a subject for me. It is a very good one,
too, and quite suitable to the occasion. Fogs are nothing more nor less
than clouds. They usually float aloft, a mile or more, high, but
sometimes drift down to the ground and lie all around us. They are so
light that they rise and fall from very slight causes, when there is no
wind. A brisk breeze soon drives them off."

"But what are clouds made of?" inquires May, who has become such a
favorite with the Professor that she never hesitates to stop him when
she wants anything explained.

"Clouds, May, are made up of small particles of water or vapor slightly
chilled. When vapor or steam is hot, it can not be seen, but is
invisible like the air. You have noticed the steam from a tea-kettle.
Near the spout it is hidden, but a little farther off, where it has got
cooled by mixing with the air, it begins to look gray, like a cloud. If
the kettle be allowed to boil a long while, so that a large quantity of
steam is formed, it will collect on the walls and window-panes, where,
becoming thoroughly chilled, it turns again to water, the same as it was
when first poured into the kettle. So it is with the clouds
out-of-doors; when the sun comes out bright and hot, it dries them up,
as we say; that is, it heats them so much that they become invisible.
Cool air mingling with them brings them into sight again; and, if cool
enough, it condenses."

"Oh dear!"

The Professor laughs. "There can be no doubt about it, May, science is
full of big words. We will say that the cool wind makes the clouds heavy
by squeezing them together, and sends them down in drops of rain. This
is called condensing."

May rewards the Professor for his simple explanation with such a bright
glance that he proceeds with an illustration.

"You have made soap-bubbles, and seen how they will float around in the
air, and sometimes be wafted clear up above the trees, until they get
broken, when they come down drops of water. The particles of vapor that
form clouds are little bubbles, or hollow spheres filled with air. When
a cold wind crushes them, they become solid, unite with one another, and
fall as rain-drops. Cold water is much heavier than air; but water made
hot by fire or by the sun, and turned into vapor, is lighter. In time of
a fog the vapor is just warm enough to have the same weight as the air,
so that it neither rises nor falls, but remains quietly near the
ground."

"Professor," remarked Joe, "did you n