[111] There is _Opus Reticulatum_ of rude and early character (more like the _Opus Incertum_ of the Emporium than the mausoleum of Augustus) at the entrance of this passage, which is mentioned by Nibby. There is a stone-quarry also at the entrance, of the hard, dark-coloured stone used for making roads, and excellent for that purpose. This is the same stone that is called _selce_ or _silex_ in Rome, and seems to be similar to the hard lava under which Herculaneum is buried, and of which there are quarries near the tomb of Cecilia Metella. [112] The persons employed by Mr. Parker went down to the bottom of this steep tunnel-passage to ascertain this. [113] Nibby, Analisi storico-topografico-antiquaria della carta de’ Dintorni di Roma, tom. i. Roma, 1837, 8vo. art. _Alsietina_. [114] Cassio, Corso delle Acque, vol. i. p. 147. [115] Nibby considers this a mistake, and is of opinion that the water was the Sabatina, not the Alsietina; but the mistake is made by Nibby himself, not by the engineers of Pope Paul, who certainly brought the water from the Lacus Alsietina and the other small lake above it (as mentioned on p. 50). By the draining of these lakes, the aqueduct is _now_ made to depend on the Lacus Alsietina only. The sources of the Alsietina are very different from those of the Sabatina. The former was taken by Augustus from the lake Alsietinus, now called Lago di Martignano, and the latter by Trajan from the sources between the lake Sabatinus and the villages of Vicarello, Bassano, and Oriolo. The Alsietina was at the lowest level of all the aqueducts, and the Sabatina at the highest. The first _specus_ was for the most part subterranean, and the other was carried upon arcades for part of its course. [116] By the more usually received computation, the second year of Caligula would be A.U.C. 791; and the year of the consulship of Sulla and Titian, A.U.C. 805. [117] It surpassed all the others _in quantity_, and being the highest, was used to supply the others when the water fell short; but the water was not so good for drinking. [118] Frontinus, c. 13. [119] That is, higher than any other aqueduct. [120] Frontinus, c. 20. [121] The road to Subiaco. [122] Frontinus, c. 14. [123] Ibid. [124] Ibid. [125] Ibid., c. 18. [126] _In Subruino_ is the reading given in the text by Dederich. _In suo rivo_ is the reading adopted by Polenus, Jocundus, and other editors. A third reading has been suggested, “In Simbrivio,” or in Simbruino, that is, referring to the Simbruine hills. This is the reading of Buecheler (1868), who follows verbatim the manuscript of Monte Cassino. The reading, however, bears little upon the evidence. [127] _Herculaneus Rivus._ This is not the same _Herculaneus Rivus_ as the one mentioned by Frontinus in connection with the Anio Vetus, in ch. 19. The same name is given in these places to different streams, all strong and rapid. [128] Frontinus, c. 15. [129] Ibid., c. 93. [130] Ibid., c. 91.