communications, written before he was twenty-one, are recorded in the Royal Society's "Catalogue of Scientific Papers," which also enumerates twenty-one others between 1859 and 1874. His wonderful keenness as an observer was signalized, while yet an undergraduate, by the discovery of a comet on the 29th day of April, 1856, and, four months after graduation, by the discovery of a planet on the 20th of October, 1857, which, however, proved to have been observed by Luther a few days before, and has been named Aglaia. His observations of Donati's comet, in 1858, possess a standard value, and his computation of the orbit is recognized as authoritative. The interest awakened by this comet prompted to the preparation of "A Popular Treatise on Comets," published early in 1860.
In 1860 Dr. Brünnow resumed the directorship of the Observatory, and young Watson was assigned to the chair of Physics in the university, which he retained for three years, when, on the final retirement of Dr. Brünnow, Watson was made Professor of Astronomy and Director of the Observatory, a position which he held and honored for sixteen years. Scarcely had he been clothed with full control of the instruments, when he resumed his remarkable career of discovery. There seemed almost a magic in his powers. Unrecognized celestial objects seemed to crowd spontaneously upon his notice. On September 14, 1863, he made his first independent planetary discovery. This was Eurynome. On January 9, 1864, he discovered the comet since known as 1,863, VI, which Respighi, as it proved, had already noted. On the 9th of October, 1865, he discovered a planet which also proved to have been announced by Peters, and has since been named Io. He discovered Minerva, August 24, and Aurora, September 6, 1867. During 1868 he added no less than six minor planets to the solar system, furnishing the only instance in which the list of planetary discoverers presents the same name four times in immediate succession.
Meantime he was engaged upon a work which might well have engrossed all his powers, and must have quite exceeded the abilities of any hut a gifted mathematical genius. It was no less than a complete digest of the results and methods of all the great writers on theoretical astronomy, and an independent development of the great principles of the science. "Having carefully read the works of the great masters," he says in his preface, "my plan was to prepare a complete work on the subject, commencing with the fundamental principles of dynamics, and systematically treating, from one point of view, all the problems presented." This broad plan, conceived by a young man of twenty-eight, and completed when twenty-nine, was executed with ability so commanding, that the work, on its appearance in 1869, was immediately accepted as an authoritative exposition of the higher principles and processes of dynamical astronomy, and was made a text-hook at Leipsic, at Paris, and at Greenwich. The same year he was sent by the General Government on an expedition to observe the