NavList:

George,
Thanks for your very informative letter.

> It's a pity Alex and I are on opposite sides
> of the Atlantic. He would

Actually I am planning a visit to England next summer:-)
I hope to be able to find a good vernier sextant on e-bay
by that time, but my general impression is that
in the last two years, the number of real sextants
in e-bay sharply decreased. (As opposite to the number
of decorative imitations that sharply increased).
Two years ago I could see 5-10 reasonable sextants
on e-bay every week.

> introduction of the micrometer sextant.

I clearly remember reading somewhere that
the wormscrew was suggested by R. Hook, as early as in
the beginning of XVIII century. Unfortunately,
I do not remember WHERE did I read this.
The text said that it is unclear whether the idea
was actually tested at that time.
We had a short discussion of this on the old
list about 2 years ago. There is no doubt that the
idea of a wormscrew was not new. And the reasons
why it was not implemented in XIX century still has
to be discovered.

If I understand correctly, the first "dividing engins"
(late XVIII century) used the wormscrew principle.
BTW, the very first one is apparently located in
Smithsonian now, but it is not on display and I was
unable to see it.

I also remember reading somewhere the following argument
in favor of the drum/worm:
Easy reading permits you to make more observations
in the given time.
Then you average them and achieve a better
result.

On my experience in reading the silver scale
(on my pocket sextant): it is very relevant
from which direction the light falls.
To read the first observation, I just turn around
and turn the sextant to find the best position.
At night I use a small but very bright LCD flashlight
(of the type sold in Celestaire) and it works very
well.

In Bremerhaven museum I saw some very interesting reflecting
circles (by Pistor and Martens), equipped with
detacheable oil lamps on each vernier:-)

Alex.

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