Rome was greater in her secular than in her sacred architecture. For here she could escape the bondage of tradition and unite engineering with art- utility and power with beauty and form- in a manner all her own. The principle of Greek architecture had been the straight line (however delicately modulated as in the Parthenon): the vertical column, the horizontal architrave, the triangular pediment. The principle of specifically Roman architecture was to be the curve. The Romans wanted grandeur, audacity, size; but they could not roof their vast buildings on rectilinear and trabeate principles except by a maze of impeding columns. They solved the problem with the arch, usually in its rounded form; with the vault, which is a prolonged arch; and with the dome, which is a rotated arch. Perhaps Roman generals and their aides had brought from Egypt and Asia a growing familiarity with arcuate shapes, and had reawakened early Roman and Etruscan traditions long overwhelmed by orthodox Greek styles. Now Rome employed the arch on so great a scale that the whole art of building took from this form a new and lasting name. By laying a web of brick ribs along the lines of strain before pouring concrete into the wooden frame of the roof, the Romans developed the articulated vault; by crossing two cylindrical or barrel vaults at right angles they produced a network of ribs and groins that could sustain a heavier superstructure and bear more lateral thrust. These were the principles of Rome’s arcuate revolution. It was in the great baths and amphitheaters that the new style reached its completion. The baths of Agrippa, Nero, and Titus were the first of a long series that culminated in the Baths of Diocletian. They were monumental buildings of concrete faced with stucco or brick, and rising to majestic heights. The interiors were richly decorated with marble and mosaic pavements, varicolored columns, coffered ceilings, paintings, and statuary. They were equipped with dressing rooms, hot and cold baths, an intermediate room of warm air, swimming pools, palaestras, libraries, reading rooms, research rooms, lounges, and probably art galleries. Most of the chambers were centrally heated by large clay pipes running under floors and within the walls. These thermae were the most spacious and sumptuous public buildings ever erected, and they have never been equaled in their class. They were part of that socialism of recreation with which the principate excused its growing monarchy. This same paternalism built the greatest theaters in history. Those of Rome were much fewer but larger than those of modern capitals. The smallest was that which Cornelius Balbus built in the Field of Mars (13 B.C.), seating 7700; Augustus rebuilt Pompey’s theater, seating 17,500; he completed another, named for Marcellus, seating 20,500. Unlike Greek theaters, these were walled, and the stands were supported by arched and vaulted masonry instead of resting on the slope of a hill. Only the stage was roofed; but often the audience was sheltered from the sun by a linen awning ( velarium ), which in Pompey’s theater covered a space 550 feet wide. Over the entrances were boxes for dignitaries and magnates. Some stages had curtains which, when the play began, were not raised aloft, but lowered into a groove. The stage was elevated some five feet. Its background usually took the form of an elaborate building which, extending from wing to wing, helped the actors to throw their voices out over the immense audiences. Seneca speaks of “stage mechanics who invent scaffolding that goes aloft of its own accord, or floors that rise silently into the air.” A change of scene was effected by revolving prisms, or by moving a set into the wings or into the loft, thereby exposing the next. Acoustics were aided by sinking hollow jars into the floor and walls of the stage. The auditorium was cooled by rivulets of water running along the passages; sometimes a mixture of water, wine, and crocus juice was conducted by pipes to the highest tiers and thence scattered over the audience as a perfumed spray. Statues adorned the interior, and large pictures were painted as scenery. Probably no theater or opera house in the world today could equal the size and splendor of Pompey’s.