y Le Verrier and Adams, which gave still wider extent to
the solar system by the discovery of the planet Neptune revolving far
outside Uranus.

We have already alluded to the difficulties which were experienced when
it was sought to reconcile the early observations of Uranus with those
made since its discovery. We have shown that the path in which this
planet revolved experienced change, and that consequently Uranus must be
exposed to the action of some other force besides the sun's attraction.

The question arises as to the nature of these disturbing forces. From
what we have already learned of the mutual deranging influence between
any two planets, it seems natural to inquire whether the irregularities
of Uranus could not be accounted for by the attraction of the other
planets. Uranus revolves just outside Saturn. The mass of Saturn is much
larger than the mass of Uranus. Could it not be that Saturn draws Uranus
aside, and thus causes the changes? This is a question to be decided by
the mathematician. He can compute what Saturn is able to do, and he
finds, no doubt, that Saturn is capable of producing some displacement
of Uranus. In a similar manner Jupiter, with his mighty mass, acts on
Uranus, and produces a disturbance which the mathematician calculates.
When the figures had been worked out for all the known planets they were
applied to Uranus, and we might expect to find that they would fully
account for the observed irregularities of his path. This was, however,
not the case. After every known source of disturbance had been carefully
allowed for, Uranus was still shown to be influenced by some further
agent; and hence the conclusion was established that Uranus must be
affected by some unknown body. What could this unknown body be, and
where must it be situated? Analogy was here the guide of those who
speculated on this matter. We know no cause of disturbance of a planet's
motion except it be the attraction of another planet. Could it be that
Uranus was really attracted by some other planet at that time utterly
unknown? This suggestion was made by many astronomers, and it was
possible to determine some conditions which the unknown body should
fulfil. In the first place its orbit must lie outside the orbit of
Uranus. This was necessary, because the unknown planet must be a large
and massive one to produce the observed irregularities. If, therefore,
it were nearer than Uranus, it would be a conspicuous object, and m