COMPASSES, instruments for transferring and marking off distances, or for drawing circles, ellipseS, etc. The common G. or dividers are simply two rods or " legs" joined together at one end by a pivot-joint, and pointed at the other; when used for drawing circles, the lower part of one of the legs is replaced by a pen or pencil Spring dividers are much in use by workmen; in these, the legs are united by a strong steel spring, the action of which is to stretch them open; but half-way down, a screw passing between the legs, regulates the degree of opening. The value of these depends upon the per manency with which they retain any degree of opening given to them, pivot C. being liable to slip.
Beam C. consist of points sliding on a long bar, to which they may be clamped at any distance from each other. They are used for greater openings than pivot C. can
safelyspan, and, when delicately made, for more accurate dividing. See GRADUATION.
Proportional C. have a point at each end of each leg, and the pivot between, thus forming a double pair of C. opposite to each other, end to end. If the pivot is midway between the points, the opening of each pair of points will be equal; if the distance from one pair of points be double that from the other, the openings will be as two to one; and so on for any ratio. When a single fixed proportion only is required, the pivot is fixed accordingly; but to adapt. them for variable proportions,.the pivot is a clamping screw, which moves an elongated interval iu the legs, and may be fixed at any point.
Triangular C. have three legs, so that the points of a triangle may be all transferred at once.
Calliper Compasses. See CALLIPERS.