sion by a fine thread and entirely disconnected from the rest of the body, it would form a simple pendulum; but in general its period would not be that of the body itself. Those nearest the axis of suspension would also vibrate in a shorter time than those farther away. As a rule it is possible, however, to find a series of particles which, vibrating as simple pendulums, would have the same period as that of the body. Their distance from the axis of suspension is evidently given by the condition

from

If a line can be drawn in the body parallel to the axis of suspension, at the distance it, and so that the plane of the two lines includes the centre of gravity, it is called the 'axis of oscillation,' with reference to the given axis of suspension. It may be shown that if the body be suspended so as to vibrate about the axis of oscillation, the former axis of suspension will be the new axis of oscillation, and the period of vibration is the same in both cases. The intersection of the axis of oscillation by a plane passing through the centre of gravity and perpendicular to the axis is called the 'centre of oscillation.'

CENTRE OF PERCUSSION (Lat. percussio, a beating, striking). If a rigid body is suspended so that it can turn freely about a fixed axis -e.g. a door-a 'line of percussion' is such a line that a blow struck the body in that direction produces no reaction of any kind on the axis. In general, when a blow is struck the body in any chance direction, the axis experiences both a twist and a sidewise push; and, if the axis is held in fixed bearings, they must be strong enough to resist these. The point where the line of percussion intersects the plane which passes through the axis of suspension and the centre of inertia is called the centre of percussion.' This point may be proved to lie on the axis of oscillation,' with reference to the given axis of suspension. If the body is hanging freely from a point, so that the centre of gravity lies vertically below it, the centre of percussion coincides with the centre of oscillation.

CENTRE OF PRESSURE. The centre of pressure of any surface immersed in a fluid is the point in which the resultant of the pressures of the fluid on the several points meets the surface. The pressure of a fluid at a point is the 'force per unit area' at that point, or the limiting value of the ratio of the force acting over a surface to the area of the surface, as the surface is taken smaller and smaller. This pressure is due to two things-the weight of the fluid and the reaction of the walls of the vessel which contains the fluid. (See HYDROSTATICS.) The pressure due to a gas is generally uniform over a surface, unless it is very large, and so the centre of pressure is the centre of gravity of the surface; but in liquids it is different. When the bottom of a vessel containing a liquid, or when a plane immersed in a liquid is horizontal, the pressures on all the points of it are the same; obviously they form a system of equal parallel forces, whose resultant will pass through the centre of

gravity of the bottom or plane. But when the plane is inclined at an angle to the surface of the liquid, the pressure is not the same at all points, being greater at the lower than at the upper points. The resultant of the forces, then, will not pass through the centre of gravity of the surface, but through a point below it. This point is the centre of pressure. In the case of a vessel with a rectangle for one side, the centre of pressure is at the distance of one-third of the height from the bottom. In the general case it must be found by calculation involving the calculus. A gun

CENTRE PINTLE CARRIAGE. carriage in which the chassis, or way on which the upper part of a coast gun moves, is attached to the pintle of the platform at its middle, and revolves around it through the entire circumference of the circle. The traverse circles are consequently continuous. By this arrangement an all round horizontal field of fire is secured. See ORDNANCE and COAST ARTILLERY.

CENTRIFUGAL (Neo-Lat. centrifugus, from Lat. centrum, centre fugere, to flee). A machine employed in the process of clarification and filtration. It consists of a circular vessel so constructed as to be capable of being made to revolve at a very high rate of speed. When the muddy liquid is placed in the vessel and the whole caused to revolve, it is found that the particles of dust, mud, or other matter fly to the circumference, leaving the liquid in the centre practically clear. By suitable arrangements the clear liquid can be drawn off. A machine of this kind is used in sugar-refining, and on a smaller scale in chemical laboratories.

CENTRIFUGAL and CENTRIP'ETAL (in botany). See INFLORESCENCE.

CENTRIFUGAL FORCE. See CENTRAL FORCES; and MECHANICS (section on Dynamics). CENTRING. The temporary framework either of timber or a combination of timber and

iron, upon which any arched structure is to be built whether of masonry or of steel.

A centre consists of a series of parallel templets called ribs, usually placed five to six feet apart, and so constructed as to have their upper edges conform in outline to the soffit or internal concave face of the arch. Transverse planks called laggings are fastened to these and upon them the voussoirs or archstones directly rest.

In building the arch the centring should always be loaded symmetrically, and hence the work proceeds simultaneously from the two sides toward the middle. The stresses produced by the laying of the masonry are longitudinal rather than transverse, since under these conditions greater efficiency is secured from the timbering.

Transverse stresses will generally break the members under a very much smaller load than may be carried by the same members under a compressive thrust.

The centres should be allowed to remain in position as long as possible even after the completion of the arch, and even after the backing has been built so that the mortar may have time to set completely. Where the arch forms one of several with intermediate piers, no centre should be removed or struck until it is symmetrically located with regard to two other and adjacent arches; as otherwise, unless the piers are abutment piers, the thrust from one side will not be

counteracted by a similar thrust from the other. A centre should be absolutely stiff and settle as little as possible, since it is upon this templet that the structure is built. Generally the entire centre may be lowered simultaneously by removing the wedges in the striking plates, which latter perform the function of keeping the lagging in position close to the arch stones. The object of removing the entire centre at once is to avoid

cross stresses in the arch itself.

In masonry arches of long span, where centres have reached their greatest development, three

methods are used:

1. Direct supports from intermediate points. 2. Inclined struts in pairs. 3. Trussed girders.

The first is used wherever practicable, and the second and third when the supports in the first method can not be brought close enough together. In arches of great span, such as that of Waterloo Bridge, London, or the Cabin John Bridge, Washington (see BRIDGE), a longitudinal pulling strain is almost inevitable, as a beam of great length would bend to some extent under a thrusting strain. In such cases great skill is demanded in the designing and construction of the joints. As an arch is built from the piers toward the keystone, the weight upon the haunches during construction tends to push the crown upward, and therefore the problem of designing a framed centring involves the resistance of this tendency, as well as the supporting of the weight of the materials. CENTRIPETAL FORCE. See CENTRAL

FORCES.

CEN TROSOME (from Gk. Kévтρov, kentron, centre oμa, sóma, body). One of the two polar centres of the spindle, or "dynamic centres" from which the "archoplasmic" threads radiate and toward which the halves of the split chromatin elements are drawn at the time of cell-division. The centrosome seems to be less frequently present in the plant than in the animal cell. See CELL; EMBRYOLOGY.

CENTUM/VIRI (Lat. nom. pl., board of a hundred men, from centum, hundred vir, man). In ancient Rome, a body of judges, three from each tribe, in charge of civil cases. Under the Empire the number was increased to 180, presided over by the Decemviri Stlitibus Iudicandis, and the sessions were held in the Basilica Julia, in the Forum. Here the younger Pliny pleaded, and the Emperor Trajan sometimes lis

tened to the arguments.

CENTURION (Lat. centurio, from centuria, company of a hundred, from centum, hundred). In the Roman Army, the captain of a centuria or company. There were sixty centurions in each legion, a junior and a senior for each of the thirty maniples; and the senior of the first maniple of the first cohort was the chief centurion, or primipilus, of the legion, and often had the real command of the entire body.

CENTURIPE, chen'too-rē'på. A city in Sicily, 28 miles northwest of Catania, formerly called Centorbi (Map: Italy, J 10). It was destroyed in 1233 by Emperor Frederick II., and the inhabitants were transplanted to his new town, Agosta (q.v.), but it was rebuilt in 1548. Population, in 1881 (commune), 9000; in 1901,

11,311.

CENTURY PLANT. See AGAVE.

CENTURY WHITE. The nickname of John White (1590-1645), author of First Century of Scandalous Malignant Priests.

or

CEORL, keôrl or chĕrl (AS., man, husband, Ger. Kerl, man, possibly ultimately connected with Skt. jūra, bridegroom, paramour), CHURL. In the Anglo-Saxon social order, a common freeman, as distinguished from an eorl (earl), who was a privileged freeman. As the term earl did not originally imply a patent of nobility, nor even lordship or dominion, but only superior birth and social position; so the title ceor implied no degradation or inferior status, but, on the contrary, marked its possessor off from the lower classes-the laet, or unfree man, and the theow, or slave. The ceorl was capable of owning land, and had a share in the common plowland and meadow-land of his community. The ceorls were the citizens of the mark (q.v.), or hundred (q.v.), composing the moot (q.v.), or general assembly of the people, and administering the local justice and government. See VILLEINAGE, and consult the authorities there referred to; also Freeman, History of the Norman Conquest of England, Vol. I. (2d ed., Oxford, Eng., 1870-76).

CE'OS. See ZEA. CEPH'AE'LIS.

See IPECACUANHA. CEPH'ALAL'GIA. See HEADACHE. CEPH'ALAS'PIS (Neo-Lat., from Gk. KE paλý, kephale, head + donis, aspis, shield). A genus of primitive armored fossil fishes of the order Osteostraci, found in the brackish and fresh-water deposits of the British and North American Devonian formations. The body was elongated, triangular in section, and covered throughout with armor. The armor of the head region consists of a single heavy principal plate on the dorsal surface, which, when viewed from above, has the form of a horseshoe or the carapace of a king-crab (Limulus). A ventral plate is opposed to the dorsal plate and there are a few small accessory plates. There are no jaws, for the mouth was of the same form as that of

the modern lamprey-eel, in which animal it is a sucking organ. Indeed, the lampreys are believed to be degenerate descendants from early heavily armored types like Cephalaspis and its associates phalaspis must have been wholly cartilaginous, among the Ostracodermi. The skeleton of Cefor no traces of it have been preserved. The body was covered by bony scale-like rings, it had a single dorsal fin, and the tail-fin was heterocereal as in the modern sharks. The best examples of Cephalaspis have been obtained in the Old Red Sandstone of Scotland and England. The American examples are of less satisfactory preservation and are found in the Devonian sandstones of the provinces of New Brunswick and Quebec, Canada. For illustration, see OSTRACODERMI. CEPHALIC INDEX. See INDEX, CEPHALIC,

etc.

CEPH'ALIZATION (from Gk. Kepaλý, kephale, head). The phenomena of progressive concentration and enlargement of the parts of the body belonging to the head; a phenomenon characteristic of segmented animals. Cephalization is only a special case of the law of division of labor. In the earlier metameric animals the head is only slightly different externally from the other segments, and consists of but two or

three metameres. Already, however, the prin cipal sense-organs are located in the head, and the central nervous system is enlarged there to form a brain. In the Arthropoda and the vertebrates the number of metameres in the head is increased, there being six head-metameres in the crayfish, probably six in insects, and from nine to thirteen in vertebrates. The head now takes on the principal sensory and psychical functions, which are abandoned by the rest of the body. The sense-organs become more and more concentrated at the head and they become larger; the nerve-centres at the head become relatively more important, and so the brain increases in size, while the interrelation of parts becomes more intimate, so that the head not only becomes larger, but also more of a unit, and the brain more efficient as a controlling factor. sef'al-ô-kôr❜då (NeoLat. nom. pl., from Gk. kepaλý, kephale, head + Xopoń, chorde, cord). A small class of chordate animals, including the lancelet. These are usually regarded as offshoots of the primary vertebrate type, although some maintain that they are pioneer or ancestral types themselves and fore

CEPHALOCHORDA,

shadow fishes. See AMPHIOXUS.

CEPH'ALODIS'CUS. An extraordinary deepsea animal, regarded as allied to Balanoglossus (q.v.), and classed, with a related genus, Rhabdopleura, among the Adelochorda, as the lowest of the phylum Chordata. They were formerly regarded as polyzoans, but resemble Balanoglossus in structure, having a proboscis, a collar and collar-cavity, and a trunk, containing a structure resembling a notochord. "Cephalodiscus has an investment in the form of a branching gelatinous structure, which is beset with numerous short, filiform processes, and contains a number of cavities occupied by zooids. The latter are not in organic continuity, so that, though inclosed in a common investment, they do not form a colony in the sense in which the word is used of the Polyzoa or hydroid polyps. They multiply by buds, but these become detached before they are mature. Rhabdopleura occurs in colonies of zooids organically connected together, and inclosed in, though not in organic continuity with, a system of branching membranous tubes." Consult Parker and Haswell, Tert-Book of Zoology (London and New York, 1897).

CEPH'ALOʻNIA, or KEPH'ALLEʼNIA (Gk. Kepaλλŋvía, from кepaλý, kephale, head + ovos, onos, donkey). The largest of the Ionian Islands (q.v.), situated off the coast of Greece, between latitudes 38 and 38° 30' N. and longitudes 20° 21′ and 20° 49′ E. (Map: Greece, B 3). Its greatest length is about 32 miles, and its total area 266 square miles. The surface is mountainous, rising near the centre to an altitude of 3714 feet. The soil is for the most part thin, and water scarce and the rainfall slight. The climate is warm and dry, but agreeable. Earthquakes are not infrequent. The inhabitants are industrious and enterprising, and have planted vineyards wherever the grape will grow. There is little available land for the cultivation of grain, but the output of fruit is considerable. The chief products are olives, currants, and other southern fruits and vegetables, wine, and oil. The population in 1896 was 70.077. The language spoken is a Greek dialect. The chief towns are Argostoli

(q.v.), the capital, and Lixuri, on the Bay of Livada, on the southwest coast.

1

Cephalonia is called, by Homer, Same or Samos. Later the island appears under the name of It Cephallenia, derived from its inhabitants. successively fell into the hands of the Athenians, Romans, Byzantines, Normans, and Venetians, from the last of whom it was repeatedly wrested by the Turks. On the fall of the Venetian Republic, in 1797, it was seized by the French. In 1809 it came into the possession of England. It was ceded to Greece in 1864. With Ithaca and some neighboring islets Cephalonia now forms a nomarchy of Greece, with an area of 315 square miles and a population (1896) of 83,363.

CEPH'ALOP'ODA (Neo-Lat. nom. pl., from Gk. Kepaλý, kephale, head + Toûs, pous, foot). The highest and most specialized class of Mollusca, characterized by the presence of a circlet of fleshy arms surrounding the mouth on the front of the head. These arms, also called 'tentacles' and 'feet,' are modifications of the 'foot' which, as the muscular organ of locomotion of the Gastropoda and the Pelecypoda, occupies a ventral position on the body of the animal in those classes. In the Cephalopoda the foot has moved forward and become divided into two sections. The anterior section forms the circlet of arms that serve both as accessory mouth-parts, to catch and hold the prey, and as organs of creeping loco

motion.

The posterior section has become developed into part of a most efficient hydrostatic organ of propulsion, called the hyponome' or siphon.' The body of the cephalopod is essentially a bag formed of the 'mantle,' which entirely incloses the visceral organs, and which is open only at the end to which the head is attached. In some genera this bag is almost spherical, and creeping locomotion is accomplished chiefly by the appendages of the head; in others, the body is elongated, and furnished with two fin-like expansions on the sides, and the animal propels itself through the water by means of the hydrostatic organ, the hyponome. The ordinary position of the cephalopod in water is quite different from that of any other mollusk. A chiton, a snail, or a clam moves with the foot down, the mouth at the anterior end, and the visceral hump uppermost on the dorsal surface. A cephalopod when creeping has a very similar position, except that the mouth is ventral. In swimming, however, cephalopods have the mouth at the anterior end, and what was the anterior side is now the dorsal side of the animal. Thus the cephalopod, in taking to an active swimming life, has tipped backward through an angle of 90°. Cephalopods are able to swim forward or backward, often with great rapidity, the most important means of locomotion being the hyponome, near the under side of the head. The mantle is at

tached to the neck only on the dorsal side; elsewhere there is free communication between the mantle-cavity and the outside. When the mantle relaxes, water fills the cavity; when it contracts it closes tight about the neck; there is no exit for the contained water save through the hyponome, and, as contraction takes place suddenly and forcibly, a jet of water is expelled from the hyponome, causing the animal to dart backward through the water.

The head of a cephalopod is roundish, more distinct from the rest of the body than it is in

other mollusks, and is generally furnished with two large and prominent eyes that are very similar in structure to those of vertebrate animals. The nervous system is very well developed, and is more centralized than in other mollusks. The ganglia are well concentrated in what may be called the brain, which lies above and around the cesophagus, and is more or less wholly included within the cartilage of the head. From this brain nerves go to the various organs and parts of the body. The senses of hearing and smell are regarded as weak, but that of taste seems to be strong. The mouth opens in the midst of the circlet of arms. It is furnished with a strong, horny beak of two jaws that move vertically like those of a parrot, but the upper jaw is the shorter of the two. In addition to the jaws, there is a lingual ribbon covered with minute teeth, like that of the odontophore of the gastropods. The digestive apparatus is very complicated. The gullet swells out into a crop, and there is a gizzard as muscular as that of a bird. The intestine, after a few convolutions, terminates in the cavity which contains the gills at the base of the hyponome, by which the water is ejected from the mantle-cavity after having supplied air for respiration. There are two gills in most living cephalopods, one on each side, lying close to the body-wall; the only exceptions to this rule are in the three species of nautilus which have four gills, a pair on each side. Each gill consists of many membranous plates fixed to two sides of a stalk. The heart in the Tetrabranchiata consists of a single ventricle only; but there are four auricles, one for each gill; the Dibranchiata have two auricles, one for each gill. In the latter group, moreover, are found two contractile reservoirs (branchial or respiratory hearts), one for each gill, by which the blood is pumped into these

organs.

The arms or feet are very numerous in the Tetrabranchiata, and are not provided with suckers, but are hollow and furnished with long, retractile tentacles. In the Dibranchiata the arms are either eight or ten in number, are furnished with suckers (acetabula), and in the ten-armed genera two of the arms are much longer than the others and differ from them in form. The suckers are muscular disks, with cartilaginous rims, capable of exact application to any object, with central cavities the bottoms of which can be retracted at will to form a vacuum, and thus render the adhesion of the sucker close and firm after the manner of a cupping-glass. The poulpe or octopus has each of its eight flexible arms crowded with 120 pairs of such suckers, and as animals of this kind, with arms several feet long, exist commonly in the tropics, they are considered dangerous neighbors. Still more for midable, however, are the hook-squids of the south seas, the two long arms of which have suckers furnished in the centres with hooks that enter the flesh of any creature upon which the squid may lay hold.

The sexes are distinct in all cephalopods. In most cases the male is smaller than the female. He differs, moreover, in the asymmetry of his arms, one of which is more or less modified to form a copulatory organ. In two or three genera this modified arm breaks off from the body during copulation, and finds its way into the mantlecavity of the female, where it empties itself of

the male sperm-cells which it carries. The first zoologist who found such an arm in the mantlecavity of a female argonaut (q.v.) mistook it for a parasitic worm and called it 'hectocotylus' (hundred cups), from the numerous suckers which it bears. When its true nature was discovered, it was called a hectocotilized' arm, and the term is still used in that connection. The eggs have a horny covering, and after their extrusion from the parent they become agglutinated into masses of various forms. The young from the first very much resemble the mature animals, except in size.

All dibranchiates are provided with a peculiar organ of defense, called an ‘ink-bag,' which is absent in the tetrabranchiates. This ink-bag is filled with a peculiar secretion, capable of being expelled at will through the hyponome to darken the surrounding water and thus facilitate the escape of the cephalopod. (See SEPIA.) The tetrabranchiate cephalopods have a chambered shell which is described in further detail below. (See also NAUTILUS.) The dibranchiates have no true external shells-the shell of the female argonaut (q.v.) being scarcely an exception — but they have an internal shell, homologous to the external tetrabranchiate shell, known as cuttlefish bone, etc., sometimes merely rudimentary, included between the folds of the mantle and apparently serving to support the organs and to give rigidity to the body while swimming.

The cephalopods are all very voracious; they are carnivorous, feeding on fish, mollusks, crustaceans, etc. Even a powerful crab is not safe from the attack of a dibranchiate cephalopod little bigger than itself; the arms of the octopus, so abundantly provided with suckers, seize the crab, trammel every movement, and the parrotlike beak is strong enough to break the hard shell. Cephalopods are all marine, and they are found in the temperate and tropical parts of all seas. Nautilus, the single modern genus of the tetrabranchiates, with three species, lives in the Indian and South Pacific oceans, where it creeps on the bottom, near the shore, in comparatively shallow water. The dibranchiates are quite abundantly represented at the present time. The members of the two living orders, Sepioidea and Octopoda, have rather different habits and corre spondingly different habitats. The Octopoda, with round, sac-like bodies and eight arms, are essentially creepers on the bottom, like the poulpe or octopus. They live in shallow water along the shore, hiding in cavities during the day and creeping about over the rocks of the bottom by night. They are also capable, when they find themselves in large bodies of water, of swimming rapidly by means of the hyponome. They are often handsomely colored creatures, but they are generally repulsive objects.

The Decapoda have long, slender bodies and ten arms, of which two are much longer than the rest. They are found in all seas, often in deep water. Many of them congregate in hordes in the open sea, swimming about with great rapidity. Unlike the octopods, they seldom creep on the bottom, the hyponome being the principal organ of locomotion. Their motions are extremely graceful, and they are often beautifully colored. In size the living cephalopods range from a few inches in length (Sepiola) to many feet. The largest invertebrate is undoubtedly a giant squid (Architeuthis) of the Newfoundland