|
|
Subject: Re: Cable repair story
From: George Huxtable (george@XXX.XXX)
Date: Mon Mar 24 2003 - 05:00:14 EST
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@XXX.XXX
====================
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@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.
================================================================
|