one consistent with stability in a system subject to perturbations. If the law of gravitation varied according to the inverse cube, instead of square, the slightest disturbance would precipitate the planet on the sun, or cause it to fly off from its control. With the existing law of gravity, though a planet were pushed out of its course by an impulse from a comet, it would pursue its new orbit with undeviating regularity; the primitive disturbance would not have a progressive tendency. When, however, we contemplate a system of many planetary bodies, the mere law of gravitation does not by any means insure stability. The complexity of the scheme can at once be understood, when we keep in view, that any given body is attracted, not merely by the sun, but by all the other bodies of the system—and that these attractions are constantly varying, according to differences of distance and direction. The consequence is, that the planet, instead of describing the orbit which it would do if undisturbed, proceeds along a very complex and irregular line. A planet thus disturbed may be compared to a person making his way through a crowd. He is jostled at every step, and pushed out of the straight course he would otherwise pursue. Still his path may be determinate enough, though made up of innumerable irregularities. The planet has, in like manner, to thread its way through the perturbing influences of the other bodies of the system, and, therefore, pursues a disturbed instead of a regular course.
