The fourth class are isolated showers, occurring over particular localities, or belts of drought and showers alternating; sometimes a general disposition to cloudy and showery weather for a longer or shorter interval over the whole country; at others, limited to particular localities in the course of the trade. Such a period occurred during the wheat harvest of 1855. This class I attribute to a general increased magnetic action, but it may be induced by an increased volume, or greater south polar magnetic intensity of the counter-trade, exciting and concentrating the regular currents of the field, and increasing their activity and energy. These also often work off south gradually, and are followed by a cold N. W. air for a day or two; showing a tendency, in the excited magnetism, to pass as a wave toward the tropics. The following diagram will give some idea of this class: [Illustration: Fig. 23.] There are sometimes very obvious local tendencies to precipitation over portions adjoining an area affected with drought, as there are other magnetic irregularities over particular areas. All these classes of storms are variant in intensity. Sometimes the general or local cloud-formation is weak, and does not produce precipitation at all; so of that which extends southerly. Probably the tropical storm are always sufficiently dense and active to precipitate. Their action is often violent over particular localities, and hence the more frequent occurrence of the tornado over the more intense area of Ohio, and other portions of the west. All violent local storms are doubtless owing to local magneto-electric activity. CHAPTER XI. The reader who has attentively perused and considered the facts stated, and the principles deduced, in the preceding pages, and is ready to make a practical application of them by careful observation, will have little difficulty in understanding the varied atmospheric conditions; and will soon be able to form a correct judgment of the immediate future of the weather, so far as his limited horizon will permit. But there are other facts and considerations, not specifically alluded to, which will materially aid him in his observations; and there is a degree of philosophical truth in the proverbs and signs, which ancient popular observation accumulated, and poetry and tradition have preserved, that meteorologists have been slow to discover or admit, but which will be obvious upon examination, and commend them to his attention. The classical reader is doubtless familiar with that part of the first Georgic of Virgil, which contains a description of the signs indicative of atmospheric changes. Much of it is beautifully poetic, and, if read in the light of a correct philosophy, is equally truthful. I copy from a creditable translation, found in the first volume of Howard's "Climate of London":