sun may be shining from a clear sky above, with perhaps the
exception of the frost clouds that we have referred to floating high
above the mountain tops.

We have now described in a general way how clouds are formed, how they
are condensed into rain, and how moisture is distributed over large
areas by these rain clouds being borne on the wings of the wind; and now
you ask, Whence the wind? In our next and following chapters we will try
to answer this question.

CHAPTER X.

WIND--WHY IT BLOWS.

We have said that globules of moisture, released by the action of the
sun's rays in the process of evaporation, tend to rise because they are
lighter than the air. Right here let it be said that all material
substances have weight; even hydrogen, the lightest known gas, has
weight, and is attracted by gravitation. If there were no air or other
gaseous substances on the face of the earth except hydrogen, it would be
attracted to and envelop the earth the same as the air now does. Carbon
dioxide is a gas that is heavier than the air. If we take a vessel
filled with this gas and pour it into another vessel it will sink to the
bottom and displace the air contained in it until the air is all driven
out. If we fill a jar with water up to a certain height and then pour a
pint of shot into it the water will be caused to rise in the vessel
because it has been displaced at the bottom by the heavier material. Now
if we remove the shot the water will recede to the level maintained
before the shot was put in. On the contrary, if we should pour an equal
bulk of cork or pith balls into the jar the water would not be
displaced, because the balls are lighter than the water and would lie on
top of it; if, however, the water is removed from the jar, the cork will
immediately go to the bottom of the jar, because the cork is heavier
than the air which has taken the place of the water. We wish to impress
upon the mind of the reader the fact, that all substances of a fluidic
nature, whether in the fluid or gaseous state, have weight, and obey the
laws of gravitation, and the heavier portions will always seek the lower
levels, and in doing this will displace the lighter portions, causing
them to rise. There is no tendency in any substance to rise of itself,
but the lighter substance rises because it is forced to do so by the
heavier, which displaces it. This law lies at the bottom of all of the
phenomena of air currents.

If we are at cert