and in a suitable position, the blue pole of an artificial magnet, or by placing near the south end the red pole of an artificial magnet, as shown in the figure. In either case, the needle will be brought back to magnetic north. This in brief is the whole theory of compass compensation, and the student will at once perceive that the whole operation of compensating is simply an application of the law of magnetic attraction and repulsion, which has already been described. 88. We will now proceed to give the student an approximate insight of how the different errors of the compass are adjusted, well realizing the fact that while the compensation, as a rule, is made, and should be made, by professional compass adjusters, it is, nevertheless, essential that a navigator should have a knowledge of how compass compensation is performed, since occasions may arise when familiarity with this important subject will prove of great value to him. METHODS OF COMPENSATION 89. Classification of Errors.-The two principal errors to adjust are the semicircular deviation and the quadrantal deviation. The semicircular error, we know, is the combined effect of the subpermanent magnetism of the ship and the induced magnetism of vertical iron; but, as a whole, and in regard to compensation, it is convenient to divide this error into two parts and consider each of these parts as a separate force, one acting in a fore-and-aft, and the other in an athwartship, direction. The first part of that error, which affects the compass needle when heading on easterly and westerly courses, is usually denoted by the letter B; while the second part, which affects the needle when heading on northerly and southerly courses, is denoted by the letter C. The quadrantal deviation, resulting from horizontal iron and affecting the compass when heading on any of the quadrantal points, is denoted by D. When compensating a compass, the parts B and C are usually attended to first, the order of procedure being as follows. W 90. Compensation of the Semicircular Deviation. For the purpose of accuracy, two chalk lines are drawn on deck, one in a fore-and-aft, the other in an athwartship, direction, so as to intersect, or cross, each other under the center of the compass to be compensated, as shown in Fig. 24. The ship is then swung with her head toward magnetic north according to some compass uninfluenced by the magnetism of the ship (for instance by a compass on shore) or by permanent marks on land, the bearing between which coincides with the magnetic meridian. If the compass in this position does not show the correct north, that is, if the compass north does not coincide with the lubber line, a deviation must exist. Assume that the northpoint is deflected to the east, as in Fig. 24 (x). Then, in order to counteract the force causing this deviation, an artificial magnet, or, as it is usually termed, a compensating bar magnet, br is placed athwartship, with its center on the foreand-aft line and with its red pole to starboard, as shown in the figure. The distance of the magnet must be determined by trial, beginning by placing the magnet at some distance from the compass and gradually approaching it until the compass shows correct magnetic north, when the magnet is secured to the deck. If the needle had been deflected to the west it is evident that the red end, or pole, of the magnet should have been placed to the port side. In case this error is large, the ship is swung around until its head is pointing correct magnetic south, and if the compass then does not indicate south, a second magnet is placed athwartship with its center on the chalk line abaft the compass, and with its blue pole to the side toward which the southern point of the needle is deflected. The part of the semicircular error denoted by C is thus corrected. 91. The ship is next brought into an east-and-west direction with its head, or bow, magnetic east, as in Fig. 24 (y). If the compass north on this course is deflected to the east (as in the figure), a magnet b' r' is placed with its center on the athwartship line, with its red pole forward, and at a suitable distance from the compass to insure that the east point of the compass will show correct magnetic east. The needle being deflected to the west, the compensating magnet is reversed. A similar operation is then performed, if necessary, with the ship's head pointing magnetic west. That part of the semicircular error denoted by B is thus corrected. 92. When compensating magnets are fastened to the deck it is necessary that their centers are exactly on the chalk lines, and perpendicular to their respective lines. It matters not where the magnets are placed as long as this principle is observed. Care should also be taken that the magnets are not reversed when replaced after having been taken up for some purpose. Compensating magnets should be made of good steel, tempered, and well magnetized; deck magnets may be from 10 to 24 inches in length and about by 1 inches in cross-section; they should be covered with tallow, and should be protected by a thin sheeting of metal. Before being used their magnetism should be some months old, in order to possess a permanent magnetic condition. 93. Compensating Binnacles. At the present day, and particularly in iron ships, magnets for compensating the semicircular deviation are seldom, if ever, fastened to the deck, but are fitted instead to slide into horizontal fore-andaft and athwartship holes within the binnacle (see m, Fig. 12). In some binnacles the magnets are arranged in such a manner as to be moved up or down, nearer to and farther from the compass, as may be required, and then secured by means of clamp screws that cannot be touched except by opening the door of the binnacle; in others, the movement of the magnet is controlled from the outside of the binnacle by means of a crank-key, thus enabling the adjuster to watch the compass while he is altering the position of the magnets and to move them the exact amount required; after the adjustment is completed the crank-key is removed and the casing locked, making it impossible for any one to tamper with the magnets. Such a binnacle is called a compensating binnacle. The principle of magnets being stored within the binnacle is precisely the same as in securing them to the deck, both the magnets for B and C being exactly parallel to the ship's deck or to the plane of the compass card when the ship is in an upright position. 94. Compensating the Quadrantal Deviation. After the semicircular deviation has been corrected or reduced to a minimum value, the quadrantal deviation is attended to. Since this error, which is caused by the magnetism of horizontal soft iron, is greatest on the quadrantal points, the ship is accordingly swung in the direction of one of these points; for example, N E, as shown in Fig. 25, and since the error is caused by soft iron, it is necessary to compensate it by using soft iron correctors. For this reason, artificial magnets are not used to compensate the quadrantal deviation; instead, soft-iron spheres are used. These spheres are so placed in the plane of the compass card as to cause opposite effect to the magnetism of horizontal iron. The error to be corrected being easterly in the NE and SW quadrants and westerly in the NW and SE quadrants in almost every ship, the spheres, or globes, are placed athwartship on the same horizontal plane and at equal distances from the center of the Magnetic the center of the compass should, however, not be less than 14 times the diameter of the card. 95. According to a previous article, the quadrantal deviation is constant in all latitudes, hence its compensation remains constant everywhere. Such, however, is not the case with that part of the semicircular error caused by the induced magnetism of vertical iron. Since this magnetism depends on the magnetic dip, it is evident that the deviation resulting from it will also depend on the magnetic dip. To distinguish this latter error from that produced by subpermanent magnetism and apply to it a proper compensation is a difficult task requiring skill, good judgment, and an intimate knowledge of the magnetic conditions of the ship. The usual method to correct or compensate this error is by means of a vertical iron bar called the Flinders bar, which is placed within the binnacle either immediately before or abaft the compass. 96. The Flinders bar is not a permanent magnet; it is made of soft iron, and, consequently, receives its magnetism by induction from the earth. Thus, in the northern hemi |