The earth is always in a state of positive electricity, and the air when pure in a state of negative electricity. Atmospheric air, however, is subject to incessant variations, and hence its “electrical equilibrium” or natural electrical state is subject to be disturbed. This equilibrium will be restored when an explosion has taken place, and thus it is that in peculiar states of the atmosphere thunder storms act a beneficial part in restoring the air to a normal condition. The intensity of electrical action is greater during the day than at night and also in summer than in winter; and diminishes from the equator to the poles.
Electricity is perpetually effecting great changes in the earth’s crust, and in very many instances acts on the principal of voltaic electricity, the action in such cases being produced by long-continued currents.
LIGHTNING—THE ELECTRICAL DISCHARGE
IN THE HEAVENS
Lightning is the dazzling light produced by an electrical discharge passing between clouds which are oppositely electrified, or between the clouds and the earth. Lightning flashes have been distinguished as zigzag or chain lightning, sheet and globular lightning.
The first has the aspect of a sharply defined chain of fire, and moves at the rate of 250,000 miles per second. Its zigzag course is attributed to the resistance of the air, condensed in the passage of the electrical discharge, which is sufficient to turn it aside frequently in the direction of less resistance.
Sheet lightning includes the expanded flashes which occur during a storm, and the heat lightning, seen on summer evenings, when no clouds are visible, which is supposed to be the reflection of a storm taking place below the horizon.
Globular lightning is seen on rare occasions, when the electrical discharge takes the form of a ball of fire, and descending with less rapidity, is visible for several seconds. In certain conditions of the atmosphere, globes or spires of electrical light, called St. Elmo’s fire, are seen tipping the extremities of bodies in contact with the earth, like church spires, or masts of ships.
All the conditions which give rise to electrical excitement in the atmosphere are much more intense in warm than in cold latitudes; hence the thunder storms of the tropical regions greatly exceed, both in frequency and in violence, those of temperate and cold climates.
THE AURORA BOREALIS, OR
NORTHERN LIGHTS
This phenomenon is frequently observed in the northern heavens. It occurs in many forms, but the most common is that of a luminous arch whose summit is in the magnetic meridian of the place of observation, and from which vivid flashes of light dart towards the zenith. A like phenomenon in the southern heavens is denominated the Aurora Australis. Auroras are most frequent and brilliant in the polar regions, and diminish in intensity towards the equator.