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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