One usually is filled with sand that passes to the second glass.
The time that takes the sand to get to the other glass is the basic unit of measurement of the hourglass. After all the sand is done it's possible to turn the hourglass over , and measure the same unit of time.
Some say that the history of the hourglass had already started from the 11th century with the Pirates. The shape of the sea clock was often appears on the Pirate's flags, and signed that every thing has an end .
The Sundial.
The Sundial is a clock, that uses the movement of the sun to deliver a shadow on an expected hour. That was one of the first clocks that was invented in the world.
The first sundial that humans invented was by Obelisk in ancient Egypt in 3500 BC. This was a stick stuck in the ground, its shadow was on the floor, and there were signs that shows the time. Because the sun rises from east to west through the south it was possible to find out the hour.
In 1500 BC the use of the clock became more advanced, meaning that units of time were smaller and more precise so the people could know the time more specifically. After that the water-clock was invented.
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