accomplished would perhaps have been deemed impossible had it not
been actually done.

The superb success which had attended Le Verrier's efforts to explain
the cause of the perturbations of Uranus, naturally led this
wonderful computer to look for a similar explanation of certain other
irregularities in planetary movements. To a large extent he
succeeded in showing how the movements of each of the great planets
could be satisfactorily accounted for by the influence of the
attractions of the other bodies of the same class. One circumstance
in connection with these investigations is sufficiently noteworthy to
require a few words here. Just as at the opening of his career, Le
Verrier had discovered that Uranus, the outermost planet of the then
known system, exhibited the influence of an unknown external body, so
now it appeared to him that Mercury, the innermost body of our
system, was also subjected to some disturbances, which could not be
satisfactorily accounted for as consequences of any known agents of
attraction. The ellipse in which Mercury revolved was animated by a
slow movement, which caused it to revolve in its plane. It appeared
to Le Verrier that this displacement was incapable of explanation by
the action of any of the known bodies of our system. He was,
therefore, induced to try whether he could not determine from the
disturbances of Mercury the existence of some other planet, at
present unknown, which revolved inside the orbit of the known
planet. Theory seemed to indicate that the observed alteration in
the track of the planet could be thus accounted for. He naturally
desired to obtain telescopic confirmation which might verify the
existence of such a body in the same way as Dr. Galle verified the
existence of Neptune. If there were, indeed, an intramercurial
planet, then it must occasionally cross between the earth and the