not be accounted for merely by the law of gravitation; but the period of rotation, or the length of the planet's day, is, also, altogether unaccounted for. It has been, therefore, a matter of interest to trace such an analogy in these rotations as might lead to the determining cause. Generally speaking, the period of rotation is greater in the planets least remote from the sun, though there are particular cases which contravene this rule. No definite law, then, can be derived from the mere element of distance. The nebular hypothesis led Kirkwood to the surmise, that the rotation might be dependent on the masses of the planets, and, with this clue to guide him, he was led to the following law: That the square of the number of rotations or days in the year of the planet, is proportional to the cube of the diameter of the sphere of attraction. The diameter of the sphere of attraction is determined in this way: The planet in question is supposed to be in conjunction with the two nearest planets, one on each side of it,—the three planets being thus in line. It is plain that there are two neutral points on each side of the middle planet, where its attraction is equally balanced by the attraction of either extreme. For example there is a point between the Earth and Venus, where, if a body were placed, it would be stationary, being equally attracted by both. A similar point exists between the Earth and Mars. The line joining these two points is called the diameter of the sphere of attraction. Now the law asserts, that
