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Henry Halboth  (hch) wrote, on 25 Jan-
"I recall posting at the time the significant, though not insurmountable,
problem of keeping the centers of buoyancy and gravity aligned exactly both
transversely and longitudinally in a floating arrangement of composite
construction, so as to insure perfect equilibrium of floatation. It does
seem rather extraneous to me to float a reflective device when the medium
of floatation itself provides an adequate horizontal reflective surface to
begin with.

I have searched in vain to find these postings."

==================

I remember perceptive postings by Henry on this topic, but just like him,
can't now recall where to find them.

I fully agree with him over the superiority of a liquid surface (preferably
Mercury) over a levelled glass, because of its automatic self-levelling.
And I agree, too, with his view that "It does seem rather extraneous to me
to float a reflective device when the medium of floatation itself provides
an adequate horizontal reflective surface to begin with."

=================

It is possible to do star navigation by reflection from a watery liquid, as
one-time listmember Kieran Kelly showed in his postings about Augustus
Gregory, who explored much of the Australian outback around the 1850's, as
described in Kelly's book "Hard Country, Hard Men". Gregory would take
sextant star-altitudes, as reflected in a pannikin of black tea, when
camped for the night, using the camp-fire light to read the sextant; then
drink the tea. No doubt, stars seemed brighter in that clear desert air,
than in our light-polluted surroundings.

Together with the list of problems with levelling a reflecting raft that I
referred to in a posting on 20 Jan, there's another serious difficulty that
I didn't mention, but Geoffrey did; the likelihood of entrapping even a
tiny water-bubble under the raft, which would spoil its balance.

Greg, on 23 Jan, wrote-

"Being able to rotate a floating mirror 180* should get the job done. If I
were to fabricate an artificial horizon using a standard flat mirror it
would involve a shallow nested oversized square floating tray (in water or
blue windshield cleaner fluid for below freezing) with a flat 4" x 4"
mirror glued precisely at the center of the floating inner tray. Floating
foam spacers on all four sides could be used to keep the inner tray sides
from contacting the outer tray."

But that hasn't overcome the problem of meniscus attraction around the
edge: just transferred it to those floating buffers he has introduced. It
looks to me as if Greg is setting himself a hard task. But if he is keen to
try it, I suggest another form of buffering, to centralise the raft in the
tank, away from the sides, might be to fix 3 or 4 bristles or hairs (from a
paintbrush, for example) poking inwards from the trough walls. As long as
these are under water, there should be no surface tension forces, and they
should contact the underwater float surface only lightly, if at all. But no
doubt there will be some ill-defined frictional forces, where any contact
occurs, so I would not claim it as a perfect solution..

Now to consider a fixed reflecting plate, using screw-adjustment tested by
a spirit level.
The point about using a dark glass plate as a adjustable reflector was to
eliminate any competing reflection from the lower surface, in case the two
surfaces were not exactly parallel. But modern floated mirror-glass in now
of much higher quality than mirrors used to be, as Bill Morris points out,
and it may be good enough to use an ordinary (perhaps selected) piece of
mirror-glass in this role, taking advantage of the extra brightness that
arises from combined reflections at both surfaces. Alternatively, if a
front surface mirror can be found in a big enough piece, that could serve
even better.

The whole assembly, including any tripod or stand and the adjusting-screw
arrangement, needs to be very stout and rigid. It must not be affected by
repositioning the spirit level. Depending on how firm the local ground may
be, it's sometmes possible to observe a shift in level as the observer,
standing close by, transfers his weight from one foot to another.

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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