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.