A short time ago, we posted about the amazing Elephant clock by the ancient inventor and engineer, al-Jazari. This video demonstrates the workings of a simpler, but nevertheless ingenious water clock.
Like most water clocks, also known as a clepsydra, the flow of water measures the passage of time. In this clock, al-Jazari placed a float on the surface of the water. Though buoyant, the float does have weight. As the water level falls, the weight also falls. Ropes (probably fine chains) and pulleys transfer the the linear pull of the weight into the rotary motion of a vertical axle. A scribe figure attached to this rotating axle will then spin slowly, the rod in his hand pointing to the hour of the day.
Several things make this water clock interesting. First, the use of the figure places it in a category with automata. Second, standing at 1.5 meters high the clock is large -- and no doubt heavy -- but still portable. That's most useful. Third, the reservoir that holds the water has been tapered in an effort to keep the flow of water at a constant rate. This would have made the clock more accurate than a straight-sided tank. Finally, this was designed 800 years ago!
You can learn more about water clocks, as well as other ancient forms of time-keeping from this volume of The Time Museum, a book which also covers sand-glasses and fire-clocks.