The water clock has three parts.On the left is a tank holding water.This tank has a narrow opening through which water drips into a second container,also a tank.Inside the second tank is a floating object,to the end of which is attached a long pole with an arrow.On the right is a cylinder with the numbers one to twelve written on it.As the water from the tank on the left drips into the second tank,the level rises and with it the lfoating object.This causes the arrow to move from one number to the next.One thus has a twelve hour clock.After twelve hours the second tank is emptied and the process is begun again.