Page:The New International Encyclopædia 1st ed. v. 04.djvu/499

CENTRE OF OSCILLATION. 427 sion by a fine thread and entirely disconneeted from the rc#t of the body, it would form a simple pendulum: but in general its period would not l)e that of the body itself. 'I'hose nearest the a.is of suspension would also vibrate in a shorter time than those farther away. As a rule it is pi)ssil)le. however, to tind 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 evi- dentlv given by the condition / _ I or I / =: M/l If a line can be drawn in the body parallel to the axis of suspension, at the distance I from it, and so that the plane of the two lines includes the centre of gravity, it is called the 'axis of oscillatiim.' 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 fonner axis of suspension will be the new axis of oscillaticm. and the period of vibration is the same in both cases. The inter- section of the axis of oscillation by a plane pass- ing through the centre of gravity and perpen- dicular to the axis is called the 'centre of oscil- lalinu.' CENTRE OF PERCUSSION ( l.at. pcrcussio, a beating, striking). If a rigid body is suspend- ed so that it can turn freely about a fixed axis — e.g. a door — a 'line of percussion' is such a line^tbat a blow struck the body in that direc- tion produces no reaction of any kind on the axis. In general, when a blow is strui-k the body in any chan<e 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 ;;or'»l^ so that the centre of gravity lies ver- tically below it. the centre of percussion coin- cides with the centre of oscillation. CENTRE OF PRESSURE. The centre of pressure of any surface immersed in a lluid is the point in which the resultant of the pressures of the lluid on the several points meets the sur- face. The pressure of a fluid at a point is the ■force per unit area' at that point, or the limit- ing value of the ratio of the force acting over .1 surface to the area of the surface, as the sur- face is taken smaller and smaller. This pres- sure is due to two things — the weight of the lluid and the reaction of the walls of the vessel which contains the (luid. (See IIvi)ROST.Tlcs.) 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 pres- sures on all the points of it are the same: obvi- ously they fonn a system of equal parallel forces, whose resultant will pass through the centre of Vol.. IV -2a CENTRING. gravity of the bottom or plane. But when the plane is inclined at an angle to the :>urface of the liquid, the pressure is not the same at all points, lieing 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 sur- face, but through a point below it. This point is the centre of pressure. Tn the case of a vessel with a rectangle for one side, the centre of pres- sure is at the distance of one-third of the height from the bottom. In the general ciise it must be found by calculation involving the calculus, CENTRE PINTLE CARRIAGE. A gun- carriaui' in which the c!ias^i>. or way on which the up])cr part of a coast gun moves, is attached to the pintle of the platform at its middle, and revolves aroiuul it through the entire circum- ference of the circle. The traverse circles are consequently continuous. By this arrangement a much greater horizontal field of fire is secured. See Ordn.xci:: and Coast Artillery. CENTRIF'UGAL (Neo-Lat. centrifugus, from l.at. cenlrimt. centre + fugete, to flee). A iracliine 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 veiy high rate of speed. Mien 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 Infi.ore.scexce. CENTRIFUGAL FORCE. See Central Foiu'es: and Mechanics (section on Di/namics). CENTRING (from centre, Fr. centre, from Lat. centrum, Gk. Ktv-pov, kentron, centre, from Kevreiv, kentein, to prick). The framework upon which an arch or vault of stone, brick, or iron is supported during its construction. The simplest form of centring is that used by ma- sons and bricklayers for the arches of windows and doors, which consist of boards cut to the required shape, upon the curved edge of which the bricks or stones of the arch are supported until they are keyed in. In building bridges or other structures where arches of great span are to be constructed, the centring is usually made of framed timbers, or timbers and iron com- bined. The arrangement of the timbers should be such that the strain upon each shall be main- ly a thrust in the direction of its length, for if the strain were transverse, a comparatively slight force would snap or bend it, and if a longitu- dinal pull, the whole structure would be no stronger than the joints holding the pieces of timber together. In arches of great span, such as that of Waterloo Bridge, London, the Cabin .lohn Bridge, Washington, or the Luxemboirg Bridge (see Bridge), a longitudinal pulling strain is almost inevitable in some parts, as a beam of great length would bend to some extent under a thrusting strain. In such cases great skill and care are demanded in the designing and construction of the Joints. -As an arch is built from the piers toward the keystone, the weight upon the liaunches during construction tends to