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

showing the source to have been 824.3 feet above the level of the sea. The other aqueducts showed a proportional declivity, between their sources and the points of distribution.

The piscinas or reservoirs, to which reference has been made, were placed at a certain distance from the city, and there the waters deposited their sediment, and there also the quantity of the supply was determined by a register ; six of the aqueducts were thus emptied into piscinas; three of these, the Julia, Marcia, and Tepula were conducted from the purifying reservoirs to the city, over the same aqueduct, as has already been noticed, one above the other. The Anio Novus, and the Claudia also flowed in separate channels over the same arches. The Anio Vetus had a separate series of arches. Neither the Virgo, Appia, nor Alsietina were supplied with piscinas.