From these early observations it was possible
to determine the track of Uranus, and it was found that the great
planet takes a period of no less than eighty-four years to accomplish
a circuit. Calculations were made of the shape of the orbit in which
it revolved before its discovery by Herschel, and these were compared
with the orbit which observations showed the same body to pursue in
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.
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