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    Re: Sextants, vernier and micrometer.
    From: George Huxtable
    Date: 2006 Oct 29, 19:02 -0000

    Alex wrote-
    
    | Actually I am planning a visit to England next summer:-)
    
    I hope Alex can navigate his way to somewhere near Oxford when that
    happens. We could enjoy a pint together, at the very least.
    
    | 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.
    
    Yes, we discussed early micrometers in the past, but that's no reason
    for not giving the idea another airing now. Hooke made a reflecting
    quadrant for taking angles, back in 1665, but this was not a
    double-reflecting instrument, so more appropriate on land than at sea.
    I don't know (and rather doubt) whether this involved any sort of
    micrometer. It seems more likely, to me, that any sort of worm-drive
    by Hooke would have been in the context of moving microscope
    specimens, but I am no expert on these matters.
    
    The first such application to an observing instrument that I know of
    was that of de Fouchy, around 1740, to an octant, shown in "Taking the
    stars" fig.66. But this was simply a way of getting finely-contolled
    shifting of the cross-wire against the scale, for interpolation.
    
    A common arrangement was a telescopic screw-strut which allowed for a
    controlled and measurable shift of the index arm, which could be
    useful for fine interpolation between coarse divisions of the arc. The
    first such implementation I know of was in a surveying circle by
    Mayer, in the 1750s, which still exists in the museum at Meldorf,
    Schleswig-Holstein, Germany, and the same method was suggested by him
    for use on his Mariner's Circle. It involved a movable "shoe", which
    could be clamped to the arc at an appropriate position, and this then
    allowed a few degrees of measured adjustment about that position,
    depending on the length of the strut. But this depended on precise
    alignment, by eye through a lens, of two scale marks, so couldn't
    really be any more precise than a good Vernier. Because it was a
    measured change in the chord of an arc, it depended on the cosine of a
    small angle, so could never be exactly linear, and would rely on a
    small correction factor, using a bit of trig.
    
    A well-known mariner's sextant was developed by Ramsden in about 1780,
    using that same principle, and referred to in "Taking the Stars". It
    can be seen in the Greenwich museum. Many astronomical quadrants had a
    similar arrangement. It's easy to see how it would be useful to
    astronomers, who in a sky-survey would want to measure the small angle
    between a star and an accurately-known reference star, if that
    difference-angle could be accommodated within the range of the
    screw-strut. That wouldn't be so useful to a mariner, however, who
    never needed to measure small differences, all his measurements being
    all the way up from the horizon, or perhaps of a lunar distance never
    less than 20 degrees. So it's reasonably clear why such an arrangement
    failed to catch on with mariners, being no real improvement on their
    Verniers. What a mariner needed was a continuous micrometer, operating
    all the way around an arc covering the whole range of the instrument,
    and the necessary technology for precise enough machining of worm and
    rack did not exist until after 1900..
    
    | If I understand correctly, the first "dividing engins"
    | (late XVIII century) used the wormscrew principle.
    
    Yes, that used a precise worm and rack, going all the way round 360
    degrees. That was the master circle,  which was made with great care
    and difficulty as a one-off device. But it wasn't practicable to
    replicate that for each working sextant.
    
    | BTW, the very first one is apparently located in
    | Smithsonian now, but it is not on display and I was
    | unable to see it.
    
    That's a real shame. The Science museum, in London, possesses a
    dividing engine, from a few years later. Again, it's not on display,
    and I have seen it when visiting (by prior arrangement) their
    back-room store.
    
    | In Bremerhaven museum I saw some very interesting reflecting
    | circles (by Pistor and Martens), equipped with
    | detacheable oil lamps on each vernier:-)
    
    Illumination must have presented real problems, until the advent of
    electricity. I remember travelling between Liverpool and Belgium on a
    2000-ton coaster, propelled by triple-expansion steam, which was, as
    far as I remember, lit throughout by oil lamps, and this was in 1950.
    
    George.
    
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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