THE STELLAR UNIVERSE. 



367 



the margin of one of his observation-books, the apparent direction of their line 

 of junction, as being parallel to that of two remarkable stars and <5 of the 

 same constellation, as seen by the naked eye ; and this note, which has been 

 rescued from oblivion by the diligence of Professor Rigaud, has proved of 

 signal service in the investigation of their orbit. They are entered also as 

 distinct stars in Mayer's catalogue ; and this affords also another means of re- 

 covering their relative situation at the date of his observations, which were 

 made about the year 1756. Without particularizing individual measurements, 

 which will be found in their proper repositories, it will suffice to remark, that 

 their whole series (which since the beginning of the present century has been 

 very numerous and carefully made, and which embraces an angular motion of 

 100, and a diminution of distance to one sixth of its former amount) is repre- 

 sented with a degree of exactness fully equal to that of observation itself, by an 

 ellipse of the dimensions and period stated in the following little table, and of 

 which the further requisite particulars are as follows : 



Perihelion passage. August 18, 1834. 



Inclination of orbit to the visual ray, - - 22 58' 



Angle of position of the perihelion projected on the heavens, - - 36 24' 



Angle of position of the line of nodes, or intersection of the plane of the > q-o o, 

 orbit with the surface of the heavens, $ 



The manner in which the periodic motion of a double star is observed, will 

 be readily apprehended by the aid of the annexed diagram, fig. 20, by which 

 Dr. Dick has represented the observations of Sir William Herschel on the 

 double star Castor. In the year 1759 Dr. Bradley had observed the position 

 of the two individuals of this star, and communicated it to Dr. Markelyne. At 

 that time, therefore, it is known that the line joining them was parallel to the 

 line joining the stars Castor and Pollux, as seen by the naked eye. The fol- 

 lowing table exhibits the angles which the same joining line made with the 

 meridian of Sir William Herschel's observatory : 



Times of the Observations. Angles of Position. 



November 1, 1759 , 56 32' 



November 5, 1779 35 29 



February 23, 1791 23 36 



December 15, 1795 18 32 



March 26, 1 800 14 3 



December 3 1, 1801 .12 12 



February 28, 1802 12 1 



March 27, 1803 10 53 



It appears, therefore, that in the interval between November, 1759, and 

 March, 1803, a portion of an orbit amounting to 45 degrees and 39 minutes 

 has been described by the smaller star round the greater, or more strictly round 

 their common centre of gravity. This would be at the rate of one degree and 

 three minutes per annum, at which rate a complete revolution would be per- 

 formed in about 343 years. 



Let the small central circle C represent the larger star Castor, and D the 

 smaller star, and let the line E F represent the direction of the two stars in a 

 line with the star Pollux at E, as observed by Dr. Bradley in 1759. In 

 November, 1779, they were found in the position C H, twenty-one degrees 

 from the position they occupied twenty years before ; in February, 1791, they 

 were thirty-three degrees from the same position. &c. ; and in March, 1803, 

 forty-six and a half degrees, giving evident indication of a regular progressive 

 motion in a circle. Since 1803 its motion has been regularly traced by Struve, 

 Sir John Herschel, and Sir J. South ; and in 1816 it was found about fifty- 

 seven degrees from its first position, and in 1830 about sixty-eight degrees, 

 still regularly progressing. In 1819 the distance of the small star from Castor 



