AN INSTRU- MENT FOR ASCERTAINING THE VARIATION OF THE COMPASS," WHICH I DENOMINATE: "THE MERIDIAN DECLINATION DIAL
Year: 1819

page_text: TO ALL TO WHOM THESE PRESENTS SHALL COME, I, GEORGE
ATKINS; of Hornsey Road, in the Parish of Islington, in the County of
Middlesex, Gentleman, send greeting...
WHEREAS His most Excellent Majesty King George the Third did,
by His Letters Patent under the Great Seal of the United Kingdom : of
Great Britain and Ireland, bearing date at Westminster, the Eighteenth day
of May, in the fifty-ninth year of His reign, 'give and grant unto me; the
said George Atkins, my exors, admors, and assigns, His especial licence,
full power, sole privilege and authority, that I, the said George: Atkins, my.
exors, adiñors, and assigns, during the term of years therein mentioned,
should and lawfully might, make, use, exercise, and vend; within. England,.
Wales, and the Town of Berwick-upon-Tweed, my Invention of "AN INSTRU-
MENT FOR ASCERTAINING THE VARIATION OF THE COMPASS," WHICH I DENOMINATE:
"THE MERIDIAN DECLINATION DIAL;" in which said. Letters Patent there is.
contained, a proviso, that if I,i the said George Atkins, shall not. particularly
describe and ascertain the nature of my said: Invention, and in what manner
the same is to be performed, by an instrument in writing under my hand.
and: seal, and cause the same to be inrolled in His Majesty's High Court:
of Chancery within six calendar months next and immediately after the:
date of the said Letters Patent, that then the said Letters. Patent, and all

page_text: liberties and advantages whatsoever thereby granted, shall utterly cease
determine, and become void, as in and by the same (relation being there-
unto had), will more fully and at large appear.
NOW KNOW YE, that in compliance with the said proviso, I, the said
George Atkins, do hereby declare that the nature of my said Invention, for
more effectually ascertaining the variation of the compass, is fully described
and explained in the following description and the Drawings hereunto annexed,
videlicet:-
My Invention consists of certain improvements in magnetic needles, by
which the magnetic meridian either at sea or on land may be ascertained
with greater accuracy, and a machine or instrument, which, in conjunction
with the magnetic needle, is calculated to ascertain with facility the magnetic
variation. One part of my improvement in magnetic needles consists in
applying two steel arms to the needles usually termed bar needles (see Drawing
Fig. 1), which arms are to be fixed by screws or rivets to the south half of the
needle, forming equidistant angles of about sixty degrees with the north end of
the needle. It is well known that an ordinary straight needle is subject to a
magnetic force or power of attraction which increases in intensity in propor-
tion as the needle diverges from its point of rest to an angle of 90° on either
side of the meridian. Hence it will be obvious, that a needle with two addi-
tional arms (having north polarity), extending from the center at considerable
angles with the north end, will give the needle much greater magnetic force,
and consequently greater steadiness at sea than any needle of the usual con-
struction. The arms of my needles are to be made of two pieces of steel wire
(exactly equal both in length and weight) a little shorter in redius, and lighter
in substance than the north end of the needle. The south end must of course
be sufficiently heavy to counterpoise the three north ends. The middle bar of
the needle, see Fig. 2, is the best form, in order to have the principal weight
near the extremities, which is a desirable point in the construction of magnetic
needles. It is adviseable to set the arms at angles not exceeding 70° from the
north end of the needle, which allows 20° for oscillation on either side the
meridian without losing any of the aggregate power of the three north arms.
Neither should the arms be fixed at less than forty-five degrees in order to
preserve their magnetism from being influenced by the north end of the bar.
My improvement of arms in the construction of magnetic needles is applicable
to all nautical and mathematical instruments in which the compass forms
a component part; but I have an additional improvement applicable to the
needles of steering or binnacle compasses, by means of which the magnetic
meridian, and consequently the variation, may be defined with great accuracy,

page_text: even in rough or stormy weather. The needle is to be made of the form
Fig. 3, or any other segment of a circle, or of an elipse, or of a polygonal
figure, having a ring or aperture at bottom to admit the central point, and two
horizontal ears at the ends, to which the card is attached. The extremities of
the needle being magnetized, and the card being fixed by pins or rivets to the
ends of the needle, two arms of the form N, N, Fig. 1 (being also magnetized),
are to be fixed by cement or otherwise to the under side of the card, at equi-
distant angles from the north end of the needle, in the manner before mentioned;
a thin strip of brass may extend across the needle from ear to ear to support
and strengthen the card. This needle may be applied to the brass box or kettle
of the ordinary steering compass, see Figure 3, K, K,) or (by inverting the
card) it may apply to the pendant or hanging compass.
It will be readily
seen from the figure of this needle that it must possess decided advantages
over the horizontal needles, in which the magnetism is diffused through the
whole bar, whilst in this needle the middle of the card is insulated, and the fluid
condensed in a vertical line near the extremities (N, N, S, S). The weight
being entirely below the centre of suspension, this needle will maintain its hori-
zontal position, and not be liable to be dismounted in the most stormy weather.
It will also be less liable to be influenced by the attraction called dip than any
needle of the ordinary construction.
The other part of my Invention consists of an instrument or machine
for ascertaining by astronomical observation the true terrestrial meridian
at sea, and by comparison of which, with the magnetic meridian determined
by the needle before mentioned, shewing on inspection the amount of the
magnetic variation.
Fig. 4 is a section of the instrument suspended in its box or case with
the cover detached. A, A, is a brass collar with a shoulder or flanch beneath,
which supports the plate B, B. Above the plate the collar receives the
limb C, C, which at one extremity carries a long sight, furnished with a
thread or wire, and standing at an elevation of about 60° when the instrument
is in use.
At the other end of the limb is a short sight, perforated down
the middle, and having a shade glass to protect the eye from the sun. The
cap over the collar has a point which carries a magnetic needle or card D, D,
which is covered by a glass and rim E, E. The card and rim may be detached
at pleasure, which enables the instrument to be used (by comparing its indi-
cations with those of the binnacle compass) on any part of the ship's deck,
even in contact with masses of iron, where the azimuth compass would be
totally deranged. The collar is furnished with two hardened steel screws, the
points of which work against the sides of a ring of hard bronze I, I. Two

page_text: other screws pass through the sides of the ring at right angles with the former,
and work against holes, in the head of the pillar F, but which cannot be shewn
in the section. The pillar may be made of brass with the head of hard bronze,
and to be firmly screwed to the bottom of the box. A' cone made of thin
brass, copper, or tin plate G, G, is to be fixed by screws to the under side of
the collar, having a ring of lead round its lower edge of sufficient weight to
poise the instrument steady horizontal at sea. The point I is an index
affixed to the box, to which any required degree of the dial plate is to be set
when the instrument is in use, and the feet K, K, serve to fix the box by pins
to any platform or other part of the ship's deck. The dial plate is to be
divided and engraved with the rhumb lines or points and degrees similar to the
card of the mariner's compass.. The instrument must be accurately adjusted
so as to maintain its horizontal level when the plate and limb are turned on the
axis, and the workmanship should be so well executed that a line drawn
through the centre of the limb shall intersect the center of the two sights, and
fall on corresponding degrees on the edge of the dial plate:..
Fig:15. is a section of. a different mode of suspension... The central collar
forms a semicircular, cap which works. on the head of the pillar F. An arm,
projecting from beneath the collar, carries a vertical: crotch A, having a slit or
notch which slides freely (but without any shake) on a steel fixed to the side of
the box. The arm B serves to counterpoise the opposite arm. The cone and
weight G..G, and the plate and. limb B and C, are similar to Fig. 4, with the
compass card and ring. removed.
In Fig. 6 the central collar carries four arms to a ring A, A, nearly
the same diameter as the plate. The ring is suspended on the points of
two steel screws, which pass through a gimbel ring B, B, the gimbel ring
also working on the points of two other screws affixed to the box at right
angles. The suspension will be. more perfect if two or more. gimbel. rings
be employed. The weight C must be sufficient to poise the instrument
steadily horizontal. The upper part of the instrument is as above described.
The small section Fig. 7 represents the instrument Fig. 5 with the steering
compass Fig. 3 fitted in different compartments of the same box of a paral-
lelogram form. This is probably a more convenient construction of the
instrument than having the horizontal needle over the center, as in Fig. 4.
It is also more convenient than having the needle in a detached box; as
Fig..3, excepting in certain cases where the proximity of iron might render it
necessary...
i
Although either of the before-mentioned. methods of construction is recom-
mended for great accuracy, yet the following economical construction of

page_text: the instrument might be sufficient for the purposes of ordinary navigation:-
A block or platform of mahogany or other well-seasoned wood is. to be turned
to nearly the size of the dial plate, the block to be suspended in the box by
the ordinary gimbel rings. The plate may be of brass or tin plate, having a
copper-plate impression of a compass card affixed. A vertical shank, about
four inches long, passes through the center of the plate and block, and at the
lower end receives the poise weight, as in Fig. 6. The limb may be made
without the short sight, and the long sight may be fixed (or made to detach)
from either the extremity of the limb or over the axis. The meridian declina-
tion dial is calculated to serve as a steering compass, and shew the magnetic
variation at the same time, and is to be used as follows:-Fix the box
on the capstan, or any platform on the ship's deck open to the sun, with
the feet standing in a line drawn paralell with the keel, the index of the
box being placed towards the ship's head; the ship being kept steady on any
given point of the card turn the dial plate to correspond; or if the instrument
be used without the needle and card, set the same point of the dial plate to the
index as that on which the ship is steering by a good binnacle compass.
Having by the usual methods ascertained the time when the sun is on the
meridian, raise the long sight to the sun, and turn the limb so as to bring the
shadow of its thread on the middle of the short limb. It is obvious that when
the sun is on the meridian this shadow will feel due north, and will indicate the
angle of variation between the north point of the plate, which is on the mag-
netic meridian, and the line of the sun's shadow. at noon, or the true meridian.
The eye is to be applied to the short sight, bringing the long sight to biscet
the object if the sun be partially obscured, or the moon or a star be observed
on their passing the meridian. If an observation cannot be taken when the
object is on the meridian, it would be sufficiently accurate within half an hour
by deducting one degree in the angle of the variation for every four minutes
past meridiau time. At any other time the sun's azimuth must be computed
to be perfectly correct, although the amplitude .tables gives the sun's place, so
as to enable this instrument to be used with sufficient accuracy at sunrise and
sunset without the trouble of computation.
The advantages of the before-mentioned instrument over the azimuth com-
pass are, that it may be used (without the heedle) even in the immediate
vicinity of iron; that it only partakes of the same horizontal motion as the ship,
whilst the arc of vibration of the compass card is frequently three times greater
than that of the ship; and in high latitudes, where the compass needle is so
greatly deranged and almost paralized in its action, the ship's course might be
determined by this instrument, without the aid of any compass, by frequently

page_text: finding the sun's agimuth, and thence deducting the ship's bearing with the
terrestial meridian. The meridian declination dial will also be found advan-
tageous in estimating angular distances, and for various other purposes
connected with land surveying and navigation.
In witness whereof, I, the said George Atkins (party hereto), have
hereunto set my hand and seal, this Sixteenth day of November, in
the year of our Lord One thousand eight hundred and nineteen.