IMPROVEMENTS IN TREATING SHIPS AND VESSELS TO CORRECT AND PREVENT COMPASS DEVIATIONS
Year: 1868

page_text: PROVISIONAL SPECIFICATION left by Henry John Girdlestone and
John Ward Girdlestone at the Office of the Commissioners of
Patents, with their Petition, on the 29th September 1868.
We, HENRY JOHN GIRDLESTONE and JOHN WARD GIRDLESTONE,
both of Duke Street, Westminster, Civil Engineers, do hereby declare
the nature of the said Invention for "IMPROVEMENTS IN TREATING SHIPS
AND VESSELS TO CORRECT AND PREVENT COMPASS DEVIATIONS," to be as
follows:-
This Invention relates to a novel method of correcting the deviation
of the compass of iron, steel, or composite ships or vessels. The cause
of compass deviation is ordinarily either the magnetism acquired during
construction, or the induced polarity of the steering gear, or some other
portion of the ship near to the compass, the particular character of this
magnetism being partially dependent on the position or bearing of the
ship while on the stocks. A method that has been hitherto very
frequently employed for the purpose of correcting the deviation of the
compass has been by compensation, where magnets commonly of from 12
to 36 inches in length made from bars of flat steel are so placed (either
horizontally on or at about the level of the deck, or more or less
vertically against the binnacle, mast, or some other portion of the ship

page_text: near to the compass) as to render the compass accurate at one or more
points, the error remaining on other points being ascertained by swinging
the ship.
In the Specification of the Patent granted to Evan Hopkins on the
23rd January 1866 (No. 227), the depolarization (that is the removal,
destruction, or neutralization of the magnetism) of iron ships, is thus
described-A powerful electro-magnet is connected with a battery, and
the currents generated thereby are made to circulate in a direction con-
trary to the acquired polarity of the vessel. The electro-magnet is
suspended over the ship's side and moved rapidly and repeatedly from
end to end over and in contact with the main plates of the ship, a single-
acting electro-magnet being placed in the culminating point of the upper
part of the stern to act as the stern pole of the vessel, the operations
being continued until the process is completed by the removal of the
polarity of the iron hull.
According to our Invention we first, by the aid of suitable compasses
or other instruments, carefully survey the magnetic condition of the hull,
deck beams, stringers, ribs, masts, steering gear, and other portions of the
vessel, and thus practically ascertain the cause of the deviation. In
some cases for the purpose of correcting and preventing the deviation of
the compass we depolarize, by means of an ordinary horse-shoe or per-
manent magnet, such of the longitudinal, transverse, and radial deck
beams, and stringers as lie either wholly or in part within a radius of
15 feet of the compass. In other cases for the same purpose instead of
depolarizing any portion of the vessel we compensate, balance, or over-
come the disturbing influence of the polarized iron or steel by polarizing
or magnetizing either the hull, ribs, deck beams, masts, stern post,
rudder head, or other parts of the vessel to such an extent and in such
a manner as shall render the compass perfectly accurate at the bearing
in which the vessel lies during the operation. The same operation may
be, if requisite, carried out on one or more fresh bearings, or any
remaining deviation there may be at any other bearing may be ascertained
and tabulated on swinging the ship.
A mode of proceeding may be thus more fully described:-When the
deviation of the compass is primarily or in the main attributable to the
influence of one or more North poles ahead or of one or more South
poles astern of the compass, or vice versa, the following method of
treating the ship may either in whole or in part, so far as is requisite and

page_text: advisable, be employed for the purpose of correcting the deviation,
namely, by imparting to the centres of one or more of the transverse
deck beams, and to the ends nearest to the compass of one or more of
the radial and longitudinal deck beams and stringers lined nearest to
the compass, to the ends of one or more of the longitudinal deck beams
and stringers running past and lying nearest to the compass, and to either
the mast, stern post, rudder head, or stanchions at or about the level of
the compass (if when there polarized they affect the compass), South
poles if ahead and North poles if astern of the compass, or vice versa.
For the purpose of what may be called compensating polarization
above described, we generally employ either an electric spiral or spirals
with battery and apparatus, such as described or referred to in the
Specification of a Patent granted to F. A. Paget on the 13th September
1867 (No. 2592), or electro-magnets with battery or ordinary horse-shoe
or permanent magnets.
The process of compensating polarization may be combined on any
ship with that of depolarization, that is to say, a compass may be
rendered accurate by polarizing one portion and depolarizing another
portion of the same ship.