LESLIE

LESLIE, Sir John, Professor of Natural Philosophy in the University of Edinburgh, the son of a poor joiner or cabinet-maker, was born at the village of Largo, in the county of Fife, on the 16th of April 1766. In his early youth, being of feeble health, he was sent, when four years old, to a school kept by an old woman, and afterwards to a higher school. He received his first lessons in mathematics from his father and his eldest brother, and a slight knowledge of Latin at a school in the neighbouring town of Leven, which he attended for six months. The first person of any note who observed his precocious attainments, was Mr Oliphant, who became minister of Largo about the time when the boy had reached his eleventh or twelfth year. Struck with his knowledge of mathematical and physical science, he kindly lent him some scientific books; and he also strongly urged him, but without avail, to resume the study of Latin, for which he entertained at this time a strong aversion. He also became known to Professors Robinson and Stewart of Edinburgh; and by their advice he was, in his thirteenth year, sent to the University of St Andrews to study mathematics under Professor Vilant. On examination by the professor, he was found already qualified for the second or senior class; and at the close of the session he obtained a prize, and attracted the notice of the Earl of Kinnoull, then chancellor of the university, who proposed to his father to defray the expense of his son's education, on the condition that he should study for the Church. The proposal was readily embraced; and as a knowledge of Latin was now imperative, he commenced doggedly to study it, and, by the aid of a private teacher, qualified himself for entering the Humanity class. He also seems to have got rid of his strong dislike to the language; for no trace of it remained in his after life, and he is extremely fond of introducing classical quotations in his writings. His continued success during his second session secured the continued patronage of the chancellor, who invited him to Duplin Castle. About the same time began his acquaintance with Playfair and Dr Small.

In 1783 or 1784, he quitted St Andrews, and, along with his fellow-student, James Ivory, proceeded to Edinburgh, where, though he formally entered the Divinity Hall, he contrived to devote his first session to the sciences, particularly to chemistry. In fact, he seems early to have relinquished all thoughts of the Church: a resolution, perhaps, hastened by the death of his patron, the Earl of Kinnoull, soon after his removal to Edinburgh. He continued to study here till the close of the session of 1787; and, as is customary with students of his limited means, devoted part of his time to private tuition. Acting as tutor to a young man nearly related to Adam Smith, he thus became known to that philosopher, who treated him kindly, and occasionally favoured him with directions as to his own pursuits. His first essay, "On the Resolution of Indeterminate Problems,"

composed about this time, was read to the Royal Society of Edinburgh by Mr Playfair, in 1788, and published in their Transactions for 1790.

In 1788 he accompanied, in the capacity of tutor, two young students of the name of Randolph, to Virginia. Here his agreeable and profitable sojourn was brought to an end, after the lapse of little more than a year, by the death of the father of his young friends; and after visiting New York and Philadelphia, he returned to his native place towards the close of the year 1789.

From some letters to his family, written about the time of his leaving America, he seems to have meditated trying his fortune in India, probably as a civil engineer; a notion that recurred afterwards without leading to any results. In January 1790, carrying, among other letters of recommendation, one from Adam Smith, he repaired to London, chiefly for the purpose of ascertaining what success he might expect from a course of lectures on natural philosophy; and the information he received soon satisfied him, as he says in a letter to one of his brothers, that "rational lectures would not succeed." He therefore employed himself for some time in writing for the Monthly Review, and in executing literary jobs delegated to him by his countryman, Dr William Thomson, author of the continuation of Watson's History of Philip the Third. In April 1790, he became tutor to his former fellow-students, the younger Wedgewoods of Etruria, in Staffordshire, where he remained till the close of 1792, in the enjoyment of a liberal salary, and of society at once agreeable and intelligent. He was at the same time assiduously employed in experimental investigations, and in completing a translation of Buffon's Natural History of Birds, which he had previously undertaken for a London bookseller. It was published in 1793, in 9 vols. 8vo. Though executed with fidelity and vigour, it was valued by himself in after life solely for having, by the sum which it procured him, laid the foundation of that pecuniary competency which his industrious and prudent habits soon enabled him, in a moderate degree, to attain.

In 1794, after a sojourn of some months in Holland, during which he acquired the German language, he returned to Largo. Here he remained for about two years, devoted to experimental researches, in the course of which he invented and perfected his Differential Thermometer: a contrivance suggested during his attempts to construct an accurate Hygrometer, and the parent, as it may be called, of his subsequent inventions,—the Hygroscopic, Photometer, Pyroscope, Ethrioscopic, and Atmometer. At a later period, when his name had attained a high degree of celebrity, he was accused of having plagiarized this invention, if not from Van Helmont, who died in 1644, at any rate from John Christopher Sturmius, who died about sixty years later. But it is now, we believe, allowed, that this is just one of those curious cases so frequently found in the history of science, in which the inventor of a contrivance, partially anticipated by another, shows, by his skillful and fruitful employment of the disputed invention, how much he surpassed, and how little he needed the help of, him whom he is ungenerously supposed to have robbed of his legitimate honours.

In the spring of 1796, Mr Leslie accompanied his friend Mr Thomas Wedgwood on a tour through the N. of Germany and Switzerland, during which, as on all similar occasions, he was no less observant of the social, moral, and economical condition of countries he visited, than of their geological, meteorological, and physical aspects. On his return, he was probably engaged in his experimental pursuits, and divided his time chiefly between London and Largo, to which place, and to the society of his family, he ever was fondly attached. Early in the summer of 1799, he set out with an old college acquaintance on a tour through Denmark, part of Norway, and Sweden, and returned to Eng-

Leslie, land about the end of November. In the following year
Sir John. appeared, in Nicholson's Philosophical Journal, "A Description of an Hygrometer and Photometer;" a paper "On the Absorbent Powers of the Different Earths;" and "Observations and Experiments on Light and Heat, with Remarks on the Inquiries of Dr Herschel on these objects." A series of more extended investigations, which he carried on while residing at Largo, resulted, in 1804, in the publication at London of his Experimental Inquiry into the Nature and Propagation of Heat; a work which may be considered as a development of his essay on Heat and Climate, read at two successive meetings of the Royal Society of London in 1793. The originality and boldness of the peculiar doctrines of the Inquiry, and the number of new and important facts disclosed by its ingenious experimental combinations, conspired to render it an object of extraordinary interest in the scientific world; and, indeed, it must ever be viewed as constituting an era in the history of that branch of physical science which forms its subject. The Royal Society of London unanimously adjudged to its author the Rumford Medal, appropriated as the reward of discoveries in that recondite province. Although paradoxical in many of its theories, defective in its arrangement, and over-ambitious in its style, this work is almost unrivalled in the entire range of physical science, for its indication of vigorous and inventive genius.

Previous to this period of his life, Leslie had twice appeared as a candidate for an academical chair; first in the University of St Andrews, afterwards in that of Glasgow, and on both occasions without success. Early in the year 1805, he became a candidate for the mathematical chair at Edinburgh, vacant through the promotion of Playfair to the chair of Natural Philosophy. His principal competitor, Dr Thomas Macknight, a man of respectable attainments, was one of the ministers of Edinburgh, and was supported by the majority of the city clergy. These, blinding perhaps their better judgment by an excessive zeal for the cause of their fellow-minister, fastened on one of Leslie's notes, in his essay on Heat, in which, after complimenting Hume's philosophical talent, he says, "that the unsophisticated notions of mankind are in perfect unison with the deductions of logic, and imply nothing more at bottom, in the relation of cause and effect, than a constant and invariable sequence." Alleging that the town-council were bound, according to the fundamental charter of the college, to act with the advice of the ministers in electing professors, they preferred a formal protest, accusing Leslie of having, in this note, "laid a foundation for rejecting all the argument that is derived from the works of God, in proof of His Being or Attributes." The patrons neglected their advice; and Leslie, to the great joy of all liberal minds, was, in March 1805, elected to the Mathematical chair. The case, however, was carried before the General Assembly; and after a memorable debate of two days, was dismissed, on the 23d of May, "as vexatious." When Leslie was informed of the charge, he excused himself in a very pointed letter, which was laid before his opponents, by stating, that his observations "referred entirely to the relation between cause and effect, considered as an object of physical examination." This was a sufficient defence of his orthodoxy, and ought to have stopped the prosecution before the Assembly.

Entertaining lofty notions of the dignity and utility of his new vocation, he discharged its duties with great ardour. Though the bent of his genius lay more to physics than to pure mathematics, he had cultivated the study of geometry with kindred relish, and with an admiration, in particular, of the analytical investigations of the ancient geometers, which led to his happiest essay in that science. Of his Course of Mathematics, the first volume, comprising Ele-

ments of Geometry, Geometrical Analysis, and Plane Trigonometry, was published, in 1809, at Edinburgh, and republished in 1811, and in 1817. His own favourite portion, on Geometrical Analysis, has been extolled by the most unsparing critics of the first part, and was speedily translated into French and German. He reproduced it, in 1821, with considerable emendations, in the second volume of his Mathematical Course, which also contains the Geometry of Curve Lines. The want of a third volume, on Descriptive Geometry and the Theory of Solids, which was part of his original design, left his Mathematical Course incomplete. An abridgement of the first volume appeared in 1828, under the title of Rudiments of Plane Geometry, including Geometrical Analysis, and Plane Trigonometry. In connection with his mathematical works, we may here mention the profound and learned treatise, published in 1817, on the Philosophy of Arithmetic,—a republication, with considerable alterations and additions, of one of the numerous articles contributed by him to the Supplement of this Encyclopaedia.

Notwithstanding the labours of the Mathematical chair, Leslie continued his experimental inquiries; and in June 1810, some experiments previously suggested in the course of his researches with his Hygrometer, led him to the discovery of that beautiful process of Artificial Congelation, by which he was enabled to produce ice, and even to freeze mercury at pleasure. This process consists in a combination of the powers of rarefaction and absorption, effected by placing a very strong absorbent under the receiver of an air-pump. Some endeavours were made, as in the case of the Differential Thermometer, to transfer the merit of this new discovery to a gentleman of the name of Nairne. The claim for him was founded on a paper published in 1777, in the Transactions of the Royal Society of London; from which it appeared that he was acquainted with the facts, that evaporation produces cold, and that sulphuric acid, the absorbent employed by Leslie, imbibes moisture. The best vindication of Leslie's claims is furnished by the admitted fact, that with Nairne's paper before them for a long course of years, the scientific world remained utterly ignorant of any such process till the date of Leslie's discovery; and even after his description of that process the most distinguished experimentalists of the capital failed in their trials of it, till it was performed there by himself in the ensuing summer!

In 1811 he performed the experiment in London before a meeting of some members of the Royal Society, and others. The discovery was announced in the same year, in the Memoirs of the French Institute; and the process itself was afterwards exhibited in presence of that body, by M. Pictet and M. Gay-Lussac. His experiments and views on this subject in 1813 he explained at considerable length in a small volume published at Edinburgh, and entitled, A Short Account of Experiments and Instruments depending on the Relations of Air to Heat and Moisture. Closely connected with the subject of this treatise, was an ingenious paper, published in 1818, in the Transactions of the Royal Society of Edinburgh, under this title: On certain Impressions of Cold transmitted from the higher Atmosphere; with a description of an Instrument adapted to Measure them. The Ethioscope is the instrument here alluded to.

In the autumn of 1814, Leslie travelled through France and the Netherlands, where the ever-fresh beauty of the scenery, combined with the meeting of such men as Humboldt, Laplace, Berthollet, and Baron Zach, tended to render that short tour of six weeks more than usually agreeable.

The celebrated Supplement of this Encyclopaedia, commenced in 1815, and completed in 1824, was greatly in-

debted to Leslie for his valuable contributions, and for the ever-ready counsel which his extensive and minute information could afford to its editor. He also contributed several articles to the Edinburgh Review, the principal of which are,—those on the Physical and Chemical Memoirs of the Society of Arcueil; on the History of the Barometer; on Delambre's work on the Arithmetic of the Greeks; on Von Buch's Travels; on Humboldt's Physical view of the Equatorial Regions, and on His Travels; and on the Attempts to discover a North-West Passage to Asia.

In the year 1819, on the death of Playfair, he was promoted to the chair of Natural Philosophy, a sphere more propitious to his peculiar genius. One of the first cares of his new situation was the extension of the apparatus for that greatly enlarged series of experiments which he thought necessary for the illustration of the course. This, indeed, was an object from the first to the last year of his incumbency ever present to his mind; and it was through his exertions that the means of experimental illustration, in the Natural Philosophy Class, were for the first time made worthy of the place. As a teacher both of mathematics and of natural philosophy, he wanted that sequence of ideas and perspicuity of exposition which can alone lead an undisciplined intellect through complicated doctrines, and thus attain the highest success in tuition. Still, his numerous experiments, the celebrity of his name, and his well-known extraordinary powers, concurred, with great simplicity and affability of manner, to secure the respect of his students, and to sustain the fame of the University. In 1823 he published, chiefly for the use of his class, his Elements of Natural Philosophy; being the first volume of a course intended to extend to three, and to exhibit a comprehensive view of the principles of that congeries of sciences which we are accustomed to class under the above term. A second edition of this volume, the only part of the plan ever completed was published in 1829, with corrections and additions.

In the summer of 1823, he accomplished his long-cherished design of visiting Italy, and especially Rome; but these objects of his tour appear to have disappointed his expectations. Two years after this, another tour which he made on the Continent, seems to have been confined to France, the Netherlands, and Holland, which for the most part he had traversed before.

The only important production of Mr Leslie's latter years, and his crowning benefaction to this work, was his Discourse on the Progress of Mathematical and Physical Science during the Eighteenth Century; which constitutes the Fifth Dissertation in the first volume of this present edition. His arrangement of his materials, and his view of the whole subject, is comprehensive, vigorous, and spirited; while the greater ease and perspicuity of its style render this the most agreeable of all his writings.

The volumes of the Edinburgh Philosophical Journal, published between 1824 and 1829, contain some small contributions from him, entitled Remarks on the Light of the Moon and of the Planets; Enumeration of the Instruments requisite for Meteorological Observations; Letter on the Coniometer; and Observations on the Theory of Compression, applied to discover the Internal Constitution of our Earth. Even these small treatises display all the characteristic boldness and poetical dress of his speculations.

Early in the year 1832, on the recommendation of Lord Brougham, then Lord High Chancellor, he was created, along with several other eminent men of science, a Knight of the Guelphic Order. He was also a member of the Royal Society of Edinburgh, and in 1820 had been elected a corresponding member of the Royal Institute of France, the only distinction of the kind that he valued.

For a few years before this period, he had passed all his intervals of leisure at Coates, a small estate which he had

purchased near Largo, and which he loved to decorate and improve. In the last days of October, whilst engaged in superintending some improvements on his grounds, he exposed himself to wet, and caught a severe cold. This was followed by erysipelas in one of his legs, which, owing to his contempt for medicine, and an excessive confidence in his strength and durability, he neglected, and again imprudently exposed himself in the fields. He soon afterwards became dangerously ill, and expired at Coates, on the evening of Saturday, Nov. 3, 1832, in the sixty-seventh year of his age.

Surpassed by a few of his contemporaries in the same walks of science, in profundity of understanding, in philosophical caution, and in logical accuracy, he rivalled them all in that creative faculty, which discovers often by an intuitive glimpse the hidden secrets of nature, and in that subtlety and reach of discernment which seizes the finest and least obvious qualities and relations of things, and ministers to new and unexpected combinations of her powers. Possessing the powers of judgment and reason in less efficiency, he was often led, in his speculations, to results glaringly inconsistent. His credulity, often shown in affairs of ordinary life, induced him too often to indulge in unwarrantable applications of mathematical reasoning, to subjects altogether foreign to that science. For example, he finds an analogy between circulating decimals and the lengthened cycles of the seasons! False, however, though many of his theoretical notions may be, his exquisite instruments, and his experimental combinations, will ever attest the utility, no less than the originality of his labours, and continue to act as aids to farther discovery. His information, ample and accurate, not only in science, but in the general range of subjects, and especially in Scottish history, becomes more remarkable from its co-existence with his great power of invention,—a union by no means common. His conversation, unlike his writings in point of expression, was simple, unaffected, and correct. It had no wit, little repartee, and no fine turns of any kind; but it had a strongly original and racy cast, and was replete with striking remarks and curious information. His moral character was slightly blemished by a tendency to avarice, unbecoming in a philosopher, and by an occasional reluctance to acknowledge kindred merit; but these defects were more than counterbalanced by his child-like simplicity, social disposition, warmth of affection, and unchanging serenity of temper. He was never married. In person he was somewhat under the middle size, and corpulent, but strong and well limbed; and though his face was large and florid, there were about his eyes and forehead the distinct traits of no ordinary man. There is a bust of him by Joseph; a portrait, of the ordinary size, taken a few years before his death, by Wilkie; and a head, drawn at an earlier period, by Henning, which presents a striking likeness.