Hennessy, Henry (1826–1901), physicist and mathematician, was born 19 March 1826, probably at 4 Mount Verdon Terrace, Cork city, second son among five sons and three daughters of John Hennessy of Ballyhennessy, hide merchant on Pope's Quay, and his wife Elizabeth, daughter of Henry Casey, Cork butter merchant. The politician and colonial governor John Pope Hennessy (qv) was his younger brother; his sister Frances later married William Kirby Sullivan (qv). He was educated privately in Cork under Michael Healy, where he was instructed in languages and mathematics to a high standard. His catholic background prevented him from entering university in Dublin. After schooling he followed his interest in mathematics, becoming largely self-taught. He became an assistant engineer on the Midland Great Western and Waterford & Kilkenny railways, and later worked for the Board of Works (1846–7).
In 1851 he was appointed first librarian to QCC. He does not seem to have been interested in remaining long in the position, and in 1853 he unsuccessfully applied for the chair of mineralogy. In 1855 he left Cork when he was offered the post of professor of physics by John Henry Newman (qv) at the newly established Catholic University of Ireland in Dublin. He made plans with William Kirby Sullivan to establish a physics laboratory at the university, but this fell through due to lack of funding. Unable to secure a charter, the university had great difficulty in attracting students, especially in the sciences. Hennessy spent some time in Oxford conducting observations with the astronomer Manuel Johnson. He was elected a fellow of the Royal Society in 1858, much to the chagrin of Johnson, who believed that Hennessy had plagiarised his experiments. When in 1857 Newman decided to establish a learned periodical to promote the science faculty of the university, Hennessy proposed the title Atlantis in reference to Francis Bacon's New Atlantis. He contributed eight papers to the periodical.
He remained concerned throughout his life with the dearth of catholic students interested in science. In November 1858 he gave the inaugural lecture to the science faculty, entitled The study of science in its relations to individuals and society (1858; 2nd ed. 1859). The lecture traced the development of the mathematical and physical sciences from antiquity and was essentially a call-to-arms for catholic students to take up science. He brought the cause of a charter for the Catholic University to the National Association for the Promotion of Social Science at Liverpool in 1858. This lecture was later published as On freedom of education (1859). He moved to the Royal College of Science in 1874 as professor of applied mathematics, serving as dean in 1880 and 1888. During the 1880s he became interested in the cause of vocational education for the labouring classes. With Arnold Graves (qv) and Michael Davitt (qv), he helped organise and plan an Irish Artisan's Exhibition to encourage technical education. He recommended that Irishmen should look to the continental technical schools for their educational models and was a member of the Society for Preservation of the Irish Language.
He published his first paper at the age of 19 in the Philosophical Magazine, proposing the photographic recording of meteorological measurements. Other articles, which dealt with the origin, structure, and formation of the earth and planets, were published in the Journal of the Geological Society of Dublin (1849) and the Proceedings of the Royal Society (1851). He maintained the fluid origin of planetary bodies and believed that all scientific evidence related to the structure of the earth was best explained by the existence of fluid matter at a high temperature enclosed within its crust. He also wrote on climate and claimed to have proved the existence of laws regulating the distribution of temperature surrounding islands, and to have deduced consequences of general application from the physical properties of water. His paper ‘On the influence of the gulf stream on the winters of the British Isles’ in the Proceedings of the Royal Society (1858) led in 1870 to his being called on to report on the temperature of waters surrounding the British Isles for the information of a commission of inquiry into Irish fisheries. His report advocated a great extension of river and canal navigation. He took an active part in the question of uniformity of weights and measures; and proposed a decimal system of weights and measures, based on the length of the earth's polar axis, a quantity that can be more accurately determined than the earth's quadrant. His scientific interests ranged through terrestrial physics, meteorology, geophysics, geology, climatology, hydraulics, and metrology, and he contributed over eighty papers to contemporary scientific journals. He was elected MRIA (1851) and served as the Academy's vice-president (1870–73).
New college regulations required his sudden retirement at Christmas 1890, aged 64, much to his own surprise and that of his colleagues. He protested against this and about the inadequate pension that he received. A memorial on his behalf, signed by the president and a large number of the fellows of the Royal Society, and professors in the principal universities of the UK, was presented to the government, but without effect. He spent some time in Switzerland and later returned to Ireland. He died 8 March 1901 at Bray, Co. Wicklow.
He married (24 April 1873) in Donnybrook, Dublin, Rosetta (Rosa) Truelock, a widow, youngest daughter of Haydn Corri, organist and composer. The Corri family was prominent in Dublin's musical life; as a young woman Rosetta Corri had taught drawing and calisthenics. She survived her husband.
Other published papers include ‘Some researches upon the connexion between the rotation of the earth and the geological changes of its surface’, Philosophical Magazine, xxvii, no. 181 (November 1845), 376–84; ‘On the changes of the earth's figure and climate, resulting from forces acting at its surface’ and ‘On the variation of gravity at the earth's surface, on the hypothesis of its primitive solidity’, Journal of the Geological Society of Dublin, iv (1849), 139–42, 147–50; ‘On the influence of the earth's figure on the distribution of land and water at its surface’, RIA Proc., iv (April 1849), 333–40; ‘Researches in terrestrial physics’, Philosophical Transactions of the Royal Society (1851), 524–7; ‘On a uniform system of weights, measures, and coins, for all nations’, Atlantis, ii (1858), 301–30; ‘On the distribution of heat over islands, and especially over the British Isles’, Atlantis, ii (1858), 396–413; On freedom of education (1859); and ‘On technical education in Ireland’, Dublin University Review (July 1886), 573–86.