The compasses represented by Fig. 282 possesses jointed legs, to admit two pointed pieces which are of the same length and shape, and also one leg having a thick conical foot; when this foot is fixed for use, the instrument appears as in the Figure. Such a foot is used when it is necessary to scribe arcs or circumferences by placing one foot of the compasses into a mouth of a hole or recess, in which case a thin compass-leg cannot be used without fixing something in the mouth of the hole; a conical foot is therefore useful to fit a number of holes of different sizes, if the legs are fixed at a proper distance in their sockets in order to scribe the arcs in an easy manner. Compasses, fitted with such a conical foot, will mark circumferences if the cone is in a hole which is either round, square, hexagonal, or of any other section. When it is necessary to use the instrument without the cone, it is taken out, and the thin pointed leg put into the socket; and, while thus fitted, the compasses are in order for scribing curves or circumferences upon gun-metal, iron, or steel, the points being properly hardened to cut into the metal whenever it may be necessary to do so.