Dictionary of National Biography, 1885-1900/Mitchell, Alexander
MITCHELL, ALEXANDER (1780–1868), civil engineer, born in Dublin on 13 April 1780, was son of William Mitchell, inspector-general of barracks in Ireland. At school he showed a marked taste for mathematics. In 1802 his eyesight, always defective in consequence of an attack of small-pox, almost totally failed him. He soon carried on, in Belfast, the joint business of brickmaking and building, from which he retired in 1832, having previously invented several machines employed in those trades. In 1842 he became known as the inventor and patentee of the Mitchell screw-pile and mooring, a simple yet effective means of constructing durable lighthouses in deep water, on mudbanks and shifting sands, of fixing beacons, and of mooring ships. For this invention he was chosen an associate of the Institution of Civil Engineers, and in 1848 was elected a member, receiving the Telford silver medal for a paper on his own invention. His system was generally approved of by engineers of eminence (Proc. of Inst. of Civ. Eng. ii. 150, vii. 108). He established himself at Belfast, and at 17 Great George Street, Westminster, as ‘Mitchell's Screw-Pile and Mooring Company.’ At the expiration of his patent in 1847 the privy council, in consideration of its merit, granted a renewal for fourteen years.
Mitchell's screw-pile was first used for the foundation of the Maplin Sand Lighthouse at the mouth of the Thames in 1838 (ib. vii. 146). In 1839 he designed and constructed, with the aid of his son, the Fleetwood-on-Wyre Lighthouse, Morecambe Bay. In the summer of 1844 a screw-pile lighthouse, serving also as a pilot station, was successfully placed by him in Belfast Lough, Carrickfergus Bay; but his attempt to construct a lighthouse on the Kish Bank, between Dublin Bay and Waterford, proved a failure. He also constructed, in the summer of 1847, a screw-pile jetty at Courtown on the coast of Wexford. After the success of screw-piles had been established, they were applied to more extensive undertakings. The great government breakwater at Portland, the long viaduct and bridges on the Bombay and Baroda railway, the whole system of Indian telegraphs, and the Madras pier, were among the works executed with this invention.
His improved method of mooring ships was likewise generally adopted. The corporation of Newcastle-upon-Tyne purchased, for 2,500l., the right of putting down screw moorings in the Tyne.
Mitchell, who retired from the Engineers' Institution in 1857 (ib. xvii. 85), settled first at Farm Hill, but latterly at Glen Devis, near Belfast, where he died on 25 June 1868. He had a family of two sons and three daughters, of whom only one, the wife of Professor Burden of Queen's College, Belfast, survived him.
He published: 1. ‘Description of a Patent Screw-pile Battery and Lighthouse,’ 8vo, Belfast, 1843. 2. ‘On Submarine Foundations, particularly the Screw-pile and Moorings,’ 8vo, London, 1848, a description of his invention, read before the Institution of Civil Engineers on 22 Feb. 1848.
[Belfast News-Letter, 29 June 1868; Men of the Time, 1868 p. 586, 1872 p. 1001; Denham's Mersey and Dee Navigation; Hugh M'Call's Ireland and her Staple Manufactures.]