280 COMPARISON OF OBSERVAXIONS OTft 



near the zenith, for determining the collimation of the qiiaw- 

 drant ; and upon Polaris, to determine the distance of the pole 

 for the co-latitude. In consequence of the erection of circidar 

 instruments in several fixed observatories, since tlie year 1790, 

 they have been employed in observing the same thirty-eight 

 fixed stars, and it is from the results of tliese observations that 

 we have been enabled to make the comparison contained in 

 the following Table. 



■ The first column contains the observations made by Dr Mas- 

 KELYNE at Greenwich, with an eight feet mural quadrant : The 

 second, contains the observations made at Armagh by Dr Ha- 

 milton : The third, those made at Palermo by M. Piazzi, with 

 a five feet circle : The fourth, contains those made by Mr 

 Pond at Westbury, with a two and a half feet cii'cle : The fifth, 

 contains the mean of the observations made by the preceding 

 circular instruments, as deduced by Mr Pond : The sixth, con- 

 tains the observations made by Dr Brinkley at Dublin, with 

 an eight feet circle : And the seventh, contains the observa^ 

 tions made at Blackheath, by the author of this paper, with a 

 four feet mural circle. 



The differences among the results obtained by these instrir- 

 ments, may arise from various causes ; — from errors in the di- 

 vision of the instrument ; — ^from the assumption of different 

 formulse of refraction, which will affect all the zenith distances 

 as well as the co-latitude; — from a probable deviation of the 

 plumb-line ; and from the uncertainty of the proper motion of 

 the stars, in reducing to the same epoch the observations made 

 from 1792 to 1810, the quantity of which is very considerable 

 in Arcturus and Sirius. The mural circle of six feet, erected 

 at Greenwich in 1812, may be expected to reconcile several of 

 these discrepancies, leaving doubtful only the small quantity, 



that 



