342 LAPLACE. 



such telescopes could not be employed on board a ship 

 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- 



