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An Account of Observations made with the Eight-Feet Astronomical 

 Circle, at the Observatory of Trinity College, Dublin, since the Be- 

 ginning of the Year 181 8, for Investigating the Effects of Parallax 

 and Aberration on the Places of certain fixed Stars ; also the Com- 

 parison of these with former Observations for determining the Effects 

 of Lunar Nutation. By the Reverend John Brinkley, D.D. F.R.S. 

 and M.R.I. A. Andrews Professor of Astronomy in the University 

 of Dublin. Read June 21, 1821. [Phil. Trans. 1821, p. 327.] 



The observations, of which the details are given in this communi- 

 cation, were instituted with a view to discover the source of the dif- 

 ferences that have existed between the observations made at Green- 

 wich and those at Dublin. Dr. Brinkley's former observations of cer- 

 tain stars pointed out a deviation of about one second from the mean 

 place, after having made all usual corrections. Mr. Pond's observa- 

 tions pointed out no such deviations. Dr. Brinkley referred the de- 

 viations which he had observed to parallax ; and his principal object 

 in the present paper is to ascertain how far that opinion is correct, 

 and to show that no change of figure in the instrument, nor any un- 

 certainties of the changes of refraction, can be admitted as the causes 

 of the effects observed. 



As, in deducing the quantity of parallax, the results must be affected 

 by any uncertainty in the constant of aberration ; and, in like manner, 

 as in investigating the constant of aberration, from observations of a 

 given star, the parallax, if any, will be involved, the author adopted 

 the following process in reducing the observations. 



The observed zenith distances of a given star were reduced to Ja- 

 nuary 1st, 1819, by the common equations, taking the constant of aber- 

 ration = 20"'25. The mean of these was taken. The correct mean 

 zenith distance was supposed equal to this mean e ; the constant 

 of aberration = 20"' 25+ x; and the semi-parallax = p. The equa- 

 tions of condition thus resulting from the respective observations, 

 thus contained three unknown quantities. These equations were 

 reduced to three by the method of making the sum of the squares of 

 the errors a minimum. The solutions of these three equations give 

 the values of e and p, and thence the values of the mean polar distance, 

 constant of aberration, and semi-parallax. 



After a very detailed account of the method of conducting his ob- 

 servations, instituted with a view of obtaining an explanation of the 

 source of the difference of the results of his former observations, and 

 of those of Mr. Pond relative to parallax, Dr. Brinkley states his in- 

 ability to detect any such explanation, or to obtain any result op- 

 posed to his former conclusions : he remarks, however, that the dis- 

 cordances between his observations and those made at Greenwich, 

 may by some be considered as showing the great precision of modern 

 observations, since the whole extent of the absolute difference is only 

 about one second. Independent, however, of the interest of the 

 question of parallax, it is important, he observes, to ascertain the 

 origin of this small difference. 



