An odometer is a device used for indicating distance traveled by a vehicle (precursor of the taximetre) . Vitruvius around 27 to 23 BC describes such a device although the actual invention may have been by Archimedes during the First Punic War. Heron also describes an odometer in chapter 34 of his Dioptra. It consisted of a box with cooperating screws and gearwheels attached to a moving vehicle.
One axial rod on one of the vehicle'w wheels carried the motion to the first eight-toothed (gear)wheel in the box, while the calibrated discs on the outer top surface of the box which where embedded on the axles indicated the distance travelled. The ratio in the proposed Heron's construction is 1:8:30:30:30 so a full rotation of the last disc corresponds to 216000 revolutions of the vehicle's wheels.
By the wheel's diametre of 1.6 metres the distance is 1080 kilometres. In a variation of the device one calibrated disc had radial holes with balls that when one of them was aligned with the corresponfing hole of the box the ball felt into a metal vessel offering easy measurement of the distance.
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