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    Re: Cable repair story
    From: George Huxtable
    Date: 2003 Mar 24, 10:00 +0000

    To my statement about cable repair
    
    >>If it was down-line from the fault, the hut would no longer have acess to
    >>time signals from the cable.Paul Baechler responded
    
    
    >Assuming we're talking about repairs to an in-service cable, the hut
    >should have access to time signals from one side of the cable at all
    >times.
    >--
    >Paul Baechler
    >pbaechle{at}bellsouth.net
    
    ====================
    
    Well, yes, one would expect there to be a remaining contact between the
    on-shore cable hut and part of the world, via the remaining part of the
    cable. But at the date we are discussing, 1895, before the days of radio, I
    suggest that timing signals would need to be generated at a
    time-observatory with an accurate master-clock kept to time using a transit
    telescope, and with automatic generation of timing pulses.
    
    If there was such an astronomical time-source available at both ends of the
    line, then Paul's suggestion works. But that would only be true in a few
    cases, and wouldn't be true if the cable link was in the course of being
    extended via isolated islands (for example). So the scenario we consider is
    that the link with Greenwich has been broken, and precise astronomical time
    is not available from the downstream end of the cable.
    
    No doubt every cable station on the way would have its own clock, including
    the onshore cable-hut we are referring to. Presumably these had been kept
    to GMT by the timing signals along the line. Once the line had been broken,
    time could have been transmitted back to our onshure cable-hut by a private
    arrangement between the operators downstream of the break. However, once
    the link from Greenwich had disappeared, those times would be no better
    than the long-term accuracy of this downstream clockwork. After a delay of
    some weeks (and Combe's account makes it clear that's the expected interval
    between breakage and repair) those clocks will presumably have drifted
    significantly in time from Greenwich. A part of the world, once linked to
    Greenwich time by the cable, has become cut off and allowed to drift.
    
    So presumably the accurate sextant-work described so well by Combe was to
    put that right. It assumes that previously, when the cable was providing
    accurate time, it was used to determine the longitude of the cable hut with
    some precision: by that same process of sextant-altitudes for time-sights,
    especially of stars, with a reflecting pool, or more likely with a
    theodolite. Once the longitude of the hut had been measured, it never
    needed measuring again (in the absence of continental drift...) until
    technology improved.
    
    With the cable-link broken, what Combe had to do was to reverse that
    operation. To take the known longitude (which for permanence may have been
    carved on the lintel of the hut), determine local time from his
    observations, and from those two, deduce Greenwich time (to the second, I
    would hope). He then had find a way to transfer that time to the ship so
    that its chronometer timings for GMT were well established once again
    (after several weeks at sea, they will have drifted off). And no doubt the
    cable operator could correct his local clock, and inform other operators
    downstream of this corrected time. This might cause many clocks and
    time-balls, strung around the world, to be reset more precisely once again.
    
    
    If the ship is anchored somewhere that's out of sight of the onshore hut,
    then Combe might wish to choose an observing station that's within sight of
    both. In which case he would need to know, by a surveyor's triangulation
    technique, how much his chosen station and the hut differed in longitude
    
    Much of this is speculation, just my rationalisation after reading Combe's
    account. Others may read it differently, and better. To Combe and his
    generation, these problems in obtaining precise time in isolated parts of
    the world were accepted as just part-of-life that didn't need explaining in
    detail. It's rather hard for us to imagine ourselves into that same
    position, now that precise time pips out at us whenever we switch on a
    radio.
    
    An additional question that has always puzzled me, though it's a bit
    off-beam as a navigation topic, is how the cable relay stations, in
    isolated parts of the world, obtained their high-voltage electrical power.
    Did this come from great strings of Leclanche cells in series? Were they
    provided with stacks of zinc and carbon anodes, and electrolyte, to keep
    them going?
    
    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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