It may seem a pity to many that the Greenwich equatorial was not pointed at the place, just to see whether any foreign object did happen to be in that neighborhood; but it is no light matter to derange the work of an observatory, and alter the plans laid out for the staff, into a sudden sweep for a new planet on the strength of a mathematical investigation just received by post. If observatories were conducted on these unsystematic and spasmodic principles they would not be the calm, accurate, satisfactory places they are. Of course, if anyone had known that a new planet was to be found for the looking, _any_ course would have been justified; but no one could know this. I do not suppose that Adams himself felt an absolute confidence in his attempted prediction. So there the matter dropped. Adams's communication was pigeonholed, and remained in seclusion eight or nine months. Meanwhile, and quite independently, something of the same sort was going on in France. A brilliant young mathematician, Urban Jean Joseph Leverrier, born in Normandy in 1811, held the post of astronomical professor at the École Polytechnique, founded by Napoleon. His first published papers directed attention to his wonderful powers; and the official head of astronomy in France, the famous Arago, suggested to him the unexplained perturbations of Uranus as a worthy object for his fresh and well-armed vigor. At once he set to work in a thorough and systematic way. He first considered whether the discrepancies could be due to errors in the tables or errors in the old observations. He discussed them with minute care, and came to the conclusion that they were not thus to be explained away. This part of the work he published in November, 1845. He then set to work to consider the perturbations produced by Jupiter and Saturn to see whether they had been accurately allowed for, or whether some minute improvements could be made sufficient to destroy the irregularities. He introduced several fresh terms into these calculations, but none of them of sufficient importance to do more than partly explain the mysterious perturbations. He next examined the various hypotheses that had been suggested to account for them. Were they caused by a failure in the law of gravitation or by the presence of a resisting medium? Were they due to some large but unseen satellite or to a collision with some comet? All these theories he examined and dismissed for various reasons. The perturbations were due to some continuous cause--for instance, some unknown planet. Could this planet be inside the orbit of Uranus? No, for then it would perturb Saturn and Jupiter also, and they were not perturbed by it. It must, therefore, be some planet outside the orbit of Uranus, and in all probability, according to Bode's empirical law, at nearly double the distance from the sun that Uranus is. Finally he proceeded to determine where this planet was, and what its orbit must be to produce the observed disturbances.