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 TOWNS AND HYDRAULIC WORKS. 371 which carried off the superfluous waters of the marshes, and the majestic arches which afforded a passage through the walls of fortified towns. Perhaps it was from the gateways of Latin and Etruscan cities that the idea of the posterns at Eryx was taken (Figs. 232 and 233). But here the arch is only apparent ; its curves are not turned by voussoirs, they are cut in the mass of the horizontal courses. All those who have studied the ruins in Tunisia agree in ascribing to Rome the keyed arches which are found at many points of the old African province, and yet from Beule's description which, by the way, is much too summary of the chambers in the foundations of the Byrsa wall, it would rppear that they were roofed with surbased spherical vaults. 1 We may then admit, until proof to the contrary, that the Carthaginians either did not use the keyed-masonry arch at all or used it very little ; but we are told by one of the most careful students of their architecture, that they obtained a similar result with the use of arches turned in a kind of concrete, " small stones set in a bath of mortar mixed with sand so fine that its grains are hardly to be distinguished, and with lime made from the same material as the small stones. To this mixture lime has given a consistence and homogeneity equal, and not seldom even superior, to that of the stone employed." * Many things lend probability to this hypothesis. At Carthage the building stone available was of very mediocre quality. It was a calcareous tufa, which rapidly lost consistency underexposure to the weather. Its durability w y as enhanced by covering those faces of any building which were turned towards the sea with a coat of tar. 3 Such a proceeding must have been rather costly, and the desire to avoid the expense must have caused concrete of one kind or another to come into very wide use. The Carthaginians made use of pise. In the first century of our era the remains of edifices in beaten earth, viz., ramparts and guard-houses, were to be seen both in Spain and Africa. 4 These the Romans did not recognize 1 BEULE, Fouilles a Carthage, p. 59. ' 2 DAUX, Recherches^ p. 117. 3 PLINY, Hist. Nat. xxvi. 48. 4 The passage in PLINY on which we found this statement is interesting enough to deserve quotation : " Quid ? Non in Africa Hispaniaque ex terra parietes, quos appellant formaceos, quoniam in forma circumdatis utrinque duabus tabulis inferciuntur verius quam instruuntur sevis durant, incorrupt! imbribus, vends, ignibus, omnique caemento firmiores ? Spectat etiamnunc speculas Hannibalis Hispania ; terrenasque turres jugis montium impositas." Hist. Nat. xxxv. 47.