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    Re: Sextant and quintant limits
    From: George Huxtable
    Date: 2008 Dec 20, 19:33 -0000

    Richard Pisko pointed us to an instrument for surveying, at the Smithsonian,
    which they called a reflecting alidade, at-
    http://americanhistory.si.edu/collections/surveying/object.cfm?recordnumber=764354
    
     which was made by Dollond (not Dolland, though his name often got
    misspelled)
    
    There's an almost identical instrument by Cary, at the National Maritime
    museum, Greenwich, at-
    http://www.nmm.ac.uk/collections/explore/object.cfm?ID=NAV0140&picture=1
    
    which they name a "Douglas reflecting protractor". (Note, this shouldn't be
    confused with the modern Douglas Protractor, a transparent marked-off
    square). It was the invention of Sir Charles Douglas, an army man, who got a
    British patent for it (3461 of 1811). I haven't discovered if there's a way
    to access UK patents without visiting the British Library, so if anyone
    knows how to do that, that would be of interest.
    
    The two items are shown opened to almost identical angles.
    
    Richard has explained what it does, as follows-
    
    "The mechanism is such that the angle between the rule edge and the
    adjusting (index) arm edge is the same as that between the line of sight
    through the aperature over the "horizon" mirror to the first object, and the
    reflected line of sight to the second object.  This is set around 25
    degrees, I think."
    
    It's a horizontal sextant, really, intended to be laid on to a piece of flat
    paper, a "plane table", as used by surveyors. There are two ruled arms,
    pivoted together, and another pivoted sight-arm, with a peep-hole (no
    telescope, as it wasn't a high-accuracy device). As far as I can work it
    out, there's a sort-of pantograph arrangement, with a peg in a sliding slot,
    so that the angle between the rules is always twice the angle between the
    planes of the two mirrors. Therefore, the angle between the rules becomes
    the same as the angle between two landmarks that are aligned in the direct
    and reflected views. So, when two marks are aligned in that way, the rulers
    allow the angles between their bearings to be drawn in on the paper
    directly, with no need to measure off the angles in degrees. However,
    there's a Vernier scale for doing so, if it's needed.
    
    Plane table surveying was much used on land, because directions measured
    around the horizon could be easily transferred to paper, using a simple
    sighting tube attached to a ruler. That worked when the plane table could be
    firmly planted on land, but it was difficult to use it at sea, when the
    yawing of the ship made all directions unstable. Instead, a marine surveyor
    had to measure relative bearings between two landmarks, for which the
    sextant was the ideal instrument. But then he had to read off the angle, and
    put an ordinary  protractor at the corresponding setting, to draw the angle
    between the landmarks. The Douglas instrument provided a short-cut, allowing
    the relative bearing to be observed, and the angle drawn in, in a single
    step
    
    Those words, above, are based on a bit of guesswork, and not on a complete
    understanding. I wonder whether Richard Pisko, who knows far more about
    surveying than I ever will, concurs with what I've said.
    
    One obvious feature of this instrument is the linear scale, with a diagonal
    section for interpolating distances, presumably using dividers. I suspect
    that this was quite unrelated to the angular measurement system Richard and
    I have described above, but it could have been useful to surveyors
    nevertheless.
    
    The arrangement of this protractor/alidade has no connection with the
    "alternative" sextant geometry that I was referring to in Navlist 7436 and
    7340, except that it shows a different way of getting round the 2:1
    multiplication between mirror angle and true angle. Indeed, an adaption of
    the geometry I propose in 7340 could have made this protractor much simpler.
    
    George
    
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    ----- Original Message ----- 
    From: "Richard M. Pisko" 
    To: 
    Sent: Saturday, December 20, 2008 6:27 AM
    Subject: [NavList 6775] Re: Sextant and quintant limitst: was: sextants on
    aeroplanes
    
    
    On Sun, 14 Dec 2008 03:41:37 -0700, George Huxtable 
    wrote:
    
    > However, it seems to me at least plausible for a 2-mirror instrument
    > (which
    > couldn't, any longer, be called a sextant) to be designed, which could
    > measure angles from zero to 180�, though nobody ever seems to have done
    > so.
    > That's material for another posting.
    
    George,
    
    You might take a look at this instrument from the Smithsonian (National
    Museum of American History)  Physical Sciences Collection - Surveying and
    Geodesy; Alidade (reflecting) by G. Dolland PH*319452: (see 35 KB
    attachment)
    
    The mechanism is such that the angle between the rule edge and the
    adjusting (index) arm edge is the same as that between the line of sight
    through the aperature over the "horizon" mirror to the first object, and
    the reflected line of sight to the second object.  This is set around 25
    degrees, I think.
    
    There is a very similar instrument in (if I recall correctly) in one of
    the British museums; opened to a wider angle, close to 60 degrees.
    
    I have not been able to find out what the inventor called the various
    parts, but I hope you can follow my description while looking at the
    illustration.
    
    -- 
    Richard . . .
    
    Using Opera 9.2.4 after the "Dog" died
    
    
    
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