98 HISTOKY OF ASTRONOMY. 



On the 18th of September, the same object was ob- 

 served by Mr. Lassell, of Liverpool. The following is 

 Mr. Lassell's account of the discovery. " On the 18th, 

 while I was attentively examining the planet Saturn, I 

 was struck with the appearance of two stars situated on 

 the line of the interior satellites. I supposed that one of 

 them, which I shall designate by c, was the most distant 

 satellite, and the other, which I shall designate by x, a 

 fixed star. In order to prepare myself for the subsequent 

 observations, I made a careful map of its position with re- 

 spect to some fixed stars in its vicinity, one of which I 

 shall designate by a. 



" On the 19th I was surprised to find that the two 

 stars, c and x, already mentioned, had receded from the 

 fixed star a, x remaining exactly on the line of the satel- 

 lites, but appearing to have approached the planet ; while 

 c, although it followed Saturn, had passed to the north of 

 the plane of the orbits of the interior satellites. 



" This appearance suggested the idea that x must be a 

 new satellite, and c lapetus. In order to verify this con- 

 jecture, I took the difference of right ascension between 

 x and a, and between c and a. The result showed that 

 in 2h. 36m, x had moved to the west of a by 2s. 47 : and 

 that in Ih. 24m, c had moved Is. 27 to the west of a ; 

 which clearly proves that the two stars x and c were in 

 motion. I measured twice at an interval of four hours, 

 the distance of x from the line passing through the in- 

 terior satellites, and satisfied myself that during this in- 

 terval, the distance experienced no sensible change. As 



