state of perpetual but slow change. At the present time the inclinations of all the orbits are decreasing, but so slowly that the inclination of Jupiter's orbit is only about six minutes less now than it was in the age of Ptolemy. The terrestrial eccentricity is decreasing at the rate of about 41·44 miles annually; and, if it were to decrease equably, it would be 37521 years before the earth's orbit became a circle. But, in the midst of all these vicissitudes, the major axes and mean motions of the planets remain permanently independent of secular changes; they are so connected by Kepler's law of the squares of the periodic times being proportional to the cubes of the mean distances of the planets from the sun, that one cannot vary without affecting the other.
With the exception of these two elements, it appears that ail the bodies are in motion, and every orbit in a state of perpetual change. Minute as these changes are, they might be supposed to accumulate in the course of ages sufficiently to derange the whole order of nature, to alter the relative positions of the planets, to put an end to the vicissitudes of the seasons, and to bring about collisions which would involve our whole system, now so harmonious, in chaotic confusion. It is natural to inquire what proof exists that nature will he preserved from such a catastrophe? Nothing
