Storm-clouds move generally in large numbers, and are generally composed of two separate masses, differently electrified--the lower one giving out negative electricity, the higher positive electricity. The flashes of lightning occur generally between these two masses, though also, less frequently, between the lower mass and the earth. It may be said that, generally speaking, storms are the result of the meeting of two masses of clouds differently electrified. For long, physicists refused to admit the validity of any other theory, and combated in particular the idea that lightning could issue from a single isolated cloud. This has, however, been established now as a fact, and in such cases the flashes have always, of course, taken place between the cloud and the earth. Marcorelle, of Toulouse, reports that on September 12, 1747, the sky being then pure and cloudless but for one round speck, there was suddenly a thunderclap and a flash which killed a woman on the spot, burning her breast but doing no damage to her clothes. Here is another interesting case. Two priests of the Cathedral of Lombey, who were standing in the area of their chapter-house, busy winnowing, saw a small cloud approaching them little by little. When it was immediately above them a flash of lightning broke out and struck a tree just beside them, splitting it from top to bottom. They heard no thunderclap. The weather was quite fine. There was no wind, and this was the only cloud in the sky. Storms are far more prevalent in some countries than in others. According to Pliny, thunder was unknown in Egypt, and, according to Plutarch, in Abyssinia. This could not be said now, however, perhaps because these lands have grown unworthy of their exemption. It might be said, however, of Peru, whose pure and limpid skies are never troubled by tempest. _Jupiter tonans_ must be a myth indeed to a people who know nothing of thunderclaps or wet days. Storms diminish in number in high latitudes, but there are local conditions which affect their distribution. Then they are particularly frequent in countries that are thickly wooded and in mountainous districts. Arago came to the conclusion, after a considerable number of observations, that, out in the open sea or among islands, there is no thunder in the north beyond the 75th degree of latitude. This is not absolutely so, but it is a fact that storms are very much rarer in the polar regions. They become more and more frequent towards the equator, and are very numerous in the tropics. On either side of the equator storms come year after year with remarkable regularity in the wet season, and at the time of the monsoons. At Guadeloupe and Martinique there is never any thunder in December, January, February, or March. In temperate climates there are scarcely any storms in winter; they begin in the spring, and attain their maximum of intensity in the heat of summer. In Italy there are thunderstorms at almost all times of the year.