AQUÆDUCT, in hydraulics and architecture, a structure formed for conveying water from one place to another, over grounds that are unequal. The word is compounded of the Latin substantive aqua water, and of ductus a channel, by which that water may be conducted.
Architects distinguish two kinds of aquæducts; the visible, and the subterraneous.—The visible are constructed in valleys or marshes, and protracted in longitude or latitude as the situation requires. They are composed of admiracula for supporting the arches and confining the stream, and of arcades.—The subterraneous are formed, by piercing the mountains, and conducting them below the surface of the earth. They are built of stone, hewn or rough; and covered above with vaults, or with flat stones, which may be termed flags: these flags shelter the waters from the heat of the sun.
They divide them still into double and triple aquæducts; that is to say, such as are supported either by two or by three ranges of arcades. Such was the aquæduct which Procopius records to have been built by Cosroes king of the Persians, for the city of Petra in Mingrelia: it had three conduits upon the same line, each elevated above the other.
Frequently aquæducts are paved. Sometimes the waters flow through a natural channel of clay. Frequently they are conveyed by pipes of lead into reservoirs of the same metal, or into troughs of hewn stone. The channels are cut with an imperceptible descent, that the current may be accelerated by its own weight. Parallel to its course, on each side, is cut a narrow foot path, where people may walk when necessary. By conduits, or grooves, the waters are conveyed into large cisterns, but not forced above their original level. To make them rise and issue from their apertures with force, they must be confined in tubes of a small diameter, and abruptly fall from a considerable declivity.
Aquæducts of every kind were long ago the wonders of Rome. The vast quantity of them which they had; the prodigious expence employed in conducting waters over arcades from one place to another, at the distance of 30, 40, 60, and even 100 miles, which were either continued or supplied by other labours, as by cutting mountains and piercing rocks; all this ought to surprise us: nothing like this is undertaken in our times: we dare not even think of purchasing public convenience at so dear a rate. Appius the censor advised and
constructed the first aquæduct. His example gave the public luxury a hint to cultivate these objects; and the force of prodigious and indefatigable labour diverted the course of rivers and floods to Rome. Agrippa, in that year when he was ædile, put the last hand to the magnificence of these works. It is chiefly in this respect that the modern so much resembles the ancient city of Rome. For this advantage, she is peculiarly indebted to Sextus V. and to Paul V. who for grandeur and magnificence emulated the masters of the universe*. There are still to be seen, in different places contiguous to Rome, striking remains of these aquæducts; arches continued thro' a long space, over which were extended the canals which carried the water to the city. The arches are sometimes low, sometimes raised to a vast height, to humour the tumidities or depressions of the ground. There are some which have two arcades, one constructed above the other; and this precaution was observed, lest the height of a single arcade, if extended as far as the situation required, might render the structure less firm and permanent. They are commonly of bricks; which by their cement cohere so strongly, that the parts are not separated without the utmost difficulty.—When the elevations of the ground were enormous, it became necessary to form subterraneous aquæducts. These carried the waters to such aquæducts as were raised above ground, in the declivity or at the foot of mountains. If the artificial channel of the water was not susceptible of a downward bias but by passing through a rock, through this they cut a passage at the same height with the superior aquæduct: such an one may be seen above the city of Tivoli, and at the place called Vicavaro. The canal which formed the course of the aquæduct is hewn out of the rock to the extent of more than a mile, about five feet in height, and four in breadth.
There is one thing, however, which deserves to be remarked. It is, that these aquæducts, which might have been directed in a straight line to the city, did not arrive at it but by frequent and winding mazes. Some have said that this oblique tract was pursued to avoid the expence which must attend the building of arcades to an extraordinary height: others, that it was their intention to diminish the impetuosity of the current; which, rolling in a straight line through an immense space, must always have increased its velocity, must have worn the canals by perpetual and forcible attrition, and of consequence afforded an impure and unwholesome draught to the inhabitants. But since there was so great a descent between the cascade of Tivoli and Rome, it is demanded why they should go to draw water from the same river at the distance of more than 20 miles higher; nay, of more than 30 miles, if we reckon the curvatures of its direction through that mountainous country. It is replied, the motive of obtaining the water more salubrious, and more limpid, was sufficient to make the Romans think their labour necessary, and their expence properly bestowed; and to those who reflect that the waters of this river were impregnated with mineral particles, and by no means wholesome, the answer will appear satisfactory.
If any one will cast his eyes upon plate 128th of the Antiquities of Father Montfaucon, he will see with how much care these immense works were constructed. From distance to distance spiramenta were left, that, if
the water should happen to be stopped by any accident, it might gradually disembogue, till they could clear its ordinary passage. There were likewise, even in the very canals which conveyed the water, cavities considerably deeper than its internal surface, into which the stream was precipitated, and where it remained stagnant till it was refined from mud and feculence; and ponds, where it might expand itself till it was purified.
The aquaduct of the aqua Marcia had an arch of 16 feet in diameter. The whole was composed of three different kinds of stone; one of them redish, another brown, and a third of an earth colour. Above, there appeared two canals; of which the highest was fed by the new waters of the Tiverone, and the lower by what they call the Claudian river. The entire edifice is 70 Roman feet high. Near this aquaduct, we have in Father Montfaucon the plan of another with three canals; the highest supplied by the water called Julia, that in the middle from Tepula, and the lowest from the aqua Marcia.
The arch of the aquaduct of the aqua Claudia is of hewn stone, very beautiful; that of the aquaduct of the aqua Neronia is of bricks: they are each of them 72 Roman feet in height.
The canal of the aquaduct which was called the aqua Appia, deserves to be mentioned for a singularity which is observed in it: for it is not, like the others, plain, nor gradual in its descent; but much narrower at the lower than the higher end.
The consul Frontinus, who superintended the aquaducts under the emperor Nerva, mentions nine of them which had each 13594 pipes of an inch in diameter. Vigerus observes, that, in the space of 24 hours, Rome received 500,000 hogheads of water.
We might likewise have mentioned the aquaduct of Drusus, and that of Riminus: but we shall satisfy ourselves with observing here, that Augustus caused all the aquaducts to be repaired; and afterwards passed to other monuments of the same kind, and still more important, which give the most striking ideas of Roman magnificence.
One of these monuments is the aquaduct of Metz, of which a great number of arcades still remain. These arcades crossed the Moselle, a river which is broad and vast at that place. The copious sources of Gorze furnished water for the representation of a sea-fight. This water was collected in a reservoir: from thence it was conducted by subterraneous canals formed of hewn stone, and so spacious that a man could walk erect in them: it traversed the Moselle upon its superb and lofty arcades, which may still be seen at the distance of two leagues from Metz; so nicely wrought and so firmly cemented, that, except those parts in the middle which have been carried away by the ice, they have resisted, and will still resist, the severest shocks of the most violent seasons. From these arcades, other aquaducts conveyed the water to the baths, and to the place where the naval engagement was mimicked.
If we may trust Colmenarius, the aquaduct of Segovia may be compared with the most admired labours of antiquity. There still remain 159 arcades, wholly consisting of stones enormously large, and joined without mortar. These arcades, with what remains of the edifice, are 102 feet high; there are two ranges of arcades, one above another. The aquaduct flows thro'
the city, and runs beneath the greatest number of houses which are at the lower end.
After these exorbitant structures, we may be in some degree believed when we speak of the aquaduct which Lewis XIV. caused to be built near Maintenon, for carrying water from the river Bueq to Versailles: it is perhaps the greatest aquaduct which now subsists in the world; it is 7000 fathoms in length, above 2560 in height, and contains 242 arcades.