An instrument for drawing a circular arc without a central point. There are various ways of performing this, but the following is the most simple, and is often practised by bricklayers and masons. Two nails are driven into the face of the wall upon which the curve is to be struck, the nails being at each extremity of the curve; two laths or straight rods are then nailed together at such an angle as that the apex shall touch the crown of the arch when the two sides are in contact with the nails at the extremities of it; then if the apex of the laths be gradually moved round from one nail to the other, the laths being kept continually in contact with the nails, a tracer placed at the apex will describe the required arch. An instrument of this kind, invented by Mr. Rotch, and rewarded by the Society of Arts, will be found very serviceable for drawing upon paper arcs of very large circles, whose centres lie beyond the limits of the drawing board. It consists of two straight rulers, connected by a joint similar to that of a sector, in the centre of which joint is a socket to carry the tracer.
The two limbs are connected to two graduated arcs, sliding upon each other, by means of which the limbs may be set to any angle so as to describe an arc containing any required number of degrees.