Pipelines and Lifelines
Water is heavy and sluggish, so moving large quantities of it from one place to another is not a simple task. While the technology behind the aqueducts that first performed this job has evolved over the millenniums, the principle has remained the same: Make gravity do the work.
The Romans are generally credited with perfecting the early aqueducts that brought water to their cities, though the Assyrians had built similar structures several centuries earlier. Traces of the most famous pre-Roman aqueduct, built by King Sennacherib for his capital city of Nineveh sometime around 700 B.C., are still visible in the north of Iraq.
Multitiered stone arcades remain the iconic image of the ancient aqueducts, but most of the routes consisted of either tunnels or pipelines at a very shallow downward slope so that the water would naturally flow from an elevated source down to the city.
In places where a valley intervened along the way, the Romans would often utilize a cheaper alternative to the arcades. Using U-shaped pipe lines known as "inverted siphons," they would route the flow down a valley and back up again, relying only on the pressure at the receiving end of the pipe to power the water back up the opposite hill. (This required the mouth of the pipe where the water emerged to be at a lower elevation than the source.)
Many major U.S. cities, including New York and Los Angeles, still use similar technology to supply water to their residents.
New York boasts that unless there is a drought, 95 percent of its water is still delivered by gravitythe cheapest and most renewable source of power on Earth.
This story appears in the June 4, 2007 print edition of U.S. News & World Report.