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LEU through the interest of Dr Fothergill, his old Quaker preceptor, got into good practice. He was admitted licentiate of the Royal College of Physicians in 1769, and in 1770 was elected fellow of the Royal Society. In the following year he became a member of the Linnæan Society. Medicine and botany have been particularly indebted to his researches. With schemes of benevolence he was much occupied; the General, Finsbury, and Surrey dispensaries, all owe their origin to him; also the sea-bathing infirmary near Margate. He kept up a constant correspondence with the most eminent literati in Europe, and was highly esteemed for his great learning and sound judgment. He belonged to no less than sixteen universities. That he was not without a sense of the humorous is evinced by the following epitaph, which he wrote for his own tombstone—

Dr. Lettsom died in London, November 1, 1815. His principal works are—"Reflections on the General Treatment and Cure of Fevers," 8vo, 1772; "The Natural History of the Tea-tree, with observations on the medical qualities of Tea, and effects of Tea Drinking," 4to, 1772; "Medical Memoirs of the General Dispensary in London," 8vo, 1774; "Improvement of Medicine in London on the Basis of Public Good," 8vo, 1775; "History of the Origin of Medicine, and of the State of Physic prior to the Trojan War," an oration, Ito, 1778; "Hints to Promote Beneficence, Temperance, and Medical Science," 3 vols. 8vo; an edition of the Works of J. Fothergill, M.D., in 3 vols. 8vo; a Life of Dr. Fothergill, 8vo; "The Naturalists' and Travellers' Companion," 8vo.—M. G. S.  LEUCHTENBERG. See.  LEUCIPPUS, a Greek philosopher, the author of the atomic theory, taught a system of pure materialism in opposition to the Eleatic school. He held the existence of a matter which filled space, and consisted of an aggregate of indivisible entities. These in themselves are immutable, but are of most diversified forms, and are susceptible of very various combinations. The changes in them produce all the changes in the universe, and these result from a law of absolute necessity. Round atoms possess the property of motion, and the soul consists of a mass of these. He flourished about 500 .—D. W. R.  LEUNCLAVIUS,, a celebrated German scholar, was born at Amelbeuern in Westphalia in 1533. His real name was Löwenclau, but he is better known under its Latinized form. He travelled in almost all the countries of Europe, acquired the Turkish language, and gathered most valuable materials for a history of the Turks. The results of his researches were published in his "Musulmanicæ Historiæ; libri xviii.," Frankfort, 1595; his "Annales Sultanorum Othomanidorum," 1596; and his "Pandectæ Historiæ Turcicæ," by which works a more solid knowledge of Turkish history was first introduced into western Europe. Leunclavius was besides an excellent Greek and Latin scholar, and was not less conversant with the law of nations. The better part of his life was passed in the service of various courts, especially that of Savoy. He was nominated to the chair of Greek at Heidelberg, but seems never to have entered upon the duties of this office. He died at Vienna in 1593. His morals, no less than his works, have met with severe censure. The latter, however, received unqualified praise from other and perhaps more competent critics. Thus Huet says with regard to Leunclavius' translation of Xenophon, "that he omits or perverts nothing; his Latin answering to the Greek word for word, and preserving the construction and arrangement; so that we find the original author complete, yet with a purity of idiom and a free and natural air not often met with." Besides Xenophon, Leunclavius translated several of the later Greek authors (Zosimus, Procopius, Dio Cassius), and edited and commented on some of the fathers.—K. E.  LEUPOLD,, a German mechanician, and the first known inventor of the non-condensing or high-pressure steam-engine, was born at Planitz, near Zwickau, on the 25th of July, 1674, and died at Leipsic on the 12th of January, 1727. He was at first bred to the occupation of a turner and cabinetmaker; then studied for a time at Jena and Wittenberg, with a view to the ecclesiastical profession; and finally established himself at Leipsic as a mechanician, and maker of mathematical and physical instruments. He became celebrated for his skill in the science and practice of mechanics, and was appointed by the elector of Saxony, in 1725, a member of the Council of Mines. He was a corresponding member of the Berlin Academy. He wrote various treatises on machinery and on mechanical questions, of which the most remarkable is his "Theatrum Machinarum Generale," in 9 vols. folio, Leipsic, 1723-27 and 1739, with a supplementary volume and index, by J. E. Scheffler, 1741. This work is a general treatise on mechanics, on machinery, and on some branches of civil engineering, as they existed in Leupold's time; including an account of some inventions of his own, and especially of a high-pressure or non-condensing steam-engine, in which the pistons of two cylinders are hung from opposite ends of a beam, and driven alternately by the admission of steam below them from a boiler, the steam which has done its work being discharged into the atmosphere. The admission and discharge of the steam are regulated by a "four-way-cock," of which contrivance also Leupold is held to have been the first inventor. The engine is represented as applied to the purpose of pumping water. It does not appear that it was ever executed; but our present knowledge assures us that if it had been executed it would have worked.—W. J. M. R.  LEUSDEN,, a distinguished Dutch philologist, was born at Utrecht in 1624, and devoted himself, first in his native town and afterwards at Amsterdam, to the study of oriental languages. In 1649 he was appointed to the professorship of Hebrew at Utrecht, the duties of which office he discharged with great distinction till his death in 1699. He has left a number of learned works, among which we mention his three Philologi—"Philologus Hebræus, Philologus Hebræo-mixtus, and Philologus Hebræo-græcus;" his "Hebrew Grammar;" his "Novum Testamentum Syriacum;" and last, but by no means least, his brilliant edition of the Hebrew Bible, the printer of which (the Jew Athias) received a gold chain and medal as a well-merited reward from the states-general.—K. E.  LEUWENHOECK,, or , a celebrated Dutch naturalist, was born at Delft, October 24, 1632. He appears to have attained some celebrity at an early age as a maker of optical instruments, the superiority of which was principally due to his skill in grinding lenses. Wunderbeck, in his History of Medicine, and several other authors, affirm that Leuwenhoeck was a physician in his native city; but there can be little doubt that this is an error. With the profession, the interests of which were so much advanced by his labours, he appears, like many other great discoverers, to have had no connection as a practitioner. It is certain, that on the title-pages of his books he gives himself no designation but that of F.R.S., and Sprengel simply calls him "a great naturalist and artist." When a lad of sixteen years, he was admitted into the counting-house of a merchant as an apprentice; but beyond the fact that he soon quitted this employment, and that he married at an early age, there is little known of his personal history. Peter the Great being in the neighbourhood of Delft in 1698, honoured him with an audience at which Leuwenhoeck submitted his microscopes to the inspection of the czar, and expounded to him the new doctrine of the circulation of the blood. Indefatigable to the last, Leuwenhoeck attained the great age of ninety-one years, dying August 26, 1723. The superiority of his microscopes gave him a great advantage over most of his contemporaries in the researches to which his life was devoted. When Harvey rediscovered the circulation of the blood—it was known almost a century before to Calvin's opponent, Michael Servetus—he had to meet the objection, learnedly and persistently urged, that if the arteries were in communication with the veins, as his doctrine implied, it was impossible the blood could nourish the system in merely passing through them. In 1686 the Dutch naturalist sent a paper to the Royal Society of London, in which he denied the communication of the smallest arteries with the veins by capillary vessels. Four years later, however, having prosecuted his microscopical researches with the advantage of improved instruments, he became a convert to Harvey's doctrine, and wrote in support of it; proving the continuity of the arteries and the veins through intervening capillaries, in which the exact boundary line between the two kinds of blood-vessels could not be distinctly traced With the great event in the history of medical science in the seventeenth century, his name is thus prominently connected. His researches did much to silence at once the cavils with which Harvey was on all sides assailed. 