The Roman Empire, at its widest extent in the second century, included forty-three provinces. They were protected against Germans, Parthians and other foes by twenty-five legions, numbering with the auxiliary forces, about three hundred thousand men. This standing army was one of Rome's most important agencies for the spread of her civilization over barbarian lands. Its membership was drawn largely from the border provinces, often from the very countries where the soldiers' camps were fixed. Though the army became less and less Roman in blood, it always kept in character and spirit the best traditions of Rome. The long intervals of peace were not passed by the soldiers in idleness. They built the great highways that penetrated every region of the empire, spanned the streams with bridges, raised dikes and aqueducts, and taught the border races the arts of civilization. It was due, finally, to the labors of the legionaries, that the most exposed parts of the frontiers were provided with an extensive system of walls and ramparts. [Illustration: THE PANTHEON The original building was the work of Agrippa, a minister of Augustus. The temple was reconstructed by Hadrian who left the Greek portico unchanged but added the rotunda and the dome. This great dome, the largest in the world, is made of solid concrete. During the Middle Ages, the Pantheon was converted into a church. It is now the burial place of the kings of Italy.] THE ROMAN ROADS The Roman system of roads received its great extension during the imperial age. The principal trunk lines began at the gates of Rome and radiated thence to every province. Along these highways sped the couriers of the Caesars, carrying dispatches and making, by means of relays of horses, as much as one hundred and fifty miles a day. The roads resounded to the tramp of the legionaries passing to their stations on the distant frontier. Travelers by foot, horseback, or litter journeyed on them from land to land, employing maps which described routes and distances. Traders used them for the transport of merchandise. Roman roads, in short, were the railways of antiquity. [11] [Illustration: THE TOMB OF HADRIAN The building was formerly topped by another of smaller size which bore a statue of the emperor. In medieval times this stately tomb was converted into a castle. It is now used as a museum. The bridge across the Tiber was built by Hadrian.] [Illustration: Map, ROMAN BRITAIN SHOWING CHIEF ROMAN ROADS] THE PAX ROMANA In her roads and fortifications, in the living rampart of her legions, Rome long found security. Except for the districts conquered by Trajan but abandoned by Hadrian, [12] the empire during this period did not lose a province. For more than two hundred years, throughout an area as large as the United States, the civilized world rested under what an ancient writer calls "the immense majesty of the Roman peace." [13] EXTENSION OF ROMAN CITIZENSHIP