Lightning is divided into two kinds, which you will recognize under the names of sheet and forked lightning. Sheet lightning is supposed to be caused by the discharge of electricity over a large space, while forked lightning consists of a ball of fire rushing with exceeding swiftness through the air, and very often destroying everything in its way.
The passage of one of these fire-balls is nearly always in a zigzag line, and so rapidly does it travel that it always presents to the eye the appearance of an unbroken line. It has not yet been possible to measure its rate of speed, but it exceeds that of light, which is 185,000 miles in a second. Some of the flashes of lightning have been estimated at more than ten miles in length, while those from five to eight miles long are not so uncommon. The brilliancy of some of these flashes is so great that cases are on record where a flash has rendered the beholder incurably blind.
The idea that electricity and lightning were one and the same seems to have been first entertained about the middle of the seventeenth century. Many experiments were made to establish the relationship, but without any decisive result, when one of our own countrymen, Benjamin Franklin, gave a new impulse to the science. After a number of experiments, he was impressed with the idea that a metal point raised to a great height in the air would form a conductor for the electricity stored in the thunder-clouds.
Too impatient to wait for the completion of a church steeple which he intended to make use of in his investigations, he prepared a kite, using silk to enable it to withstand rain, and with it made his early experiments—at first privately, because of the fear that his neighbors would ridicule an old man's kite-flying. He raised the kite during a storm, and was delighted to feel, on applying his finger to the string, a slight spark. For the first time man had succeeded in coaxing the lightning from the clouds, and playing with it. This occurred in 1752.
Scientific men everywhere now began to devote themselves to the study of electricity. It was discovered that lightning burns its way, setting fire even to metals, and melting sand into glass by momentary contact. A striking illustration of its intense heat are the fulgurites, or curious glass tubes, produced from sand by lightning as follows: In certain places, where the ground is formed of a particular kind of sand, and lightning enters it from a cloud, the expansion of the air, as the electricity rushes through, forces it back in all directions, and the heat melts it into glass at the same time. These tubes have a diameter of one or two inches, and ordinarily a length of two or three feet. The interior surface is glazed, while the outside is formed of sand. Many have been taken out of the ground entire, and placed in museums as curiosities. It is said that fulgurites twenty to thirty feet in length have been discovered.
The experiments of the men to whom we are indebted for our knowledge of these marvels of nature are not always unattended with danger. In 1753, Richman, a member of the St. Petersburg Academy of Sciences, had an iron rod for the attraction of electricity erected on his house and continued down into his study, in order to be better able to observe its effects. During a violent storm he was working at some distance from the conductor in order to be out of the way of the large sparks. He at last incautiously approached too near, when a globe of bluish fire struck him on the forehead, killing him instantly.
The following incident illustrates the danger of being in a direct line with any article of iron during a storm. A number of people were assembled in one room of a house, conversing and watching the play of the lightning, when one of their number was struck and instantly killed by a flash that came from overhead. The death of this one man and the escape of all the rest were at first regarded as one of the freaks of which lightning is frequently guilty, but a close search revealed the fact that the accident was strictly in accordance with natural laws. It was found that in the room above, there hung a saw, one end of it nearly touching the floor directly over the man's head, while in the cellar below were a number of iron tools, among them a crowbar standing in such a position that the upper end of it was directly beneath his feet. His body had therefore only been a connecting link in the chain along which the lightning had travelled.
Another incident, but of a less tragic character, is the following. During a violent thunder-storm lightning struck a farm-house; a ball of fire descended through the chimney, and rolled across the floor of a room in which three women and a child were sitting without injuring them. It then rolled out through the kitchen, passing close to the feet of a young man, and passed out through a crevice in the wall. It next appeared in the pig-sty, and killed the pig without burning the straw on which it lay.
In olden times, before the study of the natural sciences was undertaken, every occurrence out of the common was thought to be an act of Divine power. Even in our days this idea has not entirely died out, and in those countries where people are ignorant lightning is still regarded as a mark of God's anger and a visitation sent for the punishment of sin. But with the spread of scientific knowledge it has been robbed of its terrors, and in the lightning-rod a means has been given us of attracting and controlling the electric current, and thus protecting ourselves from harm.