Page:Popular Science Monthly Volume 11.djvu/40

and were called after their builders or projectors. The waters of the Marcian, the most prized for their purity and coldness, were collected from several springs. For the Anio Novus, which was unfailing as well as the most abundant of the aqueducts, the river Anio was arrested near its source by three gigantic walls at different levels, and formed into as many lakes, one below the other. Over these walls the waste-water fell in magnificent cascades, one of them over 150 feet high. The object of the lakes was to clarify the water; for the Anio, though usually a limpid stream, is liable to become muddy after a heavy rain. The sources of the Anio Novus and the Aqua Claudia are over 2,000 feet above the level of the city, and those of the Marcia and Anio Vetus are not very much lower. Descending from such a height and for distances varying in direct lines from 30 to 43 miles, the water would naturally acquire great velocity and tremendous force, which it was necessary to diminish, and that was done by making numerous angles in the conduits. The angles were made generally at every half-mile, and were points at which reservoirs (castella), or filtering-places (piscinœ), or both, with accompanying air-shafts, were built. These were surmounted by small towers. As an additional means of breaking the force of the water, the bottoms of the conduits were given a succession of short undulations. The conduits, reservoirs, and filtering-places, were lined with a cement called opus signinum, which is so compact that it will resist a hard tool. The art of making it has been lost. The conduits, always covered, were carried on arcades only where it was necessary to cross a valley or a plain above its level; for the rest of their way they ran in places upon the surface of the ground, but mostly below it. Thus of the 58 miles of the Anio Novus, 49 were underground. No two aqueducts were on the same level, and so, where their courses converged, it was both possible and convenient to carry one conduit upon another, because it was forbidden by law to erect a building within a certain number of feet on either side of an aqueduct; hence we find the Aquæ Marcia, Tepula, and Julia, carried from their point of convergence one above the other on one arcade, and the Aqua Claudia and Anio Novus on another. Each of the conduits was differently shaped, some having arched, others angular roofs. Besides the small reservoirs referred to as occurring at the angles of the conduits, there were larger ones at longer intervals. The ruins of one of these, belonging to the Aqua Marcia, are still to be seen near Carciano. It is a huge subterranean chamber divided by an arcade in the middle. Between five and seven miles from Rome were the great filtering-places to which most of the aqueducts converged. The waters, however, were not mingled, for each aqueduct had its separate chambers, though it was always within the power of the attendants (aquarii) to turn the water from one aqueduct into another at will. Of these