The second book begins with an account of different philosophical views concerning the origin of matter, and a discussion of the earliest dwellings of man. Its real theme, however, is building-material--brick, sand, lime, stone, concrete, marble, stucco, timber, pozzolano. In reference to the last (volcanic ash combined with lime and rubble to form a cement) Vitruvius writes in a way that indicates a discriminating knowledge of geological formations. Likewise his discussion of the influence of the Apennines on the rainfall, and, consequently, on the timber of the firs on the east and west of the range, shows a grasp of meteorological principles. His real power to generalize is shown in connection with his specialty, in his treatment of the sources of building-material, rather than in his consideration of the origin of matter. Similarly the fifth book begins with a discussion of the theories of Pythagoras, but its real topic is public buildings--fora, basilicas, theaters, baths, palæstras, harbors, and quays. In the theaters bronze vases of various sizes, arranged according to Pythagorean musical principles, were to be used in the auditorium to reinforce the voice of the actor. (This recommendation was misunderstood centuries later, when Vitruvius was considered of great authority, and led to the futile practice of placing earthenware jars beneath the floors of church choirs.) According to our author, "The voice arises from flowing breath, sensible to the hearing through its percussion on the air." It is compared to the wavelets produced by a stone dropped in water, only that in the case of sound the waves are not confined to one plane. This generalization concerning the nature of sound was probably not original, however; it may have been suggested to Vitruvius by one of the Aristotelian writings.