In the first place, it is quite certain that the atmosphere which surrounds our globe is almost always in a state of electrification. Further, the electrical condition of the atmosphere would seem to be as variable as the wind. It changes with the change of season; it changes from day to day; it changes from hour to hour. The charge of electricity is sometimes positive, sometimes negative; sometimes it is strong, sometimes feeble; and the transition from one condition to another is sometimes slow and gradual, sometimes sudden and violent. As a general rule, in fine, clear weather, the electricity of the atmosphere is positive, and not very strongly developed. In wet weather the charge may be either positive or negative, and is generally strong, especially when there are sudden heavy showers. In fog it is also strong, and almost always positive. In a snowstorm it is very strong, and most frequently positive. Finally, in a thunderstorm it is extremely strong, and generally negative; but it is subject to a sudden change of sign, when a flash of lightning passes or when rain begins to fall. So far I have simply stated facts, which have been ascertained by careful observations, made at different stations by competent observers, and extending over a period of many years. But as regards the process by which the electricity of the atmosphere is developed, we have, up to the present time, no certain knowledge. It has been said that electricity may be generated in the atmosphere by the friction of the air itself, and of the minute particles floating in it, against the surface of the earth, against trees and buildings, against rocks, cliffs, and mountains. But this opinion, however probable it may be, has not yet been confirmed by any direct experimental investigation. The second theory is that the electricity of the atmosphere is due, in great part at least, to the evaporation of salt water. Many years ago, Pouillet, a French philosopher, made a series of experiments in the laboratory, which seemed to show that evaporation is generally attended with the development of electricity; and, in particular, he satisfied himself that the vapor which passes off from the surface of salt water is always positively electrified. Now, the atmosphere is everywhere charged, more or less, with vapor which comes, almost entirely, from the salt water of the ocean. Hence Pouillet inferred that the chief source of atmospheric electricity is the evaporation of sea water. This explanation would certainly go far to account for the presence of electricity in the atmosphere, if the fact on which it rests were established beyond dispute. But there is some reason to doubt whether the development of electricity, in the experiments of Pouillet, was due simply to the process of evaporation, and not rather to other causes, the influence of which he did not sufficiently take into account.