ChrisRogers

If the magazine would like to post a diagram of the string technique, let me know and I will draw one up.

This technique doesn't produce a true ellipse. The edges are too flat. The only mechanical technique for creating a true ellipse is the string method: Put a temporary pin at the top of the rise on the minor axis, and swing an arc with a radius equal to 1/2 the major axis and a center point at the temporary pin. Where this arc crosses the major axis, drive another pin on each side. A string fixed at each end by the pins on the major axis with the length determined by looping around the temporary pin will now act as a guide for a pencil to trace out the ellipse. As an added bonus, you can bisect the angle created by the 2 halves of the string at any point to get a line indicating the direction of the masonry joints of the arch above.