This was thought the most remoteft planet in our fyftem; but Dr. HERSCHEL, on the 13th of March, 1781, difcovered another at a ftill greater diftance, called the GEORGIUM SIDUS, his distance from the furi is computed to be one thoufand eight hundred millions of miles, its magnitude is about eighty-nine times greater than the earth's; and that it revolves round the fun in an orbit, which is nearly circular, in about eighty-two years.

SATELLITES OR MOONS.

Befides the primary planets here mentioned, there are ten others, called fecondary planets, or fatellites, which regard their primaries as the centres, of their motions, and revolve round them in the fame manner as those primaries rovolve round the Sun.

The most confpicuous of these fatellites is the Moon, who is a conftant attendant on our Earth; and, whilft fhe accompanies it in its annual progrefs through the heavens, keeps revolving round it con tinually, by a different motion in the space of a month.

The moon's diameter is two thousand one hundred and eighty miles her distance from the Earth two hundred and forty thousand miles; and in bulk fhe is about fixty times lefs than the Earth. Jupiter has four fuch moons, and Saturn five; and from the continual change of their phafes, or appearances, it is evident that thefe alfo are opaque bodies, like the planets, and fhine only by means of the borrowed light which they receive from the Sun.

It may also be obferved, that our earth is a moon to the moon, waxing and waneing in the fame manner, but appearing about thirteen times larger, and, of courfe, affording a proportional quantity of light. When The changes to us, the Earth will appear full to her, and when she is in her first quarter to us, the Earth will be in her third quarter to her. And, as her axis is almoft perpendicular to the plane of the ecliptic, one half of her orb will be conftantly illuminated by the reflected light of the Earth in the Sun's abfence, whilft the other half will have a fortnight's darkness, and a fortnight's light, alternately.

The rotation of the moon upon her axis, is alfo performed in the fame time that the goes once round the earth, as is evident from her always prefenting the fame face to us during the whole of her monthly revolution; on which account, it is plain that the inhabitants of one half of the lunar world, are totally deprived of a fight of the earth, whilst the inhabitants of the other hemifphere have a full view of our globe, moving through the heavens at the rate of fifty-eight thousand miles an hour, and appears ing to them near thirteen times larger than that of the fun.

The reafon of the moon's rifing and fetting an hour later every night than it did the night before, is owing to its diurnal motion of about 13° 10' from weft to eaft round her orbit; which, together with the mo tion of the earth round her orbit, makes verv near an hour's difference of rifing and fetting, one night with another; but it is not fo every night, being fometimes as much again as at others, according to the different

3 K

times

times of the year, and the different parts of the ecliptic, the earth and moon may then happen to be in; as is evident from the different aspects of the moon towards the autumn, vulgarly called the harvest moon; which does not fet the next night after full, but rifes about the fame time for feveral nights together.

In an oblique fphere, all great circles interfecting the equinoctial, will, in the revolution of the fphere, interfect the horizon with different angles, at every different part thereof. Thus, with refpect to the ecliptic, when the beginning of Libra is orient, or rifing in the eaft, it then makes the greatest angle with the horrizon; when Capricorn is orient, the angle is mean; and when Aries is orient, the angle is leaft of all; therefore, when the moon is full in the beginning of Libra, one day's motion depreffes her fartheft below the horizon, and least when in the beginning of Aries; confequently, the difference of her rifing each day at the vernal equinox will be greatest, and least of all in the autumnal equinox.

The revolution of the moon through the zodiac is called a lunation, and 12 of these lunations or revolutions is a lunar year; which takes up the fpace of 354 days, 8 hours, 48' 38". The difference between this and the folar year, which contains 365 days, 5 hours, 48′ 57′′ is almost I days, which chronologers call the epact.

And because the moon's motion about her axis is performed in the fame time as about the earth, the lunarians have their natural days equal to their months.

Befides the moons, these planets are accompained with, Saturn is known to be encompaffed with a ring; which furprizing phænomenon was discovered about 132 years fince. It is faid, the inner border of the ring, from the body of Saturn, is equal to the breadth of the ring itfelf; each is computed to be, at least, 21,000 miles; though others make the interval between the ring and Saturn's body to be 210,265, and the breadth of the ring to be 29,200 miles; its thickness is unknown, being too fmall for obfervation: it hath a variety of aspects, fometimes appearing a large ellipfis, then a fmaller; fometimes only a ftraight line, and fometimes not visible at all. Thefe are the most remarkable particulars of this prodigy of nature, known by aftronomers; as to the matter, of which it doth confift, it is not known by any.

OF COMETS.

Comets, or blazing ftars, according to Sir ISAAC NEWTON, are folid, compact, fixed, and durable substances; and are a kind of planets, which move about the fun in ftated periods of time, and fhine by the light of the fun-beams reflected from them; and that the orbits are very eccentric and elliptical, but fome more and fome lefs; fo confequently their periods are longer or fhorter. The forms of three remarkable cometary orbs are defcribed in the folar fyftem before-going.

Dr.

Dr. HALLEY has determined the longest axis of the orbit of that comet, which appeared in 1680, and whofe period is 575 years, to be 1382975 parts, of which the mean diftance of the earth from the fun is 10000; therefore, fuppofing this mean diftance to be 81000000 English miles, then the length of that comet's orb will be above eleven thousand and two hundred millions of English miles.

And Sir ISAAC NEWTON has computed the heat of the aforefaid comet, when neareft the fun, to be 2000 times hotter than red-hot iron; and it is computed, that a ball of iron, as big as the globe of our earth, would, if red-hot, require 50,000 years to grow cold in; and the bodies of comets being fo much greater than our earth, can never be cold at their greatest distance: and the learned Dr. HALLEY has compiled a fet of tables, whereby the places in the zodiac, of above 20 comets, may be determined for any given time,

OF THE FIXED STARS.

Fixed Stars, are fo called, in oppofition to the planets, or moving ftars, because they always keep the fame place in the heavens, and do not seem to move for feveral ages together; yet they have an apparent motion, occafioned by a certain contrary motion of the earth, arifing from the fpheroidical figure thereof.

This motion of the fixed ftars does not exceed 50" of a degree in a year, or one degree in 70 years; therefore, to complete one revolution of a circle, is required 25,920 years; after which time the stars all return again to their former places,

The diftance of the fixed ftars is but imperfectly known; however, the famous HUGENS has computed the brightest, and of course the nearest of all the fixed ftars, viz. Syrius, to be, in appearance, 27,664 times lefs than the fun; and fince their distances are greater, as their magnitudes are leffer, therefore this ftar must be at the rate of above two millions of millions of miles; which is fo great, that a cannon ball would spend almoft 700,000 years in paffing through it; and it is probable, that all fixed ftars are equally diftant from each other, in proportion to the distance of the nearest of them from our fun.

The number of vifible fixed ftars, whofe places have been rectified by aftronomers, are these :

Hypparchus, 140 years before Christ, had a catalogue of stars,

containing

} 1022

1600

1026

1017

777

1163

1725

400

1635 Brachius

1635 Brachius

1651 John Ricciolus

1670 John Hevetius

1676 Dr. Edmund Halley $690 John Flamstead

1671 1468

1888

373

- 3000

Aftronomers have divided the stars into fix feveral fizes or magnitudes, of which the greatest or brightest of them are called ftars of the first magnitude, as Acturus, Regulus, Sirius, &c. and the next to them in brightnefs, are called the stars of the fecond magnitude; next to them in brightness, are ftars of the third magnitude, &c. till we come to ftars of the fixth magnitude, which comprehend the fmallest stars that can be difcerned with the naked eye; and in the above catalogues, the most complete contains only 3000 ftars, though affifted by the best glaffes; but the most that can be discovered by the naked eye, in the mast ferene night, are not above three or four hundred; and Dr. KEILL, in his Aftronom. Lect. vi. p. 51, 52, 53, 54, fays of the 3000 stars in Mr. FLAMSTEAD'S catalogue, it is feldom that a very good eye can reckon more than one hundred together; and the famous Mr. FLAMSTEAD himfelf afferts, that the naked eye cannot difcover above 384 ftars in the fereneft night, in both the hemifpheres.

The Galaxy, via lectea, or milky way, is a broad white tract, encompaffing the whole heavens, and extending itself in the fign of Capricorn, from the equinoctial to the tropic of Cancer, with a double path, and the reft of it is a fingle one. Some of the ancients, as ARISTOTLE imagined that this path confifted only of a certain exhalation hanging in the air; but by the observations with the telescope, made in this age, it has been discovered to confift of an innumerable quantity of fixed ftars, different in fituation and magnitude; from the confused mixture of whofe light, its white colour is fuppofed to be occafioned.

The fixed ftars are known from the planets by their fcintillation, or fparkling; for the planets have no fuch vibration, twinkling, or glimmering of light; but, generally, all the fixed ftars, more or lefs, and at fome times more than others. The cause of their scintillation is variously difcourfed of, both by philofophers and aftronomers. ARISTOTLE among the ancients, affigns the cause thereof to their remotenefs from our fight, by which they are weakly, and as it were by a trembling weariness reached. Some, again, will have the caufe of this twinkling to proceed from refraction; others affign the cause to arife from the unequal fuperfices of the fluctuating air or medium; as ftones in the bottom of a river, by the rapid motion of the water, feem to have a kind of tremulous motion: but GASSENEUS, more probably, conceives this fcintillation of the fixed ftars to proceed from that native and primogenial light they are indued with, like that of the fun's fparkling, cafting forth fuch quick darted rays, as our weaker fight cannot behold with put that trembling paffion,

GEOGRAPHY.

THE

437

PART VII.

SECTION LXXX.

HE word Geography comes from the Greek; and, in a proper fenfe, fignifies nothing more than a defcription of such parts of the furface of the earth as are really land; the other parts, which de fcribes the water, being called Hydrology. For the globe of our earth, having its external furface partly land and partly water, has been from thence always denominated the terraqueous globe, which is the foundation of the two above-mentioned fciences, Geography and Hydrology.

The figure of the earth has been long well known to be globular, or fpherical: it was originally fuppofed flat, or a plane; but this was too great an error for any perfon to continue in long; because if a perfon walks directly north or fouth, it will cause the stars to have a greater or leffer elevation above the horizon; but no alteration, in that refpect, would happen to them, in walking on a plane, though the diftance be ever fo great. This, therefore, afforded an evident proof, that the furface of the earth was of a curvilineal form; and because walking over equal spaces occafioned an equal difference in the meridian altitude of the ftars, it was a proof that the curve furface was of the spherical kind; and therefore, the body of the globe was in form of a globe or fphere.

And this was the general opinion, till the beginning of the laft century; when experiments on pendulums, the nature of gravity, a centrifugal force in revolving bodies, and fome other phyfical principles, came to be understood, there was great reason to suspect, that the figure of the earth could not poffibly be that of a globe, but that of a fpheriod as above-mentioned..

These discoveries excited a great defire among the learned, to be fatisfied (experimentally) of the true figure of the earth; which they eafily knew could not be done, without actually meafuring a degree on the furface of the earth, in feveral different parts of it; and the more remote from each other, the better. At length, by the munifi cence of Kings, and great propenfity of philofophers and mathematicians, the arduous undertaking was attempted, profecuted, and finished, with fuccefs.

NORWOOD, and others, make the circumference of the globe 25020 miles; the diameter in the equator is 7964 miles, at the poles 7930 miles; but to form a general idea of thefe things, we may, without much error, look upon the earth as a globe or fphere; and fo the di. menfions of its furface, computed in fquare miles, 60 to a degree, will be expreffed in the following table; whereby you may fee, at one view, the fuperficial content of the whole, and its feveral parts.

The