222 Biographical Memoir of Sir William Herschel. 
11. Planetary Nebulae ; and 12. Planetary Nebulae with 
Centres. He considers it highly improbable, that Double Stars 
should consist of Stars at a considerable distance ; and he thence 
supposes that they have a motion of rotation round their com- 
mon centre of gravity. The same reasoning he extends to the 
more Complicated Sidereal Systems, and he shews how three or 
more Stars may be preserved in a permanent connection, by re- 
volving in proper orbits round a common centre of motion. 
This curious paper, which possesses a very considerable degree 
of interest, is concluded with a 44 Catalogue of 500 new Nebula , 
Nebulous Stars , Planetary Nebula , and Clusters of Stars” 
The views contained in this paper received a very remarkable 
confirmation, from a new discovery of our author. In more 
than fifty of the double stars, he found that during twenty-five 
years a change had taken place, either in the distance of the two 
stars, or in the angle of position, the angle that a line joining 
the two stars forms with the direction of their daily motion. In 
Castor , t Bootis, £ Serpentis , and y Virginis , the angle of posi- 
tion had suffered a very considerable variation, without any 
change taking place in the distance of the stars, while in yLeonis , 
both the distance and the angle of position had changed, and in 
£ Her culls, the two stars had approached so near to each other, 
that five-eighths of the apparent diameter of the small star was 
actually eclipsed by the large one. With regard to Castor , the 
most remarkable of these double stars, Dr Bradley had obser- 
ved, in 1759, that the line joining the two stars was, at all times 
of the year, parallel to the line joining Castor and Pollux ; 
whereas, in 1803, Dr Herschel found that these two lines formed 
with each other an angle of 45° 39', giving for the time of a 
whole apparent revolution of the small star round Castor 342 
years and 2 months. This interesting inquiry will be found in 
the Philosophical Transactions for 1803. 
The motion of the Solar System in absolute space, the gene- 
ral direction of which he had determined in 1783, again occu- 
pied the attention of Dr Herschel, and he has published the re- 
sults of a very elaborate inquiry in the Philosophical Trans- 
actions for 1805, under the title of 44 On the Direction and Ve- 
locity of the Motion of the Sun and Solar System and, in 
the same work for 1806, 44 On the Quantity and Velocity of 
