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Fred Hebard said

>There is a link to a riveting, first-hand account of undersea cable
>repair in the late 1800s at
>http://groups.yahoo.com/group/Sextants/message/877.

I hope others had less trouble trying to access this site that I did. It
connected me to an infuriatingly longwinded procedure for signing on to
Yahoo groups, and then failed to find message 877 for me, finally crashing
my old Mac. So I have now signed off again. Maybe it was finger-trouble on
my part, but I am happy to be now quit of Yahoo groups.

However it's the sort of story that would interest me, if there was an
easier way of getting to it.

Fred adds-

>At the end of the link is a detailed description of using sun sights
>through an artificial horizon on land to synchronize the chronometers.
>The author mentions using a sextant stand, reading the sextant to the
>second, and the chronometer to the tenth of a second.

I wonder if this is what was otherwise described as a pillar-sextant. Just
to make life complicated, the word pillar-sextant can be used for either a
sextant on a stand in the form of a pillar, or otherwise a sextant with a
frame that's made up of two thin brass plates, spaced by a number of short
pillars, Some sextants can be a pillar-sextant by both these definitions at
the same time. I discuss the first usage.

Pillar sextants I have seen have tripod feet with two spirit levels at the
base, and at the top of the pillar have a pivot-axis which can be tilted
and clamped to be parallel to the Earth's axis: just like the polar axis
mounting on a telescope. With respect to this axis the sextant itself can
be tilted and clamped in two planes, so that for a lunar distance it can be
put into the plane containing both the Moon and the other-body. Then by
swinging it about the polar axis, (just like a telescope mount) the two
bodies can be held in view in the mirrors, and the angle between them
measured at leisure, in a relaxed manner.

Such pillar sextants were intended for use on land, and were hefty items
which a land-traveller would be most reluctant to carry in his baggage.
Perhaps they were intended to be set up near shore at a ship's landfall.

In those sextants the pillar-mounting and the sextant were designed
together, whereas it seems from Fred's question below that he is expecting
the pillar-mounting to clamp on to any sextant. This would be different to
anything I have seen.

>I wonder what a sextant stand would look like, specifically the part
>that cradles the instrument; ie, how does one attach the sextant to the
>tripod.

For measuring altitudes up from the horizon, rather than the tilted sextant
that a lunar distance requires, perhaps a simpler mounting would suffice,
with fewer adjustable tilts.

>I also wonder what sort of sextant would allow one to read to
>the nearest second.

I have some doubts about that claim. At that date it would be a Vernier
sextant, not a micrometer. The scale would need a powerful lens to read it,
and the scale would have to be very evenly cut. I think perhaps theodolites
in that period could be read more precisely that the standard maritime
sextant could, so the technology may have existed. Averaging a lot of
readings would help. It would be interesting to learn more.
>
>Regarding the method itself, it would seem to me that one would have to
>know one's longitude in order to solve for time.  The author mentions
>working "Observations for Time" on each of 20 or 30 observations to
>arrive at synchronization to the nearest tenth of a second for the
>chronometers.

It's not clear to me what the aim was here. You can "synchronise" any
number of chronometers (set them all to the same time) by bringing them all
together and listening for the ticks. Then they can be dispersed as needed.
It needs no astronomy. But that doesn't set them to the right time, just
the same time. Was the aim to set up a number of chronometers, at locations
with unknown longitudes, to establish those longitude differences?

>These guys also seem to be working sights to the nearest tenth or fifth
>of a minute of arc, although I infer this from the story rather than
>reading it directly.  I would imagine such accuracy would be very
>helpful in finding cable 20,000 leagues under the sea.

Yes, but it presumes that the cable was originally laid along a known line
with corresponding accuracy.

George Huxtable.

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