IMPROVEMENTS IN MATHEMATICAL INSTRUMENTS FOR LAYING OFF AND MEASURING COURSES, BEARINGS, AND DISTANCES ON CHARTS AND PLANS
Year: 1877

page_text: LETTERS PATENT to Digby Murray, of Caterham, in the County of Surrey,
for the Invention of "IMPROVEMENTS IN MATHEMATICAL INSTRUMENTS FOR
LAYING OFF AND MEASURING COURSES, BEARINGS, AND DISTANCES ON CHARTS
AND PLANS."
Sealed the 21st August 1877, and dated the 28th February 1877.
PROVISIONAL SPECIFICATION left by the said Digby Murray at the Office
of the Commissioners of Patents on the 28th February 1877.,
DIGBY MURRAY, of Caterham in the County of Surrey. IMPROVEMENTS IN
MATHEMATICAL INSTRUMENTS FOR LAYING OFF AND MEASURING COURSES,
BEARINGS, AND DISTANCES ON CHARTS AND PLANS.”¨`
According to my Invention I combine in one instrument a graduated arc, a radial
set square, and a measuring band.
I prefer to make the arc an exact semicircle rim with one diametrical bar carrying
the centre..
The rectangular sides of the set square are radial straight edges from the true
centre of the graduated semicircle, so that as they form a moveable right angle they
adapt the instrument to be used conveniently as an angular protractor over an
entire circle, although the graduated arc is only a semicircle.
The mechanical centre of the instrument is fitted with a small transparent disc,
having the cross lines and small central circle marked as is common in the pro-
tractors now used for trigonometrical plotting.
The sides of the set square are arms having their outer edges radial straight
edges (as above mentioned), continued beyond the circumference of the arc as far as
the allowable size of the instrument will permit, and connected beyond the circum-
ference by a plate fitted at its inner edge to the external edge of the graduated rim,
and by, preference made a straight edge on its outer edge, which forms the hypo-
thenuse of the set square. These prolonged radial arms of the set square give a
straight radial edge almost continuous from nearly-the centre of the instrument,
interrupted over only a small arc on either side at the central bar of the arc.
The arc is graduated in degrees, and also in points and half and quarter points,

page_text: from the middle of the arc towards either side. The points are distinctively marked
but they are not lettered, so that each. quadrant can be read for courses in two
quadrants of the entire circle or horizon by using one edge or the other of the radial
set square.
For measuring distances from the point to which the centre of the instrument is
applied, I construct the instrument with a flexible measuring band marked in dimen-
sions starting from the true centre of the instrument. The band I prefer is a steel
measuring tape of the kind now well known as pocket tapes.
Having now described the construction of the instrument, I shall next explain
how it is to be applied for the purposes set forth. I will describe only its application
to practical navigation, and it will therefrom be apparent how it can be applied as a
convenient, practically accurate, and cheaply constructed instrument for ordinary
draughtsmen's works.
For trigonometrical plotting the more elaborate instrument with Vernier scale
will however be preferable.
The charts used in navigation have, according to Mercator's Projection, straight
lines of latitude and straight lines of longitude crossing each other at right angles.
The deviated magnetic line of force directing the needle of the steering compass in a
vessel makes an angle with the terrestrial meridian or line of longitude at any given
place, which angle is the sum of the terrestrial magnetic declination at the place and
of the local deviation caused by the material of the ship; this sum is the correction
to be applied between the true course and the compass course.
The navigator has to continuously set off the true course of the vessel from suc-
cessive positions and given compass courses. My improved instrument is contrived
for use in this work.
.
To set off any given compass course from a station or place marked on the chart,
the instrument has to be applied with its diameter to a parallel of latitude adjacent
to the station, so that when one of the radial edges of the set square is set to the
correction the instrument can be moved along the line of latitude until the radial
edge cuts the given position. A pencil line is then drawn through the spot marked;
this line is the deviated magnetic meridian for the position and the vessel. The
instrument is then applied to this line, the diameter edge to the line, and the trans-
parent true centre directly over the marked spot. Holding the graduated rim
firmly on the paper, the moveable set square is then set with one of its radial edges
on the division denoting the conipass course, and a pencil line drawn by that
straight edge will be the line of the true course on the chart, and its direction will
accurately pass through the given station. The distance from the marked place of
the ship to any point on that line can then be ascertained by applying the measuring
tape while the centre of the instrument is held over the station.
-
The advantages which are obviously secured by this Invention are, that although
only a semicircle is used as a graduated arc, yet by the use of the radial set square
therewith the facility of setting off definite directions over the whole of the circle or
horizon is obtained; also, from the almost absence of arms on the graduated arc, and
the entire absence. of obstruction over the other semicircle, it is practicable to con-
tinue the radially laid off line nearly unbroken almost to the centre of the instru-
ment in most positions; further, the facility with which distances can be measured
with accuracy from the given station by means of the fitted flexible measuring
band, and the absence of complexity of parts, all render the instrument, as con-
structed according to my Invention, peculiarly applicable for the laying off work in
practical navigation, and for the ordinary work of draughtsmen where the high 5
accuracy of trigonometrical plotting is not requisite.
I construct the instrument of brass, with a steel measuring tape and a glass
marked centre, but it is obvious that other materials may be used, and that in some
of the details the arrangement may be modified without departing from the essential
principles of my Invention.

page_text: SPECIFICATION
in pursuance of the conditions of the Letters Iatent meu
by the said Digby Murray in the Great Seal Patent Office on the 24th.
August 1877.
DIGRY MURRAY, of Caterham, in the County of Surrey. " IMPROVEMENTS IN
MATHEMATICAL INSTRUMENTS FOR LAYING OFF AND MEASURING COURSES,
BEARINGS, AND DISTANCES ON CHARTS AND PLANS."
According to my Invention I combine in one instrument a graduated are A, a
radial set square B, and a measuring band D.
I prefer to make the are A an exact semicircular rim with one diametrical bar a, a,
carrying the centre C.
The rectangular sides b, b, of the set square are radial straight edges from the true
centre C of the graduated semicircles, so that as they form a moveable right angle
they adapt the instrument to be used conveniently as an angular protractor over an
entire circle, although the graduated arc is only a semicircle.
The mechanical centre of the instrument is fitted with a transparent disc d, d,
having the cross lines and small central circle marked as is common in the pro-
tractors now used for trigonometrical plotting.
The sides of the set squares b, b, are arms having their outer edges radial straight
edges (as above mentioned), continued beyond the circumference of the arc A, A, as
far as the allowable size of the instrument will permit, and connected beyond the
circumference by a plate B, B, fitted at its inner edge to the external edge of the
graduated rim, and by preference made a straight edge on its outer edge E, which
forms the hypothenuse of the set square. These prolonged radial arms of the set
square give a straight radial edge almost continuous from nearly the centre of the
instrument.
The arc A, A, is graduated in degrees, and also in points and half and quarter
points, from the middle of the arc towards either side. The points are distinctively
marked, but they are not lettered, so that each quadrant can be read for courses in
two quadrants of the entire circle or horizon by using one edge or the other of the
radial set square.
For measuring distances from the point to which the centre C of the instrument
is applied, I construct the instrument with a flexible measuring band D marked in
dimensions starting from the true centre C of the instrument. The band I prefer for
this purpose is a steel measuring tape, of suitable thickness, and similar to the kind
now well.known as pocket tapes.
Having now described the construction of the instrument, I shall next explain
how it is to be applied for the purposes set forth. I will describe only its application
to practical navigation, and it will therefrom be apparent how it can be applied as
a convenient and practically accurate instrument for ordinary draughtsman's work.
The charts used in navigation have, according to Mercator's Projection, straight
lines of latitude and straight lines of longitude crossing each other at right angles.
The deviated magnetic line of force directing the needle of the steering compass in a
vessel makes an angle with the terrestrial meridian or line of longitude at any given
place, which angle is the sum of the terrestrial magnetic declination at the place and
of the local deviation caused by the material of the ship; this sum is the correction
to be applied between the true course and the compass course.
The navigator has to continuously set off the true course of the vessel from suc-
cessive positions and given compass courses. My improved instrument is contrived
for use in this work.
To set off any given compass course from a station or place marked on the chart,
the instrument has to be applied with its diameter a, a, to a parallel of latitude
adjacent to the station, so that when one of the radial edges b, b, of the set square B
is set to the correction the instrument can be moved along the line of latitude until
the radial edge cuts the given position. A pencil line is then drawn through the
spots marked; this line is the deviated magnetic meridian for the position and the

page_text: vessel. The instrument is then applied to this line, the diameter edge a, a, to the
line, and the transparent true centre C directly over the marked spot. Holding
the graduated rim A firmly on the paper, the moveable set square B is then set
with one of its radial edges b on the division denoting the compass course,
and a
pencil line drawn by that straight edge will be the line of the compass course on the
chart, and its direction will accurately pass through the given station. The distance
from the marked place of the ship to any point on that line can then be ascertained
by applying the measuring tape D while the centre of the instrument C is held over
the station.
The advantages which are obviously secured by this arrangement are, that
although only a semicircle is used as a graduated arc, yet by the use of the radial
set square therewith the facility of setting off definite directions over the whole of
the circle or horizon is obtained; also, from the almost entire absence of arms on
the graduated arc, and the absence of obstruction over the other semicircle, it is prac-
ticable to continue the radially laid off line nearly unbroken almost to the centre of
the instrument in most positions; further, the facility with which distances can be
measured with accuracy from the given station by means of the fitted flexible mea-
suring band, and the absence of complexity of parts, all render the instrument, as
constructed according to my Invention, peculiarly applicable for the laying off work
in practical navigation, and for the ordinary work of draughtsmen where the high
accuracy of trigonometrical plotting is not requisite.
I construct the instrument of brass or other suitable material, with a flexible steel
measuring tape and a glass, horn, or other transparent marked centre.
The details of construction and the arrangement of parts may be modified and
varied without departing from the essential principles of my Invention.
What I claim as new and novel in this Invention is, the combining together in
one instrument of a graduated arc, a radial set square, and a flexible measuring
band..
In witness whereof, I, the said Digby Murray, have hereunto set my hand
and seal, this Twenty-fourth day of August, in the year of our Lord 1877.