It was a mechanism for the accurate measurement of road distances, precursor to the taximeter. It consisted of a box with co-operating worm gears and gearwheels attached to a moving vehicle. One axial rod on the hub of one of the vehicle’s 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 were incorporated 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 corresponded to 216.000 revolutions of the vehicle’s wheels. By the wheels’ diameter of 1.60 metres the distance is 1080 kilometres.
In a variation of the device, one calibrated disc had radial holes with spherules. When one of the holes was aligned with a corresponding hole of the box, a spherule fell into a metal vessel offering easy measurement of the distance. Archimedes is probably the inventor of this device. (Ioannis Tzetzes, Chiliades (Thousands), 2, 12th century A.D.).

SOURCES: Vitruvius, On Architecture; Heron of Alexandria, On Dioptra.