Clepsydra, from the Greek for "water thief" is a term used to describe any form of water clock, devices used to measure the passage of time with water. Days, months (well lunar months), seasons and years can all be measured easily by observing the world around us, but hours and seconds are human inventions, which require consistent devices to measure.
While there is evidence that the Chinese invented water clocks as early as 3000 B.C., it is clear that they were also invented by the Egyptians around 1500 B.C. In roughly 325 B.C. the Egyptian design made its way to Greece where it was given the name Clepsydra.
While primitive Clepsydras were often much more accurate than watching the sun, they were still not accurate to today’s standards, the best being accurate to within 15 minutes per day: impressive for 5000 years ago, but not acceptable today.
The two major types of Clepsydras are outflow clocks and inflow clocks. Outflow clocks are essentially containers with small holes in them to allow water to seep out at a regular rate. Rings are placed around the inside of the container to denote periods of time. An inflow clock is the opposite, water is poured into a container at a steady rate, and as water covers up rings, the passage of time is marked.
Water annoyingly tends to drip out of a Clepsydra slower, as the water level drops. To counter this, the Egyptians began to build tapered containers to keep the water flow regular. The Greeks brought something revolutionary to the design, a floating valve. If the water sank below a certain level, the float valve would add more water until it reached the high point. This kept the amount of water in the contained relatively constant so an inflow Clepsydra could operate almost indefinitely.
The most famous Clepsydra was built in China by Su Sung in the early 11th century. The clock was more then 30 feet tall and use a primitive escapement mechanism (something that would later be included in all mechanical clocks). Su Sung's clock displayed not only the time, but the position of the planets, and included animated figures that rung bells at various times.
No matter how complex you make your Clepsydra, you still have to fight evaporation, and the fact that the flow of water will never be precise over time. Clepsydras also loose accuracy when moved from a different temperature, level of humidity, or a different barometric pressure. Clepsydras are an ancient invention that are still practical for certain application and can be constructed out of scrap materials around the house. A simple Clepsydra can be made by punching a pin hole in the cap of a two liter bottle of soda once it has been emptied and cutting off the bottom of the bottle. When inverted and filled with water, the water will run out through the hole in the cap at a regular rate, and you can mark off the water level ever minute until the bottle is empty. Now, ever time you fill up the bottle, you have your very own homemade outflow Clepsydra.