Having abandoned all thoughts of the clerical profession, Leslie went over to Virginia, as tutor to the Messrs. Randolph, with whom he spent upwards of a year in America. He next proceeded to London, having introductory letters from Dr. Smith, where he proposed delivering lectures on Natural Philosophy; but in this he was disappointed. His first literary employment was on the notes to a new edition of the Bible, then in course of publication by his friend Dr. William Thomson, with whom he had become acquainted at St. Andrews. He next entered into an engagement with Murray, the bookseller, to translate Buffon's "Natural History of Birds," which was published in 1793, in nine volumes octavo. He subsequently visited Holland ; and, in 1796, proceeded on a tour through Switzerland and Germany with Mr. Thomas Wedgwood. On returning to Scotland, he stood candidate for a Chair, first in the University of St. Andrews, and afterwards in that of Glasgow, but was unsuccessful in both attempts. In 1799 he again went abroad, making the tour of Norway and Sweden, in company with Mr. Robert Gordon, whose friendship he had acquired at St. Andrews.

The first fruits of Mr. Leslie's genius for physical inquiry appeared prior to the year 1800, by the production of his celebrated "Differential Thermometer," which has been described as one of the "most beautiful and delicate instruments that inductive genius ever contrived as a help to experimental inquiry." This was followed, in 1804, by his well known "Essay on the Nature and Propagation of Heat," which was written while residing with his brothers at Largo, where the experimental discoveries were made for which the treatise is so much distinguished. The Essay immediately attracted the notice of the Royal Society, by the Council of which the Rumford medals were unanimously awarded to him.

In 1800, the Mathematical Chair in the University of Edinburgh having become vacant by the translation of Professor Playfair to the chair of Natural Philosophy, Mr. Leslie came forward as a candidate. He was opposed by Dr. Thomas M'Knight, one of the ministers of the city. In addition to the fame of his recent discoveries, Mr. Leslie was warmly recommended to the Town Council and Magistrates by testimonials from the most scientific and able men of the day. Vigorous opposition, however, was made to his election by most of the city clergy, who accused him of infidelity, and they insisted on their right to be consulted in the choice of Professors, according to the original charter of the College. They protested against the proceedings of the Council; and subsequently, on the 22nd of May, brought the affair before the General Assembly. The leaders in this opposition were of the Moderate party ; while the cause of Mr. Leslie was as warmly espoused by those usually to be found on the opposite side. The case created great excitement. Satisfactory testimonials were produced, as well as one of Mr. Leslie's own letters, confirmatory of his orthodox principles. The debate—in which the Rev. Sir Henry Moncrieff was one of the most powerful speakers in favour of the accused—was not concluded till about midnight of the second day, when his opponents were outvoted by ninety-six to eighty-four.

Mr. Leslie now took possession of the Mathematical Chair without farther opposition. Finding the class apparatus very deficient, he immediately set about remedying the defect, by making extensive collections, and adding several instruments of his own invention; and, throughout the whole period of his professorship, much of his leisure was devoted to the accomplishment of still further improvements. In 1810, by the aid of the hygrometer—one of his ingenious contrivances —he arrived at the discovery of artificial congelation, or the mode of converting water and mercury into ice, which has been characterised as a process "singularly beautiful." In 1819, on the death of Professor Playfair, he obtained the Chair of Natural Philosophy, and thereby found his sphere of usefulness extended, and a wider field for the display of his talents.

The various works produced by Mr. Leslie are as follow :—In 1809, "Elements of Geometry," which immediately became a class-book— 1813, an "Account of Experiments and Instruments depending on the Relation of Air to Heat and Moisture "—1817, "Philosophy of Arithmetic, exhibiting a Progressive View of the Theory and Progress of Calculation"—1821, Geometrical Analysis, and Geometry of Curve Lines, being volume second of a course of Mathematics, and designed as an introduction to the Study of Natural Philosophy,' for the use of his class, of which only one volume appeared—1828, "Rudiments of Geometry," a small octavo, designed for popular use. Besides these, he wrote many articles in the Edinburgh Review, in Nicholson's Philosophical Journal, in the Transactions of the Royal Society of Edinburgh, and furnished several valuable treatises on different branches of Physics in the Supplement to the Encycloycedia Britannica. In the seventh edition of that work, begun in 1829, he wrote a "Discourse on the History of Mathematics and Physical Science during the Eighteenth Century," which is allowed to be the most pleasing and faultless of all his writings.

In 1832, on the recommendation of Lord Brougham, then Lord Chancellor, Mr. Leslie was created a Knight of the Guelphic Order, and a similar honour was conferred on Herschel, Bell, Ivory, Brewster, South, Nicholas, and other individuals equally eminent for their attainments ; but he did not long enjoy the honour conferred on him. He had purchased an estate, called Coates, near his native place, where, by exposing himself to wet while superintending some improvements, he caught a severe cold, which terminated in his death on the 3rd November, 1832.

The character of Sir John has been subject to some little stricture. All have admired the inventive fertility of his genius, his extensive knowledge, and vigorous rnind. As a writer, however, his style has been criticised ; and he has been accused as somewhat illiberal in his estimate of kindred merit; while he is represented to have been credulous in matters of common life, and sceptical in science. "His faults," says his biographer, "were far more than compensated by his many good qualities—by his constant equanimity, his cheerfulness, his simplicity of character—almost infantine—his straight-forwardness, his perfect freedom from affectation, and, above all, his unconquerable good nature. He was, indeed, one of the most placable of human beings; and if, as has been thought, he generally had a steady eye in his worldly course to his own interest, it cannot be denied that he was, notwithstanding, a warm and good friend, and a relation on whose affectionate assistance a firm reliance could ever be placed." In this character we are disposed to concur. One slight blemish, however, has been overlooked—personal vanity ; for, strange to say, although in the eyes of others the worthy knight was very far from an Adonis, yet in his own estimation he was a perfect model of male beauty.