orbit and velocity, is obviously greater as the distance of the earth from the sun is less. Now the earth is nearer the sun in winter than in summer ; hence the disturbing force of the sun on the moon is greater in winter than in summer. The consequence is, in winter, when the moon is at A, she is drawn away from the earth farther than in summer ; and when at E, the earth is drawn away from the moon more than in summer. The effect is, that the distance between the moon and earth is greater in winter than in summer ; and hence the moon occupies a longer period in completing her revolution in the former season, than in the latter. The difference is about 24 minutes.

181. It is recorded in history, that an eclipse of the moon took place at Alexandria on the 22d Sept. 201 years before the Christian era, and that when the moon arose, she was so much eclipsed, that the eclipse must have begun half an hour before she rose. But accord ing to our Tables, this eclipse did not begin till ten minutes after the moon rose at Alexandria. Now had this eclipse begun and ended while the sun was below the horizon, it might have been supposed that the observer, who had no certain way of measuring time, might have been so far mistaken in the hours, that we could not rely on the accuracy of his account. But as in the case given, the sun had not set and the moon had not risen, till some time after the eclipse began ; this circumstance is such, that the observer could not be mistaken in it.

182. From this, and many other instances of discordance between ancient records and our own Tables, it is certain that the moon now describes a less orbit, and occupies less time in a revolution, than she did formerly. This fact led astronomers, and Ferguson among them, to suppose that the moon met with some resistance in her orbit, so that her projectile force was

continually diminished, and her centripetal force increased. They hence inferred that the moon would continually draw nearer and nearer to the earth by slow degrees, till at length they would fall together.

(See Ferguson's Astronomy, Nos. 163 and 322.)

183. But by examining the effects of the several planets, and especially of Jupiter and Saturn, on the form of the earth's orbit, La Place, an eminent French astronomer, has discovered that the eccentricity of the earth's orbit has been diminishing from ancient time'; and that this diminution is the cause of the acceleration of the moon's motion, which we are now considering. The subject is too intricate to admit a familiar illustration ; but it is important, as putting to rest all those fears of an ultimate wreck of this world, which were grounded on the apparently inevitable effects resulting from the principle of gravitation. La Place also discovered that this, and all other irregularities in the Solar System, generated by the mutual action of the planets, are all periodical, confined to narrow limits, and balanced by irregularities of an equal and opposite kind. After reaching a certain limit, they gradually diminish, till the system, regaining its balance, returns to that state of harmony and order, which preceded the commencement of these secular inequalities.

184. But there is no class of bodies liable to so great

disturbances as comets. The same comet is sometimes twice as far distant from the sun as Uranus, and in a different part of its orbit, twice as near to him as we are.

Hence the motion of these bodies is very variable. They cross the orbits of the planets in all directions ; and are of course accelerated, retarded, and turned out of their course, according as they actually approach these bodies. Dr. Halley computed that the disturbing power of Jupiter alone on the comet of 1682, would retard its return 511 days; and Clairaut

m.

prevent it from going as far in the second portion of time as it went in the first; the distance dg being less than Ad. This effect becomes more and more apparent, till the earth completes half its orbit at q. During this part of the orbit, the projectile force more than balances the sun's attraction. But the sun's attraction is constantly diminishing that force; till at 9, the sun's attraction more than balances it, and the earth begins to approach the sun. At q, these forces act again at right angles; but the projectile force being overbalanced, would carry the earth to o, while the sun's attraction would bring it to r; it consequently comes to

At this point these forces do not act at right angles; but the sun's attraction tends to increase the projectile force. And this effect is more and more obvious till the earth comes to A again, and these forces act at right angles. In this manner, the planets, primary and secondary, continually describe elliptical orbits.

168. Since all attraction is mutual, it is obvious that the sun does not remain entirely at rest, while the earth performs its revolution ; but must also perform a small revolution round the centre of gravity, which (on account of the smallness of the earth) cannot be far from the sun's centre. In this revolution, it is also manifest that the sun's motion must be very irregular. For while the earth is drawing him one way, some of the other planets are drawing him in an opposite or side direction.

169. In like manner, while the moon performs its revolution round the earth, the earth also describes a similar revolution round the centre of gravity. But as the difference in the solid contents and distance of these two bodies, is no way comparable with that of the sun and" earth, the centre of gravity is not very near the earth's centre, but is about 2,000 miles from the earth's surface.

170. But the planets of our solar system are not the only bodies which have orbits. It was stated above, No. 50, that many stars, which appear single to the naked eye, appear double, treble, or even quadruple, when seen through a telescope. According to the observations of distinguished astronomers, it has been found that in many cases the stars, composing these double stars, change their situation with regard to each other; and hence it is inferred that they revolve round ä сommon centre of gravity. Dr. Herschel, during a series of observations on double stars, has found that in more than fifty of them, this change of situation really takes place; and that therefore they describe orbits round a centre of gravity. Some of their periodical times he has calculated; but the accuracy of his calculations remains to be tested.

171. Besides these motions of the single stars composing double ones, it is beyond question that many of the other stars have motions peculiar to themselves. An apparent change of place in some of the stars was first discovered by Dr. Halley, by comparing their present places with their places as laid down in ancient catalogues. Other astronomers confirmed his observations, and this motion of the stars is termed their proper motion,

172. If the stars be suns and have motion, does our sun also have motion ? If so, the whole solar system of planets, primary and secondary, must partake of his motion, and be carried along with him. It will be obvious on a moment's reflection, that if we are moving towards one part of the visible heavens, the stars in that quarter will appear to recede from each other; while those in the opposite part, from which we are moving, will appear to approach each other. Now observation shows that the stars in one region of the heavens do actually appear to recede from each other, while those in the opposite region appear to draw nearer together. Hence we seem to have evidence little short of demonstration, that the sun and we with him are in a progressive motion. The constellation Hercules is the region, to which this motion appears to be directed.

173. If the stars and the sun have motion, if they describe orbits, around what do they move? Hitherto we have stated only observations and the conclusions resulting from them. But here observation has not been continued for a length of time sufficient to justify even a conjecture. All is speculation. Herschel supposes (and the supposition has simplicity and beauty, and hence probably truth) that the sun is one of an innumerable multitude of stars composing the milky-way; that all these stars with their systems have a motion round a common centre of gravity. But where this centre is, he does not pretend to conjecture. It is also probable that those whitish regions known as nebulæ, (of which Dr. Herschel has given a catalogue of 2500) are each composed of a system of stars describing orbits round a centre of gravity, like the stars of the milky-way. Still the analogy of the universe is not complete, without giving these systems of stars, these milky-ways, a progressive motion ; without supposing, that they describe each an orbit round a common centre. But here we must stop, for no more materials are given,

SECT. II.

Of the retrograde Motion of the Moon's Nodes. 174. Under the article Eclipses, it was stated, that the moon's nodes were not always in the same points of the ecliptic, but had a motion backward, contrary to the order of the signs; by, which motion the line of the nodes