Free electricity is always in the air. During clear weather it is generally positive; during cloudy weather it is negative. This electricity is carried in the air by the moisture. As dry air is a non-conductor of electricity, in fair weather the electrified particles of air are insulated and therefore acquire very little intensity. The clouds having been formed and being filled with moisture, form an excellent conductor of electricity, which acquires considerable intensity. It is a well-known physical law that two bodies having opposite electricities attract each other, and those having like charges repel each other. From this, two clouds having opposite charges rush together and produce the phenomena, called lightning, which is accompanied by an explosion called thunder. Often we see several flashes of lightning and then hear several thunder crashes, which is caused by only one section of a cloud discharging its electricity at a time. As a cloud attracts the opposite charge of electricity from the surface of the earth beneath it by inductive influence, often we see a discharge of electricity from the cloud to the earth, the charge usually being received by such objects as hills, trees, church spires, high buildings, etc. Bodies containing large quantities of moisture are susceptible to strokes of lightning, as the moisture causes them to become good conductors of electricity. Also trees on the outer edge of a forest are more liable to be struck than those farther in. There are several forms of lightning, such as zigzag, ball, sheet, and heat lightning. Zigzag lightning, as the name implies, follows an irregular course, producing a long zigzag line of light, sometimes ten miles in length, and is caused by the air producing a field of resistance to the path of electricity, causing it to seek a path of less resistance. Ball lightning appears like a large ball of fire, usually accompanied by a terrific explosion. This is the result of the bodies being charged with electricity of great intensity, and travels in a straight path, as it has enough strength to oppose any resistance placed in its path. Heat lightning is usually seen on warm evenings, especially during the summer, and very often unaccompanied by thunder, due to the great distance of the lightning clouds from where we are located, thus diminishing the intensity of the thunder. The electricity of the clouds escape in flashes so feeble as to produce no audible sound. Sheet lightning is a diffused glare of light sometimes illuminating only the edges of a cloud, and again spreading over its entire surface. Ordinary flashes of lightning last but the minutest part of a second. Thunder is the re-entrance of air into an empty space. The vacuum is created by the lightning in its passage through the air. The violence of thunder varies according to the intensity of the electrical flashes.