Ostia,[108] fifteen miles from Rome, is situated by the ancient channel of the Tiber, for this stream has changed its course somewhat, and is receding still more every day. We breakfasted in haste at a mean tavern, beyond which we could see La Rocca, a small castle strong enough in itself, but now bare of a garrison. The Popes, and notably the present one, have built along this coast high towers or look-outs, about a mile apart, to guard against the raids often made by the Turks, especially in the vintage season, and the capture of men and cattle. By the discharge of a cannon from these towers they advise one another so rapidly that the alarm is sped to Rome forthwith. Round Ostia are the salt works, situated in a wide marshy plain into which the sea flows, and these supply all the States of the Church.[109] This road from Ostia to Rome, the _Via Ostensis_, abounds in vestiges of its ancient splendour, such as causeways and ruins of aqueducts; moreover, all along it is set with mighty ruins, and for two-thirds of its length it is still paved with those large black squares of stone which they used formerly as pavement. In looking at this bank of the Tiber it is easy to believe that of old the road all the way from Rome to Ostia ran past the habitations of men. Amongst other ruins we saw, about midway and on our left, a very beautiful tomb of a Roman prætor with an inscription quite perfect thereon. In Rome, the ruins, as a rule, only manifest themselves to us by the massive solidity of their construction. The ancients built thick walls of brick, and these they lined either with strips of marble or some other white stone, or with a kind of cement, or with thick tiles set thereupon. This outside crust, on which the inscriptions were written, has almost everywhere been ruined by the lapse of years, wherefore we have now but little knowledge as to all these matters. Inscriptions still remain where the walls were originally built in solid fashion.