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. [Illustration: 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.