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XLIX. On the Repeating Circle improved according to the 

 Suggestion of Baron Zach : a?id on taking Observations 

 of the Pole-Star. By Professor Littrow, of the Imperial 

 Observatory of Vienna*. 



J. HAVE not yet spoken to you of my repeating circle, of 

 eighteen inches, by Reichenbach and Ertel, constructed here 

 at Vienna; but it is because I received it only at the beginning 

 of August (1820): I have however made a good number of 

 observations with it of the latitude. 



This circle is furnished with two excellent levels witli air- 

 bubbles ; one of them being, according to your recommenda- 

 tion, made fast to the outer circle, or circle of divisions, in 

 order to be certain of its invariable position during the con- 

 jugate observation. I have found the addition of this level very 

 necessary; for in fact, when the vernier or inner circle which 

 carries the telescope is turned, small oscillations are always 

 remarked in the bubble of the level, although strongly con- 

 nected with the circle of divisions by the clamp and its screw. 

 The bubble does not return to its place mitil the vernier circle ' 

 and its telescope are I'eturned to their old position ; — an evident 

 23roof that the circle of divisions does not continue fixed, and 

 that it does not always return to its old position after the 

 movement of the vernier-circle, which had hitherto been al- 

 wa\-s tacitly and falsely supposed. You were the first to point 

 out this defect, and long since cautioned astronomers and 

 artists respecting it : at length it has engaged their attention. 

 I have always attended to this fixed level of the circle of divi- 

 sions, which indeed renders the obsei'vations a little more 

 troublesome, but in return much more certain and accui'ate. 



In order to fix the latitude of my observatory, which has 

 appeared to me to be by no means well determined, I have 

 made choice of the ])ole-star for reasons known to all astrono- 

 mers. Though the telescope of my circle is but 20 inches in 

 length, it is such an excellent one that I can see the pole-star 

 with it, without the smallest difficulty, at any hour of the day, 

 and even at bright noon. Impatient to collect in a little time 

 a great number of results, I did not content myself with two 

 meridian transits a day, but had recourse to a method which 

 1 first j)roposed in the third volume, page 208, of Messrs. Lin- 

 denau and Hohnenljerger's Astronomical .h)inna!, and which 

 consists in taking the altitude of tiic jioic-slai at any time 

 whatever. I have since noticed with pleasure that several 



* Translated from a liCttcr, datcil Vicnn.i, November 2.3, 1820,1 > liAiioN 

 Zach, published in Iiis Corrcipondaitcc AstrunomiquCf&c. 



astronomci's 



