er. In like manner
Uranus did obeisance to both his huge neighbors on the sun's side of
his orbit. He, too, veered toward them as he passed, and they in turn
recognized the courtesy by going out of their orbits as they passed.
What, therefore, should be said of the outswinging movement of Uranus
from his orbit in that part of his course where no disturbing
influence was known to exist? Certainly _something_ must be in that
quarter of space to occasion the perturbation. What was it?

It would appear that the elder Bouvart, the French astronomer referred
to above, was the first to suggest that the disturbances in the orbit
of Uranus, throwing that planet from his pathway outward, might be and
probably were to be explained by the presence in outer space of an
unknown ultra-Uranian planet. Bouvart prepared tables to show the
perturbations in question, and declared his opinion that they were
caused by an unknown planet beyond. No observer, however, undertook to
verify this suggestion or to disprove it. Nor did Bouvart go so far as
to indicate the particular part of the heavens which should be
explored in order to find the undiscovered world. His tables, however,
do show from the perturbations of the orbits of Jupiter, Saturn and
Uranus that the same are caused by the mutual influence of the planets
upon one another.

It seems to have remained for Dr. T.J. Hussey, of Hayes, England, to
suggest the actual discovery of the unknown planet by following the
clew of the disturbance produced by its presence in a certain field of
space. Dr. Hussey, in 1834, wrote to Sir George Biddell Airy,
astronomer royal at Greenwich, suggesting that the perturbation of the
orbit of Uranus might be used as the clew for the discovery of the
planet beyond. But Sir George was one of those safe, conservative
scholars who scorn to follow the suggestions of genius, preferring
rather to explore only what is known already. He said in answer that
he doubted if the irregularity in the Uranian orbit was in such a
state of demonstration as to give any hope of the discovery of the
disturbing cause. He doubted even that there was such irregularity in
the Uranian orbit. He was of opinion that the observers had been
mistaken in the alleged detection of perturbations. So the Greenwich
observatory was not used on the line of exploration suggested by
Hussey.

Three years afterward, and again in 1842, Sir George received letters
from the younger Bouvart, again su