Parsons, William (1800–67), astronomer and 3rd earl of Rosse , was born 17 June 1800 at York, England, eldest among three sons and two daughters of Sir Lawrence Parsons (1758–1841) of Parsonstown (Birr), King's Co. (Offaly), and Alice Parsons (née Lloyd), daughter of a neighbouring landowner. When the title of earl of Rosse passed to his father in 1807, William became Lord Oxmantown. He was tutored at home, and entered TCD in 1818, but transferred to Magdalen College, Oxford, from where he graduated in 1822, first class in mathematics. In that year he was elected for King's Co. to the house of commons at Westminster, where he served until 1835.
He decided to study astronomy, following the example of William Herschel, who had built telescopes with large apertures. These made possible the study of faint objects like nebulae, but did not permit observations of high positional accuracy. In 1824 he joined the Astronomical Society and began a programme at Birr Castle aimed at the construction of large telescopes. He built reflecting mirrors of speculum metal, a very hard and brittle bronze alloy, initially assembling large mirrors from smaller pieces of bronze soldered to a brass base. He also designed and built a machine for shaping the mirror surface. It was driven by a small steam engine, constructed by him in 1827, which allowed him systematically to improve the processes of grinding and polishing. By 1830 he had published an account of his researches, which had culminated in the mounting of a 24 in. (60 cm) diameter mirror on a wooden altazimuthal stand. In 1831 he was elected FRS. In that year he became lord lieutenant of King's Co.
He married (14 April 1836) Mary Field, an heiress from Heaton, Yorkshire. Her wealth allowed him to extend his programme of work, refining the methods of casting, fabricating, and shaping mirrors. He made rapid progress. In 1840 he published details of his method, including the secret of success, which was to construct the mould for castings with a base of close packed steel strips through which gases could escape. A mirror of 3 ft (91 cm) diameter was constructed and mounted, quickly followed by the casting of a mirror of 6 ft diameter and weighing 3.5 tons. So great was the size and weight of the mirror and its 54-ft-long (16.45 m) tube that massive stone walls were required for support and orientation in an altazimuthal arrangement. He carried out this work at Birr with a team of local craftsmen whom he had trained himself. The cost was reputed to lie between £20,000 and £30,000.
One of the first objects to be studied was the nebula M51 in Canes Venatici. The light-gathering power was such that details of a spiral structure could be seen. A pencil drawing made by him was shown to a meeting of the British Association at Cambridge in June 1845, where the revelation created a sensation.
When his father died in 1841 he succeeded as 3rd earl of Rosse. In 1845 he was elected an Irish representative peer in the house of lords, and created a Knight of Saint Patrick for his scientific contributions. His mastery of engineering skills was recognised in 1849 by election as an honorary member of the Institution of Civil Engineers. In 1851 his scientific work was honoured by the award of the Royal Society's Gold Medal. The ‘Leviathan’ of Birr remained the largest telescope in the world for many years, but eventually its supremacy as an instrument for discerning faint objects was lost as advances in photography reduced the time required for exposures. Lord Rosse had attempted to use photography but the available materials were not suitable, nor could the telescope be moved with sufficient precision. Later Lady Rosse used the photographic equipment and became an accomplished photographer in her own right.
The devastation about which Lord Rosse had warned for several years came to Ireland in 1845 with the outbreak of the great famine. With Mary, countess of Rosse, he now worked to ease the hardship round Birr. He published his thoughts, which were very critical, on the behaviour of the government in London. When the crisis began to ease in 1848, he was elected president of the Royal Society (1848–54) and because this and pressure of parliamentary business meant long absences from Birr, he funded a succession of able astronomical assistants to continue the work of the observatory. Parsons's interests lay not just in science but in engineering and manufacturing, and he visited many workshops and shipyards. As president of the Royal Society he argued for continued funding of Charles Babbage's mechanical ‘difference engine’. He criticised the admiralty for not making more use of technically educated personnel, and put forward a proposal for an ironclad vessel to be used in the Crimean war. He received many honours: member of the Imperial Academy of Science, St Petersburg (1852); knight of the Légion d'honneur (1855); chancellor of the University of Dublin (1862); and honorary LLD degrees from Cambridge (1842) and Dublin (1863).
In later life he purchased a yacht in which he took his wife and children cruising, around Scotland and back to the Isle of Wight or to Spain and France. He died 31 October 1867 and was buried at St Brendan’s Church, Parsonstown. Lord and Lady Rosse (d. 1885) were very close to their children, who were tutored by the astronomical assistants at Birr, but of eleven children only four boys survived childhood. The eldest, Laurence (qv), continued an interest in astronomy. The two younger sons graduated as engineers, Clere from TCD and Charles (qv), the inventor of the steam turbine, from Cambridge. William Parsons died in Dublin after an operation to remove a tumour from his knee. His portrait, in the robes of chancellor of the University of Dublin, hangs in the Dining Hall, TCD. A statue by J. H. Foley (qv) was unveiled at Birr (21 March 1876) by Lady Rosse.