e disregarded, is Dr. T.J. Hussey, of
Hayes, England, whose mind seems to have been one of the first to
anticipate the existence of an ultra-Uranian planet. And still again,
the English astronomer royal, Sir G.B. Airy must be mentioned as a
contributor to the final result; but he is to be regarded rather as a
contributor by negation. The great actors in the thing done were
Leverrier, Adams and Galle. English authors contend strongly for
placing the names in this order: Adams, Leverrier and Galle.

Suffice it to say that when Uranus was discovered by the elder
Herschel in 1781, that world was supposed to be the outside planet of
our system. Hitherto the splendid Saturn had marked the uttermost
excursion of astronomical knowledge as it respected our solar group.
For about a quarter of a century after Herschel's discovery the world
rested upon it as a finality. The orbit of Uranus was thought to
circumscribe the whole. But in the meantime, observations of this
orbit led to the knowledge that it did not conform in all respects to
astronomical and mathematical conditions. The orbit showed
irregularities, disturbances, perturbations, that could not be
accounted for when all of the known mathematical calculations were
applied thereto. Uranus was seen to get out of his path. At times he
would lag a little, and then at other times appear to be accelerated.
Each year, when the earth would swing around on the Uranian side of
the sun, the observations were renewed, but always with the result
that the planet did not seem to conform perfectly to the conditions of
his orbit. What could be the cause of this seeming disregard of
mathematical laws?

Astronomers could not accept the supposition that there was any actual
violation of the known conditions of gravitation. Certainly Uranus was
following his orbit under the centripetal and centrifugal laws in the
same manner as the other planets. There must, therefore, be some
undiscovered disturbing cause. It had already been noted that in the
case of the infra-Uranian planets they were swayed somewhat from their
paths by the mutual influence of one upon the other. This was
noticeable in particular in the movements of Jupiter, Saturn and
Uranus. When Saturn, for instance, would be on the same side of the
sun with Jupiter, it might be noted that the latter was drawn outward
and the former inward from their prescribed curves. The perturbation
was greatest when the planets were nearest, togeth