1st. Diurnal Variations.
2nd. Annual Variations.
3rd. Secular Variations.
Let us look at these three variations from the standpoint of the magnetic lines of force which exist around the earth, and around every planet. In relation to the variations of the magnetic forces upon the surface of the earth, Faraday points out that these variations are caused by the action of the sun's rays upon the terrestrial lines of force. He uses the following figure to illustrate his meaning. Let H be the sun, E the earth--
He writes as follows: “If the magnetic and astronomical poles of our earth be supposed to be coincident, then North and South poles will also represent the North and South magnetic poles, and the different curves cutting the earth will sufficiently represent a course of magnetic lines as they occur at, or about, the surface of the earth. H represents the sun, and a the place immediately underneath it, which is also coincident with the magnetic equator. Point a will be a line of no dip, while at point b there will be dip. This dip will be increased by the action of the sun's rays, because the atmosphere under the influence of the sun's rays has expanded the air, and has thus acquired a power to affect the lines of magnetic force.“
“All the lines passing through the heated and expanded air will, because of its being a worse magnetic conductor, tend to open out, and the mass of heated air will as a whole assume the condition of diamagnetic polarity (2923). The case may be more simply stated for the facility of recollection by saying, that the effect of the sun is to raise the magnetic circles over the equatorial and neighbouring parts from their normal position, in doing which the North and South dip are simultaneously affected and increased.“
Thus it can readily be seen that every day as the earth turns round on its axis, and presents each side of the globe successively to the rays of the sun, there should be a gradual change in the intensity of the terrestrial magnetism. In Art. 2925, Faraday points out that the maximum of dip would be when the sun was at its zenith or directly overhead. With reference to the Annual Variation, Faraday points out (2882) that if the axis of the earth were perpendicular to the plane of its orbit, the intensity and direction of the magnetic forces might be considered constant, but (2883) as the axis of the earth's rotation is inclined 23° to the plane of the ecliptic, the two hemispheres will become alternately warmer and colder than each other, and then a variation in the magnetic condition may arise. The consideration of this annual variation is further considered by Faraday in the subsequent paragraphs to those already quoted, and I must refer the reader to them for fuller details; I wish simply to indicate the possible explanation of the terrestrial magnetism, in so far as that explanation is in conformity with the aspect of the Aether submitted in Chapter [IV].
With regard to secular variations Faraday points out (2880) that the temperature of the air at the equatorial parts of the earth is greater than in latitudes north and south, and as an elevation of temperature diminishes the conducting power of magnetism, so the proportion of force passing through those parts ought to be less, and that passing through the cooler parts, greater, than if the temperature were at the same degree over the whole surface of the globe.
Now with our definite conception of the aetherial lines of force traversing space, and existing on all sides of the earth, these suggestions of Faraday's obtain an increased value in relation to the varying intensity of terrestrial magnetism, and will account for the variations in a satisfactory manner from the aetherial standpoint, if taken in conjunction with the electro-magnetic character of the Aether. Of course, what applies to the earth equally applies to all the other planets, as they also are magnets according to [Art. 87], so that they will also possess their magnetic fields, with their own lines of force, and their variations in intensity and magnetic dip.