=18. Atmospheric Electricity.= The air is usually electrified, even in clear weather, although its cause is not thoroughly understood. In 1752 it was proved by Benjamin Franklin (Fig. 15), with his famous kite experiment, that atmospheric and frictional electricities are of the same nature. By means of a kite, the string being wet by the rain, he succeeded, during a thunderstorm, in drawing sparks, charging condensers, etc. [Illustration: Fig. 15.] [Illustration: Fig. 16.] =19. Lightning= may be produced by the passage of electricity between clouds, or between a cloud and the earth (Fig. 16), which, with the intervening air, have the effect of a condenser. When the attraction between the two electrifications gets great enough, a spark passes. When the spark has a zigzag motion it is called _chain lightning_. In hot weather flashes are often seen which light whole clouds, no thunder being heard. This is called _heat lightning_, and is generally considered to be due to distant discharges, the light of which is reflected by the clouds. The lightning flash represents billions of volts. [Illustration: Fig. 17.] =20. Thunder= is caused by the violent disturbances produced in the air by lightning. Clouds, hills, etc., produce echoes, which, with the original sound, make the rolling effect. =21. Lightning-Rods=, when well constructed, often prevent violent discharges. Their pointed prongs at the top allow the negative electricity of the earth to pass quietly into the air to neutralize the positive in the cloud above. In case of a discharge, or stroke of lightning, the rods aid in conducting the electricity to the earth. The ends of the rods are placed deep in the earth, Fig. 17. =22. St. Elmo's Fire.= Electrification from the earth is often drawn up from the earth through the masts of ships, Fig. 18, to neutralize that in the clouds, and, as it escapes from the points of the masts, light is produced. [Illustration: Fig. 18.] =23. Aurora Borealis=, also called Northern Lights, are luminous effects, Fig. 19, often seen in the north. They often occur at the same time with magnetic storms, when telegraph and telephone work may be disturbed. The exact cause of this light is not known, but it is thought by many to be due to disturbances in the earth's magnetism caused by the action of the sun. [Illustration: Fig. 19.] CHAPTER II. ABOUT MAGNETS AND MAGNETISM. =24. Natural Magnets.= Hundreds of years ago it was discovered that a certain ore of iron, called lodestone, had the power of picking up small pieces of iron. It was used to indicate the north and south line, and it was discovered later that small pieces of steel could be permanently magnetized by rubbing them upon the lodestone. =25. Artificial Magnets.= Pieces of steel, when magnetized, are called artificial magnets. They are made in many forms. The electromagnet is also an artificial magnet; this will be treated separately. [Illustration: Fig. 20]