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.