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>>On Sep 16, 2005, at 9:59 AM, Piccer wrote:
>>
>>>I followed the dicussion on the plastic sextant with a lot of interest.
>>>
>>>At home I have a replica of what is called a " box" sextant.
>>>I never tried to use it, but can somebody tell me if this is a serious
>>>instrument?
>>>
>>>The only reference I have found is on the website of Stanley, London :
>>>they sell  the replica's
>>>
>>>Many thanks
>>>
>>>Willem Piccer


----- Original Message -----
From: "george huxtable" 
To: 
Sent: Wednesday, September 28, 2005 12:37 AM
Subject: Re: beginner


> At 15:06 16/09/2005, Fred Hebard replied to a question from Willem Piccer
> about box sextants:
>>Willem,
>>
>>My understanding is that the replicas are not accurate and are not
>>suitable for any sort of measurement whatsoever.


00000000000000000000000000000000000000000000000000000000000000000000000000000000000000


To George et al.
Nice to have you back:
I have in my hand a book on surveying , I don't know what date it was
written but the eighth edition was extant  around 1950.  This below, is an
extract from it and I can send two small  .bmp files of the diagrams direct
to anyone who  would reply. One is a an annotated drawing  and the other is
a diagrammatic view through the telescope. Both are line drawings but
unfortunately they are not very clear.
It is apparent that the original instruments made by Stanley were intended
for serious surveying work. but I personally have seen many so called
replicas which are useless except for ornaments.  A good way to tell is if
the instrument is constructed of quality machined parts, like with good
clockwork, as distinct from parts that have be obviously stamped out and
polished up. A few examples of the later are sold by a firm called
'Nauticalia' here in the UK.




************************************

extract from 'SURVEYING' by J.Whitelaw

SURVEYING WITH ANGULAR INSTRUMENTS.



BOX SEXTANT:

      This is a very handy instrument for approximate measurement of angles,
and as made by Stanley, is shown in Fig 1. It consists of a circular brass
box 3 or 31/2 in. in diameter, and about 1 1/2 in. deep. It is protected by
a lid or cap which is screwed on the top when not in use, and is also
screwed on to the bottom to serve as a handle when using the instrument. It
may be used either with or without the telescope T, which for convenience in
carrying is fixed by the screws so that it may be unfastened when not in
use.

      In the upper part of the instrument there is an arc of a circle A
usually about 70? in extent and divided into 140?. The angle is read off by
the vernier v and the microscope M.

      Opposite the slit for the eye or telescope T and inside the box there
is a mirror having its upper half only silvered, so as to allow of one of
the two objects between which the angle is to be measured to be seen
directly through an opening in the side of the box beside this mirror.

This mirror is called the horizon glass, and is fixed perpendicular to the
plane of the instrument. The other side of the box opposite the side of the
instrument shown in the figure is left open, and there is placed there
inside the box another movable mirror which moves on the same axis as the
vernier arm v. This mirror is called the index glass, and it and the vernier
v are both moved simultaneously by the screw B which is connected to a rack
and pinion arrangement inside the box.

When the two mirrors are parallel to each other the vernier should read
zero. The angle actually read on the divided arc is the inclination of the
two mirrors to each other. This angle is half the actual angle between the
two objects, and the arc of 70? is therefore divided into 140. This is a
source of error, as of course any error of observation is doubled. The
vernier reads to 1 minute.

USE OF BOX SEXTANT.-

      To use the instrument sight directly on one of the object through the
opening opposite the lower unsilvered half of the fixed mirror, and then
rotate the other mirror by means of the screw B until the second object is
seen reflected in the upper silvered half of the fixed mirror directly over
the first object as seen through the lower unsilvered half as shown in Fig
2. The angle between the two objects is then to be read off by means of the
vernier.



MEASURING ANGLES WITH BOX SEXTANT WHEN GROUND IS NOT LEVEL.-

      In using the box sextant, unless the two objects between which the
angle is measured are on a level with the eye, the angle observed will not
be measured in a horizontal plane. As it is the horizontal projection of the
angle that is required, we may observe this by placing two ranging rods in
line with the objects so that these ranging rods are as nearly as possible
on a level with the eye. The angle between the ranging rods may then be
taken.

      If the ground does not permit of this, then the actual

angle may be measured and afterwards reduced to its horizontal projection by
the proper formula.( Unless in extreme cases, this will not usually be
necessary as the error will generally be within the limits of accuracy of
ordinary box sextant survey work.)



************************************


Regards
Clive.

   

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