Down the centre of the building, for the whole length, is a wide passage, which was called _the gulf_[10], and which was necessary for sending up the scenery, of which we read frequently, and which must have been put together below, and then sent up to the stage, as wanted, by means of this passage; there is no room for it anywhere else, and there was no place behind the scenes for the actors and workmen, as in a modern theatre. On each side of this great central passage are remains of two canals for water, each about ten feet square, and about the same height from the ground[11]; these were evidently filled with water supplied from the aqueducts, and unmistakable traces of three reservoirs for water from the aqueducts have been found in the first gallery. The four canals are not all quite of the same period, nor on the same plan; one on each side was supported on flat arches of brick of the third century, the other on large beams of wood; the places to receive the ends of the beams are left in the walls on each side of the passage for the workmen under it. These walls are of such a thickness in proportion to their height, that they were evidently made to support the great weight of water; these very thick walls served instead of the great projection of the buttresses of the usual reservoirs of the aqueducts. There is no bond or junction of any kind between this _brick_ theatre, or amphitheatre, and the magnificent _stone_ corridors and grand front of the Flavian Emperors; these have evidently been added to a brick building previously existing in the centre[12]. It had long been a matter of wonder to all the great architects of Europe how it could be possible to collect such an enormous mass of material of all kinds, and put them together in ten years; to bring that quantity of travertine stone from the quarries at Tivoli, twenty miles off, cut it, and erect it, was in itself an enormous work. This gigantic building is in round numbers 500 ft. long, 300 ft. wide, and 120 ft. high above the foundations, and the grand corridors are double all round, with the _vomitoria_, or stairs from the seats at short intervals, and there are altogether six storeys, including the _entresols_.