The finer portion of the sand of the river Nilus is not very different in its properties from the dust of Puteoli; not, indeed, that it is used for breaking the force of the sea and withstanding the waves, but only for the purpose, forsooth, of subduing[2231] the body for the exercises of the palestra! At all events, it was for this purpose that it used to be brought over for Patrobius,[2232] a freedman of the Emperor Nero. I find it stated also, that Craterus, Leonnatus, and Meleager, generals of Alexander the Great, had this sand transported along with their munitions of war. But I forbear to enlarge any further upon this subject; or indeed, by Hercules! upon those preparations of earth and wax of which the ceromata are made, so much employed by our youth in their exercises of the body, at the cost of all vigour of the mind. CHAP. 48. (14.)—FORMACEAN WALLS. And then, besides, have we not in Africa and in Spain walls[2233] of earth, known as “formacean” walls? from the fact that they are moulded, rather than built, by enclosing earth within a frame of boards, constructed on either side. These walls will last for centuries, are proof against rain, wind, and fire, and are superior in solidity to any cement. Even at this day, Spain still beholds watch-towers that were erected by Hannibal, and turrets of earth[2234] placed on the very summits of her mountains. It is from the same source, too, that we derive the substantial materials so well adapted for forming the earth-works of our camps and embankments against the impetuous violence of rivers. What person, too, is unacquainted with the fact, that partitions are made of hurdles coated with clay, and that walls are constructed of unbaked bricks? CHAP. 49.—WALLS OF BRICK. THE METHOD OF MAKING BRICKS. Earth for making bricks should never be extracted from a sandy or gravelly soil, and still less from one that is stony; but from a stratum that is white and cretaceous, or else impregnated with red earth.[2235] If a sandy soil must be employed for the purpose, it should at least be male[2236] sand, and no other. The spring is the best season for making bricks, as at midsummer they are very apt to crack. For building, bricks two years old are the only ones that are approved of; and the wrought material of them should be well macerated before they are made.