This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
We know that the segments of the true equilibrium polygon must be parallel with the rays of the force diagram which has its pole at O3, and also that it must pass through the points h and k on the line vm. The point h lies between loads 4 and 5; therefore we draw through the point h a line parallel to the ray of the force diagram from O3 to the point on the load line between load 4 and load 5. Similarly, since the point k lies between loads 16 and 17, we may draw through k a segment of the equilibrium polygon which is parallel to the ray from O3 to the point on the load line which is between load 16 and load 17. In order to avoid inaccuracy, the segments of the equilibrium polygon should be drawn from these two segments each way toward the crown of the arch and each way toward the abutments. As a check on the work, these separate sections of the equilibrium polygons should accurately meet at the top of the arch, and they should also reach the last verticals through 0 and B at points which are on the same horizontal line. It is merely a coincidence that these points 0" and B" are almost exactly at the same level as the lowest point of the skew-backs.