The construction of these was essentially different from that adopted by most earlier nations. The Romans rather avoided than cultivated the use of large blocks of stone; they invented methods by which very small materials could be aggregated together into massive and solid walls. They used mortar of great cementing power, so much so that many specimens of Roman walling exist in this country as well as in Italy or France, where the mortar is as hard as the stones which it unites. They also employed a system of binding together the small materials so employed by introducing, at short distances apart, courses of flat stones or bricks, called "bond courses," and they further fortified such walls by bands of flat materials placed edgeways after the manner popularly known as herring-bone work. The result of these methods of construction was that the Roman architect could build anywhere, no matter how unpromising the materials which the locality afforded; that he could put the walls of his building together without its being requisite to employ exclusively the skilled labour of the mason, and that both time and expense were thus saved. This economy and speed were not pushed so far as to render the work anything but durable; they had, however, a bad effect in another direction, for these rough rubble walls were habitually encased in some more sightly material, in order to make them look as though they were something finer than they really were; and accordingly, the exterior was often faced with a thin skin of masonry, and not infrequently plastered. The interior was also almost invariably plastered, but to this little exception can be taken. This casing of the exteriors was, however, the beginning of a system of what may be called false architecture, and one which led to much that was degrading to the art. The walls were in many cases, it has been already observed, gathered into strong masses, such as it is customary to term piers, in order to support the vaulted roofs at the proper points. They were often carried to a much greater height than in Greek buildings, and they played altogether a far more important part in the design of Roman buildings than they had done in that of the Greeks. _The Roofs._ As has been already stated, the Romans, in their possession of a new system of construction, enjoyed a degree of freedom which was unknown before. This system was based upon the use of the arch, and arched roofs and domes, and it enabled the Romans to produce interiors unapproached before for size and splendour, and such as have hardly been surpassed since, except by the vaulted churches of the Middle Ages,--buildings which are themselves descended from Roman originals. The art of vaulting was, in short, the key to the whole system of Roman architecture, just as the Orders were to that of the Greeks.