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Subject: Cable repair story
From: George Huxtable (george@XXX.XXX)
Date: Sun Mar 23 2003 - 14:15:32 EST
Cable repair story.
Let me hypothesise about what those celestial observations were for, in
that interesting story about repairing cables,
http://atlantic-cable.com/Article/Combe/
One of the first and most valuable uses of a telegraph cable was to
establish the precise longitude of places along its route, because the
cable supplied rather accurate time signals for GMT. To do this job, an
accurate celestial observation for local-time was needed, best from a star
or, better, stars. That having been done once, the longitude was then
deduced and noted, and that measurement didn't need to be repeated ever
again, unless the technology improved. Can we presume, then, that the
longitude of the onshore cable hut had been precisely determined, in this
way?
Subsequently, a cable break might occur, somewhere in mid-ocean. To drag a
grapnel for it, the repair vessel had first to find it, so had to return to
the line along which it had been laid. The closer the repair vessel could
get to that line, the easier the job would be. We have learned from Blish
that as it was being laid, a cable could provide time-signals on board, so
that longitudes as well as latitudes could then be accurately noted (within
the limitations allowed by anomalous dip, etc). The cable ship didn't need
to rely on its on-board chronometers, nor on lunar distances (which would
have been hopelessly inaccurate for its purpose).
For the repair vessel to return to the presumed position of the fault, to
lift the cable for repair, it then required the best possible estimate of
its own position. That included both latitude and longitude. It no longer
had access to the cable for time signals. Instead. it was necessary to rely
on the ship's on-board chronometers, and any error in those chronometers
needed to be found. Also the chronometers' rate of gain or loss needed
checking.
In any case, the repair vessel would need to call at the onshore cable hut
so that the cable engineer could determine how far along the cable the
fault had occurred. Presumably, the vessel would anchor as close as
possible to the hut, then send a boat party ashore.
If it was down-line from the fault, the hut would no longer have acess to
time signals from the cable. Because the longitude of the hut had been
predetermined accurately, a precise measurement at the hut of local time,
using the Sun (or better, a star) from the hut in a time-sight, would
provide a precise measure of Greenwich Time. This time could then be
referred to that of the ship's chronometers by carrying a hack-watch back
by boat to the ship, or perhaps by firing a gun. The anchored position of
the ship would also need to be known with respect to the hut, and so a bit
of triangulation would be called for, using a measured baseline between
points from which an observer could see both the ship and the hut. Perhaps
it was this latter part of the operation that called for the cleared area
of level ground?
Repeating the measurement of chronometer error over a few days would
establish its "rate" of gaining or losing, for correcting future clock
observations.
I'm not saying "this was how, and why, it was done". It's no more than
speculation, by someone who knows little about cable technology 100 years
ago, asking whether that explanation is plausible.
George Huxtable
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contact George Huxtable by email at george@XXX.XXX by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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