extending down the centre of this colossal building. It has also been mentioned that a very ancient wooden frame, calculated for the keel of a vessel to slide upon, remained on the ground in 1875, just within the passage at the south-east end of the building, as if the vessels used in the sham fights could be placed out of sight in the lofty central passage. This is said by those accustomed to dockyards to have all the appearance of a dry dock, or a cradle for vessels to stand upon[174]. We read of the vessels being divided into two nations or sides, there were probably six on each side, and each nation occupied one of the canals. It is probable that the surface between the two canals on either side of this central passage, just under the level of the arena (which was twenty-one feet above the brick floor), was flooded with two or three feet in water, but the keels of the vessels were in the canals. On either side of the passages before mentioned[175] are remains of other walls of tufa, with vertical grooves in them, as if for lifts; the brick walls, between those of tufa, have been introduced at a later period, and in these instances the grooves are not opposite each other. This shews that great alterations have been going on at different periods in these subterranean works, some of which are earlier than the existing building, and others considerably later[176]. In one place, near the south-east entrance, the two old stone walls, with the vertical grooves, remain in their original places facing each other, so that lifts might work up and down in them. Architects had long wondered where the builders could possibly have obtained such an immense mass of materials in so short a time, it was therefore evidently natural that they should make use of anything that served their purpose. It appears that in some parts the galleries for the spectators of the old Naumachia were thus made use of as they stood, without actually rebuilding them. The great tufa blocks of the second wall of Rome were also used as old materials for the substructure of the great stone arcades; but the builders, who had to add the upper gallery, were afraid to trust the soft tufa to carry so great a weight[177], and therefore built piers of travertine about four feet wide[178]. These piers to support the upper gallery go right through the walls of all the lower galleries, from the top to the bottom of the building (as has been said on p. 12).