s, 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 observed, also
whatever be the distance from that body to the sun, or to the planet;
finally, whatever be the sensibility of the observer's sight, all which
circumstances considerably influence the time of apparent disappearance.
The same series of ingenious and delicate observations led the author,
very curiously, to the determination of the true diameters of the
satellites, that is to say, of small luminous points, which, with the
telescopes then in use, showed no perceptible diameter.

I will rest contented with these general considerations; only remarking,
in addition, that the diaphragms used by Bailly were not intended only
to diminish the quantity of light contributi