The political aims and imperial ideas of Trajan, Hadrian and the Antonines are nobly illustrated by the modifications and enlargements they introduced in the structure and extent of the city of Rome. Trajan in his magnificent Forum and library sought to encourage the metropolitan life and literary tastes of the nation, while on his storied column he recorded their world-wide triumphs and reminded them of their enormous power. The Mausoleum of Hadrian remains to commemorate the vast and ponderous strength of his rule, and the Aurelian Column stands to attest the lofty magnificence of the Antonine dynasty. * * * * * In the reign of Commodus, between the Antonine era and the time of Severus, a great fire devastated the central districts of Rome. The restorations effected by Severus and the popular policy of his successors are commemorated in the Arch of Severus, the Portico of the Pantheon, and the huge ruins of the Baths of Caracalla. The defensive power of the Roman nation then became gradually weaker and weaker, till in sixty years after the time of Caracalla, Aurelian commenced the sad task of home fortification. His walls, which were completed a hundred years later by Honorius, still surround the greater part of Rome. During these hundred years the power of the Constantinian rule, of which the great basilica and arch remain monuments, and the warlike courage of Diocletian revived for a time the imperial spirit at Rome. The last and most familiar of the monuments which follow the transfer of power from Rome to Constantinople is the Column of Phocas in the Forum, erected when three centuries of desolation had followed the grandeur of Constantine and his dynasty. The Vatican Hill and the northern end of the Transtiberine district were not enclosed within walls till the time of Pope Leo IV. He undertook in A.D. 848 the enclosure of St. Peter’s and the Vatican Hill, thus forming that district into a separate town, which was named after the Pope Civitas Leoniana. The western wall of this enclosure may still be traced by its ruins in the garden of the Vatican palace. After the successive destructions and minor repairs of the tenth, eleventh, twelfth, and thirteenth centuries, in 1527 the architect San Gallo was employed to erect huge bastions on the wall of Rome, which he placed chiefly between the Porta Ostiense and the Porta Appia. In 1628 Pope Urban VIII. restored the walls on the left bank, and subsequently in 1642 he proceeded to erect the walls which now stand between the Porta Portese and the Porta Cavallegieri, where the arms of that Pope are still affixed to the walls. This was the final important addition to the main walls of the city. III.--ROMAN BUILDING AND ARCHITECTURE.