According to Encyclopedia Britannia, "About 15 BC, the Roman architect and engineer Vitruvius mounted a large wheel of known circumference in a small frame, in much the same fashion as the wheel is mounted on a wheelbarrow; when it was pushed along the ground by hand it automatically dropped a pebble into a container at each revolution, giving a measure of the distance traveled. It was, in effect, the first odometer."
These odometers were used in taxi carriages. Each time the wheel of the carriage turned, a pebble, a calculus, dropped from a container into another. In the end of the ride, the driver counted how many pebbles had dropped, and that determined the price of the transportation. This kind of usages of pebbles gave the word Calculus its present meaning.