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Back before the dawn of time (on Wed, 26 Mar 2003 12:46:15
-0800, to be exact), "Royer, Doug" 
wrote:

>Mr. Pisko, I have 1 thing to ask and 1 to report.It concerns the 1911 piece
>you wrote. I don't understand "one was fixed at 45* and the other read the
>angle".Could you expain this?I can't visualise what the 1st sextant is for.
>
Sorry, I was condensing and simplifying.  "Sextant-like
device" would have been more accurate.  I will include the
actual quote later, from:
, but
since the Optical Character Recognition is terrible, and
there are no illustrations, I will give my expanded
explanation first.

The first station  had a mirror or prism fixed at 45
degrees, so as to have a right angle through the reflection
and the sight toward the target.  This can be done less
accurately by a regular sextant with the arm fixed at 90
degrees, and held horizontally.

The first sextant is thus the right angle of a right angle
triangle.  The short base is 50 yards, and the second or
reading sextant is at the other end, and also held
horizontally.  For example, if reading #2 is 80 degrees
(measuring the angle between sextant #1 and the target),
then the unknown distance from sextant #1 is 50 yards times
tan 80 degrees, or 283 yards.  Well within rifle battle
sight range and thus not needed.

If #2 reads 85 degrees, the distance would be (50 x tan(85))
= 572 yards.  Still close, but maybe useful data for a
surprise mortar round or two.

At 87 degrees, 30 minutes, the distance would be 1,145 yards
and finally something the infantry might be able to use ...
or at least report to the artillery.

>>>>>>> quote from 1911 Encyclopedian Britanica >>>

The mekometer is practically a box sextant. Two instrument
are used simultaneously at the ends of a base of fixed
length. Or sextant, called the right-angle instrument, is
fitted with index an horizon glasses permanently inclined at
45�. It consequenti measures a right angle. In the other
sextant, called the readir instrument, a graduated drum
takes the place of the usual mdc arm and scale. The drum is
graduated spirally with a scale ranges. Both reading and
right-angle instruments are fitted wit a vane of gun metat
with a white strip down the centre to facilitat
observations. Telescopes of low power can be fitted to the
instri rnents, and two cords of 50 (or 253/4) yds. are
provided with wliic to measure the base.

Two observers attach the ends of the cord of fixed length
(usuall 50 yds.) to their instruments and separate until it
is taut. TI
observer with the right-angle nstrument moves into suc Meko
a position that coincidence of image will be given betwec
meter. the objective and the vane of the instrument
attheothl

end of the base, i.e. he makes ABC a right angle (fig. 3).
When the right angle is established, the observer at C turns
ti graduated drum of the reading instrument until the image
of ti vane of the right angle instrument coincides with the
direction the objective. The range AC is then read on the
drum. TI ranges on the drum are measures of the angle BAC
when the bat BC is 50 yds.
The mekometer is open to the objection which is common to a
range-finders requiring more than one observer. There is
always danger that observers may cause coincidence on
different objectiv
or on different parts of the same objective, and thus
inaccuracy in thi recordedrange must result. The instruments
are expected to give alt accuracy of less than
12 0/0 at 2000 yds. For A5
Ofijective
ranges over that distance, i.e. for usual
artillery ranges, it is
i~o- desirable to use a N., ~~Andouble base (100 yds.
i~~~in length), in which

case the range registered on the drum must be doubled. This
opera-
- tion, although slight,
Raodsng
is a distinct disadvantage, since it adds to the
time of taking a range Rightangle
�~.rrumen
and is a possible source inatrumen B
- of error. For field - - -� - - - Base P
artillery, however, a FIG. 3.
range-finder is only an
auxiliary adjunct. The true range can be found by a proces
of trial and error (see ARTILLERY) in as short a time as th
- mekometer observers take to report it. It must further bI
remembered that as shrapnel is the principal projectile of
fieli artillery, not only the correct elevation but also the
true length a time fuse has to be found. This the
range-finder cannot do. Henc

it is that the range-finder for field artillery, although a
valuabl auxiliary, is not of the same importance as in
purely defensiv positions, such as batteries for harbour
defence, and land forts.
1 The Marindin range-finder was from 1908 gradually intro
duced in the infantry to replace the mekometer. It was th
invention of Captain A. H. Marindin, of the Black Watcl
(Royal Highlanders).

The principle of the instrument is that of coincidence, as
in th Gautier Christie Le Cyre, Souchier, and Barr and
Stroud. Bu

>>>>>>>>>>>>>>>> end quote >>>>>>>>>>>>>>>>>>

The Barr and Stroud rangefinder, the same article goes on to
say, used a thin wedge of glass which slid along one arm and
changed the angle through only 3 degrees.  The one meter
base infantry version slid only about six inches, had a
minumum range of about 400 yards, maximum reading of 20,000
yards or so, and used a 14 power telescope system.  The
upright image was able to be precicely matched with an
inverted image from the other end of the instrument.

Three of these, and a crossection of the whole are shown at:


I can't find out for sure, but the overall look, layout of
the controls and eyepieces of the Wild TM2 are very similar.
You can see photographs of one, and views of the outside,
accessories,  through the eyepiece at:
<

Range accuracy tables are shown there also.


--
Richard ...

   

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