Astronomical Society. 4,59 



he has detected and corrected in several of the results of this pro- 

 found geometer, although almost insensible in a numerical point of 

 view, are valuable, as proving at once the general accuracy of his 

 investigations and the minuteness of the scrutiny they have under- 

 gone. 



The most delicate part of the whole operation, however, was the 

 choice of the several coefficients, which, if erroneously assumed, 

 would render the whole subsequent work of no value. In making 

 this assumption, Mr. Baily has exercised a degree of judgement 

 which I feel convinced will unite the suffrages of astronomers. 

 Taking a comprehensive view of the results afforded by all former 

 investigations, he has uniformly adhered to the principle, to steer 

 clear of extreme quantities, and to adopt only such as not only rest 

 on the greatest number of the best observations, but agree in their 

 values nearly with the average of all. Thus, in the case of the aber- 

 ration, the value adopted is the mean of the almost miraculously 

 coincident results of Brinkley and Struve, and agrees within two- 

 hundredths of a second with that of the extreme values assigned by 

 Bradley and Bessel. I have much satisfaction in being enabled to 

 state, that this value has been recently confirmed within a very minute 

 fraction of a second, by the praiseworthy zeal and industry of Mr. 

 Richardson of the Royal Observatory, who has compared for this 

 purpose upwards of 2000 observations, made with the two mural 

 circles of Jones and Troughton; so that this datum may be regard- 

 ed as one of the best established in Astronomy. In the same cau- 

 tious manner has Mr. Baily proceeded with the other coefficients. 

 That of precession he has taken entirely from Bessel's elaborate in- 

 vestigations compared with those of Laplace, in which the only re- 

 maining source of uncertainty, is that arising from our ignorance of 

 the mass of Venus ; the influence of which cannot possibly produce 

 an error, however, of a tenth of a second in the precession. The 

 nutation he has taken, as it results from Dr. Brinkley's observations 

 which (like his aberration) justify this partiality by holding almost 

 exactly an average value among all the different results of Bradley, 

 Mayer, Maskelyne, Laplace, and Lindenau, and can hardly be con- 

 sidered as more than a tenth of a second in error. 



This judicious choice will secure the present tables from a possi- 

 bility of ever sharing the fate of preceding labours of this sort. 

 They can never be superseded by others of greater accuracy, nor 

 fall into disuse or grow obsolete till the apparent places of the stars 

 shall have become so much altered by the effect of precession as to 

 render the computations inexact, for which a very long series of 

 years will be required.. 



But tiie distinguishing characteristic of this work, is the adoption 

 throughout of Professor Bessel's capital improvement in the system 

 of applying the corrections, by arranging the formula; in such a 

 manner tiiat all that is peculiar to each star, and permanent in mag- 

 nitude, shall stand distinctly separated from all that is ephemeral, or 

 varying from day to day ; and that in such a manner that a short 

 ephemeral table, capable of being compressed into a single pace 

 shall serve, not only for these stars, but for every star in the heavens 



3 N 2 The 



