Page:The Solar System - Six Lectures - Lowell.djvu/143



account for it. Suppose, first, the density uniform, or nearly so. Then the inner parts of the mass that went to form the planet would be traveling fastest, and their momentum would prevail over that of the outer particles and give a retrograde rotation to the whole. Suppose, however, that the density increased toward the inner side of the mass. Then the centre of inertia would be so far shifted toward the inner edge, say to N, that the sum of the moments about it of the particles from without would, owing to their distance from it, surpass that of those within and a direct rotation result.

The attraction, and thence the velocities in the Laws of force different parts of the nebula, may be well shown graphically.