IMPROVED MEANS FOR CORRECTING THE READINGS OF MARINERS COMPASSES
Year: 1884

page_text: :
Compasses.
I ARTHUR WILLIAM LOVELL REDDIE of the Office for Patents 6 Breams
Buildings Chancery Lane in the County of Middlesex Patent Agent do hereby
declare the nature of my invention for IMPROVED MEANS FOR CORRECTING THE
READINGS OF MARINERS COMPASSES, to be as follows:-
The object of the present invention is to provide means by which the readings
of mariners compasses may be speedily and geometrically corrected.
To this end the invention relates to an instrument to which the name of
"orthodrometer " may be appropriately given which is so constructed that certain
data being obtained such as by taking two distinct. readings of the compass while
the ship is on different courses, or by taking a sight and obtaining one such reading
of the compass the instrument may be accordingly set and from an inspection of
the instrument the true geographical course of the vessel may be steered.
The instrument thus enables the observer to perceive and read off simultaneously
the erroneous course indicated by the compass as well as the course corrected of all
its deviations and of the magnetic declination of the place or in other words the
true course of the vessel.
As is well known mariners compasses are subject to deviation from several
causes and its extent and direction must be ascertained in order that the vessel
may be steered on a true course.
1. There is the insignificant deviation present no matter what course is steered,.
which may be represented by A.
:
2. There is a variable deviation due to local polarity and the presence of iron
and steel in the vessel herself and this deviation may be represent by A3 and is
called semicircular because it varies according to the law:
A3B sin. 3 + C. cos 3.
3. There is the deviation which varies with the magnetic azimuth or exact
magnetic course. of the vessel axising from induend

page_text: pieces of soft iron This deviation which may be represented by 63 is termed
quadrantal, because it varies with the magnetic meridian following the law
63 = D sin 23 + E cos 23.
So that the total deviation d3 may be expressed by the algebraic sum of three
partial deviations A, A3, 83.
This deviation being a priori unknown, the equation may be stated in a general
manner as follows:-
d = A + B sin 3 + C cos 3 + D sin 23 + E cos 23
when the deviations do not exceed 20 degrees.
The parameters A, D, E. do not vary appreciably at sea, or at any rate during a
voyage when the vessel has acquired her definite magnetic condition.
B and C on the contrary do vary according to the temperature, the weather, and
the components of the terrestrial magnetism of the spot where the vessel may be.
These coefficients are consequently always indefinite at sea; whereas A, D
and E may without inconvenience retain during the whole of a voyage the
numerical values assigned to them in port prior to starting.
In principle therefore the operation of correcting the compass at sea consists of
two distinct parts
1. To ascertain experimentally the numerical values of the uncertain coefficients B
and C which are proper to the place where the vessel may be.
2. To calculate by means of these values and of those of the unalterable
coefficients A, D and E previously ascertained in port, the complete series of the
deviations corresponding to all the values of the magnetic azimuth from 0 to 360°
by the general formula:
"
d₁ = A + B sin 3 + C. cos 3 + D sin 23 + E cos 23.
Now the "orthodrometer," or instrument to which the present invention has
reference, enables these two operations to be carried out, the first in a few minutes
by means of two readings taken from the compass which it is desired to correct ;
and the second immediately.
The instrument is therefore properly speaking a portable apparatus for
calculating the deviation, which is set or adjusted according to the place where the
vessel may be at the time, by means of two observations taken from the compass
in question, and which simultaneously effects in a geometrical manner all the
numerical operations necessary for the calculation of the general formula of
deviations.
The complete theory on which the construction is based consists in the appli-
cation of a series of five theorems the details and formulæ of which need not at
present be given in full as they will be better understood when the construction of
the complete instrument has been explained.
180
It may be well however to give the leading idea of the first of these theorems
which is that if a graduated ring or disc of any radius be placed horizontally
above the magnetized needle of a compass in an excentric position such that the
coordinates of its centre, with reference to the longitudinal and transverse planes
of the ship, cutting one another at or immediately over the central axis of
suspension of the needle, may be multiples B, C, of the parameters B and O
of the semicircular deviation A3; and so that the origin of the graduation on this
ring is upon its diameter parallel to the central or guide line of the instrument,
then the line of the azimuthal plane of the compass needle will indicate exactly on
the graduated edge of this ring in all the orientations of the vessel, the course
entirely corrected of its semicircular deviation, that is to say the angular measure
of the algebraic expression 1

page_text: Proceeding upon the lines thus indicated the instrument by which the reading of
the compass. may be readily and geometrically corrected consists of the following
essential features.
A large fixed ring divided into rhumbs inversely with reference to the central or
guide line of the instrument that is to say a ring on which the degrees marking
the usual relative positions of East and West (with reference to the North and
South on a compass card) are reversed.
A small ring bearing similar degree marks as the larger ring but drawn
vertically upon and around its edge following the radial divisions of the outer ring
or circle,
This smaller ring is so mounted that its centre can be shifted with reference to
the vertical axis or centre of the large ring following two rectangular movements
This is effected by a grooved guide block situate precisely over the centre of the
large circle within which guide block a short bar is arranged to slide horizontally
in a direction coincident with or parallel to the centre line of the instrument.
This sliding bar itself carries a grooved block midway of its length, and sliding
horizontally in this grooved block in a plane at right angles to the first sliding bar
is a bridge piece or bar connected to an annular frame at diametrically opposite
points of its circumference. Within the upper and lower flanges of this annular
frame the small graduated ring above named is mounted so as to be able to be
rotated.
Thus it will be seen that the annular frame and its movable edge or ring
divided into rhumbs can be slidden in straight courses from North to South or
East to West and vice versa, and can be put into any intermediate position which
such movements allow; and also that the movable edge or graduated ring can be
turned independently of the annular frame which carries it, and that its North and
South poles may be set at any angle in a horizontal plane with respect to tho
sliding bars which are the co-ordinate axes of the annular frame and ring.
This movable graduated ring then is supported on a central stem which stands
up vertically and centrally from a cylindrical case which case is glazed over just
below the said movable ring and contains within it the large fixed graduated circle
above described and also certain pointers, mirrors and adjusting needles which will
be presently described.
Sliding on the rim of the cylindrical case and above its glazed portion is a small
mirror and adjusting needle, and a pressure screw serves to fix it in any required
position around the said rim.
The degrees upon the edge of the movable ring above described are engraved in
reverse so that they read correctly when seen in the small sliding mirror on the rim
opposite to them.
The adjusting needle in front of the said mirror and sliding with it, is to enable
any one of the degrees on the small movable ring to be accurately brought into
line with a rhumb or division of the larger fixed circle or ring. The sliding mirror
has an inclination given it to suit the observer who is slightly bending over the
instrument.
⚫On the central stem whose axis conincides with the centre of the larger graduated
circle is mounted a toothed wheel whose lower face is divided by tens from 0° to
360° in a direction opposite to that in which the hands of a watch move, and
gearing into this toothed wheel is another toothed wheel of half the diameter
mounted upon a pointer turning around the said stem means being provided for
setting and keeping the two wheels in close contact and preventing back lash.
Thus when the pointer is turned around its centre through an angle of 360° the
small wheel it carries will make two complete rotations on its axis and this small
wheel carries an adjustable vertical stud pin or crank excentric to its axis of
rotation which pin enters a longitudinal slot in an indicator hand which is also
mounted to turn around the central stem The result of this arrangement is that
as the small wheel rotates around the larger wheel its vertical stud pin is moved
around the axis of rotation of the amall wheel, and working in the slot of the

page_text: indicator hand causes that hand to vibrate to right or left; and there will be four
such vibrations of the indicator during the completion of one revolution of the small
wheel and its pointer around the central stem.
The vertical stud pin is carried by a threaded block through which passes a small
screw in a horizontal and radial direction and by this means the position or E
excentricity of the said pin can be adjusted with great precision. The pointer
carries two mirrors facing one another; the one furthest from the centre round-
which the pointer moves being inclined similarly to the mirror sliding on the rim
of the case; and between these two mirrors is situate a vertical blackened needle
or adjusting point carried by the indicator hand above named at a distance from 1
its centre of rotation and vibration equal to the radius of the small movable
graduated ring outside the glass cover of the instrument.
A key with a milled head is provided for rotating a worm gearing with a worm
wheel within the case which worm wheel is fast to the boss of the pointer above
named and hence from outside the case the said pointer may be moved around the 1
large graduated circle and set to any degree marked thereon and in thus moving
it the indicator hand will also be moved round and will assume certain positions
of angularity or coincidence with respect to the pointer due to its peculiar
connection with the said pointer.
With this instrument in all positions which the indicator hand may assume with
reference to the pointer hand the angle made by the two hands will always be
the exact measure in size and sign of the quadrantal deviation of the compass
whose readings will have served to adjust the length of radius and initial position
of the vertical stud pin on the small toothed wheel carried by the pointer which
stud pin acts as a crank pin when the pointer is turned around its centre.
When the quadrantal deviation is given on the excentric circle or frame, it is
necessary to set the indicator and pointer at the same angle when employing only
the movable graduated ring carried by the said excentric circle.
To do this the pointer carries the two mirrors above described whose plane is
normal to the plane of its axis and the indicator hand carries the vertical
blackened point or pin set at a distance from the centre of rotation equal to the
extreme radius of the excentric circle.
The pointer having a certain position, if it be set so that the direct reflection of
the blackened point and also the double reflection of its image are brought into
coincidence in the outer or inclined mirror the angle formed by the two hands will
be equal to the quadrantal deviation.
If moreover the graduated ring be turned in the annular frame of the excentric
circle so that the division mark indicating the constant factor A of the deviation
is brought in line with the adjusting point marking the extremity of the diameter
parallel to the central or guide line of the instrument the division mark which
will be seen in the outer mirror carried by the pointer as indicated above, will be
the measure of the algebraic expression :
3+ A+B sin 3 + C. cos 3 + D sin 23 + E cos 23;
that is to say, of the course corrected of the total deviation by which it is affected.
Finally if the graduated ring be turned in its annular frame so that the division
mark thereon indicating the constant A of the deviation, algebraically increased by
the magnetic declination of the spot, D, is brought into line with the setting or
adjusting point indicating the diameter parallel to the central or adjusting line of
the instrument, the division mark seen in the outer mirror carried by the pointer
when the direct and doubly reflected images of the blackened point on the
indicator hand are made to coincide, will be the measure of the algebraic
expression 1
D + 3 + A + B sin 3 + C cos 3 + D sin 23 + E cos 23 = 3 + D + d3,

page_text: Thus it will be seen that when accurately adjusted and set by means of readings
obtained from any compass, on any vessel, and in any spot, when the captain wishes
to steer the true geographical course he has marked out on his chart, he will by
means of the milled headed screw or key of the instrument move the parts so that
the division mark on the small movable ring indicating the true geographical
course, as seen in the small outer mirror carried by the pointer shall coincide with
the double reflection of the blackened point carried by the indicator hand and the
course which the pointer marks on the graduated large circle will be the false
course or extent of deviation of the compass in question which it will be necessary
to bear in mind in order to steer the vessel in the desired course.
The operation of adjusting the instrument from time to time consists substantially
in modifying the excentricity of the annular frame with its small graduated
movable ring and for this purpose it is only necessary to find out the deviation of
the compass in question in two very distinct courses of the ship.
This can be done at sea by taking one observation while the vessel keeps on her
way, and by making a short, but decided, change of course in order to obtain the
second observation or reading; and this temporary change of course takes no
longer time than is spent in taking a sight of a star.

page_text: Improved Means for Correcting the Readings of Mariners
Compasses.
I ARTHUR WILLIAM LOVELL REDDIE of the Office for Patents, 6 Bream's
Buildings, Chancery Lane in the County of Middlesex, Patent Agent, do hereby
declare the nature of the invention for "IMPROVED MEANS FOR CORRECTING THE
READINGS OF MARINERS' COMPASSES," and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:
The object of the present invention is to provide means by which the readings
of mariners compasses may be speedily and geometrically corrected.
To this end the invention relates to an instrument to which the name of
"orthodrometer" may be appropriately given, which is so constructed that certain
data being obtained, such as by taking two distinct readings of the compass while
the ship is on different courses or by taking a sight, and obtaining one such
reading of the compass the instrument may be accordingly set, and from an
inspection of the instrument the true geographical course of the vessel may be
steered.
The instrument thus enables the observer to perceive and read off simultaneously
the erroneous course indicated by the compass, as well as the course corrected of all
its deviations and of the magnetic declination of the place, or in other words the
true course of the vessel.
As is well known mariners compasses are subject to deviation from several
causes and the extent and direction of such deviation must be ascertained before
the vessel can be steered on her true course.
The deviation of the compass depends on the course of the vessel under certain
conditions which it may be well to recall.
For this purpose let Z be the magnetic azimuth or true magnetic course of the
vessel, that is to say the angle which the north half of the magnetic meridian
forms with respect to the forward half of the longitudinal plane of the vessel, this
angle being counted from 0° to 360° from the north pole towards the east; Z¹ the
false magnetic azimuth that is to say the course indicated by the compass or angle
which the north pole of the magnet needle makes with the forward half of the
longitudinal plane of the vessel, this angle being counted from 0° to 360° in the
direction of the hands of a watch, and
d3 the deviation of the magnet needle from this course Z, apart from the
magnetic meridian of the place; this angle being distinguished by the sign + when
the north pole of the needle is to the right of the north pole of the magnetic
meridian, and by the sign in the contrary case:-
-
Then the deviation d3 may be resolved into 3 distinct partial deviations as
follows:-
1. There is a constant deviation no matter what course is steered, which is
always insignificant and sometimes nil, and this deviation may be represented, as
Mr Smith has represented it, by A.

page_text: 2. There is a variable deviation due to local polarity and the presence of iron
and steel in the vessel herself, and this deviation may be represented by A3 and
is called semicircular, because it varies according to the law :-
A3
=
B sin 3+ C. cos 3.
3. There is the deviation which varies with the magnetic azimuth or exact
magnetic course of the vessel, arising from induced magnetism and peculiar to
pieces of soft iron. This deviation which may be represonted by 63 is termed
quadrantal, because it varies with the magnetic meridian following the law :-
83 = D sin 23 + E cos 23,
so that the total deviation d3 may be expressed by the algebraic sum of the three
partial deviations A, A3, 83.
This deviation being a priori unknown, the equation may be stated in a general
manner as follows:-
dx =
A + B sin 3 + C cos 3 + D sin 23 + E cos 23
when the deviations do not exceed 20 degrees.
The parameters ADE do not vary appreciably at sea, or at any rate during a
voyage when the vessel has acquired her definite magnetic condition, that is to say
a month or two after she has been fitted out and left port, they are independent
of the horizontal and vertical components of the terrestrial magnetism.
B and C on the contrary do vary according to the temperature, the weather;
and the components of the terrestrial magnetism of the spot where the vessel may
be. These coefficients are consequently always indefinite and uncertain at sea,
whereas A. D and E may without inconvenience retain during the whole of ȧ
voyage the numerical value assigned to them in port prior to starting.
In principle therefore the operation of correcting the compass at sea consists of
two distinct parts.
1. To ascertain experimentally the numerical values of the uncertain coefficients B
and C which are proper to the place where the vessel may be.
2. To calculate by means of these values, and of those of the unalterable
coefficients A D and E, whose values have been previously ascertained in port, the
complete series of the deviations corresponding to all the values of the true
magnetic azimuth Z from 0 to 360°, by the general formula:-
dx = A + B sin 3 + C cos 3 + D sin 23 + E cos 23.
Now the "orthodrometer'
"
or instrument to which the present invention has
reference enables these two operations to be carried out; the first in a few minutes
by means of two readings taken from the compass which it is desired to correct ;
and the second, immediately.
The instrument is therefore properly speaking a portable apparatus for calcu-
lating the deviation, which is set or adjusted according to the place where the
vessel may be at the time, by means of two observations taken from the compass
in question, and which simultaneously effects in a geometrical manner all the
numerical operations necessary for the calculation of the general formula of
deviations.
The construction of the instrument is based upon the application of a series of
five theorems the details and formulæ of which will be given later on as they will
be better understood when the construction of the instrument has been explained.
The instrument by which the readings of the compass may be readily and
geometrically corrected, consists of the following essential features:-
A large fixed ring A. divided into rhumbs inversely with reference to the central
or guide line N S of the instrument; that is to say a ring on which the degrees
marking the usual relative positions of east and west (with reference to the north
and south on the compass card,) are reversed.
A small ring B bearing similar degree marks to the larger ring, but drawn
vertically upon and around its edge, following the radial divisions of the outer
ring or circle,
1

page_text: i
.
This smaller ring which may be called the excentric ring, is so mounted that its
centre can be shifted with reference to the vertical axis or centre, O, of the large
ring, following two rectangular movements. This is effected by a grooved guide
block situate precisely over the centre, O, of the large circle, within which guide
block a short bar G is arranged to slide horizontally in a direction coincident with
or parallel to the centre line of the instrument.
This sliding bar G itself carries a grooved block midway of its length, and
sliding horizontally in this grooved block, in a plane at right angles to the first
sliding bar, is a bridge piece or bar G¹ connected to an annular frame, H, at
diametrically opposite points of its circumference. Within the upper and lower
flanges of this annular frame the small graduated ring B above named is mounted,
so as to be able to be rotated. Thus it will be seen that the annular frane H and
its movable edge or ring B, divided into rhumbs, can be slidden in straight courses
from north to south or east to west and vice versa, and can be put into any
intermediate position which such movements allow, and also that the movable
edge or graduated ring can be turned independently of the annular frame which
carries it, and that its north and south poles may be set at any angle in a
horizontal plane with respect to the sliding bars which are the co-ordinate axes of
the annular frame and ring.
This movable graduated ring then is supported on a central stem which stands
up vertically and centrally from a cylindrical case J which case is glazed over just
below the said movable ring and contains within it the larger fixed graduated
circle A, above described, and also certain pointers, mirrors and adjusting needles,
which will be presently described.
Sliding on the rim of the cylindrical case and above its glazed portion is a small
mirror M, and adjusting needle k, and a pressure screw L serves to fix it any
required position around the said rim.
The degrees upon the edge of the movable ring B, above described, are engraved
in reverse, so that they read correctly when seen in the small sliding mirror on the
rim opposite to them.
The adjusting needle k in front of the said mirror and sliding with it, is to enable
any one of the degrees on the small movable ring B to be accurately brought into
line with a rhumb or division of the larger fixed circle or ring A. The sliding
mirror has an inclination given it to suit the observer who is slightly bending
over the instrument and the needle k is blackened so as to give a distinct
reflection.
On the central stem whose axis coincides with the centre of the larger graduated
circle A, is mounted a toothed wheel C whose lower face is divided by tens from 0°
to 360° in a direction opposite to that in which the hands of a watch move, and
gearing into this toothed wheel is another toothed wheel D, of half the diameter,
mounted upon a pointer E turning around the said stem, means being provided for
setting and keeping the two wheels in close contact and preventing back lash.
Thus when the pointer E is turned around its centre through an angle of 360°,
the small wheel it carries will make two complete rotations on its axis, and this
small wheel D carries an adjustable vertical stud pin or crank v, excentric to its
axis of rotation, which pin v enters a longitudinal slot in an indicator hand, F,
which is also mounted to turn around the central stem. The result of this arrange-
ment is that as the small wheel D rotates around the larger wheel C, its vertical
stud pin v is moved around the axis of rotation of the small wheel, and working
in the slot of the indicator hand F causes that hand to vibrate to right or left;
and there will be four such vibrations of the indicator hand during the completion
of one revolution of the small wheel and its pointer around the central stem O.
The vertical stud pin v is carried by a threaded block through which passes a
small screw in a horizontal and radial direction, and by this means the position, or
.excentricity of the said pin with reference to the axis of the wheel D, can be
adjusted with great precision. The pointer E carries two mirrors N N¹ facing
one another, the one furthest from the centre round which the pointer moves being

page_text: In the diagram Fig. 5 it may be as well to observe that the letters m m³, M M³
indicate the positions of the mirrors N N¹ respectively of the instrument shown in
Figs. 1 and 2.
Theorem IV. If the graduated ring be turned in the annular frame of the
excentric circle B, so that the division mark indicating the constant factor A of
the deviation is brought in line with the adjusting point marking the extremity
of the diameter which is parallel to the central or guide line of the instrument,
the division mark which will be seen in the outer mirror N¹ carried by the
pointer E, as indicated above, will be the measure of the algebraic expression
3 + A + B sin 3 + 0 cos 3 + D sin 23 + E cos 23,
That is to say of the course corrected of the total deviation by which it is affected.
Theorem V. If the graduated ring be turned in its annular frame so that the
division mark thereon indicating the constant A of the deviation, algebraically
increased by the magnetic declination of the spot D, is brought into line with the
setting or adjusting point indicating the diameter parallel to the central or
adjusting line of the instrument, the division mark seon in the outer mirror N¹
carried by the pointer when the direct and doubly reflected images of the
blackened point I on the indicator hand E are made to coincide, will be the
measure of the algebraic expression :-
D+ 3+ A+B sin 3 + C cos 3 + D sin 23 + E cos 23 = 3 + D + dz
or course corrected of the variation of the compass, that is the true geographical
course (Cap du Monde) which is the indication necessary to enable the mariner to
draw or plot out upon his chart the course of the vessel.
Thus it will be seen that when accurately adjusted and set by means of readings
obtained from any compass on any vessel and in any spot, when the captain wishes
to steer the true geographical course he has marked out on his chart, he will by
means of the milled headed screw or key V, of the instrument, move the parts so
that the division mark on the small movable ring indicating the true geographical
course as seen in the small outer mirror N¹ carried by the pointer E shall coincide
with the double reflection of the blackened point I carried by the indicator hand F
and the course which the pointer marks on the graduated large circle A will be the
false course or extent of deviation of the compass in question which it will be
necessary to bear in mind and allow for, in order to navigate the vessel in the
desired course.
The instrument should be corrected or set from time to time during a voyage by:
means of readings obtained from the compass whose deviation is to be ascertained.
This operation of adjusting the instrument consists substantially in modifying
the excentricity of the annular frame with its small graduated movable ring and
for this purpose it is only necessary to find out the deviation of the compass in
question on two very distinct courses of the ship. With this knowledge the
excentric circle or ring may be shifted by successive movements in such a manner
that the observer reads in the mirror N¹ the 2 courses corrected of the observed
deviations when the pointer E has been brought successively into the two positions
corresponding to these two courses.
One of the observations or readings may be taken while the vessel keeps on her 4
proper course, and the second reading by making a short but decided change of
course. Thus it is only necessary to make one deviation of route, and this
temporary change of course need only last a few minutes or about the same time
that is required to take a sight of a star.
The essential parts of the apparatus constituting the instrument, or Orthodro-
meter, above described are essembled or combined substantially as represented in
the Drawing which is only the exact reproduction of the geometrical elements
whose dimensions and relative positions have been deduced from the application
of the theorems above enunciated.

page_text: Having now particularly described and ascertained the nature of the said
invention and in what manner the same may be performed as communicated to
me by my Foreign Correspondents I declare that what I claim is
1. The means hereinabove described for ascertaining the deviation and correcting
the readings of Mariners Compasses, the said means embodying the practical
application of the theorems above set forth.
2. Constructing orthodrometers or instruments for correcting the readings of
Mariners Compasses in the manner described with reference to the Drawings.
Dated this 2nd dex of Sentember 1885
NEWTON & SON,
Agents for the Applicant.