peared to move daily from eaft to weft, were conceived to be at no great distance from it, and to be only defigned for its ufe or ornament. Several reafons, however, occurred, which rendered this opinion improbable; it is needlefs to mention them, because we have now a fufficient proof of the figure of the earth, from the voyages of many navigators, who have actually failed round it; as for inftance that of Magellan's fhip, which was the first that circumnavigated the globe, failing weft from a port in Europe in 519, and returning to the fame, after a voyage of 1124 days, without altering its direction, except to the north or fouth, as compelled by the winds, or intervening land. The fpherical figure of the earth being fully proved, the hypothefis of its motion was evidently rendered much more probable. For while it was confidered as a plane, mankind had an obfcure notion of its being fupported, like a fcaffolding, on pillars, though they could not tell what fupported thefe. But the figure of a globe is much better adapted to motion; and a very ftrong, and, in reality, unanfwerable, argument for that motion was derived from confidering, that, if the earth did not move round the fun, not only the fun, but all the ftars and planets, must move round the earth. Now, as aftronomers, by reckonings founded on the fureft obfervations, have been able to ascertain pretty nearly the distances of the heavenly bodies from the earth and from each other, in the fame manner as every perfon acquainted with the firft elements of mathematics can measure the height of a fteeple, or any object placed on it, it appeared, that, if we conceived the heavenly bodies to move round the earth, we must fuppofe them endowed with a motion or velocity fo immenfe as to exceed all conception: whereas all the appearances in nature may be as well explained by imagining the earth to move round the fun in the space of a year, and to turn on its own axis once in 24 hours. To form a conception of these two motions of the earth, we may ima gine a ball moving on a billiard table or bowling-green: the ball proceeds forwards upon the green or table, not by fliding along like a plane upon wood, or a flate upon ice, but by turning round its own axis, which is an imaginary line drawn through the centre or middle of the ball, and ending on its furface in two points called its poles. We muft, however, remember that thefe two motions in the earth are perfectly diftinct, and not imagine that the number of revolutions caused by the rotatory motion is in proportion to the space pailed through by the progreslive, as is the cafe with the ball on the table or the bowling-green. The earth, therefore, in the space of 24 hours, moves from wett to eaft, while the inhabitants on the furface of it, like men on the deck of a fhip, who are infenfible of their own motion, and think that the banks move from them in a contrary direction, will conceive that the fun and ftars move from east to west in the fame time of 24 hours in which they, along with the earth, move from weft to eaft. This daily or diurnal motion of the earth being once clearly conceived, will enable us eafily to form a notion of its annual or yearly motion round the fun. For as that Juminary feems to have a diurnal motion round our earth, which is really occafioned by the daily motion of the earth round its own axis, fo, in the course of a year, he feems to have an annual motion in the heavens, and to rife and fet in different points of them, which is really occafioned by the annual motion of the earth in its orbit or path round the fun, which it completes in the fpace of a year. Now, as to the first of thefe motions we owe the difference of day and night, fo to the fecond we are indebted for the difference in the length of the days and nights, and in the feasons of the year. THE PLANETS.] It will eafily be conceived that what has been said with regard to the motion of the earth, is equally applicable to all the other planets. Of thefe, befide the feven already mentioned, which move round the fun, there are fourteen others which move round four of these, in the same manner as they do round the fun; and of these our earth has one, called the moon; Jupiter has four; Saturn has seven (two * of, these having been lately difcovered by Dr. Herfchel); and the Georgium Sidus two, as that excellent aftronomer has fhown. Thefe are called moons, from their refemblance to our moon; and fometimes fecondary planets, because they seem to be attendants of the Earth, Jupiter, Saturn, and the Georgium Sidus, about which they move, and which are called primary. It will be neceffary for the understanding of the following table, to explain what is meant by the inclination of the axes of the planets to their orbits, and the mean distances of the planets. We have already said that the annual motion of the earth occafioned the diverfity of feafons. But this would not happen, were the axis of the earth exactly parallel to or in a line with the axis of its orbit: decause then the fame parts of the earth would be turned towards the fun in every diurnal revolution; which would deprive mankind of the grateful viciffitudes of the feasons, arifing from the difference in length of the days and nights. This, therefore, is not the cafe :-the axis of the earth is inclined to the plane of the earth's orbit, which we may conceive by fuppofing a fpindle put through a ball, with one end of it touching the ground; if we move the ball directly forwards, while one end of the spindle continues to touch the ground, and the other points towards fome quarter of the heavens, we may form an idea of the inclination of the earth's axis to its orbit, from the inclination of the fpindle to the ground. The fame obfervation applies to fome of the other planets, as may be feen from the table. In order to understand what is meant by the mean distances of the planets from the fun, we muft obferve that the orbit, or path which a planet defcribes, were it to be marked out, would not be quite round or circular, but in the fhape of a figure called an ellipfis, which, though refembling a circle, is longer than broad. Hence the fame planet is not always at the fame diftance from the fun; and the mean diftance of it is that which is exactly betwixt its greatest and least distance. Here follows the table: * See the 80th vol. of the Philosophical Tranfa&tions. A TABLE OF THE DIAMETERS, PERIODS, &c. OF THE SEVERAL PLANETS IN THE SOLAR SYSTEM. Names of the tions of the Miles. tranfit of Ve fun. equa. orbit. The Georgian planet (or Georgium Sidus) was discovered by Dr. Herschel with his telescope of great fize and power, forty feet in length, and four and a half in diameter, in the year 1781. For this difcovery he obtained from the Royal Society the honorary recompenfe of fir Godfrey Copley's medal. Though it was not till then known as a planet, yet there are many reafons to fuppose it had been seen before, but had been confidered as a fixed ftar. But, from the steadiness of its light, from its diameter being increased by high magnifying powers, and from the change he had obferved in its fituation, Dr. Herschel firft concluded that it was a comet; but in a little time, he, with others, determined that it was a planet, from its vicinity to the ecliptic, the direction of its motion being stationary in the time, and in fuch circumftances, as correfpond with fimilar appearances in other planets.-When the moon is abfent, it may be seen by the naked eye; and the difcovery of two fatellites attending it seems to confer upon it a dignity, and to raise it into a more confpicuous fituation among the great bodies of our solar system. As the distances of the planets, when marked in miles, are a burden to the memory, aftronomers often exprefs their mean distances in a fhorter way, by fuppofing the diftance from the earth to the fun to be divided into ten parts. Mercury may then be estimated at four of fuch parts from the fun, Venus at feven, the Earth at ten, Mars at fifteen, Jupiter at fiftytwo, Saturn at ninety-five, and the Georgium Sidus at one hundred and ninety. COMETS.] The reader having obtained an idea of the planets from the table, and the previous obfervations neceffary for understanding it, muft next turn his attention to the comets, which, as they revolve round our fun, are a part of the folar fyftem. Thefe, defcending from the far diftant parts of the fyftem with great rapidity, furprise us with their fingular appearance of a train or tail, which accompanies them; become vifible to us in the lower parts of their orbits, and, after a fhort ftay, go off again to vast distances, and disappear. Though fome of the ancients had more juft notions of them, yet the opinion having prevailed that they were only meteors generated in the air, like to thofe we fee in it every night, and in a few moments vanishing, no care was taken to obferve or record their phænomena accurately, till of late. Hence this part of aftronomy is very imperfect. The general doctrine is that they are folid compact bodies, like other planets, and regulated by the fame laws of gravity, fo as to defcribe equal areas in proportional times by radii drawn to the common centre. They move about the fun in very eccentric ellipfes, and are of much greater denfity than the earth; for fome of them are heated in every period to fuch a degree as would vitrify or diffipate any fubftance known to us. Sir Ifaac Newton computed the heat of the comet that appeared in the year 1680, when nearest the fun, to be 2000 times hotter than red-hot iron, and that, being thus heated, it muft retain its heat till it comes round again, although its period should be more than 20,000 years; and it is computed to be only 575. The number of comets is very much greater than that of the planets which move in the vicinity of the fun. From the reports of hiftorians, as well as from the obfervations of late years, it has been ascertained that more than 450 have been feen previous to the year 1771; and when the attention of aftronomers was called to this object by the expectation of the return of the comet of 1759, no fewer than feven were observed in the course of seven years. From this circumftance, and the probability that all the comets recorded in ancient authors were of confiderable apparent magnitude, while the smaller were overlooked, it is reasonable to conclude that the number of comets confiderably exceeds any eftimation that might be made from the observations we now poffefs. But the number of those, whofe orbits are settled with fufficient accuracy to afcertain their identity when they may appear again, is no more than 59, reckoning as late as the year 1771. The orbits of most of these are inclined to the plane of the ecliptic in large angles, and the greater number of them approached nearer to the fun than to the earth. Their motions in the heavens are not all in the order of the figns, or direct, like thofe of the planets; but the number whose motion is retrograde is nearly equal to that of those whose motion is direct. All which have been obferved, however, have moved through the ethereal regions and the orbits of the planets, without fuffering the leaft fenfible refiftance in their motions; which fufficiently proves that the planets do not move in folid orbs. Of all the comets, the periods of three only are known with any degree of certainty, being found to return at intervals of 75, 129, and 575 years; and of these, that which appeared in 1680 is the most remarkable. This comet, at its greatest diftance, is about 11 thousand 200 millions of miles from the fun, while its leaft diftance from the centre of the fun is about 190 thousand miles; being less than one third part of the fun's femidiameter from his furface. In that part of its orbit which is nearest the fun, it flies with the amazing velocity of 880,000 miles in an hour; and the fun, as feen from it, appears 100 degrees in breadth, confequently 40,000 times as large as he appears to us. The aftonishing diftance that this comet runs out into empty space naturally fuggefts to our imagination the vaft diftance between our fun and the nearest of the fixed fiars, of whose attractions all the comets must keep clear, to return periodically and go round the fun, Dr. Halley, to whom every part of aftronomy, but this in a particular manner, is highly indebted, has joined his labours to thofe of fir Ifaac Newton on this fubject. Our earth was out of the way, when this comet laft paffed near her orbit: but it requires a more perfect knowledge of the motion of the comet, to be able to judge if it will always pafs by us with fo little effect; for it may be here obferved that the comet, in one part of its orbit, approaches very near to the orbit of our earth; fo that, in fome revolutions, it may approach near enough to have very confiderable, if not fatal, effects upon it. See Newton, Halley, Gregory, Keil, Maclaurin, Derham, Ferguson, and Whiston.) THE FIXED STARS.] Having thus briefly furveyed the folar fyftem, which, though great in itself, is fmall in comparison with the immenfity ef the universe, we next proceed to the contemplation of thole other vaft bodies, called the fixed ftars, which being of infinite ufe in the practice of geography, claim a particular notice in this work. These fixed ftars are diftinguished by the naked eye from the planets, by being lefs bright and luminous, and by continually exhibiting that appearance which we call the twinkling of the ftars. This arifes from their being fo extremely fmall, that the interpofition of the leaft body, of which there are many conftantly floating in the air, deprives us of the fight of them; when the interpofed body changes its place, we again fee the ftar; and this fucceffion being perpetual, occafions the twinkling. But a more remarkable property of the fixed ftars, and that from which they have obtained their name, is their never changing their fituation, with regard to each other; as the planets, from what we have already faid, muft evidently be always changing theirs. The ftars which are nearest to us feem largeft, and are therefore called flars of the firft magnitude. Thofe of the fecond magnitude appear lefs, being at a greater diftance; and fo proceeding on to the fixth magnitude, which includes all the fixed stars that are visible without a telescope. As to their number, though, in a clear winter's night without moonfhine, they feem to be innumerable (which is owing to their firong fparkling, and our looking at them in a confufed manner), yet when the whole firmament is divided, as it has been by the ancients, into figns and conftellations, the number that can at any time be seen with the naked eye, is not above a thousand. Since the invention of telescopes, indeed, the number of the fixed stars has been juftly confidered as immenfe; because the greater perfection we arrive at in our glasses, the more ftars always appear to us. Mr. Flamfteed, late royal aftronomer at Greenwich, has given us a catalogue of about 3000 ftars. These are called telescopic ftars, from their being invifible without the affiftance of the telescope. Dr. Herfchel, to whofe ingenuity and affiduity the aftronomical world is fo much indebted, has evinced what great difcoveries may be made by improvements in the inftruments of obfervation. In fpeaking here of his difcoveries, I fhall ufe the words of M. de la Lande; In paffing rapidly over the heavens with his new telescope, the universe "increafed under his eye; 44,000 ftars, feen in the space of a few de " grees, feemed to indicate that there were feventy-five millions in the "heavens." But what are all thefe, when compared to thofe that fill the whole expanfe, the boundless fields of æther? Indeed the immensity of the universe must contain fuch numbers as would exceed the utmost stretch of the human imagination; for who can say how far the universe extends, or point out thofe limits where the Creator " ftayed his rapid wheels," or where he "fixed the golden compafles?") The immenfe diftance of the fixed ftars from our earth, and from each other, is, of all confiderations, the moft proper for raifing our ideas of the works of God. For, notwithstanding the great extent of the earth's orbit or path (which is at leaft 190 millions of miles in diameter) round the fun, the diftance of a fixed ftar is not fenfibly affected by it; fo that the ftar does not appear to be any nearer to us when the earth is in that part |