The Romans made of the vault something more than a mere constructive device. It became in their hands an element of interior effect at least equally important with the arch and column. No style of architecture has ever evolved nobler forms of ceiling than the groined vault and the dome. Moreover, the use of vaulting made possible effects of unencumbered spaciousness and amplitude which could never be compassed by any combination of piers and columns. It also assured to the Roman monuments a duration and a freedom from danger of destruction by fire impossible with any wooden-roofed architecture, however noble its form or careful its execution. +CONSTRUCTION.+ The constructive methods of the Romans varied with the conditions and resources of different provinces, but were everywhere dominated by the same practical spirit. Their vaulted architecture demanded for the support of its enormous weights and for resistance to its disruptive thrusts, piers and buttresses of great mass. To construct these wholly of cut stone appeared preposterous and wasteful to the Roman. Italy abounds in clay, lime, and a volcanic product, _pozzolana_ (from Puteoli or Pozzuoli, where it has always been obtained in large quantities), which makes an admirable hydraulic cement. With these materials it was possible to employ unskilled labor for the great bulk of this massive masonry, and to erect with the greatest rapidity and in the most economical manner those stupendous piles which, even in their ruin, excite the admiration of every beholder. [Illustration: FIG. 48.--ROMAN WALL MASONRY. a, _Brickwork_; b, _Tufa ashlar_; r, _Opus reticulatum_; i, _Opus incertum_.]