Compass
an instrument designed to indicate directions. It is constructed on the principle that a lodestone, or magnetized needle, will swing into a north and south line if mounted on a pivot, so as to turn freely. A suspended needle and a dial are the essential features. A mariner's compass is swung in rings in such a way that the dial retains a vertical position, however the ship may pitch and toss. These rings are called gimbals. The gimbals with the compass are then placed inside a box, called the binnacle. Inside the binnacle, in the direction of the ship's bow, is a vertical black line called the lubber-line, and the steersman must keep the point of the card which marks the prescribed course always in contact with the black line. The dial is laid off in the figure of a star with thirty-two rays corresponding to the thirty-two directions recognized by a seaman. Between north and northeast, for instance, designated in the usual way by N. and NE., are three additional directions called north-by-east, north-northeast, and north-east-by-north, designated as NbE, NNE, and NEbN respectively. To box the compass is to pass around the dial, beginning at the north, and naming the thirty-two directions consecutively.
It is difficult to give proper credit for the invention or discovery of the compass. The Chinese seem to have understood its secret 3,000 years before Europeans learned to steer without the aid of the sun and the North Star. According to some authorities, Marco Polo brought back a knowledge of the compass from Cathay in 1260.
Owing to the position of the magnetic poles and the unequal distribution of iron and magnetic ores, there are few lines on the earth's surface where the needle assumes an exact north and south direction. A deviation is called variation of the compass. The compass used for land surveying differs somewhat from the mariner's compass. Engineers who are running railroad lines or surveying public lands ascertain the variation by reference to the North Star and make due allowance. In the United States, the line of no variation crosses Lake Erie into western Pennsylvania and passes southeasterly into the Atlantic, near the boundary between the Carolinas.
Dr. William Gilbert, physician to Queen Elizabeth, first explained the action of the compass. In place of the theory that the needle is drawn into position by the influence of certain stars, he showed that the earth is a magnet, and that the earth acts on the needle as one magnet acts on another. Subsequent investigation has demonstrated the truth of Gilbert's theory. Inasmuch as unlike poles of magnets attract each other, that spot in the northern hemisphere that attracts the north pole, or north end of the magnetic needle, is considered the south magnetic pole of the earth. Moreover the magnetic poles are not located at the geographic poles. In 1831 the south magnetic pole was located in Canada in latitude 70 deg. 30', longitude 95 deg. The north magnetic pole, i. e., the pole that attracts the south end of the needle, is in latitude 72 deg 35' S., longitude 152 deg. E. Explorers in polar regions find it necessary to be on guard. When carried to a point east of the south, that is to say, the Canadian magnetic pole, the needle points west, and at a corresponding point to the westward, the needle points east.
See North Star; Roger Bacon; Magnet
The variation itself could not be accounted for on the doctrine vulgarly received, that magnetism is an effluvium issuing forth from the root of the tail of the Little Bear, but was scientifically, though erroneously, expressed by Gilbert's hypothesis that earthly substance is attractive--that a needle approaching a continent will incline toward it; and hence that in the midst of the Atlantic, being equally disturbed by Europe and America, it will point evenly between both.--Draper.