What a mysterious world is that of atmospheric electricity! It is truly the New World for the scientific mind--a mine, fruitful in unknown and even unsuspected marvels, which is perpetually disclosing its riches for our admiration. One of our most valued collaborators in our researches on the nature of lightning is photography. Faithfully and unhesitatingly it registers an indestructible document of the fugitive lightning, which imprints itself on the sensitized plate, and the astronomer can afterwards examine the smallest details of the sudden apparition comfortably and at his leisure. We have already a considerable number of plates of the outline of the lightning in flight. An examination of these electric pictures is very interesting. Who knows whether, later on, when phonography is brought to perfection, it will not also register the noisy accompaniment to the electric flash? Then, with the help of the cinematograph, we could have dramatic representations of sensational storms. While the photograph unrolls all the phases of the lightning, from its emerging from the cloud to its fall to earth, before the gaze of the spectators, the phonograph will repeat the sonorous accents of the terrorizing voice of thunder. Thunder, as all the world knows, is the noise which accompanies lightning. It is produced when a change of electricity--a neutralization--takes place between two points more or less distant. The causes which provoke it are still somewhat of a mystery. The luminous rocket which flings itself precipitately from a cloud saturated with electricity, spreads itself like a trail of flames in the atmosphere where an infinity of invisible molecules are floating; these it repels. The passage of this whirlwind of fire in a centre which is greatly compressed produces a momentary void into which the surrounding air at once rushes, and it is the same all the way along the route followed by lightning. In all probability the equilibrium of the atmosphere, which is momentarily disturbed by the intrusion of the ignited matter, hastily re-establishes itself by a rush of the air which the lightning has ejected, and which is swallowed up with a crash in the opening which has been made. It is, on a large scale, a similar phenomenon to that which is produced by opening a case which has been hermetically sealed. The air rushing in makes a dull noise. Pouillet objects to this very natural explanation on the ground that the flight of a cannon-ball ought to produce a similar noise. But this objection errs in its basis, because, as regards velocity, a cannon-ball is as a tortoise as compared with the arrow of lightning, and as regards size, who can compare a few grammes of powder to the torrents of fire launched into space by the prodigious force of electricity?