sty glance nothing might seem easier than to reconcile the
phenomena of the ring with the attraction of the planet. We might
suppose that the ring stands at rest symmetrically around the planet. At
its centre the planet pulls in the ring equally on all sides, so that
there is no tendency in it to move in one way rather than another; and,
therefore, it will stay at rest. This will not do. A ring composed of
materials almost infinitely rigid might possibly, under such
circumstances, be for a moment at rest; but it could not remain
permanently at rest any more than can a needle balanced vertically on
its point. In each case the equilibrium is unstable. If the slightest
cause of disturbance arise, the equilibrium is destroyed, and the ring
would inevitably fall in upon the planet. Such causes of derangement are
incessantly present, so that unstable equilibrium cannot be an
appropriate explanation of the phenomena.

Even if this difficulty could be removed, there is still another, which
would be quite insuperable if the ring be composed of any materials with
which we are acquainted. Let us ponder for a moment on the matter, as it
will lead up naturally to that explanation of the rings of Saturn which
is now most generally accepted.

Imagine that you stood on the planet Saturn, near his equator; over your
head stretches the ring, which sinks down to the horizon in the east and
in the west. The half-ring above your horizon would then resemble a
mighty arch, with a span of about a hundred thousand miles. Every
particle of this arch is drawn towards Saturn by gravitation, and if the
arch continue to exist, it must do so in obedience to the ordinary
mechanical laws which regulate the railway arches with which we are
familiar.

The continuance of these arches depends upon the resistance of the
stones forming them to a crushing force. Each stone of an arch is
subjected to a vast pressure, but stone is a material capable of
resisting such pressure, and the arch remains. The wider the span of the
arch the greater is the pressure to which each stone is exposed. At
length a span is reached which corresponds to a pressure as great as the
stones can safely bear, and accordingly we thus find the limiting span
over which a single arch of masonry can be constructed. Apply these
principles to the stupendous arch formed by the ring of Saturn. It can
be shown that the pressure on the materials of the arch capable of
spanning an abyss of suc