Fig. 286.—Benjamin Franklin.
Before Franklin’s ever-memorable experiment with his kite established the identity of lightning and electricity, the resemblance between the two discharges had been frequently noticed. The Etruscans pretended to bring down lightning from heaven, and Tullus Hostilius, when experimenting or performing certain “ceremonies,” was killed by the electric discharge he desired to attract. But after all, we cannot attribute any knowledge of electric science to the ancients, although they were, of course, familiar with electric phenomena.
It is to Dr. Wall that testimony points as the first person who remarked the analogy between the electric spark and lightning. This was in 1708. Grey and other philosophers supported the theory, but could not establish it. To Franklin, who in June 1752 actually brought down the lightning by his kite and a key, is the actual discovery due. We have already detailed the circumstances (page 206) and need not repeat the account of the experiment.
Of course the American philosopher found numerous imitators, not always with impunity. Professor Richmann was killed by the spirit he was invoking; Lemounier and Beccaria confirmed the theory that the air was full of electricity; while Du Saussure, from his investigations on the Alps, and Volta from the invention of the pile, are most famous in the history of electricity. They applied themselves with much success to the investigation of the electric condition of the atmosphere, of which the disturbances called thunderstorms are the result.
The amount of electricity varies in the atmosphere at different times in the day and night. Towards midday and midnight the development is generally greatest, and this fact will account for the prevalence of storms during our hours of rest. Again, different kinds of clouds have different degrees of electricity, and of different kinds. Under certain conditions these clouds will give forth lightning, and a storm will begin. The more clouds the more globules, and therefore in summer, while there is more production of vapour from solutions of salts, etc., we are more likely to have the storms. We are most of us familiar with the mass of the “thunder cloud” rising in the distance, light at the upper part, very dark below, and throwing out tentacles like the octopus, coming up sometimes—frequently, indeed—“against the wind,” impelled by an upper current, or following the course of a river, which is not unusual. Below, there is perhaps an army of thin dark clouds. The nature and height of clouds have also a great deal to do with the phenomena displayed. In general, storm-clouds are positively electrified.
Fig. 287.—Cirrus cloud.
Clouds are good conductors of electricity, and yet they may be so insulated by the dry air surrounding them that they will accumulate it; and when thus charged, if they encounter other clouds charged with opposite electricity, the opposing masses will attract each other until a discharge takes place. This is what we term lightning, and under such conditions electricity, though very dazzling, is harmless. It is when the cloud comes near to the earth, and a discharge is released, that lightning is so dangerous to persons who remain in the fields. Sometimes the discharge comes from the earth to meet that from the cloud. Sheep are frequently killed by ground lightning, and once, at Malvern, we had an escape from an upward stroke. The back-stroke from a cloud is also dangerous. It may happen that the cloud has discharged itself upon the earth many miles away, but a return discharge takes place at the other end, and if that end be near the earth the consequences may be serious. As a rule, the return stroke is not so violent as the first discharge.
The colour of lightning varies very much. We have the white, the blue, the violet, and red. The colour depends upon the distance and intensity of the lightning, and the more there is of it the whiter the light. We can illustrate the varied hues of the electric “fluid” by passing a spark through the receiver of an air-pump. If the air be rarefied, or there be a vacuum, we shall perceive a blue or violet light. Therefore we may conclude that the blue and violet flashes have birth in high strata of the atmosphere.
