The Astronomer Royal on the Parallax of a Lyrcc, 281 



opinions have been founded on so many well- chosen observations, it 

 would be useless now to express an opinion, except it were based on 

 more numerous and more excellent observations, reduced with 

 greater attention to accuracy than in former instances. He states , 

 therefore, that the whole number of observations employed was 184, 

 made entirely in the year 1836, and, in general, distributed uni- 

 formly over the year (with the exception of the month of February, 

 in which no observations could be obtained) : that the obsenations 

 were divided equally between the two circles, and that nearly half of 

 them were by reflection : that the telescopes have been in the same 

 position on the circle during the whole year, with the exception of 

 a few days at its commencement : that the zenith points have been 

 determined independently every day ; and that the six microscopes 

 of each circle have been read for every one of these observations, as 

 well as for every observation assisting to determine the zenith 

 point. He then states, as an innovation on the practice of Mr. 

 Pond, that a correction for runs has been introduced, determined 

 from examinations made every week ; the necessity for which arises 

 from the circumstance that it is practically impossible to adjust the 

 microscopes, so that five turns of their micrometers shall correspond 

 exactly to the interval between two divisions on the limb, and whose 

 importance in this investigation depends on its periodic character 

 varying with the temperature, and (with some regularity) with the 

 season of the year. The author states his belief that, with this cor- 

 rection, the only appreciable defect of the mural circle is removed ; 

 and that it is thus superior to any other instrument which has been 

 employed in this investigation. The author then gives the results 

 in the form of equations, founded each on the mean of a group of 

 observations ; each set of observations (Troughton, by direct vision ; 

 Troughton, by reflection ; Jones, by direct vision ; Jones, by reflec- 

 tion) being divided into four groups, and an equation being obtained 

 from each group, expressing the polar distance in terms of the cor- 

 rection of the coefficient of aberration, and the coefficient of paral- 

 lax. Taking the mean of the results, with each circle, by direct 

 and reflected vision, the coefficient of parallax from Troughton's 

 circle appears to be -f 0"*2 ; and that from Jones's circle — 0"*1. 

 The author concludes from this, that the annual parallax is too small 

 to be sensible to our best instruments. The coefficient of aberration 

 (20" "3 6) appears, from the observations with Troughton's circle, to 

 require no sensible alteration ; from the observations with Jones's 

 circle, it appears to require the increase 0""4. The north polar di- 

 stance of a Lyrse, for Jan. 1, 1836, deduced from the whole of the 

 observations, is 51° 21' 53"-73. 



The constant quantity of the Moon's Equatorial Horizontal Pa- 

 rallax, deduced from observations made at Greenwich, Cambridge, 

 and the Cape of Good Hope, in 1832 and 1833. By Professor 

 Henderson, Astronomer Royal for Scotland. 



An abstract of this paper will be found in the Society's "Monthly 

 Notices " ; but it is proper to state here, that the most probable 



