geographic north direction also varies from point to point on the earth, at some points being east of north and at others west of north. This fact was first observed by Columbus who, as he advanced in his voyage, was alarmed to see that the compass no longer pointed as it had done when he started.

The chart on the preceding page shows the declination of the magnetic needle throughout the United States in 1900. Isogonal lines connect places where the declination is the same.

H

496. Intensity of the Earth's Magnetism.-The magnetic force of the earth at any point may be considered as the resultant of two component forces, the horizontal component H (Fig. 268), and the vertical component V. The horizontal component H is the force which is effective in directing the compass needle. The smaller this component the more feebly will the needle be affected. When a needle is balanced or suspended in the usual way so as to vibrate in a horizontal plane, its period of oscillation depends on this component only.

R

FIG. 268.

As shown in paragraph 491, the intensities at different places may be compared by causing the same magnet to vibrate first at one place and then at the other. The intensities are proportional to the squares of the number of vibrations per second. By this means the horizontal component of the intensity may be determined at any point as compared with that at some standard place.

When the horizontal intensity and the dip are both known, the resultant intensity may be found by the relation

The following table shows the value of the horizontal component and total force at certain places. Notice how the horizontal force becomes less in higher latitudes. The intensity is given in C. G. S. units, or the force in dynes upon a unit pole.

Strength of the Earth's Magnetic Field (Dynes per Unit Pole).

One of the most

497. Secular Change in the Magnetic Field. remarkable features of the earth's magnetism is that it is continually changing. The declination of the needle is slowly changing everywhere; that is, the magnetic poles are slowly shifting their positions. At the same time the dip is changing. The changes in declination and dip at London are shown by the

FIG. 269.-Secular change in dip and declination at London. (After L. A. Bauer.)

curve in figure 269. The pole of a freely suspended needle would at that place apparently move through a complete cycle of change in about 470 years. This slow change is called the secular change.

498. Diurnal Variations.- The careful study of the magnetic conditions at any place by self-recording instruments shows that there is a periodic change in the magnetic elements depending on the time of day.

The curves of figure 270 show the average variations at Kew, near London. It will be observed that the maximum

changes take place in the daytime and may be due to variations in temperature of the earth's surface.

499. Irregular Disturbances.-The magnetic needle is also often disturbed by what are called magnetic storms; these disturbances usually accompany any marked display of the aurora borealis, and they also seem to be more prevalent at times of sunspot maxima.

FIG. 270.-Diurnal changes in dip, declination and intensity at Kew.

500. The Earth a Magnet. It was suggested by Dr. William Gilbert (1600), physician in the court of Queen Elizabeth and the first to take up the scientific study of magnetism, that the earth itself was probably a great magnet, and later observations have borne out this idea. Two well-marked magnetic poles being found, one northwest of Hudson Bay in North America and the other south of Australia.

But while there is this general resemblance to a simple magnet, the direction of the magnetic force varies from place to place in a way that cannot be wholly accounted for by the supposition of simply two poles.

The magnetism of the earth seems to be due to a variety of causes, the presence in the earth of magnetic masses is a cause of local variations and may have great influence in the surface layer of the earth, but it seems probable that the temperature in the interior of the earth is too high for it to possess any very strong magnetism. Electric currents flowing in the surface of the earth and due to its varying temperature as first one side and then another is exposed to the sun, as well as currents of electricity in the upper air, probably play an important part

in determining its magnetic state. But the complete explanation has not yet been given, and any satisfactory theory must account for the remarkable secular changes in its magnetism which go on slowly and progressively year after year.

501. Gauss' Method of Measuring the Horizontal Intensity.The horizontal component of the earth's magnetic force may be measured by the following method due to Gauss. A small steel bar magnet is suspended horizontally by a fine fiber in a closed box by which it is protected from air currents. It is then set oscillating through a small arc and the period of oscillation carefully determined. This period depends on M the magnetic moment of the magnet and on H the horizontal component of the earth's magnetic force. By $491

where K is the moment of inertia of the magnet, a quantity that is determined by its mass, size, and shape, and T is the period of a complete oscillation. The product HM is thus found.

To determine the relation of H to Ma second experiment is necessary.

net NS is placed exactly east or west of P and with its axis on the east and west line, as shown in Fig. 271. If r is the distance from the center of NS to P, then the force at P due to the magnet is, as shown in §492,

Then at P the force H due to the earth and the force F due to the magnet are at right angles to each other, as shown by the arrows in the figure. The needle at P will take the direction of the resultant force R and will therefore be deflected through the angle a, but