A certain conic section supposed to be drawn in the
   tangent plane to any surface, and used to determine the accidents of
   curvature of the surface at the point of contact. The curve is similar
   to the intersection of the surface with a parallel to the tangent plane
   and indefinitely near it. It is an ellipse when the curvature is
   synclastic, and an hyperbola when the curvature is anticlastic.