Dome

Bascule bridge, detail of tower base; Amsterdam has more than one hundred kilometers of canals, about 90 islands and 1,500 bridges. The three main canals, Herengracht, Prinsengracht, and Keizersgracht, dug in the 17th century during the Dutch Golden Age, form concentric belts around the city. This 17th century canal ring has been designated a UNESCO World Heritage site. Canals are sealed from the North Sea via locks, purportedly a Dutch invention of the 14th century. To maintain sanitary conditions, the canals are flushed three times a week when the locks are open and fresh water flows in from Lake IJssel. Amsterdam has many people living on houseboats in the canals; since 2005 these are now all connected into the city sewer system. Many of the bridges are bascule bridges which can be raised to allow ship passage; center sections of the span are pivoted upward by use of counterweights. Source: Wikipedia; http://en.wikipedia.org/wiki/Main_Page (accessed 6/3/2014)