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By its means the concave compartments are set slightly back from the octagon's inner face, leaving, at the springing line, portions of the wall-head to appear as little flat ledges on each side of the angles.

In San Vitale a considerable walling is built between the line of the octagon and the springing line of the dome, while the bed for the dome is formed by introducing, in the space over the angles of the octagon, niches which are worked above to the circle on plan.

The lagging was first laid to a height of 3 ft. above the springing line on each side and the concrete dumped directly in place from runways laid on the lower chords of the arch ribs, which were placed 4 ft. apart.

The concrete was shoveled into the wall forms from the traveling platform, the lagging being placed a board at a time as the work progressed upward and the platform being elevated as required, its final position being at about springing line level.

The concrete for the portion of the pier above springing line and for the lower portions of the haunches was dumped direct from the cars.