THE OUTER PLANETS. 115 



outer satellites, Titania and Oberon, were re- 

 discovered in 1828 by his son, but the other 

 four, which he was believed to have discovered, 

 were never seen again. In 1847 two inner 

 satellites, Ariel and Umbriel, were discovered 

 by Lassell and Otto Struve respectively, their 

 existence being finally confirmed by LasselFs 

 observations in 1851. 



After the discovery of Uranus by Herschel, 

 mathematical astronomers determined its orbit 

 and calculated its position in the future. Alexis 

 Bouvard, the calculating partner of Laplace, 

 published tables of the planet's motions, founded 

 on observations made by various astronomers 

 who had considered it a star before its discovery 

 by Herschel ; but as the planet was not in the 

 exact position which Bouvard predicted, he 

 rejected the earlier observations altogether. For 

 a few years the planet conformed to the French- 

 man's predictions, but shortly afterwards it was 

 again observed to move in an irregular manner, 

 and the discrepancy between observation and 

 the calculations of mathematicians became intol- 

 erable. Did the law of gravitation not hold 

 good for the frontiers of the Solar System ? 

 Gradually astronomers arrived at the conclusion 

 that Uranus was being attracted off its course 

 by the influence of an unseen body, an exterior 



