ADDITIONS TO THE NEWTONIAN THEORY. 269 



called double stars, and which consist of two stars, 

 so near to each other that the telescope alone can 

 separate them. The elder Herschel diligently ob- 

 served and measured the relative positions of the 

 two stars in such pairs; and as has so often happened 

 in astronomical history, pursuing one object he fell 

 in with another. Supposing such pairs to be really 

 unconnected, he wished to learn, from their pheno- 

 mena, something respecting the annual parallax 

 of the earth's orbit. But in the course of twenty 

 years' observations he made the discovery (in 1803) 

 that some of these couples were turning round 

 each other with various angular velocities. These 

 revolutions were, for the most part, so slow that 

 he was obliged to leave their complete determina- 

 tion as an inheritance to the next generation. His 

 son was not careless of the bequest, and after 

 having added an enormous mass of observations 

 to those of his father, he applied himself to deter- 

 mine the laws of these revolutions. A problem 

 so obvious and so tempting was attacked also by 

 others, as Savary and Encke, in 1830 and 1832, 

 with the resources of analysis. But a problem in 

 which the data are so minute and inevitably imper- 

 fect, required the mathematician to employ much 

 judgment, as well as skill in using and combining 

 these data; and Sir John Herschel, by employing 

 positions only of the line joining the pair of stars, 

 (which can be observed with comparative exactness) 

 to the exclusion of their distances, (which cannot be 



