Improvements in Great Circle Course Indicators and Calculators.
Year: 1898

page_text: Date of Application, 27th June, 1898-Accepted, 13th Aug.,
1898
COMI LIVE SPECIFICATION.
Improvements in Great Circle Course Indicators and Calculators.
I, STEPHEN ROGERS KIRBY, Ship Builder, of 1105, Broadway, in the City of
New York, Borough of Manhattan, in the County and State of New York,
United States of America, do hereby declare the nature of my invention, and in
what manner the same is to be performed, to be particularly described and ascer-
tained in and by the following statement:-
My invention relates to a device intended to assist navigators in determining
the direction and location of a great circle between any two points in order that
they may steer their ship over the course making the shortest distance between
said points.
My invention comprises the novel features hereinafter shown and described.
Reference is to be had to the accompanying drawings forming a part of this
specification in which similar characters of reference indicate corresponding parts
in all the figures.
Figure 1 is a top plan view of my device; and
Figure 2 is a sectional elevation taken upon the line 2-2 of Figure 1.
It is a universally known fact among navigators that an arc of a great circle
is the shortest distance between two points and that when possible to do so, ships
are sailed upon the arcs of great circles. The direction and location of the great
circles have heretofore been determined principally by calculations which involve
complicated figuring, or by graphical methods which necessitate special charts
which, at their best, are but approximations to accuracy.
The object of my invention is to produce a device which shall be accurate
and which may be manipulated by any person to quickly determine the direction
of a great circle at any point. Being easily operated they will be used to
determine the direction of a circle at very short intervals and thus enable the
navigator to sail very accurately upon the great circle.
My device comprises an equatorial arc which is connected with meridian arcs,
the latter being connected to a polar pivot so that the meridians may be swung
to any point desired. The polar pivot is also mounted upon a meridian plane
which is pivoted at a point representing the centre of the earth so that the pole
may be swung in this meridian plane to adjust the device for any latitude.
Passing through a central point which represents the ship's position is a great
circle are which has a pivot located in the meridian plane and extending towards
the centre upon which said plane is pivoted, that is, towards the centre of the
earth.
A suitable frame-work A is provided upon which is mounted a standard A³.
In this standard is pivoted a plate C by a pivot C. This plate represents a
meridian plane or a plane which passes through the pole and the centre of the
earth. The plate C is preferably made to extend over about 90° or the quarter
of a circle, the pivot C being near the corner of the plate. This pivot C₁ repre-
sents the centre of the earth. Upon one side of the plate C is an arm C which
supports the polar pivot P. This pivot is in line with the central pivot (" and

page_text: represents a pole of the earth. Two meridian arcs F-F extend from the polar
pivot and at their opposite ends are connected with the ends of an equatorial
arc E. The meridian arcs, of course, extend through 90° of a circle and have a
scale marked upon their face, said scale being laid off in degrees. The equatorial
arc may be of any suitable length but is preferably made as shown in the draw-
ings, to extend through an arc of about 110°, said are being sufficient to cover
the distance between any two points between which it is ordinarily possible to
lay out a sailing course upon an arc of a great circle. This equatorial arc slides
through a groove formed in an extension or arm C carried upon the side of the
meridian plane C opposite that carrying the pole. The surface of this equatorial
arc toward the pole is exactly 90° from said pole. This are is laid out in a
scale of degrees preferably having double numbering, the numbering beginning
from opposite ends. Upon the arm C³ is secured a plate E' which has a vernier
scale. By this means the equatorial arc may be set to 5¹
Upon the upper end of the standard A is a central pivot D¹ which is in line
with the pivot C₁ and supports the great circle arc D. The centre line of this
pivot represents the ship's position at any time. The upper end of this pivot is
provided with a button or circular plate D2 which has a vernier thereon so that
the meridian arcs may be more accurately set. The reading of the vernier is to 5¹.
Clamping screws E secure the equatorial are in any fixed position, and clamping
screws a secure the meridian plane in any fixed position.
The base of the device is provided with a ring A¹ and outside of this is a
ring B which is adjustable, being rotatable about the centre of the device. This
ring B is provided with a scale or preferably with two scales, one laid out to
degrees and the other with the points of the compass. As shown in the drawings
these two scales are used, the outer scale B2, being the one which represents the
degrees, and the inner scale B¹ being the one which is laid out as a compass.
The clamping screw G serves to lock the movable ring B in any position to
which it has been adjusted. The fixed ring A' is provided with a pointer b by
which the movable ring B is adjusted. A second pointer may be attached
to the frame and extend over the outer edge of the ring, one pointer being
intended for use in reading the degrees and the other in reading the compass
points. The great circle arc is also laid out in degrees as indicated, in the
latter reading in this case to 110° on each side of the ship's position. This is
used in determining the length of the course sailed.
My device is used as follows:-
The latitude of the ship being known the meridian plane is set to a position
corresponding with this latitude. This is done in the following manner: The
equatorial arc is loosened so that one of the meridian arcs may be swung so
that its face corresponds with the meridian plane, that is, until its edge is in
line with the zero position of the vernier E³. The screw a which clamps the
meridian plane in position is loosened and the plane adjusted so that the reading
upon the meridian are corresponds with the latitude of the ship. The meridian
plane is then clamped in this position. The equatorial arc is then moved toward
either the cast or west, as the case may be, until its reading corresponds with
the difference in longitude between the port where the ship is and the port
toward which the ship is sailing. This corresponds with the port of sailing
and port of destination or the beginning and end of the voyage. From obser-
vations taken during the voyage the latitude and longitude are to be determined
and this data used in setting the device. The meridian plane having been set
for the latitude of the ship and the equatorial are having been set to represent
the difference in longitude between the two points, the great circle arc is then
turned until the edge thereof, which is provided with the scale, is exactly in line
with the reading upon the meridian arc, corresponding with the latitude of the
port of destination. This are then corresponds with a great circle passing through
the latitude and longitude of the ship and the latitude and longitude of the
port of destination. The direction of the course at that particular point will then

page_text: be obtained by reading from the scale upon the circle B. If, as is usually the
case, the compass does not at that particular point, point directly north the
variation of the compass is allowed for by either adding or subtracting the
proper number of degrees from the reading given, or may be read direct from
the device by setting the ring B to the right or left an amount corresponding
with the variation of the compass. By setting the ring B in this manner the
reading may be taken directly from the device.
As an example of this use we will suppose that the sailing port is in latitude
of 40° 30' and the port of destination in latitude 42°, and the difference in
longitude is 62° and toward the west. Setting the meridian plane to correspond
with the latitude 40° 30' and the equatorial arc to correspond with the difference
of longitude of 620, we then swing the great circle are until it corresponds with
the reading 42° on the meridian arc. We then find the compass reading to be
north 66° 30' west. This is without allowing for any variation in compass which
would have to be added or subtracted as the case may be. The different latitudes
through which this circle will take the ship may be found by freeing the equa-
torial circle and swinging the meridian arc over the great circle are. The
difference in reading upon the meridian arc will indicate the latitudes through
which the ship will pass in its course.
It will be seen that this device is accurate and is one which may be quickly
operated and understood by any ordinary navigator. Its use will enable a
navigator to keep his ship accurately upon a great circle and thus result in much
shortening of sailing courses and consequent saving of time and fuel.
The distance between the two ports upon the arc of a great circle may be read
from the great circle arc. When the instrument has been set to read the course
the number of degrees upon the great circle are indicated by the intersection
of the meridian arc therewith gives the degrees upon the great circle and the
knots may be determined by multiplying this by 60.
It is evident that my device may be constructed and successfully operated
using only one meridian arc instead of two as shown. It is, however, preferred
to use two as shown, as it is more convenient and is a stiffer mechanical
construction.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is:
1. The meridian plane pivoted about an axis lying in the equatorial plane..
2. The meridian are or ares pivoted to the meridian plane by a polar or axial
pivot.
3. The equatorial are secured to the end of the meridian are or ares and adjust-
able therewith about the polar or axial pivot.
4. The great circle are pivoted about an axis lying in the meridian plane.
5. The great circle are having at its pivot an indicator arranged to indicate
on the inner edge of the graduated meridian are which fits over said great
circle arc.
Dated this 27th day of June 1898
A. M. & WM. CLARK,
Chartered Patent Agents, 53, Chancery Lane, London.