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(20) The semicircular error, or deviation, of a compass is caused by the combined action of the subpermanent magnetism and the transient magnetism from the vertical soft iron of the ship.
(21) (a) The quadrantal deviation.
(6) The quadrantal deviation is greatest when the ship is heading on any of the quadrantal points.
(22) Consult Art. 88.
(23) Consult Art. 90.
(24) In cases where a ship is not fitted with a compensating binnacle, a common practice is to draw two lines on the deck, one in a fore-and-aft direction, the other at right angles, so that their point of intersection will come vertically below the center of the compass to be compensated. Then, with the ship's head in a correct magnetic north or south direction, a bar magnet is placed on deck, with its middle point on and perpendicular to the fore-andaft line, at such suitable distance from the compass as will cause the needle to indicate correct magnetic north. The magnet is then secured to the deck.
(25) The ship should be brought with her head in a correct magnetic east or west direction; a bar magnet is then placed on deck perpendicular to the athwartship line and with its middle point on that line; the magnet is then moved toward the compass until the needle indicates correct magnetic east or west, when it is secured to the deck.
(26) No; they need not be secured to the deck, but may be placed in any convenient position near the compass in or outside the binnacle, provided the following conditions are complied with: The center of the fore-and-aft magnets must be exactly on the vertical athwartship plane passing through the center of the compass; the center of the athwartship magnets must be exactly on the vertical fore-and-aft plane passing through the center of the compass; all magnets must be parallel to the plane of the compass card when the ship is in an upright position.
(27) Since the quadrantal deviation is greatest on the quadrantal points, the ship's head is swung in a NW, SE S W, or N E correct magnetic direction. The deviation on this point is then corrected by the two soft-iron spheres usually attached to the binnacle, by moving them from or to the compass until the error disappears.
(28) Consult Arts. 96 and 97.
(29) By subpermanent magnetism is understood the magnetic condition of a more or less enduring character possessed by a ship when launched, and which was acquired when building by induction from the earth and rendered permanent by hammering.
(30) The mean object of compensating a compass is not to entirely remove the deviation but to bring it within manageable limits or, in other words, to reduce it to a convenient minimum value. After being compensated, a a . compass is more uniformly sensitive, whereas an uncompensated compass with large deviations has a tendency of alternative sluggishness and sensitiveness certain directions of the ship's head.
(1) First method: With an object on shore, whose correct magnetic bearing is known, the ship being swung successively on each point, the deviation for the actual direction of the ship's head is found by taking the difference between the compass bearing of the object and its correct magnetic bearing. If the correct magnetic bearing of the object is not known, it may be found by taking the mean of all compass bearings.
Second method: A compass is set up on shore or in a boat some distance from the ship, where it is free from local attraction, and an observer stationed by it. The ship is then swung as before, and, as her head comes on each successive point, simultaneous bearings are taken of each compass from the other, according to a prearranged signal. The bearings by the shore compass, reversed, are the correct magnetic bearings with which the bearings observed on board are compared, and the deviation found.
(2) Consult Art. 5.
(3) No; it is not.
Consult Art. 16.
As often as an opportunity presents itself.
(a) and (6) Consult Arts. 26, 27, and 28.
(6) (a) Leeway should be applied from the wind. (6) Correct course = SE.