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CHAPTER VI.

THE STABILITY OF THE PLANETARY SYSTEM.

WHEN by the application of a single great law, the mind had succeeded in resolving the difficult problems presented by the motions of the earth and its satellite, the moon, it rose to the examination of the higher and more complicated questions of the stability of the entire system of planets, satellites, and comets, which are found to pursue their course round the sun. The number of bodies involved in this investigation, their magnitudes and vast periods of revolution, their great distances from the observer, and the exceeding delicacy of the required observations, combined with the high interest which attaches itself to the final result, have united to render this investigation the most wonderful which has ever employed the energies of the human mind.

To comprehend the dignity and importance of this great subject, let us rapidly survey the system, and moving outward to its known boundaries, mark the number and variety of worlds involved in the investigation. Beginning, then, at the great centre, the grand controlling orb, the sun, we find its magnitude such as greatly to exceed the combined masses of all its attendant planets. Indeed, if these could all be arranged in a straight line on the same side of the sun, so that their joint

effect might be exerted on that body, the centre of gravity of the entire system thus located, would scarcely fall beyond the limits of the sun's surface. At a mean distance of 36,000,000 of miles from the sun we meet the nearest planet, Mercury, revolving in an orbit of considerable eccentricity, and completing its circuit around the sun in a period of about eighty-eight of our days. This world has a diameter of only 3,140 miles, and is the smallest of the old planets. Pursuing our journey, at a distance of 68,000,000 of miles from the sun, we cross the orbit of the planet Venus. Her magnitude is nearly equal to that of the earth. Her diameter is 7,700 miles, and the length of her year is nearly 225 of our days. The next planet we meet is the earth, whose mean distance from the sun is 95,000,000 of miles. The peculiarities which mark its movements, and those of its satellite, have been already discussed. Leaving the earth, and continuing our journey outward, we cross the orbit of Mars, at a mean distance from the sun of 142,000,000 of miles. This planet is 4,100 miles in diameter, and performs its revolution around the sun in about 687 days, in an orbit but little inclined to the plane of the ecliptic. Its features, as we shall see hereafter, are more like those of the earth than any other planet. Beyond the orbit of Mars, and at a mean distance from the sun of about 250,000,000 of miles, we encounter a group of small planets, eight in number, presenting an anomaly in the system, and entirely different from anything elsewhere to be found. These little planets are called asteroids. Their orbits are, in general, more eccentric, and more inclined to the ecliptic, than those of the other planets; but the most remarkable fact is this:that their orbits are so nearly equal in size, that when projected on a common plane, they are not

enclosed, the one within the other, but actually cross each other.

We shall return to an examination of these wonderful objects hereafter. At a mean distance of 485,000,000 of miles from the sun, we cross the orbit of Jupiter, the largest and most magnificent of all the planets. His diameter is nearly 90,000 miles. He is attended by four moons, and performs his revolution round the sun in a period of nearly twelve years.-Leaving this vast world, and continuing our journey to a distance of 890,000,000 of miles from the sun, we cross the orbit of Saturn, the most wonderful of all the planets. His diameter is 76,068 miles, and he sweeps round the sun in a period of nearly twenty-nine and a half years. He is surrounded by several broad concentric rings, and is accompanied by no fewer than seven satellites or moons. The interplanetary spaces we perceive are rapidly increasing. The orbit of Uranus is crossed at a mean distance from the sun of 1,800,000,000 of miles. His diameter is 35,000 miles, and his period of revolution amounts to rather more than eighty-four of of our years. He is attended by six moons, and pursues his journey at a slower rate than any of the interior planets. Leaving this planet, we reach the known boundary of the planetary system, at a distance of about 3,000,000,000 of miles from the sun. Here revolves the last discovered planet, Neptune, attended by one, probably by two moons, and completing his vast circuit about the sun in a period of one hundred and sixty-four of our years.

-His diameter is eight times greater than the earth's, and he contains an amount of matter sufficient to form one hundred and twenty-five worlds such as ours.

Here we reach the known limit of the planetary worlds, and standing at this remote point and look

ing back towards the sun, the keenest vision of man could not descry more than one solitary planet along the line we have traversed. The distance is so great, that even Saturn and Jupiter are utterly invisible, and the sun himself has shrunk to be scarcely greater than a fixed star.

There are certain great characteristics which distinguish this entire scheme of worlds. They are all nearly globular-they all revolve on axestheir orbits are all nearly circular-they all revolve in the same direction around the sun-the planes of their orbits are but slightly inclined to each other, and their moons follow the same general laws. With a knowledge of these general facts, it is proposed to trace the reciprocal influences of all these revolving worlds, and to learn, if it be possible, whether this vast scheme has been so constructed as to endure while time shall last, or whether the elements of its final dissolution are not contained within itself, either causing the planets, one by one, to drop into the sun, or to recede from this great centre, released from its influence, pursue their lawless orbits through unknown regions of space.

Before proceeding to the investigation of the great problem of the stability of the universe, let us examine how far the law of gravitation extends its influence over the bodies which are united in the solar system. A broad and distinct line must be drawn between those phenomena, for which gravitation must render a satisfactory account, and those other phenomena, for which it is in no wise responsible. In the solar system we find, for example, that all the planets revolve in the same direction around the sun, in orbits slightly elliptical, and in planes but little inclined to each other. Neither of these three peculiarities is in any way traceable to the law of gravitation.

Start a planet in its career, and, no matter what be the eccentricity of its orbit, the direction of its movement, or the inclination of the plane in which it pursues its journey, once projected, it falls under the empire of gravitation, and ever after, this law is accountable for all its movements. We are not, therefore, to regard the remarkable constitution of the solar system as a result of any of the known laws of Nature.

If the sun were created, and the planetary worlds formed and placed at the disposal of a being possessed of less than infinite wisdom, and he were required so to locate them in space, and to project them in orbits, such that their revolutions should be eternal, even with the assistance of the known law of motion and gravitation, this finite being would fail to construct his required system.

Let it be remembered, that each and every one of these bodies exerts an influence upon all the others. There is no isolated object in the system. Planet sways planet, and satellite bends the orbit of satellite, until the primitive curves described, lose the simplicity of their character, and perturbations arise, which may end in absolute destruction. There is no chance work in the construction of our mighty system. Every planet has been weighed and poised, and placed precisely where it should be. If it were possible to drag Jupiter from its orbit, and cause him to change places with the planet Venus, this interchange of orbits would be fatal to the stability of the entire system. In contemplating the delicacy and complexity of the adjustment of the planetary worlds, the mind cannot fail to recognise the fact that, in all this intricate balancing, there is a higher object to be gained than the mere perpetuity of the system.

If stability had been the sole object, it might have been gained by a far simpler arrangement.