If we turn to Roman baths, like those of Caracalla in Rome, begun in A.D. 211, we are back on the chronological track again, but we find that the last major archaeological work upon them was done at the end of the last century, and that their chief interest today lies in the inspiration they have offered to modern architects. As for aqueducts, the last important ancient one was built under the Emperor Alexander Severus, in A.D. 226, but working back from that date we can profitably review the difficult and absorbing topographical work done in tracing the courses of the major aqueducts by a devoted Englishman and an American woman. Finally we shall review the work of another Englishman in tracing the chronology and building techniques of ancient Rome’s last great fortification, Aurelian’s Wall, begun in A.D. 271 and still in large part standing. Its alterations and repairs have been traced down to the middle of the sixth century of our era. The examples chosen should justify the Romans’ high reputation for engineering skill, and illuminate Roman history, at the same time underlining on the one hand our debt, for the facts we know and the inferences we draw, to the careful work of modern archaeologists, and on the other the catalytic effect, in the case of the baths, of Roman work upon our own architecture of the day before yesterday. [Illustration: FIG. 12.1 Roman road construction. (U.S. Bureau of Public Roads)] Roman roads (see Fig. 4.1) echoing to the measured tread of marching legions, had made a large contribution to unifying Italy by the time the last great consular highway, the Via Aemilia, opened up the Po valley from Ariminum to Placentia in 187 B.C., but their work of carrying commerce and ideas was unceasing. Of course there were roads in Italy before the Romans: the name and route of the Via Salaria, from the salt-pans at the Tiber’s mouth up the valley into the Apennines, suggest that it must have been in use since prehistoric times. The Via Latina, named not for a Roman consul but for a people potent in central Italy until the Romans broke their league in 338 B.C., must count as a pre-Roman road, and its winding course along the foothills must antedate the draining of the Pomptine marshes and the laying down of the straight course across them from Rome to Tarracina and thence to Capua of the _regina viarum_, the queen of roads, the Via Appia. It bears the name of a Roman censor of 312 B.C. This is the first of the great highways, and it deserves its fame for its bold conquest of natural obstacles, its arrow-straight course across the marshes, but its gravel surface was not replaced by stone pavement until 293 B.C., and then only as far as the suburb of Bovillae. And its course, like that of many another Roman road, was not always so arrow-straight. In the hills behind Tarracina it followed the contours; it was not until Trajan’s time that another bold stroke of engineering