ROME: VITRUVIUS 33 



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. 



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 em- 

 broidery), 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 Architecture* 



