19 



Notice of Determination of the Arc of Longitude between the 

 Observatories of Armagh and Dublin. By the Rev. T. R. 

 Robinson, D.D., &^c. 



At the Edinburgh Meeting of the Association, a Committee was 

 appointed to determine, by chronometers or signals or both, the 

 longitudes of the principal observatories of the British Isles, 

 and its members were authorized to apply to the Government 

 for any aid that might be necessary. Cambridge and Oxford 

 present no peculiar difficulty, but the observatories of Ireland 

 and Scotland, both from distance and local circumstances, are 

 less easily managed. The chronometric part of the process has, 

 however, been most effectually performed by one of our mem- 

 bers, Mr. Dent, who, in the first instance, sent twelve of his 

 chronometers from Greenwich to Edinburgh and Makerstown, 

 (the observatory of Sir Thomas Makdougall Brisbane) : they were 

 returned again to Greenwich, and Professor Henderson has de- 

 duced from the results, 



m s 



Longitude of Edinburgh +12 42*99 

 Makerstown + 10 3-66 



These results were reported at the Newcastle Meeting by Sir 

 Thomas Brisbane, and they inspired Sir W. Hamilton and my- 

 self with the desire of obtaining a similar determination of our 

 longitudes. Mr. Dent not merely placed these chronometers at 

 our disposal, with three additional, but bestowed what was even 

 more precious, his personal attendance, and assisted us in the 

 comparisons ; an advantage which could not have been pur- 

 chased, but which I notice as an instance of the aids which 

 these meetings afford to Science. 



Twelve of the chronometers were rated as before at Greenwich 

 Observatory, the three others at the Marine School ; and Mr. 

 Dent, setting out on the evening of September 20, was enabled 

 by the rapidity of railroad travelling to compare them at Dublin 

 on the morning of the 22nd, and on that of the following day at 

 Armagh, having travelled about 500 miles. His return was ef- 

 fected with equal rapidity, and I have deduced from the com- 

 parisons, 



m s 



Longitude of Dublin + 25 21-08 

 Armagh + 26 35-44 

 c2 



