316 Astronomical Society. 



titude and azimuth instrument, described circumstantially in vol. ii. 

 part i. of the Memoirs. Dr. Pearson describes the mode in which 

 the instrument was used and its errors corrected, together with the 

 methods followed in reducing the observations, and the elements 

 employed in computing the corrections for parallax, refraction, nu- 

 tation, and the sun's latitude ; and concludes with a synopsis of the 

 reduced observations, which were in number 1648, a'nd a table of 

 the mean obliquity on the 1st of January in each year, from 1750 to 

 1900, both inclusive, deduced from the above determination of the 

 annual diminution. 



" On the Parallax of a Cenfauri." By Professor Henderson. 



The two stars designated a' and a- Centauri. are situated within 

 19" of space of each other. On comparing the observations of La- 

 caille with those of the present time, it has been found that, although 

 the two stars have not sensibly changed their relative positions, each 

 has an annual proper motion of 3'6 seconds of space. It thus ap- 

 pears that they form a binary system, having one of the greatest 

 proper motions that have been observed; and from this circumstance, 

 and the brightness of the stars, it is reasonable to suppose that their 

 parallax may be sufficiently sensible to powerful instruments. 



On reducing the declinations from his observations at the Cape of 

 Good Hope, Mr. Henderson remarks, that a sensible parallax ap- 

 peared, but he delayed communicating the result until it should be 

 seen whether it Avas confirmed by the observations of right ascen- 

 sion made by Lieutenant Meadows with the transit instrument. 

 He now finds that these observations also indicate a sensible pa- 

 rallax. 



It is to be observed, that the observations both of right ascension 

 and of declination were not made for the purpose of ascertaining the 

 parallax, but of determining the mean places of the stars with a pro- 

 per degree of accuracy. Had the author been aware of the proper 

 motion at an earlier period, a much greater number of observations, 

 and of such as would have been better adapted for ascertaining the 

 parallax, would have been made, and the result thereby rendered 

 more secure. 



The right ascensions and declinations of the two stars (which are 

 always above the horizon of the Cape, and favourably placed for ob- 

 servation at all seasons,) have been determined by comparisons with 

 such of the principal or standard stars as were observed on the same 

 day. It is consequently assumed that the latter have no sensible 

 parallax. The mean places of the standard stars, or rather their 

 relative positions, are also assumed to be known ; and, in reducing 

 the observations to the beginning of 1833, the coefficient of aberra- 

 tion has been assumed =20"'5, and that of lunar nutation =9""25. 

 Recent observations make the coefficient of aberration less ; but a 

 term is introduced into the equations of condition, by which the 

 effect of a change in the aberration is immediately obtained. 



For the determination of the parallaxes, three systems of equa- 

 tions of condition are formed for each star, namely, from the ob- 

 servations of right ascension, the direct observations of declination, 



