CERTAIN MEANS OF CORRECTING OR PREVENTING THE DEVIATION OF THE COMPASS NEEDLE PROM LOCAL ATTRACTION
Year: 1855

page_text: PROVISIONAL SPECIFICATION left by Thomas Allan at the Office of the
Commissioners of Patents, with his Petition, on the 8th September 1855.
I, THOMAS ALLAN, of Adelphi Terrace, in the City of Westminster, Civil
Engineer, do hereby declare the nature of the said Invention for "CERTAIN
MEANS OF CORRECTING OR PREVENTING THE DEVIATION OF THE COMPASS NEEDLE
PROM LOCAL ATTRACTION" to be as follows:-
The object of these improvements is to counteract the deviation of ships'
compasses from the true north, arising from or caused by the induced mag-
netism in the iron on board of ships, or the iron of which ships are now so
frequently made, under all circumstances and in all latitudes. For this purpose,
as a lesser magnetic influence in the vicinity of a compass needle is equal in
effect upon it to a greater at a greater distance, I use an ascertained induced
magnetic influence as a counterpoise equivalent to counterbalance at equal
angles from the true north the magnetism of the ship's iron or force of deviation.
To ascertain this, in swinging the ship I place the ship's keel, or rather the
ascertained line of the ship's magnetism, in a line, say N.E.; I then describe a
circle around the compass of a convenient diameter, say, 6 or 8 feet, with the
pivot of the needle as a centre; I then place a counterpoise in a line N.W.
on that circle. This counterpoise may be formed of a bundle of soft iron rods,
so that it can be the more easily added to or deducted from until its magnetism

page_text: exactly counterbalances the magnetism of the ship's iron, when the needle will
point to the true north, bisecting the angle formed by the line of the keel N.E.,
and the radial line on which the counterpoise is placed N.W. After this
balance of power is precisely ascertained, the true north can at any time and in
all latitudes and positions be found by shifting the counterpoise forward or
backward until the needle bisects the angle formed by these two lines aforesaid,
or (which is the same thing) the arc of the circle comprehended between them.
The counterpoise being formed of soft iron rods, will be magnetised by induction
from the same source or cause as the iron of the ship, and will fluctuate in
intensity and proportionately with the ship's iron. These two counteracting
quantities of magnetic influence will consequently vary equally to each other in
all latitudes and positions, and the true north be ascertained when the needle
bisects the angle formed by the relative directions of these two forces.
My Invention also consists in another plan, which is still more simple, and
does not require the shifting of the counterpoise to ascertain the true north by
the bisection of the angle of the forces. In this case, if when swinging the
ship, instead of placing the counterpoise in a line N.W., I place it in a line
S.W., that is to say, in a line in continuation of the line of the keel or position
of the ship's magnetism N.E., it will, by its repulsion on the south pole of the
needle from the west, counterbalance the attraction on the north pole of
the needle from the cast. This point will thus be a fixed point, altering its
position as regards the needle co-relatively with the magnetism of the ship
without any adjustment being necessary. It will likewise in heavy weather
and a rolling sea have less effect in causing the needle to swing or oscillate,
as would otherwise be the case when placed between two similar polarities
attractively.