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From: George Huxtable (no email)
Date: Thu Jul 10 2003 - 15:18:20 EDT
Robert Eno wrote-
>All this talk of tea and wine for use as artificial horizons. Tsk. Tsk. Tea
>and wine are for drinking (especially red wine -- I recommend a Tasmanian
>Cabernet Sauvingon called "Morilla Estates"; the very best). You want a
>good artificial horizon, I would go for the dark glass ones. Sure, you have
>to level them but that takes about 30 seconds at the most; and best of all,
>you can't spill them and you don't have to boil them to make them work
>better.
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Comment by George Huxtable-
We have touched on this type of artificial horizon before, when discussing
Amundsen's polar navigation. (Mercury can freeze in Antarctica)
There's a nice picture of a Norwegian black-glass artificial horizon of
this type in Peter Ifland's book, Taking the Stars.
My interest in this type of instrument has been revived by discovering, in
the Moulton editions of Lewis & Clark's journals of their journey two
centuries ago, that the travellers frequently used a levelled glass horizon
instead of their water-reflecting horizon. One such instrument was a glass
plate mounted on a wooden half-ball that dropped into a triangular cut-out
(presumably to allow coarse adjustment) of a tripod with screw-feet.
Another used a sextant-mirror glued to a plate which sat on that same
tripod.
Presumably the screw-feet were adjusted to obtain horizontality while
trying the plate surface with a spirit-level in two opposite directions,
and then in two opposite directions as right-angles to the first, repeating
the provess as necessary.
Presumable sensitive spirit-levels were available 200 years ago for use
with the theodolites of he day. Can anyone suggest what the sensitivity of
a level of those days might have been?
One of the requirements of the procedure above must have been a lightweight
level and a rigid mounting, because the mirror must not be deflected under
the weight of the level by a signficant amount, or it would flex back into
position when the level was removed. Can anyone suggest what the level of
overall precision that might have been available to Lewis and Clark in
using such a levelled reflecting plate? Just as a starting-point, what
accuracy is achievable by modern observers using a reflecting-plate, with a
modern spirit-level?
George Huxtable.
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contact George Huxtable by email at , 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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