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Summary Article: Parsons, William (1800-1867) from The Hutchinson Dictionary of Scientific Biography

Place: Ireland, Republic of

Subject: biography, astronomy

Irish politician, engineer, and astronomer, whose main interest was in rediscovering the techniques used by William Herschel to build bigger and better telescopes. After considerable expense and dedicated effort he succeeded, and with his new instruments made some important observations, particularly of nebulae.

Parsons was born in York on 17 June 1800, but it was at his ancestral home, Birr Castle in County Offaly, that he received his early education. He then attended Trinity College, Dublin, for a year before going to Magdalen College, Oxford, in 1819, and graduating with a first-class degree in mathematics at the age of 22. A political career commensurate with his family's land ownership and title was virtually obligatory for Parsons, as the eldest son, and even while still an undergraduate he was elected to the House of Commons to represent King's County, a seat he then held at Westminster for 13 years. In 1831 he became Lord Lieutenant of County Offaly, and although he retired from parliamentary life in 1834, seven years later he was back, at the House of Lords, an elected Irish representative peer, having succeeded to the title of Rosse on his father's death. During and after the potato famine of 1846, Parsons worked to alleviate the living conditions of his tenants. It was work that Irish landowners were more or less forced to undertake when the government in London delayed aid; but his tenants were grateful to Parsons, and when he died in Monkstown on 31 October 1867, thousands attended his funeral.

The work of William Herschel was a source of fascination to Parsons. Early on he decided that he too would construct enormous telescopes and make great astronomical discoveries. Accordingly, he learned to grind mirrors, made a few small ones, and then began to seek a material capable of being cast as a large mirror. An alloy using copper and tin was considered but found difficult to cast directly. An attempt at another solution, using sectional mirrors surrounding a central disc soldered on to a brass disc, proved unsatisfactory for instruments with an aperture larger than 46 cm/18 in. Subsequently, Parsons developed a way of casting solid discs, designing a mould ventilator that permitted the even cooling of the metal forming the mirror, in an annealing oven. Thirteen years after his experiments began, Parsons was able to construct a 92-cm/36-in mirror in sections; a solid mirror of the same size was completed a year later. And in 1842 Parsons cast the first 1.8-m/72-in disc, the ‘Leviathan of Parsonstown’, which weighed nearly 4 tonnes and was incorporated into a telescope with a focal length of 16.2 m/54 ft. It took three years to put together, including setting it up on two masonry piers.

At last Parsons was ready for the observational side of his work. And during the next 13 years, he made a number of important observations. His telescope was, after all, the largest in contemporary use, and with it he researched particularly into nebulae. He was the first to remark that some were shaped in a spiral - in fact he went on to find 15 spiral nebulae - and resolved others into clusters of stars. It was he who named the famous Crab nebula.

In constructing his ‘Leviathan’, Parsons designed a mechanism (since copied by many others) for grinding and polishing metal mirrors. He also invented a clockwork drive for the large equatorial mounting of an observatory. He was even among the first to take photographs of the Moon.

His other interests included a study of problems in constructing iron-armoured ships.

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