NavList:

   

Reply

Gary LaPook wrote about Mercury-

| We've gone around before about mercury. There are very toxic organic
compounds involving mercury but you can drink pure mercury and it just
passes through you without harm. The "mad hatter" breathed mercury fumes
over long term exposures which had bad results. It has had a long history
for use in artificial horizons, ask Lewis and Clark. It does evaporate very
slowly so don't keep the bottle in you bedroom but, otherwise, the limited
exposure to fumes during the occasional use as an artificial horizon will
not cause you any harm. And there is no fluid that is more reflective than
mercury for use in an artificial horizon, just try to shoot stars with
water or oil!

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

I agree with Gary's opinions about the hazards of Mercury, but not with his
reference to Lewis and Clark.

Lewis and Clark made much use of an artificial horizon, with some sort of
"cloche" cover, presumably similar to what has been discussed here, but
with windows of "talc". These were sheets of mica, high-quality examples of
which were optically superior to the plane glass of that time.

But they used water, not Mercury, as the reflecting surface.

On July 22, 1804, Lewis listed in his journal the instruments carried by
the expedition, writing -

"3rd- An Artificial Horizon on the construction recommended and practiced
by Mr Andrw. Ellicott of Lancaster, Pennsyla., in which water is used as
the reflecting surface; believing this artificial Horizon liable to less
error than any other in my possession, I have uniformly used it when the
object observed was sufficiently bright to reflect a distinct immage; but
as much light is lost by reflection from water I found it inconvenient in
most cases to take the altitude of the moon with this horizon, and that of
a star impractible with any degree of accuracy."

He went on to describe the alternatives employed when water-reflection was
unusable; a tiltable glass-plate and a tiltable mirror, both used with a
spirit level. But these would be much more liable to error that the
self-adjusting surface of a liquid.

This, and much more,  can be found in vol 2 of "The Journals of the Lewis
and Clark Expedition, ed. Gary E Moulton, University of Nebraska Press,
1986.

It surprises me that they didn't carry, and use, Mercury for a horizon. It
was by no means a rare material, being used in large quantities in the
mining and refining of precious metals, and I would expect it to have been
available in Pittsburgh or St Louis.

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.

   

Reply