Unlike the clepsydra, the motion of the ship while sailing did not affect the hourglass.
From the en.wikipedia.org
Clepsydra, literally water thief, is the Greek word for water clock.
From the en.wikipedia.org
Today, in Beijing's Drum Tower an outflow clepsydra is operational and displayed for tourists.
From the en.wikipedia.org
However, the most common type of public and palace timekeeping device was the inflow clepsydra.
From the en.wikipedia.org
A commonly used water clock was the simple outflow clepsydra.
From the en.wikipedia.org
Shen is credited with making improved designs of the gnomon, armillary sphere, and clepsydra clock.
From the en.wikipedia.org
They introduced several types of the inflow clepsydra, one of which included the earliest feedback control system.
From the en.wikipedia.org
In both Greek and Roman times, this type of clepsydra was used in courts for allocating periods of time to speakers.
From the en.wikipedia.org
Unlike water in a clepsydra, the rate of flow of sand is independent of the depth in the upper reservoir, and the instrument is not liable to freeze.
From the en.wikipedia.org
More examples
Water clock: clock that measures time by the escape of water
A water clock or clepsydra (Greek u03BAu03BBu03B5u03C8u03CDu03B4u03C1u03B1 from u03BAu03BBu03ADu03C0u03C4u03B5u03B9u03BD kleptein, 'to steal'; u1F55u03B4u03C9u03C1 hydor, 'water') is any timepiece in which time is measured by the regulated flow of liquid into (inflow type) or out from (outflow type) a vessel where the amount is then measured.
Clepsydra is a recently described genus of diatoms, including the species Clepsydra truganiniae. It was found in Tasmania.
A water clock, especially as used in the ancient world
Water clock used by Chinese and Romans. The latter sometimes attached a bell, thus making their clocks the first to chime.
A water clock, esp. as used to measure the time allotted to orators.
A clepsydra is a vessel with a hole in the bottom and one on the top.