The pipes which the Romans commonly made were of lead, bronze, or wood. None of these could be made and cleared cheaply enough to serve for the volume of water required for household use, the baths, and the public fountains of Rome. Meanwhile slave labour was inexpensive, and the cost of building an aqueduct of any length was of little account to the Roman. When the water reached the city it was conducted into settling and distributing reservoirs and its flow regulated. Thence it was carried by pipes, mostly of lead, wherever it was required. When Agrippa was minister of water-supply he constructed in the city 700 public pools or basins and 500 fountains, drawing their supply from 130 collecting heads or reservoirs. And it is to the credit of Agrippa and of Rome that all these pools, fountains, and reservoirs were made pleasant to the eye with suitable adornment. There is mention of 400 marble columns and 300 statues, but these are to be regarded as only chief among the embellishments. The streets of Rome were commonly paved with blocks of lava quarried in the neighbourhood from the abundant deposits which had formed in a not very remote volcanic period. The materials employed for substantial building were various; in the older days red and black tufa--a stone so soft as to require protection by a layer of stucco; later the dark-brown peperino, the golden-creamy travertine, marble white and coloured, and concrete. The modern visitor to Rome who regards the ruins but superficially would naturally imagine that many of the edifices were mainly constructed of brick. In reality there was no building so composed. The flat triangular bricks, or rather tiles, which are so much in evidence, are but inserted in the face of concrete to cover the nakedness of that material. Concrete alone might serve for cores and substructures, but those parts of the building which showed were required to present a more pleasing surface. At the date of Nero this might be achieved by a fronting of marble slabs and blocks, but more commonly it was obtained by means of the triangular red or yellow tiles above mentioned. In buildings of slightly earlier date the exterior often presented a "diamond pattern" or network arrangement of square pieces of stone inserted in the concrete while it was still soft. The huge vaults and arches affected by the Romans made concrete a particularly convenient material, and nothing could better illustrate its strength than the tenacity with which it has endured the strain in the unsupported portions of the vaults of the Basilica of Constantine. Any of the more imposing buildings which were not mainly of concrete were composed of blocks of stone, held to each other by clamps soldered in with lead. Few, if any, such buildings were made entirely of marble. In the case of those composes of the other varieties of stone already named, the surface was commonly coated either with stucco or with marble facings