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.
Trifecting COMPASSES consist of two central rules, and an arch of a circle of 120 degrees, immoveable, with its radius; which is fastened with one of the central rules like the two legs of a sector, that the central rule may be carried through all the points of the circumference of the arch. The radius and rule should be as thin as possible; and 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 dove-tail fastened on it for its motion, and a hole in the centre of each rule. The use of this instrument is to facilitate the trifecting of angles geometrically; and it is said to have been invented by M. Tarragen for that purpose.
Turn-up COMPASSES. The body of this instrument is like the common compasses: but towards the bottom of the legs, without-side, are added two other points besides the usual ones; the one whereof carry 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.