302 M. Struve on the 



that contained by the silica. That in the thorina is a little 

 more than twice the oxygen in the other bases ; but both the 

 great number of the latter, and the mixture of bases having 

 one, with others having three atoms oxygen, between which no 

 multiple ratio can obtain, induce me to consider thorite as 

 an accidental mixture of several hydrated silicates. 



(To be continued.) 



Notice on the Observations of the Comet of Encke during its 

 appearance in 1828. By M. Struve. 



[From Le Recueil des Actes de la Seance Publique de TAcademie Im- 

 periale des Sciences de St. Petersbourg, 29 Dec. 1828.] 



At the annual meeting of the Imperial Academy of Sciences 

 at St. Petersburgh, December 29, 1828, a notice was com- 

 municated by M. Struve, on the Observations of the Comet 

 of Encke, made in the observatory at Dorpat, in the autumn of 

 1828, with the great refracting telescope of that observatory. 



The comet was first seen by M. Struve on the 16th of Sep- 

 tember, New Style, which was several days earlier than it 

 was seen elsewhere. It then appeared as an extremely faint 

 nebulous spot ; but early in October, and at a time when all 

 other observers complain, that, from its faintness, they were 

 unable to make satisfactory observations, the superior light of 

 the Dorpat telescope enabled M. Struve to commence a series of 

 exact determinations of its place. The micrometer attached 

 to the telescope, — the spider-threads of which are illuminated, 

 whilst the field remains dark, — is also peculiarly suited to the 

 observation of very faint objects. 



M. Struve's series of observations commence on the 13th of 

 October, and terminate on the 26th of December, comprising 

 an interval of nearly two months and a half. By submitting 

 the observations to the calculation of probabilities, he finds the 

 probable error in the determinations of right ascension and 

 declination, between the 26th of October and 10th of November, 

 to be about two seconds of arc, and from the 30th of November 

 to the 25th of December, to be less than one second. 



