IMPROVEMENTS IN INSTRUMENTS OR PROVISIONAL SPECIFICATION
Year: 1880

page_text: LETTERS PATENT to Henry James Thatcher, of Cardiff, in the County of
Glamorgan, for an Invention of "IMPROVEMENTS IN INSTRUMENTS OR
PROVISIONAL SPECIFICATION left by the said Henry James Thatcher at
the Office of the Commissioners of Patents on the 11th May 1880.
HENRY JAMES THATCHER, of Cardiff, in the County of Glamorgan. "IMPROVE-
MENTS IN INSTRUMENTS OR APPLIANCES FOR ADJUSTING AND CORRECTING
DEVIATIONS IN THE MARINER'S COMPASS."
According to my Invention I employ a plain dumb card, such as is used for
adjusting compasses, but fitted in gimbals for use at sea, having a wire line
stretched across the face or surface of the card, which line is fixed into two studs
or blocks for receiving sight pieces for taking azimuths or bearings of any distant
object. When the above mentioned wire line is turned in the direction of the sun
a shadow is thrown across the dumb card. When this shadow crosses the exact
centre of the card, which may be marked by a small black spot, the finger piece
under the line shows the exact bearing of the sun by the dumb card. There is
also another wire line provided for fixing across the card on the top of the sight
names, exactly over the wire line above described; and if it is required to take
the bearing of the sun at a high altitude when too obscured to cast a shadow, or a
star when too high for the ordinary azimuth sights, then the bearing may be taken
by means of this second wire.
This corrector is, I believe, more simple than any other known, and it may be
used for taking an azimuth by any compass aboard the ship.
The wire line across instead of a shadow pin is better, because when the sun is
very high in the heavens, as for instance is the case near the tropics, the shadow
from the shadow pin in the centre is so short as to be almost uscless, whereas the
shadow of a line stretched right across will correctly indicate the direction of the
sun even if it was nearly overhead, and by the second or upper line the bearing of
a star may be taken even at an altitude of 80 degrees.

page_text: SPECIFICATION in pursuance of the conditions of the Letters Patent filed by
the said Henry James Thatcher in the Great Seal Patent Office on the 9th
November 1880.
HENRY JAMES THATCHER, of Cardiff, in the County of Glamorgan.
MENTS IN INSTRUMENTS OR APPLIANCES FOR ADJUSTING AND
DEVIATIONS IN THE MARINER'S COMPASS."
" IMPROVE-
CORRECTING
This Invention has for its object "Improvements in Instruments or Appliances
for Adjusting and Correcting Deviations in the Mariner's Compass."
The instrument consists of a flat circular plate made of German silver, and
divided and marked precisely the same as a compass card (see Figure 1), the said
plate being fitted and arranged so as to turn horizontally on the shoulder (marked i,
Figure 2) of an upright brass rod (k, Figure 2) swinging in gimbals (a, b, c, d,
Figure 2), and having a lead weight (e, Figure 2) at the bottom so as to keep it
always perpendicular.
Below the metal card a flat brass bar (l, Figure 3) is made to revolve upon a
second shoulder (f, f, Figure 2) of the aforementioned upright rod, but made so as
to turn easily without touching the metal card. This flat brass bar has its ends
turned up at right angles to the edge of the metal card, so that when the bar is in
its place on the shoulder of the upright rod (see a, a, Figure 4), and the metal
plate (b, b, Figure 4) is also in its place, the ends of the bar (c, c, Figure 4) extend
upward above the plate about the distance of one inch, and the ends of the wire
(marked d, Figure 4) are permanently fixed into the two upright ends of the brass
bar. The perpendicular brass rod (k, Figure 2) is filed square at g, Figure 2,
immediately above where it passes through the metal plate, the said square
corresponding to and fitting into the socket of a metal hand (m, Figure 5), the said
hand being intended to remain fixed in one direction, while both the metal card
and the brass bar underneath revolve freely. Above the square the perpendicular
brass rod (k, Fig. 2) is tapped with a screw thread to receive a short thumb
screw (marked y, Figure 2), which answers the double purpose of clamping the
metal card and also showing by the speck 1, Figure 2, the exact centre of the card.
In each of the upright ends of the flat bar two dovetail slots are filed, shewn at a, a,
Figure 3, to receive the ends of two azimuth sight vanes b¹, b¹, Figure 6, exactly
the same as in an ordinary azimuth compass. These sights have also a small slot
cut at c, Figures 6, in the top of each, so as to receive the ends of a thin wire
strained across from one to the other see (a³, a¹, Figure 4).
When using the instrument the first thing to be done is to place it so that the
hand on the dial points in the direction the. ship is going; then you look at the
compass you wish to correct, and see how the ship's head is by that compass, and
then you turn the dial round until the hand points to exactly the same degree on
the metal plate as the lubber line of the compass shews the ship's head is. If the
sun is shining all that is now required is to turn the flat bar round until the
shadow of the cross wire falls upon the little black speck in the centre of the metal
card. By looking at the little indicator y, Figure 4, you will then see at once the
bearing of the sun by the metal card, and as this metal card is exactly the same as
the compass you are correcting you have the bearing of the sun by the compass.
Reference to Burdwood's azimuth tables will give the true bearing, and the
difference is of course the deviation and variation combined.
The sight vanes are for taking bearings of any object it may be found desirable
to correct the compasses by, and the cross wire over all is to take the bearings of the
North Star or any other heavenly body whose altitude is too great for the ordinary
sights.

page_text: Having thus described the nature of my said Invention, and the manner of
carrying the same into effect, I would have it understood that I claim the improved
construction of instrument for adjusting and correcting deviations in the mariner's
compass, substantially as herein before described and illustrated.
In witness whereof, I, the said Henry James Thatcher, have hereunto set my
hand and seal, this Sixth day of November, in the year of our Lord One
thousand eight hundred and eighty.
HENRY J. THATCHER. (L.S.)