This loch is situated about half-way between the Tor Fiscale and the farm-house called Roma Vecchia, rather more than three miles from Rome, and a quarter of a mile to the left of the Via Appia Nova. The Almo is a mountain stream coming down from the Alban hills, often flooded in the rainy season and dry in the hot season, with a very deep bed called a foss. This bed was convenient for the engineers who made the mill-stream, now called the Marrana, in the twelfth century, and they used it when the ground was high and the foss deep; but in other parts, when the ground was low and liable to be flooded, they banked up the stream, or made a new channel for it on a raised bank, for sometimes half a mile together, then joined the old winding bed again for perhaps another mile. At the point where the division into two streams takes place, one branch is banked up and comes through Rome, the other remains in the deep bed and receives the surplus water from the loch, made at this point, and this second stream runs through the valley of the Caffarella, and has its mouth near the church of S. Paul f. m. The Plate shews the plan at the division, one section of the loch, and the lasher. The second stream has no other beginning than this division, and the deep bed or foss can be traced in its winding course by the side of the cross-road from the Via Appia Nova to S. Urban, at the head of the valley of the Caffarella. PLATE XIV. RIVER ALMO, NOW THE MARRANA. ENTRANCE INTO ROME UNDER THE PORTA METRONIA. [Illustration: XIV. RIVER ALMO, NOW THE MARRANA. ENTRANCE INTO ROME UNDER THE PORTA METRONIA. A. ELEVATION, WITH THE BRIDGE UNDER THE GATE. B. PLAN AND SECTIONS—A. B. THE BRIDGE; C. D. SECTIONS. C. PLAN OF THE GROUND WITHOUT AND WITHIN THE WALL OF ROME.] A. Elevation, with the bridge under the gate. The lower bridge under the modern road is of the time of the Early Empire, and can be seen, though not without some little difficulty, by climbing upon the bank of the mill-stream, which now runs in the deep winding foss of the Almo, and makes a bend under the gate, as is seen in the small plan in the centre, marked M. B. Plan and sections, longitudinal and transverse, of the bridge, with the gate upon it. The longitudinal section A B has the line of it, marked on the small plan M; the transverse section is marked C D.