BAILLY A MEMBER OF THE ACADEMY OF SCIENCES.—­HIS RESEARCHES ON JUPITER’S SATELLITES.

Bailly was named member of the Academy of Sciences the 29th January, 1763.  From that moment his astronomical zeal no longer knew any bounds.  The laborious life of our fellow-academician might, on occasion, be set up against a line, more fanciful than true, by which an ill-natured poet stigmatized academical honours.  Certainly no one would say of Bailly, that after his election,

     “Il s’endormit et ne fit qu’un somme.”

     “He fell asleep and made but one nap (or sum).”

On the contrary, we cannot but be surprised at the multitude of literary and scientific labours that he accomplished in a few years.

Bailly’s earliest researches on Jupiter’s satellites began in 1763.

The subject was happily chosen.  Studying it in all its generalities, he showed himself both an indefatigable computer, a clear-sighted geometer, and an industrious and able observer.  Bailly’s researches on the satellites of Jupiter, will always be his first and chief claim to scientific glory.  Before him, the Maraldis, the Bradleys, the Wargentins had discovered empirically some of the principal perturbations that those bodies undergo, in their revolving motions around the powerful planet that rules them; but they had not been traced up to the principles of universal attraction.  The initiative honour in this respect belongs to Bailly.  Nor is this honour decreased by the ulterior and considerable improvements that the science has since received; even the discoveries of Lagrange and of Laplace have left this honour intact.

The knowledge of the satellitic motions rests almost entirely on the observation of the precise moment when each of those bodies disappears, by entering into the conical shadow, which the immense opaque globe of Jupiter projects on the opposite side from the sun.  In the course of discussing a multitude of these eclipses, Bailly was not long in perceiving that the computers of the Satellitic Tables worked on numerical data that were not at all comparable with each other.  This seemed of little consequence previous to the birth of the theory; but, after the analytical discovery of the perturbations, it became desirable to estimate the possible errors of observation, and to suggest means for remedying them.  This was the object of the very considerable work that Bailly presented to the Academy in 1771.

In this beautiful memoir, the illustrious astronomer developes the series of experiments, by the aid of which each observation may give the instant of the real disappearance of a satellite, distinguished from the instant of the apparent disappearance, whatever be the power of the telescope used, whatever be the altitude of the eclipsed body above the horizon, and consequently, whatever be the transparency of the atmospheric strata through which the phenomenon is