CHRISTIAN, lord of Zeelhem, was the second son of Constantine Huypgens, secretary and counsellor to the princes of Orange, and born at the Hague on the 14th of April 1629. He was one of those rare men who, from the most sublime theories, know how to deduce the most useful applications, and whom admirable inventions in the arts, as well as the sciences, place on the same level with the Archimedeses and the Newtons. His family, originally from Brabant, was rich and respected; and the important post which his father held under three successive princes of Orange had been filled by his grandfather, as it subsequently was by his elder brother Constantine, who in this capacity attended King William in the celebrated expedition to England, which Huygens issued in the Revolution of 1688. His father, a distinguished man of letters, whose poetical compositions had attained much celebrity, was not slow in remarking the happy qualities of his genius, and, full of paternal solicitude for his improvement, became his first instructor. He early taught his son music, arithmetic, and geography; and, about the age of thirteen, initiated him in the knowledge of mechanics, for which young Huygens showed a surprising aptitude. At fifteen he received the assistance of a master in the mathematics, a geometer of Amsterdam named Stampioen, of whom Descartes has left an unfavourable impression, but who in a short time enabled his pupil to make great progress. At the age of sixteen he was sent to Leyden to study law under the learned juristconsult Vinnius, who dedicated to him his Commentary on the Institutions; but he did not permit jurisprudence to divert him from his mathematical studies, which he prosecuted at this place, as well as afterwards at Breda, where an university had been erected, and where, being placed under the direction of his father, he resided from 1646 to 1648. In these two cities he had as masters two very able geometers, Francis Schooten and John Pell; and his first essays were so successful that they attracted the notice of Descartes, to whom the author had communicated them. The genius of this great man divined instinctively that of Huygens. "It is some time," said he, in a letter written at this period, "since Professor Schooten sent me a writing by the second son of M. de Zuylichem touching a mathematical invention which he had attempted to make; and although he has not yet found his account in the pursuit (which is not wonderful, seeing he sought a thing which no one has ever been able to find), yet he has taken such a bias towards geometry, that I feel assured he will become excellent in that science, of which I scarcely see any one who knows anything." On his part, the young geometer was filled with admiration of the great philosopher; and he wrote to Father Mersenne that, for ages, no one comparable to him had appeared. Nevertheless, Huygens had not the good fortune to see him. Descartes had quitted Holland, and when, in 1649, Huygens, having left the university, travelled with Henry Count de Nassau, he regretted much not having it in his power to pass from Denmark to Sweden, whither Descartes had repaired, out of complaisance to the imperious Christina. After this tour, he settled in his native country; and it was then that he commenced that series of inventions which have rendered his name so justly celebrated. In 1651 he published at Leyden his Theoremata de quadratura Hyperboles, Ellipsis, et Circuli, ex dato portionum gravitatis centro; a work which afforded a favourable specimen of his genius for mathematics, and gave promise of still greater excellence. In 1655 he travelled into France, and took the degree of doctor of laws at Angers. In 1658 he published at the Hague his Horologium oscillatorium, sive de motu pendulorum. In a preceding work, entitled Brevis Institutio de usu Horologiorum ad inveniendas longitudines, he had exhibited a model of a newly-invented pendulum; but as some persons, envious of his reputation, laboured to deprive him of the honour of the invention, he wrote his Horologium oscillatorium to explain the construction of his pendulum, and to show that it differed from the astronomical pendulum invented by Galileo. In 1659 he published his Systema Saturninum, sive de causis mirandorum Saturni phaenomenon, et comite ejus planeta novo. Galileo had endeavoured to explain some of the appearances exhibited by the planet Saturn; he had at first perceived two attendant stars, but some time afterwards was surprised to find that they had disappeared. Huygens, desirous to account for these changes, laboured with his brother Constantine to improve the construction of telescopes; and having at length made an instrument of this kind, possessing greater power than any which had Huygens yet been contrived, he proceeded to observe the phases of Saturn, and to record all the different aspects of that planet. The results were of equal interest and importance to the science of astronomy. He discovered a satellite of that planet, which had hitherto escaped the notice of astronomers; and, after a long course of observation, he showed that the planet is surrounded with a solid and permanent ring, which never changes its situation. In 1660, he took a second journey into France, and the year following he visited England, where he communicated the art of polishing glasses for telescopes, and was admitted as a member of the Royal Society. The air-pump, then recently invented, he materially improved; and about the same time he also discovered the laws of the collision of elastic bodies, as did afterwards our own countrymen, Wallis and Wren, who disputed with him the honour of the discovery. After a stay of some months in England, Huygens returned to France, where, in 1663, his merit became so conspicuous, that Colbert resolved to bestow on him such a pension as might induce him to establish himself at Paris. But this resolution was not carried into effect until 1665, when letters written in the king's name were forwarded to the Hague, where he then resided, inviting him to repair to Paris, and offering him a considerable pension, with other advantages. Huygens accepted the proposal of the French minister, and, from 1665 to 1681 resided at Paris, where he was admitted as a member of the Royal Academy of Sciences. During this period he was occupied with mathematical pursuits; he wrote and published several works, which were most favourably received; and he invented and perfected some useful instruments and machines. But, by continued application, his health began to be impaired; and although he had twice had recourse to his native air, and on both occasions derived benefit from the change, the improvement was only temporary; and he at length found it necessary to quit France and return to his own country. It is not improbable that his departure was accelerated by the revocation of the edict of Nantes; for although he had received an assurance that he would be permitted to enjoy the same liberty as before, without being molested on account of his opinions, he had but little encouragement to remain in a country where his religion was proscribed, and those who professed it subjected to the most cruel persecution. He accordingly left Paris in 1681; passed the remainder of his life in his own country, occupied with his usual pursuits; and died at the Hague on the 8th of June 1693, in the sixty-seventh year of his age, whilst his Cosmoteoros, a treatise on the plurality of worlds, was passing through the press. The Opera Varia of Huygens were published in 1700, 4to, and his Opuscula Posthuma appeared soon afterwards, Leyden, 1703, in 4to. The former collection, which is generally bound in four volumes, contains the greater number of the pieces which he published separately, and is divided into four parts; the first comprising the pieces relating to mechanics; the second, those relating to geometry; the third, those relating to astronomy; and the fourth, miscellaneous productions, which could not be arranged under any of the preceding heads. 'sGravesande had the care of this edition, in which he inserted various additions to the different pieces extracted from the manuscripts of Huygens. The same individual also superintended the publication of his Opera Reliqua, which appeared at Amsterdam in 1728, in two vols. 4to, and contain, 1. his Treatises on Light and Gravity; and, 2. his Opuscula Posthuma, originally printed in 1703. For a masterly account of the genius and labours of Huygens, the reader may consult the Dissertations by Professors Playfair and Sir John Leslie, prefixed to this work. This illustrious man gave his whole time to science; he loved a quiet studious life, and found sufficient enjoyment in pursuing curious and useful researches. He was modest, amiable, cheerful, and in all respects as estimable in private life as he was eminent in science. It is not a little singular, that the discovery of the real nature, or at least of the true figure, of the luminous belt or ring which encompasses the planet Saturn should have been made by the same individual who invented the pendulum clock and the micrometer.