Augustus, the enclosing wall of the Forum of Nerva, and other fragments now wholly destroyed, were the pieces of architectural art which most especially influenced the studies of the men of the fifteenth and sixteenth centuries. [Plate X] gives what now remains of the Temple of Castor, and also what remains of the Temple of Mars; but as late as the sixteenth century there was much more to be seen and studied about these ruins. The building behind the Temple of Castor in the Forum, now entirely stripped of its architectural decorations, retained its interior order of marble columns until the sixteenth century, and this building also was of great importance to the earlier restorers of antique art: it is thought by modern archæologists to have been the Temple of Augustus, which is known to have existed in this neighborhood.
The buildings named above were generally columnar in character. The memorial arch and the Pantheon are the only two of them which were certainly vaulted structures. Now, the memorial arch required only one or three simple barrel vaults, and the example of the Etruscans must have made such work as that familiar to the people of Rome, but the Pantheon is a very different thing. This, as rebuilt under Hadrian, with the rotunda which we know, must have been one of the earliest Roman buildings in solid mortar-masonry. Its walls are very thick, faced on both sides with brick, but built actually of small stones laid in strong mortar, and it is roofed with extremely massive vaulting of the same materials. Other such buildings of which large parts exist are, in the city of Rome itself, the great Halls of the Thermæ of Caracalla (probably built about 205-10 A. D.); those of the Thermæ of Diocletian, built a century later, and that of the basilica of Maxentius and Constantine on the north side of the Forum Romanum, built between 312 and about 330 A. D. In these buildings a vaulting as massive as that of the Pantheon but of wholly different shape was used. The Pantheon, a circular building, is roofed by a circular cupola[26] which is kept in place by a ponderous superstructure carried up from the haunches of the vault, so that the thrust of the cupola could not, however great it might be, affect the stability of the structure. In the great halls of the Thermæ and the basilica above named, the conditions are very different, for the groined-vaulting[27] of these halls would, if built under ordinary conditions, exert a formidable pressure outward upon all its points of support. In these Roman examples, however, there were two influences at work to save the buildings from possible injury: the skillful disposition of walls and piers to take up or absorb the thrust from each point of support, and the fact that these vaults were built in such a fashion, with horizontal beds of stone laid in strong cement mortar, that there could not be much thrust when once the mortar was dry and the vault consolidated. The vault could not thrust outward without breaking: and it was too homogeneous to break. Buildings whose actual construction was carried out in this fashion exist throughout those Mediterranean lands which once were included in the great empire. This system of building gave the world those great permanent interiors which were the first in the world’s history to be of architectural importance. Egyptians, Babylonians, Assyrians, Greeks, both those of Greece and those of the Colonies—none of these great building nations had ever conceived of interiors prepared and designed for their own sake, and as the chief part of the building. The Assyrian kings in their palaces came nearer to understanding the possible effectiveness of the interior: but even they were satisfied with long and narrow halls shaped like what we call corridors. It was left for the Romans at once to develop their system of vaulting and at the same time to improve the construction of their roofs of wood and metal, so that halls fifty feet, sixty feet, even eighty feet wide, could be built with roofs of effective and beautiful form high above the floor. Under these conditions the most splendid possible interior effects were producible. Such vast columned interiors as that of the Ulpian basilica and that of the Septa Julia must have given an effect of stately grace absolutely unknown to the modern world; the true evolution of Greek art in one direction was assuredly to be found there. On the other hand the imperial dwellings on the Palatine Hill in Rome with their numerous vaulted halls, the temples of pure Roman design, like that of Venus and that of the City of Rome, built back to back, near the Colosseum, and the great halls of the basilicas and baths, as above suggested, were capable of being adorned in a permanent and strictly architectural way as none of the buildings of earlier races had been. The basilica of Maxentius had its middle division, its nave,[28] about eighty-three feet wide and roofed with a groined vault, although the span of that vault is less than this, about seventy-eight feet, because carried by immense columns which stand free of the wall on either side. This great hall was one hundred and twenty-five feet high to the top of the vault: and it was flanked on either side by an aisle[29] made up of three rooms, each about fifty-three feet square, opening into the central hall; and the barrel-vaults[30] even of these six minor divisions rose eighty feet from the pavement. (See [Plate XI].) This building dates from the declining days of the Empire and of classical civilization, when sculpture had already become a feeble and barbarous thing, without character, and when what we consider the Byzantine feeling in matters of decoration had already obtained the mastery throughout the greater part of the Roman world. The strong hold which the system of building had upon the engineers of the empire can be judged from this fact.
