lit adjusted to coincidence with the equatorial plane of the system, he
determined the comparative radial velocities of its different parts. And
these supply a crucial test of Clerk Maxwell's theory. For if the rings
were solid, the swiftest rates of rotation should be at their outer
edges, corresponding to wider circles described in the same period;
while, if they are pulverulent, the inverse relation must hold good.
This proved to be actually the case. The motion slowed off outward, in
agreement with the diminishing speed of particles travelling freely,
each in its own orbit. Keeler's result was promptly confirmed by
Campbell,[1109] as well as by Deslandres and Belopolsky.

A question of singular interest, and one which we cannot refrain from
putting to ourselves, is--whether we see in the rings of Saturn a
finished structure, destined to play a permanent part in the economy of
the system; or whether they represent merely a stage in the process of
development out of the chaotic state in which it is impossible to doubt
that the materials of all planets were originally merged. M. Otto Struve
attempted to give a definite answer to this important query.

A study of early and later records of observations disclosed to him, in
1851, an apparent progressive approach of the inner edge of the bright
ring to the planet. The rate of approach he estimated at about
fifty-seven English miles a year, or 11,000 miles during the 194 years
elapsed since the time of Huygens.[1110] Were it to continue, a collapse
of the system must be far advanced within three centuries. But was the
change real or illusory--a plausible, but deceptive inference from
insecure data? M. Struve resolved to put it to the test. A set of
elaborately careful micrometrical measures of the dimensions of Saturn's
rings, executed by himself at Pulkowa in the autumn of 1851, was
provided as a standard of future comparison; and he was enabled to renew
them, under closely similar circumstances, in 1882.[1111] But the
expected diminution of the space between Saturn's globe and his rings
had not taken place. A slight extension in the width of the system, both
outward and inward, was indeed, hinted at; and it is worth notice that
just such a separation of the rings was indicated by Clerk Maxwell's
theory, so that there is an _a priori_ likelihood of its being in
progress. Yet Hall's measures in 1884-87[1112] failed to supply evidence
of alteration with time; and Barnard's,