, for
example. The mass of the ring system, however, must be exceedingly
small, for it does not appear to produce any disturbances in the
movements of Saturn's satellites. From the kinetic theory, therefore,
one would not expect to find any atmosphere on the rings, and the
absence of it is duly shown by spectroscopic observations.

The diameter of Saturn, roughly speaking, is about one-fifth less than
that of Jupiter. The planet is very flattened at the poles, this
flattening being quite noticeable in a good telescope. For instance, the
diameter across the equator is about 76,470 miles, while from pole to
pole it is much less, namely, 69,770.

The surface of Saturn bears a strong resemblance to that of Jupiter. Its
markings, though not so well defined, are of the same belt-like
description; and from observation of them it appears that the planet
rotates _on an average_ in a little over ten hours. The rotation is in
fact of the same peculiar kind as that of the sun and Jupiter; but the
difference of speed at which the various portions of Saturn go round are
even more marked than in the case of the Giant Planet. The density of
Saturn is less than that of Jupiter; so that it must be largely in a
condition of vapour, and in all probability at a still earlier stage of
planetary evolution.

Up to the present we know of as many as ten satellites circling around
Saturn, which is more than any other planet of the solar system can lay
claim to. Two of these, however, are very recent discoveries; one,
Phoebe, having been found by photography in August 1898, and the other,
Themis, in 1904, also by the same means. For both of these we are
indebted to Professor W.H. Pickering. Themis is said to be _the faintest
object in the solar system_. It cannot be _seen_, even with the largest
telescope in existence; a fact which should hardly fail to impress upon
one the great advantage the photographic plate possesses in these
researches over the human eye.

The most important of the whole Saturnian family of satellites are the
two known as Titan and Japetus. These were discovered respectively by
Huyghens in 1655 and by Cassini in 1671. Japetus is about the same size
as our moon; while the diameter of Titan, the largest of the satellites,
is about half as much again. Titan takes about sixteen days to revolve
around Saturn, while Japetus takes more than two months and a half. The
former is about three-quarters of a million miles distant fro