[Illustration: RUINED AQUEDUCT IN THE ROMAN CAMPAGNA.] By Hadrian’s day the aqueducts supplying the city have become wholly admirable. Time fails us to go out into the Campagna or to the distant hills and see how, by gravity alone, and without the aid of pumping engines, “copious streams are conducted great distances despite the obstacles presented by mountains, valleys, or low-lying level plains, sometimes rushing along in vast subterranean tunnels, at other times supported on long ranges of lofty arches, the remains of which [in after ages] will still be seen spanning the waste of the Campagna.” [Lanciani.] There is difficulty in making very large iron pipes capable of standing high pressure over long distances; and as a result the Roman engineers prefer to carry the water in channels lined with solid cement and borne across the open ground on a vast series of arches. Besides, most of the good water near Rome leaves a calcareous deposit; and it is much easier to clean out large channels than an underground piping system. =259. The Great Aqueducts.=--When we try to understand the water system of Rome we come upon astonishing figures for the great aqueducts. There are nine of these huge conduits in constant use. The oldest is the _Aqua Appia_, built in 312 B.C. by that tough old censor, Appius Claudius, and it starts only about eleven miles from the city, with nearly its entire bed underground; but when this supply proved inadequate the engineers had to reach much farther back into the hills to find powerful jets. An increasing proportion of the channels of the newer aqueducts has also to be on arches; for example, the _Aqua Julia_, built by Agrippa in 33 B.C., has to go back fifteen and a half miles, and six and a half of these are on arches; while the _Aqua Claudia_, built about 40 A.D., is no less than forty-six miles long with nine and a half on elevated arches. There are two others, the older _Aqua Marcia_ and the slightly newer _Aqua Anio Novus_ (taking water from the river Anio), that are not much shorter either upon the ground or in their elevated sections. Once inside the city this enormous volume of water is distributed in a most scientific manner according to a scheme worked out by the mighty Agrippa. There are 700 public pools and basins and 500 public fountains drawing their supply from 130 collecting heads or reservoirs. Only the poorest or tallest tenement houses, consequently, are bereft of a water supply, clear, sanitary, and abundant, such as most later cities can desire in vain until close upon the twentieth century.