B.-Magnetism as an extra-terrestrial influence

Magnetic effects of extra-terrestrial origin cannot be directly recognised, but the variable magnetic condition of our earth shows conclusively the part which they play. In particular, the so-called disturbances of the earth's magnetic lines of force clearly point to the influence of other heavenly bodies. Thus magnetism is a universal bond which unites world to world.

50. Influence of the sun on the magnetic elements of the earth. Above all, it is the sun, the central body of our planetary system, which exerts a magnetic influence on the earth. The effect is not a thermal one, which thus indirectly modifies the distribution of the lines of force, but a direct magnetic effect due to the sun itself, for the variations do not correspond to the changes of temperature (which, as we know, lag considerably behind the apparent motion of the sun), but follow the position of the sun without retardation.

There are three phenomena which produce sensible variations in the magnetic elements of the earth's field:

(a) Period of the annual revolution of the earth about the sun. When it is winter in our hemisphere, the earth, moving in its elliptical orbit, is nearer to the sun than in summer. At the same time the dip and total force throughout both hemispheres have a somewhat augmented value. Thus, when the earth is nearer to the sun, the lines of force enter and leave the body of the earth more nearly vertically, and are more closely crowded together. In both hemispheres also a part of the variation of the declination-angle follows the annual changes in the relative positions of sun and earth.

(b) Period of the sun's rotation.-The sun rotates about an axis in the same sense as the earth, the period of a complete rotation being twenty-four and a half days; but since the earth at the same time is travelling round the sun, it is twenty-six days before the sun presents once more the same aspect to the earth. Both the horizontal magnetic force and the daily variation of the declination-needle

have a twenty-six-day period, which must correspond to the magnetic action of the sun as a whole.

(c) Periods of solar activity.—The radiation of heat and light from the sun is not always the same in amount, but varies from time to time like the aspect of his disc. We can distinguish without difficulty between times of greater and less solar activity. The former are characterised by a great abundance of spots, faculæ and protuberances, while in the intermediate periods of comparative quiescence these phenomena are less both in number and in degree. The alternations occur pretty regularly, with a period of eleven years, while the variations of the terrestrial magnetic elements have also a component with an eleven-year period. The diurnal variation of the declination is so closely related to the variations of solar activity that its mean value for any year may be reckoned to a fraction of a minute of arc from the number of sun-spots during the year.

Again, in mean latitudes, the frequency of the aurora borealis is related to the same period. At the times of greatest activity, that is when the sun-spots are most numerous, luminous phenomena, similar in character to the aurora, are to be observed in the highest strata of the atmosphere at nearly all places on the earth's surface.

The distribution of the earth's magnetic lines of force is influenced not only by solar activity in general, but by individual spots and protuberances. Thus a specially violent eruption on the sun is almost invariably associated with considerable magnetic disturbances (magnetic storms) on the earth. So far, indeed, it has not been found possible to establish a definite correspondence between the individual phases of the solar and terrestrial phenomena; but it is certain that the sun produces magnetic effects which are appreciable here. It does not follow that the sun behaves like an immense permanent magnet rotating about an axis.

51. Magnetic theory of the sun's corona.-The above relations lead us naturally to inquire whether some of the phenomena to be observed in the sun are not to be explained

as the result of its magnetic properties. An observer looking at the earth from a distance would be able to infer the course of the magnetic lines of force from the luminous rays of the aurora. The sun possesses, in fact, something which is very similar in character and is called the 'corona.' At times of no special solar activity this forms a somewhat thin, faintly luminous envelope, enclosing the ordinarily visible body of the sun, and exhibiting no special features. At times, however, it becomes greatly extended, and acquires a striated structure, appearing as a network of curved rays, some of which projected far into the atmosphere of rarefied gas by which the sun is surrounded. Suppose the line-offorce diagram of the magnetised sphere in fig. 22 to be formed for all planes through the centre of the sphere. The whole system of lines, as seen from a distant point, will form a kind of network, having the closest resemblance to the fully developed corona of the sun. It has been shown experimentally that rarefied gases become luminous more readily when placed in a magnetic field (EBERT), the rarefied atmospheres of the sun and earth being influenced in like manner by the magnetic fields of these bodies. Thus in a certain sense the rays of the corona may be regarded as the sun's magnetic lines of force rendered visible.

52. Influence of the moon on terrestrial magnetism.--The moon, which is nearer to us than any other heavenly body, has a considerable influence on the magnetic lines of force of the earth. It gives rise to a periodic movement of the declination-needle, the disturbance exhibiting two maxima and two minima between two successive transits of the moon across the meridian of the place of observation. There is also a lunar variation of the dip and of the horizontal force.

53. Effect of the planets on terrestrial magnetism.-A relation has long been recognised between certain variations of the terrestrial magnetic elements and the positions of the nearest planets, Venus and Mars, while recently, according to Leyst, it has been established that all the planets produce magnetic effects upon the earth. Especially re

markable is the influence of Mercury, the planet which is nearest the sun, and which, unlike all the other planets, is greater in density than the earth.

Venus, like the moon, exhibits changes of phase, so that at periodically recurring times it appears as a narrow crescent, a considerable part of its dark hemisphere being turned towards us. At such times especial luminous phenomena are often observed, and these have been supposed analogous to the aurora.

To obtain information on this point, it is important to find whether our observations show a periodicity in the brightness of this phenomenon. If it should prove to be related to the elevenyear periods of solar activity like the polar lights of the planet Earth, the similarity of nature of the two phenomena would be rendered very probable.

CHAPTER IV

THE MEASUREMENT OF FIELDS OF FORCE

IN this and the following two chapters some simple calculations will be introduced. The reader who wishes for the present to confine his attention to qualitative relations may proceed at once to Chapter VI.

A.-Static methods of measuring magnetic magnitudes; the absolute system of units

We shall now consider the nature of the magnitudes which are involved in quantitative magnetic determinations, and we shall describe those methods of measurement which depend on balancing magnetic forces by gravitational forces of known amount.

54. Gravitational field of force.-At all places on the earth's surface we find that freely movable masses have a tendency to move downward, this tendency being due to their weight. Since the form of the earth is approximately spherical, this may also be expressed by saying that the weights of all bodies are forces directed towards the centre of the earth, the lines of force of the earth's gravitational field all proceeding radially from this point. They differ, however, from the lines of force of magnetic (and electrical) actions in not being related to two opposite kinds of polarity; the only known gravitational forces are attractions, never repulsions. Every body, in accordance with NEWTON's law of universal gravitation, exerts a force of attraction on all neighbouring bodies, and the lines of force emanating from such a body run counter to those of the earth, like the