Not more than two arches were built at the same time. At any moment, in those rude, warfaring periods, work might be interrupted by strife, and its progress was so very slow that it took from ten to thirty years to bring a bridge to completion, usually after a continuous fight against money troubles. Many a hint on economy was borrowed from the Romans, whose enterprise was far in advance of their current cash. Piers that look marvels of solid masonry may be nothing more than shells filled with beaten earth and gravel; and those passages through the piers at Cahors have one thing in common with the relief arches that pierce the spandrils of the Pont des Consuls at Montauban: they enabled the builders to be thrifty. [Illustration: PONT VALENTRÉ AT CAHORS-SUR-LOT A FORTIFIED BRIDGE, THIRTEENTH CENTURY] In a Persian bridge (on the way between Resht, on the Caspian Sea, and Teheran, the capital) thrift hollowed the spandrils into chambers, some of which were used by travellers. This bridge carried a rough highway over the Kâredj River, which runs down from the Elburz Mountains between Kasvîn and Teheran, and disappears in a gravelly plain. In 1874 the Kâredj Bridge was studied in measured drawings by J. Romilly Allen, and eighteen years later (November 19, 1892) the drawings were published in “The Builder,” with a most valuable description. Let us linger for a few minutes over Romilly Allen’s research, as the technique of old Persian bridge-builders has points in common both with Gothic methods and with modern practice also. Some mediæval spandrils are hollow, for example; and a very noted French architect of the eighteenth century, Perronet, not only left empty spaces behind the haunches[114] of an arch, but made tunnels in piers, after the manner used by Pope Sextus IV in the Ponte Sisto. And the bridge of Glasgow over the Clyde has tunnelled piers, so this technical detail has a long and entertaining history. In the Kâredj Bridge, then, the builders had to solve three or four difficulties that strained the usual penury of Persian finance. The river itself must have been a constant trouble while the bridge was being constructed. A rapid mountain torrent with precipitous rocky banks, it pours through a gorge of rock, and at one spot only it forms a good foundation for a wide pier; but this spot has a situation that divides the bridge into inharmonious parts, making symmetry impossible. Allen’s drawing shows both arches, one with a span of 23 ft., the other with a span of 72 ft. 9 in.; and between them is a vast pier not less than 31 ft. 9 in. wide. Forty-six feet separate the highest point of the parapet from water-level; and from water-level to the peak of the big pointed vault is thirty-seven feet. In width the bridge measures thirty feet across the outside of its parapets, and twenty-six feet across the roadway, so there is room for a great deal more wheeled traffic than Persia has yet developed along her dusty trade routes.