ADDITIONS TO THE NEWTONIAN THEORY. 467 



Sect. 5. Discovery of the Laics of Double Stars. The tu'O 



Herschels. 



Xo truth, then, can be more certainly established, than that -the law 

 of gravitation prevails to the very boundaries of the solar system. But 

 does it hold good further ? Do the fixed stars also obey this universal 

 sway ? The idea, the question, is an obvious one but where are we 

 to find the means of submitting it to the test of observation ? 



o 



If the Stars were each insulated from the rest, as our Sun appears to 

 be from them, we should have been quite unable to answer this in- 

 quiry. But among the stars, there are some which are 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 as'tronomical history, pursuing 

 one object he fell in with another. Supposing such pairs to be really 

 unconnected, he wished to learn, from their phenomena, something re- 

 specting 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 determination 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 determine 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 measured with much correctness), and by 

 inventing a method which depended upon the whole body of obser- 

 vations, and not upon selected ones only, for the determination of the 

 motion, has made his investigations by far the most satisfactory of 

 those which have appeared. The result is, that it has been rendered 

 very probable, that in several of the double stars the two stars describe 

 ellipses about each other ; and therefore that here also, at an immeas- 



