354- Zach on liepeaiing Circles. 



Both the astronomical and the geodetical departments were 

 furnished with repeating circles of 13 or li inches diameter, 

 constructed by the most celebrated instrument makers of Paris. 

 Let us see to what degree of precision the most skilful observers 

 found it possible to attain with such instruments. 



In the second volume of the " Base du Sj/sieme mctrique" 

 wiU be found all the observations of latitude made with these 

 circles upon several points of this great meridian. Let us hear 

 the declarations of the observers themselves. 



M. Delambre, after relating, page 262, the observations 

 which he made at Dunkirk of the zenith distance of the star 

 /3 Urscc Minotis, at its upper culmination, adds the following 

 remarks : 



" This second series of observations of the passage of (3 

 TJrscV Minoris, presents singularities whose cause it is difficult 

 to ascertain. If it cannot serve to determine the latitude of 

 Dunkirk, it will at least not be useless as a history of the re- 

 peating circle. I think it therefore my duty to lay it before the 

 public. Up to the 2d and 3d of March, the sei'ies presented 

 nothing extraordinary. On the ^th the observations appeared 

 very good ; they however gave a latitude 24'" in excess, which 

 can only be explained on the supposition that some derange- 

 ment of the glasses had taken place. On the 7th, the latitude 

 was 1 0" in detect, although the observations appeared very sa- 

 tisfactory ; excepting only, that I thought I perceived some- 

 thing extraordinai-y in the clamp. After the series, M. Bellet 

 examined this clamp, but could discover nothing unusual 

 about it." 



M. Delambre, after saying that he had endeavoured with all 

 possible care to discover the cause of these inequalities, after 

 having exhausted every possible conjecture and hypothesis, 

 confesses, nevertheless, that all these explanations are far from 

 appearing satisfactory to him, and concludes by saying, "that 

 the safest conclusion he could come to, was to reject the series 

 altogether;" a series which consists of more tlian 200 observa- 

 tions, made from the 2d to the 18th of March, 1796. He 

 adds at the end (p. 264), " These are the only reasons for 

 failure which I have been able to imagine, and none of them 

 satisfy me. Though these inexplicable anomalies have tor- 

 mented me extremely during a whole month, I could never 

 believe that they arose from unskilfidness; and my confidence on 

 this head has been confirmed by the example ofM. Mechain." 



In fact, M. Mechain was not more fortunate in his ex- 

 planations of the cause of these irregularities, of which he 

 remarked as great a number as M. Delambre. This is suf- 

 ficiently 



