LAii'LACE. 151 



istiiig tables of the moou eutitles Laplace to be ranked amoug the beue- 

 factors of bumaniti'.* 



Ill tbe beginuiiiy: of the year IGLl, Galileo supposed that be found iu 

 tbe eclipses of Jupiter's satellites a simple aud rigorous solution of tbe 

 famous problem of tbe longitude, and active negotiations were imme- 

 diately commenced with tbe view of introducing tbe new metbod on 

 board the numerous vessels of Spain and Holland. Tbese negotiations 

 failed. From tbe discussion, it plainly appeared tbat tbe accurate ob- 

 servation of tbe eclipses of tbe satellites would requiiie powerful tele- 

 sco[)es ; but sucb telescopes could not be employed on board a sbip 

 tossed about by tbe waves. 



Tbe metbod of Galileo seemed, at any rate, to retain all its advan- 

 tages wbeu applied on land, and to promise immense improvements to 

 geogra[)liy. Tbese expectations were found to be premature. Tbe 

 movements of tbe satellites of Jupiter are not by any means so simple 

 as tbe immortal inventor of tbe method of longitudes supposed them to 

 be. It was necessary tbat three generations of astronomers and mathe- 

 maticians should labor with perseverance in unfolding their most con- 

 siderable perturbations. It was necessary, in fine, tbat the tables of 

 those bodies should acquire all desirable aud necessary precision, tbat 

 Laplace should introduce into the midst of them the torch of mathemati- 

 cal analysis. 



In tbe present day, tbe nautical e«jhemerides contain, several years in 

 advance, the indication of the times of the eclipses aud re-appearances 

 of Jupiter's satellites. Calcubition does not yield iu precision to direct 

 observation. In this group of satellites, considered as an independent 

 system of bodies, Laplace found a series of perturbations analogous to 

 those which tbe planets experience. Tbe rapidity of the revolutions un- 

 folds, iu a "^ufticiently short s[»ace of time, changes in this system which 

 require centuries for tbeir complete development iu the solar system. 



Although tbe satellites exhibit hardly an ajipreciabie diameter even 

 when viewed in tbe best telescopes, our illustrious countryman was en- 

 abled to determine tbeir masses. Fiually, he discovered certain simple 

 relations of an extremely remarkable character between the movements 

 of those bodies, which have been called tbe laws of Laplace. Posterity 

 will not obliterate this desiguation ; it will acknowledge the propriety 

 of inscribing iu the heavens the name of so great an astronomer beside 

 that of Kepler. 



* The theoretical researches of Laplace formed the basis of Burckhardt's Luuar Tables, 

 which are chiefly employed in computiLg the places of the moon for the Nauiical Almauac 

 and other ephemerides. These tables were defaced by an empiric equation, suggested for 

 the purpose of representing au inequality of long period which seemed to affect the mean 

 longitude of the moon. No satisfactory explanation of the origin of this ii equality could' 

 be discovered by any geometer, although it formed the subject of much toilsome investiga- 

 tion throughout the present century, until at length M. Hanson 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 "27.3 years, and that of the other is 2o9 years. The maximum value of 

 the former is 27". 4, and that of the latter is 23"."<J. — Tuanslatok. 



