IM- PROVEMENTS IN THE MARINER'S COMPASS, AND IN THE MEANS 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 "IM-
PROVEMENTS IN THE MARINER'S COMPASS, AND IN THE MEANS FOR ASCERTAINING
AND CORRECTING ITS ERRORS."
ealed the 23rd June 1876, and dated the 29th March 1870.
PROVISIONAL SPECIFICATION left by the said William Thomson at the Office
of the Commissioners of Patents on the 29th March 1876.
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. "IMPROVEMENTS IN THE MARINER'S COMPASS, AND IN THE
MEANS FOR ASCERTAINING AND CORRECTING ITS ERRORS."
The objects of this Invention are-
First. With smaller needles than in compasses hitherto in practical use to obtain
as long a period of free oscillation as is suitable for working well at sea.
Second. Smallness of frictional error.
Third. Improved gimbals for supporting the compass bowl to give sufficient
steadiness, and to leave it greater freedom to take up as nearly as possible the true
level.
Fourth. Practical methods for applying correctors for the quadrantal semicircular
and heeling errors.
Fifth. An improved appliance to the compass for taking magnetic azimuths of
or stars or terrestrial objects without being impeded by the quadrantal
correctors.
sun
Sixth. Improvements in the method of correcting the compass by observation of
the sun, moon, or stars on a detached azimuth circle.
Steadiness of the compass at sea in stormy weather has hitherto been attained
by making the compass card and needles large and heavy, and sometimes also by
adding weights. The addition of weights is detrimental as increasing the frictional
error, and tending thereby to make the compass follow the motion of the ship when
she moves in azimuth. In a mathematical paper communicated to the British
Association in 1874, and published in the Philosophical Magazine for November
1874, I showed that the largeness of period obtained by great dimensions is favour-
able for steadiness. It follows from these considerations that it is desirable to
[Price 8d1

page_text: obtain as large a moment of inertia as possible with as light compass card, magnets,
and frame as may be.
The compass consists of a light aluminium boss with a central sapphire cap for
the compass to rest on, and a rim of aluminium of from 4 to 9 inches diameter,
according to the size of the compass. There is an even number of holes in the
rim (to save circumlocution I shall call the number 32, being the number I prefer
for my largest size of compass card), and half that number in the circumference of
the boss. The rim and boss are connected together by means of 16 fine silk threads
forming, as it were, 32 spokes, and the compass card is partly supported by these
threads and partly by the rim. Two or four small magnets having their corte-
sponding ends tied together by silk threads of equal lengths, so that the magnets
may be as nearly parallel as possible, are attached to the rim by means of three
silk threads. One of these threads is fastened to the centre of one extreme magnet,
and the other two threads to the ends of the other extreme magnet. The other
end of the first-mentioned thread is attached to the cast or west point of the rim,
and those of the remaining threads to two points at equal angular distance on cach
side of the opposite point. Two threads parallel to the north and south line are
stretched between points on each side of the north and south points at such
distances as just to pass clear of the boss. These threads are used to judge by eye
in adjusting the magnets to have their lengths parallel to the north and south line
on the compass card.
The following method is used for giving an equal tension to each of the threads
between the boss and the rim :-
Each of the sixteen threads is passed through a hole in the circumference of the
boss, and stretched thence into two slight notches in the top of the rim, then down
and through two contiguous holes on the middle of the rim, one end of the thread
is then knotted, and a weight attached to the other end. The rim is placed on a
suitable circular stand, and the boss is worked about until the rim is made truly
circular, and the sapphire cap is truly in the centre. The ends of the threads with
the weights attached are then cemented to the aluminium rim, the weights are cut
off, and the ends are firmly secured by means of other holes in the aluminium rim.
I thus obtain a compass which, whilst being extremely light, and yet having a
large radius of gyration, has very small frictional error with small enough magnetic
moment to give a very long period of free vibration. For example, one of my
compasses of 9 inches over all diameter, having two needles, cach 21 inches long,
weighs in all 104 grains, and at Glasgow has a period of free vibration 62 seconds,
and extreme static frictional error on either side of the true position only about
one quarter degree.
My third improvement is to swing the gimbals on knife edges instead of on
cylindrical journals as is now generally done, and to calm the vibration by the use
of a pendant in a bowl of oil or liquid, or by means of a very viscous oil or other
liquid without pendant in the manner described below. The object of this
improvement is to give greater freedom to the compass to assume a horizontal
position. A small spirit level is placed in the glazed case, in which the compass is
supported, to show whether the case and bowl are properly balanced. In compasses
ordinarily in use at sea the vibrations of the case due to the rolling of the ship
are calmed by the friction between the cylindrical journals on which the gimbals
are swung, and the brasses or bolsters by which they are supported. In consequence
of this friction the compass does not of itself take up a truly level position. With
the use of knife edges, I allow the compass to take up its truly level position, but
I lose the energy destroying power of the rubbing surfaces. Instead, 1 use a
large bowl attached to the bottom of the glazed case; the bottom of the case
forming the roof of the bowl. The bowl is nearly filled with liquid. Inside the
bowl a circular disc, with the greater part of the mass at its outer rim, is suspended
by a hook or chain of one or two links from a point a little above the centre of
gravity of the bowl. The disc, by its moment of inertia round a horizontal diameter,
tends in its vibration to lag behind, or even to go in the opposite direction to the

page_text: bowl and oil, and when there is any relative motion of the disc and bowl, energy
is consumed by the intermediate action of the oil. The correctors for the
quadrantal error are founded on the principle first given by the Astronomer Royal.
They are iron cylinders, or ovals, or bundles, of soft iron wire, placed on proper
supports fixed to the sides of the binnacle, having their axes in the proper
direction for correcting the quadrantal error, which in most ships is at right angles
or very nearly at right angles to the fore-and-aft line.
The semicircular error is corrected by steel magnets according to the general
principle also first given by the Astronomer Royal. The varying adjustment of the
correction required for variation in the ship's sub-permanent magnetism and mag-
netism induced by the vertical component of the earth's inagnetic force, I effect in
the following manner:-I use two correctors, one for the fore-and-aft force, and
the other for the thwart-ship force. Each corrector consists of two bar magnets
moveable round a common horizontal axis perpendicular to their lengths, and
through or nearly through one sets of their ends.
One of these is a compound magnet, consisting of two bars rigidly connected
parallel to each other with their poles similarly turned and with a space between
them through which the other may pass. This other has a strength equal to the
suin of the strengths of the two bars of the compound magnet, a condition which is
easily fulfilled with sufficient accuracy by making it also a compound magnet
of two bars laid close beside one another with similar poles contiguous.
One of the two outer bars and the central magnet has each a cog wheel of the
same diameter rigidly attached to its inner end. These cog wheels both revolve
round the common horizontal axis, and they are connected together by means of
other cog wheels in such a manner that when set in motion they turn in opposite
directions. The pole of the outer or split magnet, and that of the central one at
the common axis, are of contrary names, and so of course also are the poles remote
from the axis. The latter will be called the "moveable poles" or sometimes
simply "the poles" for brevity.
A circular disc with degrees marked on the outer edge is attached to the common
axis to indicate the angle of deflection of the magnets from the vertical, and there
is a clamp attached to one of the outer bars so that when they have been deflected
to any required degree they may be firmly fixed in that position. The cog wheels
attached to the magnets are so connected together that when the two outer bars
are vertical (the ship being supposed to be upright) the central magnet is also
vertical and between the two parts of the compound magnet, and when the com-
pound magnet is deflected through any angle on either side of the vertical the
central one is deflected through an equal angle but on the opposite side of the
vertical.
When the two magnets are vertical their combined action at a distance is zero,
because the split magnet is equal in strength to the central one, and the poles of
opposite names are contiguous to each other. The horizontal axis round which tho
imagnets turn for the fore-and-aft corrector is thwart-ship, and for the thwart-ship
corrector it is fore-and-aft. The correctors are placed one over the other, and both
under the compass, so that in their zero position the middle line of cach corrector is
vertically under the centre of the compass (the ship being supposed to be upright).
If the ship's magnetic force is to starboard, the magnets of the thwart-ship
corrector are turned from the vertical position in such a direction as to bring their
moveable north and south poles to the port and starboard sides respectively, or if
the ship's force is to port, the correctors are turned in the opposite directions.
Similarly if the fore-and-aft component is headward, the magnets of the fore-and-
aft corrector are turned so as to bring their north and south poles aft and forward
respectively.
The cog wheels may be dispensed with, and each magnet of the pair constituting
either corrector may be moved to the proper position, and clamped there simply
by hand.
Or a single corrector, constructed according to the above description, and having

page_text: also an azimuthal adjustment, so that the line joining its poles may be brought to
make any angle with the ship's length may be used, and is preferable on the
ground of simplicity, but is not quite so easily used when the correction is to re-
adjusted frequently.
When there is a greater amount of error to be corrected than can be conveniently
done by the continuously adjustable correctors described above, I use auxiliary
fixed correcting magnets. These magnets I make by preference of steel wire or
round rod 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 the strength of their magnetism. They are made in pairs of equal strength
chained together, and when out of use are placed in a box so fitted that they cannot
be put into it except with dissimilar poles together. The position in which I prefer
to place a couple of these correctors is as nearly as may be level with the compass,
and at equal distances from the two sides of it, so that the central point of the
compass may be nearly in the position of a point of maximum or maximum-minimum
force, so far as the correctors are concerned. When fixed fore-and-aft correctors
are required, their positions may be as close as possible under the quadrantal
correctors.
For correcting the heeling error, the binnacle contains a proper geometrical
clamp for holding firmly a magnet in a vertical position centrally below the lower
of the two pairs of adjustable semicircular correctors.
My fifth improvement consists of a convex half lens placed over the compass
between the outer circumference, and the centre with its axis directed to the
graduated circumference of the compass card, and its centre at a distance from the
divisions slightly exceeding the radius of the divided circle. Attached to the lens
there is a plane mirror for observing the image of the object whose azimuth is
required, moveable about a horizontal axis, coinciding with the diametral edge of
the lens. The lens and mirror are placed on a frame which moves round a vertical
axis through the centre of the compass card. The observer when taking an
azimuth turns the instrument round its vertical axis until the mirror and lens are
fairly opposite to the object. He then places his eye above the compass and moves
the mirror about its horizontal axis until the image of the object reflected from the
mirror is seen upon a division on the compass card seen through the lens. The
point on the compass card where the object was seen will be the bearing of the
object. As the azimuths are taken by looking down on the compass the wooden
troughs which carry the quadrantal correctors do not interfere with the operation.
My sixth improvement relates to the instrument as herein-before mentioned to
be used for finding the true north directly by observations of the sun, moon, or
other bright enough planet or star, for the purpose of ascertaining from time to time
the errors of the compass varying as that does with the alterations in the ship's
magnetism. It consists of a horizontal metallic circle supported on gimbals, having
an inner circle moveable round a vertical axis. The rim of this circle is graduated
to degrees, and may also be marked with points of the compass, the inner circle
carries with it a portion of a hollow spherical surface. This surface is marked with
a great circle corresponding to the equator, and smaller circles corresponding to
degrees of declination as far as 20 degrees on each side of the equator. These
declination circles are divided into degrees reckoned in hours and minutes of time
at the rate of 4 minutes to the degree.
The spherical surface is moveable round a horizontal axis through its centre of
gravity. A lens is placed with its centre at the centre of this spherical surface, it
is pivoted on a frame, so that its axis may be directed to any point between the
extreme declination circles of the spherical surface. To use the instrument turn
the spherical surface round on its pivots until its equatorial circle is inclined to the
vertical at an angle equal to the latitude of the place, then turn the inner circle
round its vertical axis until the image of the sun or other heavenly body through
the lens falls on a point on the spherical surface which corresponds to the apparent
solar time, or to the hour angle of the body observed, and to the declination of the

page_text: body at the time. The lens is to be turned about on its bearing until the image is
well defined. Either of the conditions above specified alone would suffice if the
instrument were perfectly accurate, and there were no atmosphere refraction and no
sensible parallax. The effects of refraction and parallax are sufficiently obviated
for all practical purposes by bringing as nearly as may be judged by eye the centre
of the image into the same azimuth with the proper time point of the proper de-
clination circle. When the instrument is correct and well levelled the image of the
centre of the sun will be seen always below the proper point by the amount of
refraction; the centre of the moon's image will be seen above the proper point by
the excess of her parallax above refraction.
To obviate the great number of circles and and divisions on the spherical surface
required by this method I vary it as follows:-Instead of the large concave
spherical surface I use only a narrow strip containing the equatorial circle, and
broad enough to mark the hours on it; and a comparatively small portion of
concave spherical surface being a sector bounded by two consecutive hour circles,
and by declination circles of 29° on each side of the equator. This sector is marked
with segments of small circles corresponding to degrees of declination, the space
between the hour circles is divided into degrees corresponding to 4 minutes of time.
This sector is connected by means of an arm with the frame of the lens, and is
moveable with the lens round the polar axis of the spherical surface. To use the
instrument set the plane of the equatorial circle for the latitude, then turn the lens
frame with attached arm and sector round the polar axis until the bounding
meridians of the sector agree with the hours marked on the equatorial circle corre-
sponding to the integral hours of apparent time (by the body observed) before and
after the time of the observation; turn the inner azimuth circle round its vertical
axis until the image of the falls on the point which corresponds to the declination
and time.
Another plan that I for avoiding expense of construction and confusion in
using the instrument caused by too great number of divisions is as follows:-
A single circle is divided for hours and four minutes of time on a narrow strip of
the spherical surface. I fix the frame containing the lens in a geometrical slide so
that the centre of the lens may move in a straight line through the centre of the
circle, and at right angles to the plane of that circle. When using the instrument,
the centre of the lens is moved along the slide to a distance from the centre of the
circle equal to the tangent of the declination; the plane of the circle is set for the
latitude, and the inner azimuth circle is turned round its vertical axis until the
image of the sun falls on the circle at the point corresponding to the time. When
it is desired that the place to which the image is to be brought should be always
the same point of a moveable screen, I attach the screen to the lens frame, and give
by hand or by clockwork nearly enough continuously the proper rotation round the
polar axis. When it is desired that the place of the image be a fixed point of a
screen, fixed relatively to the inner azimuth circle, I use a mirror inclined to the
polar axis at an angle equal to half the observed body's polar distance, and so bring
the light to a direction approximately parallel to the polar axis, and I give by hand
or clockwork (as in a heliostat) the proper rotation to the mirror round the polar
axis. I also use a mirror or lens itself silvered on one side to turn the light into
such a direction that the image may be seen directly in a field marked by cross-
wires or other convenient mark without any solid screen, or on a semi-transparent.
screen, as ground glass. The dispensing with the screen allows a moderately bright
star or even a barely visible star to be used.
To find the error of the compass, place the detached azimuth circle with the axis
of its gimbals parallel and perpendicular to the ship's length in any convenient
position for receiving the direct light from the sun or other heavenly body to be
observed. When the inner azimuthal circle is set, by observation as described above,
read on the inner circle the angle between the lubber point of the outer circle and
the true north, which is the zero of the graduation of the inner circle, and simultan-
eously, or as nearly as possible simultaneously, read on the compass card the angle

page_text: Tho
between its zero or north point and the lubber point of the compass case.
difference of these readings is the deviation of the compass from the true north.
The deviation may also sometimes be found by aid of the appliance for azimuths
attached to the compass itself, which constitutes the fifth part of my Invention; but
this method fails when the compass is screened from the heavenly body to be
observed by funnel or sail, or by deck house, as is often the case with a steering
compass, and when it is applicable it requires either a calculation by spherical trigo-
nometry or the use of azimuth tables. The detached azimuth circle constituting
the sixth part of my Invention is free from both of these objections.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said William Thomson in the Great Seal Patent Office on the 27th September
1876.
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. "IMPROVEMENTS IN THE MARINER'S COMPASS, AND IN
THE MEANS FOR ASCERTAINING AND CORRECTING ITS ERRORS."
My said Invention, which relates to improvements in the mariner's compass, and in
the means for ascertaining and correcting its errors, consists, and has for its object
as follows:-
First. With smaller needles than in compasses hitherto in practical use to obtain
as long a period of free oscillation as is suitable for working well at sea.
Second. Smallness of frictional error.
Third. Improved gimbals for supporting the compass bowl to give sufficient
steadiness, and to leave it greater freedom to take up as nearly as possible the
true level.
Fourth. Practical methods for applying correctors for the quadrantal semicircular
and heeling errors.
Fifth. An improved appliance to the compass for taking magnetic azimuths
of sun or stars or terrestrial objects without being impeded by the quadrantal
correctors.
Sixth. Improvements in the method of correcting the compass by observation
of the sun, noon, or stars on a detached azimuth circle.
Steadiness of the compass at sea in stormy weather has hitherto been obtained
by making the compass card and needles large and heavy, and sometimes also by
adding weights to the compass card. The addition of weights is detrimental by
increasing the frictional error, and tending thereby to make the compass follow the
motion of the ship when she moves in azimuth. In a mathematical paper commu-
nicated to the British Association in 1874, and published in the Philosophical
Magazine for November 1874, I showed that largeness of period of free oscillation is
favourable for steadyness. It follows from these considerations that it is desirable
to obtain as large a moment of inertia as possible with as light compass card,
magnets, and frame, as may be.
My improved compass and appliances connected with it are illustrated on the
two appended Sheets of Drawings, on which Figure 1, Sheet 1, shows one half of
my improved compass card with its magnetic needles; Figure 2 being a transverse
section of the same with the pivot upon which it is supported.
whilst
Figure 1 shows in section the manner of connecting the segments of the compass
card together at their inner ends by means of the strips a¹ of gummed paper,
Figures 1 and 1b show the manner of connecting the compass card to the rin by
tongues x formed on the card.
The compass consists of a light aluminium boss a, with a central cap b of sapphire
ruby, or other sufficiently hard material for the compass to rest on, and a rim o

page_text: aluminium from about 4 to 12 inches or more in diameter, according to the size
of the compass.
The compass card is by preference constructed with an even number of holes in
the rim (to save circumlocution I shall call the number 32, being the number I prefer
for my largest size of compass card), and the central boss has the same number of
holes in its circumference. The rim and boss are connected together by means of
32 fine silk threads c, which form as it were 32 spokes. These silk threads or
spokes c are knotted, as herein-after described, so as to retain them in the holes
both of the boss and the rim, after which they are cemented thereto by shellac or
other adhesive substance. The compass card is partly supported by these threads c
directly and partly by the rim f, as shown at Figures 1 and 2. Two or more small
magnets d, of which eight are employed in the compass shown at Figures 1 and 2,
having their corresponding ends tied together by silk threads c of equal lengths, so
that the magnets may be as nearly parallel as possible, are attached to the rim by
means of four silk threads f¹, as shown.
The following method is used for giving an equal tension to each of the threads
between the boss and the rim :-
When connecting the boss and rim together, and so as to get the boss to a truly
central position, the boss is fixed horizontally over the centre of the fixed ring,
herein-after referred to. Each of the 32 threads is passed outwards through a hole
in the rin, where one end is retained by a knot on the inside of the rim, then
upwards and over the rim inwards to a corresponding hole in the boss. Each
thread passes though this hole upwards, and then horizontally outwards over the
stout fixed ring above mentioned, and downwards with a weight attached to it
to stretch it with proper force. The rim is then worked about until it is made
truly circular and concentric with the sapphire cap. After this the threads are
cemented to the boss, after which they are drawn through the holes in the boss a
second time and again cemented and their loose ends cut away. I thus obtain a
compass which, whilst being extremely light, and yet having a large radius of
gyration, has very small frictional error with small enough magnetic moment to
give a very long period of free vibration. For example, one of my compasses of
10 inches over all diameter, having eight needles of from 3 inches to 12 inches long,
weighs in all 178 grains, and at Glasgow has a period of free vibration 38 seconds,
and extreme static frictional error on either side of the true position of less than
one quarter degree.
My third improvement is to swing the gimbals on knife edges, as shown at g and
g. Figures 3 and 4, and in enlarged detail at Figures 5, 6, 7, and 8, Sheet 1, instead
of on cylindrical journals as is now generally done, and to calm the vibration by
the use of a very viscous oil or other liquid contained in the bowl A of the compass.
The object of this improvement is to give greater freedom to the compass to assume
a horizontal position.
In compasses ordinarily in use at sea the vibrations of the case due to the rolling
of the ship are calmed by the friction between the cylindrical journals on which the
gimbals are swung and the brasses or bolsters by which they are supported. In
consequence of this friction the compass does not of itself take up a truly level
position. With the use of knife edges I allow the compass to take up its truly
level position, but I lose the energy destroying power of the rubbing surfaces.
Instead of the rubbing surfaces I use the large bowl A attached to the bottom of
the glazed case, this bottom forming the roof of the bowl, which is nearly filled
with liquid, thus when there is any inotion of the bowl energy is consumed by the
frictional or viscous action of the liquid.
The correctors for the quadrantal error are founded on the principle first given
by the Astronomer Royal. They are seen at B, Figure 4, Sheet 1, and consist
of iron balls placed in boxes C, fixed to the sides of the binnacle. The semicircular
error is corrected by steel magnets according to the general principle also 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

page_text: vertical component of the carth's magnetic force I effect in the following manner:
I use, as seen more particularly at E, F, Figure 3, Sheet 1, two similar correctors,
one for the fore-and-aft force, and the other for the thwart ship force.
Figures 9 and 10, Sheet 1, are enlarged detail views of one corrector, each of which
consists of two bar magnets G, H, moveable round a common horizontal axis I,
which passes through one of their ends. One of these, the magnet G, is a compound
magnet, consisting of two bars rigidly connected parallel to each other with their
poles similarly turned, and with a space between them through which the other
magnet H passes. The inagnet H has a strength equal to the sum of the strength
of the two bars of the compound magnet G, a condition which is easily fulfilled
with sufficient accuracy by making it also a compound magnet of two bars laid close
beside one another with similar poles contiguous. One of the two outer bars G
and the central magnet H has each a spur wheel K of the same diameter rigidly
attached to its inner end. These spur wheels both revolve round the common
horizontal axis I, one of them being placed loosely thereon, and they are connected
together by means of other spur wheels L, M, in such a manner that when set in
motion they turn in opposite directions. The poles of the outer or split magnet G,
and that of the central one H at the common axis, are of contrary names, and so of
course also are the poles remote from the axis. The latter will be called the
"moveable poles" or simply "the poles" for brevity. A circular disc N, Figure 3,
with degrees marked on the outer edge is arranged concentrically with the common
axis I of the magnets to indicate the angle of deflection of the magnets from the
vertical, and there is a clamp O attached to one of the outer bars G, so that when
they have been deflected to any required degree they may be firmly fixed in that
position. The spur wheels attached to the magnets are so connected together that
when the two outer bars G are vertical (the ship being supposed to be upright) the
central magnet H is also vertical and between the two bars of the compound
magnet G, and when the compound magnet is deflected through any angle on either
side of the vertical the central one is deflected through an equal angle, but on the
opposite side of the vertical, as shown by the dotted lines at Figures 3 and 9, Sheet 1.
When the two magnets are vertical their combined action at a distance is zero,
because the split magnet is equal in strength to the central one, and the poles of
opposite names are contiguous to each other. The horizontal axis I, round which
the magnets turn for the fore-and-aft corrector E is thwart-ship, and for the thwart-
ship corrector F it is fore-and-aft. The correctors, as shown at E and F, Figure 3,
Sheet 1, are placed one over the other, and both under the compass, so that in their
zero position the middle line of each corrector is vertically under the centre of the
compass (the ship being supposed to be upright).
If the ship's magnetic force is to starboard the magnets G, H, of the thwart-ship
corrector are turned from the vertical position in such a direction as to bring
their moveable north and south poles (by which I mean the poles similar respectively
to the earth's north and south magnetic poles) to the port and starboard sides
respectively; or if the ship's force is to port the correctors are turned in the opposite
directions. Similarly if the fore-and-aft component is headward the magnets of
the fore-and-aft corrector E are turned so to bring their north and south poles aft
and forward respectively.
The spur wheels may be dispensed with and each magnet of the pair constituting
either corrector may be moved to the proper position and clamped there simply
by hand. Or a single corrector, constructed according to the above description,
and having also an azimuthal adjustment, so that the line joining its poles may
be brought to make any angle with the ship's length, may be used, and is preferable
on the ground of simplicity, but is not quite so easily used when the correction is
to be readjusted frequently.
When there is a greater amount of error to be corrected than can be conveniently
done by the continuously adjustable correctors described above, I use auxiliary
correcting permanent magnets, as shown at P, Figure 3, and in detail at N, P, Š,
in Figure 12. These magnets I make by preference of steel wire, or round rod

page_text: steel, tempered glass, hard, and soldered in brass tubes to protect them from rust
and from shocks cr from close contiguity with other magnets, which night alter
the strength of their magnetism. These magnets are made in pairs of equal
strength, and with ends so shaped that when out of use they are to be placed in
a box so fitted that they cannot be put into it except with dissimilar poles together.
This may be easily effected by making one end of each bar magnet round and the
other end square. The position in which I prefer to place a couple of these
correctors is, as nearly as may be, level with the compass and at equal distances
from the two sides of it, so that the central point of the compass may be nearly
in the position of a point of maximum or maximum-minimum force so far as the
correctors are concerned.
When fixed fore-and-aft correctors are required their positions may be as close
as possible under the quadrantal correctors B.
For correcting the heeling error, the binnacle contains an upright with a V-groove
forming a proper geometrical clamp Q, Figure 3, for holding firmly at the proper
height a magnet R, in a vertical position centrally below the lower of the two
pairs of adjustable semicircular correctors.
At-
One
Two
My fifth improvement consists of a convex half lens placed over the compass
between the outer circumference and the centre with its axis directed to the
graduated circumference of the compass card, and its centre at a distance from the
divisions slightly exceeding the radius of the divided circle. This half lens is
shown at i, Figure 3, and in enlarged detail at Figures 13 and 14, Sheet 1.
tached to the lens there is a plane mirror j, for observing the image of the
object whose azimuth is required, moveable about a horizontal axis coinciding
with the diametral edge of the lens. The lens i and mirror j are placed on a
frame, mounted on a proper geometrical slide to give it motion round an axis
through the centre of the compass card, perpendicular to the glass cover.
rounded foot n rests in a conical hollow in the centre of the glass cover.
others P, P¹, press on the glass plate lightly, being caused to do so by the spring 8,
or by the centre of gravity of the frames being very slightly on one side of m
The observer when taking an azimuth turns this frame round its vertical axis,
until the mirror j and lens i are fairly opposite to the object. He then places his
eye above the compass and moves the mirror j about its horizontal axis until the
image of the object reflected from the mirror is seen upon a division on the compass
card seen through the lens. The point on the compass card where the object is seen
will be the bearing of the object. As the azimuths are taken by looking down on
the compass the wooden troughs which carry the quadrantal correctors do not
interfere with the operation. This appliance I call the azimuth mirror. It is
provided with a spirit level m to show whether the case and bowl are properly
balanced.
My sixth improvement relates to the adjunct as herein before mentioned, which
I call a detached azimuth circle, to be used for finding the true north directly by
observations of the sun, moon, or other bright enough planet or star for the purpose
of ascertaining from time to time the error of the compass, varying as that does with
the alterations in the ship's magnetism. It consists of a horizontal outer metallic
circle supported on gimbals having an inner circle also moveable round a horizontal
axis at right angles to the axis of movement of the outer circle. The gimbals of
both the inner and outer circle are, as in the case of the gimbals of the compass,.
formed with knife edges. The rim of the inner circle is graduated to degrees,
and may also be marked with points of the compass, and this circle carries on a
horizontal axis pivoted on it a portion of a hollow spherical surface. This surface
is marked with a great circle corresponding to the equator, and with circles corre-
sponding to degrees of declination, as far as 29 degrees on each side of the equator.
These declination circles are divided into spaces corresponding to 5 minutes of
time at the rate of a degree and a quarter to five minutes, which divisions are
numbered in hours and minutes, the points in which the declination circles are cut
by a plane perpendicular to the above mentioned horizontal axis being reckoned as

page_text: noon. A lens placed with its centre at the centre of the spherical surface is
pivoted on a frame so that its axis may be directed to any point between
the extreme declination circles of the spherical surface. To use the instrument,
turn the spherical surface round on its pivots until its equatorial circle is inclined
to the vertical at an angle equal to the latitude of the place, then turn the inner
circle round horizontally until the image of the sun or other heavenly body through
the lens falls on a point on the spherical surface, which corresponds to the apparent
solar time or to the hour angle of the body observed, and to the declination of
the body at the time. The lens is to be turned about on its bearing until the image
is well defined. Either of the conditions above specified alone would suffice if
the instrument were perfectly accurate and there were no atmospheric refraction
and no sensible parallax. The effects of refraction and parallax are sufficiently
obviated for all practical purposes by bringing as nearly as may be judged by eye
the centre of the image into the same azimuth with the proper time point of the
proper declination circle. When the instrument is correct and well levelled the
image of the centre of the sun will be seen always below the proper point by the
amount of refraction; the centre of the moon's image will be seen above the proper
point by the excess of her parallax above refraction.
To obviate the great number of circles and divisions on the spherical surface
required by this method I vary it as follows, and as shown on Sheet 2 of the
Drawings, in which Figure 1 is a transverse section; Figure 2 a plan; Figure 3
another tranverse section at right angles to Figure 1, and Figure 4 an elevation of the
instrument. Instead of the large concave spherical surface, I use only a narrow strip
A', containing the equatorial circle, and broad enough to mark the hours on it, and a
comparatively small portion of concave spherical surface, B' being a sector bounded
by two consecutive hour circles and by declination circles of 28° on each side of the
equator, as seen more particularly in the enlarged detail views, Figures 5, 6, and
7, Sheet 2. This sector B is marked with segments of small circles corresponding
to every second degree of declination, the space between the hour circles is divided
into spaces corresponding to to 5 minutes of time. This sector is connected by means
of arms C with the frame D of the lens E, and is moveable with the lens round the
polar axis F of the spherical surface. The equatorial circle A', A', pivotted on a
horizontal axis in its own plane is carried on the inner circle I, which is divided into
degrees, and may also be marked with the points of the compass as before described.
To use the instrument set the plane of the equatorial circle for the latitude of the
place by means of the divided arc G, then turn the lens frame D with attached
arms and sector round the polar axis until the bounding meridians of the sector
agree with the hours marked on the equatorial circle corresponding to the integral
hours of apparent time (by the body observed), before and after the time of the
observation turn the inner azimuth circle round its axis until the image of the body
observed falls on the point which corresponds to the declination and time.
Another plan that I have for avoiding expense of construction and confusion in
using the instrument caused by too great number of divisions is as follows:-A
single circle is divided for hours and five minutes of time on a narrow strip of the
spherical surface. 1 fix the frame containing the lens in a geometrical slide so that
the centre of the lens may move in a straight line through the centre of the circle,
and at right angles to the plane of that circle. When using the instrument the
centre of the lens is moved along the slide to a distance from the centre of the circle
equal to the tangent of the declination, the plane of the circle is set for the latitude,
and the inner azimuth circle is turned round its vertical axis until the image of the
sun falls on the circle at the point corresponding to the time. When it is desired
that the place to which the image is to be brought should be always the same point
of a moveable screen, I attach the screen to the lens frame and give by hand or
by clockwork nearly enough continuously the proper rotation round the polar
axis. When it is desired that the place of the image be a fixed point of a
screen fixed relatively to the inner azimuth circle, 1 use a mirror inclined to
the polar axis at an angle equal to half the observed body's polar distance, and

page_text: so bring the light to a direction approximately parallel to the polar axis, and I give
by hand or clockwork (as in a heliostat) the proper rotation to the mirrror round
the polar axis. I also use a mirror or lens itself silvered on one side to turn the
light into such a direction that the image may be seen directly in a field marked by
cross wires or other convenient mark without any solid screen or on a semi-
transparent screen, as ground glass. The dispensing with the screen allows a
moderately bright star, or even a barely visible star to be used.
To find the error of the compass by this instrument, arranged according to any
of the modifications herein-before described, place it with the diameter through the
lubber point of the outer azimuth circle parallel to the ship's length, in any con-
venient position for receiving the direct light from the sun or other heavenly body
to be observed. When the inner azimuth circle is set by observation as described
above, read on its inner circle the angle between the lubber point of the outer circle
and the true north, which is the zero of the graduation of the inner circle, and
simultaneously, or as nearly as possible simultaneously, read on the compass card
the angle between its zero or north point and the lubber point of the compass case.
The difference of these readings is the deviation of the compass from the true north.
The deviation may also sometimes be found by aid of the appliance for azimuths
attached to the compass itself, which constitutes the fifth part of my Invention;
but this method fails when the compass is screened from the heavenly body to be
observed by funnel or sail, or by deck house, as is often the case, with a steering
compass, and when it is applicable it requires either trigonometrical calculation or
the use of azimuth tables. The detached azimuth circle constituting the sixth part
of my Invention is free from both of these.
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 Letter Patent is―
First. The compass card constructed as herein-before described and shewn at
Figures and 2, Sheet 1, of the Drawings, by means of which smallness or
reduction of frictional error is attained in mariners' compasses, and in combination
with smaller needles than have hitherto been ordinarily employed in the mariner's
compass, as long a period of free oscillation as is suitable for working well at sea is
obtained as set forth.
Second. The combination of the knife edged bearings g and g of the gimbals, seen
at Figures 3, 4, 5, 6, 7, and 8, Sheet 1 of the Drawings, for supporting the compass
case with the bowl of viscous liquid attached below, whereby there is obtained
sufficient steadiness with very perfect freedom to take up as nearly as possible the
true level, substantially as herein-before described.
Third. The employment in compasses of a large bowl or hemispherical body A,
attached to the bottom of the glazed case containing the compass card, the said bowl
being filled or partially filled with a viscous oil or other liquid, as and for the
purpose set forth.
Fourth. Applying to the improved compass correctors for the quadrantal error,
substantially in the manner herein-before described with reference to the letters B,
C, Figure 4, Sheet 1, of the accompanying Drawings.
Fifth. Applying to the improved compass correctors for the semicircular error as
herein-before described with reference to the letters E, F, and N, P, S, Figures 3,
and 12, Sheet 1, of the Drawings.
Sixth. The arrangement and construction of the correctors herein-before described
and shown at E, F, Figure 3, and in enlarged detail at Figures 9 and 10, Sheet 1, of
the Drawings.
Seventh. Applying to the improved compass a corrector for the heeling error, in
the manner herein-before described and shewn at R, Figure 3, Sheet 1.
Eighth. The improved instrument herein-before described and shewn at Figure 3,
and in detail at Figures 13 and 14, Sheet 1, which I call the azimuth nirror, and

page_text: the application thereof to the compass, constructed in accordance with my said
Invention, and to compasses generally for taking magnetic azimuths of the sun,
or stars, or terrestrial objects, as set forth and without being impeded by the
quadrantal correctors.
Ninth. The detached azimuth apparatus herein-before described and illustrated
under one modification by the several Figures on Sheet 2 of the appended
Drawings, and the employment or application of the same for finding the error of
the compass constructed in accordance with my said Invention, and of compasses
generally by observation of the sun, moon, or stars, substantially as herein-before
set forth.
In witness whereof, I, the said William Thomson, have hereunto set my hand
and seal, this Twenty-first day of September, One thousand eight hundred
and coments civ
WILLIAM THOMSON. (L.S.)

page_text: LETTERS PATENT to Sir William Thomson, Knight, of Glasgow College, Doctor
of Laws and Professor of Natural Philosophy in tho University and College of
Glasgow, in the County of Lanark, North Britain, for the Invention of "IM-
PROVEMENTS IN THE MARINER'S COMPASS, AND IN THE MEANS FOR ASCERTAINING
AND CORRECTING ITS ERRORS."
Sealed
June 1876, and dated the 29th March 1876.
PROVISIONAL SPECIFICATION left by the said William Thomson at the Office
of the Commissioners of Patents on the 29th March 1876.
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. "IMPROVEMENTS IN THE MARINER'S COMPASS, AND IN THE
MEANS FOR ASCERTAINING AND CORRECTING ITS ERRORS."
The objects of this Invention are--
Tint. With smaller needles than in compasses hitherto in practical use to obtain
as long a period of free oscillation as is suitable for working well at sea.
Second. Smallness of frictional error.
Third. Improved gimbals for supporting the compass bowl to give sufficient
steadiness, and to leave it greater freedom to tako up as nearly as possible the true
level.
Fourth. Practical methods for applying correctors for the quadrantal semicircular
and heeling errors.
Fifth. An improved appliance to the compass for taking magnetic azimuths of
sun or stars or terrestrial objects without being impeded by the quadrantal
correctors.
Sixth. Improvements in the method of correcting the compass by observation of
the sun, moon, or stars on a detached azimuth circle.
Steadiness of the compass at sea in stormy weather has hitherto been attained
by making the compass card and needles large and heavy, and sometimes also by
adding weights. The addition of weights is detrimental as increasing the frictional
error, and tending thereby to make the compass follow the motion of the ship when
she moves in azimuth. In a mathematical paper communicated to the British
Association in 1874, and published in the Philosophical Magazine for November
1874, I showed that the largeness of period obtained by great dimensions is favour-
able for steadiness. It follows from these considerations that it is desirable to
TD 077

page_text: obtain as large a moment of inertia as possible with as light compass card, magnets,
and frame as may be.
The compass consists of a light aluminium boss with a central sapphire cap for
the compass to rest on, and a rim of aluminium of from 4 to 9 inches diameter,
according to the size of the compass. There is an even number of holes in the
rim (to save circumlocution I shall call the number 32, being the number I prefer
for my largest size of compass card), and half that number in the circumference of
the boss. The rim and boss are connected together by means of 16 fine silk threads
formning, as it were, 32 spokes, and the compass card is partly supported by these
threads and partly by the rim. Two or four. small magnets having their corre-
sponding ends tied together by silk threads of equal lengths, so that the magnets
may be as nearly parallel as possible, are attached to the rim by means of threo
silk threads. One of these threads is fastened to, the centre of one extreme magnet,
and the other two threads to the ends of the other extreme magnet. The other
end of the first-nientioned thread is attached to the east or west point of the rim,
and those of the remaining threads to two points at equal angular distance on cach
side of the opposite point. Two threads parallel to the north and south line are
stretched between points on each side of the north and south points at such
distances as just to pass clear of the boss. These threads are used to judge by eye
in adjusting the magnets to have their lengths parallel to the north and south line
on the compass card.
The following method is used for giving an equal tension to each of the threads
between the boss and the rim :-
Each of the sixteen threads is passed through a hole in the circumference of the
boss, and stretched thence into two slight notches in the top of the rim,,then down
and through two contiguous holes on the middle of the rim, one end of the thread
is then knotted, and a weight attached to the other end. The rim is placed on a
suitable circular stand, and the boss is worked about until the rim is made truly
circular, and the sapphire cap is truly in the centre. The ends of the threads with
the weights attached are then cemented to the aluminium rim, the weights are cut
off, and the ends are firmly secured by means of other holes in the aluminium rim.
I this obtain a compass which, whilst being extremely light, and yet having a
large radius of gyration, has very small frictional error with small enough magnetic
moment to give a very long period of free vibration. For example, one of my
compasses of 9 inches over all diameter, having two needles, each 21, inches long,
weighs in all 104 grains, and at Glasgow has a period of free vibration 62 seconds,
and extreme static frictional error on either side of the true position only about
one quarter degree.
My third improvement is to swing the gimbals on knife edges instead of on
cylindrical journals as is now generally done, and to calm the vibration by the use
of a pendant in a bowl of oil or liquid, or by means of a very viscous oil or other
liquid without pendant in the manner described below. The object of this
improvement is to give greater freedom to the compass to assume a horizontal
position. A small spirit level is placed in the glazed case, in which the compass is
supported, to show whether the case and bowl are properly balanced. In compasses
ordinarily in use at sea the vibrations of the case due to the rolling of the ship
are calmed by the friction between the cylindrical journals on which the gimbals
are swung, and the brasses or bolsters by which they are supported. In consequence
of this friction the compass does not of itself take up a truly level position. With
the use of knife edges, I allow the compass to take up its truly level position, but
I lose the energy destroying power of the rubbing surfaces. Instead, I use a
large bowl attached to the bottom of the glazed case; the bottom of the case
forming the roof of the bowl. The bowl is nearly filled with liquid. Inside the
bowl a circular disc, with the greater part of the mass at its outer rim, is suspended
by a hook or chain of one or two links from a point a little above the centre of
gravity of the bowl. The disc, by its moment of inertia round a horizontal diameter,
tends in its vibration to lag behind, or even to go in the opposite direction to the

page_text: bowl and oil, and when there is any relative motion of the disc and bowl, energy
is consumed by the intermediate action of the oil. The correctors for the
quadrantal error are founded on the principle first given by the Astronomer Royal.
They are iron cylinders, or ovals, or bundles, of soft iron wire, placed on proper
supports fixed to the sides of the binnacle, having their axes in the proper
direction for correcting the quadrantal error, which in most ships is at right angles
or very nearly at right angles to the fore-and-aft line.
The semicircular error is corrected by steel magnets according to the general
principle also first given by the Astronomer Royal. The varying adjustment of tho
correction required for variation in the ship's sub-permanent magnetism and mag-
netism induced by the vertical component of the earth's magnetic force, I effect in
the following manner-I use two correctors, one for the fore-and-aft force, and
the other for the thwart-ship force. Each corrector consists of two bar magnets
moveable round a common horizontal axis perpendicular to their lengths, and
through or nearly through one sets of their ends.
One of theso is a compound magnet, consisting of two bars rigidly connected
parallel to each other with their poles similarly turned and with a space between
them through which the other may pass. This other has a strength equal to the
sum of the strengths of the two bars of the compound magnet, a condition which is
easily fulfilled with sufficient accuracy by making it also a compound magnet
of two bars laid close beside one another with similar poles contiguous.
One of the two outer bars and the central magnet has each a cog wheel of the
same diameter rigidly attached to its inner end. These cog wheels both revolvo
round the common horizontal axis, and they are connected together by means of
other cog wheels in such a manner that when set in motion they turn in opposite
directions. The pole of the outer or split magnet, and that of the central one at
the common axis, are of contrary names, and so of course also are the poles remote
from the axis. The latter will be called the "moveable poles" or sometimes
simply "the poles" for brevity.
A circular disc with degrees marked on the outer edge is attached to the common
axis to indicate the angle of deflection of the magnets from the vertical, and there
is a clamp attached to one of the outer bars so that when they have been deflected
to any required degree they may be firmly fixed in that position. The cog wheels
attached to the magnets are so connected together that when the two outer bars
are vertical (the ship being supposed to be upright) the central magnet is also
vertical and between the two parts of the compound magnet, and when the com-
pound magnet is deflected through any angle on either side of the vertical the
central one is deflected through an equal angle but on the opposite side of tho
vertical.
When the two magnets are vertical their combined action at a distance is zero,
because the split magnet is equal in strength to the central one, and the poles of
opposite names are contiguous to each other. The horizontal axis round which the
magnets turn for tho fore-and-aft corrector is thwart-ship, and for the thwart-ship
corrector it is fore-and-aft. The correctors are placed one over the other, and both
under the compass, so that in their zero position the middle line of each corrector is
vertically under the centre of the compass (the ship being supposed to be upright).
If the ship's magnetic force is to starboard, the magnets of the thwart-ship
corrector are turned from the vertical position in such a direction as to bring their
moveable north and south poles to the port and starboard sides respectively, or if
the ship's force is to port, the correctors are turned in the opposite directions.
Similarly if the fore-and-aft component is headward, the magnets of the fore-and-
aft corrector are turned so as to bring their north and south poles aft and forward
respectively.
The cog wheels may be dispensed with, and each magnet of the pair constituting
either corrector may be moved to the proper position, and clamped thero simply
by hand.
Or a single corrector, constructed according to the above description, and having

page_text: "
also an azimuthal adjustment, so that the line joining its poles may be brought to
make any angle with the ship's length may be used, and is preferable on tho
ground of simplicity, but is not quite so easily used when the correction is to re-
adjusted frequently..
When there is a greater amount of error to be corrected than can be conveniently
done by the continuously adjustable correctors described above, I use auxiliary
fixed correcting magnets. These magnets I make by preference of steel wire or
round rod 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 the strength of their magnetism. They are made in pairs of equal strength
chained together, and when out of uso are placed in a box so fitted that they cannot
be put into it except with dissimilar poles together. The position in which I prefer
to place a couple of these correctors is as nearly as may bo lovel with the compass,
am at equal distances from the two sides of it, so that the central point of tho
compass may ho nearly in the position of a point of maximum or maximum-minimum
force, so far as the correctora aro concerned. When fixed forandit correctors
are required, their positions may be na closa us possible under the quadrantal
correctors.
For correcting the heeling error, the binnacle contains a proper geometrical
clamp for holding firmly a magnet in a vertical position centrally below the lower
of the two pairs of adjustable semicircular correctors.
My fifth improvement consists of a convex half lens placed over the compass
between the outer circumference, and the centre with its axis directed to tho
graduated circumference of the compass card, and its centre at a distance from the
divisions slightly exceeding the radius of the divided circle. Attached to the lens
there is a plane mirror for observing the image of the object whose azimuth is
required, moveable about a horizontal axis, coinciding with the diametral edge of
the lens. The lens and mirror are placed on a frame which moves round a vertical
axis through, the centre of the compass card. The observer when taking an
azimuth turns the instrument round its vertical axis until the mirror and lens are
fairly opposite to the object. He then places his cyo above the compass and nioves
the mirror about its horizontal axis until the image of the object reflected from tho
mirror is seen upon a division on the compass card seen through the lens. The
point on the compass card where the object was seen will be the bearing of the
object. As the azimuths are taken by looking down on the compass the wooden
troughs which carry the quadrantal correctors do not interfere with the operation.
My sixth improvement relates to the instrument as herein-before mentioned to
be used for finding the true north directly by observations of the sun, moon, or
other bright enough planet or star, for the purpose of ascertaining from time to timo
the errors of the compass varying as that does with the alterations in the ship's
magnetism. It consists of a horizontal metallic circle supported on gimbals, having
an inner circle moveable round a vertical axis. The rim of this circle is graduated
to degrees, and may also be marked with points of the compass, the inner circle
carries with it a portion of a hollow spherical surface. This surface is marked with
a great circle corresponding to the equator, and smaller circles corresponding to
degrees of declination as far as 20 degrees on each side of the equator. These
declination circles are divided into degrees reckoned in hours and minutes of time
at the rate of 4 minutes to the degree.
The spherical surface is moveable round a horizontal axis through its centre of
gravity. A lens is placed with its centre at the centre of this spherical surface, it
is pivoted on a frame, so that its axis may be directed to any point between the
extreme declination circles of the spherical surface. To use the instrument turn
the spherical surface round on its pivots until its equatorial circle is inclined to the
vertical at an angle equal to the latitude of the place, then turn the inner circle
round its vertical axis until the image of the sun or other heavenly body through
the lens falls on a point on the spherical surface which corresponds to the apparent
solar time, or to the hour angle of the body observed, and to the declination of the

page_text: body at the time. The lens is to be turned about on its bearing until the image is
well defined. Either of the conditions abovo specified alone would suffice if the
instrument were perfectly accurate, and thero were no atmosphero refraction and no
sensible parallax. The effects of refraction and parallax aro sufficiently obviated
for all practical purposes by bringing as nearly as may be judged by eye the centre
of the image into the same azimuth with the proper time point of the proper de-
clination circle. When the instrument is correct and well levelled the image of the
centre of the sun will be seen always below the proper point by the amount of
refraction; the centre of the moon's image will be seen above the proper point by
the excess of her parallax above refraction.
To obviate the great number of circles and and divisions on the spherical surface
required by this method I vary it as follows:-Instead of the large concave
spherical surface I use only a narrow, strip containing the equatorial circle, and
broad enough to mark the hours on it; and a comparatively small portion of
concave spherical surface being a sector bounded by two consecutive hour circles,
and by declination circles of 29° on each side of the equator. This sector is marked
with segments of small circles corresponding to degrees of declination, the spaco
between the hour circles is divided into degrees corresponding to 4 minutes of time.
This sector is connected by means of an arm with the frame of the lens,.and is
moveable with the lens round the polar axis of the spherical surface. To use the
instrument set the plane of the equatorial circle for the latitude, then turn the lens
frame with attached arm and sector; round the polar axis until the bounding
meridians of the sector agree with the hours marked on the equatorial circle corre-
sponding to the integral hours of apparent time (by the body observed) before and
after the time of the observation; turn the inner azimuth circle round its vertical
axis until the image of the falls on the point which corresponds to the declination
and time.
Another plan that I for avoiding expense of construction and confusion in
using the instrument caused by too great number of divisions is as follows:-
A single circle is divided for hours and four minutes of time on a narrow strip of
the spherical surface. I fix the frame containing the lens in a geometrical slide so
that the centre of the lens may move in a straight line through the centre of the
circle, and at right angles to the plane of that circle. When using the instrument,
the centre of the lens is moved along the slide to a distance from the centre of the
circle equal to the tangent of the declination; the plane of the circle is set for the
latitude, and the inner azimuth circle is turned round its vertical axis until the
image of the sun falls on the circle at the point corresponding to the time. When.
it is desired that the place to which the image is to be brought should be always
the same point of a moveable screen, I attach the screen to the lens frame, and give
by hand or by clockwork nearly enough continuously the proper rotation round the
polar axis. When it is desired that the place of the image be a fixed point of a
screen, fixed relatively to the inner azimuth circle, I use a mirror inclined to the
polar axis at an angle equal to half the observed body's polar distance, and so bring
the light to a direction approximately parallel to the polar axis, and I give by hand
or clockwork (as in a heliostat) the proper rotation to the mirror round the polar
axis. I also use a mirror or lens itself silvered on one side to turn the light into
such a direction that the image may be seen directly in a field marked by cross-
wires or other convenient mark without any solid screen, or on a semi-transparent.
screen, as ground glass. The dispensing with the screen allows a moderately bright
star or even a barely visible star to be used.
To find the error of the compass, place the detached azimuth circle with the axis
of its gimbals parallel and perpendicular to the ship's length in any convenient
position for receiving the direct light from the sun or other heavenly body to be
observed. When the inner azimuthal circle is set, by observation as described above,
read on the inner circle the angle between the lubber point of the outer circle and
the true north, which is the zero of the graduation of the inner circle, and simultan-
cously, or as nearly as possible simultaneously, read on the compass card the angle

page_text: between its zero or north point and the lubber point of the compass case. Tho
difference of these readings is the deviation of the compass from the true north.
The deviation may also sometimes be found by aid of the appliance for azimuths
attached to the compass itself, which constitutes the fifth part of my Invention; but
this method fails when the compass is screened from the heavenly body to be 5
observed by funnel or sail, or by deck house, as is often the case with a steering
compass, and when it is applicable it requires either a calculation by spherical trigo-
nometry or the use of azimuth tables. The detached azimuth circle constituting
the sixth part of my Invention is free from both of these objections.
SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said William Thomson in the Great Seal Patent Office on the 27th September
1876.
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. "IMPROVEMENTS IN THE MARINER'S COMPASS, AND IN
THE MEANS FOR ASCERTAINING AND CORRECTING ITS ERRORS."
My said Invention, which relates to improvements in the mariner's compass, and in
the means for ascertaining and correcting its errors, consists, and has for its object
as follows:
==
First. With smaller needles than in compasses hitherto in practical use to obtain
as long a period of free oscillation as is suitable for working well at sea.
Second. Sinallness of frictional error.
Third. Improved gimbals for supporting the compass bowl to give sufficient
steadiness, and to leave it greater freedom to take up as nearly as possible the
true level.
Fourth. Practical methods for applying correctors for the quadrantal semicircular
and heeling errors.
Fifth. An improved appliance to the compass for taking magnetic azimuths
of sun or stars or terrestrial objects without being impeded by the quadrantal
correctors.
Sixth. Improvements in the method of correcting the compass by observation
of the sun, moon, or stars on a detached azimuth circle.
Steadiness of the compass at sea in stormy weather has hitherto been obtained
by making the compass card and needles large and heavy, and sometimes also by
adding weights to the compass card. The addition of weights is detrimental by
increasing the frictional error, and tending thereby to make the compass follow the
motion of the ship when she moves in azimuth. In a mathematical paper commu-
nicated to the British Association in 1874, and published in the Philosophical
Magazine for November 1874, I showed that largeness of period of free oscillation is
favourable for steadyness. It follows from these considerations that it is desirable
to obtain as large a moment of inertia as possible with as light compass card,
magnets, and frame, as may be.
My improved compass and appliances connected with it are illustrated on the
two appended Sheets of Drawings, on which Figure 1, Sheet 1, shows one half of
my improved compass card with its magnetic needles; Figure 2 being a transverse
section of the same with the pivot upon which it is supported.
Figure 1 shows in section the manner of connecting the segments of the compass
card together at their inner ends by means of the strips a¹ of gummed paper, whilst
Figures 1 and 1b show the manner of connecting the compass card to the rim by
tongues formed on the card.
The compass consists of a light aluminium boss a, with a central cap b of sapphire,
ruby, or other sufficiently hard material for the compass to rest on, and a rim of

page_text: aluminium from about 4 to 12 inches or moro in diameter, according to the size
of the compass.
The compass card is by preference constructed with an oven number of holes in
the rim (to save circumlocution I shall call the number 32, being the number I prefer
for ny largest size of compass card), and the central boss has the same number of
holes in its circumference. The rin and boss are connected together by means of
32 fine silk threads c, which form as it were 32 spokes. These silk threads or
spokes care knotted, as herein-after described, so as to retain them in the holes
both of the boss and the rim, after which they are cemented thereto by shellac or
other adhesive substance. The compass card is partly supported by these threads c
directly and partly by the rim f, as shown at Figures 1 and 2. Two or more small
magnets d, of which eight are employed in tho compass shown at Figures 1 and 2,
having their corresponding ends tied together by silk threads e of equal lengths, so
that the magnets may be as nearly parallel as possible, are attached to the rim by
means of four silk threads f¹, as shown.
The following method is used for giving an equal tension to each of the threads
between the boss and the rim :-
When connecting the boss and rim together, and so as to get the boss to a truly
central position, the boss is fixed horizontally over the centre of the fixed ring,
herein-after referred to. Each of the 32 threads is passed outwards through a hole
in the rim, where one end is retained by a knot on the inside of the rim, then
upwards and over the rim inwards to a corresponding hole in the boss. Each
thread passes though this hole upwards, and then horizontally outwards over the
stout fixed ring above mentioned, and downwards with a weight attached to it
to stretch it with proper force. The rim is then worked about until it is made
truly circular and concentric with the sapphire cap. After this the threads are
cemented to the boss, after which they are drawn through the holes in the boss a
second time and again cemented and their loose ends cut away. I thus obtain a
compass which, whilst being extremely light, and yet having a large radius of
gyration, has very small frictional error with small enough magnetic moment to
give a very long period of free vibration. For example, one of my compasses of
10 inches over all diameter, having eight needles of from 3 inches to 14 inches long,
weighs in all 178 grains, and at Glasgow has a period of free vibration 38 seconds,
and extreme static frictional error on either side of the true position of less than
one quarter degree.
My third improvement is to swing the gimbals on knife edges, as shown at g and
g, Figures 3 and 4, and in enlarged detail at Figures 5, 6, 7, and 8, Sheet 1, instead
of on cylindrical journals as is now generally done, and to calm the vibration by
the use of a very viscous oil or other liquid contained in the bowl A of the compass.
The object of this improvement is to give greater freedom to the compass to assume
a horizontal position.
In compasses ordinarily in use at sea the vibrations of the case due to the rolling
of the ship are calmed by the friction between the cylindrical journals on which the
gimbals are swung and the brasses or bolsters by which they are supported. In
consequence of this friction the compass does not of itself take up a truly level
position. With the use of knife edges I allow the compass to take up its truly
level position, but I lose the energy destroying power of the rubbing surfaces.
Instead of the rubbing surfaces I use the large bowl A attached to the bottom of
the glazed case, this bottom forming the roof of the bowl, which is nearly filled
with liquid, thus when there is any motion of the bowl energy is consumed by the
frictional or viscous action of the liquid.
The correctors for the quadrantal error are founded on the principle first given
by the Astronomer Royal They are seen at B, Figure 4, Sheet 1, and consist
of iron balls placed in boxes C, fixed to the sides of the binnacle. The semicircular
error is corrected by steel magnets according to the general principle also 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 .

page_text: vertical component of the carth's magnetic force I effect in the following manner:-
I use, as seen more particularly at E, F, Figure 3, Sheet 1, two similar correctors,
one for the fore-and-aft force, and the other for the thwart ship force.
Figures 9 and 10, Sheet 1, are enlarged detail views of one corrector, each of which
consists of two bar magnets G, H, moveable round a common horizontal axis I,
which passes through one of their ends. One of these, the magnet G, is a compound
magnet, consisting of two bars rigidly connected parallel to each other with their
poles similarly turned, and with a space between them through which the other
magnet H passes. The magnet H has a strength equal to the sum of the strength
of the two bars of the compound magnet G, a condition which is easily fulfilled
with sufficient accuracy by making it also a compound magnet of two bars laid close
beside one another with similar poles contiguous. One of the two outer bars G
and the central magnet H has each a spur wheel K of the same diameter rigidly
attached to its inner end. These spur wheels both revolve round the common.
horizontal axis I, one of them being placed loosely thereon, and they are connected
together by means of other spur wheels L, M, in such a manner that when set in
motion they turn in opposite directions. The poles of the outer or split magnet G,
and that of the central one H at the common axis, are of contrary names, and so of
course also are the poles remote from the axis. The latter will be called the
"moveable poles" or simply "the poles" for brevity. A circular disc N, Figure 3,
with degrees marked on the outer edge is arranged concentrically with the common
axis I of the magnets to indicate the angle of deflection of the magnets from the
vertical, and there is a clamp O attached to one of the outer bars G, so that when
they have been deflected to any required degree they may be firmly fixed in that
position. The spur wheels attached to the magnets are so connected together that
when the two outer bars G are vertical (the ship being supposed to be upright) the
central magnet H is also vertical and between the two bars of the compound
magnet G, and when the compound magnet is deflected through any angle on either
side of the vertical the central one is deflected through an equal angle, but on the
opposite side of the vertical, as shown by the dotted lines at Figures 3 and 9, Sheet 1.
When the two magnets are vertical their combined action at a distance is zero,
because the split magnet is equal in strength to the central one, and the poles of
opposite names are contiguous to each other. The horizontal axis I, round which
the magnets turn for the fore-and-aft corrector E is thwart-ship, and for the thwart-
ship corrector F it is fore-and-aft. The correctors, as shown at E and F, Figure 3,
Sheet 1, are placed one over the other, and both under the compass, so that in their
zero position the middle line of each corrector is vertically under the centre of the
compass (the ship being supposed to be upright).
If the ship's magnetic force is to starboard the magnets G, H, of the thwart-ship
corrector Fare turned from the vertical position in such a direction as to bring
their moveable north and south poles (by which I mean the poles similar respectively
to the earth's north and south magnetic poles) to the port and starboard sides
respectively; or if the ship's force is to port the correctors aro turned in the opposite
directions. Similarly if the fore-and-aft component is headward the magnets of
the fore-and-aft corrector E are turned so to bring their north and south poles aft
and forward respectively.
The spur wheels may be dispensed with and each magnet of the pair constituting
either corrector may be moved to the proper position and clamped there simply
by hand. Or a single corrector, constructed according to the above description,
and having also an azimuthal adjustment, so that the line joining its poles may
be brought to make any angle with the ship's length, inay be used, and is preferable
on the ground of simplicity, but is not quite so easily used when the correction is
to be readjusted frequently.
When there is a greater amount of error to be corrected than can be conveniently
done by the continuously adjustable correctors described above, I use auxiliary 5
correcting permanent magnets, as shown at P, Figure 3, and in detail at N, P, S,
in Figure 12. These magnets I make by preference of steel wire, or round rod

page_text: steel, tempered glass hard, and soldered in brass tubes to protect them from rust
and from shocks cr from closo contiguity with other magnets, which might alter
the strength of their magnetism. These magnets are made in pairs of cqual
strength, and with ends so shaped that when out of use they are to be placed in
a box so fitted that they cannot be put into it except with dissimilar poles together.
This may be easily effected by making one end of each bar magnet round and the
other end square. The position in which I prefer to place a couple of these
correctors is, as nearly as may be, level with the compass and at equal distances
from the two sides of it, so that the central point of the compass may be nearly
in the position of a point of maximum or maximum-minimum force so far as the
correctors are concerned.
When fixed fore-and-aft correctors are required their positions may be as close
as possible under the quadrantal correctors B.
For correcting the lieeling error, the binnacle contains an upright with a V-groove
forming a proper geometrical clamp Q, Figure 3, for holding firmly at the proper
height a magnet R, in a vertical position centrally below the lower of the two
pairs of adjustable semicircular correctors.
Ono
Two
My fifth improvement consists of a convex half lens placed over the compass
between the outer circumference and the centre with its axis directed to the
graduated circumference of the compass card, and its centre at a distance from the
divisions slightly exceeding the radius of the divided circle. This half lens is
shown at i, Figure 3, and in enlarged detail at Figures 13 and 14, Sheet 1. At-
tached to the lens i there is a plane mirror j, for observing the image of the
object whose azimuth is required, moveable about a horizontal axis coinciding
with the diametral edge of the lens. The lens i and mirror j aro placed on a
frame, mounted on a proper geometrical slide to give it motion round an axis
through tho centro of the compass card, perpendicular to the glass cover.
rounded footrests in a conical hollow in the centre of the glass cover.
others P, P, press on the glass plate lightly, being caused to do so by the spring 8,
or by the centre of gravity of the frames being very slightly on one side of n.
The observer when taking an azimuth turns this frame round its vertical axis,
until the mirror j and lens i are fairly opposite to the object. He then places his
eye above the compass and moves the mirror j about its horizontal axis until the
image of the object reflected from the mirror is seen upon a division on the compass
card seen through the lens. The point on the compass card where the object is seen
will be the bearing of the object. As the azimuths are taken by looking down on
the compass the wooden troughs which carry the quadrantal correctors do not
interfere with the operation. This appliance I call the azimuth mirror. It is
provided with a spirit level m to show whether the case and bowl are properly
balanced.
My sixth improvement relates to the adjunct as herein-before mentioned, which
I call a detached azimuth circle, to be used for finding the true north directly by
observations of the sun, moon, or other bright enough planet or star for the purpose
of ascertaining from time to time the error of the compass, varying as that does with
the alterations in the ship's magnetism. It consists of a horizontal outer metallic
circle supported on gimbals having an inner circle also moveable round a horizontal
axis at right angles to the axis of movement of the outer circle. The gimbals of
both the inner and outer circle are, as in the case of the gimbals of the compass,
formed with knife edges. The rim of the inner circle is graduated to degrees,
and may also be marked with points of the compass, and this circle carries on a
horizontal axis pivoted on it a portion of a hollow spherical surface. This surface
is marked with a great circle corresponding to the equator, and with circles corre-
sponding to degrees of declination, as far as 20 degrees on each side of the equator.
These declination circles are divided into spaces corresponding to 5 minutes of
time at the rate of a degree and a quarter to five minutes, which divisions are
numbered in hours and minutes, the points in which the declination circles are cut
by a plane perpendicular to the above mentioned horizontal axis being reckoned as

page_text: noon. A lens placed with its centre at the centre of the spherical surface is
pivoted on a frame so that its axis may be directed to any point between
the extreme declination circles of the spherical surface. To use the instrument,
turn the spherical surface round on its pivots until its equatorial circle is inclined
to the vertical at an angle equal to the latitude of the place, then turn the inner
circle round horizontally until the image of the sun or other heavenly body through
the lens falls on a point on the spherical surface, which corresponds to the apparent.
solar time or to the hour angle of the body observed, and to the declination of
the body at the time. The lens is to be turned about on its bearing until the image
is well defined. Either of the conditions above specified alone would suflico if
the instrument were perfectly accurate and there were no atmospheric refraction.
and no sensible parallax. The effects of refraction and parallax are sufliciently
obviated for all practical purposes by bringing as nearly as may bo judged by oye
the centre of the image into the same azimuth with the proper time point of thic
proper declination circle. When the instrument is correct and well levelled the
image of the centre of the sun will be seen always below the proper point by the
amount of refraction; the centre of the moon's image will be seen above the proper
point by the excess of lier parallax above refraction.
To obviate the great number of circles and divisions on the spherical surfaco
required by this method I vary it as follows, and as shown on Sheet 2 of the
Drawings, in which Figure 1 is a transverse section; Figure 2 a plan; Figuro 3
another tranverse section at right angles to Figure 1, and Figure 4 an elevation of the
instrument. Instead of the large concave spherical surface, I use only a narrow strip
A', containing the equatorial circle, and broad enough to mark the hours on it, and a
comparatively small portion of concave spherical sturface, B' being a sector bounded
by two consecutive hour circles and by declination circles of 28° on each side of tho
equator, as seen more particularly in the enlarged detail views, Figures 5, 6, and
7, Sheet 2. This sector B' is marked with segments of small circles corresponding
to every second degree of declination, the space between the hour circles is divided
into spaces corresponding to to 5 minutes of time. This sector is connected by means
of arms C with the frame D of the lens E, and is moveable with the lens round the
polar axis F of the spherical surface. The equatorial circle A', A', pivotted on a
horizontal axis in its own plane is carried on the inner circle I, which is divided into
degrees, and may also be marked with the points of the compass as before described.
To use the instrument set the plane of the equatorial circle for the latitudo of the
place by means of the divided arc G, then turn the lens framo D with attached
arms and sector round the polar axis until the bounding meridians of the sector
agree with the hours marked on the equatorial circle corresponding to the integral
hours of apparent time (by the body observed),, before and after the time of the
observation turn the inner azimuth circle round its axis until the image of the body
observed falls on the point which corresponds to the declination and time.
Another plan that I have for avoiding expense of construction and confusion in
using the instrument caused by too great number of divisions is as follows:-A
single circle is divided for hours and five minutes of time on a narrow strip of the
spherical surface. I fix the frame containing the lens in a geometrical slide so that
the centre of the lens may move in a straight line through the centre of the circle,
and at right angles to the plane of that circle. When using the instrument the
centre of the lens is moved along the slide to a distance from the centre of the circle
equal to the tangent of the declination, the plane of the circle is set for the latitude,
and the inner azimuth circle is turned round its vertical axis until the image of the
sun falls on the circle at the point corresponding to the time. When it is desired
that the place to which the image is to be brought should be always the same point
of a moveable screen, I attach the screen' to the lens frame and give by hand or
by clockwork nearly enough continuously the proper rotation round the polar
axis. When it is desired that the place of the image be a fixed point of a
screen fixed relatively to the inner azimuth circle, I use a mirror inclined to
the polar axis at an angle equal to half the observed body's polar distance, and

page_text: so bring the light to a direction approximately parallel to the polar axis, and I give
by hand or clockwork (as in a heliostat) the proper rotation to the mirrror round
the polar axis. I also use a mirror or lens itself silvered on one side to turn the
light into such a direction that the image may be seen directly in a field marked by
cross wires or other convenient mark without any solid screen or on a semi-
transparent screen, as ground glass. The dispensing with the screen allows a
moderately bright star, or even a barely visible star to be used.
To find the error of the compass by this instrument, arranged according to any
of the modifications herein-before described, place it with the diameter through the
lubber point of the outer azimuth circle parallel to the ship's length, in any con-
venient position for receiving the direct light from the sun or other heavenly body
to be observed. When the inner azimuth circle is set by observation as described
above, read on its inner circle the angle between the lubber point of the outer circle
and the true north, which is the zero of the graduation of the inner circle, and
simultaneously, or as nearly as possible simultaneously, read on the compass card
the angle between its zero or north point and the lubber point of the compass case.
The difference of these readings is the deviation of the compass from the true north.
The deviation may also sometimes be found by aid of the appliance for azimuths
attached to the compass itself, which constitutes the fifth part of my Invention ;
but this method fails when the compass is screened from the heavenly body to be
observed by funnel or sail, or by deck house, as is often the case, with a steering
compass, and when it is applicable it requires either trigonometrical calculation or
the use of azimuth tables. The detached azimuth circle constituting the sixth part
of iny Invention is free from both of these.
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 hercin-before in part recited Letter Patent is-
First. The compass card constructed as herein-before described and shewn at
Figures 1 and 2, Sheet 1, of the Drawings, by means of which smallness or
reduction of frictional error is attained in mariners' compasses, and in combination
with smaller needles than have hitherto been ordinarily employed in the mariner's
compass, as long a period of free oscillation as is suitable for working well at sea is
obtained as set forth.
I
Second. The combination of the knife edged bearings 7 and g' of the gimbals, seen
at Figures 3, 4, 5, 6, 7, and S, Sheet 1 of the Drawings, for supporting the compass
case with the bowl of viscous liquid attached below, whereby there is obtained
sufficient steadiness with very perfect freedom to take up as nearly as possible the
truc level, substantially as herein-before described.
Third. The employment in compasses of a large bowl or hemispherical body A,
attached to the bottom of the glazed case containing the compass card, the said bowl
being filled or partially filled with a viscous oil or other liquid, as and for tho
purpose set forth.
Fourth. Applying to the improved compass correctors for the quadrantal error,
substantially in the manner herein-before described with reference to the letters B,
C, Figure 4, Sheet 1, of the accompanying Drawings.
Fifth. Applying to the improved compass correctors for the semicircular error as
herein-before described with reference to the letters E, F, and N, P, S, Figures 3,
and 12, Sheet 1, of the Drawings.
Sixth. The arrangement and construction of the correctors herein-before described
and shown at E, F, Figure 3, and in enlarged detail at Figures 9 and 10, Sheet 1, of
the Drawings.
Seventh. Applying to the improved compass a corrector for the heeling error, in
the manner herein-before described and shewn at R, Figure 3, Sheet 1.
Eighth. The improved instrument herein-before described and shewn at Figure 3,
and in detail at Figures 13 and 14, Sheet 1, which I call the azimuth mirror, and

page_text: the application thereof to the compass, constructed in accordance with my said
Invention, and to compasses generally for taking magnetic azimuths of the sun,
or stars, or terrestrial objects, as set forth, and without being impeded by the
quadrantal correctors.
Ninth. The detached azimuth apparatus hercin-before described and illustrated
under one modification by the several Figures on Sheet 2 of the appended
Drawings, and the employment or application of the saine for finding the error of
the compass constructed in accordance with my said Invention, and of compasses
generally by observation of the sun, moon, or stars, substantially as herein-before
set forth.
In witness whereof, I, the said William Thomson, have hereunto set my hand
and seal, this Twenty-first day of September, One thousand cight hundred
and seventy-six.
WILLIAM THOMSON. (L.S.)

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, 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, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Royal Letters
Patent bearing date the Twenty-ninth day of March, One thousand eight hundred
and seventy-six, did for Herself, Her heirs and successors, give and grant unto
me, the said Sir William Thomson, her special license that I, the said Sir William
Thomson, my executors, administrators, and assigns, or such others as I, the said Sir
William Thomson, my executors, administrators, and assigns, should agree with and
no others, from time to time and at all times thereafter during the term therein
expressed, should and lawfully might make, use, exercise, and vend, within the United
Kingdom of Great Britain and Ireland, the Channel Islands, and Isle of Man,
an Invention of "IMPROVEMENTS IN THE MARINER'S COMPASS AND IN THE MEANS
FOR ASCERTAINING AND CORRECTING ITS ERRORS," upon the condition (amongst
others) that I, the said Sir William Thomson, by an instrument in writing under my
hand and seal, should particularly describe and ascertain the nature of the said
Invention, and in what manner the same was to be performed, and cause the same
to be filed in the Great Seal Patent Office within six calendar months next and
immediately after the date of the said Letters Patent:
And whereas I, the said Sir William Thomson, by an instrument in writing
under my hand and seal, did declare the nature of the said Invention, and in what
manner the same was to be performed, and caused the same to be filed in the Great
Seal Patent Office on the Twenty-second day of September, One thousand eight
hundred and seventy-six :
And whereas, since filing the aforesaid Specification, I have discovered that some
of the claims as made therein may be construed as comprising more that what was
intended to be claimed as being my Invention, and therefore as they stand they are
too wide, and I am desirous of disclaiming parts of those claims, and otherwise

page_text: 4
altering the Specification in manner herein-after set forth, that is to say, altering the
fourth, fifth, and seventh, claiming clauses contained therein and printed at
page 11 of the printed copy of the said Specification, as printed by the Queen's
Printers, and published at the Great Seal Patent Office, by substituting in each
clause the word "my" for the words "applying to the," and by inserting in each
claim the word "with" after the word "compass" and the word "applied" after the
word "error."
NOW KNOW YE, that I, the said Sir William Thomson, by and with the leave
of Her Majesty's Attorney General, for the reasons aforesaid, do alter the said
Specification as printed by the Queen's Printers, and published at the Great Seal
Patent Office in one thousand eight hundred and seventy-six, by substituting in
each of the fourth, fifth, and seventh claiming clauses the word "My" for the words
Applying to the," and by inserting in each claim the word "with" after the word
compass," and the word "applied" after the word "error."
"
And in order that the effect of the said Disclaimer and Memorandum of Alter-
ation may be clearly understood, I annex hereto a copy of the said Specification
as it will appear when so altered and amended, the preamble and testing clause
being omitted.
My said Invention, which relates to improvements in the mariner's compass
and in the means for ascertaining and correcting its errors, consists and has for its
object as follows:-
First. With smaller needles than in compasses hitherto in practical use, to obtain
as long a period of free oscillation as is suitable for working well at sea.
Second. Smallness of frictional error.
Third. Improved gimbals for supporting the compass bowl, to give sufficient
steadiness, and to leave it greater freedom to take up as nearly as possible the true
level.
.
"
Fourth. Practical methods for applying correctors for the quadrantal, semicircular,
and heeling errors.
Fifth. An improved appliance to the compass for taking magnetic azimuths of
sun, or stars, or terrestrial objects without being impeded by the quadrantal
correctors.
Sixth Improvements in the method of correcting the compass by observation of
the sun, moon, or stars, on a detached azimuth circle.
Steadiness of the compass at sea in stormy weather has hitherto been obtained
by making the compass card and needles large and heavy, and sometimes also by,
adding weights to the compass card. The addition of weights is detrimental by
increasing the frictional error, and tending thereby to make the compass follow the
motion of the ship when she moves in azimuth. In a mathematical paper com-
municated to the British Association in 1874, and published in the Philosophical
Magazine for November, 1874, 1 showed that largeness of period of free oscillation
is favorable for steadiness. It follows from these considerations that it is desireable
to obtain as large a moment of inertia as possible, with as light compass card,
magnets, and frame, as may be.
My improved compass and appliances connected with it are illustrated on the
two appended Sheets of Drawings, on which Figure 1, Sheet 1, shows one half of
my improved compass card with its magnetic needles, Figure 2 being a transverse
section of the same with the pivot upon which it is supported.
Figure 1e shows in section the manner of connecting the segments of the compass
card together at their inner ends by means of the strips a¹ of gummed paper, whilst
Figures 12 and 1b show the manner of connecting the compass card to the rim by.
tongues x formed on the card.
The compass consists of a light aluminium boss a, with a central cap b of sapphire,
ruby, or other sufficiently hard material for the compass to rest on, and a rim of
aluminium from about 4 to 12 inches or more in diameter, according to the size of
the compass.
The
compass
card is by preference.constructed with an even number of holes in
:

page_text: the rim (to save circumlocution I shall call the number 32, being the number I
prefer for my largest size of compass card), and the central boss has the same
number of holes in its circumference. The rim and boss are connected together by
means of 32 fine silk threads c, which form as it were 32 spokes; these silk threads
or spokes c are knotted, as herein-after described, so as to retain them in the holes
both of the boss and the rim, after which they are cemented thereto by shellac or
other adhesive substance. The compass card is partly supported by these threads c
directly, and partly by the rim f, as shown at Figures 1 and 2. Two or more small
magnets d, of which eight are employed in the compass shewn at Figures.1 and 2,
having their corresponding ends tied together by silk threads e of equal lengths, so
that the magnets may be as nearly parallel as possible, are attached to the rim by
means of four silk threads f¹, as shown.
The following method is used for giving an equal tension to each of the threads
between the boss and the rim :-
When connecting the boss and rim together, and so as to get the boss to a truly
central position, the boss is fixed horizontally over the centre of the fixed ring
herein-after referred to. Each of the 32 threads is passed outwards through a hole
in the rim, where one end is retained by a knot on the inside of the rim, then
upwards and over the rim inwards to a corresponding hole in the boss. Each thread
passes through this hole upwards, and then horizontally outwards over the stout
fixed ring above mentioned, and downwards with a weight attached to it to stretch
it with proper force. The rim is then worked about until it is made truly circular
and concentric with the sapphire cap. After this the threads are cemented to the
boss, after which they are drawn through the holes in the boss a second time, and
again cemented, and their loose ends cut away. I thus obtain a compass which,
whilst being extremely light, and yet having a large radius of gyration, has very
small frictional error, with small enough magnetic moment to give a very long
period of free vibration. For example, one of my compasses of 10 inches over all
diameter, having eight needles of from 3 inches to 13 inches long, weighs in all
178 grains, and at Glasgow, has a period of free vibration, 38 seconds, and extreme
static frictional error on either side of the true position of less than one-quarter
degree.
My third improvement is to swing the gimbals on knife edges, as shown at g and
g1, Figures 3 and 4, and in enlarged detail at Figures 5, 6, 7, and 8, Sheet 1, instead
of on cylindrical journals, as is now generally done, and to calm the vibration by
the use of a very viscous oil or other liquid contained in the bowl A of the compass.
The object of this improvement is to give greater freedom to the compass to assumo
a horizontal position.
In compasses ordinarily in use at sea the vibrations of the case due to the rolling
of the ship are calmed by the friction between the cylindrical journals on which the
gimbals are swung and the brasses or bolsters by which they are supported. In
consequence of this friction the compass does not of itself take up a truly level
position. With the use of knife edges I allow the compass to take up its truly level
position, but I lose the energy-destroying power of the rubbing surfaces. Instead
of the rubbing surfaces I use the large bowl A attached to the bottom of the glazed
case, this bottom forming the roof of the bowl, which is nearly filled with liquid;
thus, when there is any motion of the bowl, energy is consumed by the frictional or
vicous action of the liquid.
The correctors for the quadrantal error are founded on the principle first given
by the Astronomer Royal. They are seen at B, Figure 4, Sheet 1, and consist of
iron balls placed in boxes C fixed to the sides of the binnacle. The semicircular
error is corrected by steel magnets, according to the general principle, also, 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 earth's magnetic force I effect in the
following manner :-I use, as seen more particularly at E, F, Figure 3, Sheet 1, two

page_text: similar correctors, one for the fore-and-aft force, and the other for the thwart ship
force.
Figures 9 and 10, Sheet 1, are enlarged detail views of one corrector, each of
which consists of two bar magnets G, H, moveable round a common horizontal axis
I, which passes through one of their ends. One of these, the inagnet G, is a compound
magnet, consisting of two bars rigidly connected parallel to each other, with their
poles similarly turned, and with a space between them through which the other
magnet H passes. The magnet H has a strength equal to the sum of the strength
of the two bars of the compound magnet G, a condition which is easily fulfilled
with sufficient accuracy by inaking it also a compound magnet of two bars, laid
close beside one another with similar poles contiguous. One of the two outer bars
G and the central magnet H has each a spur wheel K of the same diameter rigidly
attached to its inner end. These spur wheels both revolve round the cominon
horizontal axis I, one of them being placed loosely thereon, and they are connected
together by means of other spur wheels L, M, in such a manner that when set in
motion they turn in opposite directions. The poles of the outer or split magnet G
and that of the central one H at the common axis are of contrary names, and so of
course also are the poles remote from the axis. The latter will be called the
"moveable poles" or simply "the poles," for brevity. A circular disc N, Figure 3,
with degrees marked on the outer edge, is arranged concentrically with the common
axis I of the magnets to indicate the angle of deflection of the magnets from the
vertical, and there is a clamp O attached to one of the outer bars G, so that when
they have been deflected to any required degree they may be firmly fixed in that
position.
The spur wheels attached to the magnets are so connected together that when
the two outer bars G are vertical (the ship being supposed to be upright), the
central magnet H is also vertical, and between the two bars of the compound
magnet G, and when the compound magnet is deflected through any angle on either
side of the vertical, the central one is deflected through an equal angle, but on
the opposite side of the vertical, as shown by the dotted lines at Figure 3 and 9,
Sheet 1.
When the two magnets are vertical, their combined action at a distance is zero,
because the split magnet is equal in strength to the central one, and the poles of
opposite names are contiguous to each other. The horizontal axis I, round which
the magnets turn for the fore-and-aft corrector E is thwart-ship, and for the thwart-
ship corrector F it is fore-and-aft.
The correctors, as shown at E and F, Figure 3, Sheet 1, are placed one over the
other, and both under the compass, so that in their zero position the middle line of
each corrector is vertically under the centre of the compass (the ship being supposed
to be upright).
If the ship's magnetic force is to starboard, the magnets G, H, of the thwart-ship
corrector F are turned from the vertical position in such a direction as to bring
their moveable north and south poles (by which I mean the poles similar respectively
to the earth's north and south magnetic poles), to the port and starboard sides
respectively, or if the ship's force is to port, the correctors are turned in the opposite
directions. Similarly, if the fore-and-aft component is headward, the magnets for
the fore-and-aft corrector E are turned so as to bring their north and south poles aft
and forward respectively.
The spur wheels may be dispensed with, and each magnet of the pair constituting
either corrector may be moved to the proper position and clamped there simply by
hand, Or a single corrector, constructed according to the above description, and
having also an azimuthal adjustment, so that the line joining its poles may be
brought to make any angle with the ship's length may be used, and is preferable,
on the ground of simplicity, but is not quite so easily used when the correction is to
be readjusted frequently.
When there is a greater amount of error to be corrected than can be conveniently
done by the continuously adjustable correctors described above, I use auxiliary

page_text: correcting permanent magnets, as shown at P, Figure 3, and in detail at N, P, S, in
Figure 12. These magnets I make by preference of steel wire or round rod steel,
tempered glass hard, and soldered in brass tubes to protect them from rust and from
shocks or from close contiguity with other inagnets, which might alter the strength of
their magnetism. These magnets are made in pairs of equal strength, and with ends so
shaped that when out of use they are to be placed in a box, so fitted that they cannot
be put into it except with dissimilar poles together. This may be easily effected by
making one end of each bar magnet round, and the other end square. The position in
which I prefer to place a couple of these correctors is as nearly as may be level
with the compass, and at equal distances from the two sides of it, so that the
central point of the compass may be nearly in the position of a point of
maximum or maximum-minimun force, so far as the correctors are concerned.
When fixed, fore-and-aft correctors are required; their positions may be as close
as possible under the quadrantal correctors B.
For correcting the healing error the binnacle contains an upright with a V groove
forming a proper geometrical clamp Q, Figure 3, for holding firmly at the proper
height a magnet R in a vertical position centrally below the lower of the two pairs
of adjustable semi-circular correctors.
My fifth improvement consists of a convex half lens placed over the compass
between the outer circumference and the centre, with its axis directed to the
graduated circumference of the compass card, and its centre at a distance from the
divisions slightly exceeding the radius of the divided circle. This half lens is shown
ati, Figure 3, and in enlarged detail at Figures 13 and 14, Sheet 1. Attached to the
lens there is a plane mirror j for observing the image of the object, whose azimuth
is required moveable about a horizontal axis coinciding with the diametral edge of
the lens.
The lens and mirror jare placed on a frame mounted on a proper geometrical
slide to give it motion round an axis through the centre of the compass card
perpendicular to the glass cover. One rounded foot n rests in à conical hollow in
the centre of the glass cover. Two others, P, P¹, press on the glass plate lightly,
being caused to do so by the spring s. or by the centre of gravity of the frames
being very slightly on one side of n. The observer, when taking an azimuth, turns
this frame round its vertical axis until the mirror j and lens i are fairly opposite to
the object. He then places his eye above the compass and moves the mirror j about
its horizontal axis until the image of the object reflected from the mirror is seen
upon a division on the compass card seen through the lens. The point on the
compass card where the object is seen will be the bearing of the object. As the
azimuths are taken by looking down on the compass the wooden troughs which
carry the quadrantal correctors do not interfere with the operation. This appliance
I call the azimuth mirror. It is provided with a spirit level m to show whether
the case and bowl are properly balanced.
"
My sixth improvement relates to the adjunct as herein-before mentioned, which I
call a detached azimuth circle, to be used for finding the true north directly by
observations of the sun, moon, or other bright enough planet or star for the purpose
of ascertaining from time to time the error of the compass, varying as that does
with the alterations in the ship's magnetism. It consists of a horizontal outer
metalic circle supported on gimbals having an inner circle, also moveable round a
horizontal axis at right angles to the axis of movement of the outer circle. The
gimbals of both the inner and outer circle are, as in the case of the gimbals of the
compass, formed with knife edges.
The rim of the inner circle is graduated to degrees, and may also be marked with
points of the compass, and this circle carries on a horizontal axis pivoted on it a
portion of a hollow spherical surface. This surface is marked with a great circle
corresponding to the equator, and with circles corresponding to degrees of declination
as far as 29 degrees on each side of the equator. These declination circles are
divided into spaces corresponding to 5 minutes of time at the rate of a degree and
a quarter to five minutes, which divisions are numbered in hours and minutes, the

page_text: points in which the declination circles are cut by a plane perpendicular to the above-
mentioned horizontal axis being reckoned as noon. A lens, placed with its centre
at the centre of the spherical surface, is pivoted on a frame so that its axis may be
directed to any point between the extreme declination circles of the spherical
surface.
To use the instrument turn the spherical surface round on its pivots until its
equilatorial circle is inclined to the vertical at an angle equal to the latitude of the
place, then turn the inner circle round horizontally until the image of the sun or
other heavenly body through the lens falls on a point on the spherical surface,
which corresponds to the apparent solar time, or to the hour angle of the body
observed, and to the declination of the body at the time. The lens is to be turned
about on its bearing until the image is well defined. Either of the conditions
above specified alone would suffice if the instrument were perfectly accurate, and
there were no atmospheric refraction, and no sensible parallax.
The effects of refraction and parallax are sufficiently obviated for all practical
purposes by bringing as nearly as may be judged by eye the centre of the image
into the same azimuth with the proper time point of the proper declination circle.
When the instrument is correct and well levelled the image of the centre of the
sun will be seen always below the proper point by the amount of refraction; the
centre of the moon's image will be seen above the proper point by the excess of her
parallax above refraction."
To obviate the great number of circles and divisions on the spherical surface
required by this method I vary it as follows, and as shown on Sheet 2 of the
Drawings, in which Figure 1 is a transverse section; Figure 2, a plan; Figure 3,
another transverse section at right angles to Figure 1; and Figure 4, an elevation
of the instrument.
Instead of the large concave spherical surface I use only a narrow strip A¹, con-
taining the equatorial circle, and broad enough to mark the hours on it, and a
comparatively small portion of concave spherical surface, B', being a sector bounded
by two consecutive hour circles, and by declination circles of 28° on cach side of
the equator, as seen more particularly in the enlarged detail views, Figures 5, 6,
and 7, Sheet 2. This sector, B', is marked with segments of small circles corres-
ponding to every second degree of declination; the space between the hour circles
is divided into spaces corresponding to 5 minutes of time. This sector is connected
by means of arms C with the frame D of the lens E, and is moveable with the
lens round the polar axis F of the spherical surface.
The equatorial circle A¹, A¹, pivotted on a horizontal axis in its own plane is
carried on the inner circle I, which is divided into degrees, and may also be marked
with the points of the compass as before described.
To use the instrument, set the plane of the equatorial circle for the latitude of
the place by means of the divided are G, then turn the lens frame D, with attached
arms and sector, round the polar axis until the bounding meridians of the sector
agree with the hours marked on the equatorial circle, corresponding to the integral
hours of apparent time (by the body observed); before and after the time of the
observation turn the inner azimuth circle round its axis until the image of the body
observed falls on the point which corresponds to the declination and time.
Another plan that I have for avoiding expense of construction and confusion in
using the instrument, caused by too great number of divisions, is as follows:-A
single circle is divided for hours, and five minutes of time, on a narrow strip of the
spherical surface. I fix the frame containing the lens in a geometrical slide, so that
the centre of the lens may move in a straight line through the centre of the circle,
and at right angles to the plane of that circle.
When using the instrument the centre of the lens is moved along the slide to a
distance from the centre of the circle equal to the tangent of the declination, the
plane of the circle is set for the latitude, and the inner azimuth circle is turned
round its vertical axis until the image of the sun falls on the circle at the point
corresponding to the time.

page_text: When it is desired that the place to which the image is to be brought should be
always the same point of a moveable screen, I attach the screen to the lens frame,
and give by hand or by clock work, nearly enough continuously the proper rotation
round the polar axis.
When it is desired that the place of the image be a fixed point of a screen fixed
relatively to the inner azimuth circle, I use a mirror inclined to the polar axis at-au
angle equal to half the observed body's polar distance, and so bring the light to a
direction approximately parallel to the polar axis, and I give by hand or clockwork
(as in a heliostat) the proper rotation to the mirror round the polar axis.
I also use a mirror or lens, itself silvered on one side, to turn the light into such
a direction that the image may be seen directly in a field marked by cross wires or
other convenient mark, without any solid screen, or ou a semi-transparent screen, as
ground glass. The dispensing with the screen allows a moderately bright star, or
even a barely visible star to be used.
To find the error of the compass by this instrument, arranged according to any of
the modifications herein-before described, place it with diameter, through the lubber
point of the outer azimuth circle parallel to the ship's length, in any convenient
position for receiving the direct light from the sun or other heavenly body to be
observed.
.
When the inner azimuth circle is set by observation, as described above, read on
its inner circle the angle between the lubber point of the outer circle and the true
north, which is the zero of the graduation of the inner circle, and simultaneously, or
as nearly as possible simultaneously, read on the compass card the angle between its
zero or north point and the lubber point of the compass case. The difference of
these readings is the deviation of the compass from the true north.
The deviation may also sometimes be found by aid of the appliance for azimuths
attached to the compass itself, which constitutes the fifth part of my Invention; but
this method fails when the compass is screened from the heavenly body to be
observed by funnel or sail, or by deck house, as is often the case with a steering
compass, and when it is applicable it requires either trigonometrical calculation or
the use of azimuth tables. The detached azimuth circle, constituting the sixth part
of my Invention, is free from both these.
Having now described and particularly ascertained the nature of my said Inven-
tion, 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 compass card constructed as herein-before described, and shewn at
Figures 1 and 2, Sheet 1, of the Drawings, by means of which smallness or reduction
of frictional error is attained in mariners' compasses, and in combination with
smaller needles than have hitherto been ordinarily employed in the mariners'
compass, as long a period of free oscillation as is suitable for working well at sea is
obtained, as set forth.
Second. The combination of the knife-edged bearings g and g¹, of the gimbals,
seen at Figures 3, 4, 5, 6, 7, and 8, Sheet 1, of the Drawings, for supporting the
compass case, with the bowl of viscous liquid attached below, whereby there is
obtained sufficient steadiness, with very perfect freedom, to take up as nearly as
possible the true level, substantially as herein-before described.
Third. The employment in compasses of a large bowl, or hemispherical body A,
attached to the bottom of the glazed case containing the compass card, the said
bowl being filled or partially filled with a viscous oil or other liquid, as and for the
purpose set forth.
Fourth. My improved compass with correctors for the quadrantal error applied,
substantially in the manner herein-before described, with reference to the letters B, C,
Figure 4, Sheet 1, of the accompanying Drawings.
Fifth. My improved compass with correctors for the semi-circular error, applied

page_text: as herein-before described, with reference to the letters E, F, and N, P, S, Figures
3 and 12, Sheet 1, of the Drawings.
Sixth. The arrangement and construction of the correctors herein-before described
and shown at E, F, Figure 3, and in enlarged detail at Figures 9 and 10, Sheet 1, of
the Drawings.
Seventh. My improved compass with a corrector for the heeling error applied in
the manner herein-before described and shwen at R, Figure 3, Sheet 1.
Eighth. The improved instrument herein-before described and shewn at Figure 3,
and in detail at Figures 13 and 14, Sheet 1, which I call the azimuth mirror, and
the application thereof to the compass, constructed in accordance with my said
Invention, and to compasses generally, for taking magnetic azimuths of the sun or
stars, or terrestrial objects, as set forth and without being impeded by the quad-
rantal correctors.
Ninth. The detached azimuth apparatus herein-before described and illustrated
under one modification by the several Figures on Sheet 2 of the appended Draw-
ings, and the employment or application of the same for finding the error of the
compass constructed in accordance with my said Invention, and of compasses
generally by observation of the sun, moon, or stars, substantially as herein-before
set forth.
.. In witness. whereof, I, the said Sir William Thomson, have hereunto set
my hand and seal, this Ninth day of November One thousand eight hundred
and seventy-seven.
-
I hereby grant my fiat, giving leave to the above-named Sir William Thomson to 2:
file in the Great Seal Patent Office, with the Specification to which the same
relates, the above written Disclaimer and Memorandum of Alteration.
-
Dated this 29th day of Nov. 1877.
JOHN HOLKER

page_text: Filed 30th November 1877.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, Sm 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, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Royal Letters
Patent bearing date the Twenty-ninth day of March, One thousand eight hundred
and seventy-six, did for Herself, Her heirs and successors, give and grant unto
me, the said Sir William Thomson, her special license that I, the said Sir William
Thomson, my executors, administrators, and assigns, or such others as I, the said Sir
William Thomson, my executors, administrators, and assigns, should agree with and
no others, from time to time and at all times thereafter during the term therein
expressed, should and lawfully might make, use, exercise, and vend, within the United
Kingdom of Great Britain and Ireland, the Channel Islands, and Isle of Man,
an Invention of "IMPROVEMENTS IN THE MARINER'S COMPASS AND IN THE MEANS
FOR ASCERTAINING AND CORRECTING ITS ERRORS," upon the condition (amongst
others) that I, the said Sir William Thomson, by an instrument in writing under my
hand and seal, should particularly describe and ascertain the nature of the said
Invention, and in what manner the same was to be performed, and cause the same
to be filed in the Great Seal Patent Office within six calendar months next and
immediately after the date of the said Letters Patent:
And whereas I, the said Sir William Thomson, by an instrument in writing
under my hand and seal, did declare the nature of the said Invention, and in what
manner the same was to be performed, and caused the same to be filed in the Great
Seal Patent Office on the Twenty-second day of September, One thousand eight
hundred and seventy-six:
And whereas, since filing the aforesaid Specification, I have discovered that some
of the claims as made therein may be construed as comprising more that what was
intended to be claimed as being my Invention, and therefore as they stand they are
too wide, and I am desirous of disclaiming parts of those claims, and otherwise

page_text: altering the Specification in manner herein-after set forth, that is to say, altering the
fourth, fifth, and seventh, claiming clauses contained therein and printed at
page 11 of the printed copy of the said Specification, as printed by the Queen's
Printers, and published at the Great Seal Patent Office, by substituting in each
clause the word "my" for the words "applying to the," and by inserting in each
claim the word "with" after the word "compass" and the word "applied" after the
word "error."
NOW KNOW YE, that I, the said Sir William Thomson, by and with the leave
of Her Majesty's Attorney General, for the reasons aforesaid, do alter the said
Specification as printed by the Queen's Printers, and published at the Great Seal
Patent Office in one thousand eight hundred and seventy-six, by substituting in
each of the fourth, fifth, and seventh claiming clauses the word "My" for the words
Applying to the," and by inserting in cach claim the word "with" after the word
'compass," and the word "applied" after the word "error."
"
44
And in order that the effect of the said Disclaimer and Memorandum of Alter-
ation may be clearly understood, I annex hereto a copy of the said Specification
as it will appear when so altered and amended, the preamble and testing clause
being omitted.
My said Invention, which relates to improvements in the mariner's compass
and in the means for ascertaining and correcting its errors, consists and has for its
object as follows:-
First. With smaller needles than in compasses hitherto in practical use, to obtain
as long a period of free oscillation as is suitable for working well at sea.
Second. Smallness of frictional error.
Third. Improved gimbals for supporting the compass bowl, to give sufficient
steadiness, and to leave it greater freedom to take up as nearly as possible the true
level.
Fourth. Practical methods for applying correctors for the quadrantal, semicircular,
and heeling errors.
Fifth. An improved appliance to the compass for taking magnetic azimuths of
sun, or stars, or terrestrial objects without being impeded by the quadrantal
correctors.
Sixth. Improvements in the method of correcting the compass by observation of
the sun, moon, or stars, on a detached azimuth circle.
Steadiness of the compass at sea in stormy weather has hitherto been obtained
by making the compass card and needles large and heavy, and sometimes also by
adding weights to the compass card. The addition of weights is detrimental by
increasing the frictional error, and tending thereby to make the compass follow the
motion of the ship when she moves in azimuth. In a mathematical paper com-
'municated to the British Association in 1874, and published in the Philosophical
Magazine for November, 1874, I showed that largeness of period of free oscillation
is favorable for steadiness. It follows from these considerations that it is desireable
to obtain as large a moment of inertia as possible, with as light compass card,
magnets, and frame, as may be.
My improved compass and appliances connected with it are illustrated on the
two appended Sheets of Drawings, on which Figure 1, Sheet 1, shows one half of
my improved compass card with its magnetic needles, Figure 2 being a transverse
section of the same with the pivot upon which it is supported.
Figure 1 shows in section the manner of connecting the segments of the compass
card together at their inner ends by means of the strips a¹ of gummed paper, whilst 5
Figures 10 and 1b show the manner of connecting the compass card to the rim by
tongues formed on the card.
The compass consists of a light aluminium boss a, with a central cap 6 of sapphire,
ruby, or other sufficiently hard material for the compass to rest on, and a rim of
aluminium from about 4 to 12 inches or more in diameter, according to the size of 5
the compass.
The compass card is by preference constructed with an even number of holes in

page_text: he rim (to save circumlocution I shall call the number 32, being the number I
refer for my largest size of compass card), and the central boss has the same
umber of holes in its circumference. The rim and boss are connected together by
means of 32 fine silk threads c, which form as it were 32 spokes; these silk threads
or spokes c are knotted, as herein-after described, so as to retain them in the holes
both of the boss and the rim, after which they are cemented thereto by shellac or
other adhesive substance. The compass card is partly supported by these threads c
directly, and partly by the rim f, as shown at Figures 1 and 2. Two or more small
magnets d, of which eight are employed in the compass shewn at Figures 1 and 2,
having their corresponding ends tied together by silk threads o of equal lengths, so
that the magnets may be as nearly parallel as possible, are attached to the rim by
means of four silk threads f¹, as shown.
с
The following method is used for giving an equal. tension to each of the threads
between the boss and the rim :-
When connecting the boss and rim together, and so as to get the boss to a truly
central position, the boss is fixed horizontally over the centre of the fixed ring
herein-after referred to. Each of the 32 threads is passed outwards through a hole
in the rim, where one end is retained by a knot on the inside of the rim, then
upwards and over the rim inwards to a corresponding hole in the boss. Each thread
passes through this hole upwards, and then horizontally outwards over the stout
fixed ring above mentioned, and downwards with a weight attached to it to stretch
it with proper force. The rim is then worked about until it is made truly circular
and concentric with the sapphire cap. After this the threads are cemented to tho
boss, after which they are drawn through the holes in the boss a second time, and
again cemented, and their loose ends cut away. I thus obtain a compass which,
whilst being extremely light, and yet having a large radius of gyration, has very
small frictional error, with small enough magnetic moment to give a very long
period of free vibration. For example, one of my compasses of 10 inches over all
diameter, having eight needles of from 3 inches to 13, inches long, weighs in all
178 grains, and at Glasgow, has a period of free vibration, 38 seconds, and extreme
static frictional error on either side of the true position of less than one-quarter
degree.
My third improvement is to swing the gimbals on knife edges, as shown at gand
g1, Figures 3 and 4, and in enlarged detail at Figures 5, 6, 7, and 8, Sheet 1, instead
of on cylindrical journals, as is now generally done, and to calm the vibration by
the use of a very viscous oil or other liquid contained in the bowl A of the compass.
The object of this improvement is to give greater freedom to the compass to assume
a horizontal position.
In compasses ordinarily in use at sea the vibrations of the case due to the rolling
0 of the ship are calmed by the friction between the cylindrical journals on which the
gimbals are swung and the brasses or bolsters by which they are supported. In
consequence of this friction the compass does not of itself take up a truly level
position. With the use of knife edges I allow the compass to take up its truly level
position, but I lose the energy-destroying power of the rubbing surfaces. Instead
5 of the rubbing surfaces I use the large bowl A attached to the bottom of the glazed
case, this bottom forming the roof of the bowl, which is nearly filled with liquid;
thus, when there is any motion of the bowl, energy is consumed by the frictional or
vicous action of the liquid.
The correctors for the quadrantal error are founded on the principle first given
50 by the Astronomer Royal. They are seen at B, Figure 4, Sheet 1, and consist of
iron balls placed in boxes C fixed to the sides of the binnacle. The semicircular
error is corrected by steel magnets, according to the general principle, also, first
of the correction
given by the Astronomer Royal. The varying adir
and magnetism
required for variation in the ship's sub-permanen
55 induced by the vertical component of the earth's magnetic force I effect in the
following manner :-I use, as seen more particularly at E, F, Figure 3, Sheet 1, two
.
41

page_text: similar correctors, one for the fore-and-aft force, and the other for the thwart ship
force.
Figures 9 and 10, Sheet 1, aro enlarged detail views of one corrector, each of
which consists of two bar magnets G, H, moveable round a common horizontal axis
I, which passes through one of their ends. One of these, the magnet G, is a compound
magnet, consisting of two bars rigidly connected parallel to each other, with their
poles similarly turned, and with a space between them through which the other
magnet H passes. The inagnet H has a strength equal to the sum of the strength
of the two bars of the compound magnet G, a condition which is easily fulfilled
with sufficient accuracy by inaking it also a compound magnet of two bars, laid
close beside one another with similar poles contiguous. One of the two outer bars
G and the central magnet H has each a spur wheel K of the same diameter rigidly
attached to its inner end. These spur wheels both revolve round the cominon
horizontal axis I, one of them being placed loosely thereon, and they are connected
together by means of other spur wheels L, M, in such a manner that when set in
motion they turn in opposite directions. The poles of the outer or split magnet G
and that of the central one H at the common axis are of contrary names, and so of
courso also are the poles remote from the axis. The latter will be called the
"moveable poles" or simply "the poles," for brevity. A circular disc N, Figure 3,
with degrees marked on the outer edge, is arranged concentrically with the common
axis I of the magnets to indicate the angle of deflection of the magnets from the
vertical, and there is a clamp O attached to one of the outer bars G, so that when
they have been deflected to any required degree they may be firmly fixed in that
position.
The spur wheels attached to the magnets are so connected together that when
the two outer bars G are vertical (the ship being supposed to be upright), the
central magnet H is also vertical, and between the two bars of the compound
magnet G, and when the compound magnet is deflected through any angle on either
side of the vertical, the central one is deflected through an equal angle, but on
the opposite side of the vertical, as shown by the dotted lines at Figure 3 and 9,
Sheet 1.
When the two magnets are vertical, their combined action at a distance is zero,
because the split magnet is equal in strength to the central one, and the poles of
opposite names are contiguous to each other. The horizontal axis I, round which
the magnets turn for the fore-and-aft corrector E is thwart-ship, and for the thwart-
ship corrector F it is fore-and-aft.
The correctors, as shown at E and F, Figure 3, Sheet 1, are placed one over the
other, and both under the compass, so that in their zero position the middle line of
each corrector is vertically under the centre of the compass (the ship being supposed
to bo upright).
If the ship's magnetic force is to starboard, the magnets G, H, of the thwart-ship
corrector F are turned from the vertical position in such a direction as to bring
their moveable north and south poles (by which I mean the poles similar respectively
to the earth's north and south magnetic poles), to the port and starboard sides
respectively, or if the ship's force is to port, the correctors are turned in the opposite
directions. Similarly, if the fore-and-aft component is headward, the magnets for
the fore-and-aft corrector E are turned so as to bring their north and south poles aft
and forward respectively.
The spur wheels may be dispensed with, and each magnet of the pair constituting
either corrector may be moved to the proper position and clamped there simply by
hand. Or a single corrector, constructed according to the above description, and
having also an azimuthal adjustinent, so that the line joining its poles may bo
brought to make any angle with the ship's length may be used, and is preferable,
on the ground of simplicity, but is not quito so easily used when the correction is to
bo readjusted frequently.
When there is a greater amount of error to be corrected than can be conveniently
done by the continuously adjustable correctors described above, I use auxiliary

page_text: correcting permanent magnets, as shown at P, Figure 3, and in dotail at N, P, S, in
Figure 12. These magnets I make by preference of steel wire or round rod steel,
tempered glass hard, and soldered in brass tubes to protect them from rust and from
shocks or from close contiguity with other magnott, which might alter the strength of
their magnetism. These magnets are mado in pairs of equal strength, and with ends so
shaped that when out of use they are to be placed in a box, so fitted that they cannot
be put into it except with dissimilar poles together. This may be easily effected by
making one end of each bar magnet round, and the other end square. The position in
which I prefer to place a couple of these correctors is as nearly as may be level
with the compass, and at equal distances from the two sides of it, so that tho
central point of the compass may be nearly in the position of a point of
maximum or maximum-minimun force, so far as the correctors are concerned.
'
When fixed, fore-and-aft correctors are required; their positions may be as close
as possible under the quadrantal correctors B.
For correcting the healing error the binnacle contains an upright with a V groove
forming a proper geometrical clamp Q, Figure 3, for holding firmly at the proper.
height a magnet R in a vertical position centrally below the lower of the two pairs
of adjustable semi-circular correctors.
My fifth improvement consists of a convex half lens placed over the compass
between the outer circumference and the centre, with its axis directed to the
graduated circumference of the compass card, and its centre at a distance from the
divisions slightly exceeding the radius of the divided circle. This half lens is shown
ati, Figure 3, and iu enlarged detail at Figures 13 and 14, Sheet 1. Attached to the
lens there is a plane mirror j for observing the image of the object, whose azimuth
is required moveable about a horizontal axis coinciding with the diametral edge of
the lens.
The lens i and mirror jare placed on a frame & mounted on a proper geometrical
slide to give it notion round an axis through the centre of the compass card
perpendicular to the glass cover. One rounded foot n rests in a conical hollow in
the centre of the glass cover. Two others, P, P¹, press on the glass plate lightly,
being caused to do so by the spring s, or by the centre of gravity of the frames
being very slightly on one side of n. The observer, when taking an azimuth, turns
this frame round its vertical axis until the mirror and lens i are fairly opposite to
the object. He then places his eye above the compass and moves the mirror j about
its horizontal axis until the image of the object reflected from the mirror is seen
upon a division on the compass card seen through the lens. The point on the
coinpass card where the object is seen will be the bearing of the object. As the.
azimuths are taken by looking down on the compass the wooden troughs which
carry the quadrantal correctors do not interfere with the operation. This appliance
I call the azimuth mirror. It is provided with a spirit level m to show whether
the case and bowl are properly balanced.
My sixth improvement relates to the adjunct as herein-before mentioned, which I
call a detached azimuth circle, to be used for finding the true north directly by.
observations of the sun, moon, or other bright enough planet or star for the purpose
of ascertaining from time to time the error of the compass, varying as that does
with the alterations in the ship's magnetism. It consists of a horizontal outer
metalic circle supported on gimbals having an inner circle, also moveable round a
horizontal axis at right angles to the axis of movement of the outer circle. Tho
gimbals of both the inner and outer circle are, as in the caso of the gimbals of the
compass, formed with knife edges.
The rim of the inner circle is graduated to degrees, and may also be marked with
points of the compass, and this circle carries on a horizontal axis pivoted on it a
portion of a hollow spherical surface. This surface is marked with a great cir**
corresponding to the equator, and with circles corresponding to degrees of declin
as far as 29 degrees on each side of the equator. These declination circles aro
divided into spaces corresponding to 5 minutes of time at the rate of a degree and
A quarter to five minutes, which divisions aro numbered in hours and minutes, the

page_text: points in which the declination circles are cut by a plane perpendicular to the above-
mentioned horizontal axis being reckoned as noon. A lens, placed with its centre
at the centre of the spherical surface, is pivoted on a frame so that its axis may be
directed to any point between the extreme declination circles of the spherical
surfacc.
To use the instrument turn the spherical surface round on its pivots until its
equilatorial circle is inclined to the vertical at an angle equal to the latitude of the
place, then turn the inner circle round horizontally until the image of the sun or
other heavenly body through the lens falls on a point on the spherical surface,
which corresponds to the apparent solar time, or to the hour angle of the body
observed, and to the declination of the body at the time. Tho lens is to be turned
about on its bearing until the image is well defined. Either of the conditions
above specified alone would suffice if the instrument were perfectly accurate, and
there were no atmospheric refraction, and no sensible parallax,
The effects of refraction and parallax are sufliciently obviated for all practical
purposes by bringing as nearly as may be judged by eye the centre of the image
into the same azimuth with the proper time point of the proper declination circle.
When the instrument is correct and well levelled the image of the centre of the
sun will be seen always below the proper point by the amount of refraction; the
centre of the moon's image will be seen above the proper point by the excess of her
parallax above refraction.
To obviate the great number of circles and divisions on the spherical surface
required by this method I vary it as follows, and as shown on Sheet 2 of the
Drawings, in which Figure 1 is a transverse section; Figure 2, a plan; Figure 3,
another transverse section at right angles to Figure 1; and Figure 4, an elevation
of the instrument.
Instead of the large concave spherical surface I use only a narrow strip A¹, con-
taining the equatorial circle, and broad enough to mark the hours on it, and a
comparatively small portion of concave spherical surface, B', being a sector bounded
by two consecutive hour circles, and by declination circles of 28° on each side of
the equator, as seen more particularly in the enlarged detail views, Figures 5,6,
and 7, Sheet 2. This sector, B', is marked with segments of small circles corres-
ponding to every second degree of declination; the space between the hour circles
is divided into spaces corresponding to 5 minutes of time. This sector is connected
by means of arms C with the frame D of the lens E, and is moveable with the
lens round the polar axis F of the spherical surface.
The equatorial circle A', A', pivotted on a horizontal axis in its own plane is
carried on the inner circle I, which is divided into degrees, and may also be marked
with the points of the compass as before described.
To use the instrument, set the plane of the equatorial circle for the latitude of
the place by means of the divided are G, then turn the lens frame D, with attached
arms and sector, round the polar axis until the bouuding meridians of the sector
agree with the hours marked on the equatorial circle, corresponding to the integral
hours of apparent time (by the body observed); before and after the time of the
observation turn the inner azimuth circle round its axis until the image of the body
observed falls on the point which corresponds to the declination and time.
Another plan that I have for avoiding expense of construction and confusion in
using the instrument, caused by too great number of divisions, is as follows:-A
single circle is divided for hours, and five minutes of time, on a narrow strip of tho
spherical surface. I fix the frame containing the lens in a geometrical slide, so that
the centre of the lens may move in a straight line through the centre of the circle,
and at right angles to the plane of that circle.
When using the instrument the centro of the lens is moved along the slide to a
distance from the centre of the circle equal to the tangent of the declination, the
plane of the circle is set for the latitude, and the inner azimuth circle is turned
round its vertical axis until the image of the sun falls on the circle at the point
corresponding to the time.

page_text: When it is desired that the place to which the imago is to be brought should ba
always the same point of a moveable screen, I attach the screen to the lens frame,
and give by hand or by clockwork, nearly enough continuously the proper rotation
round the polar axis.
When it is desired that the place of the image bo a fixed point of a screen fixed
relatively to the inner azimuth circle, I use a mirror inclined to the polar axis at an
angle equal to half the observed body's polar distance, and so bring the light to a
direction approximately parallel to tho polar axis, and I give by hand or clockwork
(as in a heliostat) the proper rotation to the mirror round the polar axis.
I also use a mirror or lens, itself silvered on one side, to turn the light into such
a direction that the image may be seen directly in a field marked by cross wires or
other convenient mark, without any solid screen, or on a semi-transparent screen, as
ground glass. The dispensing with the screen allows a moderately bright star, or
oven a barely visible star to be used.
To find the error of the compass by this instrument, arranged according to any of
the modifications herein-before described, place it with diameter through the lubber
point of the outer azimuth circle parallel to the ship's length, in any convenient
position for receiving the direct light from the sun or other heavenly body to be
observed.
When the inner azimuth circle is set by observation, as described above, read on
its inner circle the angle between the lubber point of the outer circle and the true
north, which is the zero of the graduation of the inner circle, and simultaneously, or
as nearly as possible simultaneously, read on the compass card the angle between its
zero or north point and the lubber point of the compass casc. The difference of
these readings is the deviation of the compass from the true north.
The deviation may also sometimes bo found by aid of the appliance for azimuths
attached to the compass itself, which constitutes the fifth part of any Invention; but
this method fails when the compass is screened from the heavenly body to be
observed by funnel or sail, or by deck house, as is often the case with a steering
compass, and when it is applicable it requires either trigonometrical calculation or
the use of azimuth tables. The detached azimuth circle, constituting the sixth part
of my Invention, is free from both thesc.
Having now described and particularly ascertained the nature of my said Inven-
tion, 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 compass card constructed as herein-before described, and shewn at
Figures 1 and 2, Sheet 1, of the Drawings, by means of which smallness or reduction
of frictional error is attained in mariners' compasses, and in combination with
smaller needles than have hitherto been ordinarily employed in the mariners'
compass, as long a period of free oscillation as is suitable for working well at sea is
obtained, as set forth.
Second. The combination of the knife-edged bearings g and g¹, of the gimbals,
seen at Figures 3, 4, 5, 6, 7, and 8, Sheet 1, of the Drawings, for supporting the
compass case, with the bowl of viscous liquid attached below, whereby there is
obtained sufficient steadiness, with very perfect freedom, to take up as nearly as
possible the true level, substantially as herein-before described.
Third. The employment in compasses of a large bowl, or hemispherical body A,
attached to the bottom of the glazed case containing the compass card, the said
bowl being filled or partially filled with a viscous oil or other liquid, as and for the
purpose set forth.
Fourth. My improved compass with correctors for the quadrantal error applied,
substantially in the manner herein-before described, with reference to the letters B, C,
Figure 4, Sheet 1, of the accompanying Drawings.
Fifth. My improved compass with correctors for the semi-circular error, applied

page_text: asherein-before described, with reference to the letters E, F, and N, P, S, Figures
3 and 12, Sheet 1, of the Drawings.
Sixth. The arrangement and construction of the correctors herein-before described
and shown at E, F, Figure 3, and in enlarged detail at Figures 9 and 10, Sheet 1, of
the Drawings.
Seventh. My improved compass with a corrector for the heeling error applied in
the manuer herein-before described and shwen at R, Figure 3, Sheet 1.
Eighth. The improved instrument herein-before described and shewn at Figure 3,
and in detail at Figures 13 and 14, Sheet 1, which I call the azimuth mirror, and
the application thereof to the compass, constructed in accordance with my said
Invention, and to compasses generally, for taking magnetic azimuths of the sun or
stars, or terrestrial objects, as set forth and without being impeded by the quad-
rantal correctors.
Ninth. The detached azimuth apparatus herein-before described and illustrated
under one modification by the several Figures on Sheet 2 of the appended Draw-
ings, and the employment or application of the same for finding the error of the
compass constructed in accordance with my said Invention, and of compasses
generally by observation of the sun, moon, or stars, substantially as herein-before
set forth.
In witness whereof, I, the said Sir William Thomson, have hereunto set
my hand and seal, this Ninth day of November One thousand eight hundred
and seventy-seven.