That such a state exists, we have already learned. The great belts of calms that we have found between the trade-winds and the return trades and polar currents, are more appropriately called the zones of equinoctial storms. We have in them districts of general calms, with winds infringing upon either side. It is evident then that, as in the case of the fence, whose recoil-current curves the snow-drift, a whirling current of considerable magnitude may arise here at any time: hence, violent storms do arise in these regions more or less at all periods of the year. But we have seen that these zones of calms move slightly to the north or south with the course of the sun. It would then appear that at the equinoctial period, when they return from the mean position toward the extreme northern or southern limit, there would be opportunity for unusual disturbances, especially since the heavy rainfall of those periods would unusually affect the temperature of the atmosphere. That is precisely what occurs. The equinoxes are marked by storms of unusual severity, and the influence of the sudden falls of rain is so great that some eminent men believe them to be nearly the sole factor in the formation of these storms. In one case they doubtless are. If a very heavy rain be decidedly local, there is low barometer at that place. Now, if on either side there be areas of high barometer, the opposing currents flowing toward the center of low area are sufficient to meet all the conditions necessary for a cyclonic storm. As the zone of calms is comparatively narrow, it is apparent that the diameter of the area of any storm, owing to the pressure exerted by the incoming currents of wind, must be still less. Hence, the cyclone center, at its time of formation, seldom exceeds one hundred miles in diameter. As it travels away from the compressing currents that formed it, it is clear that its centrifugal force must increase; hence, its area increases, and its violence correspondingly diminishes. These facts refer to the unusually violent cyclonic storms, properly known as cyclones. But all low area storms are characterized by the upward spiral motion, though not strong enough in the case of ordinary summer rains and thunderstorms to be especially noticed. We shall see, by and by, how this spiral motion may result without the intervention of any strong opposing currents. Why and how a cyclone travels, is a question that at once propounds itself. Its motion is in accordance with a fixed law, whose operation varies only as it may be affected by unusual peculiarities in the configuration of the surface over which it travels. The reason of the motion is not so easy to explain; neither is it easy to explain why heat expands objects: but its operation is none the less certain. And so the route pursued by any storm can be readily indicated in advance. It is not a matter of mere conjecture.