The Romans were apt scholars, and possessed a wonderful capacity not only to utilize prior inventions but also to develop them. They were beyond question the most successful and masterful road-builders in the ancient world; and the perfection of their highways was one of the most potent causes of their superiority in progress and civilization. When they conquered a province they not only annexed it politically, by imposing on its people their laws and system of government, but they annexed it socially and commercially, by the construction of good roads from its chief places to one or more of the great roadways which brought them in easy and direct communication with the metropolis of the Roman world. And when their territory reached from the remote east to the farthest west, and a hundred millions of people acknowledged their military and political supremacy, their capital city was in the centre of such a network of highways that it was then a common saying, "All roads lead to Rome." From the forum of Rome a broad and magnificent highway ran out towards every province of the empire. It was terraced up with sand, gravel, and cement, and covered with stones and granite, and followed in a direct line without regard to the configuration of the country, passing over or under mountains and across streams and lakes, on arches of solid masonry. The military roads were under the pretors, and were called pretorian roads; and the public roads for travel and commercial traffic were under the consuls, and were called consular roads. These roads were kept entirely distinct; the pretorian roads were used for the marching of armies and the transportation of military supplies, and the consular roads were used for traffic and general travel. They were frequently laid out alongside of each other from place to place, very much as railroads and highways are now found side by side. The consular roads were generally twelve feet wide in the travelled pathway, with a raised footway on the side; but sometimes the footway was in the middle of the road, with a carriage-way on each side of it. The military roads were generally sixty feet wide, with an elevated centre, twenty feet wide, and slopes upon either side, also twenty feet wide. Stirrups were not then invented, and mounting stones or blocks were necessary accommodations; and hence the lines of the roads were studded with mounting-blocks and also with milestones. Some of these roads could be travelled to the north and eastward two thousand miles; and they were kept in such good repair that a traveller thereon, by using relays of horses, which were kept on the road, could easily make a hundred miles a day. Far as the eye could see stretched those symbols of her all-conquering and all-attaining influence, which made the most distant provinces a part of her dominions, and connected them with her imperial capital by imperial highways.