, either known
to be spheroidal, like the earth, the sun, the moon, Jupiter, and Venus,
or presumed to be so, like the stars. The comets might be judged to be
vaporous masses of various forms; but even these were supposed to
surround or to attend upon globe-shaped nuclear masses. Here, however,
in the case of Saturn's ring, was a quoit-shaped body travelling around
the sun in continual attendance upon Saturn, whose motions, no matter
how they varied in velocity or direction, were so closely followed by
this strange attendant that the planet remained always centrally poised
within the span of its ring-girdle. To appreciate the interest with
which this strange phenomenon was regarded, we must remember that as yet
the law of gravity had not been recognised. Huyghens discovered the
ring (or rather perceived its nature) in 1659, but it was not till 1666
that Newton first entertained the idea that the moon is retained in its
orbit about the earth by the attractive energy which causes unsupported
bodies to fall earthwards; and he was unable to demonstrate the law of
gravity before 1684. Now, in a general sense, we can readily understand
in these days how a ring around a planet continues to travel along with
the planet despite all changes of velocity or direction of motion. For
the law of gravity teaches that the same causes which tend to change the
direction and velocity of the planet's motion tend in precisely the same
degree to change the direction and velocity of the ring's motion. But
when Huyghens made his discovery it must have appeared a most mysterious
circumstance that a ring and planet should be thus constantly
associated--that during thousands of years no collision should have
occurred whereby the relatively delicate structure of the ring had been
destroyed.

Only six years later a discovery was made by two English observers,
William and Thomas Ball, which enhanced the mystery. Observing the
northern face of the ring, which was at that time turned earthwards,
they perceived a black stripe of considerable breadth dividing the ring
into two concentric portions. The discovery did not attract so much
attention as it deserved, insomuch that when Cassini, ten years later,
announced the discovery of a corresponding dark division on the southern
surface, none recalled the observation made by the brothers Ball.
Cassini expressed the opinion that the ring is really divided into two,
not merely marked by a dark stripe on its s