That which we are undertaking here is not a history of architecture, but in a sense a history of the modern way of judging of architecture. What then is the origin of those traditions and accepted doctrines upon which are based all our ways of criticising a building? This and the previous chapter are a partial answer to that question. The contribution of the Roman Imperial world to this tradition has been, by much, the greatest of all. It is upon the Roman practice that all subsequent European systems of decorative building have been founded, except the lightest and slightest—the wooden-framed houses of mediæval Europe and those of modern America, and their like. Apart from fortification, and from structures built by engineers without artistic intention, there is not a single form of building in masonry since the fifth century which has not been developed from the practice of the Imperial builders. Now it appears that those builders not only built in two different ways, but that they undertook the curious twofold task of constructing their buildings with massive walls and vaults of mortar-masonry (thereby abandoning wholly the example of the Greeks who never used mortar at all in the buildings we admire, and who had no arches nor windows nor interior designing of any sort in our modern sense), and of decorating these buildings within and without, by means of a borrowed Greek system of the Orders, which had nothing whatever to do with the actual structure. They allowed themselves to take certain liberties with the Greek Orders. They raised the column on a pedestal, they made the shaft of costly and beautiful material, of porphyry or granite or pavonazzetto marble or cipollino; and consequently, because the material was precious and also hard, they did not try to adorn the shaft with channels or flutes. They made the capital of bronze, cast hollow and gilded richly, and put such capitals around the top of the shaft as a mere ornamental jacket, concealing the actual supporting member. They built the horizontal architrave of wedge-shaped stones, making of each span between two columns a flat arch instead of a simple lintel of one block, and they protected this built-up lintel by a second arch above, a discharging arch to throw the weight upon the columns and relieve the centre of the lintel. Finally, they increased the amount of carved ornament upon all parts which seemed capable of receiving it. This they did, not only by making the sculpture of any one moulding very elaborate and rich, but also by increasing the number of sculptured mouldings. Thus in [Plate XII], there is given, that it may be compared with the carved work of Athens (see [Plate VII]) a part of the entablature of the Temple of Vespasian in the Roman Forum. And the differences between Greek and Roman practice in this respect are not limited to the amount of sculpture in a given moulding or a given monument: they affect also the very nature of the ornament itself. [Plate XII] gives one side of the imperial arch at Benevento; a monument intended primarily as a pedestal for a great group of bronze figures; the reliefs on the arch showing Trajan in war and in peace, sacrificing, conquering Dacians and Armenians. It is evident that no such use of human subject in sculpture had ever suggested itself to the Greek builders of the temples. It is historical: and it is also strictly decorative, and subordinate to the architectural design. For any similar conception arising among Greek peoples we moderns must go to buildings which were utterly unknown to the European artists who built up the neo-classic system, the men of the fifteenth and subsequent centuries. Such a building as the famous tomb of Mausolus at Halicarnassos, now Budroun, on the coast of Asia Minor, may indeed have influenced greatly the Roman architects of the time of Hadrian. That Emperor, who was a great traveller, may have seen the Mausoleum; his favorite architect may have been a student of it from childhood; but any ideas which the men who brought classic art back to modern Europe drew from that famous structure came to them through the Roman designers.
However much they might abandon the Greek use, that is to say, the rational and inevitable use of the Orders, the Roman architects still employed those Orders constantly, and in a way more splendid than anything the Greeks had attempted. The Eastern notion of adorning a town by a broad central avenue lined with colonnades two or three deep, an idea developing itself rapidly in the cities of Syria, obtained throughout the empire during its peaceful days. It appears that in the fourth century it was feasible to go afoot from almost any point in the central regions of Rome, north, south, east or west, for a mile or two, while keeping always under cover; except indeed as one crossed a street or avenue, though even at such crossings there was often the Tetrapylon, the four-fronted gateway, to carry the shelter on from portico to portico. This system of colonnaded porticoes, roofed always and enclosed very often with a solid wall on one side at least, was developed in many forms. A temple would stand in a great court surrounded by just such colonnades. A forum of a Roman town like an agora of a Greek town would be faced by colonnades on every side. For the purpose of display, great squares were opened up essentially for the purpose of surrounding them by just such porticoes. [Plate XIII] gives views of the ruins at Jerash in Syria, east of the Jordan, the remains of the city of Gerasa, whose glory seems to have been of the time of the Antonine emperors. The lower figure gives the great triple archway south of the ancient walls of Gerasa: the upper figure a view of the great oval or semi-oval space, whose shape is not determined, and which we may hardly call either a forum or an agora. [Plate XIV] gives a detail of the Forum Transitorium of Nerva, Emperor from A. D. 96 to 98. The whole enclosure was a massive wall about ninety feet high and built of huge blocks of limestone, the decorative treatment and the sculptures being on the inside and facing upon the Temple of Minerva. The figure