Astronomical Society. 0,^25 



CAPTAIN KATEk's LITTLE CIRCLE (tHREE IMCHES DIAMETEU.) 



Sept. II, by a mean of lO observations ofPolaris 55 57 15,0 



The observations were made in the passage of the Observatory, 

 leading to the centre of the building ; and Capt. Hall is enabled to 

 contradict most satisfactorily an opinion that the Edinburgh Observa- 

 tory is unstable or liable to tremors. Though the hill was covered 

 with people, the streets below traversed by carriages and waggons, 

 and workmen were carrying on their operations within the enclosure 

 of the Observatory, Capl.^flall declares that he never placed an arti- 

 ficial horizon on ground where there were so few tremors. 



4. On the longitude of the Armagh Observatory, as determined by 

 transits of the moon and moon-culminating stars, by Dr. Robinson. 



This method of determining the differences of longitude between 

 two distant places is superior to nil other methods in ease and sim- 

 plicity, and would seem to be scarcely inferior in accuracy when a 

 considerable number of observations is employed. Ever since the 

 annual lists of moon-culminating stars have been published. Dr. 

 Robinson has regularly observed them, partly to furnish correspond* 

 ing observations for other astronomers, and partly to confirm and im- 

 prove the longitude of Armagh, determined by his predecessors. He 

 was thus led to notice a source of error perhaps not sufficiently at- 

 tended to, and to adopt a correction which he conceives will add to 

 the accuracy of the results to be obtained. 



It is well known that telescopes give different diameters ot the 

 heavenly bodies, which appear too large in small and inferior instru- 

 ments. When such are used, the transit of the moon's first limb 

 will appear too early, and that of the second too late j whence, as 

 the stars are wholly unaffected, the longitudes deduced from observa- 

 tions of a single limb will be erroneous to between twenty and thirty 

 times this spurious increase of disc, compared to the disc shown by 

 the standard telescope of comparison. This error may be easily 

 eliminated when an equal number of transits of each limb can be 

 obtained, but this is obviously impossible in most cases, and incon- 

 venient in all. 



The quantity of this optical defect, as it respects the sun, is easily 

 ascertained for any telescope, by comparing the diameter as given 

 by his transit with that given in the Ephemeris, or by the standard 

 transit telescope. Sixty observations of both limbs gave the solar irra- 

 diation of the old Armagh transit compared directly with the Green- 

 wich transit -f 0',I4, and compared with that of Dublin +0%32. 



Using these comparative solar irradiations respectively as an ap- 

 proximate correction for the lunar irradiation, Armagh was found to 

 be west of Greenwich, — ,„ ^ 



by 16 corresponding observations J 1 L at Armagh and Dublin* 2(i 23,52 

 by 10 at Armagh and Greenwich 2() 26,71 



The differences of these from the true longitude (20"' 30* W. 

 nearly) previoubly determined, being so nearly proportional to, the 



• Longitude of Dublin, determined bv the Di!>hop of Clojne, 25"' 22' W. 

 N.S. Vol. 7. No. 39. March 1830. 2 G assumed 



