On base line 0 – b draw a quadrant.
Divide the quarter circle into 18 equal parts and number them 0 – 90 as shown.
With centre 0 draw arcs from all the points on the curve to terminate on line 0 – b.
Using these points draw the scale immediately below the quadrant as shown. (A piece of thin plywood or similar flat material should be obtained and the scale carefully marked on the plywood.)