The Derand–Blondel rule for abutments
Divide the intrados in three; the chord of the first third, laid down beyond the springing, fixes the thickness of the pier.
The rules it illustrates
The Derand–Blondel rule for abutments. To find how thick the pier of an arch must be, divide the intrados into three equal parts; draw a line from the springing through the first division, and produce it beyond the springing by its own length. A vertical through the end of that line marks the outer face of the pier.
Where it appears in the book
In the chapters The Arch.