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 da Vinci with the triangular, pointed teeth found in the "Antikythera mechanism". With this modification, the Vitruvius odometer functioned perfectly.