Chap. IV.
. ROMAN.
153
should be taken to discharge the weight of walls by arches consisting of wedges concen-trically arranged; and further “ that on buildings which are constructed on piers andarches, with wedges whose joints are concentric, the abutments should be wider than thepiers, that they may have more power to resist the action of the wedges, which, whenloaded, press towards the centre, and have a tendency to thrust them out.” No particularrules are laid down for their proportion, which was probably left to the judgment and ex-perience of the engineer. There is seldom much depth given to the voussoirs, which are ofequal thickness throughout; and when the semicircle was complete, the spandrills werefilled in with a concrete or rubble, which has hardened into a solid mass.
The Pons Milvius , or Ernilius, two miles from Rome, on the Flaminian Way , consistingof four large arches and two smaller, has been altered at various times. The masonry
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Fig. 107.
PONS MILVIUS.
of the arches and tower at the extremity, as shown in the figure, with the openings in thepiers, are said to remain as they were wben Constantine pursued Maxcntius, as he attemptedto escape into the city, after his terrible defeat: being, however, pressed by the crowds whowere flying to the narrow pass, he was forced into the water, and his body, weighed downby his massive armour, was afterwards found in the bed of the river. The arches vary intheir opening from 51 to 79 feet 9 inches, but the whole of the water-way is in the clear413 feet 3 inches; the breadth of the bridge is 28 feet 9 inches.
Pope Nicolas V. made some alterations in this bridge, and Piranesi, who has givenan account of the change it then underwent, describes the two arches nearest the cityas the most ancient: when the writer was at Rome , it was repaired under the directionof Valadier , who recased the ruined tower at the extremity, and gave it the character of a
V
Fig. 168.