A Method of Ascertaining Approximately the Range of an Object at Sea, and Appliances for that Purpose.
Year: 1891

page_text: Date of Application, 5th Dec., 1891
Complete Specification Left, 6th Aug., 1892-Accepted, 17th Sept., 1892
PROVISIONAL SPECIFICATION.
A Method of Ascertaining Approximately the Range of an Object at
Sea, and Appliances for that Purpose.
I, GEORGE ARTHUR FRENCH of Dover Castle, in the County of Kent,
Lieutenant Colonel in the Royal Artillery, do hereby declare the nature of this
invention to be as follows:-
A gun situated in a fort or battery facing the sea is usually provided with
an adjustable back sight or tangent scale which can be raised or lowered to suit
various ranges; it may also be provided with means of ascertaining the angle
which the axis of the gun makes with the horizon, whether that angle be
determined by a spirit level or plumb line, or by marks on the gun carriage or
objects behind the gun or otherwise. My invention relates to a method of using
these provisions of a gun and appliances therewith for ascertaining approximately
the range of an object at sea. For this purpose, in the first place, from habit of
judging distances, I estimate roughly the distance of the object, which may, for
example, appear to be 3000 yards, I then set the back sight of the gun to a
less range such as 2,600 yards and I set the gun to the corresponding elevation
that is to say to the inclination to the horizon due to that range. Looking now
along the sights of the gun I find that the line of sight meets the sea at a point
short of the object. I then, without moving the gun, lower the back sight until I
get the object in the line of sight, and looking at the back sight I see that I
have shifted it an amount equivalent to a certain number of yards range, say for
example 200 yards. On adding this to the distance for which I had in the first
place set the back sight, namely 2600 yards, I thus find the real distance of the
object to be approximately 2800 yards. Instead of using the gun itself in this
manner, a dummy or model gun or mere framing mounted as a gun, and having the
like provisions of sights and means of determining inclination, can be employed.
When I employ a gun or a dummy in this manner, I prepare in the first place
a scale of elevations for different ranges by actual trial in the following manner.
I place an object at a known distance at sea, such for instance as 2000 yards and I
set the ordinary back sight or tangent scale for that range; 1 then incline the gun
or dummy until the line of sight meets the object and I mark this inclination as
that for 2000 yards on any suitable permanent scale by which I can afterwards set
the gun or dummy to the same inclination.
In like manner I determine inclinations for other ranges and I am thus enabled
afterwards to ascertain ranges as above described with considerable accuracy.
A scale of inclinations prepared as above set forth by actual trial, is useful for
laying a gun for any known range even when the object at that range cannot be
seen. In the above description I have assumed the sea to have constant level. In
cases where there are variations of level due to tides I provide the scale of elevations
with means of correcting for such variations. Obviously guns on board ship
might be employed as described for approximate range finding when the vessel is
steady.
Dated this 5th day of December 1891.
ABEL & IMRAY,
Agents for the Applicant.

page_text: COMPLETE SPECIFICATION.
A Method of Ascertaining Approximately the Range of an Object at
Sea, and Appliances for that Purpose.
I GEORGE ARTHUR FRENCH of Dover Castle, in the County of Kent,
Lieutenant Colonel in the 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 :-
at sea.
A gun situated in a fort or battery facing the sea is usually provided with an
adjustable back sight or tangent scale which can be raised or lowered to suit
various ranges,
may also be provided with means of ascertaining the angle which
the axis of the gun makes with the horizon, whether the angle be determined by a
spirit level or plumb line, or by marks on the gun carriage or objects behind the
gun or otherwise. My invention relates to a method of using these provisions of a
gun and appliances therewith for ascertaining approximately the range of an object
For this purpose in the first place from habit of judging distances, I
estimate roughly the distance of the object, which may for example appear to be
3000 yards. I then set the back sight of the gun to a less range, such as 2600 yards,
and I set the gun to the corresponding elevation, that is to say, to the inclination
to the horizon due to that range. Looking now along the sights of the gun I find
that the line of sights meets the sea at a point short of the object. I then without
moving the gun, lower the back sight until I get the object in the line of sight, and
looking at the back sight I see that I have shifted it an amount equivalent to a
certain number of yards range, say for example 200 yards. On adding this to the
distance for which I had in the first place set the back sight, namely 2600 yards-
I thus find the real distance of the object to be approximately 2800 yards. Instead
of using the gun itself in this manner, a dummy or model gun or mere framing
mounted as a gun, and having the like provisions of sights and means of determining
inclination can be employed. Similarly, if I set the gun or dummy to more than
the estimated range, 1 subtract the amount determined by shifting the back sight
which in this case has to be raised. When I employ a gun or dummy in this
manner I prepare in the first place a scale of elevations for different ranges by
actual trial in the following manner: I place an object at a known distance at sea,
such for instance as 2000 yards, and I set the ordinary back sight or tangent scale
for that range; I then incline the gun or dummy until the line of sight meets the
object and I mark this inclination as that for 2000 yards on any suitable permanent
scale, by which I can afterwards set the gun or dummy to the same inclination.
In like manner I determine inclinations for other ranges and I am thus enabled
afterwards to ascertain ranges as above described with considerable accuracy.
A scale of inclinations prepared as above set forth by actual trial, is useful
for laying a gun for any known range, even when the object at that range cannot
In the above description I have assumed the sea to have constant level.
In cases where there are variations of level due to tides, I provide the scale of
elevations with means of correcting for such variations. Obviously guns on board
ship might be employed as described for approximate range finding when, the
vessel is steady.
be seen.
+
Figure 1 of the accompanying drawings is a side view of a gun illustrating
how a scale can be formed wherewith to operate according to my invention.
Figure 2 is a side elevation, with two end elevations to an enlarged scale of the
trunnion sight,and Figure, 3 is a front view of the vertical graduated scale, which
I prepare in the following manner. Having measured some distance at sea, such
as 1500 yards, I set the tangent sight T to the graduation 1500 and view over the
foresight the object 1500 yards distant giving the gun the necessary elevation
for this purpose.
I then, applying the eye at B, in front of the trunnion sight look
back to the scale S on which I mark 1500. I find in like manner the other

page_text: graduations of the scale S, which I employ as above described in ascertaining
ranges. Obviously for the gun, a dummy having the sights T and F, and a sight
corresponding to the trunnion sight may be substituted, a scale such as S being
formed and used in the manner described. Instead of determining a scale such
as S, a table may be formed shewing the various elevations of the gun for various
ranges, the gun being laid to any of these elevations as determined by a clinometer
applied to any part of the gun parallel to its axis. The clinometer thus employed
may be itself graduated for the different ranges and like the scale S may be
provided with means of correcting for differences of level due to tides.
Having now particularly described and ascertained the nature of my said inven-
tion and in what manner the same is to be performed I declare that what I
claim is: :-
1. The herein described method of ascertaining approximately the distance of an
object at sea, by setting a gun or a dummy or model gun to less or more than the
estimated range, and finding by the alteration of the back sight, the necessary
correction to be added to or subtracted from the estimated range.
2. For use in determining ranges, a scale of the inclinations of a gun or dummy
for various ranges as determined by directing the gun or dummy to objects at those
various ranges.
Dated this 5th day of August 1892.
ABEL & IMRAY,
Agents for the Applicant.