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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 the ‘Edinburgh Review’ (vol. xx.) In 1813 appeared his ‘Geometry of Curve Lines.’ A volume on ‘Descriptive Geometry and the Theory of Solids,’ which was to have completed this part of the course, was never published. In 1817 he produced a treatise on the ‘Philosophy of Arithmetic,’ containing an elaborate discussion of fundamental principles and much interesting information on the history of the subject.

Meanwhile he continued his researches on heat. In 1810 he successfully applied the absorbent powers of sulphuric acid to freeze water under the receiver of the air-pump. This is the first recorded achievement of artificial congelation. The fact that the principle on which it is based had been stated by Nairne in the ‘Philosophical Transactions’ as far back as 1777 does not deprive Leslie of the honour of the discovery. ‘A Short Account of Experiments and Instruments depending on the Relations of Air to Heat and Moisture,’ published at Edinburgh in 1813, contains a description of this experiment, and is full of important and original work. In 1814 he published, under the pseudonym of ‘Philotechnus,’ ‘Remarks for a Series of Years on Barometrical Scales.’ As a physicist he appears to least advantage in his communication to the Royal Society of Edinburgh, dated 1818, ‘On Certain Impressions of Cold transmitted from the Higher Atmosphere.’ The true theory of the formation of dew had already been accurately worked out by Dr. Wells. Leslie expressly dissents from his conclusions, rejecting Wells's idea of heat radiations from the earth's surface in favour of his own notion of a ‘continual darting of cold pulsations by day and night from an azure sky.’ Leslie clung with curious tenacity to his theory that cold had an objective existence distinct from heat. In 1809 he began to write for the ‘Edinburgh Review,’ to which he contributed articles on ‘The Physical and Chemical Memoirs of the Society of Arcueil’ (vol. xv.); on ‘The History of the Barometer’ (vol. xx.); on Delambre's work on ‘The Arithmetic of the Greeks’ (vol. xviii.); on ‘Von Buch's Travels’ (vol. xxii.); on Humboldt's ‘Physical View of the Equatorial Regions,’ and on his ‘Travels’ (vols. xvi. xxv.); and on ‘The Attempts to Discover a North-West Passage’ (vol. xxx.). To the supplement to the ‘Encyclopædia Britannica,’ begun in 1815 and finished in 1824, he contributed articles on Achromatic Glasses, Acoustics, Aeronautics, Andes, Angle, Trisection of Angle, Arithmetic (Palpable and Figurate), Atmometer, Barometer, Barometrical Measurements, Climate, Cold and Congelation, Dew, Interpolation, Meteorology.

In 1819 the death of Playfair was followed by Leslie's election to the chair of natural philosophy at Edinburgh without opposition. He devoted himself to improving the experimental equipment of the physical laboratory, and to the work of teaching his favourite science, but he is said to have been wanting, like so many original workers, in the power of lucid exposition. Of all his ‘great and varied gifts, none was more remarkable than the delicacy and success with which he performed the most delicate experiments, excepting perhaps his intuitive sagacity in instantly detecting the cause of an accidental failure.’

In 1820 he was made a corresponding member of the Institute of France, and published a ‘Description of Instruments for Extending and Improving Meteorological Observations.’ In 1823 he published ‘Elements of Natural Philosophy,’ vol. i. including mechanics and hydrostatics. In reference to hydrostatics Leslie had in 1802 (Phil. Mag. xiv. 193) given ‘the first correct explanation of the rise of a liquid in a tube by considering the effect of the attraction of the solid on the very thin stratum of liquid in contact with it’ (Maxwell, art. ‘Capillary Action,’ Encycl. Brit., 9th edition).

In the preface to the second edition of his work on ‘Natural Philosophy’ (1828) he says: ‘I had designed the second volume of this work to appear at the same time, but have since thought it better to wait for the results of a series of experiments projected on the constitution and power of steam.’ He appears to have been unacquainted with Carnot's work on this subject, published in 1824. This second volume never appeared.

On 22 July 1822 Leslie instituted proceedings against the proprietor of ‘Blackwood's Magazine’ for certain libels accusing him of having claimed as his own other men's discoveries, and he obtained a verdict for 100l. damages on two out of the four counts. A report of the trial was published.

To the first volume of the seventh edition of the ‘Encyclopædia Britannica’ he contributed a ‘Discourse on the Progress of Mathematical and Physical Science during the 18th Century,’ a work for which the extent of his knowledge and the ripeness of his judgment peculiarly fitted him. This was his last important work. He was knighted early in 1832, on the recommendation of Lord Brougham, and died unmarried on 3 Nov. in the same year, at Coates in Fifeshire, where he had purchased a small estate.

Other writings by Leslie not mentioned above are: ‘Tracts, Historical and Philosophical,’ 2 vols. Edinb. 1806; ‘Rudiments of Plane Geometry, including Geometrical Analysis and Plane Trigonometry, designed chiefly for Professional Men,’ 1828; chapter on ‘Climate’ in Hugh Murray's ‘Narrative of Discovery and Adventure in the Polar Regions;’ ‘Mathematical Treatises,’ 1838.

A bust of Leslie was executed by Samuel Joseph [q. v.]; a copy by Jonn Rhind is in the National Portrait Gallery, Edinburgh.

[Memoir by Macvey Napier in Encycl. Brit. 7th edit. vol. xiii.; article by Professor Napier in Caledonian Mercury, summarised in Gent. Mag. 1833, i. 85–6; Leslie's works.]

C. P.