a conduit or channel for the conveyance of water. It is derived from aqua, water, and ductus, a conduit. It is applied more particularly to those structures of masonry which have been erected for the conveyance of water across valleys, to which, however, we would rather give the name of aqueduct bridges, extending the term aqueduct to the whole conduit or channel by which the water is conveyed from one place to another. The conveyance of water for the supply of large cities has in all ages formed a very important object of public economy; and aqueducts of various kinds have been in use for this purpose from the earliest times, the remains of which have been examined by travellers in different parts of the East. Pococke describes a work of this kind erected by Solomon, for conveying water from the pools and fountains near Bethlehem to Jerusalem. "The aqueduct," he says, "is built on a foundation of stone; the water runs in round earthen pipes about 10 inches diameter, which are cased with two stones, hewn out so as to fit them, and they are covered over with rough stones well cemented together; and the whole is so sunk into the ground on the side of the hills, that in many places nothing is to be seen of it." But it was in the luxurious capital of Rome that the system of aqueducts was brought to the greatest perfection, and carried to an extent which has never been equalled even in modern times, and has justly excited admiration both from the number and magnificence of the works themselves, and the prodigious quantities of water which by these means were continually poured into the city. These aqueducts extended, some of them 30, 40, and even 60 miles from the city, in one continued covered channel of stone; carried by arcades over the widest and deepest valleys, and by tunnels running in many parts for miles through mountains and through the solid rock. "If we consider attentively," says Pliny, "the quantities of water brought into the city for the use of the public, for baths, for fish-ponds, for private houses, for artificial lakes, for gardens in the neighbourhood of the city, and for villas; if we look also at the works which have been constructed for forming a regular channel for the waters—arches raised up, mountains pierced with tunnels, and valleys filled up to a level; it must be acknowledged that there is nothing in the whole world more wonderful."

For about 400 years after the building of the city the Romans were contented with the waters of the Tiber, or what was drawn from wells or from fountains in the city and its neighbourhood. But the great increase of the population rendering a more ample supply desirable, the censor Appius Claudius was the first to introduce an aqueduct to convey the waters of distant springs into the city. About thirty-nine years after this, M. Curius Dentatus brought in an additional supply from the neighbourhood of Tibur. These examples were afterwards followed by various other public men, as the wants of the city rendered new supplies necessary. Among these were Papirius, Crassus, Marcus, Agrippa, and Augustus; and most of the succeeding emperors, even Tiberius, Claudius, Caligula, Nero, and Caracalla, esteemed it an honour to connect their names with such great and useful works.

Frontinus, who was appointed curator of the aqueducts by the emperor Nerva, has left the most ample account of them. According to him, there were nine great aqueducts by which the city was supplied. Five more were added by Nerva, and the number was afterwards augmented by succeeding emperors to twenty. Of these, the most remarkable were the Aqua Appia; the Old and New Anio; the Aqua Martia, which also conveyed the Aqua Julia and the Aqua Tepula; the Aqua Virginia; and the Aqua Claudia. The Aqua Appia was so named from the censor Appius Claudius, by whom it was constructed in the 442d year of Rome. It commenced in a field near the Via Praenestina, between the 6th and 8th mile-stones, made a circuit of 780 paces to the left, and then proceeding by a deep subterranean channel of more than 11 miles, entered the city at the Appian Way by the Porta Capena, and delivered the main body of its waters into the Campus Martius. The Old and New Anio were so called from their bringing into Rome the waters of that river. The former began above the Tiber, at the 30th mile-stone, and consisted mostly of a winding channel, carried through an extent of about 43 miles. The latter, constructed under Nero, took a higher level, running 7543 paces above ground, and then through a subterranean passage of 54,297 paces in length. The Aqua Martia, which owed its formation to Quintus Martius, rose from a spring distant 33 miles from Rome, made a circuit of three miles, and afterwards forming a vault of 16 feet diameter, it ran 38 miles along a series of arcades at an elevation of 70 feet. It had openings perforated at certain distances for discharging the collected air, and at different places deep cisterns, in which the water settled and deposited its sediment. On this account it was remarkable for its clear green colour, and is celebrated by Pliny for its coolness and salubrity. The Aqua Julia and the Aqua Tepula were brought to Rome by the same aqueduct as the Aqua Martia, but on higher levels. The whole aqueduct above the arcades was divided into three stories or channels. In the uppermost flowed the Aqua Julia, in the second the Aqua Tepula, and in the lowest the Aqua Martia. From the ruins of this combined fabric, which still subsist, it appears to have been a very superb structure. The Aqua Virginia was constructed by Agrippa, who laboured to improve and beautify Rome, and who, according to Pliny, formed in one year 70 pools, 105 fountains, and 130 reservoirs. It commenced at a very copious spring, in the middle of a marsh, at the distance of eight miles from the city, and ran about 12 miles, passing through a tunnel of 800 paces in length. The Aqua Claudia, begun by Nero and completed by Claudius, took its rise 38 miles from Rome. It formed a subterranean stream 36½ miles in length, ran 10¾ miles along the surface of the ground, was vaulted for the space of three miles, and supported on arcades through the extent of seven miles, being carried along as high a level as to supply all the hills of Rome. It was built of hewn stone, and still continues to furnish the modern city with water of the best quality, which has hence procured it the name of Aqua Felice.

In all these aqueducts the channel for the waters was carried with a regular declivity from the one end to the Aqueducts other than the aqueduct, and such as was sufficient to carry the water easily along. In some cases the declivity had the line been carried straight forward, would have been too great; it was therefore made to take a circuitous route, winding along the sides of the hills, and prolonging the length of the channel so as to reduce the degree of descent, and cause the water to run gently along. The whole of the channel was regularly built of stone or brick, and arched above to cover in the channel, excepting in those places where it was cut out of the solid rock. Along all the valleys and water-courses which lay in the way it was elevated by a series of arches, all raised to the level of the conduit, resting on massive pillars, and all built in the most solid and substantial manner, with brick, and often with hewn stone. The sketches of Roman aqueducts in Plate XLVIII., taken from Fabretti, will give a better idea than any description, of the manner in which the work was executed. Sometimes there was only a single arch, as in the two middle figures; sometimes, again, where the conduit was to be elevated higher, as in the right-hand figure, a double row of arches was raised, one above the other, for greater strength and security. The figure on the left hand shows two conduits or aqueducts carried in different levels along the same building. The upper one is the New Anio, the lower is the Aqua Claudia. When we consider the labour and difficulties attending the construction of such arches and arched channels of masonry, the spirit and enterprise which could have undertaken such works as are above enumerated, undaunted by the expense, or any of the other obstacles which lay in the way, appear astonishing. In this country, where bridges, canals, and other water-works have been carried to a great extent and perfection, we consider an aqueduct of six or eight arches a work of no small extent and importance. What would we think, then, of the aqueduct of the New Anio, extending 6½ miles in one continued series of arches, many of them upwards of 100 feet high! If we allow a similar number of arches in the length, we shall have in all more than 600; and yet this is nothing compared with the aqueduct of the Aqua Martia, extending 38 miles, and containing in all nearly 7000 arches. Even an aqueduct bridge of five or six miles in length appears incredible; and yet how can we otherwise translate Frontinus, where he states the lengths of all the aqueducts, and how much was above or under ground, and how much was built in arches? Of the New Anio, for instance, he says its conduit was 63 miles 700 paces in length. Of this 49 miles 200 paces consisted of a subterranean stream, and 9 miles 400 paces were above ground, of which last the higher part consisted of "substructionibus aut opere arcuato," in several places of great length; and nearer the city, at the 7th mile-stone, consisted of "substructione" 609 paces, and "opere arcuato" 6 miles 491 paces; and he adds, "These arches are the highest of any, being raised in some parts 109 feet." The term substructio probably means a conduit built by opening up the surface of the ground and then covering over the building with earth, as we do in such works at this day. But the term opere arcuato can only refer to a continued series of arches, and certainly conveys a vast idea of the extent and magnitude of such works.

The total quantity of water delivered into Rome by these aqueducts was altogether astonishing, and quite beyond what we have any conception of now, for either comfort or luxury. Strabo said truly, that whole rivers flowed through the streets of Rome. According to Frontinus, the nine earlier aqueducts delivered daily 14'018 quinaria, which corresponds to nearly 28 millions of cubic feet; and when all the aqueducts were in operation, the quantity must have amounted to 50,000,000 cubic feet; which, reckoning the population of the city at that time at a million, would give 50 feet daily, or 7 hogsheads to each individual. This is more than ten times the supply of London, which is now reckoned to be quite profuse.

Of the modern aqueducts in Rome the principal are the Aqua Felice, the Aqua Virginia, and the Aqua Paulina. The first was constructed by Sixtus V. It commences at Palestrina, about twenty-two miles from the city, and discharges itself at the Fontana di Termini. The Aqua Paulina was repaired by Pope Paul V. in the year 1612. It divides itself into two principal channels, one of which supplies Mont Janiculum, and the other the Vatican and its neighbourhood. It is conveyed from the district of Bracciano, about twenty miles distant; and three of its five streams are not inferior to small rivers. According to the calculation of Prouy, these three aqueducts, with some additional sources, deliver in twenty-four hours 5,305,000 cubic feet. This, among a population of 130,000, gives about 40 cubic feet for each individual, which is nearly equal in proportion to the supply of ancient Rome in the period of its utmost splendour.

But the system of aqueducts was not confined to the capital of Rome. It was gradually extended throughout the provinces of that vast empire; and every city and considerable town had its conduits and aqueducts for supplying it with water, many of which still remain to attest the magnificence with which these works were carried on. In Plates XLVIII. and XLIX. we have given a view of some of these. The first is the remains of one of the principal aqueduct bridges in the aqueduct of Antioch. It is a work of great magnitude and height, but a very rude structure. The lower part consists almost entirely of solid wall, and the upper part of a series of arches with very massive pillars. It appears, however, to be a Roman work; and there are remains of another aqueduct on a lower level, and of an older date. The water to Antioch was brought from a distance of four or five miles, from a place called Battelma, which Pococke thinks was the very spot where Daphne stood. Several springs, one of which was so large as to turn several mills, were conveyed in channels of hewn stone, and united in one main stream, which was thence conveyed along the surface of the ground in a similar channel. Across all the rivulets and valleys it was raised on arches or aqueduct bridges, some of which are very lofty, and the principal is the one exhibited in the plate, extending upwards of 700 feet in length, and upwards of 200 feet high in the deepest part. From the remains of the aqueduct in the island of Mytilene, represented from Pococke in the same plate, this appears to have been also a magnificent work. It was built of gray marble rusticated. It is much superior in point of skill to the aqueduct of Antioch, the arches being carried in two ranges throughout the building. It extended about 500 feet in length, and was about 70 or 80 feet high at the deepest part.

The aqueduct or aqueduct bridge of Pyrgos, near Constantinople, forms a portion of the extensive hydraulic works with which that capital was supplied with water after it became the seat of empire. They are described by Andreaossy in his voyage to the Black Sea, and account of the Thracian Bosphorus. It is a grand work, very remarkable both in design and execution, and affords a fair specimen of the style of such structures among the Romans in the middle ages. It consists of two branches, one of which only is seen in elevation in the plate; the other stood nearly at right angles to this, and is seen partly on the plan; it was hence called the Bended or Crooked Aqueduct, to distinguish it from another termed the Long Aqueduct, which was situated near the sources of the The branch seen in elevation extends 670 feet in length, and is 106 feet in height at the deepest part. It is composed of three rows of arches, those in each row increasing in width from the bottom to the top—an arrangement very properly introduced with the view of saving materials without diminishing the strength of the work. The two upper rows consisted of arches of semicircles, the lower of Gothic arches; and this circumstance serves to fix the date of the structure, as these last were not introduced until the 10th century. The breadth of the building at the base was 21 feet, and it diminished with a regular batter on each side to the top, where it was only 11 feet. The base also was protected by strong buttresses or counterforts, erected against each of the pillars. The other branch of the aqueduct was 300 feet long, and consisted of 12 semicircular arches.

This aqueduct serves to convey to Constantinople the waters of the valley of Belgrade, one of the principal sources from which the city is supplied. These are situated on the heights of Mount Hæmus, the extremity of the Balkan Mountains, which overhangs the Black Sea. The water rises about 15 miles from the city, and between three and four miles west of the village of Belgrade, in three sources, which run in three deep and very confined valleys. These unite a little below the village, and then are collected into a large reservoir. After flowing a mile or two from this reservoir, the waters are augmented by two other streams, and conveyed by a channel of stone to the Crooked Aqueduct. From this they are conveyed to another, which is the Long Aqueduct; and then, with various accessions, into a third, termed the Aqueduct of Justinian. From this they enter a vaulted conduit, which skirts the hills on the left side of the valley, and crosses a broad valley two miles below the Aqueduct of Justinian, by means of an aqueduct with a double row of arcades of a very beautiful construction. The conduit then proceeds onward in a circuitous route, till it reaches the reservoir of Egri Kapan, situated just without and on the walls of the city. From this they are conducted to the various quarters of the city, and also to the reservoir of St Sophia, which supplies the seraglio of the grand signior. The Long Aqueduct is more imposing by its extent than the Crooked one, but is far inferior in the regularity of design and disposition of the materials. It is evidently a work of the Turks. It consists of two rows of arcades, the lower being 48 in number, and the upper 50. The whole length was about 2200 feet, and the height 80 feet. The Aqueduct of Justinian is a very excellent work, and without doubt one of the finest monuments which remain to us of the middle ages. It consists of two rows of large arcades in the pointed style, with four arches in each. Those of the lower story have 52 feet of span, the upper ones 40 feet. The piers are supported by strong buttresses, and at different heights they have little arches passing through them, which relieve the deadness of the solid pillar. The length of this aqueduct is 720 feet, and the height 109 feet. This aqueduct, though it bears the name of Justinian, was probably erected in the time of Constantine.

Besides the waters of Belgrade, Constantinople was supplied from several other principal sources, one of which took its rise on the heights of the same mountains, three or four miles east of Belgrade. This was conveyed in a similar manner by an arched channel, elevated when it was necessary on aqueduct bridges, till it reached the northern parts of the city. It was in the course of this aqueduct that was constructed the contrivance of the souterasi or hydraulic obelisks described by Andreossy, and which has excited some attention, as being an improvement on the method of conducting water by aqueduct bridges. "The souterasi," says Andreossy, "are masses of masonry, having generally the form of a truncated pyramid or an Egyptian obelisk. To form a conduit with souterasi, we choose sources of water, the level of which is several feet higher than the reservoir by which it is to be distributed over the city. We bring the water from its sources in subterranean canals, slightly declining until we come to the borders of a valley or broken ground. We there raise on each side a souterasi, to which we adapt vertically leaden pipes of determinate diameters, placed parallel to the two opposite sides of the building. These pipes are disjoined at the upper part of the obelisk, which forms a sort of basin, with which the pipes are connected. The one permits the water to rise to the level from whence it had descended; by the other, the water descends from this level to the foot of the souterasi, where it enters another canal under ground, which conducts it to a second and to a third souterasi, where it rises and again descends, as at the last station. Here a reservoir receives it and distributes it in different directions by orifices of which the discharge is known." Again he says, "it requires but little attention to perceive that this system of conducting tubes is nothing but a series of syphons open at their upper part, and communicating with each other. The expense of a conduit by souterasi is estimated at only one fifth of that of an aqueduct with arcades." We really cannot perceive any advantage in these pyramids, further than as they serve the purpose of discharging the air which collects in the pipes. For if the water is to be conveyed in pipes across the valley, what other purpose can these columns possibly serve? They are in themselves an evident obstruction, and the water would flow more freely without any interruption of the kind. In regard to the leaden pipes, again, they would have required, with so little head pressure as is stated, to be used of very extraordinary dimensions to pass the same quantity of water as was discharged along the arched conduits. There is something, therefore, which would require explanation in this account of Andreossy regarding these pyramids, or else he has misunderstood the nature of them when he says that they supply advantageously the place of the aqueducts or arches. A tram of pipes properly laid, and of proper dimensions, might do this; but what advantage the pyramids possess further than to answer the purpose of air-cocks, is not very apparent.

The other principal source from which Constantinople is supplied, is from the high grounds six or eight miles west of the town, from which it is conducted by conduits and arches, in the same manner as the others. The supply drawn from all these sources amounts, according to Andreossy, to 400,000 cubic feet per day; about two thirds of a foot to each person of a population of 600,000. The charge of the water-works at Constantinople belongs to a body of 300 Turks and 100 Albanese Greeks, who form almost an hereditary profession.

Of the aqueducts which still remain as relics of Roman grandeur, the most remarkable are, the aqueduct of Metz; the aqueduct of Nismes, or the Pont du Gard; and the aqueduct of Segovia in Spain. "The aqueducts of Rome," says Montfaucon, "were without doubt wonderful on account of their great length—arcades continued over the space of 40 or 50 miles; their great number, with which the Campagna of Rome was filled on every side: all this surprises us. But it must be confessed that if, without considering the total extent, we only look at any of the parts which remain round Rome, there is nothing that approached the aqueducts of Metz, of Nismes, or of Segovia." The aqueduct of Metz is represented in Plate XLIX. Nearly the half of it, it will be observed, has been carried away; but there still remain a great num- Aqueducts, her of arches, and enough to give an idea of the extent of the whole. It extended across the Moselle, a very considerable river, and very broad in this place; and served to convey the delicious waters of the Gorse to the city of Metz. These waters, according to Meuripe, in his history of the bishops of Metz, printed in 1634, were so abundant that they furnished water for floating the vessels every time that a naval fight was to be exhibited. They were collected into a reservoir, and from thence conducted by subterranean canals constructed of hewn stone, and so spacious that a man could easily walk in them upright. They then passed the Moselle by means of the aqueduct, which was situated about six miles from Metz, and from thence were conducted under ground in stone channels, similar to the others, to the city, to the baths, to the place of the sea-fight, and all over the city. Judging from the drawing, this aqueduct seems to have been nearly 1000 feet in length, the arches 50 feet high at the deepest part, and 50 in number. They formed only one series, the height not requiring a double row. They were so well built and cemented together, that, excepting the middle part, which the descent of ice down the river has in the lapse of ages carried away, they have resisted, and will continue to resist, the effects of time and of the most violent seasons.

The Pont du Gard was executed by the Romans in the reign of Augustus, and was then merely an aqueduct for bringing the waters of the fountain of Hurd to Nismes. It was composed of three rows of arches filling up the valley between two mountains, between which ran the river Gardon. The first row comprehended six arches, each 60 feet span, excepting one, which was the largest, and was 75 feet span; the second row contained 12 arches of the same span as the first; and the third had 36 little arches, on the top of which was the channel for conducting the water. This bridge exhibits a decided improvement and superiority over all the other Roman aqueducts, in the lightness and striking boldness of its design. The arches are wider, and the piers in proportion lighter, than any other structure of the kind previously constructed; and had the same principle been extended so as to have formed only a single row from top to bottom, it would have equalled in the skill and disposition of its materials (circumstances in which the Roman works were almost universally wanting) any of the more judicious and elegant structures of modern times. About the year 1740, this bridge, being of no more use as an aqueduct, was converted into a road-way, by widening it, or rather building in a manner another bridge to the side of it, having all the arches of the same span and dimensions. The execution of the work was attended with considerable difficulties, but these were all successfully overcome by the French engineer Pitot.

The aqueduct of Segovia, according to Culmenares, who travelled in Spain, and has written the history of Segovia, may be compared with the most wonderful works which antiquity has transmitted to us. There still remain of it 159 arches, all built with large stones, and without any cement. There are two rows of arches, one above the other, and the whole height of the edifice is 102 feet. It runs quite across the town, and passes over the greater part of the houses which lie in the hollow.

In modern times various aqueducts have been formed after the manner of the Romans, particularly in France; and the most remarkable are those which have been constructed by Louis XIV., at vast expense, for conducting water from Versailles to Marli. Of these the famous aqueduct bridge of Maintenon, which was erected for conveying the waters of the river Eure to Versailles, is without doubt, in point of magnitude and height, the most magnificent structure of the kind in the world. In Plate XLIX., we have given a view of a portion of this work, on the same scale as the other aqueducts here represented. Had the whole been delineated on the same scale, it would have extended to four times the breadth of the plate. It extends about 4400 feet in length, being nearly seven eighths of a mile, and upwards of 200 feet in height, and contains 242 arcades, each divided into three rows, forming in all 726 arches about 50 feet span. Of the subterranean aqueducts in France the finest is that of Arcueil, which serves to conduct water to that village. It is 44,300 feet in length, or upwards of eight miles, extending from the valley of Arcueil to the castle at the gate of St Jacques, all built of hewn stone. It is about six feet in height, and has on each side a foot-path 18 inches wide; it has a declivity of one foot in 1300. Another aqueduct of this kind is that of Rocquancourt, part of the system which brings water to Versailles; it is 11,760 feet in length, or upwards of two miles, and a declivity in its whole course of only three feet. In some parts of its course it was necessary to make excavations 80 or 90 feet deep, which rendered the execution very difficult.

But in modern times, at least within the last century, the invention and improvement of the manufacture of cast iron has completely changed the mode of conducting water into cities, by the introduction of cast-iron pipes instead of the stone conduits of former times. These pipes can now be formed of almost any dimensions, and united together into a continued series, so closely as to prevent the escape of the water, even under a violent pressure arising from the altitude of the fountain-head. They enable us, therefore, to take advantage of and give effect to that grand principle in hydrostatics, that the fluid element tends continually to a level, even though it be confined in the smallest or most complicated system of pipes; so that however low it be carried in any valley, or to whatever distance, still it will rise on the opposite side to the original altitude of the fountain-head—a principle which is most important indeed in such works, seeing that by it we are not restricted, as the Romans were, almost to a perfect level in the line of the conduit. We have seen that, for the purpose of attaining this level or very gentle declivity all along the conduit, they were under the necessity of raising it by arcades continued in one unbroken series, frequently thirty or forty miles in extent; and, in addition to this, often prolonging the length of the track by a circuitous route, turning and winding for miles out of its course, for the very purpose of increasing its length.

But the use of pipes enables us to dispense with these long arcades all raised nearly to the same level with the fountain-head; because the conduit may be varied in its level to any extent, and still will rise at last to its original altitude. The pipes, therefore, are merely laid along the surface of the ground, with a cover of two or three feet of soil to place them beyond the reach of frost. To prevent, however, the frequent or abrupt alternations of rise and fall, any sudden inequalities in the ground are equalised by cuttings and embankments, but not to anything like the extent that would be required to raise the whole to a level. This, therefore, forms a capital improvement in the method of conducting water, and the greatest indeed which has ever been made in this important branch of practical mechanics. That it was not introduced by the Romans, is not to be ascribed, as many have done, to their ignorance of the hydrostatic principle, that the fluid would rise to a level in the opposite branches of the same train of pipes. Professor Leslie has shown that they were well acquainted with this principle, and has moreover obtained from Italy a portion of a leaden pipe supposed to have been used in the baths of Caracalla, which sets this matter at rest. But, from the low state of the arts at that period, they were unable to give effect to the principle. They had not the means of fabricating pipes of such a magnitude as would have been required for the enormous quantities of water consumed in Rome, and at the same time of strength sufficient to withstand the pressure from the fountain-head. Lead was the only material that could be used by them for the purpose; and besides the enormous thickness that so weak a material would have required, and the impracticability of their forming them, and uniting them together endwise, they had a decided aversion to lead, as tending to make the water unwholesome by its poisonous impregnation. The use of cast iron was quite unknown. There remained, therefore, no resource but in the aqueducts, which, though attended no doubt with vast expense, and requiring great enterprise, as well as both skill and patience, were yet attainable by these means, and formed when completed a simple and very perfect mode of effecting the object. Hence arose all those works above described which have since excited such astonishment. Now, however, when the manufacture of cast iron has been brought to such perfection, and methods contrived for uniting perfectly together all the pipes into one connected train; this improved system has been universally adopted.

The most complete and perfect works of the kind are those some time ago undertaken for the supply of Edinburgh, which, by the contrivance and direction of Mr Jardine, the company's engineer, have been executed in a style quite worthy of the city, as well as of the present advanced state of science and the arts; offering, both in the general design and in all the details, a model of propriety and skill in this species of hydraulic architecture. The Crawley Spring, from which the new supply has been derived, issues from the side of a rising ground on the southern base of one of the Pentland Hills. It is scarcely seven miles distant from Edinburgh in a straight line, but eight and three fourths in the line of the pipes, these having been carried round a considerable way to the eastward to avoid the Pentland ridge, the eastern extremity of which lies in the direct line to the city. The spring is elevated 564 feet above the level of the sea, and 360 above the level of Princes Street. There is therefore ample height to carry it over the highest parts of the town. The original issue of the spring was greatly augmented by a drain, which was carried for about half a mile above the spring, up the valley in which it is situated. The soil of this valley, consisting of an immense bed of gravel, in many parts 40 feet deep, constitutes a vast natural filter, through which the water, descending from the high grounds on each side of the valley, percolates in a high degree of purity, and being all intercepted by the drain, is by it conducted, along with all the original discharge of the spring, into a reservoir or water-house, from which the pipes take their rise, and continue in one connected train all the way to the city. In the first three miles they vary from 18 to 20 inches diameter, and descend 65 feet in a pretty regular series. In the remainder of the track they are 15 inches diameter, and descend 286 feet. The descent is not perfectly regular, being in some parts steeper than in others, according to the natural declivity of the country. In one or two instances also they undulate slightly. Near Burdiehouse, four miles from the city, they ascend a little; and, after descending rapidly to Libberton Dams, they again ascend 20 or 30 feet to the high ground on the south side of the Meadows. There are, however, no sudden inequalities, all such having been carefully avoided by levelling; for which purpose considerable embankments and cuttings of the ground have been undertaken without scruple; and as the line approaches the city, it has been carried through a tunnel 2160 feet in length under Heriot's Green, about 70 or 80 feet below the surface, and another under the Castlehill 740 feet through the solid rock, and 120 feet under the reservoir. From this main line of aqueduct a branch pipe leads off on the south side of the town to the Reservoir at Heriot's Green, near Heriot's Hospital, which aids the former source in supplying fully the southern districts; a second leads off from the same place to the eastward, and affords an additional supply to the south side of the town, directly from the pipes; and a third leads off to the Reservoir on the Castlehill, and aids it in supplying the rest of the old town. The main body of the water, however, proceeds onwards to Princes Street, along Hanover Street, and across Queen Street; and from thence branch pipes are laid through all the other streets in the new town, and from these again service pipes to each house or floor of a house. Each pipe in this aqueduct is about nine feet in length, the metal being about half an inch thick. After being cast, their soundness was proved by a forcing pump applied to each separately, and with a gage-valve loaded with a weight equivalent to a pressure of 800 feet; and if under this they betrayed the smallest flaw, they were rejected. They are all joined together with what is termed spigot and faucet joint, the end of the one being let several inches into a swelled part or socket at the extremity of the other. This forms a much more perfect joint than by flanges and bolts, as it admits of a slight degree of expansion in the pipes without opening the joints. After being entered, a ring of hemp or rope-yarn is wrapped round the end of the pipe and beat into the socket of the other, and then a mass of lead run in to fill up the opening, which the yarn prevents from running into the pipe; the lead being hard rammed, and stoved with a chisel, forms a jointing completely water-tight. Air-cocks are placed at intervals all along the pipes to let off the accumulated air, which is done by the hand at regular intervals, perhaps every three or four days. The supply of water now conveyed by this aqueduct amounts to 180 or 200 cubic feet per minute at an average. This is about five times the quantity formerly delivered into the town by all the different ponds and reservoirs from which it was then supplied, and which was besides often of a very impure and unwholesome quality. The introduction of this spring therefore gave, as may easily be conceived, wonderful relief to the inhabitants, and is now found, along with the former sources, amply sufficient for every purpose of comfort or luxury. The Crawley aqueduct, however, is capable of conveying double this quantity whenever the wants of the city shall render necessary a new supply, which it is in that case intended by the present company to draw from another excellent and copious spring termed the Black Spring, situated on the north side of the Pentland Hills. The whole expense incurred by the Joint-Stock Water Company in bringing in the Crawley water, including the expense of the great compensation reservoir for supplying the deficiency to the mills on the stream from which the Crawley was diverted, amounted to about L145,000, and the company draw an assessment on every consumer of five per cent. on the rental of his premises, which is understood to yield L6000 or L7000 a year. They are allowed to divide to the amount of six and a half per cent. on their capital, but not more.

The water from the Crawley Spring is naturally of the finest quality, and issuing continually from a source deeply seated in the ground, and also at so great an altitude, is always fresh and cool, even in the heat of summer, provided it is not allowed to stagnate in our cisterns. In this respect the supply of Edinburgh is far superior to that of Aqueduct most other towns of the same magnitude. In Glasgow, for example, the supply is sufficiently ample; but being derived all from the Clyde, it is liable to become warm and unpleasant in summer. In Liverpool, Manchester, and even in London, the supplies, though abundant, are liable to the same defects. So that on the whole we have reason to congratulate ourselves that the elevated nature of the country round our city affords sources for a supply so copious, so pure, and capable of reaching so easily, and without the aid of any machinery, the highest parts of the town.

In all places such as the above, where there is a deficiency of level to carry the water naturally to the highest parts of the town, there is no resource but in the employment of machinery. A steam-engine or other agent is applied to the working of pumps, which both draw the water from the rivers or from wells, and then propel it by force through a train of pipes to all the different parts of the town, and elevate it by branches to the highest parts that may be necessary; a plan which we believe is now executing at Aberdeen, and also at Perth.

But though the system of pipes has thus superseded the use of stone channels all raised to a level in the conveyance of water, there are still cases, such as those of canals, where the water must be kept on a perfect level, and where, therefore, aqueduct bridges are still necessary in conveying it over the valleys; and of these we have had examples in France, on the Languedoc canal. The first aqueduct bridges for canals in this country were those made by the Duke of Bridgewater, under the direction of the celebrated Brindley, and which, being quite new here, excited no small degree of astonishment. The first and largest was the aqueduct at Barton Bridge for conveying the canal across the Irwell, 39 feet above the surface of the water. It consisted of three arches, the middle one 63 feet span, and admitting under it the largest barges navigating the Irwell with sails set. 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. Since that period canal aqueducts have become more common; and many excellent examples are to be found both in England and Scotland. Of these are the aqueducts over the river Lune, on the Lancaster canal, designed by Rennie, a very excellent and splendid work of five arches, each 72 feet span, and rising 65 feet above the level of the river; and the Kelvin aqueduct, near Glasgow, which conveys the Forth and Clyde canal over the valley of Kelvin, consisting of four arches, each 70 feet span, and rising 70 feet above the level of the river. In Plate XLIX, we have given views of three other principal aqueducts, viz. the aqueducts of Pont Cysylte, of Chirk, and of Slateford near Edinburgh. Of these the Pont Cysylte by Mr Telford is justly celebrated for its magnitude, for the simplicity of the design, and the skilful disposition of the parts, combining lightness with strength in a degree seldom attempted. This aqueduct serves to convey the waters of the Ellesmere canal across the Dee and the vale of Llangollen, which it traverses. The channel for the water is made of cast iron, supported on cast iron ribs or arches, and these resting on pillars of stone. The iron being much lighter than stone arches, this is one reason why the pillars have been reduced apparently to such slender dimensions. They are quite strong enough, however, as experience has proved. The whole length of the aqueduct is about 1000 feet, and consists of 19 arches, each 45 feet span. The breadth of the pillars at the top is 8 feet, and the height of the four middle ones is 115 feet to the springing. The pillars have a slight taper, the breadth of the middle ones at the base being 15 feet.

The height from the surface of the water in the Dee to that in the canal was to be 126 feet eight inches. The channel for the water consists of cast iron plates, cast with flanges, and these screwed together with bolts; they are represented in the drawing, between the arched ribs and the railing. The lines there show the joinings of the different plates. In order to preserve as much water-way as possible, the channel is made the full width of the canal and towing path, and the latter projected over one side, and supported inside by posts resting on the bottom of the canal. The aqueduct of Chirk was designed by the same able engineer, and serves also to convey across a valley the waters of the same canal. This aqueduct was the first in which any iron was employed. Hitherto the channel for the waters had been constructed of stone, or partly of stone and partly of clay puddle, which it was generally found very difficult to keep water-tight for a length of time. It was determined, therefore, by Mr Telford to try the effect of cast iron, and to lay it at first only in the bottom. The plates were accordingly laid directly over the spandril walls, which they served to bind together, and united by flanges and screws. The sides of the channel were built with stone facings and brick hearthing laid in water-lime mortar. This plan has succeeded completely, and the quantity of masonry in the aqueduct was thereby greatly reduced. The aqueduct itself is 600 feet long, and 65 feet high above the river, consisting of ten arches, each 42 feet span. The piers are ten feet thick.

The aqueduct of Slateford serves for conveying the waters of the Edinburgh and Glasgow Union Canal across the valley of the Water of Leith at Slateford. It is an elegant structure, and very similar in the plan to that of Chirk, only that the water-channel is composed entirely, the sides as well as the bottom, of cast iron, which is moreover built in with masonry. It is about 500 feet in length, and consists of eight arches, each 45 feet span; and the height of the canal is about 70 feet above the level of the river. On this canal another aqueduct of the very same construction occurs in crossing the valley of the Almond, and having several more arches. There are, in different parts of the country, various other aqueducts, which might be described; but our limits preclude our enlarging upon them; and it is the less necessary, as, excepting the formation of the water-way, these structures differ nothing in their design or the principle of their construction from ordinary bridges, particularly those that are undertaken not so much with the view of crossing rivers as of raising up the level of the road itself entirely out of the valley,—an object now become of great importance, from the improvements which have taken place within the last half-century on all our roads, and the refined notions which have in consequence begun to prevail as to the rates of travelling, and, what conduces most essentially to this object, the levels of the road. Formerly people were content to traverse slowly all the inequalities of the country through which the road might pass, descending into the valleys, and mounting the steepest acclivities. Now, however, a road is thought imperfect, and quite behind the standard of improvement, unless every rise greater than 1 in 15 or 1 in 20 feet be cut down. In crossing the valleys, therefore, it is not enough now that we build a bridge in all respects sufficient for crossing the stream itself; we must raise it nearly to a level with the ground on each side of the valley; and this circumstance gives rise to new and very extensive works of this kind, which formerly never would have been thought of. Of these we may just instance the splendid bridge of one arch of 140 feet span, built over the Don Burn at Aberdeen, to form a new access into that town; also the beautiful bridge of Cartland Craigs, built by Mr Telford, over the little stream of the Mouse, on the new road from Glasgow to Carlisle, consisting of three arches 50 feet span, and elevated 180 feet above the bed of the stream. More recently (for in this important department of our domestic economy, which regards the perfecting of the interior communications of the country, improvements seem to be going on, not merely with rapidity, but with an accelerated progression which it is quite wonderful to contemplate) the introduction of railways opens a new and still wider field for the skill and talents of the engineer in the erection of such works. This new and improved species of road, it is well known, must be kept still more nearly on a level than any of the roads of the ordinary construction. Considerable inequalities in these may still be tolerated, but in the railroad they are quite inadmissible, and would defeat the very object of the improvement. (See RAILWAY.) In this respect the railway is somewhat similar to the old Roman aqueducts, and, where the country is low, must in like manner be elevated on a series of arcades. These sort of bridges have received the name of Viaducts; and already we have an extensive one on the Liverpool and Manchester railway, termed the Sankey Viaduct, of nine arches; and numerous others will no doubt be speedily required. But for the principles and mode of construction of these works, as well as of the aqueduct bridges, so far as the arch is concerned, we refer to the articles ARCH and BRIDGE; and for further information on the subject of aqueducts, see Julius Frontinus, De Aqueductibus Urbis Romae; Raphaelis Fa-bretti De Aquis et Aqueductibus Veteris Romae Dissertatio; Famiani Nardini Roma Vetus, lib. vii. cap. iv.; Plini Hist. Nat. lib. xxxvi. cap. xv.; Montfaucon, Antiquité Expliquée, tome iv. tab. 128; Governor Pownall's Notes and Description of Antiquities in the Provincia Romana of Gaul; Belidor's Architecture Hydraulique, containing a drawing of the aqueduct of Maintenon; also Mém. Acad. Par.; Andreossy, Voyage à l'Embouchure de la Mer Noire, ou Essai sur le Bosphore; Philosophical Transactions Abridged, vol. i.; and Link's Travels in Portugal.