different emperors, and Lanuvium was surrounded with the villas of the great. At length when a dreadful succession of Lombards, Franks, and Saracens destroyed the houses, pavements, drains, crops, plantations, and cattle which had protected the Cam- pagna 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 ] 5 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 distribu- tion 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 earlier Romans 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 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. * Stuart's Die. of Archi., art. Aqueducts. t Stuart's Archi., art. Aqueducts. PRELIMINARY ESSAY. ly At this period it would appear that after a heavy fall of rain, the greater part of the water flowing into the city by all the aqueducts, was discolored by the mixture of earthy matters, even some that were pure and limpid at their sources. This impurity was particularly observable of the two Anios that were supplied from a river, which although, flowing from a lake of great transparency, passed through a rich soil, which in seasons of rain rendered the waters turbid. Indeed, the Anio Vetus was rarely free from discolora- tion, but as it flowed on a low level it could not affect the other waters, but the Anio Novtis being of a lofty level and unfailing in its stream, was occasionally used, when the other aqueducts were falling off in their supply, to make good the deficiency, and hence all were more or less contaminated — even the Marcia, which was the delight of the Romans for its limpidity and coolness. The Emperor Nerva undertook to correct this evil. He began by classing the aque- ducts according to the goodness of their waters. The Marcia was reserved solely for drinking — others according to their qualities,