=136. Fundamental Facts regarding Atmospheric Electricity.=--The _general_ electrical condition of the atmosphere is _positive_ in relation to the surface of the earth and ocean, becoming more and more positive as the altitude increases. When the sky is overcast, and the clouds are moving in different directions, it is subject to great and sudden variations, changing rapidly from positive to negative, and the reverse. During fog, rain, hail, sleet, snow, and thunderstorms, the electrical state of the air undergoes many variations. The intensity of the electricity increases with hot weather following a series of wet days, or of wet weather coming after a continuance of dry days. The atmospheric electricity, in fact, seems to depend for its intensity and kind upon the direction and character of the prevailing wind, under ordinary circumstances. It has an annual and a diurnal variation. There is a greater diurnal change of tension in winter than in summer. By comparing observations from month to month, a gradual increase of tension is perceived from July to February, and a decrease from February to July. The intensity seems to vary with the temperature. The diurnal variation exhibits two periods of greatest and two of least intensity. In summer, the _maxima_ occur about 10 a.m. and 10 p.m.; the _minima_ about 2 a.m. and noon. In winter, the _maxima_ take place near 10 a.m. and 8 p.m.; the _minima_ near 4 a.m. and 4 p.m. The researches of Saussure, Beccaria, Crosse, Quetelet, Thompson, and FitzRoy have tended to show that during the prevalence of polar currents of air positive electricity is developed, and becomes more or less active according to the greater or less coldness and strength of wind; but with winds from the equatorial direction there is little evidence of sensitive electricity, and when observable, it is of the negative kind. Storms and gales of wind are generally attended, in places, with lightning and thunder; and as the former are very often attributed to the conflict of polar and equatorial winds, the difference of the electric tension of these winds may account for the latter phenomena. It is not our intention to enter upon the general consideration of thunderstorms; the facts which we have given may be of service to the young observer; and finally, as it is interesting to be able to judge of the locality of a thunderstorm, the following simple rule will be of service, and sufficiently accurate:--Note by a second's watch the number of seconds which elapse from the sight of the lightning to the commencement of the thunder; divide them by five, and the quotient will be the distance in miles. Thus, if thunder is heard ten seconds after the lightning was seen, the distance from the seat of the storm will be about two miles. The interval between the flash and the roll has seldom been observed greater than seventy-two seconds.