Astronomical Society. 529 



were it not that my own attention is necessarily so much distracted 

 from these objects as to make me desirous that some other observer 

 may take up the subject, and verify or disprove the variability of the 

 star in question; and, if verified, assign the period of change. 



" I may take this opportunity to point out i Orionis as a star of 

 whose variability I am almost certain." 



Extract of a letter from M. Gautier, Director of the Observatory 

 at Geneva, to the President, accompanied with the Observations of 

 Encke's Comet, made at Geneva in the months of October and No- 

 vember 1838. 



The observations were made with an equatorial of Gambey, having 

 a telescope by Cauchoix, of 4 inches aperture, and 42 feet in focal 

 length ; and the circles, which are 30 inches in diameter, giving by 

 means of verniers the arcs to every three seconds. The instrument 

 required no adjustment during the whole time of the observations, 

 and the comparison of observations of the best known stars showed 

 that it possessed great stability and accuracy of division. The me- 

 thod of observing which was practised does not, however, admit of a 

 degree of precision comparable to that which may be attained when 

 it is possible to employ a wire micrometer illuminated upon a dark 

 field. On account of the extreme feebleness of the comet's light, 

 at least during a part of the observations, a micrometer of this kind 

 could not be used, and in lieu of it a simple cross was employed, 

 formed of small thin plates of silver, about 1' of a degree in breadth, 

 visible without illumination, with a magnifying power of about 25 

 times. All the observations of the comet and stars were made hj 

 referring each body to this cross, noticing the instant of observation 

 by a sidereal clock, and reading the position of the telescope by the 

 verniers of the two circles. The thickness of the plates of the cross 

 necessarily impairs the precision of the determination, especially for 

 the dechnations ; but with a little practice and care, values were 

 obtained with sufficient exactness, and of about the same precision 

 as those of comets generally are. The right ascensions were found 

 by taking the differences of the times of observation and the horary 

 angle of the equatorial circle, and the declinations by taking the 

 complement of the polar distances indicated by the declination cir- 

 cle. The advantages of this method of proceeding are, that tlic ob- 

 servations can be multiplied in a short space of time, and that the 

 best known stars can be taken for a comparison. 



Of the three tables which accompany this communication, the 

 first contains all the observations of the comet, in number 189, cor- 

 rected only for the error of the clock, which, however. Is scarcely of 

 any consequence, as it effects equally the observations of the comet 

 and of the stars used for comparison ; the second contains the obser- 

 vations of the stars, and their comparisons with tlie mean i)osltlon3 

 deduced from the catalogue ; and the third contains the reduction of 

 the observations of the comet and their comparison with the ephe- 

 meris of Mr. liremlcker. 



In comparing the results of the observations of the comet at Bcrhn 

 and Geneva, there is a sufficient agreement on the whole, altliough 

 Phil. Mag. S. 3. Vol. 14. No. 92. Hnpitt. Juhj, 1 839. 2 M 



