ends. (The words, from their brevity, and their applicability to the colour of the paint put on magnets, are convenient it has long been the custom of tradesmen to paint with red the north-seeking end of a magnet.) In the diagrams, the red end will be distinguished by a cross-hatching and the blue end by a longitudinal hatching. (It is usual for tradesmen to mark the northseeking end by a transversal file-mark.)

Introductory Diagram explaining the representations
of Magnets.

Poles of Red Magnetism, seeking the North.

Poles of Blue Magnetism, seeking the South.

A horse-shoe magnet is merely a straight magnet bent into the horse-shoe form: it will be shewn hereafter that the properties of the two ends differ in the same manner as those of the ends of a straight magnet.

8. Method of magnetizing steel bars, and of preserving their magnetic power.

It is not easy to say how artificial magnets were formed in the first instance. Probably they may have been derived from the natural magnet; whose attractive properties, and whose power of producing temporary

magnetism in iron, have been known from a very distant age. But, magnets having been once formed, there is little difficulty in forming other magnets from them. The most convenient process for magnetizing a steel bar or compass-needle, &c. is that known by the name. "double touch." It requires the use of two magnets. The bar which is to be magnetized being laid horizontally, with some slight band to prevent it from moving, the operator takes one magnet in his right hand with (say) the red end downwards, and one in his left hand with the blue end downwards (or both in the opposite positions, according to the nature of the magnetism which he wishes to impart), and, touching the bar with the ends of the magnets near the middle of its length Fig. 1.

(see Figure 1), he draws the two magnets simultaneously to the two ends of the bar (constantly maintaining the contact) till they slip off. He raises the magnets, again places them in contact with the middle of the bar, and again slides them to the ends: and repeats this operation: the motion, while in contact, being always from the middle to the ends. The bar is thus converted into a magnet: the end of the bar which was touched by the red end of the magnet employed becomes a blue end; and vice versa. The magnetizing magnets, in

general, suffer no deterioration from this employment. The new steel magnet will retain its magnetism through an indefinitely long period: its permanency depending greatly on the quality of the steel. The steel best adapted for large magnets is that which is best for fine. cutlery and the steel should be perfectly hard through the whole length of the bar. For compass-needles, the same steel at spring-temper is found more advantageous.

It is possible, by various contrivances, as for instance by holding both the hand-magnets with the red ends downwards or both with the blue ends downwards, to create a magnet with a concentration of magnetism in the middle of its length. Such magnets however are useless, and we shall not further notice them.

Some artists prefer the following method of magnetizing simultaneously three or more bars. The bars are laid so as to form a closed circuit, and a powerful horseshoe magnet is placed upon any one bar with both its ends in contact with the bar, and is carefully carried thus round the whole circuit of bars, always in contact, and with the same end of the horse-shoe magnet always preceding. (See Figure 2.) The circuit is repeated

Fig. 2.

several times without lifting the horse-shoe magnet. The red and blue ends of the resulting magnets are respectively opposed in position to those of the horseshoe magnet. (On the mode of distinguishing the ends of a horse-shoe magnet, we shall speak shortly.)

In either process, after a time, a limit to the intensity of the communicated magnetism is reached. This is usually expressed by the phrase "magnetized to saturation."

The steel which is the most valuable for retention of magnetism is also the most favourable for reception of a strong dose of magnetism.

For preserving the magnets with full magnetic intensity, it is found prudent to place them side by side with their red and blue ends in opposite positions, and to connect the opposite ends (the red end of each with the blue end of the other) by pieces of iron in contact with both.

Valuable information connected with this subject will be found in the Encyclopædia Metropolitana, article Magnetism.

9. The terrestrial force upon a magnet is a Couple: the red end is drawn towards the north, the blue end towards the south, with equal forces. First Law of Magnetism, the Duality of Powers.

If a magnet, on which the Earth's directive power is strong, be suspended by a very long suspension-thread, and the position of the thread be noted; if then the magnet be removed and a lump of lead of equal weight

be substituted for it; the suspension thread takes exactly the same position as before. This shews that, upon the whole, there is no horizontal force tending to produce a motion of translation of the magnet; and therefore, if there is one force tending to draw the red end towards the north, there is an equal force tending to draw the blue end towards the south.

If a small magnet, as a compass-needle, be supported by two small pieces of cork and floated on water, it speedily takes the north-and-south position, but it shews no disposition to approach any side of the basin.

These experiments are very important. They shew either that there are different attractions from different parts of the Earth upon different parts of the magnet, or that attraction of the Earth on one part of the magnet is accompanied with equal repulsion on another part. We shall find that, without negativing the first of these suppositions, other experiments shew that the second is universally true: that attraction on one part of a magnet is universally accompanied with repulsion on another part. And thus we arrive at the first important law of magnetism, the "Duality of Powers." It is this duality which essentially distinguishes the force of magnetism from that of gravitation: in other respects, it will be seen, there is much similarity of their laws.

10. Action of one magnet upon another. Second Law of Magnetism. There is attraction between dis similar ends, repulsion between similar ends. This is exhibited in various ways. Idea of poles. When one