Chapter III

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ARCHSTONES, or voussoirs, they form the compressed arc of materials called the ring; in some bridges they are laid in two or three sets, forming either a double or a triple ring, 305 _footnote_. The earliest archstones were arranged in horizontal courses, 6; as in the temple of Rameses II at Abydos, 155; in the Porta dell’ Arco at Arpino, 156-7; and the Lion Gate at Mycenae; but at Gizeh, in the great pyramid of Menkaura, there is a variation from this horizontal method, 156; Some Chinese bridges have arches built without keystones, 313-14; the rings being constructed with a few segmental stones from five to ten feet long, 314; The Romans extradosed their archstones, as in their bridge at Narni, 24; and this excellent practice was followed often in the Middle Ages, 282-3; The Romans, again, more often than not, bedded their archstones dry, without mortar or cement, as in most of the arches in the Pont du Gard, 175 _footnote_; but feebler masons have failed to copy with success this Roman method, notably in the restoration of the vast Roman aqueduct at Segóvia, 184; and recently Spanish workmen, after rebuilding an arch of the Puente Trajan at Alcántara, pointed the joints of the whole bridge in order to bring the masterpiece into keeping with their own weakness, 186-7. In a few English bridges the archstones are moulded like church windows and doorways; examples, Crowland, 304-5; and the Abbot’s Bridge at Bury St. Edmunds, 305 _footnote_.

ARDASHIR, of Persian history, 202.

ARDÈCHE, in France, the Pont d’Arc at, a natural arched bridge, 6, 89.

ARGUMENTS, concerning the origin of Dartmoor Clapper Bridges, 100-5; concerning the introduction of pointed arches into French bridges, 84-93; concerning the introduction of ribbed arches into English bridges, 93-100; to excuse the evolution from military bridges into defenceless bridges, 334; to prove that every sort of strife is a phase of war, vii, and section ii,