Thunder is the sound heard when electric discharges take place between different groups of clouds or between clouds and the earth. The clouds observed before or during a thunderstorm have received the name of nimbus; they are often extensive enough to enshroud the earth in darkness, and are characterised by the dark, heavy, and leaden appearance they present. They are at all distances from the ground; they are many miles thick vertically, and have the appearance, when seen from afar, of undergoing rapid movements as though they were boiling. To what the thunderstorm owes its origin is still undecided, although many explanations have been suggested, but water has a considerable part to play in connection with atmospheric electricity. Since sound travels comparatively slowly through air, while light is almost instantaneous, it follows that we see the flash of the electric discharge, i.e. the lightning, before we hear the sound, and the difference in the time elapsing between the two is greater the farther away is the scene of discharge. If we watch a flash of lightning, we see it as a long line of light of varied shape. We may consider it as consisting of an infinite number of points of light, i.e. as being caused by an infinite number of simultaneous small discharges. But each of these points may not be at the same distance from us so the sound from the more distant point will reach us later than that from the nearer. Hence the thunder will not sound as a single report to us, but will be lengthened out into a peal. When we are very near to the storm this difference of distance produces so slight an effect that we practically hear all the little reports at the same time, and the result is a loud crash. The reflection of the sound from other clouds and from the surrounding hills also modifies the intensity of the thunder, and produces that curious roll which is often heard.