boreal pole which turns towards the south, the reason of which will be seen hereafter.

If, instead of mounting the needle on a pivot, it be attached to a piece of cork and placed in a vessel of water, so that the needle may float in a horizontal position, it will turn itself slowly around and come to rest in the same general direction as though it were balanced on a pivot. In this experiment it will be found that the needle once in the meridian, does not advance either towards the north or south. Hence we infer that the force exerted upon the needle is simply a directive one.

The force which causes a movable magnet to direct itself north and south is called the directive force.

Since the phenomenon described takes place at all points of the earth's surface, the earth has been regarded as an immense magnet, having its boreal and austral poles near the north and south poles of the earth, and a neutral line near the equator. This immense magnet acting upon the smaller magnets described, would produce all of the effects observed. When we come to explain the action of electric currents, it will be seen that there is another explanation of the directive power of the earth.

Magnetic Meridian.

Declination.

Variations.

352. When a balanced magnetic needle comes to a state of rest, it points out the line of magnetic north and south. If a plane be passed through the needle in this position and the centre of the earth, it is called the plane of the magnetic meridian, or simply the magnetic meridian.

This does not, in general, coincide with the plane of the true meridian, which is determined by a plane passing through the place and the axis of the earth. The angle which the magnetic meridian at any place makes with the

How is it shown that the magnetic force is simply directive? What is the directive force? Why has the earth been regarded as a magnet? Where are its poles (352.) What is the magnetic meridian ? What is the declination of the needle?

true meridian of the same place is called the declination of the needle. In short, the declination of the needle is its variation from true north and south. This is different at different places on the earth, and even at the same place at different times.

When the north end of the needle points to the east of true north, the declination is said to be to the east; when to the west of true north the declination is said to be to the west.

There is a line running from near Cleveland, Ohio, to Charleston, S. C., along which the needle points to the true north; this is called a line of no declination.

The line of no declination is travelling slowly to the westward at a rate which would carry it around the globe in about 1000 years. For all points of the United States east of the line of no declination, the declination of the needle is to the west; for all points to the west of it, the declination is to the east ; that is, the north end of the needle in all cases is inclined towards the line of no declination:

For all points in the United States to the east of the line of no declination, the declination is slowly increasing, whilst for all points to the west of it, the declination is slowly decreasing.

Besides this slow change in declination, the needle undergoes slight changes, some of which are pretty regular and others very irregular. In our latitude the north end of the needle moves towards the west during the early part of every day, through an angle of 10 or 15 minutes, and moves back again during the latter part of the day. This is called the diurnal variation. In the southern hemisphere this motion is reversed. There is also a small change of similar character which takes place every year, called the annual variation.

When is it to the east? To the west? What is the line of no declination? How does this line move? At what rate? Where is the declination to the west? To the east? How does the declination vary in the United States? What is the diurnal variation? The annual variation?

Irregular changes are called perturbations. They usually take place during thunder storms, during the appearance of the aurora borealis, and in general, when there is any sudden change in the electrical condition of the atmosphere.

The Compass.

353. The property possessed by magnets of arranging themselves in the magnetic meridian has been utilized in the construction of COMPASSES.

Fig. 243 represents a compass. It consists of a compassbox, having a pivot at its centre, on which is poised a delicate magnetic needle. Around the rim of the box is a graduated circle, whose diameter is somewhat less than the length of the needle, and of which the pin is the centre. The pin is of hard steel, carefully pointed; a piece of hard stone is let

What are perturbations? Illustrate. (353.) What is a Compass? Describe it.

into the needle, in which is a conical hole to rest upon the pivot, to diminish the friction between the needle and its support. In addition to the graduation on the circle, the bottom of the box is divided into sixteen equal parts, indicating the points of the compass.

This instrument under various forms is used for a great variety of purposes. It is used in navigation, in surveying, and is of importance to the traveller and explorer, to say nothing of its use in mining.

The magnetic declination at any place may easily be found when the true meridian is known. Let the compass be so placed that the line, NS, coincides with the true meridian, then when the needle comes to rest, the reading under the head of the needle will be the declination required. In the figure, if we suppose NS to be in the true meridian, the declination is 19° west.

The Dipping Needle.

354. When a steel needle, mounted as shown in Fig. 242, is carefully balanced before being magnetized, it is found, after being magnetized, to incline downwards or to dip. This dip is towards the north in our latitude, that is, the north end of the needle dips or inclines. The defect of dipping in the compass is remedied by making the other end of the needle a little heavier, by adding a movable weight, as a piece of wire wound round the needle, and capable of sliding along it.

To show the dip and to measure it, the needle is mounted in the way indicated in Fig. 244. The needle is suspended on a horizontal axis, so that it can move up and down freely, and the amount of the dip is indicated by a graduated circle or quadrant. The dip indicated in the figure is 54°, which is the angle made by the needle with

What is its use? How is the magnetic declination found at any place! (354.) What is a dipping needle? How is the compass needle prevented from dipping? How is the dip shown and measured?

the horizon. At any place the dip will be the greatest possible when the needle vibrates in the plane of the magnetic meridian,

The dip varies in passing from place to place, increasing as we approach the magnetic poles of the earth, where the dip is 90°; that is, the needle is perpendicular to the horizon.

The dip is subject to irregularities corresponding to those of the declination. The amount of the dip is an important element in forming a correct notion of the laws of terrestrial magnetism, and for this reason many observations have been made and are still making, to determine it at different places, and at different times at the same place.

Fig. 244

III. METHODS OF IMPARTING

MAGNETISM.

Magnetizing by Terrestrial Induction.

355. TO MAGNETIZE a body is to impart to it the properties of a magnet; that is, to impart to it the property of attracting magnetic bodies.

The only substances that can be permanently magnetized, are steel and the compound oxide of iron, which constitutes the loadstone. A body capable of being magnetized may be converted into a magnet by the inductive influence of

How does the dip vary? Is it subject to irregularities? (855.) What is meant by magnetizing a body? What substances can be permanently magnetized?