An instrument used for describing arcs of circles in cases where compasses cannot be employed. The most simple cyclograph is that commonly used by artificers in describing arches for the tops of doors and windows, which consists of two rods connected together at such an angle that the apex may touch the highest point of the curve, whilst the sides of the rods are in contact with points fixed in the extremities of the curve, and by turning the instrument round, keeping the two legs in contact with the points at the extremities of the curve, a tracing point in the apex will describe the required arc. An improved instrument on this principle, invented by Mr. Rotch, is described in the Transactions of the Society of Arts.