The sacred buildings of the Romans have been considered thus at length because offering the best opportunity for a characterization of the orders; yet the significance of their national architecture is not to be found in the temples, but rather in their structures for public utility and comfort. In these the technical naturally far exceeded the artistic element, and it is consequently in points of construction that the great advances of the Romans appear. In these methods they were almost wholly independent, and were by far the most important people of antiquity. Masonry of brick and hewn stones early attained great extent and perfection, furthered by the excellent materials at hand—the hard Tiburtine and Travertine limestones, the tufa so easily carved, the unequalled clay for bricks, and the famous volcanic sand and pozzuolana which, when combined with lime, harden to the firmest stone. Vaulting was generally introduced as early as the time of the kings, the walls and ceiling forming an uninterrupted mass of homogeneous materials; the vertical and horizontal members, support and covering, being blended together without marked transition. Before this system of construction was invented the spacious and monumental development of protected rooms had been possible only under great limitations; without it these chief ends of Roman architecture could not have been attained.
The building of barrel vaults with hewn stones, as observed in the Cloaca Maxima, was attended with certain difficulties; the great weight of the masonry permitted a moderately large span only when immense and cumbrous buttresses were provided. This objection was, in a great degree, obviated by the employment of bricks, but the size of the spaces covered was limited by the necessity of heavy supporting-walls at the sides. The full scope of vaulted construction was not recognized until the introduction, by the Romans, of the intersecting or cross vaults, or the so-called groined arch. This replaced the two side walls previously necessary to support the barrel vault, by piers upon the four corners, at the same time opening the covered space on all four sides. The way was thus prepared for an indefinite series of such quadrangular compartments, or bays, covering a continuous space. A third development of this principle, the hemispherical vault or cupola, was of more restricted application, having been employed only for circular buildings, or, when bisected, for apses, or semicircular additions to the plans of rectangular temples and halls. The date of the first appearance of the cross-vault can hardly have been earlier than the second century B.C.
