1414. Lightning. While the thunder-cloud is swelling, and extending its branches over a large tract of country, the lightning is seen to dart from one part of it to another, and often to illuminate its whole mass. When the cloud has acquired a sufficient extent, the lightning strikes between the cloud and the earth, in two opposite places; the path of the lightning lying through the whole body of the cloud and its branches. The longer this lightning continues, the less dense does the cloud become, and the less dark its appearance; till at length it breaks in different places, and shows a clear sky. Those thunder-clouds are said to be sometimes in a positive as well as a negative state of electricity. The electricity continues longer of the same kind, in proportion as the thunder-cloud is simple and uniform in its direction; but when the lightning changes its place, there commonly happens a change in the electricity of the atmosphere over which the clouds passed. It changes suddenly after a very violent flash of lightning; but gradually when the lightning is moderate, and the progress of the thunder-cloud slow.