Astronomical Society. 149 



Ascension of Sirius and a, Lyra, the sum of the parallaxes of the 

 two stars, and has found it to be an insensible quantity. 



The extensive series of observations of Sirius, made with the 

 mural circle of the Observatory at the Cape of Good Hope, is well 

 adapted for the investigation of the parallax, as the observations 

 possess some advantages over those made in Europe. The star is 

 near the zenith of the Cape, and the temperature is nearly the same 

 when it passes the meridian at noon in June, and at midnight in 

 December, the periods of the greatest parallaxes in declination ; so 

 that the irregularities and uncertainties of refraction, which affect 

 observations in Europe, may be supposed to disappear. 



From May 1832 to May 1833, ninety-seven observations of Sirius 

 were made by Mr. Henderson with the mural circle at the Cape 

 Observatory, of which sixty-three were made by direct vision, and 

 thirt}^-four by reflection ; and in Mr. Maclear's printed observations 

 of zenith distances, made with the same instrument, there are sixty- 

 seven observations of the double altitude of the star, made between 

 August 1836 and December 1837. Each of these series of obser- 

 vations was made in one position of the telescope upon the circle, 

 so that in each series the similar observations were referred to the 

 same divisions. 



The observations made by Mr. Henderson have been reduced in 

 the same manner as were those of a Centauri, given in his memoir 

 on the parallax of the latter star. (Monthly Notice, Vol. IV. 

 No. 19.) The declinations of Sirius have been determined by com- 

 parisons with such of the principal or standard stars as were observed 

 on the same day ; and it is consequently assumed that, in the obser- 

 vations of the stars of comparison, any errors which may arise from 

 supposing their parallaxes to be insensible, and the coefficient of 

 aberration to be correctly assumed, neutralize each other. The 

 mean declinations of the standard stars of comparison have been 

 taken from the catalogue annexed to the author's " Memoir on the 

 Declinations of the Principal Stars :" the absolute places of the stars 

 are not required, but only their relative positions with regard to 

 each other. 



Mr. Maclear observed the double altitude of Sirius, or the an- 

 gular distances between the star seen by direct vision and by re- 

 flexion at the same transit over the meridian. These are independent 

 of the observations and assumed positions of other stars, and are 

 affected by twice the amount of the parallax in declination. Ob- 

 servations of this description appear to be the best adapted of any 

 which can be made with the mural circle for the investigation of 

 small variations in the declination of a star. 



In the reductions of Mr. Henderson's series the constant of 

 aberration has been assumed = 20"'50; of Mr. Maclear's, = 20"*36; 

 in the reductions of both the coefiicient of lunar nutation has been 

 assumed = 9"* 25, and the annual precession and proper motion 

 have been taken from the Tabulae Regiomontance. 



Mr. Maclear's observations are suitable for determining the con- 

 stant of aberration ; the correction to be applied to it has been 



