The general plan of the Aqua Marcia near its source is shewn in 1972*, and the line of its course in 1981*, 1982*; the bridge for it in 1983*. In the early part of its course, above Tivoli, the new aqueduct for this water, called the Aqua Marcia Pia, is carried on a stone _specus_ upon an arcade, after the same fashion as the old one (a portion of this new arcade is shewn in No. 1553). IV. THE TEPULA, and V. THE JULIA, being carried on the same arcade as the Marcia for the seven miles into Rome, have left remains visible in some places, especially at the Gates of Rome, the Porta Maggiore (31), and the Porta Tiburtina (21, 572*); in other places, they have generally been destroyed. Near the Sette Bassi, there is a portion of the _specus_ of the Julia visible just at the surface of the ground, the other two being then subterranean, as the level is rather higher in this part than usual. This portion has been examined by Signor Moraldi, at a junction whence a branch was carried to supply a reservoir at the great villa called Sette Bassi, and there are remains of the loch in the channel to turn off the water, shewing the same arrangement as in a modern canal. (A plan and section of this is given in 696*.) The _specus_ near this point, built of concrete faced with brick, is also shewn in 1006. The _specus_ of the Marcia is always of squared stone, so that one is readily distinguished from the other. There are remains of a _castellum aquæ_ or reservoir for the Tepula, near the Porta Tiburtina, or Porta Viminalis of Frontinus, now called Porta di S. Lorenzo. This is shewn in the Plan of that Gate, Nos. 1111* and 1238*, and a view of it in No. 25. It is a remarkable building of brick of the first century, and has on a level with the _specus_ a series of small corbels projecting from it, evidently intended to carry a _hourd_, or wooden balcony as a passage for the Aquarii, and perhaps for defence also. It is incorporated in the great Wall of Aurelian. It projects slightly from the line of that wall, and the end of the _specus_, with its triangular head, is visible in the angle. V. JULIA. Between this building and the gate, but within the wall, though on the bank on which it stands, are slight remains of another _castellum aquæ_, supposed to have been for the Julia (Nos. 26, 869, 1873), which has its external face in the direction of the wall, and must have been concealed by it when that was built. This is also of the first century, as is shewn by the brickwork, and it seems to have been a _castellum aquæ_ by the disproportional size of the buttresses used to support the weight of the water, one of the invariable marks of such a structure. The other mark is the peculiar cement with which the wall is lined, called _Opus Signinum_ in Latin, _Coccio Pesto_ in Italian, which is made of broken pottery, and is the hardest cement that is known; it is often impossible to break it, even after it has been exposed to the weather for centuries.