B. Central reservoir where two streams meet and are united in one _specus_, which conveyed the water into Rome. The aperture in the rock above is not original, or it has been greatly enlarged. This was probably a well to draw water for the cattle, and to give air to the _specus_. C. Two streams coming out of a sort of double cave, and meeting at the mouth of it, from whence they are carried across the meadow in a _specus_ open at the top for a certain distance, and having the appearance of a ditch only; but the water from this spring also never fails, and these two streams united gave a certain steady supply to the aqueduct. The meadow through which these streams flow has a clay soil, and the water is always liable to be muddy after rain, and left a large deposit of clay in the _specus_, as is shewn in other plates. THE AQUEDUCTS. PLATE XVIII. AQUA APPIA, OR APPIAN AQUEDUCT. [Illustration: XVIII. THE APPIAN AQUEDUCT PASSING OVER THE PORTA CAPENA AND THROUGH THE TOWER.] DESCRIPTION OF PLATE XVIII. AQUA APPIA, OR APPIAN AQUEDUCT. Crossing the valley from the Cœlian to the Aventine upon the _agger_ of Servius Tullius and over the Porta Capena, this was the only part that was above ground, as we are told by Frontinus[234]. In this view the pavement of the Via Appia is seen in the foreground, then the wall of Servius Tullius, twelve feet thick, (as usual with the walls of the Kings;) by the side of this, to the left, is seen the arcade that carried the _specus_ of the aqueduct; this goes as far as the branch of the river Almo (now called the Marrana), which runs through the valley. On the further side of the stream the land is high, and the aqueduct is again underground, but it has been traced not only across the valley, but under the cliff on the northern side of the Pseudo-Aventine, with a branch to the left to supply the Piscina Publica, which was an enormous swimming-bath for the whole population of Rome at that period, extending as far as the north end of the Thermæ of Caracalla, where the hollow with the bank round it can still be seen, and where a great reservoir remains, lined with the cement called _opus signinum_ (or _coccio pisto_). The ruins of another great reservoir rebuilt in the time of Trajan are at the north-west corner, nearly under the Aventine, and are called the Piscina Publica. THE AQUEDUCTS. PLATE XIX. PLAN AND SECTION IN A CAVE IN THE AVENTINE. [Illustration: XIX. AQUEDUCTS—PLAN AND SECTIONS IN A CAVE IN THE AVENTINE] DESCRIPTION OF PLATE XIX. PLAN AND SECTION IN A CAVE IN THE AVENTINE. This is the same cave on the Pseudo-Aventine, nearly under the church and monastery of Santa Sabba, which has been before described when partially excavated (Plate III.[235]), but which has now, in 1875-6, been more thoroughly examined and made accessible.