620 Proceedings of the Royal Irish Academy. 



bring JS'yren's constant yery near to Stnive's and Bessel's. Although, 

 j^yren appears to have seen this himself, he has not determined the 

 exact amount of the necessary correction to his constant." A few 

 years ago, however, I^ewcomh published a note, " Eeduction of the 

 Constants of Precession found hy Bessel, Struve, and Xyren, to a 

 Common Equinox," in which he investigates the changes these three 

 constants undergo, if the star-catalogues, fi'om which they were origi- 

 nally derived, be referred to a common system of standard places, and 

 as such he adopts his " Eight Ascensions of Equatorial Eundamental 

 Stars." The improved values thus found are — 



Bessel, 50" -214, 

 Struve, 50 -236, 

 ISTyren, 50 -220. 



That we are still far from possessing a satisfactory knowledge of 

 the exact value of the Constant of Precession seems evident, par- 

 ticularly when we remember the doubtful assumptions on which 

 Struve' s value depends. I have therefore thought it would be of 

 interest to determine a new value of this important constant. 



When considering the materials available for an investigation of 

 this kind, it must be remembered that it is highly desirable not to 

 employ stars diffeiing too much in brightness, nor too small a number 

 of stars, in order that the result may be as free as possible fi'om errors 

 of observation depending on magnitude, or arising fi'om irregular 

 distribution of the stars over the heavens. I decided therefore to use 

 telescopic stars, and, of catalogues of such, none seemed to me better 

 suited for the purpose than Lalande's ''Histoire Celeste Eranqaise," and 

 Schjellerup's Catalogue of stars, the latter being the same as employed 

 by Is^yren. Lalande's observations were made during the years 1789- 

 1801, at the Observatory of the Ecole ITilitaii'e, in zones of 2° ; among 

 50,000 stars, of which his Histoii'e Celeste contains observations, there 

 are about 3300 which also occur in Schjellerup's Catalogue; and as 

 they ai'e distributed in a fairly uniform manner over the equatorial 

 belt (between ±15° decl.) throughout the twenty-four hours of Eight 

 Ascension, and were observed most carefully with an interval of about 

 sixty-six years, it seemed beyond doubt that they would furnish 

 good material for the new determination of the Constant of Precession. 



The Histoire Celeste contains only the unreduced observations 

 which were not catalogued until 1847, when the British Government 

 published the well-known catalogue which had been reduced by 

 means of Schumacher's tables of reduction under the superintendence 



5 iSTyi'en, Das Aeqiunoctium fiir 1865, p. 31 (Mem. de I'Acad. de St. Peters- 

 bom-g, 1876). 



^ Vierteljalu'schrift der astronomischen Gesellschaft, xiii. pp. 107-110. 



