Compasses, instruments for transferring and marking off distances, or for drawing circles, &c. The common compasses 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 by means of a nut, the degree of opening. The value of these depends upon the permanency with which they retain any degree of opening given to them, pivot compasses being liable to slip. A more delicate variety of the spring dividers is also made for use in mechanical and architectural drawing.
Beam Compasses 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 compasses can safely span, and, when delicately made, for more accurate dividing. See GRADUATION.
Proportional Compasses have a point at each end of each leg and the pivot between, thus forming a double pair of compasses opposite to each other, end to end. The pivot is a clamping screw moving in an elongated slit in the legs, by means of which any proportion between the ends may be attained. They are usually provided with scales of proportions for lines, solids, cubes, and circles. They are very useful in making reduced or enlarged copies of drawings, especially mechanical drawings.
Triangular Compasses have three legs, so that the points of a triangle may be all transferred at once. For Calliper Compasses, see CALLIPERS.