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.]
MITCHELL, Sir ANDREW (1708-1771), diplomatist, born at Edinburgh on 15 April 1708, was the only surviving son of William Mitchell, of an Aberdeenshire family, minister of St. Giles's, Edinburgh, and one of the king's chaplains for Scotland. Mitchell received part of his education at the university of Edinburgh. Before he was twenty-one he married his cousin, Barbara Mitchell, an only daughter, and heiress of the lands of Thurnston in Aberdeenshire. She died about 1729, having given birth to an only daughter, who did not survive infancy. At the time Mitchell was studying for the Scottish bar, but the event affected him so deeply that he never afterwards resided in Scotland for any length of time. After several years spent in foreign travel, he was entered at Leyden University 5 Oct. 1730, and having formed at Paris an intimacy with Montesquieu, he settled in London in 1735 and studied for the English bar. He was elected a member of the Royal Society in March 1735, and was called to the bar at the Middle Temple on 12 May 1738. In 1741 he was served, in right of his wife, heir to the Thurnston estates. In the following year the Marquis of Tweeddale [see Hay, John, fourth Marquis], on becoming secretary of state for Scotland, appointed him under-secretary. Quin the actor, in conversation with Mitchell, hinted that his official employment was simply that of Will helping Jack to do nothing (Walpole, v. 235), but with the breaking out of the rebellion of 1745 Mitchell's office became no sinecure. His functions ceased in 1747 with the abolition of the Scottish secretaryship of state. But he was afterwards consulted by the government respecting the affairs of Scotland, and the Duke of Newcastle aided him in what proved to be his successful candidature for Aberdeenshire. He was elected as a staunch whig in 1747. He was an intimate friend of James Thomson, the poet of the ‘Seasons,’ who, dying in 1748, left Mitchell one of his executors. He spoke occasionally in the House of Commons, and in 1751-2 he was at Brussels as one of the British commissioners appointed to negotiate a commercial treaty with Austria and the Netherlands. From 1755 to 1761 he was M.P. for the Elgin burghs, but during most of the period he was absent from England, having been appointed in 1756 British envoy to Frederick the Great.
Mitchell reached Berlin just before the breaking out of the seven years' war and the formation of an Anglo-Prussian alliance. Frederick and he became strongly attracted to each other. Mitchell was admitted to confidential intercourse with the king, whose appeals for a strict fulfilment of the engagements which England had entered into with Prussia were warmly supported by Mitchell in his correspondence with his government. Frederick willingly acceded to Mitchell's application, made in pursuance of instructions from home, to be allowed to accompany him in his campaigns, and he was often by the king's side in the battle-field and under fire. The clear and instructive narratives of military operations sent home by Mitchell interested George II, and their value has been recognised by Carlyle. Mitchell's reports of Frederick's frank and lively conversations with him abound in striking traits and anecdotes of the great king. Some remarks in one of his despatches appear to have given offence to the elder Pitt, and he was recalled, General Yorke being sent to supersede him. But Frederick insisted that Mitchell should remain, and without quitting Berlin he resumed his functions as envoy. This was in 1758, and in 1759 he was raised to the rank of plenipotentiary. While attached to Frederick and approving of his policy, Mitchell did not hesitate to speak his mind freely to him in regard both to politics and to religion. They had more than once discussions on the provi-