em of
rings was exactly edgewise to the earth was on the 3rd of October 1907.

The question of the composition of these rings has given rise to a good
deal of speculation. It was formerly supposed that they were either
solid or liquid, but in 1857 it was proved by Clerk Maxwell that a
structure of this kind would not be able to stand. He showed, however,
that they could be fully explained by supposing them to consist of an
immense number of separate solid particles, or, as one might otherwise
put it, extremely small satellites, circling in dense swarms around the
middle portions of the planet. It is therefore believed that we have
here the materials ready for the formation of a satellite or satellites;
but that the powerful gravitative action, arising through the planet's
being so near at hand, is too great ever to allow these materials to
aggregate themselves into a solid mass. There is, as a matter of fact, a
minimum distance from the body of any planet within which it can be
shown that a satellite will be unable to form on account of
gravitational stress. This is known as "Roche's limit," from the name of
a French astronomer who specially investigated the question.

There thus appears to be a certain degree of analogy between Saturn's
rings and the asteroids. Empty spaces, too, exist in the asteroidal
zone, the relative position of one of which bears a striking resemblance
to that of "Cassini's division." It is suggested, indeed, that this
division had its origin in gravitational disturbances produced by the
attraction of the larger satellites, just as the empty spaces in the
asteroidal zone are supposed to be the result of perturbations caused by
the Giant Planet hard by.

It has long been understood that the system of the rings must be
rotating around Saturn, for if they were not in motion his intense
gravitational attraction would quickly tear them in pieces. This was at
length proved to be the fact by the late Professor Keeler, Director of
the Lick Observatory, who from spectroscopic observations found that
those portions of the rings situated near to the planet rotated faster
than those farther from it. This directly supports the view that the
rings are composed of satellites; for, as we have already seen, the
nearer a satellite is to its primary the faster it will revolve. On the
other hand, were the rings solid, their outer portions would move the
fastest; as we have seen takes place in the body of the earth