, apparently, from East to West in the same time, making the vicissitudes 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, produces 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 strictly united together; and all bodies that are loose upon it closely adhere to its surface, tending directly to its centre. Hence it is, that ships are able to sail with the same facility every where (void of impediments) upon the surface of the sea, quite round the Terraqueous Globe, and that (as to sense), there is no such thing as an upper or lower part of the Earth; for let the inhabitant be in what part soever, he will there gravitate towards the Earth's centre, and imagine himself to be on the highest point of its surface; from whence he will observe the Heavens like a large vault over his head, and his Antipodes he will imagine to be direct under him, as they will also theirs for the like

reason.

According to this law of Gravity, if the Earth were at rest (and not acted upon by any other power), and its parts loose, or its surface 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 West to East, 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;

and this inclination is greatest to that part of the surface, which is at the greatest distance from the axis; and, consequently, the gra vity towards the centre is there the least, whence it will follow, that those parts which gravitate the least, must yield or give way to those that have a greater gravitation, to restore an equilibrium; and, consequently, 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 shortest diameter will be the axis itself.

It is demonstrated by the writers on mechanics, that the times of the periodical vibrations of all pendulums of equal lengths are în a certain proportion to the gravity by which they are acted upon; and it has also been demonstrated, that gravity acts in a certain proportion to the distance from its centre. Hence, by the help of pendulums, we may find the proportion of gravity upon any part of the earth and consequently, 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 surface under the equinoctial, is to the same in any parallel of latitude, in the same proportion (as near as observation could be made) that it would be, if the whole body of the Earth was composed of a fluid substance, and so formed itselfinto such a figure ás above-mentioned. Hence we may infer, that the Earth is a Spheroid; and its greatest diameter (which is under the Equinoctial) is computed to be to the lesser diameter (which is under the Poles, or the Earth's axis), as 289 to 288; and consequently, the space upon the Earth's surface, answering to a degree of a great Circle where it is the greatest (or under the Equinoctial), is to the space 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 so small, that in all astronomical and geographical cases, the figure of the Earth may be esteemed truly spherical, though the small difference from it does sensibly affect the motions of pendulums.

That the earth is round, or nearly so, will appear, not only from the circular shadow 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 observations made by persons standing upon the shore, and viewing a ship departing from the port: they first lose sight of the body of the vessel, whilst they can still see the rigging and uppermost sails: but as the ship recedes farther, they lose sight of these also, as if the whole were sunk in the deep. Again, in a ship making towards the land, the mariners first descry the tops of 'steeples, trees, &c. pointing above the water; next they see the buildings themselves; and lastly the shore, which can only be the effects of the Earth's rotundity.

Its being a globe is also confirmed by the many voyages which have been made round it from East to West; 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 Anson, in 4 years; and lately by the Captains Byron, Carteret, Cook, and Clarke, accompanied with several able mathematicians and naturalists, whose observations and discoveries do honour to this nation, as well as greatly contribute to the improvement of Geography and Navigation: they having discovered many islands in the South Seas, which were formerly unknown to Europeans.

The little unevennesses of the Earth's surface, arising from the hills and vales, are no material objection to its being considered as round since the highest hill or mountain bears not so great a proportion to the bulk of the Earth itself, as the little rising upon the coat of an orange bears to the bigness of that fruit.'

In order to describe the position of places, geographers have found it necessary 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 straight line, imagined to pass through the centre of the Earth; the extreme points are the Poles, on which the Earth is supposed to move, one called the Arctic, or North Pole, and the other the Antarctic, or South Pole.

II. The Equator is a great circle under the Equinoctial Line in the Heavens, compassing the Earth in the middle, between the two Poles, and divides it into two equal parts, called the Northern and Southern Hemispheres: from it the latitude of places is reckoned, either North or South; and on it are counted the degrees of longitude from East to West. This circle is called the 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, he makes equal day and night throughout the World.

III. The Meridians are circles which pass through the Poles of the Earth, the Zenith, and Nadir, crossing the Equator at right angles, and dividing the Earth into two equal parts, one East and the other West; and are so called, because, 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 East to West, have their several meridians of these, one is called the first or chief Meridian, from which the longitude of places is reckoned; it is of special note and use, but variously placed by geographers; some placing it at London, others at Paris, Teneriffe, &c.; and, since the Earth turns once round its axis in 24 hours, every point upon its surface describing a circle of 360 degrees in that time; therefore, any place lying 15 degrees to the East of us, has the Sun upon its meridian one hour sooner; or it is twelve o'clock with the easternmost, when it is eleven with us; and any place 15 degrees to the westward of us, has the Sun one hour after us.

IV. Latitude is the nearest distance of any place from the Equa it is measured 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 situated, either North or South of the Equator; therefore, all places that lie at the same distance from, and on the same side of, the Equator, are said to be under the same parallel of Latitude.

Parallels of Latitude are circles parallel to the Equator.

The difference of Latitude is an arch of the meridian, contained between two parallels of Latitude; or it is the least distance of the parallels of Latitude of two places, showing 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 expressed by an arch of the Equator, showing the east or west distance of the meridian of that place, from some fixed meridian, where Longitude is reckoned to begin.

Difference of Longitude is an arch of the Equator, intercepted between the meridians of two places, showing how far one of them is to the eastward or westward of the other,

Longitude begins at the meridian of some place, and is counted from thence both eastward and westward, and 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 supposed 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 Heawens opposite to it, or under our feet, is called the Nadir.

When the Sun or Stars come above the easternmost part of the Horizon, they are said to rise ; and when they descend the western part, they are said to set.

When a ship is under the Equator, both the poles are in the Horizon; and, in proportion as she sails towards either, or increase her latitude, that pole is seen proportionably above the Horizon, and the other disappears as much but when a ship is sailing towards the Equator, or decreases her latitude, she depresses the elevated pole; that is, its distance from the Horizon decreases: consequently, the latitude of a place is always equal to the elevation of the pole above the Horizon.

Note. Here the Teacher will, perhaps, find it convenient to have a Globe, or Map of the World, before him, whereon he can point out the severa! Positions, Latitudes, Longitudes, &c. to the Pupil, as that will strengthen his memory, and give him a better idea than he can possibly have by only reading them over. The same may be observed in seading the use of Gunter's Scale and the Quadrant.