stands vertical; that is, where the inclination is 90°. In 1830 the first of these, the terrestrial north pole, was found by Sir James Ross, in 96° 43' west longitude and 70° north latitude. The same observer found in the South Sea, in 76° south latitude and 168° east longitude, that the inclination was 88° 37′. From this and other observations, it has been calculated that the position of the magnetic south pole was at that time in about 154° east longitude and 75° south latitude.

Lines connecting places in which the dipping needle makes equal angles are called isoclinic lines.

The inclination is subject to secular variations, like the declination. At Paris, in 1671, the inclination was 75°; since then it has been continually decreasing, and in 1859 was 66° 14′. In London also the dip has continually diminished since 1720 by about 2·6′ per annum. In 1821 it was 7c° 3′; in 1838, 69° 17′ ; in 1854 it was 68° 31'; in 1859 it was 68° 21′; it is now 67° 50'. It is also subject to slight annual and diurnal variations; being, according to Hanstein, about 15′ greater in summer than in winter, and 4′ or 5' greater before noon than after.

CHAPTER III.

METHODS OF MAGNETISATION.

384. Magnetisation by the influence of the earth.—To magnetise a substance is to impart to it the magnetic properties of attracting particles of iron, and of turning towards the north. Magnetisation can be produced slowly by the influence of the earth, or rapidly by rubbing with a magnet; or by means of electricity, in which case the magnetisation is instantaneous.

The magnetic action of the globe is powerful enough to act as a source of magnetisation. This may be illustrated by taking a tolerably thick iron wire, and placing it in the magnetic meridian, so that it makes an angle equal to the angle of dip. In this position the earth's magnetism, acting by induction on the iron wire, decomposes the two fluids, and converts the lower end into a north pole, and the upper into a south pole. Yet this magnetisa. tion is very unstable, for if the wire be turned upside down, the

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Magnetisation by the Earth.

395

poles are inverted, for pure soft iron is destitute of coercive force. But, if while the bar is in the above position it be hammered, or if it be twisted, the pressure or the twisting it undergoes imparts to it a certain amount of coercive force, and it retains for some time the magnetisation evoked in it. If several wires thus magnetised are united so that poles of the same name are together, a tolerably powerful magnet is obtained.

It is this magnetising action of the earth which developes the magnetism frequently observed in steel and iron instruments, such as fire-irons, railings, lightning conductors, lamp posts, etc., which remain for some time in a more or less inclined position. They become magnetised with their north pole downward, just as if placed

over the pole of a powerful magnet. The magnetism of native black oxide of iron has doubtless been produced by the same causes; the very different magnetic power of different specimens being partly attributable to the different positions of the veins of ore with regard to the line of dip. The ordinary irons of commerce are not quite pure, and possess a feeble coercive force; hence a feeble magnetic polarity is generally found to be possessed by the tools in a smith's shop. Cast-iron, too, has usually a great coercive force, and can be permanently magnetised.

The turnings, too, of wrought iron and of steel produced by the powerful lathes of our ironworks are found to be magnetised.

Magnetisation by magnets. In magnetising bar magnets, and

especially magnetic needles, the method generally adopted is to rub them with powerful magnets. This principle is applied in the methods of what are called single, separate, and double touch.

The method of single touch consists in moving the pole of a powerful magnet from one end to the other of the bar to be magnetised, and repeating this operation several times, always in the same direction. The neutral fluid is thus gradually decomposed throughout all the length of the bar, and that end of the bar which

This method only

was touched last by the magnet is of opposite polarity to the end of the magnet by which it has been touched. produces a feeble magnetic power, and is, accordingly, only used for small magnets. It has further the disadvantage of frequently developing consequent points.

In the method of separate touch the steel bar is rubbed separately with the contrary poles of two magnets, proceeding in opposite directions from the centre towards the ends.

Magnetisation by double touch. In this method the two magnets are placed with their poles opposite each other in the middle of the bar to be magnetised. But, instead of moving them in opposite directions towards the two ends, as in the method of separate touch, they are kept at a fixed distance by means of a piece of wood

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Magnetic Batteries.

397

placed between them (fig. 305), and are simultaneously moved first towards one end, then from this to the other end, repeating this operation several times, and finishing

in the middle, taking care that each half of the bar receives the same number of frictions.

Magnetisation by means of electrical currents is the most powerful means of imparting magnetism, and is the one generally used for large magnets, whether bar or horse-shoe.

Arma

385. Magnetic batteries. tures.-Magnetic battery, or magazine, is the name given to a system of bars joined with their similar poles together. Sometimes the bars are straight, as represented in figs. 304 and 305, and sometimes they are curved, as in fig. 306, which represents a horse-shoe battery.

B

Magnets, whether natural or artificial, would soon lose their power if they were left to themselves, and they must therefore be provided with armatures. These names are given to pieces of soft iron which are placed in contact with the poles, such as the piece, ab, in fig. 306. The two poles of the magnet acting inductively on this piece produce in it at a a north pole, and at b a south pole, and these two poles thus produced react in turn upon the magnetised bar, and by preventing the recomposition of its two fluids cause it to retain its force. The piece, ab, is also called the keeper; to it is suspended the weight which the magnet is intended to support.

Fig. 306.

BOOK VIII.

FRICTIONAL ELECTRICITY.

CHAPTER I.

FUNDAMENTAL PRINCIPLES.

386. Electricity. Its nature.—Electricity is a powerful physical agent which manifests itself mainly by attractions and repulsions, but also by luminous and heating effects, by violent commotions, by chemical decompositions, and many other phenomena. Unlike gravity, it is not inherent in bodies, but is evoked in them by a variety of causes, among which are friction, pressure, chemical action, heat, and magnetism.

Thales, one of the Greek sages, 600 B.C., knew that when amber was rubbed with silk it acquired the property of attracting light bodies, such as feathers, pieces of straw, etc., and from the Greek form of this word (ÿλekтpov, electron) the term electricity has been derived. Six centuries after it was found, Pliny, the celebrated Roman naturalist, writes, 'When the friction of the fingers has imparted to it heat and life, it attracts pieces of straw as a magnet attracts particles of iron.' This is nearly all the knowledge left by the ancients; and it was not until towards the end of the sixteenth century that Dr. Gilbert, physician to Queen Elizabeth, called attention to this property of amber, but showed that it was not limited to amber, but that other bodies, such as sulphur, wax, glass, etc., also acquired the property of attracting light bodies when they are rubbed or struck with flannel or with catskin.

To repeat this experiment, a glass red, or a stick of sealing wax, or shellac, is held in the hand, and rubbed with a piece of flannel, or with the skin of a cat ; it is then found that the parts rubbed have the property of attracting light bodies, such as pieces of silk, wool,