A NEW OR IMPROVED INSTRUMENT FOR USE IN ASCERTAINING THE TRUE COURSE OF A SHIP FROM THE COMPASS COURSE
Year: 1876

page_text: LETTERS PATENT to William Henry Roberts, of Snodland, in the County of
Kent, for the Invention of "A NEW OR IMPROVED INSTRUMENT FOR USE IN
ASCERTAINING THE TRUE COURSE OF A SHIP FROM THE COMPASS COURSE."
Sealed the 23rd May 1876, and dated the 10th March 1876.
PROVISIONAL SPECIFICATION left by the said William Henry Roberts at
the Office of the Commissioners of Patents on the 10th March 1876.
WILLIAM HENRY ROBERTS, of Snodland, in the County of Kent.
"A NEW OR
IMPROVED INSTRUMENT FOR USE IN ASCERTAINING THE TRUE COURSE OF A SHIP
FROM THE COMPASS COURSE."
As
Heretofore in correcting a ship's course, that is, in ascertaining the true course
from the course indicated by compass, it has been usual to make the necessary
allowances for leeway, deviation, and variation by arithmetical calculation.
it is necessary to bring all the elements of the calculation to a common denomi-
nation, viz., degrees, in order to make the necessary allowances, serious errors
frequently arise and much time is occupied, which it is the object of my Invention
to obviate.
The new or improved instrument of my Invention consists of a dial similar to that
of a compass having, besides the usual divisions into points, half points, and quarter
points, a circle graduated in degrees. Movable about a pivot at the centre of the
dial is an arm having an index pointing to the points, half, and quarter points of the
compass, and carrying at its outer extremity a short are provided with a vernier
which coincides with the degree in the outer circle of the instrument, corresponding
to the point of the compass at which the index is placed. By means of this index
and vernier the various calculations are made by moving the arm a proper distance
round the dial to the right or left as the case may be, and the result is the true
course which may be read off at a glance to a quarter of a degree. An adjustable
lens or magnifier is mounted on the same pivot to facilitate reading off the minute
divisions with the required accuracy.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said William Henry Roberts in the Great Seal Patent Office on the 9th
September 1876.
WILLIAM HENRY ROBERTS, of Snodland, in the County of Kent.
"A NEW OR
IMPROVED INSTRUMENT FOR USE IN ASCERTAINING THE TRUE COURSE OF A SHIP
FROM THE COMPASS COURSE."
My Invention consists in an instrument for use in making the corrections for
leeway, deviation, and variation in order to find the true course of a ship from the
compass or magnetic course.
The object of this instrument is to obviate the loss of time and the errors to
which the present method of making these corrections by arithmetical computation
is so liable.
This instrument is illustrated in the accompanying Drawing. It comprises a
dial a (similar to that of a compass) having an inner circle divided into points
(half and quarter points) indicated by letters in the ordinary way, and an outer
circle c accurately divided into degrees, which are numbered from 0 to 90 in either
direction from the N. and S. points. dis an arm movable about a pivot C at the
contro of the dial. This arm has an arc A with an index mark fwhich points to the
inner or compass circle b; and upon a short arc B at the outer extremity of the
arm there is another index or mark h, which is exactly radial with index f, and
serves to indicate on the outer circle c the degree which corresponds to any point
of the compass at which the index f may be placed. To the right and left of
line upon the arc B there is a vernier by which the indications in the outer
circle may be read off to say a quarter of a degree. D is a lens for reading the
vernier. It is carried by an arm K also mounted on the central pivot C and
movable independently of the arm d. The arm K is hinged as shown to adjust
the lens to its focus. It will be seen that there are arrows marked on the dial
with directions annexed to show which way the arm must be moved to allow for
easterly and westerly deviation and variation, all the corrections necessary for
finding the true course being made by merely moving the arm & a proper distance
round the dial to the right or left, as the case may be.
Take the following example for instance :—
From the above find the true course.
In this example there are three corrections required to obtain the true course.
First, place the index f at the magnetic course W. by S., allow the leeway
from the wind which is at S.S.W. Place the index fa point and a half to the
right, which will give W. N. Then look to the arc B and it will be seen that
the line h stands at N. 84° 15' W. The deviation being 15° 30' westerly, it is
seen by the arrows that westerly deviation is allowed to the left, therefore moving
the arc B to the left the point will be read S. 80° 15′ W. The variation being 25°
easterly it is scen by the arrows that easterly variation is allowed to the right,
therefore moving the are B to the right, the point will now be read N. 74° 45′ W.,
which is the true course.
Having described the nature of my said Invention, and the manner of performing
the same, I declare that what I claim as my Invention, to be protected by the
herein-before in part recited Letters Patent is―

page_text: The construction and use of the instrument herein described and shown in the
Drawing annexed consisting of a dial graduated as specified, and an arm movable
upon the dial, and provided with indexes corresponding with the two circles of
divisions on the dial, as and for the purpose set forth.
In witness whereof, I, the said William Henry Roberts, have hereunto set my
hand and seal, this Eighth day of September, in the year of our Lord
One thousand eight hundred and seventy-six
WILLIAM H. ROBERTS. (L.S.)