68 Astronomical Society. 



meter and thermometer during the year, with a statement of the 

 l^revailing wind, which appears to be north-west and south. 



II. A letter from Professor Bessel to Sir J. Herschel, Bart., dated 

 KiJnigsberg, Oct. -23, 1S38. 



Esteemed Sir, — Having succeeded in obtaining a long-looked-for 

 result, and presuming that it will interest so great and zealous an 

 explorer of the heavens as yourself, I take the liberty of making a 

 communication to you thereupon. Should you consider this com- 

 munication of sufficient importance to lay before other friends of 

 Astronomy, I not only have no objection, but request you to do so. 

 With this view, I might have sent it to you through Mr. Baily; and 

 I should have preferred this course, as it would have interfered less 

 with the important affairs claiming your immediate attention on your 

 return to England. But to you I can write in my own language, 

 and thus secure my meaning from indistinctness. 



After so many unsuccessful attempts to determine the parallax 

 of a fixed star, I thought it worth while to try what might be ac- 

 complished bj' means of the accuracy which my great Fraunhofer 

 heliometer gives to the observations. I undertook to make this 

 investigation upon the star CI Cygni, which, by reason of its great 

 proper motion, is perhaps the best of all ; which affords the advan- 

 tage of being a double star, and on that account may be observed 

 with greater accuracy ; and which is so near the pole that, with the 

 exception of a small part of the year, it can always be observed at 

 night at a sufficient distance from the horizon. I began the com- 

 parisons of this star in September 1834, by measuring its distance 

 from two small stars of the 11th magnitude, of which one precedes, 

 and the other is to the northward. But I soon perceived that the 

 atmosphere was seldom sufficiently favourable to allow of the ob- 

 servation of stars so small ; and, therefore, I resolved to select 

 brighter ones, although somewhat more distant. In the year 1835, 

 researches on the length of the pendulum at Berlin took me away 

 for three months from the observatory; and when I returned, H alley's 

 Comet had made its appearance, and claimed all the clear nights. 

 In 1836, I was too much occupied with the calculations of the 

 measurement of a degree in this country, and with editing my work 

 on the subject, to be able to prosecute the observations of a Cygni 

 so uninterruptedly as was necessary, in my opinion, in order that 

 they might afford an unequivocal result. But, in 1837 these ob- 

 stacles were removed, and I thereupon resumed the hope that I 

 should be led to the same result which Struve grounded upon his 

 observations of a Lyra, by similar observations of Gl Cygni. 



I selected among the small stars which surround that double 

 star, two between the 9th and 10th magnitudes ; of which one (n) 

 is nearly perpendicular to the line of direction of the double star ; 

 the other {b) nearly in this direction. I have measured with the 

 heliometer the distances of these stars from the point which bisects 

 the distance between the two stars of 61 Cygni; as I considered this 

 kind of observation the most correct that could be obtained, I have 

 commonly repeated the observations sixteen times every night. 



