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There is a link to a riveting, first-hand account of undersea cable
repair in the late 1800s at
http://groups.yahoo.com/group/Sextants/message/877.

At the end of the link is a detailed description of using sun sights
through an artificial horizon on land to synchronize the chronometers.
The author mentions using a sextant stand, reading the sextant to the
second, and the chronometer to the tenth of a second.

I wonder what a sextant stand would look like, specifically the part
that cradles the instrument; ie, how does one attach the sextant to the
tripod.  I also wonder what sort of sextant would allow one to read to
the nearest second.

Regarding the method itself, it would seem to me that one would have to
know one's longitude in order to solve for time.  The author mentions
working "Observations for Time" on each of 20 or 30 observations to
arrive at synchronization to the nearest tenth of a second for the
chronometers.

These guys also seem to be working sights to the nearest tenth or fifth
of a minute of arc, although I infer this from the story rather than
reading it directly.  I would imagine such accuracy would be very
helpful in finding cable 20,000 leagues under the sea.

   

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