3I4 NINETEENTH CENTURY. FT. in. 



In order to disentangle this complicated series of changes, 

 it was necessary to gather together all the observations made 

 of the positions of each planet from the earliest ages, and 

 to combine the whole mathematically into one complete 

 theory, in which all the movements should be truly and 

 accurately accounted for. This Leverrier undertook, and 

 it was in the course of his work that he found some move- 

 ments of Saturn and Uranus which were not produced by 

 any of the known planets, and he was thus able to announce 

 the existence of the planet Neptune (see p. 304). He next 

 found that Mars and Mercury were pulled out of their path 

 by some matter for which he could not account. As far as 

 Mars is concerned, this matter proved to be upon our earth, 

 whose mass had been reckoned too low, but in the case of 

 Mercury the cause is not yet certain, although the supposed 

 planet of Lescarbault and the two small bodies observed by 

 Watson may probably help to explain the irregularity. 



With these and a few other trifling exceptions, Leverrier 

 succeeded in 1874 in completing his gigantic task, and in 

 computing the position of the four most distant planets, 

 Jupiter, Saturn, Uranus, and Neptune, for a period of 2000 

 years from 1880 at intervals of 500 years, so that future 

 astronomers will be able to calculate at the periods 2350, 

 2850, 3350, and 3850, whether the movements of the 

 planets have followed the same order as in times past, or 

 whether any disturbing influence has arisen. 



Chief Works consisted. Airy's ' Report on Astronomy, ' British 

 Association, 1883; J. D. Forbes's 'Progress of Mathematical and 

 Physical Science' Sixth Dissertation; ' Encyclopaedia Britannica,' 

 new edition ; Guillemin, 'The Heavens;' Herschel's 'Astronomy;' 

 Grant's ' Physical Astronomy;' ' Reports of the Astronomical Society ;' 

 'The Orbs of Heaven,' Mitchell; Proctor, 'On Shooting-Stars and 

 Meteors;' Miss Clerke, 'Astronomy of the Nineteenth Century,' 1886. 



