Having sketched the general history of each one of these aqueducts, and the circumstances under which they were erected, it remains to refer briefly to some of those details which Frontinus has handed down to us, of the mode of management of the water. These details apply equally to the whole series. First of all, the position which he and his predecessors in office held was a very important and honourable one. He had the entire responsibility of the water supply, and the absolute control of seven hundred men employed to attend to their proper working, to clean the piscinæ, and to repair the channels and pipes when needed. Of these, 240 were appointed by the City, and the remaining 460 by the Emperor. The channels in which the main stream of water was carried, and the reservoirs (castella), and the piscinæ, were always lined with a particular kind of cement, called Opus Signinum.

It has also been observed that the base of the channel is constantly broken up by inequalities, or dips, if they may be termed so, as if purposely introduced to agitate the water in its course, and, consequently, to aërate it. No doubt also the inequalities, provided they were rugged, would tend to check the passage of any earthen matter held in solution by the water. Of most importance, however, and shewing still more remarkably the engineering qualities of the builders of the Aqueducts, were the ventilating-shafts which were introduced at proper intervals. Such shafts were often used as wells also, to let buckets down for water, and steps were cut on the sides for the aquarii to descend and remove obstructions. At about every half-mile the specus forms an angle to break the force of the water, and at such points there is usually a reservoir also. Advantage was taken of these angles for the later aqueducts to be carried over the older ones, as at the Torre Fiscale and the Porta Maggiore. These precautions, combined with the system of Piscinæ, rendered the water constantly fresh, pure, and wholesome.

From some considerable remains of the Piscinæ, the system can be clearly made out. The building consisted of four chambers, two beneath and two above. Supposing, for the sake of illustration, in the annexed diagram, the letters (image of the letters A-D arranged in a square: top left: A.; top right: D.; bottom left: B.; bottom right: C.) represent the four chambers. The channel of the aqueduct, coming at a tolerably high level, enters the chamber A. Thence the water passed (possibly over a large waste-pipe) into the chamber beneath, B. Between B. and C. there were small holes communicating through the wall (possibly provided with fine grating). Through the roof of C. there was a hole, and the water passed upwards, of course finding the same level in D. at its exit, as in A. at its entrance; on leaving the piscina the water was carried off into another specus. By the aid of sluice-gates the water could be transferred direct from chamber A. to chamber D. Access was obtained by an opening to the chambers beneath, and the mud was from time to time cleaned out. Curious details of the sluices, &c., have been found; but it is not easy to determine their age, as the aqueducts have undergone so many repeated alterations.

As a typical example of the size of the specus, or channel of the Aqueducts, the Marcian may be taken, which measures in the opening five Roman feet in height, two and a-half in breadth, and the thickness of the wall on each of the two sides one foot. The roof of the Marcian being surmounted by the Tepulan and Julian, no great solidity was needed; but so far as can be ascertained, the roofs were generally of considerable thickness, to prevent, as much as possible, the heat of the sun spoiling the coolness of the water. The channels and general structure of the Anio Novus and Claudian were larger in every respect, those of the Tepulan and Julian somewhat smaller, as shewn by the accompanying diagrams.

It will be seen that the period of the construction of the nine aqueducts which have been described, embraces some four centuries, as Frontinus was employed in surveying them towards the close of the first century after Christ, and the first aqueduct was commenced more than three centuries before the Christian era. If we omit the Alsietina, which was on the other side of the river, and which ought scarcely therefore to be taken into account, the order of level follows almost exactly the order of construction, the only exception being that of the Virgo. This increase of height of level as each new aqueduct was added, is pointed out by Frontinus in his eighteenth chapter.

It will be perhaps convenient to have the several aqueducts already described represented in a tabular form, shewing the dates when they were made, and the order as to level, together with the distances which they traversed.

The aqueducts, especially those which were mostly underground, it will be observed, made long detours, as compared with the direct line of road; as, for instance, in the first on the list, the distance by road was but eight Roman miles, but that by the channel of the aqueduct was eleven miles. The total length of all the channels of the aqueducts, constructed in less than four centuries, was upwards of 285.610 Roman miles, of which 242.697 miles were cut beneath the surface, and 42.918 miles carried on substructure, arched or not, as the case required, above the surface of the ground.

Of the above, six are carried to Piscinæ on the Via Latina.

Buecheler, in his edition of Frontinus, pp. xi., xii., xiii., explains the mathematical part of chap. 78 differently from Poleni and Dederich[153].

In order to regulate these several aqueducts, so that deflection in any part should be easily ascertained, means were provided for estimating the abundance of the water, and in this respect for its minuteness and clearness the treatise of Frontinus stands unrivalled, when compared with any work of ancient or even modern times. The mode of measurement is to take the area of a vertical section of the water flowing along the specus,—the object being to test each aqueduct by itself, and to see whether its proper supply was given or not. Had he been called upon to estimate the actual supply of the water, and not the relative, other points would have had to be taken into account, namely, the average fall of the aqueduct, from which to gather its velocity. On this, however, he does not touch. It must be borne in mind, also, that when Frontinus was appointed, the system had been long in force, and he had to follow the traditions and rules of his office. He made no revolution, he was simply a reformer. He tells us that in examining the books belonging to the office, he found certain measures given to certain aqueducts; these he had to verify, and a great part of his work is taken up in the account of his operations. To begin with, he is much troubled as to the measures employed. The digitorum modulus (i.e. a pipe with a given measured orifice) is uncertain. There is the square digit and the round digit (in other words, a square pipe of which each side is one digit, and the round pipe of which the diameter is one digit). As an instance of his accurate and clear expression, it may be worth while to quote his words on this point[154]:—

In this we obtain a key to most of his calculations[156], because they depend throughout upon the computation of the area of the circle.

The base measure chosen was the “quinary,” so called (according to the most natural derivation) because its diameter was five quarter-digits. And to this basis are reduced some twenty different moduli, to which the names were given of senary, septenary, octonary, denary, duodenary, and so on up to centenary. The reduction of these seems to be tolerably accurate according to the system which he pursues, and making allowance for transcribers’ errors. But either the relation of the circumference to the diameter of a circle had not been worked out to the same extent to which it has been in later times, or Frontinus considered the ratio adopted sufficient for his purpose. One of his calculations stands thus: he is measuring the Appian at a point where it is joined by a later branch; it had been assigned in the register as giving 841 quinaries: he says—

Each centenary is shewn elsewhere to contain 81 quinaries and a fraction, and therefore 22 would give 1,792 in quinaries and a fraction. The quadragenary is marked to contain 32 quinaries and a fraction. Together, therefore, the 1,825. Now the area of each quinary, according to the system of Frontinus, would be the square of its diameter (i.e. 1¼ digit), multiplied by ¹¹⁄₁₄ (the ratio already ascertained). This equals ²⁷⁵⁄₂₂₄, which, multiplied by 1,825, produces 2,240½ digits (nearly). Finally, the area of 8¾ square feet, reduced to square digits, gives 2,240[158].

This calculation illustrates his mode of computation from absolute measure of the water; but the ordinary way was to affix the moduli, which were of brass, or rather of bronze, to certain openings. Provided the modulus was placed perfectly level, the water flowed at its ordinary rate; and when the moduli were sufficient to prevent the water rising in the reservoir or channel, and, on the other hand, not too numerous or too large to reduce the mean level of the water, the sum of them represented the amount of water.

We next come to the question of distribution. On examining the registers, Frontinus says that he found that there were assigned to the nine aqueducts 12,755 quinaries; but it appeared that the distribution amounted to 13,470 quinaries. To account for this discrepancy, the whole were re-measured. In the annexed table the measures of each aqueduct are given separately, and by adding together the results of each, as calculated from the data which Frontinus gives of the water measured at the head, the total, as the table shews, comes to 24,413 quinaries. The fact was that in process of time some of the channels had fallen into decay, and the water was wasted. In other and more numerous cases, it had been abstracted by persons for their own use without authority; in addition to which there were no doubt errors in the computation, which had crept into the books, and if we are to believe Frontinus, these errors had been turned to the profit of his predecessors.

In the table of distribution will be seen the number of quinaries distributed outside the wall of the city; next, the Regiones which they serve within the city; thirdly, the measure of water so distributed within the city, and the number of castella, or reservoirs, used for that purpose. These reservoirs, at least such as remain, will be described in the several Regiones in which they occur. That over the arch of Dolabella on the Cœlian Hill, and the so-called Castellum Aquæ Juliæ on the Esquiline, are the most prominent. Many, no doubt, were of much smaller size, and merely large cisterns, as the total number was 247.

The supply of 13,470 quinaries is thus accounted for. Outside Rome 3,164 quinaries were distributed, inside 10,306; of that outside Rome 1,718 quinaries were used “in the name of the Emperor,” that is, to supply the imperial villas and gardens, &c., while 2,345 were charged to private persons. Inside Rome 1,707 were charged to the Emperor for his palaces, &c., while private persons used 3,847 quinaries, leaving 4,401 quinaries, which were for the “Public Service.” These, together, account for the 9,955 distributed within Rome.

In the details of the distribution, we learn how much water was employed in the palaces, &c., in the service of the Emperor and his household (nomine Cæsaris), how much was carried into houses for the use of private persons, and how much was used in public buildings and the public reservoirs or fountains which were established in all convenient positions, and generally accessible to the population.

It may, perhaps, be interesting to examine more carefully the distribution for the “Public Service.” We have four classes of recipients, and we know from Frontinus how much was served to each class from each aqueduct. There were first of all 19 castra or barracks, in which the army were kept when not out on service, and in which also the guards were stationed. These required in all 279 quinaries. There were next, 95 different Public Establishments which used 2,401 quinaries, and 39 theatres and places of entertainment (munera) which used 386 more. Lastly, there were 591 open reservoirs (lacus) for the service of all comers, using 1,335 quinaries. These reservoirs were what we usually speak of as fountains, and some hundreds are in use to this day, many probably on the site of the older ones. There were very stringent laws respecting their use. Heavy penalties were inflicted upon anyone dipping a dirty bucket or other vessel into the reservoir; there were also laws respecting the “overflow,” as the fountains of course were constantly running. These were the most important to keep in order, as all the poorer classes depended entirely upon them for their supply of water.

The more wealthy had water brought into their own private reservoir in the court, with buckets and windlasses for drawing the water to the upper story, and this is the case to the present day[159].

These details, while giving us an insight into the social state of the city, go to shew how thoroughly the supply of the water was under control. The pipes supplying every one of these 744 points of distribution were all registered, and their sizes being known, the system of computation, as explained by Frontinus, was brought into play: thus every part was checked, and any irregularity could be traced to its source.

It may be asked what amount of supply, in gallons, would the water represent, in order that a comparison can be made between the supply of towns at the present time, and that of Rome at the time at which Frontinus wrote. The answer is not easy. As said before, we have not sufficient data for an accurate determination; still we may attempt an approximate estimate. It is easy to compute the section of water given by the 24,805 quinaries. This being just upon 120 square feet, we can form some notion of the vast quantity, if we picture to ourselves a stream twenty feet wide by six feet deep, constantly pouring into Rome at a fall six times as rapid as that of the river Thames. It has been computed by a French engineer that, together with one or two additional aqueducts which were added between the time of Trajan and Aurelian, the supply of water to Rome was 332,306,624 gallons daily. If we assume the population to have been a million souls—and it is scarcely probable that there were more—we find that the rate was 332 gallons per diem for each person. In our own day we consider 40 gallons sufficient, and many think this excessive, including the use of water in manufactures, &c. No wonder, with the facilities for drainage which the rapid Tiber afforded, and with this plentiful supply of really good water, the great city was rendered, in spite of overcrowding and an unhealthy situation as regards its neighbourhood, comparatively free from those epidemics which are so fatal in all densely-populated towns.

One part of the system which Frontinus seems to have been the first to adopt was this: he had 247 castella, or main reservoirs, from which the pipes were carried to the smaller cisterns and fountains, and he so arranged the supply of water to these that the several aqueducts could be interchanged. A net-work was established, so that when one channel, from whatever cause, failed, whether from requiring to be cleansed or repaired, another could supply its place. But this was not all. He tells us, cap. 87, that many of the parts of the city which were dependent on one source for their supply, were, by careful distribution of the water, able to avail themselves of a second in case of need; and he gives as an instance that the Marcian was made available in the Aventine, by being carried across from the Cœlian[160]. He also speaks of the improvements which were made in keeping the water pure. Hitherto a very slight shower had made many of the streams muddy, although at their source they were pure[161].

The least pure, however, before the time of Trajan, were the Anio Vetus and Anio Novus. This arose from the banks which, being soft, gave way, and the stream, even in fair weather, thus became muddy. He points out that hitherto, by the mixture of the waters which had been adopted to prevent failure in supply, proper skill had not been shewn, and these new streams were allowed to destroy the purity of others[162]. His object, therefore, was to keep the pure streams separate, and in addition to that, arrange that they should be used for drinking purposes only; while the water which was less grateful to the taste, and which was subject to be at times muddy, should be used for cleansing the city, or watering the gardens, or other ordinary purposes, which it might fulfil equally with the fresher and purer water. In considering the supply of water to our large towns, which is one of the great problems which at the present time has to be solved, it is not too much to say that—the plan adopted by the Emperor Trajan, by the advice of Frontinus, the head of the aquarii, and of that department of the government of Rome which had charge of the aqueducts, the importance of which in that climate can hardly be over-estimated—is worthy of more attention than it has received[163].

The Curator Aquarum.

The care of the aqueducts was assigned by Augustus to a Curator Aquarum, of whom Marcus Agrippa was the first, and held the office for life. Frontinus, to whom we are indebted (as we have seen) for nearly all that we know respecting the early history of the aqueducts, held this office under Domitian, A.D. 94; he remained in office under Nerva, wrote his treatise at that time, and died under Trajan, A.D. 107. The office continued to exist till the third century, in the time of Diocletian, when it was superseded by the magistrates called Consulares Aquarum, a title changed in the fifth century into Comes Formarum Urbis. These several magistrates had 700 servants, charged with the superintendence, repair, and distribution of the water from the aqueducts, divided into familia publica and familia Cæsaris; the former, comprising 240 persons paid by the State; the latter, 460 paid by the Emperor. Agrippa supplied the city with 700 lacus, 105 fountains, 130 castella for distributing waters, and 170 gratuitous baths for public use. These structures were adorned with 400 marble columns, and 300 statues of marble or bronze. After the devastations caused in the Gothic wars, under Vitiges and Totila, only the most important aqueducts were restored, either by Belisarius or Narses. Ruin and neglect again undid much of what had then been restored to use. At the beginning of the ninth century, the only aqueducts supplying water were the Appian [I.], and Marcian [XII.], then called Jobia or Iopia, by corruption from “Jovius,” the name assumed by Diocletian.

During both the eighth and ninth centuries, indeed, various repairs for maintaining the aqueduct of Trajan [X.], which carried water from the lake Sabatinus (di Bracciano, Acqua Paola), are mentioned by Anastasius; and in 786 the “Jovia” aqueduct was restored by Hadrian I. In the twelfth century, the Aqua Lateranensis is mentioned as still in use, being that part of the Claudian aqueduct carried upon the arcade called the Arches of Nero near the Lateran, where the specus is very conspicuous. The aqueduct of Trajan [X.], and that of Agrippa [VI.], continued to supply water, though but scantily, for about two centuries later. The Appia [I.] has long been supposed to have been gradually stopped up by the deposit of clay, and thus rendered useless[164]; and so total was the failure of all these waters in the fourteenth century, that the population (said to have been not more than 17,000 during the absence of the Papal Court at Avignon) were without any supply of running water, except from the Tiber[165]. The first restoration, in the fifteenth century, was that of Agrippa [VI.], by Nicholas V., whose aims and efforts were renewed by Sixtus IV., and by later Pontiffs. Sixtus V. [Felice Peretti] determined to restore to Rome the water then supposed to be the Marcia, but really of Hadrian [XIII.] The arcade of the Claudian [VIII.] and Anio Novus [IX.], was the most magnificent of all, and conveyed its several streams along a distance of forty-six miles. This had been restored by Vespasian and Titus, by Trajan, Septimius Severus and Constantine; but, in the greater part, especially within the first few miles from Rome, the existing stone arcade is of the time of Claudius, with repairs in the brickwork of the Flavian emperors. At the beginning of the ninth century, it was certainly still serviceable, and was known by the name Forma Claudiana. The piers of this arcade were used by the engineers of Felice to carry that specus on the part near Rome; but the greater part of the Marcian arcade, and considerable parts of the Claudian, near Rome, were used as a stone-quarry by them. The channels through which the Anio and the other streams flowed, were the first, 9 ft. in height and nearly 3 in breadth, the second 6 ft. by 3, as seen in their ruins at the Porta Maggiore. The more ancient portion is not in brickwork, but of enormous squared blocks of tufa and peperino.

The popular notion that all these aqueducts were a mere waste of money, arising from the ignorance of the ancient Romans of the simple fact that water will rise to its level, is altogether erroneous. Vitruvius gives directions for taking the levels for carrying water through valleys, where, he says, there should be standing or upright pipes (columnaria,), now called respirators, to let off the confined air (spiritus[166]), and explains how to bring the water on by earthenware, lead, and even leather pipes.

Within the last few years a large leaden pipe of great antiquity, not less than two feet in diameter, was found under the Via de’ Condotti, encased in ancient brickwork, evidently shewing that they were afraid to trust to the strength of the lead. The force of the water running in a strong current from such great distances, required to be broken at frequent intervals, by being turned at a sharp angle, and then allowed to return again to its course by another angle. This arrangement occurs continually along the line of the aqueducts, generally at each half mile, with a piscina or a castellum, or both filtering-place and reservoir, at each of these angles. The great object of constructing these magnificent aqueducts was to bring the water to the highest levels in the city, from which it descended by a succession of reservoirs (castella aquarum), some with piscinæ or filters, others with fountains, each one below the level of the preceding one. The lower town along the valley, or Campus Martius, was also supplied by other aqueducts at a much lower level. The ancient Romans had abundance of leaden pipes to convey the water to the baths or thermæ, and bronze stop-cocks, as may be seen at Pompeii, and silver ones for the emperor; but they had no pumps. From each castellum there were pipes to supply the neighbouring baths or palaces, with moderate pressure; also wells or cisterns in the courtyard of each house, which had frequently a fountain. On the Janiculum, the water of the aqueduct passing from one castellum to another, has sufficient force to turn several mill-wheels.

A letter of King Theodoric, printed in Cassiodorus[167], addressed to the Roman Senate, enjoins them to assist Joannes in his enquiries after those who have diverted the water from the aqueducts for their own uses, and those who have stolen the brass and lead, or those who have helped them. This proves that they were not destroyed by the Goths, and that they were in use in the sixth century.