The aqueduct bridge
A Pont du Gard type bridge (c. AD 40–60): three tiers of arches carrying a covered channel that falls about 1 in 4,000.
The rules it illustrates
The fall of an aqueduct. Lay the channel of an aqueduct with a steady fall of not less than a quarter of an inch in every hundred feet, cover it with a vault against the sun, and carry it through hills in tunnels and across valleys on arches.
Arches over arches. Cross a deep valley on superimposed tiers of arches, the piers of each tier standing directly over those below, the spans matched from tier to tier, and the smallest arches at the top where they carry only the channel.
Where it appears in the book
In the styles Ancient Roman.
In the chapters The Classical World.