At length when a dreadful succession of Lombards, Franks, and Saracens destroyed the houses, pavements, drains, crops, plantations, and cattle which had protected the Campagna from mephitism, it then returned to its own vicious propensity, for both the form of its surface and the order of its soil promote the stagnation of water." [Forsyth, p. &H ]

18 PRELIMINARY ESSAY.

in the Campagna, were necessarily on a low level, and the Alsietina, on the lowest of all,

was distributed in quarters about the Tiber, and other flat places. The point of distribution of the Anio Novus, above the level of the Tiber, was 158.88 feet that of the Claudia, ;

148.9 feet that of the Julia, ; 129 .4 feet and of the Marcia, ; 125.4 feet. The elevation of the Anio Vetus, above the Tiber, was 82.5 feet of the Virgo, 34.2 feet and of the ; ;

Appia, 27 A feet. The Tiber itself, at Rome, was 91.5 feet above the level of the sea.* It appears from these statements of the elevation of the different conduits, that the earlierRomans conducted the water on lower levels than their successors, either from ignorance of the mode of ascertaining and preserving the exact level, or as a precaution whereby they were enabled the better to conceal the conduits, by burying them deep in the earth and thus secure them from notice and destruction by their hostile neighbors, with whom they were in an almost constant state of war.

Moreover, at all periods, the Romans gave a greater declivity to their conduits than is practised by moderns. Vitruvius, in the passage heretofore quoted, assigns the ratio of one foot in 200. According to Scammosi the general practice of the Romans was to allow a fall of 1 in 500.t