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Bill commented on my statement-

>> For an artificial horizon, there seems no reason to ask for a reflecting
>> surface bigger than, say, 3 inches diameter, which should occupy the visual
>> field of a sextant at a convenient distance.
>
as follows-

>True if the body's elevation is high.  If low, the lip of the vessel may get
>in the way, so fluid would have to be topped off.  A larger vessel allows a
>greater range of elevations without having the fluid level "too" high.  This
>might not matter for water, but transporting oils etc...

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Response from George-

The context of the discussion was about glass-plate horizons, which require
no such lip. Bill's argument is valid only for liquid horizons. Perhaps I
should have been more specific in my text.

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

Bruce Stark wrote as follows-

>I haven't had much experience with the leveled mirror horizon. Took too long
>to set up, for one thing. My water horizon takes less than two minutes to set
>up or put away, and accuracy is assured.

I agree.

>But I don't see the weight of the spirit level as a major concern for the
>leveled mirror. I've tested by slowly lifting the level, gradually increasing
>upward pressure until it lifts. There was little or no shift in the bubble as
>long as both ends of the foot were still in contact with the glass.

As long as such tests are made, then that's fine.

>The mirror glass is thick, rigid, and well supported on the tips of three
>screws. The uncertain and vulnerable element is the table or tripod supporting
>the arrangement.

We agree about that. Any flexure of the tripod or other support, when you
remove the level, is what has to be guarded against.
>
>On another note, I seem to need a larger artificial horizon than you do,
>especially for low altitudes.

I wonder why (with a mirror system). Can't you simply approach it more
closely? You can come as close as you like with the sextant, until it
actually clangs the mirror-plate. Of course, such a close approach implies
that your weight is pressing on the ground close to the tripod, so it calls
for firm ground to ensure the levelling remains unaffected. No such problem
with a water-trough, though.

George.

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