6Water is conducted in three ways, either in streams by means of channels built to convey it, in leaden pipes or in earthen tubes, according to the following rules. If in channels, the structure must be as solid as possible, and the bed of the channel must have a fall of not less than half a foot to a length of one hundred. These channels are arched over at top, that the sun may strike on the water as little as possible. When they are brought home to the walls of the city a reservoir (castellum) is built, with a triple cistern attached to it to receive the water. In the reservoir are three pipes of equal sizes, and so connected that when the water overflows at the extremities, it is discharged into the middle one,
2in which are placed pipes for the supply of the pools and fountains, in the second those for the supply of the baths, thus affording a yearly revenue to the people; in the third, those for the supply of private houses. This is to be so managed that the water for public use may never be deficient, for that cannot be diverted if the mains from the heads are rightly constructed. I have made this division in order that the rent which is collected from private individuals who are supplied with water, may be applied by the collectors to the maintenance of the aqueduct.
3If hills intervene between the city walls and the spring head, tunnels under ground must be made preserving the fall above assigned; if the ground cut through be sandstone or stone, the channel may be cut therein, but if the soil be earth or gravel, side walls must be built, and an arch turned over, and through this the water may be conducted. The distance between the shafts over the tunnelled part is to be one hundred and twenty feet.
4If the water is to be brought in leaden pipes, a reservoir is first made near the spring, from whence to the reservoir in the city, pipes are laid proportioned to the quantity of water. The pipes must be made in lengths of not less than ten feet: hence if they be one hundred inches wide (centenariæ), each length will weigh twelve hundred pounds; if eighty inches (octogenariæ), nine hundred and sixty pounds; if fifty inches (quinquagenariæ), six hundred pounds; if forty inches (quadragenariæ), four hundred and eighty pounds; if thirty inches (tricenariæ), three hundred and sixty pounds; if twenty inches (vicenariæ), two hundred and forty pounds; if fifteen inches (quinumdenum), one hundred and eighty pounds; if ten inches (denum), one hundred and twenty pounds; if eight inches (octonum), ninety-six pounds; if five inches (quinariæ), sixty pounds. It is to be observed that the pipes take the names of their sizes from the quantity of inches in width of the sheets, before they are bent round: thus, if the sheet be fifty inches wide, before bending into a pipe, it is called a fifty-inch pipe; and so of the rest.
5An aqueduct which is made of lead, should be thus constructed; if there be a proper fall from the spring head to the city, and hills high enough to cause an impediment do not intervene, the low intervals must be brought to a level by means of substructions preserving the fall directed for channel aqueducts, or by means of a circuitous course, provided it be not too much about; but if there be long valleys, let it be laid according to the slope of the hill, and when it arrives at the bottom, let it be carried level by means of a low substruction as great a distance as possible; this is the part called the venter, by the Greeks κοιλία; when it arrives at the opposite acclivity, the water therein being but slightly swelled on account of the length of the venter, it may be directed upwards.
6If the venter were not made use of in valleys, nor the level substruction, but instead of that the aqueduct were brought to an elbow, the water would burst and destroy the joints of the pipes. Over the venter long stand pipes should be placed, by means of which, the violence of the air may escape. Thus, those who have to conduct water through leaden pipes, may by these rules, excellently regulate its descent, its circuit, the venter, and the compression of the air.
7It will moreover be expedient, when the level of the fall from the spring is obtained, to build reservoirs at distances of twenty thousand feet from each other, because if damage be done to any part, it will not then be necessary to take the whole work to pieces, and the defective places will be more easily found. These reservoirs, however, are not to be made on a descent, nor on the venter, nor on a rise, nor, generally speaking, in valleys, but only on plains.
8But if the water must be conveyed more economically, the following means may be adopted. Thick earthen tubes are to be provided, not less than two inches in thickness, and tongued at one end, so that they may fit into one another. The joints are then to be coated with a mixture of quick lime and oil, and in the elbows made by the level part of the venter, instead of the pipe, must be placed a block of red stone, which is to be perforated, so that the last length of inclined pipe, as well as the first length of the level part may be received into it. Then, on the opposite side, where the acclivity begins, the block of red stone receives the last length of the venter, and the first length of the rising pipe.
9Thus adjusting the direction of the tubes, both in the descents and acclivities, the work will never be dislodged. For a great rush of air is generated in an aqueduct, strong enough to break even stones, unless the water is softly and sparingly let down from the head, and unless in elbows or bending joints it be restrained by means of ligatures, or a weight of ballast. In other respects it is similar to one with leaden pipes. When the water is first let down from the head, ashes are put in which will stop those joints not sufficiently coated. Earthen pipes have these advantages, first as to the work; next, that if damaged any one can repair it.
10Water conducted through earthen pipes is more wholesome than that through lead; indeed that conveyed in lead must be injurious, because from it white lead is obtained, and this is said to be injurious to the human system. Hence, if what is generated from it is pernicious, there can be no doubt that itself cannot be a wholesome body.
11This may be verified by observing the workers in lead, who are of a pallid colour; for in casting lead, the fumes from it fixing on the different members, and daily burning them, destroy the vigour of the blood; water should therefore on no account be conducted in leaden pipes if we are desirous that it should be wholesome. That the flavour of that conveyed in earthen pipes is better, is shewn at our daily meals, for all those whose tables are furnished with silver vessels, nevertheless use those made of earth, from the purity of the flavour being preserved in them.
12If there be no springs from which water can be obtained, it is necessary to dig wells, on which every care is to be bestowed, and the utmost ingenuity and discretion used in the examination of the natural indications of the circumstances thereabout, inasmuch as the different sorts of soil which are met with, are many and various. That, like every other body, is composed of four elements; first of earth itself; water, whence are the springs; heat, whence sulphur, alum, and bitumen are generated; and air, whence arise great vapours, which, piercing through the pores to the opening of wells, strike upon the excavators and suffocate them by their natural influence, so that those who do not immediately escape lose their lives.
13To avoid this the following method may be adopted; a lighted lamp must be lowered; if it continue to burn, a man may safely descend, but if the strength of the vapour extinguish it, then to the right and left of the well let air holes be dug, so that as it were through nostrils, the vapour may pass off. When this is done and we come to water, the well must be lined with a wall, but in such a manner as not to shut out the springs.
14If the soil be hard, and there be no veins of water found at the bottom, we must then have recourse to cisterns made of cement, in which water is collected from roofs and other high places. The cement is thus compounded; in the first place, the purest and roughest sand that can be had is to be procured; then work must be of broken flint whereon no single piece is to weigh more than a pound, the lime must be very strong, and in making it into mortar, five parts of sand are to be added to two of lime, the flint work is combined with the mortar, and of it the walls in the excavation are brought up from the bottom, and shaped by wooden bars covered with iron.
15The walls being shaped, the earth in the middle is to be thrown out as low as the foot of the walls, and when levelled, the bottom is to be covered with the same materials to the requisite thickness. If these receptacles are made in two or three divisions, so that the water may be passed from one to another, it will be more wholesome for use; for the mud in it will be thus allowed to subside, and the water will be clearer, preserve its flavor, and be free from smell; otherwise it will be necessary to use salt for purifying it. In this book I have explained to my utmost ability the virtues and varieties of waters, their use and conveyance, and how their goodness may be ascertained; in the following book I intend to describe the principles of gnomonics and the rules of dialling.