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Dear Omar,

The instrument in the picture is a wood quadrant (octant).
You can see another picture of it in the first page of
Norie's book (online version, 1828).
According to Norie's description,
the arc was divided into 20' intervals,
that is each degree into three equal parts.
(If you look at the picture VERY carefully, you
see an arc divided into whole degrees but adjacent
to it from the outside,
there is a narrower arc that is divided into 20'
intervals.
The Nonius (Vernier) is divided into 20 equal parts whose total
length equals 21 divisions of the arc,
that is 7 degrees. So you can read the scale to 1'.

Norie has description of this Nonius in detail,
and the first paper mentioned below, contains magnified
pictures of the arc and Nonius.

The radius of the arc I cannot tell from the
picture, but my guess is
that it is 17 or 18 inches, maybe even 20 inches.
(Larger instruments were made of wood, smaller of brass.
The reason is simple: larger the arc, more precisely you
can read it. The limitation is the weight of the instrument).

The book by Bruce Bauer contains a picture of the author
with such quadrant in his hands, so you get an idea of its
size from this picture (it seems that his particular
quadrant has radius at least 20 inches). The papers I mention
below describe
specific instruments in museums and give their dimensions.

Now, I don't know for what purpose you want to build such
octant: to use it as a decoration, or to measure
angles, and in the latter case, what precision you are aiming at.

In any case, you may find useful the following articles
which describe how they were actually made in XVIII-XIX centuries:

S. Moskowitz, The world's first sextants,
A. N. Stimson, Some board of longitude instruments
in the nineteenth century.

These and other similar papers are available
on the Sextant mailing list
http://groups.yahoo.com/group/Sextants/
You have to register to download them free.

If you really want to make a working instrument, the
hardest part, as I understand is to divide the arc with
sufficient precision.
In the beginning this was done by special artisans,
masters of division, who used very sophisticated
geometric constructions, performed directly on
the arc, with several compasses
("dividers"!)
of various size. The process took several months.
If you are really interested in doing THIS,
I can provide further references. (There is a special
XVIII century literature
on the arc division of astronomical/nautical instruments.
Something is available on the web, something I have).

But then Ramsden invented a division machine, which left
these fine artisans and scientists without job.

The quadrants shown in Bowdich and Norie were probably divided
by a machine. But you are not going to build a dividing machine,
are you?
:-)
I don't know the principle on which this division machine
works (can only guess) but I know that the original machine exists
and belongs to some US museum.

Alex.

P.S. I want to cite the beginning of the paper
"Observations on the Graduation of Astronomical instruments...."
by J. Smeaton, FRS (1785):

"Perhaps no part of the science of Mechanics has been
cultivated by the ingenious with more assidity, or more
deservedly so, than the art of dividing Circles for
the purpose of Astronomy and Navigation..."


A
On Fri, 19 Nov 2004, Omar Reis wrote:

> Hello,
>
> I'm trying to build an octant similar do Bowditch's.
>
> I found a beautiful image of it on the following address:
>
> www.nathanielbowditch.org/images/sextantb.jpg
>
> I wonder if anyone knows more details about this instrument:
>
> 1) Scale radius
>
> 2) Number of minutes in the vernier ( i.e. tick divisions per degree )
>
> Regards
> Omar
>

   

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