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:--