'Pont du Gard', France, 19th century.

Lithograph by E Leroux showing the Pont du Gard near Toulouse in the south of France. The Pont du Gard was built in about 20 BC by the Romans as part of a 50 kilometre-long aqueduct to supply water to the city of Nimes. The total altitude difference between the headwaters and the city was only 17 metres, meaning that the incline of the aqueduct was just 34 centimetres per kilometre. The Pont du Gard itself is 275 metres long and 49 feet high. It was built acros the River Gard of stone without the aid of mortar and consists of a three-tiered series of arches. The lower level carried a road, while the water conduit was built on the top level. Today the Pont du Gard is a World Heritage Site.