=61. The Rules of Vitruvius for Harbors.=—In Chapter XII of his Fifth Book Vitruvius lays down certain general rules for the selection and formation of harbors, and it is known that the Romans were familiar with elaborate and effective harbor construction, as is shown by that at Ostia. He appreciates that a natural harbor is one which has “rocks or long promontories jutting out, which from the shape of the place form curves or angles,” and that in such places “nothing more is necessary than to construct portices and arsenals around them, or passages to the markets.” He then proceeds to state that if such a natural formation is not to be found, and that if “on one side there is a more proper shore than on the other, by means of building or of heaps of stones, a projection is run out, and in this the enclosures of harbors are formed.” He then proceeds to explain how “pozzuolana” and lime, in the proportion of two of the former to one of the latter, are used in subaqueous construction. He also prescribed a mode of building a masonry wall up from the bottom of an excavation made within what we should call a coffer-dam, formed, among other things, “of oaken piles tied together with chain pieces.” The Romans knew well how to select harbors and how to construct in an effective manner the artificial works connected with them, although it appears that the effects of tidal and river currents in estuaries were neither well understood in themselves nor in their transporting power of the solid material which those currents eroded. =62. The Thrusts of Arches and Earth; Retaining-walls and Pavements.=—Although the Romans possessed little or no knowledge of analytical mechanics they attained to some good qualitative mechanical conceptions. Among other things they understood fairly well the general character of the thrust of an arch and the tendency of the earth to overthrow a retaining-wall. They knew that a massive abutment was needed to receive safely the thrust of an arch, and they counterforted or buttressed retaining-walls in order to hold them firmly in place. They also realized the danger of wet earth pressing against a retaining-wall, and even made a series of offsets or teeth on the inside of the wall on which the earth rested in order to aid in holding the wall in place. Vitruvius recommends as a safeguard against the pressure of earth wet by winter rains that “the thickness of the wall must be proportioned to the weight of earth against it,” and that counterforts or buttresses be employed “at a distance from each other equal to the height of the foundations, and of the same width as the foundations,” the projections at the bottom being equal in thickness to that of the wall, and diminishing toward the top.