speculated that the device might have included technology similar to that of the Greek Antikythera mechanism. The odometer of Vitruvius was based on chariot wheels of 4 Roman feet (1.18 m) diameter turning 400 times in one Roman mile (about 1,480 m). For each revolution a pin on the axle engaged a 400-tooth cogwheel thus turning it one complete revolution per mile. This engaged another gear with holes along the circumference, where pebbles (calculus) were located, that were to drop one by one into a box. The distance traveled would thus be given simply by counting the number of pebbles. Whether this instrument was ever built at the time is disputed. Leonardo da Vinci later tried to build it himself according to the description, but failed. However, in 1981 engineer Andre Sleeswyk built his own replica, replacing the square-toothed gear designs of Leonardo with the triangular, pointed teeth found in the Antikythera mechanism. With this modification, the Vitruvius odometer functioned perfectly. Imperial China Han Dynasty and Three Kingdoms period The odometer was also independently invented in ancient China, possibly by the prolific inventor and early scientist Zhang Heng (78 AD – 139 AD) of the Han dynasty. By the 3rd century (during the Three Kingdoms Period), the Chinese had termed the device as the 'jì lĭ gŭ chē' (記里鼓車), or 'li-recording drum carriage' (Note: the modern measurement of li = ). Chinese texts of the 3rd century tell of the mechanical carriage's functions, and as one li is traversed, a mechanical-driven wooden figure strikes a drum, and when ten li is traversed, another wooden figure would strike a gong or a bell with its mechanical-operated arm. Despite its association with Zhang Heng or even the later Ma Jun (c. 200–265), there is evidence to suggest that the invention of the odometer was a gradual process in Han Dynasty China that centered around the huang men court people (i.e. eunuchs, palace officials, attendants and familiars, actors, acrobats, etc.) that would follow the musical procession of the royal 'drum-chariot'. The historian Joseph Needham asserts that it is no surprise this social group would have been responsible for such a device, since there is already other evidence of their craftsmanship with mechanical toys to delight the emperor and the court. There is speculation that some time in the 1st century BC (during the Western Han Dynasty), the beating of drums and gongs were