Remains of the water wheels used for lifting water were discovered when old mines were re-opened at Rio Tinto in Spain, Rosia Montana in Romania and Dolaucothi in west Wales. The Rio Tinto wheel is now shown in the British Museum, and the Dolaucothi specimen in the National Museum of Wales. Surveying instruments That he must have been well practised in surveying is shown by his descriptions of surveying instruments, especially the water level or chorobates, which he compares favourably with the groma, a device using plumb lines. They were essential in all building operations, but especially in aqueduct construction, where a uniform gradient was important to the provision of a regular supply of water without damage to the walls of the channel. He also developed one of the first odometers, consisting of a wheel of known circumference that dropped a pebble into a container on every rotation. Central heating He describes the many innovations made in building design to improve the living conditions of the inhabitants. Foremost among them is the development of the hypocaust, a type of central heating where hot air developed by a fire was channelled under the floor and inside the walls of public baths and villas. He gives explicit instructions how to design such buildings so that fuel efficiency is maximised, so that for example, the caldarium is next to the tepidarium followed by the frigidarium. He also advises on using a type of regulator to control the heat in the hot rooms, a bronze disc set into the roof under a circular aperture which could be raised or lowered by a pulley to adjust the ventilation. Although he does not suggest it himself, it is likely that his dewatering devices such as the reverse overshot water-wheel were used in the larger baths to lift water to header tanks at the top of the larger thermae, such as the Baths of Diocletian. The one which was used in Bath of Caracalla for grinding flour. Legacy Vitruvian Man – a drawing by Leonardo da Vinci Vitruvius Britannicus – 18th century work on British architecture named after Vitruvius. Den Danske Vitruvius – 18th century work on Danish architecture – inspired by Vitruvius Britannicus. The American Vitruvius – 20th century work on civil architecture by Werner Hegemann William Vitruvius Morrison (1794–1838), the son of Irish architect Sir Richard Morrison and himself a noted architect of great houses,