I also found the TV production of "Longitude" rewarding to
watch, although I was much more interested in the original
creativity of the Harrisons and their battles with what would
today be called "academia" and the astronomers' complaint that 
the Harrisons weren't qualified to do what they had obviously
already accomplished.

In the early days of seismology the best chronometers were large
compensated pendulums. The earliest need for a portable time source
was for timing large blasts that would be recorded by fixed stations.
For this early work ships chronometers were used. Later, attempts
at portable field seismographs were made, so the chronometers were
equipped with contact closures to output a minute and hour mark. I
have two such chronometers in my collection. They are gimbal mounted in
rugged cases. They were made in 1941 by the Hamilton Company.

These ships chronometers are about the same size as the "H-4" of
the Harrisons. They have the temperature compensated balance wheel
that solved the longitude problem. They also have some common features
of ships chronometers, such as an indicator dial to show how wound up
the clock is, with a 48 hour capacity, with a ratcheted winding key (to
keep a sturdy seaman from breaking the gear). They are difficult to
set (the glass cover has to be removed) to prevent the time from 
being changed during a voyage. OF course, they were wound daily, so they
never stopped, and were as accurate as the drift error rate that was 
determined at an observatory when they were started.

A recent Discovery show showed a set of three on a modern carrier.
(three of any doubtable instrument, which includes tiltmeters,
magnetometers, and such, provides some confidence of the data of
the two more coherent instruments). Of course, the ships's navigation
is based on modern electronics. The the mechanical chronometers are
a teaching tool, with which the sailors also learn to use a sextant to 
locate themselves. But they also provide a last ditch survival of time 
in the event that the EMPs (electromagnetic pulse) of nuclear airbursts 
in the opening salvos of armageddon wipe out all the satellites (a reason 
that GPS has 24 or more) and gets past all the hardened electronics 
aboard the ship. Without any time referenced to the meridian, even a 
modern ship can be as lost at sea as the British were in "Longitude".

Regards,
Sean-Thomas

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