"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 not say that when the sun came out bright and hot, it dried up the fog? and is not the fog the very thing that keeps the sun from coming out?" "Yes, my dear; but fogs usually gather at night, and when the sun rises in the morning, he goes to work at once to heat them up and make them disappear. But when he finds them very thick, and is hindered by cold air, he may be a good part of the day in working his way through, or he may even have to go down before he is able to show himself. Generally, however, he gets help from the wind, and then the fog goes off in a hurry." "Is there no way," asked Gus, "of knowing when the wind will spring up, and give us some clear cold weather? Ted Wynant's cousin has an ice-boat, and we are all waiting for a ride on the river."