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Patrick wrote-

| But I agree: even if we blink the concerns about safety, Hg is too
| expensive.  I am considering cutting a small piece of optical grade front
| surface mirror to a size that would fit without constriction into the
Davis
| artificial horizon, backing it with a thin sheet of cork or other buoyant
| material, and then putting it afloat in the artifical horizon.  Can
anyone
| see a problem with this idea?
|

Mercury is VERY dense (over 13) so an ounce of the stuff won't go far;
occupying about 2 millilitres. My guess is that  around 10-15 ml would be
required in the trough of a sensibly-sized art. horizon, to make it easy to
use without having to be over-careful about levelling. That would
correspond to 5 to 8 ounces. Maybe it would be possible to penny-pinch and
get way with somewhat less.

If the US allows trading in Mercury, its citizens are fortunate. That's
illegal in Europe.

Patrick's proposal for a floating mirror on water looks plausible in
theory, but I doubt if it could be made to work to sufficient precision in
practice. Any such float-raft would need to be made with great uniformity
to achieve a level to half an arc-minute, necessary to provide answers to
the nearest minute. It would call for symmetry to one part in 7,000, or so,
across any diameter. Cork is not a suitably uniform material; what
alternatives exist?

Perhaps a cut-and-try procedure could be adopted, after measuring for any
tilt, with a tiny blob of movable ballast to be attached at a determined
spot.

How can the raft be prevented from drifting to one side, and trying to
climb up the meniscus at the edge of the trough? Perhaps by tethering it
near the centre, with limp threads, such as the finest silk. Which would
introduce another set of problems.

And then there are the surface tension forces around the edge of the raft.
How can the surface condition be controlled to ensure that they are kept
exactly in balance?

Finally, how is steaming-up of the windows to be avoided?

All practical problems, which need to be overcome.

George.

contact George Huxtable, at  george{at}hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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