342 ~LAPLACE. 
such telescopes could not be employed on board a — 
tossed about by the waves. 
The method of Galileo seemed, at any rate, to retain 
all its advantages when applied on land, and to promise 
immense improvements to geography. ‘These expecta- 
tions were found to be premature. The movements of 
the satellites of Jupiter are not by any means so simple 
as the immortal inventor of the method of longitudes 
supposed them to be. It was necessary that three gen- 
erations of astronomers and mathematicians should labour 
with perseverance in unfolding their most considerable 
perturbations. It was necessary, in fine, that the tables 
of those bodies should acquire all desirable and necessary 
precision, that Laplace should introduce into the midst 
of them the torch of mathematical analysis. 
In the present day, the nautical ephemerides contain, 
several years in advance, the indication of the times of 
the eclipses and reappearances of Jupiter’s satellites. 
Calculation does not yield in precision to direct observa- 
tion. In this group of satellites, considered as an inde- 
pendent system of bodies, Laplace found a series of 
perturbations analogous to those which the planets ex- 
perience. The rapidity of the revolutions unfolds, in a 
sufficiently short space of time, changes in this system 
which require centuries for their complete development 
in the solar system. 
Although the satellites exhibit hardly an appreciable 
diameter even when viewed in the best telescopes, our 
illustrious countryman was enabled to determine their 
masses. Finally, he discovered certain simple relations 
of an extremely remarkable character between the move- 
ments of those bodies, which have been called the laws 
of Laplace. Posterity will not obliterate this designa- 
