the Obliquity of the Ecliptic, ^c. 217 



Those instruments which admit of observing each star, with- 

 out a reference to other stars, seem best adapted thereto. It is 

 not hkely that the maximum of aberration differs in different 

 stars ; yet tliis ought not to be taken for granted. 



The mean N.P.D. Jan. 1, 1818, deduced from former obser- 

 vations, have been put down as a proof of the consistency of my 

 instrument. ^ Ursae Majoris is the only star in which the dif- 

 ference is woi th notice. Whether this difference is from the 

 error of observation, or from any uncertainty in the proper mo- 

 tion of the star, it is difficult to say. Three results reduced by 

 Bradley's refraction are as follow. 



N.P.D. Jan. 1, 1816. 

 My observation, 1812 34° 6' 19",99 



Mr. Pond's observation, 1815 18 ,92 



My ob.servation, 1818 17 ,67 



A comparison of independent results is for many reasons much 

 to be desired. I offer the above principally with a view of calling 

 the attention of astronomers to such investigations. 



It appears to me, that the only method by which an explana- 

 tion of the difficulties that have occurred, from a comparison of 

 tiie Greenwich observations and of those made at this Observa- 

 tory, can be obtained, is from an extensive series of observations 

 of many stars, referring each to the apparent zenith point. I 

 am therefore pursuing such a course of observations. Conclu- 

 sions as to the existence or non-existence of parallax, from com- 

 parisons of the relative places of stars taken indiscriminately, 

 nmst be liable to much uncertainty, whether the comparisons be 

 made by polar distances or by right ascensions. The former, 

 being affected by the uncertainty of refraction, may, at first view, 

 be thought more subject to error than the latter ; but a careful 

 consideration of the circumstances attending the latter method, 

 will show that it has its pecuhar difficulties*. 



* As Mr. Besscl's determination of the maximum of aberration has been 

 referred to, it may also be right to mention his results respecting the pa- 

 rallax of certain stars. He uses transit observations of stars nearly oppo- 

 site in »i<;ht ascension (p. 1 10, &c.) Thus he finds the sum of the parallaxes 

 of S'vrius and a Lyrte insensible, and the sum of the semi-parallaxes of Pro- 

 cyoi\ a.nd « AcjuiluK, nearly 1". Thi.s method of using the transit obscrva- 

 tiona is undoubtedly far preferable to that of using them indiscriminately. 

 With respect to the observations Mr. Bessel had to compute from, I think 

 it must be allowed they were not suflBciently exact, to give much weight to 

 hii conclusions. The mt thods of observing with the transit, and of enter- 

 ii\p the observations, were then far inferior to the present. This objection, 

 however, does not apply to the observations of the pole st.-xr, and therefore 

 ooes not affect the maximum of aberration deduced from the observed right 

 a>ceiision of that star. 



Vol. 36. No. 269. Sept, 1820. E e XXXV. J\o- 



