AN IMPROVED INSTRUMENT FOR DIRECTLY VERIFYING COMPASS INDICATIONS
Year: 1875

page_text: LETTERS PATENT to Duncan McGregor, of Glasgow, in the County
of Lanark, North Britain, Chronometer Maker and Optician, for
the Invention of "AN IMPROVED INSTRUMENT FOR DIRECTLY VERIFYING
COMPASS INDICATIONS."
Sealed the 9th May 1876, and dated the 17th November 1875.
PROVISIONAL SPECIFICATION left by the said Duncan McGregor
at the Office of the Commissioners of Patents, with his Petition,
on the 17th November 1875.
I, DUNCAN MCGREGOR, of Glasgow, in the County of Lanark,
North Britain, Chronometer Maker and Optician,, do hereby declare
the nature of the said Invention for "AN IMPROVED INSTRUMENT FOR
DIRECTLY VERIFYING COMPASS INDICATIONS;" to be as follows, that is to
say :-

page_text: The instrument is designed principally for use at sea, and whilst based
on well known astronomical relations is such as to dispense with the
calculations hitherto employed for deducing results from ordinary
observations, thus avoiding the risk of error to which such calculations
are liable.
The instrument is mounted in a box having a mark for the ship's
head, the box being placed when in use with such mark directed the
same way as the ship's head. A disc with the compass points on it is
carried in gimbals in the box, and is fitted with a heavy pendent weight
to keep the disc as nearly horizontal as possible. On a vertical spindle
at the centre of the compass disc there is a tubular pillar with an arrow
at the bottom and provided with a pinching screw, and this pillar is to
be adjusted with the arrow pointing to the magnetic deviation point
proper to the locality whereat the observation is being made. On the
top of the pillar there is jointed a diametrical bar with a vertical.semi-
circular arc graduated to degrees and passed through a slot in the pillar,
and this bar is by means of the arc adjusted in a position corresponding
to the latitude of the place and the sun's declination combined. A ring
or disc is fixed to the diametrical bar at right angles to the graduated
are, and on this ring or disc, and at right angles to it, are fitted two
plates with cross slits or cross wires. When the parts are properly
adjusted the compass disc is turned round until the light of the sun
passing through one crossed slit or the shadow of one crossed wire falls
exactly on the other, the point of the compass card then opposite the
ship's head mark of the box will be the true magnetic direction of the
ship's head.
SPECIFICATION in pursuance of the conditions of the Letters Patent,
filed by the said Duncan McGregor in the Great Seal Patent
Office on the 16th May 1876.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, DUNCAN 30
MCGREGOR, of Glasgow, in the County of Lanark, North Britain,
Chronometer Maker and Optician, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her
Letters Patent, bearing date the Seventeenth day of November, in the

page_text: year of our Lord One thousand eight hundred and seventy-five, in
the thirty-ninth year of Her reign, did, for Herself, Her heirs and
successors, give and grant unto me, the said Duncan McGregor, Her
special licence, that I, the said Duncan McGregor, my executors,
administrators, and assigns, or such others as I, the said Duncan
McGregor, my executors, administrators, and assigns, should at any
time 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 In-
vention for "AN IMPROVED INSTRUMENT FOR DIRECTLY VERIPYING COMPASS
INDICATIONS," upon the condition (amongst others) that I, the said
Duncan McGregor, my executors or administrators, by an instrument
in writing under my, or their, or one of their hands and seals, 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.
NOW KNOW YE, that I, the said Duncan McGregor, do hereby
declare the nature of my said Invention, and in what manner the
same is to be performed, to be particularly described and ascertained
in and by the following statement in writing, reference being had to
the accompanying Drawings, that is to say :-
My said Invention has for its object the construction of a simple and
easily manipulated instrument for verifying compass indications by
means of observation of the sun's position.
The instrument is designed principally for use at sea, and whilst based
on well known astronomical relations is such as to dispense with the
calculations hitherto employed for deducing results from ordinary
observations, thus avoiding the risk of error to which such calculations
are liable.
The instrument comprises a card or plate having marked on it the
points of the compass, and having in connection with it certain parts
which have to be set to correspond with the latitude of the place at
which the observation is taken, and with the sun's declination at the
time of the observation. The card or plate with the parts on it is then
turned until the shadow of certain cross wires falls on a particular part, .

page_text: and then the card will show the magnetic north or the true north,
accordingly as allowance has or has not been made for the magnetic
variation at the locality in adjusting the parts on the card.
And in order that my said Invention, and the manner of performing
the same, may be properly understood I hereunto append a Sheet of
explanatory Drawings, to be herein-after referred to, and representing
one modification of my improved instrument. In these Drawings the
same reference numerals, shown in red, are used to mark the same or
like parts wherever they are repeated.
Figure 1 is a plan of the instrument; and Figure 2 is a sectional
elevation. The parts are carried by a case or box 1, which is provided
with a mark 2 for the ship's head; and when the instrument is in use.
the box should be placed with this mark directed the same way as the
ship's head. A disc or circular plate 3, with the compass points shown
on its upper surface, is carried in the box by means of a gimbal ring 4
connected to the box by two diametrically opposite pivot pins ŏ and to
a cross bar 21 carrying the card by two other pivot pins 6 at right
angles to the former in the ordinary way. The cross bar 21 which
carries the compass plate or disc 3, and which is indicated by dotted
lines in Figure 1, has a heavy weight 7 fixed to and projecting down
from it to keep the plates as nearly horizontal and as steady as possible.
On a short vertical spindle 8 fixed in the centre of the compass plate 3,
and the upper part of which is indicated by dotted lines in Figure 2,
there is set a partly tubular pillar 9, to which there is attached an
arrow 10 extending over the compass plate 3, and serving for setting
the pillar 9 relatively to the compass plate, according to the magnetic
variation of the locality where the instrument is used:
A graduated arc 11 showing degrees of variation is marked on the
compass plate for the arrow to be set by, and as the arrow shows true
north it must be set on the plate in the reverse direction to the actual
variation, so that the north point on the plate may show the magnetic
north. A pinching screw. 12 is provided for- fixing the pillar 9: to the
spindle 8, and, a groove being turned; in the latter to receive the point
of the screw it secures the pillar to the spindle. To the top of the.
pillar 9 there is jointed, so as to turn about: a horizontal axis, a vertical. a
half circle 13, which. is marked with degrees of latitude; and at its
lower part. this half circle- passes through a groove in the pillar 9;.

page_text: having beneath it on the pillar a mark at which the proper degree of
latitude on the vertical half circle 13 is to be brought.
A pinching screw 14 is provided for fixing the vertical half circle 13
when adjusted. The vertical half circle carries a circle 15, which is
jointed to it, so as to be turnable about an axis at right angles to its
own axis of adjustment on the top of the pillar 9; and this circle 15 is
graduated to degrees, and has mounted on its centre a diametrical
bar 16, which can be turned round and fixed in any position by a
screw 17. The diametrical bar 16 is formed with vanes 18, 19, pro-
jecting at right angles from its ends; and in one vane 18 there is a
circular opening crossed by wires, whilst in the other 19 there is a
small central hole, and on the inner face of it two cross marks. There
is also a point 20 at one end of the diametrical bar 16, and the bar is
to be set with this point at the proper degree for the sun's declination
at the time of making the observation.
When making an observation the adjustments for latitude and
declination having been made, as herein-before described, the compass
plate 3 is to be turned and the circle 15 raised or depressed until the
shadow cast by the cross wires at 18 falls exactly on the cross at 19 on
the cross bar 16. When this result is obtained the compass plate 3 will
show the true magnetic north and other points, and a comparison of it
with the ordinary compass will show whether and to what extent the
latter deviates. The compass plate 3 is fitted so that it can be turned
about a centre pin fixed in the cross bar 21, two pins, 22, being fixed
in it at the east and west points to turn it by, and it carries round with
it the pillar 9 and parts thereon.
A clinometer is fitted to the back of the box 1, as indicated by dotted
lines at 23 in Figure 2, and consisting of a pivoted pointer with a
loaded bottom end and a graduated arc, over which the pointer moves.
This clinometer serves to show the transverse inclination or heel of the
ship to port or starboard at the time of taking an observation, so that
differences in the magnetic deviation when the ship heels to different
extents can be observed and noted. Instead of using a separate
clinometer the gimbal ring 4 might be set on fore-and-aft pivots, and
be provided with scales of degrees at the sides, which would show the
heel by the extent to which one side or the other rises above the rim of
the box,'

page_text: It will be obvious to any one sufficiently well acquainted with the
astronomical relations on which my improved instrument is based that the.
parts may be modified in various ways whilst still retaining substantially
the same modes of action. Thus for the half circle 13 there might be
substituted a vertical circle fitting and capable of turning about a hori-
zontal axis in an external ring or holder fixed to the variation arrow 10
on the compass plate 3, such vertical circle having one half of it marked
with degrees of latitude. Then for the upper circle 15 there might be
substituted a simple diametrical bar jointed within the vertical circle,
so as to turn about an axis passing through the 90° points of that circle,
and having on it the bar 16. with its vanes 18, 19. The vertical circle
being set to the latitude of the place and clamped, the cross bar 16 is
set to the declination by means of the scale on the vertical circle.
A semicircle marked with hours may be fixed at the lower equator
point of the vertical circle, and at right angles to that circle; and a
pointer on the diametrical bar (substituted for the circle 15) will serve
to show the time of day when the diametrical bar is turned into the
position for bringing the shadow of the cross wires 18 on the cross 19.
The modification shown in the Drawings can also be used for ascer-
taining the time of day from the sun.
When an observation is made at or near noon the vertical half
- circle 13 may be set to the latitude, plus or minus the declination, as
the case may be, and without a separate setting for the declination;
but by setting the bar 16, or its equivalent, separately for the declina-
tion a chance of error is avoided, and the instrument will give an
accurate result. at any time whatever when the sun's rays can reach it.
Having thus particularly described my said Invention, and the
manner of performing the same, I have to state that I do not claim the
separate parts, excepting as herein-after stated, but that what I believo
to be novel and original and claim as the Invention secured to me by
the herein-before in part recited Letters Patent is,-
1. The combining together of the parts of the instrument, sub-
stantially in the manner and for the purposes herein-before described.
2. The combining of the vertical half circle or circle (13), graduated
to show latitude, with the pillar (9) in the manner and for the purpose
herein-before described.

page_text: 3. The combining of the declination circle (15) with the latitude
circle (13) in the manner and for the purpose herein-before described.
4. The fitting of cross wires in the vane (18) combined with the cross
marked on the vane (19), as herein-before described.
In witness whereof, I, the said Duncan M'Gregor, have here-
unto set my hand and seal, this Fifteenth day of May, in the
year of our Lord One thousand eight hundred and seventy-
six.
10 Witness,
EDMUND HUNT,
Patent Agent,
... Glasgow.
D. MCGREGOR. (L.S.)