A conduit for water, constructed of different materials, built on uneven ground, for preserving the level of water, and conveying it from one place to another. Aqueducts art- distinguished into two classes; the visible, or such as are built on arches across valleys and marshes, and the subterraneous, which are formed by piercing mountains, and conveying the water below the surface of the earth. These constructions were formed of brick, stone, etc, and covered either with vaulted roofs, or flat stones, which served to shelter the water from the influence of the sun and rain. They were, in some cases, paved, and, in others, the water was conveyed through a natural channel formed in the clay. It was also frequently conducted by leaden pipes into reservoirs of the same metal, or into troughs of hewn stone. Some aqueducts are supported on single, others on double, or triple, ranges of arches. Of the atter kind is the Pont du Gard in Languedoc, supposed to have been built by the Romans, to convey water to the city of Nismes; that at Constantinople, and that which, according to Procopius, was constructed by Cosroes, king of Persia, near Petra, in Mingrelia, and which had three conduits in the same direction, each elevated above the other.

The Romans had many magnificent aqueducts, surprising on account of their magnitude and number, as well as their construction and length; some bringing water through a distance of from thirty to one hundred miles, either upon a series of arches, or by means of excavations through mountains and rocks. This is well expressed by Pliny in the following language: "If we consider the incredible quantity of water brought to Rome for the uses of the public, for fountains, baths, fishponds, private houses, gardens, and country seats; if we represent to ourselves the arches constructed at a great expense, and carried on through a long distance; mountains levelled, rocks cut through, and valleys filled up; it must be acknowledged that there is nothing in the whole world more wonderful." The waters of the Tiber, with the wells and fountains in the neighbourhood, had supplied the wants of the Romans for four centuries and a half, when API's Claudius, the censor, advised and constructed the first aqueduct. His example was speedily followed, and the courses of rivers were thus changed and diverted towards Rome, to supply the daily increasing wants and luxuries of the Roman people.

At certain distances, vents were provided, in order that the water, which was accidentally obstructed in its progress, might be discharged till its ordinary passage was cleared; and, in the canal of the aqueduct itself, there were cavities into which the water was precipitated, and where it might remain till its mud was deposited, and the water become clear. A considerable variety was observable in the construction of aqueducts. The aqueduct of the Aqua Marcia had an arch of sixteen feet diameter. The whole was composed of three different kinds of stone; one of them reddish, another brown, and a third of an earth colour. The entire edifice is 70 Roman feet in height. Above them appeared two canals, the highest of which was fed by the waters of the Tiverone, and the lower, by what was called the Claudian River. Near this aqueduct, Montfaucon gives the plan of another with three canals; the highest supplied by the Aqua Julia, that in the middle from Tepula, and the lowest from the Aqua Martia. The arch of the aqueduct of the Aqua Claudia was of hewn stone, very beautiful; that of the aqueduct of the Aqua Meronia was of brick. Each of these was 72 feet high.

The Aqua Appia deserves notice for the singularity of its construction, it not being plain or gradual in its descent, but much narrower at the lower than at the upper end. The consul Frontinus, who superintended the aqueducts under the emperor Nerva, mentions nine, each of which had 13,594 pipes of an inch diameter; and it is stated by Vigerus, that, in the space of twenty-four hours, Rome received, by means of these erections, 500,000 hogsheads of water. In modern times, the aqueduct of Legovia is considered the most magnificent. There still remain 159 arcades, wholly consisting of stones, enormously large, and joined without mortar. These arcades, with what remain of the edifice, are 102 feet high; there are two ranges of arcades, one above the other. In 1684, Louis XIV. caused an aqueduct to be commenced near Maintenon, for carrying water from the river Eure to Versailles; but the work was abandoned in 1688. This would, probably, have been the largest aqueduct in the world, the whole length being 60,000 toises; the bridge 2,070 fathoms in length, 220 feet in height, and consisting of 632 arches. The principal ancient aqueducts now in being, are those of the Aqua Virginia, Aqua Felice, and Aqua Paulina.

The quantity of water supplied by the whole of the aqueducts in ancient Rome, is calculated to have amounted to the enormous quantity of 50,000,000 cubic feet daily; which, reckoning the population of Rome at 1,000,000, allows 50 cubic feet for the daily consumption of each individual. The supply of water to London in 1790, amounted to 2,626,560 cubic feet daily; and even at the present day, it does not exceed 4,000,000 feet This quantity, although found abundantly sufficient for our use, is little more than a twelfth part of the quantity consumed by the Romans. The daily supply of water to Paris is still less, being about 293,000 feet, or half a cubic foot for the daily use of each inhabitant. The Greeks of the lower empire simplified the general mode of conducting water, and reduced the expense to a fifth part, chiefly by the introduction of the Soulerazi, or water-balance, a sort of hydraulic obelisk. The water runs down a gentle slope in covered drains, till it reaches an obelisk constructed of masonry, and rising up one side in a narrow channel, discharges itself into a bason at the top, from which again, at a level 8 inches lower, it descends by a similar channel on the other side.

The charge of the water-works at Constantinople is entrusted to a body of 300 Turks, and certain Albanese Greeks, who make it almost an hereditary profession. The supply for each person is stated to be about two-thirds of a cubic foot, or 42 pounds daily. There are two ancient cisterns still extant in Constantinople: the subterranean cistern, built of hand-brick, vaulted, and resting on marble columns; and the cistern of 101 columns, anciently called Philoxene. The latter consists of three rows of columns, one above another, and is capable of holding five days' supply of water for the whole inhabitants of this spacious city. Among the most modern must be noticed the aqueduct of Chirk, in Denbighshire, constructed by Mr. Telford for the purpose of carrying on the navigation of the Ellesmere canal. It consists of 10 arches, supported by pyramidal stone piers, and extends to about 600 feet in length. The summit of the central arch is 65 feet above the level of the water.