Yb

Suppose we were to put a spur or prop to Xb at each corner, so as to hold it fast in the centre of Yb.

There is surely still an appearance of separation between Xb and Yb, as if the one might slip off the other.

I have said nothing yet of the proportion of the height of Yb to its width, nor of that of Yb and Xb to each other.

Under such circumstances, it is well to exhibit the lower tiers of the foundation as well as Yb and Xb.

The three courses below are octagonal, with their sides set across the angles of the innermost octagon, Yb.