104 . BAILLY. 
the ulterior and considerable improvements that the sci- 
ence 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 con- 
ical 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 con- 
sequence 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 de- 
velopes the series of experiments, by the aid of which 
each observation may give the instant of the real disap- 
pearance 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, what- 
ever be the transparency of the atmospheric strata 
through which the phenomenon is observed, also what- 
ever be the distance from that body to the sun, or to the 
planet; finally, whatever be the sensibility of the obser- 
ver’s sight, all which circumstances considerably influ- 
ence the time of apparent disappearance. The same 
series of ingenious and delicate observations led the 
author, very curiously, to the determination of the true 
