Rose of the Winds: the Origin and Development of the Compass-Card
By Silvanus P. Thompson
Excerpt
Although the construction of the compass lies outside the scope of
this inquiry, some preliminary considerations are necessary
concerning the origin of the compass itself; and these must be
stated briefly. The mariners' compass, as we know it to-day,
consists of a light circular disk or card, beneath which is
attached a magnetic needle or system of magnetic needles. The card
is provided at its centre with a small cap, by which it is poised
movably upon a pin. The whole is enclosed in a hollow box or bowl
covered with a flat lid of glass; and the compass box or bowl is
suspended within two hinged rings of brass to enable it to conserve
its proper horizontal position in spite of any tilting movement to
which it is subjected by the rolling or pitching of the ship on
which it is carried.
The card is divded out into thirty-two 'points' or 'rhumbs' of
equal angular breadth forming a rose or star, and to these are
affixed the initials of the names of the thirty-two points. The
magnet needle, or system of needles, is affixed to the card
parallel to the direction marked NS on the card, so that when left
to itself, the card, obeying the directive force which acts on the
needle, sets itself in a direction pointing (magnetically) North
and South; the several 'points' marked on the card then indicating
the several directions that the mariner may know in which way to
steer in order to follow his desired course. The card is also
usually marked at its North point with a fleur-de-lis or other
distinctive sign. The magnetic needle is controlled in its pointing
by the magnetism of the earth's globe.
It is no part of the present paper to enter upon the reasons for
the irregularities which are found to exist in the directive force
of the earth, and which produce those local 'variations' of the
compass which the mariner encounters everywhere. Neither is there
here any question considered as to the errors or perturbations due
to the presence near the compass of pieces of iron on the ship, nor
as to those due to the incidental presence of magnetism in the iron
of the ship's hull or fittings; nor, again, as to the means for
compensating those perturbations or correcting those errors.
The compass, as above described, has remained practically unchanged
in all essential particulars for...
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