Bow-compass

CIDE DICTIONARY

Bow-compass, n.

An arcograph. [1913 Webster]
A small pair of compasses, one leg of which carries a pencil, or a pen, for drawing circles. Its legs are often connected by a bow-shaped spring, instead of by a joint. [1913 Webster]
A pair of compasses, with a bow or arched plate riveted to one of the legs, and passing through the other. [1913 Webster]

For further exploring for "Bow-compass" in Webster Dictionary Online