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Each of the twin arch rings has a span of 232 ft. and a rise of 70¼ ft., is 9½ ft. thick and 21½ ft. wide at the skewback and 5½ ft. thick and 18 ft. wide at the crown.

The side forms were then placed and braced apart by the struts and concreting continued to the skewback plane indicated in Fig.

After water-proofing the sand-walls and laying the low-tension ducts, a second pair of carriages, with panels on one side only, for 60 ft. of side-wall and skewback to the 15° line, were set and braced against the core-wall.

Where brick arches were used, the core-wall skewback was concreted behind special forms set up on the sidewalks, or the arch ribs and lagging were used for forms, and the brick arch was not started until after the concrete had set.

Plate LX, the center wall and skewback were built, and the posts were removed, as shown by Fig. 2,