or negative, polarity on the starboard quarter. Again, should the vessel be built in the direction of the meridian, with her bow toward the south, her stern would possess red polarity and the bow blue polarity, as shown in (c). When built in an east-and-west direction, the side of the ship that faces the north pole will acquire red, and the other side blue, polarity. When built with her bow toward SE or SW, the red polarity will be, in the former case, on the port quarter, and in the latter case, on the starboard quarter.

79. The Semicircular Deviation.-The effect on the compass of the error produced by the magnetism acquired by the ship through hammering will be different in different positions of the ship; in other words, the error will vary between certain limits. Take, for instance, a compass that is placed in the after part of the ship. Then, if the ship's head when it was being built was approximately north, the north end of the needle will be deflected to the east when the ship is heading on points between north and west, and its maximum easterly error, or deviation, will occur when the ship is heading exactly west, as at Fig. 21 (d); when heading on points between north and east the north end of the needle will be deflected to the west, with a maximum error when the ship is heading exactly east, as at Fig. 21 (e), since in both cases the blue polarity of the stern will attract the red polarity of the north end of the magnetic needle.

80. The foregoing appertains to compasses of ships built with their bows pointing to the north. For ships built with their bows pointing to the south, the conditions are reversed; then a maximum easterly error (produced by the blue polarity of the bow) will occur when the ship is heading exactly east, and a maximum westerly error when it is heading west. In a similar manner all compasses are affected, and the nature of the error, or deviation, produced will depend on the magnetic polarity of the ship, and, consequently, on the direction in which the ship was built.

81. It should be remembered that no matter where the blue and red magnetic poles of the ship are situated (they are as a rule diametrically opposite each other) they will, when both are on the line of the magnetic meridian, produce no error whatever; but at any intermediate position they will produce an error that is maximum when the line connecting the two poles is perpendicular to the magnetic meridian. This error, or deviation, is, therefore, called the semicircular error, or deviation, since its maximum value is attained in different semicircles, and the magnetism that produces it is generally known as permanent or subpermanent magnetism, because it remains constant, or nearly so, in all latitudes; it is the magnetism of the hard iron of the ship imparted to it by the hammering received while building.

The amount of subpermanent magnetism appears to be influenced by the size of the vessel, or the quantity of iron used in its construction. Thus in vessels of large tonnage, the semicircular error is comparatively greater than in vessels of a smaller size.

82. Let us now consider the magnetism of the soft iron of the ship. The soft iron entering into the make up of an iron vessel may be conveniently divided into two classes; viz., vertical and horizontal soft iron. To the former class belongs all iron running in a vertical direction, such as frames, stanchions, etc.; to the latter, all iron running horizontally, such as the keel, deck beams, etc. All vertical soft iron becomes magnetized by induction from the earth; this magnetism, however, is only transient, changing with the latitude. Thus, in a ship north of the magnetic equator, the upper ends of all vertical soft iron will have blue polarity, and the lower ends red polarity; while south of the magnetic equator the same vertical iron will possess red polarity in the upper, and blue polarity in their lower ends. The amount of this magnetism, therefore, varies in the same proportion as the vertical magnetic force of the earth. In other words, it varies as the magnetic dip, being least at the magnetic equator, and greatest at the magnetic poles.

83. How this magnetism affects the compass needle is illustrated in Fig. 22, where the lines NS represent the direction of the magnetic meridian. Assume that a bar of soft iron possessing blue (—) polarity in its upper end is kept in a vertical position and moved around the compass with its upper end on about the same level as the suspended needle. Then, in whatever position the bar is held, it will always attract the north end of the compass needle; as a consequence, there will be no deviation when the bar is held in line of the needle, or on the magnetic meridian, as at a and c, but when held on a line perpendicular to the magnetic meridian, a maximum error is produced

a

S

FIG. 22

that is westerly when the bar is to the west of the needle, as at b, and easterly when the bar is to the east of the needle, as at d. Thus, we have another semicircular error produced by the vertical bar carried around the compass, and since all vertical soft iron in a ship is similarly influenced, it is evident that when the ship is turned about, this iron will affect the compass in exactly the same way as the subpermanent magnetism of the ship. Therefore, the semicircular deviation of a compass is not the effect of subpermanent magnetism alone, but is produced by the combined action of the subpermanent magnetism and the transient magnetism from the vertical soft iron of the ship.

The amount of error produced by the latter will vary in the same ship according to the position of the compass in relation to the surrounding vertical iron. In some positions, where the iron is symmetrically distributed fore-and-aft and across the ship, the attractions from opposite sides may neutralize each other and no error will be produced; in other positions there may be large errors. From this it is evident that the placing of a compass in an iron ship is a matter of great importance.

84. Quadrantal Deviation.-Having considered the magnetic influence of vertical soft iron we will next turn our attention to that of horizontal soft iron. By horizontal iron is meant not only that which is in the immediate neighborhood of the compass, but all horizontal iron that constitutes a part of the ship's hull, above, below, and in the plane of - the compass card. From Art. 77, we know that if a softiron bar is held in a horizontal north-and-south position it will attain magnetism by induction from the earth, but will lose it when turned in an east-and-west position. The horizontal soft iron of a ship will receive and lose its magnetism in exactly the same way; hence, it will be readily understood that when a ship is heading north or south, and east or west, or when on any of the four cardinal points, there will be no error or deviation from this kind of magnetism. But when heading on any of the other points an error will be produced that is greatest when the ship is heading on any of the quadrantal points. For this reason, the deviation produced by the transient magnetism of horizontal soft iron is called the quadrantal deviation.

85. Quadrantal deviation, which is constant in all latitudes and which does not change with the lapse of time, is generally easterly in the NE and SW quadrants and westerly in the N W and S E quadrants.

The amount of this deviation is, as a rule, not very large (rarely exceeding 3° or 4°, except in armored ships), because when the ship is heading on any of the quadrantal points the

magnetic force of the fore-and-aft and athwartship horizontal iron will to a certain extent neutralize each other and thus minimize its effect on the compass.

COMPENSATION OF COMPASSES

GENERAL PRINCIPLES

86. From the preceding remarks on the magnetic properties of an iron ship, the student will readily understand that as soon as a compass is placed on board, it is at once subjected to serious disturbances from the magnetic forces of the iron surrounding it, and if left to their influence will be rendered, in many cases, totally useless. By performing certain operations, however, the effect of these disturbances may be minimized or reduced, and under favorable (though very rare) circumstances, they may be removed altogether. This operation is called compensation, or compensating the compass.

87. The general principle of compensating a compass is to counteract the magnetic disturbances by means of magnets and soft iron placed in the immediate neighborhood of the

FIG. 23

compass, and in such positions as to cause a disturbance contrary to that caused by the iron of the ship. The magnetic needle will thus be left comparatively free. This may be illustrated as follows:

Bearing in mind that the north-seeking end of the compass needle always possesses red polarity, assume a needle to be deflected from the magnetic north n to n' by an iron mass d, Fig. 23. Then, in order to bring the needle back to its proper position, or, what is the same thing, to counteract the effect of the disturbing force d, two methods may be used, viz., either by placing near the north end of the needle,