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

A modern engineer who measured some of the remains of these buildings, in order to determine this point, states that he found the mean fall of the ancient aqueducts from the purifying piscinas, or reservoirs, to the point of distribution, to be about 1 in 663, and that from the source of the stream to these reservoirs, the mean fall was about 0.132 of an English inch to the Roman passus, equal to 58.219 English inches.

Thus the Anio Novus, the pavement of whose water channel, on its arrival at Rome, is 250.3 feet above the level of the sea, has a fall of 5.2 feet from the purifying piscinas to the point of distribution, and from its source to this reservoir a fall of 568.7 feet, thus