It will be observed that the ordinary storm has a compound motion. The whole system moves in an easterly direction, while the winds are blowing spirally about the storm center. If we should be in the track of a moving storm so that its center passed over us the winds at the beginning would blow in one direction and then there would come a subsidence until it had moved forward through the quiet zone, when we should feel the wind in the opposite direction until the area of low pressure had moved forward into the region of high pressure. The velocity of the wind will be determined by the difference of pressure between the areas and by the distance that the areas of high and low pressure are apart. The steeper the grade the more rapidly the fluid will flow. Let us now have recourse, for a moment, to Figs. 1, 2, and 3 in order that the subject may be more fully understood. In looking at these diagrams we should imagine ourselves looking South, with the left hand to the East. [Illustration: FIG. 1.] Fig. 1 shows the general direction of the air movement between two areas--one of high and the other of low pressure. The arrows show the general direction of the wind. You will notice that in the upper regions it blows in an opposite direction from the air movement on the surface of the earth. Fig. 2 shows in a general way how the wind moves spirally around both centers. Over the area of high pressure the air descends spirally from the upper regions, circling around a large area--it may be one hundred miles or more in diameter--in the direction of the movement of the hands of a clock. [Illustration: FIG. 2.] But then the wind at the high-pressure area is lighter than it is at the low, and circles outwardly until it finally moves off in the direction of a low-pressure area, gradually bending in the other direction until finally it moves the reverse of the hands of a clock--although now it is in a smaller circle, and with a more rapid motion. It moves spirally and upwardly about the low-pressure area until it reaches a point in the upper air, where it goes through the same gyrations in an opposite direction. Now imagine the whole combination moving from west to east at an average rate of thirty miles per hour, and imagine further that this system is linked to other systems that are following along, and you have some idea of the weather changes as they occur in the middle United States. By referring to Fig. 3 you will see why the wind changes its direction when a storm center passes over any point. It has not only a spiral but also a forward movement. [Illustration: FIG. 3.]