The seventh book treats of interior decoration--mosaic floors, gypsum mouldings, wall painting, white lead, red lead, verdigris, mercury (which may be used to recover gold from worn-out pieces of embroidery), encaustic painting with hot wax, colors (black, blue, genuine and imitation murex purple). The eighth book deals with water and with hydraulic engineering, hot springs, mineral waters, leveling instruments, construction of aqueducts, lead and clay piping. Vitruvius was not ignorant of the fact that water seeks its own level, and he even argued that air must have weight in order to account for the rise of water in pumps. In his time it was more economical to convey the hard water by aqueducts than by such pipes as could then be constructed. The ninth book undertakes to rehearse the elements of geometry and astronomy--the signs of the zodiac, the sun, moon, planets, the phases of the moon, the mathematical divisions of the gnomon, the use of the sundial, etc. One feels in reading Vitruvius that his purpose was to turn to practical account what he had gained from the study of the sciences; and, at the same time, one is convinced that his applications tend to react on theoretical knowledge, and lead to new insights through the suggestion of new problems. The tenth book of the so-called _De Architectura_ is concerned with machinery--windmills, windlasses, axles, pulleys, cranes, pumps, fire-engines, revolving spiral tubes for raising water, wheels for irrigation worked by water-power, wheels to register distance traveled by land or water, scaling-ladders, battering-rams, tortoises, catapults, scorpions, and ballistæ. On the subject of war-engines Vitruvius speaks with special authority, as he had served, probably as military engineer, under Julius Cæsar in 46 B.C., and had been appointed superintendent of ballistæ and other military engines in the time of Augustus. It was to the divine Emperor that his book was dedicated as a protest against the administration of Roman public works. In its pages we see reflected the life of a nation employed in conquering and ruling the world, with a genius more distinguished for practical achievement than for theory and speculation. Its author is truly representative of Roman culture, for nearly everything that Rome had of a scientific and intellectual sort it drew from Greece, and it selected that part of Greek wisdom that ministered to the daily needs of the times. In his work on architecture, Vitruvius shows himself a diligent and devoted student of the sciences in order that he may turn them to account in his own department of technology.