Page:The Solar System - Six Lectures - Lowell.djvu/145



is a maximum velocity somewhere in the centre of the nebula, which degrades on both sides, so that we should have a plan of velocities for outside and inside portions of the nebula, thus :—

Supposing the density either the same throughout or to increase toward the centre, we should have, if the various planets were formed simultaneously, a retrograde rotation for the outer, a direct rotation for the inner ones.

In addition to the ten congruities known in the time of Laplace, we must now add others from knowledge acquired since, to wit :—

XI. All the satellites turn the same face to their primaries (so far as we can judge).

XII. Mercury and probably Venus do the same to the Sun.

XIII. One law governs position and size in the solar system, and in all the satellite systems.