IMPROVING THE PROCESS FOR FINDING THE VARIATION OR ERROR OR DEVIATION FROM THE MERIDIAN ON THE COMMON STEERING COMPASS
Year: 1867

page_text: LETTERS PATENT to Robert Shortrede, Major-General on the Retired
List of Her Majesty's Indian Army, residing at Surbiton, in the County
of Surrey, for the Invention of "IMPROVING THE PROCESS FOR FINDING
THE VARIATION OR ERROR OR DEVIATION FROM THE MERIDIAN ON THE
COMMON STEERING COMPASS."
Sealed the 8th October 1867, and dated the 13th April 1867.
PROVISIONAL SPECIFICATION left by the said Robert Shortrede at the
Office of the Commissioners of Patents, with his Petition, on the
18th April 1867.
I, ROBERT SHORTREDE, Major-General on the Retired List of Her Majesty's
Indian Army, residing at Surbiton, in the County of Surrey, do hereby
declare the nature of the said Invention for "IMPROVING THE PROCESS FOR
FINDING THE VARIATION OR ERROR OR DEVIATION FROM THE MERIDIAN ON THE
COMMON STEERING COMPASS," to be as follows:-
Attached to the rim of the cover and over the top of the compass, so as
not to interfere with its ordinary use in steering, is put a band or arc, nearly
semicircular, of brass or other suitable material, having on one or both
sides a thin slit, by means of which the arc may be directed to the sun or
other heavenly body, or through which the sun's light shining over the centre

page_text: and on the edge of the card shows the compass bearing by observation ;
this being compared with the bearing, determined astronomically, gives a
difference, which is the variation, or error, or deviation of the compass
from the meridian.
SPECIFICATION in pursuance of the conditions of the Letters Patent, filed
by the said Robert Shortrede in the Great Seal Patent Office on the
12th October 1867.
TO ALL TO WHOM THESE PRESENTS SHALL COME, I, ROBERT
SHORTREDE, a Major-General on the Retired List of Her Majesty's Indian
Army, residing now at Lee, in the County of Kent, and formerly at Surbiton,
in the County of Surrey, send greeting.
WHEREAS Her most Excellent Majesty Queen Victoria, by Her Letters
Patent, bearing date the Thirteenth day of April, in the year of our Lord
One thousand eight hundred and sixty-seven, in the thirtieth year of Her
reign, did, for Herself, Her heirs and successors, give and grant unto the
said Robert Shortrede, Her special licence that I, the said Robert Shortrede,
my executors, administrators, and assigns, or such others as I, the said
Robert Shortrede, 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 Invention for "IMPROVING THE
PROCESS FOR FINDING THE VARIATION OR ERROR OR DEVIATION FROM THE MERIDIAN
ON THE COMMON STEERING COMPASS," upon the condition (amongst others)
that I, the said Robert Shortrede, 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 Robert Shortrede, 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 state-
ment-
The object being to simplify the process for finding the error on the
common steering compass, or in other terms the deviation of the magnetic

page_text: from the true meridian, I attain it by making some additions to the steering
compass as usually made, by which it becomes virtually an azimuth compass,
without interfering with its ordinary use in steering. One way of effecting
this is by attaching at opposite sides to the rim of the cover a semicircular
arc or band of a convenient width having along its middle a narrow slit by
means of which it may be directed to the sun or other heavenly body; or
through which the sun's light shining over the centre and on the edge of the
card shews by a bright streak on a dark ground the compass bearing by obser-
vation;
this being compared with the bearing, determined astronomically,
gives a difference, which is the error or deviation of the compass from the true
meridian. On a surface projecting from the rim of the bowl, or on the
rim of the cover I put graduations, which are read as usual by a zero mark
on the other rim. When the sun's light is too faint to give a distinct streak,
or in observations of moon, star, or planet, the object may be viewed through
the slit, either directly or as reflected from the glass of the cover beneath the
slit. In such cases the observation is made by taking the usual reeding of
the card at the lubber line, and also the reading on the rim giving the angle
between the lubber line and the object. According to their position, the
sum or the difference of these readings gives the compass bearing of the
object; and this compared with the true azimuth gives the error or
deviation from the meridian. As a high wind acting on the continuous are
may cause the compass to have a tremulous motion, in order to avoid or
lessen this inconvenience, in such cases I remove the arc and replace it on
the side towards the object by a short piece about an inch high, and on
the other side by a shorter piece, each piece having in it as in the are a
narrow slit; the piece towards the object being fitted with a reflector,
which may be either of the usual sort with a hinge so as to be turned
according to the altitude of the object, or it may be a portion (about an octant)
of a glass cylinder fixed horizontally, the object reflected in either of these
ways may thus be viewed through the slit or hole on the opposite side.
There is yet another way of attaining the end in view. I graduate a rim
of the bowl or cover of the common binnacle compass, putting a proper zero
mark on the other rim, by turning the cover so as to bring a bar of the
roof into the shadow of the opposite bar, the zero mark indicates the angle
between the object and the ship's head. This with the azimuth of the object
and the usual reading of the card suffices as above shewn to give the true
meridian and the deviation of the compass from it.
I claim these three several ways of adapting the common steering compass
to the purpose of an azimuth compass, and particularly I claim,-

page_text: 1st, the method of getting the direction of the sun by means of a streak
of bright light in the middle of the shadow of the are or band, thus dispensing
with the wire or hair hitherto used for bisecting the sun's disc as seen through
a wide slit.
2d, the method of using the glass cover under the arc as a reflector for
viewing the object.
.3d, the use of a cylindrical reflector fixed horizontally.
4th, the utilizing of a rim of the bowl or of the cover for carrying the only
additional graduation required.
5th, the utilizing of the bars of the roof of a binnacle compass for azimuth
observation.
In witness whereof, I, the said Robert Shortrede, have hereunto set my
hand and seal, this Twenty-seventh day of September, in the year of
our Lord 1867.
ROBERT SHORTREDE. (L.S.)