under British control and has a small garrison and a lighthouse.
PERIM′ETER (Lat. perimetros, from Gk. περίμετρος, circumference, from περί, peri, around + μέτρον, metron, measure). The length of the boundary of a plane figure. See Isoperimetric Figures.
PERINÆUM (Neo-Lat., from Lat. perinæon, perineon, from Gk. περίναιον, perinaion, περίνεον, perineon, περινός, perinos, perinæum). The part of the human body which forms the floor of the true pelvis is by anatomists divided into two portions. Of these, the anterior one, situated in front of the anus, is called the true perinæum, or urethral (or, in the female, the vaginal) portion of the perinæum; the posterior portion, which contains the anus or termination of the rectum, is called the rectal or anal portion of the perinæum. The anterior portion, or true perinæum, is triangular in form, the apex being in front; the sides, about three inches in length, are formed by the rami of the pubes and ischium, and the base by an imaginary line joining the tuberosities of the ischium, and passing about half an inch in front of the anus. Through this space the urethra passes through a layer of strong fascia—the deep perinæal fascia—to communicate with the bladder, and in this space the opening is made in the operation of lithotomy.
In the female the space usually referred to as the perinæum lies between the vagina in front and the anal orifice behind. Its most important constituent is the anterior portion of the levator ani muscle. The perinæum, including a greater or less extent of this muscle and sometimes also the sphincter ani muscle, is often torn during parturition, and requires suturing for its repair.
PERIOD (Lat. periodus, from Gk. περίοδος, a going round, circumference, circuit, cycle, sentence, period, from περί, peri, around + ὁδός, hodos, road). A term used in chronology in the same sense as cycle, to denote an interval of time after which the astronomical phenomena to which it refers recur in the same order. It is also employed to signify a cycle of cycles. The Chaldeans invented the Chaldaic period or Saros, 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. This period consists of 223 lunations, or 6585.32 days, and corresponds almost exactly to 19 ‘eclipse years.’ The eclipse year is the time required for the sun, in his apparent motion among the stars, to complete a circuit from one of the nodes (q.v.) of the lunar orbit back again to the same node. On account of the motion of the lunar nodes, the eclipse year contains only 346.62 days, and 19 such years contain 6585.78 days. The error of the Saros is thus only 0.46 day (about 11 hours) in 223 lunations, or 19 eclipse years.
Various important periods or cycles are used in the calendar (q.v.) for predicting the dates of new and full moon. These phases recur on the same dates every nineteen years (except that leap years may change the dates one day), which fact was discovered by Meton, an Athenian, who invented (B.C. 432) a lunar period of 6940 days, or 19 years, called the Metonic cycle (q.v.), also the lunar cycle. The calippic period consists of 76 years, or four Metonic periods, and is thus able to take account of leap years. The period of the heliacal or solar cycle, after which the same day of the month falls upon the same day of the week, consists of 28 Julian years of 365¼ days each. If the year had regularly consisted of 365 days—that is one day more than an exact number of weeks—it is evident that at the end of seven years the days of the month and week would again correspond; but the introduction of an intercalary day into every fourth year causes this coincidence to recur at irregular periods. (To ascertain when the same days of the week and month will recur in the Gregorian calendar, see Calendar, section on Perpetual Calendar.) The Julian period is a cycle of cycles, and consists of 7980 (28⋅19⋅15) years, after the lapse of which the solar cycle, lunar cycle, and the indiction (q.v.) commence together. The time of its commencement was arranged so that it will expire at the same time as the other three periods from which it is derived. The year B.C. 4713 is taken as the first year of the period, consequently A.D. 1 is the 4714th year of it. (See Chronology; Calendar; Cycle.) Astronomers also use the term period to designate the quantity of time required by a planet or other celestial body to complete a revolution in its orbit. (See Elements.) In this sense there may be different periods for the same body, according to the point selected as the beginning and end of the periodic orbital motion. The location of the position from which the motion is supposed to be viewed may also change the period materially.
PERIODICAL. In a wide sense, a publication issued, at more or less regular intervals, in successive numbers, which are not related to one another as volumes or parts of a single book or series of books. The word, however, is commonly employed—and is here considered—in a narrower sense which excludes on the one hand newspapers (see Newspaper), or periodical summaries of current, and especially of political, events, and on the other such periodical publications as the transactions of learned societies, year-books, almanacs, and so on. Even within these limits the term includes a great variety of publications which differ so much in object and character that concise description of them is impossible; but it may be said of them in general that they are designed to furnish either information about matters of more than ephemeral interest, or entertainment, or both. They deal either with a single subject—such as literature, or a particular science or industry—or with a group of allied subjects, or with material of the most heterogeneous character. The most important special groups of them are reviews, or periodicals devoted especially to the criticism of books, and magazines, which are designed to furnish miscellaneous and entertaining reading. In the most popular of the latter class fiction forms an important part of the contents, and pictorial illustrations, often of fine artistic quality, are frequently employed.
Early Forms. The periodical, as thus defined, originated in France in the seventeenth century, and in the form of the critical literary journal. The first example of it is also one of the most famous and the longest lived, for its publication has continued, though with many interruptions, until the present day—namely the Journal des Savants. The idea which it embodied was conceived about 1663 by the historian Mézeray, who
