Page:The Solar System - Six Lectures - Lowell.djvu/93



These flights of small bodies are so exactly in one plane that they vanish when the rings are turned edgewise to the Earth. Their lustre shows them to be relatively densely packed, so that collisions among them must be not infrequent. In consequence of this, Maxwell predicted that they would eventually be forced both out or in, and in part fall upon the ball, in part be driven farther from the planet. Certainly such must ultimately happen; but the evidence is not conclusive that either process has yet been observed.

The spectroscope shows that, unlike Saturn, they carry no air with them. This, from their minute size, was to be expected on the kinetic theory of gases and the clever deduction from it as to the atmosphere a body may retain, made by Johnstone Stoney.

To attempt to account for their dimensions and divisions might at first seem hopeless. Why A is made up of an outer and an inner portion parted by Encke's streak; why B is sundered from A by Cassini's division; and why C is sharply contrasted with B at its inner edge, sound like difficult questions. But nothing in celestial mechanics is the outcome of chance, and this is no exception to the rule.

To begin with, Roche's limit falls just at the