452

THE DIP-MAGNETIC VARIATION.

[220. that end of the needle which previously pointed towards this pole will begin to incline downwards, or to dip, until when the ends of the bar is exactly under the point n of the needle, the direction of the needle will become vertical. On bringing the opposite end of the bar towards the needle, like results may be obtained with the other end of the needle. Similar phenomena are also exhibited when a magnetic needle, poised horizontally at the equator of the earth, is carried towards either of its poles. A ncedle, therefore, which when unmagnetized is so poised as to assume a horizontal position, in the latitude of London, appears to become heavier at its marked end by the process of magnetization. An instrument by means of which the angular amount of this inclination can be accurately observed, is called a dippingneedle.

(221) Declination or Variation.-In each hemisphere there is a single point at which the dipping-needle stands vertically, i.e., where the dip is 90°. In the northern hemisphere this point was found in 1831 by Sir James Ross, situated in 96° 46′ W. lon. and 70° 5' N. lat.; the point where it would be vertical in the southern hemisphere being nearly in 75° S. lat. and 138° E. lon. The line of no dip does not coincide with the earth's equator; it forms an irregular curve inclined to it at about 12°, and crossing it in four places. This arises from the fact that the magnetic system of the earth is much more complicated than is represented in the foregoing paragraph. Instead of being single, it appears to be double, as was first pointed out by Halley, and in neither of these two systems does the magnetic axis coincide with the axis of rotation of the earth. Consequently in most places the needle does not point to the true geographical north. At the present time the needle in London points rather less than 1910 west of north. This deviation from the true north is termed the variation or declination of the needle.

In the northern hemisphere there are two lines of no declination, which pass through the point of 90° of dip, and a closed curve including part of Siberia and China. These lines of no declination have reference to a double magnetic system of which the two points of maximum force in the northern hemisphere are resultants; and these points were called by Halley magnetic poles. They do not correspond to either of the points of 90° of dip, which have also been called magnetic poles.

It is remarkable that the declination of the magnetic needle is not constant at the same spot. In the year 1657, the needle, pointed due north at London. It then gradually assumed a decli

222.] MAGNETIC DECLINATION AND INCLINATION-INTENSITY. 453

nation to the west, which continued to increase until about the year 1840, at which time the variation to the west, in London, was nearly 25°; since this period it has been gradually returning towards the east, and on January 1, 1867, it was 20° 52′ W. at Kew. The rate of its motion differs in different parts of its progress, becoming slower as it approaches the point of retrogression; at present it is about 8' annually. Independently of these gradual and progressive changes, the variation is subject to diurnal movements of very small amount: north of the magnetic equator in England and the middle latitudes, the north end of the needle moves slowly eastward in the forenoon, attaining its maximum between the hours of seven and ten A.M., and returns to its mean position at about ten in the evening. Connected with these alterations are corresponding variations in the dip, which during the last fifty years has been observed in London to diminish annually about 2'6 From observations made at the Kew Observatory, the dip, in January, 1867, was 68° 5′.

The following table, kindly supplied by Mr. G. M. Whipple, the superintendent of the Kew Observatory, shows the changes of the mean values of the declination and dip which have taken place since 1868

The total diminution of declination from 1868 to 1876 is therefore 1° 1′ 16′′, the yearly average being 7′ 39′′5, and of inclination 15′ 22′′, or 1′ 55′′25 per year on the average.

(222) Variation in the Intensity of the Earth's Magnetism.The intensity of the earth's magnetism is also found to vary at different points of the surface, but the law of its increase has not been clearly determined; the line of minimum intensity, or magnetic equator, as it is sometimes called, is in the vicinity of the geographical equator, but does not coincide either with this or with the line of no dip: it forms an irregular curve cutting both of these lines. The points of greatest intensity, moreover, do not coincide with those at which the dipping-needle is vertical.

The

454

MAGNETISM OF THE EARTH.

[222.

highest degree of intensity that has been actually measured is 2052, the lowest 0706.* Both the maximum and minimum here mentioned are in the southern hemisphere. If it be supposed that the globe be divided by a plane passing through the meridians of 100° and 260°, the western hemisphere, comprising America and the Pacific Ocean, presents a higher intensity than the eastern; but the charge of the northern and of the southern hemisphere is equal. In the northern hemisphere there are two points of maximum intensity, the most powerful being in North America, and determined by Lefroy, in 1843-44, to be situated in 52° 19′ N. lat. 92° W. lon., the intensity being 1.88. The weaker maximum was found by Hansteen in 1828-29 in Siberia, in 120 E. lon. with an intensity of 176. Sir James Ross in 1842-43, found the principal maximum in the southern hemisphere in about the meridian of 134° E. and a few degrees North of the Antarctic circle, whilst the weaker maximum in the southern hemisphere, according to Sabine, is about 130° W. The intensity of the magnetic force at London is now 1372.

The intensity of the earth's magnetism, like the variation and the dip, is found to suffer periodical changes. Besides these regular variations of the magnetism of the earth, other irregular variations have been observed. These have been termed magnetic storms: they are indicated by sudden and considerable disturbances of the magnetic instruments, of short duration, which are produced by some widely acting causes, as these disturbances have been noticed simultaneously at very distant parts of the earth's surface. In extreme cases, the diminution of the magnetic intensity during the 'storms' has amounted to a large portion of its total intensity. Sabine considers that these magnetic storms are connected with changes in the solar atmosphere, which are indicated by variations in the number and form of the spots upon the sun's disk; their epochs of maximum recurring at intervals of 1111 years, with epochs of minimum intensity occurring midway between each pair of maxima. These intervals coincide

*The unit of intensity used in the text is that proposed by Humboldt, derived from the value of a particular magnet which he employed; but in the later magnetic observations the unit of intensity employed has been that recom mended by the Royal Society, viz.: a second of time, a foot of length, and a grain of mass. The magnetic intensity upon this scale at London was, in January, 1867, 10301; and in January 1877, 10 2809: the horizontal force being at the latter date 3.8896. The two last numbers expressed in the centimetre-gramme-second system of units will be 4740 and 1793 respectively.

222.]

with the

MAGNETISM OF THE EARTH.

455

11-year epochs of maximum and minimum of the solar spots observed by Schwabe.*

Since, then, the earth may be looked upon as an immense magnet of small intensity, it is natural to expect that, under favourable circumstances, magnetic induction should arise from its influence. Such effects are indeed continually observed.

If

a soft iron bar be placed in the line of the dip, it acquires temporary magnetic properties, the lower extremity in the northern magnetic hemisphere acting as the marked pole of a magnet upon a magnetized needle, while the upper extremity acts as the unmarked pole. By reversing the position of the bar, the end which is now the lower will still possess the magnetism of the marked pole. A bar of steel which is kept in a vertical position (a line in this latitude not far removed from that of the dip), is from this cause frequently found to be permanently, though weakly, magnetic. It is to the same cause operating through the lapse of ages, in the same direction, upon the loadstone, that its polarity is to be ascribed.

If a steel bar be made to vibrate while placed in the line of the dip, as by giving it a smart blow, it is magnetized still more powerfully, and this effect may be still further increased by the induction of other masses of iron placed in contact with it. For instance, by allowing a steel bar, supported in the line of the dip, to rest upon an anvil, and striking it strongly with a hammer, it becomes decidedly magnetized. All permanent magnetism may, however, again be removed from it by placing it across the line of dip, and striking it two or three blows as before.

Iron, nickel, and cobalt are the only substances which are powerfully magnetizable; but a susceptibility to magnetism in a much feebler degree has, by the researches of Faraday and others (323), been proved to exist in a variety of other bodies. Before describing the method in which these experiments were conducted, it will be necessary to examine the leading phenomena of electricity; and these will now be considered.

* A singular corroboration of this theory is afforded by an observation of Mr. Carrington, who was watching a large spot on the sun on 1st September, 1859: suddenly, at 1 1b. 20 min. A.M., a bright spot was seen in the middle of the dark one; this appearance lasted for about ten minutes, and a corresponding disturbance in time and duration was indicated by the self-registering magnetometers at Kew. Brilliant aurora were simultaneously observed in various parts of the world.

456

TWO KINDS OF ELECTRICITY.

[223.

§ II. STATIC ELECTRICITY.

(223) Electricity is one of those subtle and all-pervading influences which are intimately connected with the operations of chemical attraction. Indeed some eminent philosophers have been disposed to regard electricity and chemical attraction in the light of different manifestations of the same cause.

For upwards of 2000 years it has been known that when amber is rubbed upon bodies such as fur, or wool, or silk, it acquires for a short time the property first of attracting light objects, such as fragments of paper or particles of bran, and afterwards of repelling them. Until about 280 years ago, amber was the only known substance by which such effects were produced. About that time Gilbert discovered that a number of other bodies, such as glass, sealing-wax, and sulphur might be made to excite similar motions. The power thus called into action has been called electricity, from λEKтpov (amber), the body in which it was first observed. Independently of its origin in friction, it has been found that electricity is liberated by chemical action, by certain vital operations, by heat, by magnetism, by compression, and in fact by almost every motion that occurs upon the face of the globe. Electricity neither increases nor diminishes the weight of bodies under its influence, and neither enlarges nor reduces their volume. It may be excited in all substances, may be communicated from one electrified or excited body to another previously in a neutral or unelectrified condition, and it may be stored up for the purposes of experiment.

FIG. 168.

(224) Two kinds of Electricity.A very simple contrivance will suffice for examining the fundamental phenomena of electricity as developed by friction:

Soften a little sealing-wax in the flame of a candle, and draw it out into a thread 8 or 10 inches (20 or 25 centimetres) long, and of the thickness of a stout knitting pin. Attach to one end of it a disk of paper about an inch in diameter, as represented in fig. 168; suspend this rod and disk by means of a paper stirrup and a few fibres of unspun silk from a glass rod fixed horizontally to some convenient support. Now rub a stick of