150 Astronomical Society. 



therefore introduced as an unknown quantity into the equations of 

 condition ; and, from the value which is obtained, we may judge of 

 the degree of accuracy with which the parallax is determined. 



On resolving, by the method of minimum squares, the two sets 

 of equations, and combining the results according to their relative 

 weights, the greatest effect of parallax in declination is found, from 

 the whole of the 231 observations, = + 0"'15 ; and the greatest 

 effect of aberration in declination, = 13""07. These quantities are 

 to the total effect of parallax and aberration in the proportion of 

 13"'13 to 20"*50, whence the final results are — 



Parallax of Sirius (or the angle subtended by the radius of 



the earth's orbit, at a distance equal to that of the star). = 0"'23 

 Constant of aberration = 20'4I 



The error of this determination of the parallax may be estimated 

 not to exceed a quarter of a second, as it is almost certain that the 

 constant of aberration is not in error to a greater amqjjnt. On the 

 whole, it may be concluded that the parallax of Sirius is not greater 

 than half a second of space, and that it is probably much less. 



A Catalogue of Twenty-seven Stars of the Pleiades. By M. Bes- 

 sel, Director of the Observatory Konigsberg. 



The catalogue was computed by M. Bessel from meridian obser- 

 vations made by himself and his assistant. Dr. Busche. It contains 

 the positions, annual precession, and its secular variations in Ai and 

 declination, together with the proper motions, and a comparison 

 with Piazzi's catalogue. 



In a letter addressed to Mr. Baily, containing the above catalogue, 

 M. Bessel announces, that the observations respecting the parallax 

 of 61 Cygni"^ have been continued through a second year; and that 

 the result of this new series will agree very nearly with that of the 

 first. The publication of the observations will be delayed for a few 

 months, in order to obtain a more certain determination of the pro- 

 per motions which the two small stars compared seem to possess ; 

 and he adds, that although the weight of the former result was suffi- 

 ciently great to leave no doubt about the real existence of the par- 

 allax, it is gratifying to see its quantity so very nearly confirmed by 

 a second series of observations. 



A Letter from M. Valz, Director of the Observatory at Mar- 

 seilles, to the President, Sir J. F. W. Herschel, Bart., relative to the 

 Variation of the Apparent Diameter of Encke's Comet, 



After adverting to some objections suggested by Sir John Her- 

 schel [Memoirs of the Royal Astronomical Society, vol. vi. p. 102t) 

 to the theory by which M. Valz explains the changes observed in 

 the apparent diameters of some comets, when near their perihelia, 

 namely, the condensation of volume produced by the pressure of an 

 ethereal medium growing more dense in the vicinity of the sun, the 



* Abstracts of M. Bessel's observations on the parallax of the star 61 

 Cygm^\\\l'be found in Lond. and Edinb. Phil. Mag, vol. xiv. p. 68. 226. — Ed. 



t An abstract of Sir J. Herschel's paper appeared in Lond. and Edinb. 

 Phil, Mag. vol. ii, p, 222.— Edit. 



