# Dictionary of National Biography, 1912 supplement/Hennessy, Henry

​HENNESSY, HENRY (1826–1901), physicist, born at Cork on 19 March 1826, was the second son of John Hennessy of Ballyhennessy, co. Kerry, by his wife Elizabeth, daughter of Henry Casey of Cork. Sir John Pope-Hennessy [q. v.] was a younger brother. Educated at Cork under Michael Healy, he received an excellent training in classics, modern languages, and mathematics. Deprived as a Roman catholic of a university education, he adopted the profession of an engineer. His leisure was from early youth devoted to mathematical research, in which he engaged quite spontaneously. From an early period he made original and valuable contributions to British and foreign scientific journals, which he continued through life. In 1849 he was made librarian of Queen's College, Cork, and in 1855, on the invitation of Cardinal Newman, he became professor of physics at the Roman catholic University, Dublin. In 1874 he transferred his services to the Royal College of Science, Dublin, where he was appointed professor of applied mathematics. His work there was of exceptional merit, and he was dean of the college in 1880 and again in 1888. Hennessy was made a member of the Royal Irish Academy in 1851, and was its vice-president from 1870 to 1873. He was also elected F.R.S. in 1858.
​In 1890 he resigned his chair under the recent compulsory rules for superannuation in the civil service at the age of 65. A memorial to the government protesting against his retirement was influentially signed but was without effect. Owing to the inadequacy of his pension he resided much abroad, but returning to Ireland under medical advice, he died on 8 March 1901, at Bray, co. Wicklow. He married Rosa, youngest daughter of Hayden Corri, and had issue.
Hennessy was remarkable for his versatile interests and scientific ingenuity. In his earliest paper, which was published in 1845, when he was only nineteen, in the 'Philosophical Magazine,' he proposed to use photography for the registration of barometric and thermometric readings. In 'Researches in Terrestrial Physics' (Phil. Trans. 1851) he argued from the figure and structure of the earth and planets, that they were of fluid origin, and that a fluid nucleus at a high temperature was enclosed within their crust. He also wrote on meteorology and on climatology (British Assoc. Rep. 1857), deducing laws which regulate the distribution of temperature in islands. The excellence of a paper 'On the Influence of the Gulf Stream' (Proc. Roy. Soc. 1857-9) led to a request to report on the temperature of the seas surrounding the British Isles for the Committee on Irish Fisheries in 1870. Among his other proposals was one for a decimal system of weights and measures founded on the length of the polar axis of the earth, a quantity capable of more accurate determination than the earth's quadrant, on which the metric system is based. Standards such as the polar foot and the polar pound, and a complete set of weights and measures on the polar system, constructed under Hennessy's supervision, are in the Museum of the Royal College of Science, Dublin. In the same museum are many models of his mechanical inventions, one of them illustrating the structure of sewers best adapted to obtain the greatest scour with due provision for a great influx of storm water (cf. 'Hydraulic Problems on the Cross-sections of Pipes and Channels,' Proc. Roy. Soc. 1888).
Hennessy, besides his papers in scientific periodicals, published separately: 1. 'On the Study of Science in its Relation to Individuals and Society,' Dublin, 1858; 2nd edit. 1859. 2. 'On the Freedom of Education' (a paper at the Social Science Congress, Liverpool, in 1858), 1859. 3. 'The Relation of Science to Modern Civilisation,' 1862.
[Men of the Time, 1899; Proc. Roy. Soc. vol. 75 (1905) p. 140; Who's Who, 1901; Pratt, People of the Period, 1897.]