Leslie, Sir John

(1766-1832), mathematician and natural philosopher

by Jack Morrell

© Oxford University Press 2004 All rights reserved

Leslie, Sir John (1766-1832), mathematician and natural philosopher, was born on 17 April 1766 at Largo, Fife, Scotland, the youngest child of Robert Leslie (d. 1804), a joiner and cabinet-maker, and his wife Anne Carstairs of Largo. Though he attended three local schools for only about a year he learned mathematics at home from his father and elder brother, Alexander, and was encouraged to study by Spence Oliphant, minister of Largo.

University and early career, 1779-1805
In 1779 Leslie entered the local University of St Andrews where he pursued mathematics under Nicholas Vilant. Next year Leslie's precociousness attracted the attention of Thomas Hay, eighth earl of Kinnoul, chancellor of the university, who paid the cost of his education with the intention that he should enter the church. While an undergraduate he became friendly with John Playfair and the wealthy Ferguson brothers, Robert and Ronald, of Raith, near Kirkcaldy, Fife. In 1785 Leslie proceeded to the University of Edinburgh where he was formally registered as a divinity student but devoted his efforts to mathematics, natural philosophy, chemistry, moral philosophy, and anatomy, attending the classes of Playfair, John Robison, Joseph Black, Dugald Stewart, and Alexander Monro secundus respectively. On Kinnoul's death in 1787 Leslie unobtrusively relinquished a clerical career for which he had little enthusiasm. In 1787-8, his last session at Edinburgh, he was befriended by Adam Smith who employed him to teach his nearest relative, David Douglas. Leslie also took other private pupils and wrote his first paper, which was communicated in 1788 by Playfair to the Royal Society of Edinburgh; the society published it in 1790. Through Playfair he met James Hutton, the geologist.

Early in 1788 Leslie began to tutor a fellow student, Thomas Randolph, of Virginia. Supported by recommendations from Playfair and Robison, Leslie went to Virginia in November 1788 to be private tutor to Thomas and his brother William. By summer 1789 this American venture had failed: Thomas became difficult and his mother's death led to the breakup of the family. Back in Scotland Leslie sought posts unsuccessfully for six months. Armed with letters of recommendation from Smith he went to London hoping to become a lecturer on natural philosophy but he lacked the necessary grander instruments and was not prepared to demonstrate trifling experiments. He also hoped to become a private mathematics tutor to a grandson of the third earl of Bute but was rejected on the insulting ground that he was a Scot. At this time Leslie was also let down by Nevil Maskelyne, the astronomer royal, who offered to make Leslie his assistant, an invitation withdrawn when the existing assistant unexpectedly recovered his health. Leslie was so desperate that he approached the ninth earl of Kinnoul, nephew of his former patron, and considered unwillingly a career in the Church of Scotland.

In 1790 Leslie's fortunes improved. Through William Thomson, a Scot who had moved to London to become a miscellaneous writer, Leslie wrote the notes for a new edition of the Bible which Thomson published. For Strahan and Cadell he translated Buffon's Histoire des oiseaux (9 vols., 1793), a task occupying two years from April 1790. This literary hack work enabled Leslie to begin acquiring that financial security which he thought was the foundation of intellectual independence. From April 1790 until the end of 1792 he was private tutor at Etruria, Staffordshire, to Thomas Wedgwood, a fellow student at Edinburgh. For this he was paid £150 per annum and enjoyed the use of Wedgwood's apparatus and library.

When Wedgwood's poor health ended the arrangement, Leslie returned to Largo, his base until 1805. Again unemployed he contemplated and sometimes pursued visionary schemes but his salvation once again was literary hack work, this time as a writer from summer 1794 for the Monthly Review edited by Ralph Griffiths to whom Leslie had been recommended by Wedgwood and Alexander Chisholme, a chemical assistant at the Wedgwood works. Leslie wrote for the Monthly Review until 1808, his most frequent contributions dating from 1795 and 1796. In these years Leslie was an unsuccessful candidate for the chairs of natural philosophy at the universities of St Andrews and Glasgow respectively. He failed at the former because he was then an extreme whig and an atheist who deplored the Erastianism of many of the Scottish clergy. His failure at St Andrews confirmed his opposition to the so-called moderate faction in the Church of Scotland and to clerical juntas and bigotry in general. In 1797 Leslie was relieved of financial worries by Thomas Wedgwood who settled an annuity of £150 on him for life. He used his independence well: by 1804 he had published six papers and his Experimental Enquiries into the Nature and Properties of Heat (1804), which brought him the Rumford medal of the Royal Society of London in 1805. Drawing on the ideas of Hutton and Boscovich, Leslie proposed that heat was a material compound and not a form of motion. His contemporaries, especially Count Rumford, who regarded Leslie as a plagiarist, ignored his theories; but they took cognizance of his instrumental innovations, such as a heat source which became known as Leslie's cube and a sensitive heat detector which he called the differential thermometer, and they recognized that he had established several basic laws of heat radiation.

Atheist mathematician, 1805-1819
In autumn 1803 Leslie was yet again pressed financially as a result of inflation and a rash investment. Once more he undertook literary drudgery. Yet again he looked to an academic post but late in 1804 failed a second time to secure the chair of natural philosophy at St Andrews and early in 1805 competed, on Playfair's advice, for the Edinburgh chair of natural philosophy to which Playfair was elected, leaving his mathematics post vacant. Leslie's fifth attempt to gain a Scottish university chair was successful: in March 1805 he was elected professor of mathematics at Edinburgh, but amid great controversy. Before his election the Edinburgh moderates had insinuated that Leslie was an atheist. They adduced note xvi of his Heat in which he had written favourably about the doctrine of the sceptical David Hume that causation was nothing more than an observed constant and invariable sequence of events. When they brought forward their own candidate, Thomas MacKnight, an Edinburgh clergyman, Stewart and Playfair protested against the uniting of clerical and academic posts and objected to the intended clerical domination of the University of Edinburgh on the pattern of St Andrews. After Leslie's election the Edinburgh moderates, determined to oust Leslie, took the affair to the general assembly of the Church of Scotland, its highest forum, which decided in May 1805 by the narrow majority of 96 to 84 that the affair be dropped and Leslie be left undeposed from his mathematics chair. In a highly acrimonious controversy, Stewart, Playfair, and Leslie settled old personal scores, but the affair had political and ecclesiastical impacts. It was one of the first successes of nineteenth-century Scottish whiggery. It also confirmed the rise in the Church of Scotland of the evangelicals who defeated their traditional moderate opponents by taking the risk of supporting Leslie, a suspected atheist, and embracing the views on causation held by the atheist Hume.

As professor of mathematics Leslie strove to render his classes more respectable and more profitable. Though he lacked the clear exposition and elocution of Playfair his audience in the 1810s never dropped below 100 and reached a maximum of 180. As a textbook writer he published at intervals parts of what he at first intended to be a complete course of mathematics. His best known text, Elements of Geometry, Geometrical Analysis, and Plane Trigonometry (1809), which was speedily translated into French and German, reached a fourth edition in 1820. Characteristically Leslie presented mathematics, with its requirement of patient and accurate reasoning, as a vital part of liberal education. Though he was capable of using continental analysis and algebra in his research publications and on occasion taught them in an advanced class, in his general teaching he followed Playfair in presenting Greek geometry and trigonometry as by far the most important types of mathematics: continental analysis and algebra, with their mechanical facility, use of negative and complex numbers, and unrigorous short cuts, were in contrast philosophically suspect and pedagogically inadequate. Leslie also maintained his interest in heat and instruments, sometimes working at the home of the Ferguson brothers and devising in 1810 a machine for making ice, the principle of which had been published in 1777 by Nairne. Leslie contributed fifteen articles on mathematics and physical science to the Supplement to the Fourth, Fifth, and Sixth Editions of the Encyclopaedia Britannica (1824) edited by his friend Macvey Napier, whom he helped greatly. From 1809 Leslie supplemented the lucid writings of Playfair in the Edinburgh Review with seven contributions on the history of science and travel in his characteristically ornate, almost affected, style.

Natural philosopher, 1819-1832
Leslie enjoyed continental travel in his vacations. On his return from Holland in summer 1819 he heard of Playfair's death. Having deputized for Playfair, who was abroad in 1816-17, Leslie was the natural choice for the Edinburgh natural philosophy chair which was more suited to his talents and widely regarded as senior to that of mathematics. He was elected professor of natural philosophy in autumn 1819, having fought off a short-lived challenge from the Revd Thomas Chalmers, the candidate of the evangelicals, led by Andrew Mitchell Thomson and including David Brewster, who objected to Leslie's notorious atheism. The fixed annual salary of Leslie's new chair was £52 compared with that of £148 for mathematics so he was more dependent for emolument on class fees. Though Leslie lacked Playfair's elegance, taste, and judgement he greatly surpassed his predecessor in two ways. His course, occupying 120 hours, was much wider in scope. It was lavishly illustrated by about a thousand experiments using new apparatus which by 1826 had cost £1600 of which Leslie provided £900. In one respect, however, Leslie was constrained by the open access to the university classes: many of his students, totalling about 150 per annum, were mathematically ignorant. In 1823 Leslie proposed to solve this problem by giving an additional class in special physics but the university senate banned it because of alleged interference with other subjects such as mathematics. At the elementary level he was temporarily successful. In 1826-7 he followed his rival, T. C. Hope, the professor of chemistry, in giving popular lectures before a mixed audience, an unseemly venture which was not repeated.

Always paying attention to his own fortunes, both intellectual and financial, in 1822 Leslie employed as counsel three leading whig lawyers, Francis Jeffrey, Henry Cockburn, and James Wellwood Moncreiff, in a legal action against Blackwood's Edinburgh Magazine for libels accusing him of dishonesty, plagiarism, impiety, and dishonouring scripture. He won damages of £100 on two of the four counts. Keen to promote his visibility Leslie wanted to display his best experiments to the royal suite when George IV paid his celebrated visit to Edinburgh in 1822 and to furnish the monarch with 6-8lbs of ice every day. Past his best as a researcher, Leslie continued writing. In 1823 he published chiefly for the use of his class Elements of Natural Philosophy, the first of three intended volumes; but, as with his mathematics series, the plan was not completed. His crowning benefaction to the Encyclopaedia Britannica (7th edn, vol. 1) was his Dissertation ... exhibiting a general view of the progress of mathematical and physical science, chiefly during the eighteenth century which brought him £200 and continued Playfair's unfinished account published in 1816.

Suspicious of learned societies as incorporated juntas, Leslie was never a fellow of the Royal Society of London which had rejected an early paper of his; elected a fellow of the Royal Society of Edinburgh in 1807 he took no part in its affairs; but he did prize his election in 1820 as a corresponding member of the Institute of France. Though subsequently distinguished pupils attended Leslie's lectures, his idiosyncrasies precluded any of them from becoming a disciple.

In later life Leslie was not prepossessing in his appearance. Lampooned in a student magazine as Edinburgh's Falstaff, he was short and fat with a florid face, his front teeth projected, and he tottered when walking. A strong and active man, he ate big meals at the end of which he was capable of devouring 2 pounds of almonds and raisins. He dressed slovenly but, in an attempt to appear engaging, the affluent and vain but grubby bachelor dyed his hair purple. His great intellectual powers were combined with a love of financial reward which even his friends thought unseemly. He could be disparaging about his fellow savants and his honesty in scientific matters was questioned by Rumford, Brewster, and Thomas Young. In his private life, however, he was a warm friend, a reliable relative, free from that affectation so characteristic of his literary style, and above all placable.

Early in 1832 Leslie was created a knight of the Royal Guelphic Order on the recommendation of Henry Brougham, the whig lord chancellor. Later that year Leslie caught a severe cold while superintending some improvements on the estate at Coates, near Cupar, Fife, which he had bought in the 1820s. Contemptuous of medicine he neglected his ailment and suffered erysipelas in one of his legs. On 31 October he imprudently worked in his grounds, soon became very ill, and died from erysipelas on 3 November 1832 at Coates. A loyal Fifer to the end, he was buried at Largo on 9 November 1832.


M. Napier, 'Memoir of Leslie', in J. Leslie, Treatises on various subjects of natural and chemical philosophy (1838), 3-46
Leslie letters, U. Edin. L.
letters, NL Scot., Leslie MSS
Chambers, Scots. (1855)
J. B. Morrell, 'The Leslie affair: careers, kirk, and politics in Edinburgh in 1805', SHR, 54 (1975), 63-82
J. B. Morrell, 'Science and Scottish university reform: Edinburgh in 1826', British Journal for the History of Science, 6 (1972-3), 39-56
R. G. Olson, 'Count Rumford, Sir John Leslie, and the study of the nature and propagation of heat at the beginning of the nineteenth century', Annals of Science, 26 (1970), 273-304
R. G. Olson, 'Scottish philosophy and mathematics: 1750-1830', Journal of the History of Ideas, 32 (1971), 29-44
Wellesley index
W. Bennet, Report of the trial by jury, Professor John Leslie against William Blackwood, for libel in Blackwood's Edinburgh Magazine (1822)
J. Kay, A series of original portraits and caricature etchings ... with biographical sketches and illustrative anecdotes, ed. [H. Paton and others], 2 vols. in 4 (1837-8)
The Scotsman (7 Nov 1832)
Edinburgh Evening Courant (8 Nov 1932)
b. cert.
W. P. Anderson, Silences that speak (1931)
private information (2004)

NRA Scotland, priv. coll., corresp. and notebooks
NRA, priv. coll., family corresp. and papers
U. Edin. L., notebooks and papers |  BL, letters to Macvey Napier, Add. MSS 34611-34615
BM, Banks MSS
Keele University Library, Wedgwood MSS
Lincs. Arch., corresp. with J. S. Langton
NL Scot., corresp. with Sir Joseph Banks
NL Scot., corresp. with Archibald Constable
U. Edin. L., letters to James Brown
U. St Andr. L., corresp. with James David Forbes

J. Henning, chalk drawing, c.1805, Scot. NPG [see illus.]
W. Wastle, etching, pubd 1819, BM, NPG; repro. in J. G. Lockhart, Peter's letters to his kinsfolk (1819)
H. Cook, stipple, pubd 1833 (after A. Chisholm), BM, NPG
J. Caw, oils (after portrait by D. Wilkie), U. Edin.
F. Chantrey, pencil drawing, NPG
J. Rhind, marble bust (after S. Joseph), Scot. NPG
portrait, repro. in Kay, A series of original portraits, no. 152

Wealth at death  
under £10,000: Chambers, Scots.

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