Lardner, Dionysius (1793–1859), scientist and writer, was born 3 April 1793 in Dublin, son of William O'Brien Lardner, solicitor, and Mary Ann Lardner (maiden name unknown). He seems to have had at least one younger brother, and may have been christened Dennis, but was calling himself Dionysius by 1812. He was taught by a Mr O'Flanagan; at the age of 14 he joined his father in his office, but hated the work, and was allowed in 1812 to enter TCD. He won sixteen college prizes and graduated BA (1816) and MA (1819). He was ordained in the Church of Ireland, and stayed on in the college as chaplain, and perhaps also giving grinds, until 1827, when he was awarded the degrees of LLB and LLD. His first publication seems to have been A series of lectures upon Locke's Essay, in 1824. He published several mathematical works, one of which, on the geometry of solids, was popular as a college textbook and frequently reprinted. He also gave a series of lectures in the RDS on the steam engine; they were the first popular account of the new technology, and the RDS awarded him a gold medal. The lectures were published in 1828, and appeared in several editions up to the 1840s. He also contributed articles on scientific and mathematical subjects to encyclopedias and reviews.
He married (1815) Cecilia Flood, who was said to be related to Henry Flood (qv), and lived in Ranelagh, where they had three children. Lardner's behaviour was the subject of local gossip; in 1820 a son born to Ann, wife of Samuel Boursiquot (the name is variously spelt) and sister of George Darley (qv), was christened Dionysius Lardner Boursiquot, and was apparently acknowledged and supported by Lardner as his son; as Dion Boucicault (qv) he became famous as a dramatist. Cecilia Lardner left her husband in October 1820 after denouncing his infidelity in the presence of guests at a dinner party, and for some time Lardner lodged with the Boursiquots in Gardiner St., and later in Mount Pleasant, in a ménage à trois which must have provoked still more scandal. In 1827 Dionysius Lardner was appointed first professor of natural philosophy and astronomy in the newly founded London University (latterly University College, London), and moved to London in 1828. According to Fraser's Magazine (1832) ‘he threw himself without delay into the thick of the 1001 fights with which that most pugnacious of universities abounded’ and ‘after giving and taking as much punishment as would have been expected from Jem Ward [a noted prizefighter] ... was fairly floored at last and obliged to quit the ring’.
Undaunted, Lardner initiated a major cooperative project to publish the Cabinet encyclopædia, to which many well-known authors contributed articles. Sir Walter Scott, Robert Southey, Mary Wollstonecraft Shelley, and Thomas Moore (qv) were among the distinguished contributors to the work, which appeared over twenty years from 1829; Lardner himself wrote or co-authored seven volumes on hydrostatics and five other technical subjects, though he eventually gave up the editorship to others. Fraser's Magazine dismissively wrote of him as a ‘literary cab-driver’, but it was by this work, a useful and very popular compilation in its day, that Lardner made his name. He was responsible also for Dr Lardner's cabinet library, of which nine volumes appeared between 1830 and 1832, and edited the thirty-eight volumes of the Edinburgh cabinet library, published between 1830 and 1844.
His pamphlets and lectures dealing with scientific topics in clear and accessible language were also of considerable importance; he was one of the first to support the work of Charles Babbage, who attempted to make a calculating machine, an early prototype of the computer. However, Lardner at first refused to accept the value of a newly patented improvement to condense steam to permit reuse of the same water in the steamship boiler, and in a lecture of 1836 stated that it would be impossible for a steamship to carry enough coal for a journey as long as an Atlantic crossing, such a journey being as impossible as travel to the moon; James Beale (qv), who was in the audience, came away determined to prove the lecturer wrong, and did so in 1838. Lardner's pronouncements on the future developments of the various technologies in which he took an interest were frequently proved wrong, and occasionally contained elementary mathematical miscalculations. After enduring a stinging public attack by Samuel Hall who had patented the condensation technique, Lardner in 1837 did advocate steamship communication via the Red Sea to India.
Lardner's conduct and personal character were publicly condemned and his career received a serious setback in March 1840, when he eloped with a ‘lady of mature years’ (Annual Register), Mrs Mary Heaviside (née Spicer). Their affair had been the cause of considerable scandal for a number of years, but after the elopement her husband, Capt. Richard Heaviside, took a court case against Lardner, and was awarded £8,000 damages for the seduction of his wife. An act of parliament in 1845 allowed the Heavisides to divorce and Lardner and Mary Heaviside were married (2 August 1846) in Paris. He had divorced his wife Cecilia in 1832, probably citing her desertion and possibly also her adultery subsequent to their separation, and the marriage was dissolved by a private act of parliament in 1839. A lecture tour (1840–45) of the US and Cuba provided an opportune reason to be away from London society, and also proved financially very rewarding. He is said to have earned £40,000, and made a large amount also from the sales of the published lectures in at least fifteen editions. Lardner's books all sold well, and most, particularly those on astronomy, appeared in several editions.
In 1845 Lardner settled in Paris. He was employed by several railway companies as an expert witness in law cases, and became particularly interested in the economic aspects of railway management. His book Railway economy (1850) contained much complex and important statistical material derived from railway companies around the world, and (as well as being perhaps the first work to embody the modern concept of the business as a firm) was a pioneering application to the railway business of accounting techniques, including some then novel, developed by French exponents of the marginalist school of accounting, such as Cournot and Jullien. Railway economy was very influential; railway managers are said to have found it ‘of encyclopædic usefulness’ (Forrester), and Lardner's formulations influenced statistical economists such as W. S. Jevons, FRS, and were utilised by political economists such as Karl Marx. Lardner embarked in 1853 on still another part work, The museum of science and art, which had almost 50,000 subscribers; it consisted of twelve volumes on such topics as the electric telegraph, and appeared until 1856. It was largely Lardner's own work, and individual volumes of the series were later republished over his name.
His contributions to science were recognised by many institutions; he was FRS and FRS of Edinburgh, fellow of the Royal Astronomical Society and MRIA, and Cambridge University awarded him the degree of LLD (1833). The adroit handling of complex currently fashionable topics characterised Lardner's work; his ambitious energy, enthusiasm for science, and graphic language helped make the technology of the day accessible for the first time to a mass readership. Lardner was also attacked and satirised, and not only on account of his private life; he was lampooned by William Makepeace Thackeray as ‘Dionysius Diddler’, denounced as an ‘ignorant and impudent empiric’ by Samuel Hall, inventor of the steamship condensing boiler, and described by a later writer as an ‘egregious ass’ (Rolt, 185); his self-confidence and stubbornness seem to have been particularly annoying.
Lardner is said to have been Paris correspondent of the Daily News. He died 29 April 1859 in Naples, Italy. He and his first wife had had three children, one of whom, a son survived him; an 18-year-old son died in January 1834. With his second wife he had two daughters, and he had at least one illegitimate son. There are two portraits in the National Portrait Gallery, London; one in oils by Edith De Lisle, and a lithograph by Daniel Maclise (qv), which may be the original of the caricature in Fraser's.