Thomas Colby

Colby, Thomas, Major-General, an eminent engineer, was born at Rochester, 1st September, 1784. He entered the Engineers when but seventeen, and two years afterwards lost his left hand by the explosion of a pistol. His unwearied zeal, patience, and untiring energy in the English Ordnance Survey brought him into notice; and he was employed on the Ordnance Survey of Ireland, which was more extended in its aims than the English survey. Accompanied by Mr. Drummond, he traversed Ireland from north to south in 1824, selecting the most suitable sites for signal stations; and in 1825 the Irish triangulation commenced on Divis Mountain, near Belfast. The compensation-bar, invented by himself and Drummond, with which they measured a base line of eight miles on the south shore of Lough Foyle, proved so perfect that it has since been used as the basis of surveys at the Cape and in India.

Completely devoted to his profession, he was indifferent to personal fame. His perfect command of temper may be judged from an incident related in his memoirs. Once while encamped on Slieve Donard, the summit of Sea Fell, in Cumberland, became visible at a distance of 111 miles, and, after many trials, an instrument was brought to bear upon it. He was on the point of successfully finishing his observation, "which," says his biographer, "would have been a geodesical triumph, as including the longest side of a triangle ever attempted, when an officer, on entering the observatory, accidentally struck his elbow, and threw the telescope off the object. A momentary ejaculation of anger escaped his lips, but though he could not again succeed, and the object was therefore lost, he never afterwards alluded to the subject."

In 1828 he married an Irish wife, and settled in Dublin during the heavy work of the Irish survey. He did not finally retire from the service until 1846, after the last Irish county map had been published. He principally lived abroad the latter portion of his life, and died at New Brighton, near Liverpool, 2nd October 1852. To his encouragement and generous sympathy, Drummond, Larcom, and other eminent men largely owed their success. He is described as low-sized, and possessed of a singularly nervous and elastic frame, which no fatigue could overcome. Lieutenant-Colonel Portlock's biography of Colby contains many interesting particulars of the progress of the Irish Survey. As completed, it comprises principally: (1) Map of Ireland, on a scale of 4 miles to the inch; (2) county maps, 1 1/2 to 3 miles to the inch; (3) Ireland, in sections of 12 x 18 miles, 1 mile to the inch; (4) Ireland in sections of 4 x 6 miles, 1/6 mile to the inch; (5) maps of parishes, 1/25 mile to the inch. These maps are also given geologically coloured. The historical and topographical information collected in the course of the Survey is principally preserved in MS. in the Library of the Royal Irish Academy.


84. Colby, Major-General, Memoir: Lieutenant-Colonel J. E. Portlock. London, 1869. Colgan, John, see No. 2.

233. Manuscript and Special Information, and Current Periodicals.