BAILLY, JEAN SYLVAIN, a celebrated French astronomer, was born at Paris on the 15th September 1736. A genius for painting, having been heredi tary in the family for four successive generations, Badly was bred to the profession of his ancestors, and made considerable progress in that delightful art. A passion, however, for poetry, and other branches a literature, distracted the attention of the young artist, and unfitted him for that intense and undeviat ing application to the practice of his art which can alone raise the painter to opulence and fame.
The friends of Bailly soon perceived that his mind was bent upon subjects foreign to his profession, and regretted that a genius so promising and ardent should be chained down to the practice of an art when it aimed at the highest flights of literature and science. An accidental acquaintance with the celebrated as- ' tronomer La Caille, determined the general train of his studies, and inspired him with the most passionate enthusiasm for the science of astronomy. His first effort in this new career was the calculation of the orbit of the famous comet of 1759, which was pub. lisped in the memoirs of the academy for that year. On the 29th January, 1763, Bailly was admitted a member of the academy of sciences, and in the same year he published three memoirs on the theory of Jupiter's satellites, and his reduction of the numerous observations made by La Caille in 1760 and 1761, on 515 zodiacal stars. These reductions were pub lished in 1763, at the beginning of the Ephemerides computed by La Caille for the years 1765-1774.
The importance which was now attached to the method of finding the longitude by the eclipses of Jupiter's satellites, turned the attention of astrono mers to the theory of these secondary planets. This. interesting subject was proposed by the academy as, the prize question for 1764, and Bailly engaged in the investigation with the utmost ardour. In the illustrious La Grange, who was almost ex actly of the same age with himself, he found a for midable and a successful rival. In applying the prob
lem of the three bodies to the satellites of Jupiter, Bailly considered only the action of one satellite upon another, while La Grange viewed the subject in a more general aspect, and took. into account the mu tual derangements of all the four satellites. The results of Bailly's investigations were published in 1766, in a separate treatise, entitled, Essais sur la Theorie des Satellites de Jupiter, sztivi des tables de leur mouvement ; which likewise contained the history of that branch of astronomy. In this treatise lie happened to mention as his own, the discovery of the cause of the variation in the inclination of the orbits of Jupiter's satellites. This•circumstance occasioned a dispute between him and La Laude, who laid claim to the same discovery. Bailly asserted his own claim in the Journal Encyclopedique for June 1773, but he had afterwards the candour to state in his history of astronomy, the opposite claims of La Laude and himself, and to leave the subject to the decision of his readers.
The difficulty of finding the exact instant of the immersions and emersions of the satellites of Jupiter, stimulated Bailly to make a number of observations on this curious subject, which he has published in an interesting paper in the memoirs of the academy for 1771. The great discrepancy which was perceived in the observation of these eclipses, obviously arose the diameters of the satellites, and from the different apertures of the telescopes with which they were ob served.* In order to determine the exact diameters. of the satellites, Bailly observed an immersion with a telescope whose aperture was so much contracted that the satellite could scarcely be seen, so that it became entirely visible when the smallest portion of its diameter had entered into the shadow. He then observed the immersion of the same satellite with the.