London in 1657 and through Paris in 1669. At the present time it is moving toward the west at the rate of about 1° in 14 years. These changes in the variation are important to the navigator, and especial attention should be given to charts on which the variation is recorded to see that they contain the latest corrections in regard to this matter.

In addition to the progressive and secular changes just mentioned, there is a diurnal, or daily, variation, the amount

of which, however, is too small to be of any importance to the mariner.

70. Magnetic Dip.-Another deflection of the magnetic needle called inclination, or magnetic dip, was discovered in 1576 by an English instrument maker. He found that a needle, however well balanced, would, after being magnetized, depart from its horizontal position, and point downwards with its north end. Further investigation in different parts of the world disclosed the fact that in the southern hemisphere the south end of the needle has a tendency to be deflected downwards, but that at places situated within the region of the torrid zone this inclination is zero, or the needle assumes a horizontal position. By connecting all places where the magnetic dip is zero, a line called the magnetic equator is established.

71. The magnetic equator does not coincide with the geographical equator, but crosses it at several places, never receding more than 16 degrees on either side. From the magnetic equator the dip of the needle increases gradually until it reaches the positions known as the magnetic poles -about 18 degrees from the geographical poles-where the needle points vertically downwards.

Thus, when going northwards from the equator, the northseeking end of the needle will commence to incline until the

Equator

FIG. 20

magnetic pole is reached, where it will assume a vertical position, as shown in Fig. 20. The south-seeking end of the needle will act in the same manner when going from the equator toward the south pole.

Sir John Ross discovered the north magnetic pole in 1831 in longitude 96° 40′ W and latitude 70° N. He found the position of the south magnetic pole in 1839 in longitude 154° E and latitude 75° 30' S.

72. 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 only generally parallel to the magnetic equator, and the term "magnetic latitude" is used simply to denote the position of places with reference to the magnetic dip.

73. Another system of lines is created by joining all points on the earth's surface where the intensity of the force of terrestrial magnetism is the same. These are called isodynamic lines.

The intensity of the earth's magnetism is usually measured by the rate of oscillations or swingings of a dipping needle.

If the needle is moved from its position of equilibrium or rest, it will revert to it after a series of oscillations according to definite laws. If the needle is removed to another place and again caused to oscillate during the same length of time as before, a different number of oscillations will be observed. Then, by noting the number of oscillations at each place, the magnetic intensity is readily found. The smaller the number of oscillations, the greater is the intensity. Near the magnetic equator the intensity is the least, and increases in amount with the distance from it. The magnetic intensity at the magnetic south pole is about three times as great as that in the Bay of Guinea.

74. To the navigator the dip and magnetic intensity are of no particular importance, but it is, nevertheless, fitting that he should have an approximate insight 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, which are fastened to the under side of the card.

DEVIATION

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 cause a deflection of the magnetic needle known as deviation. The difficulty is, of course, greatest with iron vessels, where the deviation of the needle is sometimes 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 we mean 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 will retain its magnetism permanently when once magnetized. Artificial magnets, therefore, are necessarily made of hard iron.

77. From these 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 have in its lower end a red or positive (+) pole, that, 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 that will attract the north end of the magnetic needle. If, however, the bar is reversed, we will 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 from 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 which it is subjected to, and that the character of this magnetism will depend on the direction in which the ship is built. Thus, a ship built in a north-and-south direction, with her bow toward the north, will have a red, or positive, polarity in her

forward part, as shown in Fig. 21 (a), and a 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 afterpart will repel the south end of the needle. If the vessel is being built in a northeasterly direction, as at (b), we find red, or positive, polarity on the port bow, and blue,