the great sewers of ancient Rome, which are justly reckoned among the grand works of that people. The most remarkable of these was the Cloaca Maxima, said to have been constructed by Tarquinius Priscus, and which still exists in its original state. It is built of enormous blocks of stone joined together without cement, and consists of three arches overlying each other in close contact, the innermost of which is a vault of 18 Roman palms, and about 14 feet in diameter. By means of this great cloaca, the waters brought from the adjacent hills into the Velabrum and the valley of the Forum were discharged into the Tiber, opposite one end of the Insula Tiberina. Pliny states that a wain loaded with hay could pass through it with ease; which will not seem any exaggeration when we consider the inferior size of the Roman vehicles as compared with those of modern times. Also, in speaking of the numerous subterranean passages by which the foundations of Rome were intersected, he calls it "a city suspended in air" (urbs pensilia). We are told by Dion Cassius (xlii.), that when Agrippa undertook the cleansing of the cloaca he passed through the Cloaca Maxima in a boat. This stupendous structure, even after a lapse of more than 2000 years, still serves as the common sewer of the city.

Agriportum was also celebrated for its works of this kind.