Assuming a parallel spiral, the method to be adopted is the same as that. for the projection of a helix or single spiral line on a geometrical cylinder. The points A, B, C, D, in the accompanying illustration, when projected, give the lines in the elevation, while points E and F give the diameter of the cylinder. The left-hand figure shows the geometrical outlines, and the right-hand figure the shaded result.

Projection of Spiral Curves.