The Magnetism, Electricity, And Radioactivity Of The Earth

( Originally Published 1915 )

THE preceding chapters have shown us how far the terrestrial globe resembles a living being, with its period of growth and development, its internal activity, the regular pulsation of its crust, and the convulsive shocks which agitate it at intervals. We are now to see that phenomena of circulation are produced in the terrestrial crust under the form of electric currents with associated magnetic phenomena, which are inseparable from the former. The likelihood of magnetic phenomena is obvious from what we have previously learned about the Earth's nucleus. By reason of the enormous pressures to which its various parts are subjected, it is in a condition practically equivalent to the solid state, in spite of the high temperature, and is furthermore constituted of metallic elements, among which iron predominates. It is thus not surprising that the terrestrial globe in its entirety should exhibit magnetic properties.

On the other hand, the Sun possesses a considerable electrostatic charge and therefore creates an electric field about itself. The physicist Nodon was undoubtedly the discoverer of the electric action of the solar rays; in 1885, he proved that the Sun's radiation produced a positive charge in an insulated conductor, a charge which increased with the intensity of the radiation, the phenomenon ceasing when clouds passed before the Sun. In 1905, Bernard Brunhes confirmed Nodon's results by a very beautiful series of experiments, and Nodon himself, at the Pic du Midi in 1907, clearly demonstrated the solar electrical action. Our Sun is therefore charged with electricity. This charge doubtless surpasses in magnitude any imaginable one; Arrhenius's calculations show that it is always as great as 250 thousand million coulombs.' It produces an electrostatic field.

The charge, turning rapidly around the Sun's axis of rotation, must give rise to a magnetic field, this latter being a consequence of the motion of a charge, as was discovered by Rowland. This phenomenon was disputed for a long time, but the experiments of the Roumanian physicist Vasilesco Karpen have definitely verified its existence and measured it quantitatively. The intensity of the field so created is doubtless considerable in the neighbourhood of the solar surface but certainly feeble at a distance such as that of the Earth from the Sun, for the law of decrease of magnetic action is that it is inversely as the cube of the distance. Nevertheless, even if feeble, the field undoubtedly exists, and the con-ducting nucleus of the Earth moves through it with great velocity like the armature coils of a dynamo in the field of its field magnets. There must, thus, be produced "Foucault's currents" which flow through the terrestrial mass.

This is not all, however; the positively electrified surface of the Sun sends into space small negatively charged particles; as we have previously seen, this repulsive effect being superior to the attractive force in the case of very small particles, because the pressure of radiation is relatively greater upon them. Many of these minute fragments reach the Earth's atmosphere. The effect of ultra-violet light, as the work of Lenard has shown, is to discharge these particles, and it is their negative charge which escapes in the form of what are called electrons in the language of modern physics. These electrons are excessively small, and some idea of them may be obtained from the fact that a thousand of them weigh almost as much as one atom of hydrogen, while one gram [15.4 gr.] of hydrogen contains a number of atoms represented by the figure I followed by twenty-four zeros.

By the well-known demonstration of iron filings arranging themselves along the magnetic lines of force, we know that these lines of force converge to the poles of a magnet (Figs. 23 and 24). Now, the rays of the solar corona have a deflection towards the equator from the poles similar to the lines of the magnetic figures. We may therefore suppose that the Sun behaves like a great magnet, the magnetic poles of which practically coincide with the geometrical poles.

Electrified particles shot forth from the Sun may reach the Earth, and, in the course of the present chapter, we shall see beautiful experimental verifications of this theoretical conception. These particles convey their charges, the influence of which is felt in the atmosphere and at the surface of the ground. The ions of the upper atmosphere partake of the Earth's movement ; they have been repelled by the Earth, which is similarly electrified, and remain in the higher layers of our atmosphere, where they produce electrical phenomena which have a powerful generative or modifying action on the trrestrial magnetism.

To recapitulate, the Earth moves in an electric and magnetic field due to the Sun, and receives from the Sun particles which bring charges to its surface. Any change in the intensity of the solar radiation will modify the intensity of the observed effects; also any change in the velocity of displacement of the Earth in this field will cause the indirect effects to vary. Now, Kepler's law tells us that such modifications of velocity do occur, and we may therefore expect periodical variations of electric and magnetic phenomena. Such being the theoretical considerations, we shall now examine the results of actual observations.

Terrestrial magnetism, with which we shall commence, shows itself in a simple way by the directive effect which the Earth has on a magnetised needle freely suspended at its centre of gravity. This elementary experiment, more or less modified, is the basis of all our study of the manifestations of the Earth's magnetism.

The vertical plane which contains the needle is called the magnetic meridian. It points nearly towards the north pole of the Earth, but in general does not coincide with the geographical meridian of the place, the angle between the two being called the declination. The angle made with the horizontal by the needle is known as the inclination, and the intensity of the horizontal component of the force directing the needle, which is an important quantity, is often called the horizontal intensity.

If a magnetic needle be suspended and means are furnished for the measurement of its direction with all possible precision, it may be proved experimentally that, in any one place, the magnetic elements vary during each day. These variations recur regularly every day in the year, and their mean values also vary according to the season. As regards the declination, which is actually 10° to the west at Paris (1912), this quantity passes daily through a maximum and minimum value, but the actual degree of variation is slight, attaining only a few minutes of arc. The inclination and the horizontal intensity undergo analogous variations, but these differ in sign in the respective hemispheres. The variations are greater in warm weather than in cold weather.

The declination shows annual periodical variations such as the theory indicates; every year there is a maximum and a minimum, the changes of sign, that is to say the passages through the mean value, taking place about the time of the equinoxes. These epochs also determine the change of sign of the variations of the inclination and the horizontal intensity.

No relation between the lunar period and the variation of the magnetic elements has yet been established with certainty. It is the contrary with regard to the solar spots and we here find one of the most beautiful confirmations of the theoretical views which we have enunciated at the be-ginning of this chapter. The years of maximum solar spots are those in which the variation of the declination and that of the horizontal intensity also attain their maxima, and the curves which represent these three phenomena more or less coincide with one another.

Thus, at any one place, the declination, in common with the other magnetic elements, under-goes variations of daily and annual periodicity, and also one of i i years' period, and these all correspond to the known periods of the variations of the effects resulting from the solar activity, whether on account of the varying amounts of heat received by the Earth, or by reason of the variation in the distance between the two bodies, or finally because of the change in the radiation emitted by the Sun.

But this is not all; if the declination be carefully observed, and if its mean value be taken for each year, it may be proved that it varies showly from one year to another and that this secular variation appears to be also periodic. At Paris, for example, observations of the declination have been made since the year 1540. The phenomenon itself was discovered by Christopher Columbus, in 1492, at the time of his voyage resulting in the discovery of America. Now, in 1540, the declination at Paris was to the east, that is to say the magnetised needle pointed to the eastward of the geographical north (Fig. 25) ; its value subsequently increased and passed through a maximum value a little before the year 1600.

Then it began to decrease and reached zero in 1660, in which year compasses indicated the true north in the French capital. After 1660, the declination changed sign and became westwards, increasing in value each year. At the beginning of the nineteenth century, it attained a maximum of about 24° since when, though remaining west-wards, it has continually diminished. Actually it is about Io° to the west and is thus approaching the zero value again, when it will change sign and pass to the east, where it was before 166o.

As regards the inclination, the precise results of direct observations are less decisive, and its importance, for sailors and travellers, is much less than that of the declination, which gives them a fixed direction when the stars are hidden by cloud. But, in the course of recent years, Giuseppe Folgheraiter has thrown a great light on the matter by some very original work.

Potter's clay is magnetic; a vase moulded from such clay becomes therefore magnetised by induction under the influence of the terrestrial field, that is to say it exhibits two poles so placed that the line which would join them is parallel to the direction of the dip needle. Thus the vase behaves just as a piece of soft iron would. But, if while thus subjected to the action of the Earth's magnetic field, the clay vessel is placed in an oven and baked, its magnetisation will become permanent and the two poles, situated on a line parallel to the direction of the dip needle, will remain permanently. If, therefore, we knew in what way the vase was oriented in the oven with respect to the geographical north we could deduce, by investigation, its permanent magnetism, both the direction of the magnetic meridian and the value of the inclination at the time when it was baked.

The researches of Folgheraiter were carried out on Etruscan vases, the antiquity of which is considerable. These vases, having the forms of surfaces of revolution, give no exterior indication by which we can arrive at their orientation in the oven at the time of their manufacture; they cannot thus furnish us with any information as to the declination. But as regards the inclination, which is the angle made by the magnetic needle with the horizontal, they give us a sufficiently exact value, for they were always placed on a horizontal plane in the baking oven and, whatever subsequent positions they were placed in, the angle made by their poles with the horizon can be determined by again replacing them in a horizontal position.

A very remarkable result was deduced and it was at first very much disputed, viz., that the magnetic inclination must have been zero, in Central Italy, towards the middle of the sixth century B.C., and that in the preceding years, that is to say in the course of the seventh century B.C., several specimens of the art of which remain to us, the north pole of the magnetic needle was inclined above the horizontal instead of below it as at the present time.

Bernard Brunhes, in 1906, was enabled to con-firm these conclusions of Folgheraiter by means of investigations he carried out on the permanent magnetisation of the lavas of the Puy-de-Dôme. His work dealt with the metamorphic clay of the lava of Pontfarein, in Cantal; in contact with the burning lava this clay is baked in situ as if it had been in a pottery oven. Brunhes has deduced clear indications of a change of sign of the inclination having occurred in these regions at an epoch not very different from that which Folgheraiter indicated.

Brunhes has, however, done yet more. He has made a study of the pavement of the Temple of Mercury erected on the summit of the Puy-de-Dôme and has justified the principle of the work of the Italian scientist. The paving stones which form the floor of the temple are constituted of volcanic rocks and each one has retained a permanent magnetisation. The declination deduced varies from one stone to another, as would be expected, since they are oriented in different ways, but the inclination is the same for all; the elements of their magnetisation, which date from the time of their cutting, have therefore not been affected by the subsequent variations of the terrestrial magnetism. Consequently the conclusions of Folgheraiter on the ancient values of the inclination, deduced from the study of the Etruscan vases, are perfectly legitimate.

Thus the magnetic elements not only vary in the course of each day, each year, and each eleven-year solar period, but also suffer slow variations in the course of successive centuries. Here we again realise that there is a perpetual evolution in those forces, the play of which constitutes the life of our Earth. We shall now see that these variations with time are not the only ones, but that there are also variations according to position on the Earth's surface.

A magnetised steel needle freely suspended about its centre of gravity takes up a position inclined to the horizontal. By applying a light counterpoise to the higher end, we are able to force it into the horizontal position and we may then prove that it will always direct itself towards a point on the horizon called the magnetic north, being free to move in the horizontal plane in which it is constrained to remain.

If we move over the Earth's surface, walking Always in the direction of a horizontal magnetised needle, that is, a declination needle, we shall go towards this special north, and our journey will be, not along a terrestrial meridian, but over a curved line which is called a magnetic meridian. All such magnetic meridians converge towards a point situated in Northern Canada, to which the name of the North Magnetic Pole has been given. In the southern hemisphere, there is a South Magnetic Pole, situated in Victoria Land, part of the Antarctic continent not far from the volcanic mountains Erebus and Terror. The North Magnetic Pole has been several times reached by explorers, the latest being the Danish explorer Roald Amundsen some years ago; as regards the South Magnetic Pole, Sir Ernest Shackleton deter-mined its position in 1910. It should be noted that at either of these poles a magnetised declination needle will lie equally in any direction and not only in one fixed one, whereas the inclination needle is vertical. It is this latter which enables us to determine the situation of the magnetic pole.

The magnetic poles are not fixed at the Earth's surface. They are distant from the geographical poles, but the North Magnetic Pole, although incessantly moving, never wanders far from latitude 69° north, while the mean latitude of the South Magnetic Pole is 75°. Between 1770 and 1888 the North Magnetic Pole moved from latitude 66° to latitude 71°; it has reached a point more than 600 kilometres [372 miles] nearer the terrestrial pole than at the earlier date. It now appears to be retreating again. This non-fixity of the magnetic poles, the incessant fluctuation of _their position, corresponds to the secular variation of the elements of terrestrial magnetism, and one of the phenomena is a direct consequence of the other.

In order to go from one point on the Earth to the magnetic pole we have only to follow a route always tangential to the direction of the inclination needle. For every place on the Earth the value of the declination may be measured, that is to say the angle between the directions of the needle and the geographical meridian may be found. By such means it can be shown that the declination varies from one point to another of the globe. It is a matter of the greatest importance, for sailors and travellers, to know these variations for the different parts of the Earth, since when they are unable to observe the stars in order to deduce the position they are in at any time they can only direct themselves by means of the magnetic needle. Consequently it is essential to know the difference between the magnetic north and the true north at a given place, and how this difference varies from point to point of the terrestrial surface. Magnetic maps of the Earth have been drawn up by tracing on a planisphere lines passing through points on the Earth's surface where the declination has the same value; these lines are called isogonic lines.

It is obvious that all the isogonic lines must pass through the magnetic poles. They also pass through the geographical poles, since the declination is the angle between the magnetic meridian and the geographical meridian. As all geographical meridians pass through the poles of the Earth the declination there can have any value, so that the isogonic lines must all meet together there. Fig. 26 gives an idea of such a map. Similar curves have been drawn for the inclination and the horizontal intensity, but the importance of the declination for navigation and land journeys makes the isogonic maps of more immediate interest.

There are certain peculiarities which strike one at once upon this map, for example, there are three lines of zero declination, drawn thicker than the rest.

Between the two chief thicker lines passing through points on the Earth where the declination is zero are found regions where it is west ; outside them it is east. Nevertheless there are two portions of the Earth's surface characterised by singularities. There is a closed curve on Eastern Siberia, along which the declination is zero, and in its interior the declination again becomes west. Also in the Eastern Pacific there is another closed curve corresponding to a mini-mum declination. It will be seen from these re-marks how curious the distribution of terrestrial magnetism is.

As the magnetic elements vary with time and also according to the region of the Earth considered, it is obvious that magnetic maps should be frequently remade to correspond with the new values of the elements, so that travellers should have correct data and not erroneous ones which might lead them wrongly, and even into danger.

The variations of the magnetic elements of which we have spoken up to the present have been the slow and continuous ones; there are others of a sudden character which constitute magnetic storms and perturbations.

When magnetic instruments of great precision are installed in an observatory, enabling us to indicate, and preserve by photographic registration, the least variation in terrestrial magnetism, we ordinarily observe the periodic variations that have been described above. But on certain days the needles are agitated; they tremble and exhibit quite erratic movements, their oscillations obeying no regular law. Often in such cases these irregularities and agitations of the magnetic needle are great enough to be observed in ordinary compasses. Such a phenomenon is called a perturbation or magnetic storm.

A magnetic storm always makes itself felt over a considerable portion of the Earth's surface, and very frequently its occurrence coincides with polar aurorae and with important seismic phenomena. We have seen that it is possible to conceive how the movements of the internal nucleus may affect the magnetism and produce disturbance of the crust, so that it is not surprising that these phenomena exhibit a certain degree of coincidence. We shall see later on why polar aurorae often manifest themselves at the same time as magnetic storms. It is an incontestable fact, the result of actual observation and not only of theory, that the forms of the curves representing respectively the periodicities of the solar spots, the aurorae boreales, and magnetic storms are identical; the three curves have exactly the same aspect and the same irregularities.

Independent of the general variations which the Earth's magnetic elements undergo as we pass from one point of the surface to another, local anomalies may be observed exactly analogous to the case of the value of the intensity of gravity, where we observe local irregularities arising from particular local effects at the given point in question. The crust being of varying thickness, the surface is consequently unequally distant in different parts from the central nucleus containing the metallic elements to which the earth's magnetism is due. Furthermore, as the crust itself may contain more or less magnetic mineral matter, we may readily understand how purely local variations may arise from both these causes, viz., an exceptional thickness or thinness of the crust at the place under consideration and its geological nature.

This general explanation, while it is doubtless sufficient in many cases, is, however, far from satisfactory in others. Thus, in the region of Paris, there exists a very marked local anomaly; the isogonic lines are folded on themselves in the form of an S with very serrated bends. Now it is not possible to find a magnetic cause of this anomaly in the geology of the Parisian region ; the strata are, in fact, chalk. The question arises whether the cause of the anomaly is to be sought for in the deeper strata. The S-shaped curve formed by the isogonic lines seems to be the continuation of a great fault in the district of Bray, and possibly this fault, by reason of the resulting geological modification, affects the circulation of the electric currents which, as we shall shortly see, incessantly traverse the terrestrial crust. Another suggestion is that as the Tertiary Parisian basin was in some measure a marine formation, the ocean which formerly existed there corresponded to a thinner crust, according to the theory of Lippmann, and consequently the magnetic interior of the Earth is relatively near the surface of the ground in that region. The matter has not yet been fully elucidated.

Magnetic phenomena are not the only ones that indicate the Sun's influence upon the Earth; there are electric phenomena which manifest themselves in various ways around us; the first and the most important, from a practical point of view, is the existence of earth currents. In the early days of the electric telegraph the physicist Matteucci showed that, at times, the telegraph lines indicated grave disturbances, and he remarked and called attention to the coincidence of these perturbations with magnetic storms and the appearance of polar aurora.

At the present time, the phenomenon is better known, and telegraph lines are the best possible instruments for its study. It consists of the pas-sage of currents quite different to those which circulate normally in the wires. These, being superimposed on the currents transmitting the messages, confuse the latter and produce signals which are unconnected with those despatched along the wires.

These earth currents make the bells ring and sometimes even cause a spark to pass between different parts of the receiving apparatus. The electromotive force of these currents is sometimes nearly 1000 volts, the lines they traverse being several hundred kilometres [or miles] in length. By utilising for their study the telegraphic line which connects Clermont-Ferrand with the summit of the Puy-de-Dôme, Bernard Brunhes has clearly proved the influence of the inclination of the line on the degree of their manifestations, which seem to have little connection with atmospheric phenomena, but appear on the contrary to correspond closely with magnetic phenomena. Like the latter, the earth currents follow a regular periodicity and thus show recurrent variations, but the chief perturbations have an accidental character, and almost always coincide with the appearance of polar aurore, also with magnetic storms and with important seismic disturbances. At the beginning of November, 1903, telegraphic perturbation of the kind caused by earth currents took place, producing an almost complete interruption for two days of the service in Western Europe. This very intense manifestation of the special activity of the earth currents coincided exactly with an aurora borealis, with a magnetic storm of exceptional intensity, and with an earth-quake which destroyed the town of Turchiz in Persia on November 1st. Furthermore, it is remarkable that, at the same time, a spot of extraordinary dimensions made its appearance on the Sun's surface. In order to render complete our account of terrestrial electric currents, it should be added that there are also currents between the ground and the atmosphere; a positive current appears to flow upwards from places of mean latitude and to be transmitted by the upper atmospheric layers, returning to the ground in the neighbourhood of the equatorial regions, the circuit being completed by the current traversing the ground from south to north. So there is an effect analogous to the true earth currents, and one which in certain cases is probably superimposed on them.

Thus, we have the fact that these electric manifestations, the earth currents, bear some direct relation to the solar activity, and, consequently, with all the phenomena that depend on the latter, which we have noted at the beginning of this chapter. This relation is another confirmation of those theoretical conceptions which trace to the solar energy and its fluctuations all the very varied manifestations of energy observable at the Earth's surface. But it is not only the action of the solar radiation that produces electric phenomena at the surface of our globe; another cause is to be sought in the dust particles repelled from the Sun, if sufficiently small, by the pressure of its radiation, and consequently driven forth into space. These particles give rise to another class of electric phenomena which occur in the terrestrial atmosphere.

The Sun, being in a magnetic condition, presents two magnetic poles, just as the Earth itself does.

The Solar Corona, according to the beautiful theory of Arrhenius, is composed of very minute particles which the pressure of radiation has driven far away from the Sun's surface. The coronal streamers, formed by these particles which come from the region of the Sun near the poles, are deflected under the influence of the magnetic lines of force emanating from these poles, which act upon the negatively electrified particles. On a large scale, it is exactly the same as the elementary physical experiment of the magnetic figures obtained with iron filings, demonstrating both the existence of the lines of force and the direction of the field. Under the repulsive action of the pressure of the Sun's radiation part of this solar dust arrives in the neighbourhood of the Earth. As the latter is magnetic and has two poles, it exerts an influence upon the particles. Consequently these become grouped into two streams which are directed towards the magnetic poles of the Earth, and, as in all probability the magnetic poles do not crop out at the surface of the ground, but are situated at some depth in the Earth's interior, these attracted streams are simply drawn towards a region of roughly circular form surrounding the terrestrial magnetic poles. When the multitude of arriving particles is more abundant than usual, owing to exceptional solar activity at the time, their electrification is bound to affect the Earth's magnetism.

When the dust particles enter the atmosphere, and so meet the molecules of air, they produce a phosphorescent glow exactly as if this air was subjected to the action of electric radiation arising from a piece of some radioactive substance. In other words, the negatively electrified particles driven from the Sun are discharged on entering the upper regions of the Earth's atmosphere and emit cathode rays, to which the polar aurorae are due. Professor Birkeland has attempted an experimental study of the particular circumstances of the origin of the aurora, by means of laboratory researches. He took a sphere of magnetised steel, representing the Earth, covered with a fluorescent coating. He then exposed the sphere to the action of cathode rays, the point of contact of which with the sphere was shown by the illumination of the fluorescent coating produced. Thus, he artificially reproduced luminous phenomena resembling polar aurorae, and as he used the cathode rays, which are now considered to consist of small negatively charged particles moving with considerable velocity, just like the solar dust above described, the experiment affords a beautiful confirmation of the theory of aurorae that has just been briefly given.

Another confirmation has been furnished by the remarkable observations of the Italian astronomer Ricco. If the theory is exact and if aurorae, and the electric phenomena of which the Earth is the seat, have their origin in the dust particles expelled from the Sun's surface, we should expect more important manifestations at the epochs of greatest eruptive solar activity. These epochs are those at which the solar faculae are most developed, at which periods the sun-spots also are largest and most frequent. In a word, the maxima and minima of aurorae and of magnetic perturbations should coincide with those of the Sun's activity, and observation has already shown that such is the case. Now, if the cause of aurorae is to be found in the contact of solar dust with the Earth's atmosphere, this dust, which is material substance, cannot be transmitted through space with an infinite velocity, but on the contrary must occupy a certain time in reaching us from the Sun.

It is actually possible to calculate this velocity. Let us consider a very small non-transparent particle .00016 millimetre [i mm. = .03937 in.] in diameter, a size which corresponds to the maximum value of the pressure of radiation and consequently to the greatest velocity of propulsion, and let this particle have unit density, the same as that of water. The particle will be subjected to the action of the solar gravity, which attracts it towards the Sun, and to that of the repulsive radiation pressure, which is two and a half times greater than the former. A mean velocity of 740 kilometres [450 miles] per second is thus arrived at by calculation, and this implies that the particle takes fifty-six hours to pass over the distance separating the Sun from the Earth. It should be remarked that we have assumed the density of the dust particles to be equal to that of water, but they have in all probability a less density than this, since they are probably formed of hydrocarbons containing hydrogen and helium in solution. If the particles considered have a density equal to two thirds that of water, the calculation gives the result that under the resultant repulsion force the particles would reach the Earth from the Sun in forty-five hours.

Now Ricco has found precisely an interval of 45 1/2 hours between the passage of a sun-spot over the solar meridian and the maximum amplitude of the corresponding magnetic perturbation, this result being based on half a score of clearly observed cases; in another series he arrived at an interval of 42 1/2 hours. This constitutes a remarkable concordance between calculation and observation, and justifies a feeling of pride that Man is able so to overleap the apparent limitations of his environment and attain to knowledge from which at first sight he would seem for ever debarred. The explanation of the mysterious aurorae is thus simple; they surround the pole of the line of fall of the dust particles which produce them and so appear to shine to the north of this line for places which are exterior to it, and towards the south for those which are contained within it. Physicists call this line the neutral line.

The study of the presence of the solar dust in the Earth's atmosphere enables us to understand yet another thing. The negatively charged particles driven, as we have already seen, from the Sun by the pressure of radiation, meet our atmosphere and discharge themselves to earth, producing aurorae. This discharge of negative electricity communicates to the Earth's surface, and maintains an electrostatic negative charge which constitutes what is known as atmospheric electricity. A study of this phenomenon has shown that the electrical potential increases in proportion as the point of observation is higher above the ground; in the neighbourhood of the ground the increase of potential with height is, on the average, 150 volts per metre [39.37 in.]. The Earth's charge augments when the solar spots increase.

We cannot, here, enter into a detailed description of the effects of atmospheric electricity; lightning, thunder, electrification by induction and the effects of thunder-storms, are all clearly described in works on elementary physics and in popular books. What we have to remark here is that there appears to be a direct bond between solar activity, the cause of the Earth's life, and storms, which are one of the most striking manifestations of terrestrial activity. The high clouds which float in the atmosphere, the cirri, are formed in great abundance as a consequence of the production of aurorae. We know in fact that when the air is charged with water vapour and when also a strong ionisation has been produced under the influence of the cathode rays, condensation is facilitated, or, in other words, circumstances are favourable to the formation of clouds, the ions having the property of condensing vapours. Abundance of cirri should thus accompany the maxima of solar spots. Observations during fifty years permit us to state that there is an agreement between the maxima of the number of cirri and the maxima of the number of sun-spots, the periods of both phenomena being eleven years.

Cirri may also be electrified by the action of ultra-violet rays. These rays have the property of rendering gases conductors of electricity, that is to say, of ionising them. Furthermore they discharge the negative electrification of any body they fall on, while not affecting the positive charge. Cirri, which are formed of fine needles of ice, often pass above clouds that have been inductively electrified by the proximity to the ground; in this case they are in turn subjected to the inductive influence of these clouds and their component needles are charged, negatively at one extremity, positively at the other. In these conditions if a beam of cathode rays should happen to strike them their negative charge would be dissipated, and they would remain positively charged. Here again, consequently, we trace to the Sun an electrical atmospheric phenomenon.

The importance of these electro-atmospheric phenomena is extremely great, especially as regards the Earth's animal and vegetable life, for they determine the combination of the nitrogen of the air with oxygen and hydrogen and accordingly give rise to nitrates, nitrites, and ammoniacal compounds, the great importance of which is now understood by agriculturists. These compounds of nitrogen are carried down to the soil by the action of rain, and in this way more than 400 million tons are brought down yearly.

There is, finally, another electrical property of the Earth, viz.: its radioactivity. At the commencement of the history of radioactive phenomena, which were discovered by Becquerel, in 1896, and of which radium, subsequently discovered by Mme. Curie, will facilitate the study, it was believed that only minerals which had produced radioactive bodies, for example, the pitch-blende from which uranium is extracted, possessed these remarkable properties. We now know, however, that the phenomenon is general, and that all bodies are more or less radioactive. The terrestrial crust is the seat of a radioactivity which may be demonstrated by pushing a tube into the ground for a depth of one metre [or yard] and breathing the air found there; this air is always more or less charged with emanations. It follows that the air in caves or caverns is especially so charged. All mineral waters are radioactive, as Professor Moureu has discovered; they contain these rare gases of which one, helium, has been obtained by Sir W. Ramsay as a product of the transformation of radium emanation. As this emanation is very rapidly dissipated, this explains why the greater number of these waters are only efficacious, from the therapeutical point of view, when drunk directly from the source, before the gases have had time to dissipate, while when conveyed to a distance they lose all the gases which constitute the chief cause of their curative power and become merely simple saline solutions. Professor Moureu has even shown that, excluding helium which is one of the products of radioactive emanations, there is a constant mutual ratio between the amounts of the rare gases argon, krypton, xenon, neon found both in mineral springs and in natural gaseous mixtures such as mine emanations.

This is readily understandable; at the origin of the Earth's formation by the process of condensation, these gases did not combine with any other of the elements that were successively formed, owing to the chemical inertia of the former, inhibiting the production of compounds. They therefore remained in the free state while the other elements formed combinations among themselves; consequently they have persisted unchanged through, and unaffected by, all the cataclysms and convulsions which have marked the successive states in the Earth's history.

The atmosphere in contact with radioactive soil is itself radioactive. Physicists always find there traces, of course infinitesimal, of the emanation, and freshly fallen rain or snow invariably shows signs of radioactivity; it is the same with the water of the sea. Concordant experiments have demonstrated that the activity of a gram [15.4 gr.] of radium is halved in about 2000 years; throughout that time it continues to emit 120 calories' per hour, say in round numbers, one million calories annually. If, therefore, the terrestrial globe contains a quantity of this substance in its central core it would possess a considerable reserve of internal heat. How can radium thus give out heat-energy for so long a time? Does it absorb some kind of radiation from space which it is able to transform into heat by unknown means?

The radiation of radioactive bodies comprises three species of rays : (I) the a-rays, composed of positively charged particles, travel approximately 20,000 kilometres [12,400 miles] per second; these particles are atoms of helium; (2) the B-rays, which are negative electrons whose mass is 1/1700 part of that of an atom of hydrogen, and which move with the velocity of light 1; (3) the y-rays, analogous to the X-rays.

Helium is always found in radioactive minerals and it is derived from radium emanation. Helium, therefore, seems to be an ultimate element; it occurs as the final product of the disintegration of other atoms; it represents in fact a starting point for the integration of the more complex atoms.

Radioactive substances uranium, actinium, radium set free helium. Sir William Ramsay has proved this fact in regard to radium; other experiments have enabled him to ascertain that copper is transformed into potassium, sodium, and lithium and also that lead, thorium, titanium, and silicon become transmuted into carbon under the influence of the energy set free by the radium emanation. If these experiments be confirmed, this result is of the most supreme importance as regards the theory of matter. In any case, potassium, sodium, and rubidium are feebly, but distinctly, radioactive. It thus appears that radioactivity is a general property of matter.

Heavy atoms become transformed into simpler ones, losing energy in the process. This degradation is spontaneous, and it is only when it is occurring with a slowness that renders it quite imperceptible in the duration of our existence and experiments that we consider such matter as stable.

But now arises the question as to the origin of the heavy atoms, for example those of thorium or uranium. These by breaking up and transformation can give birth to those of lesser atomic weight but cannot themselves arise from a previous degradation. It is therefore natural to suppose that they arise from an inverse process of integration of matter, starting from simple atoms such as helium, under the influence of considerable energy.

We are here led back, in this consideration of the origin of radioactivity in the Earth's crust, to the important problem of the age of our globe. We have seen in a preceding chapter that the period that has elapsed since the formation of the solid crust lies between i 000 and 2000 million years. If we try to formulate the time that has passed away since the Earth became an independent body, after its detachment from the nucleus of the solar nebula, we must reckon at least a million million years.

If the Earth had been entirely formed of uranium, a million million years would have been a more than sufficient time for the whole of it to be transformed. Now uranium is actually found in the Earth's crust, whence we must conclude that this radio-active substance is formed in the mass of our globe.

If the Earth contained 1/5,000,000,000 of a gram of radium per cubic centimetre, this would suffice to prevent its cooling; we know at the present time sufficient of the constants of radioactive material to make such a calculation. Now observation shows that the radioactive material in the terrestrial crust is, on the average, twenty times greater than this. Our Earth should there-fore be getting hotter and the deduced duration of the geological periods would be increased beyond all probable limit. Consequently we must assume, as the English scientists have done, that the whole quantity of radioactive material present in the Earth is contained in a very thin layer of the Earth's crust situated in the immediate neighbourhood of its exterior surface. The thickness is probably only a very few kilometres [or miles]. Thus we are brought to a difficulty. Either the interior of the globe contains neither uranium nor thorium or else the heavy atoms of these sub-stances are formed there by the integration of matter under the influence of the colossal pressures which obtain in the mass of the central nucleus.

Moreover, Arrhenius has considered the possibility of formation, in the central parts of bodies whose inner regions remain heated, of endothermic compounds, locking up an immense quantity of energy, truly explosive bodies in comparison with which dynamite and the picrates would be mere playthings!

In discussing the origin of radioactive substances we have thus found a remarkable consequence, viz., the necessity of supposing that the evolution of matter constitutes a cycle. There is atomic decomposition or disintegration on the one hand, and on the other there is certainly a compensating integration, which assures the permanent co-existence of all kinds of matter.

All the facts of which we have taken note in the course of this chapter point to one thing, viz., that the Earth possesses a magnetic state.

What is this state? How are the elements, to which the magnetic action of the Earth is due, distributed under the surface on which we live? How are they arranged in such a way as to show the influences of solar radiation? These are questions to which the science of Physics can at present give no definite answers.

Fortunately, however, in the absence of answers which could be furnished by some great theoretical conception that has yet to be attained, an English scientist, M. H. Wilde, of Manchester, an ingenious and expert experimenter, to whom we are indebted for the first self-exciting dynamo, has constructed a wonderful apparatus which, with an almost marvellous exactness, reproduces not only the actual distribution of magnetism on the Earth's surface but even the secular variations of this distribution in the course of centuries. This instrument has been named by its inventor, the magnetarium. Wilde was led to this conception by his quite original cosmogonical theory, and the accuracy of the magnetic results, therefore, also emphasises the value of this theory. Since, for the first time, all the peculiarities of such a complex phenomenon as that of the distribution of terrestrial magnetism and its secular variations have been artificially reproduced in the laboratory, the theoretical ideas which led to such an achievement cannot be valueless and merit the fullest attention of those scientists, who by the aid of mathematical analyses make it their province to construct theories concerning the origin and functions of worlds.

We have seen in studying the birth of the Earth that the spheroidal agglomeration of incandescent material, which at a later period constituted our planet, gradually cooled in such a way as to be-come surrounded by a superficial solidified layer. On the other hand, we also know that the Earth while traversing its orbit remains inclined to the plane of the orbit, the angle between the planes of the orbit and the Earth's equator being 231°. Wilde holds that it was not always so and that at a certain time, extremely long ago since we are considering the incandescent phase previous to the formation of the solid crust, the Earth rotated about an axis perpendicular to the plane of the ecliptic. In these circumstances the magnetic axis of the system of electric currents arising from the solar energy would be parallel to the polar axis about which the incandescent spheroidal Earth rotated. At a later period, the superficial solidification occurred, covering the mass with a rocky crust. Wilde believes that at that time the axis of rotation about which the crust turned was inclined to the ecliptic as at present, while the central nucleus continued turning about the axis of its original rotatory movement. Thus, according to the English physicist, in place of a single permanent axis, the Earth has possessed two from the time of the formation of its crust : first, the actual axis, serving as axis only for the solid envelope; secondly, the primitive axis upon which turns the igneous mass that constitutes the central nucleus of the globe. Furthermore, Wilde has arrived at the conclusion that this internal mass rotates about the primitive axis with a smaller angular velocity than that of the crust turning about the inclined axis.

Another point of the theory is that the superficial layers became magnetic as they cooled, the magnetisation taken as a whole being parallel to the inclined axis, so long as, at the time of its solidification, the exterior surface of the crust remained almost level. But from the time when the crust became subject to foldings and contractions, its resulting deformations produced a magnetisation of very great complexity.

To summarise, Wilde holds that the terrestrial magnetism is the resultant of two component elements, one connected with the actual constitution of the Earth's solid envelope, the other due to interior currents having as axis of symmetry a line inclined to the axis of rotation of the crust, slowly describing a cone around the latter, on account of the inequality of the velocities of rotation of nucleus and crust.

In order to arrive at a material representation of this complex phenomenon the English physicist took a sphere like those which form terrestrial globes. This globe was mounted, as are the greater number of those used for teaching purposes, in such a way that it rotated about an axis, the two extremities of which were supported by a copper semicircular arc, forming a semi-meridian. This arc is itself capable of sliding in its own support in such a way that any point whatever of the Earth's surface may be brought to have the same horizon and the same zenith as the place of the experiment. A rigid arm, fixed to the support of the entire apparatus, enables either a small inclination needle, or a small declination needle to be placed above the point so chosen; consequently the elements of the artificial magnetism given to this magnetic globe may be experimentally measured.

In order to give his globe magnetic properties Wilde supplied it with a series of insulated wires wound according to the parallels of latitude in such a way as to constitute a sort of spherical bobbin. It follows from the laws of electromagnetism that in this case the system behaves like a sphere magnetised in a direction parallel to its axis of rotation.

This first globe contains a second one turning about a hollow axis enclosing the axis of the outer globe. The inner globe is also covered with wire so as to resemble a spherical bobbin, but it is not wound according, to the parallels of latitude; it is so wound that the poles of the spirals are the two extremities of a diameter making an angle of 18° with the axis of rotation, viz., the difference in latitude between the North Geographical and North Magnetic Poles of the Earth. A mechanism with a differential train of wheels enables the two globes to be made to turn simultaneously in such a way that the interior globe is subjected to an angular retardation of 12° in each turn relatively to the outer globe. In these circumstances it may be shown that the system is equivalent to a magnet, the line of whose poles is inclined to the Earth's axis at an angle less than 18° and which rotates continuously about that axis, the two bobbins being traversed by suitable currents supplied to them. Determinations of the declination and inclination for different places on the globe's surface were made with the above arrangement by the little test needles, but the result did not come up to expectation.

The idea then occurred to Wilde of altering the inclination of the magnetic axis of the interior globe so as to make an angle of 23 1/2°, instead of 18°, with that of the exterior globe. In other words the two axes made an angle with each other equal to that between the planes of the terrestrial equator and the ecliptic; consequently at certain periods of the movement the magnetic axis of the interior globe became perpendicular to the plane of the terrestrial orbit. The instrument so arranged showed approximately the successive values of the magnetic elements observed at London. One complete turn of the exterior globe corresponded to an angular displacement of 12° with reference to the interior globe, which corresponds to an interval of time of thirty-two years. We, therefore, draw the conclusion that the general period of the secular variation is about 96o years.

The results, however, although giving fairly exact values for the magnetic elements at London showed notable discrepancies for other places on the Earth when compared with the actual values observed at these places. Also, the distribution of magnetic meridians and isogonal lines on the magnetarium was more regular than the actual terrestrial ones. In order to remedy this Wilde conceived the very original idea of covering those portions of the surface of the magnetarium which represented the oceans with layers of sheet iron of a suitable thickness cut to shape. The result was remarkable. Not only did the instrument reproduce exactly the actual values of the magnetic elements at the various portions of the Earth's surface, even for stations as widely separated from one another as London, the Cape, and St. Helena, but it also showed, for the same stations, the secular variations of the elements. Furthermore it even reproduced the oval of Eastern Siberia, in the interior of which the declination is westward, and also the oval of minimum declination observed in the east of the Pacific near the neighbourhood of the equator.

Thus, for the first time, a natural phenomenon of so great a complexity as that of terrestrial magnetism, has been reproduced artificially in every detail, not only as regards its distribution in space but also showing the secular variations which occur in time. We cannot look with in-difference upon the theoretical considerations which have led to a result that is so remarkably in accordance with the natural phenomenon. In particular, what is the reason of the rôle of magnetic screen played by the seas? We know that the oceans exert a very great influence upon the atmospheric circulation and climatology in general. What is the nature of their mysterious influence upon the distribution of terrestrial magnetism? Perhaps it is a consequence of a state of affairs corresponding to Lippmann's theory, viz., that the thickness of the terrestrial crust is less under the oceans, so that in these parts the internal ferruginous materials are nearer the surface of the geoid than elsewhere, and consequently play the same part as Wilde's screens.

So once again, as we have already seen in the study of gravity and the form of the Earth, and as we shall see later in connection with general meteorology, the solution of the problems of the physics of the globe will probably be found in the study of the oceans. Prince Albert of Monaco clearly foresaw this when he founded the Oceanographical Institute.

In any case, we see the importance of the initial conception of distinguishing the magnetic action of the nucleus from that of the crust. Bauer, who superintends the magnetic work of the Carnegie Institution, also believes, as a result of studying Gauss's conclusions, that the terrestrial magnetism resides almost entirely in the solid crust which envelops the nucleus.

To summarise, the Sun's action is paramount as regards the electric and magnetic phenomena of which the terrestrial globe is the seat. We do not yet know, however, whether these phenomena are wholly due to the solar field or whether the latter merely modifies them. In other words, is the Sun's action sufficient to give rise to the Earth's magnetic field or, on the other hand, does this latter pre-exist, caused by some original and as yet unknown cause, only its variations arising from the variations of the solar radiation? What is quite certain, is the actually proved connection between the variation in the number of solar spots on the one hand and magnetic variations, variations of earth currents, of magnetic storms, of the number of polar aurore, and frequently of the number of seismic phenomena. This shows that, even if we must not seek the cause of terrestrial magnetism in the Sun (which we have not yet been able to prove), we must, at any rate, look there for the cause of the variations to which its numerous phenomena are subjected.