from a statue.

We see the left hand one more in front, the right hand one more in profile. On placing them in the stereoscope we see a single image in relief, as shown in Fig. 236. This image stands out in relief, presenting all the appearance of the statue from which the pictures are taken.

The peculiar form of the stereoscope seen in Fig. 236 is that of DUBOSCO. The lenses are large, and touch each other, so that they are adapted to eyes which are at any distance apart, which is not the casc in the instrument shown in Fig. 234. In that instrument the eyes must be at a certain distance apart, which does not permit the same instrument to be used by both children and adults.

Explain DUBOSCQ's form. Its advantages.

CHAPTER VII.

MAGNETISM.

I. GENERAL

PROPERTIES OF MAGNETS.

Definition of Magnetism.

344. MAGNETISM, as a science, is that branch of Physics which treats of the properties of magnets, and of their action upon each other.

Magnets.

345. A MAGNET is a body which exercises a particular power of attraction upon iron and a few other metals. Magnets are either natural or artificial.

Natural magnets are certain ores of iron, and are generally known under the name of loadstones.

The magnet is so called from the town of Magnesia, in Lydia, where it was first noticed by the Greeks. In its natural form it consists of a mixture of two oxides of iron, with a small proportion of quartz and alumina. It is now found in considerable quantities in Sweden and Norway, as well as in many other countries.

The magnet possesses the remarkable power, when freely suspended, of directing itself towards a particular point of the horizon, and it is to this property that its importance is

(344.) What is Magnetism as a science? What is a Magnet? How many kinds of magnets are there? What are natural magnets? Whence the name? What is the constitution of a natural magnet? What remarkable property does the magnet possess?

chiefly due. It may be suspended by a thread, or by balancing it on a pivot. In practice the latter method is the one most usually adopted.

Artificial magnets are bars of tempered steel, to which the property of the natural magnet has been imparted. The artificial magnet is far more valuable than the natural magnet, and is generally used in practice.

Steel is a mixture of iron with a small quantity of carbon, and when heated and then plunged into water, it becomes exceedingly hard, and capable of retaining the magnetism that may be imparted to it.

Artificial magnets for experiment are made of oblong bars, from twelve to fifteen inches in length, as represented in Figs. 245 and 246. They are sometimes made in the form of a horse-shoe, as shown in Fig. 247. Sometimes they are made in the form of a thin long needle, as shown in Fig. 239. This is the form in which they are constructed for pointing out the direction of the magnetic meridian, as in compasses. In this form they are also used in many magnetic experiments.

Magnets may be made of soft iron or untempered steel, but they do not retain their magnetism when the exciting cause is removed. Such magnets are called temporary magnets.

Distribution of Force in Magnets.

346. The force with which a magnet attracts iron, is not the same in all of its parts. The attraction is strongest at its extremities, from which it decreases towards its middle, where it is nothing.

This may be shown by plunging one end of a magnetized bar into iron filings; on withdrawing it, the filings will be seen adhering to it in long filaments, as shown in Fig. 237.

If the entire bar be rolled in the filings, it will be found that they adhere to both ends, but not to the middle.

What is an artificial magnet? What is steel? Describe an artificial magnet. What are temporary magnets? (346.) Where is the attraction strongest? How

shoron?

The two ends, where the attraction is strongest, are called poles, and the central part, where the attraction is nothing, is called the equator, or the neutral line.

Every magnet has two poles and one neutral line, whether the magnet be natural or artificial. Sometimes, besides the two principal poles, there are other minor poles, called secondary poles. In artificial magnets these arise from inequality of temper in the steel bars, or from want of proper care in magnetizing them. We shall suppose each magnet to have but two poles.

The action of a magnet upon iron takes place through intermediate bodies. If a magnetized bar be covered with a sheet of paper, and

What are poles? Equator? What are secondary poles? Their origin! How is it shown that magnetism is exerted through intermediate bodies!

then fine iron filings be sifted uniformly over the paper, they will be seen arranging themselves in regular curves around each pole, as shown in Fig. 238. No action is observed about the neutral line, the filings falling there as on any other surface.

Hypothesis of two Magnetic Fluids.

347. If we compare the action of the two poles upon soft iron, we observe the same phenomena at both. It is not so, however, when we compare the action of two magnets upon each other. If to the same pole of a magnetic needle, ab, balanced on a pivot (Fig. 239), we present in succession the two poles of a magnetized bar, held in the hand, we observe the curious phenomena, that if the pole, a, of the needle is attracted by the pole, B, of the bar, the pole, b, will be repelled by it; if the pole, a, is repelled, the pole, b, will be attracted.

To explain these phenomena, it has been supposed that there are two magnetic fluids, that is to say, two kinds of

(347.) What is the action of one magnet upon another? What is the theory of two fluids?