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.
15Then after having beaten the walls, let all the earth between them be cleared out to a level with the very bottom of the walls. Having evened this off, let the ground be beaten to the proper density. If such constructions are in two compartments or in three so as to insure clearing by changing from one to another, they will make the water much more wholesome and sweeter to use. For it will become more limpid, and keep its taste without any smell, if the mud has somewhere to settle; otherwise it will be necessary to clear it by adding salt.
In this book I have put what I could about the merits and varieties of water, its usefulness, and the ways in which it should be conducted and tested; in the next I shall write about the subject of dialling and the principles of timepieces.