<P> The odometer of Vitruvius was based on chariot wheels of 4 feet (1.2 m) diameter turning 400 times in one Roman mile (about 1400 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 da Vinci with the triangular, pointed teeth found in the Antikythera mechanism . With this modification, the Vitruvius odometer functioned perfectly . </P> <P> 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 = 500 m / 1640 ft). 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 . </P> <P> 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 craftsmenship 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 mechanically - driven by working automatically off the rotation of the road - wheels . This might have actually been the design of one Loxia Hong (c. 110 BC), yet by 125 AD the mechanical odometer carriage in China was already known (depicted in a mural of the Xiaotangshan Tomb). </P> <P> The odometer was used also in subsequent periods of Chinese history . In the historical text of the Jin Shu (635 AD), the oldest part of the compiled text, the book known as the Cui Bao (c. 300 AD), recorded the use of the odometer, providing description (and interestingly enough attributing it to the Western Han era, from 202 BC--9 AD). The passage in the Jin Shu expanded upon this, explaining that it took a similar form to the mechanical device of the south - pointing chariot invented by Ma Jun (200--265, see also differential gear). As recorded in the Song Shi of the Song Dynasty (960 - 1279 AD), the odometer and south - pointing chariot were combined into one wheeled device by engineers of the 9th century, 11th century, and 12th century . The Sunzi Suanjing (Master Sun's Mathematical Manual), dated from the 3rd century to 5th century, presented a mathematical problem for students involving the odometer . It involved a given distance between two cities, the small distance needed for one rotation of the carriage's wheel, and the posed question of how many rotations the wheels would have in all if the carriage was to travel between point A and B . </P>

What is the meaning of odo in cars