Over the plan in this plate are two sections of the _specus_; in the one to the right the clay deposit left by the water is shewn, in the one to the left a small pipe coming into the _specus_, probably part of the system of irrigation, which was one great use of the aqueducts; the green lines winding through the quarry represent the lines of the various streams of water coming at different levels, sometimes with a very rapid descent, and all falling into the old deep _specus_ of the Appia, before that was carried across the last road that it had to pass. In this instance it was probably carried under the road, and not over the arch of the gate. At this point four roads meet, which is almost a certain indication of the site of a gate. In most cases the _specus_ was carried over the gate, but here the level seems too low, unless the old foss-way has been filled up even more than the usual fifteen feet. The _specus_ is visible again in another subterranean stone quarry under S. Prisca, in the same large vineyard formerly of the Jesuits, now of Prince Torlonia, in which, at a higher level, are also the remains of the Wall of the Latins on the Aventine, and of the Thermæ of Sura and the private house of Trajan. Another aqueduct passed over this to supply the Thermæ; this is on a very high level, and was carried on a tall arcade across the valley, from the Cœlian to the Aventine, passing over the Porta Capena and on the old _agger_. There are remains of the tall brick piers in several places, and of the arcade and _specus_ on the hill near S. Prisca, visible from the Palatine. PLATE IV. [Illustration: IV. AQUEDUCTS—AQUA APPIA, MOUTH. IN A CAVE OF THE AVENTINE, UNDER S. ALEXIO, AT THE PORTA TRIGEMINA, NEAR THE MARMORATA, EXCAVATED IN 1868.]