3e SRD:Water Clock
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This large, bulky contrivance gives the time accurate to within half an hour per day since it was last set. It requires a source of water, and it must be kept still because it marks time by the regulated flow of droplets of water. It is primarily an amusement for the wealthy and a tool for the student of arcane lore. Most people have no way to tell exact time, and there's little point in knowing that it is 2:30 P.M. if nobody else does.