LESLIE, SIR JOHN, was born on the 16th of April 1766, at Largo, a village on the coast of Fifeshire. Wheu a child lie was weak and sickly, which occasioned frequent interruptious in his elementary education. He however evinced at an early age a decided partiality for geometrical exercises, and a proportional dislike to tho study of languages, more particularly of the Latin, although iu this ho qubsequently attained considerable proficiency. With the assist ance of his elder brother Alexander, ho soon made sufficient progress in arithmetic and geometry to attract the attention of the parochial minister, through whose instrumentality he was probably presented to Professors Robison and Stuart, and by their suggestions, in 1779, sent to the University of St. Andrews. Here his abilities introduced him to the patronage of the Earl of Kinnoul, the then chancellor of the university, who proposed to defray the expeuses of his education on the condition that his father would consent to his being educated for the church. After his studies at this university during six sessions, ho removed in 1783.84 in company with James Ivory Li vonr, Jelin] to Edinburgh, where he attended the courses of several of the professors for three years, in which time ho was engaged by Dr. Adam Smith to assist in the education of his nephew Mr Douglas, afterwards Lord Reston. In 1788 he became tutor to twc Americans of the name of Randolph, junior students at the University of Edinburgh, with whom he proceeded to Virginia, and after at absence of about twelve months, during which time he visited Ness York, Philadelphia, &c., he again returned to Scotland. In the earls part of 1790 he set out for London with recommendatory letters from several individuals of literary and scientific reputation ; and among others from Dr. Adam Smith, who is said on this occasion to have given him for advice, "never to approach an author whose favour he was solicitous of gaining without first reading his works, lest the conversation should turn that way." His intention seems to have been to deliver lectures on natural philosophy, but finding, to use his own words, that "rational lectures would not succeed," he determined upon writing for periodical pub. lications as the readiest means of obtaining a subsistence. He accord. ingly began to furnish articles for the Monthly Review,' and about the same time was employed by Dr. William Thomson (whose acquaintance he had originally made at St. Andrews University) to collect and furnish notes for a Bible which was then being published in parts. From the translation of Baron's 'Natural History of Birds,' which appeared in 1793, in nine volumes 8vo, he derived sufficient pecuniary emolument to lay the foundation of his subsequent independence.
In 1791 be visited Holland, and in 1796 ho proceeded through Germany and Switzerland, in company with Mr. Thomas Wedgwood. Upon his return he became candidate for some professorship in the University of St. Andrews, and shortly after for that of natural philosophy at Glasgow, but in both instances was unsuccessful. In 1799 he again set out upon a continental tour, and travelled through Denmark, Norway, and Sweden, with Mr. Robert Gordon.
In 1805 be offered himself as a candidate for the professorship of mathematics in the University of Edinburgh, which had become vacant by the promotion of Professor Playfair to the chair of natural philosophy. At this period the only production of Mr. Leslie relative
to the pure mathematics consisted of an 'Essay on the Resolution of Indeterminate Equations,' written about the time of his quitting the university, and printed in the Edinburgh Philosophical Trans notions' for the year 1788; but be had published several papers on different branches of physics in Nicholson's 'Philosophical Journal,' and the Royal Society of London had recently awarded to him the Rumford medals for his researches on the nature and propagation of heat, an account of which had appeared the preceding year (' Experi mental Inquiry into the Nature and Properties of Heat,' 8vo, 1804). In addition to the reputation he had thus acquired, he came forward with the warmest testimonials of Drs. Maskelyne and Hutton, Sir Joseph Banks, Baron Maseres, and other persons of distinction ; but the appointment rested in the magistrates and town council of Edin burgh, subject to a clause in the charter of the university, which declares that the electors shall take advice of the clergy in the choice of professors ; and these being desirous of promoting the election of Dr. Thomas Macknight—one of their own body, and a gentleman perhaps equally qualified for the situation—they therefore determined upon opposing that of Mr. Leslie. They grounded their objection upon a note in his Inquiry into the Nature of Heat' (page 135, and note 16, p. 522), wherein he refers to Hume's Theory of Causation,' which he designates "a model of clear and accurate reasoning," whence his clerical opponents somewhat illogically inferred that he had rejected those arguments which are deducible from the observance of nature in proof of the existence and attributes of a Creator. They forthwith made a formal protest against his election, and expressed their determination, in the event of his induction into the office of professor, to prosecute for his immediate ejection. The town council notwithstanding conferred the professorship upon Mr. Leslie, and the clergy accordingly brought the affair before the General Assembly. The debate which ensued (see 'Report of the Debate,' 8vo, Edin., 1805), and which lasted for two days, was marked by strong party spirit on the side of tho plaintiffs, and by the powerful and arguments of Sir Henry Moncrieffi who conducted the defence. Near midnight on the second day (23rd of May 1805), the case was dismissed as ' vexatious.' Mr. Leslie entered immediately upon his official duties, which he continued to discharge with zeal and assiduity during the fourteen following years. In 1809, upon the death of Professor Playfair, he was called to the chair of natural philosophy, when his first care was directed to the extension of the apparatus required in the more enlarged series of experiments which he thought necessary for the Illustration of the course. "This indeed," says his biographer, Mr. Napier, "was an object of which, from the first to the last hour of his incumbency, he never lost sight ; and it is due to him to state that it was through his exertions that the means of experimental illustration, in the natural philosophy class, were first made worthy of the university." He was knighted on the 27th of Juno 1832, and died on the 3rd of November in the same year, at his seat at Coates in Fife shire, about two miles from the place of his birth.