Pont du Gard, France (No. 7)
The Pont du Gard was constructed largely without the use of mortar or clamps. It contains an estimated 50,400 tons of limestone with a volume of some 21,000 m3 (740,000 cu ft); some of the individual blocks weigh up to 6 tons. Most of the stone was extracted from the local quarry of Estel located approximately 700 metres (2,300 ft) downstream, on the banks of the Gardon River. The coarse-grained soft reddish shelly limestone, known locally as "Pierre de Vers", lends itself very well to dimension stone production. The blocks were precisely cut to fit perfectly together by friction and gravity, eliminating the need for mortar. The builders also left inscriptions on the stonework conveying various messages and instructions. Many blocks were numbered and inscribed with the required locations, such as fronte dextra or fronte sinistra (front right or front left), to guide the builders.
The method of construction is fairly well understood by historians. The patron of the aqueduct – a rich individual or the city of Nîmes itself – would have hired a large team of contractors and skilled labourers. A surveyor or mensor planned the route using a groma for sighting, the chorobates for levelling, and a set of measuring poles five or ten Roman feet long. His figures and perhaps diagrams were recorded on wax tablets, later to be written up on scrolls. The builders may have used templates to guide them with tasks that required a high degree of precision, such as carving the standardised blocks from which the water conduit was constructed.
Source: Wikipedia