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 pise 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.