urther 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.]

Now let us go back to the barometer and see what part it plays in
predicting changes in the weather. At the area of low pressure the air
is ascending, as we have seen, and, owing to the peculiar way it
ascends--by circling spirally upward around a region of comparative
calm--it creates a partial vacuum, which is more pronounced in the
center of the area. At the area of high pressure the air will be
condensed by the descending current being arrested by the earth. The
descending current--coming, as it does, from the upper and colder
regions--accounts for the cool weather that most always prevails at a
high-pressure area. In order to know how great the change of weather is
likely to be, we must know what the readings of at least two barometers
are--one at the high- and another at the low-pressure area. If the
difference between the readings of the two barometers is very great, and
the areas are comparatively close together, we may expect the change to
be sudden and violent.

"High" and "low" as applied to a barometer are only relative terms.
There is no fixed point on the index of the instrument that can be said
to be arbitrarily high or low. For this reason a single barometer is not
of much use. If it begins to fall from any point, and falls rapidly, it
indicates that an area of a much lower pressure is approaching. The same
is true of a high-pressure area, if the barometer rises rapidly from any
point.

If we study the air motions in these systems sufficiently to get at
least an inkling of the law of their movements, it becomes a very
interesting subject.

Wind from whatever cause serves a wonderfully useful purpose in the
economy of nature. Without wind, heat and moisture could not be
distributed over the face of the earth and our globe would not be a fit
habitation for man. How wonderful is the machinery of Nature, that can
first forge a world into shape and afterward decorate it with green
grass and flowers that are watered by the "early and latter rain"!

CHAPTER XII.

LOCAL WINDS.

There are so many causes that will produce air motion that i