degree than that to which mathematicians are
accustomed in their investigation of the more ordinary features of our
system. To express this law it is necessary to have recourse to the
daily movements of the satellites; these are respectively--

SATELLITE. DAILY MOVEMENT.
I. 382 deg..2.
II. 262 deg..74.
III. 190 deg..7.
IV. 131 deg..4.

The law states that if to five times the movement of the first satellite
we add that of the third and four times that of the fourth, the whole
will equal ten times the movement of the second satellite. The
calculation stands thus:--

5 times I. equals 1911 deg..0
III. equals 190 deg..7 II. 262 deg..74
4 times IV. equals 525 deg..6 10
-------- --------
2627 deg..3 equal 2627 deg..4 nearly.

Nothing can be simpler than the verification of this law; but the task
of showing the physical reason why it should be fulfilled has not yet
been accomplished.

Saturn was the most distant planet known to the ancients. It revolves in
an orbit far outside the other ancient planets, and, until the discovery
of Uranus in the year 1781, the orbit of Saturn might well be regarded
as the frontier of the solar system. The ringed planet was indeed a
worthy object to occupy a position so distinguished. But we now know
that the mighty orbit of Saturn does not extend to the frontiers of the
solar system; a splendid discovery, leading to one still more splendid,
has vastly extended the boundary, by revealing two mighty planets,
revolving in dim telescopic distance, far outside the path of Saturn.
These objects have not the beauty of Saturn; they are, indeed, in no
sense effective telescopic pictures. Yet these outer planets awaken an
interest of a most special kind. The discovery of each is a classical
event in the history of astronomy, and the opinion has been maintained,
and perhaps with reason, that the discovery of Neptune, the more remote
of the two, is the greatest achievement in astronomy made since the time
of Newton.

CHAPTER XIV

URANUS.

Contrast between Uranus and the other great Planets--William
Herschel--His Birth and Parentage--Herschel's Arrival in
England--His Love of Learning--Commencement of his Astronomical
Studies--The Construction of Telescopes--Construction of
Mirrors--The Professor of M