with less encouragement than it deserved. During the revolutionary troubles he emigrated for ten years; and on his return to France, his attachment to the principles of royalism prevented him from bringing his invention before the government until after the restoration of the Bourbons. In 1816 he obtained a patent for the propulsion of ships by steam, and organized a joint-stock company to carry it out; but the undertaking proved unsuccessful. He died at the Invalides in 1832, being then the senior captain of infantry in the French army. An account of his inventions was published by his son, the Marquis Achille de Jouffroy, in 1839.—W. J. M. R.
JOUFFROY, Theodore Simon, a French philosophical writer, who was born in 1796. His father sent him to Pontarlier, where his uncle was a professor, and afterwards he studied at Dijon, where he was fortunate enough to attract the attention of M. Roger, who procured his admission to the normal school at Paris. While there his philosophical tastes were developed by a course of lectures on philosophy by Victor Cousin. In 1817 he was appointed pupil-teacher of philosophy at the normal school, and at the same time elected to deliver a philosophical course at the Bourbon college, now called the Lycée Bonaparte. He remained at this post till 1820. The normal school was suppressed in 1822, and then he commenced a course of private instruction, and began to write for the periodical press. In 1826 he published a translation of Dugald Stewart's Essays on Moral Philosophy, to which he prefixed a valuable introduction. In 1828 he delivered a course of lectures on ancient philosophy at Paris, and the same year brought out a volume of his translation of the complete works of Thomas Reid the Scottish philosopher. This version of Reid consists of six volumes, and was not concluded till 1835. The first volume, which was the last published, contains an introduction, copious and carefully written, in which we have Jouffroy's estimate of the Scottish philosophy. Following this introduction is Stewart's Life of Reid, and a curious bibliography of Scottish philosophy from the days of Hutcheson to our own time. In the compilation of these valuable works, he was assisted by some of his Scottish friends. The third and fourth volumes of his edition of Reid's works are enriched with fragments of lectures by Royer Collard, and an introduction to them. Royer Collard was, as is well known, the first who endeavoured to give a popular exposition of the Scottish philosophy in France. In 1830 Jouffroy returned to a post in the normal school, which had been restored. He was at the same time named co-professor of the history of modern philosophy at the Faculty of Letters, and delivered a course of lectures on the law of nature, which was taken down in shorthand and published in 1835 and 1842. He succeeded M. Thurot at the college of France in 1833 as lecturer on Greek and Latin philosophy, and was elected titular member of the Academy of Sciences. The same year he published a collection of miscellaneous essays under the title of "Mélanges Philosophiques;" most of these had already appeared in periodical publications, but some were now printed for the first time. In 1835 alarming symptoms showed themselves, and he was compelled to abandon for a time his much-loved studies, and to go to Italy for the sake of his health. After a time he recovered sufficiently to return to Paris, and in 1838 exchanged his place at the college of France for that of librarian to the university, and his place at the Faculty of Letters for that of philosophy. Although not well fitted for political life, Jouffroy in 1831, and for several years after, was a member of the chamber of deputies, where, however, his voice was seldom heard. Once he is said to have saved the ministry by a speech, and once, in 1840, he was intrusted with the preparation of an address which resulted in his being forsaken by his own party. An affection of the chest, under which he had laboured for four years, carried him off, February 4, 1842. After his death Damiron published the third volume of his course on the law of nature, a new volume of "Mélanges Philosophiques," and a volume entitled "Cours d'Esthetique," &c. The writings of Jouffroy ought to have especial interest in the eyes of the admirers of the Scottish philosophers.—B. H. C.
* JOULE, James Prescott, the second son of Benjamin Joule, an extensive brewer in Salford, was born in that town on December 24, 1818. He was educated chiefly by private tuition. At the age of sixteen he received lessons in chemistry and natural philosophy from Dalton, of whom he has since proved himself the worthy successor in physical science. From 1837 till 1854 he attended closely to the business of the brewery, but occupied his leisure in scientific researches. Between 1838 and 1841 he published in Sturgeon's Annals of Electricity a series of papers, describing his experiments on an electro-magnetic engine constructed by him in 1838, and demonstrating, amongst other important results, the law of the increase of the resistance to the battery current, as the engine's velocity is augmented. A thorough investigation of the electro-magnetic engine led him to the conclusion that electro-magnetism cannot be made an economical source of motive power, so long as the material consumed in galvanic batteries is much dearer than coal. He investigated the best form of electro-magnet for lifting weights, and was thus enabled to construct electro-magnets of great power for their size. He also discovered the existence of a limit to the degree of magnetization of which iron was capable. About the same time (1840) he made the discovery that a bar of iron is lengthened or shortened by being magnetized, according as the tension upon it is less or greater. His first paper on heat was read to the Royal Society in December, 1840. In it he announced for the first time the law of the development of heat by voltaic electricity, viz., that the heating effect of a current of electricity is proportional to the resistance opposed to the current, and to the square of the quantity of transmitted electricity. In 1841 he showed that, in the electrolytic decomposition of any compound, the heat evolved by the circuit is diminished by an amount exactly equal to that evolved by the recombination of the elements of the compound decomposed. A study of these and similar phenomena naturally led him towards a correct mode of regarding heat in relation to mechanical force, and suggested a series of experiments, made in the early part of 1843, which proved that the relation between work and heat is definite and invariable; and he announced approximately the number expressing that relation, which number he for the first time denominated the "mechanical equivalent of heat." From that time till 1849 he continued to make further experimental researches with a view to determine the value of that numerical constant with precision, chiefly by measuring the heat developed by the friction of fluids; and he arrived at the result that each Fahrenheit's degree of heat developed in a pound of water, is equivalent to the mechanical energy which is able to raise seven hundred and seventy-two pounds to the height of one foot. That result has since been confirmed by the experiments of others, and by various scientific and practical applications. It has been named, by the consent of scientific men, "Joule's Equivalent," and is unquestionably the most important constant quantity in molecular physics. Mr. Joule next examined the phenomena of the heat liberated and absorbed by the compression and dilatation of elastic fluids; and showed, first, that the heat liberated by compression was the equivalent of the force used in the compression; secondly, that the heat absorbed in dilatation was the equivalent of the force occupied in displacing the atmosphere; and, thirdly, that if the dilatation was accomplished without evolving force, no thermal effect was observed. These results are, however, only very close approximations to the truth, and differ from it in proportion as the gases do not strictly fulfil the laws of theoretically perfect elastic fluids. Professor William Thomson pointed out the means of studying the extent of the deviation; and, with that eminent man as a fellow-labourer, Mr. Joule has carried on a long series of experimental investigations into the thermal effects of fluids in motion. In this, as well as in many other experimental researches by Mr. Joule and Professor Thomson, results have been obtained, which had been anticipated by means of the mechanical theory of heat. About the years 1845-47 Mr. Joule was associated with Dr. Lyon Playfair in a research on atomic volume and specific gravity. Mr. Joule is the improver of many descriptions of apparatus. From the preceding very brief and very much condensed summary of Mr. Joule's scientific labours, it will be seen that they have in the main been directed to the experimental investigation of the exact laws of correlation between the different forms of physical force, such as mechanical energy, heat, electricity, and magnetism; and it may be confidently asserted that he has done more than any other man of science of the present day towards converting that fundamental department of physics from an interesting speculation into a precise experimental science. In 1842 he was elected a member of the Literary and Philosophical Society of Manchester, of which he became president in 1860. He is one of the oldest fellows of the Chemical Society of London. He was elected a fellow of the Royal Society in 1849; has
