128 MEMOIR OF DELAMBRE. 
powerful motives of public utility and the grandeur of the subject could have 
sustained him in its prosecution. The satellites which accompany Jupiter, and 
which disappear when they enter into his shadow, were the first celestial bodies 
revealed to us by the telescope. ‘Their eclipses, so entirely analogous to those 
of the moon, recur much more frequently than the latter; for a single one of 
these bodies is obscured four times in the space of seven days. Galileo, who 
first contemplated these singular phenomena, quickly inferred that observations 
of this kind might be rendered eminently subservient to the promotion of geo- 
graphical science. In fact, it required but a knowledge of the courses of these 
satellites, and their reduction into tables of sufficient exactness, to rectify a 
multitude of enormous errors in the determination of longitude, especially in 
regard to the eastern portions of the Old Continent. It is true, indeed, that 
many causes concur to limit the use and precision of this method, but it cannot 
the less be looked upon as an invaluable source of discovery, and in its conse- 
quences as one of the most fortunate of modern inventions; for even at this day 
the navigator derives the easiest means of ascertaining the proximity of Jand 
and the relative position of the places he is approaching from a group of celes- 
tial objects, whose minute size seemed to withdraw them forever from human 
observation. Nor is this the only consideration by which this world of Jupiter 
is recommended to our admiration, and invested with an interest not inferior to 
that of any other portion of the heavens. To an attentive observation of the 
eclipses of his satellites we owe our knowledge of the fact that the action of light 
is not instantaneous; and from the same source has been derived the precise 
measure of the time which is required for its propagation from the sun to our 
globe—a capital discovery, made by Reaumer at the observatory of Paris, and 
which a subsequent theory has fully confirmed. 
» The system composed of Jupiter and his four satellites is a world apart, whose 
rapid revolutions mirror to us those which are taking place in the general system 
of the sun aud planets. Hence, the study of the inequalities of those satellites 
may be said to economize astronomic time, as they present to our view a class 
of phenomena which will require an immense series of ages for their development 
in the planetary system. 
The three first satellites of Jupiter are subjected, by their mutual action and 
that of the planet, to two very remarkable laws, not less simple or constant than 
those of Kepler. There is a reciprocal dependence between their movements 
and their positions, so that the place of two of them being known, that of the 
third is thereby determined; and as one consequence of this state of things, they 
can never be all three eclipsed at the same time. Laplace had discovered these 
laws, and had demonstrated that they are necessary results from the mutual 
action of the satellites, and that the same cause tends to perpetuate their opera- 
tion. All observation affording subsistent proof of the truth of these laws, 
Delambre made those admirable theorems of Laplace the basis of his researches. 
He had occupied several years in the composition of ecliptic tables founded upon 
them, when the Academy of Sciences proposed the same inquiries as the subject 
of a prize. This was awarded to Delambre. As he had before obtained this 
distinction on account of the planet Herschel, he now a second time earned it 
on account of those of Galileo. A short time before this he had been elected a 
member of the Academy. 
About this time the execution of a grand and difficult project was set on foot 
in France—a project whose importance had attracted the interest of all en- 
lightened nations, its object being to establish a uniform system of measures 
founded on some natural and invariable base. As the chief element of the 
French metrical system, a determinate portion of the terrestrial meridian was 
fixed upon, and a fortunate opportunity was thus offered of renewing those im- 
portant geodesic operations which have carried to probably the highest attain- 
able degree of precision our knowledge of the figure and dimensions of the 
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