186 On the Changes in Declination of Fixed Stars. 



matter quite accidental, depending on the unequal elevation or 

 depression of the object-end or eye-end of the telescope, in 

 consequence of the unequal strength of the materials. If we 

 suppose error to exist in each of the catalogues, this cause 

 must have had an opposite influence in the two cases : if we 

 compare the Greenwich observations with those of Dr. Brink- 

 ley, we shall arrive at the same conclusion ; namely, that the 

 difterences must be caused by flexure in one or both of the 

 instruments ; since here also we find that the stars in the neigh- 

 bourhood of the zenith are affected by only half the difference 

 in polar distance, that is observed in the stars near the equator ; 

 and the same conclusions may be drawn from comparing the 

 Greenwich observations with those of Mr. Bessel. The polar 

 distances of all the stars in Mr. Bessel's catalogue exceed the 

 polar distances given in the Greenwich catalogue ; while those 

 of all the stars in Dr. Brinkley's catalogue as regularly fall 

 short of my determinations. It is not fi'om the casual circum- 

 stance of my results being nearly a mean between the results of 

 those two astronomers, that I intend to claim a superior weight 

 of authority for my own ; for, were this the only ground for 

 preference, I should regard the question as yet undetermined, 

 and should think it my duty to recommend the providing of 

 new and more powerful instruments for ascertaining the truth. 

 But it appears to me that from the observations by reflection, 

 which I have lately made, and from their agreement with my 

 observations by direct vision, that I am entitled to determine 

 the share of error to which each of these two catalogues is 

 liable; not only from the general superiority of the Green- 

 wich circle, which I consider to have been thus proved, but 

 from this peculiar circumstance, that whereas in the two cata- 

 logues of Mr. Bessel and Dr. Brinkley, the errors cannot fail 

 to be the greatest in stars near the horizon ; by my method of 

 reflection, those stars which are nearest the horizon must be 

 determined the most correctly, from their double altitudes be- 

 ing measured on the smallest arc. 



In stars near the equator the catalogue of Mr. Bessel differs 

 from that of Dr. Brinkley five seconds ; and from the preced- 

 ing considerations, I think we may venture to conclude that 

 Mr. Bessel's polar distances are too great by about three se- 

 conds, and Dr. Brinkley's too small by about two : and since 

 my catalogue differs from the two former from the zenidi to 

 equator in very nearly the same proportion, there can be no 

 reason to doubt that their errors throughout are divided in 

 nearly the same ratio. 



With regard to the catalogue for the present period, which 

 accompanies this paper, I beg to state that 1 consider it only 



