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George,

It's just a guess, but I imagine the English instrument makers of that day
could turn out a bubble level as accurate as you could wish. No doubt it would
be expensive.

I believe Lewis used the water horizon unless the object was too dim to
reflect in it. Though mercury horizons were standard, the captains didn't take one,
probably for the reasons Kieran has laid out. The captains certainly had no
concern about health issues with mercury. Must have had enough in their medical
kit to fill several artificial horizon flasks. They used it to treat syphilis
and, in Dr. Rush's pills -- which the men spoke of as "thunder bolts" -- as a
laxative.

I don't have the Journals for the Fort Mandan part of the trip, but
understand Lewis chose the liquid horizon for his observations there. This when the
river and everything else was frozen solid. "Artificial horizon drinkability"
could have been a factor, since the only thing that would stay liquid in the pan
was whisky.

Others will have to answer your questions about leveled horizons. But I think
one problem with them is that it's not always easy to find, or set up, a base
that can be depended on not to shift slightly.

Bruce

   

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