The upper wall on the north side, where it remains perfect, formed the back of the wooden gallery over the corridors for the common people, and was faced with brick, but the greater part of the ancient brickwork had fallen down, and has been copied in modern times; a great deal of the back of the stone wall, left exposed, shews the hasty construction[155], in the time of Gordianus. The remains of Aqueducts and _Piscinæ_ have already been mentioned[156], but some further account of them seems to be requisite. A _piscina_ always consists of four vaulted chambers, two above and two below, and the middle wall of the two lower chambers has small holes in it, for straining the water as it passes through. The lower chamber of a _piscina_ is also known by having no windows in it, and the lining being of the water-cement (_opus signinum_). The lower chambers of two _piscinæ_ only remain; of the northern one the middle wall between the two lower chambers is the only part now visible, this is faced with _opus reticulatum_ of the time of Nero, and has the usual small holes for water-pipes through the wall. The southern one is of brickwork of the third century, of the time of Alexander Severus; of this there is much more remaining, one end with the usual boldly projecting buttresses to support the weight of water, and part of two other chambers of the reservoir. The excavations which had been made in the time of the first Napoleon and of Pope Pius VII., 1810-1814, were filled up again after drawings and plans had been made of them. They were not considered satisfactory by scholars because the excavations had not gone deep enough, having been stopped by water, as very often happens in Rome at certain periods of the year, when the springs are high. They were again suspended by the same cause in the spring and summer of 1874, but Signor Rosa, with his usual energy, obtained machinery and a steam-engine to pump the water out[157]. The whole area was found to be undermined by chambers and passages, with walls chiefly of brick, but some of tufa, with indications of several different periods[158].