Ctesibius of Alexandria rebuilt the water clock around a simple problem: a draining tank empties fastest when it is full, so the hours it marks are uneven. He added a second vessel held at a constant level, which let water trickle out at a steady rate and lift a float that pointed at the hour. Nothing kept time more accurately until the pendulum clock, roughly nineteen centuries later.
Why this oneA fix for uneven hours that held the accuracy record for nineteen centuries.