When the piers reached a height of 30 feet above the river, wooden arches were hoisted into its place between them. A wooden road was nailed to the arches and covered with a layer of earth. The finished road stood over sixty feet above the river. For major or busy bridges sometimes stone went in place of wood. The semicircular arch was usually used for bridges. Aqueducts were like a bridge but built over land and carried pipes of water instead of a roadbed. Rome alone needed 340 million gallons of water per day to supply its great arched baths and other needs. Surveyors made an imaginary line with measuring sticks and a chorobate, a leveling instrument like the level used today, to make a straight line for a profile map. The surveyors designed a plan to keep the water from rushing too fast. They made a system of a fall of six inches for every one hundred feet and often added long detours to avoid a too sudden descent. Due to the materials and construction methods, many of the aqueducts survived and are still used today. Sometimes a route of an aqueduct required that a short tunnel be dug through a hill. Every twenty yards vertical shafts were sunk from the surface of the hill to the level of the proposed aqueduct. The depths of the shafts were measured from the profile map. The engineers used the profile map to construct foundations up to the imaginary line, then built the piers for fifty feet, then put the stone arches in. The reason the arches were built high over the ground was to prevent people from stealing or poisoning the water. The pipes were made with the inner surfaces lined with hard cement to prevent leaks, then a rectangular pipe was built to cover the water pipes. When the aqueduct hit a hill underground pipes were built, the profile maps measured the depths of the new pipes to be placed. When the shafts were dug a pulley was put over the opening of the shaft to lower men, tools, and materials needed down the shafts.
