those later years when its planetary character was known. It could
not, of course, be expected that the orbit should remain unaltered;
the fact that the great planets Jupiter and Saturn revolve in the
vicinity of Uranus must necessarily imply that the orbit of the
latter undergoes considerable changes. When, however, due allowance
has been made for whatever influence the attraction of Jupiter and
Saturn, and we may add of the earth and all the other Planets, could
possibly produce, the movements of Uranus were still inexplicable. It
was perfectly obvious that there must be some other influence at work
besides that which could be attributed to the planets already known.

Astronomers could only recognise one solution of such a difficulty.
It was impossible to doubt that there must be some other planet in
addition to the bodies at that time known, and that the perturbations
of Uranus hitherto unaccounted for, were due to the disturbances
caused by the action of this unknown planet. Arago urged Le Verrier
to undertake the great problem of searching for this body, whose
theoretical existence seemed demonstrated. But the conditions of the
search were such that it must needs be conducted on principles wholly
different from any search which had ever before been undertaken for a
celestial object. For this was not a case in which mere survey with
a telescope might be expected to lead to the discovery.

Certain facts might be immediately presumed with reference to the
unknown object. There could be no doubt that the unknown disturber
of Uranus must be a large body with a mass far exceeding that of the
earth. It was certain, however, that it must be so distant that it
could only appear from our point of view as a very small object.
Uranus itself lay beyond the range, or almost beyond the range, of
unassisted vision. It could be shown that the planet by which the