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    Re: Sextants, vernier and micrometer.
    From: Alexandre Eremenko
    Date: 2006 Oct 29, 10:43 -0500

    
    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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