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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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