The movements of Uranus soon gave rise to difficulties. It was found impossible, satisfactorily, to reconcile the earlier and later observations, and in the tables of Uranus, published by BOUVARD in 1821, the earlier observations were rejected. But the discrepancies between the observed and calculated places for the planet soon began to reappear and quickly increase, and the suggestion was made that these discrepancies were due to an attraction exercised by some planet as yet unknown. Thus Mrs. Somerville in a little book on the connection of the physical sciences, published in 1836, wrote, "Possibly it (that is, Uranus) may be subject to disturbances from some unseen planet revolving about the Sun beyond the present boundaries of our system. If, after the lapse of years, the tables formed from a combination of numerous observations should still be inadequate to represent the motions of Uranus, the discrepancies may reveal the existence, nay, even the mass and orbit of a body placed for ever beyond the sphere of vision." In 1843 JOHN C. ADAMS, who had just graduated as Senior Wrangler at Cambridge, proceeded to attack the problem of determining the position, orbit, and mass of the unknown body by which on this assumption Uranus was disturbed, from the irregularities evident in the motion of that planet. The problem was one of extraordinary intricacy, but by September 1845 Adams had obtained a first solution, which, he submitted to AIRY, the Astronomer Royal. As, however, he neglected to reply to some inquiries made by Airy, no search for the new planet was instituted in England until the results of a new and independent worker had been published. The same problem had been attacked by a well-known and very gifted French mathematician, U. J. J. LEVERRIER, and {40} in June 1846 he published his position for the unseen planet, which proved to be in close accord with that which Adams had furnished to Airy nine months before. On this Airy stirred up Challis, the Director of the Cambridge Observatory, which then possessed the most powerful telescope in England, to search for the planet, and Challis commenced to make charts, which included more than 3000 stars, in order to make sure that the stranger should not escape his net. Leverrier, on the other hand, communicated his result to the Berlin Observatory, where they had just received some of the star charts prepared by Dr. Bremiker in connection with the search for minor planets. The Berlin observer, Dr. Galle, had therefore nothing to do but to compare the stars in the field, upon which he turned his telescope, with those shown on the chart; a star not in the chart would probably be the desired stranger. He found it, therefore, on the very first evening, September 23, 1846, within less than four diameters of the Moon of the predicted place. The same object had been observed by Challis at Cambridge on August 4 and 12, but he was deferring the reduction of his observations until he had