314 Life, Researches ■, and Discoveries of F. W. BesseL 



with peculiar adaptations, devised by himself, for facilitating the 

 handling, setting, and reading of the circle. On the reception of 

 this instrument, which was erected in the observatory about the 



end of 1841, a mode of determining the nadir point proposed by 



Bohnenberger, by reflexion of the wires of the instrument itself 



in mercury, by which the instrument is made its own vertical 

 collimator, was adopted and brought into constant use ; a full ac- 

 count of which, and of the extreme precision so attained, will 

 be found in Nos. 480 and 481 of the Astronomische Nachrichten. 

 The possession of this admirable instrument enabled him to re- 

 sume, with every advantage he could desire, an inquiry of the 

 greatest importance, but at the same time of the utmost delicacy, 

 which had long engaged his attention. The first suspicion of a 

 want of perfect uniformity in the proper motions of certain fixed 

 stars, among which Sirius and Procyon may be especially par- 

 ticularized, occurred to Bessel in 1834. Pond appears also, at an 

 earlier period, to have become impressed with the same idea. 

 The observations of declination made at Konigsberg, previous to 

 the erection of the Repsold circle (see Astron. Nachr. 422), had 

 tended greatly to confirm this suspicion j but it was not until a 

 series of observations of considerable extent had been made with 

 the new circle, that Bessel thought himself authorized to announce 

 it as a positive astronomical fact, no longer to be confounded 

 with possible error of observation and reduction, but as a thing 

 to be accounted for by some distinct physical cause. On the 

 nature of this cause be even hazarded a speculation, doubtless a 

 very bold one, viz. that Sirius and Procyon, in which the ob- 

 served deviation from uniformity is regarded by him as fully es- 

 tablished, are really double stars, one of the individuals only, 

 however, being luminous ; and that the variability in question 

 arises from their relative orbitual motion about their common 

 centre of gravity. Time only, and assiduous observation 

 elucidate this curious subject. 



Though very far from having exhausted the catalogue of Bes- 

 sePs purely astronomical discoveries and researches, the limits o 

 this notice require us now to pass to the mention of his high- 

 ly important investigations on subjects connected more immedi- 

 ately with our own globe, viz. geodesical measurements, the de- 

 termination of standards of weight and length, the length ot t 

 pendul * " ' " ^ • ! " *-*-• ftU 



, can 



of subjects therewith connected , * 

 •tunities of displaying a skill not ie» 



opportunities of display 



vviuuu auurueu niui opportunities or displaying <x b»» - , 



consummate as a physical experimenter, than he had alrea ) 

 shewn as a mathematician and astronomer. ^ 



His first step in this career was the determination of the \ m p. 

 of the simple pendulum at his own observatory. The P rinc ^ 

 of this determination is the observation of the times of 7%**°°-^ 

 two pendulums whose difference is precisely equal to a g* 



