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earth's surface where there is sensible dip, and bending round enter the earth again at the magnetic equator. They induce electric currents in conducting-wires, moving across them exactly the same as in artificial magnets; and when a hollow helix, or coil of copper wire, whose extremities are connected with a galvanometer, is placed in the maguetic dip, and suddenly moved across the lines of force, the needle of the galvanometer will vibrate through an arc of 80° or 90°, in consequence of the electric current induced by these lines of magnetic force in the wire, and the action is greater when a core of soft iron is placed in the helix, which becomes a temporary magnet by induction, Again, if a copper plate be connected with a galvanometer by two copper wires, one from the centre, and another from the circumference, in order to collect and convey the electricity, it is found that, when the plate is made to revolve in a plane passing through the line of the dip, the galvanometer is not affected. But as soon as the plate is inclined to that plane, electricity begins to be developed by its motion across the lines of magnetic force; it becomes more powerful as the inclination increases, and arrives at a maximum when the plate revolves at right angles to the line of dip. When the revolution is in the same direction with that of the hands of a watch, the current of electricity flows from its centre to the circumference; and when the rotation is in a contrary direction, the current sets the opposite way. Thus a copper plate, revolving at right angles to the line of the dip, becomes a new electrical machine, differing from the common plate-glass machine by the copper being the most perfect conductor, whereas glass is the most perfect non-conductor; besides insulation, which is essential to the glass machine, is fatal to the copper one. The quantity of electricity evolved by the metal does not appear to be inferior to that devolved by the glass, though very different in intensity. Even a ship crossing the lines of force must have electric currents running through her. Dr. Faraday observes that such is the facility with which electricity is generated by the magnetic lines of force, that scarcely any piece of metal can be moved without a development of it; consequently, among the arrangements of steam-engines and metallic machinery, curious electro-magnetic combinations probably exist which have never yet been noticed. Thus magnetic lines of force certainly issue from the surface of the globe.
No doubt the earth is a magnet on a vast scale, but it differs from all others in having four poles of maximum magnetic force of different intensities, the two in the northern hemisphere having a secular motion in a contrary direction from the two in the southern. They are not even symmetrically placed ; hence the magnetic intensity varies so much in the different points on the earth's surface, that the dynamic equator, or line passing through all the points of least intensity, is a very irregular curve surrounding the globe, but by no means coinciding with the terrestrial equator. In consequence of the mean action of these four forces, the north end of a magnetised needle, arranged so as to revolve in a vertical plane, dips or inclines beneath the horizon in the northern hemisphere, and the south end in the southern. The two hemispheres are separated by a line encircling the earth, called the magnetic equator, or line of no dip, in which the dipping or inclination needle is horizontal. On each side of this line the inclination increases till at last the needle becomes perpendicular to the horizon in two points, or rather small spaces, in each hemisphere, known as the magnetic poles, which are quite different from the poles of the earth's rotation. The mean action of the four poles of magnetic intensity causes the mariner's compass, or a magnetic needle suspended so as to revolve in a horizontal plane, to remain at rest when pointing to the two magnetic poles. It is then in the magnetic meridian of the place of observation, which is thus determined by the mean action of all the four magnetic forces.
These mean values of the three magnetic elements, namely, the declination, inclination or dip, and magnetic intensity, are well known to be subject to secular, annual, and diurnal variations. The secular only become sensible after some years, but the annual and diurnal variations have a double progressionthat is to say, two maximum and two minimum values in their respective periods of a year and twenty-four hours; for example, the declination needle makes two deviations to the west and two to the east in the course of twenty-four hours, and that with great regularity. Now General Sabine discovered that the double progression arises from two combined or superposed variations having different hours of maxima and minima, and that they are due to two distinctly different causes—the one being the difference in the sun's position relatively to the place of observation at the different seasons of the year, and hours of the day and night; the other being a mean annual and diurnal variation proved by General Sabine to exist in those great magnetic storms or casual disturbances which affect the magnetic elements simultaneously over enormously extensive tracts of the globe.
Moreover the General discovered that, besides these annual and diurnal variations, the magnetic storms have a variation which accomplishes its vicissitudes in ten or more nearly eleven years, the increase from year to year being gradual, till its maximum becomes twice as great as its minimum value. In consequence of this inequality in the storms or casual disturbances, each of the magnetic elements has a variation of similar period and similar maxima and minima. Now the number and magnitude of the spots on the sun had been observed by M. Schwabe, of Dessau, to increase to a maximum, and decrease again to a minimum, regularly in the very same period of between ten and eleven years ; and General Sabine found that this variation in the solar spots, and that in the magnetic elements, not only have the same periods of maxima and minima, but that they correspond in all their minutest vicissitudes. Thus a very remarkable and unexpected connexion exists between terrestrial and solar magnetism. The dual and antagonist principle is perfectly maintained in the earth's magnetism, all the phenomena and their variations being in opposite directions in the two hemispheres. (N. 226.)
No doubt the magnetic lines of force in the earth are closed curves, as in artificial magnets; but in their circuitous courses they may extend to any distance in space, or rather in the ethereal medium, even to thousands or tens of thousands of
for the ethereal medium is permeable to lines of magnetic force, or rather transmits them, otherwise the solar spots could not affect the variations of terrestrial magnetism ; besides, they pass through the Torricellian vacuum, which is nearly a void with respect to air, but not to the ethereal medium.
The atmosphere which surrounds the earth to the height of about fifty miles with sensible density, consists of three and a half parts by weight of nitrogen gas and one part of oxygen, uniformly mixed. The nitrogen is neutral whether dense or rare, hot or cold, while the oxygen is highly paramagnetic; but it loses a great part of its force when rarefied by heat; conse
quently the magnetic force of the atmosphere must increase from the equator to the poles of maximum cold; it must vary summer and winter, night and day. Its effect upon terrestrial magnetism is unknown; but it can hardly be without some influence. M. E. Becquerel observes—“If we reflect that the earth is encompassed by a mass of air equivalent in weight to a layer of mercury of 30 inches, we may inquire whether such a mass of magnetic gas, continually agitated, and submitted to the regular and irregular variations of pressure and temperature, does not intervene in some of the phenomena dependent upon terrestrial magnetism. If we calculate, in fact, what is the magnetic force of this fluid mass, we find that it is equivalent to an immense plate of iron, of a thickness little more than 250 of an inch, which covers the whole surface of the globe." Both the conducting power of the air and its density are increased by cold ; and as the sum of the magnetic forces which issue from the earth on one side of the line of no dip is equal to their sum on the other side, the intensity and concentration in our winter are coincident with a diffusion and feebleness in the opposite hemisphere, so that the line of no dip will move annually from north to south and back again. The same holds with regard to day and night. Thus the law of the conservation of force is rigorously maintained ; and it is equally so in the effect of the atmosphere on the magnetic lines of force, which refracts them as they pass through it, in one direction in summer, and in the opposite direction in winter—in one direction in the enlightened hemisphere, in the other in that which is dark. The whole of the magnetic lines about the earth are held by their mutual tension in one connected, sensitive system, which feels in every part, even to the antipodes, a change in any particular place.
It may be mentioned as a well-known fact, that apparent anomalies have been found in the diurnal variation of the declination in the high magnetic latitudes of the northern hemisphere when compared with their great regularity in other parts of the same hemisphere, and that the magnetic storms are of much greater magnitude there than in lower latitudes. Moreover, although Captain Maguire's observations at Cape Barrow, in the North Polar Ocean, show that the annual and diurnal variations of the casual disturbances or magnetic storms, as well as those of the decennial period, are maintained, yet it appears that at certain hours of the day the disturbance in the declination may be easterly at Point Barrow, and westerly at the Magnetic Observatory at Toronto, in Upper Canada, and vice versá : in fact, the magnetic storms are simultaneous at these two stations, but in opposite directions—a circumstance not yet accounted for, and may possibly be due to the increased magnetism of the air in these cold regions. The heat of the sun has no effect upon terrestrial magnetism unless possibly by its indirect action on the oxygen of the atmosphere; but hitherto it has been imperceptible. It is hardly possible that the aurora can be independent of the magnetic character of the air, since it occurs in the high latitudes, where the atmospheric magnetism is most powerful. Captain Maguire remarked that it frequently appeared at Point Barrow when the magnetic storms were at a maximum.
We are totally ignorant of the cause of terrestrial magnetism, though the powerful influence of the solar spots renders it highly probable that it will ultimately be found to originate in the sun himself. Mr. Barlow's theory of electric currents revolving round the globe is borne out by Mr. Fox's observations in the Cornish mines, which show that electro-magnetism is extremely active in metallic veins; that not only the nature of the metalliferous deposits must have been determined by their relative electrical conditions, but that the direction of the metallic veins must have been influenced by the direction of the magnetic meridians, and in fact almost all the metallic deposits in the world tend from east to west, or from north-east to south-west. However, these currents of electricity may be regarded as magnetic lines of force, and are more likely to be the effect than the cause of terrestrial magnetism. They are found to have a powerful inductive effect on the Atlantic telegraph, disturbing the needles and galvanometers at each end of the line to a considerable degree, and on the night of the 6th of September, 1858, a magnetic storm passed over the cable, which violently agitated the reflecting galvanometer in connection with the telegraphic wires.
We are equally ignorant of the cause of the secular magnetic variations, but we have no reason to believe that the earth is alone magnetic; on the contrary, the planets are probably magnets, and we know that the sun and moon are magnetic ; hence, as the magnetic, like the gravitating force, is transmitted through the ethereal medium, the induction of the sun, moon,