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LEFT PERIDOTE 184 FEBIOD PERIDOTE, a name given by jewel- ers to the gn^een transparent varieties of olivine. It is usually some shade of olive-green or leek-green. Peridote is found in Brazil, Ceylon, Egypt, and Pegu. It is a very soft gem stone, dif- ficult to polish, and, when polished, liable to lose its luster and to suffer by wear. PERIGEE, the point in the moon's orbit at which she is nearest the earth. PERIGORD, an old province of France. It formed part of the military government of Guienne and Gascony, and is now represented by Dordogne and part of Lot-et-Garonne. PERIGUEUX, a town of France, formerly capital of Perigord, now in the department of Dordogne; on the right bank of the Isle, a tributary of the Dor- dogne; 95 miles N. E. of Bordeaux. It consists of the ancient city, which is gloomy in aspect and has narrow streets, with numerous houses and other re- mains of mediseval and Renaissance ar- chitecture, and the Puy St. Front, which till 1269 was a separate and a rival town. The cathedral of St. Front is a Byzan- tine edifice, said to be a copy of St. Mark's at Venice, built in 984-1047, but spoilt by "restoration" in 1865. The museum is especially rich in Roman and other antiquities. Statues of Montague, Fenelon, and the soldiers Daumesnil and Bugeaud adorn public places in the town. Iron is mined and worked, and woolens are manufactured. The celebrated pates de Perigueux, made of partridges and trufBes, are largely exported. Perigeux, a town of the highest antiquity, is the Gallic Vesunna mentioned by Caesar. The Romans built another town on the opposite side of the river at the junction of five Roman roads. Close to the mod- ern town are the remains of a vast am- phitheater, aqueducts, baths, and tem- ples. The tower of Vesunna is the most remarkable fragment of Roman architec- ture. It is 89 feet high, 200 feet in cir- cumference, and has walls 6 feet thick, but has neither doors nor windows. Its purpose is not known. The district of Perigord is noted for its archseological finds. Pop. (1911) 33,548. PERIHELION, or PERIHELIUM, the part oi a planet's or comet's orbit where it is nearest the sun, as opposed to aphelion. One of these is said to be in perihelion when it is at the extremity of the major axis of the elliptical orbit learest the focus occupied by the sun. PERIM, a barren island, and coaling and telegraph station, belonging to Great Britain, in the Strait of Bab-el-Mandeb, at the S. entrance to the Red Sea, 97 miles W. of Aden. It is about BV2 miles long by 2% wide, and crescent shaped, the two horns embracing a deep and spacious harbor. The island was held by the British in 1799-1800, and was again occupied in 1857. In 1883 it was made a coaling station, and soon began to be a rival to Aden. PERIMETER, in geometry, the bounds or limits of any figure or body. The perimeters of surfaces or figures are lines; those of bodies are surfaces. PERIOD, in geology, one of the larg- est divisions of geological time. In this sense there are at least three periods, the Primary, the Secondary, and the Ter- tiary periods, to which a fourth or Qua- ternary one is sometimes added; also their subdivisions; as, the Glacial period. In mathematics, a number of figures con- sidered together; one of two or more sets of figures or terms marked off by points or commas placed regularly after a certain number, as in numeration, in circulating decimals, or in the extraction of roots. In music, two or more phrases ending with a perfect cadence. In pa- thology, an interval more or less fixed in point of time at which the paroxysms of a fever, etc., recur. In printing, the full stop (.) which marks the end of a sen- tence in punctuating, or indicates an ab- breviation, as Mr., Jan., B. C, etc. In rhetoric, a complete sentence from one full stop to another; a sentence so con- structed as to have all its parts mutually dependent. PERIOD, a term used in chronology in the same sense as cycle, to denote an interval of time after which the astro- nomical phenomena to which it refers re- cur in the same order. It is also employed to signify a cycle of cycles. The Chal- daeans invented the Chaldaic period, or period of eclipses, from observing that, after a certain number of revolutions of the moon round the earth, her eclipses recurred in the same order and of the same magnitude. The Egyptians made use of the dog-star, Siriacal, or Sothric period, as it is variously called, to com- pare their civil year of 365 days with the true or Julian year of 365%, days. The period consequently consisted of 1,460 Julian years, corresponding to 1,461 Egyptian years, after the lapse on vv^hich the dates in both reckonings coincided. By comparing the solar and lunar years Meton, an Athenian, invented (432 b. C.) a lunar period of 6,940 days, called from him the Metonic cycle, also the lunar cycle. The Calippic period was invented by Calippus, and concisted of four Me- tonic cycles less by one day, or 27,759 days. But as this period still gave a