A Cofferdam was a double skinned box made of planks with the space between the inner and outer planks being filled with clay. This box was floated into position and then weighted down until it sank. The water was pumped out with buckets or a device called an Archimedean screw or tympanum The Cofferdams were built, so the laborers could erect the piers without going under water. First the piles of the Cofferdam, a piece of wood sharpened like a pencil, were driven into the riverbed, then pumped. When the pumping was finished, each pier stood on a foundation of tar covered piles and was constructed of carefully cut stones on the inside. The mortar used between the stones contained pozzolana, a volcanic ash. 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



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