The triumphal arches--as that of _Titus_, already mentioned, which was built of Pentelic marble--and the commemorative columns--as the Column of _Trajan_, which stood in the forum that bears his name--were among the architectural wonders of the ancient capital of the world. The plain, named of old the _Campus Martius_, on the north-west side of the city, and bordering on the Tiber, contained, among the buildings and pleasure-grounds by which it was covered, the Pantheon, and the magnificent mausoleum of Augustus. On the south-west of the Coelian Hill, the Appian Way turns to the south-east, and passes out of the Appian Gate. It is skirted for miles with sepulchral monuments of ancient Romans, of which the circular tomb of _Metella Cæcilia_ is one of the most interesting. There are varying estimates of the population of ancient Rome. Probably the number of free inhabitants, in the early centuries of the empire, was not far from a million; and the slaves were probably almost as many. DEATH OF NERO: GALBA.--Growing jealous of the legates who commanded armies on the frontiers, _Nero_ determined to destroy them. They consequently revolted; and war between the troops of two of them issued in the death of _Vindex_, the general in Gaul. But _Galba_ was deputed to carry on the contest; and Nero, being forsaken even by his creature, _Tigellinus_, and the prætorians, at last gained courage to call on a slave to dispatch him, and died (A.D. 68) at the age of thirty. The principal events out of Italy, during his reign, were the revolt of the Britons under the brave queen _Boadicea_ (A.D. 61), and the suppression of it by _Suetonius Paulinus_; the war with the Parthians and Armenians, extending slightly the frontier of the empire; and the beginning of the Jewish war. Despite the corruption at Rome, her disciplined soldiers still maintained their superiority on the borders. OTHO: VITELLIUS.--With the death of Nero, the Augustan family came to an end. _Galba_ began the series of military emperors. A Roman of the old type, simple, severe, and parsimonious, he pleased nobody. The prætorians killed him, and elevated _Otho_, a profligate noble, to the throne; but he was obliged to contend with a rival aspirant, _Vitellius_, commander of the German legions, who defeated him, and became emperor A.D. 69. Vitellius was not only vicious, like his predecessor, but was cowardly and inefficient. The Syrian and Egyptian legions refused to obey so worthless a ruler, and proclaimed their commander, _Flavius Vespasian_, as emperor. As Vespasian's general, _Antonius_, approached Rome, _Vitellius_ renounced the throne, and declared his readiness to retire to private life. His adherents withstood him; and, in the struggle that followed between the two parties in the city, the Capitoline Temple was burned. The Flavian army took Rome, and _Vitellius_ was put to an ignominious death (A.D. 69). CHAPTER III. THE FLAVIANS AND THE ANTONINES.