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 center with a small cap, by which it is poised movable 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.