1. I will now explain the supply of water to country houses and to towns. The first stage is to fix levels. This is done by dioptrae, or water levels, or the chorobates. But the more accurate method is by the chorobates because the dioptrae and the water levels mislead. The chorobates is a straight plank about twenty feet long. At the extreme ends it has legs made to correspond, and fastened at right angles to the ends of the plank, and, between the plank and the legs, cross-pieces joined by tenons. These have lines accurately drawn to a perpendicular, and plummets hanging severally over the lines from the plank. When the plank is in position, the perpendiculars which touch equally and of like measure the lines marked, indicate the level position of the instrument.