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 no wise affect their beautiful organisation. Their graceful figures and beautiful light shall greet the eyes of the student of the heavens, when ten thousand years shall have rolled away. Thus do we find that God has built the heavens in wisdom, to declare his glory, and to show forth his handiwork. 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 performed? 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. N 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 harmony of the system. The space from the orbit of Mercury to that of Venus is 31,000,000 miles; from the orbit of Venus to that of the earth is 27,000,000 miles; from the earth's orbit to that of Mars is 50,000,000 miles; but between the orbit of Mars and that of Jupiter there exists the enormous interval of 359,000,000 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. applying a greater magnifying power, the strange star showed On Dr. Herschel unequivocal symptoms of increased dimensions. Its position among the neighbouring stars was noticed with care, and by an examination on the following evening, the stranger was found to have sensibly changed position. A few nights sufficed to establish the fact that the newly discovered body was actually a wandering star; and not for a moment dreaming of the discovery of a new pianet, erscher announced to the world that he had found a remarkable comet. Efforts were made to obtain the orbit of the stranger, on the hypothesis, that like those of all the then known comets, it was extremely elongated. Maskelyn and Lexell soon reached the conclusion that no eccentric orbit could possibly represent the motions of the newly-discovered star; and on a close and diligent examination, it was at last discovered to be a primary planet, revolving in an orbit nearly circular, and almost coincident with the plane of the ecliptic. Its motion was progressive, like the other planets, and its vast orbit was only completed at the end of eighty-four of our years. Its distance from the sun was found to be no less than 1,800,000,000 of miles, and its dimensions such that out of it might be formed more than eighty worlds as large as the earth. This great discovery excited the highest interest in the astronomical world. From the earliest ages, the mighty orbit of Saturn had been regarded as forming the boundary of the vast scheme of planets dependant on the sun. Its slow and majestic motion, its great period and distance, and the wonderful magnificence of its rings and moons, seemed to render it a fitting object to guard the frontiers of the mighty system with which it was associated. But the supremacy of Saturn was now gone for ever, and its sentinel position was usurped by Uranus, whose grand orbit expanded to twice their original dimensions the boundaries of the solar system. Far sweeping in the depths of space, this new world pursued its solemn journey, flinging back the light of its parent orb, steadily obedient to the great law of universal gravitation, which held the old planets true to their changing orbits. Another unit in the number of interplanetary spaces was thus given, and the law which might possibly regulate the distances of the planets from the sun was sought after with an interest and perseverance which could not long fail of its reward. No exact progression was indeed discovered, but the following remarkable empirical law was detected by Professor Bode: Now if 10 be assumed as the earth's distance from the sun the other terms of the series will represent very nearly the distances of the planets, thus ; 4 7 10 Mercury, Venus, Earth, 16 Mars, 28 The fifth term in the series is blank, and falls exactly in the enormous interval which exists between the orbits of Mars and Jupiter, precisely where Kepler had predicted a new planet would be found. As early as 1784, three years after the discovery of Uranus, Baron de Zach, struck with the remarkable law of Bode, even went so far as to compute the probable distance and period of the now generally suspected planet. The impression that a new world would soon be added to the system, grew deeper and stronger in the minds of astronomers, until finally, in 1800, at a meeting held at Lilienthal, by six distinguished observers, the subject was discussed with deep earnestness, and it was finally resolved that the long-suspected, but yet undiscovered world, should be made the object of strict and persevering research. The range of the Zodiac was divided into twenty-four parts, and distributed among an equal number of observers, whose duty it was to scrutinise their particular regions, and detect, if possible, any moving body which might show itself among the fixed stars. In case it were possible to note down, with perfect precision, the relative places and magnitudes of all the stars in a given region, any subsequent changes which might occur would be easily recognised. In other language, if a daguerreotype picture of any region in the heavens could be made to-night, and if at the end of the year another picture of the same region could be taken, by comparing the number of stars in the one picture with that in the second, in case any one had wandered away from its place, or a stranger had come to occupy a place within the limits of the pictured region, it would be an easy matter to ascertain either the lost star, or the newly arrived stranger. Now, although a daguerreotype picture could not be had, yet, by observation, the exact relative positions of all the visible stars might be mapped out, and a picture formed, which should become the ready means of detecting future changes. Such was the method of examination adopted by the congress of astronomers assembled at Lilienthal, in 1800. The organisation was made. Baron de Zach was elected president, and K |