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    Re: Cotter's "History of Nautical Astronomy"
    From: Nicol�s de Hilster
    Date: 2007 Jun 11, 19:55 +0200

    In NavList 1779 George Huxtable wrote:
    > "Nicol�s de Hilster" 
    > wrote-
    >
    >
    > "The book I
    > ordered was however "History of the Navigator's Sextant". I did not 
    > yet
    > receive this book, it is due to arrive later this week or beginning 
    > next
    > week."
    >
    > I have that book, and expect that you will find it worthwhile.
    >
    > Writing about the sextant, Cotter seems to be much more on 
    > home-ground, than he is on "History of Nautical Astronomy", and I'm 
    > unaware of any errors.
    >
    >   
    Cotter sure did his best on this book (The History of the Navigator's 
    Sextant). However a few things are worth mentioning here:
    
    - The most obvious one: the omission of the spiegelboog as the first 
    reflecting instrument (not really an error of course and I do not expect 
    any non-Dutch book prior to my publication on this instrument to discuss 
    the spiegelboog).
    - p.132, 1st paragraph: "In this book Bouguer recognized the superiority 
    of Hadley's quadrant, or 'Quartier Anglais' as the French called it;"
    This is a very tricky assumption as Jonas Moore wrote in 1681 (A New 
    Systeme Of The Mathematicks, p. 248): "This instrument ... was the 
    contrivance of one Captain Davis ... and therefore is often called by us 
    Davis's Quadrant, but by the French the English Quadrant.". The latter 
    will translate as Quartier Anglais. The instrument Moore is referring to 
    is the Davis Quadrant with two arcs (is shown on an image on p.249).
    - p. 133, point 3.: "3. It was light in construction compared with the 
    Davis quadrant or fore-staff".
    As I own all three of them (although the cross-staff and Davis Quadrant 
    are replica's they are made as copies using the same dimensions and 
    materials), I have put them on the scales and guess what? The older the 
    design the lighter. A cross-staff would do about 400 grams, a Davis 
    Quadrant about 725 and an Octant about 1050 grams. The cross-staff I 
    checked was a short one, but the longer ones used in those days were 
    often thinner and not much more than 50% longer.
    - p. 133, point 5: "5. Being an instrument of double reflection a small 
    movement of the index arm causes a relatively large movement of the 
    double-reflected image. This results in more accurate measurements being 
    possible..."
    The contrary is true: just imagine you had to swing the index arm 90 
    degrees in order to measure 10 degrees altitude one would be able to 
    divide these 90 degrees so finely one could read of the scale without 
    the need of a vernier. So for accuracy the reflection reduces the 
    accuracy of the instrument as we only have 45 degrees to measure 90 
    degrees. For this reason early instruments were made large in order to 
    get a decent quality scale.
    - p.189, 2nd par.: Cotter is referring to Elton's 1732 invention as 
    being an Octant (Hadley Quadrant), but that should be a Davis Quadrant 
    of course. Cotter writes: "Elton's device consisted of two spirit levels 
    set at right angles to each other and fitted perpendicularly to the 
    plane of the frame of the octant". The latter should be a Davis Quadrant 
    and as Cotter starts the paragraph with "Soon after John Hadley 
    introduced his reflecting octant...a description of a 'new artificial 
    horizon' appeared...", so this is not just a typo but he really was 
    confused.
    
    For the rest it sure is a very valuable book.
    
    Nicol�s
    
    
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