ses into
cloud.

If we could see inside of a boiler in which steam had been generated,
all the space not occupied with water would seem to be vacant, since
steam before it is condensed is as transparent as the air. We will,
however, speak of this subject more fully under the head of evaporation
and cloud formation. It is not enough that we have the air in which we
live and move, with all of its properties, as we have described:
something more is needed which is absolutely essential both to animal
and vegetable life--and this essential is motion. If the air remained
perfectly still with no lateral movement or upward and downward currents
of any kind, we should have a perfectly constant condition of things
subjected only to such gradual changes as the advancing and receding
seasons would produce owing to the change in the angle of the sun's
rays. No cloud would ever form, no rain would ever fall, and no wind
would ever blow. It is of the highest importance not only that the wind
shall blow, but that comparatively sudden changes of temperature take
place in the atmosphere, in order that vegetation as well as animal life
may exist upon the surface of the globe. The only place where animal
life could exist would be in the great bodies of water, and it is even
doubtful if water could remain habitable unless there were means
provided for constant circulation--motion.

The mobility of the atmosphere is such that the least influence that
changes its balance will put it in motion. While we can account in a
general way for atmospheric movements, there are many problems relating
to the details that are unsolved. We find that even the "weather man"
makes mistakes in his prognostications; so true is this that it is never
safe to plan a picnic for to-morrow based upon the predictions of
to-day. The chief difficulty in the way of solving the great problems
relating to the sudden changes in the weather and temperature lies in
the fact that two-thirds or more of the earth's surface is covered with
water; thus making it impossible to establish stations for observation
that would be evenly distributed all over the earth's surface. Enough is
known, however, to make the study of meteorology a most wonderfully
interesting subject.

We have already stated that air is composed of a mixture of oxygen and
nitrogen chiefly, with a small amount of carbon dioxide. So far as the
life and health of the animal is concerned we could get along without