The interior or exterior walls of the building which stood upon this gigantic base had no need to fear the infiltration of water or the attacks of enemies: their solidity might be lessened without inconvenience, by economising the stone. As a matter of fact, they are of brick, baked or crude, and stone is scarcely employed in them except for the lining and paving of a few rooms. In that case great slabs of limestone or gypsum are set upright as a plinth against the lower part of the wall, to preserve it from corrosion; they are adjusted end to end by the edge, and it was sufficient, in order to fix them, to pour between their posterior surface and the wall mortar which often only imperfectly adhered: the outer and only visible surface of these slabs was decorated with bas-reliefs which served for the adornment of the halls. As for the walls themselves, they were straight and perpendicular in contrast to those of the Egyptian buildings, which, seen from without, seem to lean inwards, and give to the whole building the appearance of a truncated pyramid. The Assyrian walls rise vertically, even when they enclose vaulted chambers, or when they form part of staged pyramids; each stage forms a perpendicular terrace, not a sloping one. It has been observed that the partitions which separate the halls sometimes look like one block set up on end; the joints and the courses of the brickwork cannot be detected, to such an extent have the constructing materials been soldered together in a perfect amalgam of beaten clay. This peculiarity, noticed by Victor Place at Khorsabad, can only be explained by admitting that the bricks were employed in the building while they were still saturated with water, and before the process of drying was finished. Their natural dampness, added to that of the clayey mortar which bound them to one another, has formed a sort of muddy paste which must have taken years to harden, but which was particularly effective against the disintegration of the wall, since it became in this way entirely homogeneous. It was the extraordinary thickness of these walls which prevented them from giving way under their own weight, and even allowed them to uphold those heavy beds of clay which form the vaults and terraces of the houses. They thus protected the halls most effectively from the ardent heat of the sun. At the present day the inhabitants of Bagdad and Mosoul take refuge, during summer, in their _sirdab_, a half-underground room with extremely thick brick walls, the single opening of which looks to the north. The people of Nineveh and Babylon, subject to the same climatic conditions, certainly acted in the same manner. As for the princes, they had, to defend them against the sun, walls from 13 ft. to 26 ft. thick, and vaultings as enormous as the walls. Nevertheless, the mode of building with clay which we have just noticed was very defective; this is the weak side of Ninevite and Babylonian buildings, and we understand why