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Fred Hebard said-

>Ah yes, the cable was broken, so they couldn't get the correct time
>piped down to them!  That's why the sun sights.   George Huxtable has
>once again cut through to daylight.

Fred, you are too kind...

>But is there evidence that correct time was being transmitted by cable?

Yes indeed, it was one of the first and most important applications of the
telegraph. Wherever it reached, longitudes were quickly obtained from the
accurate time that was distributed.

The previous method for determining longitude difference between two places
was to send an observer to and fro between them, carrying many
chronometers. Howes states that in 1845, over 60 chronometers were sent 16
times between the observatories of Altona (Hamburg) and Pulkova (St
Petersburg!

On 23 March, under the thread name "Sextant accuracy and anomalous dip" I said-

"Derek Howse, in "Greenwich Time and the Longitude" (1997), says that Airy
reported a time of passage of 1/2 second from Greenwich to Paris in 1854,
and a second (nearly) to Valentia [Ireland] in 1862."

I got that wrong: the time to Paris should have been 1/12 second, not 1/2
second. Sorry about that.

Howse's engaging book describes how the telegraph was first used for
determining longitude differences in the USA in 1844.

Airy, astronomer royal, established a system for distributing Greenwich
time by "galvanism" (electricity) as early as 1852, which was shortly
extended all round Britain via the railway telegraph lines. A link to the
Paris Observatory allowed a precise measure of the longitude difference
between London and Paris. The time distribution system ran automatically,
and provided time signals within London every hour and outside London once
or twice a day. For a minute either side of the time-signal, communication
traffic on the cable was interrupted to give it a clear passage.

The advent of the telegraph gave a great boost to the precise mapping of
the world, and ensured that time-balls at ports dropped at an exact time,
for setting ships' chronometers. No use to the navigator at sea, of course:
he would have to wait for radio.

George Huxtable.

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