Correspondence and papers regarding the operation to discover the longitude of Valentia (Valencia), Ireland, by means of electric telegraph. The papers include correspondence on the project's requirements, preparations, arrangements and expenses; material produced during the earlier Ordnance Survey of the island, including tracings of the island and of Kingstown (Dún Laoghaire); papers on the loan of equipment; papers on the method of observation; E. Dunkin's daily journal kept during the operation, 9 June-23 July 1862; a record of signals from Greenwich and their sidereal times; a description of the altazimuth instrument used; observations and computations, including zenith points for the altazimuth and zenith distances, with clock times for vertical transits; computations of clock error; computed co-latitude of Polaris; computations and comparisons of sidereal times at Valentia and Greenwich, with a synopsis of the signal observations; final results giving longitude; and the velocity of the telegraphic signals. There is also an account of the whole operation by Airy. The correspondents include J. Washington, W.G. Romaine, H. James, A.R. Clarke, C.T. Bright, E. Dunkin and C. Wheatstone.