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Bill wrote, about a mirror-on-a-raft,

"Also, does the float need a bottom?  It could be open if the containing
vessel was deep enough, which would eliminate the bottom surface, and leave
only vertical surfaces submerged that would be less likely to trap and hold
air bubbles."

I don't think that would work. With an open bottom, all the form-stability
of a flat box shape would be lost. It would be a bit similar (but the other
way up) to the loss of stability that occurs on roll-on vessels when they
get water sloshing around on the wide vehicle deck.

And Gary suggested
|
| > How about you put a bulb keel on it? A keel would provide a righting
| > moment and make it behave like a pendulous mirror horizon such as the
| > one in an MA-1 sextant.

Well yes, that's another way to to the job, but it's working quite
contrarily to Ken's aim, which (as I see it) is basically to find a way to
give water a shiny reflective surface, similar to the surface you get on
Mercury. The water surface itself provides the horizontal reference, and so
Ken's aim seems to be to provide a flat which conforms with that plane, and
does so by using the form-stability of a raft.

Gary suggests, instead, a deep ballasted arrangement, which presumably
carries a light horizontal mirror at the top. The floating structure would
then be similar to that of a floating light-buoy; which is so designed to be
independent of the surface (and so, little affected by waves). In Gary's
proposal, the water would be acting to provide flotation support to the
weight of the thing, and friction-free pivoting, together with some damping,
but not providing a horizontal reference. In other respects, it would be a
pendulum, just as Gary says.

I think both arrangements would work, to some extent, but the raft
arrangement would provide firmer levelling..

To maximise the form-stability, according to Ken's proposal, I suggest that
the ideal would be a lightweight circular mirror, sitting, not on a flat
raft, but on something like a circular quoit or small inner-tube (though
preferably rigid rather than inflatable), so its support, and stability,
come from near the rim. Not sealed to the reflector, as there must be an
escape path to avoid trapping of air underneath. An advantage of such a
shape is that it would discourage attachment of underwater air-bubbles. (It
doesn't HAVE to be circular; though; square or rectangular would do)

But if stability and precision are being sought, then any drops of liquid,
that might evaporate from a water surface and condense on the underside of
the mirror, need to be avoided. To ensure that, perhaps water would not be
the best liquid to choose; but instead, a non-volatile oil; and a reasonably
viscous one at that, to ensure good damping. And then, that takes us back
into the messiness of such oil surfaces.

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

All in all, I still reckon that Mercury is the best solution, if it can be
acquired. In the EU, there are now all sorts of regulations which make it
difficult, perhaps impossible, to do so legally. One day, officials may
appear at my front door to confiscate my Mercury barometer.

Regulations or not, the real hazards of Mercury when used out-of-doors, are
negligible, as long as spillage can be avoided. Inside, in a restricted
space without good ventilation, it may be another matter, though my
generation seems to have survived school physics labs, spattered with
Mercury in every nook and crevice. Mercury hazards have been discussed in
some detail in the archives of this list and its predecessor.

The best arrangement I've seen is a hollow permanently-sealed prism, made
from perspex (= plexiglas?) which makes a trough for the Mercury and a
"cloche" to cover it. The optical quality of the perspex material had been
checked beforehand. The only snag was that over many years the surface of
the Mercury has become dulled, and needs cleaning to restore its shine. An
arrangement, devised for cleaning that surface without breaking the seal,
would make the job perfect.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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