A variable, circular‐arc rule; An aid in constructing stereographic projections

A drafting instrument which provides a ruling edge for drawing arcs of circles the radii of which are unusually long is described. A complete range of arcs of different curvature, within the limits prescribed by the construction of the instrument, are obtainable. This instrument was developed originally to allow accurate construction of circular arcs of very low curvature, which are difficult or impossible to draw with an ordinary compass, in constructing stereographic projections. It is apparent, of course, that numerous other applications are possible. To satisfy the authors' needs, a relatively small model was constructed, but the same general construction could be used in larger models.