r
the surface of the earth, but condensed above normal in the higher
regions by the upward current, which causes an overflow back toward the
rarefied upper regions over the area of high pressure.

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 f