The Romans for many of their buildings made much use of concrete. Its chief ingredient was _pozzolana_, a sand found in great abundance near Rome and other sites. When mixed with lime, it formed a very strong cement. This material was poured in a fluid state into timber casings, where it quickly set and hardened. Small pieces of stone, called rubble, were also forced down into the cement to give it additional stability. Buildings of this sort were usually faced with brick, which in turn might be covered with thin slabs of marble, thus producing an attractive appearance. TEMPLES The triumphs of Roman architecture were not confined chiefly to sacred edifices. Roman temples, indeed, are mostly copies from the Greek. In comparison with their originals, they lack grace and refinement. There is less accuracy in the masonry fitting and far less careful attention to details of construction. A frequent departure from Greek models is found in the restriction of the rows of pillars to the front of the building, while the sides and rear are lined with "engaged" columns to give the idea of a colonnade. [34] More characteristically Roman are vaulted temples, such as the Pantheon, [35] where the circular dome is faced with a Greek portico. BASILICAS Roman basilicas, of which only the ruins are now in existence, were once found in every city. These were large, lofty buildings for the use of judges and merchants. The chief feature of a basilica was the spacious central hall flanked by a single or double row of columns, forming aisles and supporting the flat roof. At one end of the hall was a semicircular recess--the apse--where the judges held court. This arrangement of the interior bears a close resemblance to the plan of the early Christian church with its nave, choir (or chancel) and columned aisles. The Christians, in fact, seem to have taken the familiar basilicas as the models for their places of worship. [Illustration: PLAN OF THE ULPIAN BASILICA The hall measured 360 feet in length and 180 feet in width.] [Illustration: INTERIOR VIEW OF THE ULPIAN BASILICA (RESTORATION) Built by the Emperor Trajan in connection with his Forum at Rome.] AQUEDUCTS Perhaps the most imposing, and certainly among the most useful, of Roman structures were aqueducts. [36] There were sixty-eight in Italy and the provinces. No less than fourteen supplied the capital city with water. The aqueducts usually ran under the surface of the ground, as do our water pipes. They were carried on arches only across depressions and valleys. The Claudian aqueduct ran for thirty-six miles underground and for nine and a half miles on arches. Though these monuments were intended simply as engineering works, their heavy masses of rough masonry produce an inspiring sense of power.