Circles of these radii are usually marked around the jack for convenience' sake.
The resistance to motion round a curve has not been so systematically studied that any definite rule can be formulated applicable to all classes of rolling stock and all radii of curves.
It is the envelope of circles described on the central radii of an ellipse as diameters.
Thus, while halos have certain definite radii, viz.
The figure included by two radii and an arc is a " sector," e.g.