CENTRE

(Lat. centrum.) This term has numerous applications in Geometry and Mechanics: thus, centre of a circle, or of an ellipse, is the middle point of any diameter; centre of a curve is the point where two diameters intersect' each other; and in mechanics we speak of centres of attraction, conversion, equilibrium, gravity, percu ion, oscillation, . Ce'ntre of Attraction, also called Centre of Gravitation, is the point to which bodies tend in consequence of the action of gravity. Ce'ntre of Conversion is the point in a body about which it turns when a force is applied to any part of it, or unequal forces to its different parts. For example, suppose a rod laid on a table to be struck near one extremity in a direction perpendicular to its length; the rod will turn round, but there will be one point in it which remains at rest, or about which, as a centre, the other points turn. This point is the centre of conversion. Ce'ntre of Equilibrium of a system of bodies is a point such that if the system were suspended from it, the whole would remain in equilibrium. Thus, the fulcrum or point of support of a lever is its centre of equilibrium. Ce'ntre of Gravity. A term employed in Mechanics to denote a certain point in the interior of a body, or system of bodies connected with each other ia an invariable manner, so situated that any plane whatever which pa es through it divides the body into two segments of which the weights are exactly equaL Hence, if the centre of gravity of any body or system of bodies be sustained, the whole will remain at rest; for the weights on both sides of a vertical plane pa ing through the point of support being equal, the body can have no tendency to angular motion. Let there be a system of bodies A, B, C, D, E, placed horizontally in the same straight line, S A B O C D E and connected with each other in an invariable manner; and suppose O to be the centre of gravity of the system: [s. 219]