tion and travel therefore most nearly in its plane. And as a fact Mercury, the Benjamin, does differ from the others by revolving in a plane inclined some 7° to their mean, agreeing in this with the Sun's own rotation, with whose plane it was probably originally coincident (digression from it now being due to secular retrogression of the planets' nodes).4
From the relations which advance has left unchanged we pass to those phenomena which seemed to present congruities in Laplace's day, but which have since proved void owing to subsequent detection of exceptions. Time prevents my making the catalogue complete, but the reader shall be shown enough to satisfy him of the problem's complexity and to whet his desire for further research—on the part, preferably, of others.
First comes, then, the rotations of the planets upon their axes, which Laplace supposed to be all in the same
Chart showing increasing tilts of the major planets.
direction, counter to the hands of a clock; for the heavens mark time oppositely from us. All those within and including Saturn, the only ones he knew, turn, indeed, in the same sense that they travel round the Sun. But Uranus departs from that direction by a right angle, wallowing rather than spinning in his orbit; while Nep-
