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    The Quintant
    From: George Huxtable
    Date: 2002 Jun 22, 20:41 +0100

    When did the Quintant first appear, I wonder? Can Nav-L help to answer this
    question?
    
    The quintant is a development of the sextant, the scale subtending 72
    degrees (a fifth of the circle: hence its name) and measures angles up to
    144 degrees, compared with a sextant which has a scale subtending 60
    degrees (hence its name) and measures to 120 degrees.
    
    Quintants may have been intended for surveying by horizontal sextant
    angles, as much as for widely-separated lunar distances. They were being
    offered, as "surveyor's quintants" right into the 1930s.
    
    A great proponent of the quintant was Lecky, in "Wrinkles", which went
    through many editions in the later part of the 19th century. I had always
    thought of the quintant as a 19th century instrument, intended to extend
    the range over which lunar distances could be measured.
    
    However, these views of mine have been considerably shaken by the recent
    appearance of Vol 1 of "The Malaspina Expedition", a translation of
    Malaspina's journal 1789-90, edited by Andrew David et al (Hakluyt Society,
    2002).
    
    This contains references on page 240 (and also 247) to the use of a
    quintant, not for taking lunars, but for measuring the altitude of the Sun,
    when the vessel was close to land in the direction of the Sun. In that
    case, a clear horizon below the Sun could not be seen, and the observer was
    driven to observe the angle between a high sun and the horizon in the
    opposite direction, using instruments that would measure more than 90
    degrees.
    
    In translation, Malaspina's words were-
    
    "Since the short distance at which we were sailing off the coast made it
    difficult in the morning and even at noon to get a horizon clear enough for
    taking sights, the method of back-observations began to be preferable both
    for observing hour angles and meridian altitudes. Sextants, and even more a
    Wright's quintant belonging to Don Juan Vernacci were on this occasion most
    useful since the Sun, being close to the zenith, very soon reached the arc
    of elevation subtended by the arc of the quintant".
    
    Malaspina was 9 degrees south of the Sun at noon, and the Peruvian
    coastline was close north-east of him. To me it seems clear that he was
    measuring Sun altitudes up, not from the horizon below the Sun, but from
    the opposite horizon, so requiring an instrument capable of measuring more
    than 90 degrees: facing away from the Sun and measuring its altitude behind
    the back of his head.
    
    In a footnote concerning this passage, Andrew David refers to a quintant
    rather as I have described it above, and then goes on to say-
    
    "This particular instrument was possibly the modified version of the Hadley
    quadrant designed by George Wright in about 1780, in which the index mirror
    could be slewed through 45 degrees for making a back observation. For a
    full description of this instrument see Charles H Cotter, A History of the
    Navigator's Sextant Glasgow, 1983, pp 136-7."
    
    The mystery deepens a little here. Andrew David has correctly followed
    Cotter's description of the George Wright instrument, including the slewing
    of the index mirror by 45 degrees, but this seems to be contradicted in
    Cotter's own diagram, which appears to show the index mirror reversed
    back-to-front, ie slewed through 180 deg, not 45 deg. As far as I am able
    to judge, the George Wright instrument could measure altitudes between 0
    and 90 deg in one range, and then by making an adjustment, the "back"
    altitudes of 90 to 180. (The difficulty with using such back observations
    was always that of correcting for the index error.)
    
    Cotter doesn't refer to the George Wright instrument he pictures as a
    quintant, and to me that word doesn't seem applicable in any way to that
    instrument, as there's nothing in it to correspond to a fifth of a circle.
    So I doubt if the George Wright instrument pictured in Cotter is the
    "Wright's quintant" referred to by Malespina.
    
    On page 206 of the Malaspina translation, there's a list of the sextants
    carried on the vessel, including one by Wright. A translator's footnote
    refers to this as being by Thomas Wright, but might Malespina perhaps be
    referring to the same instrument as the "Wright's quintant" of page 240?
    If so, which Wright was the maker?
    
    I have enquired whether the word "quintant" is what was actually used in
    the original Spanish text, or perhaps introduced in translation as a word
    embracing wide-angle measuring instruments, and Andrew David has been kind
    enough to confirm that the original Spanish text reads "Los Sextantes, y
    aun mas un Quintante de Wright propio de Don Juan Vernacci."
    
    Malaspina's words about the observation, quoted above, provide a further
    puzzle. At the date of the observation of page 240, 23 Sept 1780, the Sun
    was close to the equator, and the vessel was at latitude 9 degrees South,
    or so. So around Noon the Sun's altitude would have been about 81 deg, and
    its elevation above the backwards horizon would have been 99 deg, well
    within the 120 deg range of a sextant. As the afternoon progressed, the
    azimuth of such a high Sun would change very quickly, and it would become
    visible above the clear horizon to the NorthWest, long before it had fallen
    enough to put its backward altitude out of reach of a sextant. During that
    day, the only time a quintant would provide an advantage over a sextant, as
    I see it, would be in enabling backward altitudes to be measured before 9am
    or so, local time.
    
    So what did Malaspina imply, when he stated that the quintant was useful
    "since the Sun, being close to the zenith, very soon reached the arc of
    elevation subtended by the arc of the quintant"? This doesn't make sense to
    me. I just don't see what advantage a quintant would have provided over a
    sextant, when the Sun was close to the Zenith, on that day or any other.
    
    The quoted passage above would to me make much more sense if Malaspina had
    intended to say something like "... the Sun, being on a path which would
    take it close to the zenith ...", rather than  "... the Sun, being close to
    the zenith ..." If that was indeed Malaspina's intended meaning, then the
    whole passage would become compatible with the existence at that date of a
    real quintant, of the type which later became familiar, measuring to 144
    degrees. It would confirm the early existence of such an instrument. I
    think the existence of a quintant, and the name "quintant", as early as
    1789, would come as something of a surprise to many historians.
    
    The advantage of a quintant would then have shown up in allowing Malaspina
    to make Sun altitude measurements that day using the back horizon, much
    earlier in the morning than would have been possible with a sextant.
    
    I wonder if Nav-L members are aware of any early quintants in museum
    collections in their area, or perhaps references to quintants in early
    editions of (for example) Bowditch. Any ideas and suggestions would be
    welcome. I have already tried the "rete" list (on the history of scientiic
    instruments).
    
    Thanks to Andrew David for providing helpful information.
    
    George Huxtable.
    
    
    
    
    
    ------------------------------
    
    george---.u-net.com
    George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Tel. 01865 820222 or (int.) +44 1865 820222.
    ------------------------------
    
    
    

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