between the Observatories of Armagh and Dublin. 125 



but in 1834 the British Association expressed a wish that the longitudes of 

 Cambridge, Oxford, Edinburgh, Dublin and Armagh should be determined by 

 the method of signals, and by chronometers. For this object it appointed a 

 committee from its astronomical members, and gave them authority to apply to 

 Government for any assistance that might be necessary. Of this Sir William 

 Hamilton and myself are members ; and I am happy to say that its work has 

 commenced in Ireland. As far as the chronometric part is concerned, there is, 

 perhaps nothing to be desired, except the personal equation of the Greenwich 

 observers, which will be determined when an opportunity offers ; and though 

 the signal-measure, which is the subject of the present communication, relates to 

 the smallest of the arcs, it is important, both on its own account, and as a means 

 of training us for more extensive lines. 



The Observatories of Armagh and Dublin are situated very unfavourably 

 for the signal-method, there being no point visible from both. About four miles 

 south of the first, a range of hills rises from 600 to 1000 feet above its level ; but 

 these are shut out from the view of Dublin, by a ridge about twelve miles to the 

 north of it, 500 feet high. Even with powerful rockets it was not easy to clear 

 these barriers ; but our difficulties were removed by the aid, and, I may add, 

 encouragement which we received from our friend Lieutenant Larcom. He 

 not only gave us whatever information we required, but added a personal atten- 

 tion to the details of our work, without which it would, perhaps, have failed. 

 Among other matters for which we have to thank him, was a diagram, in which 

 he laid down the observatories, and all the mountains which could possibly serve 

 as signal stations. To each was annexed its height, distance, azimuth at each 

 observatory, altitude affected by the average terrestrial refraction ; and when the 

 line of sight was thrown up by an intervening ridge, the height there, and the 

 elevation at which it passed the summit of the station, and which, of course, it was 

 necessary that the rocket should clear, after allowing for refraction.* This 



* It is really wonderful how completely every undulation of the ground has been registered in 

 the Survey. The altitudes sent to me, which must have been computed from the general sections, 

 agree with observation in the most extraordinary way. A fact of another kind will show such 

 members as may not be acquainted with these things the precision of the Ordnance Survey. I set 

 a telescope to the azimuth given for Slieve Gullion, and ascended the intervening hill with a 

 theodolite, which I moved till, by signal from the Observatory, it was in the Una ; then I took, with 



