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 triplicate manuscript) a general report on the north-east longitudinal series. During the Mutiny he was at Mussoorio, a hill station ten miles beyond Dehra Dun. For nearly five months he was under arms and on harassing duty.

After service with the base line at Vizagapatam, in the south, he took two years' leave to England in March 1863. Entering Jesus College, Cambridge, on 31 Oct. as a fellow commoner, he pursued mathematical studies with great aptitude under professors Adams, Challis, and Walton. With the sanction of the secretary of state he learned the new process of photo-zincography at the ordnance survey offices, Southampton, and returning to duty in India (April 1865) took out an extensive apparatus with which he established the process at survey headquarters. By this means the rapid reproduction of maps and survey sheets became possible, and the great cost and delay of sending orders to England were avoided.

Hennessey, appointed to the charge of the amalgamated computing office and calculating branch, made (1866) the comparisons of standards and determined the 10 feet standard bar of the trigonometrical survey. He also took in hand the vast accumulations of material provided by the labours of William Lambton [q. v.]. Sir George Everest [q. v.], and Sir Andrew Scott Waugh [q. v.], and with the help of a large staff reduced them to order.

Hennessey assisted his chief, General James Thomas Walker [q. v.], in the editorship of the monumental 'Account of the Operations of the Great Trigonometrical Survey of India,' of which the first volume was issued in 1870. He was a large contributor to some of the volumes, fourteen of which were issued during his tenure of office. He also wrote the report on 'Explorations in Great Tibet and Mongolia, made by A——k in 1879–82' (Dehra Dun, 1884). He was designated deputy superintendent of the trigonometrical survey in Sept. 1869, officiated as its superintendent in 1874, and after the three branches of survey operations had been amalgamated under the title of the Survey of India, he was appointed (Feb. 1883) deputy surveyor-general.

On 9 Dec. 1874, with the equatorial of the Royal Society, he observed from Mussoorie (6765 ft.) the transit of Venus (see Trans. Roy. Soc. Nos. 159 and 161, 1875). This won him the fellowship of the society (1875), to the 'Transactions' of which he had contributed in 1867, 1870, 1871, and twice in 1873. Cambridge conferred upon him the honorary M.A. dedree in 1876, and after his retirement on 1 Oct. 1884 on a special pension granted by government, he was made a C.I.E. (6 June 1885).

At Mussoorie, where he at first lived after retirement, he was an active member of the municipality, captain of the local volunteer corps, and discoverer of the spring from which the water-supply is obtained. Coming to London, he resided in Alleyn park, West Dulwich, where he died on 23 May 1910, being interred at Elmer's End cemetery.

He married at Calcutta in March 1868 Elizabeth Golden, only daughter of R. Malcolm Ashman; by her he had a son and daughter. The son, Lieut. J. A. C. Hennessey, 45th (Rattray) Sikhs, was killed in action at Jandola, Waziristan, in Oct. 1900; memorial prizes for moral worth were founded at his old school, Dulwich.

 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. 