This circuit will bring into view specimens of every construction, from the days of Servius Tullius down to the present. Aurelian took into his walls whatever he found standing in their line, and they now include some remains of the Tullian walls, the walls of the Prætorian barracks, the facing of a tank, aqueducts, sepulchral monuments, a menagerie, an amphitheatre, a pyramid, &c. Thus do they exhibit the uncemented blocks of the Etruscan style, the reticular work of the republic, the travertine preferred by the first emperors, the alternate tufa and bricks employed by their successors, and that poverty of materials which marks the declining empire. Since the first breach, made by Totila, the walls have been often and variously repaired; sometimes by a case of brick-work, filled up with shattered marbles, rubble, shard, and mortar. In some parts, the cementitious work is unfaced; here you find stones and tufa mixed; there tufa alone, laid in the Saracenic manner: the latter repairs have the brick revêtement of modern fortification. The gates of Rome, at the present day, are sixteen in number, of which only twelve are open. The wall of Romulus had but three or four; and there has been much discussion among antiquaries, as to their position. That of Servius had seven; but in the time of Pliny, (in the middle of the first century) there were no less than thirty-seven gates to the city. The twelve gates at present in use correspond to some of the principal gates of former times. Modern Rome, however, can scarcely be said to rest upon the ancient base. Scarcely two-thirds of the space within the walls are now inhabited, and the most thickly peopled district is comprised within what was anciently the open plain of the Campus Martius. On the other hand the most populous part of the ancient Rome is now but a landscape; it would almost seem, indeed, as if the city had slipped off its seven hills into the plain beneath. A remarkable change, too, has taken place in the surface of the site itself. In the valleys the ground has been raised not less than fourteen or fifteen feet. This is strikingly observable in the Forum, where there has been a great rise above the ancient level, owing partly to the accumulation of soil and rubbish brought down by the rains; but chiefly, as there is reason to believe, to that occasioned by the demolition of ancient buildings, and the practice which prevailed of erecting new structures upon the prostrate ruins. The Tiber, too, still remains; but its present appearance has been variously estimated. “The Tiber,” says Dr. Burton, “is a stream of which classical recollections are apt to raise too favourable anticipations. When we think of the fleets of the capital of the world sailing up it, and pouring in their treasures of tributary kingdoms, we are likely to attach to it ideas of grandeur and magnificence. But if we come to the Tiber with such expectations, our disappointment will be great.”