Thomson, James (1822–92), engineer and inventor, was born 16 February 1822 at 17 College Square, east Belfast, third child and eldest son among four daughters and three sons of James Thomson (qv), of a presbyterian tenant-farming family of Annaghmore, Ballinahinch, Co. Down, and his wife Margaret, eldest daughter of William Gardiner of Glasgow. His father, intrigued by sundials at an early age, became a self-taught mathematician before attending university at Glasgow, which he funded by teaching in the local school during the summer months. He was appointed professor of mathematics in the collegiate section of the Belfast Academical Institution, which existed prior to the foundation of QCB. Despite the early death of his wife (1830), he educated the children at home while carrying out his teaching duties and writing numerous mathematical textbooks.
In 1832, when James junior was 10, his father was appointed professor of mathematics at Glasgow University. The family moved to Scotland and he began attending some classes in the university with his younger brother William (Lord Kelvin, qv). He was noted for his sensitivity and cautiousness, while his brother showed greater quickness and impulsiveness. He matriculated in 1834, winning a class prize, and received his MA (1839) in mathematics and natural philosophy. Rather than continue his mathematical studies he decided to follow practical engineering. He joined (1840) the civil engineering office of John Macneill (qv) in Dublin, but returned to Glasgow unwell. Between bouts of ill health he spent further time training at the engineering department of the Lancefield spinning mill, Glasgow, the Horsley Ironworks, Staffordshire, and in the works of Messrs Fairbairn & Co., one of the first engine-makers in Britain. He returned home in 1844 with a suspected heart condition and spent the following number of years working on his inventions. In 1851 he returned to Belfast to set up as a civil engineer and develop his vortex water-wheel, which he had patented in 1850. The Belfast water commissioners appointed him engineer in 1853; however, he continued his own business on the side. He was appointed professor of civil engineering in QCB (1857), where he remained until offered the professorship of civil engineering in Glasgow university (1873). While in Belfast he got the credit for being the cleverer of the two brothers, although afterwards William became more eminent.
His talents as an inventor were apparent early in his life: at 16 years of age he devised a mechanism for feathering the floats of steamboat paddles. He went on to design and patent water-wheels, as well as invent and improve a variety of pumps and turbines, which were developed commercially. From 1848 he began publishing scientific papers, contributing to the Royal Society, the British Association, the Cambridge and Dublin Mathematical Journal, and the Irish Builder, among others. Topics included the plasticity of ice, crystallisation, and the properties of whirling fluids; the latter led to his improvements in the design of blowing fans, turbines, and centrifugal pumps. His broad interest in natural phenomena led to the publication of theories on the geological formations of the parallel roads of Lochaber in Scotland, the origin of the prismatic structure of basalt of the Giant's Causeway, and the flow of water in rivers. Two papers read before the Royal Society (March, July 1884), on the law of inertia and ‘a problem of point motions’, contained propositions of ultimate motion and rest that were suggestive of some theories of Einstein (Deane, 1924). During his life he received several honours: LLD Glasgow (1870), D.Sc. Belfast (1875), LLD Dublin (1878), and FRS (1877). He was vice-president (1877) and president (1884) of the Institution of Engineers and Shipbuilders of Scotland.
He married (December 1853) Elizabeth, only daughter of William John Hancock of Lurgan, Co. Armagh, and sister of William Neilson Hancock (qv), professor of jurisprudence and political economy in QCB; they had one son and two daughters. In both science and religion he was never satisfied with formula or creed, and always questioned and explored the underlying principles of natural phenomena. He was said by Professor John Purser (1835–1903) of QCB to ‘most fullfill the idea of a philosopher’ (Deane, 1924). Despite a modest and retiring personality, he engaged in the social life of Belfast and Glasgow. He took an interest in both scientific and social development; he advocated the acquisition of land for public parks (which led to the purchase of Ormeau Park in Belfast) and the state purchase of railways. He was an active member of the Belfast Natural History Society and the Belfast Naturalists' Field Club. Due to failing eyesight he resigned his university position in 1889. He continued his study of ‘grand currents of atmospheric circulation’, which he had worked on for many years and finally presented his results in the Bekerian lecture before the Royal Society in March 1892. Two months later he was dead from pneumonia (8 May 1892), followed by his invalid wife and second daughter a few days later.