[Illustration: =TRAVELERS GROUPED ON THE SANDS OF THE SAHARA, TERRORIZED BY AN APPROACHING SIMOON=
The dreaded Simoon of the desert is a whirlwind of terrific force that raises great gyrating clouds of sand, and sweeps forward with suffocating effect upon both man and beasts. It frequently darkens the sky at midday, and sometimes lightning accompanies it caused by the friction of the sand and air, though no rain falls. The Simoon seldom lasts more than twenty minutes.]
Electricity produces an infinity of changes in the natural world. It may be artificially elicited or called forth by friction; or by contact of certain substances and the action attendant on this contact. In the one case it is termed ordinary, and in the other case voltaic or galvanic electricity.
All substances are supposed to contain a certain portion of electricity, and if by friction or other means any substance acquires more electrical action than it would naturally possess, it is said to be positively electrified; and if less, it is said to be negatively electrified. Substances when positively electrified attract or draw toward them other substances which are in a state of negative electricity, or even those which are in a natural state, but will repel or force from them substances which are positively electrified. The sudden contact of bodies in an opposite state of electricity is attended with vivid light called the “electric spark,” and accompanied by explosion and shock.
EARTH AND AIR FORM NATURE’S ELECTRIC BATTERY
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.