certain whether these conditions actually existed in nature. The occasional disappearance of the ring, in consequence of its edge being presented to the eye of the observer, gave a capital opportunity of determin ing whether it was of uniform thickness. On these rare occasions, in the most powerful telescopes, the ring remains visible edgewise, and looks like a slender fibre of silver light drawn across the diameter of the planet. In the gradual wasting away of the two extremities of the ring, it has been remarked, that the one remains visible longer than the other. As the ring is swiftly revolving, neither extremity can, in any sense, be regarded as fixed, and hence sometimes the one, sometimes the other, fades first from the sight. An exactly uniform thickness in the ring would ren der such a phenomenon impossible, and hence we conclude, that the first condition of stability is fulfilled, the rings are not equally thick throughout.

The micrometer was now applied to detect an eccentricity in the central point of the ring. Recent examinations by Struve and Bessel have settled this question in the most satisfactory manner. The centre of the ring does not coincide with that of the planet, and it is actually performing a revolution around the centre of the planet in a minute orbit, thus forming the second delicate condition of equilibrium. The analogy of the great system is unbroken in the subordinate one. For more than two hundred years have these wonderful circles of light whirled in their rapid career under the eye of man, and freed from all external action, they are so poised that millions of years shall in nowise affect their beautiful organi zation. Their graceful figures and beautiful ligh'

shall greet the eyes of the student of the heavens when ten thousand years shall have roled away.

Thus do we find that God has built the heavens in wisdom, to declare his glory, and to show forth his handy-work. There are no iron tracks, with bars and bolts, to hold the planets in their orbits. Freely in space they move, ever changing, but never changed; poised and balancing; swaying and swayed; disturbing and disturbed, onward they fly, fulfilling with unerring certainty their mighty cycles. The entire system forms one grand complicated piece of celestial machinery;-circle within circle; wheel within wheel; cycle within cycle;-revolutions so swift as to be completed in a few hours; movements so slow that their mighty periods are only counted by millions of years. Are we to believe that the Divine Architect constructed this admirably adjusted system to wear out, and to fall in ruins, even before one single revolution of its complex scheme of wheels had been per formed? No.-I see the mighty orbits of the planets slowly rocking to and fro, their figures expanding and contracting, their axes revolving in their vast periods; but stability is there. Every change shall wear away, and after sweeping through the grand cycle of cycles, the whole system shall return to its primitive condition of perfection and beauty.

LECTURE VII.

THE DISCOVERY OF NEW PLANETS.

In the earliest ages of the world, the keen vision of the old astronomers had detected the principal members of the planetary system. Even Mercury, which habitually hovers near the sun, and whose light is almost constantly lost in the superior brilliancy of that luminary, did not escape the eagle glance of the primitive students of the stars. For many thousand years no suspicion arose in the mind, as to the existence of other planets, belonging to the great scheme, and which had remained invisible from their immense distance or their minute dimensions.— Indeed the grand investigations which have recently engaged our attention, the mutation of the planetary orbits, their perpetual oscillations and final restoration, the equilibrium of the whole system, had been prosecuted and completed before the mind gave itself seriously to the contemplation of invisible worlds.

The singularly inquisitive genius of Kepler, over whom analogy seems to have ever played the tyrant. in an examination of the interplanetary spaces, finding these to increase with regularity in proceeding outward from the sun, until reaching the space between Mars and Jupiter, which was out of all proportion too great, conceived the idea that an invisible planet revolved in this space, and thus completed the

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harmony of the system. The space from the orbit of Mercury to that of Venus is 31,000,000 of miles; from the orbit of Venus to that of the earth is 27,000,000 of miles; from the earth's orbit to that of Mars is 50,000,000 of miles, but between the orbit of Mars and that of Jupiter, there exists the enormous interval of 359,000,000 of miles. The order is again resumed between the orbits of Jupiter and Saturn, and from these slender data Kepler boldly predicted that a time would come when a planet would be found intermediate between the orbits of Mars and Jupiter, whose discovery would establish a regular progression in the interplanetary spaces. For nearly two hundred years this daring speculation was regarded as one of the wild dreams of a great, but visionary mind.

Towards the close of the eighteenth century, when the planetary orbs had been studied with great care, and a comparatively accurate knowledge of their perturbations had been reached, certain unexplained irregularities gave rise to the suspicion that the movements of Saturn might be disturbed by the action of an unknown planet revolving in a vast orbit, remote from, and far beyond that of Saturn. These speculations led to no serious results, and it was only by a fortunate accident that, on the 13th of March, 1781, Sir William Herschel noticed a small star of remarkable appearance, which happened to fall in the field of his telescope. On applying a greater magnifying power, the strange star showed unequivocal symptoms of increased dimensions. Its position among the neighboring stars was noticed with care, and by an examination on the following evening, the stranger was found to have sensibly changed position. A few