At different points on the Khorsabad mound, M. Place found other sewers, some with depressed, some with basket-handle vaults, while, at Nimroud, channels were discovered which were square in section and covered with large slabs of limestone.[291] The Assyrian architects seem, however, to have had a decided preference for the vault in such a situation. They expected it to give greater solidity, and in that they were not mistaken. The vaults of burnt brick, though set without cement, have remained unshaken and close in their joints, and the sewers they inclose are the only voids that have remained clear in the ruins of the buildings to which they belong.
Fig. 94.—Sewer at Khorsabad, with elliptical vault; compiled from Place.
We may, perhaps, be accused of dwelling too minutely upon these Assyrian vaults. We have done so because there is no question more interesting or more novel in the whole history of architecture than the true origin of the keyed vault and the different uses to which it has been put. Ottfried Müller looked upon the Etruscans as the inventors of the vault; he believed that the Greek builders learnt the secret from the early inhabitants of Italy,[292] and that the arches of the Roman Cloaca Maxima built by the Tuscan architects of the Tarquins, were the oldest that had come down to us from antiquity. The archæological discoveries of the last fifty years have singularly falsified his opinion and given an age to the vault never before suspected. Even in the days of the Ancient Empire the Egyptians seem to have understood its principle; in any case the architects of Amenophis, of Thothmes, of Rameses, made frequent and skilful use of it long before the Ninevite palaces in which we have found it were erected.[293] But the possession of stones of enormous size enabled the Egyptians to dispense to a great extent with the arch, and we need not be surprised, therefore, that they failed to give it anything like its full development. They kept it in the background, and while using it when necessary in their tombs, in the outbuildings of their temples, in their private dwellings and warehouses, they never made it a conspicuous element of their architectural system. They may well be admired for the majesty of their colonnades and the magnificence of their hypostyle halls, but not for the construction of their vaults, for the imitation of which, moreover, they gave little opportunity.
In Chaldæa and Assyria the conditions were different. Supposing the architecture of those two countries to be yet entire, should we find in it vaults rivalling in age the arch in a tomb at Abydos which Mariette attributes to the sixth dynasty?[294] Probably not. So far as we can judge, Chaldæan civilization does not date from so remote a past as that of Egypt, but it appears certain that the principles of the vault were discovered and put in practice by the Chaldees long before the comparatively modern times in which the segmental and pointed arches of Nineveh were erected. The latter alone are preserved because they have been hidden during all these centuries under the heaped-up ruins of the buildings to which they belonged, while those of Chaldæa have been carried away piece by piece, and their materials used again and again by the modern population of Mesopotamia.
In spite, however, of the absence of such direct evidence, we may affirm without fear that the Chaldæan architects soon discovered the principle of the arch, and used it at least in its simplest and least complex forms. We are led to these conclusions not only by their restriction to small units of construction—a restriction which is sure, sooner or later, to lead to the discovery in question—but also by induction from the monuments we have just been studying. The arches under the hanging gardens of Babylon, the vaults of the sewers and gateways, the domes that covered the great square chambers in the Ninevite palaces—all these were derived, we may be sure, from the ancient civilization. We cannot believe that such consummate skill in the management of a difficult matter was arrived at in a day. The purely empiric knowledge of statics it implies could only have been accumulated by a long series of more or less happy experiments.
Thus only can we explain the ease with which the Assyrian builder surmounted difficulties some of which would have puzzled a modern architect, such as the pisé vaults erected over spacious galleries without any kind of centering, and the domes over square chambers, for which some system of pendentives—that is, of arches or other intermediate forces—by which the base of the cupola could be allied to the top of the supporting wall, must have been contrived. The accurate calculation of forces between the thrust of the vaults and the strength of the retaining walls, the dexterity with which the curves employed are varied and carried insensibly one into the other, the skill with which the artificial materials are prepared for their appointed office, are also surprising. By careful moulding and manipulation the Assyrian builder made his brick voussoirs as well fitted for their work as the cut stone of our day. Each brick had its own shape and size, so that it was assigned in advance a particular place in the vault and its own part in assuring the final stability of the building. In all this we cannot avoid seeing the results of a patient and long-continued process of experiment and education carried on through many centuries in all the workshops of Mesopotamia.
The art of building vaults with small units of construction was, then, carried farther in Mesopotamia than in Egypt; it was there more frankly developed; it was there forced with greater success to supply the place of stone and timber. It was in fact more of an indigenous art in the valley of the Tigris and Euphrates than anywhere else, more inspired by the permanent and unchanging conditions of the country—in a word, more national.
In these days the historian sets himself with devotion to follow in all its involutions the long chain of thought and effort by which man has been led from his primitive barbarism to the well-being of modern civilization, and to his domination—every day more complete and more intelligent—over the minor forces of nature. It is the duty of criticism, as its methods gradually perfect themselves, to add daily to its perspicacity and powers of observation, and to lessen as much as possible the occasions, still so numerous, when the thread of evidence breaks in its hands and the true relations of facts to each other become obscured. Even yet we cannot say for certain to which nation of the ancient world the invention of the arch belongs. In those remote ages the principle may have been discovered more than once or twice in different and distant countries whose inhabitants were busied over the same task. We have no reason to believe that Chaldæa learnt the secret from Egypt, or Etruria from the East. It is none the less true, however, that the unknown architects of Babylon and Nineveh made full use of it at an earlier date and in more intelligent fashion than any of their rivals. To them must be given the credit of being the masters and art-ancestors of the men who built the Pantheon and the Church of Saint Sophia, Santa Maria del Fiore, and Saint Peter's in Rome, and more especially of those great modern engineers to whom the principle of the arch has been a chief element in their success.