THE STELLAR UNIVERSE. 



were observed, rendered it necessary to watch their progress for a 

 long period of time before their motions could be certainly or 

 accurately known; and accordingly, although these researches 

 were commenced in 1778, it was not until the year 1803 that the 

 observer had collected data sufficient to justify any positive con- 

 clusion respecting their orbital motions. In that and the following 

 year, Sir William Herschel announced to the Royal Society, in 

 two memorable papers read before that body, that there exist 

 sidereal systems consisting of two stars revolving about each other 

 in regular orbits, and constituting what he called binary stars, to 

 distinguish them from double stars, generally so called, in which 

 no such periodic change of position is discoverable. Both the 

 individuals of a binary star are at the same distance from the eye 

 in the same sense in which the planet Uranus and its attendant 

 satellites are said to be at the same distance. 



More recent observation has fully confirmed these remarkable 

 discoveries. In 1841, Madler published a catalogue of upwards 

 of 100 stars of this class, and every year augments their number. 

 These stars require the best telescopes for their observation, being 

 generally so close as to render the use of very high magnifying 

 powers indispensable. 



59. The moment the revolution of one star round another was 

 ascertained, the idea of the possible extension of the great prin- 

 ciple of gravitation to these remote regions of the universe 

 naturally suggested itself. Newton has proved in his Principia, 

 that if a body revolve in an ellipse by an attractive force directed 

 to the focus, that force will vary according to the law which 

 characterises gravitation. Thus an elliptical orbit became a test of 

 the presence and sway of the law of gravitation. If, then, it could 

 be ascertained that the orbits of the double stars were ellipses, we 

 should at once arrive at the fact that the law of which the dis- 

 covery conferred such celebrity on the name of Newton, is not 

 confined to the solar system, but prevails throughout the universe. 



60. The first distinct system of calculation by which the true 

 elliptic elements of the orbit of a binary star were ascertained, 

 was supplied in 1830, by M. Savary, who showed that the motion 

 of one of the most remarkable of these stars (f Ursce major is], 

 indicated an elliptic orbit described in 58^ years. Professor 

 Encke, by another process, arrived at the fact that the star 60 

 Ophiuchi moved in an ellipse with a period of 74 years. Several 

 other orbits were ascertained and computed by Sir John Herschel, 

 MM. Madler, Hind, Smyth, and others. 



61. The following Table is given by Sir J. Herschel, as con- 

 taining the principal results of observation in this part of stellar 

 astronomy up to 1850. 



2 



