utions,
running counter to the grand current of movement in the solar system,
are performed from east to west, in a plane inclined at an angle of 35
deg. to that of the ecliptic. Their swiftness serves to measure the mass
of the globe round which they are performed. For while our moon takes
twenty-seven days and nearly eight hours to complete its circuit of the
earth, the satellite of Neptune, at a distance not greatly inferior,
sweeps round its primary in five days and twenty-one hours, showing
(according to a very simple principle of computation) that it is urged
by a force seventeen times greater than the terrestrial pull upon the
lunar orb. Combining this result with those of Professor Barnard's[225]
and Dr. See's[226] recent measurements of the small telescopic disc of
this farthest known planet, it is found that while in _mass_ Neptune
equals seventeen, in _bulk_ it is equivalent to forty-nine earths. This
is as much as to say that it is composed of relatively very light
materials, or more probably of materials distended by internal heat, as
yet unwasted by radiation into space, to about five times the volume
they would occupy in the interior of our globe. The fact, at any rate,
is fairly well ascertained, that the average density of Neptune is about
twice that of water.

We must now turn from this late-recognised member of our system to
bestow some brief attention upon the still fruitful field of discovery
offered by one of the immemorial five. The family of Saturn, unlike that
of its brilliant neighbour, has been gradually introduced to the notice
of astronomers. Titan, the sixth Saturnian moon in order of distance,
led the way, being detected by Huygens, March 25, 1655; Cassini made the
acquaintance of four more between 1671 and 1684; while Mimas and
Enceladus, the two innermost, were caught by Herschel in 1789, as they
threaded their lucid way along the edge of the almost vanished ring. In
the distances of these seven revolving bodies from their primary, an
order of progression analogous to that pointed out by Titius in the
planetary intervals was found to prevail; but with one conspicuous
interruption, similar to that which had first suggested the search for
new members of the solar system. Between Titan and Japetus--the sixth
and seventh reckoning outwards--there was obviously room for another
satellite. It was discovered on both sides of the Atlantic
simultaneously, on the 19th of September, 1848. Mr. W. C.