+ARCADES.+ The orders, though probably at first used only as free supports in porticos and colonnades, were early applied as decorations to arcaded structures. This practice became general with the multiplication of many-storied arcades like those of the amphitheatres, the engaged columns being set between the arches as buttresses, supporting entablatures which marked the divisions into stories (Fig. 45). This combination has been assailed as a false and illogical device, but the criticism proceeds from a too narrow conception of architectural propriety. It is defensible upon both artistic and logical grounds; for it not only furnishes a most desirable play of light and shade and a pleasing contrast of rectangular and curved lines, but by emphasizing the constructive divisions and elements of the building and the vertical support of the piers, it also contributes to the expressiveness and vigor of the design. +VAULTING.+ The Romans substituted vaulting in brick, concrete, or masonry for wooden ceilings wherever possible, both in public and private edifices. The Etruscans were the first vault-builders, and the Cloaca Maxima, the great sewer of Republican Rome (about 500 B.C.) still remains as a monument of their engineering skill. Probably not only Etruscan engineers (whose traditions were perhaps derived from Asiatic sources in the remote past), but Asiatic builders also from conquered eastern provinces, were engaged together in the development of the wonderful system of vaulted construction to which Roman architecture so largely owed its grandeur. Three types of vault were commonly used: the barrel-vault, the groined or four-part vault, and the dome. [Illustration: FIG. 46.--BARREL VAULT.] The barrel vault (Fig. 46) was generally semi-cylindrical in section, and was used to cover corridors and oblong halls, like the temple-cellas, or was bent around a curve, as in amphitheatre passages. [Illustration: FIG. 47.--GROINED VAULT. _g, g_, _Groins._] The groined vault is formed by the intersection of two barrel-vaults (Fig. 47). When several compartments of groined vaulting are placed together over an oblong plan, a double advantage is secured. Lateral windows can be carried up to the full height of the vaulting instead of being stopped below its springing; and the weight and thrust of the vaulting are concentrated upon a number of isolated points instead of being exerted along the whole extent of the side walls, as with the barrel-vault. The Romans saw that it was sufficient to dispose the masonry at these points in masses at right angles to the length of the hall, to best resist the lateral thrust of the vault. This appears clearly in the plan of the Basilica of Constantine (Fig. 58). The dome was in almost all Roman examples supported on a circular wall built up from the ground, as in the Pantheon (Fig. 54). The pendentive dome, sustained by four or eight arches over a square or octagonal plan, is not found in true Roman buildings.