When we consider the phenomena of terrestrial magnetism carefully, it appears to indicate the action of a power external to the earth itself, and, as Hansteen conceives, having its origin from the action of the sun, heating, illuminating, and producing a magnetic tension, in the same manner as it produces electrical excitation and actino-chemical action.
The movements of these magnetic poles have been the subject of extensive and most accurate observation in every quarter of the globe. In London, during 1657–1662, there was no magnetic variation; the agonic line passing through it. The variation steadily increased, until, in 1815, it amounted to 24° 15' 17", since which time it has been slowly diminishing. In addition to this great variation, we have a regular annual change dependent on the position of the sun, in reference to the equinoctial and solstitial points, which was discovered by Cassini, and investigated by Arago and others. Also a diurnal variation, which movement appears to commence early in the morning, moving eastward until half-past seven, a.m., when it begins to move westward until two, p.m., when it again returns to the east, and in the course of the night reaches the point from which it started twenty-four hours before.
We have also remarkable variations in what is termed the dip of the needle. It is well known that a piece of unmagnetized steel, if carefully suspended by its centre, will swing in a perfectly horizontal position, but, if we magnetize this bar, it will immediately be drawn downwards at one end. The force of the earth’s polarity, attracting the dissimilar pole, has caused it to dip.
There is, in the neighbourhood of the earth’s equator, and cutting it at four points, an irregular curve, called the magnetic equator, or aclinic line, where the needle balances itself horizontally. As we proceed from this line towards either pole the dip increases, until, at the north and south poles, the needle takes a vertical position. The intensity of the earth’s magnetism is also found to vary with the position, and to increase in a proportion which corresponds very closely with the dip. But the intensity is not a function of the dip, and the lines of equal intensity, isodynamic lines, are not parallel to those of equal dip. We have already remarked on the diurnal variation of the declination of the needle; we know, also, that there exists a regular monthly and daily change in the magnetic intensity. The greatest monthly change appears when the earth is in its perihelion and aphelion, in the months of December and June,—a maximum then occurs; and about the time of the equinoxes a minimum is detected.[177]
The daily variation of intensity is greatest in the summer, and least in the winter. The magnetism is generally found to be at a minimum when the sun is near the meridian; its intensity increasing until about six o’clock, when it again diminishes.[178]
What striking evidences all these well-ascertained facts give of the dependence of terrestrial magnetism on solar influence! and in further confirmation of this view, we find a very remarkable coincidence between the lines of equal temperature—the isothermal lines, and those of equal dip and magnetic intensity.
Sir David Brewster first pointed out that there were in the northern hemisphere two poles of maximum cold; these poles agree with the magnetic points of convergence; and the line of maximum heat, which does not run parallel to the earth’s equator, is nearly coincident with that of magnetic power. Since Seebeck has shown us that electrical and magneto-electrical phenomena can be produced by the action of heat upon metallic bars, we have, perhaps, approached towards some faint appreciation of the manner in which the solar calorific radiations may, acting on the surface of our planet, produce electrical and magnetic effects. If we suppose that the sun produces a disturbance of the earth’s electricity along any given line, in all directions at right angles to that line, we shall have magnetic polarity induced.[179] That such a disturbance is regularly produced every time the sun rises, has been sufficiently proved by many observers.
In 1750, Wargentin noticed that a very remarkable display of Aurora borealis was the cause of a peculiar disturbance of the magnetic needle; and Dr. Dalton[180] was the first to show that the luminous rays of the Aurora are always parallel to the dipping-needle, and that the Auroral arches cross the magnetic meridian at right angles. Hansteen and Arago have attended with particular care to these influences of the northern lights, and the results of their observations are:—
That as the crown of the Aurora quits the usual place, the dipping-needle moves several degrees forward:—