Watching time with water clock is one of the oldest form. In water clock the time is measured by the help of flow of liquid that's why it is called water clock. it is one of the common and simplest form of time measuring in ancient time.
there were TWO types of water clock INFLOW and OUTFLOW water clock.
In an OUTFLOW water clock, a container is filled with water, and the water is drained slowly and evenly out of the container. This container has markings that are used to show the passage of time. As the water leaves the container, an observer can see where the water is level with the lines and tell how much time has passed.
An INFLOW water clock works in basically the same way, except instead of flowing out of the container, the water is filling up the marked container. As the container fills, the observer can see where the water meets the lines and tell how much time has passed.
It is one and only way to watch time in ancient time water clock is invented by Egyptians around 1500 years before.