The lightning discharge produces a violent concussion in the cloud, and very often a shower of rain immediately follows it. The electric conditions of the different clouds which make a storm being separately liable the one to the other, the discharge of one must lead to that of several others more or less distant. In all cases the noise is caused by the expansion of the air where a more or less partial void has been made. It is the same with firearms, _crève-vessie_, etc. One of the chief characteristics of thunder is the rolling, which is often prolonged, and reverberates on the sides of steep mountains. This voice, with its lugubrious tone, becomes grave and sometimes sinister in the revolution of space--this voice, celestial and infernal, seems to momentarily dominate the world, while the clouds are enveloped with a thousand diabolical flames. Sometimes it rings in the air with fierce calls, at others it spreads itself in dull, languorous complaints. Nevertheless, the intenseness of thunder undergoes a thousand fluctuations, and presents astonishing variations. Generally it strikes and frightens, but the curious thing is that, for the ear, in reality it is less strong than the crinkling noise of a piece of paper torn close to it. Often, too, it may be compared to the discharge of firearms, a pistol or a cannon. Thus, when the lightning penetrated Volney's apartments at Naples, the people present, among whom was Saussure, had the impression of a pistol-shot in the next room. There is a case given of M. and Mme. Boddington, who were seated on the back seat of their coach in order to enjoy the view of the country, and had given the inside seats to two servants. Suddenly there was a flash of lightning, which struck M. and Mme. Boddington and flung the postillion to a great distance. The servants were untouched, and escaped with a fright. When they got over their terror, one of them said that a very brilliant flash of lightning had been immediately followed by a noise similar to that of a heavily charged musket. He thought some one had shot the horses. His fright had stunned him so that he hardly knew what had happened. At other times thunder is accompanied by a whistling noise, but as a rule it is the rolling which predominates.