The water clock was developed alongside the sundial as two of the most accurate means of telling time in the ancient world. Though most waster clocks were less precise than sun dials, they could be used during the night, indoors and on overcast days. All clocks, be they water clocks or modern timepieces have the same basic characteristics. They use a regular action to mark off equal increments of time, and they have a marked off display for showing those regular increments.
Although ancient Greeks and Egyptians used clay vessels for water clocks, you can make one using common household items. Materials used to make a water clock include a pin, a 1-liter plastic soft drink bottle, a timer, a ruler, a permanent marker and a graduated cylinder that can hold up to 1 liter of water.
Water clocks consisted of two types, an inflow clock and an outflow clock. Inflow clocks measured time by the amount of water that flowed into them, while outflow clocks had sloping sides that allowed water to drain at a constant rate of speed. An outflow clock had markings on its interior to designate the passage of time.
Use a pin to make a hole in the bottom of the bottle. If you can, make the hole smaller than the pin. Fill the bottle up to the level where the bottle begins to narrow. This will be about to the bottle's shoulder. Hold your finger over the pin hole to prevent the water from running out. Use a permanent marker to mark the level on the outside of the bottle. Use the timer to measure the amount of time it takes for the water to drain from the bottle. Repeat this process with the water bottle half full, and then again when the water bottle is only a third full. This should give you an idea of the rate of time for which water drains from your water clock.