IMPROVEMENTS IN THE MARINER'S COMPASS AND IN APPLIANCES FOR ASCERTAINING AND CORRECTING ITS ERRORS
Year: 1876

page_text: LETTERS PATENT to Sir William Thomson, Knight, of Glasgow College,
Doctor of Laws, and Professor of Natural Philosophy in the University and
College of Glasgow, in the County of Lanark, North Britain, for the Invention
of IMPROVEMENTS IN THE MARINER'S COMPASS AND IN APPLIANCES FOR
ASCERTAINING AND CORRECTING ITS ERRORS."
Sealed the 27th February 1877, and dated the 18th December 1876.
•
PROVISIONAL SPECIFICATION left by the said Sir William Thomson at the
Office of the Commissioners of Patents on the 18th December 1876.
SIR WILLIAM THOMSON, Knight, of Clasgow College, Doctor of Laws, and
Professor of Natural Philosophy in the University and College of Glasgow, in the
County of Lanark, North Britain. "IMPROVEMENTS IN THE MARINER'S COMPASS
AND IN APPLIANCES FOR ASCERTAINING AND CORRECTING ITS ERRORS."
The objects aimed at in this Invention are greater accuracy, less expense, and
less detention in the adjusting of compasses on board ships. These objects.a10
attained by,
First. An improved instrument for taking azimuths on the same principle as
the azimuth mirror described in the Specification filed in pursuance of my previous
Letters Patent, No. 1339, A.D. 1876.
Second. The application of auxiliary deflecting magnets in the inanner and for
the purposes herein-after set forth.
Third. The application of a new form of instrument for measuring the vertical
component of the magnetic force on board ship, for the purpose of discovering and
correcting the heeling error.
Fourth. An improved form of magnetic appliances for correcting the quadrantal,
semicircular, and heeling errors.
The improvement in the azimuth mirror consists chiefly of the addition of another
lens and mirror to the instrument described in my aforesaid Specification. The
relative positions of the two mirrors and lenses are so arranged as to allow of
bearings being taken simultaneously of two objects whose azimuths differ by 180
degrees. Thus an observer is able to determine when he is in the line between two

page_text: land marks (such as two lighthouses) and can at the same time ascertain the bearing
of this line by his compass. From the known magnetic bearing of this line he is
able to find the error of his compass for the course that the vessel is steering.
Another improvement consists in placing the axis round which the mirror is
pivoted above or below the plane of the half lens instead of along the horizontal
diameter of the half lens, as shewn in the Drawing of my aforesaid Specification.
This improvement allows of the interval of no vision between mirror and lens to be
made much narrower, and so to make the image of the object to be seen much more
distinctly on the degree scale of the compass card than it can be in the arrangement
shown and described in the Specification of my aforesaid Patent.
The application of auxiliary deflecting magnets is for the purpose of measuring
the horizontal directive magnetic force on board the ship as the ship's head is
turned in different directions, and from these observations ascertaining by a proper
application of the formulas of Archibald Smith, published in the Admiralty Compass
manual, the semicircular and quadrantal errors.
The instrument which I use for measuring the horizontal directive force consists
of two frames, movable round a common horizontal axis. Each frame is con-
structed of two arins, connected together at both ends. The frames are at right
angles to the axis and are inclined to the vertical in opposite directions.
There are four magnets, each about one inch long, one attached to each of the
inclined arms of the two frames. The magnets attached to one frame have both
their true north poles at the ends farthest from the axis, whilst the magnets
belonging to the other frame have their true north poles at the ends nearest to the
axis. The frames are prolonged upwards beyond the axis, and the upper parts are
connected together by means of a right and left handed screw working in gimballed
nuts. This is for altering the distance between the poles of the magnets, and so
increasing or diminishing the power of the instrument. The frames are supported
and kept in a fixed position by means of a framework consisting of an H shaped
brass or other non-magnetic metal stand, with a pair of upright brass rods on each
side, which act as guides for the ends of the horizontal axis to slide up and down
in when the distance between the poles of the magnets is altered. The jointed
double frame carrying the magnets is supported like a painter's easel on three legs.
The lower horizontal bar of one frame is prolonged beyond the sides of the frame,
and the prolongations serve for two of the legs which rest on the sides of the
stand. A fork at the centre of the lower bar of the other frame, riding on a
pointer attached to the centre of the H and parallel to its sides, forms the third foot.
When in use the instrument is mounted on the compass bowl, so as to be free
to nove round an axis perpendicular to the plane of the glass and passing through
its centre. This I affect by means of a geometrical slide in the following manner:
The stand is supported on three feet, one of which is central and attached to the
cross bar of the H. When used the central foot rests in a conical hollow in the
centre of the glass of the compass case and the other two legs rest on the glass.
The screw for adjusting the distance between the poles of the magnets is a micro-
meter screw with a divided indicating disc attaclied. A divided scale between
the cross bars, constituting the lower edges of the frames, counts the turns of the
screw, and the circumference of the disc is divided to show tenths of a turn.
The instrument is used in the following manner:-When the ship's iron is sym-
metrical on the two sides and the ship's compass is placed amidships, the ship's.
head is put successively on the different cardinal points as shown by the compass
to be adjusted, north, south, east, and west, in any order. The deflector is so
adjusted as to deflect the north point through an angle of 85° when the pointer is
over the east or west point of the card. This is done for each of the cardinal points,
and the readings of the deflector noted. When the four readings thus found do
not agree there will be either quadrantal or semicircular error, or both.
If when the ship's head is north the reading is greater (indicating a greater
directive force) than when her head is south, the difference shows that the fore-
and-aft component of the ship's magnetism is headwards. To correct this the

page_text: fore-and-aft correctors must be adjusted, so as to apply a sternward force to balance
the headward force. When this is done, the readings with the ship's head north
and south for a deflection of 85° are the same, and are nearly the mean of the two
former readings. Similarly, if when the ship's head is east, the reading is greater
than when her head is west, the difference shows that the thwart-ship component
of the ship's magnetism is to port, and the thwart-ship magnets must be applied
to give a force to starboard and so counterbalance the force to port. When this
is done, the deflector readings with the ship's head east and west for a deflection
of 85° are the same and are nearly the mean of the two former readings for these
points. When the correctors have now been adjusted so that the reading for a
deflection of 85° when the ship's head is north is the same as when her head is
south, and the reading with ship's head cast the same as with her head west, the
semicircular error will be completely corrected. If, however, the reading when the
ship's head is north or south is greater than when her head is cast or west there
will be a quadrantal error, which is to be corrected by means of the soft iron
globes.
When the compass is not placed symmetrical with respect to the ship's iron
the deflector is to be used with different bearings of the ship's head, so as to find
the bearings which give maximum and minimum directive force after the directive
force has been equalized on two pairs of opposite bearings by the magnetic correctors;
and then iron globes are to be applied on the two sides of the compass with their
centres in the direction, which was magnetic north and south, when the directive
force was minimum and at the proper distance to correct the quadrantal error
which is found either by trial or by theory from the known amount of the
quadrantal error.
When the ratio between the horizontal force on shore and the horizontal forco
on board any ship is known, and whon the quadrantal error has been corrected,
the semicircular error may be corrected by means of the deflector without the
necessity of moving the ship, provided the true bearing of the ship's head be
known.
From mathematical reasoning, and from experiment, it is found that the ratio
between the horizontal force on board and the horizontal force on shore (called
in the Admiralty Manual) remains the same for the same ship, so that having once
found this ratio for any ship the compass can very easily be corrected at any time
without swinging the ship.
The following is the procedure for adjusting the compass when this ratio is
known The compass card and bowl are carried on shore to a place as free from
the influence of iron as possible, and are fixed with the glass of the case level. The
deflector to is set so as to give a deflection such that the ratio of the sine of this
deflection to the sine of 85° is the same as the ratio of the horizontal force ou board to
the horizontal force on shore. The compass card and bowl are then brought on
board to the binnacle, and the correctors are adjusted so that without the deflector
the compass card may indicate correctly the true bearing of the ship's head, and
with the deflector applied, may rest with a deflection of 85° from this position with
the scale reading the same as on shore. When this is accomplished the semicircular
error will have been corrected completely.
Instead of the adjustable deflector it may sometimes be found more convenient
to use magnets of different strengths. In that case I use one or other of a number
which are kept in a suitable box. These deflectors consist each of one bar magnet,
or of two or more parallel bar magnets, fixed to a light frame. These deflectors
when in use are so mounted on the compass bowl as to be free to move round a
vertical axis perpendicular to the glass, and passing through the hollow in the
centre of the glass. This is accomplished exactly in the same manner as that
already described for the adjustable deflector, or the same may be done by the
method of torsion herein-after described.
For measuring the vertical component of magnetic force on board I use a
dipping needle, like a pair of needles of the compass card, constructed as follows:-

page_text: Two hardened steel wires, like a pair of needles of the compass card, are joined
together at their centres by a bridge, in which there are two iridium points in a
line at right angles to the length of the bars; one of these points rests in a
sapphire cup, and the other is a sapphire V. A stiff cross bar is connected to the
wires close to the bridge. The instrument is balanced very accurately, so that the
needles are horizontal when they are supported on the iridium points; the needles
are then magnetized.
The vertical directive force on the needles is measured by the torsion of very
fine copper or other metal wife. This wire is stretched level with the points of
support, perpendicular to the length of the magnets, and at a greater distanco from
the points of support than half the length of the magnets. One end of the wire
is fixed, and the other is attached to the centre of a torsion head, with divided
circle turning round a horizontal axis. A fine glass rod is attached to the centre
of the stretched wire by aid of a light stirrup, through which the wire is threaded,
so as to be bent nearly half round the rod; the rod is balanced at right angles to
the wire. This glass rod projects on one side of the wire just long enough to reach
to the cross bar between the magnets. The instrument is placed with the magnets
in the north and south line, and the milled head is turned, so as to put torsion on
to the wire, until by means of the glass lever pressing against the cross bar between
the magnets the magnets are brought to a level position; the amount of torsion
required measures the vertical force on the needles.
I also use an alternative construction of a simpler form, in which a light magnet,
simple or compound, is supported upon a wire stretched horizontally, and under
varying magnetic influences is balanced with its magnetic axis horizontal, or approxi-
mately horizontal, by torsion of the free stretched portion of the wire ou one side
or other of the magnet.
The same arrangement, with the magnet brought to balance and having its
magnetic axis vertical, may be used for measuring horizontal components of the
magnetic force.
The same arrangement with the wire vertical, or a simple or compound magnet,
such as the actual group of needles of the compass card, simply hanging by a fine
metallic wire, may be used for measuring horizontal component of magnetic force
alternately with the deflector herein-before described. This arrangement, whether
for measuring the vertical or horizontal components of inagnetic force on board
ship, must be mounted on gimbals in a glazed case, and put in the place of the
gimballed compass bowl, so that the centre of its simple or compound magnet shall
be, as nearly as may be, in the proper position for the centre of the set of needles
of the compass card. The compass having already been corrected the vertical force
on board should be made to bear the same ratio to the vertical force on shore that
the horizontal force on board bears to the horizontal force on shore by sliding up
or down the magnet for the heeling error herein-after described.
The semicircular error is corrected by steel magnets according to the general
principle first given by the Astronomer Royal.
The varying adjustment of the correction required for variation in the ship's
sub-permanent magnetism and magnetisin induced by the vertical component of 4
the earth's magnetic force I effect in the following manner-I use two sets of
magnets, one set placed fore-and-aft for correcting the fore-and-aft component
of the ship's magnetism, and the other set thwartship for correcting the thwartship
component. The thwartship set of magnets consists of two bar magnets; thesc
magnets are made each of several steel wires or round rods of steel, tempered glass, 5
hard, and soldered in brass tubes to protect them from rust and from shocks, or
from close contiguity with other magnets, which might alter their magnetism.
Near one end of each magnet there is a projecting ring for the purpose of fixing
the magnet in position. One of the thwartship magnets is fixed in one of two
positions at the foreside of the binnacle, one position is preferably at or about 5
twenty-five inches below the level of the needles of the compass card, and the other
at the bottom of the binnacle. The other magnet is placed in a cradle attached to

page_text: a geometrical slide which is free to move up and down the after side of the binnacle
from a position about 25 inches below the level of the needles of the card to the
bottom of the binnacle.
The method of securing the magnet in position is the same both for the sliding
and fixed magnets. The magnet rests in two V's, one near each end, and is pressed
into the angle of the V's by means of a spring pressing on the centre. There is a
slit in the centre of each V, in one or other of which the projecting ring can lie,
according to the direction in which the magnet is placed; the magnet is thus kept
from moving longitudinally.
When the greatest power is needed the fixed magnet is placed in the uppermost
position, and in the same direction as the magnet of the slide. When the force
with this arrangement and the slide in its lowest position is too strong the fixed
magnet is to be placed in its lowest position and the slide run up till the proper
degree of force is obtained. If, with the slide in its lowest position, the force is still
too strong the fixed magnet is to be turned in the reversed direction and the slide
run up till the proper degree of force is obtained. Whatever degree of force is
obtained by any particular position of the two magnets the same force in the
reverse direction is obtained by reversing the direction of each of the magnets.
The set of magnets for the fore-and-aft component consists of four, similar to the
thwartship magnets, but shorter. I use a pair of magnets, one on each side of the
centre of the binnacle for the fixed magnets, and a pair in cradles, one on each side
of the centre for the sliding magnet of the fore-and-aft corrector; this arrange-
ment in pairs being chosen in order that there may be no difference in their effect
when the ship rolls or heels to one side or the other. There are two fixed positions
for one pair of magnets; ono in which the two magnets are placed one on each side
of the centre of the binnacle on the same level, and at a distance of about 25 inches
from the level of the needles of the compass card; in the second position the
magnets are placed one on each side of the centre at the bottom of the binnacle.
The other pair of magnets is placed in the cradles, one at each side of the centre.
The cradles are attached to a stool which has three fect pressing against a vertical
board in close contiguity to the tube containing the heeling orror magnets, herein-
after described. Two of the feet are in a vertical groove at one side of the board,
and the third presses against a plane surface, thus forming a geometrical slide. In
the centre of the board there is a vertical groove with two guides, which project
slightly over the edges of the groove. A long plate slides up and down the groove
with a screw rod, which passes between the guides and through the centre of the
stool. A nut on the end of the screw rod clamps the stool in any position. The
stool, with cradles attached, is thus by a geometrical slide capable of moving up
and down, so that the magnets may be clamped in any position from the highest
position, (about 25 inches below the level of the needles of the compass card,) to the
position at the bottom of the binnacle.
The method of supporting both the fixed and moveable magnets, and the means
for obtaining the different degrees of magnetic force are similar to those heroin-
before described for the thwartship magnets.
Scales are provided for both the slides, the divisions of which are made from
calculation or experiment, so that the increase or diminution of the force caused by
raising or lowering the slides with the magnets may be the same for the same
number of divisions at any part of the scale.
For the thwartship corrector the scale is placed in the centre of one side of the
binnacle, and a glass window in the side allows of the position of the slide on the
scale being seen from the outside of the binnacle. A window in the door allows of
the position of the fore-and-aft slide being observed; or the following alternative
process consisting of a method of continuous magnetic adjustment which I introduced
in 1858 for my telegraphic signal galvanometer, and which is described in the
thirteenth part and represented in Figure five of my Patent for electric telegraphis
dated 20th February, A.D. 1858, No. 329, may be adopted.
I use only one corrector for the semicircular error, similar in construction to the

page_text: fore-and-aft corrector herein-before described, but with the longer magnets of the
aforesaid thwartship corrector. I also use the heeling magnet herein-after described.
The whole frane in which the semicircular and heeling correctors are mounted can
be turned round a vertical axis through the centre of the binnacle, and clamped in
any position.
A vertical scale at one side of the binnacle, divided as before described, with
divisions corresponding to equal increments of force indicates the degree of power
according to different positions of the edge of a circle moving up and down with the
slide. This circle, or another circle carried round with the frame, is divided to
degrees so as to indicate the azimuth relatively to the ship's head of the line of the
correcting force with which the system of correcting magnets acts on the compass.
This which a practical method of realizing one of the two principles of arrangement
of compensating magnets given by the Astronomer Royal has the disadvantage of
not being so easily used with the aid of calculation as the method by fore-and-aft
and thwartship correctors, but it has the advantage of requiring somewhat less of
mechanism and a smaller number of magnets, and the still greater advantage of
being better adapted for the provision of abundant magnetic power with continuous
scale to zero, and so avoiding in almost every case the necessity for turning any of
the inagnets otherwise than by turning the the frame carrying the whole system of
magnets round its vertical axis.
For correcting the heeling error the binnacle contains a vertical brass tube fixed
under the centre of the compass card, in which a magnet cased in brass is placed
capable of being moved up and down and of being clamped in any position. The
brass case of the vertical magnet has four slight projections constituting feet, two at
some distance above, and two at an equal distance below a screw stem projecting
horizontally from the centre of the length of the inagnet. The four feet pressing on
the curved surface of the tube form a geometrical slide.
The screw stem slides up and down in a slit which runs from top to bottom of
the brass tube. A nut on the screw clamps the magnet in any required position.
For cases in which the ship's iron is not symmetrical, or in which the binnacle is
not placed amidships, in order to correct the quadrantal error I use masses of soft
iron, preferably globes, placed with the line joining their centres in an oblique
direction, being the line which is magnetic north and south when the ship's head is
in such an azimuth as to make the directive force a minimum after the semicircular
error has been corrected. This position will be determined either by my method
herein-before described for measuring the horizontal force or by calculation, according
to the formulas of the Admiralty Manual from observation of the residual deviations
after the semicircular error has been corrected. For this purpose I may use the
same binnacle as that which would serve in the ordinary case of the compass being
placed symmetrically with respect to the ship's iron, but before fixing it to the deck
I turn it in azimuth until the line joining the centre of the globes is in the proper
position for correcting the quadrantal error, and I turn the mounting at the top of
the binnacle bearing the gimbals so as to bring the lubber line into the proper
position; or, allowing the axes of the gimbals to have positions not parallel to the
ship's length and breadth, J simply paint the lubber line in the proper position for the
ship's head after the binnacle has been properly set for correcting the quadrantal
error; but excepting in respect to expense of construction, the method which I
prefer is to use a round binnacle or a round topped binnacle carrying a frame of
brass or wood on which the iron globes are to be placed, and which may be turned
round to bring them into any required azimuth.
The improvements in the compass card consist as follows :-
First. Of the substitution of fine silk-worm gut or fine metallic wires for the silk
threads of the compass card described in the Specification filed in pursuance of my
former Letters Patent, No. 1339, A.D. 1876. Aly reasons for substituting such
other materials for silk threads are to avoid the disadvantage of the loose shreds of 5
the silk which have sometimes been found to interfere with the freedom of motion
of the compass card, and because the silk threads are liable to alter in length with

page_text: a change in the temperature and in the moisture of the atmosphere of the compass
casc. Both these defects are entirely obviated by using fine copper wire; for
example, No. 50 or No. 60 copper wire weighing from 1% to 1% of a grain per foot.
The ends of the copper wire are drawn through holes in the aluminium rim and
boss of the compass card, and are properly bent and soldered to secure them.
Second. In drilling holes through the ends of the magnetic needles for the
purpose of fixing the threads or wires by which they are attached to the aluminium
rim.
Third. The method of securing the paper with the points to the aluminium rim,
which is as follows:-A long strip of thin gummed paper, preferably one and a half
times as broad as the strip of aluminium which is to form the rim is wrappod once and
a half round the aluminium, and then secured by means of the gum; the aluminium
strip with paper is then made into a circle and the thread or wire spokes fixed
between the rim and the boss, and the paper with the points is gummed to the
paper on the aluminium.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Sir William Thomson in the Great Seal Patent Office on the 15th
June 1877.
SIR WILLIAM THOMSON, Knight, of Glasgow College, Doctor of Laws, and
Professor of Natural Philosophy in the University and College of Glasgow, in the
"IMPROVEMENTS IN THE MARINER'S COMPASS
County of Lanark, North Britain.
AND IN APPLIANCES FOR ASCERTAINING AND CORRECTING ITS ERRORS."
The objects aimed at under this Invention are greater accuracy, less expense, and
less detention in the adjusting of compasses on board ship. These objects are
attained by the instruments or apparatus herein-after described, and shown upon
the annexed Sheets of Drawings in the manner set forth.
The first part of the Invention consists of an improved instrument for taking
azimuths (on the same principle as the azimuth mirror set forth in the Specification,
and shewn in the relative Drawings, filed in pursuance of a proviso to that effect in
my previous Letters Patent, No. 1339, A.D. 1876.
The second part of the Invention consists in the apparatus for the application
of auxiliary deflecting magnets, in the manner and for the purposes herein-after set
forth.
The third part of the Invention consists of an instrument for measuring the
vertical component of magnetic force on board ship, for the purpose of discovering
and correcting the heeling errors.
The fourth part of the Invention consists of an improved magnetic appliance for
correcting the quadrantal, semicircular, and heeling errors.
And in order that my said improvements and the manner of applying them in
practice may be fully understood, I have hereunto appended three explanatory
Sheets of Drawings, on Sheet 1 of which Figure 1 is a side elevation of ay
improved azimuth mirror; Figure 2 is a plan of the same with the moveable parts Å
and B in the position corresponding to Figure 1; Figure 3 is a side elevation of the
upper part of my improved azimuth mirror with the moveable parts A and B shut
back one within the other; Figure 4 is a plan of the instrument with the parts A,
and B in position, corresponding to Figure 3; and Figure 5 is an end elevation corre-
sponding to Figure 1.
Under this improved form of my azimuth mirror, the axis a on which the mirror U
is pivoted is above the plane of the lens which is contained within the inclined
tubular leg G, and the position whereof is indicated by the dotted lines marked D,
in Figures 1 and 5, Sheet 1, instead of along the diameter of the half lens, as shown
in the Drawings annexed to my aforesaid Specification. This arrangement allows

page_text: of the interval of no vision between mirror and lens to be reduced to a minimum
by reason of the inner straight edge of the mirror cutting out of sight with a sharp
or fine line that portion of the lens which it hides, and thereby the image of the
object is seen much more distinctly on the degree scale of the compass card, than it
can be in my aforesaid former arrangements.
The advantage resulting from the narrowing of the interval of no vision may
also obviously be obtained by pivoting the mirror below the lens.
My improved azimuth mirror is provided with two moveable pieces A and B, in
which are carried the dark coloured glass shades E, F, respectively. These shades
are for obscuring any overbright body, and one or both of them are used, according
to the degree of obscuration necessary. In other respects my improved azimuth
mirror is substantially similar to the arrangement shown at Figures 13 and 14,
Sheet 1, of the Drawings appended to my aforesaid Specification.
My improved azimuth mirror may also be duplicated, that is to say, it may be
constructed with a second leg carrying mirror, shades, and lens, the two legs being
at equal distances on opposite sides of the central pivot on which the instrument
turns, the second leg and attachments being inclined in the opposite directions to
that of the single azimuth mirror, shown at Figures 1 to 5 (Sheet 1), and at the
same angle. This arrangement allows of bearings being taken simultaneously of
two objects whose azimuths differ by 180 degrees or thereabouts. Thus an observer
is able to determine-when he is in the line between two landmarks (such as two
lighthouses), and can at the same time ascortain the bearing of this line by his
compass. From the known magnetic bearing of this line he is able to find the
error of his compass for the course that the vessel is steering,
With respect to the pivoting of the axis a of the mirror C, I prefer to effect that
in the manner more particularly shown at Figure 5ª (Sheet 1), that is to say, by
forming the bearing of the axis or pivot of two straight sides at right angles to
cach other or nearly so (one side being horizontal the other vertical), and pressing
the axis or pivot a into this bearing by means of a spring . This arrangement
affords means of ready and accurate adjustment of the mirror by the use of a filo
on either of the straight sides of the bearings, so that when placed on a fixed hori-
zontal glass plate it shall give the same azimuth for two distant points in one vertical
line, and the correct azimuth for one of them (preferably one in a horizontal
direction from the instrument). To make this adjustment a glass plate with a
conical hollow like that in the cover of the compass bowl is fixed horizontally, upon
which the instrument is to be placed. A fine thread is stretched from a point
vertically under the middle of the central foot of the instrument horizontally to a
fixed point at least several yards distant. Vertically over this fixed point another
point is fixed at such a height as to be seen from the instrument at an altitude of
30° or more. The thread must be at or about the same distance below the glass as
is the circumference of the circle of the compass card. The file is used on one of
the horizontal bearings of the mirror, to make the lower point as seen in the mirror
coincide with the horizontal thread seen through the lens at the point where it is
cut by a circle corresponding to that of the compass card. The mirror is next set
to bring down the image of the upper fixed point to the circumference of this 4
circle (of which a short are in black on white paper may conveniently be fixed in
proper position below the glass), and the file is now applied to one of the vertical
faces of the bearings to bring the image of the upper fixed point to lic on the hori-
zontal thread.
The instrument for applying auxiliary deflecting magnets, and which I call the 5
"deflector" is for the purpose of measuring the horizontal directive magnetic force
on board ship, as the ship's head is turned in different directions, and from these
observations ascertaining, by the proper application of the formulas of Archibald
Smith, published in the Admiralty "Compass Manual," the semicircular and
quadrantal errors. My instrument which I use for this purpose is shown in side 5
elevation at Figure 6, in end elevation at Figure 7, in full plan at Figure 8, and in
plan of the under or H shaped frame at Figuro 9 (Sheet 1).

page_text: This instrument consists of two frames A moveable about a common horizontal
axis B. Each frame A is constructed of two arms connected together at both ends,
as seen at Figures 7 and 8 (Sheet 1). The frames A are at right angles to the
axis B, and are inclined to the vertical in opposite directions, so that they cross one
another as seen more particularly at Figure 6 (Sheet 1).
There are four magnets, each about one inch long, one attached to each of the
lower inclined arms of the two frames A.
The magnets attached to one frame A have both their true north poles at the
ends furthest from the axis, while the magnets belonging to the other frame A, have
their true north poles at the ends nearest to the axis. The frames are prolonged
upwards beyond the axis, and the upper parts are connected together by means of
a right and left handed screw C, working in gimballed nuts D. This screw C and
gimballed nuts D are for altering the distance between the poles of the inaguots
carried in the lower part of the frames A, and so increasing or diminishing the
power of the instrument upon the compass needles by varying the distance of the
poles of the magnets from the needles. The frames A are supported and kept in a
fixed position by means of a framework consisting of an H shaped brass or other
non-magnetic metal stand D', more particularly seen in detail at Figure 9 (Sheet 1),
with a pair of upright brass guides on each side, in which the ends of the horizontal
axis B slide up and down, as the distance between the poles of the magnets is
altered. The jointed frames A carrying the magnets are supported like a painter's
casel, on three legs x, x1, x2, which move horizontally when the distance between
the poles of the inagnets attached to the frames A. is altered. The lower horizontal
bar of the frame A is prolonged beyond the sides of the frame, and the prolongations
serve for two of the logs and ¹, which rest on the upper sides of the H stand. A
fork at the centre of the lower bar of the other frame riding on a pointer E
attached to the centre of the H stand and parallel to its sides forms the third foot a*.
When in use the instrument is mounted on the compass bowl so as to be free to
move round an axis perpendicular to the plane of the glass, and passing through its
centre. This I effect by means of a geometrical slide in the following manner :-
The stand is supported on three feet, one of which, y, is central, as shown more par-
ticularly at Figure 6 (Sheet 1), and attached to the cross bar of the stand.
When used the central foot rests upon a conical hollow in the centre of the glass of
the compass case, its centre of gravity being over the central foot. A light spring F
is applied to bear a small part of the weight, and cause the foet y¹ and y² to press
lightly on the glass.
The screw for adjusting the distance between the poles of the magnets is a
micrometer, with a divided indicating disc a attached. A divided scale H, between
the cross bars constituting the lower edges of the frames, counts the turns of the
screw, and the circumference of the disc is divided to shew tenths of a turn. The
moveable or adjustable frames A, and their attachments are drawn down on to the
under frame D by the tension of the two elastic bands I passed over the axis B and
under the central bar of the lower frame D'.
The instrument is used in the following manner:-When the ship's iron is
symmetrical on the two sides and the ship's compass is placed amidships, the
ship's head is put successively on the different cardinal points, as shown by the
compass to be adjusted, north, south, east, and west, in any order. The deflector
is so adjusted by raising or lowering the magnets by turning the micrometer
screw C as to deflect the north point through an angle of 85° when the pointer is
over the east or west point of the card. This is done for each of the cardinal points
and the readings of the "deflector" noted. When the four readings thus found
do not agree there will be either quadrantal or semicircular error, or both.
If when the ship's head is north the reading is less (indicating a less directive
force in virtue of the ship's magnetism) than when her head is south, the differenco
shows that the fore-and-aft component of the ship's magnetism is sternwards, or
vice versa; if when the ship's head is north the directive force on the compass is
greater than when her head is south the fore-and-aft component is headwards. To

page_text: correct this the fore-and-aft correctors must be adjusted so as to apply a headward
or sternward force to balance the ship's headward or sternward force, or the
revolving corrector herein-after described must be turned and moved on its slide so
as to give the requisite headward or sternward force. When this is done the
readings with the ship's head north and south for a deflection of S5° are the same,
and are nearly the mean of the two former readings. Similarly, if when the ship's
head is east the reading is greater than when her head is west, the difference shows
that the thwartship component of the ship's magnetisin is to port, and the thwart-
ship magnets, either those described in my aforesaid Specification or those herein-
after described, must be applied to give a force to starboard, and so counterbalance 1
the force to port, or the revolving magnetic corrector hercin-after described must be
turned and moved on its slide so as to give the requisite thwartship force. When
this is done the deflector readings with the ship's head cast and west for a deflection
of 85° are the same, and are nearly the mean of the two former readings for these
points. When the correctors have now been adjusted so that the reading for a
deflection of 85° when the ship's head is north is the same as when her head is
south, and the reading with the ship's head cast, the same as with her head west,
the semicircircular error is completely corrected. If now the reading when the
ship's head is north or south is greater than when the head is east or west, there is
a quadrantal error, which is to be corrected by means of the soft iron globes B,
shown on Figure 4 (Sheet 1) of any aforesaid Specification, and Figure 14 (Sheet 1)
of the Drawings hereunto appended.
When the compass is not placed symmetrically with respect to the ship's iron
the deflector is to be used with different bearings of the ship's head so as to find
the bearings which give the maximum and minimum directive force of the ship's S
magnetism after the directive force has been equalized on two pairs of opposite
bearings by the magnetic correctors, and then iron globes are to be applied on the
two sides of the compass, with their centres in the direction which was magnetic
north and south when the directive force was minimum, and at the proper distance
to correct the quadrantal error, which is found either by trial or by theory from the
known amount of the quadrantal error.
When the ratio between the horizontal force on shore and the horizontal forco
on board any ship is known, and when the quadrantal error has been corrected,
the semicircular error may be corrected by means of the deflector without
the necessity of moving the ship, provided the truc bearing of the ship's head be
known.
From mathematical reasoning and from experiment it is found that the ratio
between the horizontal force on board and the horizontal force on shore (called > in
the Admiralty Manual) remains the same for the same ship, so that having once
found this ratio for any ship the compass can very easily be corrected at any time,
without swinging the ship.
The following is the procedure for adjusting the compass when this ratio is
known The compass card and bowl are carried on shore to a place as free from
the influence of iron as possible, and are fixed with the glass of the case level.
The deflector is set so as to give a deflection such that the ratio of the sine of
this deflection to the sine of 85° is the same as the ratio of the horizontal force on
board to the horizontal force on shore. The compass card and bowl are then
brought on board to the binnacle and the correctors are adjusted so that without
the deflector the compass card may indicate correctly the true bearing of the ship's
head, and with the deflector applied may rest with a deflection of 85° from this
position with the scale, reading the same as on shore. When this is accomplished
the semicircular error will have been corrected completely.
Instead of the adjustable deflector it may sometimes be found more convenient
to use magnets of different strengths. In that case I use one or other of a number
which are kept in a suitable box. These deflectors consist each of one bar magnet,
or of two or more parallel bar magnets fixed to a light frame. These deflectors
when in use are to mounted on the compass bowl as to be free to move round a

page_text: vertical axis perpendicular to the glass, and passing through the hollow in the
centre of the glass. This is accomplished exactly in the same manner as that
already described for the adjustable deflector, or the same may be done by the
method of torsion hercin-after described.
For mensuring the vertical component of the ship's magnetic force on board I use
a dipping needle, consisting of a light magnet, simple or compound, supported upon
a wire stretched horizontally, and which under varying magnetic influence is
balanced with its magnetic axis horizontal by varying the torsion of the free
stretched portion of the wire on one or both sides of the magnet which it supports.
This last-named apparatus is shown in plan at Figure 10, and in transverse section
at Figure 11 (Sheet 1) of the appended Drawings, and details thereof, as herein-
after described, are shown at Figure 12 to 12 (Sheet 1). This apparatus consists
of a circular casing A, having a flat bottom B. This casing is supported on
gimbals Cresting in bearings in the ring D, which is also provided with gimbals E,
which when the instrument is used rest in the bearings in the binuacle of the
compass bowl gimbals, the compass bowl being removed to allow of the instrument
for incasuring the vertical component of the ship's magnetism.
A compass card with its circumference divided into degrees is fixed on the bottom
of the casing, as shown at Figure 10, and at a short distance above it a wire F is
stretched across a diameter of the circle carrying at its centre the magnetic
needle G. The position of the needle with the gimbals is such that its centro
shall when placed in the binnacle occupy the position as nearly as may be of the
centre of the needles of the compass card. At one end the wire F is held by a light
spring H, at the other it is attached to a micrometer torsion head I, above which a
pointer is situated, the reading of which on the divided rim of the micrometer
torsion head indicates the degree of torsion applied to that portion of the wire F
situated between the magnetic needle G and the torsion head I. For the purpose of
reading off the ratio of the vertical component of the ship's magnetic force a vertical
glass scale J is carried within the casing A. This glass scale and its attachments
are shown in detail on an enlarged scale at Figure 12, 12, and 12h of which
Figure 12 is a plan of the scale and its attachments; Figure 12 a front elevation of
the same, showing the divisions of the scale; Figure 12, a side elevation. The
lower part of the frame carrying the scale is fitted with a portion of a lens K (also
shewn in enlarged detail at Figures 12 and 12d, whose upper surface is convex, the
lower surface being flat and inclined, and constituting a reflecting surface, whilst the
front surface is plane and vertical. By looking through the upper surface of this
lens the position of the needle on the vertical scale is read off. This instrument
may be turned round the gimbals C, C, so as to bring the stretched wire vertical,
and in that position may be used for measuring or comparing horizontal components
of magnetic force. The compass having already been corrected for quadrantal and
semicircular error the vertical force on board should be made to bear the same ratio
to the vertical force on shore that the horizontal force on board bears to the
horizontal force on shore, by sliding up or down the magnet for the heeling error,
as herein-after described.
The semicircular error is corrected by steel magnets, according to the general
principle first given by the Astronomer Royal.
The varying adjustment of the correction required for variation in the ship's sub-
permanent magnetism and magnetism induced by the vertical component of the
carth's magnetic force, I effect in the following manner, reference being had to
Figures 13 to 16 inclusive (Sheet 1) of the appended Drawings, of which Figure 13
is a section, with parts in elevation, of the binnacle and compass fore and aft;
Figure 14, a section partly in elevation at right angles thereto; Figure 15, a half
horizontal section on the line a, b, Figure 13; and Figure 16, a detailed view :-
I use two sets of magnets, one set placed fore and aft, for correcting the fore-and-aft
component of the ship's magnetism, and the other set thwartship for correcting tho
thwartship component.
The thwartship set of magnets consists of two bar magnets marked A and B,

page_text: Figures 13 and 14, Sheet 1. These magnets are made each of several steel wires or
round rods of steel tempered glass, hard and soldered in brass tubes, to protect them
from rust, and from shocks or from close contiguity with other magnets, which
might alter their magnetism. Near one end of each magnet there is a projecting
ring or belt, (as more particularly seen at Figure 15, (Sheet 1,) where the thwartship
magnet B is shewn placed at the lower position in the binnacle.) This ring or
belt C marks the true south pole of the inagnet, and is used for the purpose of
fixing the magnet in the proper position in the sliding cradle when it is placed
there. One of the thwartship magnets is fixed in one of the two positions at the
forcside of the binnacle; one position is preferably at or about twenty-five inches
below the level of the needles of the compass card, the position in which the
magnet B is shown at Figure 13, and the other at the bottoni of the binnacle in the
cradle X; the other thwartship magnet A is placed in a cradle attached to a
geometrical slide D, shown separately in front elevation at Figure 16, which is
free to move up and down the after side of the binnacle from a position about
twenty-five inches below the level of the needles of the card to the bottom of the
binnacle.
The method of securing the magnet in position is the same both for the sliding
and fixed magnets. The magnet rests in two V's or curved brackets, one near cach
end, and is pressed into the angle or curved hollow of these by means of a spring or
set screw E pressing on the centre. There is a slit in each V or curved hollow,
in one or other of which the foregoing ring or belt can lic, according to the
direction in which the magnet is placed; the magnet is thus kept from moving
longitudinally.
When the greatest corrective power is needed the fixed magnet is placed in the
uppermost position, namely, that which the magnet B occupies in Figure 13, and in
the same direction as the magnet A of the slide D. When the force with this
arrangement and the slide in its lowest position is too strong the fixed magnet B is
to be placed in its lowest position, namely, at X, Figure 13, and the slide D run up
with its magnet A. until the proper degree of force is obtained. The slide D is
provided with a punching nut E for fixing it at any required height in the binnacle,
which height is indicated by the scale F. If with the slide D and magnet A in its
lowest position the force is still too strong the fixed magnet is to be turned in
the reverse direction, and the slide D run up until the proper degree of force is
obtained. Whatever degree of force is obtained by any particular position of the
two magnets A and B the same force in the reverse direction is obtained by
reversing the direction of each of the magnets. The slide B has attached to it a
hook D fastened to a spring provided with a pin a at the back, which enters one of
the holes in the plate a, vertical guide b, and so suspends the magnet. This spring
hook D' also enables the slide D with the magnets to be easily moved upwards or
downwards in the guide b, by the finger being placed under it and drawing the
pin a out of the hole, in which it is for the time entered, in which condition the slide
and magnet are suspended on the finger, whilst being adjusted nearer to or further
from the compass needle.
The set of magnets for the fore-and-aft component consist of four, similar to the
thwartship magnets, but shorter. These fore-and-aft magnets are marked Gand Ci¹,
Figures 13 and 14, (Sheet 1,) of the appended Drawings. I use a pair of magnets G,
one on each side of the centre of the binnacle for the fixed agnots, and a pair G'in
cradles H, one on each side of the centre for the sliding magnet of the fore-and-aft
corrector; this arrangement in pairs being chosen in order that there may be no
difference in their effect when the ship rolls or heels to one side or the other. There
are two fixed positions for one pair of magnets; one in which the two magnets aro
placed one on cach side of the centre of the binnacle on the same level, and at a
distance of about twenty-five inches from the level of the needles of the compass
card, as shewn at Figures 13 and 14 (Sheet 1); in the second position the magnets
are placed one on each side of the centro at the bottom of the binnacle, in the
cradles or rests I, Figures 13 and 14, (Sheet 1.) The other pair of magnets is, as

page_text: aforesaid, placed in the two cradles H, one at each side of the centre. Tho
cradles H are attached to a block I capable of moving upwards and downwards
on a vertical post J, and of being clamped thereto at any required level. The
post J is in close contiguity to the central tube K containing the heeling error
magnets herein-after described. The block 1' is formed with three feet, not shewn
in the Drawings, by which it rests against the vertical post J; two of the feet are
in a vertical groove at one side of the post, and the third presses against a plane
surface, thus forming a geometrical slide. In the centre of the post J there is a
vertical groove with two guides L, which project slightly over the edges of the
groove. A long plate slides up and down the groove with a scrow rod which
passes between the guides and through the centre of the block 11. A nut N on the
end of the screw rod clamps the block I' in any position; the block I¹ with crudles
attached is thus by a geometrical slide capable of moving up and down, so that the
magnets may be clamped in any position from the highest position about twenty-
five inches below the level of the needles of the compass card to the position at the
bottom of the binnacle.
The method of supporting both the fixed and the moveable magnets and the
means for obtaining the different degrees of magnetic force are similar to those
herein-before described for the thwartship magnets, namely by V's or curved
hollows and set screws, and the hook O serves the purpose of the hook D¹ of the
thwartship slide herein-before described.
Scales are provided for both the slides, the divisions of which are made froin
calculation or experiment, so that the increase or diminution of the magnetic force
caused by raising or lowering the slides with the magnets may be the same for the
same number of divisions at any part of the scale, the space between each division
increasing as the scale is read in a downwards direction.
For the thwartship correctors the scale is placed in the centre of one side of the
binnacle, as shown in Figure 13, and a glass window P in the side allows of
the position of the slide or the scale being seen from the outside of the binnacle.
A window W in the door allows of the position of the fore-and-aft slide being
observed.
For correcting the heeling error the binnacle contains the vertical brass tube K,
herein-before mentioned, fixed under the centre of the compass card, in which a
magnet cased in brass is placed capable of being moved up and down and of being
secured in any position. The brass case of the vertical magnet has four slight pro-
jections constituting feet, two at some distance above, and two at an equal distance
below, the screw stem projecting horizontally from the centre of the length of the
magnet. The four feet pressing on the curved surface of the interior tube form a
geometrical slide. The screw stem slides up and down in a slit which runs from top
to bottom of the brass tube. A nut Q on the screw clamps the magnet in any
required position.
The following alternative arrangements consisting of a method of continuous
magnetic adjustment which I introduced in 1858 for my telegraphic signal galvano-
neter, and which is described in the thirteenth part, and represented in Figure 5 of
the Drawings appended to my Specification for electric telegraphs, dated 20th
February, A.D. 1858, No. 329, is in some respects preferable to that herein-before
described.
In this alternative instrument only one corrector is used for the semicircular
error, similar in construction to the fore-and-aft corrector herein-before described, but
with provision being made in the cradle for adding a second or third pair of magnets
when required. I also use the heeling magnet herein-before described. The frame
in which the semicircular corrector is mounted is fitted on a collar as to be capable
of being turned round and clamped in any position on a central brass tube within
which the heeling corrector is placed, as herein-after described.
A. vertical scale at one side of the binnacle divided with divisions corresponding
to equal proportionate increments of force indicates the degree of power, according
to different positions of the edge of a circle or a pointer moving up and down with

page_text: the slide. A circle carried round with the cradle is divided to degrees so as to
indicate the azimuth relatively to the ship's head of the line of the correcting force
with which the system of correcting inagnets acts on the compass. This, which is a
practical method of realizing one of the two principles of compensating magnets
given by the Astronomer Royal has the disadvantage of not being so easily used
without the aid of calculation as the method by fore-and-aft and thwartship cor-
rectors, but it has the advantage of requiring somewhat less of mechanism and a
sinaller number of magnets, and the still greater advantage of being adapted for the
provision of abundant magnetic power with continuous scale, nearly enough to zero,
to avoid in almost every case the necessity for placing a reverse magnet in the
bottom of the binnacle.
For cases in which the ship's iron is not symmetrical, or in which the biunacle is
not placed amidships, in order to correct the quadrantal error I use masses of soft
iron, preferably globes, placed with the line joining their centres in an oblique
direction, being the line which is magnetic north and south when the ship's head is
in such an azimuth as to make the directive force a minimum after the semicircular
error has been corrected. This position will be determined either by my method
herein-before described for measuring the horizontal force or by calculation, according
to the formulae of the Admiralty Manual, from observation of the residual deviations
after the semicircular error has been corrected. For this purpose I may use the
same binnacle as that which would serve in the ordinary case of the compass being
placed symmetrically with respect to the ship's iron, but before fixing it to the deck
I turn it in azimuth until the line joining the centre of the globes is in the proper
position for correcting the quadrantal error, and I turn the mounting at the top of
thic binnacle bearing the gimbals so as to bring the lubber line into the proper
position; or, allowing the axis of the gimbals to have a position not parallel to the
ship's length and breadth, I simply point the lubber line in the proper position for
the ship's head after the binnacle has been properly set for correcting the quadrantal
error. But I prefer to use a round binnacle or a round topped binnacle carrying a
frame of brass or wood on which the iron globes are to be placed, and which may be
turned round to bring them into any required azimuth. This form is also very
convenient for the alternative method of correcting the semicircular error by one
corrector herein-after described.
A binnacle fitted in accordance with this last part of my present improvements
is shewn on Sheet 2 of the appended Drawings, of which Figure 1 is a front elevation,
with the binnacle partly in section, for the purpose of showing the interior fittings
thereof. Figure 2 is a plan of the binnacle (which is in this case a circular binnacle)
with the helmet and compass bowl removed, shewing the magucts and their attach-
ments which constitute the corrector.
Figure 3 is an enlarged detailed view of the horizontal magnetic corrector
and the parts by which it is adjusted, Figure 4 being a plan corresponding to
Figure 3.
The magnetic corrector magnets B are mounted in the cradle A, attached to a
tubular collar C, sliding and turning on a tube C³, fixed in the centre of the binnacle,
being the tube wherein the heeling error magnet is contained; the magnets B are
secured in the cradle A by the hinged shutter M closing against the ring or belt on
each magnet. The cradle is constructed to carry six magnets B, three on each side
of the central tube, and one pair, or two, or three pairs of magnets are used according
to the amount of error to be corrected. After the corrections necessary to be made
have been ascertained in the manner herein-before described, the cradle with the
magnets is moved up or down and turned round until it is in the proper position to
give a magnetic force on the compass equal and opposite to that produced by the
ship's iron. The outer vertical edge of the circle E attached to the cradle is divided
into degrees, that is to say, from 0 to 360, for the purpose of shewing the angular
position of the magnets.
To facilitate adjustment of the compass for any course of the ship an auxiliary
or guide circle F is mounted on the circle E. The guide circle F has two scales

page_text: upon it, one of degrees from 0 to 360 a second, showing 1317 times the logarithmic
cusecants of the corresponding angles. When the compass has been corrected on
any particular course the number representing this course is to be added to the
number pointed to by the index on the corrector circle, and the guide circle is to be
turned round until the index marks the sun, or the sum diminished by 360° if it
exceeds 360°. In moving the corrector to adjust the compass on any other course
without disturbing the correction on the previous course the corrector must be
turned round so as to produce the same difference of reading on the second scale of
the guide circle, as is produced on the vertical scale by raising or lowering the cor-
rector. In the side of the binnacle a vertical scale (not seen in the Drawings) is
placed (within a window) graduated to show according to the distance of the cor-
rector from the compass bowl numbers indicating the amount of the force exerted
by it on the needles. The graduation is such that one division corresponds to a
change of 3 in the amount of the force. Thus when the correction is already
approximately reached, if the ship be placed so that the force of the corrector is
perpendicular to the direction of the needles, and if then the corrector is raised or
lowered by one scale division, the compass is thereby deflected through the same
angle as it is by turning the corrector through one degree, when the ship is placed
so that the force of the corrector is approximately in the direction of the needles.
The circle E is fitted with a horizontal circular rim G, over which an arm H. extends
with a clamping claw under. This arm H is attached to the collar C, upon which
the cradle with the correcting magnets is carried, and its outer end is situated
between two guides I fixed to the interior of the circular binnacle; these guides
prevent the arm E from being turned in a horizontal plane, but do not interfere
with its freedom of vertical movement. The arm H is provided with a clamping
screw J, which being pressed upon the rim G, securely holds the cradle with the
correctors in any required angular position without interfering with its vertical
motion. The central tube C₁ is drilled with a vertical row of holes, and the collar
C has a lever K carrying a pin which enters one or other of the holes according
to its position up or down. This lever is acted on by a powerful spring L to make
sure of the pin entering and remaining in the hole. To give sufficient strength,
and to guard the heeling magnet from being obstructed by the pin two concentric
tubes are used with a small space between them. In the upper part of the tube C
a small hole a is situated through which one end of a cord or brass chain is passed.
To the other end of this cord or chain the heeling error magnet is attached, so that
by means of this cord or chain the position of the magnet within the tube C¹ is
raised or lowered, according to the correction required, and it is fixed in the required
position by fastening its cord or chain to a cleat inside the binnacle.
An alternative inethod which may be used has the advantage of giving con-
tinuous up-and-down motion to the corrector; it consists of having a screw cut on
the outer side of the central tube, and mounting a long nut on this screw with a
pointer in the guiding slot herein-before described, and providing a second collar
outside this nut, carrying the cradle as heroin-before described.
On Sheet 3 of the appended Drawings is shown a modified arrangement for
applying magnetic correctors to ship's compasses suitable for being used in the
wheel house in cases where there is not room for conveniently placing a binnacle
therein. Figure 1 (Sheet 3) is an elevation of this arrangement, and Figure 2 is a
plan thereof. The compass card and bowl are placed in the centre, as shewn at A
at both Figures, and the soft iron balls or globes B are on each side thereof, they
being fitted with clamping screws for holding them to the cups or pedestals C. On
cach side of the soft iron balls or globes B are two boxes D, which contain the fore-
and-aft and thwartship corrector mangets E and F respectively. The fore-and-aft
magnets E are carried in cradles G, the cradles being arranged to slide on guides
placed lengthwise in the boxes D. The cradles are also provided with clamping
screws H for the purpose of securing the fore-and-aft magnets at the required dis-
tance from the compass bowl, and above the thwartship magnet F' there is also

page_text: another clamping screw I for fixing thwartship magnets in the position corre
sponding to the amount of correction required.
The improvements in the compass card consists,—
First. Of the substitution of fine silkworm gut or fine metallic wires, or other
suitable material, instead of the silk thread of the compass card described in the
Specification filed in pursuance of my former Letters Patent. My reasons for sub-
stituting such other materials for silk threads are to avoid the disadvantage of the
loose shreads of the silk which might possibly sometimes be found to interfere with
the freedom of motion of the compass card, and because the silk threads are liable to
alter in length with a change in the temperature and in the moisture of the atmo-
sphere of the compass case. Both these defects are entirely obviated by using fine
metallic wire, preferably platinum; the ends of the wire are drawn through holes
in aluminium rim and boss of the compass card, and are properly bent and soldered
to secure them.
Another improvement consists in drilling holes through the ends of the magnetic
needles for the purpose of fixing the threads or wires by which they are attached
to the aluminium rim.
An improved method of securing the paper marked with the points to the
aluminium rim of the compass card consists as follows:-A long strip of thin
gummed paper, preferably one and a half times as broad as the strip of aluminium
which is to form the rim, is wrapped once and a half round the aluminium, and
then secured by means of the gum. The aluminium strip with the paper is then
made into a circle, and the threads or wire spokes fixed between the rim and boss,
and the paper with the points is gummed to the paper on the aluminium. For
large size (such as 12-inch cards or 15-inch cards) I use, instead of a flat vertical rim
of aluminium, a slip of thin sheet aluminium, beat into a tube of which the edges
nearly meeting need not be united, and thus obtain sufficent stiffness with propor-
tionately less weight of metal.
Having now described and particularly ascertained the nature of my said
Invention, and the system, mode, or manner in or under which the same is or may
be used or practically carried into effect, I would observe in conclusion that what
I consider to be novel and original, and therefore claim as the Invention secured
to me by the herein-before in part recited Letters Patent is,-
First. The improvements in my azimuth mirror, consisting in placing the axis of
the mirror either above or below the lens, so as to reduce the interval of no vision
to a minimum, and the duplicating of the instrument so as to allow of the bearings
of two objects being simultaneously taken, whose azimuths differ by 180 degrees, or
thereabouts, substantially as herein-before described, and shown at Figures 1 to 5º
(Sheet 1) of the appended Drawings.
Second. The instrument for applying auxiliary deflecting magnets for the purpose
of measuring the horizontal directive magnetic force on board ship, and the arrange-
inents for varying or adjusting the distance of the poles of the magnets therein,
substantially as herein-before described, and shown at Figures 6 to 9 inclusive
(Sheet 1) of the appended Drawings.
Third. The instrument for incasuring the vertical component of the ship's
magnetic force, wherein the force tending to make a needle dip is measured by
the torsion of a stretched horizontal wire bearing the needle, substantially as
hercin-before described, and shown at Figures 10 to 12d, Sheet 1, of the appended
Drawings.
Fourth. The two mechanical arrangements in the interior of the binnacle for
carrying out two methods of the Astronomer Royal for correcting the semicircular
error, substantially as herein-before described, and shown at Figures 13 to 16
inclusive on Sheet 1, and Figures 1 to 4 inclusive, Sheet 2, of the annexed Drawings,
in particular the moveable cradles for carrying the magnets, the catches and ver-
tical row of holes, as well as the screw for adjusting and suspending the cradles on
the central tube.

page_text: Fifth. The arrangement for carrying the iron balls, so that they may be turned
round in azimuth to correct an oblique quadrantal error, or be moved inwards or
outwards, and clamped in the required position.
Sixth. The vertical scale for the fore-and-aft and thwartship correcting magnets
divided into spaces, which increase as the scale is read in a downwards direction, so
that a constant increment or diminution of the correcting force is applied obtained at
all parts of the scale by moving the correcting magnets through an equal number
of divisions of the scale, substantially as herein-before described.
Seventh. The vertical scale for the rotating corrector, graduated so that when the
force of the corrector is perpendicular to the direction of the needles, the raising
or lowering of the corrector through one division of the scale causes the compass
card to be deflected through the same angle as it would be by turning the corrector
through one degree when the ship is so placed that the force of the corrector is
approximately in the direction of the needles, as herein-before described.
Eighth. The auxiliary or guide circle F mounted on the circlo E with the two
scales thereon, and divided as herein-before described.
In witness whereof, I, the said Sir William Thomson, have hereunto set my
hand and seal, this Fourth day of May, in the year of our Lord One thousand
eight hundred and seventy-seven.