IMPROVEMENTS IN COMPASSES FOR IRON SHIPS
Year: 1881

page_text: PROVISIONAL SPECIFICATION left by David McCallum at the Office of the
Commissioners of Patents on the 6th January 1881.
DAVID M'CALLUM, of the Octagon, Plymouth, in the County of Devon.
" IMPROVEMENTS IN COMPASSES FOR IRON SHIPS."
This Invention has for its object improvements in compasses for iron ships. A
certain amount of electricity hangs about every iron ship, which inore or less
deranges the compass and causes deviations which no amount of mechanical skill or
scientific investigation has yet been able to grapple with. After many years of
study and close application to the nature of magnetism, I have at last been enabled
to produce a compass which will supply all that is required for the purpose of
detecting the amount of deviation which may occur in any iron ship.
The following is a description of my compass for iron ships.
The whole of the apparatus is contained in and attached to a box measuring
about 10 or 12 inches square, more or less, and 10 inches high.
A round piece of wood, which may be mahogany or oak, and may be made heavy
by attaching a piece of lead under, is swung in gimbels; in the centre of this round
piece of wood a steel or bell metal point passes through on which an ordinary ship's
imagnetic needle and card are arranged to move freely; and at the top of the
thimble above the magnet a small piece of brass is screwed on, to which there is
also screwed a metal knife point, the wedge point of which is so placed as to point
always to the north, the point being in a line with the north pole of the magnet.
Over this point there is placed a strip of paper through which a hole is cut by the
point and in shape like it by a process hereinafter described. Two reels which
may be of wood or brass about one and a half inch or two inch long are attached to
two sides of the box; they contain a strip of paper. These reels are placed at a
convenient distance, so as to allow the strip of paper to be drawn down on to the
knife point which the magnet freely moves. A lever of brass is screwed to one
side of the box and reaches closely to the other side with freedom to move up and
down. To the end of this lever there is attached a spring detent which when the
lever is drawn down passes over one tooth of a ratchet wheel at every pull; the
ratchet wheel is attached to the reel that receives the marks. The lever is drawn
up again by means of a vulcanized india rubber spring, one end of which is screwed
to the box, and the other end fixed to the lever.

page_text: At the centre of the lever on each side of the knife point 4 or more pieces
of brass about 1 inch long are fixed so as to allow the strip of paper to
pass between, and thus secure the penetration of the point through the paper when
the lever is pulled down. The tension of the paper is also secured by means of a
spring at the reel which gives off the paper. At the end of the lever there is also 5
fastened a few inches of small chain which passes through the bottom of the box ;
to the end of it a brass ring is attached.
MANNER OF WORKING THE APPARATUS.
On the round piece of heavy wood under the compass card there is a pointer of
brass which is moved by hand. The object of this in the first place is to find
out at what height the ship's magnetism reaches, which will be hereinafter
described.
A mast on board each ship shall be selected which may be the most convenient,
and in the event of the masts being all iron a temporary wooden mast or spar must
be placed so as to take the place of an ordinary timber mast, as all local attraction
must be carefully avoided. Any other arrangement may be made which may be
deemed the most suitable and the box placed in any position which practical
experience may dictate. The box which contains the apparatus must be hoisted by
means of a block arrangement so high in each ship as to be above the influence of
the magnetism of the ship. As some ships will require the apparatus higher than
others, the way to find out how high each vessel requires it is to place the pointer
exactly with the card on deck, then pull up the box, stopping at various heights
and making marks in the paper by means of a small line or twine cord which is
fastened to the ring which is attached to the chain fastened to the end of lever
inside the box. As soon as the proper height is obtained which shows no deviation,
the position is secured in the following manner :-
A piece of board the width of the box must be attached to the mast or spar so
as to secure the stability of the box, and two pieces of wood or brass fastened at
the top of this flat piece of wood, so that when the box is drawn up it may go so
far and no farther and thus be jammed up against these projecting pieces. The
box may be pulled up by means of an ordinary block and pulley, but in practical
operation it may be found necessary to introduce a similar plan to that which is
adopted for hoisting lights on board of ships. In either case when the box is in
position the small cord must be pulled as often as may be found necessary, and
when the box is lowered and opened the true course is shewn by the manner in
which the cutter has pointed. Instead of the small cord being pulled by hand on
deck the cord may be dispensed with and a train of wheels attached which -would
work a lever of another form, the marks being made as above.