handkerchief does not get dry.
By this time the perspiration is running off your face and hands, and
your underclothes are getting drenched with perspiration.

[Illustration: FIG. 48. An evaporating dish.]

You hurry into the house, change your clothes, bathe and wipe yourself
dry with a towel. When you find that your wet things are not drying,
and that your dry ones are rapidly becoming moist, you hastily build
a fire and hang your clothes beside it. No use, your clothes remain as
wet as ever. If you get them very hot the moisture in them will boil
and turn to steam, of course, but the steam will all turn back to
water as soon as it cools a little and the drops will cling to your
clothes and to everything around the room. You will have to get used
to living in wet clothes. You won't catch cold, though, since there is
no evaporation to use up your heat.

But the water problem outside is not one of mere inconvenience.
It never rains. How can it when the water from the oceans cannot
evaporate to form clouds? Little by little the rivers begin to run
dry--there is no rain to feed them. No fog blows in from the sea; no
clouds cool the sun's glare; no dew moistens the grass at night; no
frost shows the coming of cold weather; no snow comes to cover the
mountains. In time there is no water left in the rivers; every lake
with an outlet runs dry. There are no springs, and, after a while,
no wells. People have to live on juicy plants. The crops fortunately
require very little moisture, since none evaporates from them or from
the ground in which they grow. And the people do not need nearly as
much water to drink.

Little by little, however, the water all soaks too deep into the
ground for the plants to get it. Gradually the continents become great
deserts, and all life perishes from the land.

All these things would really happen, and many more changes besides,
if water did not evaporate. Yet the evaporation of water is a very
simple occurrence. As the molecules of any liquid bounce around, some
get hit harder than others. These are shot off from the rest up into
the air, and get too far away to be drawn back by the pull of the
molecules behind. This shooting away of some of the molecules is
evaporation. And since it takes heat to send these molecules
flying off, the liquid that is left behind is colder because of the
evaporation. That is why you are always cold after you leave the
bathtub until you are dry. The water that