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, together.  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.