Royal Astronomical Societi/. 207 



two years. These observations also establish the conclusion that 

 the moon exerts a greater influence on the amount of atmospheric 

 pressure at the periods of her perigee than at those of her apogee. 



ROYAL ASTRONOMICAL SOCIETY. 

 [Continued from p. 139.] 

 Jan. 8, 1847. — Reduction of the Observations of Halley's Comet, 

 made at the Cambridge Observatory in the years 1835 and 1836! 

 By the Astronomer Royal. 



These observations were chiefly made with the 5 -foot equatoreal. 

 They were reduced as far as was practicable at the time, and the 

 corrections for refraction and parallax applied. In this state they 

 were published in the Cambridge Observations for 1835. 



The instrumental errors, though pretty well known, and known 

 to be small, were not then corrected. It' was thought better to put 

 oflf tliis part of the complete reduction until correct places of the 

 small stars of comparison could be obtained. In 1836 and 1837 

 most of these stars were observed at Greenwich, and they are suffi- 

 ciently numerous to establish all the results which can be drawn 

 from the observations of the comet. Some stars could not be seen, 

 though carefully looked for. 



In exhibiting the results two methods have been followed. First, 

 the place of the comet has been determined differentially with respect 

 to small stars, which have themselves been subsequently determined. 

 Secondly, the error of the instrument has been computed from the 

 observations of known stars, admitting of accurate observation, and 

 the error so deduced has been applied to correct the instrumental 

 observation of the comet. A comparison of these results will often 

 show which is to be preferred. 



The final results are compared with the Ephemeris circulated by 

 the Superintendent of the Nautical Almanac, under the date Dec. 30, 

 1835, which is the same, as to places unaffected by perturbation! 

 with the Ephemeris in theAppendix to the Nautical Almanac of 1839! 

 In this Ephemeris the effects of aberration are fully included, con- 

 trary to the supposition of the Astronomer Royal in the Camb'rido-e 

 Observations, 1835, p. Ixiv. " 



It has not been thought advisable to deduce the elements of Hal- 

 ley's comet from these observations. That task will be best per- 

 formed by some future Rosenberger, who, with all the observations 

 from both hemispheres before him, may hope to bring a satisfactory 

 result out of the whole mass. 



The work is divided into sections ; — 



1. A catalogue of stars of comparison. 



2. Errors of Ephemeris in K.A. from differential observations 

 only. 



3. Computation of the index errors of the equatoreal in R.A. 



4. Errors of the Ephemeris in R.A. from the data of the previous 

 section. 



