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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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