Il8 COMPARATIVE ARCHITECTURE. walls free for window openings. If the oblong compartment or hall were very wide, and the side walls had to be pierced by large openings, it was divided into square bays -generally three in number — and covered with groined vaults, that is to say, a longitudinal half-cylinder, of the diameter of the hall, intersected by three half-cylinders of similar diameter. (c.) Hemispherical domes or cupolas (ciipa = cup) (Nos. 54 and 55), were used for covering circular structures as in the Pantheon. Semi-domes were employed for exedrae and other recesses (No. 46 k). The great coherence of concrete formed of " Pozzolana " (see page 112) and lime was important ; by its use, vaults and domes of enormous size were constructed. Most of these were cast in one solid mass with no lateral thrust on the walls, thus having the form, without the principle, of the arch, which, if formed of radiating voussoirs of brick or stone, would possibly have pushed out the walls. As Prof. Middleton has pointed out, the Roman use of concrete for vaults was more striking and daring than for walls, and had an important effect on the general forms of Roman architecture. The use of buttresses had not been systematized, and it would have been impossible to vault the enormous spans if the vaulting had been composed of brick or of masonry as in mediaeval times. The Roman concrete vault was quite devoid of external thrust and covered its space with the rigidity of a metal lid, or inverted porcelain cup. The construction of the Pantheon dome appears to be excep- tional (page 134). In many cases (No. 46), as in the Baths of Caracalla and Basilica of Constantine, brick arches or ribs probably used as temporary centres are embedded in the concrete vaults at various points, especially at the " groins," but these are sometimes super- ficial, like the brick facing to walls, and only tail a few inches into the mass of concrete vault, which is frequently as much as 6 feet thick. The decoration of Roman buildings had little connection with the architecture proper, for a Roman edifice built of concrete could receive a decorative lining of any or every kind of marble, having no necessary connection with the general structure, such deco- ration being an independent sheathing giving a richness to the walls both internal and external. Roman architecture had the character, therefore, of a body clothed in many instances with rich materials forming a rational and appropriate finish to the structure, and differing essentially from Greek architecture. Besides the use of many colored marbles other means of decorating wall surfaces are briefly stated here. Cements and
