the dome. Even after more than 2,000 years some Roman structures still stand, due in part to sophisticated methods of making cements and concrete. Roman roads are considered the most advanced roads built until the early 19th century. The system of roadways facilitated military policing, communications, and trade. The roads were resistant to floods and other environmental hazards. Even after the collapse of the central government, some roads remained usable for more than a thousand years. Roman bridges were among the first large and lasting bridges, built from stone with the arch as the basic structure. Most used concrete as well. The largest Roman bridge was Trajan's bridge over the lower Danube, constructed by Apollodorus of Damascus, which remained for over a millennium the longest bridge to have been built, both in overall span and length. The Romans built many dams and reservoirs for water collection, such as the Subiaco Dams, two of which fed the Anio Novus, one of the largest aqueducts of Rome. They built 72 dams just on the Iberian peninsula, and many more are known across the Empire, some still in use. Several earthen dams are known from Roman Britain, including a well-preserved example from Longovicium (Lanchester). The Romans constructed numerous aqueducts. A surviving treatise by Frontinus, who served as curator aquarum (water commissioner) under Nerva, reflects the administrative importance placed on ensuring the water supply. Masonry channels carried water from distant springs and reservoirs along a precise gradient, using gravity alone. After the water passed through the aqueduct, it was collected in tanks and fed through pipes to public fountains, baths, toilets, or industrial sites. The main aqueducts in the city of Rome were the Aqua Claudia and the Aqua Marcia. The complex system built to supply Constantinople had its most distant supply drawn from over 120 km away along a sinuous route of more than 336 km. Roman aqueducts were built to remarkably fine tolerance, and to a technological standard that was not to be equalled until modern times. The Romans also made use of aqueducts in their extensive mining operations across the empire, at sites such as Las Medulas and Dolaucothi in South Wales. Insulated glazing (or "double glazing") was used in the construction of public baths. Elite housing in cooler climates might have hypocausts, a form of central heating. The Romans were the first culture to assemble all essential components of the