AN IMPROVED RANGE FINDER OR INSTRUMENT FOR MEASURING AND ASCERTAINING DISTANCE
Year: 1879

page_text: LETTERS PATENT to Francis Weldon, of Trichinopoly, in the Empire of India,
Major in the Madras Staff Corps, for the Invention of "AN IMPROVED RANGE
FINDER OR INSTRUMENT FOR MEASURING AND ASCERTAINING DISTANCE."
Sealed the 7th October 1879, and dated the 9th April 1879.
PROVISIONAL SPECIFICATION left by the said Francis Weldon at the Office
of the Commissioners of Patents on the 9th April 1879.
#f
FRANCIS WELDON, of Trichinopoly, in the Empire of India, Major in the Madras
Staff Corps.
AN IMPROVED RANGE FINDER OR INSTRUMENT FOR MEASURING AND
ASCERTAINING DISTANCE."
My improved range finder or instrument consists of two reflectors, each adjusted
to show an angle of 88 degrees, 34 minutes, 3 seconds, fixed like those of an optical
square, or same may be formed of a reflecting prism, the faces of which are ground
to show the same angle.
The reflectors or reflecting prism are firmly secured in a case, open on one side.
Above each reflector is an opening through which objects in front of the observer
are viewed direct.
When using the instrument it is to be held with the open side towards the eye,
but slightly turned in the direction of the object to be reflected, so that on looking
into the instrument only one reflector is visible, in which will be seen objects to the
right or left of the observer. To determine an angle between any two objects, one
in front, viewed directly through the opening above the reflector, and the other on
the right, seen in the reflector, it will be necessary to move to the right or left until
the reflected object is seen to coincide with the object viewed directly. Slightly
tilting the instrument will enable an apparent contact of both objects to be made
when the angle is correctly determined. In measuring distances this operation is
called "angle finding."
To make an angle with a given line and a distant object it will be necessary to
move backwards or forwards along that line until the two points that determine it
are seen to coincide with the object. In measuring distances this is called "base-
finding."
By this instrument I show that in the isosceles triangle, the base angles of which
are 88 degrees, 34 minutes. 3 seconds, the base is to the side as 1 to 20, so that in

page_text: range finding, when by means of an angle glass or prism set to 88 degrees,
34 minutes, 3 seconds, you find the length of base proportional to the sides of the
isosceles triangle formed by a distant object and the extremities of the base; it is
only to multiply that length of base by 20 to find the distance of the object.
The important point consists in my using two fixed and known angles, or perhaps
I should say three known angles in measuring distances, instead of one known
angle and one known side or base, which entails complicated mechanism to measure
the other angle, with corresponding liability to derangement and inaccuracy in
manipulation, which is the great drawback of all other methods.
The advantages of this system of range finding lie not only in the fact that the
angles are permanently fixed, thus affording the simplest form of instrument, but
that they are fixed at such an angle as enables a difference of, say, 20 yards of
distance to be readily appreciated by the eye of the observer; in other words, it
ensures that the base shall always subtend an angle of not less than 2 degrees,
51 minutes, 54 seconds, or the least that can be trusted to give a satisfactory
result.
A distance of three thousand yards can be measured by this method and instru-
ment with the same accuracy as a distance of three hundred yards, the bases of each
being proportional to the distance; thus, the base for three thousand would be one
hundred and fifty yards, and for three hundred it would be fifteen yards, no more
nor less than is actually necessary to ensure accuracy.
Although I mention 88 degrees, 34 minutes, 3 seconds, as being the most con-
venient angle for this system, giving as it does a base of 1 in 20, still I do not
confine myself to that proportion alone; for instance, the base angles being fixed at
87 degrees, 8 minutes, 30 seconds would give a base of 1 in 10, and so forth.
The principal point to bear in mind is that by my arrangements the two base
angles, whatever their value may be, are fixed and known beforehand, the base only
being variable and proportional to the distance.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Francis Weldon in the Great Seal Patent Office on the 9th October
1879.
FRANCIS WELDON, of Trichinopoly, in the Empire of India, Major in the Madras
Staff Corps. "AN IMPROVED RANGE FINDER OR INSTRUMENT FOR MEASURING AND
ASCERTAINING DISTANCES."
The improved instrument for range finding, according to my improved system
hereafter explained, consists of two reflectors, each adjusted to show an angle of
eighty eight degrees, thirty four minutes, three seconds, fixed like those of an
optical square; or same may be formed of a reflecting prism the faces of which are
ground to show the same angles. The reflectors or reflecting prism are firmly

page_text: "
secured in a case open on one side. Above each reflector is an opening through
which objects in front of the observer are viewed direct. When using the instru-
ment, it is to be held with the open side towards the eye, but slightly turned in the
direction of the object to be reflected, so that on looking into the instrument only
one reflector is visible, in which will be seen objects to the right or left of the
observer. To determine an angle between any two objects, one in front, viewed
directly through the opening above the reflector, and the other on the right, seen
in the reflector, it will be necessary to move to the right or left until the reflected
object is seen to coincide with the object viewed directly. Slightly tilting the
instrument will enable an apparent contact of both objects to be made when the
angle is correctly determined. In measuring distances this operation is called
angle finding." To make an angle with a given line and a distant object
it will be necessary to move backwards or forwards along that line until the two
points that determine it are seen to coincide with the object. In measuring
distances this is called "base finding." By this instrument I show that in the
isosceles triangle, the base angles of which are eighty eight degrees, thirty four
minutes, three seconds, the base is to the side as one to twenty, so that in range
finding, when by means of an angle glass or prism set to eighty eight degrees, thirty
four minutes, three seconds, you find the length of base proportional to the sides of
the isosceles triangle formed by a distant object and the extremities of the base, it
is only necessary to multiply that length of base by twenty to find the distance of
the object. The important point consists in my using two fixed and known angles,
or, perhaps, I should say, three known angles, in measuring distances, instead of
one known angle and one known side or base, which usually entails complicated
mechanism to measure the other angle, with corresponding liability to derangement
and inaccuracy in manipulation, which is the great drawback of all other methods.
The advantages of this system of range finding consist not only in the fact that the
angles are permanently fixed, thus affording the simplest form of instrument, but
that they are fixed at such an angle as enables a difference of, say, twenty yards of
distance to be readily appreciated by the eye of the observer; in other words, it
ensures that the base shall always subtend an angle of not less than two degrees,
fifty one minutes, fifty four seconds, or the least that can be trusted to give a satis-
factory result. A distance of three thousand yards can be measured by this method
and instrument with the same accuracy as a distance of three hundred yards, the
bases of each being proportional to the distance. Thus, the base for 3000 yards
would be 150 yards, and for 300 it would be 15 yards, no more nor less than is
actually necessary to ensure accuracy. Although I mention 88 degrees, 34 minutes,
3 seconds, as being the most convenient angle for this system, giving as it does a
base of 1 in 20, still I do not confine myself to that proportion alone; for instance,
the base angles being fixed at 87 degrees, 8 minutes, 30 seconds, would give a
base of 1 in 10, and so forth. The principal point to bear in mind is that by
my arrangements the two base angles, whatever their value may be, are fixed and
known beforehand, the base only being variable and proportional to the distance.
DESCRIPTION OF THE DRAWINGS.
Fig. 1, side elevation of improved instrument that I use for range finding.
Fig. 2, front elevation; Fig. 3, sectional plan. In these Figs. A is metal case in
which reflectors B, B, are secured and adjusted; C, handle for holding the instru-
ment; D, D, openings in metal case above each of the reflectors; E, screws for
adjusting the instrument to the required angle. One of the reflectors is shown in
dotted lines in Fig. 3 as when adjusted to a different angle. To find the range
with two men, No. 1 goes to the right and finds the augle, No. 2 goes to the left
and finds the base as follows:-The two men stand facing the object, No. 1 on the
right, No. 2 on the left; they will carefully note the object and select some pro-
minent point on it to be observed in the angle glass. They will then turn outwards
and note the particular spot on the ground in front of each that coincides with the

page_text: reflected object, and march straight on that spot until they reach the estimated
. length of base.
They now turn inwards, and No. 2 stands steady while No. 1 makes an angle
with him and the object, and when that is effected he stands steady. No. 2 now
sees in the angle glass whether the reflected object falls on the right or left of
No. 1; if on the right, No. 2 must retire. to increase his distance from No. 1; if on
the left, he must advance to lessen his distance. After each move No. 2 will halt
and stand steady for No. 1 to make a fresh angle. In this manner Nos. 1 and 2
will alternately move and halt until they get the object fairly reflected on to each
other. The distance between them is then measured to determine the range of the
object; this can be done either by pacing or by a line of thin twine wound on a
reel, marked off and numbered at every five yards, representing so many hundred
yards of range. To find the range single-handed the observer plants a picket
where he stands or at any convenient spot, and goes to the right with another
picket, having ascertained the direction in which he should go, as before explained;
he then faces No. 1 picket and moves to the right or left until he reflects the object
on to No. 1 picket, when he marks the spot with No. 2 picket, and having secured
the end of the line close to it runs out the line in the direction of No. 1 and beyond
it on the line of both pickets until he sees the object fairly reflected on to both
pickets, the range of the object is then read off the line. To adjust the instrument,
the simplest way is to lay out the required angle with a theodolite or sextant that
is itself in perfect adjustment and make the angle glass agree with that angle.
But when an angle of 88 degrees, 34 minutes, 3 seconds, cannot be determined
beforehand it will be necessary to measure carefully with a chain a certain
distance, say, one thousand yards, from some well defined object, such as a spire
or a flag staff, then with a base of one twentieth that distance (50 yards) mark
the angle made by the angle glass with the picket at the end of the measured
line. The observer then goes to the picket marking the end of the line, and if the
angle glass is in adjustment he will see the spire or flag staff reflected on the other
end of the base. If the reflection does not fall on the other end of the base the
observer must advance or retire until the object is seen to coincide with the base
line, and adjust the instrument to half the difference between the true length of
base and that shown by the angle glass.
Having now particularly described and ascertained the nature of the said Inven-
tion, and in what manner the same is to be performed or carried out in practice, I
hereby declare that I claim the Invention of the improved range finder or instru-
ment for measuring and ascertaining distances, substantially as herein before set
forth and described, it being understood that I do not limit myself to the precise
details of construction hereinbefore described, so long as the essential points and
characteristics of the Invention as above described are retained and adopted.
In witness whereof, I, the said Francis Weldon, have hereunto set my hand
and seal, this Twenty sixth day of August, in the year of our Lord One
thousand eight hundred and seventy nine.