Improvements in the Method of and Apparatus for Estimating Distances Specially Applicable for Military Purposes.
Year: 1892

page_text: Date of Application, 30th July, 1892
Complete Specification Left. 30th May, 1893-Accepted. 29th July, 1893
PROVISIONAL SPECIFICATION.
Improvements in the Method of and Apparatus for Estimating
Distances Specially Applicable for Military Purposes.
I DOUGLAS GORDON PRINSEP of 45 Glasshouse Street, Regent Street,
London in the County of Middlesex a Captain in Her Majesty's Royal Artillery,
do hereby declare the nature of this invention to be as follows:-
This invention for improvements in the method of and apparatus for estimating
distances specially applicable for military purposes has for its object to enable an
observer to estimate distances with great facility and accuracy in the field without
having recourse to direct mathematical calculation and consists essentially of
specially designed and constructed instruments for observing and measuring the
angles of a special system of triangulation and for mechanically calculating the
results therefrom.
:-
One method of estimating distances according to this invention is as follows
Let it be required to find the distance between one point A and another point C
commonly called the distant point or range. Take any arbitrarily selected point B
to form a triangle ABC such that (preferably) the angle A B C is not greater
than 120° nor less than 60°. Measure the angle A B C and set off at A the
angle CAD equal to the supplement of the angle A B C, and from B measure the
side B D of the supplementary triangle B A D parallel to the side A. C. The two
A B
triangles A B D A B C are similar triangles so that A C is equal to
BD
Το
find the length of the side A B, a triangle BA E, which may be either a right
angled or isosceles triangle, is constructed with a known predetermined base BE,
and the angle B A E being obtained by observation the side A B is determined or
the vertex angle may be a constant and the base variable.
The first part of my invention consists of a specially constructed instrument for
measuring the required angles in which an index glass and horizon glass are
arranged similarly to those of an ordinary box sextant. An index arm attached to
the index mirror operates through a projecting pin or other device two measuring
arms or their equivalents, in such manner that when the observed angle is a right
angle they occupy their normal or zero positions. Any deviation either in excess
or defect of this angle causing one of the measuring arms, or their equivalents to
be carried outwardly, the other remaining in its normal position against a central
stop, in which relative positions they can be secured by a suitable clamping device.
The mode of using the instrument is as follows:-
The measuring device being unclamped the index mirror and arm are moved
until the required objects as A and C are covered, one or the other measuring arm
being displaced by it; the two arms are then clamped together. The index arm is
now rotated until its pin or stud strikes the measuring arm which was abutting
against its central stop and carries it outwards (the two arms rotating together)
until the previously displaced arm strikes the central stop. It will thus be seen
that the index arm is now as much on one side of its central position as it was on
the other side when the observation was taken, that is to say, the angle now
subtended is the supplement of that first observed. The observer now proceeds to
a second angle of the triangle to set off the angle CAD thus obtained equal to the
supplement of the aforesaid angle A. B. C. The vertex angle of the supplementary
triangle B A E can be read or set off by means of a vernier on the index arm, as in
the ordinary sextant.
Instead of the apparatus bercinbefore described I sometimes use a telescope and

page_text: an adjustable object glass or glasses in conjunction with an horizon glass and mirror
for measuring the angles required by single reflection.
The second part of my invention consists of a modified logarithmic slide rule
arranged for a given constant base E B and so divided that on placing the number
on the slide corresponding to the length of the side BD to coincide with the
number representing the vertex angle of the triangle BAE the length A C
required is indicated on a scale by means of a fiducial mark on the end of the said
slide, or its equivalent.
For the sake of compactness the slide rule is preferably arranged in circular
form.
Dated this 30th day of July 1892.
COMPLETE SPECIFICATION.
Improvements in the Method of and Apparatus for Estimating
Distances Specially Applicable for Military Purposes.
I DOUGLAS GORDON PRINSEP, of 45 Glasshouse Street, Regent Street, London,
in the County of Middlesex, a Captain in Her Majesty's Royal Artillery, do
hereby declare the nature of this invention and in what manner the same is to
be performed to be particularly described and ascertained in and by the following
statement :-
This invention for improvements in the method of and apparatus for estimating
distances specially applicable for military and surveying purposes has for its object
to enable an observer to estimate distances with great facility and accuracy in the
field, without having recourse to direct mathematical calculation; and consists
essentially of a specially designed and constructed instrument or instruments for
observing and measuring the angles of a special system of triangulation and for
mechanically calculating the results therefrom.
In the accompanying illustrative drawings :-
-
Figure 1 represents a diagram of a system of triangulation for estimating
distances according to this invention.
Figure 2 is a plan of a range finding instrument for observing and measuring
angles and calculating the distances.
Figure 3 is a horizontal section of the same, inverted to show the method of
fixing and manipulating the index and horizon glasses.
Figures 4 and 5 are vertical section and plan of the cover of the instrument
showing a logarithmic slide apparatus for calculating the required distances
mechanically.
Figures 6 and 7 represent a sectional elevation and a sectional plan respectively
of a modified arrangement of range finder, and
Figures 8 and 9 represent, in an open and closed condition, a form of picquet for
use with my system of range finding, and Figure 10 is a horizontal section of the
same.
Referring to Figure 1 of the drawings: Let it be required to find the distance
between one point A and another point C commonly called the distant point or range.
Take any arbitrarily selected point B to form a triangle ABC such that
(preferably) the angle ABC is not greater than 120° nor less than 60°. Measure
the angle ABC and set off at A the angle CAD equal to the supplement of the
angle ABC
A BF, and from B measure the side BD, of the supplementary
triangle BAD, parallel to the side A. C. Then the two triangles A BD, ABC
=
are similar triangles so that: BD: A B
A
A B²
A B A C and therefore A. C
=
BD

page_text: To find the length of the side A B, a triangle BAE, which may be either a
right angled or isosceles triangle, is constructed with a known predetermined
base B E and, the angle BAE being obtained by observation, the side A B is
determined; or the vertex angle may be a constant and the base variable.
The first part of my invention (Figures 2 to 5 inclusive) consists of a specially
constructed instrument, for measuring the required angles, in which an index
glass a and horizon glass b are arranged similarly to those of an ordinary box
sextant. An index arm c attached to the index mirror a operates, through a
projecting pin d, between two measuring arms ef in such manner that when the
observed angle is a right angle they occupy their normal or zero positions as shown,
each arm being pressed against a stop y by a spring which is connected to both
arms efas shown, any deviation either in excess or defect of a right angle causing
one of the measuring arins to be carried outwardly, the other remaining in its
normal position against the central stop g, in which relative positions they can be
secured by a clamping screw i, spring hand limb 7.
The end of the index arm e is provided with a vernier scale to read off the angles
on the graduated limb or scale m. The index mirror a is secured to a disc n itself
mounted on a disc to whose axis the index arm is secured. The latter disc is
provided with a toothed quadrant o gearing with a pinion p operated by the milled
head g. The first mentioned disc n, to which the mirror a is directly attached, is
provided with an arm r carrying a rotary nuts engaging with a tangent screw t
passing through a bracket v secured to or forming part of the quadrant o so that,
while the mirror and index arm are rotated as a whole by the pinion p and
quadrant o, the mirror may be caused to take a limited motion of rotation,
independently of the quadrant by means of the tangent screw t. A micrometer
head won the screw serves to indicate by the pointer the amount of such
independent rotation; and a spring on the said screw t insures the accurate
position of the mirror by keeping all the parts pressed in one direction and so
preventing backlash.
The horizon glass is adjusted by means of the tangent screw.
On the back of the casing a a is arranged a circular logarithmic slide rule by
means of which the side AC, Figure 1, may be found from the side BD and
angle EA B.
The principle of this slide rule will be better understood on reference to Fig. 5^
which represents a rule similar to that shown in Figs. 4 and 5 but arranged in a
straight line.
The chief object is to reduce the work in the field to simple mechanical
manipulation.
BD
As in the common slide rule this consists of scales spaced as the logarithms of
the numbers they represent so that by sliding one scale against another, logarithmic
functions are mechanically performed, that is, a piece of one scale measured from
zero is simply added to or deducted from the other, by which multiplication or
division is performed, after which the resultant number is read off. Thus on the
bottom scale A' which corresponds with the inner scale on the circular rule are
BAE
marked the points corresponding to the values of Cosec.
2
angles.
E B
2
for various
A second scale on the slide B₁ is divided similarly to A' but of half the
dimensions so that, the divisions being logarithmic proportions, a distance repre-
senting a number taken from zero on A¹ would when applied to B read the square
of that number.
The scale C is similar and equal to scale B₁ but the divisions are made to

page_text: scale A' the ordinary logarithmic divisions are replaced by divisions in such positions
on the scale as to represent the values of
EB
BAE
2
Cosec..
2
The fiducial mark on scale B' serves to indicate the reading on C1 and its position
is determined by setting the scales A' and B' to any reading and then marking it to
indicate the correct reading on C¹.
For convenience the flat straight scales are replaced by a circular arrangement
shown in Figs. 4 and 5 which comprises a circular ring a b, provided with finger
studs a c to enable it to be readily rotated, and which is retained in position by a
fixed disc ad. The edges of the disc a d, ring a b, and plate ae are divided in the
above described ratios, so that on placing the division on the ring ab, representing
the length of the side B D, opposite the division on the disc ad corresponding to
the observed angle E A. B, the fiducial mark or arrow head af on the said ring
indicates on the plate ae the length of the range A. C in yards.
The mode of using the instrument is as follows:-
:-
The angle measuring arms being unclamped, and the observer standing at B, and
picquets having been placed at any two points A and B, C being the distant point
and AC the distance required to be found, the index mirror and arm are rotated by
the pinion g until the objects A and C are co-incident, one or other measuring arm
being displaced by it. The arms are then clamped in this position by the clamping
screw on the limb l.
The index arm is now rotated in a contrary direction until its pin or stud d
touches the other measuring arm, which it then carries out from the central stop g,
and until the first moved arm touches the same central stop, when it can go no
further by reason of the clamping screw i preventing further movement. It will
thus be seen that the index arm is now as much on one side of the central stop as
it was on the other side when the observation was taken, that is to say, if the index
arm first traversed an angle corresponding to a reflected angle ẞ from its central
position when measuring an angle a by vision (and a + B = 90°), then by reversing
the motion the arm would traverse an angle corresponding to a reflected angle = 2ẞ
and so register an angle of vision =a+2=180°-
-a or, in other words, the
supplement of a.
=
The observer now proceeds to the second point A of the triangle ABC to set
off the angle CAD: the supplement of A B C and obtained as above described,
the assistant at the same time stretching a tape graduated in any selected units on
the line BD parallel to A C, and where his hand holds the tape at D when the
point D coincides with C in the instrument, he reads the length BD.
The assistant then moves to E with the tape stretched from B to E, and at a
known predetermined length, and held so that EBA is an isosceles (triangle,
although a right angled triangle may be employed), when the observer first
rotates the index glass of the instrument until the glasses are parallel and then
measures and reads on the micrometer wheel the vertex angle EA B of the
triangle E A B.
The centre scale of the circular slide rule is then rotated until the length B D is
set to the angular measure B A E on the inner scale when the fiducial mark a f
will point on the outer scale to the length of A C or the distance required in yards
or other units. Or the vertex angle may be made a constant and the base E B be
the variable and in such case the inner scale would be graduated to linear units as
required.
The telescopic range finder illustrated in Figures 6 and 7 consists essentially
of a high power telescope ag behind the object glass ah of which is placed a half
silvered horizon glass ai connected to a train of wheels al am an ao so, that,
while the half silvered glass travels round through any angle, an extra object

page_text: glass ap also travels through twice that angle, and a ray of light is thus always
reflected up the telescope tube, and an image reflected from the half silvered
glass is seen at the eye hole superimposed on that seen by direct vision through the
unsilvered portion of the half silvered glass.
The lower toothed wheel ao carries this second object glass, and an index arm a q,
and has also a stop ar which can move between two radial arms as at and an
arc av connecting them, which latter can be clamped at any required position;
and a stop a to is placed on the bed plate a between the two radial arms as, at,
so that when the said arms are touching it on each side, and the pin ar of the
index arm ag is between them, the mirror is inclined at 45° to the axis of
the telescope, and the second object glass is at right angles to the axis of the
telescope and, consequently, a magnified image at right angles would be seen in
the eye piece over that seen by direct vision.
3
The arc a being unclamped, and the actuating wheel moved, any angle is
reflected and the are being then clamped, and the motion reversed, a similar
angle on the other side of the 90° stop will be registered 180°
angle first measured.
=
-
a; a being the
The radial arms are placed for convenience under the bed plate ax and the
stop on the index arm works in a slot ay cut out in the bed plate. All the stops
are adjustable, and the usual focussing tube to the eye piece and a focussing rack
to the object glasses may be added as required.
A second index mirror az is placed at a suitable position to measure the small
vertex angles by double reflection, the angles being read off on the scale bd
by the vernier ba on the arm be as will be readily understood.
In the picquet illustrated in Figures 8 and 9 the triangular wings of canvas be, bf
are fastened to the staff bg which has formed in it a slot bh in which is placed a
pivotted stretcher bar bi. When opened the ends of the stretcher bi lodge in
pockets and hold the wings be, bf open as shown in Figure 8.
Figure 9 shows the same in its closed or rolled up state.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed I declare that what
I claim is
1. The method of estimating distances described in the foregoing Specification,
substantially as described with reference to Figure 1 of the drawings.
2. In apparatus for measuring distances: the employment of an index
mirror having an index arm working in conjunction with measuring arms, for
automatically obtaining the supplement of the angle directly observed, substantially
as described.
3. In a method of and apparatus for estimating distances: the means for
automatically obtaining the supplement of an angle directly observed comprising,
a pair of measuring arms normally pressed by a spring against a central stop, an
index arm connected to and partaking of the motion of the index mirror and
carrying a projection which displaces the said measuring arms from their zero
position, a clamping device for securing the said measuring arms relatively to one
another, when so displaced until the supplementary angle thus cbtained is set off
and a horizon glass, substantially as described.
4. In apparatus for estimating distances: the means for measuring the small
vertex angles comprising a rotatable disc adjustably connected to the index arm and
carrying the index mirror in combination with a micrometer headed screw for
operating the same and indicating the angle observed substantially as described.
. The arrangement and construction of the logarithmic slide rule for mechanically
calculating the length of range substantially as described with reference to
Figures 4 and 5 of the drawings.
6. The arrangement and combination of parts constituting my improved
apparatus for estimating distances, the whole constructed and operating

page_text: substantially as described with reference to Figures 2 to 5 of the drawings
inclusive.
:
7. In a telescopic range finder such as is illustrated in Figures 6 and 7 of the
drawings the employment of a telescope in combination with rotary horizon and
object glasses so connected that the object glass rotates through twice the angle
of the horizon glass substantially as described and for the purpose specified.
8. In apparatus for estimating distances: the employment of a rotary horizon
glass ai, and object glass ap having index arm ag and stop ar in combination
with the measuring arms as, at and connecting clamping quadrant av and stop a w
substantially as described.
9. In apparatus for estimating distances: the combination of a telescope ag
mirror a, and index mirror az with arm bc and vernier ba substantially as
described.
10. The arrangement and construction of apparatus for estimating distances
combined and operating substantially as described with reference to Figures 6 and 7
of the annexed drawings.
11. The combination of parts constituting my improved picquet arranged and
constructed substantially as described with reference to Figures and 9 of the
drawings hereunto annexed.
Dated this 30th dow of Mov 1902
WHEATLEY & MACKENZIE,
40, Chancery Lane, London, W.C., Agents.