SECT, viii.] ASTRONOMICAL TABLES. 73 



lysis fails, because the series expressing the co-ordinates of 

 the bodies become extremely complicated, and do not con- 

 verge when applied to comets and the telescopic planets. 

 This difficulty has been overcome by Sir John Lubbock, 

 who has the honour of having completed the theory of 

 planetary motion, which becomes every day of more im- 

 portance, from the new planets that have been discovered, 

 and also with regard to comets, many of which return to 

 the sun at regular intervals, and from whose perturbations 

 the masses of the planets will be more accurately deter- 

 mined, and the retarding influence of the ethereal medium 

 better known. 



