SIDEREAL ASTRONOMY. 339 



those who have not familiarised themselves with the methods, 

 as well as results, of this high attainment. The researches on 

 the Double Stars, begun by Mayer in 1778, and since prose- 

 cuted with such admirable zeal and success by the two 

 Herschels, Struve, and other astronomers of our own day, 

 furnish a present record of at least 6,000 multiple stars ; of 

 which number about one-third were discovered by Sir J. 

 Herschel in the southern hemisphere. Of these very many 

 are doubtless only optically double that is, nearly in the 

 same line of visual direction, but at very different distances, 

 and having no actual relation to each other. But about 700 

 have been shown to undergo such changes of relative position 

 as to prove their physical connection in revolution; and 

 binary star-systems are now recognised, not merely by 

 proximity and by these changes, but also in many cases by 

 actual computation of the elements of the orbits described 

 by one or other of the connected stars. This computation, 

 showing in some instances remarkable excentricities of orbit 

 and long periods of revolution, has now been extended to 

 about sixteen double stars. One of these, f Herculis, has 

 already twice completed its circuit of thirty years under 

 observation, and presented the actual phenomenon of the 

 occultation of one fixed star by another an eclipse as 

 absolute as any of those of which we keep record in our own 

 system. The calculation of two particular periods of revo- 

 lution of double stars at more than 500 or 600 years 

 respectively, may give some idea of the scale by which are 

 measured these remote movements in space ; and we cannot 

 better illustrate the grandeur and completeness of the research 

 than by stating that Bessel, having determined the distance 

 of 61 Cygni, a double star, was able from this and from the 

 orbital motions already ascertained, to deduce the mass and 

 weight of the two stars thus connected by mutual attraction.* 



* Even since the first edition of this volume was published, a remarkable 



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