There are some curious electrical phenomena, such as St. Elmo’s Fire, already noticed under Electricity; and in some parts of America, in very hot weather, such a light is perceived to issue from trees as the fire glides through the forest. Many instances are on record concerning the luminosity of pointed sticks, and even of the tails and manes of horses in certain conditions of the atmosphere, and of the universal power of electricity and its pervading influence in nature. The benefits conferred by thunderstorms in purifying the air, and in the production of ozone and nitric acid, are very great, and apart from the magnificent phenomena exhibited, are well worth our attention, though beyond our reach.
Fig. 726.—Aurora Borealis.
Terrestrial magnetism, however, is still more puzzling in its action than is electricity, and the study of the needle, its destination, inclination, and intensity, which are marked upon charts, just as are the weather reports of the Times, is an interesting one. These magnetic maps are termed the charts of Isoclinic and Isodynamic lines. The declination of the magnetic needle from the true north is its deviation from that point, and the “inclination” is its dip towards the horizon. The line of its direction being known as the magnetic meridian, its divergence from this line constitutes its declination. There are places where it does not deviate, and these, in direction north and south, are called lines of “no variation.” There are also places in the equatorial regions where the needle does not “dip.” The line connecting such places is termed the Magnetic Equator, and north or south of this the needle dips respectively to north or south in degrees coinciding with the distance from the equator.
The earth, then, acts as a magnet, and attracts the needle, but the magnetic poles are not identical with the terrestrial poles. The north magnetic pole was reached in 1831 by Sir James Ross, when the dip was only one minute less than 90°, and the south magnetic pole was very nearly reached also by him in 1840. The magnetic equator passes between these two points.
Fig. 727.—Paraselenæ, or mock moons.
It is to magnetic atmospherical disturbance that the aurora is due. These northern (or southern) phenomena are extremely brilliant and diversified. In temperate regions the aurora does not present such grand forms as in the extreme north. There the spectacle is astonishingly beautiful. The sky at first clouds over, and mist is developed. Humboldt has eloquently described the aurora borealis, and the beautiful changes of light, the constant movement, flashes, etc., denoting a “magnetic” storm, as electrical discharges indicate an electric storm, although the area affected by the former is far more extensive than that of the latter, and there is no thunder accompanying the magnetic storm, with the production of which the electricity of the earth is unassociated. To the continuous flow of this electricity the aurora is due, and the flashes are only the electric current descending towards the earth. But the true reason of the phenomena may have to be yet discovered, for nothing absolutely certain is known as to the origin of the aurora.
Amongst the numerous effects of refraction and reflection of light the Rainbow is most common and the most beautiful. If we hold a chandelier “drop” in the sunlight, we shall see a brilliant representation of the rainbow on the wall or on the carpet. The three colours—red, yellow, and blue—mingle or shade away into seven—red, orange, green, blue, yellow, indigo, and violet. These colours are all found in the rainbow.
