104 BAILLY. 



the ulterior and considerable improvements that the sci- 

 ence has since received ; even the discoveries of Lag-range 

 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 



