Lightning

a large spark of electricity passing from cloud to cloud or from a cloud to the earth. In 1751 Franklin showed that a stroke of lightning is to be compared with the discharge of a Leyden jar. The two clouds or the cloud and the earth may be regarded as the two coatings, inner and outer, of the jar, while the air is the glass that separates them.

Observers divide discharges of lightning into three classes, sheet, ball, and chain or bolt lightning. The first is a momentary illumination of the face of a cloud or of the atmosphere. It is merely the reflection of lightning, otherwise concealed. Fiery balls or globes, possibly of air or moisture lighted by electricity, have been known to roll or float along near the surface of the earth and to explode without effect. St. Elmo's fire, a flame occasionally seen clinging to the tips of masts of ships, has been referred to this form of discharge. Something of the sort has been seen, it is claimed, on mountain peaks, having the form of a lambent flame several inches in length.

It is held that ordinary discharges of lightning, sometimes seemingly miles in length, are in reality a series of short discharges succeeding each other too rapidly for the eye to separate them. The zigzag course of a bolt is the more readily understood with this explanation. The crash produced by the passage of a stroke is an atmospheric disturbance produced by heat. The air in the path of the bolt is heated and expanded, forcing the adjacent air violently outward. The waves set up by the returning particles meet at the line of passage with a loud noise like the report of a cannon. It is not to be understood that the lightning forces the air out, or forms a vacuum in the line of its passage, but that it swells the air suddenly. The roll of thunder is the echo from cloud to cloud.

Of late, protection by lightning rods has been discredited. Insurance companies consider that rods invite lightning and are entirely inadequate to conduct it to the ground. When used, they should be as numerous and have as much surface as the owner can afford. A strip is equal to a solid rod of the same surface. An ideal protection would be a covering of a heavy wire netting or broad strips of metal, extending into the moist earth. Lightning never passes through a metal covering or leaves a conductor for a building if the conductor has surface enough to carry the current.

According to a bulletin of the United States Department of Agriculture issued in 1901, the loss by lightning for the preceding year was as follows: 563 persons were killed; 820 were injured; 4,257 head of livestock were killed in the field; and 6,256 buildings were injured. The total loss of property by lightning was $3,146,475.

See Electricity; Franklin