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    Re: Malaspina
    From: George Huxtable
    Date: 2007 Oct 6, 13:27 +0100

    Alex showed an interest in the quintant I referred to, taken with the
    Malspina expedition.|
    | Is there a picture of this quintant somewhere on
    | the Internet? I've seen several quintants in
    | the book "Taking the stars" and in the museums and
    | on e-bay. And it is frequently claimed that they
    | were made for wide spaced Lunars.
    | My own SNO has theoretical range 140 degrees which is
    | closer to a quintant than to a sextant with 120
    | or 125 degrees.
    | And I tried several Lunars when the distance was more
    | than 130. My experience shows that this is practically
    | impossible with ordinary arrangement of the mirrors.
    | I think this explains why quintants are relatively rare.
    I have copied to Alex, offlist, a scan of a pamphlet by Gabriel Wright, in
    the British Library, which had been kindly passed to me by Ken Meldrew. I
    don't have the date, but its title is "Explanation of a new constructed
    quadrant". It deals with various instruments of modified design by Wright,
    octant, sextant, and (for the first time, I think) quintant. The major
    change was to make the index mirror a bit rotatable with respect to the
    index arm. I'm a bit sceptical about the virtues of doing that. If anyone
    else would like a copy, just ask. It's about a megabyte. Gabriel Wright's
    was the quintant that went with Malaspina.
    I've seen a Wright quintant, not on public display, but in a back-room at an
    outstation of the Science Museum in London, as ref. 1923-475. This was
    several years ago, and my memory is a bit hazy now. As I remember, it was
    complete, but in poor condition. You couldn't see through the optics, and
    some of the movements would have needed undue force to shift them. It had a
    mechanism for moving the index mirror with respect to the index arm, and
    Wright's paper might explain how that was used and why.
    There are a several limits on the angle that can be measured with a
    sextant-type instrument.
    1. Mechanical. At some point the index arm will clang against some stop,
    usually the support for the horizon mirror.
    2. The arc. At some point the divisions on the arc end. Often, sextants were
    divided with an arc far in excess of what the index could actually reach,
    perhaps so that at first glance the instrument might seem "superior", to a
    customer. However, on Vernier instruments it was essential to provide a
    considerable overlap, against which the Vernier scale was read when
    measuring large angles. So Vernier sextants commonly required an arc going
    to 135 deegrees, to allow them to read out angles up to 120 degrees.
    3. Optical. As the upper limit of angle, for which an instrument was
    designed, is neared, the apparent vertical aperture of the view through the
    index mirror  narrows, like a letter-box. This is because light reflections
    off the index mirror are then at a glancing angle. At some point it becomes
    unusable, because the view shrinks to nothing, or nearly so. To some extent
    this is overcome, from the octant to the sextant to the quintant, by
    reducing the tilt of the direction at which light passes between the
    mirrors, by pushing the horizon mirror further forward.
    4. Observational. In measuring lunar distances, as the angle between two
    bodies in the sky increases, it becomes more and more tricky for an observer
    to get and keep them both into sextant view, as he adjusts the plane of the
    frame to sweep one object to brush past the other. The correct plane,
    joining the observer with the two objects, becomes less and less obvious.
    Alex's posting, above, says that  he found it difficult to use the full
    theoretical range of his sextant, but it's not clear (to me) what the cause
    of that limitation was.
    Wide-span lunars are needed only for angles between Moon and Sun, never for
    stars, because as there's such a wide range of stars to choose from, it's
    always easy to find a better alternative when the angle gets awkwardly big.
    Maskelyne limited his star lunars to a maximum of about 70 degrees. But
    there's only one Sun. Increasing the possible angle from 90, with an octant,
    to 120, with the recently-introduced sextant, allowed Sun lunars to be
    available for another four days in the month. Perhaps, if Maskelyne had
    widened his Sun predictions further, the quintant, to 144, might have
    arrived sooner. Perhaps, if the quintant had appeared earlier, it might have
    influenced the range of Sun lunars published in the almanac. It was a
    chicken-and-egg situation. But in my view Maskelyne chose a sensible limit
    (which stuck) because of the observational difficulties mentioned above.
    A strong proponent of the quintant was Lecky, in "Wrinkles", but I never
    really understood why, because he despised lunars. I suspect it may have
    been because the quintant was then the most expensive instrument you could
    get, "the best", and there was a bit of instrumental snobbery involved.
    But in the late 1800s, the quintant became the preserve of the
    hydrographers, fot taking horizontal angles when surveying, and cut-down
    instruments for hydrographic purposes were available from Plath, with limits
    on the shades and telescopes that came with the kit.
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    To post to this group, send email to NavList@fer3.com
    To unsubscribe, send email to NavList-unsubscribe@fer3.com

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