the earth, or more rapidly by being rubbed by another magnet, or finally, by the action of electricity, in which case the operation is instantaneous.

The magnetic ores of iron may exist as magnets in the natural state, or they may possess no trace of magnetic action. But they are highly susceptible to magnetic influence, and once magnetized, they retain their magnetic action by virtue of their strong coercive force.

Natural magnets owe their magnetism to the slow action of the earth, which separates the two fluids in them. The magnetic action of the earth is so great as to be used successfully in forming artificial magnets.

To use this principle, we place a thin bar of iron in the magnetic meridian and incline it to the horizon by an angle equal to the dip. In this position the earth acts upon it by induction, driving the austral fluid to the lower end (in our latitude), and the boreal fluid to the upper end.

The magnetism thus induced is only temporary, for if the bar be moved from its position, the two fluids return to a state of equilibrium. If, however, when the bar is in position, it be struck smartly by a hammer, or if it be violently twisted, sufficient coercive force may be developed to retain the induced magnetism for a time.

Magnetizing by Friction.

356. Bars of steel, and needles for compasses, are usually magnetized by rubbing them with other magnets. The three methods are called the methods by single touch, by separate touch, and by double touch.

To magnetize a steel bar by single touch, we hold the

Are the magnetic ores of iron always magnets? To what is the natural magnetization of these ores due? How are bars magnetized by this principle? (356) How may bars of steel be magnetized? Explain the method of single touch.

body to be magnetized in one hand, and with the other we pass over it a powerful bar magnet, as shown in Fig. 245. After several repetitions of this process, the steel is found to possess all the properties of a magnet. These properties

are the more durable in proportion to the hardness of the steel.

To magnetize a steel bar by separate touch, we rub it in one direction with one pole of a magnetized bar, and in the opposite direction with the opposite pole.

To magnetize a body by double touch, we make use of two magnetized bars, which are placed with their opposite poles in contact with the bar at its middle point, being only separated by a small interval, as shown in Fig. 246; the combined bars are then moved alternately in opposite directions to the two ends of the bar, and the operation is repeated several times. Care must be taken to apply the same number of touches to each end of the bar.

Bundles of Magnets.-Armatures.

357, A BUNDLE OF MAGNETS consists of a group of magnetized bars united, so that their poles of the same. name may be coincident.

Sometimes these bundles are composed of straight bars, like that shown in Fig. 245, and sometimes they are curved in the shape of a horse-shoe, as shown in Fig. 247.

Magnets, if abandoned to themselves, would lose in a short time much of their power; hence it is, that armatures are employed.

An ARMATURE is a piece of soft iron, placed in contact with the poles of a magnet. Thus, ab, in Fig. 247, is an

armature.

(357.) What is a Bundle of Magnets? What is an Armature?

The poles, acting by induction upon the armature, convert

a into an austral, and b into a boreal pole. These two poles reacting upon the poles of the magnet, AB, prevent the recomposition of the two fluids, and thus preserve its magnetism. The armature is sometimes called a keeper.

If weights be attached to the keeper till it separates from the magnet, we can, from the number of pounds applied, judge of the power of the magnet.

For many kinds of magnetic experiment the horse-shoe form is preferable. It is also the form best adapted to the application of an armature or keeper.

The most powerful horse-shoe magnets are formed by means of electrical currents. Magnets of this kind have been constructed by Prof. HENRY, of the Smithsonian Institution, capable of sustaining a weight of more than a ton and a quarter.

Fig. 247.

A keeper? How can we judge of the power of a magnet? What are the ad vantages of the horse-shoe magnet ?

358. ELECTRICITY, as a science, is that branch of Physics which treats of the laws of attraction and repulsion exhibited by bodies under certain circumstances. Such phenomena are called electrical phenomena. The name electricity is derived from the Greek elektron, which means amber.

Discovery of Electrical Properties.

359. Six hundred years before the commencement of the Christian era, THALES, of Miletus, knew that when yellow amber was vigorously rubbed with wool, it acquired the property of attracting light bodies, such as small pieces of paper, barbs of quills, straws and the like. Comparing this action to suction, the ancients said that amber had a power of suction, and sucked light bodies towards it. In consequence of the rarity of amber, whose origin is even in our day unknown, they went so far as to say, that it was formed from the tears of an Indian bird, grieved at the death of King MELEAGER.

Six centuries later, PLINY, an eminent Roman naturalist, writes: "When the friction of the fingers imparts heat and life to yellow amber, it attracts straws, just as the magnet attracts iron." This

(358.) Define Electricity as a science. What are electrical phenomena? Whence the name? (359.) Give an outline of the history of electrical discoveries.