In the same manner the voice executes its movements in concentric circles; but while in the case of water the circles move horizontally on a plane su… - Vitruvius

Hence, as the line of sight to the upper part is the longer, it makes that part look as if it were leaning back. But when the members are inclined to the front, as described above, they will seem the beholder to be plumb and perpendicular.

Both kinds should be constructed of the smallest stones, so that the walls, being thoroughly puddled with the mortar, which is made of lime and sand, may hold together longer. If the stones used are soft and porous, they are apt to suck the moisture out of the mortar and so to dry it up. But when there is abundance of lime and sand, the wall, containing more moisture, will not soon lose its strength, for they will hold it together. But if the moisture is sucked out of the mortar by the porous rubble, and the lime and sand separate and disunite, the rubble can no longer adhere to them and the wall will in time become a ruin.

For siege works against bold and venturesome men should be constructed on one plan, on another against cautious men, and on still another against the cowardly.