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affect the compass, which has therefore to be corrected by placing compensating masses of iron near it, or by fixing it high upon a mast.

135. The Earth a Magnet.—Gilbert made the great discovery that the compass needle points north and south because the earth is itself also a great magnet. The magnetic poles of the earth are, however, not exactly at the geographical north and south poles. The magnetic north pole of the earth is more than 1000 miles away from the actual pole, being in lat. 70° 5' N., and long. 96° 46 W. In 1831, it was found by

Sir J. C. Ross to be situated in Boothia Felix, just within the Arctic Circle. The south magnetic pole of the earth has never been reached; and by reason of irregularities in the distribution of the magnetism there appear to be two south magnetic polar regions.

136. Declination.-In consequence of this natural distribution the compass-needle does not at all points of the earth's surface point truly north and south. Thus, in 1881, the compass-needle at London points at an angle of about 18°33' west of the true north. This angle between the "magnetic meridian " 1 and the geographical meridian of a place is called the magnetic Declination of that place The existence of this declination was discovered by Columbus in 1492, though it appears to have been previously known to the Chinese, and is said to have been noticed in Europe in the early part of the 13th century by Peter Pellegrinus. The discovery is also claimed, though on doubtful authority, for Sebastian Cabot of Bristol. The fact that the declination differs at different points of the earth's surface, is the undisputed discovery of Columbus.

In order that ships may steer by the compass, mag

1 The Magnetic Meridian of any place is an imaginary plane drawn through the zenith, and passing through the magnetic north point and magnetic south point of the horizon, as observed at that place by the pointing of a horizontally-suspended compass-needle.

netic charts (Art. 139) must be prepared, and the declination at different places accurately measured. The upright pieces P P', on the "azimuth compass" drawn in Fig. 65, are for the purpose of sighting a star whose position may be known from astronomical tables, and thus affording a comparison between the magnetic meridian of the place and the geographical meridian, and of measuring the angle between them.

137. Inclination or Dip.-Norman, an instrumentmaker, discovered in 1576 that a balanced needle, when magnetised, tends to dip downwards toward the

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north. He therefore constructed a Dipping-Needle, capable of turning in a vertical plane about a horizontal axis, with which he found the "dip" to be (at London) an angle of 71° 50'. A simple form of Dipping-needle is shown in Fig. 66. The dip - circles used in the magnetic observatory at Kew are much more exact and delicate instruments. It was,

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however, found that the dip, like the declination, differs at different parts of the earth's surface, and that it also undergoes changes from year to year. The "dip" in London for the year 1881 is 67° 39'. At the north magnetic pole the needle dips straight down. The following table gives particulars of the Declination,.

Inclination, and total magnetic force at a number of important places, the values being approximately true for the year 1880.

TABLE OF MAGNETIC DECLINATION AND INCLINATION (for Year 1880.)

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138. Intensity.—Three things must be known in order to specify exactly the magnetism at any place; these three elements are:

The Declination;

The Inclination, and

The Intensity of the Magnetic Force.

The magnetic force is measured by one of the methods mentioned in the preceding Lesson. Its direction is in the line of the dipping-needle, which, like every magnet, tends to set itself along the lines-of-force. It is, however, more convenient to measure the force not in its total intensity in the line of the dip, but to measure the horizontal component of the force,—that is to say, the force in the direction of the horizontal compass-needle, from which the total force can be

calculated if the dip is known.1 Or if the horizontal and vertical components of the force are known, the total force and the angle of the dip can both be calculated.2 The horizontal compor ent of the force, or "horizontal intensity," can be ascertained either by the method of Vibrations or by the method of Deflexions. The mean horizontal force of the earth's magnetism at London in 1880 was 18 dyne-units, the total force (in the line of dip) is 47 dyne-units. The distribution of the magnetic force at different points of the earth's surface is irregular, and varies in different latitudes according to an approximate law, which, as given by Biot, is that the force is proportional to √ + 3 sin2, where is the magnetic latitude.

139. Magnetic Maps.-For purposes of convenience it is usual to construct magnetic maps, on which such data as these given in the Table on p. 115 can be marked down. Such maps may be constructed in several ways. Thus, it would be possible to take a map of England, or of the world, and mark it over with lines such as to represent by their direction the actual direction in which the compass points; in fact to draw the lines of force. A more useful way of marking the map is to find out those places at which the declination is the same, and to join these places by a line. The Magnetic Map of England which forms the Frontispiece to these Lessons is constructed on this plan. At Bristol the compass-needle in 1888 will point 19° to the west of the geographical north. The declination at Torquay, at Stafford, at Leeds, and at Hartlepool, will in that year be the same as at Bristol. Hence a line joining these towns may be called a line of equal declination, or an Isogonic line. It will be seen from this map that the declination is greater in the north-west of England than

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1 For if H = Horizontal Component of Force, and I = Total Force, and ✪ = angle of dip, I = H÷cos @.

2 For H2+V2= 12, where V = Vertical Component of Force.

in the south-east. We might similarly construct a magnetic map, marking it with lines joining places where the dip was equal; such lines would be called Isoclinic lines. In England they run across the map from west-south-west to east-north-east. On the globe

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the isogonic lines run for the most part from the north magnetic pole to the south magnetic polar region, but, owing to the irregularities of distribution of the earth's magnetism, their forms are not simple. The isoclinic lines of the globe run round the earth like the parallels

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