Jantar Mantar is an assembly of stone-built astronomical instruments, designed to be used with the naked eye. There were five Jantar Mantars in India, all of them built at the command of the Rajah Jai Singh II, who had a keen interest in mathematics, architecture and astronomy; four remain, as the Jantar Mantar at Mathura was torn down just before the revolt of 1857. The largest example is the equinoctial sundial seen of the picture belonging to Jaipur's assembly.