RESEARCHES ON JUPITER'S SATELLITES. 341 



youthful and which communicated itself to able coad- 

 jutors, Laplace solved the celebrated problem of the 

 longitude more completely than could have been hoped 

 for in a scientific point of view, with greater precision 

 than the art of navigation in its utmost refinement de- 

 manded. The ship, the sport of the winds and tem- 

 pests, has no occasion, in the present day, to be afraid 

 of losing itself in the immensity of the ocean. An in- 

 telligent glance at the starry vault indicates to the pilot, 

 in every place and at every time, his distance from the 

 meridian of Paris. The extreme perfection of the ex- 

 isting tables of the moon entitles Laplace to be ranked 

 among the benefactors of humanity.* 



In the beginning of the year 1611, Galileo supposed 

 that he found in the eclipses of Jupiter's satellites a sim- 

 ple and rigorous solution of the famous problem of the 

 longitude, and active negotiations were immediately com- 

 menced with the view of introducing the new method on 

 board the numerous vessels of Spain and Holland. 

 These negotiations failed. From the discussion it plainly 

 appeared that the accurate observation of the eclipses 

 of the satellites would require powerful telescopes ; but 



* The theoretical researches of Laplace formed the basis of Burck- 

 hnrdt's Lunar Tables, which are chiefly employed in computing the 

 places of the moon for the Nautical Almanac and other Ephemerides. 

 These tables were defaced by an empiric equation, suggested for the 

 purpose of representing an inequality of long period which seemed to 

 affect the mean longitude of the moon. No satisfactory explanation 

 of the origin of this inequality could be discovered by any geometer, 

 although it formed the subject of much toilsome investigation 

 throughout the present century, until at length M. Hansen found it 

 to arise from a combination of two inequalities due to the disturbing 

 action of Venus. The period of one of these inequalities is 273 years, 

 and that of the other is 239 years. The maximum value of the 

 former is 27"-4, and that of the "latter is 23" -2. Translator. 



