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    Re: telegraphic longitude article
    From: Paul Hirose
    Date: 2003 Dec 26, 23:09 -0800

    To measure the clock offset and propagation delay, the break-circuit
    chronometers at both stations were connected to the telegraph circuit
    and their pulses compared.
    Suppose the chronometer at station A is ten seconds behind B, and
    there's zero propagation delay. From the point of view of A, the B
    pulses arrive ten seconds early. But B observes A's pulses arriving 10
    seconds late.
    This symmetry doesn't occur if there's a propagation delay. Let's say
    it's 0.1 second. Now A sees the pulses from B arrive 9.9 seconds
    early. On the other hand, B sees the A pulses arrive 10.1 seconds
    In reality, it was probably not technically feasible to put both
    chronometers on line simultaneously. Instead, chronographs at A and B
    would simultaneously record the pulses on rotating paper drums, but
    only one chronometer at a time would be on line. By analyzing the
    graphs in the office, the true clock offset could be recovered.
    I think the chronometers produced one-second pulses, with the pulse at
    the top of each minute inhibited to allow specific seconds to be
    identified in the chronograph trace. I've seen one of these
    chronometers for sale on eBay. It had been used by the U.S. Coast &
    Geodetic Survey.
    In later years, the chronograph and break-circuit chronometer
    continued in use, but you didn't need the telegraph link to a second
    observing station anymore. Radio time ticks recorded on the
    chronograph provided the time reference.

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