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For an artificial horizon, there seems no reason to ask for a reflecting
surface bigger than, say, 3 inches diameter, which should occupy the visual
field of a sextant at a convenient distance.

So I'm rather horrified to see a glass plate levelled by means of such an
enormous spirit-level as has been mentioned to the list; 8 inches or even
bigger!. If the user carefully adjusts the plate using such a level perched
on it, can he then be quite sure that there will be no change, not even a
fraction of a minute, when the weight of that spirit-level is removed?

I mentioned in a previous posting the small spirit-level glass capsules
that exist as replacements for theodolite levels, and quoted from memory
those that had been acquired by a friend, as being 20 arc-seconds per
division. My memory was a bit at fault there. The levels were acquired by
my friend and neighbour, and fellow listmember, Clive Sutherland,
. Their sensitivity was actually 30
arc-seconds per division, not 20 as I suggested. The scale divisions were
2mm apart, and it's possible to estimate level to better than 1 such
division. Clive paid approx ?10 for each level (roughly $20), from a
Supplier in Kew, London.. Higher sensitivity versions (20 arc-sec per div)
exist, but cost rather more.

These glass capsules are (Clive's estimate) about one and a quarter inches
long, with pointy ends, and three-eighths diameter. To be useful, they need
to be fixed into some sort of tiny shoe, accurately glued into place such
that when reversed the reading remains the same. This is a non-trivial
operation!.

Clive recommends this address for useful information-

www.leveldevelopments.com

George.

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contact George Huxtable by email at george@huxtable.u-net.com, 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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