Pont du Gard
This aqueduct is one of the great sights of southern France, the best preserved Roman aqueduct in Europe. It is a UNESCO World Heritage Monument. There is no entrance charge. It was built on the orders of Marcus Agrippa, the son-in-law of Augustus, around 20 BC as part of a conduit to bring water from the river Eure at Uzès to the major Roman city of Nimes. The site is truly spectacular. It lies in a wilderness setting among luxuriant vegetation above the gorges of the river Gard (or Gardon). If you have time and the weather is good you can swim here. Picnicking is also possible. There is parking on both sides of the river. You can cross from one side to the other over the road, a medieval addition, on the lowest level of arches (pedestrians only). There are cafes, loos and souvenir shops on both sides of the river. The rive gauche is probably the best side for the views. The little visitor centre, which is interesting enough but not unmissable, is on the rive droite. Crossing over the top level is officially discouraged, especially in very high winds when it becomes dangerous. The path may also be blocked off due to restoration works or new regulations. If you can do it, you should. It's a fantastic feeling.
The aqueduct is composed of three superimposed rows of arches. The masonry blocks are so accurately cut and the distribution of the load so precisely calculated that it requires no mortar or cement to hold it together. Some of the blocks weigh as much as 6 tonnes. It is about 900 ft. long and stands 150 ft. above the river. The water flowed in a channel across the top level, covered but with some parts open to the air for the purposes of ventilation. The construction workers were legionnaires and slaves.
The river Eure at Uzès, the point where the waters were drawn off, is 56 ft. higher up than Nimes. The distance to be covered was 31 miles. The water had to flow at a slow and constant rate because otherwise limestone deposits would build up and the canal would either be blocked or destroyed. The canal's descent over the whole distance is a precise and constant 21 inches per mile. This way 44 million gallons of water would flow daily into Nimes under control and in perfect safety. The sheer accuracy of these precision calculations, made 2,000 years ago in country that remains today something of a wilderness, coupled with their perfect execution is mind-boggling. The Pont du Gard, as well as being a thing of great beauty, is a work of engineering genius of the highest quality. The aqueduct functioned for 900 years. It only ceased in use at that time because locals were using blocks from the connecting conduits either side of the Gard for their own building purposes (you can see clearly how the north side has been partially destroyed). It was still potentially structurally sound.
(Incidentally, the Romans knew about the principle of siphon conduits, ie. that water will flow uphill as long as the starting point is higher up than the finishing point, but elevated aqueducts like this one or Segovia in Spain were often cheaper to build and always more spectacular.)
