10 M. Arago on Double Stars. 



garded as a double star, are of 1782. The duration of the pe- 

 riod beinp: fifty-eight years, the stellary satellite of | will have 

 accomplished its entire revolution, under our eyes, in 1840. 



It has just now been remarked (p. 1.), that if it happened that 

 the plane in which the oibit of the lesser star is contained should 

 pass through the earth, — that if this orbit, to use a mechanic''s 

 term, should present itself to us, by its edge ; the satellite star 

 would appear to move, sometimes in one course, and sometimes 

 in the opposite, but always ahng the straight line passing 

 through the greater star. This variety has offered itself to the 

 attention of astronomers. 



According to Sir W. Herschel, the star t of Serpentarius is 

 double. At the epoch at which this great astronomer formed the 

 first catalogue of multiple stars, the two distinct stars which com- 

 pose T were sensibly separated. At present they are so thorough- 

 ly confounded, — they lie so exactly the one over the other, that 

 Struve himself, though using the great telescope of Fraunhofer, 

 has not been able to discover the slightest semblance of its being 

 double. What would Bradley, Lacaille, and Mayer have said, 

 if, in their day, any one had taken upon him to assert that, in 

 that firmament which they had so thoroughly examined, there 

 were occultations of some stars by others. 



^ of Orion has presented the counter-part to tt of Serpentarius. 

 At present it is a double star very easily cognizable. Herschel 

 formerly put it down in his catalogue as decidedly simple. 



In y of Virgo, the plane of the orbit has such an inclination 

 to the visual line proceeding from the south, that the distance 

 of the satellite star from the central star which, in 1756 was 6". 5, 

 has been reduced, in 1829, to 1". 8. Since this latter date, the 

 distance is again sensibly increased. 



The branch of astronomy which treats of the displacement of 

 the stellary system, is but of yesterday. We are not, then, to be 

 astonished, if little be known concerning the relative movements 

 oi triple stars. Already, however, observations have shewn, 

 that in ^ of the Crab, the two inferior stars revolve round the 

 principal one. Regarding -t^of Casiopeia, which is composed of 

 one star rather brilliant, and of two smaller ones extremely 

 near each other, it will probably be discovered that these latter 

 revolve round each other, and at the same time revolve toge-; 

 ther round their more brilliant companion. 



