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Last edited 04 Jul 2016
John Theophilus Desaguliers
This article is part of ICE's Engineer biographies series.
DESAGULIERS, John Theophilus, FRS (1683 - 1744) , lecturer and writer on experimental philosophy, was born on 12 March 1683 at La Rochelle and came as a child to England with his Huguenot parents, Jean and Marguerite Desaguliers. For a time his father was minister of the French Chapel in London and then opened a school in Islington; after his father's death in 1699 he taught at the school.
In 1705, he entered Christ Church, Oxford, graduating in 1709 and the following year he transferred to Hart Hall (later Hertford College) where he took over the pioneer lectures on experimental philosophy previously given by John Keill, FRS (1671 - 1721). Still at Oxford, he married Joanna Tuckey in October 1712.
Soon afterwards he began a course of lectures on 'mechanical and experimental philosophy' given at a booksellers near Temple Bar, London, for which he charged an attendance fee of 2 guineas; they proved to be successful. By 1715 he settled in Channel Row, Westminster, where he delivered his regular courses for more than twenty years.
Desaguliers had already been elected a Fellow of the Royal Society in 1714, and in 1716 he became curator of experiments there, a post he held until 1743, receiving £30-50 a year according to the number of experiments performed.
He published more than fifty papers in Philosophical Transactions and was awarded the Copley Medal for distinguished research three times, in 1734, 1736 and 1741. The University of Oxford awarded him the degree of DCL in 1719.
In 1716, having been ordained some years earlier, he was appointed chaplain to the Earl of Carnarvon (later Duke of Chandos) who was at that time building his great house at Canons near Edgware. The project involved a waterworks supplying both fountains and the house and Desaguliers made observations on the flow of water in a long pipeline. This led to his interest in hydraulics, and in 1718 he translated Mariotte's Traite du Movement des Eaux, first published in 1686.
He met Henry Beighton through a shared interest in the steam engine erected in 1714-1715 at Griff colliery in Warwickshire by Thomas Newcomen. Beighton went on to build a similar engine, with improved valve gear, at Washington Fell, Co. Durham, in 1718; it incorporated a lever safety valve suggested to him by Desaguliers the year before. At the same time Desaguliers devised an improved form of the steam engine invented by Thomas Savery.
Seven of these were erected to his design for pumping water to fountains, the first in the gardens of Peter the Great at St. Petersburg. Desaguliers was consulted by the Edinburgh authorities in 1721 on the Comiston aqueduct, a lead pipe 3 miles long from the springs to a receiving reservoir in the city. He realised that the problem of insufficient supply resulted principally from air trapped in the 'eminences' of the pipeline, and solved it by introducing hand-operated air valves. Further thought led to the invention of an automatic valve in 1726; in its development he collaborated with Newcomen and his 'operator', Joseph Hornblower.
Desaguliers had the honour of presenting a course of lectures before George I at Hampton Court. Soon after the accession of George II in 1727 he was appointed chaplain to Frederick, Prince of Wales, and it was under his patronage that Desaguliers published in 1734 and 1744 the two volumes of the Course on Experimental Philosophy; both were reissued in 1745 and again in 1763. Meanwhile he had translated from the Latin the Mathematical Elements of Natural Philosophy by W. J. Gravesande (1682-1742), Professor at Leiden University, in successive editions of 1720, 1721, 1726 and 1737.
As for Desaguliers's contributions, they are summed up in his own words, written towards the end of his life: 'I am still acting in my Province, which has been for many years to explain the Works of Art, as well as the Phaenomena of Nature'. In 'explaining the Works of Art' he became one of the founders of engineering science: the first English writer to analyse machines on the basis of statics and elementary dynamics; to give experimental results on the flow of water through an orifice and through a rectangular notch (useful in gauging streams); and to give a formula for the velocity of flow under a sluice gate under a head (v = ^2gh, where g is the gravitational constant (32.15ft./s2)).
He gives rules for the size of elm pipes to sustain various heads and, similarly, for lead pipes. Characteristically, he also describes an apparatus to test the bursting pressure of a lead pipe and advises a factor of safety of four in design. Well aware of the losses in a pipeline due to fluid friction, he gives rules for the up-sizing of pipes according to length. He also describes and illustrates many machines from actual examples: cranes, pile-drivers, a Newcomen engine, and so on. A valuable feature of the Course is the provision by 'my ingenious and very good friend Mr Henry Beighton' of descriptions of several of the best watermills then in existence. These accounts are well illustrated by Beighton's own drawings and include an analysis of their performance.
Desaguliers refers to 'Mr. Charles Labelye, formerly my Disciple and Assistant, and since that time appointed Engineer o f the Works of Westminster Bridge'. The two men met in 1725, soon after Labelye came to England. Following the latter's submission to Desaguliers of calculations regarding the 'fall' in water level between the piers of his proposed Westminster Bridge, both men gave evidence on the bridge to the House of Commons in February 1736, Desaguliers also to the Lords in April. In a similar manner John Grundy submitted in 1734 to Desaguliers for comment a paper on fen drainage.
Though not a great originator, Desaguliers played an important role in the dissemination of scientific knowledge and the application of science in engineering. In this capacity he was joined by his friend Petrus van Musschenbrock (1682 - 1761), Professor at Utrecht, who in 1729 published the first accurate tests to measure the strength of timber and iron in tension, compression and bending, and in France by Bernard de Forest Belidor (1698 - 1761), whose Science des Ingenieurs also appeared in 1729.
When construction of Westminster Bridge eventually started in 1738 with Labelye as engineer, Desaguliers had to move, owing to the demolition of Channel Row to make room for the approach to the bridge. From then on he lived in lodgings over the Piazza in Covent Garden where he carried on his lectures.
He died on 29 February 1744 and was buried in the Savoy Chapel. His elder son held a living in Norfolk after graduating from Hart Hall, Oxford, and his younger son, Thomas, became lieutenant-general of the Royal Artillery.
- 1718: The Motion of Water, translated from the French of Edme Mariotte
- 1720: Mathematical Elements of Natural Philosophy,translated from the Latin of W. J.Gravesande (later editions 1721, 1726, 1737)
- 1734: A Course of Experimental Philosophy, vol. 1
- 1744: A Course of Experimental Philosophy, vol. 2 (both vols, reissued 1745 and 1763).
Written by A. W. SKEMPTON
This text is an extract from A Biographical Dictionary of Civil Engineers in Great Britain and Ireland, published by ICE in 2002. Beginning with what little is known of the lives of engineers such as John Trew who practised in the Tudor period, the background, training and achievements of engineers over the following 250 years are described by specialist authors, many of whom have spent a lifetime researching the history of civil engineering.
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