Trisecting COMPASSES consist of two central rules, and an arch of circles of 120 degrees, immovable, with its radius, which is fastened with one of the central rules, like the two legs of a sector, in order that the central rule may be carried through all the points of the circumference or the arch. The radius and rule should be as thin as possible; the rule fastened to the radius should be hammered cold, to attain the greater elasticity; and the breadth of the central rule should be triple that of the radius. There must also be a groove in this rule, with a dovetail fastened on it for its motion, and a hole in the centre of each rule. The use of this instrument is to facilitate the trisection of angles geometrically.