AN IMPROVEMENT IN SHIP'S COMPASSES
Year: 1885

page_text: 11.
I, ALEXANDER MITCHELL, of 32 Seagate, Dundee Forfarshire, do hereby declare
the nature of my invention for "AN IMPROVEMENT IN SHIP'S COMPASSES
follows:
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to be as
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In adjusting ships compasses it is usually found necessary to employ a number
of permanent magnets which are set in various positions around the compass in
order to correct large deviations from the true magnetic directions at the cardinal
points and masses of soft iron for the quadrantal points, a mean correction being
obtained at which in general a small deviation occurs at a greater or less number
of the points of the compass. As an " aide-memoire" to the sailing master of the
ship on which the compass is fitted, the compass adjuster provides a deviation; card
showing the extent of deviation at the different points and the sailing master in
steering the ship, instead of sailing the course as indicated by the compass, sails,
that course plus or minus the number of degrees of deviation shown on the card
for that point. As masters have to set the course in all sorts of weather and
sometimes are excited when doing so, mistakes are apt to be made such as putting
the deviation easterly instead of westerly and vice versa, thus doubling the error.
To obviate such mistakes I employ a compass card upon which I fit movable points
and I set the movable point to the requisite extent on one side or the other of the
normal position according to the deviation shown for that point on the deviation
card and the same with all other points thus making the correction once for all.
For example if the course to be sailed is true magnetic north and the deviation of
the compass for that point is 5 degrees east I set the movable north point 5 degrees
east of true north and steer by that point. The movable points may be secured to
the compass card in any convenient way. They may be formed of sheet brass or
of card-board or other suitable material and be fitted to slide round a ring fitted
concentrically on the card. In general it will suffice to make all the whole points
movable, in which case the half-points, and if necessary, the quarter points may be
stamped upon an annular ring of paper lightly' attached to the compass card near
its periphery, or in any other suitable way.

page_text: An Improvement in Ships' Compasses.
I, ALEXANDER MITCHELL of 32 Seagate, Dundee, Forfarshire, do hereby declare
the nature of this invention and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement:-
In adjusting ships compasses it is usually found necessary to employ a number
of permanent magnets which are set in various positions around the compass in
order to correct large deviations from the true magnetic directions at the cardinal
points and masses of soft iron for the quadrantal points a mean correction being
obtained at which in general a small deviation occurs at a greater or less number
of the points of the compass. As an "aide-memoire" to the sailing master of the
ship on which the compass is fitted the compass adjuster provides a deviation card
showing the extent of deviation at the different points and the sailing master in
steering the ship instead of sailing the course as indicated by the compass sails
that course plus or minus the number of degrees of deviation shown on the card
for that point. As masters have to set the course in all sorts of weather and
sometimes are excited when doing so mistakes are apt to be made such as putting
the deviation easterly instead of westerly and vice versa thus doubling the error.
To obviate such mistakes I employ a compass card upon which I fit movable points
and I set each movable point to the requisite extent on one side or the other of its
normal position according to the deviation shown for such point on the deviation
card, thus making the correction once for all. For example if the course to be
sailed is true magnetic north and the deviation of the compass for that point is
5 degrees east I set the movable north point 5 degrees west of its normal position
and steer by that point. The movable points may be secured to the compass card
in any convenient way. They may be formed of sheet brass or of card-board or
other suitable material and fitted to slide round a ring fitted concentrically on the
card. In general it will suffice to make all the whole points movable, in which case
the half-points, and if necessary, the quarter points may be stamped upon an
annular ring of paper lightly attached to the compass card near its periphery or in
any other suitable way.
In the accompanying drawings which' illustrate my invention figure 1 is a plan
and figure 2 a cross section of the improved compass card and figures 3 and 4 are
detached views on an enlarged scale of details. As represented in these drawings
the compass card A is graduated in the usual way, but in lieu of having the
several "points" marked thereon the points are movable thereon and are formed
of strips B of thin sheet metal or other material which in the present instance are
secured to a grooved or slotted ring C on the upper face of the card by means of
screws. By undoing the screws each of the several strips B forming the " points
may be moved to one side or the other of the position of the ordinary fixed points
and the object of the invention thereby attained. As shown particularly in
figures 3 and 4 a bent wire b attached to each of the movable points may serve
to indicate the "half-points" and by being slightly bent by hand may be corrected
"

page_text: for deviation. In other respects the compass card is or may be of the usual
formation and is carried in any of the ordinary ways.
Having now particularly described and ascertained the nature of my said
invention, and in what manner the same is to be performed, I declare that what
I claim is :-
That improvement in ships' compasses which consists in providing the compass
card with movable "points" substantially as and for the purpose set forth.