If we admit that architects made no use of columns to afford intermediate support to the heavy roofs, we may at first be inclined to believe that wooden ceilings were only used in very narrow apartments, for we can hardly give a length of more than from twenty-four to twenty-seven feet to beams that were called upon to support a thick covering of beaten earth as well as their own weight.[222] Perhaps, however, the skill of their carpenters was equal to increasing the span and rigidity of the beams used by a few simple contrivances. One of these is shown in our Fig. 60, a diagram composed by M. Chipiez to give an idea of the different methods of construction used by, or, at least, at the command of, the Assyrian builder. All the rooms were surmounted by flat roofs, and our horizontal sections show how these roofs were accommodated to the domes or the timber ceilings by which they were supported. On the left of the engraving semicircular vaults are shown, on the right a timbered roof. The arrangement of the latter is taken from an Etruscan tomb at Corneto, where, however, it is carried out in stone.[223] A frame like this could be put together on the spot and offered the means of covering a wider space with the same materials than could be roofed in by a horizontal arrangement. Further back rises one of those domes over square substructures whose existence seems to us so probable. Behind this again opens one of the courts by which so much of the area of the palace was occupied. The composition is completed by a wall with parapet and flanking towers. [Illustration: FIG. 59.--Fortress; from Layard's _Monuments_, 1st Series.] After considering the method employed for roofing the palace apartments, we come naturally to investigate their system of illumination. In view of the extravagant thickness of their walls it is difficult to believe that they made use of such openings as we should call windows. The small loop-holes that appear in some of the bas-reliefs near the summits of towers and fortified walls were mere embrasures, for the purpose of admitting a little air and light to the narrow chambers within which the defenders could find shelter from the missiles of an enemy and could store their own arms and engines of war (see Fig. 59). The walls of Khorsabad even now are everywhere at least ten feet high, and in some parts they are as much as fifteen, twenty, and five-and-twenty feet, an elevation far in excess of a man's stature, and they show no trace of a window. Hence we may at least affirm that windows were not pierced under the same conditions as in modern architecture.[224] [Illustration: FIG. 60.--Crude brick construction; compiled by Charles Chipiez.]