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    Cable repair story
    From: George Huxtable
    Date: 2003 Mar 23, 19:15 +0000

    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
    
    
    
    
    
    ================================================================
    contact George Huxtable by email at george---.u-net.com, 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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