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receding more that 16° on either side. From the magnetic equator the dip of the needle increases gradually until at positions called the magnetic poles - about 18° from the geographical poles - where the needle points vertically
downwards. Thus, when going northwards from the equator, the north-seeking end of the needle will commence to incline until the north magnetic pole is reached,
where it will assume a vertical position, Equator
as shown in Fig. 19. Similarly will the south end of the needle act when going from the equator toward the south pole.
In 1831, Sir John Ross discovered the north magnetic pole in latitude 70° N and longitude 96° 40' W. The same observer, in 1839, found the position of the south magnetic pole in latitude 75° 30' S and longitude 154° E.
73. The magnetic dip, like the variation, is subject to continual and progressive changes, both secular and periodical. Lines that are drawn intermediate to the poles and equator, connecting all points where the inclination of the magnetic needle is the same, are called lines of equal inclination, also isoclinic lines.
These lines are somewhat analogous to the geographical latitude parallels, but do not coincide with them; they are, as a rule, nearly parallel to the magnetic equator, and the term "magnetic latitude" is used simply to denote the position of a place with reference to the magnetic dip.
Another system of lines is created by joining all points on the earth's surface where the intensity of the earth's magnetic force is the same; these lines are called isodynamic lines.
To the navigator the dip and magnetic intensity is of no particular importance, but it is nevertheless fitting that he should have some knowledge of the nature of these subjects. In the mariner's compass the effect of the dip is counteracted by small copper slides that are attached to the needle, or by other suitable weights fastened to the under side of the compass card.
75. In connection with the practical use of the mariner's compass a serious difficulty arises from the disturbing influences of the magnetism of the ship, which causes a deflection of the compass needle from the magnetic meridian known as deviation. The difficulty is, of course, greatest with iron and steel vessels, where the deviation of the needle is frequently so great as to render the compass almost useless, unless extraordinary precautions are taken to control its effect.
76. Soft Iron and Hard Iron. – In order to explain the cause of deviation, it may be advisable before going further into this subject to state that in reference to magnetism there are two classes of iron, namely, soft iron and hard iron.
By soft iron is meant such iron as becomes magnetized immediately on being exposed to the influence of some magnetic body, but which has no power to retain the magnetism thus acquired when removed from the sphere of influence of the magnetic body.
By hard iron is meant such iron as does not become magnetized by induction when exposed to the influence of a magnetic body, but which retains its magnetism permanently when once magnetized. Artificial magnets are therefore necessarily made of hard iron.
77. From the preceding definitions it is evident that when a bar of soft iron is held in a north-and-south direction, with its northerly end somewhat inclined downwards, it will be feebly magnetized by induction from the earth and will have in its lower end a red, or positive (+), pole, which, according to the law of attraction and repulsion, will repel the north end of the magnetic needle; and in its upper end a blue, or negative (-), pole, which will attract the north end of the magnetic needle. If, however, the bar is reversed, we shall find that the polarity of the bar is changed also, its lower and northerly end always being of red polarity. Again, if the bar is held in a north-and-south horizontal position, it will still retain a red polarity in its north end and a blue polarity in its south end, although the magnetism in this position is of less intensity. If the bar is held horizontally in an east-and-west position, it will lose its magnetism entirely.
But soft iron can be transformed into hard iron by hammering it, and we know, also, that magnetism can be imparted to an iron bar by simply subjecting it to blows from a hammer. A soft iron bar, therefore, after being hammered will retain its magnetism and will act as a magnet, irrespective of its position, the nature of its polarity depending on the direction in which it was held when being hammered. Thus, if held in the direction of the meridian, its north end will have red, and its south end blue, polarity, and this polarity will remain constant no matter how the bar is turned.
78. Magnetic Property of Iron and Steel Vessels. Since the iron of which a ship is constructed is of an intermediate character of soft and hard iron, it is evident that, while being built, the ship will acquire a magnetic character, partly through induction from the earth and partly through the great amount of hammering to which it is subjected, and the character of this magnetism will depend on the direction in which the ship is built. Thus, a ship built in a north-andsouth direction, that is, with her bow toward the north, will have red, or positive, polarity in her forward part, as shown in Fig. 20 (a), and blue, or negative, polarity in her stern. The effect of this will be that the magnetism in the ship's forward part will repel the north end of the compass needle and, similarly, the magnetism in the after part will repel the south end of the needle. If the vessel is being built in a northeasterly direction, as at (6), we shall find red, or positive, polarity on the port bow, and blue, 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 will 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 its bow to southeast or southwest, 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, or 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. If the ship's head