f the body is at the planet's equator. Its action
on the matter of the ring is, of course, very much less, because of the
increased distance, but still a force is exerted on every part of the
ring which is comparable with the familiar force of terrestrial gravity.
The outer edge of the outer ring lies about 83,500 miles from the
planet's centre, the inner edge of the inner ring (I speak throughout of
the ring-system as known to Sir W. Herschel and Laplace) about 54,500
miles from the centre, the breadth of the system of bright rings being
about 29,000 miles. Between the planet's equator and the inner edge of
the innermost bright ring there intervenes a space of about 20,000
miles. Roughly speaking, it may be said that the attraction of the
planet on the substance of the ring's inner edge is less than gravity at
Saturn's equator (or, which is almost exactly the same thing, is less
than terrestrial gravity) in about the proportion of 9 to 20; or, still
more roughly, the inner edge of Saturn's inner bright ring is drawn
inwards by about half the force of gravity at the earth's surface. The
outer edge is drawn towards Saturn by a force less than terrestrial
gravity in the proportion of about 3 to 16--say roughly that the force
thus exerted by Saturn on the matter of the outer edge of the
ring-system is equivalent to about one-fifth of the force of gravity at
the earth's surface.

It is clear, first, that if the ring-system did not rotate, the forces
thus acting on the material of the rings would immediately break them
into fragments, and, dragging these down to the planet's equator, would
leave them scattered in heaps upon that portion of Saturn's surface. The
ring would in fact be in that case like a mighty arch, each portion of
which would be drawn towards Saturn's centre by its own weight. This
weight would be enormous if Bessel's estimate of the mass of the
ring-system is correct. He made the mass of the ring rather greater than
the mass of the earth--an estimate which I believe to be greatly in
excess of the truth. Probably the rings do not amount in mass to more
than a fourth part of the earth's mass. But even that is enormous, and
subjected as is the material of the rings to forces varying from
one-half to a fifth of terrestrial gravity, the strains and pressures
upon the various parts of the system would exceed thousands of times
those which even the strongest material built up into their shape could
resist. The sys