Mitchell, Alexander (1780–1868), civil engineer, was born 13 April 1780 in South William St., Dublin, eighth son among thirteen children of William Mitchell, inspector general of barracks in Ireland, and Jane Mitchell (née Fergusson or Ferguson), remembered in the family as ‘Sparkling Jenny’. Both parents were from Belfast. William Mitchell took his family back to the north after his father's death in 1788, and Alexander attended a school near their house at Pine Hill. A few years later, after William Mitchell died, Alexander's mother moved to a smaller house, Eglinby Cottage, formerly owned by Barry Yelverton (qv) in Ballymacarrett, near Belfast.
Alexander attended Belfast Academy, where he did well, especially in science, though it became clear that his sight was failing. An attack of smallpox had damaged the optic nerve, and by his early twenties he could no longer read or write, though he continued to study mechanics and mathematics with the help of his wife and later his children. He took an active part in social and intellectual life; he was a member of the Belfast Anacreontic Society, the Belfast Literary Society, and the Belfast Natural History and Philosophical Society, and in 1810 he was on the committee of the Linen Hall Library, Belfast. He was able to run the family brick-making business, and designed several improved machines for the brickworks; he also built a number of houses in Alfred St., Belfast.
In 1832 he retired from business and began work on a novel idea to make it possible to build lighthouses on sandbanks and other unstable ground; the lack of such navigational aids made inshore waters particularly dangerous. He had developed the idea when a sail he had invented blew away on Belfast Lough and screwed itself firmly into the mud; he applied this technique to the existing technology of straight-pile lighthouses, where the structure was supported on wooden or iron legs, thus dissipating the damaging effects of wave action. Mitchell developed a cast-iron screw pile, with which, helped by his teenage son John, he carried out experiments in secret and at night on a sandbank in Belfast Lough, and in 1833 he patented his system of lighthouse construction. In 1838 he and his son built the first screw-pile lighthouse on the notorious Maplin Sands at the mouth of the Thames, off the Essex coast. They left it in place for some months to make sure it would survive winter gales, and went to Fleetwood, on the Lancashire coast, where they built a lighthouse at the mouth of the River Wyre. It was built using 16-ft-long (4.9 m) wrought-iron piles with 3-ft-diameter (0.9 m) screws, and was first lighted in June 1840. The lighthouse at Maplin was then completed, and lighted in February 1841. It was the first lighthouse anywhere to use a screw-pile construction and to be made entirely of iron.
In 1844 Mitchell constructed a lighthouse at Carrickfergus, Co. Antrim, to make the approach to Belfast Lough safer; on this project, wishing to benefit the town of Belfast, he charged only for materials and labour. Bad weather foiled an attempt to place a light on the Kish Bank, south of Dublin, but at least eighteen lighthouses were successfully built around the British coast, and his system was also widely used to provide moorings for ships. In 1847 he worked at Courtown, Co. Wexford, and in 1848 he built two lighthouses in Dundalk Bay. A lighthouse was built at Queenstown (Cobh), Co. Cork, and a huge breakwater was built using his system at Portland, Dorset. A number of bridges and railway viaducts, a pier at Madras, and lighthouses were built by other contractors in India, and his system was also used in important projects in the US; Mitchell was consultant to the first screw-pile lighthouse built there between 1848 and 1850, in Delaware Bay.
In 1837 he was made an associate of the Institute of Civil Engineers, and in 1848 was elected a member. He was awarded their Telford medal for a paper on his invention. The privy council granted him the almost unprecedented privilege of a fourteen-year extension of his patent from 1847, and in 1855 he received a silver medal at the Paris exhibition. He also received a medal at the Crystal Palace exhibition, and was awarded the CBE.
When Mitchell was completely blind and over 60 years old, he still supervised all the company's projects himself; he often climbed up ladders and out on to scaffolds to inspect work high above the sea, saying that it was safer for him to do so than for a sighted person, who would have been terrified by the height. His fingers had become so sensitive that he could detect flaws in workmanship, even badly seasoned timber, which would have been unnoticed by other surveyors. He also developed, and in 1854 patented, an improvement to the screw propeller; Mitchell made no charge for its use by the Screw Steamship Company. He built himself a novel kind of boat, which he called a Zangada and sailed round Belfast Lough, and was also a keen gardener and musician. He wrote papers on meteorites, runic inscriptions, and forest trees, which were read on his behalf to local societies.
His mother had disapproved strongly of his marriage at the age of 21 to their neighbour Mary Banks, who was two years older than her husband, but the marriage was very happy until her death in 1864. Two sons and two daughters also died before Mitchell; he went to live at Glen Divis with his only surviving child, his daughter Margaret and her husband, Professor William Burden of QCB, and died there 25 June 1868. He was a lifelong member of Rosemary St. presbyterian church, and was buried in Clifton St. graveyard. A bust of Mitchell by Samuel Ferris Lynn (qv) was presented by F. J. Bigger (qv) to the city of Belfast.