and she alone yields a satisfactory reply. Finally, if curiosity leads us to inquire whether the length of the day and night, the revolution of the earth on its axis, be uniform, or whether it may not have changed by a single second in a thousand years, we go to the moon for an answer, and in each and every instance her replies to all these profound and mysterious questions are clear and satisfactory. How wonderful the structure of the universe! How gigantic the power of the human intellect! If all the stars of heaven were struck from existence; if every planet and satellite which the eye and the telescope descry, inside and beyond the earth's orbit, were swept away for ever, and the sun, earth, and moon, alone remained for the study of man, and as evidences of the being and wisdom of God, in the exquisite adjustments or this system, in the reciprocal influences of its three bodies, in their vast cycles of configuration, in their relative masses, magnitudes, distances, motions, and perturbations, there would remain themes sufficient for the exercise of the most exalted genius, and proof of the being of God, so clear and positive, that no sane mind could comprehend it and disbelieve.

LECTURE VI.

THE STABILITY OF THE PLANETARY SYSTEM.

HEN, 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 courses 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 nearly 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 inclosed, 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