Such a method of construction is only adapted to buildings of small dimensions; it could not be used for chambers with wide roofs, or where any great weight was to be upheld. The arches upon which, according to both Strabo and Diodorus,[282] the hanging gardens of Babylon were supported, must have been real centred arches. As to whether they were of pise, like those of Khorsabad, the Greek writers tell us nothing. From what we know of the habits of the Chaldaean builder we may conclude that they were true arches with voussoirs either of bricks burnt in the kiln, or so well dried that they were almost as hard and durable as those that had passed through the fire. This conjecture is confirmed by the fact that the structures in question lasted till the Macedonian conquest. Strabo and Diodorus speak of the great temple of Bel as so ruinous that its original height could not be guessed, even approximatively. It was otherwise with the hanging gardens. Of these they give the measurements, on plan, of the platforms and piers, together with their heights, and the heights of the arches. We should find it difficult to explain the preciseness of these measurements and their agreement one with another, unless we supposed that both writers had some exact authority, such as one of the companions or historians of Alexander, to refer to. The kings of Persia lived at Babylon for a part of the year. These princes may well have been indifferent to the preservation of the national fanes, they may even have hastened their destruction, as Xerxes is said to have done, in order to punish and humiliate the rebellious Babylonians. But in their own interest they would see that proper care was taken of those hanging gardens by which their stay in the city would be rendered more pleasant than it would otherwise have been, from whose lofty platforms their watchful eyes could roam over the city and the adjoining plain, and follow the course of the great river until it disappeared on the south amid groves of waving palm. After the rise of Seleucia and Ctesiphon, however, the gardens would rapidly hasten to decay, but they must have been solidly built in the first instance to last as long as they did. The pise vaults of the Ninevite palaces could never have stood so well. In spite of the layers of lead and bitumen which, as Diodorus tells us, were spread upon their terraces, the summer rains must in time have found their way into their walls and set up a process of disintegration which could have but one end. Real brick with good mortar could alone resist such influences, and those, no doubt, were the materials used in the Babylonian gardens. If their substructures should ever be found and laid open, we have little doubt that arches as carefully built as those of the Assyrian ruins will be brought to light.