The hodometer, as a vehicle provided with gearing to measure the distance travelled, cannot be reckoned among the more important surveying instruments of antiquity. But while its interest lies more in its mechanical complexities than in its contribution to the surveyor's work, it does deserve brief discussion.

We have two detailed specifications, by Vitruvius and by Hero. In Vitruvius'version (Source 59), which was ‘handed down by our predecessors’, the road wheel is one 400th of a mile in circumference. Attached to its hub is a disc carrying a single tooth. At every revolution of the wheel this tooth advances a vertical pinion, which carries 400 teeth, by one tooth. Similarly at every revolution of the vertical pinion a projecting tooth advances a horizontal pinion by one tooth. In the horizontal pinion is a series of holes, one corresponding to each tooth and each containing a pebble. After a mile, therefore, the road wheel has revolved 400 times, the vertical pinion has revolved once, and the horizontal pinion advances by one tooth. The first hole containing a pebble now coincides with a hole in the casing below, which allows the pebble to drop through into a bronze bowl, giving audible notice that a mile has been completed. At the end of the journey the number of pebbles in the bowl indicates the miles travelled.