Royal Astronomical Society, 397 



The supposition of a uniform change can, however, be regarded as 

 an approximation to the truth, only when the interval of time be- 

 tween the first and last reading is very small, in comparison with the 

 interval between the successive maxima and minima, in the fluctua- 

 tions of the irregular movement. Hence, we may conclude, that it is 

 important, in the first place, to employ three readings in preference 

 to any greater number ; and, secondly, that it is desirable that the 

 time of vibration of the magnet itself should be as small as possible, 

 consistently with the accuracy of its indications in other respects. 



ROYAL ASTRONOMICAL SOCIETY. 

 [Continued from p. 61.] 



January 14, 1842. — I. Observations of Halley's Comet, made at the 

 Observatory of Geneva in the years 1835 and 1836. By M. Miiller, 

 under the direction of M. Gautier, Director of the Observatory. Com- 

 municated by Sir J. F. W. Herschel, Bart. 



These observations were made on fifty-two nights, beginning with 

 August 31, 1835, and ending with May 7, 1836 ; of which thirty- 

 one were before the perihelion passage of the comet, and twenty- 

 one after the passage. The instrument used is an equatoreal of 

 Gambey, whose telescope has an object-glass of four French inches 

 diameter, and of forty-two French inches focal length. The decli- 

 nation circle and the hour circle of the instrument are each thirty 

 inches in diameter ; the former being divided to every three minutes 

 of a degree, and by means of its verniers giving arcs of 3" ; and the 

 latter being divided in time, and by means of its verniers giving the 

 fifth part of seconds of time. The times were taken with a clock by 

 Lepaute, which was every evening compared with the transit clock. 

 The index corrections, obtained chiefly by observations of stars found 

 in the Astronomical Society's Catalogue, and whose observed places 

 were compared with the places taken from that Catalogue, and from 

 Pond's Catalogue of 1112 stars, were very consistent throughout the 

 whole series of observations, and show that the firmness of the in- 

 strument, as well as its state of adjustment, were highly satisfactory. 

 Absolute observations of both elements were obtained in every in- 

 stance by reading off both circles ; this method being preferred by 

 M. Gautier to differential observations with a micrometer. A reticu- 

 lar micrometer, made of fine plates of metal, was used, the faintness 

 of the comet scarcely ever admitting of any illumination of the field. 



In the reduction of the observations, the mean refractions were 

 computed for all the observations of the comet and the comparison- 

 stars ; and the instrumental right ascensions and north polar di- 

 stances are given, cleared of the effects of them. The index cor- 

 rections obtained from all the observations of stars are also given. 

 It is, however, left to those who may be desirous of using the ob- 

 servations of the comet to apply them, and also the effects of paral- 

 lax, to the observed places. 



The height of the observatory above the level of the sea (above 

 400 metres) caused the comet to be visible at this observatory longer 

 than at most other places in Europe ; and the author hopes that the 



