For some reason or other Northern England welcomed in bridges the bluff economy of ribbed arches, while neglecting the more gracious thrift of Early English or pointed vaults. These are easier to build because they need lighter centres or arched scaffolds, and their thrust being less powerful than that of round-headed arches, they require less bulk in the piers. Some writers say that pointed arches interfere with sailing-boats, but this depends on the size of their spans. At Montauban there is room enough for ordinary boat traffic under the Pont des Consuls.
The Pont Valentré at Cahors has ogivale arches, and in one fine drawing Brangwyn studies the technique of their construction. For instance, the embattled piers are triangular, and each of them is pierced transversely by a bay or passage, which is put on a level with the springing of every arch. Below this bay are three holes; and another line of holes runs across the under surface of the arch beneath the springing.[113] Now, these holes and the bays have a great technical interest, they remind us how the Pont Valentré was built in the thirteenth century. With their help simple scaffolds were erected. The first step was to thrust fir saplings through the holes in a pier till they jutted out on each side; then they were covered with planks and used as footbridges by the workmen, and also as resting-places for barrow-loads of dressed stone, which were lifted up by movable cranes. The service of the masons was effected through the bay in a pier, and the centring of every arch was fixed in those other holes which Brangwyn has represented in his vivacious water-colour.
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.
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.
From this description it is evident that the builders had a stiff job. Timber for centring has ever been scarce in Persia[115]; so in Persian bridge-building the usual plan is to set up a light scaffold just strong enough to bear its own weight and a few rings of brickwork. After a single rib of bricks has been made, other bricks are dabbed against the first set, more being added at the abutment ends than in the centre of an arch; and so, as the work goes on, the arch grows to be self-supporting, like a cantilever bridge. When the middle part of the span has been covered over, the remaining courses at each side are completed with bricks set at right angles to the others. In looking upward at the under surface of a Persian vault a pontist sees that the courses of brick go in two directions, one parallel to the central axis of the bridge and another at right angles. Such is the Persian method of building a brick arch; its main object is to evade, without too much risk, the cost of heavy centring, timber being so difficult to get and so expensive to carry about.[115]
In the four bridges that Romilly Allen studied, between Resht and Teheran, the building was brickwork, and the bricks were rather like Roman tiles; they measured 10 in. by 10 in. by 2½ in. At Kâredj the mortar joints were about ¾ in. thick, so that twenty-four courses of bricks with their mortar joints built a wall about 6 ft. 2 in. high. At the thinnest part of the big arch there were only three bricks, giving a thickness of 2 ft. 6 in.; further on there were five bricks, and two more were added at the abutments, where the walls were 7 ft. 6 in. thick. Here is much economy, for thick joints of mortar are not praiseworthy ([p. 175]); and thrift is very noticeable in other details of the workmanship. Beneath the roadway were two chambers with pointed barrel vaults, which were built partly to relieve the haunches of the big arch, and partly to save materials. On one side of the arch the chamber was about 12 ft. high; its length, varying with the curve of the voussoirs, and extending across the abutment, ranged from 27 ft. to 49 ft. On the pier side of the big arch the chamber was not so long, but its height was 12 ft.; and the pier itself was chambered in its upper part and pierced below with a Tudor-like arch about 14 ft. wide and 11 ft. 6 in. high. The chamber above had a cone-shaped roof, and at each side of it were three square-headed windows that measured 3 ft. 6 in. wide by 6 ft. high. I am speaking in the past tense, for I know not whether this bridge is still in use; but now we will return to the present tense in a short quotation from Romilly Allen:—