Spring COMPASSES, or dividers, those with an arched head, which by its spring opens the legs; the opening being directed by a circular screw fastened to one of the legs and let through the other, worked with a nut. These compasses are made of hardened steel.
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.
Turn-up COMPASSES. The body of this instrument is like the common compasses; but towards the bottom of the legs, outside, are added two other points besides the usual ones, the one of which carries a drawing pen point, and the other a port-crayon, both adjusted so as to turn round, and be in the way of use, or out of it, as occasion requires. These compasses have been contrived to save the trouble of changing the points.