274 EIGHTEENTH CENTURY. pt. HI. 



the movement of our earth In its orbit, had another peculiar 

 curved motion, as if they were both moving round some point 

 half-way between them. This movement was so slow that it 

 was twenty-five years before he could be sure about it ; but at 

 the end of this time he was able to tell the Royal Society 

 that these double or binary stars, as they are called, are not 

 one behind the other, but are actual pairs of stars moving 

 round and round each other, as if they were connected by a 

 rod suspended by its centre, and then set revolving ! 



To understand how great a discovery this was, it is neces- 

 sary to bear in mind that Newton had only been able to 

 prove that gravitation acts between the sun and the planets ; 

 but here was a reason for believing that even in the far-off stars, 

 millions of miles away from our system, the same force is 

 holding distant suns together, and keeping them in their 

 orbits. This great discovery has been still more clearly 

 proved by later investigations, and groups of two, three, and 

 even more stars are now known, in which these bodies re- 

 volve round a common centre, held together by the force 

 of gravitation. 



Herschel studies Star-clusters and Nebulae, 1786. — The 

 next discovery which Herschel made was quite as remark- 

 able as that of the binary stars. As long ago as the time of 

 Ptolemy (loo B.C.) five curious stars had been observed, 

 which he called * cloudy stars,' because they looked as if 

 they were covered by a mist ; and the number of these 

 cloudy masses had been increased by different astronomers 

 as time went on. When Herschel turned his attention to 

 them he discovered so many that, in 1786, he published a 

 catalogue of no less than a thousand, and added fifteen hun- 

 dred more a few years later ! Some of these bodies, such as 

 the bright spot called the * beehive,' in the constellation 



