A plan and section of the ground between the two gates, called by Frontinus Esquilina (S. Lorenzo) and Viminalis (Maggiore), shew the difference of level, and the three aqueducts passing underground in the middle between the two gates, and carried on arches at both ends near the gates (see No. 1938*). The plan of the ground at the sources of the Tepula and Julia is given in 1980*. VI. THE VIRGO (NOW CALLED AQUA DI TREVI) WAS MADE IN THE YEAR OF ROME 732, B.C. 21. This aqueduct has its sources in the meadows of Lucullus, on the banks of the river Anio, on the old Via Collatia, eight miles from Rome, about a mile further than the Aqua Appia, not at the same level, but comparatively near the surface. There are several springs, each of which has its own separate reservoir just below the surface of the ground; in some of them the vault is scarcely perceived. These are also called conduit-heads (864, 863, 862). From each of these small reservoirs a conduit runs into the central reservoir (860, 861), which is considerably larger, circular in form, surrounded by a wall, lined with the cement called _coccio pesto_, and one part of this central reservoir under the road now remains. This is near Salone, with its medieval tower. (See the Plan, 1155*.) From this large central reservoir the surplus water is carried off by short conduits into the country ditches, and so into the river Anio. The main _specus_ into Rome begins at the central reservoir, and runs generally underground along the line of the old Via Collatina, now called Lunghezza. The course of the aqueduct can be clearly traced by the small pyramidal or conical structures over the wells at regular intervals, called Respirators (660*), or which might have been called Ventilators, as they give air to the _specus_ below. It runs in the high bank of the old road for two or three miles, behind the ruin called Torre d’ Scavi, supposed to be the Thermæ of the Gordiani, in a direct line towards the Porta Maggiore; but about half-a-mile before arriving there, it turns sharp to the north along the bank of the great _foss_ or valley, and being underground is traced by the Respirators. Further to the north, beyond S. Agnes, at some little distance, the old line can be traced in the catacomb of S. Priscilla on the Via Salaria, where the _specus_ is visible, half filled up with the deposit of clay (1109*, 1466). In the road on the bank on which the Wall of Aurelian is built, near the Porta Salaria, it can be traced by the low arcade at the foot of the wall which is built upon it (5).