Thomson, James (1786–1849), professor of mathematics, was born 13 November 1786 in Annaghmore, near Ballynahinch, Co. Down, third son and fifth child of James Thomson and Agnes Thomson (née Nesbit), farmers. James was taught to read by his older sisters, worked his way unaided through an old mathematical textbook, and made a functioning sundial from first principles. His family had supported the United Irishmen, and Thomson retained an interest in liberal, even radical politics; he was friendly with William Drennan (qv), and in February 1825 published in the Belfast Magazine an eyewitness account of the battle of Ballynahinch in 1798, which had greatly affected him. He was sent to a school kept by a seceder minister, Samuel Edgar, father of John Edgar (qv). Since he intended to become a presbyterian minister, in October 1810 he went to Glasgow University to matriculate, walking most of the way; he was employed in Edgar's school part of the year to make money for his own support. He studied medicine as well as theology, graduated MA (1812), and in 1814 was offered a job as headmaster of the arithmetical school in the newly established Belfast Academical Institution. In 1815, when it achieved the status of a college to educate presbyterian ministers, Thomson was appointed professor of mathematics; he voluntarily gave up his professorial salary of £50 in 1823, when the Institution was in financial difficulties. He was a successful teacher, who did much to establish the reputation of the school, and in 1829 he received the degree of LLD from Glasgow.
Thomson was known for his arithmetic and geography textbooks, by which he made a good deal of money; A treatise on arithmetic, first published in 1819, passed through at least seventy-two editions in the course of the nineteenth century, and his Introduction to modern geography had at least twenty-seven editions. He published textbooks on trigonometry and calculus, geometry and algebra; he was concerned to provide teaching materials that were as value-free as was possible in the period, because he strongly supported the concept of practical education for all denominations. He also published mathematical papers in the journal of the Royal Society of Edinburgh and in the proceedings of the Royal Society of London.
In 1832, two years after his wife's death, he took his young family with him to a new home in Glasgow, where he had been appointed professor of mathematics. He was somewhat disappointed by his new position; the environment was much less stimulating than what he had been used to in Belfast, and he also discovered that the professorial salary was less than he had been led to expect. He organised afternoon classes in mathematics and geography for young ladies, by which, since they were novel and very popular, he was able to increase his earnings. He also worked with other reform-minded professors to bring about important changes in the structures and ethos of Glasgow University. In the 1840s, he was consulted by the prime minister, Robert Peel (qv), about the planned colleges in Ireland, and was offered the vice-presidency of QCB when it was established in 1845. Much to the regret of supporters in Belfast, he turned down the post, as he would have had to take a drop in income if he had accepted.
He married (1817) Margaret, daughter of William Gardner, who was from Glasgow, and had come to Belfast to visit her cousin William Cairns, professor of elocution in the Institution. The Thomsons had four sons and three daughters; one son died as an infant, a daughter died young, and a son died of cholera contracted during his medical training. Relationships within the family were intensely affectionate; James Thomson was of great importance in the education and intellectual development of his famous sons, James Thomson (qv) and William Thomson, Lord Kelvin (qv), and when he died after suffering an attack of cholera on 12 January 1849, his four surviving children were heartbroken.