heavens was determined, and many of its properties and peculiarities described, before it had been seen by human eye for the discovery was made, not by accident or improved instruments, but by one of the rarest combinations of observation and science that ever did honour to the human intellect. And it adds to the interest of this discovery, that the investigations which led to it were simultaneously and successfully prosecuted, and quite independently of each other, by a French savant in Paris, and an English mathematician at Cambridge.1

We have now therefore eight Planets; and several of them, as will be seen in the Tabular Series, are attended by Satellites or secondary planets, which, while they accompany their primary in its revolutions round the Sun, have also a motion peculiar to themselves, each round its own primary.

Our Moon is a body of this description, 2160 miles in diameter. Like all the other Planets primary and secondary, it is opaque, but capable, like them, of reflecting a portion of the Sun's light. The Moon revolves round the Earth in one month at the mean distance of 237,000 miles. If the plane of the

1 Leverrier and Adams. See Appendix, Note A. at the end of the Introduction.

"Learners are apt to boggle at the astronomical use of the adjective mean. It is derived immediately from the French "moyen," and remotely from the Latin medium, "middle," which is the real signification of the term. If a body moved, round a fixed point, in a circle, there would be no need, in describing its motions, to use the

Moon's orbit coincided with the plane of the Earth's, there would be a total eclipse of the moon every month at the full, and either a total or annular1 eclipse of the sun just before new moon. The lunar eclipse would take place when the Earth, being interposed between the Moon and the Sun, projected a cone of darkness which the moon would require two hours to travel through. The solar eclipse would occur when the Moon was directly between the Earth and Sun. But such monthly eclipses do not take place, because the orbit of the Moon is inclined to that of the Earth at an angle of 5° 9";2 and this is enough to raise her above or depress her below the cone of the Earth's shadow at all times, except when she happens to be full or new at the time of her nodes, that is, when the orbits of the two bodies intersect each other, and the centres of all the three bodies, sun, moon, and earth, and earth, are in the same straight line. That eclipses of the Sun are more rarely seen than those of the Moon is owing to the fact, that a body so comparatively small as the Moon, whose diameter is only that of the Earth's, can never intercept

term mean distance, because its distance would be always the same -the radius, to wit, of the circle. But if it moves in an oval or ellipse, as all the planets do, its distance from the fixed point is constantly changing; and the mean distance is the middle term between the greatest and the least.

Total, when the moon was nearest to the earth, and annular, when she was farthest.

2 See Appendix. Note B.

the Sun's light by her projected shadow from more than a small portion of the Earth's surface, and that only for a short space of time.

But besides the eight Planets named, reckoning the Earth as one of them, there are certain other bodies moving between the orbits of Mars and Jupiter, which fulfil the condition of planetary existence, by revolving round the Sun in independent elliptical orbits of their own; but which are at the same time, comparatively so small,—the largest of them being greatly inferior in bulk to our Moon,1that they ought not to be classed with the great Planets of the system, the Dî majorum gentium, but arranged under a separate head with the title of 'Planetoids.' It is a prevailing notion among astronomers, that they are the fragments of a burst planet, which, before the catastrophe, revolved in the space between Mars and Jupiter,-where indeed such a body had been long desiderated, to preserve the law of proportionate distance of the Planets from each other.

To the Planets, primary and secondary, and the Planetoids, all revolving round the Sun as their common centre, there must be added, in order to complete our conception of the Solar System, a very considerable number, not less than many hundreds, of Comets; concerning whose nature, uses, and courses, we are

1 At railway speed one might make the circuit of the globe Ceres, which is the largest of them, in a few hours.

still very much in the dark. They move in long or very eccentric ellipses. The comet which Sir Isaac Newton observed in 1680 approached the Sun so closely in its perihelion as to be greatly within the orbit of Mercury, and in its remotest point or aphelion, was calculated to be 150 times farther from the sun than the earth is.1 A Comet appeared two years after, which Halley identified by careful observation and happy conjecture with the comet of 1531 and 1607; and accordingly he foretold its re-appearance after a period of 76 or 77 years :-a prediction which was verified in 1759; and again in 1835. This is the only comet of which many successive returns have been accurately observed, and the nature of whose orbit is well understood, with the exception of three small telescopic comets, which are called 'planetary' because their orbits are included within that of Jupiter. They are known by the names of their discoverers, Encke's, Biela's, and Faye's.

All these bodies, then, taken together,—the Sun, Planets, Planetoids, Moons, and Comets-constitute the system of which our little world is a part. Their movements in space are all produced by an impulse originally communicated to each, which is controled and regulated by the continual action of a centripetal force, exerted by, and directed to, the Sun. The original impulse, acting alone, would have carried the

1 That is, 150 times 95,000,000 of miles,-a distance which it baffles imagination to conceive.

CENTRIFUGAL AND CENTRIPETAL FORCES. XV

planets straight forward into infinite space, in the line of their first projection; it being an axiom in physics, that any body to which motion is communicated will continue to move on for ever, uniformly and constantly, in the direction given, unless it be affected by some disturbing cause. The centripetal force, again, supposing it acted alone on the planets at rest, would make them fall into the body of the Sun with a continually accelerated motion; just as a stone let fall descends to the earth. And the descent of the planet, like the fall of the stone, would be merely an instance of the great truth established by Sir Isaac Newton, that all bodies whatever mutually attract each other, with a force directly proportional to their quantity of matter as measured by weight, and inversely to the square of the distance. Hence it follows that the sun tends or gravitates to the planets, as well as the planets to the sun; and both sun and planets revolve round a common central point within the system; a point, however, which, owing to the prodigiously greater mass of matter in the sun, is within his body, and never distant from his centre so much as one-fourth of the solar diameter. "If we tie two stones together by a string, and fling them aloft, we see them circulate about a point between them, which is their common centre of gravity; but if one of them be greatly more ponderous than the other, this common centre will be proportionally nearer to that one, and even within