of Rothes remarried Bennet Langton [q. v.], the friend of Dr. Johnson.
[Douglas's Peerage of Scotland (Wood), ii. 434.]
LESLIE, Sir JOHN (1766–1832), mathematician and natural philosopher, born at Largo in Fifeshire, on 16 April 1766, was youngest child of a joiner and cabinet-maker, by his wife Anne Carstairs. In spite of delicate health and scanty opportunities, his education was sufficiently advanced in his thirteenth year for him to be sent to the university of St. Andrews. After his first session Thomas Hay, eighth earl of Kinnoull [q. v.], chancellor of the university, offered to pay the expenses of his education there, with a view to his qualifying himself for the church. Leslie remained at St. Andrews till 1783 or 1784, when he entered at Edinburgh as a student of divinity. James (afterwards Sir James) Ivory [q. v.] was his fellow-student, and for some time shared rooms with him. Leslie soon found that he preferred scientific to theological studies, and in 1787, on the death of his patron, the Earl of Kinnoull, abandoned his intention of entering the church. He remained at Edinburgh till 1787 and took pupils, through one of whom he made the acquaintance of Adam Smith. In 1788 his paper ‘On the Resolution of Indeterminate Problems’ was communicated by Playfair to the Royal Society of Edinburgh, and published in its ‘Transactions.’ The year 1789 he spent in Virginia, as tutor to two young Americans named Randolph. On his return he went to London in search of fortune. He had planned a course of lectures on natural philosophy, but finding that ‘rational lectures would not succeed,’ he wrote articles for the ‘Monthly Review’ and for his countryman, Dr. William Thomson. From this employment he obtained release through an invitation of the Wedgwoods, who had been his fellow-students at Edinburgh, to reside with them and superintend their studies. Accordingly from April 1790 to the end of 1792 he lived at Etruria, Staffordshire. Here he translated Buffon's ‘Natural History of Birds’ for a London bookseller, and wrote his first physical paper, ‘Observations on Electrical Theories.’ Indignant at the delay of the Edinburgh Philosophical Society in publishing it, he recalled it, and it appeared thirty-three years later in the ‘Edinburgh Philosophical Journal’ (vol. xi.)
His engagement at Etruria ended, Leslie spent a few months in Holland, and then, returning to Largo, devoted ten years to study and experimental research. He invented several instruments for use in the sciences of heat and meteorology, of which the differential thermometer may be taken as the type. His life at Largo was diversified by visits to London and by continental travel. In 1796 a tour through the north of Germany and Switzerland, in company with Thomas Wedgwood, gave him opportunities for observing the Swiss glaciers. In 1799 he made a circuit of the capitals of northern Europe. In his later life hardly a year passed without a visit to the continent.
The result of his researches appeared in 1804 in his ‘Experimental Inquiry into the Nature and Properties of Heat,’ dedicated to his friend Thomas Wedgwood. It is an important contribution to the scientific study of the subject; the experimental methods and results were sound and fruitful, and at the same time attractively simple; and his hypotheses based thereon, though proved inadequate by later discoveries, were nevertheless a substantial advance on those current at the time. It is by his discoveries in relation to the radiation of heat, first announced in this volume, that the name of Leslie is now most widely known. His work obtained speedy recognition from the Royal Society of London, which awarded him the Rumford medal in 1805. In the same year Professor Playfair exchanged the chair of mathematics at Edinburgh for that of natural philosophy, and Leslie was elected to the vacant chair in March 1805, in spite of the united opposition of the ministers of Edinburgh, who, on the ground that he had quoted with approval in his book some of Hume's remarks on causation, professed to see in him a champion of freethought. The controversy was angrily continued till the end of May, when a general assembly of the national church put an end to it (cf. A Summons of Awakening, or the Evil Tendency and Danger of Speculative Philosophy exemplified in Mr. Leslie's Inquiry into the Nature of Heat and Mr. Malthus' Essay on Population, and in that speculative System of Common Law which is at present administered in these Kingdoms, anon., Hawick, 1807).
Leslie justified his election to a chair of pure mathematics by publishing at intervals parts of what he at first intended to be a complete course of mathematical study. In 1809 appeared ‘Elements of Geometry, Geometrical Analysis and Plane Trigonometry,’ a work conspicuous for freshness and originality of treatment, though not always happy in its departure from traditional methods. It attracted considerable attention, was translated into French and German, reached a fourth edition in 1820, and had an article (from the pen of Playfair) devoted to it in
