Popular Science Monthly/Volume 11/May 1877/Aqueducts
| AQUEDUCTS. |
THE remains of the lofty arcades upon which the aqueducts of ancient Rome were carried to the city have been justly classed among the finest and most picturesque ruins of the Roman Empire. Stretching across the plain eastward of the city, and towering high above the landscape, they are the first objects to fix the gaze and command the admiration of the stranger approaching the home of the Cæsars, and to fill his mind with visions of the strength and grandeur of the nation which mastered the world two thousand years ago. But these ruins speak not only of the mechanical skill and physical greatness of that vanished people, but also of their refinement and their acquaintance with the deeply-hidden laws of hygiene; for they well knew what has become known to us only after a lapse of twenty centuries, after the measurement of the heavens, and the discovery of the steam-engine, that for every large city an abundant supply of pure, fresh water is indispensable to the preservation of health. At the zenith of her grandeur, Rome had eleven distinct aqueducts, whose aggregate discharge was equivalent to a stream twenty feet wide by six deep, with a fall six times as rapid as that of the river Thames, The daily supply was in the proportion of 332 gallons to each inhabitant, and it was distributed to the palaces and humbler dwellings in every part of the city, as well as to innumerable fountains, many public wells and large reservoirs, to the numerous baths, and to several artificial lakes, where the emperors held their naumachiæ, or sham naval battles. These eleven constituted the most extensive and perfect system of aqueducts that has been possessed by any city even up to the present time. Their combined length was over 300 miles, 50 of which were above-ground either upon low substructures or more imposing arcades. The loftiest arcade was that belonging to the Aqua Claudia and the Anio Novus; it was in one place 109 feet high.[1] In respect to height of arcades, however, the aqueducts of Rome were less remarkable than several built by the emperors, about the same time, for certain provincial cities of the empire, and others of more recent times. Thus the Emperor Agrippa built an aqueduct for the city of Nemausus (Nimes) in France, and carried it across the river Gard upon an arcade 180 feet high, and about 900 feet long. This splendid structure, still perfect, is now called the Pont du Gard, and is an object of attraction and astonishment to modern travelers. It consists of a triple row of arches, which in the two lower tiers are of wide span, and in the upper one narrow. This arcade "has no rival for lightness and boldness of design among the existing remains of works of this class carried out by the ancient Romans." It is constructed entirely of freestone, to the covering of the upper row of arches. The stones were laid without cement, and each was raised by the lewis, the holes in which it was inserted being still visible exactly over the centre of gravity in every stone. Still more remarkable for height is one of the bridges of the aqueduct of Antioch, also built by the Romans. It is 700 feet long and 200 feet high. The lower part consists of a solid wall pierced by two arches, in the centre—one upon the ground, the other directly above. Along the top is a row of narrow arches. The design and workmanship of this structure are very rude. But in later times arcades of even greater height have been built. The arcade Delle Torri, near Spoleto, built in the seventh or eighth century a.d., is about 300 feet high and over 700 long. It consists of ten arches between lofty columns, and is remarkable as an early example of the pointed arch, as well as for lightness of design. The arcade of the Roquefavour Aqueduct across the river Arc is 262 feet high, and 1,287 feet long. This aqueduct supplies the city of Marseilles with water from the river Durance, 51 miles distant. It was constructed between 1839 and 1847, and has eight and a half miles of tunnels passing through three chains of limestone mountains. But the most imposing arcade in the world, as regards the combined effect of height and length, is that of Maintenon. It is about five-sixths of a mile long, and over 200 feet high. Louis XIV. built it for an aqueduct he projected to convey the water of the Eure from Pont Gouin to Versailles, a distance of about 33 miles. This great enterprise was abandoned in 1688, after an expenditure of four years and 22,000,000 francs. The design contemplated one arcade over three miles long, which in its highest part was to have been formed of three tiers of arches.
At the time that it was built, the Anio Novus, probably of all aqueducts in the world, drew its water from the most distant source. True, the conduit of the Aqua Marcia, one of the most important of the aqueducts of Rome, was longer, but its source was only 39 miles from the city, while that of the Anio was 42; the conduit of the one was 61 miles and 710 paces long, of the other 58 miles and 700 paces. There are at Carthage the remains of an aqueduct which is said to have been over 50 miles long, but it is impossible to tell whether it was built by the Carthaginians proper, or by the Romans who, long after the destruction of the old city, founded a new one on its ruins. The accompanying cut represents the remains, near Undena, of one of the arcades of this aqueduct. It comprised 1,000 arches, many of which were over 100 feet high. The ancient Peruvians are said to have built the most remarkable aqueducts in the world for length. Garcilasso speaks of one that was 360 miles long, and another 450, but these were for irrigating purposes, and they wound around the mountains and followed the surface of the valleys instead of crossing them on arcades, and therefore differed essentially from the aqueducts that we have been considering. And, besides, the statements as to their length should not be received without caution, for, at the time that the Spaniards first visited the country, their belief in the marvelous had been very greatly enlarged by the discovery of a new world.
Fig. 1.—The Aqueducts. The Crossing in the Campagna near the Piscinæ and Roma Vecchia.
The longest aqueduct proper is that now building to convey the waters of the Somme-Soude, Soudon, and Dhuis, to Paris. It will be about 110 English miles long. The aqueduct of Roquefavour, already referred to, is 60 miles long, the longest in actual use.
The Romans appear to have got their knowledge of aqueduct building, like most of their other knowledge, from the Greeks; for, while their first aqueduct, the Aqua Appia, was not constructed until 441 years after the building of the city, or 312 b. c, the Greeks had built aqueducts at Megara and Samos as early as 625 b. c, and at Athens in 560 b. c. But there is this difference, that the Greeks did not use arcades, which, however, were not rendered necessary by the topography of the country. At Samos, a tunnel four-fifths of a mile long, eight feet high and eight wide, was cut through a hill between the city and the water-source. A channel three feet wide was built within the tunnel, and an opening of the same width made to the surface from end to end, so that the fresh air came in contact with the water, which flowed into a conduit of masonry at the lower end, and thence directly to the baths, fountains, etc., of the city. This work was constructed by Eupalinus, who had previously gained celebrity by building the aqueduct at Megara. At Athens the water-supply was drawn by subterranean conduits from Mounts Hymettus, Pentelicus, and Parnes, and received into reservoirs outside the city. Two conduits came from Mount Hymettus, and passed under the bed of the river Ilissus. Of course, it was necessary to supply fresh air to the water flowing through these subterranean channels, and that was done by piercing them with shafts at intervals of about fifty yards. Subterranean channels were also used to distribute the water through the city; they were of different forms, being round or square, and in some of them pipes of baked clay were laid. It is somewhat remarkable that these beneficent works were constructed by the wisdom of rulers who have come down to us branded as tyrants. The tyrants Theagenes of Megara, Polycrates of Samos, and Pisistratus of Athens, were the men who caused them to be built. Some of those old aqueducts still continue to supply Athens with water. The aqueduct of Syracuse which still supplies the city with an abundance of water, and which is remarkable for having a tunnel under the sea, between the city and the mainland, was built some time prior to the Athenian invasion, 412 B. C. for Thucydides mentions that it was partially destroyed by the invaders. But far more ancient than any yet referred to is the one at Jerusalem, built by Solomon, to conduct the water from the reservoirs, or "pools," that bear his name, to the city, a distance of six miles. It was formed by an earthen pipe ten inches in diameter, incased in stone and laid underground. It is still in use.
The periodical overflow of the Nile, the Tigris, and Euphrates, enabled the peoples of Egypt and Babylonia to store up vast quantities of water in artificial lakes, of which the Mœris in Egypt is a celebrated example, and the water was utilized as required, by surface-conduits or canals.
Let us now turn back to the aqueducts of Rome, and examine somewhat the details of construction. A recently-published work on the aqueducts comprehended in the archæology of Rome, by John Henry Parker, C. B., affords much interesting information in this connection. The facts are ascertained partly from the work of Sextus Julius Frontinus, who was superintendent of the aqueducts (curator aquarum) under the Emperors Nerva and Trajan (A. D. 94–107), and partly from explorations of the courses and remains of the aqueducts made by Mr. Parker himself. Of the eleven aqueducts already referred to, ten approached the city from the east and one from the west. Of the ten on the east, four had their sources near Subiaco, in a spur of the Apennines beyond Tivoli; the others took their rise in the lower lands nearer Rome. Two of these, the Anio Vetus and the Anio Novus, were fed by the river Anio, as is indicated by their names; the others received their waters from springs or small lakes, and were called after their builders or projectors. The waters of the Marcian, the most prized for their purity and coldness, were collected from several springs. For the Anio Novus, which was unfailing as well as the most abundant of the aqueducts, the river Anio was arrested near its source by three gigantic walls at different levels, and formed into as many lakes, one below the other. Over these walls the waste-water fell in magnificent cascades, one of them over 150 feet high. The object of the lakes was to clarify the water; for the Anio, though usually a limpid stream, is liable to become muddy after a heavy rain. The sources of the Anio Novus and the Aqua Claudia are over 2,000 feet above the level of the city, and those of the Marcia and Anio Vetus are not very much lower. Descending from such a height and for distances varying in direct lines from 30 to 43 miles, the water would naturally acquire great velocity and tremendous force, which it was necessary to diminish, and that was done by making numerous angles in the conduits. The angles were made generally at every half-mile, and were points at which reservoirs (castella), or filtering-places (piscinœ), or both, with accompanying air-shafts, were built. These were surmounted by small towers. As an additional means of breaking the force of the water, the bottoms of the conduits were given a succession of short undulations. The conduits, reservoirs, and filtering-places, were lined with a cement called opus signinum, which is so compact that it will resist a hard tool. The art of making it has been lost. The conduits, always covered, were carried on arcades only where it was necessary to cross a valley or a plain above its level; for the rest of their way they ran in places upon the surface of the ground, but mostly below it. Thus of the 58 miles of the Anio Novus, 49 were underground. No two aqueducts were on the same level, and so, where their courses converged, it was both possible and convenient to carry one conduit upon another, because it was forbidden by law to erect a building within a certain number of feet on either side of an aqueduct; hence we find the Aquæ Marcia, Tepula, and Julia, carried from their point of convergence one above the other on one arcade, and the Aqua Claudia and Anio Novus on another. Each of the conduits was differently shaped, some having arched, others angular roofs. Besides the small reservoirs referred to as occurring at the angles of the conduits, there were larger ones at longer intervals. The ruins of one of these, belonging to the Aqua Marcia, are still to be seen near Carciano. It is a huge subterranean chamber divided by an arcade in the middle. Between five and seven miles from Rome were the great filtering-places to which most of the aqueducts converged. The waters, however, were not mingled, for each aqueduct had its separate chambers, though it was always within the power of the attendants (aquarii) to turn the water from one aqueduct into another at will. Of these filtering-places, those of the Claudia and Anio Novus were under-ground, and now appear simply as mounds. The others were above-ground, but covered over. From this point two magnificent arcades, the Marcian and the Claudian, extended to the city—the one carrying three aqueducts, the other two. They were not more than 100 yards apart, and the Marcian was 30 feet high, the Claudian 50. The filtering-places were of peculiar construction and admirable design. They consisted of four chambers, two on a level with the conduit, and two directly below (Fig. 2). The water flowing into the first descended
Fig. 2.—Section of the Piscina of the Anio Novus, at the Entrance into Rome in a Tower of the Wall of Aurelian and of the Gardens of the Sessorian Palace.
through an opening in the floor to the second, whence it flowed on through a perforated wall or grating to the third, ascending from that through an opening in the roof to the fourth, where it found its original level and reentered the conduit. A stairway descending to an opening afforded access into the chambers beneath, and by the assistance of sluice-gates the water could be turned directly from the first chamber into the fourth, so that the mud could be cleaned out of the chambers below. It is remarkable that this ingenious device for filtering has not occurred to modern aqueduct-builders, for its simplicity and utility are conspicuous.
The details of distribution are interesting, but we have not space to go far into them. There were 247 main reservoirs in the city, from which the water was distributed to 19 barracks for the use of the army, 95 public establishments, 39 theatres and places of amusement, and 591 open reservoirs for the public. That was in the time of Frontinus. The number of open reservoirs was afterward increased. Heavy penalties were inflicted for dipping a dirty vessel into one of these reservoirs. Of the total supply, a little over one-third was given to the public, and the remainder divided pretty evenly between private and imperial purposes. The wealthy had water brought into reservoirs within the courts of their residences, whence it was raised to the upper stories in buckets worked by windlasses. This method of supplying the upper stories is in use at the present time. The Romans had no pumps. Why the water was not conveyed upward in pipes does not appear, except that in regard to the more elevated parts of the city it was probably not brought in at a high enough level. They possessed lead pipes of different sizes, and stopcocks of bronze and silver, for these have been found in various places; and that they were perfectly familiar with the principle of hydraulics, that water may be returned to its original level, is proved not only by the construction of the filtering-places already described, but also by the fact that they actually applied the principle on a stupendous scale. Besides, there is a work of Vitruvius extant which recognizes and gives directions for conveying water on this principle. An aqueduct constructed by the Emperor Claudius, for the ancient city of Lugdunum (now Lyons), possessed two inverted siphons, by which the water was carried across deep valleys. There is no doubt that they were acquainted, too, with the poisonous action of lead on water; but, if that deterred them from raising the water, it shows they were more careful in guarding against unhealthful influences than we moderns are, for lead pipes are in general use to distribute water through our houses to-day.
The aqueducts were placed under the care of a curator aquarum, and afterward, in the time of Diocletian, under several magistrates, called consulares aquarum. The actual attendants numbered 700, and were divided into the familia publica and the familia Cæsaris. The former, 240 in number, were paid by the state; the latter, 460, by the emperor. With regard to the cost of building the aqueducts, it seems to have been defrayed, in the majority of cases, out of government funds; but it is recorded in an inscription on the Porta Maggiore, a gate of the city over which the conduits of the Claudia and Anio Nevus were carried, that those two aqueducts were built by the Emperor Claudius at his own expense. This gate affords a clew to the reason why arcades instead of solid walls were used to carry the aqueducts across the plains: it was not solely for economy's sake, nor for beauty's; but while those considerations, no doubt, were entertained, the main object was, to avoid interference with the freedom of travel.
The aqueducts were all destroyed in the Gothic wars under Vitiges and Totila, but the most important of them were restored either by Belisarius or Narses. These, however, fell gradually into decay, and ultimately became useless. Pope Paul III. (1540) restored to use the aqueduct on the west side of Rome; and Sixtus V. (1585) restored the aqueduct of Trajan by mistake for the Marcian. These two, the former called Paola, and the latter Felice, continued to be the only means of supply until 1870, when the real Marcian was restored by a company of Englishmen and Romans. The water is brought as far as Tivoli in a stone conduit, and the rest of the way in cast-iron pipes. It has sufficient pressure to supply the tops of all the houses.
Fig. 3.—Reservoir of Aqua Marcia. (Interior.)
Reference has been made to the aqueduct now building for Paris. The supply of water required is 22,000,000 gallons per day, and the aqueduct was designed to convey that quantity. It is intended for household use only, the existing supply, which is abundant for other purposes, having become somewhat polluted. It was calculated that the aggregate yield of the three rivers which are to be turned to account would be 28,000,000 gallons per day; but subsequent observation has shown that in very dry seasons it falls considerably below the quantity required. It was therefore found necessary to sink wells or shafts into the chalky formation in which the rivers have their rise, to artificially increase the supply. The plan comprises conduits for collecting the waters from the several sources called "conduits of derivation," and a main aqueduct to which these converge. The former are together about 50 English miles long, and the latter is 110. The main aqueduct has a fall of 73-2/3 feet, and terminates in service-reservoirs at Belleville 83-2/3 feet above the level of the city. Along the course of the main and subsidiary conduits are 17 bridges, 3¾ miles of arcade, 4⅓ miles of siphon, and 17⅓ miles of tunnel. The work is done under the direction and supervision and in accordance with the designs of M. Belgrand. The total estimated cost, including $900,000 paid for injury to riparian rights, is $5,200,000.
The aqueduct which supplies the city of Manchester (England) with water is remarkable for its system of impounding reservoirs, comprising seven, with dams varying from 70 to 100 feet in height. The work was begun in 1848, and had not been completed in 1874, although it was far enough advanced to supply the city with water. The city of Glasgow is supplied by the Loch Katrine Aqueduct, 35 miles long, which conveys the water of the famous lake of that name. It consists of a conduit of masonry 8 feet deep, 8 wide, and 27 miles long, and two lines of cast-iron pipes, between the city and the service-reservoirs, 8 miles long. The conduit between Loch Katrine and the service-reservoirs is for the most part a tunmnel through solid rock. It crosses some ravines on stone or iron arcades, and others by siphons. It is capable of discharging 50,000,000 gallons per day. It was opened by the queen with appropriate ceremonies in October, 1859. The work was begun in 1855, and finished in 1860. The cost, exclusive of facilities for distribution, was $3,340,000.
In the autumn of 1873 was finished the aqueduct designed by Herr Carl Junker, of Vienna, and constructed by Mr. Antonio Gabrielli, of London, to convey the water of two springs (the Kaiserbrunn and Stixenstein), situated at the foot of the Styrian Alps, to Vienna, a distance of 56½ miles. The conduit, which varies in size from 4½ x 2½ feet to 6½ x 4 feet, and is faced with polished cement, to facilitate the flow of the water, is always six feet below the surface of the earth or embankment through which it is carried. The object aimed at is to keep the water cold in summer and from freezing in winter. It has several splendid arcades, chief among which are one at Baden, another at Mödling, and a third at Liesing. The former is 96 feet high, about 2,000 feet long, and comprises 43 arches. The aqueduct delivers about 20,000,000 gallons a day. It was begun in 1869, and its cost was $10,000,000.
But, in regard to water-supply, the Roquefavour Aqueduct, referred to previously, is by long odds the most remarkable. The conduit is 7 feet deep, 30 wide at the top, and 10 at the bottom. It discharges 11 tons of water per second, or about 285,000,000 gallons per day. The water is used for the city of Marseilles, and to irrigate 25,000 acres around it.
In our own country there are several noted aqueducts—as the Cochituate at Boston, the Washington Aqueduct, and Croton at New York. The method employed by the cities of Chicago and Milwaukee to obtain their water-supply is unique. The water of Lake Michigan is brought into the city by a tunnel from a sufficient distance off to insure its being pure, and is then pumped up into service-reservoirs, whence it is distributed in the usual manner. The Chicago tunnel is three miles long, that of Milwaukee is shorter.
The Washington Aqueduct leads from a reservoir which impounds the river-water at the Potomac Falls, is 16 miles long, and supplies the cities of Washington and Georgetown. Its capacity is 70,000,000 gallons per day. The water is conveyed in a brick-and-rubble masonry conduit, of circular form, to the service-reservoir five miles from the city, and the rest of the way in three large cast-iron pipes capable of delivering 30,000,000 gallons per day. This aqueduct was constructed at the expense of the United States Government, and cost $3,000,000. It has several fine bridges, of which the most notable is the one across Cabin John Creek. This is a single granite arch, 100 feet high and 220 long. Another remarkable example of the wide, single arch occurs on the Lisbon Aqueduct, finished in 1738. It is 115 feet wide and 250 high.
By far the finest aqueduct in America is the Croton. This was begun in 1837, and finished in 1842, at a cost of $8,575,000, without the means of distribution, which cost $1,800,000 more. The length of conduit from the impounding to the receiving-reservoirs in Central Park is 38¼ miles, for 33 of which the conduit is built of stone, brick, and cement, arched above and below, 8 feet 5 inches high, 6¼ feet wide at the bottom, and 7⅔ at the top. The water crosses Harlem River in two cast-iron pipes 3 feet in diameter, and one wrought-iron pipe 7 feet 6 inches in diameter, laid upon High Bridge, a magnificent granite arcade 1,460 feet long and 114 high. It comprises 15 arches, 7 of which have 50 feet span, and 8, those over the river, 80 feet. The fall is 1.10 foot per mile, the velocity of the water 1½ mile an hour, and the possible discharge 115,000,000 gallons per day.
For the first six years after the completion of the aqueduct, the quantity of water used was only 18,000,000 gallons per day, but it has now increased to over 88,000,000. The supply is drawn from Croton River, a small stream that flows into the Hudson, a short distance above Sing Sing. The river was arrested by a dam 40 feet high, and made to form what is now called Croton Lake. The mouth of the aqueduct is 12 feet below the surface of the lake, whereby it is protected from freezing up in winter, and the water is obtained pure and cool in summer. The lake has an area of 400 acres, and usually affords a daily supply of 50,000,000 gallons; but this fell off, during a severe drought seven or eight years ago, to only 27,000,000, and since then another source has been added by damming up the western branch of Croton River. The receiving-reservoirs, two in number, are located in Central Park: the "old" covers 35 acres, and holds 150,000,000 gallons; the "new" covers 100 acres, and holds 1,030,000,000. The distributing-reservoir is situated 2¼ miles farther down, between Fifth and Sixth Avenues, Fortieth and Forty-second Streets. Its walls are 45 feet high, and they inclose a little more than 4 acres. The water is brought down in five lines of iron pipe, two of which are 30 inches in diameter, two three feet, and one four feet. The distributing-pipes, ramifying throughout the city, are about 340 miles long. The "mains" are laid near the sidewalks on either side of the streets, and at every crossing are provided with branches for supplying the adjacent buildings. These branches are provided with stopcocks for turning off the water when necessary. The higher parts of the city lying north of Manhattan Valley are supplied from a tower and reservoir recently built on high ground near One Hundred and Seventy-third Street and Tenth Avenue, to which the water is raised by powerful pumps. The reader will have been struck with the similarity between this aqueduct and those of ancient Rome; it remains to be shown that there is one other point of resemblance, in the air-shafts that are built at intervals of a mile. They rise 14 feet above the ground, and, like the old Roman ones, are in the form of towers. Every third one is provided with a door and way of access into the conduit. But the conduit is without the filtering-places and the angles. The conduit does, indeed, make several curves of 500 feet radius, but these are for changing the course of the aqueduct to avoid obstacles, instead of for breaking the force of the water, which in fact is unnecessary, the inclination being, as already shown, insignificant. The level of Croton Lake is about 115 feet above that of Manhattan Valley, and when the old reservoir in Central Park was yet building, the citizens of New York were afforded the magnificent spectacle of a vertical column of water shooting up over 100 feet from the bottom of the valley.
In connection with our subject, though not strictly belonging to it, may be mentioned the fact that canals are in many places carried across valleys and rivers upon bridges. Examples have long existed on the Languedoc Canal in France. The first in England was the Barton Bridge, which carries a canal across the river Irwell 39 feet above the surface. It was constructed by Brindley, for the Duke of Bridgewater. Says a contemporary English writer: "It was commenced in September, 1760; and in July of the following year the spectacle was first presented, in this country, of vessels floating and sailing across the course of the river, while others in the river itself were passing under them." The Lancaster Canal has one of five arches of 72 feet span each, and 65 feet high, across the river Lune. Later and more celebrated examples, though, are those of Pont-y-Cysylte and Chirk in Wales. The former, constructed by Mr. Telford, "is justly celebrated for its magnitude, simplicity of desgin, and skillful disposition of parts, combining lightness with strength in a degree seldom attempted. It consists of cast-iron arches resting on pillars of stone; the length is 1,000 feet, the number of arches 19, and the height 126 feet." In this country these bridges are numerous, there being no less than 32 on the Erie Canal. The finest of them are two across the Mohawk River, a third at Richmond over the Seneca River, and a fourth across the Genesee at Rochester. The latter is a splendid stone arcade 920 feet long, having six cut-stone arches of 52 feet span. A wire suspension-bridge of seven spans, each 160 feet long, conveys the Pennsylvania Canal across the Alleghany River at Pittsburg.
- ↑ The Roman foot was 11.6496 English inches; 5 feet made one passus; 1,000 passus one mile, or 1,618 English yards.