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planets would appear to separate from the earth in the space of a day; and thus finding how many days will be necessary for their separation to the distance of a whole circle. Thus, if the daily motion of the earth be 59′ 8′′-3548′′, and that of Herschel, 42′′.88, the earth will appear to recede from him with a velocity equal to the difference of their motions, viz. 3505".12 per day. Then say, as 3505".12: 1 day :: 1,296,000" the seconds in a circle: 370 days nearly, which must pass from the time of one conjunction, or opposition to the next. In this manner, the intervals between the conjunctions of any of the other planets may be found from their daily motions, which are as follows; of Mercury, 4° 5′ 32′′; of Venus, 1° 36′ 8′′; of Mars, 31′ 27′′; of Jupiter; 4′ 59′′; of Saturn, 2′ 1′′; of Herschel, 42.88". In the same manner the mean conjunctions of the planets are found, which may differ some days from the true times, on account of the unequal motions of the planets in their elliptical orbits. These times are therefore to be corrected from the astronomical tables of their motions, by computing their real distances from each other, as seen from the sun, at the times of their mean distances; and from their known velocities at that time, finding the time of their true conjunctions.

The planets are all opake, globular, and rough bodies; as it is owing to the roughness of their surfaces that they reflect light enough from the sun to make them visible to us. For if their surfaces were smooth and well polished, the image of the sun reflected from them would be no more than a point, and the planet would become invisible; but being rough, they reflect the light in all directions, and from every point, in sufficient quantities, to make them appear in their proper dimensions.

COMETS.

THERE is still remaining the most numerous class of bodies belonging to the solar system, which revolve round the sun in very eccentric orbits, in long periods, and in all different directions; some from west to east, others from east to west; some from north to south, and others in a contrary course; and which are usually denominated comets. They are solid and opake bodies, usually attended with long and shining trains, sometimes extending a hundred degrees from the comet, and projected in a direction opposite to the sun. As they revolve in very eccentric ellipses, they take amazing excursions through the regions of space, and then return, after a long absence, down through the planetary orbits, and many of them approach very near to the center of force, when they move with incredible velocity. They are visible only for a small part of their periods, viz. while they are performing that part of their orbits, which lies within the planetary regions; and hence it is so extremely difficult to ascertain their periodical returns with precision, from the few observations that can be made of their motions, while they are visible. We have therefore no method left for this purpose, but by consulting the histories of comets, and by noting the years in which comets have appeared, that have a similar course through the heavens; and if we find any such appearing after equal periods, and in similar orbits, we have then some probable grounds to conclude that it was the same comet which returned to the sun after these periods. By these means, Dr. Halley first predicted the return of the comet of 1759, whose period appears to be 75.5 years, as he had observed that it had appeared before, in a

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similar course, in the years 1308, 1456, 1531, 1607, 1682. From the equality of periods and similitude of appearances, he also concluded that the comet of 1532 appeared again in 1661, after a period of 129 years; and if his conjecture be right, we may expect to see it again in January 1790 or 1791. That the comet, which appeared in the 44th year before Christ, appeared (in the consulate of Orestes and Lampadius) in 531, in 1106, and 1680, after a period of 575 years. In the same manner it is concluded, that the comet of 204, 570, 1299 and 1664, was the same, returning after a period of 365 years. It is also probable, that the comets of 1264 and 1556 may be one and the same comet, returning after a period of 292 years; and also that the comets of 1596 and 1699 may be the same comet, returning after a period of 103 years; and may be expected again in 1802. Although the astronomical elements of between 40 and 50 comets have been calculated in parabolic orbits by Dr. Halley and others, yet we cannot believe, from these elements, that the periodical times of any of them have been ascertained, excepting those above mentioned, and even these are doubtful, until their future returns confirm the predictions. As the comets descend within the planetary regions, they may approach near to the planets in crossing their orbits, and thereby suffer some alterations in their velocities and periods, and also in the inclination of their orbits to the ecliptic, which will ever make it a very difficult matter to predict their returns with precision.

To the naked eye, the head of a comet generally appears like a cloudy star, shining with a dull and obscure light, though some have been observed to shine with a vivid light equal to that of stars of the first magnitude, and some to have even surpassed Jupiter in

splendour. But when viewed through a good telescope, the head appears a solid globe, surrounded with a large and gross atmosphere, and most commonly attended with a long and shining train. Various have been the opinions of philosophers concerning the tails of comets; although all acknowledge that they depend, some way or other, upon the action of the sun, for this plain reason, that they are projected from the comet opposite to the sun, and are observed to grow longer upon the comet's nearer approach to him. Sir Isaac Newton, whose opinion, even when delivered as a conjecture, is justly revered by every philosopher, supposed, that the tails of the comets were their atmospheres, greatly rarefied by the heat of the sun, and thereby made specifically lighter than the ether, which he supposes might fill the planetary regions, and regard the sun as its atmosphere, and gravitate towards it. He supposed that this ether, being most rarefied near to the sun, would constantly ascend from it, and thereby carry up with it the reflecting matter of which the tail is composed. This solution, however, is objected to, from the improbability of the ascent of this vapour from the comet; being so much swifter than the motion of the comet, as to cause the tail to ascend before it, when the comet of 1680, at its nearest approach to the sun, moved with the amazing rapidity of 880,000 miles in an hour. Rowning therefore supposes, that the atmospheres of the comets extend as far as their tails, and that the part which forms the tail is distinguished from the rest, by the rays of the sun being most copiously reflected from it. The supposed extent of the atmospheres is an objection to this solution. It is more probable to me, that the atmospheres of the comets are of a moderate size, and are conden

sed about them in their aphelia; but that when they descend towards the sun, these atmospheres expand, and exhibit the appearance of a coma surrounding the comet when it first begins to be visible; and that when it comes close to the sun, the whole atmosphere becomes so rarefied, as to be carried forward by the action of the sun's rays in their own direction, and by the reflexion of the light, to afford the appearance which is denominated the tail or train of the comet.

The tails of comets appear to be bent a little towards that part which the comet has left, being well defined on the convex side, and always in the plane of their orbits.

When the perihelion distances and periods of comets are known, their aphelion distances are also found from the proportion between the cubes of the mean distances and the squares of the periodical times. Thus the comet of 1759, whose perihelion distance is 58.8 of such parts as the mean distance of the earth contains 100, and period 75.5 years, will be found by that analogy to be at the mean distance of 1786.4; from the double of which, viz. 3572.8, subtract 58.8, and there remains 3514 for the aphelion distance, which is about twice the distance of Herschel from the sun, or 35 times the distance of the earth. By a similar process, the aphelion distance of the comet of 1680, whose period is 575 years, and perihelion distance but 0.612, comes out to be near 138 times the mean distance of the earth, that is, nearly 13,800 millions of miles from the sun; whereas at its nearest distance from the sun, it was not above one third of his semidiameter from his surface; or about half a million of miles from his center. At this distance, the sun appeared to this comet 40 thousand times as large

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