68 



CENTRE-BOARD 



CEPHALASPIS 



curve called a parabola. Again, the moon is held 

 in her orbit round the earth by the action of 

 gravity, which is constantly preventing her from 

 going off in the line of the tangent to her path at 

 any instant. 



In connection with this subject we have to make 

 some remarks on what is called centrifugal force. 

 We have seen that force must always be applied to 

 make a body move in a curved path. Such ,a force 

 is called a centrifugal force, the old erroneous 

 notion being that bodies have a tendency to fly 

 outwards from the centre about which they are 

 revolving. The use of the term will, however, cause 

 no inconvenience, provided we interpret it merely 

 as indicating that, to keep a body moving in a 

 curve instead of in its natural straight line, a force 

 directed towards the centre of curvature is always 

 required. 



Many familiar illustrations of the action of the 

 so-called centrifugal force will occur to the reader. 

 A ball fastened to the end of a string, and whirled 

 round, will, if the motion is sufficiently rapid, nt 

 last break the string, and fly off in a tangential 

 path. This is due to the fact that the cohesion of 

 the particles of the string are no longer able to 

 supply the force necessary to keep the ball moving 

 in its circular path. For a similar reason a fly- 

 wheel or a grindstone bursts when it is made to 

 rotate too rapidly. It is found that at a curve on 

 a railway it is the outer of the two rails which is 

 most worn. This is due to the fact that the outer 

 rail has to supply the force necessary to keep the 

 trains moving in curved paths. A glass of water 

 may be whirled so rapidly that, even when the 

 mouth is downwards, the excess of the centrifugal 

 force over the weight of the water is sufficient to 

 prevent the water from falling out. The cen- 

 trifugal force increases with the velocity. As a 

 matter of fact, it can be shown that when a body 

 moves in a circle of radius r, with velocity v, its 



centrifugal force is - By means of this formula 



it can be proved that about ^^th of its weight is 

 required merely to keep a body on the earth's 

 surface at the equator. By this amount the weight 

 of a body is diminished. Now 289 is equal to 17 2 . 

 Hence it follows that if the earth were to rotate 

 seventeen times as fast as it does now, the 

 attraction of gravitation would only just be able at 

 the equator to keep bodies from flying off its surface. 

 If the rotating body be plastic, it will swell out in 

 all directions perpendicular to the axis of rotation, 

 and assume the form of an oblate spheroid. For 

 the same reason the earth itself has assumed 

 the form of an oblate spheroid, a result which is 

 seen on a greater scale in the case of Jupiter and 

 Saturn on account of their larger size and more 

 rapid rotation. 



Centre-board. See YACHT, Vol. X. p. 770. 



Centrifugal and Centripetal are terms 

 used in Botany to designate two different kinds of 

 leaf development or inflorescence, the former term 

 being applied when the development proceeds from 

 the apex towards the base of the axis or leaf, and 

 the latter when it is from the base upwards towards 

 the apex. See LEAF, INFLORESCENCE. 



Centrifugal Force. See CENTRE. 



Centripetal Force. See CENTRE. 



Centum'yiri ( ' a hundred men ' ), a college of 

 justice in ancient Rome, which had jurisdiction in 

 civil cases. It has been supposed that the body- 

 was originally made up of three delegates from each 

 of the thirty-five tribes. There were 180 members 

 in the time of Augustus, and under the emperors it 

 increased in importance, as it became the only 

 scene left for the display of judicial eloquence and 

 of legal knowledge. 



Centurion ( Lat. centurio, from centum, ' a 

 hundred'), a Roman officer commanding a century 

 or company of foot-soldiers. There were sixty 

 centurions in a Legion (q.v. ). 



Ceorl, a word which occurs frequently in the 

 laws before the Norman Conquest under somewhat 

 varying senses, but substantially meaning an ordi- 

 nary freeman not of noble birth. His position 

 gradually sank in social status until it hardly 

 differed from that of the serf, save that the ceorl had 

 the right of choosing his own master in accordance 

 with the law of Athelstan, which required every 

 landless man to find himself a lord. He still 

 remained ' law- worthy,' and paid his wer-gild of 

 two hundred shillings ; but part of his freedom had 

 disappeared, and ultimately nis condition developed 

 into the complete villenage characteristic of feudal- 

 ism. On the other hand, ceorls who possessed land 

 often contrived to force their way into a higher 

 social class, that of the thegns, a kind of nobility 

 of service who may be roughly put as equivalent to 

 the knights of the period after the Conquest. A 

 ceoii with 5 hides ( 600 acres ) of land was ' thegn- 

 worthy.' The name ceorl does not occur in 

 Domesday the very degradation of the meaning 

 of the word churl in modern usage is but a part of 

 the historical degradation of the social class which 

 it denoted. 



Ceos (sometimes called by the Italianised name 

 of Zea or Tzia ), one of the Cyclades, in the ^Egean 

 Sea, 14 miles off' the Attic coast. It is 13 miles 

 long, 8 broad, and 39 sq. m. in area. The central 

 and culminating point is Mount Elias, 1863 feet 

 high. It is fairly fertile, raising fruit, wine, honey, 

 and valonia. The population is 4311, of whom 4295 

 belong to the capital, Zea or Ceos. In ancient 

 times Ceos was noted as the birthplace of the poets 

 Simonides and Bacchylides, and the physician 

 Erasistratus ; and the Cean laws were famous for 

 their excellence. 



Ceplialaspis, a genus of fossil Ganoid fishes, 

 of which six species have been described, two 

 belonging to the Upper Silurian, and four to the 

 Devonian measures. The head was protected by 

 a large ganoid plate, sculptured externally with 

 circular radiating markings. The shield was pro- 

 duced into a horn at each posterior corner, and bore 

 a median and posterior dorsal spine. Agassiz 

 gave the name cephalaspis ('buckler-headed') from 

 this extraordinary covering, which has very much 

 the appearance of, and was formerly supposed to 

 be, the cephalic shield of an Asaphus or Trilobite. 

 The body was covered with rhomboidal enamelled 

 scales, and furnished with dorsal and pectoral fins : 

 it terminated in a large unsymmetrical tail. In a 

 graphic description of this fossil in his Old Red 

 Sandstone, Miller thus sketches the general appear- 

 ance of the animal : ' Has the reader ever seen a 

 saddler's cutting-knife a tool with a crescent- 

 shaped blade, and the handle fixed transversely in 

 the centre of its concave side ? In general outline, 

 the cephalaspis resembles this tool ; the crescent- 

 shaped blade representing the head, the transverse 

 handle the body.' The endo-skeleton was mainly 

 cartilaginous, retaining the notochord through life. 

 The flexible body, assisted by the large tail and 

 the fins, would give the cephalaspis the power of 

 moving rapidly through the water. Being a pre- 

 daceous fish, it must have been a formidable enemy 

 to its associates in the Palaeozoic seas, for, besides 

 its power of rapid motion, the sharp margin of its 

 shield probably did the work of a vigorously hurled 

 javelin, as in the sword-fish. Pteraspis, Asterolepis 

 (20 to 30 feet in length), Scaphaspis, Auchenaspis, 

 and a number of other genera, are united in the 

 same family as Cephalaspis. See Ray Lankester,. 

 A Monograph of the Fishes of the Old Bed Sand- 



