On correcting Errors of the Astronomical Circle by Opposite Read- 

 ings. By T. R. Robinson, D. D. M. R. I. A. Professor of As- 

 tronomy at Armagh. 



Read December 5, 1825. 



While engaged in making arrangements for obtaining the in- 

 struments which the munificence of the Primate has ordered for this 

 Observatory,* it became an object of anxious enquiry whether the 

 existing models of the astronomical circle were in any respects defec- 

 tive, and how far they might be susceptible of improvement. But I 

 found it difficult to come to any conclusion on several important 

 points ; though I had the advantage of consulting men who are justly 

 considered the brightest ornaments of Science, and of inspecting the 

 noblest instruments in existence. Each observer, as he becomes 

 habituated to his instrument, learns to correct, to avoid, or perhaps 

 to overlook its defects, while he perceives the inaccuracies of other 

 constructions in too strong a light : and the very close agreement 

 between the results of all that are carefully used, shews that in 

 truth the art of making and dividing instruments is nearly perfect. 

 Yet the diversity of opinion which I found, made it desirable not 

 to rely on authority alone, but if possible, to be guided by certain 



* A transit instrument and circle. The princely-minded founder of the Observatory, died 

 before it was completed, and from circumstances which need not be detailed, those which he 

 had ordered, were not obtained by the Observatory. It has at present a transit of 2 inches 

 aperture and 3 feet focal length; but no instrument for polar distances except Troughton'i 

 equatorial, which, though excellent, is by no means capable of entering into the field with large 

 ineridian circles. 



