1. The arch.
2. The cross-vault.
3. The cupola.
Through the arch, which was used by the Etruskans, but which under the Romans became a most important part of architecture, the art of constructing was suddenly freed of all hindrances. Courts could be formed with more ease, and the ground-plan drawn with greater variety.
The cross-vault may be considered a specially Roman form. Two cylindrical arches were constructed intersecting each other at right angles in a quadrangular space. The intersection takes place at the two diagonal lines, thus uniting the opposite corners. The arches in their cross-form rise from four points of support, and divide the arch into four curved triangles, called calottes.
The cupola, a third form of the vault, was called forth in the Romans by their preference for circular buildings. The ancient mound was revived by them. It was the application of the arch to a circular ground-plan. A powerful larger or smaller member of decoration was produced, interrupting the flatness of walls, and allowing a rich variety of statuary to be used as ornament. This refers to the use of the half-cupola for semicircular niches and the apsidæ. These merely geometrical constructions would have been monotonous and without grace, like our railway stations; and would have been classed as works of engineering, but never of art; had not the Romans employed Greek forms in the shape of columns and pilasters, to adorn them. The Doric, Ionic, Attic, and Korinthian styles were all used, and a so-called composite or Roman order was added. The composite stood in the same relation to the proto-Korinthian, as Plautus to Aristophanes. The Romans, with arrogant clumsiness, placed a coarser and more contracted form of the Ionic capital on two rows of richly carved acanthus leaves. This was a bad composition, and left room for much improvement, which was effected more than fifteen hundred years later.
Nothing is known of art during the mythical period of Roman history.
During the first centuries of the Republic, the Etruskan style is said to have been prevalent, introduced by Tarquin. Square wooden huts with a rounded and tumulus-like roof of straw, were the first palaces of the Roman citizens. Only after the conquest of Greece 183 B.C., Grecian architecture and sculpture were introduced. From this date we may classify Roman art, under Greek, Assyrian, and Egyptian influences, into the following periods.
a. The first begins in the second century B.C., under the Republic, after the destruction of Korinthum by Mummius, and ends with the reign of the Augustan house, and the beginning of the rule of the Flavians, 69 A.D. This was the Græco-Roman Period of architecture and sculpture.
b. The second period commences with the Flavians, 69 A.D. After the tyrannical reigns of Tiberius and Nero, the cosmopolitan Imperial state acquired an apparent stability in new social forms, which found expression in gigantic artistic constructions. This period, which lasted down to Septimus Severus, 193 A.D., has three distinct phases.
α. A phase of powerful development under the Flavians.
β. A phase of brilliant success under Trajan (98–117 A.D.)