An Instrument for use in Military Operations for Ascertaining the Amount of Cover from Fire afforded by a Parapet or by Natural Rise in the Ground, or the He of a Structure or Parapet necessary to afford such Protection.
Year: 1891

page_text: Date of Application, 29th Jan., 1891
Complete Specification Left, 21st Oct., 1891-Accepted, 9th Jan., 1892
PROVISIONAL DIVIFICATION.
An Instrument for use in Military Operations for Ascertaining the
Amount of Cover from Fire afforded by a Parapet or by Natural
Rise in the Ground, or the Height of a Structure or Parapet
necessary to afford such Protection.
I ARTHUR JOHN WATSON of the Junior United Service Club Charles Street
in the County of Middlesex Major Suffolk Regiment do hereby declare the nature
of this invention to be as follows:-
An instrument constructed according to my invention consists of a plate or board
upon which are centered upon a common centre two arms extending to a segmental
scale arranged as part of a circle struck from the centre upon which the said arms
turn. This scale is marked with degrees. One of the arms is provided with a
scale of distances (say in yards) corresponding to the depth from front to rear of
the ground for which cover is required. This arm which I will call the "plane of
site
arm is adjusted in accordance with the general plane of the ground whether
level or rising or descending the said arm being adjusted to one side or the other of
the central line of the scale according to whether the ground rises or falls and to
the central line if the ground be level. The plane of the ground can be ascertained
by providing the instrument with any suitable clinometer such as a plumb bob
hanging from the centre on which the arms turn by a line or cord passing across
the segmental scale the top edge of the plate or board being used as a sight line for
observing an object at a distance. On the surface over which the arms move are
ruled lines parallel with the central line of the scale and at a distance apart
corresponding to the scale on the plane of site arm. The second arm which I will
call the "fire tangent" arm is adjusted on the scale in accordance with the angle of
the "fire tangent" from the most distant or most commanding firing point from
which the enemy can fire. By reading off on the scale on the arm used for getting
the plane of sight the distance for which protection is needed then the distance
between the two arms at this point as read on the scale of parallel lines and at right
angles to them will shew the height of structure or parapet which is necessary for
giving the required extent of protection to the rear of the structure or parapet or
the same instrument it will be evident can be similarly used for indicating what
protection is already afforded by an existing parapet structure or rise of ground.
On the back of the plate or board I mark tables shewing angles of drop for different
kinds of projectiles as may be necessary say for the Martini-Henry bullet or for the
lowest bullet of shrapnel fire of any particular gun or guns,
In using the instrument from the proposed position of the parapet or crest of the
hill to be defended find with the clinometer the elevation or depression of the point
from which the enemy can deliver the most searching fire and get the distance
which can be done with a range finder. Then take from the tables the angle of
drop of the projectile under consideration. To the angle of drop add the angle of
elevation or deduct the angle of depression of the place of firing and there will be
obtained the angle of inclination of the fire tangent which is the line or plane below
which no bullet can pass say within twenty five yards in rear of the parapet or
crest.
Let the arms to correspond to the angles of the fire tangent and plane of site
when the distance between the arms measured on the divisions of the scale of
parallel lines appearing between the said arms and at right angles to the lines will

page_text: give the required height for the top of the parapet or structure in excess of the
height to which the gorge is to be protected from fire.
Dated this 29th day of January 1891.
J. H. JOHNSON & Co.,
47, Lincoln's Inn Fields, London, W.C., Agents.
COMPLETE SPECIFICATION.
An Instrument for use in Military Operations for Ascertaining the
Amount of Cover from Fire afforded by a Parapet or by Natural
Rise in the Ground, or the Height of a Structure or Parapet
necessary to afford such Protection.
1 ARTHUR JOHN WATSON of the Junior United Service Club Charles Street
in the County of Middlesex, Major Suffolk Regiment 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 :--
An instrument constructed according to my invention is illustrated in face view
by Figure 1 on the accompanying drawing. It consists of a plate or board A upon
which are centred upon a common centre two arms B and B extending to a
segmental scale C arranged as part of a circle struck from the centre b upon which
the said arms B and B turn. This scale C is marked with degrees. One of the
arms that B is provided with a scale of distances (say in yards) corresponding to
the depth from front to rear of the ground for which cover is required. This
arm B which I will call the "plane of site" arm is adjusted in accordance with the
general plane of the ground whether level or rising or descending the said arm
being adjusted to one side or the other of the central or zero line of the scale
according to whether the ground rises or falls and to the said central or zero line if
the ground be level. The plane of the ground can be ascertained by providing the
instrument with any suitable clinometer such as a plumb bob & hanging from the
centre b, on which the arms turn, by a line or cord 2 passing across the segmental
scale C the top edge of the plate or board A being used as a sight line for observing
an object at a distance. On the surface over which the arms move are ruled lines e
parallel with the central line of the scale and at a distance apart corresponding to
the scale on the plane of site arm. The second arm (B) which I will call the fire
tangent arm is adjusted on the scale in accordance with the angle of the "fire
tangent" from the most distant or most commanding firing point from which the
enemy can fire.
By reading off on the scale on the arm B² used for getting the
plane of sight the distance for which protection is needed then the distance between
the two arms B and B at this point as read on the scale of parallel lines (e) and at
right angles to them will shew the height of structure or parapet which is necessary
for giving the required extent of protection to the rear of the structure or parapet,
or the same instrument it will be evident can be similarly used for indicating what
protection is already afforded by an existing parapet structure or rise of ground.
On the back of the plate or board AI may mark tables shewing angles of drop
for different kinds of projectiles as may be necessary say for the Martini-Henry
bullet or for the lowest bullet of shrapnel fire of any particular gun or guns.
22
Figure 2 is a diagram explanatory of the method of using the instrument, in
which diagram D represents the position of the piece of ground or site for
which a parapet is required to be erected for the purpose of defending it from
the enemy's fire at E, which in this example is supposed to be elevated above the
plane of site D. First find by means of the clinometer d d the angle of elevation
of the point E above D. which I will assume to be .. Next find, by means of a
range finder, the distance of E from D and find from the tables the angle of drop of
the projectiles for this range, which for example I will assume to be y
on the level
as shown at y. To the angle of drop (°) add the angle of elevation (a) which

page_text: will give (0°) the angle of inclination of the fire tangent. Adjust the
arm B of the instrument on the scale C to the angle on the left of zero point, and
then ascertain by means of the clinometer the inclination of the plane of site D
which I will assume to be 0 from the horizontal. Adjust the arm B2 of the
instrument to 10 on the scale C to the left of the zero point. The arms B and B2
being thus set to the angles of the fire tangent and the inclination of the plane of
site suppose it be required to protect by the parapet a distance in yards behind the
parapet as represented at c the height of the parapet required is found by reading
off on the scale of parallel lines e in a line at right angles to the said lines and
extending from the division corresponding to the distance e marked on the arm B³
to the edge of the arm B the number of divisions embraced by the arms which
gives the height of the parapet F required in feet.
If the firing point E be below the plane of site D the angle of depression of the
place of firing must be deducted from the angle of drop to obtain the angle of
inclination of the fire tangent otherwise the process is the same as before described
to find the required height of the protecting parapet.
The amount of protection afforded by an existing parapet or crest of a hill may
be ascertained in a similar manner, for example as shewn by the diagram Fig. 3
let F represent the crest of a hill, D the ground protected thereby, and E the
enemy's point of fire, which for example I will assume to be at an angle of a below
the plane of site D. First ascertain the height of the mound F from the plane of
site D which I will assume to be feet. Ascertain the distance of the point of
fire E, the inclination of the plane of site D and the angle of drop as before
described which latter I will assume to be y° and the inclination of the plane of site
an angle of inclining upwards from the base of the mound.
=
Subtract from the angle of drop (y"), the angle of depression (2), which will
give (0) the tangent of fire." Adjust the arm B to the degree marked on
the scale C corresponding to to the left of the zero point and adjust the arm B²
to the point corresponding to w marked on the right of the zero point of the said
scale C then find at what point on the arm B in a line at right angles to the
parallel lines e will embrace f feet only between the arms Band B2 and this point on
the arm B will give the distance e in yards behind the mound F protected thereby
from the enemy's fire at E.
Having now particularly described and ascertained the nature of the said inven-
tion and in what manner the same is to be performed I declare that what I
claim is :-
:―
An instrument for the purpose described constructed substantially as hereinbefore
explained.
41 in 21st Jure of October 1801