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HUY IIUYGENS 97 from the lowest plant or animalcule to the highest being, are fundamentally of one char- acter. Prof. Huxley is a corresponding mem- ber of the principal foreign scientific societies, and has received honorary degrees from the universities of Breslau and Edinburgh. His works are as follows : " The Oceanic Hydro- zoa" (1857); "Evidence as to Man's Place in Nature" (1863); "Lectures on the Elements of Comparative Anatomy" (1864); "Lessons in Elementary Physiology" (1866); "An In- troduction to the Classification of Animals" (1869) ; " Lay Sermons, Addresses, and Re- views" (1870); and "Critiques and Address- es" (1873). He is the author also of a large number of papers published in the journals of the royal, the Linnfflan, the geological, and the zoological societies, and in the memoirs of the geological survey of Great Britain. BUY, a town of Belgium, in the province and 16 m. S. W. of the city of Liege, at the entrance of the Hoyoux into the Meuse ; pop. in 1866, 11,055. It has a handsome Gothic church, a college, manufactories of paper, leather, and faience, distilleries, and an active trade. The former abbey of Neufmoutier contained the tomb of Peter the Hermit, by whom it had been founded ; in 1858 a statue was erected in his honor in the garden of the abbey. In the neighborhood there are mines of iron, zinc, and coal, and several mineral springs. HCYGENS (incorrectly HUYGHENB), Christian, a Dutch natural philosopher, born at the Hague, April 14, 1629, died there, July 8, 1695. He was the second son of Constantine Huygens, secretary and counsellor of the stadtholders Frederick Henry, William II., and William III. His father taught him the rudiments of educa- tion and the elements of mechanics. At the age of 15 he became the pupil of Stampion, and at 16 he was sent to Leyden to study law with Vinnius, who dedicated to him his first commentary on the Institutes of Justinian. He there also pursued mathematical studies, and afterward at Breda in the university, which was under the direction of his father. In 1650, after a journey to Denmark with Henry, count of Nassau, he began those mathematical and physical researches which afterward made him famous. In 1651 he published at Leyden his first work, on the quadrature of the hyperbola, the ellipse, and the circle, and in 1654 a paper entitled De Circuit Magnitudine irwenta nova. In 1655 Huygens went for the first time to France, and received the degree of doctor of laws from the faculty of the academy of An- gers. On his return to Holland he turned his attention to the construction of telescopes, in connection with his elder brother Constantine. With one of these instruments, having a focal length of 10 ft., and more powerful than any ever before made, he discovered the first (now called the fourth) satellite of Saturn, and pub- lished the discovery at the Hague in 1656. During the next year he wrote a paper on the calculus of probabilities. Pascal and Fermat had already written upon the subject, but the treatise of Huygens was more profound, and 50 years afterward James Bernoulli employed it as an introduction to his Ars Conjectandi. It was also translated into Latin by his former tutor Schooten under the title He Ratioeiniis in Ludo Alece, by which it is also known in 's Gravesande's edition of Huygens's works. Schooten published it in his Exercitationes Mathematics, to demonstrate, as he says, the utility of algebra. About this time Huygens sent a paper to Wallis on the area of the cis- soid, and to Pascal a calculation for hyperbolic conoids, and spheroids in general, and on the quadrature of a portion of a cycloid, in which papers he employed methods having the high- est characteristics of original thought. But his attention was not wholly devoted to mere- ly theoretical mathematics, for about this time he introduced one of the most practical and important of all inventions. Galileo had ob- served the isochronism of small vibrations of the pendulum, and had employed it as a mea- surer of time, but his method required an as- sistant to count the oscillations, and was of course far from being exact. To keep the pendulum in motion and cause it to register its successive vibrations was one of the problems which Huygens attempted, and which he suc- ceeded in solving by the invention of the pen- dulum clock, a description of which, under the title of Horologium, he dedicated to the states general of Holland in 1658. (See CLOCKS AND WATCHES.) In 1659 he constructed a tele- scope of 22 ft. focal length, in which he used a combination of two eye pieces, and again examined Saturn, making the discovery of the ring of the planet. The singular appearance which it sometimes presents of being accom- panied by two luminous bodies, one on either side, had been observed by Galileo, but his telescope had not sufficient power to permit him to discover its cause. Huygens's instru- ment enabled him to make out that the phe- nomenon in question, which at regular times appeared and disappeared, was produced by the oblique position of the ring with regard to the earth and to the sun. From an analysis of the phenomenon he predicted the disap- pearance of the ring in 1671, and the predic- tion was verified. He published an account of these observations at the Hague in 1659, in a volume also containing an account of sev- eral other discoveries, such as that of the great nebula in the sword of Orion, the bands upon the disks of Jupiter and Mars, and the fact that the fixed stars have no sensible magnitude. It was also accompanied by a description of a method for measuring the diameter of the planets. The micrometer used by him has been superseded by others, but it served the pur- pose of making correct measurements. In 1660 he visited France and England, and soon after published his celebrated theorems on the laws of the impact of bodies, in which most of the principles of the laws of motion are es-